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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. Melt segregation in plagioclase-poikilitic mesosiderites

    NASA Technical Reports Server (NTRS)

    Hewins, R. H.; Harriott, T. A.

    1986-01-01

    The Budalan and Mincy mesosiderites contain a poikilitic-plagioclase matrix with orthopyroxene chadacrysts and interstitial-subophitic inverted pigeonite. Orthopyroxene chadacrysts in both mesosiderites are uniformly more aluminous than orthopyroxene clasts, suggesting that they were not derived from clasts by metamorphism. Interstitial inverted pigeonite is more ferroan than adjacent orthopyroxene in the matrix, consistent with the crystallization of a melt with the sequence orthopyroxene followed by pigeonite. The magnesium chadcrysts in Mincy could not have formed from a melt in equilibrium with the clasts but could have crystallized from impact melt. The most Mg chadacrysts are enclosed in large reversely zoned plagioclase crystals as a result of the undercooling in melt-lacking plagioclase clasts and associated nuclei. Mincy contains both plagioclase-poor and plagioclase-rich regions, explained by a separation of silicate melt into pools. Reckling Peak A80258, a plagioclase-poikilitic mesosiderite with a very high chadacryst/plagioclase ratio, resembles Mincy material from which melt has been extracted. It is suggested that the origin of the plagioclase-poikilitic mesosiderites is impact melting of a metal-silicate mixture.

  4. Thermal expansion of plagioclase feldspars

    NASA Astrophysics Data System (ADS)

    Tribaudino, M.; Angel, R. J.; Cámara, F.; Nestola, F.; Pasqual, D.; Margiolaki, I.

    2010-12-01

    The volume thermal expansion coefficient and the anisotropy of thermal expansion were determined for nine natural feldspars with compositions, in terms of albite (NaAlSi3O8, Ab) and anorthite (CaAl2Si2O8, An), of Ab100, An27Ab73, An35Ab65, An46Ab54, An60Ab40, An78Ab22, An89Ab11, An96Ab4 and An100 by high resolution powder diffraction with a synchrotron radiation source. Unit-cell parameters were determined from 124 powder patterns of each sample, collected over the temperature range 298-935 K. The volume thermal expansion coefficient of the samples determined by a linear fit of V/ V 0 = α( T - T 0) varies with composition ( X An in mol %) as: αV = 2.90left( 4 right) × 10^{ - 5} - 3.0left( 2 right) × 10^{ - 7} *X_{text{An}} + 1.8left( 2 right) × 10^{ - 9} *X_{text{An}}2 Two empirical models for the non-linear behaviour of volume with temperature give a better fit to the experimental data. The change with composition in the a° parameter of the non-linear Holland-Powell model V/ V 0 = 1 + a°( T - T 0) + 20a° (√ T - √ T 0) is: a^circ = 4.96left( 5 right) × 10^{ - 5} - 4.7left( 2 right) × 10^{ - 7} *X_{text{An}} + 2.2left( 2 right) × 10^{ - 9} *X_{text{An}}2 For the Berman model, V/ V 0 = a 1( T - T 0) + a 2*( T- T 0)2, the parameters change with composition as: begin{aligned}& a1 = 2.44left( {15} right) × 10^{ - 5} - 3.1left( 6 right) × 10^{ - 7} *X_{text{An}}\\& quad + 1.8left( 5 right) × 10^{ - 9} *X_{text{An}}2 \\& a2 = 9left( 1 right) × 10^{ - 9} - 4left( 2 right) × 10^{ - 11} *X_{text{An}} \\ The thermal expansion of all plagioclases is very anisotropic, with more than 70% of the volume expansion being accommodated by a direction fairly close to the (100) plane normal, whereas perpendicular directions exhibit smaller, and in some cases slightly negative or zero, thermal expansion.

  5. 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.

  6. 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.

  7. Thermochemistry of the high structural state plagioclases

    NASA Astrophysics Data System (ADS)

    Newton, R. C.; Charlu, T. V.; Kleppa, O. J.

    1980-07-01

    The enthalpies of solution of a suite of 19 high-structural state synthetic plagioclases were measured in a Pb 2B 2O 5 melt at 970 K. The samples were crystallized from analyzed glasses at 1200°C and 20 kbar pressure in a piston-cylinder apparatus. A number of runs were also made on Amelia albite and Amelia albite synthetically disordered at 1050-1080°C and one bar for one month and at 1200°C and 20 kbar for 10 hr. The component oxides of anorthite, CaO, Al 2O 3 and SiO 2, were remeasured. The ΔH of disorder of albite inferred in the present study from albite crystallized from glass is 3.23 kcal, which agrees with the 3.4 found by HOLM and KLEPPA (1968). It is not certain whether this value includes the ΔH of a reversible displacive transition to monoclinic symmetry, as suggested by HELGESONet al. (1978) for the Holm-Kleppa results. The enthalpy of solution value for albite accepted for the solid solution series is based on the heat-treated Amelia albite and is 2.86 kcal less than for untreated Amelia albite. The enthalpy of formation from the oxides at 970 K of synthetic anorthite is -24.06 ± 0.31 kcal, significantly higher than the -23.16 kcal found by CHARLUet al. (1978), and in good agreement with the value of -23.89 ± 0.82 given by ROBIEet al. (1979), based on acid calorimetry. The excess enthalpy of mixing in high plagioclase can be represented by the expression, valid at 970 K: ΔHex(±0.16 kcal) = 6.7461 XabX2An + 2.0247 XAnX2Ab where XAb and XAn are, respectively, the mole fractions of NaAlSi 3O 8 and CaAl 2Si 2O 8. This ΔH ex, together with the mixing entropy of KERRICK and DARKEN'S (1975) Al-avoidance model, reproduces almost perfectly the free energy of mixing found by ORVILLE (1972) in aqueous cation-exchange experiments at 700°C. It is likely that Al-avoidance is the significant stabilizing factor in the high plagioclase series, at least for XAn≥ 0.3. At high temperatures the plagioclases have nearly the free energies of ideal one

  8. 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.

  9. Europium anomaly in plagioclase feldspar: experimental results and semiquantitative model.

    PubMed

    Weill, D F; Drake, M J

    1973-06-01

    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. PMID:17806582

  10. 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.

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

    NASA Astrophysics Data System (ADS)

    Wark, D. A.

    1987-02-01

    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.

  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. Plagioclase-Hosted Magnetite Inclusions From the Bushveld Complex

    NASA Astrophysics Data System (ADS)

    Feinberg, J. M.; Scott, G. R.; Renne, P. R.; Wenk, H.

    2004-12-01

    Gabbros from the Main Zone of the 2.064 Ga Bushveld Complex have long been known to possess unusually stable magnetizations due to the presence of high coercivity, exsolved magnetite inclusions in plagioclase and clinopyroxene. The paleomagnetic pole for these rocks has been used to anchor apparent polar wander paths for the Kaapval craton during the Early-Mid Proterozoic. To better understand the rock magnetic properties of silicate-hosted magnetite inclusions, oriented paleomagnetic samples of gabbro were collected from quarries near Belfast and Rustenberg, South Africa, sampling the eastern and western limbs of the Complex, respectively. Plagioclase composition at both sites ranges from An55 (rims) to An65 (cores) based on optical and electron microprobe data. Four kinds of inclusions are present within the plagioclase: elongate magnetite needles, nanometer-scale magnetite particles (responsible for the "cloudy" appearance of some crystals), translucent brown hematite/ilmenite platelets, and colorless euhedral inclusions of pyroxene and/or feldspar. Magnetite inclusions are most abundant at the cores of the plagioclase crystals. Orientations of the needles and the platelets are crystallographically controlled by the silicate host. Although the elongation direction of the magnetite inclusions can occur in any of five possible orientations, only two or three of these directions dominates each plagioclase crystal. Alternating field demagnetization of bulk samples (NRM = 1.5 x 101 A m-1) shows univectorial remanence with average median destructive fields (MDF) of 115 mT (Belfast) and 90 mT (Rustenberg). AF demagnetization of single plagioclase crystals (NRM = 100 A m-1) also shows single component remanence with average MDFs >150 mT. The NRM coercivity spectra of single plagioclase crystals are indistinguishable from that of the bulk samples. When normalized to their abundance in bulk samples the magnetite-bearing plagioclase fully accounts for the NRM of Bushveld

  15. 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.

  16. The origin of olivine-rich troctolites/plagioclase-dunites

    NASA Astrophysics Data System (ADS)

    Garapic, G.; Faul, U.; Kruckenberg, S. C.; Wiejaczka, J.; Newton, J. C.

    2015-12-01

    Olivine-rich troctolites or plagioclase dunites are a type of rock commonly found in oceanic crust and peridotite massifs that does not fit into a IUGS classification. Part of the reason is that their origin is poorly constrained, in particular whether these rocks are cumulates or residual mantle rocks. To avoid implications for origin or process Blackman et al. 2006 defined as olivine-rich troctolites rocks that contain > 70% olivine,with plagioclase, cpx and spinel. We examined this type of rock in Krivaja peridotite massif in Bosnia-Herzegovina where it occurs as massive outcrops with an area of several tens of square kilometres. The plagioclase dunites are underlain by peridotites that contain plagioclase patches indicative of melt migration. These peridotites are progressively depleted of pyroxene and cross-cut by gabbro veins. The plagioclase dunites have Mg# predominantly from 89 - 90 and Ni contents from 2500 - 3500 ppm, similar to the peridotites. EBSD mapping of whole thin sections shows orientation distribution functions (odf) that are unlike any of the fabric types observed in naturally or experimentally deformed rocks with a strong maximum in (001) near the foliation plane (although this plane is poorly defined) and weak girdles of the other two axes perpendicular to it. For comparison, we also mapped known cumulates from the Rum and Stillwater layered intrusions with somewhat lower Mg# of 84 - 86. The odf of these samples show a strong maximum of (010) perpendicular to the foliation plane and weak girdles of the other two axes in the plane. This fabric type has been explained by crystal settling and compaction in a magma chamber. Together these observations show that the plagioclase dunites from Krivaja, as well as rocks with similar characteristics e.g. from the Mid-Atlantic Ridge (Drouin et al. 2010) and Italy (Renna and Tribuzio, 2011) are not cumulates but are of mantle origin. An important characteristic of the plagioclase dunites is that the

  17. 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.

  18. 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.

  19. Numerical Simulation of Plagioclase Growth During Magma Storage and Ascent

    NASA Astrophysics Data System (ADS)

    Melnik, O. E.; Gorokhova, N. V.; Plechov, P.

    2012-12-01

    A mathematical model of plagioclase growth from a magmatic melt is developed. Plagioclase is one of the main rock-forming minerals. Plagioclase is composed of molecules of two types: anorthite and albite. Crystallization occurs as a result of component (albite and anorthite) diffusion to the boundary of the growing crystal and opposite diffusion of residual melt components. The model is based on multicomponent diffusion equations with the real dependencies between crystal and melt compositions, pressure and temperature. Mass conservation is applied as a boundary condition on a crystal-melt interface. Crystal growth rate depends on local magma undercooling in the vicinity of the crystal. We assume that the growth is in 1D and that the crystal is located in a cell with symmetry (no mass flux) conditions on the cell boundaries. Diffusion of components within the crystal is negligibly slow. Based on this model variation of plagioclase composition with respect of monotonic cooling or pressure drop is studied. It is shown that due to interplay between diffusion of melt components to crystal-melt interface and crystal growth rate crystal composition can vary in a non-monotonic way in response to linear decrease in temperature. A condition of presence of plagioclase zonation is obtained as a result of parametric studies. A growth of rims on plagioclase crystals related to magma ascent during dome building eruptions on Bezymianny volcano (Kamchatka) is simulated. Bezymianny Volcano supplies material to the Earth surface about every half a year. Eruptions of 2000, 2006 and 2007 were selected to determine the magma ascent conditions as the most typical for the last activity cycle. The role of initial conditions, crystal growth rate, the ratio between anorthite and albite diffusion rates and ascent conditions on the rim composition is investigated. The ascent conditions of magma for Bezymianny Volcano eruptions were reconstructed. Simulations suggest ascent from a pressure of

  20. 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

  1. 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

  2. Plagioclase dissolution during CO₂-SO₂ cosequestration: effects of sulfate.

    PubMed

    Min, Yujia; Kubicki, James D; Jun, Young-Shin

    2015-02-01

    Geologic CO2 sequestration (GCS) is one of the most promising methods to mitigate the adverse impacts of global climate change. The performance of GCS can be affected by mineral dissolution and precipitation induced by injected CO2. Cosequestration with acidic gas such as SO2 can reduce the high cost of GCS, but it will increase the sulfate's concentration in GCS sites, where sulfate can potentially affect plagioclase dissolution/precipitation. This work investigated the effects of 0.05 M sulfate on plagioclase (anorthite) dissolution and subsequent mineral precipitation at 90 °C, 100 atm CO2, and 1 M NaCl, conditions relevant to GCS sites. The adsorption of sulfate on anorthite, a Ca-rich plagioclase, was examined using attenuated total reflectance Fourier-transform infrared spectroscopy and then simulated using density functional theory calculations. We found that the dissolution rate of anorthite was enhanced by a factor of 1.36 by the formation of inner-sphere monodentate complexes between sulfate and the aluminum sites on anorthite surfaces. However, this effect was almost completely suppressed in the presence of 0.01 M oxalate, an organic ligand that can exist in GCS sites. Interestingly, sulfate also inhibited the formation of secondary mineral precipitation through the formation of aluminum-sulfate complexes in the aqueous phase. This work, for the first time, reports the surface complexation between sulfate and plagioclase that can occur in GCS sites. The results provide new insights for obtaining scientific guidelines for the proper amount of SO2 coinjection and finally for evaluating the economic efficiency and environmental safety of GCS operations. PMID:25549263

  3. Grain-Recycling Zoning of Plagioclase and Metamorphic Fractionation

    NASA Astrophysics Data System (ADS)

    Pearce, M. A.; Wheeler, J.

    2008-12-01

    Quartzo-feldspathic gneisses make up much of the continental crust often having enjoyed a complex thermal history. Determining peak metamorphic conditions using conventional equilibrium thermodynamics is difficult because there are too many degrees of freedom. Zoned minerals are problematic, because of uncertainties in the exact equilibrium assemblage at any particular time, but provide a time-dependent measure of changes in equilibrium conditions. Zoning can arise due to diffusion of atoms into a homogeneous lattice from grain boundarys or through mineral growth under changing pressure, temperature or bulk rock composition. Conventional growth zoning considers a porphyroblast (commonly garnet) growing in an effectively homogeneous matrix with the growth rate controlled by reactions that produce new porphyroblast material. However, simulations of zoning developed by grain growth in a monophase domains of more complex rocks show boundary migration rates control the zoning geometry as shrinking grains are cannibalised by growing grains. This new grain-recycling zoning develops because chemical reactions change the composition of the material that is swept by the grain boundary without changing the mineral. A model of this process has been created using the Gibbs free energy minimisation software Theriak-Domino controlled by our custom written Matlab control program. This program assumes an initially homogeneous composition of equigranular plagioclase then uses the experimentally determined normal growth law for plagioclase to calculate the grain-size increase for a given time-step thus giving the amount of material swept. Assuming this is all available for reaction at the same time, the rest of the plagioclase is removed from the bulk composition, the equilibrium plagioclase composition calculated, and added to the growing grain. This fractionation alters the range of plagioclase compositions available over a given temperature range, changing PT estimates obtained

  4. 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

  5. Spectral properties of plagioclase and pyroxene mixtures and the interpretation of lunar soil spectra

    NASA Technical Reports Server (NTRS)

    Crown, David A.; Pieters, Carle M.

    1987-01-01

    Pyroxene and plagioclase mixtures' spectral characteristics are investigated with a view to particle size and mineral proportion effects in the cases of variously proportioned labradorite/enstatite compositional mixtures. The amount of plagioclase that will allow its band to be observed in discrete absorption is particle size-dependent; plagioclase is detectable in the flattening of the pyroxene reflectance peak that lies between the 0.9 and 1.9 micron absorption bands, if sufficient plagioclase is present. Attention is given to the combined effects of mineral proportions and particle size for plagioclase/pyroxene mixtures in the case of analogs of mature and immature lunar highland soils.

  6. Weathering of plagioclase across variable flow and solute transport regimes

    NASA Astrophysics Data System (ADS)

    Pacheco, Fernando A. L.; Van der Weijden, Cornelis H.

