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Sample records for plagioclase

  1. Elasticity of plagioclase feldspars

    NASA Astrophysics Data System (ADS)

    Brown, J. Michael; Angel, Ross J.; Ross, Nancy L.

    2016-02-01

    Elastic properties are reported for eight plagioclase feldspars that span compositions from albite (NaSi3AlO8) to anorthite (CaSi2Al2O8). Surface acoustic wave velocities measured using Impulsive Stimulated Light Scattering and compliance sums from high-pressure X-ray compression studies accurately determine all 21 components of the elasticity tensor for these triclinic minerals. The overall pattern of elasticity and the changes in individual elastic components with composition can be rationalized on the basis of the evolution of crystal structures and chemistry across this solid-solution join. All plagioclase feldspars have high elastic anisotropy; a* (the direction perpendicular to the b and c axes) is the softest direction by a factor of 3 in albite. From albite to anorthite the stiffness of this direction undergoes the greatest change, increasing twofold. Small discontinuities in the elastic components, inferred to occur between the three plagioclase phases with distinct symmetry (C1>¯, I1>¯, and P1>¯), appear consistent with the nature of the underlying conformation of the framework-linked tetrahedra and the associated structural changes. Measured body wave velocities of plagioclase-rich rocks, reported over the last five decades, are consistent with calculated Hill-averaged velocities using the current moduli. This confirms long-standing speculation that previously reported elastic moduli for plagioclase feldspars are systematically in error. The current results provide greater assurance that the seismic structure of the middle and lower crusts can be accurately estimated on the basis of specified mineral modes, chemistry, and fabric.

  2. Plagioclase flotation and lunar crust formation

    NASA Technical Reports Server (NTRS)

    Walker, D.; Hays, J. F.

    1977-01-01

    Anorthitic plagioclase floats in liquids parental to the lunar highlands crust. The plagioclase enrichment that is characteristic of lunar highlands rocks can be the result of plagioclase flotation. Such rocks would form a gravitationally stable upper crust on their parental magma.

  3. Plagioclase mineralogy of olivine alkaline basalt

    NASA Technical Reports Server (NTRS)

    Hoffer, J. M.

    1973-01-01

    A geological and mineralogical study of the Potrillo volcanics is reported. The investigation consisted first of field mapping to establish and identify the different rock types and volcanic features in order to determine the geological history. Next, samples were collected and analyzed petrographically to determine suitable rocks from the various stratigraphic units for study of plagioclase. Samples selected for further study were crushed and the plagioclase extracted for the determination of composition and structural state. These results were then related to the petrology and crystallization of the basalt.

  4. Effects of shock pressures on calcic plagioclase

    NASA Technical Reports Server (NTRS)

    Gibbons, R. V.; Ahrens, T. J.

    1977-01-01

    Calcic plagioclase single crystals were subjected to shock loading up to a pressure of 496 kbar; optical and electron microscope studies were conducted to investigate the shock-induced effects on the mineral, which is found in terrestrial and lunar rocks and in meteorites. It was observed that up to 287 kbar pressure, the recovered samples are essentially crystalline, while samples subjected to pressures between 300 and 400 kbar are almost 100% diaplectic glasses, suggesting shock transformation in the solid state. Samples shock-loaded to pressures greater than 400 kbar yielded glasses with refractive indices similar to those of thermally fused glass. It is concluded that planar features, absent in all the specimens, may not be definitive shock indicators, but may be linked to local heterogeneous dynamic stresses experienced by plagioclase grains within shocked rocks.

  5. Effects of shock pressures on calcic plagioclase

    NASA Technical Reports Server (NTRS)

    Gibbons, R. V.; Ahrens, T. J.

    1977-01-01

    Calcic plagioclase single crystals were subjected to shock loading up to a pressure of 496 kbar; optical and electron microscope studies were conducted to investigate the shock-induced effects on the mineral, which is found in terrestrial and lunar rocks and in meteorites. It was observed that up to 287 kbar pressure, the recovered samples are essentially crystalline, while samples subjected to pressures between 300 and 400 kbar are almost 100% diaplectic glasses, suggesting shock transformation in the solid state. Samples shock-loaded to pressures greater than 400 kbar yielded glasses with refractive indices similar to those of thermally fused glass. It is concluded that planar features, absent in all the specimens, may not be definitive shock indicators, but may be linked to local heterogeneous dynamic stresses experienced by plagioclase grains within shocked rocks.

  6. Glomerophyric plagioclase in epizonal plutons: synneusis or peritectic reaction relationship

    SciTech Connect

    Hogan, J.P.

    1985-01-01

    Glomerophyric plagioclase, a common texture in epizonal granites often attributed to synneusis, may record the peritectic liquidus relationship in the ternary feldspar system. This texture represents a crystallization process rather than a coincidental union of phenocrysts in the melt. Petrologic studies of the Northport monzogranite, Maine, yield P-T conditions of 1.5-1.0 Kb, 703/sup 0/C. Plagioclase crystallized early in the Northport pluton with dendritic, discontinuously zoned cores (An/sub 40/) and normal oscillatory zoned margins (An/sub 35/ to An/sub 10/). Magmatic resorption of the plagioclase resulted in embayment of the margins and dendritic cores. Textural evidence suggests plagioclase and alkali feldspar did not coprecipitate. Alkali feldspar crystallization was initiated concurrent with resorption of plagioclase along the peritectic portion of the liquidus boundary. A model for the formation of glomerophyric plagioclase clusters, derived from the study of the Northport pluton, suggests that a narrow halo of plagioclase like component in the surrounding melt, produced during resorption, envelops individual plagioclase phenocrysts. Overlap of these enrichment halos shifts the bulk composition of the intervening melt off the boundary line into the plagioclase field, resulting in crystallization of a thin layer of plagioclase which bonds the resorbed phenocrysts together.

  7. Uncommon behavior of plagioclase and the ancient lunar crust

    NASA Astrophysics Data System (ADS)

    Nekvasil, Hanna; Lindsley, Donald H.; DiFrancesco, Nicholas; Catalano, Tristan; Coraor, Aron E.; Charlier, Bernard

    2015-12-01

    Calcic plagioclase, the dominant mineral of the anorthositic lunar crust, fails to show the Na enrichment during cooling that is typical of magmatic plagioclase. We show that this enigmatic behavior may arise during fractionation of highly calcic plagioclase at depths greater than ~70 km in the lunar magma ocean because of the development of a negative azeotropic configuration at high anorthite contents that impedes and may even reverse the standard plagioclase albite enrichment with dropping temperature. This result supports a high-pressure origin of this plagioclase consistent with the lunar magma ocean model. It also provides a new mechanism for forming lunar lithologies with sodic plagioclase from a highly Na-depleted Moon through gravitational settling of spinel and refines the compositional characteristics of the late stage residual liquids of the lunar magma ocean.

  8. Europium anomaly in plagioclase feldspar - Experimental results and semiquantitative model.

    NASA Technical Reports Server (NTRS)

    Weill, D. F.; Drake, M. J.

    1973-01-01

    The partition of europium between plagioclase feldspar and magmatic liquid is considered in terms of the distribution coefficients for divalent and trivalent europium. A model equation is derived giving the europium anomaly in plagioclase as a function of temperature and oxygen fugacity. The model explains europium anomalies in plagioclase synthesized under controlled laboratory conditions as well as the variations of the anomaly observed in natural terrestrial and extraterrestrial igneous rocks.

  9. Petrogenesis of calcic plagioclase megacrysts in Archean rocks

    NASA Technical Reports Server (NTRS)

    Phinney, W. C.; Morrison, D. A.

    1986-01-01

    Anorthositic complexes with large equidimensional plagioclase grains of highly calcic composition occur in nearly all Archean cratons. Similar plagioclase occur as megacrysts in many Archean sills, dikes, and volcanic flows. In the Canadian Shield these units occur throughout the Archean portions of the entire shield and are particularly common as dikes over an area of a few 100,000 sq km in Ontario and Manitoba during a period of at least 100 m.y. in many different rock types and metamorphic grades. The plagioclase generally occurs in three modes: as inclusions in mafic intrusions at various stages of fractionation, as crystal segregations in anorthosite complexes, or as megacrysts in fractionated sills, dikes, and flows. Most occurrences suggest that the plagioclase was formed elsewhere before being transported to its present location. The evidence seems to be quite clear that occurrences of these types of calcic plagioclase require: (1) ponding of a relatively undifferentiated Archean tholeiitic melt at some depth; (2) isothermal crystallization of large, equidimensional homogeneous plagioclase crystals; (3) separation of the plagioclase crystals from any other crystalline phases; (4) further fractionation of melt; (5)transport of various combinations of individual plagioclase crystals and clusters of crystals by variously fractionated melts; and (6) emplacement as various types of igneous intrusions or flows.

  10. An analytical and experimental study of zoning in plagioclase

    NASA Technical Reports Server (NTRS)

    Smith, R. K.; Lofgren, G. E.

    1983-01-01

    A detailed electron microprobe study has been conducted on natural and experimentally grown zoned plagioclase feldspars. Discontinuous, sector, and oscillatory chemical zoning are observed superimposed on continuous normal or reverse zoning trends. The relative accuracy of 3 element (Na, Ca, K) microprobe traverses was found statistically to be 2 mole percent. Comparison of microprobe data on natural zoned plagioclase with zoned plagioclase grown in controlled experiments has shown that it may be possible to distinguish zonal development resulting from physio-chemical changes to the bulk magma from zoning related to local kinetic control on the growth of individual crystals.

  11. Electron microprobe study of lunar and planetary zoned plagioclase feldspars: An analytical and experimental study of zoning in plagioclase

    NASA Technical Reports Server (NTRS)

    Smith, R. K.; Lofgren, G. E.

    1982-01-01

    Natural and experimentally grown zoned plagioclase feldspars were examined by electron microprobe. The analyses revealed discontinuous, sector, and oscillary chemical zoning superimposed on continuous normal or reverse zoning trends. Postulated mechanisms for the origin of zoning are based on either physical changes external to the magma (P, T, H2O saturation) or kinetic changes internal to the magma (diffusion, supersaturation, growth rate). Comparison of microprobe data on natural zoned plagioclase with zoned plagioclase grown in controlled experiments show that it may be possible to distinguish zonal development resulting from physio-chemical changes to the bulk magma from local kinetic control on the growth of individual crystals.

  12. Partition coefficients for calcic plagioclase - Implications for Archean anorthosites

    NASA Technical Reports Server (NTRS)

    Phinney, W. C.; Morrison, D. A.

    1990-01-01

    In most Archean cratons, cumulates of equant plagioclase megacrysts form anorthositic complexes, including those at Bad Vermilion Lake (Ontario). In this paper, partition coefficients (Ds) of REEs between natural high-Ca plagioclase megacrysts and their basaltic matrices were determined, using a multiple aliquot techique, and megacrystic plagioclases occurring in anorthosites were analyzed for the same components which, in conjunction with their Ds, were applied to calculations of melts in equilibrium with anorthosites. The REE's Ds were found to agree well with experimentally determined values and to predict equilibrium melts for Archean anorthosites that agree well with coeval basaltic flows and dikes. The Ds also appear to be valid for both the tholeiitic and alkali basalts over a wide range of mg numbers and REE concentrations. It is suggested that the moderately Fe-rich tholeiites that are hosts to plagioclase megacrysts in greenstone belts form the parental melts for megacrysts which make up the Bad Vermilion Lake Archean anorthositic complex.

  13. Plagioclase-rich inclusions in carbonaceous chondrite meteorites - Liquid condensates?

    NASA Technical Reports Server (NTRS)

    Wark, D. A.

    1987-01-01

    The characteristics and formation of coarse-grained, plagioclase-rich inclusions are investigated. The textures, mineralogical compositions, and initial Al-26/Al-27 ratios for the plagioclase-rich inclusions are described. It is observed that plagioclase-rich inclusions in carbonaceous chondrites are either Ca-Al-rich inclusions (CAIs) composed of 30-60 vol pct anorthite, and less than 35 vol pct Ti-Al-pyroxene and melilite, or CA chondrites composed of plagioclase, pyroxene, olivine, spinel, and melilite. It is observed that CA chondrules are chemically and mineralogically the most similar components shared by carbonaceous and ordinary chondrites. The textural changes observed in the inclusions are examined. The data reveal that the CAIs have three textural groups: coarse anorthite laths, equigranular anorthite and Ti-Al-pyroxene, and lacy Ti-Al-pyroxene and fine-grained anorthite.

  14. Application of New Partition Coefficients to Modeling Plagioclase

    NASA Technical Reports Server (NTRS)

    Fagan, A. L.; Neal, C. R.; Rapp, J. F.; Draper, D. S.; Lapen, T. J.

    2017-01-01

    Previously, studies that determined the partition coefficient for an element, i, between plagioclase and the residual basaltic melt (Di plag) have been conducted using experimental conditions dissimilar from the Moon, and thus these values are not ideal for modeling plagioclase fractionation in a lunar system. However, recent work [1] has determined partition coefficients for plagioclase at lunar oxygen fugacities, and resulted in plagioclase with Anorthite contents =An90; these are significantly more calcic than plagioclase in previous studies, and the An content has a profound effect on partition coefficient values [2,3]. Plagioclase D-values, which are dependent on the An content of the crystal [e.g., 2-6], can be determined using published experimental data and the correlative An contents. Here, we examine new experimental data from [1] to ascertain their effect on the calculation of equilibrium liquids from Apollo 16 sample 60635,2. This sample is a coarse grained, subophitic impact melt composed of 55% plagioclase laths with An94.4-98.7 [7,8], distinctly more calcic than of previous partition coefficient studies (e.g., [3-6, 9-10]). Sample 60635,2 is notable as having several plagioclase trace element analyses containing a negative Europium anomaly (-Eu) in the rare-earth element (REE) profile, rather than the typical positive Eu anomaly (+Eu) [7-8] (Fig. 1). The expected +Eu is due to the similarity in size and charge with Ca2+, thereby allowing Eu2+ to be easily taken up by the plagioclase crystal structure, in contrast to the remaining REE3+. Some 60635,2 plagioclase crystals only have +Eu REE profiles, some only have -Eu REE profiles, and some +Eu and -Eu analyses in different areas on a single crystal [7, 8]. Moreover, there does not seem to be any core-rim association with the +Eu or -Eu analyses, nor does there appear to be a correlation between the size, shape, or location of a particular crystal within the sample and the sign of its Eu anomaly, which

  15. Modeling the re-equilibration processes between melt inclusions and their host plagioclase megacrysts in Plagioclase Ultraphyric Basalts

    NASA Astrophysics Data System (ADS)

    Drignon, M. J.; Nielsen, R. L.; Kotash, A.; Tepley, F. J., III

    2016-12-01

    Plagioclase ultraphyric basalts (PUB) are characterized by >15% plagioclase "phenocrysts". Based on isotopic data and petrologic studies, we have interpreted the crystal cargo to be disrupted troctolite cumulates (Lange et al., 2013). However, questions regarding their origins remain, including their depth of formation. CO2 contents in melt inclusions (MIs) can be used to estimate the crystallization depth; however, due to the effects of post entrapment crystallization, plagioclase hosted MIs need to be homogenized. To understand the effects of homogenization on the composition of MIs, we conducted time-series experiments to track chemical changes. Our results show that the composition of plagioclase hosted MIs is time dependent. We interpreted this dependence to be due to the relaxation of the crystal structure. This relaxation can be physically observed in olivine-hosted MIs by looking at dislocation creep mechanisms around the MIs (Schiavi et al., 2016), and will be investigated in plagioclase megacrysts by acid etching. The rate of change of the plagioclase hosted MIs from PUB suggest that the rate of relaxation in plagioclase hosted MIs is significantly slower than for olivine hosted MIs (days vs hours). A consequence of this relaxation is that pressure estimates would be minimum pressures because the size of the inclusion would increase, reducing the internal pressure and this CO2 based depth estimates. It is important to note that this process is faster in olivine than plagioclase, and that this type of relaxation would happen naturally during ascent. Another mean for estimating the original composition of MIs is to model the re-equilibration processes that back-calculates the chemistry of the MIs using the chemistry of the host. This is readily done for olivine (Danyushvesky et al., 2000; 2002) because the FeO-MgO exchange reaction is independent of temperature. Our aim is to develop such a model for plagioclase hosted MIs using a CaO-Na2O exchange reaction

  16. Linking Plagioclase Zoning Patterns to Active Magma Processes

    NASA Astrophysics Data System (ADS)

    Izbekov, P. E.; Nicolaysen, K. P.; Neill, O. K.; Shcherbakov, V.; Plechov, P.; Eichelberger, J. C.

    2015-12-01

    Plagioclase, one of the most common and abundant mineral phases in volcanic products, will vary in composition in response to changes in temperature, pressure, composition of the ambient silicate melt, and melt H2O concentration. Changes in these parameters may cause dissolution or growth of plagioclase crystals, forming characteristic textural and compositional variations (zoning patterns), the complete core-to-rim sequence of which describes events experienced by an individual crystal from its nucleation to the last moments of its growth. Plagioclase crystals in a typical volcanic rock may look drastically dissimilar despite their spatial proximity and the fact that they have erupted together. Although they shared last moments of their growth during magma ascent and eruption, their prior experiences could be very different, as plagioclase crystals often come from different domains of the same magma system. Distinguishing similar zoning patterns, correlating them across the entire population of plagioclase crystals, and linking these patterns to specific perturbations in the magmatic system may provide additional perspective on the variety, extent, and timing of magma processes at active volcanic systems. Examples of magma processes, which may be distinguished based on plagioclase zoning patterns, include (1) cooling due to heat loss, (2) heating and/or pressure build up due to an input of new magmatic material, (3) pressure drop in response to magma system depressurization, and (4) crystal transfer between different magma domains/bodies. This review will include contrasting examples of zoning patters from recent eruptions of Karymsky, Bezymianny, and Tolbachik Volcanoes in Kamchatka, Augustine and Cleveland Volcanoes in Alaska, as well as from the drilling into an active magma body at Krafla, Iceland.

  17. A REE-in-plagioclase-clinopyroxene thermometer for crustal rocks

    NASA Astrophysics Data System (ADS)

    Sun, Chenguang; Liang, Yan

    2017-04-01

    A REE-in-plagioclase-clinopyroxene thermometer has been developed on the basis of the temperature- and composition-dependent rare-earth element (REE) partitioning between coexisting plagioclase and clinopyroxene. This two-mineral exchange thermobarometer is constructed using parameters from lattice strain models for REE + Y partitioning in plagioclase and in clinopyroxene that were independently calibrated against experimentally determined mineral-melt partitioning data. An important advantage of this REE-based thermometer is that it can provide accurate temperatures through linear least-squares analysis of REE + Y as a group. Applications of the REE-in-plagioclase-clinopyroxene thermometer to volcanic and cumulate rocks show that temperatures derived from the new thermometer agree well with independently constrained magma crystallization temperatures, which adds confidence to applications of the REE-exchange thermometer to natural rocks with a wide spectrum of composition (i.e., from basalt to rhyolite). However, systematic temperature differences appear between the REE- and Mg-exchange thermometers for the volcanic and cumulate rocks. Through numerical simulations of diffusion in plagioclase-clinopyroxene systems, we demonstrate that (1) due to their slower diffusion rates, REE in minerals preferentially records crystallization or near-crystallization temperatures of the rock, and that (2) Mg is readily rest to lower temperatures for rocks from intermediately or slowly cooled magma bodies but records the initial crystallization temperatures of rocks from rapidly cooled magmas. Given their distinct diffusive responses to temperature changes, REE and Mg closure temperatures recorded by the two thermometers can be used in concert to study thermal and magmatic histories of plagioclase- and clinopyroxene-bearing rocks.

  18. High-Resolution Diffusion Chronometry of Volcanic Plagioclase Crystals

    NASA Astrophysics Data System (ADS)

    Saunders, K.; Blundy, J.; Dohmen, R.; Kilburn, M.

    2010-12-01

    Plagioclase crystals have long been recognised as an archive of magmatic evolution. Recent studies, have exploited the zoned nature of plagioclase crystals using the chemical and textural heterogeneity of zones to identify the magmatic process(es) that formed these zones, unravelling the magmatic history. Using the minor and trace chemical gradients between two adjacent crystal zones has further enhanced some of these studies through the application of diffusion models to calculate the timescales between the growth of these zones and subsequent quenching of the sample during eruption. However, the calculated timescales for plagioclase crystals from modelling of Sr, Mg and Ba are limited typically by the spatial resolution of the analytical method employed to obtain the chemical profile. For example, laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and secondary ion mass spectrometry (SIMS) spot analyses of crystals are generally in the region of 10-20 microns and this limits the timescales that can be calculated to years for most minor and trace elements. To obtain diffusion timescales in the region of months and days, the spatial resolution of the analytical technique used must have micron or sub-micron scale, respectively. NanoSIMS allows relative concentration gradients of elements to be obtained at the sub-micron scale within crystals. Quantification of secondary ion counts is challenging, however, diffusion modelling relies on the concentration gradient without the need for quantification, thus allowing us to exploit this technique. Plagioclase crystals from Mount St. Helens, USA, provide an ideal framework in which to test this method. To this end we have measured relative concentrations of Ca, Na, Si, Sr, Ba, Li, Ti, Mg and Fe in plagioclase crystals by NanoSIMS, across compositional interfaces at 300-600 nanometre resolution. Allied with the traditional techniques of scanning electron microscopy and electron probe microanalyser, this

  19. Phase transformations and exsolution in lunar and terrestrial calcic plagioclases.

    NASA Technical Reports Server (NTRS)

    Heuer, A. H.; Radcliffe, S. V.; Lally, J. S.; Christie, J. M.

    1972-01-01

    A coarse domain structure had been discovered in plagioclase in an electron microscopic study of lunar rocks reported by Christie et al. (1971). The domain structures are described in more detail, and their significance in the light of current knowledge concerning the crystal structures of the plagioclases is discussed. An exsolution texture is also described. On the basis of the scale of exsolution it appears that the Apollo 11 and 12 basalts examined cooled more rapidly than the Apollo 14 basalt, which in turn cooled more rapidly than the Stillwater rock.

  20. Formation of plagioclase-bearing peridotite and plagioclase-bearing wehrlite and gabbro suite through reactive crystallization: an experimental study

    NASA Astrophysics Data System (ADS)

    Saper, Lee; Liang, Yan

    2014-03-01

    Plagioclase-bearing peridotites are commonly associated with gabbroic rocks sampled around the Moho Transition Zone. Based on mineral chemistry, texture, and spatial relations, the formation of plagioclase-bearing peridotites has been attributed to impregnation of basalt into residual peridotites. We conducted reactive dissolution and crystallization experiments to test this hypothesis by reacting a primitive mid-ocean ridge basalt with a melt-impregnated lherzolite at 1,300 °C and 1 GPa and then cooling to 1,050 °C as pressure decreased to 0.7 GPa. Crystallization during cooling produced lithologic sequences of gabbro-wehrlite or gabbro-wehrlite-peridotite, depending on reaction time. Wehrlitic and peridotitic sections contain significant amounts of plagioclase interstitial to olivine and clinopyroxene and plagioclase compositions are spatially homogeneous. Clinopyroxene in the wehrlite-peridotite section is reprecipitated from the melt and exhibits poikilitic texture with small rounded olivine chadacrysts. Mineral composition in olivine and clinopyroxene varies spatially, both at the scale of the sample and within individual grains. Olivine grains that crystallized close to the melt-peridotite interface are enriched in iron due to their proximity to the basaltic melt reservoir. Consistent with many field studies, we observed gradual spatial variation in olivine and clinopyroxene composition across a lithologically sharp boundary between the gabbro and wehrlite-peridotite. Plagioclase compositions show no obvious dependence on distance from the melt-rock interface and were precipitated from late-stage trapped melts. Compositional trends of olivine, pyroxene, and plagioclase are consistent with previous experimental results and natural observations of the Moho Transition Zone. Different lithological sequences form based primarily on the melt-rock ratio, composition of the melt and host peridotite, and thermochemical conditions, but are expected to grade from

  1. Synthesis for Lunar Simulants: Glass, Agglutinate, Plagioclase, Breccia

    NASA Technical Reports Server (NTRS)

    Weinstein, Michael; Wilson, Stephen A.; Rickman, Douglas L.; Stoeser, Douglas

    2012-01-01

    The video describes a process for making glass for lunar regolith simulants that was developed from a patented glass-producing technology. Glass composition can be matched to simulant design and specification. Production of glass, pseudo agglutinates, plagioclase, and breccias is demonstrated. The system is capable of producing hundreds of kilograms of high quality glass and simulants per day.

  2. Porphyry copper enrichment linked to excess aluminium in plagioclase

    NASA Astrophysics Data System (ADS)

    Williamson, B. J.; Herrington, R. J.; Morris, A.

    2016-03-01

    Porphyry copper deposits provide around 75%, 50% and 20% of world copper, molybdenum and gold, respectively. The deposits are mainly centred on calc-alkaline porphyry magmatic systems in subduction zone settings. Although calc-alkaline magmas are relatively common, large porphyry copper deposits are extremely rare and increasingly difficult to discover. Here, we compile existing geochemical data for magmatic plagioclase, a dominant mineral in calc-alkaline rocks, from fertile (porphyry-associated) and barren magmatic systems worldwide, barren examples having no associated porphyry deposit. We show that plagioclase from fertile systems is distinct in containing `excess’ aluminium. This signature is clearly demonstrated in a case study carried out on plagioclase from the fertile La Paloma and Los Sulfatos copper porphyry systems in Chile. Further, the presence of concentric zones of high excess aluminium suggests its incorporation as a result of magmatic processes. As excess aluminium has been linked to high melt water contents, the concentric zones may record injections of hydrous fluid or fluid-rich melts into the sub-porphyry magma chamber. We propose that excess aluminium may exclude copper from plagioclase, so enriching the remaining melts. Furthermore, this chemical signature can be used as an exploration indicator for copper porphyry deposits.

  3. Synthesis for Lunar Simulants: Glass, Agglutinate, Plagioclase, Breccia

    NASA Technical Reports Server (NTRS)

    Weinstein, Michael; Wilson, Stephen A.; Rickman, Douglas L.; Stoeser, Douglas

    2012-01-01

    The video describes a process for making glass for lunar regolith simulants that was developed from a patented glass-producing technology. Glass composition can be matched to simulant design and specification. Production of glass, pseudo agglutinates, plagioclase, and breccias is demonstrated. The system is capable of producing hundreds of kilograms of high quality glass and simulants per day.

  4. A newly calibrated plagioclase-liquid hygrometer for rhyolites

    NASA Astrophysics Data System (ADS)

    Lange, R. A.; Waters, L.

    2012-12-01

    Rhyolite is the most differentiated silicate magma type on Earth and makes up some of the largest explosive eruptions (100-1000's km3), including those at Yellowstone and Long Valley calderas. Understanding the origin and evolution of large-volume rhyolitic magmatic systems is of considerable interest because their formation must fundamentally re-constitute and differentiate continental crust. The mineral phases in rhyolites often provide a rich opportunity to examine pre-eruptive temperatures, oxidation states, and melt water concentrations, as well as time scales for melt accumulation in the upper crust. Given the wealth of information that can be derived from various mineral phases in rhyolites, it is perhaps surprising that so few phase-equilibrium experiments exist for natural high-silica rhyolite melt compositions. The limitation in the experimental data set for rhyolites was a problem encountered by Lange et al. (2009) during their calibration of the plagioclase-liquid hygrometer. Available high-quality experiments (e.g., those with glass totals, including H2O, ≤ 97%) were restricted to those with plagioclase ≥ An37. Results from the experimental study of Couch et al. (2003) on a low-SiO2 rhyolite (71.4 wt%) were included in the Lange et al. (2009) calibration, whereas experiments on a rhyolite (75.2 wt% SiO2) from a more recent study (Tomiya et al., 2010) were published afterwards. Therefore, application of the 2009 plagioclase-liquid hygrometer to rhyolites with sodic plagioclase (plagioclase-liquid hygrometer well calibrated for rhyolites. Moreover, at fluid-saturated, shallow crustal conditions (100-250 MPa) plagioclase often saturates at higher pressures than quartz in many rhyolites, and therefore the

  5. Trace element partitioning between plagioclase and silicate melt: The importance of temperature and plagioclase composition, with implications for terrestrial and lunar magmatism

    NASA Astrophysics Data System (ADS)

    Sun, Chenguang; Graff, Michelle; Liang, Yan

    2017-06-01

    Trace element partition coefficients between anorthitic plagioclase and basaltic melts (D) have been determined experimentally at 0.6 GPa and 1350-1400 °C in a lunar high-Ti picritic glass and a mid-ocean ridge basalt (MORB). Plagioclases with 98 mol% and 86 mol% anorthite were produced in the lunar picritic melt and MORB melt, respectively. Based on the new experimental partitioning data and those selected from the literature, we developed parameterized lattice strain models for the partitioning of monovalent (Na, K, Li), divalent (Ca, Mg, Ba, Sr, Ra) and trivalent (REE and Y) cations between plagioclase and silicate melt. Through the new models we showed that the partitioning of these trace elements in plagioclase depends on temperature, pressure, and the abundances of Ca and Na in plagioclase. Particularly, Na content in plagioclase primarily controls divalent element partitioning, while temperature and Ca content in plagioclase are the dominant factors for REE partitioning in plagioclase. From these models, we also derived a new expression for DRa/DBa that can be used for Ra-Th dating on volcanic plagioclase phenocrysts, and a new model for plagioclase-melt noble gas partitioning. Applications of these partitioning models to fractional crystallization of MORB and lunar magma ocean (LMO) indicate that (1) the competing effect of temperature and plagioclase composition leads to small variations of plagioclase-melt DREE during MORB differentiation, but (2) the temperature effect is especially significant and can vary anorthite-melt DREE by over one order of magnitude during LMO solidification. Temperature and plagioclase composition have to be considered when modeling the chemical differentiation of mafic to felsic magmas involving plagioclase.

  6. Plagioclase-melt equilibria. [crystallization from magmatic liquid

    NASA Technical Reports Server (NTRS)

    Drake, M. J.

    1976-01-01

    Results of experiments investigating the crystallization of plagioclase from natural and synthetic melts are presented and are analyzed in terms of empirical and semiquantitative mixing models for the melt. Elemental partition constants were determined from the results and from other published data. Activities of the melt components were modeled by assuming that the melt consists of two independent quasi-lattices of network-forming and network-modifying components, each of which is an ideal solution of its respective component. The semiquantitative analysis supports the suggestion that Na(+) is strongly associated with tetrahedrally-coordinated Al in the melt. It is shown that it is possible to predict the composition of plagioclase crystallizing under equilibrium conditions from a dry melt of known composition and known temperature at low total pressure.

  7. Plagioclase-melt equilibria. [crystallization from magmatic liquid

    NASA Technical Reports Server (NTRS)

    Drake, M. J.

    1976-01-01

    Results of experiments investigating the crystallization of plagioclase from natural and synthetic melts are presented and are analyzed in terms of empirical and semiquantitative mixing models for the melt. Elemental partition constants were determined from the results and from other published data. Activities of the melt components were modeled by assuming that the melt consists of two independent quasi-lattices of network-forming and network-modifying components, each of which is an ideal solution of its respective component. The semiquantitative analysis supports the suggestion that Na(+) is strongly associated with tetrahedrally-coordinated Al in the melt. It is shown that it is possible to predict the composition of plagioclase crystallizing under equilibrium conditions from a dry melt of known composition and known temperature at low total pressure.

  8. Skaergaard Liquidus Temperatures and the Frailty of Plagioclase Thermometry

    NASA Astrophysics Data System (ADS)

    Morse, S. A.

    2010-12-01

    Because of its refractory nature and low diffusivity, plagioclase is the only mineral likely to record liquidus temperatures of mafic magmas. As such, it has become a talisman of such thermometry, but with limited success. Precise thermal information can perhaps best be obtained experimentally by finding the unique cotectic assemblage of all relevant mineral compositions with melt at relevant pressure. By repeating such experiments at more evolved compositions a relevant plagioclase thermometric history should be obtained. This principle of cotectic calibration was that used as a starting point by Morse (2008). By contrast, unfiltered literature results of experimental plagioclase - liquid determinations were used by Thy et al. (2009), with a T-X regression through all the data including evolved compositions, to describe the Skaergaard liquidus. Implicit in this exercise was the hypothesis that all the data represented stable equilibria and that low-melting components at static melting behaved the same as if in fractional crystallization. Tests for stable equilibrium in such a database [including that of Putirka (2005) and many others as e.g. in LEPR (lepr.ofm-research.org)] can usefully start with an examination of plagioclase loop width versus temperature or plagioclase composition. The loop width of a binary solution against either T or X is described by a slightly skewed parabola anchored at zero at both ends and rising to a broad maximum near An60 in the case of plagioclase. The peak width in XAn (Sol - Liq) has a value of 0.32 at 1 atm in Di-An-Ab and 0.24 at An-57 in a MORB fractional crystallization exercise. Values falling outside this range downward are likely to reflect metastable compositions. All of the 54 data points in one source used by Thy et al. (2009) fall below this range and are scattered to as low as 0.04. Such a shotgun scatter defines metastability and the loss of calibration. Moderate scatter at much higher values of loop width in the LEPR

  9. Oxidation state of iron in plagioclase from lunar basalts.

    NASA Technical Reports Server (NTRS)

    Hafner, S. S.; Virgo, D.; Warburton, D.

    1971-01-01

    Determination of the oxidation state of iron in the plagioclase from the coarse-grained basalts 10044 and 12021, using Mossbauer spectroscopy. The location of iron in the crystal structure was also investigated. The spectra show that iron is in the high-spin ferrous state, and they located at least two distinct positions with different coordination numbers. Some excess resonant absorption is probably due to Fe(3+), although the Fe(3+) doublet could not be positively resolved.

  10. Oxidation state of iron in plagioclase from lunar basalts.

    NASA Technical Reports Server (NTRS)

    Hafner, S. S.; Virgo, D.; Warburton, D.

    1971-01-01

    Determination of the oxidation state of iron in the plagioclase from the coarse-grained basalts 10044 and 12021, using Mossbauer spectroscopy. The location of iron in the crystal structure was also investigated. The spectra show that iron is in the high-spin ferrous state, and they located at least two distinct positions with different coordination numbers. Some excess resonant absorption is probably due to Fe(3+), although the Fe(3+) doublet could not be positively resolved.

  11. Origin of plagioclase-olivine inclusions in carbonaceous chondrites

    SciTech Connect

    Sheng, Y.J.; Hutcheon, I.D.; Wasserburg, G.J. )

    1991-02-01

    Plagioclase-Olivine Inclusions (POIs) are an abundant group of chondrule-like objects with igneous textures found in carbonaceous chondrites. POIs consist of plagioclase, olivine, pyroxene, and spinel, and cover a wide range of compositions between Type C Ca-Al-rich Inclusions (CAIs) and ferromagnesian chondrules. POIs are distinguished from CAIs by the absence of melilite, lack of refractory siderophile-rich opaque assemblages, more sodic plagioclase, and abundance of olivine and aluminousenstatite. Rare accessory minerals including armalcolite, zirconolite, rutile, and sapphirine are found in several POIs. The petrographic and chemical properties of POIs indicate that they are not condensates or evaporative residues but formed by melting or partial melting of pre-existing solids. Seven of fourteen POIs contain isotopically fractionated Mg, and despite their textures these POIs are not isotopically homogeneous. A comparison of the essential characteristics of POIs and CAIs suggests that the major processes leading to formation of POIs - including condensation, dust/gas fractionation, aggregation of chemically and isotopically disparate materials, and partial melting - are common to most CAIs and chondrules. We present a scenario for the formulation of these objects and conclude that the homogeneity of the final assemblage - CAI, POI, or chondrule - is primarily a reflection of the thermal history rather than the nature of precursor materials.

  12. Raman Study of Shock Effects in Plagioclase Feldspar from the Mistastin Lake Impact Structure, Canada

    NASA Astrophysics Data System (ADS)

    Xie, T. X.; Shieh, S. R. S.; Osinski, G. R. O.

    2016-08-01

    This study mainly uses Raman spectroscopy with a 514 nm laser to study anorthosite from Mistastin Lake Impact Crater, Canada, which mainly contains plagioclase with composition of An 28-55, to better understand shock processes in plagioclase feldspar.

  13. A New Thermodynamic Model for the Plagioclase-Liquid Hygrometer

    NASA Astrophysics Data System (ADS)

    Lange, R. A.

    2006-12-01

    A new thermodynamic model for the plagioclase-liquid hygrometer is presented, which is based on the equilibrium exchange of the An (CaAl2Si2O8) and Ab (NaAlSi3O8) components between plagioclase solid solution and magmatic liquid solution. The reaction is: CaAl2Si2O8 (plag) + NaAlSi3O8 (liq) = CaAl2Si2O8 (liq) + NaAlSi3O8 (plag). The equilibrium for this reaction as a function of temperature, pressure, and composition is calculated from: DeltaG(T,P)/RT = 0 = DeltaH°(T)/RT - DeltaS°(T)/R + lnK + intDeltaV°(P)/RT, where R is the universal gas constant and K is the equilibrium constant. In this equation, DeltaH°(T) and DeltaS°(T) are equal to the difference between the enthalpy and entropy of fusion of pure An and pure Ab, respectively, as a function of temperature. These data are obtained from the drop calorimetric experiments of Stebbins et al. (1983) and Richet and Bottinga (1984). Similarly, DeltaV°T(P) is the difference between the volume of fusion of pure An and that of pure Ab at temperature T as a function of pressure. The volumetric data for the crystalline phases are taken from Wruck et al. (1991), Berman (1988), Fei (1995), Downs et al. (1993), and Angel (2004). The volumetric data for the liquid phase are taken from Lange (1997), Kress et al. (1988) and Ai and Lange (2005). The THERMOCALC model of Powell et al. (1998) is used to calculate the activity- composition relation for crystalline plagioclase. For the liquid components, the ideal contribution to their activity terms is taken as the mole fraction of CaO for the An component and NaO0.5 for the Ab component. The ln ratio of their activity coefficient terms is equated to: aXH2O + b + cXSiO2, where a, b, and c are parameters fitted from regression against hydrous liquid-plagioclase equilibria. The fact that XH2O strongly affects the liquid activity coefficient terms for albite and anorthite forms the basis of the plagioclase hygrometer. The thermodynamic model outlined above was calibrated against

  14. Plagioclase alteration in anorthositic wall rocks surrounding eclogite facies pseudotachylites

    NASA Astrophysics Data System (ADS)

    Petley-Ragan, Arianne; Austrheim, Håkon; Dunkel, Kristina G.; Jamtveit, Bjørn

    2017-04-01

    Plagioclase within the wall rock of eclogite facies pseudotachylites contains microstructures that may be unique to deep-seated earthquakes. The Bergen Arcs of western Norway are made up of partially eclogitized granulite terranes that were exhumed from depths of 55 km in the Caledonian collision zone. Pseudotachylites found within the granulite consist of a predominant matrix of omphacite (XJd 0.23) and kyanite with grossular-rich garnet (XGrs 0.21) and local zoisite, phengite and amphibole. Plagioclase dominates the granulite mineralogy (XAn 0.38) and is partially altered along grain boundaries, planar fractures and cleavage planes. The alteration is observed as < 30 µm clusters of alkali feldspar and zoisite scattered along grain boundaries and throughout the grains, with kyanite, labradorite (XAn 0.60), oligoclase (XAn 0.20) and quartz within planar fractures, and individual zoisite needles along cleavage planes. Within the planar fractures a mosaic structure is observed with an interconnected network of labradorite separating < 5 µm polygons of oligoclase. Kyanite needles are also occasionally observed as sets of parallel lamellae that emanate from the planar fractures. These microstructures are characterized by electron backscatter diffraction. Thermodynamic modelling indicates that the wall-rock assemblage kyanite-quartz-zoisite-K-feldspar formed from more saline fluids (aH2O<0.1) than the fluids involved in the formation of phengite-bearing assemblages in the pseudotachylites. The evidence gained from detailed geochemical and microstructural analyses of plagioclase within these rocks may provide new tools for recognizing lower crustal earthquakes.

  15. Origin of strongly reversed rims on plagioclase in cumulates

    NASA Astrophysics Data System (ADS)

    Morse, S. A.; Nolan, Kathleen M.

    1984-06-01

    Narrow reversed rims on plagioclase are ubiquitous in troctolites and olivine gabbros of the Kiglapait intrusion and may be a common feature of all such cumulates. The rims occur at plag/plag, plag/ol, and less strongly at plag/aug grain boundaries. They are optically obvious at ΔAn < 10 mol.% and can reach ΔAn = 32 mol.% or more. In parallel, K/Na drops sharply. Although ubiquitous from sample to sample, the reversed rims are only locally present at grain boundaries even for the same pair of crystals in contact; they are prominent in linear networks suggesting the last trace of intercumulus liquid. A subsolidus origin is ruled out by the absence of reactants at plag/plag and plag/ol boundaries and by the local rather than pervasive development of rims. The rims are required to grow from intercumulus liquid, in which the partition of An component between crystals and liquid increases with the trapped augite component of the liquid. Calculations from published experimental data show that ΔAn > 30 can easily be achieved by such a process. It is also probable that the trapped liquid is part of an An-rich boundary layer generated by solute rejection during adcumulus growth. The ability of the rims to sustain steep K/Na gradients despite a long subsolidus cooling history proves that the K sbnd Na exchange rate is vanishingly small over a geologic time scale in An-rich feldspar, suggesting that at low K content the potassium is site-bound to the tetrahedral Al/Si distribution. Reversed rims therefore provide important information on diffusion limits as well as on the late-stage solidification history of plagioclase-rich cumulates. Moreover, they demonstrate that plagioclase geothermometry cannot be divorced from effects of liquid composition and structure as monitored, for example, by augite content.

  16. Albitization of plagioclase kinetics in the low temperature region.

    NASA Astrophysics Data System (ADS)

    Perez, R. J.; Boles, J. R.

    2004-05-01

    We propose an empirically derived kinetic model that estimates the extent of reaction for albitization of plagioclase as a function of time, reactive surface area, and saturation index in the low temperature region, from 0 to 200 C. We use a pseudo-second order kinetic formulation, consistent with rate laws and the Arrhenius equation. We derived the apparent activation energy and frequency factor by fitting the extent of albitization measured in 11 samples from the San Joaquin basin. Subsequently, we tested the model and the kinetic parameters against two independent albitization trends, one from the Texas Gulf Coast basin and one the Denver basin of Colorado. Our results indicate that albitization of detrital plagioclase fit, in all three basins, an activation energy of 70 kJ and frequency factor of 2.2E-7 mol/cm2s. The rate dependence on temperature is consistent with experimental values for albite crystal growth and with empirically derived precipitation rates of other diagenetic silicate reactions, such as illitization of smectite and quartz precipitation. The parameters and fit suggest that albitization can be modeled as a surface controlled reaction, primarily dependent on temperature.

  17. Thermal infrared spectroscopy and modeling of experimentally shocked plagioclase feldspars

    USGS Publications Warehouse

    Johnson, J. R.; Horz, F.; Staid, M.I.

    2003-01-01

    Thermal infrared emission and reflectance spectra (250-1400 cm-1; ???7???40 ??m) of experimentally shocked albite- and anorthite-rich rocks (17-56 GPa) demonstrate that plagioclase feldspars exhibit characteristic degradations in spectral features with increasing pressure. New measurements of albite (Ab98) presented here display major spectral absorptions between 1000-1250 cm-1 (8-10 ??m) (due to Si-O antisymmetric stretch motions of the silica tetrahedra) and weaker absorptions between 350-700 cm-1 (14-29 ??m) (due to Si-O-Si octahedral bending vibrations). Many of these features persist to higher pressures compared to similar features in measurements of shocked anorthite, consistent with previous thermal infrared absorption studies of shocked feldspars. A transparency feature at 855 cm-1 (11.7 ??m) observed in powdered albite spectra also degrades with increasing pressure, similar to the 830 cm-1 (12.0 ??m) transparency feature in spectra of powders of shocked anorthite. Linear deconvolution models demonstrate that combinations of common mineral and glass spectra can replicate the spectra of shocked anorthite relatively well until shock pressures of 20-25 GPa, above which model errors increase substantially, coincident with the onset of diaplectic glass formation. Albite deconvolutions exhibit higher errors overall but do not change significantly with pressure, likely because certain clay minerals selected by the model exhibit absorption features similar to those in highly shocked albite. The implication for deconvolution of thermal infrared spectra of planetary surfaces (or laboratory spectra of samples) is that the use of highly shocked anorthite spectra in end-member libraries could be helpful in identifying highly shocked calcic plagioclase feldspars.

  18. Diversity in the Visible-NIR Absorption Band Characteristics of Lunar and Asteroidal Plagioclase

    NASA Technical Reports Server (NTRS)

    Hiroi, T.; Kaiden, H.; Misawa, K.; Kojima, H.; Uemoto, K.; Ohtake, M.; Arai, T.; Sasaki, S.; Takeda, H.; Nyquist, L. E.; Shih, C.-Y.

    2012-01-01

    Studying the visible and near-infrared (VNIR) spectral properties of plagioclase has been challenging because of the difficulty in obtaining good plagioclase separates from pristine planetary materials such as meteorites and returned lunar samples. After an early study indicated that the 1.25 m band position of plagioclase spectrum might be correlated with the molar percentage of anorthite (An#) [1], there have been few studies which dealt with the band center behavior. In this study, the VNIR absorption band parameters of plagioclase samples have been derived using the modified Gaussian model (MGM) [2] following a pioneering study by [3].

  19. Infrared reflectance spectra (2.2-15 microns) of plagioclase feldspars

    NASA Technical Reports Server (NTRS)

    Nash, Douglas B.; Salisbury, John W.

    1991-01-01

    Laboratory results show that (1) the Christiansen frequency (CF) feature in mid-infrared reflectance spectra of powders can be used to accurately distinguish plagioclase composition, and (2) the wavelength position of the CF is not affected by vitrification of the plagioclase. Although the CF position does not distinguish glass from crystalline forms of plagioclase, other features (combination-tone, overtone, restrahlen bands) in the mid-IR spectra of plagioclase can be used for that purpose. These results have important implications for application of thermal emission spectroscopy to mapping the surface composition of regolith-covered planetary bodies like the moon, Mars, and asteroids.

  20. Ion microprobe mass analysis of plagioclase from 'non-mare' lunar samples

    NASA Technical Reports Server (NTRS)

    Meyer, C., Jr.; Anderson, D. H.; Bradley, J. G.

    1974-01-01

    The ion microprobe was used to measure the composition and distribution of trace elements in lunar plagioclase, and these analyses are used as criteria in determining the possible origins of some nonmare lunar samples. The Apollo 16 samples with metaclastic texture and high-bulk trace-element contents contain plagioclase clasts with extremely low trace-element contents. These plagioclase inclusions represent unequilibrated relicts of anorthositic, noritic, or troctolitic rocks that have been intermixed as a rock flour into the KREEP-rich matrix of these samples. All of the plagioclase-rich inclusions which were analyzed in the KREEP-rich Apollo 14 breccias were found to be rich in trace elements. This does not seem to be consistent with the interpretation that the Apollo 14 samples represent a pre-Imbrium regolith, because such an ancient regolith should have contained many plagioclase clasts with low trace-element contents more typical of plagioclase from the pre-Imbrium crust. Ion-microprobe analyses for Ba and Sr in large plagioclase phenocrysts in 14310 and 68415 are consistent with the bulk compositions of these rocks and with the known distribution coefficients for these elements. The distribution coefficient for Li (basaltic liquid/plagioclase) was measured to be about 2.

  1. Subsolidus REE partitioning between pyroxene and plagioclase in cumulative eucrites: An ion microprobe investigation

    SciTech Connect

    Pun, A.; Papike, J.J.; Layne, G.D.

    1997-12-01

    In an attempt to elucidate the relationship between the cumulate eucrites and the noncumulate (main series) eucrites, we have examined the trace-element systematics of plagioclase grains that coexist with inverted pigeonites of the cumulative eucrites. Analyses were done by secondary ion mass spectrometry (SIMS). Plagioclase compositions are uniform in trace-element abundances within grains from each cumulate eucrite, but are variable between the four analyzed: Moama, Moore County, Serra de Mage, and Binda. Shapes of CI-normalized trace-element abundance patterns of the plagioclase grains of the cumulates are typical of plagioclase, showing a LREE-enriched pattern with positive Eu anomalies reflecting the site preference of Eu{sup 2+} in the feldspar structure. Using trace-element concentrations of the pyroxenes (orthopyroxene, pigeonite, augite; Pun and Papike, 1995) and plagioclase (this study), we calculate the subsolidus partition coefficients (Ds) for the REEs and Y between plagioclase and pyroxens in the examined cumulate eucrites. Trace-element systematics suggest that igneous trace-element signatures have been altered by subsolidus exchange for both the plagioclase and pyroxene. Calculated hypothetical parental-melt patterns determined form pyroxene and plagioclase are different and can differ by a factor of 4-5 for individual REEs. 36 refs., 4 figs., 4 tabs.

  2. The origin of amorphous rims on lunar plagioclase grains: Solar wind damage or vapor condensates

    NASA Technical Reports Server (NTRS)

    Keller, Lindsay P.; Mckay, David S.

    1991-01-01

    A distinctive feature of micron sized plagioclase grains from mature lunar soils is a thin (20 to 100 nm) amorphous rim surrounding the grains. These rims were originally described from high voltage electron microscope observations of lunar plagioclase grains by Dran et al., who observed rims up to 100 nm thick on plagioclase grains from Apollo 11 and 12 soils. These rims are believed to be the product of solar wind damage. The amorphous rims were studied on micron sized plagioclase grains from a mature Apollo 16 soil using a JEOL 200FX transmission electron microscope equipped with an energy dispersive x ray spectrometer. It was found that the amorphous rims are compositionally distinct from the interior plagioclase and it is proposed that a major component of vapor condensates is present in the rims.

  3. Magmatic interactions as recorded in plagioclase phenocrysts of Chaos Crags, Lassen Volcanic Center, California

    USGS Publications Warehouse

    Tepley, F. J.; Davidson, J.P.; Clynne, M.A.

    1999-01-01

    The silicic lava domes of Chaos Crags in Lassen Volcanic National Park contain a suite of variably quenched, hybrid basaltic andesite magmatic inclusions. The inclusions represent thorough mixing between rhyodacite and basalt recharge liquids accompanied by some mechanical disaggregation of the inclusions resulting in crystals mixing into the rhyodacite host preserved by quenching on dome emplacement. 87Sr/86Sr ratios (~0.7037-0.7038) of the inclusions are distinctly lower than those of the host rhyodacite (~0.704-0.7041), which are used to fingerprint the origin of mineral components and to monitor the mixing and mingling process. Chemical, isotopic, and textural characteristics indicate that the inclusions are hybrid magmas formed from the mixing and undercooling of recharge basaltic magma with rhyodacitic magma. All the host magma phenocrysts (biotite, plagioclase, hornblende and quartz crystals) also occur in the inclusions, where they are rimmed by reaction products. Compositional and strontium isotopic data from cores of unresorbed plagioclase crystals in the host rhyodacite, partially resorbed plagioclase crystals enclosed within basaltic andesite inclusions, and partially resorbed plagioclase crystals in the rhyodacitic host are all similar. Rim 87Sr/86Sr ratios of the partially resorbed plagioclase crystals in both inclusions and host are lower and close to those of the whole-rock hybrid basaltic andesite values. This observation indicates that some crystals originally crystallized in the silicic host, were partially resorbed and subsequently overgrown in the hybrid basaltic andesite magma, and then some of these partially resorbed plagioclase crystals were recycled back into the host rhyodacite. Textural evidence, in the form of sieve zones and major dissolution boundaries of the resorbed plagioclase crystals, indicates immersion of crystals into a hotter, more calcic magma. The occurrence of partially resorbed plagioclase together with plagioclase

  4. Effects of changing H2O concentrations and viscosities on plagioclase crystallization in a rhyolite obsidian: experiments and plagioclase speedometry (Invited)

    NASA Astrophysics Data System (ADS)

    Waters, L.; Andrews, B. J.; Lange, R. A.

    2013-12-01

    H2O-saturated phase equilibrium and decompression experiments on a rhyolite obsidian (73 wt% SiO2) from Medicine Lake Volcano, CA demonstrate the effect of changing melt H2O concentrations and melt viscosity on plagioclase crystallization. The natural sample is saturated in plagioclase + orthopyroxene + ilmenite + magnetite + apatite + zircon, despite low phenocryst abundances (<2.3%) and no microlite crystallization. Eruptive temperature and oxygen fugacity (×1σ), on the basis of Fe-Ti oxide thermometry, are 852 × 12°C and ΔNNO +0.3 × 0.1. Plagioclase compositions range from 33-53 mol% An. Given the low crystallinity and absence of significant cooling, the progressive loss of dissolved melt H2O during ascent best explains the broad range in phenocryst composition and the low crystallinity. Phase equilibrium experiments were conducted at temperatures and pressures ranging from 750-950°C and 50-300 MPa, respectively. Experiments were conducted in a Ni-rich pressure vessel (Waspaloy) with Ni filler rod, which produces an intrinsic fO2 of ΔNNO +1 × 0.5 (Geshwind & Rutherford, 1992) and pressurized with H2O (where Ptotal= PH2O). The results of the phase equilibrium experiments show that the most anorthitic plagioclase crystallized at ~3.95 wt% H2O and the most albitic at ~3.49 wt% H2O. Plagioclase crystallization in the natural sample ceased at relatively high melt H2O content (3.49 wt%), which corresponds to a viscosity of 4.85 log10 Pa s (Hui & Zhang, 2007). To evaluate the effect of decompression rate on plagioclase crystallization, experiments were conducted on the rhyolite at two different continuous decompression rates, 3.0 MPa/hr and 0.8 MPa/hr. Two decompression experiments were conducted for each rate over two pressure interals:150 to 89 MPa and from 150 to 58 MPa. The results from our study are combined with the results of single- and multi-step decompression experiments on rhyolites/rhyodacites from Geshwind & Rutherford (1995), Couch et al., (2003

  5. Heterogeneous bubble nucleation on pyroxene and plagioclase in andesite magmas

    NASA Astrophysics Data System (ADS)

    Pleše, P.; Higgins, M.; Brun, F.; Casselman, J.; Fife, J.; Mancini, L.; Lanzafame, G.; Baker, D. R.

    2016-12-01

    Understanding bubble nucleation and growth has long been considered a key to improving our knowledge of magmatic evolution, and aiding our goal of predicting violent volcanic eruptions. The role crystals play as heterogeneous nucleation sites for bubbles has become an active area of research because they have the potential to reduce the supersaturation necessary for bubble growth. The nucleation sites of bubbles in magmas are elusive because they cannot be directly observed in natural volcanic systems. Studies are generally conducted on natural, post-eruption samples or quenched experimental charges, but both provide only a view of the final state and provide little information on how this state was achieved. We directly observed bubble nucleation and growth by 4D in-situ synchrotron X-ray tomography of bubble nucleation and growth at the Swiss Light Source. Experiments were conducted on previously prepared, hydrous, crystal-bearing andesitic melts to observe bubble nucleation and track bubble growth and movement. We collected 3D images every 0.5 s while heating hydrated melts at 1 atm. We observed that bubbles first nucleated heterogeneously at clinopyroxene/melt and near plagioclase/melt interfaces, rather than homogeneously within the melt. Heterogeneous nucleation on one oxide crystal and homogeneous nucleation within the melt occurred significantly after nucleation on the silicates. The measured bubble-crystal contact angle was not constant and decreased with time. Bubbles grew much larger than the crystals in the experiments, producing textures similar to those seen in some natural volcanic samples. Our results show that the presence of silicate phases in magmas must be taken into account when discussing bubble nucleation in magmatic systems.

  6. Visible/near-infrared spectra of experimentally shocked plagioclase feldspars

    USGS Publications Warehouse

    Johnson, J. R.; Horz, F.

    2003-01-01

    High shock pressures cause structural changes in plagioclase feldspars such as mechanical fracturing and disaggregation of the crystal lattice at submicron scales, the formation of diaplectic glass (maskelynite), and genuine melting. Past studies of visible/ near-infrared spectra of shocked feldspars demonstrated few spectral variations with pressure except for a decrease in the depth of the absorption feature near 1250-1300 nm and an overall decrease in reflectance. New visible/near-infrared spectra (400-2500 nm) of experimentally shocked (17-56 GPa) albite- and anorthite-rich rock powders demonstrate similar trends, including the loss of minor hydrated mineral bands near 1410, 1930, 2250, and 2350 nm. However, the most interesting new observations are increases in reflectance at intermediate pressures, followed by subsequent decreases in reflectance at higher pressures. The amount of internal scattering and overall sample reflectance is controlled by the relative proportions of micro-fractures, submicron grains, diaplectic glass, and melts formed during shock metamorphism. We interpret the observed reflectance increases at intermediate pressures to result from progressively larger proportions of submicron feldspar grains and diaplectic glass. The ensuing decreases in reflectance occur after diaplectic glass formation is complete and the proportion of genuine melt inclusions increases. The pressure regimes over which these reflectance variations occur differ between albite and anorthite, consistent with thermal infrared spectra of these samples and previous studies of shocked feldspars. These types of spectral variations associated with different peak shock pressures should be considered during interpretation and modeling of visible/near-infrared remotely sensed spectra of planetary and asteroidal surfaces.

  7. Trace Elements in High-Ca Pyroxene & Plagioclase in the Bilanga Diogenite: Implications for the Magmatic Evolution of Diogenites

    NASA Astrophysics Data System (ADS)

    Domanik, K. J.; Shearer, C. K.; Hagerty, J.; Kolar, S. E.; Drake, M. J.

    2003-03-01

    The trace element contents of high-Ca pyroxene and plagioclase were measured in trapped intercumulus melt and exotic clasts in the Bilanga Diogenite. Results indicate significant crystallization of orthopyroxene and plagioclase.

  8. Magma mixing, recharge and eruption histories recorded in plagioclase phenocrysts from El Chichon Volcano, Mexico

    USGS Publications Warehouse

    Tepley, F. J.; Davidson, J.P.; Tilling, R.I.; Arth, Joseph G.

    2000-01-01

    Consistent core-to-rim decreases of 87Sr/86Sr ratios and coincident increases in Sr concentrations in plagioclase phenocrysts of varying size (~ 1 cm to 2 mm) are reported from samples of the 1982 and pre-1982 (~ 200 ka) eruptions of El Chichon Volcano. Maximum 87Sr/86Sr ratios of ~ 0.7054, significantly higher than the whole-rock isotopic ratios (~ 0.7040-0.7045), are found in the cores of plagioclase phenocrysts, and minimum 87Sr/86Sr ratios of ~ 0.7039 are found near some of the rims. Plagioclase phenocrysts commonly display abrupt fluctuations in An content (up to 25 mol %) that correspond to well-developed dissolution surfaces The isotopic, textural and compositional characteristics suggest that these plagioclase phenocrysts grew in a system that was periodically recharged by higher-temperature magma with a lower 87Sr/86Sr ratio and a higher Sr concentration. Rim 87Sr/86Sr ratios in plagioclase phenocrysts of rocks from the 200 ka eruption indicate that, at that time, the magma had already attained the lowest recorded 87Sr/86Sr value of the system (~ 0.7039). In contrast, cores from plagioclase phenocrysts of the 1982 eruption, inferred to have grown in the past few thousand years, have the highest recorded 87Sr/86Sr ratios of the system. Collectively, the Sr isotopic data (for plagioclase and whole rock), disequilibrium textural features of the phenocrysts, known eruption frequencies, and inferred crystal-residence times of the plagioclases are best interpreted in terms of an intermittent magma chamber model. Similar processes, including crustal contamination, magma mixing, periodic recharge by addition of more mafic magma to induce plagioclase disequilibrium (possibly triggering eruption) and subsequent re-equilibration, apparently were operative throughout the 200 ky history of the El Chichon magma system.

  9. Plagioclase feldspars - Visible and near infrared diffuse reflectance spectra as applied to remote sensing

    NASA Technical Reports Server (NTRS)

    Adams, J. B.; Goullaud, L. H.

    1978-01-01

    Visible and near IR diffuse reflectance spectra of plagioclase feldspars are characterized by absorption features caused by minor amounts of Fe(2+) that occur bound in the crystal structure. It is found that identification of terrestrial feldspars by remote sensing appears to be feasible for the compositional range An50 to An80, providing that other minerals do not mask the feldspar signatures. Determination of plagioclase composition using the wavelength of the Fe(2+) band may be possible for lunar samples, where the plagioclase can be assumed to be more calcic than An65.

  10. The Petrographic Distinction between Basalt and Andesite Based upon the Arrested Fractionation of Plagioclase Phenocrysts.

    ERIC Educational Resources Information Center

    Garlick, G. Donald; Garlick, Benjamin J.

    1987-01-01

    Discusses the need to take into account the effects of arrested fractional crystallization in the petrographic classification of volcanic rocks containing plagioclase phenocrysts. Describes the development and use of a computer program to accomplish this task graphically. (TW)

  11. The Petrographic Distinction between Basalt and Andesite Based upon the Arrested Fractionation of Plagioclase Phenocrysts.

    ERIC Educational Resources Information Center

    Garlick, G. Donald; Garlick, Benjamin J.

    1987-01-01

    Discusses the need to take into account the effects of arrested fractional crystallization in the petrographic classification of volcanic rocks containing plagioclase phenocrysts. Describes the development and use of a computer program to accomplish this task graphically. (TW)

  12. Oxygen isotopic determinations of sequentially erupted plagioclases in the 1974 magma of Fuego Volcano, Guatemala

    USGS Publications Warehouse

    Rose, W.I.; Friedman, I.; Woodruff, L.G.

    1980-01-01

    Plagioclases in the 1974 high-Al basalt from Fuego Volcano have ??O18 values of +6.0 to +8.5 per mil. Meteoric water cannot have played a significant role in Fuego's magma. Large, weakly zone clear phenocrysts had ??O18 values in the accepted mantle range, while patchyzoned and oscillatory-zoned plagioclases inferred to have formed later and shallower levels have slightly heavier oxygen isotopic ratios. ?? 1980 Intern. Association of Volcanology and Chemistry of the Earth's Interior.

  13. Plagioclase-Rich Itokawa Grains: Space Weathering, Exposure Ages, and Comparison to Lunar Soil Grains

    NASA Technical Reports Server (NTRS)

    Keller, L. P.; Berge, E.

    2017-01-01

    Regolith grains returned by the Hayabusa mission to asteroid 25143 Itokawa provide the only samples currently available to study the interaction of chondritic asteroidal material with the space weathering environment. Several studies have documented the surface alterations observed on the regolith grains, but most of these studies involved olivine because of its abundance. Here we focus on the rarer Itokawa plagioclase grains, in order to allow comparisons between Itokawa and lunar soil plagioclase grains for which an extensive data set exists.

  14. VERY High Temperature Hydrothermal Record in Plagioclase of BLACK Gabbros in Oman Ophiolite

    NASA Astrophysics Data System (ADS)

    Boudier, F. I.; Mainprice, D.; Nicolas, A. A.

    2014-12-01

    The lower crustal section in Oman ophiolite includes 'black gabbros' that have escaped the common medium-low temperature hydrous alteration. Their plagioclases are totally fresh, but contain in their mass, nebulous inclusions most times below the resolution of optical microscope, or expressed as solid silicate phases clinopyroxene and pargasitic amphibole, up to 10 µm sized, having T equilibrium above 900°C with their host plagioclase. These gabbros have a well-expressed magmatic foliation, relayed by plastic strain marked by stretched olivine crystals, and pinching twins in plagioclase. In addition to major elements analyses, the crystallographic relationships of these Mg silicate inclusions to their host plagioclase are explored by Electron Back Scattering Diffraction (EBSD) processing. - Diopsidic clinopyroxene inclusions are dominant over pargasitic amphibole that tend to locate close to the margins of host plagioclase (Fig 1). Some inclusions are mixed clinopyroxene-amphibole, separated by a non-indexed phase that could represent a pyribole-type structure, suggesting transformation from clinopyroxene to amphibole during cooling. High chlorine content in the amphibole sign the seawater contamination at least during the development of this phase. - Preliminary statistical pole figures (Fig. 2) in the six joined plagioclase grains studied, show that both plagioclase and diopside inclusions have a strong crystal preferred orientation (CPO) connected such that the strong [010]pl maximum coincide with the strong [100]di. In addition, a coincidence appears between three sub-maxima of [100]pl and [001]di. These interesting relationships are refined. It is inferred that clinopyroxene developed through corrosion of the plagioclase by a Mg-bearing hydrous fluid, penetrating possibly via twin interface and diffusing at T~1100°C, upper limit of clinopyroxene stability in hydrous conditions. Development of pargasite implies increasing hydration during cooling.

  15. Deconvolution of mixtures with high plagioclase content for the remote interpretation of lunar plagioclase-rich regions

    NASA Astrophysics Data System (ADS)

    Serventi, Giovanna; Carli, Cristian; Sgavetti, Maria

    2016-07-01

    Anorthositic rocks are widespread on the lunar surface and have probably been formed by flotation of PL over a magma ocean. A large portion of pristine rocks are characterized by a low Mg/(Mg+Fe) ratio, and have been classified as ferroan anorthosite, and recently, after observation from SELENE Spectral Profiler,pure anorthosites regions with more than 98% PL have been recognized. In this paper, we analyze a set of mixtures with PL content similar to the ferroan anorthosites and to the pure anorthosite regions, using the Origin Software and the Modified Gaussian Model. We consider three plagioclases with varying FeOwt% contents (PL1, PL2 and PL3)andthree mafic end-members (1) 100% orthopyroxene, (2) 56% orthopyroxene and 44% clinopyroxene, and (3) 100% olivine (OL). The spectral parameters considered here are: band depth, band center, band width, c0 (the continuum intercept) and c1 (the continuum offset). Here we have shown that in pyroxene (PX)-bearing mixtures, the PX is distinguishable even in mixtures with only 1% PX and that PX band at ca. 900 nm is always deeper than PL1 band while PL2 and PL3 are deeperthan OPX 900 nm band from 95, 96% PL. In OL-bearing mixtures, OL detection limit is 2% when mixed with PL1, and 3% and 4% if mixed with PL2 and PL3. We also demonstrated how spectral parameters vary with PL%, and, generally, increasing the PL content: (1) 1250 nm band depth decreases when mixed with OL, while it deepens in mixtures with PX; (2) 1250 nm band centers generally move towards longer wavelength for PL1-bearing mixtures, while do not show significant variations considering PL2/PL3-mixtures; (3) 1250 nm band width of PL1 in E1 and E5-mixtures substantially widens while in other mixtures it only slightly varies. Here we also proposed an application to a real case, from Proclus crater, revealing how studying terrestrial analogues is fundamental to infer hypothesis on the mineralogical composition of a planetary surface, but also how the spectral

  16. Laser Raman Spectroscopic Characterization of Shocked Plagioclase from the Lonar Impact Crater, India.

    NASA Astrophysics Data System (ADS)

    Chakrabarti, R.; Basu, A. R.; Peterson, J.; Misra, S.

    2004-12-01

    We report Raman spectra of shocked plagioclase grains from the Lonar impact Crater of India. The Lonar Crater, located in the Buldana district of Maharashtra, India (19° 58'N, 76° 31'E), is an almost circular depression in the 65Ma old basalt flows of the Deccan Traps. Age estimates of this impact crater range from 10-50ka. Tektite and basalt samples were collected for this study from the rim of the crater, which is raised about 20 meters above the surrounding plains. For comparison, a Manicouagan maskelynite and an unaltered mid-oceanic ridge basalt with plagioclase laths were also analyzed. Polished thin sections of all these samples were first petrographically studied. The MORB plagioglase as well as the plagioclase from Lonar host-basalts show first order interference colors and distinct multiple lamellar twinning. The Manicouagan maskelynite is isotropic under crossed-polars. The Lonar tektite samples characteristically demonstrate spherules which are identified by their perfectly circular cross-section and isotropic nature. The spherules also contain fragments of the host basalt with plagioclase laths showing lamellar twinning. The groundmass within the spherules shows lath shaped plagioclase grains, most of which show varying degrees of isotropism due to maskelynitization. Raman scattering measurements were performed using the 514.5 nm line of an argon ion laser at an intensity of 40 kW/cm2. An inverted microscope (Nikon TE3000) with 50x objective (NA 0.55) was used for confocal imaging. A holographic notch filter removed residual laser scatter and the Raman scattering was detected by a silicon CCD at -90° C (Princeton Instruments Spec10-400R). Raman spectra were collected from ~250 cm-1 through 2000 cm-1. Raman spectra of crystalline unshocked plagioclase feldspars from the MORB and the Lonar host basalt show strongest peaks at 265 cm-1, 410 cm-1, 510 cm-1 and 1110 cm-1. The results remain the same for different points in a single grain but vary slightly

  17. Crystal size distributions of plagioclase in lavas from the July-August 2001 Mount Etna eruption

    NASA Astrophysics Data System (ADS)

    Fornaciai, Alessandro; Perinelli, Cristina; Armienti, Pietro; Favalli, Massimiliano

    2015-08-01

    During the 2001 eruption of Mount Etna, two independent vent systems simultaneously erupted two different lavas. The Upper Vents system (UV), opened between 3100 and 2650 m a.s.l., emitted products that are markedly porphyritic and rich in plagioclase, while the Lower Vents system (LV), opened at 2100 and 2550 m a.s.l., emitted products that are sparsely porphyritic with scarce plagioclase. In this study, the crystal size distributions (CSDs) of plagioclase were measured for a series of 14 samples collected from all the main flows of the 2001 eruption. The coefficient of R 2 determination was used to evaluate the goodness of fit of linear models to the CSDs, and the results are represented as a grid of R 2 values by using a numerical code developed ad hoc. R 2 diagrams suggest that the 2001 products can be separated into two main groups with slightly different characteristics: plagioclase CSDs from the UVs can be modeled by three straight lines with different slopes while the plagioclase CSDs from the LVs are largely concave. We have interpreted the CSDs of the UVs as representing three different populations of plagioclases: (i) the large phenocrysts (type I), which started to crystallize at lower cooling rate in a deep reservoir from 13 to 8 months before eruption onset; (ii) the phenocrysts (type II), which crystallized largely during continuous degassing in a shallow reservoir; and (iii) the microlites, which crystallized during magma ascent immediately prior to the eruption. The plagioclase CSD curves for the LVs lava are interpreted to reflect strong and rapid changes in undercooling induced by strong and sudden degassing.

  18. Plagioclase-Ultraphyric Basalts of the Northern Galapagos and Plume-Ridge Interaction

    NASA Astrophysics Data System (ADS)

    Barr, J. A.; Harpp, K. S.; Geist, D. L.

    2004-05-01

    The northern islands of the Galapagos Archipelago (Wolf, Darwin, Pinta, Marchena, and Genovesa) are located between 50-100 km south of the Galapagos Spreading Center (GSC) and constitute a distinct geologic province from the main island chain. One of the most notable petrologic characteristics shared by the northern islands is the predominance of basaltic flows containing up to 60% high-An plagioclase feldspar megacrysts (An 80-95; up to 3 cm long), a feature not observed in the rest of the archipelago. Lavas from Wolf, Darwin, and Genovesa Islands were examined using XRF, ICP-MS, and electron microprobe to characterize the basalts and determine the magmatic history recorded by the large plagioclase crystals. Megacrysts of high-An plagioclase, accompanied by minor clinopyroxene (<5%) and/or olivine and orthopyroxene phenocrysts, dominate 30-60% of the volume of the plagioclase ultraphyric lavas, with clinopyroxene, plagioclase, and Fe-Ti oxides comprising the groundmass. The plagioclase crystals exhibit complex oscillatory zoning, glomerocrystic textures, quench rims of lower An content than the cores, abundant melt inclusions, and inclusions of Cr-Spinel (absent on Genovesa). Thermodynamic calculations (by MELTS) suggest that the megacrysts are not in equilibrium with the groundmass, which corroborates textural evidence for a xenocrystic origin for the megacrysts. According to statistical analyses, the plagioclase megacryst compositions from each island are not significantly different from each other. Nevertheless, preliminary electron microprobe analyses of rehomogenized plagioclase-hosted melt inclusions indicate that island-specific evolutionary trends are preserved in the crystals from each volcano. Our results suggest that a common feature of the northern islands is that they all formed from high temperature Al2O3 rich basalts, conducive to the formation of significant volumes of An-rich plagioclase. The composition of the mantle source, however, differs

  19. Is plagioclase removal responsible for the negative Eu anomaly in the source regions of mare basalts

    SciTech Connect

    Shearer, C.K.; Papike, J.J. )

    1989-12-01

    The nearly ubiquitous presence of a negative Eu anomaly in the mare basalts has been suggested to indicate prior separation and flotation of plagioclase from the basalt source region during its crystallization from a lunar magma ocean (LMO). Are there any mare basalts derived from a mantle source which did not experience prior plagioclase separation Crystal chemical rationale for REE substitution in pyroxene suggests that the combination of REE size and charge, M2 site characteristics of pyroxene, fO{sub 2}, magma chemistry, and temperature may account for the negative Eu anomaly in the source region of some types of primitive, low TiO{sub 2} mare basalts. This origin for the negative Eu anomaly does not preclude the possibility of the LMO as many mare basalts still require prior plagioclase crystallization and separation and/or hybridization involving a KREEP component.

  20. Is plagioclase removal responsible for the negative Eu anomaly in the source regions of mare basalts?

    NASA Technical Reports Server (NTRS)

    Shearer, C. K.; Papike, J. J.

    1989-01-01

    The nearly ubiquitous presence of a negative Eu anomaly in the mare basalts has been suggested to indicate prior separation and flotation of plagioclase from the basalt source region during its crystallization from a lunar magma ocean (LMO). Are there any mare basalts derived from a mantle source which did not experience prior plagioclase separation? Crystal chemical rationale for REE substitution in pyroxene suggests that the combination of REE size and charge, M2 site characteristics of pyroxene, fO2, magma chemistry, and temperature may account for the negative Eu anomaly in the source region of some types of primitive, low TiO2 mare basalts. This origin for the negative Eu anomaly does not preclude the possibility of the LMO as many mare basalts still require prior plagioclase crystallization and separation and/or hybridization involving a KREEP component.

  1. SCR and GCR exposure ages of plagioclase grains from lunar soil

    NASA Technical Reports Server (NTRS)

    Etique, P.; Baur, H.; Signer, P.; Wieler, R.

    1986-01-01

    The concentrations of solar wind implanted Ar-36 in mineral grains extracted from lunar soils show that they were exposed to the solar wind on the lunar surface for an integrated time of 10E4 to 10E5 years. From the bulk soil 61501 plagioclase separates of 8 grain size ranges was prepared. The depletion of the implanted gases was achieved by etching aliquot samples of 4 grain sizes to various degrees. The experimental results pertinent to the present discussion are: The spallogenic Ne is, as in most plagioclases from lunar soils, affected by diffusive losses and of no use. The Ar-36 of solar wind origin amounts to (2030 + or - 100) x 10E-8 ccSTP/g in the 150 to 200 mm size fraction and shows that these grains were exposed to the solar wind for at least 10,000 years. The Ne-21/Ne-22 ratio of the spallogenic Ne is 0.75 + or - 0.01 and in very good agreement with the value of this ratio in a plagioclase separate from rock 76535. This rock has had a simple exposure history and its plagioclases have a chemical composition quite similar to those studied. In addition to the noble gases, the heavy particle tracks in an aliquot of the 150 to 200 mm plagioclase separate were investigated and found 92% of the grains to contain more than 10E8 tracks/sq cm. This corresponds to a mean track density of (5 + or - 1) x 10E8 tracks/sq cm. The exploration of the exposure history of the plagioclase separates from the soil 61501 do not contradict the model for the regolith dynamics but also fail to prove it.

  2. Uptaking of plagioclase xenocryst into H2O-rich rear-arc basaltic magma

    NASA Astrophysics Data System (ADS)

    Hamada, M.

    2015-12-01

    Kuritani et al. (2013, Mineral. Petrol.) and Kuritani et al. (2014, Contrib. Mineral. Petrol.) estimated genetic conditions of primary arc magmas beneath the Iwate volcano (a frontal arc volcano in the northeast Japan arc) and the Sannome-gata volcano (a rear-arc volcano in the northeast Japan arc) based on analyses of volcanic rocks and numerical simulation. They estimated that H2O concentrations of primary melts are 4-5 wt.% beneath the Iwate volcano and 6-7 wt.% beneath the Sannnome-gata volcano, respectively. Their arguments mean that primary melts beneath frontal-arc volcanoes and rear-arc volcanoes are both H2O-rich, yet there has been no direct evidence to support their arguments at the Sannnome-gata volcano because volcanic rocks are either almost aphyric and/or almost no melt inclusions were found. Hydrogen concentration in nominally anhydrous minerals serves as a hygrometer of arc basaltic melts (e.g., Hamada et al. 2013, Earth Planet. Sci. Lett.). In this study, hydrogen concentration of plagioclase as a crustal xenocryst was analyzed to estimate H2O concentration of basaltic melt coexisted with plagioclase before the eruption. Plagioclase xenocrists were separated from crushed scoria which erupted from the Sannome-gata volcano 20,000-24,000 years ago. Composition of the plagioclase core is homogeneous and ranges from An30 through An35. The rim is 150 to 200-μm-thick dusty zone whose composition is around An60, suggesting that the rim crystallized rapidly from degassed basaltic melt. The profiles of infrared absorption area per unit thickness across the plagioclase core were obtained using Fourier Transform InfraRed spectrometer (FTIR). The inner core contains hydrogen of about 60 wt. ppm H2O, and hydrogen concentration elevates at outer core. Hydrogen concentration at the outermost core of plagioclase is >200 wt. ppm H2O, suggesting that plagioclase xenocrists were taken by hydrous melt (H2O>5 wt.%; Hamada et al. 2014, Earth Planet. Sci. Lett.) and

  3. Compositional controls on spinel clouding and garnet formation in plagioclase of olivine metagabbros, Adirondack Mountains, New York

    USGS Publications Warehouse

    McLelland, J.M.; Whitney, P.R.

    1980-01-01

    Olivine metagabbros from the Adirondacks usually contain both clear and spinel-clouded plagioclase, as well as garnet. The latter occurs primarily as the outer rim of coronas surrounding olivine and pyroxene, and less commonly as lamellae or isolated grains within plagioclase. The formation of garnet and metamorphic spinel is dependent upon the anorthite content of the plagioclase. Plagioclase more sodic than An38??2 does not exhibit spinel clouding, and garnet rarely occurs in contact with plagioclase more albitic than An36??4. As a result of these compositional controls, the distribution of spinel and garnet mimics and visually enhances original igneous zoning in plagioclase. Most features of the arrangement of clear (unclouded) plagioclase, including the shells or moats of clear plagioclase which frequently occur inside the garnet rims of coronas, can be explained on the basis of igneous zoning. The form and distribution of the clear zones may also be affected by the metamorphic reactions which have produced the coronas, and by redistribution of plagioclase in response to local volume changes during metamorphism. ?? 1980 Springer-Verlag.

  4. Iron in Plagioclase: Synthesis Experiments with Applications to Lunar Reflectance Spectroscopy

    NASA Astrophysics Data System (ADS)

    Cheek, L.; Parman, S. W.; Pieters, C. M.

    2010-12-01

    Plagioclase is the most abundant mineral on the Moon’s surface, comprising >90% of the anorthositic rocks that dominate the crust as primary crystallization products of the lunar magma ocean. Understanding the compositional variability of plagioclase across the lunar surface may inform various aspects of the Moon’s evolution, such as the extent of homogeneity of magma ocean crystallization, or the effect of low-grade metamorphism on mineral chemistry during prolonged cooling. Information about plagioclase compositions can be obtained remotely from near-infrared reflectance (NIR) data, which is sensitive to the particular coordination environment of Fe2+ cations in the mineral structure. As high spatial and spectral resolution NIR data have only recently begun to come available for the Moon, robust laboratory characterizations of the compositional controls on the optical properties of plagioclase are now necessary. Here we present preliminary results of experiments to synthesize various plagioclase compositions under lunar conditions. Powdered synthetic anorthite and albite glasses were used as endmember starting materials, and mixed in varying proportions with controlled amounts of Fe2O3. The mixtures were homogenized by melting at 1550C in a platinum capsule, and quenched in air after 2 hours. Samples were then sintered at 1400C for at least 24 hours in a gas-mixing furnace (CO/CO2) at a pO2~2^-10. Electron microprobe analyses confirm that the anorthite and albite starting endmembers can be combined and homogenized with sufficient accuracy to generate well-controlled plagioclase compositions, and that the sintering process allows the Fe2+ to be incorporated into the plagioclase homogenously. No contaminating phases have been observed. While previous laboratory and remotely-sensed NIR reflectance data typically show only one prominent Fe2+ absorption band (near 1250 nm) [e.g. Adams and Goullaud, 1978], the spectra of samples presented here display an additional

  5. High-temperature hydrothermal alteration of tje Boehls Butte anorthosite: Origin of a bimodal plagioclase assemblage

    SciTech Connect

    Mora, Claudia I; Riciputi, Lee R; Cole, David; Walker, Karen

    2008-01-01

    The Boehls Butte anorthosite consists predominantly of an unusual bimodal assemblage of andesine and bytownite anorthite. Oxygen isotope compositions of the anorthosite were profoundly altered by high temperature, retrograde interaction with meteorichydrothermal fluids that varied in composition from isotopically evolved to nearly pristine meteoric water. Oxygen isotope ratios of bulk plagioclase separates are in the range ?7.0 to -6.2% V-SMOW, however, secondary ion mass spectrometry indicates spot-sized isotope values as low as -16%. Typical inter- and intra-plagioclase grain variability is 3 6%, and extreme heterogeneity of up to 20%is noted in a few samples. High-temperature hydrothermal alteration of intermediate plagioclase is proposed to explain the origin of bytownite anorthite in the anorthosite and creation of its unusual bimodal plagioclase assemblage. The anorthite-forming reaction created retrograde reaction-enhanced permeability which, together with rapid decompression, extension, and unroofing of the anorthosite complex, helped to accommodated influx of significant volumes of meteoric-hydrothermal fluids into the anorthosite.

  6. Shock effects in plagioclase feldspar from the Mistastin Lake impact structure, Canada

    NASA Astrophysics Data System (ADS)

    Pickersgill, Annemarie E.; Osinski, Gordon R.; Flemming, Roberta L.

    2015-09-01

    Shock metamorphism, caused by hypervelocity impact, is a poorly understood process in feldspar due to the complexity of the crystal structure, the relative ease of weathering, and chemical variations, making optical studies of shocked feldspars challenging. Understanding shock metamorphism in feldspars, and plagioclase in particular, is vital for understanding the history of Earth's moon, Mars, and many other planetary bodies. We present here a comprehensive study of shock effects in andesine and labradorite from the Mistastin Lake impact structure, Labrador, Canada. Samples from a range of different settings were studied, from in situ central uplift materials to clasts from various breccias and impact melt rocks. Evidence of shock metamorphism includes undulose extinction, offset twins, kinked twins, alternate twin deformation, and partial to complete transformation to diaplectic plagioclase glass. In some cases, isotropization of alternating twin lamellae was observed. Planar deformation features (PDFs) are notably absent in the plagioclase, even when present in neighboring quartz grains. It is notable that various microlites, twin planes, and compositionally different lamellae could easily be mistaken for PDFs and so care must be taken. A pseudomorphous zeolite phase (levyne-Ca) was identified as a replacement mineral of diaplectic feldspar glass in some samples, which could, in some instances, also be potentially mistaken for PDFs. We suggest that the lack of PDFs in plagioclase could be due to a combination of structural controls relating to the crystal structure of different feldspars and/or the presence of existing planes of weakness in the form of twin and cleavage planes.