    2012-02-01

    SummaryThe study area is situated in a fault zone with fractured granites and metasediments. In a conceptual model, infiltrating water first passes the bedrock cover of soil and saprolite and then partly enters the fractures. Weathering reactions of minerals occur in small pores and fissures in the bedrock cover zone to continue in the larger fractures. Pumping tests were carried out in a number of boreholes to measure the drawdown as a function of pumping time. From the results, values of transmissivity ( T) could be derived. In combination with the storage coefficient ( S) for similar fault zones, the hydraulic diffusivity ( D = T/ S) could be computed. Water samples, collected from the boreholes, represent fluid packets with a history of weathering reactions in the bedrock cover and in the larger fractures. The major element composition of these samples was used by means of the SiB mass balance algorithm ( Pacheco and Van der Weijden, 1996) to calculate the moles L -1 of dissolved plagioclase (oligoclase with An ≈ 0.20) and the moles L -1 of secondary phases (gibbsite, halloysite, smectite) precipitated along the flow paths of the samples. These results were then used to calculate the net dissolved silica concentrations ( [HSiO40]) related to dissolution of plagioclase followed by precipitation of each of the secondary phases. An interpretation of a plot of each of these [HSiO40] 's versusD is that at D < 0.7 m 2 s -1, dissolution of plagioclase is followed by precipitation of halloysite in the large fractures of the fault zone (open system), whereas at D ⩾ 0.7 m 2 s -1 precipitation of both halloysite and smectite occurs in the rock matrix with small fissures and pores (semi-open system). Before being pumped, the percolating fluids travelled 0.01-13.7 years. During these periods, plagioclase weathered at rates ( W Pl) of 10 -(12.9±1.1) moles m -2 s -1, which are approximately 2.2 orders of magnitude higher than solid-state weathering rates reported in

  7. Visible/Near-Infrared Spectra of Experimentally Shocked Plagioclase Feldspars

    NASA Technical Reports Server (NTRS)

    Johnson, J. R.

    2003-01-01

    Minerals subjected to high shock pressures exhibit structural changes with increasing pressure (e.g., fractures, deformations, formation of diaplectic glass, and complete melting [1-6]). Petrologic and thermal infrared spectroscopic studies have shown that diaplectic glass (maskelynite) formation in feldspars occurs between 25-45 GPa, while significant melting occurs above 45 GPa [7- 12]. Past studies of visible/near-infrared spectra of shocked plagioclase feldspars demonstrated few variations in spectral features with pressure except for a decrease in the absorption feature near 1250 nm and an overall decrease in albedo [13-17]. We report new visible/near-infrared spectra of albite- and anorthiterich rocks experimentally shocked from 17-56 GPa.

  8. Plagioclase twin laws in lunar highland rocks - Possible petrogenetic significance

    NASA Technical Reports Server (NTRS)

    Dowty, E.; Keil, K.; Prinz, M.

    1974-01-01

    Plagioclases in different types of lunar highland rocks (all highly feldspathic) are twinned according to different laws and in different styles. Carlsbad and Carlsbad-albite twins, presumed to be growth twins, occur mainly in rocks which show igneous texture, and which have not been severely brecciated. These two twin laws appear to be absent from cataclastic rocks, including cataclastic anorthosite, possibly because the original twins were preferentially broken up in cataclasis (the composition plane being a plane of weakness). Pericline and lamellar albite twins, presumed to be deformation twins (except for some albite growth twins) occur in all types of rocks, and obvious deformation features, such as bending of lamellae, are well shown in many cataclastic rocks. Surprisingly, some Carlsbad and Carlsbad-albite twins are found in rocks with granoblastic texture, which presumably recrystallized in the solid state.

  9. Origin of plagioclase-olivine inclusions in carbonaceous chondrites

    NASA Astrophysics Data System (ADS)

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

    1991-02-01

    The origin of plagioclase-olivine inclusions (POIs) from three CV chondrites and one ungrouped chondrite was investigated by examining the chemical, mineralogical, and isotopic characteristics of a group of POIs from these chondrites. Results of these analyses demonstrate that the mixing and the partial melting processes in these inclusions were superimposed on more ancient isotopically heterogeneous material. A comparison of the essential characteristics of POIs and CAIs suggests that the major processes leading to the formation of POIs (such as condensation, dust/gas fractionation, aggregation of chemically and isotopically disparate materials, and partial melting) are common to most CAIs and chondrules. A scenario for the origin of POIs is proposed, showing that the homogeneity of the final assemblage (whether a POI, a CAI, or a chondrite) is primarily a reflection of the thermal history rather than the nature of precursor materials.

  10. The Petrogenetic significance of Plagioclase megacrysts in Archean rocks

    NASA Technical Reports Server (NTRS)

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

    1988-01-01

    The petrogenetic significance of plagioclase megacryst-bearing Archean rocks was considered. It was suggested that these developed in mid-to upper-crustal magma chambers that were repeatedly replenished. Crystallization of megacrysts from a primitive liquid that evolves to an Fe-rich tholeiite (with LREE enrichment) is nearly isothermal and is an equilibrium process. Cumulates probably form near the margins of the chambers and liquids with megacrysts are periodically extracted and can appear as volcanics. Some flows and intrusives are found in arc-like settings in greenstone belts. Megacrystic dikes represent large volumes of melt and dike swarms such as the Metachawan swarm of Ontario suggest multiple sources of similar compositions. A complex series of melt ponding and migration are probable and involve large amounts of liquid.

  11. 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.

  12. 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.

  13. 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].

  14. 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.

  15. 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.

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

    USGS Publications Warehouse

    Tepley, F. J., III; 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

  17. 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

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

    NASA Astrophysics Data System (ADS)

    Johnson, Jeffrey R.; Hörz, Friedrich

    2003-11-01

    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.

  19. 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.

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

    USGS Publications Warehouse

    Tepley, F. J., III; 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.

  1. 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)

  2. 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

  3. 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.

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

    USGS Publications Warehouse

    Rose, W.I., Jr.; 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.

  5. Space weathering on Mercury: Simulation of plagioclase weathering

    NASA Astrophysics Data System (ADS)

    Sasaki, Sho; Hiroi, Takahiro; Helbert, Jorn; Arai, Tomoko

    ions may also increase the size of nanophase iron. The other possibility for attenuating space weathering on Mercury would be deeper mixing depth. The surface mixing by impacts on Mercury is greater than that on the Moon, because of higher impact flux and velocity of incoming meteoroid bodies. The difference of space weathering between the Moon and Mercury might be also due to the compositional effect. Mercury surface is considered to be plagioclase-rich like the highland of the Moon. We started experimental simulation of space weathering on Mercury (and the Moon) using anorthite samples. Although pure anorthite is in lack of iron, addition of iron-bearing material could alter the anorthite reflectance. Our experiments show that laser irradiation on pure anorthite should not alter its spectrum largely. The addition of small amount of pyroxene can change the anorthite reflectance upon laser irradiation (darkening/reddening of visible spectrum). Addition of 5 References: [1] Keller, L. P. and McKay, D. S. (1993), Science 261, 1305. [2] Sasaki, S. et al. (2001) Nature 410, 555. [3] Sasaki, S. and Kurahashi, E. (2004) Adv. Space Res. 33, 2152. [4] McClintock, W. E. et al. (2008) Science 321, 62. [5] Noble, S. K. and Pieters, C. M. (2001) Mercury: Space Environ. Surface Interior,8012.

  6. 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.

  7. Plagioclase sub-species in Chinese loess deposits: Implications for dust source migration and past climate change

    NASA Astrophysics Data System (ADS)

    He, Tong; Liu, Lianwen; Chen, Yang; Sheng, Xuefen; Ji, Junfeng

    2016-01-01

    Plagioclase mineral sub-species in the Lingtai Section in central Chinese Loess Plateau are examined using Mineral Liberation Analyzer techniques, showing that loess and paleosol samples exhibit similar patterns in terms of plagioclase feldspar sub-species content. This suggests that both loess and paleosol units have preserved their primary Ca-bearing plagioclase compositions of loess source regions. Weighted average CaO (%) in Ca-bearing plagioclase lies within a narrow range and is equivalent to the average plagioclase composition for upper continental crust. This fact supports the hypothesis that Chinese loess deposits are the result of a thorough mixing of dust sources. The sum of Ca-bearing plagioclase content exhibits a general increasing trend superimposed by glacial-interglacial oscillations. In combination with observed plagioclase data in the deserts, the variations of Ca-bearing plagioclase minerals might be used as a proxy for dust source migration and climate changes in the loess source regions. Furthermore, linear relationship between lithogenic magnetic susceptibility (MS) component input and contents of Ca-bearing plagioclase in loess units revises a MS proxy for reconstructing paleo-monsoon precipitation history. The revised MS and plagioclase sub-species records help in understanding the mechanism of glaciation across northern Tibetan Plateau.

  8. Nonlinear partitioning of OH between Ca-rich plagioclase and arc basaltic melt

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    The hydrogen in nominally anhydrous minerals (NAMs) is becoming a new proxy for dissolved H2O in silicate melts. Plagioclase is one of the NAMs which accommodates hydrogen as OH. Here, we report experimental results on the partitioning of OH between Ca-rich plagioclase and arc basaltic melt. We carried out hydrous melting experiments of arc basaltic magma at 350 MPa using an internally-heated pressure vessel. Starting material was hydrous glass (0.8 wt.%≦H2O≦4.5 wt.%) of an undifferentiated rock from Miyakejima volcano, a frontal-arc volcano in Izu-arc (MTL rock: 50.5% SiO2, 18.1% Al2O3, 4.9% MgO). A grain of Ca-rich plagioclase (≈ 1 mg, about An95, FeOt ≈ 0.5 wt.%) and ≈ 10 mg of powdered glasses were sealed in Au80Pd20 alloy capsule and kept at around the liquidus temperature. Liquidus phase of MTL rock at 350 MPa is always plagioclase with 0 to 4.5 wt.% H2O in melt, and therefore, a grain of plagioclase and hydrous melt are nearly in equilibrium. Oxygen fugacity during the melting experiments was not controlled; the estimated oxygen fugacity was 3 log unit above Ni-NiO buffer. Experiments were quenched after 24-48 hours. Concentrations of H2O in melt and concentration of OH in plagioclase were analyzed by infrared spectroscopy. Obtained correlation between H2O concentration in melt and OH concentration in plagioclase is nonlinear; partition coefficient in molar basis is ≈ 0.01 with low H2O in melt (≤ 1 wt.%), while it decreases down to ≈ 0.005 with increasing H2O in melt (Fig.1). The OH concentration of Ca-rich plagioclase (about An90) from the 1986 summit eruption of Izu-Oshima volcano, also a frontal-arc volcano in Izu arc, shows variation ranging from <50 ppm H2O through 300 ppm H2O as a result of polybaric degassing (Hamada et al. 2011, EPSL 308, 259-266). Melting experiments of hydrous basalts constrained that An90 plagioclase crystallizes form H2O-rich melt (up to 6 wt.% H2O). In consistent with previous studies, our experiments demonstrate

  9. 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.

  10. Significance of major and minor element variations in plagioclase in sodic ferrogabbro and breccia matrix in lunar highlands sample 67915

    NASA Technical Reports Server (NTRS)

    Weiblen, P. W.; Day, W. C.; Miller, J. D., Jr.

    1980-01-01

    Attention is given to the significance of the results of a study of Ca, K, Ti, Fe, Mn, and Mg variations in plagioclase in highlands sample 67915,84. This polymict breccia from Outhouse Rock station 11 at the Apollo 16 site has been selected for study because it contains a wide variety of clast types, including a differentiated type-sodic ferrogabbro. It is found that the data on Ca, K, Ti, Fe, Mn, and Mg in plagioclase show no evidence of reaction between sodic ferrogabbro and breccia matrix clasts. Two groups of plagioclase compositions have been recognized in the breccia matrix. The data suggest that the prebreccia characteristics of plagioclase compositions have been preserved in 67915. Data on Mg/(Mg+FE) ratios suggest that the sodic ferrogabbro and the intermediate-Ca plagioclase clasts could be related to the Mg-rich plutonic rock suite and the high-Ca plagioclase clasts to the ferroan anorthosites.

  11. 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.

  12. 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

  13. Comparative Planetary Mineralogy: Basaltic Plagioclase from Earth, Moon, Mars and 4 Vesta

    NASA Technical Reports Server (NTRS)

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

    2003-01-01

    Major, minor and trace element analysis of silicates has allowed for the study of planetary basalts in a comparative planetary mineralogy context. We continue this initiative by exploring the chemistry of plagioclase feldspar in basalts from the Earth, Moon, Mars and 4 Vesta. This paper presents new data on plagioclase from six terrestrial basalt suites including Keweenawan, Island Arc, Hawaiian, Columbia Plateau, Taos Plateau, and Ocean Floor; six lunar basalt suites including Apollo 11 Low K, Apollo 12 Ilmenite, Apollo 12 Olivine, Apollo 12 Pigeonite, Apollo 15 Olivine, and Apollo 15 Pigeonite; two basaltic martian meteorites, Shergotty and QUE 94201; and one unequilibrated eucrite, Pasamonte.

  14. 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.

  15. 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.

  16. 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.

  17. TEM observation of bacteria-induced plagioclase dissolution and secondary mineral formation

    NASA Astrophysics Data System (ADS)

    Tamura, T.; Kyono, A.; Nishimiya, Y.

    2015-12-01

    Silicate minerals are the most common minerals in the earth's crust. Bacteria are also distributed throughout the earth's surface environment. The silicate minerals are known to be dissolved by organic acids and polysaccharides known as bacteria metabolites. The metabolic activity of bacteria therefore plays an important role in the interaction between dissolution of the silicate minerals and formation of secondary minerals. However, little is known about the secondary mineral formation process associated with the bacterial metabolism. To clarify the bacterial effect on the mineral dissolution and the secondary mineral formation, we closely investigated the effect of bacterial activity on surface texture modification and chemical composition changes of plagioclase which is the most abundant silicate mineral in the earth's crust. The bacteria were isolated from soil and then added in a suitable medium with several plagioclase fragments (Ab100% and An100%). It was incubated for 10 days. Al and Si concentrations in the medium were measured by ICP-AES to monitor the dissolution of the plagioclase. Secondary mineral formation during the incubation was observed by TEM, EDS and SAED methods. The authors will give the experiment results and discuss the effect of bacterial activity on the plagioclase dissolution and the secondary mineral formation in detail.

  18. 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.

  19. Hydrogen concentration in plagioclase as a hygrometer of arc basaltic melts: Approaches from melt inclusion analyses and hydrous melting experiments

    NASA Astrophysics Data System (ADS)

    Hamada, Morihisa; Ushioda, Masashi; Fujii, Toshitsugu; Takahashi, Eiichi

    2013-03-01

    The partition coefficients of hydrogen between plagioclase and basaltic melt were determined by two approaches. For the first part of this study, plagioclase-hosted melt inclusions in mid-ocean ridge basalt (MORB) from the Rodriguez Triple Junction in the Indian Ocean were analyzed. The hydrogen concentration in plagioclase is less than 60 wt ppm water, and the average H2O concentration in melt inclusions is 0.3 wt%. Therefore, the apparent partition coefficient of hydrogen between plagioclase and melt is ≈0.01 on a molar basis. For the second part of this study, hydrous melting experiments of arc basaltic magma were performed at 0.35 GPa using an internally-heated pressure vessel at f≈NNO+3. The starting material was hydrous basaltic glass with H2O ranging from 0.8 to 5.5 wt%. A grain of Ca-rich plagioclase (≈1 mg) and 10 mg of powdered basaltic glass were sealed in a Au80Pd20 alloy capsule, and then kept at near the crystallization temperature of plagioclase as a liquidus phase to attain an equilibrium of hydrogen between plagioclase and melt. Combining the results of these two parts of this study, we formulated two linear equations to correlate the hydrogen concentration in plagioclase and the H2O concentration in melt. When H2O in melt is ≤1 wt%, hydrogen in plagioclase (wt ppm water)≈80×H2O in melt (wt%). When H2O in melt is≥4 wt%, hydrogen in plagioclase (wt ppm water)≈40×H2O in melt (wt%). Hydrogen concentration in plagioclase lies between two equations when H2O in melt ranges from 1 to 4 wt%. In accordance with these formulations, the partition coefficients of hydrogen between plagioclase and basaltic melt switches from 0.01±0.005 under H2O-poor conditions (≤100 wt ppm water in plagioclase, ≤1 wt% H2O in melt) to 0.005±0.001 under H2O-rich conditions (≥150 wt ppm water in plagioclase, ≥4 wt% H2O in melt). Such switches of hydrogen partitioning with an increase in H2O can be related to change of the atomic site for hydrogen in the

  20. 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

  1. Anomalous Plagioclase Compositions from Apollo 16 Impact Melts: Modification of Impact Melts on the Lunar Surface

    NASA Astrophysics Data System (ADS)

    Fagan, A.; Neal, C. R.

    2011-12-01

    A study of Apollo 16 impact melts has been initiated as part of the NASA Lunar Science Institute's Center for Lunar Science and Exploration. The investigation uses a novel approach to investigate the crystallization conditions for the igneous textured samples returned by Apollo 16. This approach uses a combination of crystal size distributions and mineral chemistry to develop a crystal stratigraphy approach to examine crystallization conditions. The Apollo 16 impact melts are enriched in Al2O3 (~25-28%) and are highly feldspathic with only minor amounts of pyroxene, olivine, and opaque minerals. Initial studies focused on sample 60635 where previous whole rock analyses show that, surprisingly this sample possesses a small negative Eu anomaly [1,2]. This is intriguing given the feldspathic nature of the sample (~70% modal plagioclase), which should impart a strong positive Eu anomaly on the whole rock signature. The major element compositions of the plagioclases are relatively homogeneous with a range of An94-98 and an average of An97.2. Plagioclase trace element compositions exhibit a much wider compositional range with some crystals having the predicted positive Eu anomaly but other showing an unexpected negative Eu anomaly. There are about the same number of crystals with a negative Eu anomaly as there are with a positive anomaly and both are equally distributed through the thin section (60635,2). Intriguingly, 5 plagioclase crystals from this sample contain both positive and negative Eu anomalies in different sections of the crystals. Subsequent analyses of other impact melt samples show that a further 4 samples from the LM/Station 10 site also contain plagioclase with both positive and negative Eu anomalies (60235,5; 60335,13; 60615,8; 60618,3 and ,4), along with one sample from Station 4 (64817,3). The hypothesis we are considering is that the impact melt(s) represented by these anomalous samples were generated by impact into typical lunar highlands materials

  2. The effect of organic acids on plagioclase dissolution rates and stoichiometry

    NASA Astrophysics Data System (ADS)

    Welch, Susan A.; Ullman, William J.

    1993-06-01

    The rates of plagioclase dissolution in solutions containing organic acids are up to ten times greater than the rates determined in solutions containing inorganic acids at the same acidity. Initial rates of dissolution are poorly reproduced in replicate experiments. After a day, however, the rates of plagioclase dissolution calculated from the rates of silicon release are reproducible and constant for up to nineteen days. Steady-state rates of dissolution are highest (up to 1.3 × 10 -8 mol/m 2/sec) in acidic solutions (pH ≈ 3) and decrease (to 1 × 10 -11 mol/m 2/sec) as acidity decreases toward neutral pH. The polyfunctional acids, oxalate, citrate, succinate, pyruvate, and 2-ketoglutarate, are the most effective at promoting dissolution. Acetate and propionate are not as effective as the other organic acids but are nonetheless more effective than solutions containing only inorganic acids. The degree of ligand-promoted enhancement of dissolution rate (rate in organic-containing solution/rate in inorganic solution at the same pH) decreases as acidity increases, indicating that the ligand-promoted dissolution mechanism becomes relatively more important as the rate of proton-promoted dissolution decreases. The stoichiometry of release to solution indicates that dissolution is selective even after the rates of dissolution become constant. As in previously published studies, Na and Ca are rapidly released from the plagioclase feldspar, leaving a surface enriched in Si and/or Al. The ratio of Al/Si released to solution indicates that the stoichiometry of the residual plagioclase surface is a function of pH and the nature of the organic ligand. The ligands which remove Al in preference to Si from the dissolving mineral surface are also those which enhance overall plagioclase dissolution rates.

  3. Degassing driving crystallization of plagioclase phenocrysts in lava tube stalactites on Mount Etna (Sicily, Italy)

    NASA Astrophysics Data System (ADS)

    Lanzafame, Gabriele; Ferlito, Carmelo

    2014-10-01

    Basaltic lava flows can form tubes in response to the cooling of the outer surface. We collected lava stalactites (frozen lava tears) and sampled lava from the ceilings of three lava tubes on Mount Etna. Comparison of the petrographic characters between ceiling lavas and relative stalactites reveals surprising differences in the groundmass textures and crystal compositions. Major and trace element contents in stalactites show only a slight increase in alkali and SiO2 compared to ceiling lava, whereas significant differences exist in composition and textures between plagioclases within the ceiling lava and those within the stalactites, being in the last case definitively more An-rich. We advance the hypothesis that the high temperature reached in the cave caused the exsolution of the volatiles still trapped in the dripping melt. The volatiles, mainly H2O, formed bubbles and escaped from the melt; such a water-loss might have promoted the silicate polymerization in the stalactites resulting in the growth of An-rich plagioclase phenocrysts. Our results have important implications: in fact plagioclase phenocrysts are usually associated with intratelluric growth and are often considered as the main petrologic evidence for the existence of a magma chamber. The textural and chemical features of plagioclases in stalactites prove that phenocryst growth in syn to post-eruptive conditions is plausible and clearly explains the relatively low viscosity of many phenocryst-rich lava flows on Mount Etna, as well as on many other volcanoes around the world. Therefore, we can conclude that plagioclase phenocrysts cannot exclusively be considered as having originated within a magma chamber.

  4. 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.

  5. Timescale of emplacement of the Panzhihua gabbroic layered intrusion recorded in giant plagioclase at Sichuan Province, SW China

    NASA Astrophysics Data System (ADS)

    Cheng, Lilu; Zeng, Ling; Ren, Zhongyuan; Wang, Yu; Luo, Zhaohua

    2014-09-01

    Giant plagioclase crystals carried into the Panzhihua gabbroic layered intrusion from a deeper magma chamber can help constrain the timescales of emplacement of the Panzhihua intrusion in the Emeishan large igneous province (LIP). In this study, we present the petrographic textures and chemical compositions of giant plagioclase and fine-grained gabbro samples along a ~ 50 m horizontal outcrop of the low zone of the Panzhihua deposit. The giant plagioclase gabbro (GPG) dykes mostly intrude into the fine-grained gabbros without significant contact. Both types of gabbros have the similar bulk-rock major and trace element compositions. However, the mineral composition shows that most of the plagioclase megacrysts contain less An than the plagioclase in fine-grained gabbro samples. In situ analyses of Sr isotopes from core-to-rim transects of plagioclase megacrysts are constant, indicating that there are no recycled crystals. The textural characteristics of samples combined with petrological modeling using MELTS suggest that the plagioclase megacrysts should crystallize and grow in a deeper magma chamber. Textual studies of the GPG indicate that these plagioclase megacrysts mostly plot as straight lines on a classic crystal size distribution (CSD) diagram. For a plagioclase growth range of 10- 11-10- 10 mm/s, the plagioclase should have a growth time of 530-8118 years. In a 100 km3 magma chamber releasing thermal energy at a rate of 1000 MW, the Panzhihua intrusion should reach 50% crystallization after ~ 2400 years. The growth time recorded by the megacrysts in the GPG and numerical modeling may indicate that the emplacement and crystallization of the Panzhihua intrusion may have taken place in thousands of years. In the Emeishan LIP, therefore the combination of large volumes of Fe-rich magma flux and efficient metal precipitation led to the formation of a giant Fe-Ti-V oxide deposit in a short period.

  6. Investigation of Plagioclase Reactivity in Wet Supercritical CO2 by In Situ Infrared Spectroscopy

    NASA Astrophysics Data System (ADS)

    Thompson, C.; Gauglitz, K.; Loring, J.; Schaef, T.; Miller, Q.; Johnson, K. T.; Wang, Z.; Rosso, K. M.; McGrail, P.

    2013-12-01

    Increasingly, CO2 capture and subsequent storage in deep geologic reservoirs is being implemented as a viable approach for reducing anthropogenic emissions of CO2 into the atmosphere. Several mechanisms may act to secure the injected CO2, including hydrodynamic confinement, dissolution into reservoir fluids, retention of CO2 as a separate phase in pore spaces, and carbonation of reservoir minerals. This latter mechanism is the most permanent, but it requires the presence of reactive minerals and potentially significant amounts of time for the reactions to proceed. Plagioclase feldspars are highly abundant in the earth's crust and are present in the caprocks and storage formations of many target reservoirs. Although the dissolution kinetics and carbonation reactions of feldspars have been well studied in the aqueous phase, comparatively little work has focused on plagioclase reactivity in the CO2-rich fluid at conditions relevant to geologic carbon sequestration. In this study, we used in situ infrared spectroscopy to investigate the carbonation potential of a powdered plagioclase sample similar to labradorite [(Ca,Na)(Al,Si)4O8] that had been isolated from a Hawaiian basalt. Experiments were carried out at 50 °C and 91 bar by circulating a stream of dry or wet supercritical CO2 (scCO2) past a sample overlayer deposited on the window of a high-pressure infrared flow cell. Water concentrations ranged from 0% to 125% relative to saturation, and transmission-mode absorbance spectra were recorded as a function of time for 24 hours. In experiments with excess water, a controlled temperature gradient was used to intentionally condense a film of liquid water on the overlayers' surfaces. No discernible reaction was detected when the samples were exposed to anhydrous scCO2. When water was added, a thin film of liquid-like water was observed on the samples' surfaces, and up to 0.3% of the plagioclase was converted to a carbonate phase. Calcite is the most likely reaction

  7. 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.

  8. Orientations of Exsolved Magnetite Inclusions in Clinopyroxene and Plagioclase Determined With Electron Backscatter Diffraction (EBSD)

    NASA Astrophysics Data System (ADS)

    Feinberg, J. M.; Wenk, H.; Scott, G. R.; Renne, P. R.

    2003-12-01

    Crystallographically oriented magnetite inclusions occur as subsolidus exsolution features in slowly cooled mafic rocks and are of interest to paleomagnetism because of their highly stable magnetic remanence. Many inclusions in both clinopyroxene and plagioclase exist as elongate laths with generalized dimensions of 1 μ m x 2 μ m x 50 μ m. Of initial interest is the formation temperature and orientation of these elongate inclusions with respect to their silicate hosts. In this study, the electron backscatter diffraction (EBSD) technique is used to determine orientation relationships across exsolution boundaries for magnetite inclusions > 0.5 μ m in diameter in host crystals of both clinopyroxene and plagioclase. Magnetite inclusions in clinopyroxenes from the Early Cretaceous Messum Complex of Namibia occur as two arrays growing within (010) of clinopyroxene and elongated subparallel to either the [100] or [001] axes. Inclusions subparallel to [100]cpx have [-110]mag // [010]cpx, (-1-11)mag // (-101)cpx, and [112]mag // [101]cpx. Inclusions subparallel to [001]cpx have [-110]mag // [010]cpx, (111)mag // (100)cpx, and [-1-12]mag // [001]cpx. Both arrays of inclusions are oriented such that planes of roughly close-packed oxygen atoms in both phases, \\{111\\} in magnetite and (-101) and (100) in clinopyroxene, are aligned. These EBSD-derived orientation relationships agree well with previous TEM and X-ray diffraction studies on similar materials, and are consistent with a high-temperature exsolution origin for the magnetite and coexistent amphibole. In contrast to clinopyroxene, the orientation relationships between plagioclase and its exsolved magnetite inclusions have so far been unexplored. Such inclusions are responsible for stable magnetic remanence in both layered intrusions and oceanic gabbros. Exsolved magnetite inclusions in plagioclase crystals from anorthosites in the Early Jurassic Freetown Complex of Sierra Leone have been investigated. Preliminary

  9. Plagioclase nucleation and growth kinetics in a hydrous basaltic melt by decompression experiments

    NASA Astrophysics Data System (ADS)

    Arzilli, Fabio; Agostini, C.; Landi, P.; Fortunati, A.; Mancini, L.; Carroll, M. R.

    2015-12-01

    Isothermal single-step decompression experiments (at temperature of 1075 °C and pressure between 5 and 50 MPa) were used to study the crystallization kinetics of plagioclase in hydrous high-K basaltic melts as a function of pressure, effective undercooling (Δ T eff) and time. Single-step decompression causes water exsolution and a consequent increase in the plagioclase liquidus, thus imposing an effective undercooling (∆ T eff), accompanied by increased melt viscosity. Here, we show that the decompression process acts directly on viscosity and thermodynamic energy barriers (such as interfacial-free energy), controlling the nucleation process and favoring the formation of homogeneous nuclei also at high pressure (low effective undercoolings). In fact, this study shows that similar crystal number densities ( N a) can be obtained both at low and high pressure (between 5 and 50 MPa), whereas crystal growth processes are favored at low pressures (5-10 MPa). The main evidence of this study is that the crystallization of plagioclase in decompressed high-K basalts is more rapid than that in rhyolitic melts on similar timescales. The onset of the crystallization process during experiments was characterized by an initial nucleation event within the first hour of the experiment, which produced the largest amount of plagioclase. This nucleation event, at short experimental duration, can produce a dramatic change in crystal number density ( N a) and crystal fraction ( ϕ), triggering a significant textural evolution in only 1 h. In natural systems, this may affect the magma rheology and eruptive dynamics on very short time scales.

  10. 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. PMID:10801125

  11. Tectonically emplaced ultra-depleted lithospheric mantle records garnet, spinel and plagioclase facies events

    NASA Astrophysics Data System (ADS)

    Czertowicz, Thomas; Scott, James; Palin, Mike

    2014-05-01

    The poorly studied Anita Ultramafites in western New Zealand represent a several km wide slice of lithospheric mantle that was tectonically emplaced onto the Gondwana supercontinent margin. The peridotites are almost exclusively spinel facies dunite and harzburgite, although spinel-orthopyroxene symplectites indicate the former presence of Cr-garnet. Pyroxenite dikes are uncommon, and there is no sign of an ophiolitic type structure. Olivine (~Fo93) and chromite (~Cr# 70) attest to extreme degrees of melt depletion, likely under hydrous conditions. The rocks were decompressed and equilibrated at the spinel facies. The ultramafites were then refertilised by a fluid that was rich in Si, Ca, K, OH and LREE, and probably equates to a low-degree silicate melt. The occurrence of negative and positive Eu and Sr anomalies in amphibole points to the influence of plagioclase, and suggests that refertilisation occurred at a very shallow lithospheric level. An added complication is that the peridotite was metamorphosed to upper amphibolite facies in the Cretaceous after tectonic emplacement. This generated talc, tremolite and chlorite. P-T conditions from adjacent gneisses indicate that this event occurred at ~ 10-12 kbar in association with crustal thickening. Thus, the peridotite may have been pushed back out of plagioclase facies conditions, partially melted, and re-equilibrated back in the spinel facies. The Anita Ultramafics therefore record a sequence of attempts to equilibrate at garnet - spinel - plagioclase - spinel facies, before final exhumation.

  12. 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.

  13. 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.

  14. 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

  15. Laser ICP-MS study of trace element partitioning between olivine, plagioclase, orthopyroxene and melt

    NASA Astrophysics Data System (ADS)

    Laubier, M.; Grove, T. L.; Langmuir, C. H.

    2010-12-01

    Trace element contents in natural minerals such as plagioclase, olivine, clinopyroxene and orthopyroxene can be used to correct melt compositions for fractionation and estimate the redox state of magmas. In order to appropriately address fractionation in the different magmatic settings, high precision, fO2 controlled measurements at low pressures and variable temperature of mineral-melt trace element partition coefficients are needed. This study consists of 0.1 Mpa experiments on two starting compositions, a normal MORB AII96-18-1 from the Kane Fracture Zone and a basaltic andesite 85-44 from Mt Shasta. The starting compositions were enriched in 13 elements by addition of elemental standard solutions. The experiments were carried out at MIT in a vertical-tube DelTech quenching furnace with a CO2-H2 gas atmosphere. Both isothermal and cooling rate experiments, as well as non-doped experiments, were performed. The range in temperature was 1150-1175°C for the MORB and 1155-1190°C for the basaltic andesite. The different fO2 conditions were QFM, NNO, and NNO+2. Glasses, olivine, plagioclase and orthopyroxene crystals in the run products were analyzed by laser ICP-MS using a variety of calibration glass and mineral standards. Clinopyroxene crystals were too small to be analyzed accurately for their trace element contents. Fe and Na loss in our experiments is generally low and unlikely to have significant effects on the liquidus temperature. For the MORB composition, the average values for the Ol/Melt exchange distribution coefficient KD Fe-Mg (0.28±0.01), Cpx/Melt KD Fe-Mg (0.24±0.01) and Plg/Melt KD Ca-Na (0.94±0.03) attest for the achievement of chemical equilibrium. Similarly, for the basaltic andesite, the average values are Ol/Melt KD Fe-Mg = 0.28±0.01, Cpx/Melt KD Fe-Mg = 0.291±0.005, Opx/Melt KD Fe-Mg = 0.275±0.015 and Plg/Melt KD Ca-Na = 1.03±0.1. Cooling experiments systematically show different phase proportions, mineral compositions and partition

  16. 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.