  7. FeO and MgO in plagioclase of lunar anorthosites: Igneous or metamorphic?

    NASA Technical Reports Server (NTRS)

    Phinney, W. C.

    1994-01-01

    The combined evidence from terrestrial anorthosites and experimental laboratory studies strongly implies that lunar anorthosites have been subjected to high-grade metamorphic events that have erased the igneous signatures of FeO and MgO in their plagioclases. Arguments to the contrary have, to this point, been more hopeful than rigorous.

  8. Effects of Plagioclase Chemistry and Modal Abundance on Spectral Properties of Multimineral Fe,Mg Mixtures

    NASA Astrophysics Data System (ADS)

    Serventi, G.; Carli, C.; Sgavetti, M.; Pompilio, L.

    2012-03-01

    In this abstract we show plagioclase effects on three different Fe,Mg mixtures. The spectra of these mixtures were analyzed via decomposition with an EGO algorithm in order to determine band spectral parameters, particularly in the 1.2-μm region.

  9. Spectral variability of plagioclase-mafic mixtures (3): Quantitative analysis applying the MGM algorithm

    NASA Astrophysics Data System (ADS)

    Serventi, Giovanna; Carli, Cristian; Sgavetti, Maria

    2015-07-01

    Among the techniques to detect planet's mineralogical composition remote sensing, visible and near-infrared (VNIR) reflectance spectroscopy is a powerful tool, because crystal field absorption bands are related to particular transitional metals in well-defined crystal structures, e.g., Fe2+ in M1 and M2 sites of olivine (OL) or pyroxene (PX). Although OL, PX and their mixtures have been widely studied, plagioclase (PL), considered a spectroscopically transparent mineral, has been poorly analyzed. In this work we quantitatively investigate the influence of plagioclase absorption band on the absorption bands of Fe, Mg minerals using the Modified Gaussian Model - MGM (Sunshine, J.M. et al. [1990]. J. Geophys. Res. 95, 6955-6966). We consider three plagioclase compositions of varying FeO wt.% contents and five mafic end-members (1) 56% orthopyroxene and 44% clinopyroxene, (2) 28% olivine and 72% orthopyroxene, (3) 30% orthopyroxene and 70% olivine, (4) 100% olivine and (5) 100% orthopyroxene, at two different particle sizes. The spectral parameters considered here are: band depth, band center, band width, c0 (the continuum intercept) and c1 (the continuum offset). In particular, we show the variation of the plagioclase and composite (plagioclase-olivine) band spectral parameters versus the volumetric iron content related to the plagioclase abundance in mixtures. Generally, increasing the vol. FeO% due to the PL: (1) 1250 nm band deepens with linear trend in mixtures with pyroxenes, while it decreases in mixtures with olivine, with trend shifting from parabolic to linear increasing the olivine content in end-member; (2) 1250 nm band center moves towards longer wavelengths with linear trend in pyroxene-rich mixtures and parabolic trend in olivine-rich mixtures; and (3) 1250 nm band clearly widens with linear trend in olivine-free mixtures, while the widening is only slight in olivine-rich mixtures. We also outline how spectral parameters can be ambiguous leading to an

  10. Role for syn-eruptive plagioclase disequilibrium crystallisation in basaltic magma ascent dynamics

    NASA Astrophysics Data System (ADS)

    La Spina, Giuseppe; Burton, Mike; de'Michieli Vitturi, Mattia; Arzilli, Fabio

    2017-04-01

    Magma ascent dynamics in volcanic conduits play a key role in determining the eruptive style of a volcano. The lack of direct observations inside the conduit means that numerical conduit models, constrained with observational data, provide invaluable tools for quantitative insights into complex magma ascent dynamics. The highly nonlinear, interdependent processes involved in magma ascent dynamics require several simplifications when modelling their ascent. For example, timescales of magma ascent in conduit models are typically assumed to be much longer than crystallisation and gas exsolution for basaltic eruptions. However, it is now recognized that basaltic magmas may rise fast enough for disequilibrium processes to play a key role on the ascent dynamics. The quantification of the characteristic times for crystallisation and exsolution processes are fundamental to our understanding of such disequilibria and ascent dynamics. Using observations from Mount Etna's 2001 eruption and a magma ascent model we are able to constrain timescales for crystallisation and exsolution processes. Our results show that plagioclase reaches equilibrium in 1-2 h, whereas ascent times were 1 h. Furthermore, we have related the amount of plagioclase in erupted products with the ascent dynamics of basaltic eruptions. We find that relatively high plagioclase content requires crystallisation in a shallow reservoir, whilst a low plagioclase content reflects a disequilibrium crystallisation occurring during a fast ascent from depth to the surface. Using these new constraints on disequilibrium plagioclase crystallisation we also reproduce observed crystal abundances for different basaltic eruptions: Etna 2002/2003, Stromboli 2007 (effusive eruption) and 1930 (paroxysm) and different Pu'u' O'o eruptions at Kilauea (episodes 49-53). Therefore, our results show that disequilibrium processes play a key role on the ascent dynamics of basaltic magmas and cannot be neglected when describing basaltic

  11. Plagioclase preferred orientation and induced seismic anisotropy in mafic igneous rocks

    NASA Astrophysics Data System (ADS)

    Ji, Shaocheng; Shao, Tongbin; Salisbury, Matthew H.; Sun, Shengsi; Michibayashi, Katsuyoshi; Zhao, Weihua; Long, Changxing; Liang, Fenghua; Satsukawa, Takako

    2014-11-01

    Fractional crystallization and crystal segregation controlled by settling or floating of minerals during the cooling of magma can lead to layered structures in mafic and ultramafic intrusions in continental and oceanic settings in the lower crust. Thus, the seismic properties and fabrics of layered intrusions must be calibrated to gain insight into the origin of seismic reflections and anisotropy in the deep crust. To this end, we have measured P and S wave velocities and anisotropy in 17 plagioclase-rich mafic igneous rocks such as anorthosite and gabbro at hydrostatic pressures up to 650 MPa. Anorthosites and gabbroic anorthosites containing >80 vol% plagioclase and gabbros consisting of nearly equal modal contents of plagioclase and pyroxene display distinctive seismic anisotropy patterns: Vp(Z)/Vp(Y) ≥ 1 and Vp(Z)/Vp(X) ≥ 1 for anorthosites while 0.8 < Vp(Z)/Vp(Y) ≤ 1 and 0.8 < Vp(Z)/Vp(X) ≤ 1 for gabbros. Amphibolites lie in the same domain as gabbros, but show a significantly stronger tendency of Vp(X) > Vp(Y) than the gabbros. Laminated anorthosites with Vp(X) ≈ Vp(Y) ≪ Vp(Z) display a strong crystal preferred orientation (CPO) of plagioclase whose (010) planes and [100] and [001] directions parallel to the foliation. For the gabbros and amphibolites characterized by Vp(X) ≈ Vp(Y) > Vp(Z) and Vp(X) > Vp(Y) > Vp(Z), respectively, pyroxene and amphibole play a dominant role over plagioclase in the formation of seismic anisotropy. The Poisson's ratio calculated using the average P and S wave velocities from the three principal propagation-polarization directions (X, Y, and Z) of a highly anisotropic anorthosite cannot represent the value of a true isotropic equivalent. The CPO-induced anisotropy enhances and decreases the foliation-normal incidence reflectivity at gabbro-peridotite and anorthosite-peridotite interfaces, respectively.

  12. Volatile and light lithophile elements in high-anorthite plagioclase-hosted melt inclusions from Iceland

    NASA Astrophysics Data System (ADS)

    Neave, David A.; Hartley, Margaret E.; Maclennan, John; Edmonds, Marie; Thordarson, Thorvaldur

    2017-05-01

    Melt inclusions formed during the early stages of magmatic evolution trap primitive melt compositions and enable the volatile contents of primary melts and the mantle to be estimated. However, the syn- and post-entrapment behaviour of volatiles in primitive high-anorthite plagioclase-hosted melt inclusions from oceanic basalts remains poorly constrained. To address this deficit, we present volatile and light lithophile element analyses from a well-characterised suite of nine matrix glasses and 102 melt inclusions from the 10 ka Grímsvötn tephra series (i.e., Saksunarvatn ash) of Iceland's Eastern Volcanic Zone (EVZ). High matrix glass H2O and S contents indicate that eruption-related exsolution was arrested by quenching in a phreatomagmatic setting; Li, B, F and Cl did not exsolve during eruption. The almost uniformly low CO2 content of plagioclase-hosted melt inclusions cannot be explained by either shallow entrapment or the sequestration of CO2 into shrinkage bubbles, suggesting that inclusion CO2 contents were controlled by decrepitation instead. High H2O/Ce values in primitive plagioclase-hosted inclusions (182-823) generally exceed values expected for EVZ primary melts (∼ 180), and can be accounted for by diffusive H2O gain following the entrainment of primitive macrocrysts into evolved and H2O-rich melts a few days before eruption. A strong positive correlation between H2O and Li in plagioclase-hosted inclusions suggests that diffusive Li gain may also have occurred. Extreme F enrichments in primitive plagioclase-hosted inclusions (F/Nd = 51-216 versus ∼15 in matrix glasses) possibly reflect the entrapment of inclusions from high-Al/(Al + Si) melt pools formed by dissolution-crystallisation processes (as indicated by HFSE depletions in some inclusions), and into which F was concentrated by uphill diffusion since F is highly soluble in Al-rich melts. The high S/Dy of primitive inclusions (∼300) indicates that primary melts were S-rich in comparison

  13. Plagioclase-melt (dis)equilibrium due to cooling dynamics: Implications for thermometry, barometry and hygrometry

    NASA Astrophysics Data System (ADS)

    Mollo, Silvio; Putirka, Keith; Iezzi, Gianluca; Del Gaudio, Pierdomenico; Scarlato, Piergiorgio

    2011-07-01

    The compositional variation of plagioclase and the partitioning of major elements between plagioclase and melt have been experimentally measured as a function of the cooling rate. Crystals were grown from a basaltic melt at a pressure of 500 MPa under (i) variable cooling rates of 0.5, 2.1, 3, 9.4, and 15 °C/min from 1250 °C down to 1000 °C, (ii) quenching temperatures of 1025, 1050, 1075, 1090, and 1100 °C at the fixed cooling rate of 0.5 °C/min, and (iii) isothermal temperatures of 1000, 1025, 1050, 1075, 1090, and 1100 °C. Our results show that euhedral, faceted plagioclases form during isothermal and slower cooling experiments exhibiting idiomorphic tabular shapes. In contrast, dendritic shapes are observed from faster cooled charges. As the cooling rate is increased, concentrations of Al + Ca + Fe + Mg increase and Si + Na + K decrease in plagioclase favoring higher An and lower Ab + Or contents. Significant variations of pl-liqKd are also observed by the comparison between isothermal and cooled charges; notably, pl-liqKdAb-An, pl-liqKdCa-Na and pl-liqKdFe-Mg progressively change with increasing cooling rate. Therefore, crystal-melt exchange reactions have the potential to reveal the departure from equilibrium for plagioclase-bearing cooling magmas. Finally, thermometers, barometers, and hygrometers derived through the plagioclase-liquid equilibria have been tested at these non-equilibrium experimental conditions. Since such models are based on assumption of equilibrium, any form of disequilibrium will yield errors. Results show that errors on estimates of temperature, pressure, and melt-water content increase systematically with increasing cooling rate (i.e. disequilibrium condition) depicting monotonic trends towards drastic overestimates. These trends are perfectly correlated with those of pl-liqKdCa-Na, pl-liqKdAb-An, and pl-liqKdFe-Mg, thus demonstrating their ability to test (dis)equilibrium conditions.

  14. Role of syn-eruptive plagioclase disequilibrium crystallization in basaltic magma ascent dynamics.

    PubMed

    La Spina, G; Burton, M; De' Michieli Vitturi, M; Arzilli, F

    2016-12-12

    Timescales of magma ascent in conduit models are typically assumed to be much longer than crystallization and gas exsolution for basaltic eruptions. However, it is now recognized that basaltic magmas may rise fast enough for disequilibrium processes to play a key role on the ascent dynamics. The quantification of the characteristic times for crystallization and exsolution processes are fundamental to our understanding of such disequilibria and ascent dynamics. Here we use observations from Mount Etna's 2001 eruption and a magma ascent model to constrain timescales for crystallization and exsolution processes. Our results show that plagioclase reaches equilibrium in 1-2 h, whereas ascent times were <1 h. Using these new constraints on disequilibrium plagioclase crystallization we also reproduce observed crystal abundances for different basaltic eruptions. The strong relation between magma ascent rate and disequilibrium crystallization and exsolution plays a key role in controlling eruption dynamics in basaltic volcanism.

  15. Role of syn-eruptive plagioclase disequilibrium crystallization in basaltic magma ascent dynamics

    NASA Astrophysics Data System (ADS)

    La Spina, G.; Burton, M.; de'Michieli Vitturi, M.; Arzilli, F.

    2016-12-01

    Timescales of magma ascent in conduit models are typically assumed to be much longer than crystallization and gas exsolution for basaltic eruptions. However, it is now recognized that basaltic magmas may rise fast enough for disequilibrium processes to play a key role on the ascent dynamics. The quantification of the characteristic times for crystallization and exsolution processes are fundamental to our understanding of such disequilibria and ascent dynamics. Here we use observations from Mount Etna's 2001 eruption and a magma ascent model to constrain timescales for crystallization and exsolution processes. Our results show that plagioclase reaches equilibrium in 1-2 h, whereas ascent times were <1 h. Using these new constraints on disequilibrium plagioclase crystallization we also reproduce observed crystal abundances for different basaltic eruptions. The strong relation between magma ascent rate and disequilibrium crystallization and exsolution plays a key role in controlling eruption dynamics in basaltic volcanism.

  16. Role of syn-eruptive plagioclase disequilibrium crystallization in basaltic magma ascent dynamics

    PubMed Central

    La Spina, G.; Burton, M.; de' Michieli Vitturi, M.; Arzilli, F.

    2016-01-01

    Timescales of magma ascent in conduit models are typically assumed to be much longer than crystallization and gas exsolution for basaltic eruptions. However, it is now recognized that basaltic magmas may rise fast enough for disequilibrium processes to play a key role on the ascent dynamics. The quantification of the characteristic times for crystallization and exsolution processes are fundamental to our understanding of such disequilibria and ascent dynamics. Here we use observations from Mount Etna's 2001 eruption and a magma ascent model to constrain timescales for crystallization and exsolution processes. Our results show that plagioclase reaches equilibrium in 1–2 h, whereas ascent times were <1 h. Using these new constraints on disequilibrium plagioclase crystallization we also reproduce observed crystal abundances for different basaltic eruptions. The strong relation between magma ascent rate and disequilibrium crystallization and exsolution plays a key role in controlling eruption dynamics in basaltic volcanism. PMID:27941750

  17. Orthopyroxene-plagioclase fragments in the lunar soil from apollo 12.

    PubMed

    Fuchs, L H

    1970-08-28

    Rock fragments consisting of orthopyroxene-calcic plagioclase assemblages appear to be more common in Apollo 12 soil samples than in the breccias or soil from Apollo 11 and are mineralogically and chemically different from any of the crystalline rocks returned by either Apollo 11 or Apollo 12. Compositionally, these fragments are orthopyroxenites and feldspathic orthopyroxenites. They are probably not fragments of meteorites; other considerations point to a near-surface lunar origin.

  18. Anorthosites and anorthosites: Contrasting plagioclase-rich rocks in the Archaean and Proterozoic

    SciTech Connect

    Owens, B.E. . Dept. of Earth Planetary Sciences)

    1993-03-01

    Anorthosites -- rocks consisting predominantly of plagioclase feldspar -- have figured prominently in at least two distinct intervals of Earth history: the late-Archaean and mid-Proterozoic. Archaean anorthosites (AA) are a key component of high-grade gneiss terranes, where they typically form laterally extensive deformed sheets or sills up to a km thick. In contrast, Proterozoic anorthosites (PA) form plutons or plutonic complexes, and are most abundant in a quasi-continuous belt across NE N. America. In addition to these temporal and structural contrasts, AA and PA display markedly different mineralogical and geochemical properties, including, respectively: (1) equant plagioclase megacrysts vs. tabular megacrysts; (2) highly calcic compositions vs. intermediate to alkalic compositions; (3) amphibole vs. olivine or orthopyroxene as the dominant mafic mineral; (4) the presence of chromite, locally in ore-grade layers vs. massive, cross-cutting Fe-Ti oxide ores; (5) low levels of Sr and Ba vs. high to extreme levels; (6) high levels of ferromagnesian trace elements vs. low levels; (7) Ga/Al values typical of basaltic plagioclase vs. much lower values; and (8) moderately vs. extremely fractionated REE patterns. Given these contrasts, it appears that the only property AA and PA share is their plag-rich nature, suggesting that there must be more than one process (and probably multiple processes) capable of producing anorthosite.

  19. Recycled oceanic crust observed in 'ghost plagioclase' within the source of Mauna Loa lavas

    PubMed

    Sobolev; Hofmann; Nikogosian

    2000-04-27

    The hypothesis that mantle plumes contain recycled oceanic crust is now widely accepted. Some specific source components of the Hawaiian plume have been inferred to represent recycled oceanic basalts, pelagic sediments or oceanic gabbros. Bulk lava compositions, however, retain the specific trace-element fingerprint of the original crustal component in only a highly attenuated form. Here we report the discovery of exotic, strontium-enriched melt inclusions in Mauna Loa olivines. Their complete trace-element patterns strongly resemble those of layered gabbros found in ophiolites, which are characterized by cumulus plagioclase with very high strontium abundances. The major-element compositions of these melts indicate that their composition cannot be the result of the assimilation of present-day oceanic crust through which the melts have travelled. Instead, the gabbro has been transformed into a (high-pressure) eclogite by subduction and recycling, and this eclogite has then been incorporated into the Hawaiian mantle plume. The trace-element signature of the original plagioclase is present only as a 'ghost' signature, which permits specific identification of the recycled rock type. The 'ghost plagioclase' trace-element signature demonstrates that the former gabbro can retain much of its original chemical identity through the convective cycle without completely mixing with other portions of the former oceanic crust.

  20. Trace Elements in High-Ca Pyroxene and Plagioclase in the Bilanga Diogenite: Implications for the Magmatic Evolution of Diogenites

    NASA Technical Reports Server (NTRS)

    Domanik, K. J.; Shearer, C. K.; Hagerty, J.; Kolar, S. E.; Drake, M. J.

    2003-01-01

    High-Ca pyroxene and plagioclase are typically present as minor phases in diogenites. However, although the trace element content of diogenite orthopyroxene has been measured in a number of studies; almost no trace element data is available for the high-Ca pyroxene and plagioclase with which it routinely coexists in these meteorites. These data could provide insights into the nature and evolution of the melts from which diogenites crystallized in the HED parent body. In this study we have obtained initial measurements of several REEs in high-Ca pyroxene, plagioclase, and orthopyroxene in the Bilanga. Measurement of additional incompatible trace element concentrations in these phases is currently in progress.

  1. Strontium isotope ratio variations in plagioclase phenocrysts from the Imnaha basalt

    NASA Astrophysics Data System (ADS)

    Eckberg, A. E.; Wolff, J. A.; Ramos, F. C.; Hart, G. L.

    2006-12-01

    Of all the Columbia River flood basalt formations, the Imnaha Basalt best captures the isotopic signature of the mantle plume source (see abstract by Wolff, Ramos and Patterson). It is of interest, therefore, to understand the extent to which the plume signature is compromised by magma-lithosphere interaction during transport to the surface. Whole rock 87Sr/86Sr ratios of Imnaha lavas span a narrow range of 0.7038 - 0.7042, and convey a false impression of relative isotopic uniformity. Plagioclase phenocrysts analyzed by LA-MC- ICPMS exhibit much greater variations of 0.7038 - 0.7061 for the whole formation as represented by 8 flows that we have studied. Individual plagioclase phenocrysts in the Imnaha lavas typically fall into two Sr isotope ratio populations: 0.7038 - 0.7044, and 0.7045 - 0.7051. The former population, which encompasses the range of 87Sr/86Sr in whole rocks, is more abundant among a subgroup of flows that have long been recognized as more primitive (Hooper et al., 1984), and is dominated by tabular phenocrysts, some of which are zoned with less radiogenic cores and more radiogenic rims. The less radiogenic end of this range is considered to be representative of the mantle source for the Imnaha Basalt. The latter more radiogenic group of plagioclase grains is texturally more diverse, and is more abundant in flows with relatively evolved bulk compositions. Relatively radiogenic Sr (87Sr/86Sr >0.7044) among Columbia River basalts (whole rocks) is characteristic of post-Imnaha formations, including the very voluminous Grande Ronde flows, and is thought to reflect the involvement of lithospheric components derived from crust and/or mantle. Four of the 8 Imnaha flows so far investigated contain both populations of plagioclase. These results are consistent with a model where magmas residing in crustal magma chambers experience interaction with high- 87Sr/86Sr host rock before and/or during crystallization of plagioclase and are recharged by mantle

  2. Prediction of plagioclase-melt equilibria in anhydrous silicate melts at 1-atm

    NASA Astrophysics Data System (ADS)

    Namur, Olivier; Charlier, Bernard; Toplis, Michael J.; Vander Auwera, Jacqueline

    2012-01-01

    Many models for plagioclase-melt equilibria have been proposed over the past 30 years, but the focus is increasingly on the effects of water content and pressure. However, many geological and petrological applications concern low pressure and low water systems, such as the differentiation of large terrestrial basaltic magma chambers, and lunar and asteroidal magmatism. There is, therefore, a justified need to quantify the influence of anhydrous liquid composition on the composition of equilibrium plagioclase at 1-atm. With this in mind, a database of over 500 experimentally determined plagioclase-liquid pairs has been created. The selected low pressure, anhydrous, experiments include both natural and synthetic liquids, whose compositions range from basalt to rhyolite. Four equations are proposed, derived from this data. The first is based on a thermodynamically inspired formalism, explicitly integrating the effect of temperature. This equation uses free energies and activities of crystalline anorthite available from the literature. For the activity of anorthite in the liquid phase, it is found that current models of the activity of individual oxides are insufficient to account for the experimental results. We have therefore derived an empirical expression for the variation of anorthite activity in the liquid as a function of melt composition, based upon inversion of the experimental data. Using this expression allows the calculation of plagioclase composition with a relative error less than 10%. However, in light of the fact that temperature is not necessarily known for many petrological applications, an alternative set of T-independent equations is also proposed. For this entirely empirical approach, the database has been divided into three compositional groups, treated independently for regression purposes: mafic-ultramafic, alkali-rich mafic-ultramafic, and intermediate-felsic. This separation into distinct subgroups was found to be necessary to maintain errors

  3. Mantle Melting in the Plagioclase-Spinel Transition Zone; Reconciling Experiments and Thermodynamic Models

    NASA Astrophysics Data System (ADS)

    Smith, P. M.; Asimow, P. D.

    2003-12-01

    The slope of the solidus of plagioclase and spinel bearing lherzolite is important as it controls mantle melting behaviour during isentropic decompression. The invariant point in CaO-MgO-Al2}O{3}-SiO_{2 (CMAS), where the reaction of plagioclase lherzolite to spinel lherzolite intersects the solidus, expands to form a divariant surface in CaO-MgO-Al2}O{3}-SiO_{2}-Na_{2O-FeO (CMASNF). Experimental results in two five component systems, CMASN and CMASF, suggest that the temperature of the surface increases slightly with pressure, so that the plagioclase-spinel transition interval could be the site of enhanced melting consistent with seismic observations (Presnall et al., 2000 and references therein). In contrast, calculations using the MELTS (Ghiorso & Sack, 1995) and pMELTS (Ghiorso, 1998) algorithms predict a strong or weak temperature drop respectively as pressure is increased, giving a pronounced cusp in the solidus where melts would freeze (Asimow et al., 1995). We are currently using an integrated experimental and theoretical approach to explore the equilibrium between plagioclase, spinel, olivine, orthopyroxene, clinopyroxene and liquid to see if these results may be reconciled. Piston-cylinder experiments are being carried out in CMASNF and sub-systems. Using the parameterisation of Walter & Presnall (1994) we have chosen a suitable bulk composition in CMASN that should intersect the univariant melting reaction over a range of pressures (11-15 kbar) whilst maximizing the modal proportion of each phase above and below the reaction. To account for interlaboratory differences in pressure-calibration we are redetermining the CMAS invariant point. We are also developing calibration software suitable for a new thermodynamic model of peridotite melting. The CMASNF system is outside the calibrated composition ranges of MELTS and pMELTS, which were constructed exclusively from natural system data. By using experiments from CMAS, CMASN and CMASF in our calibration

  4. Time and temperature dependence of the re-equilibration processes in plagioclase-hosted melt inclusions

    NASA Astrophysics Data System (ADS)

    Drignon, M. J.; Nielsen, R. L.; Tepley, F. J., III

    2015-12-01

    Interpretation of compositional data from melt inclusions and their host requires that we understand the effects of the two main post-entrapment processes. First, the crystals grow from the melt inclusion at the time the host lava erupts and cools. Second, the melt inclusion and its host undergo diffusive re-equilibration. These processes have been well established for olivine in olivine-hosted melt inclusions (Danyushevsky et al., 2000; 2004). No similar study has been done for plagioclase-hosted inclusions. To better understand post entrapment crystallization in these crystals, we have conducted a set of time-series experiments on plagioclases in Plagioclase Ultraphyric Basalts (PUBs, Nielsen et al., 1995; 1998; 2011; Sours-Page et al., 2000; Lange et al., 2012; 2013). Plagioclase crystals were heated near the entrapment temperature (1200-1230°C) for 30 min, 1h, 4 h and 4 days. Based on the S content of the melt inclusions, ~80% of the melt inclusions retain their integrity with respect to S after homogenization. It is based on the assumption that S will degas if the inclusion has ruptured, that all S is present as S=, and that the melts are initially sulfide saturated. This is not true for the major element composition of the melt inclusions. For run times of 30 min to 4 hours, the melt inclusions do not show significant compositional changes. However, after 4 days, the average composition of the re-homogenized melt inclusions is shifted toward higher MgO values and lower Al2O3 concentrations regardless of run temperature. These results suggest that re-homogenization techniques held at high temperature for less than 4 hours will return information that is most relevant to the re-homogenization of the post-entrapment crystals. The long term "drift" of the major elements suggests that diffusive re-equilibration with the host crystal takes place at a rate that is too slow to be seen during typical re-homogenization times used (e.g. 20 min-2 hrs.). Key words

  5. Water in Olivine, Clinopyroxenen and Plagioclase of Lunar Meteorites of the NWA 773 Clan by IR Micro-Spectrosocpy

    NASA Astrophysics Data System (ADS)

    Kayama, M.; Nakashima, S.; Tomioka, N.; Ohtani, E.; Seto, Y.; Nagaoka, H.; Ozawa, S.; Sekine, T.; Miyahara, M.; Miyake, A.; Götze, J.; Tomeoka, K.

    2016-08-01

    Water in olivine, clinopyroxene and plagioclase of gabbroic lunar meteorites of Northwest Africa 2977 and 6950 and gabbroic/basaltic brecciated lunar meteorite of NWA 2727 were characterized by in-situ Fourier-transform infrared micro-spectroscopy.

  6. Monoclinic and Tetragonal Plagioclase (An54) in Shock Veins from the Central Uplift of the Manicouagan Impact Structure

    NASA Astrophysics Data System (ADS)

    Spray, J. G.; Boonsue, S.

    2016-08-01

    This work documents the discovery of monoclinic- and tetragonal-structured plagioclase. Critically, the high-pressure polymorphs possess the same composition as the non-shocked triclinic phase (An54).

  7. Origin of biotite-hornblende-garnet coronas between oxides and plagioclase in olivine metagabbros, Adirondack region, New York

    USGS Publications Warehouse

    Whitney, P.R.; McLelland, J.M.

    1982-01-01

    Complex multivariant reactions involving Fe-Ti oxide minerals, plagioclase and olivine have produced coronas of biotite, hornblende and garnet between ilmenite and plagioclase in Adirondack olivine metagabbros. Both the biotite (6-10% TiO2) and the hornblende (3-6% TiO2) are exceptionally Titanium-rich. The garnet is nearly identical in composition to the garnet in coronas around olivine in the same rocks. The coronas form in two stages: (a) Plagioclase+Fe-Ti Oxides+Olivine+water =Hornblende+Spinel+Orthopyroxene??Biotite +more-sodic Plagioclase (b) Hornblende+Orthopyroxene??Spinel+Plagioclase =Garnet+Clinopyroxene+more-sodic Plagioclase The Orthopyroxene and part of the clinopyroxene form adjacent to olivine. Both reactions are linked by exchange of Mg2+ and Fe2+ with the reactions forming pyroxene and garnet coronas around olivine in the same rocks. The reactions occur under granulite fades metamorphic conditions, either during isobaric cooling or with increasing pressure at high temperature. ?? 1983 Springer-Verlag.

  8. Evolution of the lunar crust: SIMS study of plagioclase from ferroan anorthosites

    SciTech Connect

    Papike, J.J.; Fowler, G.W.; Shearer, C.K.

    1997-06-01

    The lunar crust, down to a depth of {approximately}65 km, is composed of older (>4.5 Ga) ferroan anorthosites and younger (4.43-4.17 Ga) Mg-suite lithologies which include dunites, troctolites, and norites. The anorthosites are generally inferred to represent floating cumulates in a lunar magma ocean (possible depth 800 km, moon`s radius {approximately}1,738 km). The cumulates that are inferred to be located near the base of the magma ocean are dominantly olivine and pyroxene. The last dregs of the magma ocean are enriched in incompatible elements and have been named KREEP (K, rare earth elements, P). KREEP, formed in this manner, is probably concentrated near the crust/mantle boundary at {approximately}70 km depth. We are attempting to characterize melts parental to ferroan anorthosites and Mg-suite norites by analyzing REEs (La, Ce, Nd, Sm, Eu, Dy, Er, Yb) and Ba, Sr, and Y in their cumulus plagioclase. If the cumulus grains have not been compromised by postcrystallization effects and if we know the relevant mineral/melt partition coefficients (Ds) we can invert the trace element data for plagioclase to parental melt compositions. Melts parental to ferroan anorthosites are estimated to contain REE at concentrations ten to fifty times chondrites. Melts parental to the earlier crystallizing anorthosites (lower REE) have virtually no Eu anomaly, while melts parental to later crystallizing anorthosites (higher REE) have small negative Eu anomalies. This is qualitatively consistent with the fractionation of Eu relative to other REE by crystallization of plagioclase with large positive Eu anomalies. Melts parental to the Mg-suite have much higher total REE and very large negative Eu anomalies. 42 refs., 5 figs., 4 tabs.

  9. Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry

    NASA Astrophysics Data System (ADS)

    Holland, Tim; Blundy, Jon

    1994-05-01

    Amphibole thermodynamics are approximated with the symmetric formalism (regular solution model for within-site non-ideality and a reciprocal solution model for cross-site-terms) in order to formulate improved thermometers for amphibole-plagioclase assemblages. This approximation provides a convenient framework with which to account for composition-dependence of the ideal (mixing-on-sites) equilibrium constants for the equilibria: A) edenite+4quartz=tremolite+albite B) edenite+albite=richterite+anorthite For A and B all possible within-site and cross-site interactions among the species □-K-Na-Ca-Mg-Fe2+-Fe3+-Al-Si on the A, M4, M1, M3, M2 and T1 amphibole crystallographic sites were examined. Of the 36 possible interaction energy terms, application of the symmetric formalism results in a dramatic simplification to eight independent parameters. Plagioclase nonideality is modelled using Darken's quadratic formalism. We have supplemented an experimental data set of 92 amphibole-plagioclase pairs with 215 natural pairs from igneous and metamorphic rocks in which the pressure and temperature of equilibration are well constrained. Regression of the combined dataset yields values for the eight interaction parameters as well as for apparent enthalpy, entropy and volume changes for each reaction. These parameters are used to formulate two new thermometers, which perform well (±40°C) in the range 400 1000°C and 1 15 kbar over a broad range of bulk compositions, including tschermakitic amphiboles from garnet amphibolites which caused problems for the simple thermometer of Blundy and Holland (1990). For silica-saturated rocks both thermometers may be applied: in silica-undersaturated rocks or magmas thermometer B alone can be applied. An improved procedure for estimation of ferric iron in calcic amphiboles is presented in the appendix.

  10. Laboratory emissivity measurements of the plagioclase solid solution series under varying environmental conditions

    NASA Astrophysics Data System (ADS)

    Donaldson Hanna, K. L.; Thomas, I. R.; Bowles, N. E.; Greenhagen, B. T.; Pieters, C. M.; Mustard, J. F.; Jackson, C. R. M.; Wyatt, M. B.

    2012-11-01

    New laboratory thermal infrared emissivity measurements of the plagioclase solid solution series over the 1700 ˜ 400 cm-1 (6-25 μm) spectral range are presented. Thermal infrared (TIR) spectral changes for fine-particulate samples (0-25 μm) are characterized for the first time under different laboratory environmental conditions: ambient (terrestrial-like), half-vacuum (Mars-like), vacuum, and vacuum with cooled chamber (lunar-like). Under all environmental conditions the Christiansen Feature (CF) is observed to vary in a systematic way with Na-rich end-member (albite) having a CF position at the highest wave number (shortest wavelength) and the Ca-rich end-member (anorthite) having a CF position with the lowest wave number (longest wavelength). As pressure decreases to <10-3 mbar four observations are made: (1) the CF position shifts to higher wave numbers, (2) the spectral contrast of the CF increases relative to the RB, (3) the spectral contrast of the RB in the ˜1200-900 spectral range decreases while the spectral contrast of the RB in the ˜800-400 spectral range either increases or remains the same and (4) the TF disappears. A relationship between the wavelength position of the CF measured under simulated lunar conditions and plagioclase composition (An#) is developed. Although its exact form may evolve with additional data, this linear relationship should be applied to current and future TIR data sets of the Moon. Our new spectral measurements demonstrate how sensitive thermal infrared emissivity spectra of plagioclase feldspars are to the environmental conditions under which they are measured and provide important constraints for interpreting current and future thermal infrared data sets.

  11. Diffuse reflectance spectra of orthopyroxene, olivine, and plagioclase as a function of composition and structure

    NASA Technical Reports Server (NTRS)

    Zolensky, M. E.; Le, L.; Galindo, C.; Morris, R.; Lauer, V.; Vilas, F.

    1993-01-01

    Although many similarities exist between meteorite spectra and 'primitive' asteroids, there are unexplained discrepancies. These discrepancies do not appear to arise from grain size effects. Assuming that primitive meteorites did in fact originate from the 'primitive' asteroids, we believe that there are two testable explanations for the observed spectral discrepancies: compositional or structural differences. We have begun to synthesize and collect reflectance and Mossbauer spectra of pertinent materials, beginning with olivine, pyroxene, and plagioclase (all found in primitive meteorites), and to assess the possible effects composition may have on spectral features. Our study focuses on the combination of composition and structural effects.

  12. Plagioclase in Dacites of the Current Mt. St. Helens Eruption: Constraints for Magma Origin and Dynamics

    NASA Astrophysics Data System (ADS)

    Streck, M. J.; Broderick, C.; Thornber, C. R.; Clynne, M. A.; Pallister, J. S.

    2005-12-01

    The current, now year-long eruption of Mt. St. Helens is remarkable for a variety of reasons. Nearly solid, gas-poor dacite lava is extruding at the surface. Earthquakes are limited to the upper 3 km with most occurring less than 1 km below the surface; no deeper seismicity has been observed. Emissions of SO2, H2S, CO2 are extremely low, suggesting eruption of degassed magma. No direct evidence for mingling of more mafic magma has been observed. We investigated plagioclase crystals in 2004-5 dacite and 1980-86 dacite dome rocks to shed light on the nature and origin of the current magma. Polarized light and Nomarski microscopy in combination with detailed microprobe traverses were used to characterize single plagioclase crystals, focusing on areas near their rims. In addition, we mapped all plagioclase crystals within an area of ~1/3 of each thin section and grouped them according to easily distinguishable mineral features. Although labor-intensive, this new approach was devised to generate data statistically significant enough to compare plagioclase crystal populations of crystal-rich samples. Mappable features within crystals include: i) zones of acicular opx microlitic inclusions (commonly occurring near the rim), ii) resorption surfaces, iii) spatial association of i) and ii). Our key results from the crystal mapping indicate that sequential samples of 2004/5 have remained comparable, but are distinct from 1980-86 dacite dome samples. We interpret this result as strong evidence that the 2004-5 dacite is new magma and not 'left-over' 1980's dacite. The most interesting result from combining compositional profiling with texture is that acicular opx inclusions occur at the lowest observed An content of 40-33 regardless of position within crystal and that overgrowth on resorption surfaces are typically 10-20 mol% higher in An than within opx-rich zones resulting in single to multiple oscillations of An content towards the rim. These features could possibly be

  13. Diffuse reflectance spectra of orthopyroxene, olivine, and plagioclase as a function of composition and structure

    NASA Technical Reports Server (NTRS)

    Zolensky, M. E.; Le, L.; Galindo, C.; Morris, R.; Lauer, V.; Vilas, F.

    1993-01-01

    Although many similarities exist between meteorite spectra and 'primitive' asteroids, there are unexplained discrepancies. These discrepancies do not appear to arise from grain size effects. Assuming that primitive meteorites did in fact originate from the 'primitive' asteroids, we believe that there are two testable explanations for the observed spectral discrepancies: compositional or structural differences. We have begun to synthesize and collect reflectance and Mossbauer spectra of pertinent materials, beginning with olivine, pyroxene, and plagioclase (all found in primitive meteorites), and to assess the possible effects composition may have on spectral features. Our study focuses on the combination of composition and structural effects.

  14. Ion microprobe magnesium isotope analysis of plagioclase and hibonite from ordinary chondrites

    NASA Technical Reports Server (NTRS)

    Hinton, R. W.; Bischoff, A.

    1984-01-01

    Ion and electron microprobes were used to examine Mg-26 excesses from Al-26 decay in four Al-rich objects from the type 3 ordinary hibonite clast in the Dhajala chondrite. The initial Al-26/Al-27 ratio was actually significantly lower than Al-rich inclusions in carbonaceous chondrites. Also, no Mg-26 excesses were found in three plagioclase-bearing chondrules that were also examined. The Mg-26 excesses in the hibonite chondrites indicated a common origin for chondrites with the excesses. The implied Al-26 content in a proposed parent body could not, however, be confirmed as a widespread heat source in the early solar system.

  15. Change in Magma Dynamics at Okataina Rhyolite Caldera revealed by Plagioclase Textures and Geochemistry

    NASA Astrophysics Data System (ADS)

    Shane, P. A. R.

    2015-12-01

    A fundamental reorganization of magma dynamics at Okataina volcano, New Zealand, occurred at 26 ka involving a change from smaller volume, high-temperature rhyodacite magmas to a lower eruptive tempo of larger volume, low-temperature, rhyolite magmas. Zircon studies demonstrate the presence of a periodically active, long-lived (100,000 yr) magmatic reservoir. However, there is little correlation between periods of zircon crystallization and eruption events. In contrast, the changing magmatic dynamics is revealed in plagioclase growth histories. Crystals from the ~0.7 ka Kaharoa eruption are characterized by resorbed cores displaying a cellular-texture of high-An (>40) zones partially replaced by low-An (<30) zones, surrounded by a resorption surface and a prominent normal-zoned rim (An50-20). Elevated An, Fe, Mg, Sr and Ti follow the resorption surface and display rimward depletion trends, accompanied by Ba and REE enrichment. The zonation is consistent with fractional crystallization and cooling. The cores display wide trace element diversity, pointing to crystallization in a variety of melts, before transport and mixing into a common magma where the rims grew. Plagioclase from the ~36 ka Hauparu eruption display several regrowth zones separated by resorption surfaces, which surround small resorbed cores with a spongy cellular texture of variable An content (An 40-50). The crystals display step-wise re-growth of successively higher An, Fe, Mg and Ti content, consistent with progressive mafic recharge. Two crystal groups are distinguished by trace element chemistry indicating growth in separate melts and co-occurrence via magma-mingling. The contrasting zoning patterns in plagioclase correspond to the evolutionary history of magmatism at Okataina. Emptying of the magma reservoir following caldera eruption at 46 ka reduced barriers to mafic magma ascent. This is recorded by the frequent resorption and recharge episodes in Hauparu crystals. Subsequent re

  16. Ion microprobe magnesium isotope analysis of plagioclase and hibonite from ordinary chondrites

    NASA Astrophysics Data System (ADS)

    Hinton, R. W.; Bischoff, A.

    1984-03-01

    In a search for 26Mg excesses generated by 26Al decay the authors analysed four Al-rich objects from the type 3 ordinary chondrites using an ion microprobe. They report here the presence of 26Mg excesses of up to 100% in an unusually pure hibonite clast from the Dhajala chondrite; this 26Mg excess is the first to be found in an ordinary chondrite. No 26Mg excesses were observed in the three plagioclase-bearing chondrules analysed. It is concluded that 26Al may not have been sufficiently plentiful to act as a major heat source in condensed Solar System bodies.

  17. Stellate Plagioclase and Aligned Olivine of the Central Layered Series Peridotite, Isle of Rum, Scotland: Evidence of Magma Flow

    NASA Astrophysics Data System (ADS)

    Labonte, F.; Fowler, A. D.; Cheadle, M.; Emeleus, H.

    2004-05-01

    Donaldson et al. (1973, Nature, 243, 69-70) described what they termed poikilo-macro-spherulitic plagioclase as consisting of metre-scale bifurcating rays of plagioclase within feldspathic peridotite. They concluded that the plagioclase rays were continuous and likely represented in-situ crystal growth. In our specimens, the plagioclase rays, which are apparent in stark relief on weathered surfaces, are composed of numerous discrete plagioclase crystals, each on the order of ~2-10mm. The plagioclase appears to be interstitial to aligned olivine crystals. Accordingly we undertook a detailed examination of the rock texture at the thin section scale using petrography, Electron Microprobe (EMP), Crystal Size Distribution (CSD) and Electron Backscatter Diffracttion (EBSD). Olivine is present in two morphologies, equant euhedral to subhedral crystals typically ranging from ~0.01-2 mm, and as quasi-tabular crystals. The latter are conspicuous in outcrop and range in scale from ~1-15mm. Olivine composition varies from Fo78-Fo84. Crystals poikilitically enclosed in clinopyroxene generally have a lower Fo content than those intergrown with plagioclase. Plagioclase crystals are anhedral and may have a crude chemical banding varying from An55 to An80. Clinopyroxene is interstitial as oikocrysts, ranging from ~10-30mm. The CSD analysis demonstrates that the olivines have a shape-preferred orientation (SPO). A large proportion of the CSD plots are characterised by a downturn at the smallest crystal sizes, indicating that an initial original linear CSD was modified perhaps by ripening, or compaction-pressure-solution. EBSD shows the overall SPO of the crystals corresponds with their crystallographic-preferred orientation (CPO), i.e. the crystallographic orientation of individual crystals coincides with their shape. Olivine morphology is quasi-tabular, characterised by a very short crystallographic b-axis compared to the other two axes. Pole figures for quasi-tabular crystals show

  18. Plagioclase-peridotites recording the incipient stage of oceanic basin formation: new constraints from the Nain ophiolites (central Iran)

    NASA Astrophysics Data System (ADS)

    Pirnia Naeini, Tahmineh; Arai, Shoji; Saccani, Emilio

    2016-04-01

    Pargasite-bearing plagioclase peridotites of Nain have recorded a complex tectono-magmatic and -metamorphic history. Based on texture, as well as mineral major and trace element chemistry, two stages of facies exchange have been documented in the peridotites, namely, from plagioclase to spinel and vice versa. The earliest event that influenced the peridotites was a low degree of flux melting (<4%) which occurred at the pargasite stability field. Melting resulted in a small fraction of highly enriched melt as well as a depleted chemistry of the pargasites and coexisting pyroxenes. The produced melt fraction was scanty, and consequently hardly mobile. It crystallized in-situ in the peridotites in form of plagioclase. After the melting stage, cooling caused the peridotites to recrystallize in spinel facies. Recrystallization obliterated all textural features associated with formation of magmatic plagioclase in the peridotites. However, lines of evidence are documented by the chemistry of the spinel-facies mineral assemblage, which strongly suggest the pre-existence of plagioclases. This evidence includes a positive Eu anomaly in pargasites and pyroxenes, as well as a Zr negative anomaly in orthopyroxenes. The spinel-facies assemblage subsequently recrystallized in the plagioclase stability field. Recrystallization occurred in various degrees. Strong recrystallization resulted in the formation of modally typical, but atypically enriched, harzburgite from the lherzolite. Remarkable textural characteristics of peridotites that indicate subsolidus origin for plagioclases include: (1) plagioclase heterogeneous distribution, (2) plagioclase exclusive concurrence with spinel and, (3) plagioclase modal positive correlation with olivine but negative correlation with pyroxenes and pargasite. The transition from spinel to plagioclase-facies was associated with the following compositional variations in the peridotite minerals: (1) Al, Mg, Ni decrease and Cr, Ti, Fe increase in

  19. The Effects of Microbial Surface Attachment on the Dissolution Kinetics of Plagioclase Feldspar

    NASA Astrophysics Data System (ADS)

    Conrad, P. G.; Luttge, A.

    2003-12-01

    The rate of mineral dissolution can be influenced by the attachment of microbes to a mineral surface. We have previously reported the effect of Shewanella oneidensis (MR-1) biofilm formation on the dissolution kinetics of calcite, dolomite and rhodochrosite, where the organisms completely control dissolution kinetics by recognizing high-energy surface sites overlying screw dislocations and attaching to those sites, inhibiting the opening of etch pits and significantly retarding dissolution. Note that calcite and dolomite are not known to possess nutritional significance for MR-1; and while this facultative anaerobe can reduce both Mn and Fe, our experiments were all conducted with aerobic organisms. In recent experiments, we have observed that this inhibitory effect requires the cells to be alive; dead cells do not prevent the opening of etch pits and subsequent dissolution of the carbonate crystals under conditions in which they would otherwise dissolve. Now we report on the effects of the MR-1 biofilm formation on the more slowly dissolving end-member plagioclase feldspar, anorthite (CaAl2Si2O8). The influence of organisms on feldspar weathering rates, in particular the Ca-rich plagioclases, has been the subject of considerable interest because of their crustal abundance and role in the maintenance of atmospheric CO2. (Schwartzman and Volk, 1989, Schwartzman, 1995 and others). Our methodology of direct observation of microbial attachment, biofilm propagation and effects on mineral dissolution provides a quantitative measure of biological contribution to weathering of single crystal minerals for subsequent use in biogeochemical modeling.

  20. A Clinopyroxene-Plagioclase Geobarometer for A-type Silicic Volcanic Rocks

    NASA Astrophysics Data System (ADS)

    Wolff, J.; Iveson, A. A.; Davis, K.; Johnson, T. A.; Gahagan, S.; Ellis, B. S.

    2015-12-01

    Constraining the crustal storage depths of magmas is important in understanding volcanism. The reaction: anorthite (pl) = Ca-Tschermak's (cpx) + silica (Q or liq) has a large volume change and hence offers potential as a geobarometer, but has not been extensively exploited as such. One of the chief barriers to its wide application is consistent estimation of melt silica activity for assemblages that lack quartz. We have skirted this problem by confining attention to metaluminous silicic compositions (SiO2 > 60% by weight), for which silica activity during crystallization is presumed to be close to 1, and calibrated the barometer for the range 0 - 2 GPa using the LEPR database and additional experiments from the literature. Additional improvement is obtained by excluding hydrous phase-bearing assemblages. Despite the analytical uncertainties present in older experimental investigations, with knowledge of temperature, and clinopyroxene, plagioclase and host melt compositions, pressures for amphibole- and biotite-free dacites and rhyolites can be estimated to ±0.17 GPa (1 sigma). The limitations of the barometer render it most applicable to intraplate, A-type rhyolites. Application to one such system, the Snake River Plain rhyolites, indicate that both melt-hosted phenocrysts and clinopyroxene-plagioclase aggregate grains found in these rhyolites formed at low pressures, <0.5 GPa. This is consistent with isotopic evidence for a shallow crustal origin for Snake River Plain rhyolites.

  1. Trace element partitioning between high-An plagioclase and basaltic to basaltic andesite melt at 1 atmosphere pressure

    NASA Astrophysics Data System (ADS)

    Tepley, Frank J., III; Lundstrom, Craig C.; McDonough, William F.; Thompson, Amy

    2010-07-01

    We determined plagioclase-melt partition coefficients for 18 elements by performing controlled cooling rate, 1-atmosphere experiments using both natural and synthetic basaltic (51 wt.% SiO 2) and basaltic andesite (56 wt.% SiO 2) powders in a vertical quench furnace. The experiments produced An 69 to An 87 composition plagioclase. Three starting powders were Gorda Ridge basalt, synthetic diopside (40%)-albite (28%)-anorthite (32%) mixture, and Arenal volcano (Costa Rica) basaltic andesite. The Gorda and synthetic powders were doped at both low concentrations (20-200 ppm) and high concentration (200-5000 ppm), whereas the Arenal powder was doped only at high concentrations resulting in two doped Gorda powders (low: NP, and high: SDP), two doped diopside/albite/anorthite powders (low: DAD, high: SDD) and one doped Arenal powder (high: AR99-2). Trace elements concentrations in both glass and plagioclase were measured by secondary ion mass spectrometry (SIMS) and/or by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Results for the partitioning of trace elements between plagioclase and melt at different doping levels demonstrate both adherence to Henry's Law and good agreement between the different analytical techniques. In general, plagioclase-melt partition coefficients determined in the An 69-73 range are similar to other published values, however, some of those occurring for plagioclase compositions > An 75 are distinctly lower than those predicted by current regression formulations. We applied a two-lattice melt model to these data to account for differences in melt composition and temperature and found that there were no aberrations associated with partition coefficients. A new set of regression formulations is determined involving the newly determined dataset for plagioclase with An contents between 75 and 87.

  2. The Trace Element Diversity of Anorthitic Plagioclase and Melt Inclusions in MOR Basalts

    NASA Astrophysics Data System (ADS)

    Weinsteiger, A. B.; Nielsen, R. L.; Kent, A. J.

    2009-05-01

    This investigation centered on the spatial and compositional relationship between anorthitic feldspars and included melts. Such phenocrysts are characteristic of primitive lavas in suites of high Al MORB most often associated with slow to intermediate spreading ridges close to fracture zones. This study uses the relationship between the melt inclusions and host feldspar to evaluate the degree to which the melt inclusions represent original trapped liquid, versus the degree to which melt inclusion compositions are the product of entrapment or post-entrapment processes, and to evaluate their relationship to MORB magmas. Feldspar phenocryst from several samples from the Southeast Indian and Gorda Ridges were analyzed for major and minor components. Feldspar and Melt inclusion major element compositions show little variation with An content from 86 to 92, with individual phenocrysts rarely ranging more than +/- 2 An%. One of the striking characteristics of plagioclase hosted inclusions in MORB is the wide range of Ti contents from single phenocrysts [1, 2, 3]. The origin of this chemical signal has been attributed to a number of processes, including post entrapment diffusive re-equilibration [4], diffusive process at the time of entrapment [3], local disequilibrium events [5], and reactions in melt channels [6]. Measurement of the trace element content of the feldspar host allowed us to evaluate some of those models. Our results show that the trace element content of plagioclase exhibits an even greater range of composition than that exhibited in the melt inclusions. Concentration in the feldspar changes extremely rapidly as one traverses the crystal, mirroring the variations seen in the melt inclusions. Each feldspar crystal appears to have a distinct history, and each sample contains a complex crystal cargo of phenocrysts. Partition coefficient "proxies" were calculated for Mg, Ti, Fe, Sr, Ba, Y, Zr, La, Ce, Pr, Eu, and Pb by analyzing plagioclase spots directly

  3. Chromite-Plagioclase Assemblages as a New Shock Indicator; Implications for the Shock and Thermal Histories of Ordinary Chondrites

    NASA Technical Reports Server (NTRS)

    Rubin, Alan E.

    2006-01-01

    Chromite in ordinary chondrites (OC) can be used as a shock indicator. A survey of 76 equilibrated H, L and LL chondrites shows that unshocked chromite grains occur in equant, subhedral and rounded morphologies surrounded by silicate or intergrown with metallic Fe-Ni and/or troilite. Some unmelted chromite grains are fractured or crushed during whole-rock brecciation. Others are transected by opaque veins; the veins form when impacts cause localized heating of metal-troilite intergrowths above the Fe-FeS eutectic (988 C), mobilization of metal-troilite melts, and penetration of the melt into fractures in chromite grains. Chromite-plagioclase assemblages occur in nearly every shock-stage S3-S6 OC; the assemblages range in size from 20-300 microns and consist of 0.2-20-micron-size euhedral, subhedral, anhedral and rounded chromite grains surrounded by plagioclase or glass of plagioclase composition. Plagioclase has a low impedance to shock compression. Heat from shock-melted plagioclase caused adjacent chromite grains to melt; chromite grains crystallized from this melt. Those chromite grains in the assemblages that are completely surrounded by plagioclase are generally richer in Al2O3, than unmelted, matrix chromite grains in the same meteorite. Chromite veinlets (typically 0.5-2 microns thick and 10-300 microns long) occur typically in the vicinity of chromite-plagioclase assemblages. The veinlets formed from chromite-plagioclase melts that were injected into fractures in neighboring silicate grains; chromite crystallized in the fractures and the residual plagioclase-rich melt continued to flow, eventually pooling to form plagioclase-rich melt pockets. Chromite-rich chondrules (consisting mainly of olivine, plagioclase-normative mesostasis, and 5-15 vol.% chromite) occur in many shocked OC and OC regolith breccias but they are absent from primitive type-3 OC. They may have formed by impact melting chromite, plagioclase and adjacent mafic silicates during higher

  4. Chromite-Plagioclase Assemblages as a New Shock Indicator; Implications for the Shock and Thermal Histories of Ordinary Chondrites

    NASA Technical Reports Server (NTRS)

    Rubin, Alan E.

    2006-01-01

    Chromite in ordinary chondrites (OC) can be used as a shock indicator. A survey of 76 equilibrated H, L and LL chondrites shows that unshocked chromite grains occur in equant, subhedral and rounded morphologies surrounded by silicate or intergrown with metallic Fe-Ni and/or troilite. Some unmelted chromite grains are fractured or crushed during whole-rock brecciation. Others are transected by opaque veins; the veins form when impacts cause localized heating of metal-troilite intergrowths above the Fe-FeS eutectic (988 C), mobilization of metal-troilite melts, and penetration of the melt into fractures in chromite grains. Chromite-plagioclase assemblages occur in nearly every shock-stage S3-S6 OC; the assemblages range in size from 20-300 microns and consist of 0.2-20-micron-size euhedral, subhedral, anhedral and rounded chromite grains surrounded by plagioclase or glass of plagioclase composition. Plagioclase has a low impedance to shock compression. Heat from shock-melted plagioclase caused adjacent chromite grains to melt; chromite grains crystallized from this melt. Those chromite grains in the assemblages that are completely surrounded by plagioclase are generally richer in Al2O3, than unmelted, matrix chromite grains in the same meteorite. Chromite veinlets (typically 0.5-2 microns thick and 10-300 microns long) occur typically in the vicinity of chromite-plagioclase assemblages. The veinlets formed from chromite-plagioclase melts that were injected into fractures in neighboring silicate grains; chromite crystallized in the fractures and the residual plagioclase-rich melt continued to flow, eventually pooling to form plagioclase-rich melt pockets. Chromite-rich chondrules (consisting mainly of olivine, plagioclase-normative mesostasis, and 5-15 vol.% chromite) occur in many shocked OC and OC regolith breccias but they are absent from primitive type-3 OC. They may have formed by impact melting chromite, plagioclase and adjacent mafic silicates during higher

  5. Crystallization processes in the Rocky Hill granodiorite Pluton, California: An interpretation based on compositional zoning of plagioclase

    NASA Astrophysics Data System (ADS)

    Loomis, Timothy P.; Welber, Paul W.

    1982-12-01

    The Rocky Hill Pluton was chosen for study of plagioclase zoning because it has an apparently simple geometry and one-stage emplacement history, documented by Putman and Alfors (1969). The pluton is divided texturally into a hypidiomorphic granular rim fades and porphyritic core facies, both comprising zoned plagioclase phenocrysts averaging 2 4 mm in diameter. Major features of compositional zoning profiles of plagioclase were found to be consistent within a sample, showing resorbed core regions followed by a sharp drop of ca. 10% An to a plateau region, and ending with normal zoning. The width of the plateau region is narrow in samples near the pluton rim but increases systematically toward the pluton core, and the outer part of the plateau develops reverse zoning in the core facies. A review of the effects of pressure, water content of the melt, and temperature on equilibrium and disequilibrium crystallization processes concludes that only variations of temperature and water content during disequilibrium crystallization of plagioclase phenocrysts can produce the observed zoning patterns. Experimental data suggest that the trend toward finer grain size toward the interior of the pluton may be due to increasing water content rather than due to increasing cooling rate. The compositional zoning of plagioclase and textural variations in the pluton can be explained by a model in which the pluton crystallized by migration of a partly crystalline “mush zone” inward from the edge of the plutonic chamber while convection within the residual magma: (1) redistributed water excluded from the mush zone and (2) slowed the cooling rate of the mush zone. The reverse zoning of plagioclase growing in the convecting magma changed to normal zoning after capture in the mush zone because the cooling rate increased and water was removed. Comparison of the results from our study of Rocky Hill with published descriptions of plutons and experimental and theoretical studies of

  6. Revisiting 26Al-26Mg systematics of plagioclase in H4 chondrites

    NASA Astrophysics Data System (ADS)

    Telus, M.; Huss, G. R.; Nagashima, K.; Ogliore, R. C.

    2014-06-01

    Zinner and Göpel found clear evidence for the former presence of 26Al in the H4 chondrites Ste. Marguerite and Forest Vale. They assumed that the 26Al-26Mg systematics of these chondrites date "metamorphic cooling of the H4 parent body." Plagioclase in these chondrites can have very high Al/Mg ratios and low Mg concentrations, making these ion probe analyses susceptible to ratio bias, which is inversely proportional to the number of counts of the denominator isotope (Ogliore et al.). Zinner and Göpel used the mean of the ratios to calculate the isotope ratios, which exacerbates this problem. We analyzed the Al/Mg ratios and Mg isotopic compositions of plagioclase grains in thin sections of Ste. Marguerite, Forest Vale, Beaver Creek, and Sena to evaluate the possible influence of ratio bias on the published initial 26Al/27Al ratios for these meteorites. We calculated the isotope ratios using total counts, a less biased method of calculating isotope ratios. The results from our analyses are consistent with those from Zinner and Göpel, indicating that ratio bias does not significantly affect 26Al-26Mg results for plagioclase in these chondrites. Ste. Marguerite has a clear isochron with an initial 26Al/27Al ratio indicating that it cooled to below 450 °C 5.2 ± 0.2 Myr after CAIs. The isochrons for Forest Vale and Beaver Creek also show clear evidence that 26Al was alive when they cooled, but the initial 26Al/27Al ratios are not well constrained. Sena does not show evidence that 26Al was alive when it cooled to below the Al-Mg closure temperature. Given that metallographic cooling rates for Ste. Marguerite, Forest Vale, and Beaver Creek are atypical (>5000 °C/Myr at 500 °C) compared with most H4s, including Sena, which have cooling rates of 10-50 °C/Myr at 500 °C (Scott et al.), we conclude that the Al-Mg systematics for Ste. Marguerite, Forest Vale, and Beaver Creek are the result of impact excavation of these chondrites and cooling at the surface of the

  7. Cryptic young zircon and young plagioclase in the Kaharoa Rhyolite, Tarawera, New Zealand: Implications for crystal recycling in magmatic systems

    NASA Astrophysics Data System (ADS)

    Klemetti, E. W.; Cooper, K. M.

    2007-12-01

    We measured in-situ 238U-230Th zircon and bulk plagioclase 238U-230Th-226Ra disequilibria in rhyolite lava and tephra from the ~1315 AD Kaharoa eruption of Tarawera Volcano, New Zealand in order to constrain its history of chemical evolution. These data suggest that zircon records a protracted history (10s of kyr) whereas plagioclase is dominantly young (few kyr), but both phases crystallize up to the eruption. The Kaharoa eruptive period at Tarawera consists of ~2.5 km3 of crystal-rich rhyolite (74-75 wt% silica) lava and ~5 km3 of coeval tephra deposits, making it the largest silicic eruption in New Zealand in the last 1,000 years. 238U-230Th disequilibria measurements of zircon determined via SHRIMP-RG analyses produce an array of ages, with three main populations: (1) within error of eruption age; (2) 15-80 k.y.; (3) 100-175 k.y. Very few analyzed zircon fall within error of eruption age and little difference is seen in the age distribution of zircon between the lava and tephra. In contrast, 238U-230Th and 230Th-226Ra plagioclase ages appear to be within error of eruption age however this age is complicated zircon inclusions in the plagioclase. This contamination by zircon is seen in the 238U-230Th disequilibria and trace element data, where the addition of zircon pushes the bulk plagioclase separate towards more U-enriched values and high Zr values. However, the (230Th)/(232Th) ratios for the separates are the same as the whole rock values, indicating that any zircon in the bulk separate must be young (eruption age). This finding is also borne out in 230Th-226Ra disequilibria, where zero-age zircon contamination is reflected in increased (230Th)/[Ba] with no change in (226Ra)/[Ba]. In both cases, as little as 1 ppm of zero-age zircon contamination is needed to create these patterns. This signal of young plagioclase and zircon growth is in contrast to the protracted history seen in the SHRIMP-RG zircon data. This suggests that young zircon growth in the Kaharoa

  8. Ion microprobe, electron microprobe and cathodoluminescence data for Allende inclusions with emphasis on plagioclase chemistry

    NASA Technical Reports Server (NTRS)

    Hutcheon, I. D.; Steele, I. M.; Smith, J. V.; Clayton, R. N.

    1978-01-01

    Three Type B inclusions from the Allende meteorite have been analyzed. A grain-to-grain characterization of mineral chemistry and isotopic content was made possible by the use of a range of techniques, including luminescence and scanning electron microscopy and electron and ion microprobe analysis. Cathodoluminescence was used in fine-grained, optically opaque regions to distinguish between sub-micrometer phases, such as garnet and Si-rich material, subsequently identified by electron probe and scanning electron microscope analyses. Four types of luminescence patterns, due to twinning, primary sector zoning, alteration of boundaries and fractures, and shock effects, were identified in Allende plagioclase. Luminescence color exhibited a strong correlation with Mg content and provided a guide for an electron probe quantitative map of Mg and Na distributions. Ion microprobe studies of individual grains revealed large excesses of Mg-26.

  9. High geomagnetic intensity during the mid-Cretaceous from Thellier analyses of single plagioclase crystals.

    PubMed

    Tarduno, J A; Cottrell, R D; Smirnov, A V

    2001-03-02

    Recent numerical simulations have yielded the most efficient geodynamo, having the largest dipole intensity when reversal frequency is low. Reliable paleointensity data are limited but heretofore have suggested that reversal frequency and paleointensity are decoupled. We report data from 56 Thellier-Thellier experiments on plagioclase crystals separated from basalts of the Rajmahal Traps (113 to 116 million years old) of India that formed during the Cretaceous Normal Polarity Superchron. These data suggest a time-averaged paleomagnetic dipole moment of 12.5 +/- 1.4 x 10(22) amperes per square meter, three times greater than mean Cenozoic and Early Cretaceous-Late Jurassic dipole moments when geomagnetic reversals were frequent. This result supports a correlation between intervals of low reversal frequency and high geomagnetic field strength.

  10. Crystal Zoning in Deccan Giant Plagioclase Basalts: Implications for a Short Duration of Deccan Trap Eruption.

    NASA Astrophysics Data System (ADS)

    Borges, M. R.; Sen, G.; Hart, G. L.; Wolff, J. A.

    2006-12-01

    The lower formations in the Western Ghats Type Section of the 65 Ma Deccan Traps are characterized by spectacular Giant Plagioclase Basalt (GPB) flows that cap individual formations. These flows have plagioclase phenocrysts that reach up to 5 cm in length and constitute as much as 40% by mode of the rock. The lavas that host these giant crystals are also the most differentiated members of their respective formations. In this study, we present new data on crystal sizes, elemental and Sr-isotopic compositions of GPB crystals from the oldest Jawhar Formation, infer their petrogenesis, and discuss their implications for the duration of the Deccan eruption in the Western Ghats. Surprisingly, liquid line of descent calculations (COMAGMAT) suggest that a high-MgO lava from a younger formation represents the most likely parent for the lavas belonging to this oldest formation. Replenishment of magma chambers by less evolved magma along with fractionation and contamination in the conduit system may give rise to the differences in chemistry within and among formations. Individual giant crystals are compositionally near-homogeneous with small (2-3%) changes in An-content, and show oscillatory zoning. Though the crystals are near-homogeneous in major and trace elements, cathodoluminescence imaging reveals distinct core and rim zonations in some crystals. These cores and rims also differ markedly in their 87Sr/86Sr ratio, with ratios of ~0.7096 in the cores and ~0.7106 in the rims. 87Sr/86Sr of the core is similar to that of the groundmass plagioclase in a flow that directly underlies the GPB, whereas the rims have the same composition as the groundmass plagioclase of the GPB flow itself. Thus, the cores of the giant phenocrysts grew in the magma that fed the older flow and their rims grew in the mixed and differentiated batch of magma that later erupted as a GPB flow. Assuming that crystal growth continued uninterrupted, the growth time of the crystal would be the incubation

  11. Ion microprobe, electron microprobe and cathodoluminescence data for Allende inclusions with emphasis on plagioclase chemistry

    NASA Technical Reports Server (NTRS)

    Hutcheon, I. D.; Steele, I. M.; Smith, J. V.; Clayton, R. N.

    1978-01-01

    Three Type B inclusions from the Allende meteorite have been analyzed. A grain-to-grain characterization of mineral chemistry and isotopic content was made possible by the use of a range of techniques, including luminescence and scanning electron microscopy and electron and ion microprobe analysis. Cathodoluminescence was used in fine-grained, optically opaque regions to distinguish between sub-micrometer phases, such as garnet and Si-rich material, subsequently identified by electron probe and scanning electron microscope analyses. Four types of luminescence patterns, due to twinning, primary sector zoning, alteration of boundaries and fractures, and shock effects, were identified in Allende plagioclase. Luminescence color exhibited a strong correlation with Mg content and provided a guide for an electron probe quantitative map of Mg and Na distributions. Ion microprobe studies of individual grains revealed large excesses of Mg-26.

  12. Elastic properties of plagioclase aggregates and seismic velocities in the moon

    NASA Technical Reports Server (NTRS)

    Wang, H.; Todd, T.; Richter, D.; Simmons, G.

    1973-01-01

    The compressional velocities of Apollo 16 gabbroic anorthosites in which the cracks have been closed match the seismic velocity of 7 km/sec in the 25 to 65 km depth region of the moon beneath the Imbrium Basin. The intrinsic velocities of plagioclase aggregates indicate that a velocity of 7 km/sec in a highly calcic gabbroic anorthosite is consistent only with a very small pyroxene component. Because mare basalts and gabbroic-anorthosites both have intrinsic velocities of 7 km/sec, the laboratory velocity data do not require a compositional change from basalt to anorthosite at the 25 km discontinuity. The laboratory velocity data only imply that the 25 km seismic discontinuity is one of microcrack density. The physical rather than the chemical or mineralogical state is constrained.

  13. Characterisation of grain-size, shape and orientation of plagioclase in the Rooi Rand dyke swarm, South Africa

    NASA Astrophysics Data System (ADS)

    Hastie, Warwick William; Watkeys, Michael K.; Aubourg, Charles

    2013-01-01

    Magmatic (type-A) fabric co-exists with dyke-orthogonal (type-B) fabric in both the plagioclase and opaque grain fractions in dykes of the Rooi Rand dyke swarm (RRDS). We present new data from the RRDS pertaining to the size, shape, texture and orientation of plagioclase. Texturally, the samples range from intersertal to sub-ophitic and phenocrystic (plagioclase-phyric). More than 90% of plagioclase grains are < 33 μm in size and the modal size is 12.3 μm. The smallest, most abundant grains have modest shape ratios (1.40 < r < 1.95). Larger grains have relatively low shape ratios although grain-size and shape in the RRDS are essentially independent. Additionally, we investigate the angle between individual grains and the respective dyke margins in (sub-) horizontal thin-sections. This apparent imbrication angle, Ai, in conjunction with three dimensional fabric shape tensors and grain-size and shape data, shows that the most common grain-size class, with modest shape ratios (1.80-2.20), is predominantly associated with type-A fabric. The interaction of grains attempting to rotate in even-textured samples appears to have resulted in the lack of correlation between shape and size parameters and the orientation or intensity of the fabric, whilst grains with increasing r values show a tendency towards type-B fabric. Based on textural information and crystal size distribution, we suggest that plagioclase < 80 μm in size grew as a result of a late-stage nucleation event, becoming increasingly anisotropic as a result of relatively rapid undercooling. Thus, late-stage, rapid nucleation of the plagioclase groundmass significantly affected the final fabric that developed, and the interaction of large, anisotropic grains has not played as significant a role in the development of type-B fabric as previously thought.

  14. Strong climate and tectonic control on plagioclase weathering in granitic terrain

    USGS Publications Warehouse

    Rasmussen, C.; Brantley, S.; Richter, D.D.B.; Blum, A.; Dixon, J.; White, A.F.

    2011-01-01

    Investigations to understand linkages among climate, erosion and weathering are central to quantifying landscape evolution. We approach these linkages through synthesis of regolith data for granitic terrain compiled with respect to climate, geochemistry, and denudation rates for low sloping upland profiles. Focusing on Na as a proxy for plagioclase weathering, we quantified regolith Na depletion, Na mass loss, and the relative partitioning of denudation to physical and chemical contributions. The depth and magnitude of regolith Na depletion increased continuously with increasing water availability, except for locations with mean annual temperature <5??C that exhibited little Na depletion, and locations with physical erosion rates <20gm-2yr-1 that exhibited deep and complete regolith Na depletion. Surface Na depletion also tended to decrease with increasing physical erosion. Depth-integrated Na mass loss and regolith depth were both three orders of magnitude greater in the fully depleted, low erosion rate sites relative to other locations. These locations exhibited strong erosion-limitation of Na chemical weathering rates based on correlation of Na chemical weathering rate to total Na denudation. Sodium weathering rates in cool locations with positive annual water balance were strongly correlated to total Na denudation and precipitation, and exhibited an average apparent activation energy (Ea) of 69kJmol-1 Na. The remaining water-limited locations exhibited kinetic limitation of Na weathering rates with an Ea of 136kJmol-1 Na, roughly equivalent to the sum of laboratory measures of Ea and dissolution reaction enthalpy for albite. Water availability is suggested as the dominant factor limiting rate kinetics in the water-limited systems. Together, these data demonstrate marked transitions and nonlinearity in how climate and tectonics correlate to plagioclase chemical weathering and Na mass loss. ?? 2010 Elsevier B.V.

  15. Sims Analysis of Water Abundance and Hydrogen Isotope in Lunar Highland Plagioclase

    NASA Technical Reports Server (NTRS)

    Hui, Hejiu; Guan, Yunbin; Chen, Yang; Peslier, Anne H.; Zhang, Youxue; Liu, Yang; Rossman, George R.; Eiler, John M.; Neal, Clive R.

    2015-01-01

    The detection of indigenous water in mare basaltic glass beads has challenged the view established since the Apollo era of a "dry" Moon. Since this discovery, measurements of water in lunar apatite, olivine-hosted melt inclusions, agglutinates, and nominally anhydrous minerals have confirmed that lunar igneous materials contain water, implying that some parts of lunar mantle may have as much water as Earth's upper mantle. The interpretation of hydrogen (H) isotopes in lunar samples, however, is controversial. The large variation of H isotope ratios in lunar apatite (delta Deuterium = -202 to +1010 per mille) has been taken as evidence that water in the lunar interior comes from the lunar mantle, solar wind protons, and/or comets. The very low deuterium/H ratios in lunar agglutinates indicate that solar wind protons have contributed to their hydrogen content. Conversely, H isotopes in lunar volcanic glass beads and olivine-hosted melt inclusions being similar to those of common terrestrial igneous rocks, suggest a common origin for water in both Earth and Moon. Lunar water could be inherited from carbonaceous chondrites, consistent with the model of late accretion of chondrite-type materials to the Moon as proposed by. One complication about the sources of lunar water, is that geologic processes (e.g., late accretion and magmatic degassing) may have modified the H isotope signatures of lunar materials. Recent FTIR analyses have shown that plagioclases in lunar ferroan anorthosite contain approximately 6 ppm H2O. So far, ferroan anorthosite is the only available lithology that is believed to be a primary product of the lunar magma ocean (LMO). A possible consequence is that the LMO could have contained up to approximately 320 ppm H2O. Here we examine the possible sources of water in the LMO through measurements of water abundances and H isotopes in plagioclase of two ferroan anorthosites and one troctolite from lunar highlands.

  16. Communication between cation environments in aluminosilicate frameworks: incommensurately modulated crystal structure of an e-plagioclase.

    PubMed

    Fredrickson, Rie T; Fredrickson, Daniel C

    2016-10-01

    Despite being one of the most common minerals in the earth's crust the crystal structure of intermediate e-plagioclase remains only partially understood, due in a large part to its complex diffraction patterns including satellite reflections. In this article we present a detailed analysis of the structure of e-plagioclase (An44) using single-crystal X-ray diffraction measured at ambient and low temperature (T = 100 K), in which the full modulated structure is successfully refined. As in earlier studies, the diffraction pattern exhibits strong main a-reflections and weak e-satellite reflections. The average structure could be solved in terms of an albite-like basic cell with the triclinic centrosymmetric and non-centrosymmetric space groups P \\bar 1 and P1 (treated in its C \\bar 1 and C1 setting, respectively, to follow conventions in the literature), while the incommensurately modulated structure was modeled in (3 + 1)D superspace, employing both the centro- and non-centrosymmetric superspace groups X \\bar 1(αβγ)0 and X1(αβγ)0, where X refers to a special (3 + 1)D lattice centering with centering vectors (0 0 ½ ½), (½ ½ 0 ½), and (½ ½ ½ 0). Individual positional and occupational modulations for Ca/Na were refined with deeper insights being revealed in the non-centrosymmetric structure model. Through the structural details emerging from this model, the origin of the modulation can be traced to the communication between Ca/Na site positions through their bridging aluminosilicate (Si/Al)O4 tetrahedra.

  17. A melting model for variably depleted and enriched lherzolite in the plagioclase and spinel stability fields

    NASA Astrophysics Data System (ADS)

    Till, Christy B.; Grove, Timothy L.; Krawczynski, Michael J.

    2012-06-01

    Here we develop a lherzolite melting model and explore the effects of variations in mantle composition, pressure, temperature, and H2O content on melt composition. New experiments and a compilation of experimental liquids saturated with all of the mantle minerals (olivine, orthopyroxene, clinopyroxene, plagioclase and/or spinel) are used to calibrate a model that predicts the temperature and major element composition of a broad spectrum of primary basalt types produced under anhydrous to low H2O-content conditions at upper mantle pressures. The model can also be used to calculate the temperature and pressure at which primary magmas were produced in the mantle, as well as to model both near-fractional adiabatic decompression and batch melting. Our experimental compilation locates the pressure interval of the plagioclase to spinel transition on the solidus and shows that it is narrow (˜0.1 GPa) for melting of natural peridotite compositions. The multiple saturation boundaries determined by our model provide a method for assessing the appropriate mineral assemblage, as well as the extent of the fractional crystallization correction required to return a relatively primitive liquid to equilibrium with the mantle source. We demonstrate that an inaccurate fractionation correction can overestimate temperature and depths of melting by hundreds of degrees and tens of kilometers, respectively. This model is particularly well suited to examining the temperature and pressure of origin for intraplate basaltic volcanism and is used to examine the petrogenesis of a suite of Holocene basaltic lavas from Diamond Crater in Oregon's High Lava Plains (HLP).

  18. Fe and Ti in Plagioclase From the Lac Saint-Jean Anorthosite: Significance of the Si4O8 and AlAl3SiO8 End-Members in Plagioclase

    NASA Astrophysics Data System (ADS)

    Murakami, H.; Lalonde, A. E.

    2004-05-01

    Plagioclase phenocrysts from the Proterozoic Lac Saint-Jean massif anorthosite have a distinctive black color that is attributed to myriads of oriented μ m-size Fe-Ti oxides. These inclusions have variable composition within the ilmenite-hematite solid solution and are responsible for the black color of the plagioclase. In some plagioclase specimens the fine inclusions have coarsened by Ostwald ripening to acicular grains of hematite ~10 μ m in length. In addition, coarser crystals or clusters of crystals of ilmenite or magnetite, ~1 mm in diameter, are observed and on occasion are rimmed by late amphibole that formed presumably by late deuteric hydration. Electron microprobe and X-ray fluorescence analyses of the plagioclase reveal significant contents of the Si4O8 (up to 2.2 mole %) and AlAl3SiO8 (up to 6.6 mole %) end members. A positive relationship is observed between the content of these two end members. Furthermore, the ratio of Si4O8 to AlAl3SiO8 is between 3 to 4. We believe that the high content of Si4O8 and AlAl3SiO8 in plagioclase, and the presence of minute Fe-Ti oxide inclusions are both products of a late postmagmatic oxidation via the following chemical reactions: 1) 4Ti2+Fe2+Si3O8 + 2O2 -> 4FeTiO3 + 3 Si4O8 2) 12Fe2+Fe2+Si3O8 + 4O2 -> 8Fe3O4 + 9 Si4O8 3) 8Fe2+Al2Si2O8 + 2O2 -> 4Fe2O3 + 3 Si4O8 + 4AlAl3SiO8 4) 4Ti2+Al2Si2O8 + 4Fe2+Al2Si2O8 + 2O2 -> 4FeTiO3 + 3 Si4O8 + 4AlAl3SiO8 These reactions indicate that significant amounts of Fe2+ and Ti2+ were originally present in the tetrahedral and cavity sites of the plagioclase. We conclude that plagioclase in the Lac Saint-Jean anorthosite crystallized under fairly reducing conditions and later underwent postmagmatic oxidation when the plutonic mass was uplifted and came into contact with crustally-derived oxygenated fluids.

  19. Partition coefficients for iron between plagioclase and basalt as a function of oxygen fugacity - Implications for Archean and lunar anorthosites

    NASA Technical Reports Server (NTRS)

    Phinney, W. C.

    1992-01-01

    As a prelude to determinations of the content of total iron as FeO(T) in melts in equilibrium with calcic anorthosites, the partition coefficients (Ds) for FeO(T) between calcic plagioclase and basaltic melt were determined, as a function of oxygen fugacity (f(O2)), for a basaltic composition that occurs as matrices for plagioclase megacrysts. Results showed that, at the liquidus conditions, the value of D for FeO(T) between calcic plagioclase and tholeiitic basalt changed little (from 0.030 to 0.044) between the very low f(O2) of the iron-wustite buffer and that of the quartz-fayalite-magnetite (QFM) buffer. At fugacities above QFM, the value for D increased rapidly to 0.14 at the magnetite-hematite buffer and to 0.33 in air. The increase in D results from the fact that, at f(O2) below QFM, nearly all of the Fe is in the Fe(2+) state; above QFM, the Fe(3+)/Fe(2+) ratio in the melt increases rapidly, causing more Fe to enter the plagioclase which accepts Fe(3+) more readily than Fe(2+).

  20. Plagioclase-rich Objects in the Ungrouped E3 Chondrite LEW 87234: Petrology and Al-Mg Isotopic Data

    NASA Astrophysics Data System (ADS)

    Grossman, J. N.; MacPherson, G. J.; Hsu, W.; Zinner, E. K.

    1995-09-01

    LEW 87234 (paired with LEW 87223, LEW 87057, and LEW 87220) is an unusual E3 chondrite with unique textural and compositional properties [1]. Although it was described as an anomalous EL3 with an unusual history [2], and it does have such characteristic features as only ~0.5% Si in metal and an EL3 size-frequency distribution of chondrules, LEW87234 also contains extraordinarily high abundances of several classes of objects that are rare or absent in EL3's. It is better classified as an ungrouped E3 chondrite. Among its unusual components are a wide variety of Ca-Al-rich objects, including chondrules that contain primary (igneous) calcic plagioclase, isolated plagioclase grains, and plagioclase-rich rims around metal grains; some of these occurrences were first noted by [3]. Besides being noteworthy features of this meteorite in their own right, these aluminous objects represent the first opportunity to obtain Al-Mg isotopic data on material from an enstatite chondrite, albeit an odd one. We found ~12 Al-rich chondrules in three thin sections of LEW 87234 by detailed x-ray mapping, all containing crystalline plagioclase. A large metal grain with a thick, plagioclase-rich rim was found. A large (~230 micrometers max.), isolated, triangular fragment of a single plagioclase crystal was also found in LEW 87234. Similar analyses of single thin sections of MAC 88136, ALH 85119 and PCA 91020, all EL3 chondrites, revealed only "normal" chondrules with aluminous mesostases (also present in LEW 87234), and no plagioclase-rich objects or rims. Petrographic analysis showed that LEW 87234 has 28 vol.% chondrules that are larger than 250 micrometers (in thin section), similar to the EL3 group mean (including EET 90299 and the 3 listed above) of 32 vol.%, but much higher than the EH3 mean of only 12 vol.% large chondrules (for Qingzhen, Kota-Kota, and ALHA 77295). Four plagioclase-rich chondrules from one thin section of LEW 87234 and the plagioclase fragment were analyzed. The

  1. Partition coefficients for iron between plagioclase and basalt as a function of oxygen fugacity - Implications for Archean and lunar anorthosites

    NASA Technical Reports Server (NTRS)

    Phinney, W. C.

    1992-01-01

    As a prelude to determinations of the content of total iron as FeO(T) in melts in equilibrium with calcic anorthosites, the partition coefficients (Ds) for FeO(T) between calcic plagioclase and basaltic melt were determined, as a function of oxygen fugacity (f(O2)), for a basaltic composition that occurs as matrices for plagioclase megacrysts. Results showed that, at the liquidus conditions, the value of D for FeO(T) between calcic plagioclase and tholeiitic basalt changed little (from 0.030 to 0.044) between the very low f(O2) of the iron-wustite buffer and that of the quartz-fayalite-magnetite (QFM) buffer. At fugacities above QFM, the value for D increased rapidly to 0.14 at the magnetite-hematite buffer and to 0.33 in air. The increase in D results from the fact that, at f(O2) below QFM, nearly all of the Fe is in the Fe(2+) state; above QFM, the Fe(3+)/Fe(2+) ratio in the melt increases rapidly, causing more Fe to enter the plagioclase which accepts Fe(3+) more readily than Fe(2+).

  2. Multiphase inclusions in plagioclase from anorthosites in the Stillwater Complex, Montana: implications for the origin of the anorthosites

    USGS Publications Warehouse

    Loferski, P.J.; Arculus, R.J.

    1993-01-01

    Multiphase inclusions, consisting of clinopyroxene+ilmenite+apatite, occur within cumulus plagioclase grains from anorthosites in the Stillwater Complex, Montana, and in other rocks from the Middle Banded series of the intrusion. The textures and constant modal mineralogy of the inclusions indicate that they were incorporated in the plagioclase as liquid droplets that later crystallized rather than as solid aggregates. Their unusual assemblage, including a distinctive manganiferous ilmenite and the presence of baddeleyite (ZrO2), indicates formation from an unusual liquid. A process involving silicater liquid immiscibility is proposed, whereby small globules of a liquid enriched in Mg, Fe, Ca, Ti, P, REE, Zr and Mn exsolved from the main liquid that gave rise to the anorthosites, became trapped in the plagioclase, and later crystallized to form the inclusions. The immiscibility could have occurred locally within compositional boundaries around crystallizing plagioclase grains or it could have occurred pervasively throughout the liquid. It is proposed that the two immiscible liquids were analogous, n terms of their melt structures, to immiscible liquid pairs reported in the literature both in experiments and in natural basalts. For the previously reported pairs, immiscibility is between a highly polymerized liquid, typically granitic in composition, and a depolymerized liquid, typically ferrobasaltic in composition. In the case of the anorthosites, the depolymerized liquid is represented by the inclusions, and the other liquid was a highly polymerized aluminosilicate melt with a high normative plagioclase content from which the bulk of the anorthosites crystallized. Crystallization of the anorthosites from this highly polymerized liquid accounts for various distinctive textural and chemical features of the anorthosites compared to other rocks in the Stillwater Complex. A lack of correlation between P contents and chondrite-normalized rare earth element (REE) ratios

  3. Diffusion-Reaction Between Basaltic Andesite and Gabbro at 0.5 GPa: an Explanation for Anorthitic Plagioclase?

    NASA Astrophysics Data System (ADS)

    Lundstrom, C. C.; Boudreau, A. E.; Pertermann, M.

    2004-12-01

    Despite the remarkably smooth variation in bulk composition of erupted lavas at Arenal volcano (1968-2003), mineral compositions vary widely. Plagioclase ranges from An52 to An95 while Cr2O3 in CPX varies from 0.7 to 0.05 wt % (Streck et al., 2003). To address the question "how do bulk compositions remain near-steady-state while crystal compositions vary widely," we have performed 2 diffusion-reaction experiments in the piston cylinder at 0.5 GPa. These juxtaposed Arenal basaltic andesite AR-8 at 1200° C with a Stillwater Complex gabbro, lying in a thermal gradient toward the piston. In one experiment, we synthesized a glass-plagioclase (An67-75) aggregate of AR-8 in a graphite-Pt-Ti capsule at P-T, polished one end, dried tracer solutions of 45Ca, 6Li, 84Sr and 136Ba on its surface, and juxtaposed it with gabbro for 13 days. Profiles of bulk composition as a function of distance from the interface show that AR-8 gains Al2O3, MgO and CaO from the gabbro and loses Na2O, K2O, SiO2 and FeO to it. Notably, a plagioclase rich (65%) layer develops at the interface between the two materials as CPX disappears. This layer and the compositional profiles are reproduced by diffusion-reaction models using IRIDIUM (Boudreau, 2003). Plagioclase at the interface develops a texture of homogeneous anorthitic cores (An90) that abruptly shift to 10μ m rims having compositions (An67) in Na-Ca exchange equilibrium with the co-existing melt. A beta track map shows that 45Ca is incorporated into the plagioclase cores while SIMS analyses indicate isotopic equilibration between core and melt. Thus, these anorthitic plagioclase result from diffusion-reaction with efficient chemical communication between the melt and the plagioclase core. Microchannels cutting through the rim, rather than solid-state diffusion, appear to control re-equilibration. Other observations from the experiment parallel Arenal lavas: Mg# variation in OPX is small in both experiments and lavas while profiles of Cr

  4. Application of the Plagioclase-Liquid Hygrometer to the Bishop Tuff: Consistency with Melt Inclusion H2O Contents

    NASA Astrophysics Data System (ADS)

    Jolles, J.; Lange, R. A.

    2015-12-01

    High-silica (74-77 wt% SiO2) rhyolites are the most evolved magmas on Earth and constitute some of the largest eruptions (1000s of km3). Of these, one classic example is the Bishop Tuff, a 760 ka eruption of >670 km3 of high-silica rhyolite erupted from Long Valley caldera, CA. Documenting dissolved H2O contents is crucial for understanding its origin and evolution. Analyses of water contents measured in quartz-hosted melt inclusions from the Bishop Tuff (Wallace et al. 1999; Anderson et al. 2000) show that the Early and Middle Bishop Tuff (Ig1Eb, Ig2Ea) have higher water contents (≤ 6.3 wt% H2O) than the Late Bishop Tuff (Ig2NWa; ≤ 5.2 wt%). Our work utilizes the revised plagioclase-liquid hygrometer (Waters & Lange, 2015), which is applicable to rhyolite, to evaluate internal consistency between Fe-Ti oxide temperatures, the plagioclase hygrometer, and melt inclusion H2O analyses. Two-oxide thermometry (Ghiorso & Evans, 2008), using all possible Fe-Ti oxide pairs (between 56 and 1500 pairs for individual samples), was carried out on 2-3 pumice clasts for each sampled eruptive unit. Resulting temperatures (°C ± 1σ) for individual clasts are: 705 ± 12, 728 ± 10 for unit Ig1Eb; 710 ± 12, 728 ± 11 for unit Ig2Ea; 752 ±10, 776 ± 8, 778 ± 7 for unit Ig2NWa; 791 ± 7, 795 ± 8 for unit Ig2Nb. The compositions of the most calcic plagioclase phenocrysts in the Early and Middle units are An17-19, whereas in the Late units they are An29-30. When the Fe-Ti oxide temperatures, whole rock analyses, and plagioclase compositions are incorporated into the plagioclase hygrometer, they give water contents at the onset of plagioclase crystallization of 6.6-6.9 wt% for the Early and Middle units and 4.8-4.9 wt % for the Late units. These results show internal consistency between melt inclusion analyses of water, Fe-Ti oxide thermometry, and the plagioclase-liquid hygrometer; they further support a temperature gradient across the Early, Middle, and Late Bishop Tuff units.

  5. Crystal origins and magmatic system beneath Ngauruhoe volcano (New Zealand) revealed by plagioclase textures and compositions

    NASA Astrophysics Data System (ADS)

    Coote, Alisha C.; Shane, Phil

    2016-09-01

    The textural variation and compositional zoning of plagioclase in pre-historic and historic basaltic andesite lava flows from Ngauruhoe volcano reveals extensive crystal recycling from a multi-level magma system. Most phenocrysts have a calcic ( An80-90) resorbed core with diffuse or no zonation that is depleted in Fe and Mg. Some cores display patchy zonation from replacement by high An crystallization prior to resorption. The cores are mantled by oscillatory-zoned rims of lower An content (< An60), and are enriched in Fe and Mg. Rim zones vary in relative thickness and textural complexity, and include sieve-textured bands, and/or cyclic calcic growth following dissolution events. A subordinate crystal population display similar features, but lack a resorbed core. These latter crystals display overall rimward enrichment in An, Fe and Mg. The resorbed cores crystallized from magmas more mafic than those erupted at Ngauruhoe, and slow cooling and prolonged storage resulted in loss of An zoning patterns and depletion of Fe and Mg by diffusion. These crystals are likely to have originated from deep cumulates or intrusions, and were subsequently entrained in ascending magmas. Patchy-textured cores were produced during decompression in a water under-saturated magma and staged ascent. The diversity in crystal cores reflect different conduits and ascent histories. The crystal rims grew in a more differentiated magma reservoir, and are in equilibrium with the erupted melt. Most of the zoning patterns in the rim zone require water pressure and/or temperature changes. These changes could have been caused by convective self-mixing in a closed system and/or the intrusion of hydrous melts of similar bulk composition. Other crystals display rimward elemental enrichments consistent with mafic recharge. Previously reported rimward enrichment in 87Sr-86Sr compositions can be explained by the re-cycled origin of the crystal cores and progressive crustal assimilation at shallower

  6. Lunar rock types - The role of plagioclase in non-mare and highland rock types

    NASA Technical Reports Server (NTRS)

    Hubbard, N. J.; Gast, P. W.; Nyquist, L. E.; Rhodes, J. M.; Bansal, B. M.; Wiesmann, H.; Shih, C.-Y.

    1973-01-01

    Some nonmare and highland rock types (14310 type KREEP and very high Al2O3 basalts) have the internal chemical variations expected for a plagioclase-liquid system. The observed Eu variations in these rock types suggest a D(Eu) 1/p of 0.6 to 0.7. The Sr variations suggest a D(Sr) 1/p of about 0.6, with values as low as 0.35 suggested for some materials from sample 14063. Common Apollo 14 KREEP and Apollo 15 KREEP do not show internal Sr, Eu, Al2O3 variations consistent with the D(Eu, Sr) 1/p values derived for 14310 type KREEP. Major element and experimental data indicate that olivine or pyroxene is a large, perhaps dominant, controller of chemical variations within common Apollo 14 KREEP. The application of these distribution coefficients to pure anorthosites like 15415 yields the model dependent conclusion that the silicate liquids with which such anorthosites may have been chemically equilibrated have not yet been analyzed and perhaps not directly sampled.

  7. Oxygen isotope heterogeneity of arc magma recorded in plagioclase from the 2010 Merapi eruption (Central Java, Indonesia)

    NASA Astrophysics Data System (ADS)

    Borisova, Anastassia Y.; Gurenko, Andrey A.; Martel, Caroline; Kouzmanov, Kalin; Cathala, Annick; Bohrson, Wendy A.; Pratomo, Indyo; Sumarti, Sri

    2016-10-01

    Chemical and isotopic compositions of magmatic crystals provide important information to distinguish between deep juvenile and crustal contributions. In this work, high-resolution multicollector secondary ion mass spectrometry data reveal strong variations of δ18O values in three plagioclase crystals (800-1700 μm) from two representative basaltic andesite samples of the 2010 Merapi eruption (Central Java, Indonesia). The δ18O values (from 4.6‰ to 7.9‰) are interpreted to reflect oxygen isotope heterogeneity in the melt composition during plagioclase growth. The lowest δ18O values (4.6-6.6‰) are found in anorthite-rich cores (An82-97), whereas higher δ18O values (5.7-7.9‰) are found in anorthite-poorer zones (An33-86), typically in crystal rims. Combining these new plagioclase δ18O data with δ18O of calc-silicate crustal xenoliths erupted between 1994 and 1998, the composition of glass inclusions hosted by the anorthite-rich plagioclase (An82-92), available experimental data, and the results of thermodynamic modeling using the Magma Chamber Simulator code, we conclude that the abundant anorthite-rich cores crystallized from a mantle-derived hydrous basaltic to basaltic trachyandesite melt that recharged a deeper (200-600 MPa) magma storage zone, whereas lower anorthite zones crystallized at shallower levels (100-200 MPa). The oxygen isotope variations in the plagioclase are explained by a two-stage model of interaction of the hydrous, mafic mantle-derived magma (1) with old crustal rocks depleted in 18O due to high temperature alteration that yielded the low δ18O values in the anorthite-rich cores at deep levels (13-20 km), and later (2) with 18O-enriched carbonate material that yielded the high δ18O values in anorthite-poorer zones at shallow levels (∼4.5-9 km). Thermodynamic modeling is consistent with ∼18 wt.% assimilation of crustal calc-silicate material at 925-950 °C and 100-200 MPa by the 2010 Merapi basaltic andesite magma prior to

  8. Synthetic and natural plagioclases: iron variations and its influence on VNIR reflectance spectra

    NASA Astrophysics Data System (ADS)

    Carli, Cristian; Orlando, Andrea; Borini, Daniele; Giuli, Gabriele; Serventi, Giovanna; Pratesi, Giovanni; Sgavetti, Maria

    2017-04-01

    Besides being one of the most important rock-forming phases, plagioclase (pl) is a common surface mineral in several Solar System bodies. In particular, pl is present in meteorites and lunar samples, both in lunar Highland, where it is the dominant phase, and Maria samples. Moreover, pl has been detected in Martian meteorites, as well as in HEDs. In visible and near-infrared (VNIR) reflectance spectroscopy this phase is characterized by a crystal field (C.F.) absorption band in iron-bearing samples. In particular, Burns (1993) summarized the electronic absorptions due to iron, pointing out: 1) a broad absorption around 1.25 μm related to a C.F. transition due to Fe2+ replacing Ca2+ in seven-fold coordinated sites; 2) narrow absorptions around 0.4 μm related to tetrahedrally coordinated Fe3+ ions replacing Al in the tetrahedral sites. A better understanding of the spectral properties of Fe2+-pl can be an important tool to investigate the spectral influence of pl in regolith material in which it can be mixed with variable amount of other components with variable abundance. This goal can be reached working on synthetic pl with variable FeO and An contents, which must be well characterized to be sure about the attribution of absorption bands seen in reflectance spectra, as well as working on well characterized (in term of An, iron amount and Fe2+/Fe3+) terrestrial pl. Future rover mission will have onboard hyperspectral instrument working in the VNIR with relative high spatial resolution and, so, with the possibility of measured pl crystals. For this reason, working more in detail on iron bearing plagioclase can be an important task. Here, we present our results on synthetic An90 mol% pl with different iron contents (0, 0.5 and 1.0 FeO wt%) with the aim to investigate the effects of iron substitution on the VNIR spectra of pl. Reagent-grade oxides and carbonates reactants used as starting materials were thoroughly mixed to ensure homogeneity. Each experimental charge

  9. Effect of infiltrated water on rheology of plagioclase feldspar under lower crustal condition

    NASA Astrophysics Data System (ADS)

    Kido, M.; Muto, J.; Koizumi, S.; Nagahama, H.

    2016-12-01

    Fluids in the deep crust have an important role in deformation of lithosphere and seismicity. In this study, we performed deformation experiments to reveal rheological properties of plagioclase feldspars as a main constituent of crustal materials with infilitrated water. Axial compression tests on synthetic polycrystalline anorthite (An) were performed in a Griggs-type deformation apparatus at temparature of 900 °C, strain rates of roughly about 10-5 s-1 and various confining pressures of 0.8-1.4 GPa. Distilled water was added on samples before tests. Times for infiltration of water into samples were changed to investigate the variation of strength associated with diffusion of water. Strengths of wet An tended to decrease with infiltration time or strain magnitude. If other conditions such as temperature, time and strain being the same, strengths increase with confining pressures. Recovered samples show that deformation was concentrated in the lower part of samples. Differential stresses were significantly lower than predicted values by a previous flow law for wet An obtained by low pressure gas apparatus ( 0.4 GPa, Rybacki et al., 2006). This implies that the effect of water on mechanical behavior in higher pressure might be larger than those predicted by lower pressure experiments. Ideal water concentration and strength profile of internal of samples were estimated by one-dimensional model of grain boundary diffusion. Estimated strength of internal part of samples was significant higher than measured stresses. There is possibility that cataclastic flow partially occurred in samples. In addition, deformation-enhanced fluid flow probably occurred. In conclusion, strength of wet An depends on water infiltration time, strain magnitude and confining pressure. The results suggest that the strength of fluid-rich regions in the lower crust becomes lower than that predicted by previous studies.

  10. Mechanism of plagioclase dissolution in acid solution at 25[degrees]C

    SciTech Connect

    Oxburgh, R.; Drever, J.I.; Sun, Yan-Ting )

    1994-01-01

    The dissolution kinetics of three plagioclase feldspars (An[sub 13], An[sub 46], and An[sub 76]) were studied in flow-through reactors over the pH range 3-7. In accordance with the surface complexation model, dissolution rate was described by the equation Rate = [chi][sub a]A exp(E[sub a]/kT) (C[sup 5][sub H])[sup n], where (C[sup 5][sub H]) is the concentration of protonated surface sites, [chi][sub a] is the mole fraction of these that are activated, A is the Arrhenius pre-exponential factor, and E[sub a] is the activation energy for the reaction. The reaction order with respect to surface charge (n) at pH < 5 is a function of mineral composition, and increases with increasing anorthite content. Orders of 0.46, 1.2, and 2.0 are obtained for oligoclase, andesine, and bytownite, respectively. This indicates that the reaction intermediate is more highly protonated in feldspars of higher aluminum content. The nonintegral reaction orders obtained demonstrate the importance of several surface species in the dissolution reaction. Reaction rates increase with increasing anorthite content. In acid solution this reflects the linear dependence of E[sub a] on the number of Al-O-Si bonds involved in the rate-determining step. Under conditions of low surface protonation (in near-neutral solutions), dissolution is independent of surface charge. As reaction occurs at neutral alumina sites, the rate is approximately proportional to the anorthite (Al) content of the feldspar.

  11. Modeling H, Na, and K diffusion in plagioclase feldspar by relating point defect parameters to bulk properties

    NASA Astrophysics Data System (ADS)

    Zhang, Baohua; Shan, Shuangming; Wu, Xiaoping

    2016-02-01

    Hydrogen and alkali ion diffusion in plagioclase feldspars is important to study the evolution of the crust and the kinetics of exsolution and ion-exchange reactions in feldspars. Using the available PVT equation of state of feldspars, we show that the diffusivities of H and alkali in plagioclase feldspars as a function of temperature can be successfully reproduced in terms of the bulk elastic and expansivity data through a thermodynamic model that interconnects point defect parameters with bulk properties. Our calculated diffusion coefficients of H, Na, and K well agree with experimental ones when uncertainties are considered. Additional point defect parameters such as activation enthalpy, activation entropy, and activation volume are also predicted. Furthermore, the electrical conductivity of feldspars inferred from our predicted diffusivities of H, Na, and K through the Nernst-Einstein equation is compared with previous experimental data.

  12. Chemical bonding and electronic structures of microcline, orthoclase and the plagioclase series by X-ray photoelectron spectroscopy.

    PubMed

    Kloprogge, J Theo; Wood, Barry J

    2015-02-25

    A detailed analysis was undertaken of the X-ray photoelectron spectra obtained from microcline, orthoclase and several samples of plagioclase with varying Na/Ca ratio. Comparison of the spectra was made based on the chemical bonding and structural differences in the Al- and Si-coordination within each specimen. The spectra for Si 2p and Al 2p vary with the change in symmetry between microcline and orthoclase, while in plagioclase an increase in Al-O-Si linkages results in a small but observable decrease in binding energy. The overall shapes of the O 1s peaks observed in all spectra are similar and show shifts similar to those observed for Si 2p and Al 2p. The lower-VB spectra for microcline and orthoclase are similar intermediate between α-SiO2 and α-Al2O3 in terms of binding energies. In the plagioclase series increasing coupled substitution of Na and Si for Ca and Al results in a change of the overall shape of the spectra, showing a distinct broadening associated with the presence of two separate but overlapping bands similar to the 21 eV band observed for quartz and the 23 eV band observed for corundum. The bonding character for microcline and orthoclase is more covalent than that of α-Al2O3, but less than that of α-SiO2. In contrast, the plagioclase samples show two distinct bonding characters that are comparable with those of α-SiO2 and α-Al2O3. Copyright © 2014 Elsevier B.V. All rights reserved.

  13. Constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the Samail ophiolite

    NASA Astrophysics Data System (ADS)

    VanTongeren, J. A.; Hirth, G.; Kelemen, P. B.

    2015-10-01

    Oceanic crust represents more than 60% of the earth's surface and despite a large body of knowledge regarding the formation and chemistry of the extrusive upper oceanic crust, there still remains significant debate over how the intrusive gabbroic lower oceanic crust is accreted at the ridge axis. The two proposed end-member models, the Gabbro Glacier and the Sheeted Sills, predict radically different strain accumulation in the lower crust during accretion. In order to determine which of these two hypotheses is most applicable to a well-studied lower crustal section, we present data on plagioclase lattice preferred orientations (LPO) in the Wadi Khafifah section of the Samail ophiolite. We observe no systematic change in the strength of the plagioclase LPO with height above the crust-mantle transition, no dominant orientation of the plagioclase a-axis lineation, and no systematic change in the obliquity of the plagioclase LPO with respect to the modal layering and macroscopic foliation evident in outcrop. These observations are most consistent with the Sheeted Sills hypothesis, in which gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with increasing depth in a Gabbro Glacier. Our data support the hypothesis of MacLeod and Yaouancq (2000) that the rotation of the outcrop-scale layering from sub-horizontal in the layered gabbros to sub-vertical near the sheeted dikes is due to rapid vertical melt migration through upper gabbros close to the axial magma chamber. Additionally, our results support the hypothesis that the majority of extensional strain in fast spreading ridges is accommodated in partially molten regions at the ridge axis, whereas in slow and ultra-slow ridges large shear strains are accommodated by plastic deformation.

  14. 238U-230Th-226Ra disequilibria in plagioclase from recent mixed magmas at Mount Hood: constraints on crystal storage timescales and eruption triggering processes

    NASA Astrophysics Data System (ADS)

    Eppich, G. R.; Cooper, K. M.; Kent, A. J.; Koleszar, A. M.

    2010-12-01

    Uranium-series crystal ages, interpreted within a textural and geochemical framework, can provide insight into crystal storage timescales and eruption triggering processes. Mount Hood is a mixing-driven volcano that consistently erupts magmas of intermediate composition. Mixed magmas incorporate plagioclase derived from mafic and silicic end-member magmas (Kent AJR; Darr C; Koleszar AM; Salisbury MJ; Cooper KM. 2010. Preferential eruption of andesitic magmas through recharge filtering. Nature Geosci.). We measured 238U-230Th-226Ra disequilibria for four plagioclase size fractions, groundmass separates, and mafic inclusions from the Timberline (1500 a) and Old Maid (215 a) eruptive sequences. Measured (230Th)/(238U) was 1.126-1.143 for Timberline plagioclase and 1.127-1.143 for Old Maid plagioclase. Measured (226Ra)/(230Th) was 1.22-1.62 for Timberline plagioclase and 1.27-1.43 for Old Maid plagioclase. Corrections were performed for the presence of groundmass in the >500 µm plagioclase separates, and large plagioclase + groundmass in the <500 µm plagioclase separates. Small (<500 µm) plagioclase, derived from mafic magma, records enriched present-day (226Ra)/Ba relative to equilibrium with liquid proxies (groundmass and mafic inclusions), leading to undefined Ra-Th model ages. However, the measured disequilibria require that the majority of plagioclase in the separate is young (<<10 ka). Large (>500 µm) plagioclase, derived from the silicic mixing end-member, records (238U)/(230Th) disequilibrium and minor (226Ra)/(230Th) disequilibrium. Ra-Th model ages are 5-10 ka. Rims of large plagioclase crystallized within weeks of eruption (Kent et al., 2010), suggesting that the 5-10 ka Ra-Th model ages are averages of young rim growth and old cores, which could be >10 ka. Crystallization timescales of large plagioclase cores are longer than timescales of repose between eruptions, suggesting that the crystals must have been present yet remained untapped during older

  15. Pristine moon rocks - An alkali anorthosite with coarse augite exsolution from plagioclase, a magnesian harzburgite, and other oddities

    NASA Technical Reports Server (NTRS)

    Warren, P. H.; Jerde, E. A.; Kallemeyn, G. W.

    1990-01-01

    Results are presented on the analyses of 18 samples of pristine rocks obtained from the primarily mare Apollo 12 site and from the primarily highland Apollo 14 site, as well as samples from the nonmare Apollo 15 site. It was found that, while two of anorthosites from Apollo 12 were similar in composition to most other anorthosites from the west-central near region, the texture of an alkali anorthosite featured a long and narrow crystal of augite surrounded by a single crystal of plagioclase, clearly suggesting that the augite formed by exsolution out of the plagioclase. Another Apollo 12 rocklet was an unusual magnesian harzburite, with subequal amounts of enstatite and olivine, traces of Cr-Fe spinel, and FeNi metal, but no plagioclase; the bulk composition was found to be remarkably Ir-rich (53 percent) for a pristine rock, and the texture was also unusual. Apollo 14 samples included several uncommonly Al-rich and REE-poor impact melt breccias.

  16. Insight Into the Magmatic Evolution of Fernandina Volcano, Galapagos, From Olivine- and Plagioclase-Hosted Melt Inclusions

    NASA Astrophysics Data System (ADS)

    Hedfield, E. J.; Geist, D. J.

    2003-12-01

    Lavas erupted subaerially from Fernandina Volcano have previously been characterized as being evolved, well-mixed, and homogeneous (Allan and Simkin 2000), obscuring details of earlier stages of magmatic evolution. In this study, phenocrysts and melt inclusions from recent submarine lava samples and representative subaerial lava samples are investigated to gain insight into earlier stages of Fernandina's magmatic evolution. Olivine is present as three populations: unzoned high-forsterite Type I olivine, unzoned low-forsterite Type II olivine, and normally-zoned Type III olivine with Type I cores and Type II rims. Melt inclusions were rehomogenized at 1180C and drop-quenched to remove the effects of post-entrapment crystal growth. Type I olivine contain highly variable melt inclusions (K2O/TiO2 from 0.03 to 0.29, compared to glass and whole rock values of 0.15 plus or minus 0.01). The variability of the inclusions can be produced by 1 to 2 percent fractional melting of a MORB-like mantle source. Plagioclase and Type II olivine contain more homogeneous melt inclusions that define a trend of fractional crystallization of olivine, plagioclase, and clinopyroxene from a parental melt represented by an average Type I melt inclusion. Type I olivine textures and olivine/host glass disequilibrium suggest that this olivine crystallized from primitive, compositionally-diverse magmas and is exotic. Type II olivine most likely grew in the evolving liquid prior to eruption but after homogenization by mixing. All olivine types consistently demonstrate an equilibrium relationship with their reheated melt inclusions. For Type I olivine, the olivine/inclusion equilibrium and the exotic origin suggest that olivine phenocrysts resided at a temperature close to the trapping/crystallization temperature until shortly before eruption. For Type II olivine, the equilibrium with its inclusions and its host glass indicates that crystal growth occurred shortly before eruption. Post

  17. Plagioclase textures and compositional zoning in recent Mt. Etna lavas: physical-chemical constrains of the shallow feeding system

    NASA Astrophysics Data System (ADS)

    Giacomoni, P.; Coltorti, M.; Ferlito, C.; Bonadiman, C.; Lanzafame, G.

    2013-12-01

    Textures and compositional zoning of plagioclase reflect changes in the physical and chemical conditions of the host magma, thus representing an useful tool to investigate the processes occurring in the feeding system. An open-conduit volcano such as Mt. Etna is the perfect site to investigate interactions between basaltic melts with different volatiles content and the effect of degassing on mineral phase nucleation and growth. In this context, an accurate textural study on more than 130 samples from recent Mt. Etna eruptions (2001, 2002-2003, 2004-2005 and 2006) was carried out. A classification scheme was developed, taking into account the different portions of the crystals constituted by euhedral or rounded (clear, patchy, sieved and dusty) cores and dusty or melt inclusion alignment rims, divided by oscillatory-zoned overgrowth. In this way, each crystal portion could be attributed to a phase of growth or dissolution that occurs at specific P-T-fO2 and H2O content. Thermo-barometric and crystal-melt equilibrium equations were in fact applied to calculate the intensive parameters such as P, T, fO2 and the amount of dissolved water content for each portion of a single crystal [1, 2, 3]. Equilibrium conditions were checked assuming primary melt composition for plagioclase cores and whole rock for plagioclase rims. The obtained data were compared with MELTs modeling aiming at constraining plagioclase composition and theoretical stability field. Results indicate that Mt. Etna plumbing system is rather continuous, excluding the presence of relevant well-defined magma chambers. This feeding system is persistently filled by magmas with similar composition in terms of major and trace element but different dissolved H2O, which constrain the depth and composition of plagioclase cores. A small number of these cores (patchy) nucleate at 14 km of depth, whereas most of them (clear, sieved and dusty) nucleate between 7 and 4 km below the summit. At this depth, mixing probably

  18. Magma mixing and degassing recorded in plagioclase from the shallow magma body at Stromboli (Aeolian Archypelago, Italy)

    NASA Astrophysics Data System (ADS)

    Landi, P.; Metrich, N.; Bertagnini, A.; Rosi, M.

    2003-04-01

    Stromboli has produced nearly-aphyric pumice and crystal-rich scoriae since the beginning of its persistent strombolian activity, 1400-1800 years ago. In spite of their contrasting texture, the highly vesiculated pumice and the dense scoriae rich in mm-sized crystals have virtually the same bulk rock composition, ranging from HK-basaltic andesite to HK-basalt. It is thought that the crystal-rich magma at the origin of the dense scoriae derives from the volatile-rich melt, emitted as pumice, via low pressure crystallization induced by water loss. Such characteristics make the shallow, crystal-rich body, a study-case to investigate the mechanisms of crystallization during rapid degassing. We have studied different crystal-rich products, both scoriae and lavas, of the 1985-2000 period of activity. All of them, contain 47-55 vol% euhedral phenocrysts of plagioclase, the dominant phase, clinopyroxene and olivine embedded in a glassy to hypocrystalline, homogeneous shoshonitic groundmass. Crystallization history and magma dynamics are mainly discussed on the basis of the chemical and textural zoning of the plagioclase phenocrysts. They consist of alternating, concentric layers of An-rich and Ab-rich plagioclase. The Ab-rich layers (An64-An70) are characterized by a small-scale (1-5 mm) oscillatory zoning and appear to be in equilibrium with a liquid with the composition of the glassy matrix. The An-rich zones (An70-An88) are patchy zoned, show sieve texture with abundant micrometric glass inclusions and voids, and overgrow on dissolution surfaces. We propose that zoning of the plagioclase reflects successive intrusions of volatile-rich melts in the shallow crystal-rich mush. The high H_2O content of ascending melt blobs stabilizes An-rich plagioclase. Sieve textures result from rapid crystallization occurring under supercooling conditions, which are induced by rapid degassing of the volatile-rich magma blobs when they react with the shallow crystal-rich magma, at low

  19. Plagioclase and pyroxene hosted melt inclusions in basaltic andesites of the current eruption of Arenal volcano, Costa Rica

    NASA Astrophysics Data System (ADS)

    Streck, Martin J.; Wacaster, Sue

    2006-09-01

    In this study, we investigated melt inclusions hosted in pyroxene and plagioclase to find direct evidence for the composition of melt components in the pre-eruptive magmas of the current eruption of Arenal volcano and to further shed light on the petrogenetic history of remarkably uniform basaltic andesitic bulk compositions. Composition of melt inclusions ranges widely regardless of whether an inclusion is hosted by plagioclase or by pyroxene and whether crystals with inclusions came from tephra samples or from slower emplaced lava flows. However, inclusions from each type of host mineral have distinct evolutionary trends most consistent with being mainly introduced by post-emplacement crystallization of the enclosing host, which is also supported by the composition of groundmass glasses. At the least-modified compositions, plagioclase and pyroxene inclusion trends overlap allowing for identification of melt compositions entrapped by both phases and, thus, strongly suggesting that these melt compositions existed in the reservoir prior to entrapment. Most of these are "dacitic" (61 to 64 wt.% SiO 2) and strongly match phenocryst-poor dacitic magmas of earlier eruptive phases of Arenal [Borgia, A., Poore, C., Carr, M.J., Melson, W.G., Alvarado, G.E., 1988. Structural, stratigraphic, and petrologic aspects of the Arenal-Chato volcanic system, Costa Rica: evolution of a young stratovolcanic complex. Bull Volcanol, 50, 86-105], suggesting earlier dacitic magmas may have been generated by melt-extraction processes. Correction for host crystallization of some inclusions also suggests that melt components as mafic as ˜ 53 wt.% may have been entrapped. All melt components inferred to have existed in the magmatic reservoirs prior to entrapment have low Mg# (38-45) yielding evidence for liquid compositions required for crystallizing the bulk of observed pyroxene and likely also olivine. Water rich (> 5 wt.%, by difference) and probably undegassed (S = 400-1600 ppm, Cl =

  20. Documenting magmatic processes at Filicudi Island, Aeolian Arc, Italy: Integrating Quantitative Modeling, Plagioclase Textural and In Situ Compositional Data

    NASA Astrophysics Data System (ADS)

    Harris, M. A.; Bohrson, W. A.; Mayfield, A.

    2012-12-01

    Although numerous studies have documented how compositional diversity develops in magmas on Earth, controversy exists regarding the roles that recharge, assimilation and fractional crystallization (RAFC) play in magma evolution. Filicudi Island is one of seven major islands of the Aeolian archipelago in the southern Tyrrhenian Sea, Italy. Previous whole rock geochemical studies of Filicudi Island have related its magmatic and compositional evolution from calc-alkaline basalt (51 wt. % SiO2) to high-K andesite (62 wt. % SiO2) to fractional crystallization and assimilation processes occurring within several small, separated magma chambers (Santo, 2000). New data collected on samples from Filicudi include whole rock elemental and isotopic data, plagioclase textural and in situ elemental and Sr isotope data, and MELTS modeling. Integration of data allows documentation of the roles and relative chronology that RAFC played in magmatic evolution, as well as elucidates aspects of Filicudi's magma plumbing-system structure. Best-fit MELTS results based on several hundred simulations indicate that magmas from Filicudi evolved in a polybaric magma plumbing system. At deeper levels (3.5-4 kbars), fractional crystallization of Mg-rich clinopyroxene, olivine, and spinel played a dominant role, while high initial H2O contents (3-4.5 wt.%) acted to suppress plagioclase crystallization. Upon ascent, in a shallower part of the magmatic system (0.5-1.2 kbars), magma potentially devolatilized, allowing texturally monotonous plagioclase that was relatively An (90-98 mol %) and Fe (0.6-1.0 wt. %) rich and Sr-depleted (900-1300 ppm) to crystallize. As fractional crystallization continued in the deeper chamber, melts that ascended to the shallow part of the system became progressively more silicic as evidenced by plagioclase that became less An (75-90 mol %) and Fe (0.5-0.6 wt. %) rich, and more Sr-rich (1500-1900 ppm). Similarly, continued fractional crystallization at shallow levels

  1. Bingham fluid behavior of plagioclase-bearing basaltic magma: Approach from laboratory viscosity measurements

    NASA Astrophysics Data System (ADS)

    Ishibashi, H.; Sato, H.

    2010-12-01

    be a critical factor for the onset of yield stress. Keywords: magma, viscosity, Bingham fluid, yield stress, plagioclase

  2. Thermal Infrared Spectroscopy of Experimentally Shocked Plagioclase and Basalt and Applications to Mars

    NASA Astrophysics Data System (ADS)

    Johnson, J. R.

    2007-12-01

    Laboratory thermal infrared emission spectra (250-1400 cm-1) of experimentally shocked (17-60 GPa) plagioclase feldspars (bytownite, andesine, and albite), basalt, and basaltic andesite demonstrate the disordering of mineral lattices and increasing glass content with increasing shock pressure. These effects cause loss of spectral detail and shifts in absorption feature positions. Disordering in the feldspar structure begins at pressures >15-20 GPa, and diaplectic glass (maskelynite) formation is complete between ~30-45 GPa. As pressures increase the mutual existence of crystalline phases and diaplectic glasses cause the characteristic, fourfold (tetrahedral), strong coordination bonds of Si and Al in feldspars to alter to weaker, less polymerized bonds that approach sixfold (octahedral) coordination. This influences the characteristic vibrational frequencies in the thermal infrared. For example, the bands near 400-550 cm-1 are caused by bending vibrations in the Si-O-Al planar ring structures in tectosilicates and diaplectic glasses. Si-O-Si octahedral bending vibrations cause absorptions between about 700-450 cm-1 and SiO6 octahedral stretching vibrations occur between 750-850 cm-1. Absorptions in the 900-1200 cm-1 region are due to Si-O antisymmetric stretch motions of silica tetrahedra. Many of these spectral features persist to higher pressures in albite compared to bytownite, possibly due to the relatively lower Al content in albite. With increasing Ca content, the main absorption band of highly shocked albite shifts from ~1050 cm-1 to ~1000 cm-1 for andesine and ~950 cm-1 for bytownite. However, the other main absorption in highly shocked feldspars near 450-460 cm-1 varies little with Ca content. Linear mixing models demonstrate that mineral and glass spectra cannot replicate shocked bytownite spectra beyond shock pressures of 20-25 GPa, coincident with the onset of diaplectic glass formation. Similar models of shocked basalt also exhibit increased errors

  3. Wishstone to Watchtower: Alteration of Plagioclase-rich Rocks in Gusev Crater, Mars

    NASA Astrophysics Data System (ADS)

    Ruff, S. W.; Hamilton, V. E.

    2014-12-01

    The Mars Exploration Rover Spirit encountered a remarkable diversity of rocks during its traverse of the Columbia Hills in Gusev crater, manifested both as variations in primary mineralogy and in secondary alteration. The Wishstone and Watchtower Classes represent examples where the less altered (Wishstone) and more altered versions (Watchtower) were recognized as members of an alteration series identified by variations in geochemistry and Fe-bearing mineral phases [1-3]. Work by [1] demonstrated a geochemical relationship consistent with two-component mixing between a high Al2O3, TiO2, CaO, Na2O, P2O5 end-member and a second end-member enriched in MgO, Zn, S, Br, and Cl. The first end-member probably is Wishstone Class, with Watchtower Class intermediate between it and an unrecognized second end-member lithology [1]. New results using mirror-dust corrected spectra from Spirit's Miniature Thermal Emission Spectrometer [4] affirm the dominant plagioclase component in Wishstone and an amorphous component resembling basaltic glass in Watchtower identified previously [5]. But now we recognize a suite of rocks spanning the full range of alteration, including one dubbed Bruce that appears to be an alteration spectral end-member [4]. The spectra of some rocks with an intermediate level of alteration are well modeled as a simple two-component mixture of Wishstone and Bruce spectra. This is consistent with a style of alteration that progressively obscures spectral contributions of the host rock minerals and is inconsistent with a surface coating. The Bruce spectrum is poorly modeled by primary and secondary phases including phyllosilicates and amorphous silicates. This suggests a style of alteration not recognized in terrestrial settings. Based on the similarity of the Bruce spectrum to TES type 2, this style of alteration may be more widespread on Mars. The conditions that produced this alteration are poorly constrained, hence the relationship to habitability is unknown at

  4. The origin of felsic microgranitoid enclaves: Insights from plagioclase crystal size distributions and thermodynamic models

    NASA Astrophysics Data System (ADS)

    Alves, Adriana; Pereira, Giovanna de Souza; Janasi, Valdecir de Assis; Higgins, Michael; Polo, Liza Angelica; Juriaans, Orlando Stanley; Ribeiro, Bruno Vieira

    2015-12-01

    Magma mixing is widely recognized in contemporary petrology as one of the primary igneous processes. Microgranitoid enclaves (MEs) are considered to be remnants of such mixing processes, and the term has a well-established genetic implication. However, microgranitoid enclaves span a wide range of compositions, and felsic varieties are also frequently reported. Nd-Sr isotope and textural data from felsic microgranitoid enclaves (FMEs), mafic microgranitoid enclaves (MMEs) and host granites from the Salto pluton, Itu Granitic Province, show that the cm-sized MMEs are dioritic, have medium-grained igneous textures and xenocrysts of alkali feldspar and quartz. The FMEs are cm- to meter-sized, have spheric shapes, show corrugated contacts with the host granites, and have resorbed feldspars and deformed quartz crystals interpreted as xenocrysts set in a fine-grained groundmass. Compared to the host granites, both MME and FME samples have increased FeO, MgO, TiO2, P2O5 and Zr contents, but their Sr and Nd isotope signatures are identical: FME 87Sr/86Sri = 0.7088-0.7063, εNdi = - 10.0 to - 10.2; MME 87Sr/86Sri = 0.7070, εNdi = - 10.5; host granite 87Sr/86Sri 0.7056-0.7060, εNdi = - 10.2 to - 10.3. These indicate that the enclaves derive from a similar source, although the melts from which they formed were probably hotter and chemically more primitive than their host granites. Crystal size distributions (CSDs) of plagioclase in samples drilled from rinds and cores of three FMEs show that the rind samples are systematically finer-grained than the samples from the cores, which indicates that the FMEs cooled inwards and contradict interpretations that the FMEs are autoliths. Thermal modeling suggests that a slightly more primitive, hotter magma would be thermally equilibrated with an evolved resident melt within weeks after mixing/mingling. Upon thermal equilibrium, the FMEs would have an increased crystal cargo, and the resulting touching framework would impart a solid

  5. FTIR Analysis of Water in Pyroxene and Plagioclase in ALH 84001 and Nakhlites

    NASA Technical Reports Server (NTRS)

    Peslier, A. H.; Cintala, M. J.; Montes, R.; Cardenas, F.

    2016-01-01

    with crustal reservoirs or hydrothermal fluids. Here, nominally anhydrous minerals (pyroxene, olivine, plagioclase, or maskelynite) in orthopyroxenite ALH 84001 and selected nakhlites are analyzed for water and major elements, in order to determine 1) whether they contain any water; 2) if they do, what controls its distribution (crystallization, degassing, hydrothermal or impact processes); and 3) if any of these measurements can be used to infer the water contents of the parent magma and their mantle sources. A shock-reverberation experiment was also performed on terrestrial orthopyroxenes (opx) to simulate the heavily shocked conditions of ALH 84001 (> 31 GPa [17]).

  6. Ion microprobe investigation of plagioclase and orthopyroxene from lunar Mg-suite norites: Implications for calculating parental melt REE concentrations and for assessing postcrystallization REE redistribution

    SciTech Connect

    Papike, J.J.; Fowler, G.W.; Shearer, C.K.; Layne, G.D.

    1996-10-01

    The lunar Mg-suite, which includes dunites, troctolites, and norites, makes up to 20-30% of the Moon`s crust down to a depth of {approximately}60 km. The remainder is largely anorthosite. This report focuses on norites (which consist mostly of orthopyroxene and plagioclase) because we have found that both phases are effective recorders of their parental melt compositions. In an earlier report, we analyzed orthopyroxene from twelve samples (three from Apollo 14, two from A-15, and seven from A-17) by orthopyroxene from twelve samples (three from Apollo 14, two from A-15, and seven from A-17) by SIMS for eight REE (La, Ce, Nd, Sm, Eu, Dy, Er, Yb). Inversion of these data to estimated melt compositions yielded extremely high REE concentrations similar to KREEP. In this study, we report SIMS REE data for plagioclase from these same twelve samples. The major objective of this study is to estimate parental REE concentrations from both orthopyroxene and plagioclase data to see if both data inversions produce concordant from both orthopyroxene and plagioclase data to see if both data inversions produce concordant melt compositions and thus better constrain the composition of melts parental to Mg-suite norites. The estimated REE concentrations from both phases show some evidence of slight postcrystallization REE redistribution. Comparison of the observed ratio of REE for pyroxene/plagioclase to the ratio of the Ds for pyroxene/plagioclase is consistent with REE redistribution which involves LREE diffusing from pyroxene into plagioclase and HREE diffusing from plagioclase into pyroxene. However, apparently these postcrystallization exchanges have not seriously affected our ability to estimate melt REE concentrations. 34 refs., 7 figs., 5 tabs.

  7. Water content in arc basaltic magma in the Northeast Japan and Izu arcs: an estimate from Ca/Na partitioning between plagioclase and melt

    NASA Astrophysics Data System (ADS)

    Ushioda, Masashi; Takahashi, Eiichi; Hamada, Morihisa; Suzuki, Toshihiro

    2014-12-01

    The variation in water content of arc basaltic magmas in the Northeast Japan arc and the Izu arc was estimated using a simple plagioclase phenocryst hygrometer. In order to construct a plagioclase phenocryst hygrometer optimized for arc basalt magmas, we have conducted high-pressure melting experiments of relatively primitive basalt from the Miyakejima volcano, a frontal-arc volcano in the Izu arc. As a result of the experiments, we found that the Ca/Na partition coefficient between plagioclase and hydrous basaltic melt increases linearly with an increase in H2O content in the melts. We then selected from literature geochemical data sets of relatively primitive basaltic rocks with no evidence of magma mixing and the most frequent Ca-rich plagioclase phenocrysts from 15 basaltic arc volcanoes including both frontal-arc and rear-arc volcanoes. In the 15 volcanoes studied, plagioclase phenocrysts of high anorthite content (An > 90) were commonly observed, whereas plagioclase phenocrysts in rear arc volcanoes usually had a lower anorthite content (90 > An > 80). In all volcanoes studied, the estimated H2O content of basaltic magma was at least 3 wt.% H2O or higher. The magmas of volcanoes located on the volcanic front have about 5 wt.% H2O in magma whereas those from the rear-arc side are slightly lower in H2O content.

  8. Water content in arc basaltic magma in the Northeast Japan and Izu arcs: an estimate from Ca/Na partitioning between plagioclase and melt

    NASA Astrophysics Data System (ADS)

    Ushioda, M.; Takahashi, E.; Hamada, M.; Suzuki, T.

    2015-12-01

    The variation in water content of arc basaltic magmas in the Northeast Japan arc and the Izu arc was estimatedusing a simple plagioclase phenocryst hygrometer. In order to construct a plagioclase phenocryst hygrometeroptimized for arc basalt magmas, we have conducted hydrous melting experiments of relatively primitive basaltfrom the Miyakejima volcano, a frontal-arc volcano in the Izu arc. As a result of the experiments, we found that theCa/Na partition coefficient between plagioclase and hydrous basaltic melt increases linearly with an increase in H2Ocontent in the melts. We then compiled published geochemical data sets of relatively primitive basaltic rocks with no evidence of magma mixing and the most frequent Ca-rich plagioclase phenocrysts from 15 basaltic arc volcanoesincluding both frontal-arc and rear-arc volcanoes. In the 15 volcanoes studied, plagioclase phenocrysts of high anorthitecontent (An > 90) were commonly observed, whereas plagioclase phenocrysts in rear arc volcanoes usually had a loweranorthite content (90 > An > 80). In all volcanoes studied, the estimated H2O content of basaltic magma was at least3 wt.% H2O or higher. The magmas of volcanoes located on the volcanic front have about 5 wt.% H2O in magmawhereas those from the rear-arc side are slightly lower in H2O content.

  9. The importance of plagioclase in the reflectance spectra of Fe, Mg mixtures: a better understanding of spectra from Lunar and Hermean terrains.

    NASA Astrophysics Data System (ADS)

    Serventi, G.; Sgavetti, M.; Carli, C.; Pompilio, L.

    2012-04-01

    Spectra obtained on Lunar highland and on Mercury show low contrast features. We suggest the interference of adjacent absorptions bands due to different minerals as a possible cause of low contrast spectra. While the combined effects of Fe2+ absorptions in various clinopyroxene, orthopyroxene and olivine mixtures have been widely studied, the spectroscopic effects of plagioclase have been considered only for <0,26 wt% FeO-bearing compositions, even if plagioclase is considered an important constituent of Lunar and Hermean terrains. Here we consider mixtures composed by various abundances of multimineral grains and plagioclase, separated from cumulate rocks of a layered intrusion belonging to the anorthosite kindred. Three different Fe, Mg multimineral compositions have been considered. The first is olivine-free and consists of clinopyroxene En45-Wo46 (43.9%) and orthopyroxene En77 (56.1%). The second one is olivine-poor and includes orthopyroxene En86 (70%) and olivine Fo87 (30%). The third one, olivine-rich, is composed by orthopyroxene En82 (28.2%), clinopyroxene En45-Wo46 (3.4%), olivine Fo84 (68.4%). Two distinct plagioclase compositions, having FeO wt.% concentration of 0.36 (medium-iron), and 0.5 (rich-iron) were systematically mixed to each starting assemblage. The amount of plagioclase in the mixtures ranges between 30% and 90%. Mixtures with grain sizes of 63-125 μm and 125-250 μm were prepared. Bidirectional reflectance spectra (i=30°, e=0° angle phase) on these mixtures were acquired at the SLAB (Spectroscopy Laboratory, Iasf-INAF, Roma) in the VIS-NIR range (0.3-2.5 µm). Preliminary results show that increasing plagioclase content produces higher albedo and lower spectral contrast. In olivine-free mixtures, plagioclase produces a flattening in the 1.2 µm region at about 70% of medium-iron plagioclase and 50% of iron-rich plagioclase; for higher content of plagioclase a clear absorption band appears. In olivine-poor mixtures, the presence of

  10. Contrasting Sr isotope ratios in plagioclase from different formations of the mid-Miocene Columbia River Basalt Group

    NASA Astrophysics Data System (ADS)

    Starkel, W. A.; Wolff, J.; Eckberg, A.; Ramos, F.

    2008-12-01

    Many early Columbia River Basalt flows of the Steens and Imnaha Formations are characterized by abundant, texturally complex, coarse plagioclase phenocrysts. In Imnaha lavas, the feldspars typically have more radiogenic 87Sr/86Sr than whole rock and matrix, and may exhibit complex isotopic zoning that is not correlated with An content. Imnaha plagioclase grains are interpreted as variably-contaminated crystals produced when high-crystallinity mid-crustal basaltic intrusions exchanged interstitial melt with adjacent partly-melted crustal rock; this isotopically variable debris was then remobilized by subsequent intrusion of mantle-derived basalt and brought to the surface as an isotopically heterogeneous mixture. In contrast, plagioclase grains in the texturally very similar Steens lavas are isotopically near-homogeneous and 87Sr/86Sr is not significantly displaced from that of the bulk rock. This is consistent with magma- crust interaction at low degrees of crustal melting during the early stages of the Columbia River flood basalt episode, where Steens and Imnaha lavas were erupted from distinct magma systems hosted by different types of crust that exerted different degrees of isotopic leverage on the mantle-derived magmas [1]. Thermal input to the Steens system declined at the same time as the Imnaha magmatic flux increased to ultimately produce the huge outpouring of Grande Ronde lavas, which are mixtures of mantle- and crust-derived liquids, the latter produced during high degrees of crustal melting during the time of peak magmatic flux. [1] Wolff et al. (2008) Nature Geoscience 1, 177-180.

  11. Sr Isotopic Variation in Plagioclase Phenocrysts of the Heise Volcanic Field, Eastern Snake River Plain, Idaho USA

    NASA Astrophysics Data System (ADS)

    Phillips, W. M.; Schwartz, D. M.; Ellis, B. S.

    2012-12-01

    Feldspars within single eruptive units of rhyolites of the central Snake River Plain are tightly grouped into unimodal Sr isotope populations. Wolff et al. (2011) suggested that this Sr isotopic homogeneity is characteristic of Snake River-type rhyolitic volcanism, and reflects unusually high magma temperatures and low water contents. We test this hypothesis with new Sr data from plagioclase phenocrysts from the Heise Volcanic Field, a large nested caldera complex in the eastern Snake River Plain. We sampled the oldest unit (Tuff of Blacktail Creek, 6.6 Ma) and youngest unit (Kilgore Tuff, 4.5 Ma) at their type sections. To assess within unit variability, we also sampled widely separated exposures of the units across the caldera complex. Plagioclase crystals were separated magnetically and by hand-picking. Sr isotopes were analyzed in 9 to 66 grains per sample by LA-MC-ICPMS at the Washington State University GeoAnalytical Lab. Blacktail Creek samples have tight unimodal distributions with 87Sr/86Sr modes between 0.7126 and 0.7128 that support the Wolff et al. hypothesis. The Kilgore samples show considerably more variability. While all Kilgore samples have a similar principal mode between 0.7116 and 0.7118, additional minor modes are generally present. The Kilgore results are surprising given oxygen isotope evidence for magma homogeneity prior to eruption, crystal residence times of ~110 kyr, and magma temperatures of ~800-900°C (Watts et al., 2011). Under such temperatures, Sr isotopic homogeneity in plagioclase is likely achieved in 5 mm grains within <10 kyr. The observed Sr isotope heterogeneity in Kilgore may result from isolation of magma batches until shortly before eruption. References: Wolff et al., 2011, Geology 39(10), 931-934; Watts et al. 2011, J. Petrology 52(5), 857-890.

  12. Constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the Samail ophiolite

    NASA Astrophysics Data System (ADS)

    VanTongeren, J. A.; Hirth, G.; Kelemen, P. B.

    2015-12-01

    The debate over the processes of igneous accretion of gabbroic lower crust at submarine spreading centers is centered on two end-member hypotheses: Gabbro Glaciers and Sheeted Sills. In order to determine which of these two hypotheses is most applicable to a well-studied lower crustal section, we present newly published data (VanTongeren et al., 2015 EPSL v. 427, p. 249-261) on plagioclase lattice preferred orientations (LPO) in the Wadi Khafifah section of the Samail ophiolite, Oman. Based on our results we provide five critical observations that any model for the accretion of the lower oceanic crust must satisfy: (1) There is a distinctive change in the orientation of the outcrop-scale layering from near-vertical to sub-horizontal that is also reflected in the plagioclase fabrics in the uppermost ~1000-1500 m of the gabbroic crust; (2) The distinction between the upper gabbros and lower gabbros is not a geochemical boundary. Rather, the change in outcrop-scale orientation from near-vertical to sub-horizontal occurs stratigraphically lower in the crust than a change in whole-rock geochemistry; (3) There is no systematic difference in plagioclase fabric strength in any crystallographic axis between the upper gabbros and the lower gabbros; (4) Beneath the abrupt transition from sub-vertical to sub-horizontal fabric, there is no systematic change in the geographic orientation of the plagioclase fabric, or in the development of a dominant lineation direction within the upper gabbros or the lower gabbros; (5) In the lower gabbros, the obliquity between the (010) and the modal layering remains approximately constant and indicates a consistent top to the right sense of shear throughout the stratigraphy. Our observations are most consistent with the Sheeted Sills hypothesis, in which the majority of lower crustal gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with

  13. Rapid crystal recycling at Krafla Volcano, Iceland, inferred from oxygen-isotope and trace- element compositions and U-Th-Ra disequilibria in plagioclase

    NASA Astrophysics Data System (ADS)

    Cooper, K. M.; Sims, K. W.; Eiler, J. M.; Banerjee, N. R.

    2008-12-01

    The Icelandic central volcano of Krafla exhibits increasing assimilation of hydrothermally-altered crust with increasing differentiation of magmas, as evidenced by decreasing δ18O with decreasing MgO (Nicholson et al., 1991, J Pet 32, p.1005). The Krafla Fires eruption (1975-84) produced two different magma compositions simultaneously: quartz tholeiites near the center of the volcano, and olivine tholeiites north of the central volcano (Gronvold et al., 2008, Goldschmidt abstract). Examination of crystals in these magmas has the potential to provide information about the nature and timescales of mixing of distinct magmas and assimilation of crustal material at Krafla. We present oxygen-isotope compositions, trace-element compositions, and 238U-230Th-226Ra disequilibria measured in plagioclase crystals from samples of lavas erupted during two phases of the Krafla Fires eruption (ol tholeiite erupted Jan-Feb 1981, and qz tholeiite erupted Nov 1981). Oxygen-isotope data for multiple size fractions of plagioclase show a decrease in δ18O with increasing crystal size for the ol tholeiite (from ~4.1 permil to 3.5 permil), whereas there is no clear relationship between plagioclase size and oxygen-isotope composition in the qtz tholeiite (all size fractions average 4.1-4.3 permil). Furthermore, all measured plagioclase have δ18O lower than would be in equilibrium with the whole rock measurements (by up to 1.5 permil). These data imply that (1) few or none of the measured crystals precipitated from the host liquids, and (2) the crystals were entrained in the host magmas shortly prior to eruption, allowing them to maintain oxygen isotopic disequilibrium and heterogeneity within the crystal populations. These inferences are corroborated by trace element compositions measured in plagioclase by laser-ablation ICPMS, as the majority of analyzed points have Ba and Sr concentrations inconsistent with equilibrium partitioning between crystals and liquid. Furthermore, in the case

  14. Cooling and crystallization of rhyolite-obsidian lava: Insights from micron-scale projections on plagioclase microlites

    NASA Astrophysics Data System (ADS)

    Sano, Kyohei; Toramaru, Atsushi

    2017-07-01

    To reveal the cooling process of a rhyolite-obsidian flow, we studied the morphology of plagioclase microlites in the Tokachi-Ishizawa lava of Shirataki, northern Hokkaido, Japan, where the structure of the lava can be observed from obsidian at the base of the flow to the innermost rhyolite. Needle-like micron-scale textures, known as ;projections;, occur on the short side surfaces of the plagioclase microlites. Using FE-SEM we discovered a positive correlation between the lengths and spacings of these projections. On the basis of the instability theory of an interface between melt and crystal, and to understand the length and spacing data, we developed a model that explains the positive correlation and allows us to simultaneously estimate growth rates and growth times. Applying the model to our morphological data and the estimated growth rates and growth times, we suggest that the characteristics of the projections reflect the degree of undercooling, which in turn correlates with lava structure (the obsidian at the margin of the flow experienced a higher degree of undercooling than the interior rhyolite). The newly developed method provides insights into the degree of undercooling during the final stages of crystallization of a rhyolitic lava flow.

  15. Experimental determination of plagioclase dissolution rates as a function of its composition and pH at 22 °C

    NASA Astrophysics Data System (ADS)

    Gudbrandsson, Snorri; Wolff-Boenisch, Domenik; Gislason, Sigurdur R.; Oelkers, Eric H.

    2014-08-01

    The steady-state, far-from-equilibrium dissolution rates of nine distinct plagioclases ranging in composition from An2 to An89 were measured in mixed flow reactors at 22 ± 2 °C and pH from 2 to 11. The dissolution rates of all plagioclases based on silica release show a common U-shaped behaviour as a function of pH, where rates decrease with increasing pH at acid condition but rise with increasing pH at alkaline conditions. Consistent with literature findings, constant pH plagioclase dissolution rates increase with increasing anorthite content at acidic conditions; measured anorthite dissolution rates are ∼2.5 orders of magnitude faster than those of albite at pH ∼2. Perhaps more significantly, rates are independent of plagioclase composition at alkaline conditions. Interpretation and data fitting suggests that plagioclase dissolution rates are consistent with their control by the detachment of Si-rich activated complexes formed by the removal of Al from the mineral framework. Taking account of this mechanism and transition state theory yields equations describing plagioclase dissolution rates (r+) as a function of both the mineral and aqueous fluid compositions found in natural Earth surface systems. For pH ⩾ 6 rates are consistent with Log(r+/(mol/cm/s))=0.35Log(aH3/aAl)-11.53 and for pH < 6 rates are consistent with Log(r+/(mol/cm/s))=nacidLog(aH3/aAl)+0.033An%-14.77 where An% represents the percent anorthite in the plagioclase solid solution, ai corresponds to the activity of the ith aqueous species, and nacid is given by nacid=0.004An%+0.05 .

  16. Magma evolution during the last 23 ky at Popocatepetl Volcano: insights from Sr, Nd, and Pb isotopes in plagioclase, pyroxene and pumice matrix

    NASA Astrophysics Data System (ADS)

    Sosa, G.; Gardner, J. E.; Lassiter, J. C.

    2009-12-01

    In the last 23 ky Popocatepetl, a dominately andesitic volcano, has erupted at least 5 large volumes of dacitic magma. A common question is how are these dacites related to the andesites and how silicic magmas are affected by magma mixing and crustal assimilation through its eruptive history. The isotopic composition of magma is the cumulative product of its genesis and evolution, including magma mixing and crustal assimilation. To clarify how magma evolution influence Popocatepetl magmatic system we did a textural analysis of plagioclase, measured their compositional variability, analyzed Sr, Nd and Pb isotopes on plagioclase, pumice groundmass, and pyroxene of dacitic Plinian eruptions from the last 23 ky and compare them with older andesites from the same magmatic system. Plagioclase textures and compositional transects were analyzed in order to investigate the evolutionary processes responsible for crystal growth. Sr, Nd, and Pb isotopes were analyzed for each major phase to corroborate observations from plagioclase textural-compositional analysis and to investigate which processes dominate the final isotopic composition of the magmas. Our results show that plagioclase from different eruptions record variable textures and compositions that can be explained by different thermo-compositional fluctuations in the magma reservoir. In order to investigate if magmas have been affected by compositional changes or just thermal fluctuations we analyzed their isotopic compositions. Isotopically, plagioclase, pyroxene and groundmass from dacites from Popocatepetl Plinian eruptions display considerable ranges in 87Sr/86Sr (.70395-.70457) and ɛNd (2.2-5.1), whereas Pb isotopic ratios display more narrow ranges (206Pb/204Pb 18.62-18.68, 207Pb/204Pb 15.56-15.62, 208Pb/204Pb 38.30-38.49). Those dacites younger than 5 ky all pyroxene, plagioclase and groundmass are in isotopic equilibrium for Sr and Pb, suggesting that sieve textures and compositional variability of plagioclase

  17. Formation of plagioclase-bearing peridotite and a peridotite-wehrlite-gabbro suite through melt-rock reaction: An experimental study

    NASA Astrophysics Data System (ADS)

    Saper, L.; Liang, Y.

    2012-12-01

    Plagioclase-bearing peridotites are observed among abyssal peridotites, massif peridotites, and mantle sections of ophiolites of lherzolite subtype. Formation of plagioclase-bearing peridotites is often attributed to basalt impregnation into host harzburgite or lherzolite in a thermal boundary layer. During transport through asthenospheric mantle, melt generated in the deep mantle will inevitably interact with the overlying mantle column through reactive dissolution and may leave geochemical imprints on plagioclase-bearing peridotites. To assess the role of melt-rock reaction on the formation of plagioclase-bearing peridotites and its implications for lithosphere composition, we conducted dissolution experiments in which a 88% spinel lherzolite + 12% basalt starting mixture was juxtaposed against a primitive MORB in a graphite-lined molybdenum capsule. The reaction couples were run at 1300°C and 1 GPa for 1 or 24 hrs, and then stepped cooled to 1050°C and 0.7 GPa over the next several days. Cooling promotes in situ crystallization of interstitial melts, allowing us to better characterize the mineral compositional trends produced and observed by melt-rock reaction and crystallization. A gabbro and a plagioclase-bearing peridotite were observed in the two halves of the reaction couple after the experiments were completed. The peridotite from the 24 hr reaction experiment is mostly composed of subhedral to euhedral olivines (10-50 μm in size, Mg# 75-83), poikilitic clinopyroxene (~100 μm in size, Mg# 73-83) with olivine and spinel chadocrysts, and interstitial plagioclase (An# 68-78) and melt. In a control experiment quenched after a 24 hour reaction at 1300°C the basalt completely dissolved the pyroxenes and spinels leaving a residue of rounded olivine grains (10-100 μm in size) surrounded by a relatively large melt fraction. Textural results from the step-cooling experiments suggest the following crystallization sequence from the olivine+melt mush: olivine

  18. Degassing-Induced Crystallization of Plagioclase in Hydrous Rhyolite Liquids: Evidence from Obsidian Samples from the Mexican and Cascades Volcanic Arcs

    NASA Astrophysics Data System (ADS)

    Waters, L.; Lange, R. A.

    2010-12-01

    Rhyolites are among the most differentiated magmatic liquids on Earth, and in order to understand their origin, it is necessary to constrain temperatures and melt water concentrations during phenocryst growth. In this study, we present a detailed petrologic study of obsidian samples (71-75 wt% SiO2) from the Mexican and Cascades volcanic arcs. Despite low phenocryst abundances (2-10%), the samples are each multiply saturated with plagioclase + titanomagnetite + ilmenite ± orthopyroxene ± clinopyroxene ± hornblende ± biotite ± sanidine ± quartz. Pre-eruptive temperatures have been calculated from equilibrium pairs of titanomagnetite and ilmenite using the thermometer of Ghiorso and Evans (2008); values range from ~797-837°C. The plagioclase hygrometer (Lange et al., 2009) was applied to the sparse plagioclase crystals in each sample using these temperatures, leading to maximum melt water concentrations (based on the most calcic plagioclase in each sample) that range from 7.3 to 6.0 wt% H2O. These results require that all of these magmas were fluid-saturated at depths ≥ 9.5 km, respectively, and that during adiabatic ascent to the surface they would have degassed and lost water from the melt phase. In many of the rhyolites, we document a wide and continuous range of plagioclase compositions (e.g., 52-27 mol% An), despite low crystal (phenocryst + microphenocryst) abundances (< 3%), which can be attributed to crystallization under variable melt water concentrations, owing to degassing upon ascent. Analyzed Sr and Ba concentrations in these plagioclases lead to partition coefficients that are consistent with a phenocryst origin. The lowest recorded melt water concentrations in the glassy rhyolites, based on their most sodic plagioclases, are also relatively high and range from ~5.0-6.4 wt%. These results suggest that there may have been a kinetic barrier to plagioclase crystallization as the ascending magmas degassed and lost water, leading to a rapid increase

  19. Integrating MELTS Modeling with In Situ Textural and Chemical Plagioclase Data to Document Ascent and Storage Conditions of Four Historical Mount Etna Magmas

    NASA Astrophysics Data System (ADS)

    Moses, M. N.; Bohrson, W. A.

    2009-12-01

    Mount Etna, Europe’s most active volcano, erupts hydrous basalts that are abundant in plagioclase. Integrating plagioclase chemical and textural data with results from MELTS (Ghiorso & Sack, 1995) models can provide information about the ascent and storage history of associated magmas. Textures for 4 historical samples (1646, 1651, and the 1669 residing and recharge magmas) imaged by Nomarski Differential Interference Contrast microscopy reveal 2 dominant core textures, patchy and oscillatory. Most crystals are characterized by unzoned rims. Electron microprobe data show that patchy core values have modes at An70 and An80, whereas oscillatory zoned cores have a single mode at An60. Rim anorthite values range from An60 to An40. 343 MELTS fractional crystallization simulations were conducted in isentropic and polybaric modes. The 1763 lava was used as the parental magma and has 6.6 wt.% MgO, 47.1 wt.% SiO2. PTX conditions for simulations include initial water contents from 3 wt.% to anhydrous, pressures of 7, 5, and 2 kb, oxygen fugacity of buffers QFM-1 and NNO. Model results were compared to whole rock major element data for historical lavas, as well as An data from the four samples noted above. The 7 and 5 kb models successfully fit the major element trends but did not crystallize observed plagioclase compositions. The best-fit models were polybaric, and used pressure (P)-temperature (T) gradients (dP/dT) of 2 bars/0.5°C for a starting P of 2 kb. Because water saturated at a higher T in the 2 kb models, plagioclase also saturated at a higher T. Thus, the 2 kb models yielded more abundant plagioclase and had a greater compositional variation that more closely resembled the natural dataset. MELTS results for plagioclase from 2 kb polybaric runs with 2.7-2.0 wt. % initial water correlate well with dominant An contents of 85 to 70 in patchy zoned cores from the 1651 and 1669 residing and recharge magmas. Oscillatory and unzoned plagioclase cores typically have

  20. Experimental quantification of P-T conditions of mantle refertilisation at shallow depth under spreading ridges and formation of plagioclase + spinel lherzolite

    NASA Astrophysics Data System (ADS)

    Chalot-Prat, Françoise; Falloon, Trevor J.; Green, David H.

    2014-05-01

    We studied the first-order melting process of differentiation in the Earth, and the major process of rejuvenation of the upper mantle after melting related to plate spreading (Chalot-Prat et al, 2010; 2013). We conducted experiments at High Pressure (0.75 and 0.5 GPa) and High Temperature (1260-1100°C) to obtain magma compositions in equilibrium with the mineral assemblages of a plagioclase + spinel lherzolite. These PT conditions prevail at 17-30km below axial oceanic spreading ridges. We used a "trial and error" approach in a system involving nine elements (Cr-Na-Fe-Ca-Mg-Al-Si-Ti-Ni). This approaches as closely as possible a natural mantle composition, Cr being a key element in the system. Our objectives were : • to determine experimentally the compositions of melts in equilibrium with plagioclase + spinel lherzolite, with emphasis on the role of plagioclase composition in controlling melt compositions; • to test the hypothesis that MORB are produced at shallow depth (17-30kms) • to quantify liquid- and mantle residue compositional paths at decreasing T and low P to understand magma differentiation by "percolation-reaction" at shallow depth in the mantle; • to compare experimental mantle mineral compositions to those of re-fertilised oceanic mantle lithosphere outcropping at the axis of oceanic spreading ridges, enabling quantification of the pressure (i.e. depth) and temperature of the re-fertilisation process that leads to formation of plagioclase and indicates the minimum thickness of the lithosphere at ridge axes. In the normative basalt tetrahedron, liquids plot on two parallel cotectic lines from silica-oversaturated (basaltic andesite at 0.75 GPa or andesite at 0.5 GPa) at the calcic end to silica-undersaturated compositions (trachyte) at the sodic end. The lower the pressure, the greater the silica oversaturation. Besides the plagioclase solid solution has a dominant role in determining the solidus temperature of plagioclase + spinel lherzolites

  1. A Plagioclase Ultraphyric Basalt group in the Neogene flood basalt piles of eastern Iceland: Volcanic architecture and mode of emplacement

    NASA Astrophysics Data System (ADS)

    Oskarsson, B. V.; Riishuus, M. S.

    2013-12-01

    3D photogrammetry in conjunction with ground mapping was applied in order to assess the architecture of a Plagioclase Ultraphyric Basalt (PUB) group in eastern Iceland, namely the Grænavatn group. The ~10 Myr old group is exposed in steep glacially carved fjords and can be traced over 60 km along strike. Two feeder dikes have been found and show that the group erupted along the trend of the dike swarm associated with the Breiddalur central volcano. The group has 9--14 flows where thickest, and thins to about 3--4 flows up-dip to the east within the distance of 15-20 km from the source. We have estimated the volume of the group to exceed 40 km3. The flows have mixed architecture of simple and compound morphology. The flow lobes have thicknesses from 1--24 m and many reach lengths over 1000 m. The surface morphology varies from rubbly to scoriaceous, but is dominantly of pahoehoe style. The internal structure of the lava flows is well preserved and the flows display abundant vesicle cylinders. The modal percentage of An-rich plagioclase macrocrysts varies from 25--50 % and they are in the range of 5--30 mm. The aspect ratio of the group and the nature of the flows indicate fissure-fed eruptions. A thick flow found at the base of the group in various locations seems to record the largest eruption episode in the formation of the group. This phase is also the most abundant in macrocryst. An asymmetric buildup is seen in one location and may have characterized the general buildup of the group. The general morphology of the lava flows suggests low viscous behavior, at odds with the high crystal content. Petrographic observations and mineral chemistry shows that the plagioclase macrocrysts are very calcic (An80-85) and in disequilibrium with the groundmass and plagioclases therein (An50-70). Thus the apparent lava rheology and emplacement of the PUBs was likely achieved due to fast ascent of the magma through the crust and transfer of heat from the primitive macrocrysts

  2. Optical absorption, TL and IRSL of basic plagioclase megacrysts from the pinacate (Sonora, Mexico) quaternary alkalic volcanics.

    PubMed

    Chernov, V; Paz-Moreno, F; Piters, T M; Barboza-Flores, M

    2006-01-01

    The paper presents the first results of an investigation on optical absorption (OA), thermally and infrared stimulated luminescence (TL and IRSL) of the Pinacate plagioclase (labradorite). The OA spectra reveal two bands with maxima at 1.0 and 3.2 eV connected with absorption of the Fe3+ and Fe2+ and IR absorption at wavelengths longer than 2700 nm. The ultraviolet absorption varies exponentially with the photon energy following the 'vitreous' empirical Urbach rule indicating exponential distribution of localised states in the forbidden band. The natural TL is peaked at 700 K. Laboratory beta irradiation creates a very broad TL peak with maximum at 430 K. The change of the 430 K TL peak shape under the thermal cleaning procedure and dark storage after irradiation reveals a monotonous increasing of the activation energy that can be explained by the exponential distribution of traps. The IRSL response is weak and exhibits a typical decay behaviour.

  3. Lingunite-a high-pressure plagioclase polymorph at mineral interfaces in doleritic rock of the Lockne impact structure (Sweden)

    PubMed Central

    Agarwal, Amar; Reznik, Boris; Kontny, Agnes; Heissler, Stefan; Schilling, Frank

    2016-01-01

    Lingunite nanocrystals and amorphous plagioclase (maskelynite) are identified at the contacts between augite and labradorite wedge-shaped interfaces in the doleritic rocks of the Lockne impact structure in Sweden. The occurrence of lingunite suggests that the local pressure was above 19 GPa and the local temperature overwhelmed 1000 °C. These values are up to 10 times higher than previous values estimated numerically for bulk pressure and temperature. High shock-induced temperatures are manifested by maskelynite injections into microfractures in augite located next to the wedges. We discuss a possible model of shock heterogeneity at mineral interfaces, which may lead to longer duration of the same shock pressure and a concentration of high temperature thus triggering the kinetics of labradorite transformation into lingunite and maskelynite. PMID:27188436

  4. Mercury: Mid-infrared (7.3 - 13.5 microns) spectroscopic observations showing features characteristic of plagioclase

    NASA Technical Reports Server (NTRS)

    Kozlowski, R. W. H.; Sprague, A. L.; Witteborn, F. C.; Cruikshank, D. P.; Wooden, D.; Snyder, K. D.

    1994-01-01

    Mid-infrared spectroscopic observations of the surface of Mercury are reported for the wavelength range 7.3 to 13.5 microns. The observed spectral radiance emanated from equatorial and low latitude regions between 110-130 deg Mercurian longitude. The area is primarily an intercrater plain. The spectra show distinct and recognizable features, the principal Christiansen emission peak being the most prominent. The Christiansen feature strongly suggests the presence of plagioclase (Ca,Na)(Al,Si)AlSi2O8, (in particular labradorite: Ab(50) - Ab(30)). In addition we have studied the effects of thermal gradients to gain insight into the effects of thermal conditions on the spectral radiance of rock samples. This simulates the thermophysical effects as the rotating surface of Mercury is alternately heated and cooled. The spectral features of the samples are retained; however, the relative and absolute amplitudes vary as illustrated by laboratory reflectance and emittance spectra from quartzite.

  5. Plagioclase and epidote buffering of cation ratios in mid-ocean ridge hydrothermal fluids: Experimental results in and near the supercritical region

    SciTech Connect

    Berndt, M.E.; Seyfried, W.E. Jr. ); Janecky, D.R. )

    1989-09-01

    Experiments have been performed with Na-Ca-K-Cl fluids of seawater chlorinity and diabase, basalt, and plagioclase bearing mineral mixtures at 350-425{degree}C and 250-400 bars to help constrain hydrothermal alteration processes at mid-ocean ridges. Dissolved Ca, Na, and pH for all experiments responded systematically to differences in dissolved SiO{sub 2} concentrations and the compositions of plagioclase reactants. Diabase alteration at low fluid/rock mass ratios (0.5 to 1) produces fluids undersaturated with respect to quartz during hydration of primary olivine and orthopyroxene, whereas basalt alteration under similar conditions yields fluids slightly supersaturated with respect to quartz during breakdown of glass to smectite and amphibole. Fluid chemistry in all experiments appears to approach a partial equilibrium state with the albite and anorthite components in plagioclase and approaches a pH consistent with plagioclase alteration to epidote. Trace element data from vent fluids, specifically B and Sr, together with major element chemistry, provides evidence that the reaction zone for black-smoker fluids at mid-ocean ridges is composed of only slightly altered diabase and is characterized by small amounts of epidote, nearly fresh plagioclase and clinopyroxene, and partially to completely hydrated olivine and orthopyroxene. Using equilibrium between plagioclase, the dominant reactant, and epidote, the dominant reaction product in experiments, the authors estimate that temperatures in reaction zones are in excess of 375{degree}C for most vent systems. These temperatures are higher than measured vent temperatures, suggesting that hot spring fluids commonly loose heat during ascent to the sea floor.

  6. Deformation of the deep crust: Insights from physiochemical characteristics of deformation microstructures of plagioclase and quartz in gneiss from the Salt Mylonite Zone, Western Gneiss Region, Norway

    NASA Astrophysics Data System (ADS)

    Renedo, R. N.; Piazolo, S.; Whitney, D. L.; Teyssier, C. P.

    2014-12-01

    The deformation behavior of quartz and feldspar controls the rheology of large parts of the continental crust. One way to understand the deformation of these abundant and important minerals in the deep crust is to compare their behavior within and outside of naturally-deformed shear zones. To this end, three samples of quartzofeldspathic gneiss within and adjacent to the Salt Mylonite Zone, a discrete ductile deformation zone in the ultrahigh-pressure Western Gneiss Region of Norway, are investigated in terms of microstructure, chemical composition, and fabric. The three samples represent the microstructural variation across the shear zone including grain size variation, layered (quartz ribbon bearing) v. non-layered gneiss, and variation in modal abundance. Layered gneiss is composed of one grain thick, laterally continuous quartz ribbons with plagioclase and accessory phases in the intervening regions. Non-layered gneiss consists of isolated quartz (individual grains or clusters of up to four grains) within an interconnected network of plagioclase and accessory phases. In layered gneiss, quartz preserves a well-developed crystallographic preferred orientation consistent with dominant activation of the prism and rhomb slip systems, and plagioclase preserves a nearly random fabric. Quartz fabrics from layered shear zone gneiss are stronger than those of quartz from layered gneiss outside of the shear zone. In non-layered gneiss, plagioclase develops a fabric that is consistent with activation of the (001) <010> slip system whereas quartz exhibits a random fabric. Plagioclase in all samples is zoned from Na-richer cores to Ca-richer rims (reverse zoning); zoning is weaker outside of the shear zone (average core-to-rim ΔAn 6%) than within the shear zone (average core-to-rim ΔAn 10%). Results suggest a change in plagioclase and quartz deformation mechanisms occurred during decompression and shear zone development owing to strain/strain-rate variation.

  7. 40Ar/39Ar and cosmic ray exposure ages of plagioclase-rich lithic fragments from Apollo 17 regolith, 78461

    NASA Astrophysics Data System (ADS)

    Das, J. P.; Baldwin, S. L.; Delano, J. W.

    2016-01-01

    Argon isotopic data is used to assess the potential of low-mass samples collected by sample return missions on planetary objects (e.g., Moon, Mars, asteroids), to reveal planetary surface processes. We report the first 40Ar/39Ar ages and 38Ar cosmic ray exposure (CRE) ages, determined for eleven submillimeter-sized (ranging from 0.06 to 1.2 mg) plagioclase-rich lithic fragments from Apollo 17 regolith sample 78461 collected at the base of the Sculptured Hills. Total fusion analysis was used to outgas argon from the lithic fragments. Three different approaches were used to determine 40Ar/39Ar ages and illustrate the sensitivity of age determination to the choice of trapped (40Ar/36Ar)t. 40Ar/39Ar ages range from ~4.0 to 4.4 Ga with one exception (Plag#10). Surface CRE ages, based on 38Ar, range from ~1 to 24 Ma. The relatively young CRE ages suggest recent re-working of the upper few centimeters of the regolith. The CRE ages may result from the effect of downslope movement of materials to the base of the Sculptured Hills from higher elevations. The apparent 40Ar/39Ar age for Plag#10 is >5 Ga and yielded the oldest CRE age (i.e., ~24 Ma). We interpret this data to indicate the presence of parentless 40Ar in Plag#10, originating in the lunar atmosphere and implanted in lunar regolith by solar wind. Based on a chemical mixing model, plagioclase compositions, and 40Ar/39Ar ages, we conclude that lithic fragments originated from Mg-suite of highland rocks, and none were derived from the mare region.

  8. Microstructural Evolution of an Extensional Shear Zone: the Transition from Dislocation Creep to Grain Boundary Sliding in Naturally Deformed Plagioclase

    NASA Astrophysics Data System (ADS)

    Miranda, E.; Hirth, G.; John, B. E.

    2014-12-01

    We present microstructural, LPO, and misorientation data from EBSD analyses to interpret the deformation mechanisms of naturally deformed plagioclase in an amphibolite-facies extensional shear zone within oceanic crust. Textural data and mineral chemistry data for thermometry were acquired on samples of gabbro mylonite collected from the footwall of the Atlantis Bank oceanic core complex; we focus on a monophase plagioclase layer with a high ratio of recrystallized matrix grains to porphyroclasts. Sample microstructures are subdivided into three regions: seven porphyroclasts, recrystallized grains immediately adjacent to those porphyroclasts, and the population of recrystallized matrix grains. All porphyroclasts exhibit subgrain development and show clustering of low-angle (3-10º) misorientation axes within the {010} plane, consistent with slip on {010}. However, only one porphyroclast is oriented for operation of the {010}<001> easy slip system. The small recrystallized grains immediately adjacent to the porphyroclasts do not show a host control relationship with their respective porphyroclasts, and these grains are also smaller than the subgrains of the porphyroclasts. Recrystallized matrix grains are fine-grained (mean grain size 8 μm) and slightly elongate parallel to foliation, with local misorientations concentrated along grain boundaries and junctions. They exhibit a weak, nonrandom LPO suggesting the activity of the {111}<110> slip system, and their neighbor-pair misorientations are shifted towards higher angles. We interpret the distinctive relationships between the three regions as evidence of a transition from dislocation creep to dislocation-accommodated grain boundary sliding (DisGBS). Porphyroclast subgrains and misorientation axes suggest the operation of dislocation creep, but the lack of host control in the adjacent recrystallized grains precludes grain size reduction through subgrain rotation recrystallization alone. High driving force bulge

  9. Constraints on crystal storage timescales in mixed magmas: Uranium-series disequilibria in plagioclase from Holocene magmas at Mount Hood, Oregon

    NASA Astrophysics Data System (ADS)

    Eppich, Gary R.; Cooper, Kari M.; Kent, Adam J. R.; Koleszar, Alison

    2012-02-01

    Uranium-series crystal ages, interpreted within a textural and geochemical framework, can provide insight into crystal storage timescales, especially in cases where crystals may derive from multiple sources. We report here 230Th-226Ra model ages of two distinct populations of plagioclase from low silica dacites from Mount Hood, Oregon, a volcano where previous studies show that the compositions of erupted magmas are controlled by magma recharge, mixing, and incorporation of plagioclase derived from mafic and silicic end-member magmas. We have measured trace element concentrations and 238U-230Th-226Ra disequilibria in four plagioclase size fractions from the Timberline (1500 a) and Old Maid (215 a) eruptive sequences. After correction for groundmass and apatite contamination, average 230Th-226Ra model ages of large (> 500 μm) plagioclase are > 4.5 ka (Timberline) and > 5.5 ka (Old Maid), with ages of cores that are > 10 ka in each case, indicating that plagioclase derived from silicic magmas crystallized thousands of years before eruptions. These model ages are longer than timescales of repose between eruptions, indicating that these crystals resided in the sub-surface over multiple eruptions, likely stored in a silicic crystal mush zone that periodically interacts with mafic recharge magmas, remobilizing a fraction of the large plagioclase crystals during each eruptive event. After correction for large plagioclase contamination, small (< 500 μm) plagioclase, derived from mafic magmas, have high (226Ra)/Ba relative to equilibrium with liquid proxies (groundmass and mafic inclusion), leading to 230Th-226Ra model ages that are <~3 ka for Old Maid and undefined for Timberline separates. However, the preservation of significant 230Th-226Ra disequilibria require that the majority of crystals in the separate are young (<<10 ka). The high (226Ra)/[Ba] could potentially be explained by rapid crystallization immediately prior to and/or during mixing events, consistent with

  10. Trace element and Pb isotope composition of plagioclase from dome samples from the 2004-2005 eruption of Mount St. Helens, Washington: Chapter 35 in A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006

    USGS Publications Warehouse

    Kent, Adam J.R.; Rowe, Michael C.; Thornber, Carl R.; Pallister, John S.; Sherrod, David R.; Scott, William E.; Stauffer, Peter H.