  17. A shock-metamorphic model for silicate darkening and compositionally variable plagioclase in CK and ordinary chondrites

    SciTech Connect

    Rubin, A.E. )

    1992-04-01

    Silicate darkening in ordinary chondrites (OC) is caused by tiny grains of metallic Fe-Ni and troilite occurring mainly within curvilinear trails that traverse silicate interiors and decorate or, in some cases, cut across silicate grain boundaries. Highly shocked OC tend to have greater degrees of silicate darkening than lightly shocked OC; this indicates that silicate darkening is probably a result of shock metamorphism. The low Fe-FeS eutectic temperature (988C) renders metal and troilite susceptible to melting and mobilization during shock heating. Unshocked OC tend to have plagioclase with uniform compositions; shocked OC tend to have plagioclase with more variable (albeit still stoichiometric) compositions. The low impedance of plagioclase to shock compression makes it particularly susceptible to melting and mobilization; this is consistent with the molten appearance of plagioclase in highly shocked OC (e.g., Rose City and Paragould). CK chondrites also have compositionally variable plagioclase. The common association of silicate darkening with compositionally variable plagioclase is consistent with the hypothesis that both are products of shock metamorphism. Some CK and OC chondrites exhibit light shock effects in olivine that are consistent with equilibrium peak shock pressures that are too low to account for the silicate darkening or opaque shock veins in these meteorites. Therefore, the olivine in these chondrites may have been annealed after intense shock produced these effects. A few CK chondrites that contain olivine with undulose or mosaic extinction (e.g., LEW87009 and EET83311) may have been shocked again, after annealing.

  18. Effect of Phase Transition of Intermediate Plagioclase on Elastic Wave Velocity

    NASA Astrophysics Data System (ADS)

    Kono, Y.; Ishikawa, M.; Arima, M.

    2005-12-01

    We measured compressional (Vp) and shear (Vs) wave velocities of plagioclases with albite, labradorite, and bytownite composition up to 700°C at 1 GPa with a piston cylinder high-pressure apparatus. The Vp and Vs measurements were conducted with the pulse reflection method. Simultaneous Vp and Vs measurements were carried out using 10°Y-cut LiNbO3 transducer. The discontinuous change in temperature derivative of Vp and Vs at 400°C is observed in the labradorite while the albite and bytownite show linear Vp and Vs change with elevating temperature up to 700°C. The temperature derivative of Vp (dVp/dT) and Vs (dVs/dT) is -0.9x10-4 km s-1 °C-1 and -1.2x10-4 km s-1 °C-1 in the albite, respectively. In the bytownite dVp/dT is -1.2x10-4 km s-1 °C-1 and dVs/dT is -0.9x10-4 km s-1 °C-1. The dVp/dT and dVs/dT in the labradorite is -0.7x10-4 and -1.2x10-4 km s-1 °C-1 below 400°C, respectively. Above 400°C dVp/dT and dVs/dT increase to -3.0x10-4 and -3.3x10-4 km s-1 °C-1, respectively. The XRD analyses of the labradorite before and after high P-T measurements show that the discontinuous change in temperature derivative of Vp and Vs in the labradorite would be the result of phase transition of plagioclase. The present data suggest that Vp and Vs of intermediate plagioclase above 400°C could not be evaluated by the simple mixing theory using the low temperature Vp and Vs values of end-member plagioclases (albite and anorthite).

  19. A Plagioclase-Augite Inclusion in Caddo County: Low-Temperature Melt of Primitive Achondrites

    NASA Astrophysics Data System (ADS)

    Takeda, H.; Baba, T.; Saiki, K.; Otsuki, M.; Ebihara, M.

    1993-07-01

    Caddo County is an IAB-iron meteorite with silicate inclusions. Its silicate inclusion with olivine and clinopyroxene as major minerals was studied by INAA by Palme et al. [1]. We report mineralogy and INAA data of an inclusion with abundant augite and plagioclase, which will provide us with information on the missing materials removed from sourse regions of the primitive achondrites. The PTS of Caddo County, 7.5 x 5.5 mm in size has been studied by mineralogical techniques including electron probe microanalysis (EPMA) and scanning electron microscopy (SEM) with digital imaging utilities. A thick slice of the specimen next to the PTS was crushed and separated into metal- rich and a silicate-rich fractions for determination of lithophile and sidelophile element concentrations by INAA. The PTS shows aligned textures of silicate inclusions in metallic matrix, suggesting that some parts are intruded into this place. Augite (Aug) and plagioclase fragments are rounded and anhedral, and numerous metal veins fill interstices of the silicate grains. The silicate parts are mainly composed of augite and plagioclase with additional small amounts of minor orthopyroxene (Opx). Olivine is present only as small grains. Crystals of augite reach up to 0.8 x 0.4 mm in size and plagioclase crystals up to 0.6 x 0.3 mm in size. Many plagioclase grains in the metal matrix within one region shear a common crystallographic orientation as indicated by polysynthetic twin lamellae. The Opx distributes in a limited region and Opx and Aug are in contact with curved boundaries even within a grain. The pyroxene compositions are Aug (Ca44.5Mg52.5Fe3) and Opx (Ca2Mg91.5Fe6.5). The plagioclase compositions (Or3Ab80An17) are Na rich and show small ranges from grain to grain. The compositions of olivine (Fa97.5) are similar to that of winonaites [2]. The modal abundances are: Aug 20 vol%, plagioclase 28%, Opx 3%, metallic irons etc. 49% (excluding minor olivine and Opx). Na, Al and REE are enriched in

  20. A study of strontium diffusion in plagioclase using Rutherford backscattering spectroscopy

    NASA Astrophysics Data System (ADS)

    Cherniak, Daniele J.; Watson, E. Bruce

    1994-12-01

    Strontium chemical diffusion has been measured in plagioclase of three intermediate compositions (between An 23 and An 67) under anhydrous, 1-atmosphere conditions. A strontium oxide-aluminosilicate powder mixture was used as the diffusant source material, with Rutherford backscattering spectroscopy (RBS) used to measure diffusion profiles. Over the temperature range 725-1075°C, the following Arrhenius relations were obtained for diffusion normal to (001) ( D in m 2 s -1): Oligoclase (An 23): log D = (-6.07 ± 0.58) + (-273 ± 13 kJ mol -1)/RT . Andesine (An 43): log D = (-6.75 ± 0.33) + (-265 ± 8 kJ mol -1)/RT . Labradorite (An 67): log D = (-7.03 ± 0.37) + (-268 ± 8 kJ mol -1)/RT . In the labradorite, diffusion of Sr normal to (010) was somewhat slower (by ~0.7 log units) than diffusion perpendicular to (001). The activation energy for diffusion normal to (010) appears to follow a trend similar to that observed for diffusion perpendicular to (001). In oligoclase, diffusion normal to (001) also appears to be somewhat faster than diffusion normal to (010), but the differences are less pronounced than in labradorite. No significant crystallographic anisotropy was noted in the andesine. The results show a clear trend of increasing Sr diffusivity with decreasing An content of the plagioclase. Profiles of Ca and Na, measured with RBS and Nuclear Reaction Analysis (NRA) respectively, indicate that Sr is exchanging with both of these species in the plagioclase. In each feldspar composition, decreases in Ca and Na in the near-surface region are nearly proportional to their bulk concentrations, indicating that there is no significant substitutional preference of Sr for either cation in those feldspars investigated. NRA measurements of Al, showing a corresponding uptake of Al and Sr, suggest that a coupled exchange, possibly Sr +2 + Al +3 → Na +1 + Si +4, is taking place when Sr exchanges with Na. Chemical diffusion of Sr in calcic plagioclase is dominated by Sr +2

  1. Thermal Emission Spectrometer Mosaics of Impact Craters: Progress on Shocked Plagioclase Detections

    NASA Technical Reports Server (NTRS)

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

    2003-01-01

    High shock pressures disorder the mineral lattice of plagioclase feldspars and cause weakening and shifts in thermal infrared absorption bands related to an increase in diaplectic glass content, particularly at shock pressures above 20-25 GPa [1- 9]. We are continuing our investigation of spectral deconvolutions of TES mosaics and mosaics using standard TES orbits for eight impact crater areas. Using a combination of mineral laboratory spectra and spectra of experimentally shocked bytownite feldspars [5] to deconvolve the TES spectra, we find that locations of shocked feldspar detections are not restricted to ejecta near large craters.

  2. 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.

  3. 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.

  4. 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

  5. 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

  6. Evolution of groundwater chemical composition by plagioclase hydrolysis in Norwegian anorthosites

    NASA Astrophysics Data System (ADS)

    Banks, David; Frengstad, Bjørn

    2006-03-01

    The Precambrian Egersund anorthosites exhibit a wide range of groundwater chemical composition (pH 5.40-9.93, Ca 2+ 1.5-41 mg/L, Na + 12.3-103 mg/L). They also exhibit an evolutionary trend, culminating in high pH, Na-rich, low-Ca groundwaters, that is broadly representative of Norwegian crystalline bedrock aquifers in general. Simple PHREEQC modelling of monomineralic plagioclase-CO 2-H 2O systems demonstrates that the evolution of such waters can be explained solely by plagioclase weathering, coupled with calcite precipitation, without invoking cation exchange. Some degree of reaction in open CO 2 systems seems necessary to generate the observed maximum solute concentrations, while subsequent system closure can be invoked to explain high observed pH values. Empirical data provide observations required or predicted by such a model: (i) the presence of secondary calcite in silicate aquifer systems, (ii) the buffering of pH at around 8.0-8.3 by calcite precipitation, (iii) significant soil gas CO 2 concentrations ( PCO 2 > 10 -2 atm) even in poorly vegetated sub-arctic catchments, and (iv) the eventual re-accumulation of calcium in highly evolved, high pH waters.

  7. 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.

  8. 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

  9. 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

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

    PubMed

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

    2001-03-01

    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. PMID:11230692

  11. 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.

  12. 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.

  13. Black Butte Dacitic Dome, Ca: A Study of Ascent Rates Using Amphibole and Plagioclase Reactions and Crystal Size Distribution

    NASA Astrophysics Data System (ADS)

    McCanta, M.; Rutherford, M.; Hammer, J.

    2001-12-01

    Magma ascent rate can be quantified petrologically utilizing measurement of (1) decompression-driven amphibole breakdown rims, (2) compositional changes in plagioclase phenocryst rims and (3) crystal size distribution (CSD) of groundmass plagioclase microlites, that are known to have grown during ascent and cooling. Measurements of these parameters in both natural and experimental samples allows for an investigation into the relative rates and correlations between amphibole breakdown, plagioclase growth (of both phenocrysts and microlites) and magma ascent. Black Butte, Ca provides a good opportunity to study the effects of ascent rate as determined using the above three methods. Black Butte consists of four overlapping dacitic domes on the southwest flank of Mt. Shasta. At Black Butte the natural phase assemblage consists of amphiboles with well developed breakdown rims, homogeneous An74 plagioclase phenocryst cores decreasing monotonically to An55 rims, and extensive groundmass plagioclase microphenocryst growth. Amphibole breakdown rim widths in natural samples are 30-45 μ m, consistent over all four domes. It is important to note that unlike any other studied dome, these samples exhibit a lack of variable rim thicknesses in any one section, indicating little to no magma mixing in the conduit. Plagioclase phenocryst rim widths, defined by a compositional change, are 10-30 μ m. Textural analysis of the groundmass indicates that crystallization of plagioclase microlites accounts for ~57 volume%, with little variation between samples. CSD plots are approximately log-linear over the size range 1-250 μ m. The characteristic microphenocryst size, volumetric number density, and growth rate are 33μ m, 1.5E-5, ~1.3E-5, respectively. Isothermal, constant rate decompression (P1=200 MPa, P2=2 MPa, dP/dt=6 MPa/day) experiments were run to determine the conditions necessary to produce the rims measured in the natural samples. The starting material was a partially crushed

  14. Crystal-chemical controls on the partitioning of Sr and Ba between plagioclase feldspar, silicate melts, and hydrothermal solutions

    NASA Astrophysics Data System (ADS)

    Blundy, Jonathan D.; Wood, Bernard J.

    1991-01-01

    The isothermal (750°C) experiments of LAGACHE and DUJON (1987) reveal that the partitioning of Sr between plagioclase feldspar and hydrothermal solutions is a funtion of the anorthite (An) content of the plagioclase, indicating that crystal chemistry may exert a powerful influence on trace element partitioning. In order to compare these results with those on trace element partitioning between plagioclase and silicate melts we have compiled from the literature a large dataset of experimental and volcanic distribution coefficients ( D's) for Sr (and Ba). These data, which span a compositional range from lunar basalt to high silica rhyolite and a temperature range of over 650°C, show a relationship between DSr (and DBa) and mole fraction An ( XAn) which is similar to that exhibited by the hydrothermal results obtained at constant temperature. Plots of In DSr and In DBa versus XAn are linear with negative slope, indicating that both elements are more compatible in albite than anorthite. In terms of molar distribution coefficients ( D Sr∗) the hydrothermal and silicate melt data display an identical linear relationship between RT In D Sr∗ (where T is the absolute temperature in K and R is the gas constant, 8.314 JK -1 mol -1) and XAn. We conclude therefore that crystal chemistry provides the dominant control on partitioning of Sr and Ba into plagioclase and that the effects of temperature, pressure, and fluid composition are minor. Apparent relationships between DSr (and DBa) and the reciprocal temperature (1/ T) are artefacts of the linear relationships between XAn and 1/ T in the experimental studies. By defining a Henry's law standard state for the silicate melts and hydrothermal solutions, and considering plagioclases to be ternary regular solutions, we are able to relate the observed relationships between RT In D i∗ (where i is Ba or Sr) and XAn to the excess free energies of the trace element partitioning reactions between plagioclase and melt or

  15. Calc-alkalic vs. tholeiitic series revisited: new insight from isotopic micro-analyses of plagioclase phenocrysts

    NASA Astrophysics Data System (ADS)

    Tatsumi, Y.; Takahashi, T.; Shimizu, N.

    2004-12-01

    Major, trace and Sr isotopic compositions of plagioclase phenocrysts in calc-alkalic and tholeiitic rocks from Zao volcano, NE Japan were mapped with micro-analyses techniques. Plagioclase in tholeiitic rocks possesses a constant Sr isotopic ratio of ~0.7042, regardless with its Ca/Na ratio or the degree of magmatic differentiation. On the other hand, Sr isotopic ratios of plagioclase in calc-alkalic rocks are lower (0.7034-0.7041) than those for tholeiites, and more importantly, increase with decreasing Ca/Na ratios. Generally accepted mechanisms for production of those two types of arc magmas are fractional crystallization of a mantle-derived basalt magma for tholeiitic rocks and mixing between tholeiitic, mafic magmas and crust-derived, felsic magmas for calc-alkalic rocks. However, the above observation does not support this. Alternatively, tholeiitic magmas could be produced by anatexis of isotopically enriched, lower crustal material; calc-alkalic andesites are mixing products between a mantle-derived, isotopically depleted basalt magma and crust-derived, tholeiitic felsic magmas. Trace element characteristics of such mantle-derived, primary O^calc-alkalicO basalt are deduced based on plagioclase compositions and inferred distribution coefficients.

  16. Valid garnet biotite (GB) geothermometry and garnet aluminum silicate plagioclase quartz (GASP) geobarometry in metapelitic rocks

    NASA Astrophysics Data System (ADS)

    Wu, Chun-Ming; Cheng, Ben-He

    2006-06-01

    At present there are many calibrations of both the garnet-biotite (GB) thermometer and the garnet-aluminum silicate-plagioclase-quartz (GASP) barometer that may confuse geologists in choosing a reliable thermometer and/or barometer. To test the accuracy of the GB thermometers we have applied the various GB thermometers to reproduce the experimental data and data from natural metapelitic rocks of various prograde sequences, inverted metamorphic zones and thermal contact aureoles. We have concluded that the four GB thermometers (Perchuk, L.L., Lavrent'eva, I.V., 1983. Experimental investigation of exchange equilibria in the system cordierite-garnet-biotite. In: Saxena, S.K. (ed.) Kinetics and equilibrium in mineral reactions. Springer-Verlag New York, Berlin, Heidelberg. pp. 199-239.; Kleemann, U., Reinhardt, J., 1994. Garnet-biotite thermometry revised: the effect of Al VI and Ti in biotite. European Journal of Mineralogy 6, 925-941.; Holdaway, M.J., 2000. Application of new experimental and garnet Margules data to the garnet-biotite geothermometer. American Mineralogist 85, 881-892., Model 6AV; Kaneko, Y., Miyano, T., 2004. Recalibration of mutually consistent garnet-biotite and garnet-cordierite geothermometers. Lithos 73, 255-269. Model B) are the most valid and reliable of this kind of thermometer. More specifically, we prefer the Holdaway (Holdaway, M.J., 2000. Application of new experimental and garnet Margules data to the garnet-biotite geothermometer. American Mineralogist 85, 881-892.) and the Kleemann and Reinhardt (Kleemann, U., Reinhardt, J., 1994. Garnet-biotite thermometry revised: the effect of Al VI and Ti in biotite. European Journal of Mineralogy 6, 925-941.) calibrations due to their small errors in reproducing the experimental temperatures and good accuracy in successfully discerning the systematic temperature changes of the different sequences. In addition, after applying the GASP barometer to 335 natural metapelitic samples containing one kind

  17. 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.