    2008-01-01

    Plagioclase crystals from gabbronorite inclusions in three dacite samples have markedly different trace-element and Pbisotope compositions from those of plagioclase phenocrysts, despite having a similar range of anorthite contents. Inclusions show some systematic differences from each other but typically have higher Ti, Ba, LREE, and Pb and lower Sr and have lower 208Pb/206Pb and 207Pb/206Pb ratios than coexisting plagioclase phenocrysts. The compositions of plagioclase from inclusions cannot be related to phenocryst compositions by any reasonable petrologic model. From this we suggest that they are unlikely to represent magmatic cumulates or restite inclusions but instead are samples of mafic Tertiary basement from beneath the volcano.

  11. Luminescence Spectroscopical Properties of Plagioclase Particles from the Hayabusa Sample Return Mission: An Implication for Study of Space Weathering Processes in the Asteroid Itokawa.

    PubMed

    Gucsik, Arnold; Nakamura, Tomoki; Jäger, Cornelia; Ninagawa, Kiyotaka; Nishido, Hirotsugu; Kayama, Masahiro; Tsuchiyama, Akira; Ott, Ulrich; Kereszturi, Ákos

    2017-02-01

    We report a systematic spectroscopical investigation of three plagioclase particles (RB-QD04-0022, RA-QD02-0025-01, and RA-QD02-0025-02) returned by the Hayabusa spacecraft from the asteroid Itokawa, by means of scanning electron microscopy, cathodoluminescence microscopy/spectroscopy, and micro-Raman spectroscopy. The cathodoluminescence properties are used to evaluate the crystallization effects and the degree of space weathering processes, especially the shock-wave history of Itokawa. They provide new insights regarding spectral changes of asteroidal bodies due to space weathering processes. The cathodoluminescence spectra of the plagioclase particles from Itokawa show a defect-related broad band centered at around 450 nm, with a shoulder peak at 425 nm in the blue region, but there are no Mn- or Fe-related emission peaks. The absence of these crystal field-related activators indicates that the plagioclase was formed during thermal metamorphism at subsolidus temperature and extreme low oxygen fugacity. Luminescence characteristics of the selected samples do not show any signatures of the shock-induced microstructures or amorphization, indicating that these plagioclase samples suffered no (or low-shock pressure regime) shock metamorphism. Cathodoluminescence can play a key role as a powerful tool to determine mineralogy of fine-grained astromaterials.

  12. Orientation-related electrical conductivity of hydrous olivine, clinopyroxene and plagioclase and implications for the structure of the lower continental crust and uppermost mantle

    NASA Astrophysics Data System (ADS)

    Yang, Xiaozhi

    2012-02-01

    Orientation-related electrical conductivity of H-annealed olivine, clinopyroxene and plagioclase single crystals have been measured at 10 kbar and 200-800 °C with an end-loaded piston cylinder apparatus and a Solartron-1260 Impedance/Gain Phase analyzer in the frequency range of 106-0.1 Hz. The complex spectra usually show an arc over the whole frequency range at low temperature, with much scatter of data-points at low frequencies for samples of large resistance, and an arc plus a short tail in the high and low frequency range, respectively, at high temperature. The arc is due to grain interior conduction, and the short tail is related to electrode effects. The conduction is dominated by H-related point defects. The results show negligible anisotropy in electrical conductivity for hydrous olivine and clinopyroxene but large anisotropy for hydrous plagioclase. Electrical anisotropy in the lower continental crust may be caused by the fabrics of the constitutive granulites due to lattice-preferred orientation (LPO) of hydrous plagioclase and banded pyroxene- and plagioclase-rich microstructure. Electrical anisotropy in the uppermost mantle cannot be produced by hydrous olivine (and pyroxenes in peridotites), but may be related to mantle macro-heterogeneity due to the local presence of Fe3 +- and H2O-rich augites and other pyroxenites in the form of dykes/veins and/or remnants of recycled crustal materials in the form of stretched lenses.

  13. Plagioclase zonation styles in hornblende gabbro inclusions from Little Glass Mountain, Medicine Lake volcano, California: Implications for fractionation mechanisms and the formation of composition gaps

    USGS Publications Warehouse

    Brophy, J.G.; Dorais, M.J.; Donnelly-Nolan, J.; Singer, B.S.

    1997-01-01

    The rhyolite of Little Glass Mountain (73-74% SiO2) is a single eruptive unit that contains inclusions of quenched andesite liquid (54-61% SiO2) and partially crystalline cumulate hornblende gabbro (53-55% SiO2). Based on previous studies, the quenched andesite inclusions and host rhyolite lava are related to one another through fractional crystallization and represent an example of a fractionation-generated composition gap. The hornblende gabbros represent the cumulate residue associated with the rhyolite-producing and composition gap-forming fractionation event. This study combines textural (Nomarski Differential Interference Contrast, NDIC, imaging), major element (An content) and trace element (Mg, Fe, Sr, K, Ti, Ba) data on the style of zonation of plagioclase crystals from representative andesite and gabbro inclusions, to assess the physical environment in which the fractionation event and composition gap formation took place. The andesite inclusions (54-61% SiO2) are sparsely phyric with phenocrysts of plagioclase, augite and Fe-oxide??olivine, +/-orthopyroxene, +/-hornblende set within a glassy to crystalline matrix. The gabbro cumulates (53-55% SiO2) consist of an interconnected framework of plagioclase, augite, olivine, orthopyroxene, hornblende and Fe-oxide along with highly vesicular interstitial glass (70-74% SiO2). The gabbros record a two-stage crystallization history of plagioclase + olivine + augite (Stage I) followed by plagioclase+orthopyroxene + hornblende + Fe-oxide (Stage II). Texturally, the plagioclase crystals in the andesite inclusions are characterized by complex, fine-scale oscillatory zonation and abundant dissolution surfaces. Compositionally (An content) the crystals are essentially unzoned from core-to-rim. These features indicate growth within a dynamic (convecting?), reservoir of andesite magma. In contrast, the plagioclase crystals in the gabbros are texturally smooth and featureless with strong normal zonation from An74 at the

  14. Dry and Wet Friction of Plagioclase: Pure Cataclastic Flow(CF) vs. CF with Concurrent Pressure Solution

    NASA Astrophysics Data System (ADS)

    He, C.; Tan, W.

    2015-12-01

    To distinguish different deformation mechanisms at hydrothermal conditions, friction experiments of plagioclase under nominally dry conditions were compared with that at hydrothermal conditions documented in a previous study[He et al.,2013]. Preliminary result[Tan and He, 2008] shows that the rate dependence of plagioclase under confining pressure of 150MPa and nominally dry conditions is velocity strengthening at temperatures of 50-600oC, in contrast to the full velocity weakening at hydrothermal conditions. Here a) we conducted data fitting to the rate and state friction law to compare with the hydrothermal case; b) microstructural comparison was performed to understand the difference between the dry and wet conditions in the operative deformation mechanisms. The evolution effect (b value) under dry conditions exhibits much smaller values than that at wet conditions, and in contrast to the increasing trend at wet conditions, b values under dry conditions have a decreasing trend as temperature increases, from ~0.007 at 300oC down to 0 at 600oC. The direct effect (a value) at dry conditions has a peak of ~0.01 at 300oC and decreases to a level of 0.007-0.008 at higher temperatures, in contrast to the increasing trend seen at hydrothermal conditions. In the dry case, microstructure at temperatures of 300-600oC transitions gradually from a fabric characterized by localized Riedel shear zones to pervasive shear deformation, with the grain size reduced to a level of 1-3 micron in a submicron matrix in the latter case, corresponding to a lower porosity. The close association between porosity evolution and that of state variable revealed in previous studies[Morrow and Byerlee, 1989; Marone et al.,1990] suggests that the porosity change contributes largely to the evolution effect in addition to plasticity at intergranular contacts, probably due to gradual switching between different densities of packing. Our dry experiments indicate a cataclastic flow where the evolution

  15. Incommensurate density modulation in a Na-rich plagioclase feldspar: Z-contrast imaging and single-crystal X-ray diffraction study.

    PubMed

    Xu, Huifang; Jin, Shiyun; Noll, Bruce C

    2016-12-01

    Plagioclase feldspars are the most abundant mineral in the Earth's crust. Intermediate plagioclase feldspars commonly display incommensurately modulated or aperiodic structures. Z-contrast images show both Ca-Na ordering and density modulation. The local structure of lamellae domains has I1-like symmetry. The neighboring lamellae domains are in an inversion twinning relationship. With a state-of-the-art X-ray diffraction unit, second-order satellite reflections (f-reflections) are observed for the first time in andesine (An45), a Na-rich e-plagioclase. The f-reflections indicate a structure with a density modulation which is close to a Ca-rich e-plagioclase. The similarity between this e-andesine structure and previously solved e-labradorite structure is confirmed. Refinement of the structure shows density modulation of ∼ 7 mol % in compositional variation of the anorthite (An) component. The results from Z-contrast imaging and low-temperature single X-ray diffraction (XRD) provide a structure consistent with density modulation. The discovery of f-reflections in Na-rich e-plagioclase extends the composition range of e1 structure with density modulation to as low as at least An45, which is the lower end of the composition range of Bøggild intergrowth. The new result supports the loop-shaped solvus for Bøggild intergrowth, below which is a homogenous stable area for e1 structure in the phase diagram. The phase transition between e2 structure without density modulation and e1 structure with density modulation should happen at low temperature. There is a change in modulation period accompanying the phase transition, as well as higher occupancy of Al in the T1o site. The andesine with density modulation also indicates extremely slow cooling of its host rock.

  16. Increased mantle heat flow with on-going rifting of the West Antarctic rift system inferred from characterisation of plagioclase peridotite in the shallow Antarctic mantle

    NASA Astrophysics Data System (ADS)

    Martin, A. P.; Cooper, A. F.; Price, R. C.

    2014-03-01

    The lithospheric, and shallow asthenospheric, mantle in Southern Victoria Land are known to record anomalously high heat flow but the cause remains imperfectly understood. To address this issue plagioclase peridotite xenoliths have been collected from Cenozoic alkalic igneous rocks at three localities along a 150 km transect across the western shoulder of the West Antarctic rift system in Southern Victoria Land, Antarctica. There is a geochemical, thermal and chronological progression across this section of the rift shoulder from relatively hot, young and thick lithosphere in the west to cooler, older and thinner lithosphere in the east. Overprinting this progression are relatively more recent mantle refertilising events. Melt depletion and refertilisation was relatively limited in the lithospheric mantle to the west but has been more extensive in the east. Thermometry obtained from orthopyroxene in these plagioclase peridotites indicates that those samples most recently affected by refertilising melts have attained the highest temperatures, above those predicted from idealised dynamic rift or Northern Victoria Land geotherms and higher than those prevailing in the equivalent East Antarctic mantle. Anomalously high heat flow can thus be attributed to entrapment of syn-rift melts in the lithosphere, probably since regional magmatism commenced at least 24 Myr ago. The chemistry and mineralogy of shallow plagioclase peridotite mantle can be explained by up to 8% melt extraction and a series of refertilisation events. These include: (a) up to 8% refertilisation by a N-MORB melt; (b) metasomatism involving up to 1% addition of a subduction-related component; and (c) addition of ~ 1.5% average calcio-carbonatite. A high MgO group of clinopyroxenes can be modelled by the addition of up to 1% alkalic melt. Melt extraction and refertilisation mainly occurred in the spinel stability field prior to decompression and uplift. In this region mantle plagioclase originates by a

  17. A SIMS (secondary ion mass spectrometry) and XPS (X-ray photoelectron spectroscopy) study of dissolving plagioclase

    SciTech Connect

    Muir, I.J. ); Bancroft, G.M.; Nesbitt, H.W. ); Shotyk, W. )

    1990-08-01

    In an earlier report, the authors showed that altered layers formed on the surface of dissolving labradorite feldspar grains, and that the thickness of these layers (up to hundreds of angstroms) is strongly dependent on the pH of the reactant solution. In this paper, they show that the thickness of these altered layers also depends strongly on the composition of the plagioclase feldspar. Secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) have been used to characterize these altered layers. During dissolution, Ca and Al are removed from the solid material to form an altered layer residually enriched in Si with very similar profiles for Ca and Al. In acidic solutions (pH 3.5) for 90 days, the altered layers increase in thickness from a few hundred angstroms to many hundred angstroms in the order: albite < oligoclase < labradorite < bytownite. These results emphasize the central role of hydrolysis of the bridging Si-O-Al bonds in the initial weathering process. Analysis by scanning electron microscopy (SEM) does not provide any evidence for the growth of secondary phases. Extensive dissolution features (etch pits) were observed on many of the reacted surfaces.

  18. Microstructural evidence for the transition from dislocation creep to dislocation-accommodated grain boundary sliding in naturally deformed plagioclase

    NASA Astrophysics Data System (ADS)

    Miranda, Elena A.; Hirth, Greg; John, Barbara E.

    2016-11-01

    We use quantitative microstructural analysis including misorientation analysis based on electron backscatter diffraction (EBSD) data to investigate deformation mechanisms of naturally deformed plagioclase in an amphibolite gabbro mylonite. The sample is from lower oceanic crust exposed near the Southwest Indian Ridge, and it has a high ratio of recrystallized matrix grains to porphyroclasts. Microstructures preserved in porphyroclasts suggest that early deformation was achieved principally by dislocation creep with subgrain rotation recrystallization; recrystallized grain (average diameter ∼8 μm) microstructures indicate that subsequent grain boundary sliding (GBS) was active in the continued deformation of the recrystallized matrix. The recrystallized matrix shows four-grain junctions, randomized misorientation axes, and a shift towards higher angles for neighbor-pair misorientations, all indicative of GBS. The matrix grains also exhibit a shape preferred orientation, a weak lattice preferred orientation consistent with slip on multiple slip systems, and intragrain microstructures indicative of dislocation movement. The combination of these microstructures suggest deformation by dislocation-accommodated GBS (DisGBS). Strain localization within the recrystallized matrix was promoted by a transition from grain size insensitive dislocation creep to grain size sensitive GBS, and sustained by the maintenance of a small grain size during superplasticity.

  19. High-resolution quantitative imaging of plagioclase composition using accumulated backscattered electron images: new constraints on oscillatory zoning

    NASA Astrophysics Data System (ADS)

    Ginibre, Catherine; Kronz, Andreas; Wörner, Gerhard

    2001-08-01

    Oscillatory zoning in plagioclase is investigated at small scale (<=10 µm) using accumulated backscattered electron (BSE) images as high-resolution imaging method. Combined with electron microprobe quantitative analysis, gray-value profiles across these images can be calibrated for An-content with a resolution of 0.5 mol% An. Applied to oscillatory-zoned crystals, this new application of BSE imaging allows better characterization of zoning patterns along a profile and quantification of wavelength, amplitude, and shape of the oscillations. We also obtain high-resolution information on the morphology of growth zones boundaries. This approach allows us to better classify the different types of "oscillations" and concentric zoning. Dissolution is more frequent than usually recognized. Major resorption surfaces crosscut several growth zones. Irregular 5-10-µm saw-tooth zones are delimited by faint wavy dissolution surfaces and must be distinguished from small-scale oscillations (<=1-3 µm) with straight boundaries. This suggests at least two mechanisms for the formation of these zoning patterns: faint oscillations are probably caused by local kinetic control whereas wavy dissolution surfaces involve magma chamber dynamics.

  20. High-resolution quantitative imaging of plagioclase composition using accumulated backscattered electron images: new constraints on oscillatory zoning

    NASA Astrophysics Data System (ADS)

    Ginibre, Catherine; Kronz, Andreas; Wörner, Gerhard

    Oscillatory zoning in plagioclase is investigated at small scale (<=10 µm) using accumulated backscattered electron (BSE) images as high-resolution imaging method. Combined with electron microprobe quantitative analysis, gray-value profiles across these images can be calibrated for An-content with a resolution of 0.5 mol% An. Applied to oscillatory-zoned crystals, this new application of BSE imaging allows better characterization of zoning patterns along a profile and quantification of wavelength, amplitude, and shape of the oscillations. We also obtain high-resolution information on the morphology of growth zones boundaries. This approach allows us to better classify the different types of "oscillations" and concentric zoning. Dissolution is more frequent than usually recognized. Major resorption surfaces crosscut several growth zones. Irregular 5-10-µm saw-tooth zones are delimited by faint wavy dissolution surfaces and must be distinguished from small-scale oscillations (<=1-3 µm) with straight boundaries. This suggests at least two mechanisms for the formation of these zoning patterns: faint oscillations are probably caused by local kinetic control whereas wavy dissolution surfaces involve magma chamber dynamics.

  1. Calcic amphibole thermobarometry in metamorphic and igneous rocks: New calibrations based on plagioclase/amphibole Al-Si partitioning and amphibole/liquid Mg partitioning

    NASA Astrophysics Data System (ADS)

    Molina, J. F.; Moreno, J. A.; Castro, A.; Rodríguez, C.; Fershtater, G. B.

    2015-09-01

    Dependencies of plagioclase/amphibole Al-Si partitioning, DAl/Siplg/amp, and amphibole/liquid Mg partitioning, DMgamp/liq, on temperature, pressure and phase compositions are investigated employing robust regression methods based on MM-estimators. A database with 92 amphibole-plagioclase pairs - temperature range: 650-1050 °C; amphibole compositional limits: > 0.02 apfu (23O) Ti and > 0.05 apfu Al - and 148 amphibole-glass pairs - temperature range: 800-1100 °C; amphibole compositional limit: CaM4/(CaM4 + NaM4) > 0.75 - compiled from experiments in the literature was used for the calculations (amphibole normalization scheme: 13-CNK method).

  2. Significance of Elevated Contents of Si4O8 and AlAl3SiO8 End Members in Black Clouded Plagioclase From Anorthosite: Evidence for Oxidation-Induced Postmagmatic Exsolution of Fe-Ti Oxide Inclusions From Plagioclase

    NASA Astrophysics Data System (ADS)

    Murakami, H.; Lalonde, A. E.

    2004-12-01

    Plagioclase phenocrysts from large Proterozoic massif anorthosites that have escaped metamorphic recrystallization commonly have a distinctive black color that is attributed to myriads of minute Fe-Ti oxide inclusions. These inclusions are responsible for the black color of the feldspar. The orientation of these inclusions along specific crystallographic directions in the plagioclase is strong evidence that these formed by exsolution from the plagioclase. Plagioclase from the Proterozoic Lac Saint-Jean anorthosite in Québec is a good example of such clouded feldspar. In this plagioclase we recognize four types of Fe-Ti oxide inclusions: 1) Myriads of fine, submicroscopic and unidentifiable dust-like inclusions, presumably Fe-Ti oxides, and responsible in great part for the black color. 2) Small ˜1 μ m sized crystals of hemo-ilmenite that occur in regions of the plagioclase that are free of type 1 dust, suggesting that these originated by Ostwald ripening from the dusty material. 3) Larger aligned and acicular crystals of hematite, ˜10 μ m in diameter and with good rhombohedral sections. These occur, like type 2 inclusions, in zones free of type 1 dust suggesting again that they result from Ostwald ripening. 4) Millimetric inclusions of ilmenite or magnetite of magmatic origin, often rimmed by amphibole or biotite, signs of late hydration. As part of our study, EPMA analyses of plagioclase from Lac Saint-Jean were done. In addition, a large single crystal, ˜30 cm in diameter was analyzed by bulk XRF. Our results show that plagioclase contains significant amounts of the Si4O8 (up to 6.6 mole %) and AlAl3SiO8 (up to 2.2 mole %) endmembers. A positive relationship is observed between the content of these two endmembers. Furthermore, the ratio of Si4O8 to AlAl3SiO8 is approximately 3:4. We believe that the high content of Si4O8 and AlAl3SiO8 in plagioclase, and the presence of minute Fe-Ti oxide inclusions are both products of a late postmagmatic oxidation via the

  3. The mode of emplacement of Neogene flood basalts in Eastern Iceland: The plagioclase ultraphyric basalts in the Grænavatn group

    NASA Astrophysics Data System (ADS)

    V. Óskarsson, Birgir; B. Andersen, Christina; S. Riishuus, Morten; Sørensen, Erik Vest; Tegner, Christian

    2017-02-01

    Plagioclase ultraphyric basalt lava with high fraction of solids have a mode of emplacement that is poorly understood. In this study we conduct detailed mapping of a PUB group in eastern Iceland, namely the Grænavatn group, and assess the group architecture, flow morphology and internal structure with additional constraints from petrography, petrology and crystal size distribution, to derive information on emplacement dynamics of plagioclase ultraphyric basalts. We also derive information on the plumbing system of the group with reference to the source of the macrocysts. The group is exposed in steep glacially carved fjords and can be traced for more than 70 km along strike. The flows have mixed architecture of simple and compound flows. Individual flow lobes have thicknesses in the range of 1-24 m and many reach widths and lengths exceeding 1000 m. The flows vary from rubbly to slabby pahoehoe, but are predominantly of pahoehoe type. The aspect ratio of the group and the nature of the flows indicate fissure-fed eruptions. The plagioclase macrocrysts (5-30 mm) are An-rich, exhibit bimodal size distribution and the modal proportions within the group varies from 15-40%. Clinopyroxene macrocrysts are also present ranging from 1-6%. The lowermost flow is thickest and carries the greatest crystal cargo load. The morphology of the lava flows suggests low viscous behavior, at odds with the high crystal content. The very calcic plagioclase macrocrysts (An80-85) are in disequilibrium with the groundmass and plagioclase microlaths therein (An50-70), meaning that the crystal-laden magmas quickly ascended from deeper crustal levels to the surface. The flows with highest crystal content may have maintained high temperatures by heat exchange with the primitive macrocrysts in the flows and developed non-Newtonian behavior such as shear thinning. Such conditions would have enabled the flows to advance rapidly during episodes with high effusion rates forming the simple flows, and

  4. Textures and compositions of olivine and plagioclase in primitive basalts of the Poison Lake chain in the Lassen Region of the southern Cascades

    NASA Astrophysics Data System (ADS)

    Garrido, A.; Lewis, G. T.; Teasdale, R.; Wenner, J. M.

    2016-12-01

    The Poison Lake chain (PLC) is located 30 km east of the Lassen Volcanic Center, at the eastern boundary of the southern Cascades and the western margin of the Basin and Range. PLC basalts erupted from monogenetic cinder cones and are subdivided into 9 geochemical groups, ranging in age from 100±10 ka (1). One group, "the basalts of old railroad grade" (bg), is split into four units that have phenocrysts of euhedral olivine and fine grained plagioclase. Point counts indicate samples contain 5-10% olivine phenocrysts (cores with Fo: 84-88%) and 7-20% plagioclase phenocrysts (cores with An: 84-87%, except bg1 with cores ≥ An72%) that make three Textural Types. Texture Type I includes the most primitive bg units (bg3 and bg4) and features embayed olivine with homogeneous cores (Fo: 85-88%) with slightly lower forsterite rims (Fo: 80-84%) and normal zoned plagioclase (cores: An 85-87%; rims: An 70-75%). Unit bg1 represents Texture Type II with glomerocrysts of embayed, resorbed olivine with homogeneous cores (Fo: 84-86%; rims Fo: 81-82%) and plagioclase that are embayed, resorbed, swallow-tailed and normal zoned (cores: An 72-88%; rims: An 55-70%), some of which are also oscillatory zoned. Texture Type III includes bg2 samples with embayed, resorbed, and normal zoned olivine (cores Fo: 84-88%; rims: Fo: 78-80%) and embayed, resorbed, and normal zoned plagioclase (cores: An 79-85%; rims: An 76-84%), with some oscillatory zoned crystals. Texture Types are the result of compositional or temperature variations in the magma. Cooling experiments can be used to test temperatures of crystallization and development of textures. MELTS (2) models predict olivine crystallization started at 1252C, which can be used as a starting point for crystallization experiments, followed by heating and cooling experiments to replicate development of embayed textures. (1) Muffler et al., 2011 (2) Ghiorso and Sack, 1995

  5. Timescales of magma processes occurred prior to recent Campi Flegrei caldera eruptions: first results from diffusion profiles on plagioclase phenocrysts

    NASA Astrophysics Data System (ADS)

    D'Antonio, Massimo; Arienzo, Ilenia; Fedele, Lorenzo; Iovine, Raffaella; Carmine Mazzeo, Fabio; Civetta, Lucia; Orsi, Giovanni; Wörner, Gerhard

    2015-04-01

    Knowledge of the timescales of magma rising and stagnation, as well as mingling/mixing processes occurring in the shallow plumbing system of an active volcano is crucial for volcanic hazard assessment and risk mitigation. Among few recently developed methodologies, high-precision, high spatial resolution analysis of major-, minor- and trace elements on zoned phenocrysts through electron microprobe techniques represents a powerful tool to provide good estimates of timescales of pre-eruptive magma rising, stagnation and/or mingling/mixing processes. To this purpose, volcanic rock samples of trachytic composition representative of the Agnano-Monte Spina eruption (4.7 ka CAL BP) occurred at the Campi Flegrei caldera (southern Italy) have been selected. The investigation has been carried out in the framework of Project V2 - Precursori di Eruzioni, funded by the Italian Dipartimento per la Protezione Civile - Istituto Nazionale di Geofisica e Vulcanologia. The investigated rock samples are pumice fragments from which double-polished, 100 µm thick thin sections have been prepared for analytical purposes. Back-scattered electrons (BSE) images have been acquired at the scanning electron microscope (SEM), in order to identify the plagioclase phenocrysts suitable to be analyzed successively, selected among those that best display their zoning. After a careful observation of the BSE images, major-, minor- and selected trace element contents have been determined through combined energy-dispersive and wavelength-dispersive system electron microprobe analyses (EDS-WDS-EMPA) on transects crossing the growth zones of the selected phenocrysts. This methodology has allowed reconstructing the diffusion profile of some key-elements through the growth zones of the investigated phenocrysts. Successively, the diffusion profiles have been combined with textural features obtained through BSE images in order to obtain diffusion models aimed at estimating the timescales of crystals

  6. Analysis of Silicate Melt Inclusions in Plagioclase Phenocrysts in Prehistoric Tephra ˜1400 Years B.P. From Augustine Volcano, Alaska.

    NASA Astrophysics Data System (ADS)

    Tappen, C. M.; Webster, J. D.; Mandeville, C. W.

    2003-12-01

    Augustine volcano, located in southern Cook Inlet, Alaska, has been historically active, erupting 6 times in the last 200 years. Eruptions first began prior to 40,000 years B.P. (Begét and Kienle, 1992). There are a minimum of 6 prehistoric tephra layers, G (oldest), I, H, C, M and B (youngest), present on Augustine Island (Waitt et al, 1996). In this study, we analyzed glassy silicate melt inclusions in plagioclase phenocrysts from tephra layer H ( ˜1400 years B.P.) for major and minor and some trace elements (Cl, F, S, Ba, and Sr) by electron microprobe. We use the data to determine the chemical variation of melt inclusions in specific locations within zoned plagioclase phenocrysts. Plagioclase phenocrysts (0.5 to 4 mm long) exhibit unzoned, oscillatory or patchy zoned regions. Unzoned phenocryst cores lack melt inclusions. Patchy zonation occurs in cores and is sometimes found in intermediate zones between the core and rim. Planar oscillatory zones are distinguished in BSE images by light (An56-90) and dark (An46-55) bands. In some phenocrysts light and dark layers differ only by 1% An. Most phenocrysts show 2-3 repeated oscillating pairs of light and dark plagioclase compositional layers. Normal and reverse zoning are apparent in phenocrysts. Rims tend to be more calcic than the cores, varying from 1-5% An. Large melt inclusions (60 to 70 μ m long) are located in patchy zoned cores. Small melt inclusions (2 to 10 μ m long) are located at the contact of high calcic and low calcic oscillatory layers. All melt inclusions are trapped along compositional boundaries and occur in the more calcic plagioclase. Petrography suggests that melt inclusions may have been formed by partial dissolution of a less calcic plagioclase layer. The composition of the melt inclusions are rhyolitic (71 to 75% SiO2). The chlorine concentrations range from 3020 to 6100 ppm with the more chlorine enriched concentrations occurring in the outer rims of the phenocryst. Sr and Ba vary from

  7. Linking in-situ Hf isotopes in zircon with in-situ Pb isotopes in plagioclase: a microanalytical approach to characterize Archean anorthosite petrogenesis

    NASA Astrophysics Data System (ADS)

    Souders, K.; Sylvester, P.; Myers, J.

    2011-12-01

    Multiple isotope systems are often used to distinguish petrogenetic processes and determine the age and source of magmatic systems. Advances in laser ablation multi-collector ICPMS instrumentation have allowed Earth scientists to determine accurate and precise isotope ratios of minerals in-situ. Most studies have focused on measuring isotopes that are abundant within a mineral (e.g. Hf in zircon) but the integration of multiple ion counters into the collector configuration of MC-ICPMS instruments has provided the ability to measure isotope ratios of minor elements (e.g. Pb in plagioclase) in-situ. These abilities allow for an alternative approach to igneous petrogenesis. Instead of isotopic analysis of bulk samples, in-situ methods can be utilized to target specific domains preserved in individual minerals. Analysis of co-magmatic minerals in igneous rocks using multiple isotopic systems can be linked to solve a range of petrologic problems. As an example, we present in-situ analyses by LA-MC-ICPMS for Pb isotope compositions of preserved igneous plagioclase megacrysts and Hf isotope compositions of zircon grains from the 2936 Ma Fiskenæsset and 2914 Ma Nunataarsuk anorthosite complexes, southwestern Greenland, two of the best-preserved Archean anorthosites in the world. For both Fiskenæsset and Nunataarsuk, the initial Pb isotope compositions of plagioclase megacrysts and the initial ɛHf compositions of zircon grains extend beyond analytical uncertainty suggesting multiple sources contributed to the parent magma for both anorthosite complexes. Initial ɛHf of zircon grains from both anorthosite complexes fall between depleted mantle and a less radiogenic crustal source with a total range up to 5 ɛHf units. Plagioclase Pb isotope compositions from both anorthosite complexes share a depleted mantle end member yet diverge from this point: Fiskenæsset toward a high-μ, more radiogenic Pb crustal composition and Nunataarsuk toward a low-μ, less radiogenic Pb

  8. Time scales of magma recharge and crystal growth rate determined from Mg and Ti zoning in plagioclase phenocrysts from the Upper Toluca Pumice, Mexico

    NASA Astrophysics Data System (ADS)

    Dohmen, Ralf; Smith, Victoria C.; Arce, Jose Luis; Blundy, Jonathan D.

    2010-05-01

    Major and trace element zoning in plagioclase phenocrysts has the potential to stores information on the temporal evolution of the chemical environment during crystal growth, i.e. the surrounding melt composition as well as the intensive parameters temperature (T) and pressure (P), provided that equilibrium partitioning accompanies growth. However, the problem is complicated by the fact that diffusion of mobile elements changes their initial concentrations due to re-equilibration with the surrounding melt at later stages, making estimation of the pre-diffusive element profiles fraught with uncertainty. Here we present a new approach that combines the information from immobile (e.g., Ca, Ti) and mobile (e.g., Mg) elements in plagioclase to unravel the growth history and time scales of magma recharge events from the 10.5 ka Upper Toluca plinian eruption of Nevado de Toluca volcano, Mexico. Since trace elements are less sensitive to intensive parameters their variations in plagioclase phenocrysts have been used to identify open-system processes in silicic systems [1]. These phenocrysts preserve complex element patterns, such as oscillatory zoning and overgrowths, indicating multiple magma recharging events. Based on available diffusion data major elements and, for example, the trace element Ba, are effectively unchanged since crystallization, but the mobility of Mg [2] is large enough to alter the initial concentration at later growth stages. We made attempts to model the Mg zoning using two endmember cases for the growth history of the plagioclase. In the model the growth rate can either be constant until the final crystal diameter is reached or involve various short growth stages with diffusion relaxation breaks in-between. The corresponding moving boundary problem of the diffusion equation was solved numerically using the method of finite differences and a front-tracking method [3]. A particular challenge of the modelling is to estimate the initial Mg concentration

  9. Non-Newtonian behavior of plagioclase-bearing basaltic magma: Subliquidus viscosity measurement of the 1707 basalt of Fuji volcano, Japan

    NASA Astrophysics Data System (ADS)

    Ishibashi, Hidemi

    2009-03-01

    Laboratory measurements of viscosity were done for basalt erupted in 1707 AD from Fuji volcano, Japan, using a concentric cylinder rotational viscometer at temperatures of 1297-1157 °C, 1 atm pressure, and fO 2 near the Ni-NiO buffer. On cooling, elongated plagioclase crystals with a mean length/width ratio of ca. 8.5 appeared at 1237 °C, followed by olivine at 1157 °C. At progressively lower temperatures, the total crystal volume fraction increased monotonously to ca. 0.25; viscosity increased from 38.9 to 765 Pa s at a shear strain rate of 1 s - 1 . This basalt magma behaves as a Newtonian fluid at temperatures greater than 1217 °C, but shear-thinning behavior occurs at temperatures less than 1197 °C because of the suspended plagioclase crystals. This behavior is well approximated as a power law fluid. At the onset of shear thinning, the crystal volume fraction was between 0.06 and 0.13, which is attributed to the pronounced lath-shape of plagioclase crystals. The relative viscosity increases monotonously with increase of crystal volume fraction at a constant shear strain rate, and with decrease of shear strain rate at a constant crystal volume fraction. A modified form of the Krieger-Dougherty equation is introduced herein. It enables us to describe the dependencies of relative viscosity on both the crystal volume fraction and shear strain rate, and consequently the onset of shear-thinning behavior.

  10. A mechanistic understanding of plagioclase dissolution based on Al occupancy and T-O bond length: from geologic carbon sequestration to ambient conditions.

    PubMed

    Yang, Yi; Min, Yujia; Jun, Young-Shin

    2013-11-14

    A quantitative description of how the bulk properties of aluminosilicates affect their dissolution kinetics is important in helping people understand the regulation of atmospheric CO2 concentration by silicate weathering and predict the fate and transport of geologically sequestered CO2 through brine-rock interactions. In this study, we employed a structure model based on the C1 space group to illustrate how differences in crystallographic properties of aluminosilicates, such as T-O (Tetrahedral site-Oxygen) bond length and Al/Si ordering, can result in quantifiable variations in mineral dissolution rates. The dissolution rates of plagioclases were measured under representative geologic carbon sequestration (GCS) conditions (90 °C, 100 atm of CO2, 1.0 M NaCl, and pH ∼ 3.1), and used to validate the model. We found that the logarithm of the characteristic time of the breakdown of Al-O-Si linkages in plagioclases follows a good linear relation with the mineral's aluminum content (nAl). The Si release rates of plagioclases can be calculated based on an assumption of dissolution congruency or on the regularity of Al/Si distribution in the constituent tetrahedra of the mineral. We further extended the application of our approach to scenarios where dissolution incongruency arises because of different linkage reactivities in the solid matrix, and compared the model predictions with published data. The application of our results enables a significant reduction of experimental work for determining the dissolution rates of structurally related aluminosilicates, given a reaction environment.

  11. A Mechanism For Production Of Calc-alkalic And Tholeiitic Magma Series In Zao Volcano, NE Japan (II) - Sr Isotope Micro-analysis Of Plagioclase Phenocrysts

    NASA Astrophysics Data System (ADS)

    Takahashi, T.; Hirahara, Y.; Tatsumi, Y.; Kimura, J.; Ban, M.

    2006-12-01

    It was discussed from the investigation of bulk rock chemical compositions and isotopes ratio that the origin magma or material of calc-alkalic series (CA) and tholeiitic series (TH) from Zao volacano, NE Japan was several necessity (Hirahara et al., 2006). Consequently, we paid attention to Sr isotope ratio of phenocrystic minerals in volcanic rocks, because it can be thought to recorded such magmatic processes, and proposed a mechanism for producing these two magma series based on data obtained by Sr isotopic micro-analyses of plagioclase in volcanic rocks from Zao Volcano. The Sr isotope micro-analyses were performed by two methods. One is the Laser Ablation Multicollector Inductively Coupled Plasma Mass Spectrometry. The ablate crater size is 0.2mm. Other one is combined method of microdrilling and Thermal Ionization Mass Spectrometey. The microdrilling is the sampling technique of drilling a sample mechanically with a small drill and collecting the sample powder milled. The diameter at the tip of the drills used for sampling is 0.1 and 0.27mm. The collected sample powder was dissolved with acid, and Sr was separated using micro-columns Sr selective resin. Sr isotope measurement was carried out on the Thermal Ionization Mass Spectrometer. Core part of plagioclases in CA has widely An% and Sr isotope ratio (52 ~ 93 and 0.7035 ~ 0.7045), and there are divided into several types by the isotopical and compositional characteristics. Especially, plagioclase of most high An% (90 ~ 93) type in CA shows the lowest Sr isotope ratio (0.7035 ~ 0.7037). On the other hand, plagioclase in TH possesses relatively narrow range of An% and Sr isotope ratio (85 ~ 95 and 0.7042 ~ 0.7045), and there is a tendency that Sr isotope ratio slightly increase with decreasing An%. Results of Sr isotope micro-analyses show that CA formed by magma mixing between isotopically depleted basalt magma and isotopically enriched felsic magma. On the other hand, it shows that the primary basalt magma

  12. Mantle and crustal sources of Archean anorthosite: a combined in situ isotopic study of Pb-Pb in plagioclase and Lu-Hf in zircon

    NASA Astrophysics Data System (ADS)

    Souders, A. Kate; Sylvester, Paul J.; Myers, John S.

    2013-01-01

    Isotopic analyses of ancient mantle-derived magmatic rocks are used to trace the geochemical evolution of the Earth's mantle, but it is often difficult to determine their primary, initial isotope ratios due to the detrimental effects of metamorphism and secondary alteration. We present in situ analyses by LA-MC-ICPMS for the Pb isotopic compositions of igneous plagioclase (An75-89) megacrysts and the Hf isotopic compositions of BSE-imaged domains of zircon grains from two mantle-derived anorthosite complexes from south West Greenland, Fiskenæsset and Nunataarsuk, which represent two of the best-preserved Archean anorthosites in the world. In situ LA-ICPMS U-Pb geochronology of the zircon grains suggests that the minimum crystallization age of the Fiskenæsset complex is 2,936 ± 13 Ma (2σ, MSWD = 1.5) and the Nunataarsuk complex is 2,914 ± 6.9 Ma (2σ, MSWD = 2.0). Initial Hf isotopic compositions of zircon grains from both anorthosite complexes fall between depleted mantle and a less radiogenic crustal source with a total range up to 5 ɛHf units. In terms of Pb isotopic compositions of plagioclase, both anorthosite complexes share a depleted mantle end member yet their Pb isotopic compositions diverge in opposite directions from this point: Fiskenæsset toward a high-μ, more radiogenic Pb, crustal composition and Nunataarsuk toward low-μ, less radiogenic Pb, crustal composition. By using Hf isotopes in zircon in conjunction with Pb isotopes in plagioclase, we are able to constrain both the timing of mantle extraction of the crustal end member and its composition. At Fiskenæsset, the depleted mantle melt interacted with an Eoarchean (~3,700 Ma) mafic crust with a maximum 176Lu/177Hf ~0.028. At Nunataarsuk, the depleted mantle melt interacted with a Hadean (~4,200 Ma) mafic crust with a maximum 176Lu/177Hf ~0.0315. Evidence from both anorthosite complexes provides support for the long-term survival of ancient mafic crusts that, although unidentified at the

  13. Experimental and natural partitioning behaviour of trace-elements between hydrous evolved melts, amphibole, plagioclase, and clinopyroxene at shallow crustal conditions

    NASA Astrophysics Data System (ADS)

    Iveson, A. A.; Webster, J. D.; Rowe, M. C.; Neill, O. K.

    2016-12-01

    New experimental data for crystal-melt partitioning behaviour of a suite of trace-elements are presented. Hydrous rhyo-dacitic starting glasses from Mt. Usu, Japan, were doped with Li, Sc, Cr, Mn, Ni, Cu, Zn, Ga, Rb, Sr, Y, Nb, Mo, Ba, W, and Pb. Aqueous solutions were added such that the volatile phase(s) coexisting with amphibole, plagioclase, and clinopyroxene at run conditions buffered the S, F, and Cl contents of the melts. Internally-heated pressure vessel experiments were conducted at 750-850 °C, 1.0-4.0 Kbar, and ƒO2 ≈ NNO-NNO+2 log units. Major- and minor-element concentrations in the phenocrysts and glasses were analysed by EPMA, and trace-element contents by SIMS and/or LA-ICP-MS. The long run durations, homogeneous glasses, and minimal compositional zonation of crystals suggest that near-equilibrium conditions were achieved. Results of multiple phenocryst and glass analyses show that Nernst-type crystal-melt partition coefficients for these elements range from strongly incompatible e.g. Dmineral/melt ≈ 0 for Nb into plagioclase, to moderately incompatible e.g. Dmineral/melt ≈ 0.75 for Ga into amphibole, to strongly compatible e.g. Dmineral/melt > 50 for Ni into amphibole and clinopyroxene. Furthermore, unlike other elements investigated, partitioning of Li between phenocrysts and melt is similar for all three phases, with average DLicpx/melt ≈ 0.26 > DLiplag/melt ≈ 0.24 > DLiamph/melt ≈ 0.19. Relative to major-element composition of crystalline phases, the temperature, pressure, and ƒO2 conditions do not appear to strongly affect this behaviour. The incorporation of F and Cl into amphiboles is also consistent with the Fe-F and Mg-Cl crystallographic avoidance principles. Importantly, across two orders of magnitude in concentration, partitioning behaviours of all analysed trace-elements appear to obey Henry's Law. The experimental data are integrated with new amphibole, plagioclase, and pyroxene analyses from eruptive products of Augustine

  14. Evolution of the Craters of the Moon Lavas from primitive Snake River Plain basalts: inferences from plagioclase-melt thermobarometers and whole rock compositions

    NASA Astrophysics Data System (ADS)

    Vaid, N.; Putirka, K.; Kuntz, M.

    2005-12-01

    The volcanic rocks of the Craters of the Mon Lava field provide an ideal laboratory for testing models of magma transport and evolution. Their compositions, relative ages and volumes are well known, as are the fractionation processes leading to their evolution (Leeman, 1976). The COM is somewhat distinctive in the Snake River Plain (SRP) region, due to its evolved character, and an apparent compositional segregation from associated SRP basalts. Some have suggested that the high Fe liquids of the COM demand an origin separate from that of SRP basalts, possibly involving an Fe-enriched mantle, while others have suggested that the COM lavas may be derived by fractionation at moderate depths (30 km). In either case, there are important implications in regard to mantle composition and the nature and distribution of thermal energy. We use plagioclase-melt pairs and an analysis of whole rock compositions in attempt to test models of COM magmatic evolution. Plagioclase-melt thermobarometers provide rough estimates of crystallization depths, and show that COM and SRP lavas partially crystallized at similar depths of 14 +/- 6 km. However, plagioclase crystallization temperatures for SRP basalts (1400 +/- 25 K; Kings Bowl, Cerro Grande, North and South Robbers) exceed temperatures for COM lavas (1358 +/- 45 K) by 40 K. Our data also show that fractional crystallization (ol + plag) can explain the evolution of surrounding SRP basalt flows, and that the most evolved SRP basalts approach primitive COM lava compositions. The most primitive of COM magmas appear to be characterized by the appearance of apatite + magnetite as fractionating phases. Our results thus confirm the geochemical model of Leeman (1976) and the physical model of Kuntz (1992), with the added insight that SRP basalts are parental to the more evolved COM lavas, through low-pressure fractional crystallization in the upper crust. The principal differences between SRP and COM magmas appear to relate more to the

  15. 238U-230Th-226Ra disequilibria in dacite and plagioclase from the 2004-2005 eruption of Mount St. Helens: Chapter 36 in A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006

    USGS Publications Warehouse

    Cooper, Kari M.; Donnelly, Carrie T.; Sherrod, David R.; Scott, William E.; Stauffer, Peter H.

    2008-01-01

    230Th)/(232Th) measured for the 1980s reference suite. However, (230Th)/(232Th) for plagioclase separates for dome samples erupted during October and November 2004 are significantly different from corresponding whole-rock values, which suggests that a large fraction (>30 percent) of crystals in each sample are foreign to the host liquid. Furthermore, plagioclase in the two 2004 samples have U-series characteristics distinct from each other and from plagioclase in dacite erupted in 1982, indicating that (1) the current eruption must include a component of crystals (and potentially associated magma) that were not sampled by the 1980-86 eruption, and (2) dacite magmas erupted only a month apart in 2004 contain different populations of crystals, indicating that this foreign component is highly heterogeneous within the 2004-5 magma reservoir.

  16. Mineralogy of Inverted Pigeonite and Plagioclase in Cumulate Eucrites Y-980433 and Y-980318 with Reference to Early Crust Formation of the Vesta-Like Body

    NASA Technical Reports Server (NTRS)

    Takeda, H.; Ohtake, M.; Hiroi, T.; Nyquist, L. E.; Shih, C.-Y.; Yamaguchi, A.; Nagaoka, H.

    2011-01-01

    On July 16, the Dawn spacecraft became the first probe to enter orbit around asteroid 4 Vesta and will study the asteroid for a year before departing for Ceres. The Vesta-HED link is directly tied to the observed and inferred mineralogy of the asteroid and the mineralogy of the meteorites [1]. Pieters et al. [2] reported reflectance spectra of the Yamato- (Y-)980318 cumulate eucrite as a part of their study on the Asteroid-Meteorite Links in connection with the Dawn Mission. Pyroxenes and calcic plagioclase are the dominant minerals present in HED meteorites and provide multiple clues about how the parent body evolved [1]. The differentiation trends of HED meteorites are much simpler than those of the lunar crust

  17. The role of very fine particle sizes in the reflectance spectroscopy of plagioclase-bearing mixtures: New understanding for the interpretation of the finest sizes of the lunar regolith

    NASA Astrophysics Data System (ADS)

    Serventi, Giovanna; Carli, Cristian

    2017-09-01

    The lunar surface consists of a regolith layer that covers the underlying bedrocks, and is generally characterized by particulates <1 cm. Lunar soil is the fine fraction of the regolith, and is generally between 60 and 80 μm. Sizes <10 μm, accounting for ca. 5-20% of the soil, were recognized and petrologically classified. The coarsest sizes of the regolith are chemically and mineralogically similar, while the finest fractions are more feldspathic, probably due to easier fracturing of plagioclase than mafic minerals. Due to the more feldspathic nature of the very fine lunar soils, in this paper, we quantitatively investigate the influence of very fine (<10 μm) plagioclase on the absorption bands of mafic minerals using the Modified Gaussian Model. We considered two plagioclases with different iron content and two mafic end-members (1) 56% orthopyroxene and 44% clinopyroxene, and (2) 30% orthopyroxene and 70% olivine. We also compared our results with the deconvolution of the same mixtures at coarser sizes. Our results mainly show that: (1) fine sizes act principally on reflectance and on spectral contrast (with the former increasing and the latter decreasing); (2) very fine plagioclase has a blue slope in the Near Infrared and very shallow 1250 nm band depth, close to zero; (3) consequently, the plagioclase band is always shallower than mafic bands; (4) in mixtures with olivine, the composite band center always shows the typical olivine value, differently from coarser mixtures; and (5) mafic materials have a blue slope in the Short Wavelength Infrared Region, a more V-shaped 1 μm pyroxene absorption and the 1 μm mafic band centers are shifted by ca. 40 nm vs. coarse sizes, reflecting a different weight within the crystal field absorption of the mafic component in very fine size. We also evidenced that a coarse plagioclase could be overestimated, while a very fine one could be underestimated if compared with the 63-125 μm size.

  18. Melt inclusions hosted in olivine and plagioclase phenocrysts from the Antarctic-Phoenix Ridge basalts: Implications for origins of N- and E-type MORB parent magmas

    NASA Astrophysics Data System (ADS)

    Choi, S.; Schiano, P.; Chen, Y.; Devidal, J.

    2011-12-01

    The Antarctic-Phoenix Ridge (APR) is a fossil spreading center in the Drake Passage, Antarctic Ocean. Extinction of spreading occurred at the time of chron C2A (ca. 3.3 Ma). Enriched (E-type) mid-ocean ridge basalts (MORB) coexist with normal (N-type) MORB in the axial region of the APR, which is far from any known hotspots. The E-type basalts are relatively young (<3.1 Ma) compared to the N-type ones (>3.5 Ma). The E-type basalts are characterized by elevated K2O/TiO2 and (La/Sm)N ratios (K2O/TiO2 = 0.4 - 0.7; (La/Sm)N = 2.2 - 3.4) compared with the N-type basalts (K2O/TiO2 = 0.1 - 0.2; (La/Sm)N = 0.7 - 0.8). The E-type basalts are considered to have been generated by low-degree of partial melting of an enriched mantle source. In order to understand the spaciotemporal primitive melt diversity, we used silicate melt inclusions hosted in primitive olivine (Fo87-89) and plagioclase (An85-89) phenocrysts from the N-type APR basalts. Rehomogenized melt inclusions have been analyzed by electron and LA-ICPMS for major and trace elements. The homogenization temperature varies from 1205 to 1237oC for olivine hosts, and 1206 to 1247oC for plagioclase hosts. Melt inclusions have more primitive compositions than the host lavas. Melt inclusions hosted in olivine display Mg#s ranging from 67.7 to 71.9, and those in plagioclase from 67.5 to 74.1. All melt inclusions from the N-type APR basalts exhibit light rare earth element depleted patterns. They have K2O/TiO2 and (La/Sm)N ratios ranging from 0.1 to 0.3, and 0.4 to 0.9, respectively, which are overlapping with the values for N-type APR basalts. Furthermore, the melt inclusions have elevated (Lu/Hf)N and (Sm/Nd)N ratios compared with the E-type APR basalts. It should be pointed out that the N-type APR basalts do not contain any of enriched melt inclusions. These observations indicate that the N- and E-type APR basalts were produced at different depths. The chemistry of melt inclusions with N-type basalts is consistent with a

  19. Investigations of the phase relations among e1, e2 and C{\\bar {\\bf 1}} structures of Na-rich plagioclase feldspars: a single-crystal X-ray diffraction study.

    PubMed

    Jin, Shiyun; Xu, Huifang

    2017-10-01

    The subsolidus phase relations of plagioclase feldspar solid solution have been puzzling mineralogists and petrologists for decades, mainly due to the complicated structures of intermediate plagioclase at low temperature. The crystal structures of 12 Na-rich plagioclase samples are investigated by single-crystal X-ray diffraction analyses. The samples studied cover a compositional range from An21 to An49 (An is anorthite, CaAl2Si2O8), as well as a wide variety of origins, from extremely slow-cooled gabbroic rocks to pegmatite and metamorphic rocks. The structures fall into three different types: C{\\bar 1}, e2 and e1, with an obviously increasing trend in the ordering states of the structures. The phase transitions from C{\\bar 1} to e2 and e2 to e1 are both continuous in nature, as no abrupt structure change is required for the transformation. However, the structural difference between C\\bar 1 and e1 is large enough to create a miscibility gap causing the Bøggild intergrowth. As the plagioclase structure becomes more and more ordered, Al-Si reorganization in the framework would occur before the ordering of Ca and Na in M sites. Dramatic variations of Na occupancy would only appear in e1 structure with density modulation. This result confirms that Al-Si ordering is the major driving force of the formation of e-plagioclase structure. The composition of the lower end of the Bøggild intergrowth is precisely constrained to An44-An45, based on the structural differences between two samples from the same pegmatite crystal. The modulation periods and directions of e-plagioclase are dependent on the conditions at which e-ordering starts to happen, other than the composition of the plagioclase. However, the three components (δh, δk and δl) of the q vector show strong linear correlations among one another, indicating some crystallographic constraint on the modulation direction which might be independent from the composition. The detailed subsolidus phase relations

  20. Spinel inclusion in olivine and plagioclase crystals: a marker for primary crystals and a tracer for crystal migration in a differentiating magma reservoir

    NASA Astrophysics Data System (ADS)

    Hoshide, T.

    2012-04-01

    Crystal accumulation is an important process in magmatic differentiation. Although whether cumulus crystals were intratelluric phenocrysts or newly-formed crystals after magma emplacement is an important issue concerning mechanisms of magmatic differentiation, there has been no effective criteria for the judgement (e.g., Simkin, 1967; Gibb & Henderson, 2006; Latypov & Chisvakova). We demonstrate, from a study of the Murotomisaki Gabbroic Intrusion, that Cr-spinel inclusions in silicates may be used as a useful marker to identify the intratelluric phenocrysts and also as a tracer for studying crystal redistribution in a differentiating magma body. The Murotomisaki Gabbroic Intrusion is a 210-m-thick sill-like magma body emplaced in Tertiary sediments in Shikoku, Japan. Several olivine-rich zones have been identified in a lower 100-m zone of the intrusion. Olivine and plagioclase phenocrysts in the chilled margin commonly contain tiny Cr-spinel inclusions, which have uniform composition regardless of the host phases. Cr-spinel inclusions also occur in olivine and plagioclase in the lower 40-m cumulates (AC subzone), whereas no Cr-spinel inclusions occur in the silicates from the upper 40-100 m zone (GR subzone). For samples from the AC subzone, olivine-hosted spinel inclusions show increases in TiO2, FeO and Fe2O3 contents and decreases in Cr2O3, Al2O3 and MgO as compared with those from the chilled margin. Al2O3 and Fe2O3 contents of spinel inclusions in olivine do not show clear correlations with depth, but spinel inclusions located near the edge of the host olivine have more TiO2 and less Cr2O3 than those located in more central part of the olivine. It is noteworthy that TiO2 contents of the spinel inclusions gradually decrease, with increasing distance from the edge of host olivine, and approach the chilled marginal value. From the above observations, we infer that the olivine-hosted Cr-spinel inclusions from the AC subzone originally had the same compositions as

  1. A coupled model between mechanical deformation and chemical diffusion: An explanation for the preservation of chemical zonation in plagioclase at high temperatures

    NASA Astrophysics Data System (ADS)

    Zhong, Xin; Vrijmoed, Johannes; Moulas, Evangelos; Tajcmanová, Lucie

    2016-04-01

    Compositional zoning in metamorphic minerals have been generally recognized as an important geological feature to decipher the metamorphic history of rocks. The observed chemical zoning of, e.g. garnet, is commonly interpreted as disequilibrium between the fractionated inner core and the surrounding matrix. However, chemically zoned minerals were also observed in high grade rocks (T>800 degree C) where the duration of metamorphic processes was independently dated to take several Ma. This implies that temperature may not be the only factor that controls diffusion timescales, and grain scale pressure variation was proposed to be a complementary factor that may significantly contribute to the formation and preservation of chemical zoning in high temperature metamorphic minerals [Tajcmanová 2013, 2015]. Here, a coupled model is developed to simulate viscous deformation and chemical diffusion. The numerical approach considers the conservation of mass, momentum, and a constitutive relation developed from equilibrium thermodynamics. A compressible viscoelastic rheology is applied, which associates the volumetric change triggered by deformation and diffusion to a change of pressure. The numerical model is applied to the chemically zoned plagioclase rim described by [Tajcmanová 2014]. The diffusion process operating during the plagioclase rim formation can lead to a development of a pressure gradient. Such a pressure gradient, if maintained during ongoing viscous relaxation, can lead to the preservation of the observed chemical zonation in minerals. An important dimensionless number, the Deborah number, is defined as the ratio between the Maxwell viscoelastic relaxation time and the characteristic diffusion time. It characterizes the relative influence between the maintenance of grain scale pressure variation and chemical diffusion. Two extreme regimes are shown: the mechanically-controlled regime (high Deborah number) and diffusion-controlled regime (low Deborah number

  2. Generation and evolution of magma beneath the East Pacific Rise: Constraints from U-series disequilibrium and plagioclase-hosted melt inclusions

    NASA Astrophysics Data System (ADS)

    Zhang, Guo-Liang; Zeng, Zhi-Gang; Beier, Christoph; Yin, Xue-Bo; Turner, Simon

    2010-06-01

    Mid-ocean ridge basalt (MORB) samples from the East Pacific Rise (EPR 12°50'N) were analyzed for U-series isotopes and compositions of plagioclase-hosted melt inclusions. The 226Ra and 230Th excesses are negatively correlated; the 226Ra excess is positively correlated with Mg# and Sm/Nd, and is negatively correlated with La/Sm and Fe 8; the 230Th excess is positively correlated with Fe 8 and La/Sm and is negatively correlated with Mg# and Sm/Nd. Interpretation of these correlations is critical for understanding the magmatic process. There are two models (the dynamic model and the "two-porosity" model) for interpreting these correlations, however, some crucial parameters used in these models are not ascertained. We propose instead a model to explain the U-series isotopic compositions based on the control of melt density variation. For melting either peridotite or the "marble-cake" mantle, the FeO t content, 230Th excess and La/Sm ratio increases and Sm/Nd decreases with increasing pressure. A deep melt will evolve to a higher density and lower Mg# than a shallow melt, the former corresponds to a long residence time, which lowers the 226Ra excess significantly. This model is supported by the existence of low 226Ra excesses and high 230Th excesses in MORBs having a high Fe 8 content and high density. The positive correlation of 226Ra excess and magma liquidus temperature implies that the shallow melt is cooled less than the deep melt due to its low density and short residence time. The correlations among Fe 8, Ti 8 and Ca 8/Al 8 in plagioclase-hosted melt inclusions further prove that MORBs are formed from melts having a negative correlation in melting depths and degrees. The negative correlation of 226Ra excess vs. chemical diversity index (standard deviation of Fe 8, Ti 8 and Ca 8/Al 8) of the melt inclusions is in accordance with the influence of a density-controlled magma residence time. We conclude that the magma density variation exerts significant control on

  3. Alternating augite-plagioclase wedges in basement dolerites of Lockne impact structure, Sweden: A new shock wave-induced deformation feature

    NASA Astrophysics Data System (ADS)

    Agarwal, A.; Reznik, B.; Alva-Valdivia, L. M.; Srivastava, D. C.

    2017-03-01

    This paper reports peculiar alternating augite-plagioclase wedges in basement dolerites of Lockne impact structure, Sweden. The combined microscopic and spectroscopic studies of the micro/nanoscale wedges reveal that these are deformation-induced features. First, samples showing wedges, 12 out of 18 studied, are distributed in the impact structure within a radius of up to 10 km from the crater center. Second, the margins between the augite and labradorite wedges are sharp and the {110} prismatic cleavage of augite develops into fractures and thereafter into wedges. The fractures are filled with molten labradorite pushed from the neighboring bulk labradorite grain. Third, compared to the bulk labradorite, the dislocation density and the residual strain in the labradorite wedges are significantly higher. A possible mechanism of genesis of the wedges is proposed. The mechanism explains that passing of the shock waves in the basement dolerite induced (i) formation of microfractures in augite and labradorite; (ii) development of the augite prismatic cleavages into the wedges, which overprint the microfracture in the labradorite wedges; and (iii) thereafter, infilling of microfractures in the augite wedges by labradorite.

  4. Elastic anisotropy of layered rocks: Ultrasonic measurements of plagioclase-biotite-muscovite (sillimanite) gneiss versus texture-based theoretical predictions (effective media modeling)

    NASA Astrophysics Data System (ADS)

    Ivankina, T. I.; Zel, I. Yu.; Lokajicek, T.; Kern, H.; Lobanov, K. V.; Zharikov, A. V.

    2017-08-01

    In this paper we present experimental and theoretical studies on a highly anisotropic layered rock sample characterized by alternating layers of biotite and muscovite (retrogressed from sillimanite) and plagioclase and quartz, respectively. We applied two different experimental methods to determine seismic anisotropy at pressures up to 400 MPa: (1) measurement of P- and S-wave phase velocities on a cube in three foliation-related orthogonal directions and (2) measurement of P-wave group velocities on a sphere in 132 directions The combination of the spatial distribution of P-wave velocities on the sphere (converted to phase velocities) with S-wave velocities of three orthogonal structural directions on the cube made it possible to calculate the bulk elastic moduli of the anisotropic rock sample. On the basis of the crystallographic preferred orientations (CPOs) of major minerals obtained by time-of-flight neutron diffraction, effective media modeling was performed using different inclusion methods and averaging procedures. The implementation of a nonlinear approximation of the P-wave velocity-pressure relation was applied to estimate the mineral matrix properties and the orientation distribution of microcracks. Comparison of theoretical calculations of elastic properties of the mineral matrix with those derived from the nonlinear approximation showed discrepancies in elastic moduli and P-wave velocities of about 10%. The observed discrepancies between the effective media modeling and ultrasonic velocity data are a consequence of the inhomogeneous structure of the sample and inability to perform long-wave approximation. Furthermore, small differences between elastic moduli predicted by the different theoretical models, including specific fabric characteristics such as crystallographic texture, grain shape and layering were observed. It is shown that the bulk elastic anisotropy of the sample is basically controlled by the CPO of biotite and muscovite and their volume

  5. The Late Cretaceous igneous rocks of Romania (Apuseni Mountains and Banat): the possible role of amphibole versus plagioclase deep fractionation in two different crustal terranes

    NASA Astrophysics Data System (ADS)

    Vander Auwera, Jacqueline; Berza, Tudor; Gesels, Julie; Dupont, Alain

    2016-04-01

    We provide new whole-rock major and trace elements as well as 87Sr/86Sr and 143Nd/144Nd isotopic data of a suite of samples collected in the Late Cretaceous volcanic and plutonic bodies of the Apuseni Mts. (Romania) that belong to the Banatitic Magmatic and Metallogenic Belt, also called the Apuseni-Banat-Timok-Srednogorie belt. The samples define a medium- to high-K calc-alkaline differentiation trend that can be predicted by a three-step fractional crystallization process which probably took place in upper crustal magma chambers. Published experimental data indicate that the parent magma (Mg# = 0.47) of the Apuseni Mts. trend could have been produced by the lower crustal differentiation of a primary (in equilibrium with a mantle source) magma. The Late Cretaceous magmatic rocks of the Apuseni Mts. and Banat display overlapping major and trace element trends except that Sr is slightly lower and Ga is higher in the Apuseni Mts. parent magma. This difference can be accounted for by fractionating plagioclase-bearing (Apuseni Mts.) or amphibole-bearing (Banat) cumulates during the lower crustal differentiation of the primary magma to the composition of the parent magma of both trends. This, together with results obtained on the Late Cretaceous igneous rocks from the Timok area in Eastern Serbia, further suggests variation of the water content of the primary magma along and across the belt. The Apuseni Mts. versus the Banat samples display different isotopic compositions that likely resulted from the assimilation of two distinct crustal contaminants, in agreement with their emplacement in two separate mega-units of Alpine Europe.

  6. Timing of degassing and plagioclase growth in lavas erupted from Mount St. Helens, 2004-2005, from 210Po-210Pb-226Ra disequilibria: Chapter 37 in A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006

    USGS Publications Warehouse

    Reagan, Mark K.; Cooper, Kari M.; Pallister, John S.; Thornber, Carl R.; Wortel, Matthew; Sherrod, David R.; Scott, William E.; Stauffer, Peter H.

    2008-01-01

    Disequilibrium between 210Po, 210Pb, and 226Ra was measured on rocks and plagioclase mineral separates erupted during the first year of the ongoing eruption of Mount St. Helens. The purpose of this study was to monitor the volatile fluxing and crystal growth that occurred in the weeks, years, and decades leading up to eruption. Whole-rock samples were leached in dilute HCl to remove 210Po precipitated in open spaces. Before leaching, samples had variable initial (210Po) values, whereas after leaching, the groundmasses of nearly all juvenile samples were found to have had (210Po) ≈ 0 when they erupted. Thus, most samples degassed 210Po both before and after the magmas switched from open- to closed-system degassing. All juvenile samples have (210Pb)/(226Ra) ratios within 2 δ of equilibrium, suggesting that the magmas involved in the ongoing eruption did not have strong, persistent fluxes of 222Rn in or out of magmas during the decades and years leading to eruption. These equilibrium values also require a period of at least a century after magma generation and the last significant differentiation of the Mount St. Helens dacites. Despite this, the elevated (210Pb)/(226Ra) value measured in a plagioclase mineral separate from lava erupted in 2004 suggests that a significant proportion of this plagioclase grew within a few decades of eruption. The combined dataset suggests that for most 2004-5 lavas, the last stage of open-system degassing of the dacite magmas at Mount St. Helens is confined to the period between 1-2 years and 1-2 weeks before eruption, whereas plagioclase large enough to be included in the mineral separate grew around the time of the 1980s eruption or earlier.

  7. Magmatic evolution of lunar highland rocks estimated from trace elements in plagioclase: A new bulk silicate Moon model with sub-chondritic Ti/Ba, Sr/Ba, and Sr/Al ratios

    NASA Astrophysics Data System (ADS)

    Togashi, Shigeko; Kita, Noriko T.; Tomiya, Akihiko; Morishita, Yuichi

    2017-08-01

    The compositions of host magmas of ferroan anorthosites (FAN-host magmas) were estimated from secondary ion mass spectrometry analyses of plagioclase in lunar highland rocks. The evolution of the magmas was investigated by considering phase relations based on the MELTS algorithm and by re-examining partition coefficients for trace elements between plagioclase and melts. Data little affected by post-magmatic processes were selected by using plagioclase with relatively primitive Sc and Co contents. The FAN-host magma contained 90-174 ppm Sr, 40-119 ppm Ba and 0.5-1.3% TiO2, and had sub-chondritic Sr/Ba and Ti/Ba ratios. It is difficult to account for the formation of FAN-host magma on the basis of magma evolution processes of previously proposed bulk silicate Moon models with chondritic ratios for refractory elements at global scale. Therefore, the source of the FAN-host magma must have had primordial sub-chondritic Sr/Ba and Ti/Ba ratios. The FAN-host magmas were consistent in refractory elements with the estimated host mafic magma for feldspathic crust based on lunar meteorites, and some very-low-Ti mare rocks from lunar meteorites. Here, we propose an alternative bulk silicate Moon model (the cBSM model), which is enriched in crustal components of proto-bodies relative to the present whole Earth-Moon system.

  8. Magnetic, Chemical and Mössbauer Tracking of the Solid Solution of Fe and Exsolution of Magnetite in Plagioclase of the 2.05 Ga Bushveld Complex, South Africa

    NASA Astrophysics Data System (ADS)

    McEnroe, S. A.; Tegner, C.; Robinson, P.; Dyar, M. D.; Church, N. S.

    2016-12-01

    Mapping magnetic fields is a primary tool to explore Earth and other planets. Understanding the nature of oxide exsolutions (or lack of) in silicate minerals gives constraints to interpret the magnetic signatures of planetary crusts when combined with chemical mapping techniques. Exsolution of magnetite (mgt) from silicates provides a stable recorder of the field at the time of exsolution. We explore relationships between plagioclase chemistry and mgt exsolution by EMP, XRF, Mössbauer, and magnetic measurements. These are compared to chemical and magnetic properties of the bulk rock samples. We examine links between chemical evolution and magnetic signatures. 28 gabbroic samples, with similar total FeO (0.2-0.6 wt%) in plagioclase contain Fe in very different forms: as Fe in solid solution, or exsolved, as micron to submicroscopic needles of mgt. Minor chemical substitution of Fe in plagioclase can be represented by three logical but structurally implausible end members: Fe2+[Fe2+Si3]08 with maximal Fe2+; Fe3+[Fe3+3 Si]08 with maximal Fe3+; and an intermediate component Fe2+[Fe3+2Si2]08 equivalent to 1 mgt (4 Ox) + 2 quartz (4 Ox). Mössbauer measurements of Fe3+/FeT on 28 plagioclase separates indicate that the 1st and 3rd end members describe 20 compositions, so that exsolution of mgt and volumetrically equivalent quartz, though depleting total Fe, would increase the proportion of Fe2+. Five sample compositions are colinear with the 3rd, and only 3 samples appear to contain a significant proportion of the 2nd. Volume of exsolved magnetite, 0 to 0.09%, was determined from Ms values, and its magnetic state by Preisach and FORC measurements. Plagioclase (+rock) results are in three groups, where An indicates similar magma evolution: A) An74-58, FeOwt% 0.21-.39; B) An69-58, FeO 0.35-.63; C) An66-57; FeO 0.23-.42. A) with low Fe is dominantly paramagnetic with Fe in solid solution. B) with highest Fe lacks primary magnetite, but has abundant exsolved mgt

  9. Immiscible Fe- and Si-rich silicate melts in plagioclase from the Baima mafic intrusion (SW China): Implications for the origin of bi-modal igneous suites in large igneous provinces

    NASA Astrophysics Data System (ADS)

    Liu, Ping-Ping; Zhou, Mei-Fu; Ren, Zhongyuan; Wang, Christina Yan; Wang, Kun

    2016-09-01

    The Emeishan large igneous province (ELIP) in SW China is characterized by voluminous high-Ti and low-Ti basalts and spatially associated Fe-Ti oxide-bearing mafic-ultramafic and syenitic/granitic intrusions. The Baima layered mafic intrusion in the central part of the ELIP is surrounded by syenitic and granitic rocks and contains a Lower Zone of interlayered Fe-Ti oxide ores, troctolites and clinopyroxenites and an Upper Zone of isotropic olivine gabbros and gabbros (UZa) and apatite gabbros and Fe-Ti-P oxide ores (UZb). Polycrystalline mineral inclusions, for the first time, were observed in primocryst plagioclase from the basal part of the UZa through to the top of the UZb and consist mostly of clinopyroxene, plagioclase, magnetite, ilmenite and apatite with minor orthopyroxene, sulfide and hornblende. These minerals are commonly anhedral and form irregular shapes. Daughter plagioclase usually crystallizes on the walls of host primocryst plagioclase and has An contents typically 3-6 An% lower than the host plagioclase. Daughter clinopyroxene has similar Mg# but lower TiO2 and Al2O3 contents than primocryst clinopyroxene. These polycrystalline mineral inclusions are considered to crystallize from melts contemporaneous with host plagioclase. The compositional differences between daughter and primocryst minerals can be attributed to equilibrium crystallization in a closed system of the trapped melt inclusions in contrast to fractional crystallization and possible magma replenishment in an open system typical for primo-cumulates of large layered intrusions. Heated and homogenized melt inclusions have variable SiO2 (33-52 wt%), CaO (7-20 wt%), TiO2 (0.1-12 wt%), FeOt (5-20 wt%), P2O5 (0.2-10 wt%) and K2O (0-2.2 wt%). The large ranges of melt compositions are interpreted to result from heterogeneous trapping of different proportions of immiscible Si-rich and Fe-Ti-rich silicate liquids, together with entrapment of various microphenocrysts. The separation of micrometer

  10. Formation conditions of leucogranite dykes and aplite-pegmatite dykes in the eastern Mt. Capanne plutonic complex (Elba, Italy): fluid inclusion studies in quartz, tourmaline, andalusite and plagioclase

    NASA Astrophysics Data System (ADS)

    Bakker, Ronald J.; Schilli, Sebastian E.

    2016-02-01

    Leucogranite and aplite-pegmatite dykes are associated with the Mt. Capanne pluton (Elba) and partly occur in the thermally metamorphosed host rock (serpentinites). Crystallization conditions of these dykes in the late magmatic-hydrothermal stage are estimated from fluid inclusion studies and mineralogical characterisation, obtained from detailed microthermometry, Raman spectroscopy, and electron microprobe analyses. Fluid inclusion assemblages are analysed in andalusite, quartz, and plagioclase from the leucogranite dykes, and in tourmaline and quartz from the aplite-pegmatite dykes. The fluid inclusion assemblages record multiple pulses of low salinity H2O-rich magmatic and reduced metamorphic fluid stages. Magmatic fluids are characterized by the presence of minor amounts of CO2 and H3BO3, whereas the metamorphic fluids contain CH4 and H2. The highly reduced conditions are also inferred from the presence of native arsenic in some fluid inclusions. Several fluid inclusion assemblages reveal fluid compositions that must have resulted from mixing of both fluid sources. In leucogranite dykes, magmatic andalusite contains a low-density magmatic CO2-rich gas mixture with minor amounts of CH4 and H2. Accidentally trapped crystals (mica) and step-daughters (quartz and diaspore) are detected in some inclusions in andalusite. The first generation of inclusions in quartz that crystallized after andalusite contains a highly reduced H2O-H2 mixture and micas. The second type of inclusions in quartz from the leucogranite is similar to the primary inclusion assemblage in tourmaline from the aplite-pegmatite, and contains up to 4.2 mass% H3BO3, present as a sassolite daughter crystal or dissolved ions, in addition to a CO2-CH4 gas mixture, with traces of H2, N2, H2S, and C2H6. H2O is the main component of all these fluids ( x = 0.91 to 0.96) with maximally 7 mass% NaCl. Some accidentally trapped arsenolite and native arsenic are also detected. These fluids were trapped in the

  11. The stability of Cl-CO3-scapolite relative to plagioclase + CaCO3 + CaSO4 in the presence of NaCl brines as a function of P-T-XNaCl

    NASA Astrophysics Data System (ADS)

    Harlov, D. E.; Budzyn, B.

    2008-12-01

    Cl-CO3-scapolite [(Na,Ca)4[Al3 (Al,Si) 3 Si3 O24](Cl, CO3 , SO4 )] occurs as a common partial to total alteration of plagioclase in deep-crustal xenoliths, skarns, marbles, gabbros, metabasites, calc-silicate gneisses, as well as in quartzofeldspathic granulite-facies rocks in general (Moecher and Essene, 1990, J Petrol 31, 997). Alteration of plagioclase to Cl-CO3-scapolite is presumed due to metasomatism by CO2-NaCl-H2O fluids (Satish-Kumar and Santosh, 1998, Geol Mag 135, 27). Previous experimental work on CO3-scapolite has focused on reversing the equilibrium 3 CaAl2 Si2 O8 + CaCO3 = Ca4 Al6 Si6 O24 CO3 in either pure CO2 (Goldschmidt and Newton, 1977, Am Mineral 62, 1063) or in CO2-H2O (Huckenholz and Seiberl, 1989 Abs IGC 28, 2.79). These experiments have determined that the anorthite- calcite-scapolite equilibrium is nearly pressure-invariant in P-T space (200 to 1500 MPa) occurring at approximately 790 to 820 °C (Huckenholz and Seiberl, 1989). In this study, a series of experiments, involving the equilibrium 3 Plagioclase(An60) + 0.5 CaCO3 + 0.5 CaSO4 = [(Na,Ca)4[Al3 (Al,Si)3 Si3 O24](Cl, CO3, SO4 )] plus an NaCl brine (10/90, 20/80, 30/70, and 50/50 molar NaCl/H2O) have been done at 500, 1000, and 1500 MPa and 600 to 900 °C. Natural plagioclase and scapolite, along with synthetic calcite and anhydrite, were lightly ground together in equi-molar amounts in ethanol. The mineral mix (10 mg) + NaCl brine (5 mg), or pure H2O (1.5 mg), were loaded into 3 mm diameter/1.3 mm long Pt capsules which were arc-welded shut, folded, and placed horizontally in a CaF2 setup (with graphite oven), such that the thermocouple tip touched the Pt capsule, or placed in a hydrothermal autoclave (600 and 700 °C; 500 MPa) with an internal thermocouple. A series of duplicate experiments for the same mineral mix, at the same P-T conditions, were done utilizing pure H2O as the flux. The H2O-only experiments duplicated the P-T reversals of Huckenholz and Seiberl (1989). In contrast

  12. Melt inclusions in olivine and plagioclase phenocrysts from Antarctic-Phoenix Ridge basalts: Implications for origins of N- and E-type MORB parent magmas

    NASA Astrophysics Data System (ADS)

    Choi, Sung Hi; Schiano, Pierre; Chen, Yang; Devidal, Jean-Luc; Choo, Mi Kyung; Lee, Jong-Ik

    2013-03-01

    The Antarctic-Phoenix Ridge (APR) is a fossil spreading center in the Drake Passage, Antarctic Ocean. Spreading ceased in chron C2A (ca. 3.3 Ma). Although the APR is a normal ridge that is not influenced by a hotspot, enriched (E-type) mid-ocean ridge basalt (MORB) coexists with normal (N-type) MORB in the ridge's axial region. The E-type APR basalt is relatively young (< 3.1 Ma) compared to the N-type basalt (> 3.5 Ma). The E-type basalt is characterized by elevated K2O/TiO2 (= 0.4-0.8) and (La/Sm)N (= 2.2-3.4) ratios, relative to the N-type basalt (K2O/TiO2 = 0.1-0.3; (La/Sm)N = 0.7-0.8). To better understand the compositional variation in the APR basalts and their mantle source regions through time, silicate melt inclusions in primitive olivine (Fo87-89) and plagioclase (An85-89) phenocrysts from the N-type APR basalt were studied. Rehomogenized melt inclusions were analyzed by electron microprobe and LA-ICPMS for major and trace elements. The melt inclusions are more primitive than the host basalt, with Mg#s from 67.5 to 74.1. All inclusions exhibit patterns that are depleted in the light rare earth elements. The inclusions have K2O/TiO2 from 0.1 to 0.3 and (La/Sm)N ratios from 0.4 to 0.9; these values overlap with those of the N-type APR basalt. Furthermore, the melt inclusions have elevated (Lu/Hf)N and (Sm/Nd)N ratios compared to the E-type basalts. The N-type APR basalts do not contain any melt inclusions that are enriched in incompatible elements. The E-type basalt was generated by a low degree of partial melting of a relatively incompatible-element-enriched mantle source. In contrast, chemistries of melt inclusions and N-type basalts are compatible with high degrees of partial melting of an increasingly depleted mantle source. Assuming a veined or otherwise heterogeneous mantle, the absence of E-type inclusions from the N-type host has implications for cyclic magmatic activity beneath the APR. Multi-stage mantle melting and melt extraction from a

  13. The thermodynamics of the I overline{1} - P overline{1} phase transition in Ca-rich plagioclase from an assessment of the spontaneous strain

    NASA Astrophysics Data System (ADS)

    Tribaudino, Mario; Angel, Ross J.

    2012-10-01

    In-situ powder diffraction measurements between 90 and 935 K on four anorthite-rich plagioclase samples (An100, An96Ab4, An89Ab11 and An78Ab22) were used to determine the detailed evolution of these samples through the I overline{1} -P overline{1} phase transition. The c-type reflections indicative of P overline{1} symmetry were detected only in An100, An96Ab4, whereas deviations in the evolution of the unit-cell parameters with temperature were observed in all samples, most prominently in the β unit-cell angle. The c-type reflections disappear at 510 and 425 K in An100 and An96Ab4 respectively, and their intensity decreases according to a tricritical trend I2 ∝ ( {T - T_{{c}} } ) . The cell parameter changes were used to determine the spontaneous strains arising from the transition which were modelled with Landau theory, allowing for low-temperature quantum saturation, in order to determine the thermodynamic behaviour. In An100 tricritical behaviour was observed [ T c = 512.7(4) K; θs = 394(4)] in good agreement with previous studies, and the c-type superlattice reflections indicative of P overline{1} symmetry persist up to the T c determined from the spontaneous strain, and then disappear. The evolution of the spontaneous strain in An96Ab4 is tricritical at low temperatures [ T c = 459(1) K, θs = 396(5)] up to the temperature of disappearance of c-type reflections, but becomes second order beyond 440 K. In An89Ab11 the strain displays second-order behaviour throughout [ T c = 500(1) and θs = 212(5)], and the c-type reflections are not detected in the powder diffraction patterns at any temperature. The apparent discrepancy between the absence of c-type reflections in temperature ranges where the cell parameters display significant spontaneous strain is resolved through consideration of the sizes of the anti-phase domains within the crystals. It is deduced that the tricritical phase transition occurs in well-ordered crystals with large domains in which the

  14. Abundance and Charge State of Implanted Solar Wind Transition Metals in Individual Apollo 16 and 17 Lunar Soil Plagioclase Grains Determined In Situ Using Synchrotron X-ray Fluorescence

    SciTech Connect

    Kitts, K.; Sutton, S.; Newville, M.

    2007-03-06

    We report (1) a new method for determining the relative abundances in situ of Cr, Mn, Fe and Ni in implanted solar wind in individual Apollo 16 and 17 lunar plagioclases via synchrotron X-ray fluorescence and (2) the charge states of these metals. By virture of its mass alone, the Sun provides a representative composition of the solar system and can be used as a background against which to gauge excesses or deficiencies of specific components. One way of sampling the Sun is by measuring solar wind implanted ions in lunar soil grains. Such measurements are valuable because of their long exposure ages which compliment shorter time scale collections, such as those obtained by the Genesis spacecraft. Kitts et al. sought to determine the isotopic composition of solar Cr by analyzing the solar wind implanted into plagioclase grains from Apollo 16 lunar soils. The isotopic composition of the solar wind bearing fraction was anomalous and did not match any other known Cr isotopic signature. This could only be explained by either (1) an enrichment in the solar wind of heavy Cr due to spallation in the solar atmosphere or (2) that the Earth and the various parent bodies of the meteorites are distinct from the Sun and must have formed from slightly different mixes of presolar materials. To help resolve this issue, we have developed a wholly independent method for determining the relative abundances of transition metals in the solar wind implanted in individual lunar soil grains. This method is based on in situ abundance measurements by microbeam x-ray fluorescence in both the implantation zone and bulk grains using the synchrotron x-ray microprobe at the Advanced Photon Source (GSECARS sector 13) at Argonne National Laboratory. Here, we report results for Apollo 16 and 17 plagioclase grains. Additionally, a micro-XANES technique was used to determine charge states of the implanted Cr, Mn, Fe and Ni.

  15. Mercury'S Dark Plains West Of Caloris Basin--high Ca Clinopyroxene, Na-rich Plagioclase, Mg-rich Olivine, Tio2: Caloris Basin--k-spar, High Ca Clinopyroxene, Tio2, Na-rich Plagioclase, Hornblende And Mg- And Ca-rich Garnets

    NASA Astrophysics Data System (ADS)

    Sprague, Ann L.; Donaldson Hanna, K. L.; Kozlowski, R. W. H.; Helbert, J.; Maturilli, A.