  18. 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

  19. Magnetic and plagioclase fabrics in Early Cretaceous mafic dykes from Namibia

    NASA Astrophysics Data System (ADS)

    Wiegand, M.; Greiling, R. O.; Kontny, A. M.; Trumbull, R. B.

    2011-12-01

    Mafic dyke swarms are major components of the South Atlantic Large Igneous Province, which originated during the Cretaceous break up of Africa and South America. We present data from the major Henties Bay-Outjo dyke swarm (HOD) in coastal and inland NW Namibia where dykes have been emplaced into the Neoproterozoic Damara mobile belt. Most of the dykes strike at a high angle to the coast (NE-SW), but minor proportions are coast-parallel (NNW-SSE), N-S or NNE-SSW. Depending on dyke thickness (ca. 0.1 m to >100 m), the rocks are variably fine- to medium grained with chilled margins. With few exceptions the dyke compositions are tholeiitic basalt to basaltic andesite. Main minerals are plagioclase, clinopyroxene, olivine, Fe-Ti oxides, and accessory apatite and sulphides. For the magnetic study, we investigated 1140 standard cylinders from 41 dykes. The samples were studied microscopically and the following magnetic properties were determined: bulk susceptibility, anisotropy of magnetic susceptibility (P'), field dependence, hysteresis, Natural Remanent Magnetization (NRM) and Anisotropy of Anhysteretic Remanent Magnetization (AARM). Magnetic susceptibilities vary between 0.16 and 151*10-3 SI units. Microscopic studies and measurements of the temperature-dependent susceptibility identify magnetite and titanomagnetite as the dominant magnetic minerals. Hysteresis loops show that the magnetic domain states of titanomagnetite are pseudo-single or a mixture of single and multi-domain. Anisotropy is mostly low, with P' values between 1.01 and 1.15. Rare values of up to 1.33 reflect a strong magmatic flow fabric. The shapes of the AMS ellipsoids range from prolate to neutral and oblate. Two main fabric types (normal and inverse) can be recognized. We used the normal fabric type samples to interpret magmatic flow fabrics and attribute steep magnetic lineations of normal fabrics to vertical magma flow and shallow magnetic lineations to horizontal flow along the dyke planes. In

  20. 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

  1. 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.

  2. Crystallization temperature determination of Itokawa particles by plagioclase thermometry with X-ray diffraction data obtained by a high-resolution synchrotron Gandolfi camera

    NASA Astrophysics Data System (ADS)

    Tanaka, Masahiko; Nakamura, Tomoki; Noguchi, Takaaki; Nakato, Aiko; Ishida, Hatsumi; Yada, Toru; Shirai, Kei; Fujimura, Akio; Ishibashi, Yukihiro; Abe, Masanao; Okada, Tatsuaki; Ueno, Munetaka; Mukai, Toshifumi

    2014-02-01

    The crystallization temperatures of Itokawa surface particles recovered by the space probe Hayabusa were estimated by a plagioclase geothermometer using sodic plagioclase triclinicity. The Δ131-index required for the thermometer, which is the difference in X-ray diffraction peak positions between the 131 and 13¯1 reflections of plagioclase, was obtained by a high-resolution synchrotron Gandolfi camera developed for the third generation synchrotron radiation beamline, BL15XU at SPring-8. Crystallization temperatures were successfully determined from the Δ131-indices for four particles. The observed plagioclase crystallization temperatures were in a range from 655 to 660 °C. The temperatures indicate crystallization temperatures of plagioclases in the process of prograde metamorphism before the peak metamorphic stage.

  3. Magma mixing recorded by Sr isotopes of plagioclase from dacites of the Quaternary Tengchong volcanic field, SE Tibetan Plateau

    NASA Astrophysics Data System (ADS)

    Gao, Jian-Feng; Zhou, Mei-Fu; Robinson, Paul T.; Wang, Christina Yan; Zhao, Jun-Hong; Malpas, John

    2015-02-01

    The Tengchong volcanic field east of the Burma arc in SW China comprises numerous Quaternary volcanoes. The volcanic rocks can be grouped into four units, numbered 1-4 from oldest to youngest. Units 1, 3 and 4 are composed of trachybasalt, basaltic trachyandesite and trachyandesite, respectively, whereas Unit 2 consists of hornblende dacite. Primary minerals in the dacite include plagioclase, clinopyroxene, amphibole and magnetite, all set in a glassy groundmass. Some of the dacites are moderately to highly weathered and show effects of low temperature alteration, with loss on ignition (LOI) up to 9.6 wt% and positive correlations of LOI with Al2O3, but negative correlations with SiO2, CaO and Na2O. Fresh dacites (LOI <3 wt%) belong to the calk-alkaline series and have pronounced negative Ti, Nb and Ta anomalies. They have whole-rock 87Sr/86Sr ratios ranging from 0.7090 to 0.7101, εNd values from -10.8 to -11.8, and εHf values from -5.1 to -6.8. Many of the large plagioclase crystals in the fresh dacites have reverse zoning, ranging from An55 in the cores to An72 in the rims. The smaller phenocrysts have relatively uniform An values between 56 and 50. One plagioclase crystal has an An value of 33.5. Plagioclase in the fresh dacite has relatively high Sr contents and 87Sr/86Sr ratios ranging from 0.7050 to 07125 except for one value of 0.7516, whereas plagioclase in the altered dacite has low Sr contents and highly variable Sr isotopic compositions (0.7039-0.7138), indistinguishable from those of the fresh rocks. Both An values and Sr isotopic compositions of the plagioclase indicate mixing of mantle- and crustally-derived magmas. We therefore propose that mantle-derived basaltic magma caused partial melting of the lower-middle crust and that mixing of the mafic and felsic magmas produced the dacite in staging magma chambers. The dacites contain minerals crystallized from both the mafic and felsic magmas, as well as a few xenocrysts plucked from the country rocks.

  4. 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

  5. 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.

  6. Preferred lattice orientations of plagioclase from amphibolite and greenschist facies rocks near the Insubric Line (Western Alps)

    NASA Astrophysics Data System (ADS)

    Kruhl, Jörn H.

    1987-04-01

    Preferred lattice orientations of plagioclase, measured by the universal stage, from deformed amphibolite and greenschist facies rocks near the Insubric Line (Western Alps), reflect the type and orientation of the strain system: either (001) or (010) are oriented parallel or subparallel to the schistosity and form more or less complete girdles around the stretching direction. By the preferred orientation patterns dominant (010) [001] slip is indicated and, additionally, slip on (001) parallel to the a direction, possibly at somewhat higher temperatures and/or at a higher An content of the plagioclase. It is suggested that obliquities of the symmetry-planes of the orientation patterns against the principle axes of strain are related to the sense of shearing.

  7. 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. PMID:25261735

  8. 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.

  9. 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.

  10. 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]).

  11. 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

  12. Anomalous isotopes and trace element zoning in plagioclase peridotite xenoliths of Oahu (Hawaii): implications for the Hawaiian plume

    NASA Astrophysics Data System (ADS)

    Sen, Gautam; Yang, Huai-Jen; Ducea, Mihai

    2003-02-01

    Survival of plagioclase in the residual melting column during melting can have a significant impact on the melting process beneath a mid-oceanic ridge [Asimow et al., Phil. Trans. R. Soc. London Ser. A 355 (1997) 255-281]. Here we investigate the origin of plagioclase that occurs in some rare mantle xenoliths from Oahu, Hawaii. The xenoliths are harzburgitic with less than 2 modal% clinopyroxene and are characterized by strong foliation and porphyroclastic texture. Olivine and orthopyroxene are common porphyroclasts; and only one xenolith (77PAII-9) contains a single large clinopyroxene porphyroclast with thick exsolved orthopyroxene lamellae. The strongly foliated groundmass shows well-developed triple-point junctions and is dominantly composed of olivine (ol 85-90 opx 7-14 cpx <1-2 plag 3-5 spinel trace). Spinel grains are small and dispersed through the groundmass and show extreme variation in Cr/Al ratio within individual thin sections, indicating that they are out of equilibrium with the other phases in these xenoliths. A highly anorthitic plagioclase (An 92-96) occurs only in the groundmass and its modal abundance (˜3-5%) is too high relative to the abundance of clinopyroxene (commonly <1%) for it to be a residual phase, implying that plagioclase may have an exotic origin. The porphyroclasts show strong compositional zoning near the rims and appear to be relict phases (as are all the spinel grains) that had once equilibrated with melts within the stability field of spinel peridotite (pressure ˜1-3 GPa). Clinopyroxene neoblasts and the single porphyroclast in PAII-9 are all characterized by lithosphere-like strongly depleted light rare earth element (chondrite-normalized) patterns. The clinopyroxene porphyroclast in PAII-9 is zoned in Al, Eu, Cr, and Na. The porphyroclastic ortho- and clinopyroxenes give a homogenized (host+exsolution) temperature of 1300°C, which is inferred to be the temperature at which the porphyroclast cores equilibrated with a MORB

  13. 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.

  14. 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.

  15. 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.

  16. Melting of plagioclase + spinel lherzolite at low pressures (0.5 GPa): An experimental approach to the evolution of basaltic melt during mantle refertilisation at shallow depths

    NASA Astrophysics Data System (ADS)

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

    2013-07-01

    The presence of plagioclase + spinel lherzolites among ocean floor samples and in some ophiolite complexes invites speculation on their origin and relationships to processes of magmatism and lithosphere refertilisation beneath mid-ocean ridges. In an experimental approach to their petrogenesis, we have determined the compositions of liquids and co-existing minerals in the six phase assemblage [liquid + olivine + orthopyroxene + clinopyroxene + plagioclase + spinel] at 0.5 GPa and 1100 °C to 1200 °C. In our experimental approach we maintained the olivine Mg# [Mg / (Mg + Fe)] close to 90 (i.e., 88.8-95.5) but varied plagioclase from anorthite to albite. The major variations in liquid compositions are related to plagioclase composition. Liquids have much lower MgO and FeO and higher SiO2 and Al2O3 than liquids in the 6-phase plagioclase + spinel lherzolite at 0.75 GPa and 1 GPa. Liquids are quartz-normative (silica-oversaturated) for plagioclase that are more calcic than An40 but nepheline-normative (critically silica-undersaturated) for plagioclase that are more sodic than An25. Liquid compositions are quite unlike natural MORB glasses with similar Mg# (i.e., compatible with parental magmas from lherzolitic mantle with Mg# ≈ 90). Our study provides no support for models of MORB petrogenesis which suggest extraction of near-solidus melts from plagioclase lherzolite at low pressure. Similarly, referring to numerical models of melting volumes beneath mid-ocean ridges (Langmuir et al., 1992; McKenzie and Bickle, 1988) in which melt increments are calculated for different sites and these increments pooled to form MORB, our data argue that melts equilibrated with plagioclase ± spinel lherzolite at < 1 GPa cannot be significant components of such ‘pooled melt’ focussed from within the melting volume. The compositions of minerals from plagioclase ± spinel lherzolite at Lanzo (northern Italy; Piccardo et al., 2007) are compared with our experimental assemblages at 0

  17. 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

  18. 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 .

  19. Plagioclase deformation in upper-greenschist facies meta-pegmatite mylonites from the Austroalpine Matsch Unit (Eastern Alps, Italy)

    NASA Astrophysics Data System (ADS)

    Eberlei, Tobias; Habler, Gerlinde; Abart, Rainer; Grasemann, Bernhard

    2014-05-01

    Feldspars are common rock forming minerals as they are stable over a wide range of bulk rock compositions and metamorphic conditions within the Earth's crust. The deformation mechanisms of feldspar play an important role in rheological models for the crust and therefore have received considerable attention in studies on natural rocks and in experimental studies. The interaction of frictional and viscous deformation mechanisms and the onset of crystal plastic deformation in feldspars occur over a broad range of pressures and temperatures. In this work, we present new microstructural, textural and mineral chemical data of plagioclase from Permian metapegmatites within the Austroalpine Matsch Unit in Southern Tyrol (Italy). These crystalline basement rocks were deformed and metamorphosed at conditions close to the greenschist/amphibolites facies transition at 480±26°C during the Cretaceous (Habler et al., 2009). The investigated samples have been collected from meter-scale shear zones which typically occur at boundaries of lithological subunits. The southern tectonic boundary of this unit is commonly referred to as the "Vinschgau Shear Zone" (Schmid & Haas, 1989). We applied the Electron Backscatter Diffraction method to investigate the grain- and subgrain-boundaries and the nature of effective deformation mechanisms in plagioclase. Large albite porphyroclasts in the mylonitic Permian metapegmatites show grain internal traces of dissolution surfaces and the formation of new, strain-free grains with straight grain boundary segments and partly 120° grain boundary triple junctions in dilatant sites. The aggregates of new grains neither have a lattice preferred orientation nor a crystallographic orientation relation with the adjacent clast, and are characterized by the lack of grain internal deformation, suggesting that these are new precipitates rather than clast-fragments or recrystallized subgrains. Furthermore, the porphyroclasts show cracks and kinks, associated

  20. 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

  1. Plagioclase-free andesites from Zitácuaro (Michoacán), Mexico: petrology and experimental constraints

    NASA Astrophysics Data System (ADS)

    Blatter, Dawnika L.; Carmichael, Ian S. E.

    Approximately 150km west of Mexico City in the central part of the Mexican Volcanic Belt (MVB) near Zitácuaro, Mexico, young volcanism has produced shield volcanoes, large volume silicic deposits, and fault-related basalt and andesite lava flows and cinder cones. This paper concerns a small cluster of Pleistocene andesite cones and flows which can be separated into two distinct groups: high-magnesium andesites (>6% MgO, 57-59% SiO2), conveniently called basaltic andesites, with phenocrysts of orthopyroxene and augite, or augite and olivine; and andesites (60-62% SiO2, <4.6% MgO), which have phenocrysts of orthopyroxene and augite, and ghosts of relict hornblende. Remarkably, plagioclase phenocrysts are absent, and evenly distributed but sparse (0.5-3.5%) quartz xenocrysts are present in all the lavas. In order to establish the conditions under which early crystallizing plagioclase is suppressed in these lavas, water saturated experiments up to 3 kbars were performed on one of the basaltic andesites. The conditions required to reproduce the phenocryst assemblages (either olivine + augite or opx + augite) are temperatures in excess of 1000°C, with water saturated liquids (>3 wt%) at pressures of about 1 kbar. Compared to basaltic andesites of western Mexico, the Zitácuaro basaltic andesites have 2wt% lower Al2O3 concentrations, which causes plagioclase to precipitate at significantly lower temperatures, and it therefore follows the crystallization sequence: olivine, augite, and orthopyroxene. Based on ubiquitous quartz xenocrysts, with glassy rhyolitic inclusions, a reasonable conclusion is that substantial mixing of a quartz-bearing rhyolitic magma with a parental basaltic andesite has occurred at low pressure (shallow depth), and this would account for the low Al2O3 concentrations in the Zitácuaro basaltic andesites. Whatever the mechanism of incorporation, the quartz xenocrysts are evidence of contamination of basaltic magma with more siliceous material, thus

  2. 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). Statistical analysis reveals a significant dependence of DAl/Siplg/amp on pressure, temperature, Al fraction in amphibole T1-site, XAlT1, and albite fraction in plagioclase, XAb, leading to the barometric expression:

  3. 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

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

    PubMed

    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

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

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

    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.

  6. 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

  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. 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.

  9. 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.

  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.

    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. 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

  12. Petrology and Geochemistry of Plagioclase-Phyric Basaltic Lava Flows on St. George Island, Alaska: Evidence for a Genetic Link Between Magmatic Centers of the Pribilof Islands

    NASA Astrophysics Data System (ADS)

    Deraps, M. R.; Feeley, T. C.; Underwood, S. J.; Winer, G. S.

    2006-12-01

    St. George and St. Paul Islands, the two largest Pribilof Islands (Alaska), are located 70 km apart in the Bering Sea 400-450 km north of the Aleutian arc front. The islands are centers of alkaline basaltic volcanism associated with the diffuse Bering Sea basalt province. On St. George an extensive history of volcanism (~2.6-1.4 Ma; Mukasa and Andronikov, JGR, submitted) is exposed in high sea cliffs and several NE-SW and E-W trending fault blocks that define the present topography of the island. In contrast, St. Paul is a potentially active volcanic center (~550-3 ka) with numerous young tephra cones and associated lava flows that show minimal erosion and deformation by faulting. Volcanic rocks erupted on St. George are mainly olivine- + clinopyroxene-phyric basalts and trachy-basalts with high to moderate MgO contents (12-8 wt%). In addition, distinct plagioclase-phyric lavas are present at high stratigraphic levels. The plagioclase-phyric lavas are relatively evolved with 47-51 wt% SiO2 and 6.6-4.9 wt% MgO. Chondrite-normalized REE patterns are LREE-enriched with little to no Eu anomalies (Eu/Eu* = 1.03-1.06), indicating that the evolved compositions and plagioclase-rich nature of the flows are not due to phenocryst accumulation. Instead, production of plagioclase-phyric lavas on St. George likely involved a complex set of differentiation processes, including fractional crystallization and magma mixing. This assertion is supported by major and trace element compositions of the rocks, which define distinct trends on chemical variation diagrams relative to less evolved plagioclase-poor rocks. For example, whereas MgO contents of most St. George lavas as a group decrease with increasing SiO2 contents, these elements show a positive linear correlation for plagioclase-phyric lavas. The linear trends of these samples on chemical variation diagrams are best explained by mixing of at least two distinct evolved endmember magmas. Major element mass balance calculations

  13. Grain Growth Pinning and Strain Localization: Implications for Plagioclase Flow Laws and Strength of the Lower Crust

    NASA Astrophysics Data System (ADS)

    Mehl, L.; Hirth, G.