    2008-09-01

    We identify mineral phases and approximate abundances on Mercury's surface for spectral measurements made over Caloris Basin and the dark plains to the west. Our results are obtained by fitting spectra obtained with the Mid-Infrared Spectrometer and Imager (MIRSI) at the Infrared Telescope Facility (IRTF) using an established spectral deconvolution algorithm (Ramsey 1996, Ph.D. Dissertation, ASU; Ramsey and Christiansen 1998, JGR). We have assembled several laboratory mineral spectral libraries (JHU, Salisbury et al. 1987, Open-File Report 87-263, USGS; JPL, http://speclib.jpl.nasa.gov; RELAB, http://www.planetary.brown.edu/relab; ASU, Christensen et al. 2000, JGR; BED, Helbert et al. 2007, Adv. Space Res.; USGS, Clark et al. 2007, USGS digital spectral library) with a wide range of known mineral compositions with grain sizes ranging from the finest separates, 0 - 45 µm, incrementally increasing to 250 - 400 µm. Head et al. and Murchie et al. (Science, 2008) show ample evidence for one or more episodes of extrusive volcanism in around Caloris Basin. Our spectral fitting suggests similarities and differences between Caloris infill and the dark plains to the west. Both contain high-Ca clinopyroxene, Mg-rich orthopyroxene (Sprague et al. 1998, Icarus), Na-rich to intermediate plagioclase (Sprague et al. 1994, Icarus), and TiO2 likely in the form of rutile. Sanidine appears to be the dominate K-spar in Caloris Basin, but not in the dark plains (Donaldson Hanna et al. 2008, EGU Abs). A slight improvement in spectral fitting was made to one spectrum from Caloris by including a Na- and K-rich hornblende. In addition small abundances of pyrope and grossular (Ca- and Mg-rich garnets) are apparently present in Caloris Basin infill. This indicates extrusive volcanic episodes moved lava to the surface quickly before entrained garnets from the upper mantle could dissolve and equilibrate with the source magma. This work was funded by NSF AST0406796.

  16. Plagioclase populations and zoning in dacite of the 2004-2005 Mount St. Helens eruption: constraints for magma origin and dynamics: Chapter 34 in A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006

    USGS Publications Warehouse

    Streck, Martin J.; Broderick, Cindy A.; Thronber, Carl R.; Clynne, Michael A.; Pallister, John S.; Sherrod, David R.; Scott, William E.; Stauffer, Peter H.

    2008-01-01

    We propose that crystals with no dissolution surfaces are those that were supplied last to the shallow reservoir, whereas plagioclase with increasingly more complex zoning patterns (that is, the number of zoned bands bounded by dissolution surfaces) result from prolonged residency and evolution in the reservoir. We propose that banding and An zoning across multiple bands are primarily a response to thermally induced fluctuations in crystallinity of the magma in combination with recharge; a lesser role is ascribed to cycling crystals through pressure gradients. Crystals without dissolution surfaces, in contrast, could have grown only in response to steady(?) decompression. Some heating-cooling cycles probably postdate the final eruption in 1986. They resulted from small recharge events that supplied new crystals that then experienced resorption-growth cycles. We suggest that magmatic events shortly prior to the current eruption, recorded in the outermost zones of plagioclase phenocrysts, began with the incorporation of acicular orthopyroxene, followed by last resorption, and concluded with crystallization of euhedral rims. Finally, we propose that 2004-5 dacite is composed mostly of dacite magma that remained after 1986 and underwent subsequent magmatic evolution but, more importantly, contains a component of new dacite from deeper in the magmatic system, which may have triggered the new eruption.

  17. Multiple plagioclase crystal populations identified by crystal size distribution and in situ chemical data: Implications for timescales of magma chamber processes associated with the 1915 eruption of Lassen Peak, CA

    USGS Publications Warehouse

    Salisbury, M.J.; Bohrson, W.A.; Clynne, M.A.; Ramos, F.C.; Hoskin, P.

    2008-01-01

    Products of the 1915 Lassen Peak eruption reveal evidence for a magma recharge-magma mixing event that may have catalyzed the eruption and from which four compositional members were identified: light dacite, black dacite, andesitic inclusion, and dark andesite. Crystal size distribution, textural, and in situ chemical (major and trace element and Sr isotope) data for plagioclase from these compositional products define three crystal populations that have distinct origins: phenocrysts (long axis > 0??5 mm) that typically have core An contents between 34 and 36 mol %, microphenocrysts (long axis between 0??1 and 0??5 mm) that have core An contents of 66-69, and microlites (long axis < 0??1 mm) with variable An core contents from 64 to 52. Phenocrysts are interpreted to form in an isolated dacitic magma chamber that experienced slow cooling. Based on textural, compositional, and isotopic data for the magma represented by the dacitic component, magma recharge was not an important process until just prior to the 1915 eruption. Average residence times for phenocrysts are in the range of centuries to millennia. Microphenocrysts formed in a hybrid layer that resulted from mixing between end-member reservoir dacite and recharge magma of basaltic andesite composition. High thermal contrast between the two end-member magmas led to relatively high degrees of undercooling, which resulted in faster crystal growth rates and acicular and swallowtail crystal habits. Some plagioclase phenocrysts from the dacitic chamber were incorporated into the hybrid layer and underwent dissolution-precipitation, seen in both crystal textures and rim compositions. Average microphenocryst residence times are of the order of months. Microlites may have formed in response to decompression and/ or syn-eruptive degassing as magma ascended from the chamber through the volcanic conduit. Chemical distinctions in plagioclase microlite An contents reveal that melt of the dark andesite was more mafic than

  18. Mineralogy, Petrology, Chemistry, and Ar-39 - Ar-40 and Ages of the Caddo County IAB Iron: Evidence for Early Partial Melt Segregation of a Gabbro Area Rich in Plagioclase-Diopside

    NASA Technical Reports Server (NTRS)

    Takeda, Hiroshi; Bogard, Donald D.; Mittlefehldt, David W.; Garrison, Daniel H.

    2000-01-01

    We found coarse-grained gabbroic material rich in plagioclase and diopside in the Caddo County IAB iron meteorite. The polished thin sections studied were made from areas rich in Al and Ca detected by a micro-focus X-ray fluorescence (XRF) mapping technique. The gabbro is not a clast within a breccia, but rather this area is located mainly at silicate-metal boundaries only a few cm away from an area with fine-grained, ultramafic silicate similar to winonaites. Medium-grained orthopyroxene and olivine are found in transitional areas showing no disturbance of their crystalline textures. A vein-like region, starting at the area rich in fine-grained mafic silicate, extends towards the gabbroic area with a gradual increase in abundance of plagioclase and diopside. This texture and our accumulated knowledge of the formation mechanism of IAB/winonaltes meteorites, suggest that the gabbroic materials were formed by inhomogeneous segregation of partial melts of chondritic source materials. Compositional data on two mineralogically distinct samples of the gabbro-rich portion of the inclusion were obtained by INAA. Compared to an average of LAB silicate inclusions or winonaites, the Caddo County gabbro is enriched in the incompatible lithophile elements Na, Ca, Sc, REE and Hf, which is consistent with a melt origin for the gabbro. The cosmogenic space exposure age of Caddo County (511 Ma) is significantly younger than exposure ages of some other IAB meteorites, An 39Ar-40Ar age determination of the gabbroic material indicates a series of upward steps in age from 4.516 Ga to 4.523 Ga, with a few high temperature ages up to 4.54 Ga. The older age could approximate the primary recrystallization age of silicates. The stepped Ar age spectrum may indicate differences in Ar closure temperatures during slow cooling of -2-20'C/Myr in the parent body. Alternatively, the younger Ar-Ar ages may date a shock event which occurred while Caddo County was hot and which also created textures

  19. Mineralogy, Petrology, Chemistry, and Ar-39 - Ar-40 and Ages of the Caddo County IAB Iron: Evidence for Early Partial Melt Segregation of a Gabbro Area Rich in Plagioclase-Diopside

    NASA Technical Reports Server (NTRS)

    Takeda, Hiroshi; Bogard, Donald D.; Mittlefehldt, David W.; Garrison, Daniel H.

    2000-01-01

    We found coarse-grained gabbroic material rich in plagioclase and diopside in the Caddo County IAB iron meteorite. The polished thin sections studied were made from areas rich in Al and Ca detected by a micro-focus X-ray fluorescence (XRF) mapping technique. The gabbro is not a clast within a breccia, but rather this area is located mainly at silicate-metal boundaries only a few cm away from an area with fine-grained, ultramafic silicate similar to winonaites. Medium-grained orthopyroxene and olivine are found in transitional areas showing no disturbance of their crystalline textures. A vein-like region, starting at the area rich in fine-grained mafic silicate, extends towards the gabbroic area with a gradual increase in abundance of plagioclase and diopside. This texture and our accumulated knowledge of the formation mechanism of IAB/winonaltes meteorites, suggest that the gabbroic materials were formed by inhomogeneous segregation of partial melts of chondritic source materials. Compositional data on two mineralogically distinct samples of the gabbro-rich portion of the inclusion were obtained by INAA. Compared to an average of LAB silicate inclusions or winonaites, the Caddo County gabbro is enriched in the incompatible lithophile elements Na, Ca, Sc, REE and Hf, which is consistent with a melt origin for the gabbro. The cosmogenic space exposure age of Caddo County (511 Ma) is significantly younger than exposure ages of some other IAB meteorites, An 39Ar-40Ar age determination of the gabbroic material indicates a series of upward steps in age from 4.516 Ga to 4.523 Ga, with a few high temperature ages up to 4.54 Ga. The older age could approximate the primary recrystallization age of silicates. The stepped Ar age spectrum may indicate differences in Ar closure temperatures during slow cooling of -2-20'C/Myr in the parent body. Alternatively, the younger Ar-Ar ages may date a shock event which occurred while Caddo County was hot and which also created textures

  20. Compositional variations across a dunite - harzburgite - lherzolite - plagioclase lherzolite sequence at the Trinity ophiolite: Evidence for multiple episodes of melt flow and melt-rock reaction in the mantle.

    NASA Astrophysics Data System (ADS)

    Morgan, Z. T.; Liang, Y.; Kelemen, P.

    2004-12-01

    In the preceding report we showed experimentally that the dunite-harzburgite-lherzolite (DHL) sequence found in the mantle sections of ophiolite could be formed by reactive dissolution of lherzolite in a basaltic liquid. The most striking results of our lherzolite dissolution experiments are the sharp mineralogical boundaries between adjacent lithologies and simple monotonic composition variations in minerals across the DHL sequence. Here we present a detailed compositional traverse across a dunite (3.64 m wide) - harzburgite-lherzolite (5.64 m) - plagioclase lherzolite (> 10 m) sequence (referred to as DHL-PL) at the Trinity ophiolite that shows complicated composition trends and melt flow history. With the exception of a small (1 m wide) anomalous region within the dunite, less than 1 m away from the dunite-harzburgite contact, the Mg#s of olivine (90), cpx (92.8), opx (90.4 in harzburgite) and spinel (40), as well as Al2O3 and TiO2 abundance in cpx, opx and spinel are essentially constant from dunite to lherzolite. The CaO content in olivine (0.02%), opx (1%) and cpx (23.5%) are also uniform throughout the harzburgite-plagioclase lherzolite sequence. However, the Mg# of olivine and opx, Al2O3, TiO2 and Cr2O3 in cpx and opx, as well as Na2O in cpx increase 2 meters into the plagioclase lherzolite. In addition, asymmetric concentration gradients are observed for CaO in olivine and Cr2O3, Al2O3, MgO, and FeO in spinel. These asymmetric concentration gradients are mostly in the dunite-side of the dunite-harzburgite contact. And finally, the 1 m wide anomalous region within the dunite is characterized by elevated Mg# and NiO in olivine, Al2O3, TiO2, Cr2O3, and REE in cpx, and very distinct elemental abundance in spinel. The composition variations reported here are substantially different from those of Quick (1981) who measured a smaller (1 m) DHL-PL sequence at the Trinity ophiolite. Together these two Trinity data sets show a large variation in DHL-PL chemistry

  1. Microbial extracellular polysaccharides and plagioclase dissolution

    NASA Astrophysics Data System (ADS)

    Welch, S. A.; Barker, W. W.; Banfield, J. F.

    1999-05-01

    Bytownite feldspar was dissolved in batch reactors in solutions of starch (glucose polymer), gum xanthan (glucose, mannose, glucuronic acid), pectin (poly-galacturonic acid), and four alginates (mannuronic and guluronic acid) with a range of molecular weights (low, medium, high and uncharacterized) to evaluate the effect of extracellular microbial polymers on mineral dissolution rates. Solutions were analyzed for dissolved Si and Al as an indicator of feldspar dissolution. At neutral pH, feldspar dissolution was inhibited by five of the acid polysaccharides, gum xanthan, pectin, alginate low, alginate medium, alginate high, compared to an organic-free control. An uncharacterized alginate substantially enhanced both Si and Al release from the feldspar. Starch, a neutral polysaccharide, had no apparent effect. Under mildly acidic conditions, initial pH ≈ 4, all of the polymers enhanced feldspar dissolution compared to the inorganic controls. Si release from feldspar in starch solution exceeded the control by a factor of three. Pectin and gum xanthan increased feldspar dissolution by a factor of 10, and the alginates enhanced feldspar dissolution by a factor of 50 to 100. Si and Al concentrations increased with time, even though solutions were supersaturated with respect to several possible secondary phases. Under acidic conditions, initial pH ≈ 3, below the pK a of the carboxylic acid groups, dissolution rates increased, but the relative increase due to the polysaccharides is lower, approximately a factor of two to ten. Microbial extracellular polymers play a complex role in mineral weathering. Polymers appear to inhibit dissolution under some conditions, possibly by irreversibly binding to the mineral surfaces. The extracellular polysaccharides can also enhance dissolution by providing protons and complexing with ions in solution.

  2. Chapter G: Tentative Correlation Between CIPW Normin pl (Total Plagioclase) and Los Angeles Wear in Precambrian Midcontinental Granites-Examples from Missouri and Oklahoma, with Applications and Limitations for Use

    USGS Publications Warehouse

    Davis, George H.

    2004-01-01

    The normative chemical classification of Cross, Iddings, Pirsson, and Washington (CIPW) is commonly used in igneous petrology to distinguish igneous rocks by comparing their magmatic chemistries for similar and dissimilar components. A potential use for this classification other than in petrologic studies is in the rapid assessment of aggregate sources, possibly leading to an economic advantage for an aggregate producer or user, by providing the opportunity to determine whether further physical testing of an aggregate is warranted before its use in asphalt or concrete pavement. However, the CIPW classification currently should not be substituted for the physical testing required in specifications by State departments of transportation. Demands for physical testing of aggregates have increased nationally as users seek to maximize the quality of the aggregate they purchase for their pavements. Concrete pavements are being laid with increased thicknesses to withstand increasing highway loads. New pavement mixes, most notably Superior Performance Asphalt Pavement ('Superpave'), are designed for additional service life. For both concrete and asphalt, the intent is to generate a durable pavement with a longer service life that should decrease overall life-cycle costs. Numerous aggregate producers possess chemical-composition data available for examination to answer questions from the potential user. State geological surveys also possess chemical-composition data for stone sources. Paired with the results of physical testing, chemical- composition data provide indicative information about stone durability and aggregate strength. The Missouri Department of Transportation has noted a possible relation among coarse-grained Precambrian granites of the midcontinental region, correlating the results of abrasion testing with the contents of normative minerals, also known as normins, calculated from chemical composition data. Thus, normin pl ( total plagioclase) can predict, by

  3. Shocked Plagioclase Signatures in Thermal Emission Spectrometer Data of Mars

    NASA Technical Reports Server (NTRS)

    Johnson, J. R.; Staid, M. I.; Titus, T. N.

    2002-01-01

    Deconvolution of TES (Thermal Emission Spectrometer) data using a spectral library that includes spectra of experimentally shocked anorthosite (bytownite) suggests that shocked materials can be identified on Mars at low to intermediate abundances (10 - 20%) over a range of pressures. Additional information is contained in the original extended abstract.

  4. Shocked plagioclase signatures in Thermal Emission Spectrometer data of Mars

    USGS Publications Warehouse

    Johnson, J. R.; Staid, M.I.; Titus, T.N.; Becker, K.

    2006-01-01

    The extensive impact cratering record on Mars combined with evidence from SNC meteorites suggests that a significant fraction of the surface is composed of materials subjected to variable shock pressures. Pressure-induced structural changes in minerals during high-pressure shock events alter their thermal infrared spectral emission features, particularly for feldspars, in a predictable fashion. To understand the degree to which the distribution and magnitude of shock effects influence martian surface mineralogy, we used standard spectral mineral libraries supplemented by laboratory spectra of experimentally shocked bytownite feldspar [Johnson, J.R., Ho??rz, F., Christensen, P., Lucey, P.G., 2002b. J. Geophys. Res. 107 (E10), doi:10.1029/2001JE001517] to deconvolve Thermal Emission Spectrometer (TES) data from six relatively large (>50 km) impact craters on Mars. We used both TES orbital data and TES mosaics (emission phase function sequences) to study local and regional areas near the craters, and compared the differences between models using single TES detector data and 3 ?? 2 detector-averaged data. Inclusion of shocked feldspar spectra in the deconvolution models consistently improved the rms errors compared to models in which the spectra were not used, and resulted in modeled shocked feldspar abundances of >15% in some regions. However, the magnitudes of model rms error improvements were within the noise equivalent rms errors for the TES instrument [Hamilton V., personal communication]. This suggests that while shocked feldspars may be a component of the regions studied, their presence cannot be conclusively demonstrated in the TES data analyzed here. If the distributions of shocked feldspars suggested by the models are real, the lack of spatial correlation to crater materials may reflect extensive aeolian mixing of martian regolith materials composed of variably shocked impact ejecta from both local and distant sources. ?? 2005 Elsevier Inc. All rights reserved.

  5. Shocked plagioclase signatures in Thermal Emission Spectrometer data of Mars

    NASA Astrophysics Data System (ADS)

    Johnson, Jeffrey R.; Staid, Matthew I.; Titus, Timothy N.; Becker, Kris

    2006-01-01

    The extensive impact cratering record on Mars combined with evidence from SNC meteorites suggests that a significant fraction of the surface is composed of materials subjected to variable shock pressures. Pressure-induced structural changes in minerals during high-pressure shock events alter their thermal infrared spectral emission features, particularly for feldspars, in a predictable fashion. To understand the degree to which the distribution and magnitude of shock effects influence martian surface mineralogy, we used standard spectral mineral libraries supplemented by laboratory spectra of experimentally shocked bytownite feldspar [Johnson, J.R., Hörz, F., Christensen, P., Lucey, P.G., 2002b. J. Geophys. Res. 107 (E10), doi:10.1029/2001JE001517] to deconvolve Thermal Emission Spectrometer (TES) data from six relatively large (>50 km) impact craters on Mars. We used both TES orbital data and TES mosaics (emission phase function sequences) to study local and regional areas near the craters, and compared the differences between models using single TES detector data and 3×2 detector-averaged data. Inclusion of shocked feldspar spectra in the deconvolution models consistently improved the rms errors compared to models in which the spectra were not used, and resulted in modeled shocked feldspar abundances of >15% in some regions. However, the magnitudes of model rms error improvements were within the noise equivalent rms errors for the TES instrument [Hamilton V., personal communication]. This suggests that while shocked feldspars may be a component of the regions studied, their presence cannot be conclusively demonstrated in the TES data analyzed here. If the distributions of shocked feldspars suggested by the models are real, the lack of spatial correlation to crater materials may reflect extensive aeolian mixing of martian regolith materials composed of variably shocked impact ejecta from both local and distant sources.

  6. The formation of plagioclase chains during convective transfer in basaltic magma

    PubMed

    Philpotts; Dickson

    2000-07-06

    The basaltic rock in the lower part of the thick Holyoke lava flow in Connecticut and Massachusetts has been shown to have a remarkable texture, with crystals of feldspar linked together in a continuous three-dimensional network of chains. Heating experiments have revealed that this network persists to temperatures where the rock is 75% liquid, and therefore the network was interpreted to have formed at an early stage of crystallization and to have played an important role in the compaction of crystal mush in the lower part of the flow. Despite the texture's importance to our understanding of how such basalt flows form, the origin of the texture has remained uncertain. Here we show that, although the network is present in the lower third of the flow, it was actually formed in the upper solidification front and was transported down in plumes of dense crystal mush. Convection of this type has been postulated for intrusive magma chambers, but corroborative field evidence has been equivocal, especially in lava lakes and flows. Preservation of the roof-generated texture in the lower part of a thick flood-basalt flow therefore constitutes important evidence for the role of convection in the solidification and differentiation of a simple magma sheet.

  7. Use of olivine and plagioclase saturation surfaces for the petrogenetic modeling of recrystallized basic plutonic systems

    NASA Technical Reports Server (NTRS)

    Hanson, G. N.

    1983-01-01

    During petrogenetic studies of basic plutonic rocks, there are at least three major questions to be considered: (1) what were the relative proportions of cumulate crystals and intercumulus melt in a given sample? (2) what is the composition and variation in composition of the melts within the pluton? and (3) what is the original composition of the liquids, their source and evolution prior to the time of emplacement? Use of both saturation surfaces can place strong limits on the compositions of potential cumulate phases and intercumulus melts. Consideration of appropriate trace elements can indicate whether a sample is an orthocumulate, adcumulate or mesocumulate. Thus, when trace element and petrographic data are considered together with the saturation surfaces, it should be possible to begin to answer the three major questions given above, even for strongly recrystallized basic plutons.

  8. Partitioning of Eu and Sr between coexisting plagioclase and K-feldspar.

    NASA Technical Reports Server (NTRS)

    Nagasawa, H.

    1971-01-01

    Minerals were separated by an EM approach and with the aid of liquids of great density. An analysis of K, Rb, Ca, Sr, Ha, and rare earth elements was conducted by means of a mass spectrometer isotope dilution technique. The behavior of the divalent europium ions during the partition process was found to be very similar to that of divalent strontium ions, taking into consideration data of the partition coefficients between coexisting feldspars in acidic rocks.

  9. Oxalate adsorption at a plagioclase (An47) surface and models for ligand-promoted dissolution

    USGS Publications Warehouse

    Stillings, L.L.; Drever, J.I.; Poulson, S.R.

    1998-01-01

    Previous work on adsorption of oxalate at aluminosilicate surfaces suggests that maximum adsorption occurs through a bidentate attachment of the organic ligand, at near-neutral pH. Rates of ligand-promoted dissolution are expected to be greatest at this pH as well. We tested this model by measuring oxalate adsorption on the surface of andesine (An47), in solutions of pH 3- 5 and total oxalate concentrations of 0-8 mM. Contrary to expectation, the greatest adsorption density of 24 ??mol m-2 total oxalate was observed at pH 3 and 8 mM total oxalate. Adsorption is dependent upon the activities of both oxalate (C2O42-) and bioxalate (HC2O4-) in solution and can be modeled with either a two-term Langmuir or a two-term Freundlich isotherm. A Freundlich adsorption model provided the best fit to rate data because it was not constrained to a finite number of adsorption sites, as was the Langmuir model. The two-term ligand adsorption model was incorporated into a rate model: R(tot) = k(H-)[H(ads)/+](L) + k(HOx-)[HOx(ads)/-] + k(Ox2- )[Ox2(ads)/-] where R(tot) is the net dissolution rate of the feldspar, [i(ads)] is the concentration of species i adsorbed to the surface, and k(i) is the rate constant for release of the surface complex. The model was fit to data for oxalate-promoted dissolution of andesine, resulting in estimates for the rate constants of k(HOx-) = 1.16 x 10-12, k(Ox2-) = 1.05 x 10-12, and k(H-) = 9.61 x 10-13 mol of feldspar (??mol of i) (??mol of i)-1 s-1.Previous work on adsorption of oxalate at aluminosilicate surfaces suggests that maximum adsorption occurs through a bidentate attachment of the organic ligand, at near-neutral pH. Rates of ligand-promoted dissolution are expected to be greatest at this pH as well. We tested this model by measuring oxalate adsorption on the surface of andesine (An47), in solutions of pH 3-5 and total oxalate concentrations of 0-8 mM. Contrary to expectation, the greatest adsorption density of 24 ??mol m-2 total oxalate was observed at pH 3 and 8 mM total oxalate. Adsorption is dependent upon the activities of both oxalate (C2O42-) and bioxalate (HC2O4-) in solution and can be modeled with either a two-term Langmuir or a two-term Freundlich isotherm. A Freundlich adsorption model provided the best fit to rate data because it was not constrained to a finite number of adsorption sites, as was the Langmuir model. The two-term ligand adsorption model was incorporated into a rate model: Rtot = kH(+)[Hads+]L +kHOx(-) [HOxads-]+kOx(2-) [Oxads 2-] where Rtot is the net dissolution rate of the feldspar, [iads] is the concentration of species i adsorbed to the surface, and ki is the rate constant for release of the surface complex. The model was fit to data for oxalate-promoted dissolution of andesine, resulting in estimates for the rate constants of kHOx(-) = 1.16??10-12, kOx(2-) = 1.05??10-12, and kH(+) = 9.61??10-13 mol of feldspar (??mol of i)-1 s-1.

  10. Solubility of iron among plagioclase, clinopyroxene and silicate liquid in the system CaO-MgO-Al/sub 2/O/sub 3/ - SiO/sub 2/-Fe-O

    SciTech Connect

    Virgo, D.; Mysen, B.O.

    1985-01-01

    The 1-atm liquidus of the system diopside-anorthite + 10wt% ferric iron is lower (approx. 30/sup 0/C) for end-member compositions but the temperature and projected (Fe-free) composition of the minimum melt are unchanged. The effect of fO/sub 2/ on the liquidus curve is minimal. The major element compositions, the structural state of iron and the ferrous/ferric ratio of existing crystal-liquid pairs have been determined using electron microprobe analyses and /sup 57/Fe Mossbauer spectroscopy. For anorthite-liquid pairs, the partition coefficient, D(Fe) (equal to (Fe in xtal)/(Fe in liquid)) decreases with increasing temperature and with increasing anorthite/diopside ratio, at fixed fO/sub 2/. The effect of bulk composition on D(Fe) predominates over the relatively weak temperature dependence (Longhi et al. 1976). Significantly, at fixed temperature and anorthite/diopside ratio, D(Fe) decreases linearly with fO/sub 2/ from 0.25 (log fO/sub 2/=-0.68) to 0.05 (log fO/sub 2/=-6.0). The increase in D(Fe) with increasing fO/sub 2/ is accompanied by an increase in the ferric iron content of the feldspar lattice. The mechanisms for incorporation of iron in synthetic anorthite are Fe(3+)-Al under oxidizing conditions and Fe(2+), Si-2Al under reducing conditions. The use of D(Fe) as an oxygen barometer is considered. D(Fe) for the system diopside-Fe/sub 2/O/sub 3/ is not strongly fO/sub 2/-dependent at constant temperature. The incorporation of iron in liquidus pyroxene is via the Fe(2+)-Ca substitution. Ferric iron acts as an inert diluent in these melts. Tschermak's substitution is discussed for igneous, ferric-bearing clinopyroxenes.

  11. An experiment-based model for the petrogenesis of high-alumina basalts

    NASA Technical Reports Server (NTRS)

    Beard, J. S.; Lofgren, G. E.

    1992-01-01

    To understand magmatism at convergent margins, one must know the origin of their characteristic plagioclase-rich high-alumina basalts (HABs). Wet melting experiments on basalts at 3 kilobars yielded high-alumina liquids and a coexisting mineral assemblage with plagioclase crystallization in these melts, while mafic minerals underwent limited crystallization or even resorption. These results suggest that hydrous HAB liquids, presumably formed by fractionation of a hydrous basalt at depth, precipitate voluminous plagioclase as pressure drops during ascent and eruption. Plagioclase accumulation is not necessarily required to explain the petrogenesis of plagioclase-rich HAB.

  12. Genesis of compositional characteristics of Stillwater AN-I and AN-II thick anorthosite units

    NASA Technical Reports Server (NTRS)

    Haskin, Larry A.; Salpas, Peter A.

    1992-01-01

    Compositional variations among samples of the thick Stillwater anorthosite units are used here to model the mechanism by which the anorthosite formed. Suspended plagioclase, on reaching a volumetric crystal concentration of about 65 percent, formed a plagioclase framework here called cumulus plagioclase (CP). Within the interstices among the crystals, some plagioclase and pyroxene precipitated, producing what is here called adcumulus plagioclase and pyroxene (APP). The final materials consists mainly of plagioclase and pyroxene formed from complete crystallization of trapped supporting liquid and called intercumulus plagioclase and pyroxene (IPP). Where the CP framework was relatively open initially, bulk liquid communicated with the interior of the framework to produce APP. Where this communication with the bulk liquid stopped or was prevented initially, the intercumulus liquid froze. Where the CP framework was more tightly packed initially, a relatively higher proportion of IPP to APP was produced.

  13. Genesis of the cumulate eucrites Serra de Mage and Moore County - A geochemical study

    NASA Technical Reports Server (NTRS)

    Ma, M.-S.; Schmitt, R. A.

    1979-01-01

    An instrumental neutron activation analysis of element abundances in whole rock and plagioclase separates of the Serra de Mage chondrite has been conducted; plagioclase data and plagioclase mineral/liquid partition coefficients are employed to determine the REE abundances in the derivative equilibrium magmas from which Serra de Mage and Moore County chondrite plagioclases crystallized. No simple genetic relationship between the two chondrites appears likely through the route of fractional crystallization. A difference in degrees of partial melting of a similar source material (different from the source proposed for noncumulate eucrites) could be invoked to relate the two chondrites.

  14. Mineralogy of Apollo 15415 ?genesis rock' - Source of anorthosite on moon.

    NASA Technical Reports Server (NTRS)

    Steele, I. M.; Smith, J. V.

    1971-01-01

    Results of electron microprobe analyses of plagioclase points and pyroxene grains of Apollo 15415 ?genesis rock.' It is pointed out that no evidence of cumulate textures has yet appeared to support suggestions of extensive crystal-liquid differentiation producing an anorthositic crust or a lunar crust composed of a mixture of plagioclase-rich rock, basalts and minor ultramafic material, which require that plagioclase crystals float in a basaltic liquid. The plagioclase in 15415 does not show cumulate texture either. It is noted that it remains to be seen whether rock 15415 is correctly named the ?genesis rock.'

  15. Rare earth element contents and multiple mantle sources of the transform-related Mount Edgecumbe basalts, southeastern Alaska

    USGS Publications Warehouse

    Riehle, J.R.; Budahn, J.R.; Lanphere, M.A.; Brew, D.A.

    1994-01-01

    Pleistocene basalt of the Mount Edgecumbe volcanic field (MEF) is subdivided into a plagioclase type and an olivine type. Th/La ratios of plagioclase basalt are similar to those of mid-ocean-ridge basalt (MORB), whereas those of olivine basalt are of continental affinity. Rare earth element (REE) contents of the olivine basalt, which resemble those of transitional MORB, are modelled by 10-15% partial melting of fertile spinel-plagioclase lherzolite followed by removal of 8-13% olivine. It is concluded that olivine basalt originated in subcontinental spinel lherzolite and that plagioclase basalt may have originated in suboceanic lithosphere of the Pacific plate. -from Authors

  16. Shock and thermal metamorphism of basalt by nuclear explosion, nevada test site.

    PubMed

    James, O B

    1969-12-26

    Olivine trachybasalt metamorphosed by nuclear explosion is classified into categories of progressive metamorphism: (i) Weak. Plagioclase is microfracruree, and augite contains twin lamellae. (ii) Moderate. Plagioclase is converted to glass, amd mafic minerals show intragranular deformation (undulatory extinction, twin lamellae, and, possibly, defomation lamellae), but rock texture is preserved. (iii) Moderately strong. Plagioclase glass shows small-scale flow, mafic minerals are fractured and show intragranular deformation, and rocks contain tension fractures. (iv) Strong. Plagioclase glass is vesicular, augite is minutely fractured, and olivine is coarsely fragmented, shows moscaic extinction, distinctive lamellar structures, and is locally recrystallized. (v) Intense. Rocks are converted to inhomogeneous basaltic glass.

  17. Shock and thermal metamorphism of basalt by nuclear explosion, Nevada test site

    USGS Publications Warehouse

    James, O.B.

    1969-01-01

    Olivine trachybasalt metamorphosed by nuclear explosion is classified into categories of progressive metamorphism: (i) Weak. Plagioclase is microfractured, and augite cotainis fine twin lamellae. (ii) Moderate. Plagioclase is converted to glass, and mafic minerals show intragranular deformation (undulatory extinction, twin lamellae, and, possibly, deformation lamellae), but rock texture is preserved. (iii) Moderately strong. Plagioclase glass shows small-scale flow, mafic minerals are fractured and show intragranular deformation, and rocks contain tension fractures. (iv) Strong. Plagioclase glass is vesicular, augite is minutely fractured, and olivine is coarsely fragmented, shows mosaic extinction, distinctive lamellar structures, and is locally recrystallized. (v) Intense. Rocks are converted to inhomogeneous basaltic glass.

  18. The interplay and effects of deformation and crystallized melt on the rheology of the lower continental crust, Fiordland, New Zealand

    NASA Astrophysics Data System (ADS)

    Miranda, Elena A.; Klepeis, Keith A.

    2016-12-01

    Microstructural, electron backscatter diffraction (EBSD), and misorientation analyses of a migmatitic granulite-facies orthogneiss from the exhumed lower crust of a Cretaceous continental arc in Fiordland, New Zealand show how deformation was accommodated during and after episodes of melt infiltration and high-grade metamorphism. Microstructures in garnet, omphacite, plagioclase, and K-feldspar suggest that an early stage of deformation was achieved by dislocation creep of omphacite and plagioclase, with subsequent deformation becoming partitioned into plagioclase. Continued deformation after melt infiltration resulted in strain localization in the leucosome of the migmatite, where a change of plagioclase deformation mechanism promoted the onset of grain boundary sliding, most likely accommodated by diffusion creep, in fine recrystallized plagioclase grains. Our results suggest three distinctive transitions in the rheology of the lower crust of this continental arc, where initial weakening was primarily achieved by deformation of both omphacite and plagioclase. Subsequent strain localization in plagioclase of the leucosome indicates that the zones of former melt are weaker than the restite, and that changes in deformation mechanisms within plagioclase, and an evolution of its strength, primarily control the rheology of the lower crust during and after episodes of melting and magma addition.

  19. Installation Restoration Program. Preliminary Assessment: 162nd Combat Communications Group, Mt. Disappointment Air National Guard Station, California Air National Guard, Los Angeles, California

    DTIC Science & Technology

    1991-01-01

    intermediate in composition between syenite and I diorite, containing approximately equal amounts of alkali feldspar and plagioclase. 3 MOTTLED [soil] - a soil...including I streams, rivers, ponds, and lakes. SYENITE - Plutonic rock containing orthoclase and microcline with small I amounts of plagioclase

  20. Beneficiation of Stillwater Complex Rock for the Production of Lunar Simulants

    NASA Technical Reports Server (NTRS)

    Rickman, D. L.; Young, C.; Stoeser, D.; Edmunson, J.

    2014-01-01

    The availability of pure, high calcium plagioclase would be a significant asset in any attempt to manufacture high-quality lunar simulants. A suitable plagioclase product can be obtained from materials obtained from the Stillwater Complex of Montana. The access, geology, petrology, and mineralogy of the relevant rocks and the mill tailings are described here. This study demonstrates successful plagioclase recovery from mill tailings produced by the Stillwater Mine Company. Hydrogen peroxide was used to remove carboxymethyl cellulose from the tailing. The characteristics of the plagioclase products are shown and locked grains are identified as a limit to achievable purity. Based on the experimental results, flowsheets were developed showing how these resources could be processed and made into 'separates' of (1) high calcium plagioclase and (2) orthopyroxene/clinopyroxene with the thought that they would be combined later to make simulant.

  1. Using Apollo 17 high-Ti mare basalts as windows to the lunar mantle

    NASA Technical Reports Server (NTRS)

    Neal, Clive R.; Taylor, Lawrence A.

    1992-01-01

    The Apollo 17 high-Ti mare basalts are derived from source regions containing plagioclase that was not retained in the residue. Ilmenite appears to remain as a residual phase, but plagioclase is exhausted. The open-system behavior of the type B2 basalts results in slightly higher Yb/Hf and La/Sm ratios. The nature of the added component is not clear, but may be a KREEP derivative or residue. The recognition of plagioclase in the source(s) of these basalts suggests that the location of the source region(s) would be more likely to be less than 150 km (i.e., closer to the plagioclase-rich crust), which would allow incorporation of plagioclase into the source through incomplete separation of crustal feldspar.

  2. Petrology of crystalline matrix breccias from Apollo 17 rake samples

    NASA Technical Reports Server (NTRS)

    Warner, R. D.; Taylor, G. J.; Keil, K.

    1977-01-01

    The petrology, mineralogy, and composition of 13 crystalline matrix breccias from the Apollo 17 rake samples are described. The breccias have matrices of uniform modal mineralogy (plagioclase, 50-54%; olivine and pyroxene, 41-46%) but diverse textures. Clast characteristics, including the proportion of plagioclase, the composition in comparison with matrix, and the textures of different clast types, are reported. The majority of bulk matrix compositions plot on or very near the plagioclase-olivine cotectic in the system olivine-anorthite-silica. If the matrix compositions represent impact total melts, the inferred cotectic control requires that the source material was itself an igneous differentiate with compositions along the plagioclase-olivine cotectic. Alternatively, the proximity of the matrix compositions to the plagioclase-olivine cotectic could be accounted for if the breccia matrices represent a suite of impact-generated partial melts.

  3. Origin of coronas in metagabbros of the Adirondack mts., N. Y

    USGS Publications Warehouse

    Whitney, P.R.; McLelland, J.M.

    1973-01-01

    Metagabbros from two widely separated areas in the Adirondacks show development of coronas. In the Southern Adirondacks, these are cored by olivine which is enclosed in a shell of orthopyroxene that is partially, or completely, rimmed by symplectites consisting of clinopyroxene and spinel. Compositions of the corona phases have been determined by electron probe and are consistent with a mechanism involving three partial reactions, thus: (a) Olivine=Orthopyroxene+(Mg, Fe)++. (b) Plagioclase+(Mg, Fe)+++Ca++=Clinopyroxene+Spinel+Na+. (c) Plagioclase+(Mg, Fe)+++Na+=Spinel+more sodic plagioclase+Ca++. Reaction (a) occurs in the inner shell of the corona adjacent to olivine; reaction (b) in the outer shell; and (c) in the surrounding plagioclase, giving rise to the spinel clouding which is characteristic of the plagioclase in these rocks. Alumina and silica remain relatively immobile. These reactions, when balanced, can be generalized to account for the aluminous nature of the pyroxenes and for changing plagioclase composition. Summed together, the partial reactions are equivalent to: (d) Olivine + Anorthite = Aluminous orthopyroxene + Aluminous Clinopyroxene + Spinel (Kushiro and Yoder, 1966). In the Adirondack Highlands, coronas between olivine and plagioclase commonly have an outer shell of garnet replacing the clinopyroxene/spinel shell. The origin of the garnet can also be explained in terms of three partial reactions: (e) Orthopyroxene+Ca++=Clinopyroxene+(Mg, Fe)++. (f) Clinopyroxene+Spinel+Plagioclase+(Mg, Fe)++=Garnet+Ca+++Na+. (g) Plagioclase+(Mg, Fe)+++Na+=Spinel + more sodic plagioclase+Ca++. These occur in the inner and outer corona shell and the surrounding plagioclase, respectively, and involve the products of reactions (a)-(d). Alumina and silica are again relatively immobile. Balanced, and generalized to account for aluminous pyroxenes and variable An content of plagioclase, they are equivalent to: (h) Orthopyroxene+Anorthite+Spinel=Garnet (Green and

  4. Grain size reduction due to fracturing and subsequent grain-size-sensitive creep in a lower crustal shear zone in the presence of a CO2-bearing fluid

    NASA Astrophysics Data System (ADS)

    Okudaira, Takamoto; Shigematsu, Norio; Harigane, Yumiko; Yoshida, Kenta

    2017-02-01

    To understand rheological weakening in the lower continental crust, we studied mylonites in the Paleoproterozoic Eidsfjord anorthosite, northern Norway. The zones of anorthositic mylonites range from a few millimeters to several meters thick, and include ultramylonites and protomylonites. They contain syn-kinematic metamorphic minerals, including Cl-bearing amphibole and scapolite. Thermodynamic analysis reveals that syn-deformational hydration reactions occurred at ∼600 °C and ∼700 MPa under CO2-bearing conditions. The protomylonites contain many fragmented plagioclase porphyroclasts. The fractures in porphyroclasts are filled with fine-grained plagioclase, suggesting that fracturing is a common mechanism of grain size reduction. The anorthite contents of fine-grained polygonal matrix plagioclase are different from those of porphyroclastic plagioclase, suggesting that the matrix grains nucleated and grew during syn-kinematic metamorphism. Plagioclase aggregates in the matrices of mylonites do not exhibit a distinct crystallographic preferred orientation, which implies that the dominant deformation mechanism was grain-size-sensitive creep. Consequently, in the lower crustal anorthositic mylonites, grain size reduction occurred via fracturing, rather than through dynamic recrystallization, leading to grain-size-sensitive creep. The syn-kinematic recrystallization of minor phases at plagioclase grain boundaries may suppress the growth of plagioclase and contribute to the development of grain-size-sensitive creep.

  5. Lunar ferroan anorthosite petrogenesis: clues from trace element distributions in FAN subgroups

    USGS Publications Warehouse

    Floss, C.; James, O.B.; McGee, J.J.; Crozaz, G.

    1998-01-01

    The rare earth elements (REE) and selected other trace elements were measured in plagioclase and pyroxene from nine samples of the lunar ferroan anorthosite (FAN) suite of rocks. Samples were selected from each of four FAN subgroups previously defined by James et al. (1989). Plagioclase compositions are homogeneous within each sample, but high- and low-Ca pyroxenes from lithic clasts typically have different REE abundances from their counterparts in the surrounding granulated matrices. Measured plagioclase/low-Ca pyroxene concentration ratios for the REE have steeper patterns than experimentally determined plagioclase/low-Ca pyroxene partition coefficients in most samples. Textural and trace element evidence suggest that, although subsolidus equilibration may be responsible for some of the discrepancy, plagioclase compositions in most samples have been largely unaffected by intermineral redistribution of the REE. The REE systematics of plagioclase from the four subgroups are broadly consistent with their deviation through crystallization from a single evolving magma. However, samples from some of the subgroups exhibit a decoupling of plagioclase and pyroxene compositions that probably reflects the complexities inherent in crystallization from a large-scale magmatic system. For example, two anorthosites with very magnesian mafic minerals have highly evolved trace element compositions; major element compositions in plagioclase also do not reflect the evolutionary sequence recorded by their REE compositions. Finally, a noritic anorthosite breccia with relatively ferroan mafic minerals contains several clasts with high and variable REE and other trace element abundances. Although plagioclase REE compositions are consistent with their derivation from a magma with a KREEPy trace element signature, very shallow REE patterns in the pyroxenes suggest the addition of a component enriched in the light REE.

  6. Cryptic trace-element alteration of Anorthosite, Stillwater complex, Montana

    USGS Publications Warehouse

    Czamanske, G.K.; Loferski, P.J.

    1996-01-01

    Evidence of cryptic alteration and correlations among K, Ba, and LREE concentrations indicate that a post-cumulus, low-density aqueous fluid phase significantly modified the trace-element contents of samples from Anorthosite zones I and II of the Stillwater Complex, Montana. Concentrations of Ba, Ca, Co, Cr, Cu, Fe, Hf, K, Li, Mg, Mn, Na, Ni, Sc, Sr, Th, Zn, and the rare-earth elements (REE) were measured in whole rocks and plagioclase separates from five traverses across the two main plagioclase cumulate (anorthosite) zones and the contiguous cumulates of the Stillwater Complex in an attempt to better understand the origin and solidification of the anorthosites. However, nearly the entire observed compositional range for many trace elements can be duplicated at a single locality by discriminating between samples rich in oikocrystic pyroxene and those which are composed almost entirely of plagioclase and show anhedral-granular texture. Plagioclase separates with high trace-element contents were obtained from the pyroxene-poor samples, for which maps of K concentration show plagioclase grains to contain numerous fractures hosting a fine-grained, K-rich phase, presumed to be sericite. Secondary processes in layered intrusions have the potential to cause cryptic disturbance, and the utmost care must be taken to ensure that samples provide information about primary processes. Although plagioclase from Anorthosite zones I and II shows significant compositional variation, there are no systematic changes in the major- or trace-element compositions of plagioclase over as much as 630 m of anorthosite thickness or 18 km of strike length. Plagioclase in the two major anorthosite zones shows little distinction in trace-element concentrations from plagioclase in the cumulates immediately below, between, and above these zones.

  7. An integrated NIR and TIR approach to the plagiocase-rich regions on the Moon using M3 and Diviner data

    NASA Astrophysics Data System (ADS)

    Donaldson Hanna, K. L.; Pieters, C. M.; Mustard, J. F.; Cheek, L. C.; Wyatt, M. B.; Thomas, I. R.; Bowles, N. E.; Greenhagen, B. T.; Lucey, P. G.; Paige, D. A.

    2011-10-01

    Recent near infrared (NIR) observations from SELENE [1], [2] and M3 [3] have uniquely identified Fe-bearing crystalline plagioclase regions on the Moon. These results are significant because they validate earlier NIR observations [4], [5] as well as characterize the widespread distribution of crystalline plagioclase across the lunar surface. The identification of Fe-bearing crystalline plagioclase in the NIR comes from a broad absorption band at approximately 1.3 μm due to electronic transitions of Fe2+ in the crystal structure. While previous NIR lab studies of plagioclase have suggested that the band depth and center position of the 1.3 μm feature may vary with Fe and An content [6], [7], the relationship between NIR spectral properties of plagioclase and its composition (An#) has yet to be quantified. Regions of nearly pure crystalline plagioclase (<5% olivine and pyroxene) as identified in NIR spectra are ideal areas to investigate the utility of thermal infrared (TIR) Diviner data to constrain plagioclase compositions. Diviner has three spectral bands near the 8 μm region chosen specifically to measure the peak of the Christiansen Feature (CF), an emissivity maximum indicative of composition [8]. New lab measurements of varying compositions of the plagioclase solid solution series demonstrate that the CF position is diagnostic of composition and linearly related to An# even when measured under a simulated lunar environment [9]. Thus an integrated NIR and TIR approach will enable plagioclase compositions to be mapped across the lunar surface and is significant for identifying rock types (e.g. ferroan anorthosites versus Alkali-suite rocks) and may ultimately constrain their method of formation (e. g. magma ocean crystallization or plutons).

  8. Understanding Magmatic Plumbing System Dynamics at Fernandina Island, Galapagos

    NASA Astrophysics Data System (ADS)

    Varga, K. C.; McGuire, M.; Geist, D.; Harpp, K. S.

    2015-12-01

    Fernandina is the most active Galápagos volcano, and is located closest to the seismically defined hotspot. Allan and Simkin (2000) observed that the subaerial edifice is constructed of homogeneous basalts (Mg# = 49 ± 2) with highly variable plagioclase phenocryst contents and sparse olivine. Geist et al. (2006) proposed a magmatic plumbing system in which the volcano is supplied by interconnected sills, the shallowest of which is density-stratified: olivine and pyroxene are concentrated at greater depths, whereas less dense plagioclase mush is higher in the sill. Consequently, olivine-rich lava erupts laterally during submarine events, but plagioclase-rich lava supplies subaerial vents. To test this hypothesis, we examine lavas erupted in 1995, 2005, and 2009. These SW flank eruptions emerged alternatively from en echelon radial fissures on the lower flanks and circumferential fissures near the caldera rim. The 1995 radial fissure unzipped downslope and then formed a cone 4 km from the coast, sending flows to the ocean. In 2005, circumferential fissures erupted five flows south of the 1995 fissure. As in 1995, the 2009 fissures opened down the SW flank before focusing to a cone near the 1995 vents, producing 6 km-long flows that also reached the ocean. By correlating plagioclase crystal size distribution and morphologies with single event chronological sequences, we examine Fernandina's magmatic plumbing system. Modal plagioclase in 1995 lava decreases (20% to <5%) throughout the middle eruptive phase. Early 2005 samples are nearly aphyric (Chadwick et al., 2010), with 1-2% plagioclase. The 2009 eruption has reduced plagioclase, similar to mid-1995 samples. Preliminary observations suggest that less plagioclase-rich mush is being flushed out during early-to-medial event sequences, whereas plag phenocrysts are transported more during later phases. Plausible plumbing dynamics suggest a zone of plagioclase-rich mush that is eroded and incorporated into radial

  9. Mineralogic and petrologic study of lunar anorthosite slide 15415,18.

    PubMed

    Hargraves, R B; Hollister, L S

    1972-01-28

    The anorthosite slide 15415,18 contains 98 percent subhedral plagioclase (97 mole percent anorthite), two pyroxenes: diopsidic augite (46 percent wollastonite, 39 percent enstatite, 16 percent ferrosilite) with subsidiary (100) lamellae and grains of hypersthene (2.5 percent wollastonite, 58 percent enstatite, 39.5 percent ferrosilite), and traces of ilmenite. The pyroxene occurs interstitial to, and as small grains enclosed within, plagioclase. The textures and compositions of the phases appear compatible with an origin by concentration and adcumulus growth of plagioclase from a gabbroic anorthosite (or hyperaluminous) magma in a "plutonic" environment.

  10. Rb-Sr age of the Civet Cat clast, 72255, 41. [radioactive age determination for lunar rocks

    NASA Technical Reports Server (NTRS)

    Compston, W.; Gray, C. M.

    1974-01-01

    Plagioclase rich clasts, orthopyroxene rich matrix, purified pyroxene, and plagioclase from the Civet Cat clast define a Rb-Sr isochron age of 4.18 + or - 0.04 x 10 to the 9th power yr and an initial Sr-87/Sr-86 ratio of 0.69922 + or - 0.00005. The fit of all data to the line is within error except for plagioclase 3, and blank corrections are essentially negligible. The decay constant used is 1.39 x 10 to the minus 11th power yr.

  11. Shock Metamorphism of the Dhofar 378 Basaltic Shergottite

    NASA Technical Reports Server (NTRS)

    Mikouchi, T.; McKay, G.

    2006-01-01

    Shock metamorphism is one of the most fundamental processes in the history of Martian meteorites, especially shergottites, which affect their mineralogy and chronology. The formation of "maskelynite" from plagioclase and shock melts is such major mineralogical effects. Dhofar 378 is one of the recently found desert shergottites that is mainly composed of plagioclase and pyroxene. This shergottite is important because of its highly shocked nature and unique plagioclase texture, and thus has a great potential for assessing a "shock" age of shergottites. We have been working on a combined study of mineralogy and chronology of the same rock chip of Dhofar 378. This abstract reports its mineralogical part.

  12. Composition of the mount st. Helens ashfall in the moscow-pullman area on 18 may 1980.

    PubMed

    Hooper, P R; Herrick, I W; Laskowski, E R; Knowles, C R

    1980-09-05

    Mineralogical and chemical analyses of the ashfall from Mount St. Helens on 18 May 1980 indicate that there were two distinct ashes. The early dark ash is composed principally of plagioclase and lithic fragments of plagioclase and glass with titanium-rich magnetite and some basaltic hornblende and orthopyroxene. The later pale ash, four-fifths by weight of the whole fallout, is 80 percent glass with plagioclase as the principal crystalline phase. Quartz and potassium feldspar are rare to absent in both ashes. Chemical analyses of nine ash fractions and of the glass in each type emphasize the differences between the two ash types and their chemical homogeneity.

  13. A Little Island With A Big Secret: Isla Rábida, Galápagos

    NASA Astrophysics Data System (ADS)

    Bercovici, H.; Geist, D.; Harpp, K. S.; Almeida, M.; Mahr, J.; Pimentel, R.; Cleary, Z.

    2016-12-01

    The Galápagos Archipelago is a hotspot island chain 1000 km west of Ecuador, where the vast majority of the lavas are basaltic. Four volcanoes in the archipelago, Rábida, Santiago, Pinzón, and Alcedo, erupt rhyolites and trachytes. Isla Rábida, a small island 50 km east of the mantle plume center, is the focus of this project. It is 5 km2 in area, and lavas range from 0.9 to 1.1 Ma. About 25% of the rocks in our suite are intermediate to felsic, extending from Mg#=2 to 57. Major and trace element data indicate the evolved rocks formed by advanced crystallization of basaltic magma. One of the unique aspects of Rábida is the cumulate xenolith suite ranging from olivine gabbro to ferroan granite. The basalts have 6 to 58 modal% plagioclase phenocrysts, which we interpret as mixtures of melt and accumulated plagioclase mush at the margins of the shallow reservoir. Thus, Rábida erupts material that has undergone different extents of crystallization and crystal sorting from pure melts, to melt-mush hybrids, to solidified cumulates. This hypothesis is evaluated by comparing plagioclase compositions from the xenoliths and the lavas. Plagioclases in two of the lavas, one with Mg#=57 and the other with Mg#=36, have similar compositions and zonation patterns to each other. There is on average less than 4% change in anorthite content from the core of the plagioclases in the basalts to the rim, with the compositions overall varying between An22 and An37. Both melts likely picked up the crystals from the same plagioclase mush before eruption. In comparison to plagioclases in an olivine-gabbro xenolith from Rábida, those in the lavas are less zoned, suggesting that the lavas' plagioclases experienced a different growth environment. Plagioclases in the xenolith are normally zoned, with cores averaging An37 and rims averaging An32. The xenolith's plagioclases also have more diverse compositions than those in the lavas. The normal zoning in the xenolith's plagioclase is

  14. The global distribution of pure anorthosite on the Moon.

    PubMed

    Ohtake, Makiko; Matsunaga, Tsuneo; Haruyama, Junichi; Yokota, Yasuhiro; Morota, Tomokatsu; Honda, Chikatoshi; Ogawa, Yoshiko; Torii, Masaya; Miyamoto, Hideaki; Arai, Tomoko; Hirata, Naru; Iwasaki, Akira; Nakamura, Ryosuke; Hiroi, Takahiro; Sugihara, Takamitsu; Takeda, Hiroshi; Otake, Hisashi; Pieters, Carle M; Saiki, Kazuto; Kitazato, Kohei; Abe, Masanao; Asada, Noriaki; Demura, Hirohide; Yamaguchi, Yasushi; Sasaki, Sho; Kodama, Shinsuke; Terazono, Junya; Shirao, Motomaro; Yamaji, Atsushi; Minami, Shigeyuki; Akiyama, Hiroaki; Josset, Jean-Luc

    2009-09-10

    It has been thought that the lunar highland crust was formed by the crystallization and floatation of plagioclase from a global magma ocean, although the actual generation mechanisms are still debated. The composition of the lunar highland crust is therefore important for understanding the formation of such a magma ocean and the subsequent evolution of the Moon. The Multiband Imager on the Selenological and Engineering Explorer (SELENE) has a high spatial resolution of optimized spectral coverage, which should allow a clear view of the composition of the lunar crust. Here we report the global distribution of rocks of high plagioclase abundance (approaching 100 vol.%), using an unambiguous plagioclase absorption band recorded by the SELENE Multiband Imager. If the upper crust indeed consists of nearly 100 vol.% plagioclase, this is significantly higher than previous estimates of 82-92 vol.% (refs 2, 6, 7), providing a valuable constraint on models of lunar magma ocean evolution.

  15. Lunar anorthosite paradox - An alternative explanation

    NASA Technical Reports Server (NTRS)

    Drake, M. J.

    1975-01-01

    In most lunar terrae samples a trend of decreasing Ab-content of plagioclase with increasing Fa-content of olivine is observed. This covariance of composition is the opposite of the trend observed in terrestrial layered intrusions, and contradicts Bowen's reaction series. The 'anomalous' trend is considered in terms of olivine-melt and plagioclase-melt equilibria. The composition of plagioclase crystallizing from a melt depends directly upon the activity of silica in the melt, while the composition of olivine does not. It is proposed that the inverse correlation of plagioclase and olivine compositions in most lunar terrae rocks is a predictable consequence of crystallization from a lunar bulk composition which is poorer in silica than the bulk compositions of terrestrial layered intrusions. The data of Roedder and Weiblen (1974) lend support to this hypothesis.

  16. Implications for the origins of pure anorthosites found in the feldspathic lunar meteorites, Dhofar 489 group

    NASA Astrophysics Data System (ADS)

    Nagaoka, Hiroshi; Takeda, Hiroshi; Karouji, Yuzuru; Ohtake, Makiko; Yamaguchi, Akira; Yoneda, Shigekazu; Hasebe, Nobuyuki

    2014-12-01

    Remote observation by the reflectance spectrometers onboard the Japanese lunar explorer Kaguya (SELENE) showed the purest anorthosite (PAN) spots (>98% plagioclase) at some large craters. Mineralogical and petrologic investigations on the feldspathic lunar meteorites, Dhofar 489 and Dhofar 911, revealed the presence of several pure anorthosite clasts. A comparison with Apollo nearside samples of ferroan anorthosite (FAN) indicated that of the FAN samples returned by the Apollo missions, sample 60015 is the largest anorthosite with the highest plagioclase abundance and homogeneous mafic mineral compositions. These pure anorthosites (>98% plagioclase) have large chemical variations in Mg number (Mg# = molar 100 × Mg/(Mg + Fe)) of each coexisting mafic mineral. The variations imply that these pure anorthosites underwent complex formation processes and were not formed by simple flotation of plagioclase. The lunar highland samples with pure anorthosite and the PAN observed by Kaguya suggest that pure anorthosite is widely distributed as lunar crust lithology over the entire Moon.

  17. Effects of kinetics on the crystallization of quartz normative basalt 15597 - An experimental study

    NASA Technical Reports Server (NTRS)

    Grove, T. L.; Raudsepp, M.

    1978-01-01

    Equilibrium experiments on a residual liquid composition from the dynamic crystallization study on quartz normative basalt (QNB) 15597 are used to evaluate the effects of kinetics on the crystallization of pyroxene-porphyritic basalts. The results indicate that the liquids coexisting with pyroxene phenocryst rims before the precipitation of groundmass plagioclase + pyroxene have left the equilibrium liquid line of descent and metastably penetrated the plagioclase primary phase volume. The precipitation of groundmass plagioclase + pyroxene brings the basalt system closer to equilibrium with respect to the residual liquid and crystals. At rapid cooling rates pyroxene phenocryst growth produces substantial compositional gradients in the residual silicate liquid at the pyroxene/liquid interface. The compositional gradients enhance the nucleation and growth of groundmass phases at the phenocryst interface, because effects related to constitutional supercooling for these phases (e.g., plagioclase, pyroxene, etc.) favor nucleation at the interface.

  18. Installation Restoration Program. Preliminary Assessment: 261st Combat Communications Squadron, Sepulveda Air National Guard Station, California Air National Guard, Sepulveda, California.

    DTIC Science & Technology

    1991-01-01

    of the smectite group comprising expanding-lattice clay minerals when wetted. MONZONITE - Plutonic rock intermediate in composition between syenite ... SYENITE - Plutonic rock containing orthoclase and microcline with smallm amounts of plagioclase feldspar. SYNCLINORIUM - A composite synclinal structure of

  19. Installation Restoration Program. Preliminary Assessment: 216th Engineering Installation Squadron and 234th Combat Communications Squadron, Hayward Air National Guard Station, California Air National Guard, Hayward, California

    DTIC Science & Technology

    1991-01-01

    Plutonic rock intermediate in composition between syenite and diorite, containing approximately equal amounts of alkali feldspar and plagioclase...exposed at the ground surface, including streams, rivers, ponds, and lakes. SYENITE - Plutonic rock containing orthoclase and microcline with small

  20. Recycling, Remobilization, and Eruption of Crystals from the Lassen Volcanic Center

    NASA Astrophysics Data System (ADS)

    Schrecengost, K.; Cooper, K. M.; Kent, A. J.; Huber, C.; Clynne, M. A.

    2016-12-01

    The Lassen Volcanic Center recently produced two relatively small dacitic eruptions (0.03 km3 -1.4 km3) with a complex mixing history. Preliminary data for the 1915 Lassen Peak (LP) and the 1103±13 ybp Chaos Crags (CC) eruptions indicate complex mixing between a remobilized crystal mush (hornblende, biotite, sodic plagioclase, quartz) and basalt or basaltic andesite. U-series bulk ages represent crystallization of plagioclase at an average age of either a single event or a mixture of different plagioclase populations that crystallized during distinct crystallization events separated in time. We present 238U-230Th disequilibria for the LP light dacite and black dacite along with three stages (upper pyroclastic flow deposit, Dome B, and Dome F) of the CC eruption. Initial 230Th/232Th activity ratios for the LP plagioclase are higher than the LP host liquid and modeled equilibrium zero-age plagioclase towards the CC host liquid composition. The LP plagioclase data are inconsistent with crystallization from the LP host liquid. Therefore, at least a portion of the plagioclase carried by the LP eruptive products are antecrystic originating from an older and/or isotopically distinct host liquid composition. Moreover, LP bulk plagioclase is consistent with crystallization from the CC host liquid, suggesting that both eruptions are sourced from a similar host reservoir (i.e., crystal mush). Hornblende and biotite from the LP eruption have isotopic ratios that are consistent with zero age crystallization from the LP liquid composition, suggesting that they are younger and originate from a different magma than the plagioclase, with mixing between the magmas prior to eruption. However, it is more likely that hornblende, biotite, and plagioclase with varying average crystal ages were remobilized and erupted from a common crystal mush reservoir during the LP and CC eruptions. These data are consistent with zircon 238U-230Th model ages [1] that emphasize the importance of local

  1. Expanding the REE Partitioning Database for Lunar Materials

    NASA Technical Reports Server (NTRS)

    Rapp, Jennifer F.; Draper, David S.

    2014-01-01

    Positive europium anomalies are ubiquitous in the plagioclase-rich rocks of the lunar highlands, and complementary negative Eu anomalies are found in most lunar basalts. This is taken as evidence of a large-scale differentation event, with crystallization of a global-scale lunar magma ocean (LMO) resulting in a plagioclase flotation crust and a mafic lunar interior from which mare basalts were later derived. However, the extent of the Eu anomaly in lunar rocks is variable. Some plagioclase grains in a lunar impact rock (60635) have been reported to display a negative Eu anomaly, or in some cases single grains display both positive and neagtive anomalies. Cathodoluminescence images reveal that some crystals have a negative anomaly in the core and positive at the rim, or vice versa, and the negative anomalies are not associated with crystal overgrowths. Oxygen fugacity is known to affect Eu partitioning into plagioclase, as under low fO2 conditions Eu can be divalent, and has an ionic radius similar to Ca2+ - significant in lunar samples where plagioclase compositions are predominantly anorthitic. However, there are very few experimental studies of rare earth element (REE) partitioning in plagioclase relevant to lunar magmatism, with only two plagioclase DEu measurements from experiments using lunar materials, and little data in low fO2 conditions relevant to the Moon. We report on REE partitioning experiments on lunar compositions. We investigate two lunar basaltic compositions, high-alumina basalt 14072 and impact melt breccia 60635. These samples span a large range of lunar surface bulk compositions. The experiments are carried out at variable fO2 in 1 bar gas mixing furnaces, and REE are analysed by and LA-ICP-MS. Our results not only greatly expand the existing plagioclase DREE database for lunar compositions, but also investigate the significance of fO2 in Eu partitioning, and in the interpretation of Eu anomalies in lunar materials.

  2. Petrology of igneous lithic clasts from polymict eucrites ALHA76005 and ALHA77302

    NASA Technical Reports Server (NTRS)

    Simon, S. B.; Papike, J. J.

    1983-01-01

    A total of seven lithic clasts from the polymict eucrites ALHA76005 and ALHA77302 have been studied petrographically and analyzed with the electron microprobe. All clasts are composed predominantly of pyroxene and plagioclase, + or - ilmenite, troilite, Fe-Ni metal, mesostasis, and silica. Pyroxene compositions in unequilibrated clasts and clast bulk compositions, calculated by modal recombination, indicate that the clasts originally crystallized under similar conditions and that they may be genetically related to each other by fractionation of pigeonite and plagioclase.

  3. Petrologic evidence for a plutonic igneous origin of anorthositic norite clasts in 67955 and 77017

    NASA Technical Reports Server (NTRS)

    Ashwal, L. D.

    1975-01-01

    Clasts of anorthositic norite in 67955 and 77017, and several other samples, are suggested to represent material which crystallized from a silicate magma in a plutonic environment. Although no statement can be made regarding the ultimate origin of the parent melt, its composition was relatively felsic, starting within the primary field of crystallization of either plagioclase or spinel. The sequence of crystallization was: plagioclase (and/or spinel) olivine - low-Ca pyroxene - high-Ca pyroxene.

  4. Maskelynite: Formation by explosive shock

    USGS Publications Warehouse

    Milton, D.J.; De Carli, P. S.

    1963-01-01

    When high pressure (250 to 300 kilobars) was applied suddenly (shock-loading) to gabbro, the plagioclase was transformed to a noncrystalline phase (maskelynite) by a solid-state reaction at a low temperature, while the proxene remained crystalline. The shock-loaded gabbro resembles meteorites of the shergottite class; this suggests that the latter formed as a result of shock. The shock-loading of gabbro at 600 to 800 kilobars raised the temperature above the melting range of the plagioclase.

  5. Phase equilibria along a basalt-rhyolite mixing line: implications for the origin of calc-alkaline intermediate magmas

    NASA Astrophysics Data System (ADS)

    Ussler, William; Glazner, Allen F.

    1989-02-01

    One-atmosphere, anhydrous phase equilibria determined for alkali basalt/high-silica rhyolite mixtures provide a model for crystallization of natural calc-alkaline mixed magmas. The compositional trend defined by these mixtures mimics the trends of many continental calc-alkaline volcanic suites. As with many naturally occurring suites, the mixtures studied straddle the low-pressure olivine-plagioclase-augite thermal divide. Magma mixing provides a convenient method for magmas to cross this thermal divide in the absence of magnetite crystallization. For the mixtures, Mg-rich olivine (Fo82 87) coexists alone with liquid over an exceptionally large range of temperature and silica content (up to 63 wt% SiO2). This indicates that the Mg-rich olivines found in many andesites and dacites are not necessarily out of equilibrium with the host magma, as is commonly assumed. Such crystals may be either primary phenocrysts, or inherited phenocrysts derived from a mafic magma that mixed with a silicic magma. For the bulk compositions studied, the distribution of Fe and Mg between olivine and liquid ( K D ) is equal to 0.3 and is independent of temperature and composition. This result extends to silicic andesites the applicability of K D arguments for tests of equilibrium between olivine and groundmass and for modeling of fractional crystallization. In contrast, the distribution of calcium and sodium between plagioclase and liquid varies significantly with temperature and composition. Therefore, plagioclase-liquid K D s cannot be used for fractional crystallization modeling or as a test of equilibrium. Calcic plagioclase from a basalt will be close to equilibrium with andesitic mixtures, but sodic plagioclase from a rhyolite will be greatly out of equilibrium. This explains the common observation that calcic plagioclase crystals in hybrid andesites are generally close to textural equilibrium with the surrounding groundmass, but sodic plagioclase crystals generally show remelting

  6. Curved branching crystals and differentiation in comb-layered rocks

    NASA Technical Reports Server (NTRS)

    Lofgren, G. E.; Donaldson, C. H.

    1975-01-01

    An investigation is conducted concerning two common features of comb layered rocks. Attention is given to the curvature of oriented, elongate, branching crystals and the tendency to form highly differentiated layers. Crystallization studies of plagioclase show that some degree of supercooling is necessary to produce the skeletal, curved, and branching plagioclase crystal morphologies found in comb-layered rocks and that curved crystals can be grown without the presence of a directed stress.

  7. Serpentinization and alteration in an olivine cumulate from the Stillwater Complex, Southwestern Montana

    USGS Publications Warehouse

    Page, N.J.

    1976-01-01

    Some of the olivine cumulates of the Ultramafic zone of the Stillwater Complex, Montana, are progressively altered to serpentine minerals and thompsonite. Lizardite and chrysotile developed in the cumulus olivine and postcumulus pyroxenes; thompsonite developed in postcumulus plagioclase. The detailed mineralogy, petrology, and chemistry indicate that olivine and plagioclase react to form the alteration products, except for H2O, without changes in the bulk composition of the rocks. ?? 1976 Springer-Verlag.

  8. Magnetic and microscopic features of silicate-hosted Fe-oxide inclusions in an oceanic gabbro section

    NASA Astrophysics Data System (ADS)

    Till, Jessica

    2015-04-01

    The magnetic mineralogy of oceanic gabbros is typically dominated by magnetite, which occurs in several forms: as a cumulus or intercumulus phase, as a secondary phase formed through alteration, or as exsolved inclusions in plagioclase and pyroxene. This study characterizes the contribution of magnetic inclusions in plagioclase and pyroxene to the bulk rock remanence and examines changes in the distribution of remanence carriers with crustal depth. Selected samples were taken throughout a 1500-m-long section of drilled oceanic gabbro cores collected from the Oceanic Drilling Program Site 735B at Atlantis Bank on the Southwest Indian Ridge. Hysteresis parameters and curves of isothermal remanence acquisition were measured for plagioclase and clinopyroxene mineral separates and compared with whole rock measurements for samples from various depths to determine the relative contributions of each to the bulk sample remanence properties. In whole-rock samples, bulk saturation magnetization decreases and coercivity distributions become dominated by harder magnetic components with increasing depth. The changes in rock magnetic properties with depth are interpreted to result from variations in composition as well as cooling rates. Coercivity distributions in both plagioclase and pyroxene systematically shift to higher coercivities with increasing depth in the section, although the change is more pronounced in plagioclase, indicating that the size distributions of magnetic inclusions in plagioclase become progressively finer. First-order reversal curves for plagioclase separates provide a striking example of non-interacting single-domain particles. Variations in exsolution textures and compositions of the inclusions were also investigated by microanalysis and electron microscopy. Microscopic examination revealed unexpected complexity in the structure of exsolution features, with several oxide phases commonly present as inclusions in plagioclase and multiple generations of

  9. Maskelynite: Formation by Explosive Shock.

    PubMed

    Milton, D J; de Carli, P S

    1963-05-10

    When high pressure (250 to 300 kilobars) was applied suddenly (shock-loading) to gabbro, the plagioclase was transformed to a noncrystalline phase (maskelynite) by a solid-state reaction at a low temperature, while the proxene remained crystalline. The shock-loaded gabbro resembles meteorites of the shergottite class; this suggests that the latter formed as a result of shock. The shock-loading of gabbro at 600 to 800 kilobars raised the temperature above the melting range of the plagioclase.

  10. Calcic myrmekite in anorthositic and gabbroic rocks

    SciTech Connect

    Schiffries, C.M.; Dymek, R.F.

    1985-01-01

    Myrmekite is a common feature of granitic plutonic rocks and quartzo-feldspathic gneisses, but it is rarely reported in anorthositic and gabbroic rocks. The authors have identified myrmekitic intergrowths of quartz and calcic plagioclase in a variety of plagioclase-rich cumulate rocks, including samples from a number of massif anorthosites and layered igneous intrusions. It appears that calcic myrmekite has been frequently overlooked, and is a common accessory feature in these rock types. Chemical and textural characteristics of myrmekite in the St-Urbain massif anorthosite (Quebec) and the Bushveld Igneous Complex (South Africa) have several features in common, but this myrmekite appears to be fundamentally different from that described by most previous investigators. Whereas myrmekite typically consists of a vermicular intergrowth of sodic plagioclase and quartz that occurs adjacent to alkali feldspar, the intergrowths in these rocks contain highly calcic plagioclase and lack the intervening alkali feldspar. In addition, the plagioclase in the myrmekite is more calcic than that in the surrounding rock. The boundary between the myrmekite and the host material is generally extremely sharp, although reverse zoning of host plagioclase may obscure the contact in some cases. The textural and chemical evidence is consistent with a replacement origin for these intergrowths; the proportion of quartz in the myrmekite is in close agreement with the predicted amount of silica that is generated by the theoretical replacement reaction. It appears that water played a key role in the replacement process.

  11. The Case for a Heat-Pipe Phase of Planet Evolution on the Moon

    NASA Technical Reports Server (NTRS)

    Simon, J. I.; Moore, W. B.; Webb, A. A. G.

    2015-01-01

    The prevalence of anorthosite in the lunar highlands is generally attributed to the flotation of less dense plagioclase in the late stages of the solidification of the lunar magma ocean. It is not clear, however, that these models are capable of producing the extremely high plagioclase contents (near 100%) observed in both Apollo samples and remote sensing data, since a mostly solid lithosphere forms (at 60-70% solidification) before plagioclase feldspar reaches saturation (at approximately 80% solidification). Formation as a floating cumulate is made even more problematic by the near uniformity of the alkali composition of the plagioclase, even as the mafic phases record significant variations in Mg/(Mg+Fe) ratios. These problems can be resolved for the Moon if the plagioclase-rich crust is produced and refined through a widespread episode of heat-pipe magmatism rather than a process dominated by density-driven plagioclase flotation. Heat-pipes are an important feature of terrestrial planets at high heat flow, as illustrated by Io's present activity. Evidence for their operation early in Earth's history suggests that all terrestrial bodies should experience an early episode of heat-pipe cooling. As the Moon likely represents the most wellpreserved example of early planetary thermal evolution in our solar system, studies of the lunar surface and of lunar materials provide useful data to test the idea of a universal model of the way terrestrial bodies transition from a magma ocean state into subsequent single-plate, rigid-lid convection or plate tectonic phases.

  12. Metamorphism of San Antonio Terrane metapelites, San Gabriel Mountains, California

    SciTech Connect

    Archuleta, L.; Ishimatsu, J.; Schneiderman, J.S. . Geology Dept.)

    1993-04-01

    Pelitic schists and gneisses from the San Antonio terrane in the eastern San Gabriel Mountains consist of garnet, biotite, plagioclase, quartz, sillimanite, cordierite, hercynite [+-] alkali feldspar. Large garnet porphyroblasts contain quartz, plagioclase and sillimanite inclusions. Cordierite occurs as haloes around garnet porphyroblasts and as small subgrains always associated with hercynite and together replacing sillimanite blades. Hercynite additionally appears to have nucleated on the edges of sillimanite blades. Contrary to previous investigations, hercynite appears to be a late mineral phase. Reaction textures described above have been used to calculate a set of net-transfer reactions that can be used (1) to characterize all possible exchanges of matter between minerals in the system and (2) to construct a reaction space for the system. Fourteen thin sections with large garnet porphyroblasts and abundant biotite were used for microprobe analysis. Detailed probe analyses show well-developed zoning in the plagioclase and alkali feldspar whose character varies depending on location in the thin section relative to neighboring minerals. Generally, large plagioclase porphyroblasts display normal zoning and are not as calcium-rich as plagioclase inclusions in the garnet. Garnet porphyroblasts have flat zoning profiles due to high temperatures of metamorphism. Pressures and temperatures of metamorphism have been calculated from these assemblages using garnet-biotite geothermometry and quartz-garnet-aluminosilicate-plagioclase geobarometry.

  13. Shape of pinch and swell structures as a viscosity indicator: Application to lower crustal polyphase rocks

    NASA Astrophysics Data System (ADS)

    Gardner, Robyn L.; Piazolo, Sandra; Daczko, Nathan R.

    2016-07-01

    Pinch and swell structures occur where a more competent layer in a weaker matrix is subjected to layer-parallel extension. In this contribution, we use numerical models to explore the use of pinch and swell structure shape symmetry and asymmetry as a determinant of relative viscosity between layers. Maximum asymmetry is attained when the matrix viscosity on one side is subtly weaker than the competent layer, while the other side is significantly weaker. Our numerical results are directly applied to asymmetrically developed pinch and swell structures in exposed lower continental crust. Here, shape geometries observed in a shear zone comprised of plagioclase-dominated, garnet-dominated and mixed amphibole-plagioclase-dominated bands, reveals that the plagioclase-dominated band is the most competent band and is marginally stronger (2×) and significantly stronger (10-40×) than the fine grained garnet-dominated and mixed amphibole-plagioclase-dominated band, respectively. Based on the experimentally determined viscosity of a plagioclase-dominated material and quantitative microstructural analysis, the viscosity range of the natural rock bands is 2.8 × 1015 to 1.1 × 1017 Pa s. Consequently, the assumption that the experimentally-derived plagioclase flow law is an appropriate proxy for the middle to lower continental crust may lead to a viscosity over-estimation by up to forty times.

  14. Two stage melt-rock interaction in the lower oceanic crust of the Parece Vela basin (Philippine sea), evidence from the primitive troctolites from the Godzilla Megamullion

    NASA Astrophysics Data System (ADS)

    Sanfilippo, A.; Dick, H. J.; Ohara, Y.

    2011-12-01

    Godzilla Megamullion is a giant oceanic core complex exposed in an extinct slow- to intermediate-spreading segment of the Parece Vela Basin (Philippine sea) [1; 2]. It exposes lower crust and mantle rocks on the sea-floor, offering a unique opportunity to unravel the architecture and the composition of the lower oceanic lithosphere of an extinct back arc basin. Here we present data on primitive troctolites and associated olivine-gabbros from the breakaway area of the Godzilla Megamullion. On the basis of the olivine/plagioclase volume ratio, the troctolites are subdivided into Ol-troctolites (Ol/Pl >1) and Pl-troctolites (Ol/Pl<1), which show evident textural differences. Ol-troctolites have rounded to polygonal olivine, subhedral plagioclase, and poikilitic clinopyroxene. This texture suggests chemical disequilibrium between the olivine and a melt crystallizing plagioclase and clinopyroxene. We interpret these rocks as reaction products of a dunite matrix with transient basaltic melts [e.g. 3; 4]. Pl-troctolites have euhedral plagioclase and poikilitic olivine and clinopyroxene. Irregular shapes and inverse zoning of the plagioclase chadacrysts within the olivine indicate disequilibrium between existing plagioclase and an olivine-clinopyroxene saturated melt. The occurrence of plagioclase chadacrysts within clinopyroxene ranging from irregular to euhedral in shape suggests crystallization of new lower-Na plagioclase with the clinopyroxene. Olivine oikocrysts in the Pl-troctolites have low-NiO olivine in equilibrium with a high-MgO melt. The Pl-troctolites, then, may be the product of reaction between a plagioclase cumulate and a basaltic melt produced by mixing the high-MgO melt residual to the formation of the Ol-troctolites with new magma. The effect of melt-rock reaction in the Pl- and Ol- troctolites explains the sharp decrease in plagioclase An with respect to Mg# in clinopyroxene and olivine. Furthermore, the melt is shifted towards lower Na, which is

  15. Evidence for Degassing-Induced Crystallization in Peripheral Andesite and Dacite Flows along the Flanks of Volcan Sanganguey, Mexico

    NASA Astrophysics Data System (ADS)

    Crabtree, S.; Lange, R. A.

    2006-12-01

    Crystal-poor andesite and dacite flows have erupted peripherally, along the flanks of the andesitic stratovolcano of Volcan Sanganguey in western Mexico, which is part of the TransMexican Volcanic Belt. This arc volcanism is a result of subduction of the Rivera plate beneath North America along the Middle America Trench. These crystal-poor, glassy flows are in marked contrast to the highly crystalline andesite flows erupted from the central vent of V. Sanganguey. This study focuses on electron microprobe data obtained from twenty-nine individual plagioclase crystals and ten crystal clusters within three crystal-poor (1-9 %) lava flows ranging in composition from andesite to dacite (61-68 wt % SiO2). The compositions of individual plagioclase crystals were found to be relatively homogeneous and unzoned, although the population of plagioclase crystals within a single sample (e.g., XAL-134) varied by up to 35 mol % in anorthite (e.g., An70- 45). The broad range of compositions observed, coupled with the unzoned nature of the grains studied suggests that these plagioclase crystals may have formed in response to degassing during rapid ascent to the surface. The plagioclase hygrometer of Lange and Frey (2006) was applied to these crystal-poor andesites and dacites to calculate the range of water concentrations indicated by the variable plagioclase compositions. In XAL-134, the continuum of plagioclase compositions from An70 to An45 leads to a range in calculated water contents from ~5.1 to 1.4 wt % H2O, which is fully consistent with a scenario of degassing-induced crystallization. Further support for this interpretation is found in the textures of the plagioclase crystals, which are often riddled with melt inclusions and show evidence of rapid growth due to large undercoolings. The results of this study indicate that degassing-induced crystallization can produce a wide range in plagioclase composition in crystal-poor andesites and dacites, and that magma mingling is not

  16. Dissolution kinetics of oceanic lower-crustal cumulate-minerals and the potential effect of the melts on ascending magmas

    NASA Astrophysics Data System (ADS)

    Kvassnes, A. J.; Grove, T. L.; Dick, H. J.

    2003-12-01

    The most primitive Atlantis Bank (SWIR) olivine-gabbros have augite oikocrysts surrounding more evolved plagioclase chadacrysts. In addition, this coarse type of augite commonly shows reverse zoning. The observations motivated an experimental study. We investigated the kinetics of melting of the grain-boundaries between mineral-pairs commonly found in lower ocean-crust, and discovered rapid melting rates and melt-compositions that may explain the phenomenon. In our study, An62 and An54 plagioclase were melted together with Fo73 or Fo82 and with Mg#86 augite. The experiments were distributed over a melting interval of 1240-1330° C and 1180-1300° C respectively. No melting was observed below 1210° C (augite - An54) and 1255° C (Fo82 - An54). Plagioclase is buoyant in the melt, therefore the minerals were melted with the heavier mineral on top, to preserve a short distance between them. The duration of the experiments varied from 30 minutes to 24 hours. The solidus for the plagioclase-clinopyroxene-olivine system was determined to be 1150° C, and the solidus for augite-plagioclase and olivine-plagioclase was inferred to be 5° C and 40degC higher, respectively, on the basis of previous studies. Olivine, the mineral that experiences the fastest internal solid-state diffusion, has very narrow (tens of μ ms) or no observable diffusion gradient along the actively melting surface, indicating that the melting rate is similar to or faster than the diffusion rate for Fe/Mg in olivine. Some recrystallization occurred in the melt close to olivine, away from the most active melt interface. Plagioclase and clinopyroxene grains melted without internal diffusion of major elements in the crystals. Augite starts disintegrating internally at the highest temperatures, but does not show any sign of preferential melting of exsolution lamellae or preferential melting of different crystal faces. Plagioclase show a very narrow (10μ m) jagged reaction zone, but no significant

  17. Structure and petrology of the La Perouse gabbro intrusion, Fairweather Range, southeastern Alaska.

    USGS Publications Warehouse

    Loney, R.A.; Himmelberg, G.R.

    1983-01-01

    The gabbro was intruded during the Middle Tertiary into a Mesozoic granulite-facies metamorphic environment dominated by strike-slip fault movement, compression and possible minor subduction. The asymmetric funnel form of the intrusion is due to subsidence from magmatic loading at high T, coupled with control from pre-existing structures, and not from tectonic compression. The intrusion is 12 X 27 km and has exposed cumulate layering of approx 6000 m. Probe analyses of olivines (24), Ca-poor pyroxenes (28), augites (22) and plagioclases (35) are tabulated. Cumulus mineral compositions in the basal cumulates are: olivine Fo86-71, plagioclase An81-63, bronzite Ca3Mg82Fe15 - Ca4Mg75Fe21, augite Ca45Mg47Fe8 - Ca42Mg48Fe10. The layered gabbro above the basal cumulates consists dominantly of lenticularly interlayered plagioclase-augite-orthopyroxene-olivine, plagioclase-augite- olivine and plagioclase-orthopyroxene-augite cumulates, the composition ranges being olivine Fo75-50, plagioclase An78-42, orthopyroxene and inverted pigeonite Ca2.8Mg76.4Fe20.8 - Ca1.4Mg31.0Fe67.6, augite Ca43.1Mg46.9Fe10.0 - Ca40.5Mg27.1Fe32.4. The most iron-rich pyroxene and albite-rich plagioclase occur in a zone near the margin of the intrusion and are probably related to exchange reactions with the country rock. It is considered that the gabbro did not accumulate by simple fractional crystallization of a single or even several large batches of magma, but by numerous influxes of previously fractionated magma from a deeper reservoir. Conditions of crystallization are interpreted as approx 1055oC, 5.4 kbar and fO2 near the wustite-magnetite buffer.-R.A.H.

  18. Gabbroic xenoliths from the northern Gorda Ridge: Implications for magma chamber processes under slow spreading centers

    SciTech Connect

    Davis, A.S.; Clague, D.A. )

    1990-07-10

    On the basis of textures and mineral compositions, the xenoliths are divided into five types: Type A xenoliths consist of large clinopyroxene oikocrysts, enclosing euhedral to subhedral plagioclase. Olivine and glass are present only in trace amounts. Type B xenoliths consist of olivine and plagioclase ({plus minus}spinel) with abundant intersertal glass. Partially resorbed olivine with deeply embayed margins in places subophitically encloses small plagioclase crystals. Olivine and large plagioclase crystals often contain glass inclusions and/or spinel. Type C xenoliths are a composite of types A and B, with areas of large clinopyroxene oikocrysts adjoining areas of loosely joined clusters of olivine and plagioclase in glass. Type D is represented by a single large xenolith that, except for containing a larger proportion of crystals, appears identical to the host lava. Type E is represented by a single small xenolith that is mineralogically similar to type B but with a distinct, fine-grained, tightly interlocking texture, formed by small olivine oikocrysts enclosing euhedral plagioclase, in a small amount of intersertal glass. Chemistry of mineral phases in conjunction with textural features suggests that the xenoliths formed in different parts of a convecting magma chamber that underwent a period of closed system fractionation. The chamber was filled with a large proportion of crystalline mush when new, more primitive, and less dense magma was injected and mixed incompletely with the contents in the chamber, forming the hybrid host lava. Plagioclase-rich leucogabbro layers in ophiolites attest to similar processes in magma chambers of past spreading centers.

  19. Cogenetic Rock Fragments from a Lunar Soil: Evidence of a Ferroan Noritic-Anorthosite Pluton on the Moon

    NASA Technical Reports Server (NTRS)

    Jolliff, B. L.; Haskin, L. A.

    1995-01-01

    The impact that produced North Ray Crater, Apollo 16 landing site, exhumed rocks that include relatively mafic members of the lunar ferroan anorthositic suite. Bulk and mineral compositions indicate that a majority of 2-4 mm lithic fragments from sample 67513, including impact breccias and monomict igneous rocks, are related to a common noritic-anorthosite precursor. Compositions and geochemical trends of these lithic fragments and of related samples collected along the rim of North Ray Crater suggest that these rocks derived from a single igneous body. This body developed as an orthocumulate from a mixture of cumulus plagioclase and mafic intercumulus melt, after the plagioclase had separated from any cogenetic mafic minerals and had become concentrated into a crystal mush (approximately 70 wt% plagioclase, 30 wt% intercumulus melt). We present a model for the crystallization of the igneous system wherein "system" is defined as cumulus plagioclase and intercumulus melt. The initial accumulation of plagioclase is analogous to the formation of thick anorthosites of the terrestrial Stillwater Complex; however, a second stage of formation is indicated, involving migration of the cumulus-plagioclase-intercumulus-melt system to a higher crustal level, analogous to the emplacement of terrestrial massif anorthosites. Compositional variations of the lithic fragments from sample 67513 are consistent with dominantly equilibrium crystallization of intercumulus melt. The highly calcic nature of orthocumulus pyroxene and plagioclase suggests some reaction between the intercumulus melt and cumulus plagioclase, perhaps facilitated by some recrystallization of cumulus plagioclase. Bulk compositions and mineral assemblages of individual rock fragments also require that most of the mafic minerals fortned in close contact with cumulus plagioclase, not as separate layers. The distribution of compositions (and by inference, modes) has a narrow peak at anorthosite and a broader, larger

  20. Cogenetic Rock Fragments from a Lunar Soil: Evidence of a Ferroan Noritic-Anorthosite Pluton on the Moon

    NASA Astrophysics Data System (ADS)

    Jolliff, B. L.; Haskin, L. A.

    1995-06-01

    The impact that produced North Ray Crater, Apollo 16 landing site, exhumed rocks that include relatively mafic members of the lunar ferroan anorthositic suite. Bulk and mineral compositions indicate that a majority of 2-4 mm lithic fragments from sample 67513, including impact breccias and monomict igneous rocks, are related to a common noritic-anorthosite precursor. Compositions and geochemical trends of these lithic fragments and of related samples collected along the rim of North Ray Crater suggest that these rocks derived from a single igneous body. This body developed as an orthocumulate from a mixture of cumulus plagioclase and mafic intercumulus melt, after the plagioclase had separated from any cogenetic mafic minerals and had become concentrated into a crystal mush (approximately 70 wt% plagioclase, 30 wt% intercumulus melt). We present a model for the crystallization of the igneous system wherein "system" is defined as cumulus plagioclase and intercumulus melt. The initial accumulation of plagioclase is analogous to the formation of thick anorthosites of the terrestrial Stillwater Complex; however, a second stage of formation is indicated, involving migration of the cumulus-plagioclase-intercumulus-melt system to a higher crustal level, analogous to the emplacement of terrestrial massif anorthosites. Compositional variations of the lithic fragments from sample 67513 are consistent with dominantly equilibrium crystallization of intercumulus melt. The highly calcic nature of orthocumulus pyroxene and plagioclase suggests some reaction between the intercumulus melt and cumulus plagioclase, perhaps facilitated by some recrystallization of cumulus plagioclase. Bulk compositions and mineral assemblages of individual rock fragments also require that most of the mafic minerals fortned in close contact with cumulus plagioclase, not as separate layers. The distribution of compositions (and by inference, modes) has a narrow peak at anorthosite and a broader, larger

  1. Shearing within lower crust during progressive retrogression: structural analysis of gabbroic rocks from the Godzilla Mullion, an oceanic core complex in the Parece Vela backarc basin

    NASA Astrophysics Data System (ADS)

    Harigane, Y.; Michibayashi, K.; Ohara, Y.

    2008-12-01

    Microstructural and petrological analyses of gabbroic rocks sampled from the Godzilla Mullion, located along the Parece Vela Basin spreading ridge (Parece Vela Rift), Philippine Sea. We reveal the development of a ductile shear zone in the lower crust. The shear zone is interpreted to represent a detachment fault within an oceanic core complex. Microstructures indicative of intense deformation, characterized by porphyroclastic textures consisting dominantly of coarse plagioclase porphyroclasts and lesser clinopyroxene porphyroclasts in a fine-grained matrix, are observed within samples of gabbroic rocks dredged near the breakaway area of the Godzilla Mullion (dredge site D6). Samples are classified into three types based upon the grain size of fine-grained plagioclase in the matrix: coarse (80--130micron), medium (25micron), and fine (~10micron). Although the chemical composition of plagioclase porphyroclasts is consistently An 40--50 among all sample types, the compositions of fine grains in the matrix vary with decreasing grain size, being An 40--50 for the coarse-type, An 30--40 for the medium-type, and An 5--30 for the fine-type. This finding implies that the composition of fine-grained plagioclase in the matrix is related to the following retrograde reaction that occurred during deformation: clinopyroxene + plagioclase + Fe-Ti oxide + fluid - hornblende + plagioclase. Plagioclase crystal-preferred orientations also show a gradual change with grain size and plagioclase composition, varying from a (010)[100] pattern for the coarse-type, (010)[100] and (001)[100] patterns for the medium-type, and a weak (001)[100] pattern or random orientations for the fine-type. These patterns are interpreted to result from a change in the deformation mechanism of plagioclase from dislocation creep to grain-size-sensitive creep with decreasing temperature, thereby leading to strain softening and localization during cooling. Although secondary amphibole occurs ubiquitously within

  2. Lithospheric strength in the active boundary between the Pacific Plate and Baja California microplate constrained from lower crustal and upper mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Chatzaras, Vasileios; van der Werf, Thomas; Kriegsman, Leo M.; Kronenberg, Andreas; Tikoff, Basil; Drury, Martyn R.

    2017-04-01

    The lower crust is the most poorly understood of the lithospheric layers in terms of its rheology, particularly at active plate boundaries. We studied naturally deformed lower crustal xenoliths within an active plate boundary, in order to link their microstructures and rheological parameters to the well-defined active tectonic context. The Baja California shear zone (BCSZ), located at the western boundary of the Baja California microplate, comprises the active boundary accommodating the relative motion between the Pacific plate and Baja California microplate. The basalts of the Holocene San Quintin volcanic field carry lower crustal and upper mantle xenoliths, which sample the Baja California microplate lithosphere in the vicinity of the BCSZ. The lower crustal xenoliths range from undeformed gabbros to granoblastic two-pyroxene granulites. Two-pyroxene geothermometry shows that the granulites equilibrated at temperatures of 690-920 oC. Phase equilibria (P-T pseudosections using Perple_X) indicate that symplectites with intergrown pyroxenes, plagioclase, olivine and spinel formed at 3.6-5.4 kbar, following decompression from pressures exceeding 6 kbar. FTIR spectroscopy shows that the water content of plagioclase varies among the analyzed xenoliths; plagioclase is relatively dry in two xenoliths while one xenolith contains hydrated plagioclase grains. Microstructural observations and analysis of the crystallographic texture provide evidence for deformation of plagioclase by a combination of dislocation creep and grain boundary sliding. To constrain the strength of the lower crust and upper mantle near the BCSZ we estimated the differential stress using plagioclase and olivine grain size paleopiezomtery, respectively. Differential stress estimates for plagioclase range from 10 to 32 MPa and for olivine are 30 MPa. Thus the active microplate boundary records elevated crustal temperatures, heterogeneous levels of hydration, and low strength in both the lower crust and

  3. Geochemistry of apollo 15 basalt 15555 and soil 15531.

    PubMed

    Schnetzler, C C; Philpotts, J A; Nava, D F; Schuhmann, S; Thomas, H H

    1972-01-28

    Major and trace element concentrations have been determined by atomic absorption spectrophotometry, colorimetry, and isotope dilution in Apollo 15 mare basalt 15555 from the Hadley Rille area; trace element concentrations have also been determined in plagioclase and pyroxene separates from basalt 15555 and in soil 15531 from the same area. Basalt 15555 most closely resembles in composition the Apollo 12 olivine-rich basalts. The concentrations of lithium, potassium, rubidium, barium, rare-earth elements, and zirconium in basalt 15555 are the lowest, and the negative europium anomaly is the smallest, reported for lunar basalts; this basalt might be the least differentiated material yet returned from the moon. Crystallization and removal of about 6 percent of plagioclase similar to that contained in the basalt would account for the observed europium anomaly; if plagioclase is not on the liquidus of this basalt, a multistage origin is indicated. Mineral data indicate that plagioclase and pyroxene approached quasi-equilibrium. Most of the chemical differences between basalt 15555 and soil 15531 would be accounted for if the soil were a mixture of 88 percent basalt, 6 percent KREEP (a component, identified in other Apollo soils, rich in potassium, rare-earth elements, and phosphorus) and 6 percent plagioclase (anorthosite?).

  4. Magma evolution in the Nain Complex, Labrador, and implications for the origin of anorthosite

    SciTech Connect

    Berg, J.H.

    1985-01-01

    Analyses of over 50 chilled margins of plutons and contemporaneous dikes in the anorthositic Nain Complex reveal a well-defined trend of liquid evolution along a plagioclase-olivine cotectic. Fractional crystallization of olivine and plagioclase has resulted in extreme enrichment of TiO/sub 2/ and FeO/sub T/ and depletion of Al/sub 2/O/sub 3/ in the more evolved compositions. The contemporaneous Harp Dikes and Seal Lake Volcanics of southern Labrador also fall along this trend. Although rare, other dikes in the Nain complex are very olivine-rich and the olivine is quench-textured. Modeling of major and trace elements indicates that the leucotroctolitic liquids can be derived from the melatroctolites by olivine removal, and that the more evolved cotectic liquids can be derived from the leucotroctolitic liquids by removal of o1 + plag in a 25:75 ratio. If the equilibrium saturation surface of plagioclase is ignored, continued olivine removal from the leucotroctolites is capable of producing compositions very similar to compositions of hypothesized anorthositic magmas. In the Nain Complex, field evidence for supersaturation of plagioclase is abundant. Since all of the compositions discussed above are low in normative diopside, it may be that such liquids are less capable of nucleating plagioclase than liquid with more normal diopside contents, thus permitting the supersaturation.

  5. The petrogenesis of sodic island arc magmas at Savo volcano, Solomon Islands

    NASA Astrophysics Data System (ADS)

    Smith, D. J.; Petterson, M. G.; Saunders, A. D.; Millar, I. L.; Jenkin, G. R. T.; Toba, T.; Naden, J.; Cook, J. M.

    2009-12-01

    Savo, Solomon Islands, is a historically active volcano dominated by sodic, alkaline lavas, and pyroclastic rocks with up to 7.5 wt% Na2O, and high Sr, arc-like trace element chemistry. The suite is dominated by mugearites (plagioclase-clinopyroxene-magnetite ± amphibole ± olivine) and trachytes (plagioclase-amphibole-magnetite ± biotite). The presence of hydrous minerals (amphibole, biotite) indicates relatively wet magmas. In such melts, plagioclase is relatively unstable relative to iron oxides and ferromagnesian silicates; it is the latter minerals (particularly hornblende) that dominate cumulate nodules at Savo and drive the chemical differentiation of the suite, with a limited role for plagioclase. This is potentially occurring in a crustal “hot zone”, with major chemical differentiation occurring at depth. Batches of magma ascend periodically, where they are subject to decompression, water saturation and further cooling, resulting in closed-system crystallisation of plagioclase, and ultimately the production of sodic, crystal and feldspar-rich, high-Sr rocks. The sodic and hydrous nature of the parental magmas is interpreted to be the result of partial melting of metasomatised mantle, but radiogenic isotope data (Pb, Sr, Nd) cannot uniquely identify the source of the metasomatic agent.

  6. Th-230 - U-238 series disequilibrium of the Olkaria basalts Gregory Rift Valley, Kenya

    NASA Technical Reports Server (NTRS)

    Black, S.; Macdonald, R.; Kelly, M.

    1993-01-01

    U-Th disequilibrium analyses of the Naivasha basalts show a very small (U-238/Th-230) ratios which are lower than any previously analyzed basalts. The broadly positive internal isochron trend from one sample indicates that the basalts may have source heterogeneities, this is supported by earlier work. The Naivasha complex comprises a bimodal suite of basalts and rhyolites. The basalts are divided into two stratigraphic groups each of a transitional nature. The early basalt series (EBS) which were erupted prior to the Group 1 comendites and, the late basalt series (LBS) which erupted temporally between the Broad Acres and the Ololbutot centers. The basalts represent a very small percentage of the overall eruptive volume of material at Naivasha (less than 2 percent). The analyzed samples come from four stratigraphic units in close proximity around Ndabibi, Hell's Gate and Akira areas. The earliest units occur as vesicular flows from the Ndabibi plain. These basalts are olivine-plagioclase phyric with the associated hawaiites being sparsely plagioclase phyric. An absolute age of 0.5Ma was estimated for these basalts. The next youngest basalts flows occur as younger tuft cones in the Ndabibi area and are mainly olivine-plagioclase-clinopyroxcene phyric with one purely plagioclase phyric sample. The final phase of activity at Ndabibi resulted in much younger tuft cones consisting of air fall ashes and lapilli tufts. Many of these contain resorbed plagioclase phenocrysts with sample number 120c also being clinopyroxene phyric. The isotopic evidence for the basalt formation is summarized.

  7. Precambrian tholeiitic-dacitic rock-suites and Cambrian ultramafic rocks in the Pennine nappe system of the Alps: Evidence from Sm-Nd isotopes and rare earth elements

    USGS Publications Warehouse

    Stille, P.; Tatsumoto, M.

    1985-01-01

    Major element, trace element and Sm-Nd isotope analyses were made of polymetamorphic hornblendefelses, plagioclase amphibolites and banded amphibolites from the Berisal complex in the Simplon area (Italy, Switzerland) to determine their age, origin and genetic relationships. In light of major and rare earth element data, the hornblendefelses are inferred to have originally been pyroxene-rich cumulates, the plagioclase amphibolites and the dark layers of the banded amphibolites to have been tholeiitic basalts and the light layers dacites. The Sm-Nd isotope data yield isochron ages of 475??81 Ma for the hornblendefelses, 1,018??59 Ma for the plagioclase amphibolites and 1,071??43 Ma for the banded amphibolites. The 1 Ga magmatic event is the oldest one ever found in the crystalline basement of the Pennine nappes. The Sm -Nd isotope data support the consanguinity of the tholeiitic dark layers and the dacitic light layers of the banded amphibolites with the tholeiitic plagioclase amphibolites and the ultramafic hornblendefelses. The initial e{open}Nd values indicate that all three rock types originated from sources depleted in light rare earth elements. We suggest that plagioclase and banded amphibolites were a Proterozoic tholeiite-dacite sequence that was strongly deformed and flattened during subsequent folding. The hornblendefelses are thought to be Cambrian intrusions of pyroxene-rich material. ?? 1985 Springer-Verlag.

  8. Shock-induced melting in anorthositic rock 60015 and a fragment of anorthositic breccia from the 'picking pot' /70052/. [meteoritic impacts on moon

    NASA Technical Reports Server (NTRS)

    Sclar, C. B.; Bauer, J. F.

    1974-01-01

    Microscopic and chemical evidence are presented to support the contention that shock-generated incipient grain-boundary melting of plagioclase occurred in an anorthositic lunar rock and that shock-generated plagioclase liquid was present along grain boundaries during post-shock adiabatic expansion in a fragment of anorthositic breccia. The first contention is supported by microtextural relationships in the rock, the composition of its metal particles (most iron with some cobalt and less nickel), and glass inclusions with vapor bubbles. The second contention is supported by angular irregular voids in the fragment as well as the occurrence of oriented glass filaments in some of the voids. It is shown that shock-generation of 'cataclastic anorthosite' and high-temperature plagioclase liquids can explain the exceptionally young lead and argon ages of the anorthositic rock. The results of the breccia study indicate that shock lithification of plagioclase-rich particulate material from the highland regolith is due to grain-boundary melting of plagioclase.

  9. Interaction between two contrasting magmas in the Albtal pluton (Schwarzwald, SW Germany): textural and mineral-chemical evidence

    NASA Astrophysics Data System (ADS)

    Michel, Lorenz; Wenzel, Thomas; Markl, Gregor

    2017-07-01

    The magmatic evolution of the Variscan Albtal pluton, Schwarzwald, SW Germany, is explored using detailed textural observations and the chemical composition of plagioclase and biotite in both granite and its mafic magmatic enclaves (MMEs). MMEs probably formed in a two-step process. First, mafic magma intruded a granitic magma chamber and created a boundary layer, which received thermal and compositional input from the mafic magma. This is indicated by corroded "granitic" quartz crystals and by large "granitic" plagioclase xenocrysts, which contain zones of higher anorthite and partly crystallized from a melt of higher Sr content. Texturally, different plagioclase types (e.g. zoned and inclusion-rich types) correspond to different degrees of overprint most likely caused by a thermal and compositional gradient in the boundary layer. The intrusion of a second mafic magma batch into the boundary layer is recorded by a thin An50 zone along plagioclase rims that crystallized from a melt enriched in Sr. Most probably, the second mafic intrusion caused disruption of the boundary layer, dispersal of the hybrid magma in the granite magma and formation of the enclaves. Rapid thermal quenching of the MMEs in the granite magma is manifested by An30 overgrowths on large plagioclase grains that contain needle apatites. Our results demonstrate the importance of microtextural investigations for the reconstruction of possible mixing end members in the formation of granites.

  10. Sm-Nd Age and Nd- and Sr- Isotopic Evidence for the Petrogenesis of Dhofar 378

    NASA Technical Reports Server (NTRS)

    Nyquist, L. E.; Ikeda, Y.; Shih, C.-Y.; Reese, Y. D.; Nakamura, N.; Takeda, H.

    2006-01-01

    Dhofar 378 (hereafter Dho 378) is one of the most ferroan lithologies among martian meteorites, resembling the Los Angeles basaltic shergottite in lithology and mineral chemistry, although it is more highly shocked than Los Angeles. All plagioclase (Pl) grains in the original lithology were melted by an intense shock in the range 55-75 GPa. Clinopyroxenes (Cpx) sometimes show mosaic extinction under a microscope showing that they, too, experienced intense shock. Nevertheless, they zone from magnesian cores to ferroan rims, reflecting the original lithology. Cpx grains also often contain exsolution lamellae, showing that the original lithology cooled slowly enough for the lamellae to form. Because all plagioclase grains were melted by the intense shock and subsequently quenched, the main plagioclase component is glass (Pl-glass) rather than maskelynite. Like Los Angeles, but unlike most basaltic shergottites, Dho 378 contains approximately equal modal abundances of Cpx and Pl-glass. The grain sizes of the original minerals were comparatively large (approximately 1 mm). The original plagioclase zoning has been severely modified. Following shock melting, the plagioclase melts crystallized from the outside inward, first forming outer rims of Cpx-Pl intergrowths (approximately 10 micrometers) followed by inner rims (10's to 100 micrometers) of An(sub 40-50) feldspar, and finally Pl-gl cores of compositions An(sub 33-50) with orthoclase compositions up to Or(sub 12).

  11. Post-igneous redistribution of components in eucrites

    NASA Technical Reports Server (NTRS)

    Phinney, W. C.; Lindstrom, D. J.; Mittlefehldt, D. W.; Martinez, R. R.

    1993-01-01

    In our analyses, we utilize a microdrilling technique that removes 40 to 100 micron diameter cores from mineral grains in thin sections analyzed by microprobe. The cores are then analyzed by INAA using the technique of Lindstrom. Three eucrites were selected for application of this analytical technique: monomict breccias Pasamonte and Stannern and unbrecciated EET90020. Pasamonte is among the most unequilibrated of the eucrites on the basis of zoning in pyroxenes and is considered to be an igneous rock not significantly affected by metamorphism. Stannern has igneous texture but its pyroxenes indicate some re-equilibration, although little, if any, recrystallization. EET90020 has a granulite texture and has been substantially recrystallized. Our sample of Pasamonte contains several clasts of different grain sizes ranging from glass to fine grained with diabasic texture containing lathy plagioclase, unexsolved pigeonite, and mesostasis. Cores were taken of the glass and from minerals and mesostases in six lithic clasts which normally allowed sampling of more than one phase per clast. Our sample of Stannern is also a breccia but with little difference in grain size between clasts and matrix. The plagioclase and pigeonite are blocky, twinned, and exsolved and coexist with a bit of mesostasis. Cores were taken of plagioclase and pigeonite with no attempt to distinguish separate clasts. EET90020 is a granular mixture of twinned plagioclase and pigeonite having rather uniform size and many triple junctions. Several cores were taken of both phases. Both clear and cloudy grains of plagioclase and pyroxene were sampled in all three eucrites.

  12. Post-igneous redistribution of components in eucrites

    NASA Technical Reports Server (NTRS)

    Phinney, W. C.; Lindstrom, D. J.; Mittlefehldt, D. W.; Martinez, R. R.

    1993-01-01

    In our analyses, we utilize a microdrilling technique that removes 40 to 100 micron diameter cores from mineral grains in thin sections analyzed by microprobe. The cores are then analyzed by INAA using the technique of Lindstrom. Three eucrites were selected for application of this analytical technique: monomict breccias Pasamonte and Stannern and unbrecciated EET90020. Pasamonte is among the most unequilibrated of the eucrites on the basis of zoning in pyroxenes and is considered to be an igneous rock not significantly affected by metamorphism. Stannern has igneous texture but its pyroxenes indicate some re-equilibration, although little, if any, recrystallization. EET90020 has a granulite texture and has been substantially recrystallized. Our sample of Pasamonte contains several clasts of different grain sizes ranging from glass to fine grained with diabasic texture containing lathy plagioclase, unexsolved pigeonite, and mesostasis. Cores were taken of the glass and from minerals and mesostases in six lithic clasts which normally allowed sampling of more than one phase per clast. Our sample of Stannern is also a breccia but with little difference in grain size between clasts and matrix. The plagioclase and pigeonite are blocky, twinned, and exsolved and coexist with a bit of mesostasis. Cores were taken of plagioclase and pigeonite with no attempt to distinguish separate clasts. EET90020 is a granular mixture of twinned plagioclase and pigeonite having rather uniform size and many triple junctions. Several cores were taken of both phases. Both clear and cloudy grains of plagioclase and pyroxene were sampled in all three eucrites.

  13. Th-230 - U-238 series disequilibrium of the Olkaria basalts Gregory Rift Valley, Kenya

    NASA Technical Reports Server (NTRS)

    Black, S.; Macdonald, R.; Kelly, M.

    1993-01-01

    U-Th disequilibrium analyses of the Naivasha basalts show a very small (U-238/Th-230) ratios which are lower than any previously analyzed basalts. The broadly positive internal isochron trend from one sample indicates that the basalts may have source heterogeneities, this is supported by earlier work. The Naivasha complex comprises a bimodal suite of basalts and rhyolites. The basalts are divided into two stratigraphic groups each of a transitional nature. The early basalt series (EBS) which were erupted prior to the Group 1 comendites and, the late basalt series (LBS) which erupted temporally between the Broad Acres and the Ololbutot centers. The basalts represent a very small percentage of the overall eruptive volume of material at Naivasha (less than 2 percent). The analyzed samples come from four stratigraphic units in close proximity around Ndabibi, Hell's Gate and Akira areas. The earliest units occur as vesicular flows from the Ndabibi plain. These basalts are olivine-plagioclase phyric with the associated hawaiites being sparsely plagioclase phyric. An absolute age of 0.5Ma was estimated for these basalts. The next youngest basalts flows occur as younger tuft cones in the Ndabibi area and are mainly olivine-plagioclase-clinopyroxcene phyric with one purely plagioclase phyric sample. The final phase of activity at Ndabibi resulted in much younger tuft cones consisting of air fall ashes and lapilli tufts. Many of these contain resorbed plagioclase phenocrysts with sample number 120c also being clinopyroxene phyric. The isotopic evidence for the basalt formation is summarized.

  14. Petrology of deep crustal xenoliths from the eastern Snake River Plain, Idaho

    NASA Astrophysics Data System (ADS)

    Matty, D. J.

    Xenoliths, collected from certain hybrid lava flows and vents at three localities in the eastern Snake River Plain region of southern Idaho, were derived from underlying crystal terrains which experienced granulite-facies metamorphism at approximately 3.0 AE. Lithologicaly, charnockite, opdalite, enderbite, and norite metaigneous xenoliths are predominent, but biotite-garnet gneiss and cognate noritic xenoliths derived from fractional crystallization of the host lavas also occur. The metamorphic charnockite-norite xenoliths are characterized by mineral assemblages containing compositionally homogeneous plagioclase + hypersthene + /-quartz +/-alkali feldspar +/-iron-titanium oxides +/-clinopyroxene +/-biotite. Iron-titanium oxide, two-feldspar, and two-pyroxene geothermometry calculations indicate that metamorphism of the xenoliths occurred at about 700 to 800 C. Corresponding pressures, estimated from garnet-plagioclase-orthopyroxene-quartz and plagioclase-clinopyroxene-quartz relationships, range from approximately 4 to 8 kbar.

  15. Experiments and Spectral Studies of Martian Volcanic Rocks: Implications for the Origin of Pathfinder Rocks and Soils

    NASA Technical Reports Server (NTRS)

    Rutherford, Malcolm J.; Mustard, Jack; Weitz, Catherine

    2002-01-01

    The composition and spectral properties of the Mars Pathfinder rocks and soils together with the identification of basaltic and andesitic Mars terrains based on Thermal Emission Spectrometer (TES) data raised interesting questions regarding the nature and origin of Mars surface rocks. We have investigated the following questions: (1) are the Pathfinder rocks igneous and is it possible these rocks could have formed by known igneous processes, such as equilibrium or fractional crystallization, operating within SNC magmas known to exist on Mars? If it is possible, what P (depth) and PH2O conditions are required? (2) whether TES-based interpretations of plagioclase-rich basalt and andesitic terrains in the south and north regions of Mars respectively are unique. Are the surface compositions of these regions plagioclase-rich, possibly indicating the presence of old AI-rich crust of Mars, or are the spectra being affected by something like surface weathering processes that might determine the spectral pyroxene to plagioclase ratio?

  16. Shock compression of a recrystallized anorthositic rock from Apollo 15

    NASA Technical Reports Server (NTRS)

    Ahrens, T. J.; Gibbons, R. V.; O'Keefe, J. D.

    1973-01-01

    Hugoniot measurements on 15,418, a recrystallized and brecciated gabbroic anorthosite, yield a value of the Hugoniot elastic limit (HEL) varying from 45 to 70 kbar as the final shock pressure is varied from 70 to 280 kbar. Above the HEL and to 150 kbar, the pressure-density Hugoniot is closely described by a hydrostatic equation of state constructed from ultrasonic data for single-crystal plagioclase and pyroxene. Above 150 kbar, the Hugoniot states indicate that a series of one or more shock-induced phase changes are occurring in the plagioclase and pyroxene. From Hugoniot data for both the single-crystal minerals and the Frederick diabase, we infer that the shock-induced high-pressure phases in 15,418 probably consists of a 3.71 g/cu cm density, high-pressure structure for plagioclase and a 4.70 g/cu cm perovskite-type structure for pyroxene.

  17. Geologic setting and petrology of Apollo 15 anorthosite /15415/.

    NASA Technical Reports Server (NTRS)

    Wilshire, H. G.; Schaber, G. G.; Jackson, E. D.; Silver, L. T.; Phinney, W. C.

    1972-01-01

    The geological setting, petrography and history of this Apollo 15 lunar rock sample are discussed, characterizing the sample as coarse-grained anorthosite composed largely of calcic plagioclase with small amounts of three pyroxene phases. The presence of shattered and granulated minerals in the texture of the rock is traced to two or more fragmentation events, and the presence of irregular bands of coarsely recrystallized plagioclase and minor pyroxene crossing larger plagioclase grains is traced to an earlier thermal metamorphic event. It is pointed out that any of these events may have affected apparent radiometric ages of elements in this rock. A comparative summarization of data suggests that this rock is the least-deformed member of a suite of similar rocks ejected from beneath the regolith at Spur crater.

  18. Short-lived oxygen diffusion during hot, deep-seated meteoric alteration of anorthosite

    PubMed

    Mora; Riciputi; Cole

    1999-12-17

    Heterogeneous oxygen isotope compositions of plagioclase from the Boehls Butte anorthosite include some of the most oxygen-18-depleted values (to -16 per mil) reported for plagioclase in meta-igneous rocks and indicate high-temperature (T > 500 degrees C) isotopic exchange between plagioclase and nearly pristine meteoric fluid. Retrograde reaction-enhanced permeability assisted influx of meteoric-hydrothermal fluids into the deep-seated anorthosite. Isotopic gradients of about 14 per mil over 600 micrometers in single crystals require short-lived (about 10(4) years) diffusional exchange of oxygen and locally large effective water:rock ratios, followed by rapid loss of water and cessation of oxygen diffusion in the anorthosite.

  19. Geochemistry of the lunar highlands as revealed by measurements of thermal neutrons

    PubMed Central

    Beck, Andrew W.; Lawrence, David J.

    2016-01-01

    Abstract Thermal neutron emissions from the lunar surface provide a direct measure of bulk elemental composition that can be used to constrain the chemical properties of near‐surface (depth <1 m) lunar materials. We present a new calibration of the Lunar Prospector thermal neutron map, providing a direct link between measured count rates and bulk elemental composition. The data are used to examine the chemical and mineralogical composition of the lunar surface, with an emphasis on constraining the plagioclase concentration across the highlands. We observe that the regions of lowest neutron absorption, which correspond to estimated plagioclase concentrations of >85%, are generally associated with large impact basins and are colocated with clusters of nearly pure plagioclase identified with spectral reflectance data. PMID:27830110

  20. Martian soil component in impact glasses in a Martian meteorite.

    PubMed

    Rao, M N; Borg, L E; McKay, D S; Wentworth, S J

    1999-11-01

    Chemical compositions of impact melt glass veins, called Lithology C (Lith C) in Martian meteorite EET79001 were determined by electron microprobe analysis. A large enrichment of S, and significant enrichments of Al, Ca, and Na were observed in Lith C glass compared to Lithology A (Lith A). The S enrichment is due to mixing of plagioclase- enriched Lith A material with Martian soil, either prior to or during impact on Mars. A mixture of 87% Lith A, 7% plagioclase, and 6% Martian soil reproduces the average elemental abundances observed in Lith C. Shock melting of such a mixture of plagioclase-enriched, fine-grained Lith A host rock and Martian soil could yield large excesses of S (observed in this study) and Martian atmospheric noble gases (found by Bogard et al., 1983) in Lith C. These mixing proportions can be used to constrain the elemental abundance of phosphorus in Martian soil.

  1. Crustal structure beneath Montserrat, Lesser Antilles, constrained by xenoliths, seismic velocity structure and petrology

    NASA Astrophysics Data System (ADS)

    Kiddle, E. J.; Edwards, B. R.; Loughlin, S. C.; Petterson, M.; Sparks, R. S. J.; Voight, B.

    2010-04-01

    Noritic anorthosite, gabbroic anorthosite and hornblende-gabbro xenoliths are ubiquitous in the host andesite at Montserrat. Other xenoliths include quartz diorite, metamorphosed biotite-gabbro, plagioclase-hornblendite and plagioclase-clinopyroxenite. Mineral compositions suggest a majority of the xenoliths are cognate. Cumulate, hypabyssal and crescumulate textures are present. A majority of the xenoliths are estimated to have seismic velocities of 6.7-7.0 km/s for pore-free assemblages. These estimates are used in conjunction with petrological models to constrain the SEA CALIPSO seismic data and the structure of the crust beneath Montserrat. Andesitic upper crust is interpreted to overlie a lower crust dominated by amphibole and plagioclase. Xenolith textures and seismic data indicate the presence of hypabyssal intrusions in the shallow crust. The structure of the crust is consistent with petrological models indicating that fractionation is the dominant process producing andesite at Montserrat.

  2. Shooting stars: Our guide to the early solar system`s formation

    SciTech Connect

    O`Reilly, J.

    1995-11-01

    Plagioclase grains were studied from the Allende meteorite, sample 916, to determine a chronology of events that occurred within the first ten million years of the solar system`s formation. Radiometric dating of the {sup 26}-Al-{sup 26}Mg system was accomplished on the ion microprobe mass spectromer. The excess {sup 26}-Mg in core plagioclase grains of calcium-aluminum rich inclusions (CAIs) provided a time of original condensation for {sup 26}-Al of {approximately}4.55 million years ago, a hundred million years prior to the formation of the planets. This data has been found to correlate with other excess {sup 26}-Mg samples. Measurements of plagioclase in the CAI`s periphery dated 1.52 million years later, suggesting an interesting history of collision and melting.

  3. Phase equilibrium modelling of granite magma petrogenesis: A. An evaluation of the magma compositions produced by crystal entrainment in the source

    NASA Astrophysics Data System (ADS)

    Garcia-Arias, Marcos; Stevens, Gary

    2017-04-01

    S-type granites show strong positive or negative correlations for several elements and element ratios plotted against maficity (FeO + MgO), and the maficity of the granites is usually higher than that of experimental melts from metasedimentary sources. These compositional variations appear over a large range of scales within the same pluton and may reflect a strong influence of source-controlled processes in the composition of granites, i.e., the entrainment of a mafic mineral fraction to the melt, prior to co-segregation of the minerals and melt as magmas. This paper presents a thermodynamically constrained study of the magma major element compositions produced by the entrainment of a fraction of the mineral assemblage which coexisted with melt in the source, either the peritectic assemblage, the non-peritectic assemblage or mixtures of both, in several metasedimentary source compositions from 650 to 950 °C at 0.8 GPa. The compositions of the modelled magmas have been compared to a large dataset of S-type granites. Several potential factors that may control the composition of the magmas have been considered: melt loss, batch and fractional melting, spatially restricted equilibration of plagioclase during partial melting and the nature and amount of the entrained mineral assemblage. The results of the investigation show that, irrespective of the melting process, melts are too felsic to account for the more mafic granite compositions. When the entrainment of only the peritectic mineral fraction is considered, the modelled magmas are able to reproduce the geochemical trends defined by the granites with increasing maficity for most major elements. However, Ca-rich plagioclase must be present in the entrained mineral assemblage to reproduce the Ca content of granites. Ca-rich plagioclase is produced only when plagioclase equilibration is strongly spatially restricted, with a core of unreacted plagioclase not participating in the melting reactions and a rim of

  4. Melting behavior and phase relations of lunar samples

    NASA Technical Reports Server (NTRS)

    Hays, J. F.

    1976-01-01

    An attempt was made to show that feldspar would float during melting. Large anorthite crystals were placed beneath a silicate glass representative of liquid in which plagioclase accumulation is thought to have occurred. In less than 3 hours at 1,300 C, the crystals rose to the top in a Pt crucible 3 cm deep equilibrated in air and in a Mo crucible 1.5 cm deep equilibrated in an H2/CO2 gas stream of log PO2 = -10.9 (below Fe/FeO). These results suggest that lunar crustal formation by feldspar flotation is possible without special recourse to differential sinking of plagioclase versus mafic minerals or selective elutriation of plagioclase.

  5. Geochemistry of the lunar highlands as revealed by measurements of thermal neutrons.

    PubMed

    Peplowski, Patrick N; Beck, Andrew W; Lawrence, David J

    2016-03-01

    Thermal neutron emissions from the lunar surface provide a direct measure of bulk elemental composition that can be used to constrain the chemical properties of near-surface (depth <1 m) lunar materials. We present a new calibration of the Lunar Prospector thermal neutron map, providing a direct link between measured count rates and bulk elemental composition. The data are used to examine the chemical and mineralogical composition of the lunar surface, with an emphasis on constraining the plagioclase concentration across the highlands. We observe that the regions of lowest neutron absorption, which correspond to estimated plagioclase concentrations of >85%, are generally associated with large impact basins and are colocated with clusters of nearly pure plagioclase identified with spectral reflectance data.

  6. Partial melting of amphibolite to trondhjemite at Nunatak Fiord, St. Elias Mountains, Alaska

    SciTech Connect

    Barker, F.; McLellan, E.L.; Plafker, G.

    1985-01-01

    At Nunatak Fiord, 55km NE of Yakutat, Alaska, a uniform layer of Cretaceous basalt ca. 3km thick was metamorphosed ca. 67 million years ago to amphibolite and locally partially melted to pegmatitic trondhjemite. Segregations of plagioclase-quartz+/-biotite rock, leucosomes in amphibolite matrix, range from stringers 5-10mm thick to blunt pods as thick as 6m. They tend to be parallel to foliation of the amphibolite, but crosscutting is common. The assemblage aluminous hornblende-plagioclase-epidote-sphene-quartz gave a hydrous melt that crystallized to plagioclase-quartz+/-biotite pegmatitic trondhjemite. 5-10% of the rock melted. Eu at 2x chondrites is positively anomalous. REE partitioning in melt/residum was controlled largely by hornblende and sphene. Though the mineralogical variability precludes quantitative modeling, partial melting of garnet-free amphibolite to heavy-REE-depleted trondhjemitic melt is a viable process.

  7. Mechanical processess affecting differentiation of protolunar material

    NASA Technical Reports Server (NTRS)

    Kaula, W. M.

    1977-01-01

    Mechanisms prior to lunar formation are sought to account for the loss of volatiles, the depletion of iron, and the enrichment of plagioclase. Some of the same mechanisms are necessary to account for achondritic, stony-iron, and iron meteorites. Collisions seem marginally capable of providing the heat to accomplish the differentiation into iron, magnesian silicates, and plagioclase. Once this differentiation is accomplished, the subsequent mechanical history should have been sufficient to sort material according to composition in the protolunar circumterrestrial cloud. Effects operating include the correlation of body size with mechanical strength; the lesser ability of the cloud to trap the larger, denser infalling bodies; the more rapid drawing into the Earth of the larger moonlets; and the higher energy orbits for dominantly plagioclase smaller pieces broken off by collision.

  8. New opportunities of pegmatites enrichment by optical sorting

    NASA Astrophysics Data System (ADS)

    Chertov, Aleksandr N.; Gorbunova, Elena V.; Alekhin, Artem A.; Korotaev, Valery V.; Skamnitskaya, Lyubov S.; Bubnova, Tatiana P.

    2015-05-01

    The paper presents the research results of pegmatites from Karelian deposits. The aim of this research was to find selective features of microcline, biotite, muscovite, quartz, and plagioclase for determining the opportunity of their selection from original ore by optical sorting method which based on color differences of analyzed objects. Studies have shown that the solution of the problem of these minerals separation is possible in 3 stages. In the first stage groups "microcline", "muscovite and biotite", "quartz and plagioclase," are separated according to the values of channels hue H and lightness L in the color model HLS. In the second stage biotite and muscovite are separated from each other by the values of the channel hue H and saturation S. Finally, in the third stage couple "quartz - plagioclase" are separated. But these minerals are indistinguishable from each other by color, so it's proposed to separate them by selective feature "surface structure."

  9. Rock-forming Minerals Transformations in Conditions of Stepwise Shock Compression of Qwartz-feldspar-biotite-garnet Schist from Southern Ural

    NASA Astrophysics Data System (ADS)

    Belyatinskaya, Irina; Feldman, Vilen; Milyavskiy, Vladimir; Borodina, Tatiana

    2011-06-01

    Samples for experiments with use of recovery assemblies of planar geometry have been taken from Southern Ural (Russia). The maximal shock pressures in the samples were attained upon a few reverberations of the waves between the walls of the recovery ampoule (stepwise shock compression) and were equal 26, 36 and 52 GPa. Shock-metamorphic transformations of rock-forming minerals (garnet, biotite, quartz, potash feldspar and plagioclase) have been studied with the use of optical and scanning electron microscopy (SEM) methods, microprobe analysis, and X-ray phase analysis (XPA). The strongest transformations (mechanical and chemical) were observed in potash feldspar and plagioclase. These minerals undergo strong amorphization at 26 GPa already. Plagioclase chemical transformations are equal to results of earlier stepwise shock compression experiments. Biotite also reveals strong mechanical (kink-bands, partial melting) and chemical (for melted biotites only) transformations. Garnet reveals mechanical transformations only. The work was supported by RFBR (09-05-00211).

  10. Two-garnet rodingite from Amador County, California

    USGS Publications Warehouse

    Duffield, W.A.; Beeson, M.H.

    1973-01-01

    Two distinct phases of garnet have been discovered in rodingite from Amador County, Calif. The two garnets are hydrogrossular and (hydro?) grossular-andradite. Only one, generally hydrogrossular, has been reported in rodingitcs studied by other workers. The rodingite of this study formed from a mafic dike with abundant euhedral plagioclase laths. The hydrogrossular is concentrated within the areas of these laths and is volumetrically about as abundant. The (hydro?) grossular-andradite is concentrated in the groundmass and as incursions into the plagioclase laths. The garnets apparently grew during one general episode of metasomatism, and their spatial distribution and compositions were controlled principally by the unequal distribution of iron and aluminum caused by the presence of plagioclase laths (and mafic minerals?) in the original unaltered dike.

  11. Repeated shock and thermal metamorphism of the Abernathy meteorite

    NASA Technical Reports Server (NTRS)

    Lambert, P.; Lewis, C.; Moore, C. B.

    1984-01-01

    Based on the example of Abernathy (L6 chondrite), it is shown how petrographic investigation can be used to unravel the nature, chronology and conditions of superposed metamorphic events in chondrites. Features considered include the texture of the rock, optical characteristics of olivine, pyroxene and plagioclase, refractive index of plagioclase, metallographical characteristics and microhardness of Fe-Ni alloys. It is deduced that Abernathy has been involved in at least six metamorphic events since the formation of the chondrite. Four distinct shock events and two separate reheating events have been identified. The chronology of these events is established. The conditions for the last four events are reasonably well constrained. These include severe reheating (T greater than 1200 C); severe shock causing complete melting of plagioclase and local melting of the rock (P between 90 and 110 GPa, T between 1250 and 1350 C); mild shock (P between 10 and 25 GPa, T less than 500 C); and reheating below 800 C.

  12. Phenocryst compositional diversity as a consequence of degassing induced crystallization

    NASA Astrophysics Data System (ADS)

    Frey, H. M.; Lange, R. A.

    2006-12-01

    In volcanic arc lavas, compositional diversity in phenocryst populations has commonly been attributed to magma mingling or mixing. However, the amount of dissolved water in the magma appears to have a significant effect on composition of the phenocrysts that crystallize from the melt. Tens of plagioclase and pyroxene phenocrysts were analyzed from six crystal-poor (<6 vol%) andesite and dacite scoria cones on the flanks of Volcán Tequila in western Mexico. The compositions and phase assemblages in the crystal-poor lavas are remarkably similar to that of the crystal-rich lavas (15-30 vol%) from the main edifice and flank flows of Volcán Tequila. Both lava types have plagioclase phenocrysts that span a wide compositional range, up to 45 mol% anorthite. In the crystal-rich lavas, individual phenocrysts have significant compositional variation, from oscillatory zoning of tens of mol% to relatively homogenous composition cores with a 5-10 um rim of significantly different composition. In contrast, plagioclase in the crystal-poor lavas has compositional variation within the population, but not individual phenocrysts. The plagioclase have little core to rim zoning and remarkable euhedral shapes, irrespective of composition. They are often riddled with melt inclusion channels, which broadly parallel the long axis of the crystal. These textures have been recognized in plagioclase crystallization experiments to be the result of rapid and large degrees of undercooling during crystallization. In the crystal-poor lavas, there is no textural evidence to suggest the phenocrysts were ever out of equilibrium with the host magma, so an alternative to magma mingling/mixing must be considered. The composition of plagioclase is dependent on several parameters, but varies most strongly with H2O content. Because of this relationship, a new plagioclase hygrometer (Lange and Frey, 2006) calibrated on plagioclase compositions from water-saturated experiments in the literature, can be used

  13. Kosmin granodiorite and its variable enclaves

    NASA Astrophysics Data System (ADS)

    Gruszczyńska, Joanna; Pietranik, Anna

    2010-05-01

    Kośmin granodiorite represents Variscan magmatic activity in Poland, Sudetes. It is ca. 335 Ma and can be correlated with a similar magmatic episode in Czech Republic, Germany and Austria [1]. The geochemical data such as high contents of MgO and K2O, enrichment in LILE, high ratios of LREE/HREE, enrichment in Cr and Ni and low Na2O contents are similar to other K-rich intrusions in the Bohemian Massif [1]. Kośmin granodiorite contains variable dioritic, sjenitic and monzodioritic enclaves which potentially record processes of magma generation and differentiation typical for potassic magmatism. Three analyzed samples of the host granodiorite have similar composition (58 wt. % SiO2, 5 wt. % MgO, 4 wt. % K2O, 43 ppm Ni, 470 ppm Sr, 9 La/LuN). The mineral composition is dominated by K-feldspar and plagioclase, with subordinate hornblende, biotite, quartz and zircon and apatite as accessory minerals. An in plagioclase varies from 33 to 42% and average SrO content in plagioclase is ca. 650-850 ppm. The enclaves in the granodiorite can be divided into 3 groups according to their mineralogy and chemical composition: (1) Monzodiorites are the dominating type and are characterized by a range of chemical compositions (50-55 wt. % SiO2, 6-10 wt. % MgO, 4-5 wt. % K2O, 30-200 ppm Ni, 300-450 ppm Sr, 5-11 La/LuN). They consist of plagioclase, pyroxene, amphibole, biotite, K-feldspar, quartz and often contain variable amounts of plagioclase phenocrysts. The phenocrysts have variable An contents from 32 to 70%, but the highest An is observed in thin (ca. 100 μm) patchy zones close to the plagioclase rims. The SrO contents in plagioclase are ca. 900 ppm and are sometimes higher in the patchy zones up to 1100 ppm. The structure and chemical composition of phenocrysts is consistent with their crystallization in the host granodioritic magma and later transport and resorption in the monzodioritic magma. (2) Syenites comprise ca. 10% of enclaves and are characterized by very high K2O

  14. Petrographic and geochemical comparisons between the lower crystalline basement-derived section and the granite megablock and amphibolite megablock of the Eyreville B core, Chesapeake Bay impact structure, USA

    USGS Publications Warehouse

    Townsend, G.N.; Gibson, R.L.; Horton, J.W.; Reimold, W.U.; Schmitt, R.T.; Bartosova, K.

    2009-01-01

    The Eyreville B core from the Chesapeake Bay impact structure, Virginia, USA, contains a lower basement-derived section (1551.19 m to 1766.32 m deep) and two megablocks of dominantly (1) amphibolite (1376.38 m to 1389.35 m deep) and (2) granite (1095.74 m to 1371.11 m deep), which are separated by an impactite succession. Metasedimentary rocks (muscovite-quartz-plagioclase-biotite-graphite ?? fibrolite ?? garnet ?? tourmaline ?? pyrite ?? rutile ?? pyrrhotite mica schist, hornblende-plagioclase-epidote-biotite- K-feldspar-quartz-titanite-calcite amphibolite, and vesuvianite-plagioclase- quartz-epidote calc-silicate rock) are dominant in the upper part of the lower basement-derived section, and they are intruded by pegmatitic to coarse-grained granite (K-feldspar-plagioclase-quartz-muscovite ?? biotite ?? garnet) that increases in volume proportion downward. The granite megablock contains both gneissic and weakly or nonfoliated biotite granite varieties (K-feldspar-quartz-plagioclase-biotite ?? muscovite ?? pyrite), with small schist xenoliths consisting of biotite-plagioclase-quartz ?? epidote ?? amphibole. The lower basement-derived section and both megablocks exhibit similar middleto upper-amphibolite-facies metamorphic grades that suggest they might represent parts of a single terrane. However, the mica schists in the lower basement-derived sequence and in the megablock xenoliths show differences in both mineralogy and whole-rock chemistry that suggest a more mafi c source for the xenoliths. Similarly, the mineralogy of the amphibolite in the lower basement-derived section and its association with calc-silicate rock suggest a sedimentary protolith, whereas the bulk-rock and mineral chemistry of the megablock amphibolite indicate an igneous protolith. The lower basement-derived granite also shows bulk chemical and mineralogical differences from the megablock gneissic and biotite granites. ?? 2009 The Geological Society of America.

  15. Petrographic and geochemical comparisons between the lower crystalline basement-derived section and the granite megablock and amphibolite megablock of the Eyreville-B core, Chesapeake Bay impact structure

    USGS Publications Warehouse

    Townsend, Gabrielle N.; Gibson, Roger L.; Horton, J. Wright; Reimold, Wolf Uwe; Schmitt, Ralf T.; Bartosova, Katerina

    2009-01-01

    The Eyreville B core from the Chesapeake Bay impact structure, Virginia, USA, contains a lower basement-derived section (1551.19 m to 1766.32 m deep) and two megablocks of dominantly (1) amphibolite (1376.38 m to 1389.35 m deep) and (2) granite (1095.74 m to 1371.11 m deep), which are separated by an impactite succession. Metasedimentary rocks (muscovite-quartz-plagioclase-biotite-graphite ± fibrolite ± garnet ± tourmaline ± pyrite ± rutile ± pyrrhotite mica schist, hornblende-plagioclase-epidote-biotite-K-feldspar-quartz-titanite-calcite amphibolite, and vesuvianite-plagioclase-quartz-epidote calc-silicate rock) are dominant in the upper part of the lower basement-derived section, and they are intruded by pegmatitic to coarse-grained granite (K-feldspar-plagioclase-quartz-muscovite ± biotite ± garnet) that increases in volume proportion downward. The granite megablock contains both gneissic and weakly or nonfoliated biotite granite varieties (K-feldspar-quartz-plagioclase-biotite ± muscovite ± pyrite), with small schist xenoliths consisting of biotite-plagioclase-quartz ± epidote ± amphibole. The lower basement-derived section and both megablocks exhibit similar middle- to upper-amphibolite-facies metamorphic grades that suggest they might represent parts of a single terrane. However, the mica schists in the lower basement-derived sequence and in the megablock xenoliths show differences in both mineralogy and whole-rock chemistry that suggest a more mafic source for the xenoliths. Similarly, the mineralogy of the amphibolite in the lower basement-derived section and its association with calc-silicate rock suggest a sedimentary protolith, whereas the bulk-rock and mineral chemistry of the megablock amphibolite indicate an igneous protolith. The lower basement-derived granite also shows bulk chemical and mineralogical differences from the megablock gneissic and biotite granites.

  16. QUE 94204: A primitive enstatite achondrite produced by the partial melting of an E chondrite-like protolith

    NASA Astrophysics Data System (ADS)

    Izawa, Matthew R. M.; Flemming, Roberta L.; Banerjee, Neil R.; Matveev, Sergei

    2011-11-01

    Queen Alexandra Range (QUE) 94204, an enstatite achondrite, is a coarse-grained, highly recrystallized, chondrule-free and unbrecciated rock dominated (about 70 vol%) by anhedral, equigranular crystals of orthoenstatite of nearly endmember composition (Fs0.1-0.4, Wo0.3-0.4) with interstitial plagioclase, kamacite, and troilite. Abundance of approximately 120° triple junctions and the close association of metal-sulfide and plagioclase-rich melts indicate that QUE 94204 has undergone limited partial melting with inefficient melt extraction. Mineral chemistry indicates a high degree of thermal metamorphism. Kamacite in QUE 94204 contains between 2.09 and 2.55 wt% Si, similar to highly metamorphosed EL chondrites. Plagioclase has between 4.31 and 6.66 wt% CaO, higher than other E chondrites but closer in composition to plagioclase from metamorphosed EL chondrites. QUE 94204 troilite contains up to 2.55 wt% Ti, consistent with extensive thermal metamorphism of an E chondrite-like precursor. Results presented in this study indicate that QUE 94204 is the result of low degree, (about 5-20 vol%, probably toward the lower end of this range) partial melting of an E chondrite protolith. Textural and chemical evidence suggests that during the metamorphism of QUE 94204, melts formed first at the Fe,Ni-FeS cotectic near approximately 900 °C, followed by plagioclase-pyroxene silicate partial melts near approximately 1100 °C. Neither the Fe,Ni-FeS nor the plagioclase-pyroxene melts were efficiently segregated or extracted. QUE 94204 belongs to a grouplet of similar "primitive enstatite achondrites" that are analogous to the acapulcoites-lodranites, but that have resulted from the partial melting of an E chondrite-like protolith.

  17. Are Variations in TiO2 Contents in Anorthite-Hosted MORB Melt Inclusions Controlled by Magma Compositions or Boundary Layer Phenomena?

    NASA Astrophysics Data System (ADS)

    Weinsteiger, A. B.; Kent, A. J.; Nielsen, R. L.; Tepley, F. J.

    2007-12-01

    Anorthitic, AN greater than 90, plagioclase-phyric pillow basalts have been recovered from almost every Mid Ocean Ridge (MOR). These phenocrysts typically contain bands of primitive (high MgO) melt inclusions (MI's). Previous workers have described large variations in the abundance of Ti in melt inclusions within a single anorthite crystal. The origin of this anomalous Ti is uncertain. It has been ascribed to both variations in Ti (and by inference other elements) in melts from which plagioclase crystallize or to processes relating specifically to the formation of melt inclusions (e.g. diffusion controlled boundary layer effects). The goal of this study was to discriminate between these possibilities by comparing the Ti contents of melt inclusions and co-existing plagioclase. If it can be shown that the Ti content of plagioclase can be related to the Ti content of MI's through simple partitioning relationships, then this implies that inclusions represent melts with variable Ti contents. In this case, MI's potentially preserve a range of "unmixed" magma compositions that occur in the MOR magma environment. Data collected from MI's and high anorthite phenocrysts illustrate the same results for both Gorda Ridge and Southeast Indian Ridge. The range of Ti concentrations in MI's within individual phenocrysts varies from near zero (e.g. homogeneous) up to a factor of two at a given MI Mg number and constant anorthite value of plagioclase. Host feldspar analyzed next to the inclusion is characterized by a greater range of Ti concentration than that measured in the MI band. Calculated Ti distribution coefficients, based on the MI/host feldspar pairs are constant as a function of MI TiO2 and melt inclusion size. We interpret this to show that MI's represent real trapped melts, from which the plagioclase crystallized. Furthermore, MI compositions were not affected by diffusion or crystal nucleation and growth processes.

  18. Disequilibrium dihedral angles in dolerite sills

    USGS Publications Warehouse

    Holness, Marian B.; Richardson, Chris; Helz, Rosalind T.

    2012-01-01

    The geometry of clinopyroxene-plagioclase-plagioclase junctions in mafic rocks, measured by the median dihedral angle Θcpp, is created during solidification. In the solidifying Kilauea Iki (Hawaii) lava lake, the wider junctions between plagioclase grains are the first to be filled by pyroxene, followed by the narrower junctions. The final Θcpp, attained when all clinopyroxene-plagioclase-plagioclase junctions are formed, is 78° in the upper crust of the lake, and 85° in the lower solidification front. Θcpp in the 3.5-m-thick Traigh Bhàn na Sgùrra sill (Inner Hebrides) is everywhere 78°. In the Whin Sill (northern England, 38 m thick) and the Portal Peak sill (Antarctica, 129 m thick), Θcpp varies symmetrically, with the lowest values at the margins. The 266-m-thick Basement Sill (Antarctica) has asymmetric variation of Θcpp, attributed to a complex filling history. The chilled margins of the Basement Sill are partially texturally equilibrated, with high Θcpp. The plagioclase grain size in the two widest sills varies asymmetrically, with the coarsest rocks found in the upper third. Both Θcpp and average grain size are functions of model crystallization times. Θcpp increases from 78° to a maximum of ∼100° as the crystallization time increases from 1 to 500 yr. Because the use of grain size as a measure of crystallization time is dependent on an estimate of crystal growth rates, dihedral angles provide a more direct proxy for cooling rates in dolerites.

  19. Superposition of replacements in the mafic granulites of the Jijal complex of the Kohistan arc, northern Pakistan: dehydration and rehydration within deep arc crust

    NASA Astrophysics Data System (ADS)

    Yamamoto, Hiroshi; Yoshino, Takashi

    1998-09-01

    A deep-level crustal section of the Cretaceous Kohistan arc is exposed in the northern part of the Jijal complex. The occurrence of mafic to ultramafic granulite-facies rocks exhibits the nature and metamorphic evolution of the lower crust. Mafic granulites are divided into two rock types: two-pyroxene granulite (orthopyroxene+clinopyroxene+plagioclase±quartz [1]); and garnet-clinopyroxene granulite (garnet+clinopyroxene+plagioclase+quartz [2]). Two-pyroxene granulite occurs in the northeastern part of the Jijal complex as a relict host rock of garnet-clinopyroxene granulite, where the orthopyroxene-rich host is transected by elongated patches and bands of garnet-clinopyroxene granulite. Garnet-clinopyroxene granulite, together with two-pyroxene granulite, has been partly replaced by amphibolite (hornblende±garnet+plagioclase+quartz [3]). The garnet-bearing assemblage [2] is expressed by a compression-dehydration reaction: hornblende+orthopyroxene+plagioclase=garnet+clinopyroxene+quartz+H 2O↑. Subsequent amphibolitization to form the assemblage [3] is expressed by two hydration reactions: garnet+clinopyroxene+plagioclase+H 2O=hornblende+quartz and plagioclase+hornblende+H 2O=zoisite+chlorite+quartz. The mafic granulites include pod- and lens-shaped bodies of ultramafic granulites which consist of garnet hornblendite (garnet+hornblende+clinopyroxene [4]) associated with garnet clinopyroxenite, garnetite, and hornblendite. Field relation and comparisons in modal-chemical compositions between the mafic and ultramafic granulites indicate that the ultramafic granulites were originally intrusive rocks which dissected the protoliths of the mafic granulites and then have been metamorphosed simultaneously with the formation of garnet-clinopyroxene granulite. The results combined with isotopic ages reported elsewhere give the following tectonic constraints: (1) crustal thickening through the development of the Kohistan arc and the subsequent Kohistan-Asia collision

  20. Crystal-poor, multiply saturated rhyolites (obsidians) from the Cascade and Mexican arcs: evidence of degassing-induced crystallization of phenocrysts

    NASA Astrophysics Data System (ADS)

    Waters, Laura E.; Lange, Rebecca A.

    2013-09-01

    A detailed petrological study is presented for six phenocryst-poor obsidian samples (73-75 wt% SiO2) erupted as small volume, monogenetic domes in the Mexican and Cascade arcs. Despite low phenocryst (+microphenocryst) abundances (2-6 %), these rhyolites are each multiply saturated with five to eight mineral phases (plagioclase + orthopyroxene + titanomagnetite + ilmenite + apatite ± zircon ± hornblende ± clinopyroxene ± sanidine ± pyrrhotite). Plagioclase and orthopyroxene phenocrysts (identified using phase-equilibrium constraints) span ≤30 mol % An and ≤15 % Mg#, respectively. Eruptive temperatures (±1 σ), on the basis of Fe-Ti two oxide thermometry, range from 779 (±25) to 940 (±18) °C. Oxygen fugacities (±1 σ) range from -0.4 to 1.4 (±0.1) log units relative to those along the Ni-NiO buffer. With temperature known, the plagioclase-liquid hygrometer was applied; maximum water concentrations calculated for the most calcic plagioclase phenocryst in each sample range from 2.6 to 6.5 wt%. This requires that the rhyolites were fluid-saturated at depths ≥2-7 km. It is proposed that the wide compositional range in plagioclase and orthopyroxene phenocrysts, despite their low abundance, can be attributed to changing melt water concentrations owing to degassing during magma ascent. Phase-equilibrium experiments from the literature show that higher dissolved water concentrations lead to more Fe-rich orthopyroxene, as well as more calcic plagioclase. Loss of dissolved water leads to a progressive increase in melt viscosity, and phenocrysts often display diffusion-limited growth textures (e.g., dendritic and vermiform), consistent with large undercoolings caused by degassing. A kinetic barrier to microlite crystallization occurred at viscosities from 4.5 to 5.0 log10 Pa s for these rhyolites, presumably because the rate at which melt viscosity changed was high owing to rapid loss of dissolved water during magma ascent.

  1. Evaluating soil mineral and geochemical variations in Alaska using a geo-tectonic framework

    NASA Astrophysics Data System (ADS)

    Wang, B.; Hults, C.; Woodruff, L. G.; Cannon, W.; Gough, L.

    2016-12-01

    A geo-tectonic framework for Alaska provides context for understanding mineral and geochemical variability along a 1400 km-long soil transect across the state. Soils in Alaska are typically poorly developed, suggesting that bulk mineralogy and geochemistry characteristics will reflect primary minerals inherited from the parent material. Most of the parent material along the transect is alluvial, glacial, or eolian unconsolidated sediment derived from a variety of source rocks. The diversity of potential parent-material source rocks necessitates a regional geologic framework for understanding the minerology and geochemical variability because the transect crosses major structural features and distinct lithotectonic domains. We parsed sites into five tectonic affinities. When the soil mineralogy is plotted on a quartz (Q) - plagioclase feldspar (P) - K-feldspar (K) diagram all samples are displaced toward the quartz-plagioclase axis. Soil mineralogy in samples from the Arctic Coastal Plain, Arctic foothills, and the Brooks Range are typically dominated by quartz (>90%) and have a chemical index of alteration (CIA) of 70 to 80. Low quartz (< 40%) and low CIA values (40 - 50) are typical in samples from the Alaska Range and the Copper River plateau. When the Q-P-K diagram is compared to one of Q-alkali feldspar- CaP it becomes evident that Na-plagioclase minerals are the dominant plagioclase feldspar minerals in soils from the Chugach Mountains while the Copper River plateau has a greater proportion of Ca-plagioclase among all plagioclase minerals. Sample mineralogy is consistent with known lithotectonic assemblages in the suspected source region of the soil parent-material, suggesting the differing CIA indexes reflect an inherited signature that reflects the geologic history of the source rocks. This framework aided interpretation and illustrates the well-established principle that parent-material composition must be considered when evaluating soil processes.

  2. Reclassification of Villalbeto de la Peña—Occurrence of a winonaite-related fragment in a hydrothermally metamorphosed polymict L-chondritic breccia

    NASA Astrophysics Data System (ADS)

    Bischoff, Addi; Dyl, Kathryn A.; Horstmann, Marian; Ziegler, Karen; Wimmer, Karl; Young, Edward D.

    2013-04-01

    The Villalbeto de la Peña meteorite that fell in 2004 in Spain was originally classified as a moderately shocked L6 ordinary chondrite. The recognition of fragments within the Villalbeto de la Peña meteorite clearly bears consequences for the previous classification of the rock. The oxygen isotope data clearly show that an exotic eye-catching, black, and plagioclase-(maskelynite)-rich clast is not of L chondrite heritage. Villalbeto de la Peña is, consequently, reclassified as a polymict chondritic breccia. The oxygen isotope data of the clast are more closely related to data for the winonaite Tierra Blanca and the anomalous silicate-bearing iron meteorite LEW 86211 than to the ordinary chondrite groups. The REE-pattern of the bulk inclusion indicates genetic similarities to those of differentiated rocks and their minerals (e.g., lunar anorthosites, eucritic, and winonaitic plagioclases) and points to an igneous origin. The An-content of the plagioclase within the inclusion is increasing from the fragment/host meteorite boundary (approximately An10) toward the interior of the clast (approximately An52). This is accompanied by a successive compositionally controlled transformation of plagioclase into maskelynite by shock. As found for plagioclase, compositions of individual spinels enclosed in plagioclase (maskelynite) also vary from the border toward the interior of the inclusion. In addition, huge variations in oxygen isotope composition were found correlating with distance into the object. The chemical and isotopical profiles observed in the fragment indicate postaccretionary metamorphism under the presence of a volatile phase.

  3. Groundmass crystallisation and cooling rates of lava-like ignimbrites: the Grey's Landing ignimbrite, southern Idaho, USA

    NASA Astrophysics Data System (ADS)

    Ellis, B. S.; Cordonnier, B.; Rowe, M. C.; Szymanowski, D.; Bachmann, O.; Andrews, G. D. M.

    2015-10-01

    Constraining magmatic and eruptive processes is key to understanding how volcanoes operate. However, reconstructing eruptive and pre-eruptive processes requires the ability to see through any post-eruptive modification of the deposit. The well-preserved Grey's Landing ignimbrite from the central Snake River Plain provides an opportunity to systematically investigate the post-eruptive processes occurring through a single deposit sheet. Despite overall compositional homogeneity in both bulk and glass compositions, the Grey's Landing ignimbrite does preserve differences in the abundance of Li in plagioclase crystals which are strongly associated with the host lithology. Li abundances in plagioclase from the quickly cooled upper and basal vitrophyres are typically low (average 5 ppm, n = 262) while plagioclase from the microcrystalline interior of the deposit has higher Li contents (average 33 ppm, n = 773). Given that no other trace elemental parameter in plagioclase varies, we interpret the variability in Li to reflect a post-depositional process. Groundmass crystallisation of a rhyolite like Grey's Landing requires ˜50 % crystallisation of sanidine and variable amounts of a silica-rich phase (quartz, tridymite, cristobalite) and plagioclase to satisfy mass balance. We suggest the low affinity of Li for sanidine causes migration of groundmass Li into plagioclase during crystallisation. Even within the microcrystalline interior of the deposit, the morphology of the groundmass varies. The more marginal, finer-grained regions are dominated by cristobalite as the SiO2-rich phase while tridymite and quartz are additionally found in the more slowly cooled, coarser-grained portions of thick sections of the ignimbrite. Numerical models of cooling and crystallisation tested against field observations indicate that the groundmass crystallisation occurred relatively rapidly following emplacement (a maximum of a few years where the ignimbrite is thickest). These numerical

  4. Early active sun - Radiation history of distinct components in fines

    NASA Technical Reports Server (NTRS)

    Crozaz, G.; Taylor, G. J.; Walker, R. M.; Seitz, M. G.

    1974-01-01

    Plagioclase feldspars were separated from lunar soil samples and their compositions were determined by electron-microprobe analysis followed by etching and track counting in an effort to find effects of early solar activity. The feldspars were assigned on this basis to three major lithologies: mare basalts, anorthositic rocks, and KREEP rock. The results are in sharp contrast to Poupeau et al.'s (1973) observations on track densities in plagioclase crystals in the Luna 16 soil: no evidence is found for an early active sun, although the evidence does not preclude this possibility, either.

  5. Activity composition relationships in silicate melts. Final report

    SciTech Connect

    Glazner, A.F.

    1990-12-31

    Equipment progress include furnace construction and electron microprobe installation. The following studies are underway: phase equilibria along basalt-rhyolite mixing line (olivine crystallization from natural silicic andensites, distribution of Fe and Mg between olivine and liquid, dist. of Ca and Na between plagioclase and liquid), enthalpy-composition relations in magmas (bulk heat capacity of alkali basalt), density model for magma ascent and contamination, thermobarometry in igneous systems (olivine/plagioclase phenocryst growth in Quat. basalt), high-pressure phase equilibria of alkali basalt, basalt-quartz mixing experiments, phase equilibria of East African basalts, and granitic minerals in mafic magma. (DLC)

  6. Shearing within lower crust during progressive retrogression: Structural analysis of gabbroic rocks from the Godzilla Mullion, an oceanic core complex in the Parece Vela backarc basin

    NASA Astrophysics Data System (ADS)

    Harigane, Yumiko; Michibayashi, Katsuyoshi; Ohara, Yasuhiko

    2008-10-01

    Microstructural and petrological analyses of gabbroic rocks sampled from the Godzilla Mullion, located along the Parece Vela Basin spreading ridge (Parece Vela Rift), Philippine Sea, reveal the development of a ductile shear zone in the lower crust. The shear zone is interpreted to represent a detachment fault within an oceanic core complex. Microstructures indicative of intense deformation, characterized by porphyroclastic textures consisting dominantly of coarse plagioclase porphyroclasts and lesser clinopyroxene porphyroclasts in a fine-grained matrix, are observed within samples of gabbroic rocks dredged near the breakaway area of the Godzilla Mullion (dredge site D6). Samples are classified into three types based upon the grain-size of fine-grained plagioclase in the matrix: coarse (80-130 µm), medium (25 µm), and fine (˜ 10 µm). Although the chemical composition of plagioclase porphyroclasts is consistently An 40-50 among all sample types, the compositions of fine grains in the matrix vary with decreasing grain-size, being An 40-50 for the coarse-type, An 30-40 for the medium-type, and An 5-30 for the fine-type. This finding implies that the composition of fine-grained plagioclase in the matrix is related to the following retrograde reaction that occurred during deformation: clinopyroxene + plagioclase + Fe-Ti oxide + fluid → hornblende + plagioclase. Plagioclase crystal-preferred orientations also show a gradual change with grain-size, varying from a (010)[100] pattern for the coarse-type, (010)[100] and (001)[100] patterns for the medium-type, and a weak (001)[100] pattern or random orientations for the fine-type. These patterns are interpreted to result from a change in the deformation mechanism of plagioclase from dislocation creep to grain-size-sensitive creep with decreasing temperature, thereby leading to strain softening and localization during cooling. Although secondary amphibole occurs ubiquitously within all samples, the chemical composition

  7. Supercooling on the lunar surface - A review of analogue information

    NASA Technical Reports Server (NTRS)

    Donaldson, C. H.; Johnston, R.; Drever, H. I.

    1977-01-01

    Terrestrial analog studies of the phase petrology of supercooled melts and rapid crystal growth are reviewed for possible light shed on lunar crystallization, supercooling, and petrogenic processes, in particular rapid consolidation of lavas extruded on the lunar surface, and impact liquids. Crystallization of major constituent minerals (olivine, pyroxene, plagioclase) in dendritic or skeletal forms is found much more characteristic of lunar igneous rocks than of terrestrial counterparts. Olivine and pyroxene occur often as skeletal phenocrysts, and their stage of crystallization is crucial to the genesis and cooling history of porphyritic lavas. Widespread occurrence of glass and of immature radiate crystallization, particularly of highly zoned pyroxenes and zoned plagioclase, is noted.

  8. Geochemistry of Apollo 15 basalt 15555 and soil 15531.

    NASA Technical Reports Server (NTRS)

    Schnetzler, C. C.; Philpotts, J. A.; Nava, D. F.; Schuhmann, S.; Thomas, H. H.

    1972-01-01

    Data are presented on major and trace element concentrations determined by atomic absorption spectrophotometry, colorimetry, and isotope dilution in Apollo 15 mare basalt 15555 from the Hadley Rille area, as well as on trace element concentrations determined in plagioclase and pyroxene separates from basalt 15555 and in soil 15531 from the same area. Most of the chemical differences between basalt 15555 and soil 15531 could be accounted for if the soil were a mixture of 88% basalt, 6% KREEP (a component, identified in other Apollo soils, rich in potassium, rare-earth elements, and phosphorus), and 6% plagioclase.

  9. Determination of Planetary Basalt Parentage: A Simple Technique Using the Electron Microprobe

    NASA Technical Reports Server (NTRS)

    Papike, J. J.; Karner, J. M.; Shearer, C. K.

    2003-01-01

    Previous studies have demonstrated the usefulness of major and minor elements in silicate phases to understand differences among basaltic systems and the influence of different planetary environments on basalt chemistry (e.g., Papike [1]). Intriguing data displays presented by Papike [1] include a plot of Mn vs. Fe (atoms per formula unit, afu) for pyroxene and olivine and a plot showing the anorthite content of plagioclase from different planetary basalts. Here we combine portions of these plots (Fig. 4) and provide all new data for olivine and plagioclase.

  10. Activity composition relationships in silicate melts

    SciTech Connect

    Glazner, A.F.

    1990-01-01

    Equipment progress include furnace construction and electron microprobe installation. The following studies are underway: phase equilibria along basalt-rhyolite mixing line (olivine crystallization from natural silicic andensites, distribution of Fe and Mg between olivine and liquid, dist. of Ca and Na between plagioclase and liquid), enthalpy-composition relations in magmas (bulk heat capacity of alkali basalt), density model for magma ascent and contamination, thermobarometry in igneous systems (olivine/plagioclase phenocryst growth in Quat. basalt), high-pressure phase equilibria of alkali basalt, basalt-quartz mixing experiments, phase equilibria of East African basalts, and granitic minerals in mafic magma. (DLC)

  11. The Correo and Suwanee Spring meteorites - Two new ordinary chondrite finds

    NASA Technical Reports Server (NTRS)

    Rubin, A. E.; Taylor, G. J.; Keil, K.; Nelson, G.

    1981-01-01

    Two new ordinary chondrites were found about 40 km west of Albuquerque, New Mexico. Correo is an H4 chondrite with distinct chondrules and major olivine (Fo 81.4), orthopyroxene (En 82.3) and plagioclase (An 12). Suwanee Spring is an L5 chondrite with few distinct chondrules and a highly recrystallized matrix. Major minerals are olivine (Fo 75.4), orthopyroxene (En 77.7) and plagioclase (An 9). The metallic Ni-Fe phases of both meteorites are typical of slowly cooled ordinary chondrites.

  12. Origin of 78235, a lunar norite cumulate

    NASA Technical Reports Server (NTRS)

    Winzer, S. R.; Nava, D. F.; Lum, R. K. L.; Schuhmann, S.; Schuhmann, P.; Philpotts, J. A.

    1975-01-01

    A chemical and petrographic study is reported of the phases from the rock 78235 which was returned on the Apollo 17 mission. Petrographic analysis of the thin sections from the bounder confirm its cumulate origin. In order to develop further the crystallization history for 78235, its subsequent shock history, and its relationship to other lunar crustal rocks, orthopyroxene, plagioclase, glass, and whole-rock samples were prepared and analyzed for major, minor, and trace elements. It is speculated that an early fractional crystallization event producing a layer of orthopyroxene-plagioclase cumulate with varying amounts of trapped liquid took place within 20 km of the surface of the moon.

  13. Geochemistry of Apollo 15 basalt 15555 and soil 15531.

    NASA Technical Reports Server (NTRS)

    Schnetzler, C. C.; Philpotts, J. A.; Nava, D. F.; Schuhmann, S.; Thomas, H. H.

    1972-01-01

    Data are presented on major and trace element concentrations determined by atomic absorption spectrophotometry, colorimetry, and isotope dilution in Apollo 15 mare basalt 15555 from the Hadley Rille area, as well as on trace element concentrations determined in plagioclase and pyroxene separates from basalt 15555 and in soil 15531 from the same area. Most of the chemical differences between basalt 15555 and soil 15531 could be accounted for if the soil were a mixture of 88% basalt, 6% KREEP (a component, identified in other Apollo soils, rich in potassium, rare-earth elements, and phosphorus), and 6% plagioclase.

  14. Experimental calibration of Forsterite-Anorthite-Ca-Tschermak-Enstatite (FACE) geobarometer for mantle peridotites

    NASA Astrophysics Data System (ADS)

    Fumagalli, P.; Borghini, G.; Rampone, E.; Poli, S.

    2017-06-01

    The crystallization of plagioclase-bearing assemblages in mantle rocks is witness of mantle exhumation at shallow depth. Previous experimental works on peridotites have found systematic compositional variations in coexisting minerals at decreasing pressure within the plagioclase stability field. In this experimental study we present new constraints on the stability of plagioclase as a function of different Na2O/CaO bulk ratios, and we present a new geobarometer for mantle rocks. Experiments have been performed in a single-stage piston cylinder at 5-10 kbar, 1050-1150 °C at nominally anhydrous conditions using seeded gels of peridotite compositions (Na2O/CaO = 0.08-0.13; X Cr = Cr/(Cr + Al) = 0.07-0.10) as starting materials. As expected, the increase of the bulk Na2O/CaO ratio extends the plagioclase stability to higher pressure; in the studied high-Na fertile lherzolite (HNa-FLZ), the plagioclase-spinel transition occurs at 1100 °C between 9 and 10 kbar; in a fertile lherzolite (FLZ) with Na2O/CaO = 0.08, it occurs between 8 and 9 kbar at 1100 °C. This study provides, together with previous experimental results, a consistent database, covering a wide range of P- T conditions (3-9 kbar, 1000-1150 °C) and variable bulk compositions to be used to define and calibrate a geobarometer for plagioclase-bearing mantle rocks. The pressure sensitive equilibrium: Mg_{2}SiO_{4}^Ol\\limits_{Forsterite} + CaAl_{2}Si_{2}O_{8}^{Pl\\limits_{Anorthite} = CaAl_{2}SiO_{6}^{Cpx}\\limits_{Ca-Tschermak} + Mg_{2}Si_{2}O_{6}^{Opx}\\limits_{Enstatite}, has been empirically calibrated by least squares regression analysis of experimental data combined with Monte Carlo simulation. The result of the fit gives the following equation: P=7.2( ± 2.9)+0.0078( ± 0.0021)T{{ }}+0.0022( ± 0.0001)T ln K, {R^2}=0.93, where P is expressed in kbar and T in kelvin. K is the equilibrium constant K = a CaTs × a en/ a an × a fo, where a CaTs, a en, a an and a fo are the activities of Ca-Tschermak in

  15. An experimental study on the shallow-level migmatization of ferrogabbros from the Fuerteventura Basal Complex, Canary Islands

    NASA Astrophysics Data System (ADS)

    Koepke, Jürgen; Berndt, Jasper; Bussy, Francois

    2003-08-01

    Spectacular shallow-level migmatization of ferrogabbroic rocks occurs in a metamorphic contact aureole of a gabbroic pluton of the Tierra Mala massif (TM) on Fuerteventura (Canary Islands). In order to improve our knowledge of the low pressure melting behavior of gabbroic rocks and to constrain the conditions of migmatization of the TM gabbros, we performed partial melting experiments on a natural ferrogabbro, which is assumed as protolith of the migmatites. The experiments were performed in an internally heated pressure vessel (IHPV) at 200 MPa, 930-1150 °C at relatively oxidizing conditions. Distinct amounts of water were added to the charge. From 930 to 1000 °C, the observed experimental phases are plagioclase (An 60-70), clinopyroxene, amphibole (titanian magnesiohastingsites), two Fe-Ti oxides, and a basaltic, K-poor melt. Above 1000 °C, amphibole is no longer stable. The first melts are very rich in normative plagioclase (>70 wt.%). This indicates that at the beginning of partial melting plagioclase is the major phase which is consumed to produce melt. In the experiments, plagioclase is stable up to high temperatures (1060 °C) showing increasing An content with temperature. This is not compatible with the natural migmatites, in which An-rich plagioclase is absent in the melanosomes, while amphibole is stable. Our results show that the partial melting of the natural rocks cannot be regarded as an "in-situ" process that occurred in a closed system. Considerable amounts of alkalis probably transported by water-rich fluids, derived from the mafic pluton underplating the TM gabbro, were necessary to drive the melting reaction out of the stability range of plagioclase. A partial melting experiment with a migmatite gabbro showing typical "in-situ" textures as starting material supports this assumption. Crystallization experiments performed at 1000 °C on a glass of the fused ferrogabbro with different water contents added to the charge show that generally high

  16. Phase equilibrium constraints on the howardite-eucrite-diogenite association

    NASA Technical Reports Server (NTRS)

    Longhi, John; Pan, Vivian

    1988-01-01

    Model determinations of fractional crystallization and equilibrium partial melting in the 0-10-kbar range have been performed for a series of compositions relevant to diogenite and eucrite petrogenesis. Olivine is found to react with diogenite parent liquids along the plagioclase-absent olivine/low-Ca pyroxene liquidus boundary under conditions of both fractional and equilibrium crystallization up to about 2 kbar. Olivine also reacts with eucritic liquids saturated with plagioclase and low-Ca pyroxene to pressures in excess of 2 kbar. The ability of simple fractional crystallization at 2 kbar to account for mineralogical and chemical features of the diogenite-eucrite association is discussed.

  17. The Correo and Suwanee Spring meteorites - Two new ordinary chondrite finds

    NASA Astrophysics Data System (ADS)

    Rubin, A. E.; Taylor, G. J.; Keil, K.; Nelson, G. J.