    2005-12-01

    The rheology of the lower crust remains poorly understood despite its importance for understanding the strength of plates, driving forces for plate motion, and how strain is translated up to and/or down from the brittle upper crust. In this study we evaluate the applicability of experimental flow laws using insights from the analysis of naturally deformed gabbros from lower oceanic crust (Hole 735B, Southwest Indian Ridge). For comparison with experimentally-derived flow laws, we evaluate stress, temperature, and deformation mechanism in the SWIR samples. Strain is localized in discrete shear zones, and we assume a strain rate of 10-12 to 10-14 s-1. Differential stress ranges from ~20 to 150 MPa, estimated by grain size piezometry with the empirical relationship of Twiss (1977). Syn-deformation temperatures are 800-950 °C, calculated by Fe-Mg-Ca exchange between recrystallized orthopyroxene-clinopyroxene pairs using QUILF (Andersen et al., 1993). Finally, deformation mechanisms were analyzed by measuring the lattice preferred orientation (LPO) with an electron back scattered detector (EBSD) on a scanning electron microscope. Monophase layers within the shear zones have LPOs indicative of deformation by dislocation creep at all grain sizes (down to 12.5 μm). Polyphase layers have random or very weak fabrics implying dominance of diffusion creep, even in samples with a relatively coarse grain size (well within the dislocation creep field). Plagioclase grains in the polyphase layers are smaller than those in monophase layers indicating that grain growth is apparently inhibited by clinopyroxene at grain boundaries. The variation in plagioclase grain size fits models that account for the size and abundance of a pinning phase. We conclude: 1) Flow law parameters for dry An100 (Rybacki and Dresen, 2000) agree well with natural polyphase gabbro shear zones. This is a relatively `strong' flow law that predicts viscosities higher than olivine flow laws for a wet upper

  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. Crystal residence times from trace element zoning in plagioclase reveal changes in magma transfer dynamics at Mt. Etna during the last 400 years

    NASA Astrophysics Data System (ADS)

    Viccaro, Marco; Barca, Donatella; Bohrson, Wendy A.; D'Oriano, Claudia; Giuffrida, Marisa; Nicotra, Eugenio; Pitcher, Bradley W.

    2016-04-01

    Trace element zoning in plagioclase of selected alkaline lavas from the historic (1607-1892 AD) and recent (1983-2013 AD) activity of Mt. Etna volcano has been used to explore the possible role that volcano-tectonics exert on magma transfer dynamics. The observed textural characteristics of crystals include near-equilibrium textures (i.e., oscillatory zoning) and textures with variable extent of disequilibrium (patchy zoning, coarse sieve textures and dissolved cores). Historic crystals exhibit lower K concentrations at lower anorthite contents, a feature in agreement with the general more potassic character of the recent lavas if compared to the historic products. Historic plagioclases have statistically higher Ba and lower Sr concentrations than the recent crystals, which result in different Sr/Ba ratios for the two suites of plagioclase. Variations in the anorthite content along core-to-rim profiles obtained on crystals with different types of textures for both the historic and recent eruptive periods were evaluated particularly versus Sr/Ba. At comparable average An contents, crystals characterized by oscillatory zoning, which are representative of near-equilibrium crystallization from the magma, display distinct Sr/Ba ratios. We suggest that these features are primarily related to recharge of a new, geochemically-distinct magma into the storage and transport system of the volcano. In addition to distinct trace element and textural characteristics of plagioclase, Sr diffusion modeling for plagioclase suggests that residence times are generally shorter for crystals found in recently erupted lavas (25-77 years, average 43 years) compared to those of the historic products (43-163 years, average 99 years). Shorter residences times correlate with gradual increases in eruption volume and eruption frequency rates through time. We attribute these features to an increasing influence, since the 17th century, of extensional tectonic structures within the upper 10 km of

  16. Formation of andesite melts and Ca-rich plagioclase in the submarine Monowai volcanic system, Kermadec arc

    NASA Astrophysics Data System (ADS)

    Kemner, Fabian; Haase, Karsten M.; Beier, Christoph; Krumm, Stefan; Brandl, Philipp A.

    2015-12-01

    Andesites are typical rocks of island arcs and may either form by fractional crystallization processes or by mixing between a mafic and a felsic magma. Here we present new petrographic and geochemical data from lavas of the submarine Monowai volcanic system in the northern Kermadec island arc that display a continuous range in composition from basalt to andesite. Using petrology, major, trace, and volatile element data, we show that basaltic magmas mostly evolve to andesitic magmas by fractional crystallization. Our thermobarometric calculations indicate that the formation of the large caldera is related to eruption of basaltic-andesitic to andesitic magmas from a magma reservoir in the deeper crust. Small variations in trace element ratios between the caldera and the large active cone imply a homogeneous mantle source. Contrastingly, resurgent dome melts of the caldera stagnated at shallower depths are more depleted and show a stronger subduction input than the other edifices. The Monowai basaltic glasses contain less than 1 wt % H2O and follow typical tholeiitic fractionation trends. High-An plagioclase crystals observed in the Monowai lavas likely reflect mixing of H2O-saturated melt batches with hot and dry tholeiitic, decompression melt batches. The result is a relatively H2O-poor mafic magma at Monowai implying that partial melting of the mantle wedge is only partly due to the volatile flux and that adiabatic melting may play a significant role in the formation of the parental melts of the Monowai volcanic system and possibly other arc volcanoes.

  17. Effects of Al/Si ordering on feldspar dissolution: Part II. The pH dependence of plagioclases' dissolution rates

    NASA Astrophysics Data System (ADS)

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

    2014-02-01

    The rate of mineral dissolution in an aquatic environment is sensitive to the pH of the contacting solution. The pH dependence of mineral dissolution rate has been interpreted by the Transition State Theory-Surface Complexation Model (TST-SCM) formalism in terms of pH-sensitive variability in surface chemistry. In this study, we provide an alternative interpretation for the experimentally observed nonlinear pH dependence of feldspar dissolution rates. The interpretation is based on a new formalism for feldspar dissolution which, while compatible with the TST-SCM formalism, incorporates the effects of both surface chemistry and bulk chemistry on feldspar dissolution into the quantification of dissolution rate. The pH dependence of dissolution rate varies from one feldspar specimen to another because different TOT linkages within one solid matrix can respond differently to the attack of proton. Our results suggest that the pH dependence of feldspar dissolution rate is not a constant in general, and could be affected by pH, substitutional Al/Si ordering, chemical composition of the specimen, and the relative rapidness of linkage hydrolysis according to different mechanisms. The rate law proposed in this study is able to capture the experimentally observed pH dependence of the dissolution rates of a series of plagioclases, including albite, andesine, labradorite, bytownite, and anorthite. The effectiveness of the newly proposed formalism for feldspar dissolution, hence, suggests that dissolution reactions of minerals are combinations of surface renewal and heterogeneous chemical reactions. The currently widely used TST-SCM-based rate laws can be further improved by taking into account the effects of bulk chemistry and surface renewal in the prediction of mineral dissolution rates. An improved formalism for mineral dissolution will be mineral-specific, and will reflect the effects of the temporal decay in the availability of reactive surface sites as well as the

  18. Albitization of plagioclase crystals in the Stevens sandstone (Miocene), San Joaquin Basin, California, and the Frio Formation (Oligocene), Gulf Coast, Texas. A TEM/AEM study

    SciTech Connect

    Hirt, W.G. ); Wenk, H.R. ); Boles, J.R. )

    1993-06-01

    Conventional Transmission Electron Microscopy (CTEM) and Analytical Electron Microscopy (AEM) studies of partially albitized plagioclase crystals taken from drill cores from the Stevens sandstone (Miocene), San Joaquin, California, and the Frio Formation (Oligocene), Gulf Coast, Texas, reveal that replacement of Ca-rich plagioclase cores by nearly pure albite (Ab[sub 96]-Ab[sub 100]) occurs along submicroscopic ([minus]15 nm wide) en echelon (001) and (110) cleavages. The cleavages are the result of changes in the localized stress regime created by dissolution of adjacent phases. Photomicrographs show albite-lined brittle cleavage crosscutting albitized semibrittle fractures. Such crosscutting relationships can be explained by a reduction in effective stress associated with the albitization process. On a macroscopic scale, this reduction in effective stress implies that the transition from hydrostatic to lithostatic pressure is discontinuous. 30 refs., 7 figs.

  19. 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

  20. 26Al in plagioclase-rich chondrules in carbonaceous chondrites: Evidence for an extended duration of chondrule formation

    NASA Astrophysics Data System (ADS)

    Hutcheon, I. D.; Marhas, K. K.; Krot, A. N.; Goswami, J. N.; Jones, R. H.

    2009-09-01

    The 26Al- 26Mg isotope systematics in 33 petrographically and mineralogically characterized plagioclase-rich chondrules (PRCs) from 13 carbonaceous chondrites (CCs) - one ungrouped (Acfer 094), six CR, five CV, and one CO - reveal large variations in the initial 26Al/ 27Al ratio, ( 26Al/ 27Al) 0. Well-resolved 26Mg excesses (δ 26Mg) from the in situ decay of the short-lived nuclide 26Al ( t1/2 ˜ 0.72 Ma) were found in nine chondrules, two from Acfer 094, five from the CV chondrites, Allende and Efremovka, and one each from the paired CR chondrites, EET 92147 and EET 92042, with ( 26Al/ 27Al) 0 values ranging from ˜3 × 10 -6 to ˜1.5 × 10 -5. Data for seven additional chondrules from three CV and two CR chondrites show evidence suggestive of the presence of 26Al but do not yield well defined values for ( 26Al/ 27Al) 0, while the remaining chondrules do not contain excess radiogenic 26Mg and yield corresponding upper limits of (11-2) × 10 -6 for ( 26Al/ 27Al) 0. The observed range of ( 26Al/ 27Al) 0 in PRCs from CCs is similar to the range seen in chondrules from unequilibrated ordinary chondrites (UOCs) of low metamorphic grade (3.0-3.4). However, unlike the UOC chondrules, there is no clear trend between the ( 26Al/ 27Al) 0 values in PRCs from CCs and the degree of thermal metamorphism experienced by the host meteorites. High and low values of ( 26Al/ 27Al) 0 are found equally in PRCs from both CCs lacking evidence for thermal metamorphism (e.g., CRs) and CCs where such evidence is abundant (e.g., CVs). The lower ( 26Al/ 27Al) 0 values in PRCs from CCs, relative to most CAIs, are consistent with a model in which 26Al was distributed uniformly in the nebula when chondrule formation began, approximately a million years after the formation of the majority of CAIs. The observed range of ( 26Al/ 27Al) 0 values in PRCs from CCs is most plausibly explained in terms of an extended duration of ˜2-3 Ma for the formation of CC chondrules. This interval is in sharp

  1. P- T conditions of crystallization and origin of plagioclase-mantled alkali feldspar megacrysts in the Mesozoic granitoids in the Qinling orogen (China)

    NASA Astrophysics Data System (ADS)

    Wang, Xiaoxia; Wang, Tao; Haapala, Ilmari; Mao, Jingwen

    2008-07-01

    The Qinling orogen between the North China and South China cratons was intruded at 211-217 Ma by calc-alkaline quartz monzonitic to monzogranitic plutons characterized by I- to A-type geochemistry and in many places contain plagioclase-mantled alkali feldspar megacrysts (rapakivi texture sensu lato). The felsic rocks contain mafic to intermediate magmatic enclaves suggestive of mingling and mixing of mafic and felsic magmas. The P- T conditions of crystallization have been determined for early mineral assemblages (inner parts of alkali feldspar megacrysts and their plagioclase, quartz, amphibole and biotite inclusions) and late assemblages (matrix minerals) of the rapakivi-textured granitoids. Al contents in amphibole from the early and late mineral assemblages yield pressures of 1.2-3.0 and 0.7-3.0 kbar, respectively, and indicate only minor pressure change between the crystallization of the early and late assemblages. Amphibole-plagioclase thermometry gives temperatures mainly of the order of 900 to 1000 °C for both the early and late assemblages indicating nearly isothermal conditions. Feldspar thermometers yield lower temperatures. Relative abundances of minerals and their chemical compositions indicate that the late mineral assemblages tend to be richer in MgO, Na 2O and CaO than the early assemblages. Rapakivi texture is interpreted in this case mainly as a result of compositional changes related to the hybridization between granitic and more mafic magmas. Small release of pressure during crystallization of the magmas may have contributed to the origin of the mantled alkali feldspar megacrysts.

  2. 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

  3. Magma generation and evolution processes of calc-alkalic and tholeiitic suites in Azuma volcano, NE Japan - Sr isotope micro-analysis study of plagioclase phenocrysts

    NASA Astrophysics Data System (ADS)

    Takahashi, T.; Hirahara, Y.; Kimura, J.; Chang, Q.; Tatsumi, Y.

    2009-12-01

    Magma generation and evolution processes of island-arc tholeiitic (TH) and calc-alkalic (CA) suites in the NE Japan arc have been thought that the former is produced by fractional crystallization from mantle-derived basalt magma and the latter is formed by magma mixing between basic and felsic magmas. However, based on Sr isotope micro-analyses of plagioclase phenocrysts in basalt and andesite lavas from Zao volcano, Tatsumi et al. (2008) argued that TH basalt was formed by melting of lower-crustal amphibolite, and CA was formed by magma mixing between mantle-derived basalt magma and the crust-derived basaltic andesite to dacite magmas. We present another example from Azuma volcano in the NE Japan arc which verifies our model. Sr isotope micro-analyses were performed by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS, NEPTUNE coupled to 193nm excimer laser system) and the combined method of micro-milling and thermal ionization mass spectrometry (MM-TIMS, micro-mill system and TRITON). The sampling crater size of LA-ICP-MS and MM-TIMS are 0.2mm and 0.27mm in both diameter and depth, respectively. Inner part of plagioclase phenocrysts in TH basalts have narrow range in An% and Sr isotope ratios (80 ~ 95% and 0.7058 ~ 0.7062). In contrast, plagioclases in CA basalts and andesites have widely ranges in An% and Sr isotope ratio (48 ~ 94%, 0.7039 ~ 0.7056). This observed variations are similar to that found in Zao samples, except that Sr isotope ratio is overall higher than that in Zao (Zao TH: 0.7043 ~ 0.7045, Zao CA: 0.7035 ~ 0.7045). We conclude that the magma generation and evolution model proposed by Tatsumi et al. (2008) is applicable to the Azuma TH and CA suites. Difference in the Sr isotope ratio of plagioclase between Azuma and Zao, particularly found in each TH suite lavas, can be explained by the difference in crust composition beneath the volcanoes.

  4. Toward quantification of strain-related mosaicity in shocked lunar and terrestrial plagioclase by in situ micro-X-ray diffraction

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

    Studies of shock metamorphism of feldspar typically rely on qualitative petrographic observations, which, while providing invaluable information, can be difficult to interpret. Shocked feldspars, therefore, are now being studied in greater detail by various groups using a variety of modern techniques. We apply in situ micro-X-ray diffraction (μXRD) to shocked lunar and terrestrial plagioclase feldspar to contribute to the development of a quantitative scale of shock deformation for the feldspar group. Andesine and labradorite from the Mistastin Lake impact structure, Labrador, Canada, and anorthite from Earth's Moon, returned during the Apollo program, were examined using optical petrography and assigned to subgroups of the optical shock level classification system of Stöffler (1971). Two-dimensional μXRD patterns from the same samples revealed increased peak broadening in the chi dimension (χ), due to strain-related mosaicity, with increased optical signs of deformation. Measurement of the full width at half maximum along χ (FWHMχ) of these peaks provides a quantitative way to measure strain-related mosaicity in plagioclase feldspar as a proxy for shock level.