    1981-03-01

    Two new ordinary chondrites were found about 40 km west of Albuquerque, New Mexico. Correo is an H4 chondrite with distinct chondrules and major olivine (Fo 81.4), orthopyroxene (En 82.3) and plagioclase (An 12). Suwanee Spring is an L5 chondrite with few distinct chondrules and a highly recrystallized matrix. Major minerals are olivine (Fo 75.4), orthopyroxene (En 77.7) and plagioclase (An 9). The metallic Ni-Fe phases of both meteorites are typical of slowly cooled ordinary chondrites.

  18. Pb isotopes in anorthositic breccias 67075 and 62237 - A search for primitive lunar lead

    NASA Technical Reports Server (NTRS)

    Premo, W. R.; Tatsumoto, M.; Wang, J.-W.

    1989-01-01

    The lunar Pb isotopic evolution is explored based on an investigation of the U-Th-Pb systematics of the primitive Apollo 16 anorthositic breccias 76075 and 62237. The isotope data for acid-leached residues from 67075 lie within or on a triangle formed by plagioclase, opaques, and a pyroxen-olivine mixture, and the isotope data for 62237 lie within or on a triangle formed by plagioclase, olivine, and pyroxene. The results suggest that the moon had high first-stage values for the Pb evolution equations from the onset.

  19. Shock metamorphism in lunar samples.

    PubMed

    von Engelhardt, W; Arndt, J; Müller, W F; Stöffler, D

    1970-01-30

    Indications of shock metamorphism produced by pressures up to the megabar region have been observed in the fine material and the breccias, but very rarely in the coarser fragments of crystalline rocks. These indications are deformation structures in plagioclase and pyroxene, diaplectic plagioclase glasses, and glasses formed by shock-induced melting of lunar rocks. Two sources of shock waves have been distinguished: primary impact of meteorites and secondary impact of crater ejecta. There are two major chemical types of shock-induced melts. The differences in chemistry may be related to impact sites in mare and highland areas.

  20. Pb isotopes in anorthositic breccias 67075 and 62237 - A search for primitive lunar lead

    NASA Technical Reports Server (NTRS)

    Premo, W. R.; Tatsumoto, M.; Wang, J.-W.

    1989-01-01

    The lunar Pb isotopic evolution is explored based on an investigation of the U-Th-Pb systematics of the primitive Apollo 16 anorthositic breccias 76075 and 62237. The isotope data for acid-leached residues from 67075 lie within or on a triangle formed by plagioclase, opaques, and a pyroxen-olivine mixture, and the isotope data for 62237 lie within or on a triangle formed by plagioclase, olivine, and pyroxene. The results suggest that the moon had high first-stage values for the Pb evolution equations from the onset.

  1. Groundmass Crystallization of A'a and Pahoehoe flows of Cerro Azul, Galapagos and Mauna Loa, Hawaii

    NASA Astrophysics Data System (ADS)

    Teasdale, R.; Geist, D.; Wanless, D.; Cashman, K.

    2001-12-01

    Crystallinities of 1998 lavas from Cerro Azul, Galapagos are distinguished from 1843 Mauna Loa, Hawaii lavas by higher modal proportions of groundmass plagioclase crystals. In both cases, a'a lava was erupted first, followed by emplacement of pahoehoe flows. The 1843 Mauna Loa eruption lasted 90 days and produced both a'a and inflated pahoehoe flows (1). The 1998 Galapagos flows were emplaced over the course of 37 days, and the transition from a'a to pahoehoe flow morphology corresponds with decreased eruption rate during the last week of the eruption. Samples were collected along the length of flows from both volcanoes. Modal groundmass plagioclase increases down-flow for each flow type from each volcano. A'a and pahoehoe flows of Cerro Azul consistently have higher crystallinities (41% and 14%) than equivalent flow types at Mauna Loa (29% and 7%). Whereas Cerro Azul groundmass crystals are exclusively plagioclase, the groundmass of Mauna Loa lavas consists of plagioclase and pyroxene crystals, similar to Kilauea lavas. Proportions of plagioclase crystals and flow morphologies are consistent with predictions that groundmass crystal morphology plays a significant role in the development of crystal networks and yield strength, which instigates the development of a'a flow morphology (2, 3). Galapagos volcanoes tend to have more a'a than their Hawaiian counterparts Mauna Loa and Kilauea. Volcano slopes do not appear to control the proportion of each flow type (4); rather, abundance of groundmass plagioclase appears to be the distinguishing factor. An important distinction between the two systems is that Galapagos lavas have consistently higher Al2O3 content than Hawaiian volcanoes. High Al203 results in a comparatively high abundance of groundmass plagioclase, and may partly explain the prevalence of a'a. We note that while the relative abundance of a'a and pahoehoe flow morphologies on other volcanoes is rarely quantified, Mount Etna is also distinguished by abundant

  2. Petrologic characteristics of the 1982 and pre-1982 eruptive products of El Chichon volcano, Chiapas, Mexico.

    USGS Publications Warehouse

    McGee, J.J.; Tilling, R.I.; Duffield, W.A.

    1987-01-01

    Studies on a suite of rocks from this volcano indicate that the juvenile materials of the 1982 and pre-1982 eruptions of the volcano have essentially the same mineralogy and chemistry. Data suggest that chemical composition changed little over the 0.3 m.y. sample period. Modally, plagioclase is the dominant phenocryst, followed by amphibole, clinopyroxene and minor phases including anhydrite. Plagioclase phenocrysts show complex zoning: the anorthite-rich zones are probably the result of changing volatile P on the magma and may reflect the changes in the volcano's magma reservoir in response to repetitive, explosive eruptive activity.-R.E.S.

  3. Decompression-Induced Crystallization of Hydrous Basalt

    NASA Astrophysics Data System (ADS)

    Teasdale, R.; Brooker, R. A.

    2014-12-01

    Decompression-induced crystallization of hydrous basalt during magma ascent from 1.5 kb (150 MPa) is quantified using isothermal decompression TZM experiments. The starting composition is a synthetic glass based on the 1921 Kilauea basalt, with 1% H2O added. In all cases, the liquidus phase is aluminous spinel, followed by clinopyroxene, then plagioclase. The plagioclase liquidus temperatures for isobaric (equilibrium) experiments range from 1175°C (at 1.5 kb) to 1217°C (at 200b), which are 35-75°C hotter than predicted by MELTS (Ghiorso & Sack 1995). Experiments were decompressed at 1kb/hr and quenched at 800, 400, 200, or 100b for three temperatures (1160°, 1150°, and 1140°C). Plagioclase crystals formed during decompression have long axes that range from less than 1 micron to 20 microns. Increasing decompression yields larger plagioclase crystal sizes and aspect ratios for experiments at equal temperatures. However, the number of crystals does not vary systematically, indicating that crystallization is dominated by growth rather than nucleation during decompression. Plagioclase compositions for experiments were measured with University of Bristol's Electron Microprobe and the Hyperprobe with Field Emission Gun. Plagioclase compositions from equilibrium experiments (An60-An80) span the range of those from decompression experiments (An60-An73). Equilibrium experiments generated higher An compositions at lower pressures (500b) than at higher pressure (1.5kb) but do not systematically vary with temperature. Variations in plagioclase compositions are minimal above H2O saturation (100-200°C, based on Papale et al., 2006). Below H2O saturation, An content decreases slightly, by approximately 4% An. One application of this work is better characterization of groundmass crystallization in hydrous basalt as it traverses the conduit during eruption. This work also provides a means of distinguishing groundmass plagioclase related to decompression from crystals

  4. Determination of Planetary Basalt Parentage: A Simple Technique Using the Electron Microprobe

    NASA Technical Reports Server (NTRS)

    Papike, J. J.; Karner, J. M.; Shearer, C. K.

    2003-01-01

    Previous studies have demonstrated the usefulness of major and minor elements in silicate phases to understand differences among basaltic systems and the influence of different planetary environments on basalt chemistry (e.g., Papike [1]). Intriguing data displays presented by Papike [1] include a plot of Mn vs. Fe (atoms per formula unit, afu) for pyroxene and olivine and a plot showing the anorthite content of plagioclase from different planetary basalts. Here we combine portions of these plots (Fig. 4) and provide all new data for olivine and plagioclase.

  5. Dynamic crystallization and kinetic melting of the lunar soil

    NASA Technical Reports Server (NTRS)

    Lofgren, G. E.; Smith, D. P.; Brown, R. W.

    1978-01-01

    Dynamic crystallization and kinetic melting tests have been conducted for a lunar soil that resembles the bulk composition of the matrices of melt-rock breccias. The melting tests indicate that plagioclase and olivine can exist for finite times above their equilibrium liquidus temperatures; within 20-30 C below its liquidus temperature the sample is choked with crystals. A range of textures from radiate to porphyritic pyroxene-plagioclase granophyric to subophitic to intergranular to intersertal to very fine-grained poikilitic is found in the dynamic crystallization experiments. The number of nuclei, as deduced from the textures, increases as the melting temperature deceases from above the liquidus to below.

  6. National Dam Safety Program. Leatherwood Creek Number 3 Dam (Inventory Number VA 08904), Roanoke River Basin, Henry County, Virginia. Phase I Inspection Report.

    DTIC Science & Technology

    1981-06-01

    an it e f ortntiolt 5 whhijs of probable Paleozc~ic _age. At this loCCaltv, he formation ranges from a grite to a ) syenite . Quartz, whicht is an...Tsecain granite, ranges from 3 to 16 percent., In the syenite ,plagioclase feldspar bectines an important mineral. It occurs up to 25 percent of the...volume of rocks. This high plagioclase syenite is on the higher elevations of the left abutment. The Lloyd SOil that is on the left side of the emergency

  7. Petrology of a fine-grained igneous rock from the sea of tranquillity.

    PubMed

    Weill, D F; McCallum, I S; Bottinga, Y; Drake, M J; McKay, G A

    1970-01-30

    All phases in a thin section of sample 10022 have been analyzed by electron microprobe. Augite grains show strong iron enrichment in the outer 15 to 20 microns. Pigeonite cores occur within augite grains. The plagioclase has an anorthite content of between 73 and 81 mole percent and is high in Si and low in Al compared to stoichiometric feldspar. Residual phases include microcrystalline Fe-rich "pyroxene," plagioclase, K-rich alkali feldspar, silica, and rare areas rich in P and Zr with concentrations of Ba, Y, and rare earth elements. The density, viscosity, and crystallization history of the lava of sample 10022 are discussed.

  8. Mineral chemical compositions of late Cretaceous volcanic rocks in the Giresun area, NE Turkey: Implications for the crystallization conditions

    NASA Astrophysics Data System (ADS)

    Oǧuz, Simge; Aydin, Faruk; Uysal, İbrahim; Şen, Cüneyt

    2016-04-01

    This contribution contains phenocryst assemblages and mineral chemical data of late Cretaceous volcanic (LCV) rocks from the south of Görele and Tirebolu areas (Giresun, NE Turkey) in order to investigate their crystallization conditions. The LCV rocks in the study area occur in two different periods (Coniasiyen-Early Santonian and Early-Middle Campanian), which generally consist of alternation of mafic-intermediate (basaltic to andesitic) and felsic rock series (dacitic and rhyolitic) within each period. The basaltic and andesitic rocks in both periods generally exhibit porphyritic to hyalo-microlitic porphyritic texture, and contain phenocrysts of plagioclase and pyroxene, whereas the dacitic and rhyolitic rocks of the volcanic sequence usually show a vitrophyric texture with predominant plagioclase, K-feldspar, quartz and lesser amphibole-biotite phenocrysts. Zoned plagioclase crystals of the mafic and felsic rocks in different volcanic periods are basically different in composition. The compositions of plagioclase in the first-stage mafic rocks range from An52 to An78 whereas those of plagioclase from the first-stage felsic rocks have lower An content varying from An38 to An50. Rim to core profile for the zoned plagioclase of the first-stage mafic rocks show quite abrupt and notable compositional variations whereas that of the first-stage felsic rocks show slight compositional variation, although some of the grains may display reverse zoning. On the other hand, although no zoned plagioclase phenocryst observed in the second-stage mafic rocks, the compositions of microlitic plagioclase show wide range of compositional variation (An45-80). The compositions of zoned plagioclase in the second-stage felsic rocks are more calcic (An65-81) than those of the first-stage felsic rocks, and their rim to core profile display considerable oscillatory zoning. The compositions of pyroxenes in the first- and second-stage mafic-intermediate rocks vary over a wide range from

  9. The petrologic history of the Sanganguey volcanic field, Nayarit, Mexico: Comparisons in a suite of crystal-rich and crystal-poor lavas

    NASA Astrophysics Data System (ADS)

    Crabtree, Stephen M.; Waters, Laura E.

    2017-04-01

    To evaluate if intermediate magmas erupting from Volcán Sanganguey (Mexico) and the surrounding volcanic field are formed by mixing of basalts and rhyolites or if they initially exist as intermediate liquids, a detailed petrological study is presented for eight andesite and dacite magmas. Six of the samples erupted from the central edifice (four andesites and two dacites) are crystal-rich (≤ 50 vol%), whereas the remaining two samples (one andesite and one dacite) erupted from monogenetic vents in the peripheral volcanic field and are crystal poor (≤ 5 vol%). Despite the variation in crystallinity, all samples are multiply saturated in five to seven mineral phases (plagioclase + orthopyroxene + titanomagnetite + ilmenite + apatite ± clinopyroxene ± hornblende). In all samples, plagioclase spans a 30-40 mol% An range in composition and orthopyroxene spans a range in Mg# of 5-10. Pre-eruptive temperatures and oxygen fugacites (relative to the NNO buffer) range from 853 (± 24) to 1085 (± 16) °C and - 0.1 (± 0.1) to 0.9 (± 0.1) ∆ NNO, on the basis of Fe-Ti two oxide thermometry. Application of the plagioclase-liquid hygrometer to the samples reveals maximum H2O contents that range from 1.7-6.2 wt%. Comparison with phase equilibrium experiments demonstrates that all plagioclase and orthopyroxene compositions in the crystal-poor samples could have grown from their respective whole rock compositions. Comparison of crystal rich samples with phase equilibrium experiments reveals the presence of sodic xenocrysts which reflect resorption textures and an estimated excess plagioclase crystal cargo of > 6 vol%. The excess plagioclase crystal cargo is not distinguishable from phenocrystic plagioclase based on composition or texture, suggesting that they were also grown in intermediate melts, and are therefore described as antecrystic. No calcic plagioclase xenocrysts (> An79) typical of hydrous arc basalts are observed, thus it is likely that the excess plagioclase

  10. Differential rates of feldspar weathering in granitic regoliths

    USGS Publications Warehouse

    White, A.F.; Bullen, T.D.; Schulz, M.S.; Blum, A.E.; Huntington, T.G.; Peters, N.E.

    2001-01-01

    Differential rates of plagioclase and K-feldspar weathering commonly observed in bedrock and soil environments are examined in terms of chemical kinetic and solubility controls and hydrologic permeability. For the Panola regolith, in the Georgia Piedmont Province of southeastern United States, petrographic observations, coupled with elemental balances and 87Sr/86Sr ratios, indicate that plagioclase is being converted to kaolinite at depths > 6 m in the granitic bedrock. K-feldspar remains pristine in the bedrock but subsequently weathers to kaolinite at the overlying saprolite. In contrast, both plagioclase and K-feldspar remain stable in granitic bedrocks elsewhere in Piedmont Province, such as Davis Run, Virginia, where feldspars weather concurrently in an overlying thick saprolite sequence. Kinetic rate constants, mineral surface areas, and secondary hydraulic conductivities are fitted to feldspar losses with depth in the Panola and Davis Run regoliths using a time-depth computer spreadsheet model. The primary hydraulic conductivities, describing the rates of meteoric water penetration into the pristine granites, are assumed to be equal to the propagation rates of weathering fronts, which, based on cosmogenic isotope dating, are 7 m/106 yr for the Panola regolith and 4 m/106 yr for the Davis Run regolith. Best fits in the calculations indicate that the kinetic rate constants for plagioclase in both regoliths are factors of two to three times faster than K-feldspar, which is in agreement with experimental findings. However, the range for plagioclase and K-feldspar rates (kr = 1.5 x 10-17 to 2.8 x 10-16 mol m-2 s-1) is three to four orders of magnitude lower than for that for experimental feldspar dissolution rates and are among the slowest yet recorded for natural feldspar weathering. Such slow rates are attributed to the relatively old geomorphic ages of the Panola and Davis Run regoliths, implying that mineral surface reactivity decreases significantly with

  11. Best Fit for 'Bounce'

    NASA Technical Reports Server (NTRS)

    2004-01-01

    The mineralogy of 'Bounce' rock was determined by fitting spectra from a library of laboratory minerals to the spectrum of Bounce taken by the Mars Exploration Rover Opportunity's miniature thermal emission spectrometer. The minerals that give the best fit include pyroxene, plagioclase and olivine -- minerals commonly found in basaltic volcanic rocks -- and typical martian dust produced by the rover's rock abrasion tool.

  12. Assessing the shock state of the lunar highlands: Implications for the petrogenesis and chronology of crustal anorthosites.

    PubMed

    Pernet-Fisher, J F; Joy, K H; Martin, D J P; Donaldson Hanna, K L

    2017-07-19

    Our understanding of the formation and evolution of the primary lunar crust is based on geochemical systematics from the lunar ferroan anorthosite (FAN) suite. Recently, much effort has been made to understand this suite's petrologic history to constrain the timing of crystallisation and to interpret FAN chemical diversity. We investigate the shock histories of lunar anorthosites by combining Optical Microscope (OM) 'cold' cathodoluminescence (CL)-imaging and Fourier Transform Infrared (FTIR) spectroscopy analyses. In the first combined study of its kind, this study demonstrates that over ~4.5 Ga of impact processing, plagioclase is on average weakly shocked (<15 GPa) and examples of high shock states (>30 GPa; maskelynite) are uncommon. To investigate how plagioclase trace-element systematics are affected by moderate to weak shock (~5 to 30 GPa) we couple REE+Y abundances with FTIR analyses for FAN clasts from lunar meteorite Northwest Africa (NWA) 2995. We observe weak correlations between plagioclase shock state and some REE+Y systematics (e.g., La/Y and Sm/Nd ratios). This observation could prove significant to our understanding of how crystallisation ages are evaluated (e.g., plagioclase-whole rock Sm-Nd isochrons) and for what trace-elements can be used to differentiate between lunar lithologies and assess magma source compositional differences.

  13. Formation of anorthosite-Gabbro rhythmic phase layering: an example at North Arm Mountain, Bay of Isands ophiolite

    USGS Publications Warehouse

    Komor, S.C.; Elthon, D.

    1990-01-01

    Rhythmically layered anorthosite and gabbro are exposed in a 4-10-m thick interval at the base of the layered gabbro unit on North Arm Mountain, one of four massifs that compose the Bay of Islands ophiolite, Newfoundland. The rhythmically layered interval is sandwiched between thick layers of adcumulate to orthocumulate uniform gabbro. Calculated fractional crystallization paths and correlated cryptic variation patterns suggest that uniform and rhythmically layered gabbros represent 20-30% in situ crystallization of two distinct magma batches, one more evolved and the other more primitive. When the more primitive magma entered the crystallization site of the NA300-301 gabbros, it is estimated to have been ~40??C hotter than the resident evolved magma, and may have been chilled by contact with a magma chamber margin composed of uniform gabbro. In this model, chilling caused the liquid to become supercooled with respect to plagioclase nucleation temperatures, resulting in crystallization of gabbro deficient in plagioclase relative to equilibrium cotectic proportions. Subtraction of a plagioclase-poor melagabbro enriched the liquid in normative plagioclase, which in turn led to crystallization of an anorthosite layer. -from Authors

  14. KREEP basalt petrogenesis: Insights from 15434,181

    NASA Astrophysics Data System (ADS)

    Cronberger, Karl; Neal, Clive R.

    2017-05-01

    Returned lunar KREEP basalts originated through impact processes or endogenous melting of the lunar interior. Various methods have been used to distinguish between these two origins, with varying degrees of success. Apollo 15 KREEP basalts are generally considered to be endogenous melts of the lunar interior. For example, sample 15434,181 is reported to have formed by a two-stage cooling process, with large orthopyroxene (Opx) phenocrysts forming first and eventually cocrystalizing with smaller plagioclase crystals. However, major and trace element analyses of Opx and plagioclase coupled with calculated equilibrium liquids are inconsistent with the large orthopyroxenes being a phenocryst phase. Equilibrium liquid rare earth element (REE) profiles are enriched relative to the whole rock (WR) composition, inconsistent with Opx being an early crystallizing phase, and these are distinct from the plagioclase REE equilibrium liquids. Fractional crystallization modeling using the Opx equilibrium liquids as a parental composition cannot reproduce the WR values even with crystallization of late-stage phosphates and zircon. This work concludes that instead of being a phenocryst phase, the large Opx crystals are actually xenocrysts that were subsequently affected by pyroxene overgrowths that formed intergrowths with cocrystallizing plagioclase.

  15. 40Ar/39Ar dating of Quaternary feldspar: examples from the Taupo Volcanic Zone, New Zealand

    USGS Publications Warehouse

    Pringle, M.S.; McWilliams, M.; Houghton, B.F.; Lanphere, M.A.; Wilson, C.J.N.

    1992-01-01

    Using a continuous laser and resistance furnace, we have measured ages on Quaternary plagioclase with an absolute precision of about ??30 ka and on Quaternary sanidine with a relative precision of better than 1%. Such precision was achieved by using low-temperature heating steps to remove much of the nonradiogenic argon contamination. Plagioclase is one of the most common mineral phases in volcanic rocks; thus, these procedures will be widely applicable to many problems for which precise radiometric age control has not been available. We studied plagioclase and plagioclase-sanidine concentrates from the oldest and the three largest silicic ash-flow deposits of the Taupo Volcanic Zone, New Zealand, one of the world's largest and most active volcanic systems. The results are in close agreement with new magnetostratigraphic data, suggesting that existing fission-track age determinations significantly underestimate the age of older units, and shift the inception of Taupo Vaolcanic Zone volcanism back to at least 1600 ka. -from Authors

  16. Trace Element Distribution in an Al-rich Chondrule from the Mokoia CV3 Chondrite

    NASA Technical Reports Server (NTRS)

    Jones, R. H.; Shearer, C. K.; Schilk, A. J.

    2001-01-01

    We have studied an Al-rich chondrule from Mokoia. SIMS analyses of plagioclase and pyroxene show that the bulk chondrule REE pattern was originally like group II CAIs. The chondrule must have had precursor material that was a condensation product. Additional information is contained in the original extended abstract.

  17. Controls on variation of calcite content in arkose beds of the Sangre de Cristo Formation, (Pennsylvanian-Permian) Colorado

    SciTech Connect

    Wysong, J.R.; Bain, R.J. . Dept. of Geology)

    1994-04-01

    Arkosic conglomerates and sandstones of the Pennsylvanian-Permian Sangre de Cristo Formation of south-central Colorado were deposited on alluvial plains and nearshore marine shelves adjacent to the highlands of the Ancestral Rocky Mountains. Thin limestone units occur locally, however calcite content of arkoses varies independent of these limestones. The thinly bedded to laminated arkoses contain abundant detrital orthoclase and plagioclase feldspars, micas and quartz. Authigenic clay (kaolinite) and calcite occur both as void-filling cement and replacement of feldspars. Fine-grained arkoses possess more calcite and authigenic clay than their coarse-grained counterparts. Calcite occurs as plagioclase replacement in fine-grained samples whereas in coarse-grained rocks it fills interstitial voids. Calcite content in fine-grained arkoses is low where laminae are preserved and increases with the presence of bioturbation. Diagenetic processes responsible for calcite and clay content of these arkoses were controlled by several factors including original sediment texture, composition, and grain orientation. Plagioclase has been altered to produce calcite and clay more than orthoclase. Permeability of coarse-grained rocks was higher and resulted in primarily void-filling cement. In fine-grained arkoses, permeability was less and water remained in contact with grains longer thereby altering plagioclase. Aligned mica grains of laminae retarded flow and impeded diagenetic alteration whereas bioturbation disrupted grain orientation thereby enhancing diagenesis.

  18. Occurrence and Mineral Chemistry of High Pressure Phases, Potrillo Basalt, Southcentral New Mexico. M.S. Thesis Final Technical Report, 1 Jun. 1980 - 31 May 1982

    NASA Technical Reports Server (NTRS)

    Sheffield, T. M.

    1982-01-01

    The presence of an older plagioclase-rich basalt and a younger olivine-rich basalt were confirmed by modal and chemical analysis. Chemical analysis also confirmed the presence of flows that are tholeiitic in composition and could be remnants of an original tholeittic parent magma. Eruptions from different levels of a differentiated magma chamber are proposed to account for the two members.

  19. Geochemistry and Ar-Ar muscovite ages of the Daraban Leucogranite, Mawat Ophiolite, northeastern Iraq: Implications for Arabia-Eurasia continental collision

    NASA Astrophysics Data System (ADS)

    Mohammad, Yousif O.; Cornell, David H.; Qaradaghi, Jabbar H.; Mohammad, Fahmy O.

    2014-06-01

    Daraban Leucogranite dykes intruded discordantly into the basal serpentinized harzburgite of the Mawat Ophiolite, Kurdistan region, NE Iraq. These coarse grained muscovite-tourmaline leucogranites are the first leucogranite dykes identified within the Mawat Ophiolite. They are mainly composed of quartz, K-feldspar, plagioclase, tourmaline, muscovite, and secondary phologopite, while zircon, xenotime, corundum, mangano-ilemnite and cassiterite occur as accessories.

  20. The Mineralogy and Geochemistry of Manganese Nodules From the Southern Ocean

    DTIC Science & Technology

    1968-02-01

    accumulations. Quartz, plagioclase, montmorillonite , and phillipsite are almost invariably present, while clinoptilolite ii and amphibole occur less... Montmorillonite Diffraction Data ........... 125 16. Phillipsite Diffraction Data .. ......... ... 126 17. Sources of X-ray Diffraction Data...concretion. (Crust from ELTANIN 5-4; nucleus probably phillipsite- montmorillonite ; glacial erratics incorporated in ferro- manganese oxide crust.) 0 cm I

  1. The metamorphosed molybdenum vein-type deposit of the Alpeinerscharte, Tyrol (Austria) and its relation to Variscan granitoids

    NASA Astrophysics Data System (ADS)

    Melcher, F.; Prochaska, W.; Raith, J. G.; Saini-Eidukat, B.

    1996-05-01

    The molybdenite deposit of the Alpeinerscharte (Austria) is situated in Variscan greenschist- to amphibolite-facies metamorphosed granodiorites and granites of the western Tauern Window. These granitoids represent strongly fractionated calc-alkaline I-type magmas with minor S-type components and reveal post-orogenic affinities. Molybdenum contents (average 4.3 ppm) are slightly above the general background of average granites. Molybdenite mineralization is restricted to narrow quartz veins and quartz vein selvages which are presently composed of biotite and (almandine-grossular) garnet. These selvages show geochemical features typical of intermediate argillic alteration in a hydrothermal system postdating granite intrusion: instability of plagioclase causes removal of Na, Ba, Sr, Pb and Eu, while K and Ca remain nearly constant. Rare earth elements (apart from Eu) and metals are extremely enriched. Application of Fe-Mg exchange (garnet-biotite) and oxygen isotope (quartz-garnet, quartz-plagioclase) geothermometers to vein selvage mineral assemblages reveals temperatures of the late-Alpine (35 55 Ma) metamorphic overprint (˜540 °C, 7 10 kbar). Leucocratic rocks composed of mainly orthoclase and plagioclase are occasionally spatially related to molybdenite-bearing veins; they are interpreted as episyenites formed by hydrothermal alteration of the host granite. This episyenitic alteration is characterized by a mass loss of ca. 30%, relative enrichment of plagioclase components, extreme depletion of Si, and minor depletion of Fe, Zn, Cu and Mo.

  2. The characteristics of lower crust and upper mantle in the Cima volcanic field deduced from xenolith studies

    NASA Astrophysics Data System (ADS)

    Cardon, K. P.; Anthony, E.

    2015-12-01

    A lithospheric model based on mineral chemistry, textures, and temperatures is used to interpret the seismic structure of the upper mantle and lower crust observed under the Cima Volcanic Field, CA. Seismic velocities calculated from xenolith compositions are used in conjunction with petrologic information to interpret geophysical models of the area. The lower crust is composed of mafic compositions and contains a high percentage of quenched partial melt. The combination of quenched partial melt and mafic composition explains the relatively low seismic velocities observed in seismic models. The mafic composition is consistent with a rift environment. Melt compositions, some with > 60 wt% SiO2 are found in all types of Cima xenoliths, although pyroxenites and gabbros contain the largest amount. Pyroxenite from the uppermost mantle transitions into gabbroic compositions and plagioclase rich lithologies in the crust. Temperatures calculated for peridotite xenoliths range from ~ 950 to 1030˚ C. Plagioclase bearing samples have the lowest temperatures and are interpreted as residing in the immediate sub-Moho mantle. Plagioclase bearing lherzolite structurally overlies spinel bearing peridotite. Strain accumulation is most prevalent in plagioclase bearing peridotite and virtually absent from pyroxenites and gabbros. Seismic velocities calculated for peridotite xenoliths are faster than pyroxenite and gabbroic samples. Despite the chemical heterogeneity and complex history of the Moho transitional are most mantle is composed dominantly by peridotite. Very little lithosphere, rhelologically speaking, remains under the volcanic field. We interpret lithospheric dismemberment to be caused by hot mantle working northward from the Gulf of California.

  3. Spatially-Correlated Mass Spectrometric Analysis of Microbe-Mineral Interactions

    SciTech Connect

    Jill R. Scott; Beizhan Yan; Daphne L. Stoner

    2006-11-01

    A new methodology for examining the interactions of microbes with heterogeneous minerals is presented. Imaging laser-desorption Fourier transform mass spectrometry was used to examine the colonization patterns of Burkholderia vietnamiensis (Burkholderia cepacia) G4 on a heterogeneous basalt sample. Depth-profile imaging found that the bacterium preferentially colonized the plagioclase mineral phases within the basalt.

  4. Origin of mineralogical zoning in an intermediate subvolcanic pluton by crystal settling and conductive cooling

    SciTech Connect

    Hildebrand, R.S.

    1985-01-01

    The Rainy Lake Intrusive Complex is one of nine sheet-like intermediate plutons that are compositionally, temporally, and spatially linked to andesitic stratovolcanoes in the western Great Bear Magmatic Zone, an early Proterozoic continental magmatic arc. The pluton, exposed in oblique cross section, has a flat roof and is 1.5 km thick by 12 km long. The observed distribution of plagioclase phenocrysts in the body closely resembles the sinusoidal olivine distribution patterns found in lava lakes and some mafic sills. The shape of the curve indicates that the volume fraction plagioclase phenocrysts in the pluton at the time of intrusion ranged for 10% near the roof to 50% near the base. Calculations indicate that the plagioclase phenocrysts were denser than the surrounding melt. The effects of different crystal concentrations on the viscosity, and hence settling velocities, were computed as a function of height in the pluton; settling velocities ranged, at the time of intrusion, from 160 meters per year near the top to less than 1 meter per year near the base. The observed sinusoidal distribution curve for plagioclase can be explained by the interplay between crystal settling and the solidification of the pluton from the top downwards and the bottom upwards. This suggests that whole body convection did not play a major role in the cooling of the pluton.

  5. Geological-Seismological Evaluation of Earthquake Hazards at Prompton and Francis E. Walter Damsites, Pennsylvania.

    DTIC Science & Technology

    1986-09-01

    granitic gneiss, migmatite, subordinate calc - silicate rock. bhg Biotite hornblende granite. hg Hornblende granite and granitic gneiss, with subordinate...leucogranite. mb Calcitic and dolomitic marble, variably siliceous ; in part with calc - silicate rock and amphibolite. usg Quartz plagioclase gneiss; may...quartzite, quartz-feldspar gneiss, calc - silicate rock. Pennsylvania Pennsylvanian Pottsville Group Pp Predominantly sandstones and conglomerates with

  6. Some observations on the stoichiometry of feldspar hydrolysis in granitic soil

    Treesearch

    James L. Clayton

    1988-01-01

    Weathering rates of orthoclase and plagioclase were computed from mass balances of Na, K, and Ca in three forested watersheds in the Idaho batholith. On the basis of stand conditions, two watersheds were assumed to have no net gains or losses of cations in biomass, and increases in biomass were measured in the third watershed. Balanced feldspar hydrolysis reactions...

  7. REE Partitioning in Lunar Minerals

    NASA Technical Reports Server (NTRS)

    Rapp, J. F.; Lapen, T. J.; Draper, D. S.

    2015-01-01

    Rare earth elements (REE) are an extremely useful tool in modeling lunar magmatic processes. Here we present the first experimentally derived plagioclase/melt partition coefficients in lunar compositions covering the entire suite of REE. Positive europium anomalies are ubiquitous in the plagioclase-rich rocks of the lunar highlands, and complementary negative Eu anomalies are found in most lunar basalts. These features are taken as evidence of a large-scale differentiation event, with crystallization of a global-scale lunar magma ocean (LMO) resulting in a plagioclase flotation crust and a mafic lunar interior from which mare basalts were subsequently derived. However, the extent of the Eu anomaly in lunar rocks is variable. Fagan and Neal [1] reported highly anorthitic plagioclase grains in lunar impact melt rock 60635,19 that displayed negative Eu anomalies as well as the more usual positive anomalies. Indeed some grains in the sample are reported to display both positive and negative anomalies. Judging from cathodoluminescence images, these anomalies do not appear to be associated with crystal overgrowths or zones.

  8. Component mobility at 900 °C and 18 kbar from experimentally grown coronas in a natural gabbro

    NASA Astrophysics Data System (ADS)

    Keller, Lukas M.; Wunder, Bernd; Rhede, Dieter; Wirth, Richard

    2008-09-01

    Several approximately 100-μm-wide reaction zones were grown under experimental conditions of 900 °C and 18 kbar along former olivine-plagioclase contacts in a natural gabbro. The reaction zone comprises two distinct domains: (i) an irregularly bounded zone with idiomorphic grains of zoisite and minor corundum and kyanite immersed in a melt developed at the plagioclase side and (ii) a well-defined reaction band comprising a succession of mineral layers forming a corona structure around olivine. Between the olivine and the plagioclase reactant phases we observe the following layer sequence: olivine|pyroxene|garnet|partially molten domain|plagioclase. Within the pyroxene layer two micro-structurally distinct layers comprising enstatite and clinopyroxene can be discerned. Chemical potential gradients persisted for the CaO, Al 2O 3, SiO 2, MgO and FeO components, which drove diffusion of Ca, Al and Si bearing species from the garnet-matrix interface to the pyroxene-olivine interface and diffusion of Mg- and Fe-bearing species in the opposite direction. The systematic mineralogical organization and chemical zoning across the corona suggest that the olivine corona was formed by a "diffusion-controlled" reaction. We estimate a set of diffusion coefficients and conclude that LAlAl < LCaCa < ( LSiSi, LFeFe) < LMgMg during reaction rim growth.

  9. Regolith breccia consisting of H and LL chondrite mixture

    NASA Technical Reports Server (NTRS)

    Yanai, Keizo; Kojima, Hideyasu

    1993-01-01

    Antarctic meteorite Yamato-8424 (Y-8424) is a regolith breccia that is homogenized mixture of H and LL chondrite components. The breccia consists mainly of a fine-grained material with mineral fragments of olivine, pyroxene, and Fe-Ni metal with traces of plagioclase.

  10. Feldspar diagenesis in Neogene sediments, northern Gulf of Mexico

    SciTech Connect

    Milliken, K.L.; Gold, P.B.; Land, L.S.

    1985-02-01

    Alteration of feldspars in the youngest of the Gulf Coast Cenozoic sands and sandstones is dominated by dissolution and albitization. Volumetrically significant amounts of alteration are only observed below burial depths of about 4.5 km in sands of Pliocene and Miocene age. Only trivial amounts of plagioclase dissolution were observed in Pleistocene units. In general, plagioclase exhibits minor amounts of secondary dissolution at all depths, and greater among in the deepest samples. Potassium feldspar is subject to very little dissolution to depths of about 3.5 km; by 4.5 km K-feldspar removal is virtually complete. Albitization affects only plagioclase and appears to be operative, in these sediments, over temperatures of 110/sup 0/C-140/sup 0/C. Compared to older Cenozoic units elsewhere around the Gulf of Mexico feldspar dissolution and albitization in Neogene sands have advanced to a lesser degree, at least in the sense that they affect a smaller proportion of the total section. Comparison of pre-alteration feldspar composition, temperatures of alteration, and geothermal gradients for Gulf Coast sandstones of different ages suggests that the main controls on feldspar alteration are temperature, pre-alteration plagioclase composition, and possibly the amount of fluid flow. Time per se seems to be a factor of negligible importance, at least over time spans greater than 10/sup 6/ yr. Thus, the lesser volume of Neogene sand affected by feldspar dissolution and albitization can be attributed primarily to the lower geothermal gradients of the northern Gulf.

  11. Quantitative textural investigation of trachyandesites of Damavand volcano (N Iran): Insights into the magmatic processes

    NASA Astrophysics Data System (ADS)

    Zadsaleh, Mohsen; Pourkhorsandi, Hamed

    2016-08-01

    Damavand volcano is a dormant stratovolcano in northern Iran in the middle of the Alborz Mountains. Investigation of the magmatic processes responsible for the eruption of the volcano and the conditions of the magma chamber is important in order to understand the volcanism of this system. Owing to their higher abundance and younger age, trachyandesitic rocks are the main components of this volcano. To get insights into the crystallization of these rocks, we carried out a quantitative and qualitative petrographic study of three main volcanic units erupted between 63 and 66.5 years ago. Crystal Size Distribution (CSD) studies can reveal details about magmatic processes. Measuring 4732 individual plagioclase crystals and conducting a CSD study, revealed a non-straight and concave-up CSD curve for nearly all of the studied volcanic units which suggests the occurrence of similar physico-chemical processes responsible for their magmatism. Plagioclase crystals occur as microlites and phenocrysts; the phenocrysts show either oscillatory zoning or sieve textures. Each segment of the CSD curves are consistent with a particular plagioclase texture in all the studied volcanic units. The presence of different plagioclase textures and the concave-up shape of the CSD curves suggests the variation of the physico-chemical conditions of the magma chamber during the magmatism of the Damavand in this time period. Mixing of magmas with different crystal populations can be an alternative for this phenomena.

  12. Crystallization Experiments of the Martian Meteorite QUE94201: Additional Constraints on Its Formation Condition

    NASA Technical Reports Server (NTRS)

    Koizumi, E.; McKay, G.; Mikouchi, T.; Le, L.; Schwandt, C.; Monkawa, A.; Miyamoto, M.

    2002-01-01

    We focused on the Al/Ti ratio in synthetic pyroxenes as a marker for the onset of plagioclase crystallization and discuss the effects of oxygen fugacity on the Kd(Fe/Mg)ol/gl in our experiments using the same composition of QUE94201. Additional information is contained in the original extended abstract.

  13. Rare earth element abundances in rocks and minerals from the Fiskenaesset Complex, West Greenland. [comparison with lunar anorthosites

    NASA Technical Reports Server (NTRS)

    Henderson, P.; Fishlock, S. J.; Laul, J. C.; Cooper, T. D.; Conard, R. L.; Boynton, W. V.; Schmitt, R. A.

    1976-01-01

    The paper reports activation-analysis determinations of rare-earth-element (REE) and other trace-element concentrations in selected rocks, plagioclase, and mafic separates from the Fiskenaesset Complex. The REE abundances are found to be very low and atypical in comparison with other terrestrial anorthosites. The plagioclases are shown to be characterized by a deficiency in heavy RE elements relative to light ones and a positive Eu anomaly, while the mafic separates are enriched in heavy rare earths and have no Eu anomaly, except in one sample. It is found that the bulk and trace-element abundances of the plagioclases are similar to those observed in some lunar anorthosites, but the degree of Eu anomaly is less in the plagioclases. The data are taken as confirmation of the idea that fractionation processes were involved in the origin of the Complex, and it is concluded that the Complex may have been produced from a magma generated by partial melting of a garnet-bearing source.

  14. Estimation of trace element concentrations in the lunar magma ocean using mineral- and metal-silicate melt partition coefficients

    NASA Astrophysics Data System (ADS)

    Sharp, Miriam; Righter, Kevin; Walker, Richard J.

    2015-04-01

    This study uses experimentally determined plagioclase-melt D values to estimate the trace element concentrations of Sr, Hf, Ga, W, Mo, Ru, Pd, Au, Ni, and Co in a crystallizing lunar magma ocean at the point of plagioclase flotation. Similarly, experimentally determined metal-silicate partition experiments combined with a composition model for the Moon are used to constrain the concentrations of W, Mo, Ru, Pd, Au, Ni, and Co in the lunar magma ocean at the time of core formation. The metal-silicate derived lunar mantle estimates are generally consistent with previous estimates for the concentration of these elements in the lunar mantle. Plagioclase-melt derived concentrations for Sr, Ga, Ru, Pd, Au, Ni, and Co are also consistent with prior estimates. Estimates for Hf, W, and Mo, however, are higher. These elements may be concentrated in the residual liquid during fractional crystallization due to their incompatibility. Alternatively, the apparent enrichment could reflect the inappropriate use of bulk anorthosite data, rather than data for plagioclase separates.

  15. Constraints on Magma Storage Conditions Beneath Grenada, Lesser Antilles as Inferred from the Petrology of Plutonic Xenoliths

    NASA Astrophysics Data System (ADS)

    Stamper, C.; Blundy, J. D.; Arculus, R. J.; Melekhova, E.

    2013-12-01

    Grenada is the southernmost island in the Lesser Antilles island arc, a chain of volcanoes celebrated for its diversity of magma composition and its propensity to erupt plutonic xenoliths, many with cumulate textures. The cumulate assemblages are dominated by mafic phases, with hornblende and clinopyroxene dominant over orthopyroxene, and include the only known plagioclase-free examples in the Lesser Antilles. Experimental and thermobarometric evidence shows that the entire cumulate suite can be generated in a narrow pressure (0.2 - 0.5 GPa) range with different assemblages resulting from small variations in melt chemistry and temperature. Temperature estimates for individual assemblages are consistent with the observed crystallisation sequence of olivine -> clinopyroxene -> hornblende -> plagioclase. The crystallisation sequence requires elevated magmatic H2O contents (~7 wt % H2O) sufficient to suppress plagioclase saturation, leading to an offset between the forsterite content of olivine and anorthite content of plagioclase, a characteristic of many island arc settings. Grenada cumulates are notably different from those originating from its northerly neighbour, St Vincent, demonstrating the effect of small scale changes of melt composition and magma storage conditions between these islands. The unusual geochemical characteristics of the southernmost portion of the Lesser Antilles arc may be due to the proximity of a large transform fault defining the southern margin of the Caribbean plate.

  16. Petrographic-mineralogical investigation of magmatic rocks from the Sea of Fertility

    NASA Technical Reports Server (NTRS)

    Tarasov, L. S.; Shevaleyevskiy, I. D.; Nazarov, M. A.

    1974-01-01

    Petrographic and mineralogical features of fragments of magmatic rock of regolith from the Sea of Fertility are examined. The textures and mineral composition vary in relation to the type of rock. More than 50 X-ray spectral analyses of minerals (olivine, pyroxenes, plagioclases, and ores) were made; their chemical composition varies even within the limits of individual rock fragments.

  17. Mechanisms of myrmekite formation: case study from the Weinsberg granite, Moldanubian zone, Upper Austria

    NASA Astrophysics Data System (ADS)

    Abart, Rainer; Heuser, David; Habler, Gerlinde

    2014-11-01

    Myrmekites have attracted the attention of petrographers over more than a century, and several genetic models have been proposed. We report on myrmekites from the Weinsberg granite of the Moldanubian zone of Upper Austria. Based on petrographic evidence, fluid-mediated replacement of alkali feldspar by myrmekite during the sub-solidus evolution of the granite is inferred. The replacement was metasomatic on the scale of the myrmekite domains requiring addition of sodium and calcium and removal of potassium from the reaction site. In contrast, silica and aluminum were conserved across the reaction front. Myrmekite formation appears to have been synchronous with and related to the hydration of orthopyroxene and concomitant replacement of primary magmatic plagioclase by biotite at around 500 °C. The evolution of the myrmekite microstructure and a peculiar composition zoning of the plagioclase constituting the myrmekite matrix is qualitatively explained by a model for discontinuous precipitation, which accounts for chemical segregation by diffusion within the reaction front and the propagation of the reaction front with finite mobility as potentially rate limiting processes. Constraints on the underlying reaction rates are derived from the preserved microstructure and chemical pattern. Crystal orientation imaging by electron backscatter diffraction reveals grain-internal deformation, which is primarily concentrated in the quartz and less pronounced in the plagioclase matrix of the myrmekite. This is interpreted as a growth feature related to different transformation strain at the segments of the myrmekite reaction front, where quartz and plagioclase are formed.

  18. Variation in the structure of the surface layer of a heterogeneous solid (gneiss) on a shear

    NASA Astrophysics Data System (ADS)

    Vettegren', V. I.; Ponomarev, A. V.; Arora, K.; Mamalimov, R. I.; Shcherbakov, I. P.; Patonin, A. V.

    2017-07-01

    The structure of surface layers with a thickness of 1 μm formed at destruction of gneiss is studied by means of photoluminescent and infrared spectroscopy. It is found that, in this layer, feldspar (plagioclase and microcline) crystals are completely destroyed and replaced by montmorillonite.

  19. Installation Restoration Program Preliminary Assessment Granite Mountain Radio Relay Station, Alaska

    DTIC Science & Technology

    1989-04-01

    green aggregate of chlorite, epidote, calcite, and sodic plagioclase. Many small unmapped intrusive bodies of hybrid diorite, syenite , and monzonite...groundwater, surface water, soil, and air). MONZONITE - A group of plutonic rocks, intermediate in composition between syenite and diorite, containing...manufacture of soda, batteries, and a great variety of industrial operations. SYENITE - A group of plutonic rocks containing alkali feldspar (usually

  20. Mantle Origin of Olivine-rich Troctolites in a Rift Environment

    NASA Astrophysics Data System (ADS)

    Faul, U.; Garapic, G.; Michibayashi, K.

    2014-12-01

    Olivine-rich troctolites from Krivaja-Konjuh massif in the Dinarides (Bosnia and Herzegovina) represent a unique natural laboratory to study melt segregation and retention in originally fertile rift-related mantle rocks. Petrographic and chemical analyses of coexisting minerals (olivine and interstitial clinopyroxene, spinel and plagioclase) show that olivine and spinel are of mantle origin, and interstitial clinopyroxene and plagioclase are products of crystallization of trapped melt. Geochemical and microstructural observations (dissolution of pyroxene) indicate that this trapped melt started off at larger depth and that it was silica undersaturated when it reached the plagioclase stability field. The original mantle olivine remained and texturally equilibrated with the melt, which is evident from the high Ni content and euhedral crystal habit in thin sections. This suggests that the melt that crystallized the interstitial phases, clinopyroxene and plagioclase, was saturated in olivine and probably similar to MORB-type melt. The interstitial phases show no sign of deformation. A mantle origin of olivine is corroborated by EBSD data that show a progressive misorientation across individual olivine grains due to previous episodes of deformation within the mantle. By contrast, grains originating from cumulate crystallization should show little internal deformation. Therefore we suggest that olivine-rich troctolites are not crystallization products, but rather products of reaction of melt with the lherzolitic mantle. The evidence of trapped melt in those rocks provides a new insight into the crust-mantle transition at slow spreading ridges and continental rifts where olivine-rich troctolites have been observed.

  1. Evidence for the compaction of feldspar-rich cumulates in the Pleasant Bay layered intrusion, coastal Maine

    SciTech Connect

    Horrigan, E.K. )

    1993-03-01

    The Pleasant Bay intrusion is roughly 12 km by 20 km. It consists of prominent rhythmic layers, up to 100 m thick, that grade from chilled gabbro on the base, to coarse-grained gabbroic, dioritic, or granitic rocks on the top. These layers were formed by multiple injections of basalt into a large chamber of silicic magma. The focus of this study is on one layer that is about 100 m thick, and is overlain by another basally chilled gabbroic layer at least 50 m thick. Silicic pipes and veins extend upward into the overlying gabbroic chill. The lower part of the layer has dominant calcic plagioclase, An60, augite, and olivine, with subordinate hornblende and biotite. The uppermost part has dominant sodic plagioclase, An20, and two pyroxenes with subordinate quartz, K-feldspar and hornblende. SiO[sub 2] and MgO vary from 49% and 5% at the base to 58% and 1% at the top, respectively. The top 7 m of this layer are characterized by variably deformed minerals. The deformation grades from bent biotite and plagioclase near the bottom to sutured plagioclase at the top. Pockets of undeformed quartz and K-feldspar in the uppermost rocks demonstrate that interstitial liquid was present during a after compaction. The pipes and veins probably represent trapped liquid and some crystals that were expelled into the overlying gabbroic chill.

  2. Petrology and crystal chemistry of poikilitic anorthositic gabbro 77017. [lunar rocks

    NASA Technical Reports Server (NTRS)

    Mccallum, I. S.; Mathez, E. A.; Okamura, F. P.; Ghose, S.

    1974-01-01

    Aspects of mineralogy are considered, taking into account the occurrence and the characteristics of plagioclase, pyroxene, and olivine. Attention is also given to oxides, opaque minerals, and glass components. Questions regarding the temperature of formation and the origin of the considered lunar poikilitic rocks are discussed. It is pointed out that the presented hypothesis may not be applicable to other poikilitic lunar rocks.

  3. Silicate mineralogy at the surface of Mercury

    NASA Astrophysics Data System (ADS)

    Namur, Olivier; Charlier, Bernard

    2017-01-01

    NASA's MESSENGER spacecraft has revealed geochemical diversity across Mercury's volcanic crust. Near-infrared to ultraviolet spectra and images have provided evidence for the Fe2+-poor nature of silicate minerals, magnesium sulfide minerals in hollows and a darkening component attributed to graphite, but existing spectral data is insufficient to build a mineralogical map for the planet. Here we investigate the mineralogical variability of silicates in Mercury's crust using crystallization experiments on magmas with compositions and under reducing conditions expected for Mercury. We find a common crystallization sequence consisting of olivine, plagioclase, pyroxenes and tridymite for all magmas tested. Depending on the cooling rate, we suggest that lavas on Mercury are either fully crystallized or made of a glassy matrix with phenocrysts. Combining the experimental results with geochemical mapping, we can identify several mineralogical provinces: the Northern Volcanic Plains and Smooth Plains, dominated by plagioclase, the High-Mg province, strongly dominated by forsterite, and the Intermediate Plains, comprised of forsterite, plagioclase and enstatite. This implies a temporal evolution of the mineralogy from the oldest lavas, dominated by mafic minerals, to the youngest lavas, dominated by plagioclase, consistent with progressive shallowing and decreasing degree of mantle melting over time.

  4. Experimental results on decompression crystallization in an Aleutian basaltic-andesite

    NASA Astrophysics Data System (ADS)

    Larsen, J. F.; Gardner, J. E.

    2005-12-01

    In 1991-92, Westdahl volcano in the Aleutians erupted basaltic andesitic magma, producing lava fountaining and ash clouds to 2 km asl, resulting in a lava flow that extended about 7 km from vent. In order to understand the magmatic ascent rates during that relatively non-explosive eruption, this study compares plagioclase and orthopyroxene microlite textures in the basaltic andesite to crystallization kinetics of plagioclase and pyroxene in laboratory experiments. The experiments used melt-rich basaltic andesite that was hydrated at 150 MPa, 1000° C, and an oxygen fugacity of NNO. Before decompression, the starting material was mainly water-saturated melt that contained a few percent of Fe-Ti oxides and pargasite. Pieces of the starting material were then decompressed rapidly to 25 MPa, and held for various times. Preliminary results show that when held at low pressure for less than 1 hour no crystallization occurred, except for some growth around existing pargasite and oxides. In runs held for about an hour both enstatite and plagioclase nucleate and grow, and are skeletal. In runs held from 4 to 24 hours, plagioclase growth dominates, as enstatite remains a minor phase. The single-step decompressions will be compared with runs using multiple steps to examine how the kinetics change. We can then compare the results with observed crystallization textures in the Westdahl lava and tephra to infer the path and rate of magma ascent during that eruption.

  5. Evidence for Oxygen-Isotope Exchange in Chondrules and Refractory Inclusions During Fluid-Rock Interaction on the CV Chondrite Parent Body

    NASA Astrophysics Data System (ADS)

    Krot, A. N.; Nagashima, K.

    2016-08-01

    Plagioclase in chondrules, CAIs and AOAs from the carbonaceous chondrite Kaba (CV3.1) experienced oxygen-isotope exchange with a metasomatic fluid responsible for the formation of magnetite, fayalite and Ca,Fe-rich silicates on the CV parent body.

  6. Petrographic-mineralogical investigation of magmatic rocks from the Sea of Fertility

    NASA Technical Reports Server (NTRS)

    Tarasov, L. S.; Shevaleyevskiy, I. D.; Nazarov, M. A.

    1974-01-01

    Petrographic and mineralogical features of fragments of magmatic rock of regolith from the Sea of Fertility are examined. The textures and mineral composition vary in relation to the type of rock. More than 50 X-ray spectral analyses of minerals (olivine, pyroxenes, plagioclases, and ores) were made; their chemical composition varies even within the limits of individual rock fragments.

  7. Increasing Interaction of Alkaline Magmas with Lower Crustal Gabbroic Cumulates over the Evolution of Mt. Taylor Volcanic Field, New Mexico

    NASA Astrophysics Data System (ADS)

    Schmidt, M. E.; Crumpler, L. S.; Schrader, C.

    2010-12-01

    The Mount Taylor Volcanic Field at the southeastern edge of the Colorado Plateau, New Mexico erupted diverse alkaline magmas from ~3.8 to 1.5 Ma (Crumpler, 1980; Perry et al., 1990). The earliest eruptions include high silica topaz rhyolites of Grants Ridge (plagioclase, quartz, biotite) and Si-under saturated basanites and trachytes at Mt Taylor stratovolcano. Mt. Taylor was later constructed of stacks of thick, trachyandesitic to rhyolitic lava flows that were subsequently eroded into a ~4-km across amphitheatre opening toward the southeast. Early Mt. Taylor rhyolitic lavas exposed within the amphitheatre contain quartz, plagioclase, hornblende, and biotite (± sanidine) phenocrysts. Later cone-building trachydacite to trachyandesite lavas are crystal-rich with plagioclase and augite megacrysts (± hornblende, ± quartz) and record an overall trend of decreasing SiO2 with time. The last eruptions ~1.5 Ma from the stratovolcano (Perry et al. 1990) produced thick (>70 m), viscous lava flows that contain up to 50% zoned plagioclase phenocrysts. While SiO2 decreased among the silicic magmas, the degree of silica saturation increased among peripheral basaltic magmas from basanite to ne-normative hawaiite to hy-normative basalts. Evidence of increasing crustal contamination within the basalts includes zoned plagioclase megacrysts, augite and plagioclase cumulate texture xenoliths with accompanying xenocrysts. These textures within the basalts combined with abundant, complex plagioclase among the cone-building silicic magmas imply interaction and mixing with gabbroic cumulate mush in the lower crust beneath Mt. Taylor Volcano. Contemporaneous basanitic to trachytitc volcanism in the northern part of the volcanic field at Mesa Chivato (Crumpler, 1980) was more widely distributed, smaller volume, and produced mainly aphyric magmas. The lower crustal gabbroic cumulates either do not extend northward beneath Mesa Chivato, or they were not accessed by lower magma flux rate

  8. The origin of layered gabbros from the mid lower ocean crust, Hess Deep, East Pacific Rise

    NASA Astrophysics Data System (ADS)

    Cheadle, M. J.; Brown, T. C.; Ceuleneer, G.; Meyer, R.

    2014-12-01

    IODP Exp. 345 Holes U1415 I & J cored a ~30m thick unit of conspicuously layered gabbroic rocks from the lower plutonic crust at Hess Deep. These rocks likely come from >1500m below the dike gabbro transition and thus provide an unique opportunity to study the origin of layering and the formation of relatively deep, fast spread plutonic crust formed at the EPR. Here we report the initial results of a comprehensive high-resolution petrologic, geochemical and petrographic study of this unit, which focuses on a fairly continuous 1.5m long section recovered at Hole I. The rocks consist of opx-bearing olivine gabbro, olivine gabbro and gabbro and exhibit 1-10cm scale modal layering. Some layers host spectacular 2-3 cm diameter cpx oikocrysts encapsulating partially resorbed plagioclase laths. Downhole variations in mineral chemistry are complicated. Olivine, cpx and opx Mg#'s partly reflect equilibration and show a subtle metre-scale variation (1-2 Mg#), whereas, for example, plagioclase anorthite, and cpx TiO2 contents reveal a more complicated 10-20 cm-scale variation (2-4 An, and 0.2 TiO2). Mineral zonation, for all but Mg# in equilibrated olivine, is of higher magnitude than downhole variations in average mineral compositions. Trace element geochemistry reveals rather homogeneous plagioclase and opx compositions; however cpx exhibits variation at the mineral scale. Cpx shows an increased range of, and highest REE concentrations, in the more olivine rich, near cotectic, composition gabbros, whereas the more plagioclase rich, cumulates show no variation of, and low REE, concentrations.Plagioclase fabrics are moderate to weak and partially modally controlled, but the strength of the plagioclase crystallographic preferred orientation (CPO) varies dramatically, within the 1.5m core showing a significant part of the variation recorded by Oman ophiolite plutonic crust. Plagioclase shape preferred orientation and CPO match well suggesting that diffusion enabled compaction

  9. Gabbro layering induced by simple shear in the Oman ophiolite Moho transition zone

    NASA Astrophysics Data System (ADS)

    Jousselin, David; Morales, Luiz F. G.; Nicolle, Marie; Stephant, Aurore

    2012-05-01

    We investigate the origin of modal layering in gabbro lenses of the Moho transition zone in the Oman ophiolite with a microstructural study. Gabbro lenses exhibit a shape preferred orientation of plagioclase crystals, that are euhedral and devoid of any intracrystalline deformation. This texture and field kinematic indicators show a strong simple shear deformation in magmatic conditions. The parallelism of the lineation in gabbros and the plastic lineation of the host dunite indicate that their development is contemporaneous, and that the magmatic features are passively coupled to the solid-plastic flow of the host mantle. We also found undeformed-isotropic and weakly deformed-roughly layered gabbros. The host rock is plagioclase and clinopyroxene impregnated dunite; it contains euhedral plagioclase suggesting that some grains crystallized in suspension. There is no clear boundary between the gabbro and the host rock; on the contrary, the limit is diffuse, with increasing plagioclase and clinopyroxene content from the dunite to the gabbro over tens of meters. This suggests that the gabbro corresponds to melt impregnated dunite. We defined 4 gabbro types, from unlayered (type 1) to well layered (type 4) with progressively more continuous and distinct layers. We characterized deformation with crystal shape and crystallographic preferred orientation (CPO) measurements. Unlayered samples have a random fabric; type 2 have a very weak shape fabric and planar CPO, defined by olivine and plagioclase (010) planes; type 3 have a weak fabric, with a lineation defined by the [001] olivine axis and the [100] plagioclase axis, which reflects the shape fabric; type 4 have a strong shape fabric and CPO, with a lineation defined by the [100] olivine and plagioclase axis. We interpret the progression from isotropic to clear S-L fabrics as a result of increasing deformation imposed by the flowing host mantle. The type 2 gabbros must result from compaction; with increasing simple

  10. Fluid induced metamorphism and strength of the middle to lower continental crust - field and textural examples from Bergen Arcs, Western Norway

    NASA Astrophysics Data System (ADS)

    Austrheim, H.; Putnis, A.; Putnis, C. V.

    2011-12-01

    Fluids may change the rheology of the lithosphere both by changing the deformation mechanism of minerals and by inducing metamorphic reactions. In the present account the influence of fluid-induced metamorphic reactions on the mineralogical and structural evolution of a thickened continental crust is described from anorthositic granulites in the Lindås Nappe, Bergen Arcs, Norway, where the Grenvillian age (~930 My) granulites (T 800°C, P≤10kbar) are transformed to Caledonian age (~420My) eclogite (~650°C and ≤ 20kbar) and amphibolite facies assemblages. The anorthosite complex ranges in composition from pure anorthosite via gabbroic anorthosite to gabbro with lenses of peridotite and pyroxenites which allow us to study the mineral reactions and assess relative rock strength in a wide range of compositions. The complex is locally banded with up to meter thick garnet-pyroxene rich layers alternating with plagioclase rich layers. In other localities the granulite facies structure is defined by oriented disc-shaped corona textures in a plagioclase rich matrix. The eclogites (garnet, omphacite, amphibole, kyanite, white micas ± plagioclase) and amphibolites (plagioclase, hornblende, kyanite, and white micas) are formed along fluid pathways such as fractures and shear zones. Breccias, where rotated blocks of granulites are surrounded by anastamosing eclogite- and amphibolite facies shear zones, outcrop over areas of km2. Pseudotachylytes are developed in the granulites while the hydrated rocks in the shear zone respond by ductile deformation. A hierachy of rock strength can be inferred from these field observations. Notably the relict granulites form rotated angular blocks within the shear zones suggesting that granulites, independent of composition, are stronger than hydrous eclogitites and amphibolites. The garnet pyroxenite layer forms rigid blocks in eclogites suggesting that the mafic parts of the granulite complex must have been stronger than the

  11. Albedo and Christiansen Feature Relationships among Remotely Sensed Extreme Lunar Compositions: Olivine and Silica Rich Locations

    NASA Astrophysics Data System (ADS)

    Song, E.; Lucey, P. G.; Ohtake, M.; Greenhagen, B. T.; Glotch, T. D.

    2012-12-01

    Data from the Diviner Lunar Radiometer experiment is sensitive to the ratio of mafic minerals and plagioclase, as well as the presence of significant quantities of silicic mineral such as alkali feldspar or quartz through the position of the Christiansen Feature (CF) that is sensitive to these minerals [1]. Recently [2] reported widespread occurrences of abundant olivine, and suggested that these locations were plagioclase poor, and hence may expose portions of the lunar mantle. We have extracted CF positions from several of the locations of [2] and find that some of them have extreme CF positions indicating high olivine abundances and low plagioclase abundances. The CF is also sensitive to space weathering, where maturity moves the CF position to longer wavelengths, complicating the compositional interpretation. To mitigate this we also examine the 750nm albedo of the locations and compare these to several types of controls. Albedo is derived from a 500-m resolution mosaic of Kaguya Multiband Imager data that has been corrected for topographic shading to a uniform unit of radiance factor illuminated at 30 degrees. We find that the Apollo landing sites form a linear trend when CF is plotted against albedo, consistent with their relative iron contents and plagioclase contents. However, the olivine-rich locations often fall very significantly off this trend, with only a minority falling on the Apollo trend defined principally by pyroxene-plagioclase ratio. The locations with the most extreme CF values form a parallel trend that is consistent with olivine-plagioclase variation. Using CF values for pure minerals from [3], the most extreme values of CF correspond to an olivine content of 80%, close to the dunite composition reported by [2]. Silica rich regions from [4] also fall well off the trend, toward low CF values. This is consistent with the presence of silica-rich minerals, and the only moderate albedo of these locations (between the albedos of Apollo 14 and 16

  12. Eruption styles of Quaternary basalt in the southern Sierra Nevada Kern Plateau recorded in outcrop and mineral-scale stratigraphies

    NASA Astrophysics Data System (ADS)

    Browne, B. L.; Becerra, R. A.

    2015-12-01

    The Kern River Plateau in the southern Sierra Nevada contains Quaternary basalt (~0.1 km3) and rhyolite (~2 km3) that ascended through ~30 km of Mesozoic granitic crust. Basaltic vents include from oldest to youngest: Little Whitney Cone, Tunnel and South Fork Cones, and unglaciated Groundhog Cone. Little Whitney Cone is a 120-m-high pile of olivine-CPX-phyric scoria overlying two columnar jointed lava flows extending to the south and east. Tunnel Cone formed through a Hawaiian style eruption along a 400-m-long north-south trending fissure that excavated at least three 25-65-m-wide craters. Crater walls up to 12 meters high are composed of plagioclase-olivine-phyric spatter-fed flows that dip radially away from the crater center and crumble to form steep unconsolidated flanks. South Fork Cone is a 170-m-tall pile of plagioclase-olivine-phyric scoria that formed as a result of Strombolian to violent Strombolian eruptions. It overlies the South Fork Cone lava, the largest lava flow of the Kern Plateau (~0.05 km3), which flowed 7.5 km west into the Kern River Canyon. Scoria and ash fall deposits originating from South Fork Cone are found up to 2 km from the vent. Groundhog Cone is a 140-m-tall cinder and spatter cone breached on the north flank by a 0.03 km3 lava flow that partially buried the South Fork Cone lava and extends 5 km west to Kern River Canyon. Trends in mineral assemblage, texture, composition, and xenocryst abundance exist as a function of eruption style. Scoria and spatter deposits typically have (1) elevated olivine/plagioclase ratios, (2) oscillatory zoned (An63-An72) plagioclase phenocrysts surrounded by unzoned rims and (3) abundant xenocrysts, where up to 20% of plagioclase >200 micron diameter in some samples are granitoid xenocrysts with resorbed and/or reacted textures overprinted by abrupt compositional changes. In contrast, lava flow samples have (1) reduced olivine/plagioclase ratios and (2) plagioclase aggregates with oscillatory zoned

  13. Intermittent generation of mafic enclaves in the 1991-1995 dacite of Unzen Volcano recorded in mineral chemistry

    NASA Astrophysics Data System (ADS)

    Sato, Hiroaki; Holtz, Francois; Botcharnikov, Roman E.; Nakada, Setsuya

    2017-04-01

    Mafic enclaves in the 1991-1995 dacite of Unzen volcano show chemical and textural variability, such as bulk SiO2 contents ranging from 52 to 62 wt% and fine- to coarse-grained microlite textures. In this paper, we investigated the mineral chemistry of plagioclase and hornblende microlites and distinguished three enclave types. Type-I mafic enclaves contain high-Mg plagioclase and low-Cl hornblende as microlites, whereas type-III enclaves include low-Mg plagioclase and high-Cl hornblende. Type-II enclaves have an intermediate mineral chemistry. Type-I mafic enclaves tend to show a finer-grained matrix, have slightly higher bulk rock SiO2 contents (56-60 wt%) when compared with the type-III mafic enclaves (SiO2 = 53-59 wt%), but the overall bulk enclave compositions are within the trend of the basalt-dacite eruptive products of Quaternary monogenetic volcanoes around Unzen volcano. The origin of the variation of mineral chemistry in mafic enclaves is interpreted to reflect different degree of diffusion-controlled re-equilibration of minerals in a low-temperature mushy dacitic magma reservoir. Mafic enclaves with a long residence time in the dacitic magma reservoir, whose constituent minerals were annealed at low-temperature to be in equililbrium with the rhyolitic melt, represent type-III enclaves. In contrast, type-I mafic enclaves result from recent mafic injections with a mineral assemblage that still retains the high-temperature mineral chemistry. Taking temperature, Ca/(Ca + Na) ratio of plagioclase, and water activity of the hydrous Unzen magma into account, the Mg contents of plagioclase indicate that plagioclase microlites in type-III enclaves initially crystallized at high temperature and were subsequently re-equilibrated at low-temperature conditions. Compositional profiles of Mg in plagioclase suggest that older mafic enclaves (Type-III) had a residence time of 100 years at 800 °C in a stagnant magma reservoir before their incorporation into the mixed

  14. Disequilibrium texture and compositional relation of Ca-amphiboles in metagabbro near Tsundupalle Greenstone Belt: implications for fluid aided reactions

    NASA Astrophysics Data System (ADS)

    Hazra, Manoshi; Nandy, Jinia; Dutta, Upama

    2017-04-01

    Metamorphosed gabbro from Tsundupalle greenstone belt, Eastern Dharwar Craton contain clinopyroxene (Cpx) and plagioclase (Pl) as primary minerals. In most cases, clinopyroxene in the metagabro is pseudomorphed by amphibole, rare cases relict clinopyroxene occurs within its core. Plagioclase is highly altered and contains inclusions of secondary minerals in it. A blue green amphibole is found rimming the pseudomorph and plagioclase contact. EPMA analysis confirms three broad compositional varieties for amphibole, namely actinolite (Act), ferrohornblende (F.Hbl) and tschermakite (Tsch). Microscopic observation combined with BSE images reveal the mutual relation between the amphiboles precisely. Pseudomorph of clinopyroxene is dominated by randomly oriented actinolite grains whereas patches of ferrohornblende are found replacing actinolite and clinopyroxene (when present). Tschermakite is found in contact with plagioclase. It forms a rim at the boundary between plagioclase and actinolite/relict clinopyroxene. Large ferrohornblende and tschermakite grains are also found in these rocks where the latter amphibole is seen replacing the former. This feature along with replacement of actinolite by ferrohornblende, tschermakite (near plagioclase) indicate that actinolite was the first amphibole to form, followed by ferrohornblende. Tschermakite appeared at the last stage of amphibolitization. Replacement/reaction textures between amphiboles clearly represent disequilibrium relation. The different calcic amphiboles in the rock formed as a result of thermodynamic incompatibility of earlier compositions rather than miscibility gap. Depending on the mutual relations between the different phases following reactions can be drawn: a) Cpx+H2O=Act; 2) Cpx/Act±H2O=F.Hbl; 3) Cpx/Act+Pl±H2O=Tsch. Formation of amphibole from pyroxene clearly illustrates an open system. Graphical representations indicate coexisting actinolite and hornblende conversion mainly involved edenitic and

  15. The magmatic and thermal history of the Dufek Complex, Antarctica

    NASA Astrophysics Data System (ADS)

    Carnes, J. D.; Cheadle, M. J.; Gee, J. S.; Grimes, C. B.; Swapp, S. M.

    2011-12-01

    The Jurassic (~180Ma) Dufek Complex in the Pensacola Mountains of Antarctica is arguably one of the largest layered mafic intrusions in the world, with a minimum areal extent of 6600km2. However the mechanisms by which it grew are unclear. Gradually varying, decreasing upward trends in plagioclase and pyroxene mineral compositions suggest that it grew by a few large (kilometers thick) injections of magma, but it's hard to understand how these thick "vats" of magma are rheologically stable in the continental crust. Alternatively, it could have grown by repeated replenishments of small volumes of magma, the evidence for which is somehow hidden in the broad scale mineral composition trends. This research documents the thermal and magmatic history of a well-sampled 104m representative section of the Aughenbaugh Gabbro. To test whether magma replenishment is a major factor in constructing this intrusion, we have systematically documented changes in lithology, mineral compositions, and texture across a series of 10-70 cm thick, xenolith bearing, sharp based, modally graded pyroxene-rich layers, which might represent magmatic replenishment events. The section consists of gabbronorites and norites with 19-84 vol.% plagioclase and 14-63 vol.% inverted pigeonite (IP). Clinopyroxene (cpx) is intercumulus and varies from 0-18 vol.%. Modal layering is rare. The rocks have a relatively fine grain size of 0.5-2.25 mm and generally exhibit poorly equilibrated textures. Plagioclase compositions range from An62.0-An66.0 ± 0.3 in grain cores, and An60.9-An65.6 ± 0.3 in grain rims. The An content of the cumulus plagioclase decreases with increasing plagioclase modal abundance. The Mg# of IP ranges from 56.9-62.6 ± 0.3 and shows no zonation, as it has re-equilibrated. The Mg# of cpx varies from 64.9-66.4 ± 0.3. The Mg# of both the IP and the cpx increases in the pyroxene-rich layers. True dihedral angles were measured using a universal stage. The mean dihedral angle of plagioclase-plagioclase

  16. Textural analysis of obsidian lava flow in Shirataki, Northern Hokkaido, Japan

    NASA Astrophysics Data System (ADS)

    Sano, K.; Toramaru, A.; Wada, K.

    2013-12-01

    Formation process of obsidian is poorly understood and it is thought that gas loss (outgassing) plays an important role. Glass formation needs the high-effective undercooling resulted from a high ascent and decompression rates, which process increases magma viscosity. The vesiculation, crystallization, and outgassing processes of such a highly viscous magma is also unclear. In this study, we conducted textural and chemical analyses for Tokachi-Ishizawa (TI) obsidian lava one of Shirataki rhyolite lava, Hokkaido, northern part of Japan, in order to elucidate the magma ascent process. At TI lava, the interior structure of the lava can be observed, right from the outer obsidian layer to the inner rhyolite layer. That is, TI lava is an appropriate subject for textural analysis focused on the interior of obsidian lavas In Shirataki rhyolite lava area there are monogenetic volcanoes composed of 10 obsidian lava flow units, which were erupted at 2.2Ma. The TI lava is about 50 m in height and stratigraphic sequence from the bottom is a brecciated perlite layer, obsidian layer (7m), banded obsidian layer, and rhyolite layer. In this study, we define the obsidian and rhyolite based on the difference in appearance of specimen and rock texture, especially crystallinity. Rhyolite has perlitic cracks on glass, and contains the crystalline materials (i.e. spherulite and lithophysae). Banded obsidian layer, which is located between the obsidian and rhyolite layer, is composed of obsidian and rhyolite. In this study, we focused on the texture of flow bands and plagioclase microlites in glassy part of obsidian and rhyolite layers. The flow bands can be identified based on the color of glass (dark and clear), and have a contrast in abundance of oxide and transparent tiny crystals, which are plagioclase nanolites (<15μm) and micro-spherulites (<20μm). We newly defined micro-spherulite, which shows radial growth of crystals like a spherulite. The plagioclase nanolites were identified

  17. Mineralogy of new Antarctic achondrites with affinity to Lodran and a model of their evolution in an asteroid

    NASA Technical Reports Server (NTRS)

    Takeda, Hiroshi; Mori, Hiroshi; Hiroi, Takahiro; Saito, Jun

    1994-01-01

    We studied five new Antartic achondrites, MacAlpine Hills (MAC) 88177, Yamato (Y)74357, Y75274, Y791491 and Elephant Moraine (EET)84302 by mineralogical techniques to gain a better understanding of the mineral assemblages of a group of meteorites with an affinity to Lodran (stony-iron meteorite) and their formation processes. This group is being called lodranites. These meteorites contain major coarse-grained orthopyroxene (Opx) and olivine as in Lodran and variable amounts of FeNi metal and troilite etc. MAC88177 has more augite and less FeNi than Lodran; Y74357 has more olivine and contains minor augite; Y791491 contains in addition plagioclase. EET84302 has an Acapulco-like chondritic mineral assembladge and is enriched in FeNi metal and plagioclase, but one part is enriched in Opx and chromite. The EET84302 and MAC88177 Opx crystals have dusty cores as in Acapulco. EET84302 and Y75274 are more Mg-rich than other members of the lodranite group, and Y74357 is intermediate. Since these meteorites all have coarse-grained textures, similar major mineral assemblages, variable amounts of augite, plagioclase, FeNi metal, chromite and olivine, we suggest that they are related and are linked to a parent body with modified chondritic compositions. The variability of the abundances of these minerals are in line with a proposed model of the surface mineral assemblages of the S asteroids. The mineral assemblages can best be explained by differing degrees of loss or movements of lower temperature partial melts and recrystallization, and reduction. A portion of EET84302 rich in metal and plagioclase may represent a type of component removed from the lodranite group meteorites. Y791058 and Caddo County, which were studied for comparison, are plagioclase-rich silicate inclusions in IAB iron meteorites and may have been derived by similar process but in a different body.

  18. Feldspar megacrysts from the Santa Angélica composite pluton - Formation/transformation path revealed by combined CL, Raman and LA-ICP-MS data

    NASA Astrophysics Data System (ADS)

    Słaby, Ewa; De Campos, Cristina P.; Majzner, Katarzyna; Simon, Klaus; Gros, Katarzyna; Moszumańska, Izabela; Jokubauskas, Petras

    2017-04-01

    The studied feldspar megacrysts from the Santa Angélica hybrid rock unit feature complex growth morphologies and geochemical compositions. Early crystallization formed zoned K-Na alkali-feldspar and unzoned oligoclase-andesine. The chemical composition of the zoned alkali-feldspar reflects crystallization in contact with different magma batches, and the chemical composition of the plagioclase indicates growth from a homogeneous magma. Further feldspar development produced mineral chemistry and growth morphology patterns (in both alkali feldspar and plagioclase) that indicate a multi-stage process caused by the combination of chaotic mixing and replenishment. Alkali feldspar and plagioclase megacrysts show dissolution and regrowth textures. Their compositional fields are not separated. Different crystallized and recrystallized domains pass smoothly from one to another. A multiple reequilibration process limited the ordering of the feldspar domain structure. A younger generation of feldspar strongly enriched in K and Na grew over the dissolved margins of the alkali feldspar and plagioclase megacrysts, filling all embayments. Some domains of the alkali feldspar megacrysts recrystallized, resulting in a K- and Ba-rich composition. The trace element patterns of these domains define relatively consistent trends, which may indicate a new stage of equilibrium. In turn, the trace element patterns of previously crystallized and recrystallized domains are irregular, pointing to blurring of the original compositional relationship. Some voids in the megacrysts, formed due to dissolution, are cemented with quartz. The sequential early to late crystallization, recrystallization, and alteration processes are clearly revealed by Polytopic Vector Analysis (PVA). This analysis shows the changes in the alkali-feldspar and plagioclase compositions in two different directions: an initial shift from megacryst crystallization environment towards alkali-rich magma and a second shift

  19. Europium mass balance in polymict samples and implications for plutonic rocks of the lunar crust

    SciTech Connect

    Korotev, R.L.; Haskin, L.A. )

    1988-07-01

    From correlations of SM concentration and Sm/Eu ratio with Th concentration for a large number of polymict samples from various locations in the lunar highlands and the value of 0.91 {mu}g/g for the mean Th concentration of the highlands surface crust obtained by the orbiting gamma-ray experiments. The authors estimate the mean concentrations of Sm and Eu in the lunar surface crust to be between 2 and 3 {mu}g/g Sm and 0.7 and 1.2 {mu}g/g Eu. The compositional trends indicate that there is no significant enrichment or depletion of Eu, on the average, compared to Sm relative to chondritic abundances, i.e., there is no significant Eu anomaly in average upper crust. Although rich in plagioclase ({approximately}70%), the upper crust does not offer evidence for a gross vertical separation of plagioclase from the final liquid from which it crystallized. This and the chondritic ratio of Eu/Al in average highlands material imply that the net effect of the processes that led to formation of the lunar crust was to put most of the Al and incompatible elements in the crust. Among plutonic rocks, only plagioclase in rocks from the magnesian suite can supply the excess Eu in the polymict rocks. Owing to the intermediate value of the mean Mg/Fe ratio of the crust, a significant fraction of the mafic rocks of the lunar highlands must have lower Mg/Fe ratios than the norites and troctolites of the magnesian-suite of plutonic rocks. A large fraction of the plagioclase in the lunar crust is associated not with ferroan anorthosite, but with more mafic rocks. There is little evidence in the Eu data that the lunar crust ever consisted of a thick shell of nearly pure plagioclase, as envisioned in some formulations of the magma ocean model of its formation.

  20. Evidence for a lower crustal origin of high-Al orthopyroxene megacrysts in Proterozoic anorthosites

    SciTech Connect

    Wiebe, R.A.

    1985-01-01

    Nodules and xenocrysts dominated by high-Al orthopyroxene (HAO) occur in strongly chilled Proterozoic basaltic dikes which cut the Nain anorthosite complex, Labrador. HAO (En 73-68, Al/sub 2/O/sub 3/ = 6.5-4.5) lacks exsolution; it occurs both as anhedral xenocrysts up to 10 cm in diameter and with euhedral plagioclase (An55) in ophitic nodules. Rarely, olivine occurs with HAO and Al-spinel with plagioclase. Scarce Fe-rich nodules contain: (1) opx + pig, (2) aug + pig, and (3) coarsely exsolved ulvospinel. Pyroxene pairs yield T's of 1250 to 1170/degree/C, whereas coexisting lamellae in exsolved ulvospinel yield T's between 1145 and 1120/degree/C, with fO/sub 2/ near the WM buffer. If all nodules came from a similar depth, the rare occurrence of olivine with plagioclase suggests a maximum pressure of about 11 kb. The high subsolidus T's of the nodules contrasts with the low T of the host anorthosites at the time of dike emplacement and hence indicates a deep source for the nodules. HAO is nearly identical in composition to the high-Al orthopyroxene megacrysts with exsolved plagioclase (HAOM) found in most Proterozoic anorthosites. Many nodules of plagioclase and HAO also have textures comparable to ophitic occurrences of HAOM in anorthosite. Rafting of cotectic nodules from the lower crust could explain occurrences of HAOM in shallow-level anorthosites. The nodules and xenocrysts are samples of lower crustal cumulates. Their compositions suggest that they were produced by magmas similar to those that were parental to the anorthosites. They lend support to models which derive anorthosites by fractional crystallization of basaltic magma.

  1. Occurrence of chromian, hercynitic spinel ("pleonaste") in Apollo-14 samples and its petrologic implications

    USGS Publications Warehouse

    Roedder, E.; Weiblen, P.W.

    1972-01-01

    Many isolated grains of a reddish pleonaste-type spinel occur in fines and metabreccia samples, particularly 14 319. Electron microprobe analyses (104) of spinels and their associated phases include 58 of pleonaste which show Mg/(Mg + Fe) 0.44-0.62 and Cr/(Cr + Al) 0.017-0.134 (atomic), plus minor amounts of other ions, and differ greatly from almost all previously recorded lunar spinels; almost no spinels of intermediate composition were found. Two types of compositional zoning exist: a diffuse primary one with cores lower in Ti, and a narrow secondary one from reaction with matrix yielding rims higher in Cr, Ti, and Mn. At contacts with breccia matrix there is a narrow corona of almost pure plagioclase (An80-An94), free of opaque minerals and pyroxene. Two types of solid inclusions found in the pleonaste are calcic plagioclase, and tiny spherical masses of nickel-rich sulfide. Similar pleonaste occurs in crystalline rock clasts, mainly with plagioclase; one clast (A) consists only of coarse olivine, plagioclase, and pleonaste, with granulated grain boundaries suggestive of deformation. From composition and texture, this clast is one possible candidate for the mafic cumulate counterpart of the "anorthositic" crust. Another clast (B), also made solely of olivine, plagioclase and pleonaste, is itself a breccia. These data suggest a two-stage brecciation process: 1) disruption (probably pre-Imbrian) of a deep-seated pleonaste-bearing source rock like A and reconsolidation to form a breccia without addition of pyroxene, ilmenite or other minerals; and 2) disruption of this breccia to yield breccia clast B which was then incorporated into the Fra Mauro formation. ?? 1972.

  2. The Sr-isotopic stratigraphy of the Northern Limb of the Bushveld Complex, South Africa

    NASA Astrophysics Data System (ADS)

    Mangwegape, Mpho; Roelofse, Frederick; Mock, Timothy; Carlson, Richard W.