  5. Spectral emissivity measurements of Mercury's surface indicate Mg- and Ca-rich mineralogy, K-spar, Na-rich plagioclase, rutile, with possible perovskite, and garnet

    NASA Astrophysics Data System (ADS)

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

    2009-03-01

    Mid-infrared 2-D spectroscopic measurements from 8.0 to 12.7 μm of Mercury were taken using Boston University's Mid-Infrared Spectrometer and Imager (MIRSI) mounted on the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, 7-11 April 2006. Measurements reported here cover radar bright region C, a dark plains region west of Caloris Basin, and the interior of Caloris Basin. By use of spectral deconvolution with a large spectral library composed of many mineral compositions and grain size separates, we fitted, or "unmixed", the Mercury spectra. We find mineral suites composed of magnesium-rich orthopyroxene and olivine, Ca-, Mg-, Na-rich clinopyroxene, potassium feldspar, and Na-bearing plagioclase feldspar. Both Ca- and Mg-rich garnet (pyrope and grossular, respectively) are apparently present in small amounts. Opaque minerals are required for spectral matching, with rutile (TiO 2) repeatedly providing the "best fit". However, in the case of the radar bright region C, perovskite also contributed to a very good fit. Caloris Basin infill is rich in both potassium feldspar and Na-rich plagioclase. There is little or no olivine in the Caloris interior smooth plains. Together with the high alkali content, this indicates that resurfacing magmas were low to intermediate in SiO 2. Data suggest the dark plains exterior to Caloris are highly differentiated low-iron basaltic magmas resulting in material that might be classified as oligoclase basalts.

  6. Inherited argon in a Pleistocene andesite lava: 40Ar/39Ar incremental-heating and laser-fusion analyses of plagioclase

    NASA Astrophysics Data System (ADS)

    Singer, B. S.; Wijbrans, J. R.; Nelson, S. T.; Pringle, M. S.; Feeley, T. C.; Dungan, M. A.

    1998-05-01

    By using 40Ar/39Ar incremental-heating and laser-fusion techniques, xenocrystic plagioclase was discovered in a late Pleistocene andesitic lava that erupted through the Andean Cordillera. Inherited argon in the xenocrysts is as much as ˜450 times older than the host lava, the age of which is independently known, and is an obstacle to dating the lava by using a whole-rock sample. The xenocrysts are impossible to identify from petrography or chemical parameters such as their K/Ca ratios. Holocrystalline groundmass, carefully separated from plagioclase and other phenocrysts, gives an accurate 40Ar/39Ar age for the lava. The xenocrysts could not have been degassed for more than several days in the magma and probably were assimilated from Paleozoic rocks buried under kilometers of Mesozoic and Tertiary arc rocks composing the Cordillera in central Chile. The required magma ascent velocity, on the order of kilometers/day, is extraordinarily high compared to the 10-4 km/day minimum implied by the 226Ra excess in continental arc lavas. These data permit magma migration and storage in the mantle and lower crust for as much as thousands of years, followed abruptly by rapid ascent to the surface.

  7. Garnet-orthopyroxene-plagioclase-quartz barometry: refinement and application to the English River subprovince and the Minnesota River valley

    NASA Astrophysics Data System (ADS)

    Perkins, Dexter; Chipera, Steve J.

    1985-03-01

    Recent experimental, theoretical, and thermodynamic studies permit better calibration of two reactions for geobarometry: grossular+pyrope+quartz=anorthite+enstatite grossular+almandine+quartz=anorthite+ferrosilite If both reactions are applied using the same thermodynamic data and activity models they should yield the same pressure for a given garnet-pyroxene-plagioclase-quartz assemblage. Application to a variety of high-grade terrains generally yielded excellent results. However, poor results have been obtained for high-Fe rocks which can be traced to errors in activity models for garnet and/or pyroxene. Either a two-site ideal mixing model for orthopyroxene (cf. Wood and Banno 1973) underestimates enstatite activity for high-Fe orthopyroxenes or the Ganguly and Saxena (1984) model overestimates pyrope activity in low-Mg garnets. Application of both barometers to a variety of high-grade terrains gives the following average pressures: Terrain Mg-reaction Fe-reaction Adirondacks, New York 11.2 8.0 Buksefjorden, Greenland 9.5 8.89 Doubtful Sound, New Zealand 12.6 12.0

  8. 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

  9. 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.

    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.

  10. 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

  11. Shock melting of K-feldspar and interlacing with cataclastically deformed plagioclase in granitic rocks at Toqqusap Nunaa, southern West Greenland: Implications for the genesis of the Maniitsoq structure

    NASA Astrophysics Data System (ADS)

    Keulen, Nynke; Garde, Adam A.; Jørgart, Tommy

    2015-11-01

    Folded sheets of Mesoarchaean, leucocratic plagioclase-K-feldspar-mesoperthite-bearing granitic rocks in the Toqqusap Nunaa area of the Maniitsoq structure, West Greenland, are characterised by their very fine grain sizes and microstructures without normal igneous or planar/linear tectonic fabrics. Quartz forms equidimensional and branching, ductilely deformed aggregates and bifurcating panels with protrusions, constrictions and chains of ball-shaped grains with healed, radiating intergranular fractures. Plagioclase (An10-20) was cataclastically deformed and comminuted, whereas K-feldspar and mesoperthite are devoid of cataclastic microstructures. K-feldspar forms dispersed, highly irregular grains with numerous cusps and saddles, indicating almost ubiquitous direct (shock) melting of this mineral. It is commonly located along former fractures in plagioclase, resulting in an 'interlaced' feldspar microstructure with contact shapes indicating subsequent melting of plagioclase directly adjacent to K-feldspar. Mesoperthite forms separate, rounded, and irregular grains with protrusions and cusped margins indicating crystallisation from melts. Some mesoperthite grains are texturally and compositionally heterogeneous and contain internal lenses of K-feldspar and/or plagioclase. Other mesoperthite grains comprise coarsened, 'unzipped' areas, presumably due to localised, fluid-controlled dissolution-reprecipitation processes. The ternary feldspar precursor of the mesoperthite is interpreted as having crystallised from variably effectively mixed K-feldspar shock melts and plagioclase contact melts. Direct melting of K-feldspar, but no whole-rock melting, requires shock metamorphism with a short-lived temperature excursion to above the melting temperature of K-feldspar (~ 1300 °C). The presence of three different feldspar species and absence of chemical zonation, magmatic mantling, or metamorphic coronas furthermore hinders interpretations solely by means of endogenic

  12. 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

  13. Peraluminous websterite and granulite xenoliths from the Chyulu Hills volcanic field, Kenya: Plagioclase-rich cumulates re-equilibrated at uppermost mantle and crustal conditions?

    NASA Astrophysics Data System (ADS)

    Ulianov, A.; Kalt, A.; Pettke, T.

    2004-12-01

    Basanites of the Chyulu Hills volcanic field, Kenya, contain a suite of meta-igneous peraluminous spinel-garnet olivine websterite, Mg-Al sapphirine-bearing and Ca-Al hibonite-bearing granulite xenoliths. The websterites are the most mafic and magnesian members of this sequence. The Mg-Al sapphirine-bearing granulites are more Si- and Al-rich and less magnesian. They consist of ortho- and clinopyroxene, corundum, spinel, sapphirine, sillimanite, plagioclase and garnet. The Ca-Al hibonite-bearing granulites are most enriched in Si and Al. They are dominated by clinopyroxene and plagioclase and may contain hibonite closely associated with spinel, mullite, sapphirine and sillimanite. Hibonite, which is very rare in terrestrial rocks, is the earliest mineral in the crystallization sequence. All rocks are poor in REE, HFSE and Cr and enriched in LILE and Ni. They follow a magmatic fractionation trend and form a cumulate sequence finally equilibrated in the range of uppermost mantle (websterites) and crustal (granulites) depths. The websterites could have formed by high-pressure metamorphism of low-pressure troctolite-like cumulates and appear similar to some websteritic lithologies from exhumed high-pressure ultramafic complexes (Kornprobst, 1990; Morishita et al., 2003). The final P-T conditions for most websterites correspond to 920-1000 C / 17-22 kbar, whereas the final equilibration in granulites occurred at ca. 600-740 C / <8 kbar, the pressure limit being defined by the stability field of sillimanite. The geodynamic interpretation of spatially close granulite terranes (Moeller et al., 1998) coupled with the petrological evidence suggest that the studied rocks were metamorphosed and may have formed in the environment of a Pan-African active continental margin. Moeller, A., Mezger, K., Schenk, V. (1998). Journal of Petrology 39, 749-783. Kornprobst, J., Piboule, M., Roden, M., Tabit, A. (1990). Journal of Petrology 31, 717-745. Morishita, T., Arai, S., Gervilla, F

  14. Chemistry, reaction mechanisms and micro-structures during retrograde metamorphism of gedrite-biotite-plagioclase bearing rocks from Bergslagen, south-central Sweden

    NASA Astrophysics Data System (ADS)

    Ferrow, Embaie A.; Ripa, Magnus

    1991-01-01

    The Proterozoic rhyolitic volcanics constituting the foot-wall rocks in the Stollberg ore-field, Bergslagen, south-central Sweden, locally contain gedrite altered to chlorite and serpentine, biotite altered to chlorite and plagioclase altered to epidote. The intergrowths between the host gedrite and the chlorite/serpentine inclusions are oriented with the a ∗ of gedrite parallel to the c ∗ of serpentine and chlorite. The biotite has been altered to chlorite by brucitization of both the K-interlayer and talc-like layer. In both cases the net change in volume during chloritization is small. The assumption that Al is conserved during alteration of gedrite and biotite agrees very well with the micro-structures and orientation relations observed by transmission electron microscopy. Normalizing the chlorite to 1.00 mole, the overall chemical change that took place during the retrograde metamorphism of the Stollberg rocks can be written as: 0.84Ged+0.14Bio+0.65Mg+4.76H 2O+0.42H=1.00Chl+0.57Alb+0.72Fe+0.01Na+0.12K+0.01Ti+0.05Mn+0.95H 4SiO 4 The reaction results in ca 9% increase in volume for the solid phases. Thus, a slightly acidic Mg-rich fluid started the reaction and, upon leaving the system, the metasomatic fluid was enriched in Na, Fe, K, and Si.

  15. Crystallography of some lunar plagioclases

    USGS Publications Warehouse

    Stewart, D.B.; Appleman, D.E.; Huebner, J.S.; Clark, J.R.

    1970-01-01

    Crystals of calcic bytownite from type B rocks have space group U with c ??? 14 angstroms. Bytownite crystals from type A rocks are more sodic and have space group C1, c ??? 7 angstroms. Cell parameters of eight bulk feldspar separates from crystalline rocks indicate that the range of angle gamma is about 23 times the standard error of measurement, and its value might be useful for estimation of composition. Cell parameters of seven ilmenites are close to those of pure FeTiO3.

  16. 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.

  17. 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

    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.

  18. 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

  19. 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

  20. 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

  1. 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.

  2. 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.

  3. 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.

    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.

  4. 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

  5. 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

  6. Microbial extracellular polysaccharides and plagioclase dissolution

    SciTech Connect

    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 {approx} 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 {approx} 3, below the pK{sub 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.

  7. 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

  8. Anisotropy of magnetic susceptibility in alkali feldspar and plagioclase

    NASA Astrophysics Data System (ADS)

    Biedermann, Andrea R.; Pettke, Thomas; Angel, Ross J.; Hirt, Ann M.

    2016-04-01

    Feldspars are the most abundant rock-forming minerals in the Earth's crust, but their magnetic properties have not been rigorously studied. This work focuses on the intrinsic magnetic anisotropy of 31 feldspar samples with various chemical compositions. Because feldspar is often twinned or shows exsolution textures, measurements were performed on twinned and exsolved samples as well as single crystals. The anisotropy is controlled by the diamagnetic susceptibility and displays a consistent orientation of principal susceptibility axes; the most negative or minimum susceptibility is parallel to [010], and the maximum (least negative) is close to the crystallographic [001] axis. However, the magnetic anisotropy is weak when compared to other rock-forming minerals, 1.53 × 10-9 m3 kg-1 at maximum. Therefore, lower abundance minerals, such as augite, hornblende or biotite, often dominate the bulk paramagnetic anisotropy of a rock. Ferromagnetic anisotropy is not significant in most samples. In the few samples that do show ferromagnetic anisotropy, the principal susceptibility directions of the ferromagnetic subfabric do not display a systematic orientation with respect to the feldspar lattice. These results suggest that palaeointensity estimates of the geomagnetic field made on single crystals of feldspar will not be affected by a systematic orientation of the ferromagnetic inclusions within the feldspar lattice.

  9. 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.

  10. 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.

  11. Heterogeneous plagioclase compositions in the Maralinga CK4 chondrite

    NASA Technical Reports Server (NTRS)

    Keller, Lindsay P.

    1993-01-01

    One of the characteristic features of CK chondrites is the wide compositional range displayed by feldspar grains in matrix relative to the narrow range of compositions exhibited by the highly equilibrated olivines and pyroxenes. Recently, it was suggested that these heterogeneous feldspar compositions may have been strongly influenced by shock metamorphisms. It is shown that the apparent range of feldspar compositions in Maralinga probably results from annealing during parent body thermal metamorphism rather than shock. The majority of matrix feldspars in Maralinga are typically 50 microns in size and are compositionally zoned, with oligoclase cores (approximately An40) and bytownite rims (approximately An80). The contact between core and rim is sharp and abrupt and is readily observed in backscattered scanning electron microscopy (SEM) images.

  12. Anisotropy of magnetic susceptibility in alkali feldspar and plagioclase

    NASA Astrophysics Data System (ADS)

    Biedermann, Andrea R.; Pettke, Thomas; Angel, Ross J.; Hirt, Ann M.

    2016-01-01

    Feldspars are the most abundant rock-forming minerals in the Earth's crust, but their magnetic properties have not been rigorously studied. This work focuses on the intrinsic magnetic anisotropy of 31 feldspar samples with various chemical compositions. Because feldspar is often twinned or shows exsolution textures, measurements were performed on twinned and exsolved samples as well as single crystals. The anisotropy is controlled by the diamagnetic susceptibility and displays a consistent orientation of principal susceptibility axes; the most negative or minimum susceptibility is parallel to [010], and the maximum (least negative) is close to the crystallographic [001] axis. However, the magnetic anisotropy is weak when compared to other rock-forming minerals, 1.53*10-9 m3 kg-1 at maximum. Therefore, lower abundance minerals such as augite, hornblende or biotite often dominate the bulk paramagnetic anisotropy of a rock. Ferromagnetic anisotropy is not significant in most samples. In the few samples that do show ferromagnetic anisotropy, the principal susceptibility directions of the ferromagnetic subfabric do not display a systematic orientation with respect to the feldspar lattice. These results suggest that paleointensity estimates of the geomagnetic field made on single crystals of feldspar will not be affected by a systematic orientation of the ferromagnetic inclusions within the feldspar lattice.

  13. 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.

  14. A re-examination of relationships among pyroxene-plagioclase achondrites.

    NASA Technical Reports Server (NTRS)

    Jerome, D. Y.; Goles, G. G.

    1971-01-01

    In light of the new data of Jerome (1970), a brief discussion of genetic relationships between eucrites, howardites and diogenites is presented. Howardites are not direct products of magmatic crystallization, as has sometimes been postulated, but instead are interpreted as polymict breccias resulting from the mechanical mixture of eucrites and diogenites. Testing of this model by means of chemical arguments is made complicated however by the probable presence of a chondritic component in howardites and by a sequence analogous to terrestrial differentiation series, among the eucrites themselves.

  15. 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.

  16. 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.

  17. 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.

  18. 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.'

  19. 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.

  20. 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. PMID:17758716

  1. 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.

  2. 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.

  3. 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.

  4. Hydrogen concentration in plagioclaes as a hygrometer of magmas: Approaches from melt inclusion analyses and hydrous melting experiments

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    Plagioclase is one of the nominally anhydrous minerals (NAMs) which accommodates hundreds wt. ppm of hydrogen. Hydrogen in igneous plagioclase (OH) can act as a proxy of dissolved H2O in silicate melt. In order to use it as a practical hygrometer of magmas, we studied partitioning of hydrogen between plagioclase and basaltic melt dis-solving H2O (0.3˜5.5 wt.%) by two approaches: analyses of plagioclase-hosted melt inclusions (H2O≈0.3 wt.%) from mid-ocean ridge basalt (MORB) and hydrogen partitioning experiments between An96 plagioclase and hydrous basaltic melt (0.8 wt.%≦H2O≦5.5 wt.%) at 0.35 GPa. Concentration hydrogen in plagioclase and concentration of H2O in basaltic glasses were analyzed by infrared spectroscopy. As a first series of this study, plagioclase-hosted melt inclusions in MORB (50.5% SiO2, 15.1% Al2O3, 7.4% MgO) from the Rodriguez triple junction in the Indian Ocean were analyzed. The hydrogen concentration of plagioclase is less than 50 wt. ppm water, and no correlation between hydrogen concentration and anorthite content is recognized. Average H2O concentrations in melt inclusions is 0.3 wt.%. Therefore, apparent partition coefficient of hydrogen between plagioclase and melt is ≈ 0.01 in molar basis. As a second series of this study, hydrous melting experiments of arc basaltic magma were performed at 350 MPa using internally-heated pressure vessel. Starting material was hydrous glass (0.8 wt.%≦H2O≦5.5 wt.%) of an undifferentiated rock from Miyakejima volcano, a frontal-arc volcano in Izu-arc (MTL rock: 50.5% SiO2, 18.1% Al2O3, 4.9% MgO). A grain of Ca-rich plagioclase (≈1 mg, about An96) and 10 mg of powdered glasses were sealed in Au80Pd20 alloy capsule, and then kept at temperature at around the liquidus. Liquidus phase of MTL rock at 350 MPa is always plagioclase with 0 to 5.5 wt.% H2O in melt, and therefore, a grain of plagioclase and hydrous melt were nearly in equilibrium. Oxygen fugacity (fO2) during the melting experiments

  5. 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

  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. 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.