    2016-01-01

    We present a compilation of Sr-isotopic data obtained on plagioclase from 27 samples covering the entire stratigraphy of the Northern Limb of the Bushveld Complex as determined by LA-MC-ICPMS. The lower parts of the Main Zone in the Northern Limb are characterised by significant variations in Sr-isotopic compositions (87Sr/86Sr ˜0.7087 ± 0.0005 (1-σ)) coupled with very limited differentiation as exemplified by plagioclase An%, suggesting construction of the lower Main Zone through the repeated influx of magmas. Fairly constant Sr-isotopic compositions of plagioclase within the Upper and upper Main Zones (87Sr/86Sr ˜0.7073 ± 0.0003) coupled with a broad normal differentiation trend are suggestive of fractionation processes being the dominant factor in the petrogenesis of these zones. The present results argue against the laterally extensive troctolite horizon of the Northern Limb having a Critical Zone parentage or being the direct equivalent of the Pyroxenite Marker in the Northern Limb. It may, however, be an analogue of the Pyroxenite Marker in that it occurs at the approximate level where the last voluminuous influx of magma into the Northern Limb is inferred to have taken place. The nature of the magmas that gave rise to the lower parts of the Main Zone in the Northern Limb deserves further investigation in light of the fact that plagioclase here shows considerable variation both within and between individual co-existing plagioclase crystals that may point to the intrusion of crystal mushes as opposed to aphyric liquids.

  3. Experimental petrology constraints on the recycling of mafic cumulate: a focus on Cr-spinel from the Rum Eastern Layered Intrusion, Scotland

    NASA Astrophysics Data System (ADS)

    Leuthold, Julien; Blundy, Jonathan D.; Brooker, Richard A.

    2015-08-01

    Reactive liquid flow is a common process in layered intrusions and more generally in episodically refilled magma chambers. Interaction between newly injected melt and cumulates, or crystal mushes, perturbs the liquid line of descent of the melt and modifies mineral chemistry and texture. We present insights into the effects of assimilation of mafic cumulate rocks (gabbro, troctolite) by cogenetic Mg-rich basalt liquid using one-atmosphere, controlled fO2 phase equilibrium experiments on picritic parental liquid to the Rum layered intrusion, Scotland. For picrite-only experiments at fO2 = QFM, Cr-spinel (Cr# = Cr/[Cr + Al + Fe3+] = 0.43; Fe# = Fe2+/[Mg + Fe2+] = 0.32) saturates at 1320 °C, olivine (Fo88) at ~1290 °C, plagioclase (An77) at 1200 °C, and clinopyroxene (Mg#: 0.81) at 1180 °C. In melting experiments on picrite + gabbro mixtures, plagioclase (1230 °C, An80) and clinopyroxene (1200 °C, Mg#: 0.85) saturation temperature and mode are increased significantly. Cr-spinel in these experiments has a distinctive, low Fe#. In melting experiments on picrite + troctolite mixtures, plagioclase (An86) saturates at 1240 °C and clinopyroxene (Mg#: 0.81) at 1170 °C. Al-rich spinel crystallizes at high temperature (>1220 °C) and becomes more Cr-rich upon cooling, reaching the highest Cr# = 0.47 at 1180 °C (0.54 at QFM-1.2). The experimental results confirm that plagioclase and clinopyroxene stability plays a major role in determining the composition of coexisting spinel. Comparing our experimental results to the Rum Eastern Layered Intrusion, we propose a model for the precipitation of spinel from picrite-troctolite hybrid melt that is compatible with the observed olivine, plagioclase, and clinopyroxene chemistry.

  4. Interaction of chemical and physical processes during deformation at fluid-present conditions: a case study from an anorthosite-leucogabbro deformed at amphibolite facies conditions

    NASA Astrophysics Data System (ADS)

    Svahnberg, Henrik; Piazolo, Sandra

    2013-03-01

    We present microstructural and chemical analyses of chemically zoned and recrystallized plagioclase grains in variably strained samples of a naturally deformed anorthosite-leucogabbro, southern West Greenland. The recorded microstructures formed in the presence of fluids at mid-crustal conditions (620-640 °C, 7.4-8.6 kbar). Recrystallized plagioclase grains (average grain size 342 μm) with a random crystallographic orientation are volumetrically dominant in high-strain areas. They are characterized by asymmetric chemical zoning (An80 cores and An64 rims) that are directly associated with areas exhibiting high amphibole content and phase mixing. Analyses of zoning indicate anisotropic behaviour of bytownite plagioclase with a preferred replacement in the < {0 10} rangle direction and along the (001) plane. In areas of high finite strain, recrystallization of plagioclase dominantly occurred by bulging recrystallization and is intimately linked to the chemical zoning. The lack of CPO as well as the developed asymmetric zoning can be explained by the activity of grain boundary sliding accommodated by dissolution and precipitation creep (DPC). In low-strain domains, grain size is on average larger and the rim distribution is not related to the inferred stress axes indicating chemically induced grain replacement instead of stress-related DPC. We suggest that during deformation, in high-strain areas, pre-existing phase mixture and stress induced DPC-caused grain rotations that allowed a deformation-enhanced heterogeneous fluid influx. This resulted in local plagioclase replacement through interface-coupled dissolution and precipitation and chemically induced grain boundary migration, accompanied by bulging recrystallization, along with neocrystallization of other phases. This study illustrates a strong interaction and feedback between physical and chemical processes where the amount of stress and fluids dictates the dominant active process. The interaction is a cause of

  5. Mineralogy of new Antarctic achondrites with affinity to Lodran and a model of their evolution in an asteroid

    NASA Technical Reports Server (NTRS)

    Takeda, Hiroshi; Mori, Hiroshi; Hiroi, Takahiro; Saito, Jun

    1994-01-01

    We studied five new Antartic achondrites, MacAlpine Hills (MAC) 88177, Yamato (Y)74357, Y75274, Y791491 and Elephant Moraine (EET)84302 by mineralogical techniques to gain a better understanding of the mineral assemblages of a group of meteorites with an affinity to Lodran (stony-iron meteorite) and their formation processes. This group is being called lodranites. These meteorites contain major coarse-grained orthopyroxene (Opx) and olivine as in Lodran and variable amounts of FeNi metal and troilite etc. MAC88177 has more augite and less FeNi than Lodran; Y74357 has more olivine and contains minor augite; Y791491 contains in addition plagioclase. EET84302 has an Acapulco-like chondritic mineral assembladge and is enriched in FeNi metal and plagioclase, but one part is enriched in Opx and chromite. The EET84302 and MAC88177 Opx crystals have dusty cores as in Acapulco. EET84302 and Y75274 are more Mg-rich than other members of the lodranite group, and Y74357 is intermediate. Since these meteorites all have coarse-grained textures, similar major mineral assemblages, variable amounts of augite, plagioclase, FeNi metal, chromite and olivine, we suggest that they are related and are linked to a parent body with modified chondritic compositions. The variability of the abundances of these minerals are in line with a proposed model of the surface mineral assemblages of the S asteroids. The mineral assemblages can best be explained by differing degrees of loss or movements of lower temperature partial melts and recrystallization, and reduction. A portion of EET84302 rich in metal and plagioclase may represent a type of component removed from the lodranite group meteorites. Y791058 and Caddo County, which were studied for comparison, are plagioclase-rich silicate inclusions in IAB iron meteorites and may have been derived by similar process but in a different body.

  6. Mineralogy of new Antarctic achondrites with affinity to Lodran and a model of their evolution in an asteroid

    NASA Astrophysics Data System (ADS)

    Takeda, Hiroshi; Mori, Hiroshi; Hiroi, Takahiro; Saito, Jun

    1994-11-01

    We studied five new Antartic achondrites, MacAlpine Hills (MAC) 88177, Yamato (Y)74357, Y75274, Y791491 and Elephant Moraine (EET)84302 by mineralogical techniques to gain a better understanding of the mineral assemblages of a group of meteorites with an affinity to Lodran (stony-iron meteorite) and their formation processes. This group is being called lodranites. These meteorites contain major coarse-grained orthopyroxene (Opx) and olivine as in Lodran and variable amounts of FeNi metal and troilite etc. MAC88177 has more augite and less FeNi than Lodran; Y74357 has more olivine and contains minor augite; Y791491 contains in addition plagioclase. EET84302 has an Acapulco-like chondritic mineral assemblage and is enriched in FeNi metal and plagioclase, but one part is enriched in Opx and chromite. The EET84302 and MAC88177 Opx crystals have dusty cores as in Acapulco. EET84302 and Y75274 are more Mg-rich than other members of the lodranite group, and Y74357 is intermediate. Since these meteorites all have coarse-grained textures, similar major mineral assemblages, variable amounts of augite, plagioclase, FeNi metal, chromite and olivine, we suggest that they are related and are linked to a parent body with modified chondritic compositions. The variability of the abundances of these minerals are in line with a proposed model of the surface mineral assemblages of the S asteroids. The mineral assemblages can best be explained by differing degrees of loss or movements of lower temperature partial melts and recrystallization, and reduction. A portion of EET84302 rich in metal and plagioclase may represent a type of component removed from the lodranite group meteorites. Y791058 and Caddo County, which were studied for comparison, are plagioclase-rich silicate inclusions in IAB iron meteorites and may have been derived by similar process but in a different body.

  7. A first find of retrogressed eclogites in the Odenwald Crystalline Complex, Mid-German Crystalline Rise, Germany: evidence for a so far unrecognised high-pressure metamorphism in the Central Variscides

    NASA Astrophysics Data System (ADS)

    Will, Thomas M.; Schmädicke, Esther

    2001-11-01

    Metabasic rocks were recently found in the Böllsteiner Odenwald, being part of the Variscan Mid-German Crystalline Rise (MGCR), that give evidence of a so far unrecognised eclogite-facies metamorphic event and testify, for the first time, to high-pressure metamorphism in the MGCR, the assumed suture zone of the European Variscides. Eclogite-facies metamorphism is indicated by both widespread clinopyroxene-plagioclase symplectites—interpreted as breakdown products of omphacite—and the composition of symplectitic clinopyroxene with measured jadeite contents of up to 27 mol%, extending into the omphacite field. Reintegration of numerous clinopyroxene-plagioclase symplectites implies minimum jadeite contents of the former omphacite of at least 38 mol%. For the eclogite stage, the four-phase assemblage omphacite-garnet-quartz-rutile can be reconstructed. A post-eclogitic overprint led to the formation of symplectitic intergrowths of clinopyroxene and plagioclase, amphibole-plagioclase coronas around garnet and domains with recrystallised amphibole and plagioclase. Preliminary P- T estimates for the eclogite-facies metamorphism indicate minimum pressures of some 16-17 kbar and temperatures of approximately 700±50 °C. Geothermobarometry for the subsequent symplectitic breakdown of omphacite yields some 14 kbar and 700 °C. P- T estimates on retrograde amphibolite-facies domains and on prograde mineral assemblages preserved in garnet cores point to a clockwise P- T path experienced by these rocks. The eclogites formed from a tholeiitic protolith, that may have been genetically linked to a continental extension zone or a young oceanic ridge or back-arc environment.

  8. A generalized garnet-forming reaction for metaigneous rocks in the Adirondacks

    USGS Publications Warehouse

    McLelland, J.M.; Whitney, P.R.

    1980-01-01

    A generalized reaction is presented to account for garnet formation in a variety of Adirondack metaigneous rocks. This reaction, which is the sum of five partial reactions written in aluminum-fixed frames of reference, is given by: 4(y+1+w)Anorthite+4 k(y+1+2 w)Olivine +4(1-k)(y+1+2 w)Fe-oxide+(8(y+1) -4 k(y+1+2 w))Orthopyroxene = 2(y+1)Garnet +2(y+1+2 w)Clinopyroxene+4 wSpinel where y is a function of plagioclase composition, k refers to the relative amounts of olivine and Fe-oxide participating in the reaction, and w is a measure of silicon mobility. When mass balanced for Mg and Fe, this reaction is found to be consistent with analyzed mineral compositions in a wide range of Adirondack metaigneous rocks. The reaction applies equally well whether the garnets were formed directly from the rectants given above or went through an intermadiate stage involving the formation of spinel, orthopyroxene, and clinopyroxene. The actual reactions which have produced garnet in both undersaturated and quartz-bearing rocks are special cases of the above general reaction. The most important special cases appear to be those in which the reactants include either olivine alone (k=1) or Fe-oxide alone (k=0). Silicon is relatively immobile (w =2) in olivine bearing, magnesium-rich rocks (k???1), and this correlates with the increased intensity in spinel clouding of plagioclase in these rocks. Silicon mobility apparently increases in the more iron-rich rocks, which also tend to contain clear or lightly clouded plagioclase. In all the rocks studied the most common composition of metamorphic plagioclase is close to An33 (i.e., y=1). Plagioclase of lower anorthite content may be too sodic to participate in garnet formation at the P-T conditions involved. ?? 1980 Springer-Verlag.

  9. Granulite facies lower crustal xenoliths from the Eifel, West Germany: petrological and geochemical aspects

    NASA Astrophysics Data System (ADS)

    Loock, G.; Stosch, H.-G.; Seck, H. A.

    1990-06-01

    Petrographic, petrological and geochemical data for 16 mafic meta-igneous, granulite facies lower crustal xenoliths from the East Eifel were collected in order to develop a model for the lower crustal history for this region. The xenoliths consist of plagioclase±amphibole±clinopyroxene±garnet±orthopyroxene±scapolite + opaque minerals±apatite±rutile±zircon. Garnet has reacted to a variable extent with plagioclase and clinopyroxene to form a corona of plagioclaseII+ amphibole + orthopyroxeneII. Pyroxenes and plagioclases show complex zoning patterns with regard to Al and Ca which can be interpreted in terms of P, T history. Decreasing temperature and pressure conditions are recorded by decreasing Al in clinopyroxene rims coexisting with increasing anorthite contents in plagioclase rims and the breakdown of garnet. In addition, a young heating event that affected the granulites to different degrees is inferred from the complementary Ca-zoning patterns in clino- and orthopyroxenes. Rare earth element (REE) patterns of whole rocks together with the trends displayed and fractionated liquids. REE analyses of the mineral separates display equilibrium partitioning patterns for amphibole and clinopyroxene, although isotopic data show that amphibole contains externally-derived Sr and Nd components not recognized in other minerals. At least a 4-stage history for the granulites is recorded: (1) intrusion and crystal fractionation of basaltic magmas in the lower crust, probably accompanied by crustal assimilation, (2) granulite facies metamorphism, (3) a decrease in temperature and pressure, and (4) a later heating event. The complicated thermal history is reflected in Sm-Nd mineral isochron ages which range from about 170 Ma down to about 100 Ma and cannot be assigned to distinct geological events. These ages correlate with inferred temperatures; the low ages are measured for xenoliths with the highest temperatures. In some cases the young heating event is likely to be

  10. Iron transfer and temporal variation in the Soufriere Hills lavas (1995-2007)

    NASA Astrophysics Data System (ADS)

    Christopher, T. E.; Humphreys, M.; Genareau, K.; Barclay, J.; De Angelis, S. H.; Plail, M. J.

    2012-12-01

    The Soufriere Hills eruption has been ongoing for 16 years to date; the eruption is defined by episodic lava output with the extrusion phases having durations ranging from months to years, separated by periods of quiescence. The erupted lavas are mostly andesitic (SiO2 56-62 wt %) with minor enclaves (1-8 vol% for the eruption) of a basaltic to basaltic andesite composition (SiO2 49-56 wt %). The andesite generally contains 30-46% phenocrysts, up to 75% of which may be plagioclase; 28% amphibole, 7% orthopyroxene and 3% Fe-Ti oxides. Less than 1% clinopyroxene microphenocrysts and accessory apatite can also be present. Amphibole is absent from the groundmass yielding a groundmass of plagioclase, two pyroxenes and Fe-Ti oxides. The enclaves are normally ellipsoidal with chilled margins and are crystal poor with microlites and microphenocrysts of plagioclase, amphibole, orthopyroxene, Fe-Ti oxides and clinopyroxene. Phenocrysts inherited from the andesite are present in the enclaves most of which are plagioclase, most enclaves contain a diktytaxitic frame work of predominantly plagioclase and pargasitic amphibole. Both magmas have rhyolitic glass (SiO2 70-82 wt %) with the enclave glass slightly enriched in K2O wt% and TiO2 wt%. Evidence of bidirectional crystal transfer between both magma types and reheating of the andesite is present. The bulk andesite composition varies with each eruptive phase with only the iron content doing so systematically. The bulk iron content of the andesites define a decreasing trend from phase I (1995-1998) to phase II (1999-2003) and phase III (2005-2007). This trend is mirrored by the syn-eruptive enclave bulk compositions indicating iron transfer from the intruding basalt to the andesite.

  11. Geothermometry and Geobarometry of Kepsut-Dursunbey Volcanites (NW Anatolia, Turkey): Implications for Magma Chamber Depth and Crystallization Conditions

    NASA Astrophysics Data System (ADS)

    Kamaci, O.; Altunkaynak, S.

    2012-04-01

    The Kepsut-Dursunbey volcanic field (NW Turkey) contains various products of the Neogene magmatic activity. Two distinct volcanic suites have been recognized in the study area; Kepsut volcanic suite (KVS) and the Dursunbey volcanic suite (DVS). Both KVS and DVS are Early Miocene in age and representative for the post-collisional magmatism in western Anatolia. Mainly andesites, latites, basaltic andesites and pyroclastic rocks of KVS were formed during the early stage of volcanism in the study area. Without a major interruption in the volcanism, partly contemporanously, KVS followed by felsic volcanic products of DVS. Rhyolithic domes, lavas and extensive pyroclastic units were formed during this felsic volcanic activity. Both DVS and KVS display textural and chemical evidence for interaction of mafic and silicic magmas. This is evidenced by disequilibrium textures such as existence of rounded plagioclase phenocrysts with reaction rims (regrowth), hornblend -mantled clinopyroxenes (corona texture), synneusis, sieve textured plagioclase, patchy zones and oscilatory zoning in plagioclase phenocrysts. Basic lavas of KVS also contains xenocrysts represented by biotite mantled quartz and asidic plagioclase ovoids and glomerocrysts entrained from melt zones near base of the crust. The observed disequilibrium textures in both suites are provide evidence for magma mixing or Assimilation and fractional crystallization (AFC). Calculated pressures and temperatures using the amphibole-plagioclase geothermometer and Al-in-hornblend geobarometer give values of 5.7-7.0 kbar and 927-982 °C for the KVS and 3.7-5.3 kbar and 783-787°C for the DVS, indicating crystallization in magma chambers at deep and mid crustal levels; 21 km and 12 km, respectively. A main deep magma reservoir served as a source for the intermediate to basic lavas of KVS. The existence of similar mixing-compatible (disequilibrium) textures in both KVS and DVS support the establishment and evolution of KDVF magma

  12. Groundmass crystallization in dacite dykes taken in Unzen Scientific Drilling Project (USDP-4)

    NASA Astrophysics Data System (ADS)

    Noguchi, Satoshi; Toramaru, Atsushi; Nakada, Setsuya

    2008-07-01

    Groundmass textural and compositional analyses of the drilled dacite dykes of the Unzen Scientific Drilling Project (USDP-4) identify the feeder dyke of the 1990-1995 eruption and elucidate the crystallization process of dykes at depth. In the drilling depth range of 1582-1996 m ("conduit zone"), four dacite dykes were recognized. The groundmasses of all but one of these dykes have textures ranging from cryptocrystalline to microcrystalline aggregate of crystals < 10 µm across forming an equigranular mosaic of plagioclase, alkali feldspar, quartz, and pyrite. The samples include a small number of coarser-grained plagioclase microlites (20 µm to 0.3 mm long). The compositions of groundmass consisting only of grains < 10 µm plotted at the lower pressure (< 50 MPa) ternary minimum in the Qz'-Ab'-Or' system suggests that the crystallization of plagioclase, alkali feldspar, and quartz took place nearly simultaneously. The compositions of coarser plagioclase microlites and groundmass, the plagioclase microlite textures, and the phenocryst assemblages show significant differences from historical lavas exposed in the summit area. This implies the possibility that most of the dacite dykes are not feeder dykes for the lavas at the summit and remained beneath the surface, perhaps because of high viscosity associated with high SiO 2. One sample C14-1-1 collected 1977 m, has a texture, composition, and phenocryst assemblage nearly identical to that of the dome lava of the 1990-1995 eruption, differing only in the presence of hydrothermal alternation. At this time we cannot definitely conclude that C14-1-1 was the feeder dyke for the 1990-1995 eruption until we can elucidate the time scale and the conditions governing hydrothermal alternation.

  13. Megacrystic pyroxene basalts sample deep crustal gabbroic cumulates beneath the Mount Taylor volcanic field, New Mexico

    NASA Astrophysics Data System (ADS)

    Schmidt, Mariek E.; Schrader, Christian M.; Crumpler, Larry S.; Rowe, Michael C.; Wolff, John A.; Boroughs, Scott P.

    2016-04-01

    Distributed over the ~ 2.3 m.y. history of the alkaline and compositionally diverse Mount Taylor Volcanic Field (MTVF), New Mexico is a widespread texturally distinct family of differentiated basalts that contain resorbed megacrysts (up to 3 cm) of plagioclase, clinopyroxene, and olivine ± Ti-magnetite ± ilmenite ± orthopyroxene. These lavas have gabbroic cumulate inclusions with mineral compositions similar to the megacrysts, suggesting a common origin. Gabbroic and megacrystic clinopyroxenes form positive linear arrays in TiO2 (0.2-2.3 wt.%) with respect to Al2O3 (0.7-9.3 wt.%). Plagioclase (An41-80) from representative thin sections analyzed for 87Sr/86Sr by laser ablation ICP-MS range from 0.7036 to 0.7048. The low 87Sr/86Sr plagioclases (0.7036 to 0.7037) are associated with high Ti-Al clinopyroxenes. Likewise, the higher 87Sr/86Sr plagioclases (0.7043 to 0.7047) are associated with the low-Al clinopyroxenes. Taken together, the pyroxene and plagioclase megacrysts appear to track the differentiation of a gabbroic pluton (or related plutons) from alkaline to Si-saturated conditions by fractional crystallization and crustal assimilation. Clinopyroxene-liquid geobarometry calculations suggest that crystallization occurred near the crust-mantle transition at an average of ~ 1200 °C and 12-13 kbar. The distribution of the megacrystic pyroxene basalts suggests that a gabbroic intrusive body underlies subregions of the MTVF that have generated silicic magmas. The gabbro is interpreted to be a significant heat and mass input into the lower crust that is capable of driving the petrogenesis of diverse silicic compositions (through fractionation and crustal assimilation), including mugearites, trachytes, trachy-andesites and dacites, high-Si rhyolites, and topaz rhyolites of the MTVF.

  14. Petrogenesis of Sierra Nevada plutons inferred from the Sr, Nd, and O isotopic signatures of mafic igneous complexes in Yosemite Valley, California

    NASA Astrophysics Data System (ADS)

    Nelson, Wendy R.; Dorais, Michael J.; Christiansen, Eric H.; Hart, Garret L.

    2013-02-01

    Mafic complexes in the central Sierra Nevada batholith record valuable geochemical information regarding the role mafic magmas play in arc magmatism and the generation of continental crust. In the intrusive suite of Yosemite Valley, major and trace element compositions of the hornblende-bearing gabbroic rocks from the Rockslides mafic complex and of the mafic dikes in the North America Wall are compositionally similar to high-alumina basalt. Of these rocks, two samples have higher Ni and Cr abundances as well as higher ɛNd values than previously recognized for the intrusive suite. Plagioclase crystals in rocks from the North America Wall and the Rockslides have prominent calcic cores and sharply defined sodic rims, a texture commonly associated with mixing of mafic and felsic magmas. In situ analyses of 87Sr/86Sr in plagioclase show no significant isotopic difference from the cores to the rims of these grains. We propose that the high 87Sr/86Sr (~0.7067) and low ɛNd (~-3.4) of bulk rocks, the homogeneity of 87Sr/86Sr in plagioclase, and the high δ18O values of bulk rocks (6.6-7.3 ‰) and zircon (Lackey et al. in J Petrol 49:1397-1426, 2008) demonstrate that continental crust was assimilated into the sublithospheric mantle-derived basaltic precursors of the mafic rocks in Yosemite Valley. Contamination (20-40 %) likely occurred in the lower crust as the magma differentiated to high-alumina basalt prior to plagioclase (and zircon) crystallization. As a consequence, the isotopic signatures recorded by whole rocks, plagioclase, and zircon do not represent the composition of the underlying lithospheric mantle. We conclude that the mafic and associated felsic members of the intrusive suite of Yosemite Valley represent 60-80 % new additions to the crust and include significant quantities of recycled ancient crust.

  15. Solidification of interstitial melt in a gabbroic crystal mush: the Skaergaard intrusion, Greenland

    NASA Astrophysics Data System (ADS)

    Namur, Olivier; Humphreys, Madeleine C. S.; Holness, Marian B.; Veksler, Ilya V.

    2013-04-01

    The Eocene Skaergaard intrusion of East Greenland occupies a box-shaped, fault-bounded chamber, approximately 8km × 11km × 4km, at the contact between Precambrian gneisses and a thick overlying sequence of Eocene plateau lavas. The intrusion is divided into three main units: the Layered Series (LS) which crystallized on the floor, the Upper Border Series (UBS) which crystallized from the roof, and the Marginal Border Series (MBS) which grew inwards from the vertical walls. The order of appearance of cumulus phases in the LS is considered to be: plagioclase + olivine (HZ, LZa) + augite (LZb) + Fe-Ti oxides (LZc) - olivine (MZ) + olivine (UZa) + apatite (UZb) + ferro-hedenbergite (UZc). The Marginal Border Series (MBS) is subdivided in an analogous manner to the LS into HZ*, LZa*, LZb*, LZc*, MZ*, UZa* and UZb*. We measured plagioclase compositional profiles in the LS and MBS for major (EPMA) and trace (ion probe and LA-ICP-MS) elements. Plagioclase profiles in the MBS show a relatively simple pattern of continuously decreasing An-content from core to rim. Compatible trace elements evolve similarly to An, while incompatible trace elements continuously increase. In contrast, compositional profiles in the LS are highly complex. In the lowest stratrigraphic unit (LZa), the core of the plagioclase grains has a mantle of decreasing An-content with an external rim of constant composition at An55-58. Compatible and incompatible elements decrease and increase respectively from core to rim, signifying decoupling of major and trace elements in the external rim. In LZb, similar profiles are observed except that the external rim has a composition of An50-51. From LZc to UZa, plagioclase grains are mostly unzoned. However, where zoned they also show an external rim of constant composition, but this time at An40. From UZb to UZc, plagioclase grains are unzoned or show normal zoning without external constant composition rims. We suggest that the external rims at An58, An50 and An

  16. The Jelly Sandwich Bites Back: A Case Study of the Viscosity Contrast Between the Lower Crust and Upper Mantle From the Oman Ophiolite

    NASA Astrophysics Data System (ADS)

    Homburg, J. M.; Hirth, G.; Kelemen, P. B.

    2008-12-01

    The viscosity contrast between the lower crust and upper mantle controls key tectonic processes such as lithospheric coupling between the crust and mantle and the origin and transmission of plate driving forces. Recent reviews have emphasized data that suggest a strong, mafic lower crust overlies a relatively weak, peridotite upper mantle in continental plates, in contrast to the "Jelly Sandwich" concept in which weak lower crust is underlain by a relatively strong shallow mantle. While experimental data provide a basis to predict viscosity contrast, this involves substantial extrapolation from laboratory to natural conditions. Pertinent geologic observations of synchronously deformed gabbro and peridotite are not well documented. We analyzed microstructures in deformed gabbronorite dikes (~10-50 cm thick) and host harzburgite from the Oman ophiolite. Outcrop scale observations demonstrate that deformation is highly localized in 1-50 mm wide mylonitic shear bands formed within portions of the gabbronorite that are plagioclase rich. We constrained the conditions of deformation using pyroxene thermometry, petrology, grain size piezometry and EBSD analyses of lattice preferred orientation (LPO). Syn-deformation temperature ranged from ~600- 800°C, based on Fe-Mg-Ca exchange between recrystallized orthopyroxene-clinopyroxene pairs using QUILF (Andersen et al., 1993) and tremolite/actinolite thermal stability. Pressure was ~300 MPa based on structural thickness of the section. We observe a range of plagioclase grain sizes indicating that strain localized into progressively smaller volumes within the gabbronorite as deformation continued within the finest, most localized plagioclase grain size indicating a stress of ~80 MPa. In contrast, olivine grain size in peridotite corresponds to a stress of ~10 MPa, indicative of earlier, high T, distributed deformation. Plagioclase exhibits a LPO throughout the analyzed samples, though variations in orientation and strength of

  17. Composition and petrogenesis of oxide-, apatite-rich gabbronorites associated with Proterozoic anorthosite massifs: examples from the Adirondack Mountains, New York

    NASA Astrophysics Data System (ADS)

    McLelland, James; Ashwal, Lewis; Moore, Laura

    1994-03-01

    Mafic dikes and sheets rich in Fe, Ti-oxides and apatite are commonly associated with Proterozoic massif anorthosites and are referred to as oxide-apatite gabbronorites (OAGN). Within the Adirondacks, field evidence indicates that during middle to late stages of anorthositic evolution, these bodies were emplaced as magmas with unspecified liquid-crystal ratios. Sixty whole rock analyses of Adirondack OAGN and related rocks define continuous oxide trends on Harker variation diagrams (SiO2=37 54%). Similar trends exist for Sr, Y, Nb, Zr, and REE and together suggest a common origin via fractional crystallization. A representative parental magma (plagioclase-rich crystal mush) has been chosen from this suite, and successive daughter magmas have been produced by removal of minerals with compositions corresponding to those determined in actual rocks. Least squares, mass balance calculations of major element trends indicate that removal of intermediate plagioclase (˜An40 50) plus lesser amounts of pyroxene account for the compositional variation of this suite and produce very low sums of the squares of the residuals (R2 s>0.25). The extracted mineral phases correspond volumetrically and compositionally to those of the anorthositic suite, and the model succeeds in accounting for the observed OAGN trends. The major element model is utilized to calculate trace elejent concentrations for successive magmas, and these agree closely with observation. We conclude that, beginning with a plagioclase-rich crystal mush, the extraction of intermediate plagioclase (˜An40 50) drives residual magmas to increasingly Fe-, Ti-, and P-rich and SiO2-poor conditions characteristic of Fenner-type fractionation. The crystallization sequence is plagioclase→plagioclase+orthopyroxene→plagioclase+orthopyroxene (pigeonite)+augite. Fe, Ti-oxides begin to crystallize near the end of the sequence and are followed by apatite and fayalitic olivine which appears in place of pigeonite. Augitic

  18. Rehydration reactions and microstructure development in lower crustal granulites from the Bergen Arcs, Norway

    NASA Astrophysics Data System (ADS)

    Erickson, Timmons; Reddy, Steven; Clark, Chris; Hand, Martin; Bhowany, Kamini; Prent, Alex

    2017-04-01

    An investigation of the feedbacks generated between lower crust-derived fluids and deformation microstructures formed within retrogressed granulites from the Bergen Arcs on the west coast of Norway will be presented. We hope to assess the role of deformation microstructures in assisting fluid infiltration into nominally impermeable lower crustal rocks, the role of fluids in driving mineral reactions and thus weakening the rock strength, and the interplay between these mechanisms. Granulite wall-rock adjacent to an amphibolite facies shear zone near Isdal, Norway has been sectioned, texturally mapped using electron backscatter diffraction (EBSD) and chemically mapped using energy dispersive x-ray spectrometry (EDS). The granuilte protolith is made up of a Precambrian anorthosite - gabbro assemblage of plagioclase and coronas of garnet around clinopyroxene. Local alteration of the granulite to eclogite and amphibolite occurred during the Caledonian orogen and has been attributed to the infiltration of fluids during the high strain event (Mukai et al., 2014). In thin section a thin ( 75 µm) rim of pargasite amphibole can be seen between the garnet and plagioclase, while the rim of amphibole is thicker (600 µm) when between the clinopyroxene and plagioclase. Plagioclase is coarse grained (mms in diameter) and displays prominent growth twins within the undeformed regions of the granulite. However, within a sheared domain of the granulite the grain size has been significantly reduced (max diameter = 74 µm) as has the growth twinning. The plagioclase from the sheared domain also displays a crystallographic preferred orientation (CPO) which does not appear to be inherited from the 'parent' grains. Within the strained domain there is also an increase in the reaction of garnet to pargasite, which also displays a strong CPO. These textural relationships offer the opportunity to study the active mechanisms during hydration of the lower crust and evaluate the relationships

  19. Hyperextension of continental to oceanic-like lithosphere: The record of late gabbros in the shallow subcontinental lithospheric mantle of the westernmost Mediterranean

    NASA Astrophysics Data System (ADS)

    Hidas, Károly; Varas-Reus, Maria Isabel; Garrido, Carlos J.; Marchesi, Claudio; Acosta-Vigil, Antonio; Padrón-Navarta, José Alberto; Targuisti, Kamal; Konc, Zoltán

    2015-05-01

    We report gabbroic dikes in the plagioclase tectonite domains of the Ojén and Ronda massifs (Betic Cordillera, southern Spain), which record crystallization at low-pressure syn-, or slightly postkinematic to the late ductile history of the Betic Peridotite in the westernmost Mediterranean. We present mineral major and trace element compositional data of discordant gabbroic dikes in the Ojén massif and gabbroic patches in the Ronda massif, complemented by the whole rock and electron backscattered diffraction (EBSD) data of the Ojén occurrence. In the Ojén massif, gabbro occurs as 1-3 centimeter wide discordant dikes that crosscut the plagioclase tectonite foliation at high angle. These dikes are composed of cm-scale igneous plagioclase and clinopyroxene crystals that show shape preferred orientations subparallel to the lineation of the host peridotite and oblique to the trend of the dike. Intrusion of Ojén gabbro dikes is coherent with the stress field that formed the high temperature, ductile plagioclase tectonite foliation and then attests for a mantle igneous event prior to the intracrustal emplacement of the massif. In the Ronda massif, gabbroic rocks crystallized in subcentimeter wide anastomozing veins, or as interstitial patches in the host dunite. They are mostly composed of plagioclase and clinopyroxene. Plagioclase composition is bytownitic in the Ojén, and andesinic in the Ronda massif. Clinopyroxene in both places shows identical, light Rare-Earth Element (LREE) depleted trace element patterns. The calculated trace element composition of melts in exchange equilibrium with the studied igneous clinopyroxenes reflects LREE-enriched character coupled with negative Eu anomaly, and indicates that gabbro-forming melts in Ronda and Ojén share a common melt source with an island arc tholeiitic affinity. Geothermobarometric data and liquidus mineralogy indicate that gabbro crystallization occurred at shallow depths (0.2-0.5 GPa) in a 7-16 km thick

  20. Short Timescales for Crustal Residence, Transport and Contamination of Flood Basalt Magma: Crystal Isotope Stratigraphy of the Columbia River Basalt Group.

    NASA Astrophysics Data System (ADS)

    Tollstrup, D. L.; Ramos, F. C.; Wolff, J. A.

    2002-12-01

    Geochemical studies of continental flood basalt magmas provide evidence for contributions from one or more enriched reservoirs. There is, however, no consensus on the role of continental crust as a major source of enriched signatures. With its stratigraphy defined and mapped at the scale of individual flows, the Columbia River Basalt Group (CRBG) is the most thoroughly studied continental flood basalt province in the world. Its tectonic position (overlying both thin accreted Mesozoic crust and thick ancient cratonic crust) makes the CRBG ideal for isolating the contribution of crust in the petrogenesis of continental flood basalts. Many flows are plagioclase-phyric. Because plagioclase in basaltic magmas can be assumed to have grown at crustal pressures, growth layers in plagioclase phenocrysts record changes in the chemical and isotopic composition of the magma occurring at crustal depths. We have initiated a micro-sampling study utilizing laser ablation multicollector ICP-MS (ThermoFinnigan Neptuner) to analyze 87Sr/86Sr variability in plagioclase and clinopyroxene phenocrysts (where present) and associated groundmass. Initial results are: 1) plagioclase and clinopyroxene phenocrysts within CRBG lavas are overall less radiogenic than host groundmass and 2) plagioclase phenocrysts are commonly zoned from less radiogenic cores to more radiogenic rims. The rims may have similar compositions to, or be less radiogenic than, host groundmass. One-dimensional diffusion modeling applied to observed 87Sr/86Sr zoning and crystal/groundmass gradients constrains phenocryst residence times, and the timescale of crustal-level petrogenetic events that modified CRBG magmas. Residence times for phenocrysts in their final host liquid may be as little as 10 years prior to quenching. These results require that the 87Sr/86Sr composition of the CRBG magmas increased rapidly with time at crustal pressures during and after phenocryst growth. This could result from mixing between magmas

  1. Gabbroic and Peridotitic Enclaves from the 2008 Kasatochi Eruption, Aleutian Islands, Alaska

    NASA Astrophysics Data System (ADS)

    Kentner, A.; Nadin, E. S.; Izbekov, P. E.; Nye, C. J.; Neill, O. K.

    2012-12-01

    Kasatochi volcano of the Andreanof Islands in the western Aleutian Arc violently erupted over a two day period from August 7-8, 2008. The eruption involved multiple explosive events generating pyroclastic flows, which included abundant mafic and ultramafic enclaves that have since weathered out and accumulated in talus along the coast. These and other mafic enclaves sampled by modern island arc lavas provide insight into subduction magmatism because they emerge from a section of the subduction system that is less likely than shallower zones to be modified by magmatic processes such as mixing, assimilation, or fractionation. We present new whole rock, clinopyroxene, amphibole, plagioclase, and melt compositions from Kasatochi enclaves of the 2008 eruption. The highly crystalline (~40 vol. % phenocryst content), medium-K basaltic andesite host rock contains ~52-55 wt. % SiO2 and 0.6-0.9 wt. % K2O, and is composed of plagioclase, ortho- and clinopyroxene, amphibole, and Ti-magnetite in a microlite-rich groundmass. Upon eruption, this magma sampled two distinct enclave populations: gabbro and peridotite. The gabbro has abundant amphibole (mostly magnesio-hastingsite) and plagioclase with minor clinopyroxene, olivine, and magnetite, while the peridotite is composed of olivine with minor amounts of clinopyroxene and orthopyroxene. There is little textural variation amongst the peridotitic samples collected, but the gabbroic samples vary from layered to massive and cover a range in grain size from fine-grained to pegmatitic. The layered gabbros display centimeter-scale bands of alternating plagioclase- and amphibole-rich layers, with a strong preferential alignment of the amphibole grains. The coarser-grained samples are very friable, with ~10% pore space; disaggregation of these upon host-magma ascent likely formed the amphibole and plagioclase xenocrysts in the andesitic host. Based on the textural and compositional differences, we divide the enclaves into four groups

  2. Interaction between felsic and mafic magmas in the Salmas intrusive complex, Northwestern Iran: Constraints from petrography and geochemistry

    NASA Astrophysics Data System (ADS)

    Ghaffari, Mitra; Rashidnejad-Omran, Nematollah; Dabiri, Rahim; Santos, José Francisco; Mata, João; Buchs, David; McDonald, Iain; Appel, Peter; Garbe-Schönberg, Dieter

    2015-11-01

    The Salmas plutonic complex, in the northernmost part of Sanandaj-Sirjan Zone of Iran, provides evidence for magma interaction processes. The complex contains mafic-intermediate, hybrid and felsic rocks which intruded into the Paleozoic metamorphic complex. They show typical relationships described in many mafic-felsic mingling and mixing zones worldwide, such as mafic microgranular enclaves (in felsic and hybrid rocks), mafic sheets, and hybrid rocks. The mafic microgranular enclaves (MMEs) are characterized by fine-grained, equigranular and hypidiomorphic texture and some special types of microscopic textures, e.g., quartz xenocrysts, oscillatory-zoned plagioclase, small lath-shaped plagioclase in large plagioclase, spike zones in plagioclase and spongy-cellular plagioclase textures, rounded plagioclase megacrysts blade-shaped biotite, acicular apatite. The mafic sheets and MMEs in granites (MME-Gr), which indicated magma mingling structures, show ISr values and εNd(i) similar to diorites. The hybrid rocks and their mafic enclaves (MME-H) show isotope signatures similar to each other. Granites have isotope signatures [higher 87Sr/86Sr(i) (0.70788-0.71075) and lower εNd(i) (-2.4 to -4.2)] distinct to those of the all rock types and MMEs. Major, trace and REE modeling show that hybrid rocks are generated via 40-60% mixing of mafic (dioritic) and felsic (granitic) end-members. All the geochemical data suggest that underplating of dioritic magma, which has been produced by fractional crystallization of gabbros, under the lower crust caused its melting to make felsic (granitic) magma. Injection of dioritic magma into the base of the felsic magma chamber and a limited mixing of two end-members, the lower crust-derived magma and mantle-derived melts, formed hybrid magma and their enclaves. Injections of new mafic magma pulses into hybrid magma generated mafic enclaves into them. The injections of denser dioritic magma pulses into a felsic magma chamber and spreading

  3. A New Hygrometer based on the Europium Anomaly in Clinopyroxene Phenocrysts in Arc Volcanic Rocks

    NASA Astrophysics Data System (ADS)

    Plank, T.; Benjamin, E.; Wade, J.; Grove, T.

    2004-12-01

    Water is arguably the most important chemical component in arc magmas, affecting everything from liquidus temperatures to crystal fractionation trends to melt rheology. Water concentrations in arc magmas provide a first-order constraint on water contents in the mantle wedge, and the mechanisms of wet mantle melting. However, measuring the water content of primary arc magmas has been difficult, or in some cases impossible, due to the near complete degassing of volcanic rocks, and the scarcity of olivine-hosted melt inclusions in many arc volcanoes. We have thus developed a new hygrometer using the composition of clinopyroxene phenocrysts, which are common in arc basalts and andesites. The hygrometer is based on the well-known suppression of plagioclase by water dissolved in the melt, and the effect on the rare earth element (REE) pattern of coexisting phases, such as clinopyroxene. Dry melts saturate in plagioclase early, and the preferential partitioning of Eu2+ in plagioclase causes a negative Eu anomaly to develop in coexisting melts and clinopyroxene. In wet magmas, clinopyroxene crystallizes before plagioclase, and so initially appears with a negligible Eu anomaly. Clinopyroxenes then record water content in the delayed development of their negative Eu anomaly, caused by the delayed appearance of plagioclase along the cotectic with increasing water. We have tested this model using tephras from the 1723 eruption of Irazu, the ET3 unit of Arenal and the 1995 eruption of Cerro Negro volcanoes in Central America, with known water contents of ˜ 3, 4 and 5 wt%, respectively, based on olivine-hosted melt inclusions. Clinopyroxene phenocrysts separated from these samples vary in Mg# from 86-72, and in some cases span the entire liquid line of descent of the volcano. REE patterns were determined by laser ablation ICPMS on 150 micron spots. A marked increase in the magnitude of the negative Eu anomaly occurs in clinopyroxenes with Mg# < 84 in Irazu, < 82 in Arenal and

  4. Mineralogy of Meteorite Groups

    NASA Technical Reports Server (NTRS)

    Rubin, Alan E.

    1997-01-01

    Approximately 275 mineral species have been identified in meteorites, reflecting diverse redox environments, and, in some cases, unusual nebular formation conditions. Anhydrous ordinary, carbonaceous and R chondrites contain major olivine, pyroxene and plagioclase; major opaque phases include metallic Fe-Ni, troilite and chromite. Primitive achondrites are mineralogically similar. The highly reduced enstatite chondrites and achondrites contain major enstatite, plagioclase, free silica and kamacite as well as nitrides, a silicide and Ca-, Mg-, Mn-, Na-, Cr-, K- and Ti-rich sulfides. Aqueously altered carbonaceous chondrites contain major amounts of hydrous phyllosilicates, complex organic compounds, magnetite, various sulfates and sulfides, and carbonates. In addition to kamacite and taenite, iron meteorites contain carbides, elemental C, nitrides, phosphates, phosphides, chromite and sulfides. Silicate inclusions in IAB/IIICD and lIE iron meteorites consist of mafic silicates, plagioclase and various sulfides, oxides and phosphates. Eucrites, howardites and diogenites have basaltic to orthopyroxenitic compositions and consist of major pyroxene and calcic plagioclase and several accessory oxides. Ureilttes .are made up mainly of calcic, chromian olivine and low-Ca clinopyroxene embedded in a carbonaceous matrix; accessory phases include the C polymorphs graphite, diamond, lonsdaleite and chaoite as well as metallic Fe-Ni, troilite and halides. Angrites are achondrites rich in fassaitic pyroxene (i.e. , AI-Ti diopside); minor olivine with included magnesian kirschsteinite is also present. Martian meteorites comprise basalts, Iherzolites, a dunite and an orthopyroxenite. Major phases include various pyroxenes and olivine; minor to accessory phases include various sulfides, magnetite, chromite and Ca-phosphates. Lunar meteorites comprise mare basalts with major augite and calcic plagioclase and anorthositic breccias with major calcic plagioclase. Several meteoritic

  5. Petrogenesis and tectonic implications of gabbro and plagiogranite intrusions in mantle peridotites of the Myitkyina ophiolite, Myanmar

    NASA Astrophysics Data System (ADS)

    Xu, Yang; Liu, Chuan-Zhou; Chen, Yi; Guo, Shun; Wang, Jian-Gang; Sein, Kyaing

    2017-07-01

    Centimeter-size intrusions of gabbros and plagiogranites occur in mantle peridotites of the Myitkyina ophiolite, Myanmar. The gabbros mainly consist of plagioclase and clinopyroxene, whereas orthopyroxene occasionally occurs. The plagiogranites are mainly composed of plagioclase, quartz and amphibole, with small amount of accessory minerals, such as zircon, apatite and rutile. Plagioclase in the gabbros varies from andesine to anorthite (An37-91), whereas plagioclase in the plagiogranites is less calcic (An1-40). Clinopyroxene in the gabbros is pervasively altered to hornblende. The gabbros contain 42.97-52.88 wt% SiO2, which show negative correlations with Al2O3, CaO and MgO, but positive correlations with Na2O, P2O5 and TiO2. Microtextural relations reveal the crystallization of clinopyroxene prior to plagioclase in the Myitkyina gabbros. This suggests that the gabbros were crystallized from hydrous melts, which is also supported by the occurrence of orthopyroxene and anorthitic plagioclase in some gabbros. The gabbros have slightly enriched Sr-Nd isotopes, with initial 87Sr/86Sr ratios of 0.703938-0.706609 and εNd(t) values of + 2.4-+7.2, and relatively variable Hf isotopes, with εHf(t) values of + 13.4-+24.9. A subduction component is required to explain the decoupled Nd-Hf isotopes of the gabbros. Binary mixing suggests that addition of ca 2% subducted sediments to a depleted mantle can account for the Nd-Hf decoupling. Therefore, both petrological and geochemical data of the gabbros support that the Myitkyina ophiolite was originated in a supra-subduction zone setting. The plagiogranites have compositions of tonalites and trondhjemites, containing 56.93-77.93 wt% SiO2, 1.27-10.79 wt% Na2O and 0.05-0.71 wt% K2O. They are slightly enriched in LREE over HREE and display positive anomalies in Eu, Zr, Hf but negative Nb anomalies. Very low TiO2 contents (0.03-0.2 wt%) of the plagiogranites suggest that they were not products of fractional crystallization of MORB

  6. Small crystals, big implications: glomerocrysts and their connections to large silicic magma reservoirs. Examples from Yellowstone rhyolite lavas.

    NASA Astrophysics Data System (ADS)

    Girard, G.

    2016-12-01

    Characterizing the nature and physical state of the magma reservoirs associated to large silicic volcanoes is critical to assess their potential to erupt. Yellowstone is among the most active of these systems, having produced 3 caldera-forming eruption sequences and >40 rhyolite lava flows over the last 2.15 Ma. The geochemical evolution of the most recently erupted lava flow sequences, the Upper Basin and Central Plateau members, argues for the existence of one large magma reservoir, from which melts are periodically extracted and erupted as lava with <10% phenocrysts. This study focuses on two eruptions closely postdating the second and third caldera collapse events, the 1.28 Ma Osborne Butte lava dome, and the 0.6 Ma East Biscuit Basin lava flow. These more phyric units have complex mineralogy with multiple crystal textures and geochemistry. In particular, they exhibit plagioclase-clinopyroxene-oxides glomerocrysts with grain size < 1mm. These coexist with larger, euhedral, isolated clinopyroxene and plagioclase. In both lavas, glomerocrysts are the components with the most primitive compositions. In the East Biscuit Basin lava, plagioclase from glomerocrysts ranges from An23 to An46 with 560 ± 30 ppm Sr (1σ) while isolated sieved plagioclase has An20-25 with similar Sr. Clinopyroxene in glomerocrysts is En25-31Fs31-43Wo34-38 while isolated crystals are En21-23Fs40-41Wo37-38. Rare earth elements also are more enriched in the isolated clinopyroxene. In the Osborne Butte lava, the glomerocrysts also are the most primitive components with An19-23 and 245 ± 40 ppm Sr in plagioclase, while larger isolated plagioclase has diverse but generally more evolved compositions (An14-24 and 35-280 ppm Sr). The more primitive compositions and plutonic textures of these glomerocrysts, together with their presence being restricted to lavas erupted closely after caldera collapse (when the volcano subsurface is the most disturbed) argue for their origin from the magma body from

  7. Disequilibrium dihedral angles as a proxy for cooling rate: new opportunities for decoding the effects of liquid migration in dolerites and basalts.

    NASA Astrophysics Data System (ADS)

    Holness, Marian; Richardson, Chris; Philpotts, Anthony

    2013-04-01

    The geometry of clinopyroxene-plagioclase-plagioclase junctions in mafic rocks, measured by the median dihedral angle, Θcpp, is created during solidification, with junction geometry a function of the initial impingement angle of the two plagioclase grains together with the relative rates of growth of augite and plagioclase. Rapid solidification results in Θcpp ~78˚, whereas more slowly cooled rocks have higher Θcpp. Θcpp varies symmetrically across dolerite sills, with the lowest values at the margins. Simple thermal models of sills, based on a crystallization interval of 1200-1000°C and including consideration of latent heat, suggest that Θcpp ~78° signifies crystallisation times of less than a few years. The symmetrical variation of Θcpp across the sills is in marked contrast to the variation of average plagioclase grain size - generally the coarsest rocks are in the upper third of the sills. The straightforward mapping of Θcpp onto crystallization times means dihedral angles provide a robust measure of cooling rates, in contrast to the more commonly used method based on crystal size distributions which is limited by an incomplete knowledge of crystal growth rates. While sills lose heat equally from both top and bottom surfaces, lava flows and lakes primarily cool from the upper surface, especially when flooded with water. This is reflected in a highly asymmetric Θcpp variation, with maximum values close to the floor. Comparison of average plagioclase grain size, calculated extent of compaction and Θcpp through the thickest part of the Holyoke Flood-Basalt Flow, sampled at North Branford and Tariffville, demonstrates the slowest-cooled parts of the body (i.e. that part with maximum Θcpp and % compaction) underlie those of maximum grain size. We interpret the horizon with the coarsest grain size to contain significant volumes of relatively evolved liquids, derived by compaction of the underlying layers, in which crystallization continued to

  8. Protracted weakening during lower crustal shearing along an extensional shear zone

    NASA Astrophysics Data System (ADS)

    degli Alessandrini, Giulia; Menegon, Luca; Giuntoli, Francesco

    2017-04-01

    This study investigates grain-scale deformation mechanisms in the mafic lower continental crust, with particular focus on the role of syn-kinematic metamorphic reactions and their product - symplectites - in promoting grain size reduction, phase mixing and thus strain localization. The investigated extensional shear zone is hosted in the Finero mafic-ultramafic complex in the Italian Southern Alps. Field and microstructural observations indicate that strain partitioned in gabbroic layers where the primary mineralogical assemblage contained amphibole, forming ultramylonites. These ultramylonites are characterized by isolated porphyroclasts of amphibole, garnet, clinopyroxene and orthopyroxene, embedded in a matrix of plagioclase (ca. 39 vol%) + amphibole (25 vol%) + clinopyroxene (18 vol%) + orthopyroxene (11 vol%) + Fe-Ti oxides (6 vol%) ± apatite (<1 vol%). Matrix grain-size is consistently below 30 μm for all phases. EBSD results are consistent with deformation by grain-size sensitive creep. Amphibole shows a CPO with [001] axes preferentially aligned parallel to the stretching lineation, which we interpret as oriented grain growth during heterogeneous nucleation of amphibole. Pyroxenes and plagioclase lack a CPO and evidence for dislocation creep and dynamic recrystallization. Protracted shearing was initiated by syn-kinematic metamorphic reactions: garnet porphyroclasts formed orthopyroxene + plagioclase symplectites and amphibole porphyroclasts formed pyroxene + plagioclase symplectites. The latter reaction indicates that strain localization initiated with dehydration reactions leading to primary amphibole breakdown into pyroxene and plagioclase, now preserved in the ultramylonite. Geothermobarometry using plagioclase-amphibole pairs in the ultramylonites indicate temperature conditions of ca. 800˚ C and pressures from 8 to 6kbar. This suggests that protracted shearing in the ultramylonites occurred at decreasing pressure and nearly constant T. We suggest

  9. Textural and microstructural development of the Barro Alto Complex: implications for seismic anisotropy

    NASA Astrophysics Data System (ADS)

    Silveira, Camila; Lagoeiro, Leonardo; Barbosa, Paola; Cavalcante, Geane Carolina; Ferreira, Filippe; Suita, Marcos; Conte, Thailli

    2017-04-01

    Crustal rheology is associated with the behavior of its constituents in response to stress and strain, while the seismic anisotropy is a property that can correlate these parameters. Seismic properties are strongly related to the microstructures and crystallographic preferred orientation (CPO) of the rocks. In this work, we study CPO-derived seismic anisotropy of metamorphosed gabbro-norites from the Barro Alto (Brazil central) layered complex. The EBSD technique was employed to analyze the crystallographic orientation of the main mineral assembly, diopside and feldspar. The Barro Alto complex belongs to the Tocantins Structural Province, developed between the Amazon and São Francisco cratons, during the Neoproterozoic Brasiliano orogenic cycle. This complex was formed by a mafic-ultramafic layered intrusion mylonitized and metamorphosed under granulite facies conditions. The mylonitic foliation shows compositional segregation into felsic and mafic bands. The samples are composed of porphyroclasts of plagioclase and diopside in a fine matrix of plagioclase, clinopyroxene, orthopyroxene and, less commonly, amphibole and biotite. The plagioclase porphyroclasts exhibit undulose extinction and core-mantle structure. In fine matrix samples the poles to a(100), b(010) and c(001) are randomly distributed in both phases. However, for increasing matrix grain size plagioclase grains shows maxima of a(100) poles sub-parallel to the foliation and b(010) normal to the foliation. The low value of the J index (2.4 for plagioclase and 1.8 for diopside) indicates poorly developed fabric. Misorientation profiles showing high frequency of small angle boundaries are typical of recrystallization by subgrain rotation mechanisms. The microstructural and CPO analyses suggest deformation controlled by diffusive processes. The CPO models were compared to models described in the literature, based on the anorthite + diopside assembly, since these are the major phases, and thus control the

  10. A preliminary study of the fabrics in the Skaergaard Layered Series and implications for the significance of compaction

    NASA Astrophysics Data System (ADS)

    Vukmanovic, Z.; Holness, M. B.; Mariani, E.

    2016-12-01

    Gabbroic cumulates often have foliations and/or lineations defined by shape-preferred orientations of cumulus grains (SPO). These fabrics are commonly interpreted as a product of crystal alignment by flowing magma or by slumping of a non-cohesive mush. Conversely it has also been argued that cumulate fabrics are secondary and formed during compaction via dislocation creep and/or solution-reprecipitation creep. The dominant plagioclase slip system, (010)[001], creates a crystallographic preferred orientation (CPO) defined by the alignment of (010) planes, with [001] parallel to lineation. Solution-reprecipitation results in a CPO with (010) planes aligned parallel to the principal compressive stress, and preferential mineral growth on (010) planes to form an SPO defined by grains elongated perpendicular to (010). In the Skaergaard Layered Series, the shape of cumulus plagioclase grains changes systematically from highly tabular to equant up the stratigraphy. Foliations, defined both by a plagioclase SPO (with tabular grains aligned horizontally) and an associated CPO ((010) parallel to foliation), are strongest lower in the stratigraphy and reduce in strength upwards. Lineations are generally absent or weak. Evidence for crystal plasticity is limited to bending of some plagioclase crystals and small numbers of low angle boundaries in all phases. Compositional zoning is present on all plagioclase growth faces in the lower part of the stratigraphy, inconsistent with preferential solution - reprecipitation during compression. There are no fabrics or microstructures that can be attributed to solution-reprecipitation, and evidence for only minor microstructural modification by dislocation creep. Plagioclase grain shape and strength of foliations are approximately anti-correlated with incompatible element concentration. It has been argued that the upwards decrease in incompatible element concentration in the Skaergaard Layered Series is due to an upwards increasing

  11. Cyclic units in the Somerset Dam layered gabbro intrusion, southeastern Queensland, Australia

    NASA Astrophysics Data System (ADS)

    Mathison, Charter I.

    1987-06-01

    The well-preserved Somerset Dam intrusion probably represents a small, relatively shallow, subvolcanic magma chamber. The 500-m-thick exposed sequence consists of 22 macrolayers which are defined by sharp phase, modal and textural contacts. At least six cyclic units, 30-150 m thick, are exposed, and the sequence from the base to the top of a cyclic unit is inferred to be leucogabbro (plagioclase cumulate), troctolite (plagioclase-olivine cumulate), olivine gabbro (plagioclase-augite-olivine cumulate), and oxide gabbro (plagioclase-augite-(olivine)-magnetite-ilmenite cumulate). Mineral compositions in a typical cyclic unit show a reversed fractionation trend in the sequence leucogabbro-troctolite, and a normal fractionation trend from troctolite (the least fractionated rock type) to the oxide gabbro (the most fractionated rock type). The most sensitive parameters for defining the cryptic trends are An in plagioclase, Fo and Ni in olivine, and Cr in magnetite and augite. Whole-rock compositions also show marked changes, and Fe, Ti, V, S and Cu increase and Al, {Mg}/{Fe}, Cr and Ni decrease from troctolite to oxide gabbro. Despite the remarkable similarity of successive cyclic units, significant differences exist between them in the sequences of layers, thicknesses of individual layers and of the cyclic units, mineral compositions and cryptic patterns, average level of fractionation and the size of the reversals. Unit 3 is particularly unusual because it is the least fractionated and consists of two incomplete subunits. Unit 1, the lowest exposed, is the most fractionated. These differences between the units cannot be explained in terms of a closed system, and are strong evidence for an open system involving periodic injections of magma. The formation of a cyclic unit appears to reflect the dominant control of the order of crystallisation from a batch of replenished magma, which is essentially plagioclase first, followed by olivine, augite, magnetite and ilmenite, and

  12. Coronitic textures in mafic xenoliths from Puy Beaunit (French Massif Central): Evidence for pyrometamorphism on cumulate rocks from a deep layered complex

    NASA Astrophysics Data System (ADS)

    Berger, J.; Femenias, O.; Demaiffe, D.

    2003-04-01

    The Puy Beaunit quaternary maar (Chaîne des Puys) is well known for its large diversity of xenoliths. Besides the typical crustal (granites and granulites) and mantle-derived (peridotites sensu lato) xenoliths, we have also identified magmatic mafic xenoliths, some of them displaying mm- to cm- scale layering. These mafic rocks are mainly gabbronorites with minor pyroxenites and anorthositic gabbros. It has been proposed that these mafic xenoliths are derived from a differentiated complex emplaced at the crust-mantle boundary during Permian times (257+/- 6Ma, zircon U-Pb SIMS data). These xenoliths have undergone a pyrometamorphic (HT-BP) event while they were brought up to the surface by the quaternary lava. Beside the classical polygonal texture, the xenoliths display four types of coronitic and symplectitic textures: 1) plag-opx-oxide symplectite between plagioclase and clinopyroxene; 2) brown-orange glass with skeletal and euhedral olivine around orthopyroxene; 3) poikilitic clinopyroxene with numerous inclusions of euhedral plagioclase and oxide (this texture is also observed around some clinopyroxene); 4) plag-opx-cpx-oxide symplectite around amphibole relics. Mafic samples with coronites are relatively poor in SiO2 (41 to 47wt% SiO2) when compared to polygonal mafic xenoliths (49 to 51wt% SiO2). It appears that the type of corona in a given sample depends on the composition of the primary (=magmatic) pyroxenes, plagioclase and ghost phase (amphibole?). Xenoliths with primary Fe-rich pyroxenes (opx: En59-67, cpx: En36-39) show poikilitic clinopyroxene textures. Xenoliths with primary An-poor plagioclase (An55) show coronas with olivine-bearing glass around orthopyroxene. Xenoliths with primary highly calcic plagioclase (An85-95) show plag-opx-oxide symplectite. Xenoliths with polygonal texture contain Mg-rich orthopyroxene (En70-80) and labrador plagioclase (An65-70). Thermobarometers based on the composition of amphibole relics give magmatic P

  13. Phenocryst complexity in andesites and dacites from the Tequila volcanic field, Mexico: resolving the effects of degassing vs. magma mixing

    NASA Astrophysics Data System (ADS)

    Frey, Holli M.; Lange, Rebecca A.

    2011-08-01

    The petrology of five phenocryst-poor (2-5%) andesites and dacites, all of which were erupted from different short-lived, monogenetic vents, is compared to that of phenocryst-rich (10-25%) andesites erupted from the adjacent stratovolcano, Volcán Tequila, in the Mexican arc. Despite differences in phenocryst abundances, these magmas have comparable phase assemblages (plagioclase + orthopyroxene + titanomagnetite + ilmenite + apatite ± augite ± hornblende), and similarly wide variations in phenocryst compositions, coupled to complex zoning patterns. For the phenocryst-poor lavas, equilibrium pairs of two Fe-Ti oxides lead to a narrow range of calculated temperatures for each sample that range from 934 (±24) to 1,073 (±6)°C and oxygen fugacities that range from +0.1 to +0.7 log units relative to the Ni-NiO buffer. Application of the plagioclase-liquid hygrometer to each sample at these calculated temperatures leads to maximum melt water concentrations of 4.6-3.1 wt% during plagioclase crystallization, indicating that the magmas were fluid saturated at depths ≥6.4-4.5 km. There is a wide, continuous range in the composition of plagioclase (≤44 mol% An) and orthopyroxene (≤16% Mg#) phenocrysts in each sample, which is consistent with a loss of dissolved water (≤2.8 wt%) from the melt phase during degassing as the magmas ascended rapidly to the surface. Evidence is presented that shows the effect of dissolved water is to reduce the activity of MgO relative to FeO in the melt phase, which indicates that degassing will also affect the Mg# of pyroxene phenocrysts, with higher melt water concentrations favoring Fe-rich pyroxene. Both plagioclase and orthopyroxene commonly display diffusion-limited growth textures (e.g., skeletal and hopper crystals, large interior melt hollows, and swallow tails), which are consistent with large undercoolings produced by degassing-induced crystallization. Therefore, degassing is proposed as a possible cause for the phenocryst

  14. 40Ar/39Ar and K/Ar dating of low grade metamorphism: examples on metabasites from Central Chile

    NASA Astrophysics Data System (ADS)

    Aguirre, L.; Feraud, G.; Fuentes, F.; Delbar, M.; Morata, D.

    2003-04-01

    Dating low to very low-grade burial metamorphic assemblages is often difficult because of (1) few mineral phases compositionally suitable to apply the 40Ar/39Ar and K-Ar methods, and (2) small amount in which these phases are commonly found. K-feldspar adularia, sericitic mica, and celadonite are the best known K-bearing secondary minerals. We present some successful attempts to analyse two distinct secondary phases from a same volcanic formation that allow to test the validity of the measured ages. These ages have been also compared with the crystallisation age of the volcanic rocks in which the secondary phases were lately developed. Adularia and sericite were selected from basic lava flows from a 3 to 13 km thick Cretaceous sequence from the Coastal Range of central Chile, at two different locations: the Bustamante Hill (west from Santiago), and the Cordón de Chacana, c. 80 km further north. Adularia came from a low-variance assemblage with pumpellyite, chlorite and low-albite contained in amygdules whereas sericite was present in milky-white strongly sericitized plagioclase crystals. While small clusters of rare fresh plagioclase grains from lava flows from Bustamante and Chacana displayed concordant plateau ages 119.4 ± 2.4 (2 sigma) and 118.7 ± 0.6 Ma, respectively, the adularia from the same formations gave sensibly younger ages around 94 Ma (high temperature steps), and 96.8 ± 0.2 Ma (plateau age) in Bustamante and Chacana, respectively. Sericite ages were measured in situ into single crystals of strongly transformed plagioclases. The relative proportion of sericite and plagioclase corresponding to each degasing step was monitored by measuring the Ca/K ratio (deduced from 37ArCa/39Ar_K). While intermediate ages were measured on some sericite of both sites (corresponding to a variable but permanent contribution of plagioclase on each step), a plateau age of 97.0 ± 1.6 Ma (concordant with adularia) could be obtained on a strongly sericitized plagioclase

  15. Origin of High Electrical Conductivity in the Lower Continental Crust: A Review

    NASA Astrophysics Data System (ADS)

    Yang, Xiaozhi

    2011-11-01

    Electromagnetic measurements have demonstrated that the lower continental crust has remarkable electrical anomalies of high conductivity and electrical anisotropy on a global scale (probably with some local exceptions), but their origin is a long-standing and controversial problem. Typical electrical properties of the lower continental crust include: (1) the electrical conductivity is usually 10-4 to 10-1 S/m; (2) the overlying shallow crust and underlying upper mantle are in most cases less conductive; (3) the electrical conductivity is statistically much higher in Phanerozoic than in Precambrian areas; (4) horizontal anisotropy has been resolved in many areas; and (5) in some regions there appear to be correlations between high electrical conductivity and other physical properties such as seismic reflections. The explanation based on conduction by interconnected, highly conductive phases such as fluids, melts, or graphite films in grain boundary zones has various problems in accounting for geophysically resolved electrical conductivity and other chemical and physical properties of the lower crust. The lower continental crust is dominated by mafic granulites (in particular beneath stable regions), with nominally anhydrous clinopyroxene, orthopyroxene, and plagioclase as the main assemblages, and the prevailing temperatures are mostly 700-1,000°C as estimated from xenolith data, surface heat flow, and seismic imaging. Pyroxenes have significantly higher Fe content in the lower crust than in the upper mantle (peridotites), and plagioclase has higher Na content in the lower crust than in the shallow crust (granites). Minerals in the lower continental crust generally contain trace amounts of water as H-related point defects, from less than 100 to more than 1,000 ppm H2O (by weight), with concentrations usually higher than those in the upper mantle. Observations of xenolith granulites captured by volcano-related eruptions indicate that the lower continental crust is

  16. Can cathodoluminescence of feldspar be used as provenance indicator?

    NASA Astrophysics Data System (ADS)

    Scholonek, Christiane; Augustsson, Carita

    2016-05-01

    We have studied feldspar from crystalline rocks for its textural and spectral cathodoluminescence (CL) characteristics with the aim to reveal their provenance potential. We analyzed ca. 60 rock samples of plutonic, volcanic, metamorphic, and pegmatitic origin from different continents and of 16 Ma to 2 Ga age for their feldspar CL textures and ca. 1200 feldspar crystals from these rocks for their CL color spectra. Among the analyzed rocks, igneous feldspar is most commonly zoned, whereby oscillatory zoning can be confirmed to be typical for volcanic plagioclase. The volcanic plagioclase also less commonly contains twin lamellae that are visible in CL light than crystals from other rock types. Alkali feldspar, particularly from igneous and pegmatitic rocks, was noted to be most affected by alteration features, visible as dark spots, lines and irregular areas. The size of all textural features of up to ca. 150 μm, in combination with possible alteration in both the source area and the sedimentary system, makes the CL textures of feldspar possible to use for qualitative provenance research only. We observed alkali feldspar mostly to luminesce in a bluish color and sometimes in red, and plagioclase in green to yellow. The corresponding CL spectra are dominated by three apparent intensity peaks at 440-520 nm (mainly blue), 540-620 nm (mainly green) and 680-740 nm (red to infrared). A dominance of the peak in the green wavelength interval over the blue one for plagioclase makes CL particularly useful for the differentiation of plagioclase from alkali feldspar. An apparent peak position in red to infrared at < 710 nm for plagioclase mainly is present in mafic rocks. Present-day coastal sand from Peru containing feldspar with the red to infrared peak position mainly exceeding 725 nm for northern Peruvian sand and a larger variety for sand from southern Peru illustrates a discriminative effect of different source areas. We conclude that the provenance application

  17. Construction of Layered Mafic Intrusions by Repeated Emplacement of Crystal Mushes

    NASA Astrophysics Data System (ADS)

    Ashwal, Lewis

    2013-04-01

    A widely accepted physical model for the origin of Proterozoic massif type anorthosites (e.g. Nain, Rogaland, Adirondacks) involves emplacement of plagioclase-rich mushes that ascended to shallow crustal emplacement sites from deep (~Moho) staging chambers in which ~An50 crystals floated due to density relations at high pressure. Supporting evidence includes large (up to ~1 m) grain size, compositional homogeneity of plagioclase with variable Mg# (caused by trapped liquid effects), and protoclastic textures. Isotopic disequilibrium (Sr, Nd, Pb) between cumulus plagioclase and post-cumulus pyroxene result from progressive contamination with continental components. This is dramatically demonstrated at Nain (Labrador), where the anorthositic crystal mushes (~1.3 Ga) were emplaced into early Archean (~3.8 Ga) country rocks. High-Al, high-pressure orthopyroxene megacrysts are commonly dragged upward in feldspathic mushes to shallow emplacement sites, where they exsolved plagioclase lamellae. Can a similar model be applied to layered mafic intrusions (LMI)? Many LMI (e.g. Bushveld, Stillwater, Dufek, Duluth) contain thick horizons of anorthosite with compositionally uniform plagioclase, in some cases throughout >1000 m of stratigraphy. This is best interpreted as representing repeated emplacement of plagioclase-rich mushes from one or more deeper crystallizing magma chambers, although the depths of these sub-chambers in the lithosphere are as yet uncertain. In the Bushveld Complex (South Africa), where deep drill cores have allowed near-continuous measurements of mineral compositions and geophysical properties, the results reveal a subtle cyclicity, invisible in outcrops, over scales of 50 - 100 m, commonly associated with broad reversals in mineral compositional trends. Each of these can be interpreted as a blending zone involving a new addition of crystal-laden magma. Much of Bushveld stratigraphy, and that of other LMI contains plagioclase : pyroxene demonstrably

  18. Mush!

    NASA Astrophysics Data System (ADS)

    Ashwal, L. D.

    2012-12-01

    Evidence for emplacement as crystal-laden mushes is abundant in mafic magma systems (i.e. products of broadly basaltic magmatism), including anorthosite complexes, layered mafic intrusions and a variety of sills and dikes. Some of the best examples involve the way feldspar becomes concentrated into anorthositic rocks. Proterozoic anorthosite massifs (e.g. Nain, Rogaland, Adirondacks), whose bulk compositions are characteristically hyperfeldspathic, are best interpreted as resulting from emplacement of plagioclase-rich mushes that ascended to shallow crustal emplacement sites from deep (~Moho) staging chambers in which ~An50 crystals floated due to density relations at high pressure. Supporting evidence includes large (up to ~1 m) grain size, compositional homogeneity of plagioclase with variable Mg# (caused by trapped liquid effects), and protoclastic textures. Isotopic disequilibrium between cumulus plagioclase and post-cumulus pyroxene result from progressive contamination with continental components. This is dramatically demonstrated at Nain (Labrador), where the anorthositic crystal mushes (~1.3 Ga) were emplaced into early Archean (~3.8 Ga) country rocks. High-Al, high-pressure orthopyroxene megacrysts are commonly dragged upward in feldspathic mushes to shallow emplacement sites, where they exsolved plagioclase lamellae (Bybee & Ashwal, this meeting). Archean calcic anorthosites (e.g. Fiskenaesset, W Greenland) and related sills, dikes and flows that contain homogeneous megacrysts up to 10s of cm across of ~An80 also must have formed by mush emplacement, although probably from shallower staging chambers in oceanic rather than continental crust. Many layered mafic intrusions (e.g. Bushveld, Stillwater, Dufek, Duluth) contain thick horizons of anorthosite in which plagioclase compositions are uniform, in some cases throughout >1000 m of stratigraphy. This is best interpreted as representing repeated emplacement of plagioclase-rich mushes from one or more deeper

  19. Rare earth element evidence for the petrogenesis of the banded series of the Stillwater Complex, Montana, and its anorthosites

    USGS Publications Warehouse

    Loferski, P.J.; Arculus, R.J.; Czamanske, G.K.

    1994-01-01

    A rare earth element (REE) study was made by isotope-dilution mass spectrometry of plagioclase separates from a variety of cumulates stratigraphically spanning the Banded series of the Stillwater Complex, Montana. Evaluation of parent liquid REE patterns, calculated on the basis of published plagioclase-liquid partition coefficients, shows that the range of REE ratios is too large to be attributable to fractionation of a single magma type. At least two different parental melts were present throughout the Banded series. This finding supports hypotheses of previous workers that the Stillwater Complex formed from two different parent magma types, designated the anorthosite- or A-type liquid and the ultramafic- or U-type liquid. -from Authors

  20. Role of replacement in the genesis of anorthosite in the Boehls Butte area, Idaho.

    USGS Publications Warehouse

    Hietanen, A.

    1986-01-01

    In this area in N Idaho, three large and numerous small lenses of layered to massive anorthosite consisting of two, and locally three, types of plagioclase with minor hornblende and mica occur in aluminium silicate-rich garnet mica schist. In most of this anorthosite, megacrysts of andesine with bytownite inclusions are embedded in a fine-grained groundmass of bytownite or anorthite; locally, labradorite occurs rather than andesine. Some labradorite laths show Carlsbad twinning and rims of andesine around anorthite inclusions. Along the contacts, lenses of fine-grained bytownite anorthosite with some hornblende or garnet and quartz are common. These lenses could represent calcic parent rocks converted to two-plagioclase rocks by partial replacement of bytownite by andesine. -R.A.H.

  1. Lunar ferroan anorthosite 60025 - Petrology and chemistry of mafic lithologies

    NASA Technical Reports Server (NTRS)

    James, O. B.; Mcgee, J. J.; Lindstrom, M. M.

    1991-01-01

    Eleven splits from the mafic-mineral-rich part of anorthosite 60025 were studied in order to establish the exact nature and causes of compositional variations in the minerals of lunar ferroan anorthosites. All splits were analyzed by INAA, and five were studied intensively by petrologic techniques. All splits were found to have similar cataclastic textures and show textural evidence of at least two episodes of deformation. The whole-rock split contains mafic minerals having a wide range of compositions and is probably polymict. It is suggested that the rare-earth patterns for all splits can be duplicated safactorily, assuming that the equilibrium liquids had flat, or nearly flat, chondrite-normalized rare-earth patterns. The plagioclases in all splits were found to be identical. Data obtained indicate that in ferroan anorthosites An content in plagioclase and mg' of associated mafic minerals are not strongly correlated.

  2. Rare earth elements in minerals of the ALHA77005 shergottite and implications for its parent magma and crystallization history

    NASA Technical Reports Server (NTRS)

    Lundberg, Laura L.; Crozaz, Ghislaine; Mcsween, Harry Y., Jr.

    1990-01-01

    Analyses of mineral REE and selected minor and trace elements were carried out on individual grains of pyroxenes, whitlockite, maskelynite, and olivine of the Antarctic shergottite ALHA77005, and the results are used to interpret its parent magma and crystallization history. The results of mineral compositions and textural observations suggest that ALHA77005 is a cumulate with about half cumulus material (olivine + chromite) and half postcumulus phases. Most of the REEs in ALHA77005 reside in whitlockite whose modal concentration is about 1 percent. Mineral REE data support previous suggestions that plagioclase and whitlockite crystallized late, and that low-Ca pyroxene initiated crystallization before high-Ca pyroxene. The REE patterns for the intercumulus liquid, calculated from distribution coefficients for ALHA77005 pyroxene, plagioclase, and whitlockite, are in very good agreement and are similar to that of Shergotty.

  3. Origin of High-Alumina Basalt, Andesite, and Dacite Magmas.

    PubMed

    Hamilton, W

    1964-10-30

    The typical volcanic rocks of most island arcs and eugeosynclines, and of some continental environments, are basalt, andesite, and dacite, of high alumina content. The high-alumina basalt differs from tholeiitic basalt primarily in having a greater content of the components of calcic plagioclase. Laboratory data indicate that in the upper mantle, below the level at which the basaltic component of mantle rock is transformed by pressure to eclogite or pyroxenite, the entire basaltic portion probably is melted within a narrow temperature range, but that above the level of that transformation plagioclase is melted selectively before pyroxene over a wide temperature range. The broad spectrum of high-alumina magmas may represent widely varying degrees of partial melting above the transformation level, whereas narrow-spectrum tholeiite magma may represent more complete melting beneath it.

  4. Effect of water on the composition of partial melts of greenstone and amphibolite

    NASA Technical Reports Server (NTRS)

    Beard, James S.; Lofgren, Gary E.

    1989-01-01

    Closed-system partial melts of hydrated, metamorphosed arc basalts and andesites (greenstones and amphibolites), where only water structurally bound in metamorphic minerals is available for melting (dehydration melting), are generally water-undersaturated, coexist with plagioclase-rich, anhydrous restites, and have compositions like island arc tonalites. In contrast, water-saturated melting at water pressures of 3 kilobars yields strongly peraluminous, low iron melts that coexist with an amphibole-bearing, plagioclase-poor restite. These melt compositions are unlike those of most natural silicic rocks. Thus, dehydration melting over a range of pressures in the crust of island arcs is a plausible mechanism for the petrogenesis of islands arc tonalite, whereas water-saturated melting at pressure of 3 kilobars and above is not.

  5. Petrology and geochemistry of mafic and ultramafic cumulates occurring as xenoliths in volcanic rocks from Polish part of Central European Volcanic Province.

    NASA Astrophysics Data System (ADS)

    Dajek, Michał; Matusiak-Małek, Magdalena; Puziewicz, Jacek; Ntaflos, Theodoros

    2015-04-01

    Mafic xenoliths coexisting with the peridotitic ones in rocks from Polish part of Cenozoic European Volcanic Province have been scarcely examined. (Bakun-Czubarow and Białowolska, 2003, Mineralogical Society of Poland- Spec. Pap. and references therein; Matusiak, 2006, Min. Polonica- Spec. Pap.; Puziewicz et al., 2011, JoP). In this study we present new results on mafic and ultramafic xenolithic rocks from the Wilcza Góra, Winna Góra, Góra Świątek, Mnisia Górka and Grodziec volcanic rocks in the Złotoryja-Jawor Volcanic Complex. The studied xenoliths are either plagioclase-free (clinopyroxenite, websterite) or plagioclase-bearing (anorthosite, gabbro, olivine-bearing gabbro and norite). Both the types may occur in the same volcanic rock. The cumulative xenoliths are smaller than peridotitic ones, blackish and show clear cumulative, coarse grained textures. Beside the rock-forming phases, the xenoliths occasionally contain spinel, sulfides and amphibole. Usually clinopyroxene grains occurring in gabbros are strongly corroded or disintegrated, while other phases are well-preserved. Contacts between xenolith and host volcanic rock are usually sharp with subhedral crystals of clinopyroxene growing at the xenolith surface. The mineral grains are usually zoned and chemical equilibrium between phases is scarce. Clinopyroxene in plagioclase-free rocks has composition of diopside with occasionally elevated Al, Ti and Cr contents. It's mg# varies from 0.89 to 0.79. It is slightly to moderately enriched in LREE; the REE patterns are concave, and the normalized values vary significantly between localities. It shows negative Sr anomaly, depth of Ti anomaly is variable. Orthopyroxene is Al-rich enstatite with mg# varying from 0.85 down to 0.75. Orthopyroxene in websterites is LREE depleted and show strong positive Ti and Zr-Hf anomalies. Opaques are ilmenite - Ti-magnetite solid solution and/or sulfides Clinopyroxene forming plagioclase-bearing rocks also has composition

  6. Composition and maturity of the 60013/14 core

    NASA Technical Reports Server (NTRS)

    Korotev, Randy L.; Morris, Richard V.; Lauer, Howard V., Jr.

    1993-01-01

    The 60013/14 double drive tube (62 cm deep) is one of three regolith cores taken 35-40 m apart in a triangular array on the Cayley plains at station 10' (LM/ALSEP), Apollo 16. This trio, which includes double drive tube 60009/10 (59 cm deep) and deep drill core 60001-7 (220 cm), is the only such array of cores returned from the Moon. The top 45 cm of 60013/14 is mature, as is surface reference soil 60601 taken nearby. Maturity generally decreases with depth, with soil below 45 cm being submature. The zone of lowest maturity (34 is less than or equal to I(sub s)/FeO is less than 50) extends from 46 to 58 cm depth, and corresponds to the distinct region of light-colored soil observed during core processing. In the other two cores, most of the compositional variation results from mixing between fine-grained, mature soil with 10-11 micro-g/g Sc and coarse-grained ferroan anorthosite consisting of greater than 99% plagioclase with less than 0.5 micro-g/g Sc. This is most evident in 60009/10 which contains a high abundance of plagioclase at about 54 cm depth (minimum Sc: 3-4 micro-g/g); a similar zone occurs in 60001-7 at 17-22 cm (MPU-C), although it is not as rich in plagioclase (minimum Sc: 6-7 micro-g/g). Compositional variations are less in 60013/14 than in the other two cores (range: 7.9-10.0 micro-g/g Sc), but are generally consistent with the 'plagioclase dilution' effect seen in 60009/10, i.e., most 60013/14 samples plot along the mixing line of 60009/10. However, a plagioclase component is not the cause of the lower maturity and lighter color of the unit at 46-58 cm depth in 60013/14. Many of the samples in this zone have distinctly lower Sm/Sc ratios than typical LM-area soils and plot off the mixing trend defined by 60009/10. This requires a component with moderately high Sc, but low-Sm/Sc, such as feldspathic fragmental breccia (FFB) or granulitic breccia. A component of Descartes regolith, such as occurs at North Ray Crater (NRC) and which is rich in FFB

  7. Composition of the lunar upper crust estimated from Kaguya spectral data

    NASA Astrophysics Data System (ADS)

    Ohtake, M.; Matsunaga, T.; Takeda, H.; Yokota, Y.; Yamamoto, S.; Moroda, T.; Ogawa, Y.; Hiroi, T.; Nakamura, R.; Haruyama, J.

    2010-12-01

    The magma ocean hypothesis has been the most widely accepted mechanism explaining the generation of the lunar highland crust. This hypothesis