  8. Thermoluminescence of the mineral components in granite

    SciTech Connect

    Schwartzman, R.G.; Kierstead, J.A.; Levy, P.W.

    1982-01-01

    The thermoluminescence (TL) of the minerals in Climax Stock (Nevada, USA) granite has been studied. The principal mineral constituents are plagioclase, quartz, potassium feldspar and biotite. Pyrite, sphene apatite and zircon occur at one percent or less. All exhibit TL except biotite. The TL kinetics were determined for plagioclase, quartz, potassium feldspar and pyrite. Plagioclase and potassium feldspar exhibit second order and pyrite first orker kinetics. Natural TL of quartz follows second order and artificial TL first order kinetics. However, in these four minerals unrealistic kinetic parameters are often obtained; thus more general kinetics, e.g. interactive kinetics, may apply. 8 figures.

  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. PMID:17731366

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  15. 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. PMID:19741704

  16. 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.

  17. 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.

  18. 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.

  19. 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

  20. 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.

  1. 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.

  2. 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.

  3. 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.

  4. 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

  5. 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

  6. 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.

  7. 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

  8. A new approach to relate kinetic parameters, composition and temperatures of igneous minerals: the case of Villarrica Volcano 1971 eruption

    NASA Astrophysics Data System (ADS)

    Contreras, C.; Parada, M. A.; Morgado, E. E.; Castruccio, A.

    2014-12-01

    This contribution provides a new approach to calculate the plagioclase composition of a given igneous rock based on kinetics parameters of crystallization obtained from CSD analyses of plagioclase. The procedure we use includes the following steps: 1) determination of the effective rates of crystal growth and nucleation based on the maximum time interval of mineral crystallization obtained from the CSD segment with the minimum slope and extrapolated to rest of the segments; 2) calculation of the total crystallization time; 3) determination of the time-dependent variation of plagioclase volume fraction based on crystal population density and CSD slopes; 4) calculation of the ideal rates of crystal growth and nucleation; 5) determination of temperature of plagioclase crystallization as a function of the ideal crystal growth rate; 6) determination of plagioclase composition as a function of the ideal crystal growth rate. We apply our model for plagioclase of 17 samples of The Villarrica Volcano 1971 eruption lava flow. All CSD show two segments with a break at 0.3 mm. The first segment is interpreted as the syn-eruptive crystallization and the second as the within-reservoir crystallization. An interval of 1.5 years as a maximum crystallization time for the syn-eruptive stage was estimated from Cashman's (1993) expression. Crystal growth rates in the reservoir stage took place during two stages. The first stage varies from 10^-8.9 to 10^-9.4 mm s-1 and the second one varied from 10^-9.4 to 10^-9.1 mm s-1. Finally, the crystal growth rate as high as 10^-7.8 mm s-1 were estimated for the syn-eruptive stage. Nucleation rates varies exponentially from 10^-13 to 10^-8 s-1 in the reservoir stage and from 10^-8 to 10^-1 s^-1 in the syn-eruptive stage. The total crystallization interval varies from 120 to 390 years. A mean total volume fraction of plagioclase of 0.55 was obtained (0.98 for all minerals) whereas a mean value of 0.054 was obtained at the onset of the eruption (0

  9. Melt inclusion data from the July/August 2006 and May 2010 eruptions of Tungurahua volcano, Ecuador

    NASA Astrophysics Data System (ADS)

    Myers, M.; Geist, D.; Rowe, M. C.; Harpp, K. S.; Dufek, J.

    2011-12-01

    Tungurahua volcano, located in the eastern Cordillera of the Ecuadorian Andes, began a new eruptive sequence in 1999 after ~80 years of inactivity, which persists as of August 2011. This study focuses on the pyroclastic flow-forming eruptions of 2006 and May 2010, using melt inclusions within plagioclase and pyroxene to complement the whole rock compositions presented by Samaniego et al. (2011). Melt inclusions in pyroxene from the 2006 eruption have higher silica concentrations (61-64 wt%) than those hosted within plagioclase (59-62 wt%), the matrix glass (61-62 wt%), and the bulk rock (58 wt%). The 2010 eruption shows a similar but more exaggerated trend, with pyroxene-hosted melt inclusions having silica concentrations ranging from 58-67 wt%, plagioclase between 61-63 wt% and glass between 59-62 wt%. These differences are not attributable to post-entrapment modification. The more evolved inclusions in the 2010 pyroxenes can be created through fractional crystallization of the 2006 glass compositions, using pressures ~2-3 kb and water concentrations ~3 wt%, as simulated by MELTs. Phenocrysts in the 2006 samples have reabsorbed rims and sieve textures, whereas few of the 2010 plagioclases are sieve textured. The 2006 and 2010 melt inclusion compositions plot within the trend formed by the whole rock data from historical eruptions, suggesting that they originated from similar petrogenetic processes. The 2006 pyroxene-hosted melt inclusions have aluminum concentrations above the whole rock trend, whereas the 2010 fall below; otherwise, rocks and inclusions from the two eruptions have similar compositions. Because the pyroxene-hosted melt inclusions contain more evolved melt than the plagioclase-hosted melt inclusions, it is possible that: 1) plagioclase crystallized at higher temperature, along with iron-titanium oxides, increasing the silica concentrations in liquids that were captured by pyroxene, followed by a mafic intrusion that triggered the eruption or 2

  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. 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.

  12. 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?). PMID:17731364

  13. 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

    2016-07-01

    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.

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. Mineralogy of the Ibitira eucrite and comparison with other eucrites and lunar samples

    NASA Technical Reports Server (NTRS)

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

    1976-01-01

    The mineralogy of the Ibitira meteorite is studied using single-crystal X-ray and energy-dispersive electron-probe techniques. It is shown that this strongly metamorphosed unbrecciated vesicular eucrite has a primary variolitic and secondary hornfelsic texture dominated by equant grains of pale-brown pyroxene (60%) set in an irregular matrix of plagioclase (30%). Minor phases are detected which indicate igneous crystallization, a prolonged metamorphic history, and vapor deposition. The following sequence of events is suggested as the simplest explanation for the mineralogical features observed: (1) crystallization of the meteorite in a lava flow; (2) prolonged annealing, reduction of primary spinel, and vapor deposition; (3) strong shock sufficient to modify the texture of the plagioclase grains; and (4) sufficient annealing to allow crystallization of the plagioclase as a subparallel mosaic and partial conversion to the low structural state. Evidence for this sequence is examined in detail, particularly in relation to the general features of eucrites, howardites, and mesosiderites

  19. 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.

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

    NASA Technical Reports Server (NTRS)

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

    1999-01-01

    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. Epidote-amphibolite to amphibolite facies transition in the southern Appalachian Piedmont: P-T conditions across the garnet and calc-silicate isograds

    SciTech Connect

    Chalokwu, C.I. )

    1989-06-01

    Amphibolites occur in three distinct localities (Hudson Rapids, Davis Mill, and Lindsey Creek) in the west-central Georgia part of the Uchee belt, southern Appalachian Piedmont. Epidote-amphibolite facies assemblage (plagioclase (An{sub 24-36}) + hornblende + epidote + sphene + quartz) at the Davis Mill occurrence gives way to the amphibolite assemblage (calcic plagioclase (An{sub 33-88}) + hornblende + garnet + epidote + ilmenite + quartz) at Hudson Rapids. Mineral chemistries indicate that the transition was controlled by increasing pressure and unusual bulk compositions. Equilibrium mineral assemblages record temperatures ranging from 610 to 780 C at the garnet isograd, and from 570 to 634 C close to the calc-silicate isograd. Estimated pressures range from 7 to 9.25 {plus minus} 2 kbar. These P-T conditions are consistent with a thickened crustal section caused by emplacement of the Piedmont allochthon, followed by isobaric cooling (or isothermal compression), and the formation of symplectites of epidote + amphibole around plagioclase + clinopyroxene.

  2. 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.

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

    NASA Astrophysics Data System (ADS)

    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.

  4. 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.

  5. Observations of silicate reststrahlen bands in lunar infrared spectra

    NASA Technical Reports Server (NTRS)

    Potter, A. E., Jr.; Morgan, T. H.

    1982-01-01

    Thermal emission spectra of three lunar sites (Apollo 11, Descartes Formation, and Tycho central peak) are measured in the 8-14 micron spectral range. Transmission and instrument effects are accounted for by forming ratios of the Descartes and Tycho spectra to the Apollo 11 spectrum. The ratio spectra are compared with ratios of published laboratory spectra of returned lunar samples and also with ratio spectra calculated using the Aronson-Emslie (1975) model. The comparisons show pyroxene bands in the Descartes ratio spectrum and plagioclase bands in the Tycho ratio spectrum. The Tycho spectrum is found to be consistent with the existence of fine plagioclase dust (approximately 1 micron) at the rock surface and a higher-than-usual sodium content of the plagioclase.

  6. 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. PMID:10600738

  7. 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.

  8. 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.

  9. 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.

  10. Theoretical prediction of phase relationships in planetary mantles

    NASA Technical Reports Server (NTRS)

    Wood, B. J.; Holloway, J. R.

    1982-01-01

    Thermodynamic and phase equilibrium data are used to generate an internally consistent set of enthalpies and entropies for important components of the CaO-MgO-Al2O3-SiO2 (CMAS) system in silica-undersaturated compositions. The addition of Na and Fe(2+) to the CMAS system produces shifts in the plagioclase, spinel and garnet stability fields. While the Morgan and Anders (1979) model Martian composition has stability fields of plagioclase and garnet lherzolite, and a small spinel lherzolite field at temperatures below 900 C, the Martian mantle composition of McGetchin and Smyth (1978) would not contain orthopyroxene, and a low pressure assemblage of plagioclase-spinel wehrlite would be replaced by garnet-spinel wehrlite at higher pressure. In both cases, the Fe-Mg ratio would be substantially greater than that found in primitive terrestrial basalts.

  11. 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.

  12. 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.

  13. 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?

  14. 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

  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, USA

    USGS Publications Warehouse

    Townsend, G.N.; Gibson, R.L.; Horton, J.W., Jr.; 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.

  16. 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, Jr.; 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.

  17. 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.

  18. 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.

  19. 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. PMID:17737107

  20. 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)

  1. 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)

  2. The Maryville meteorite - A 1983 fall of an L6 chondrite

    NASA Astrophysics Data System (ADS)

    Shervais, J. W.; Taylor, L. A.; Cirlin, E.-H.; Jarosewich, E.; Laul, J. C.

    1986-03-01

    The Maryville chondrite fell on January 28, 1983 in eastern Tennessee. Compositions of olivine (Fa 24.5), orthopyroxene (Fs 20.8), plagioclase (An 10.6), along with the bulk composition and siderophile concentrations, indicate L-group classification. The presence of highly equilibrated minerals, strongly recrystallized matrix, and the development of large, clear plagioclase grains suggest petrologic type 6 classification. Subsequent to metamorphism the meteorite was subjected to high transient pressures that converted some feldspar to glass, deformed the silicates, and caused small amounts of melting to occur. The effects of this shock event correspond to shock facies 'd' or 'e'.

  3. Melt/peridotite interaction in the Southern Lanzo peridotite: Field, textural and geochemical evidence

    NASA Astrophysics Data System (ADS)

    Piccardo, G. B.; Zanetti, A.; Müntener, O.

    2007-03-01

    This paper presents field, petrographic-structural and geochemical data on spinel and plagioclase peridotites from the southern domain of the Lanzo ophiolitic peridotite massif (Western Alps). Spinel lherzolites, harzburgites and dunites crop out at Mt. Arpone and Mt. Musinè. Field evidence indicates that pristine porphyroclastic spinel lherzolites are transformed to coarse granular spinel harzburgites, which are in turn overprinted by plagioclase peridotites, while strongly depleted spinel harzburgite and dunite bands and bodies replace the plagioclase peridotites. On the northern flank of Mt. Arpone, deformed, porphyroclastic (lithospheric) lherzolites, with diffuse pyroxenite banding, represent the oldest spinel-facies rocks. They show microstructures of a composite subsolidus evolution, suggesting provenance from deeper (asthenospheric) mantle levels and accretion to the lithosphere. These protoliths are locally transformed to coarse granular (reactive) spinel harzburgites and dunites, which show textures reminiscent of melt/rock reaction and geochemical characteristics suggesting that they are products of peridotite interaction with reactively percolating melts. Geochemical data and modelling suggest that <1-5% fractional melting of spinel-facies DMM produced the injected melts. Plagioclase peridotites are hybrid rocks resulting from pre-existing spinel peridotites and variable enrichment of plagioclase and micro-gabbroic material by percolating melts. The impregnating melts attained silica-saturation, as testified by widespread orthopyroxene replacement of olivine, during open system migration in the lithosphere. At Mt. Musinè, coarse granular spinel harzburgite and dunite bodies replace the plagioclase peridotites. Most of these replacive, refractory peridotites have interstitial magmatic clinopyroxene with trace element compositions in equilibrium with MORB, while some Cpx have REE-depleted patterns suggesting transient geochemical features of the

  4. 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.

  5. Timing and tectonic setting of the Sijiaying banded iron deposit in the eastern Hebei province, North China Craton: Constraints from geochemistry and SIMS zircon U-Pb dating

    NASA Astrophysics Data System (ADS)

    Cui, Minli; Zhang, Lianchang; Wu, Huaying; Xu, Yingxia; Li, Wenjun

    2014-11-01

    The North China Craton (NCC), one of the oldest continental blocks in Asia, has a complicated evolutionary history with the age of the old crust up to 3.8 Ga and records the important geological events of the Earth. The Sijiaying BIF, the largest banded iron formation located in eastern Hebei province, in the east part of NCC, records an important thermo-tectonic event; Previous studies focused on the geological description, but the timing and tectonic setting have been unclear so far. The Sijiaying BIF is hosted in Neoarchean metamorphic rocks, which includes biotite-leptynite, hornblende plagioclase-gneiss and biotite plagioclase-gneiss. Using major element contents and ratios of the host rocks, the protoliths of the hornblende plagioclase-gneiss and biotite plagioclase-gneiss are shown to be dacite, whereas those of the biotite-leptynites are sedimentary rocks. Based on geology and geochemistry of the host rocks, we infer that the Sijiaying BIF is an Algoma type deposit. SIMS zircon U-Pb dating shows: igneous zircons from the biotite plagioclase-gneiss and hornblende plagioclase-gneiss have U-Pb ages of 2535 ± 8 Ma and 2543 ± 14 Ma, respectively; zircons from the biotite-leptynite have an age of 2537 ± 13 Ma. We infer that the ages of 2543-2535 Ma represent the time of the Sijiaying BIF. PAAS-normalized REY profiles of the ore samples are characterized by LREE depletion and HREE enrichment, positive La and Eu anomalies and an average Y/Ho weight ratio of 32, indicating a mixture of submarine hydrothermal fluids and deep seawater. REE data was combined with the δ18O analyzed by previous researcher of the ore from individual magnetite bands to infer that the Sijiaying BIF precipitated from hydrothermal fluids discharging on the sea floor. The hornblende plagioclase-gneiss and biotite plagioclase-gneiss that are enriched in LILE and LREE and depleted in high field strength elements (HFSE) Nb, Ta and Ti, have geochemical signatures that are similar to those of

  6. 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.

  7. 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.

  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

  9. 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

  10. Kinetics of crystal evolution as a probe to magmatism at Stromboli (Aeolian Archipelago, Italy)

    NASA Astrophysics Data System (ADS)

    Agostini, C.; Fortunati, A.; Arzilli, F.; Landi, P.; Carroll, M. R.

    2013-06-01

    The kinetics of hydrated basaltic melts erupted during the present activity at Stromboli have been studied to estimate the growth and dissolution rate of plagioclase. Specifically, a high-K basalt composition (PST-9) has been studied to investigate magma and eruption dynamics at Stromboli volcano by combining crystallization kinetics of plagioclase and CSD measurements on natural samples from literature (Armienti et al., 2007; Fornaciai et al., 2009). A series of water-saturated decompression experiments over a range of final water pressure (Pf = 75-5 MPa) at constant temperature (1075 °C) show that plagioclase is systematically present from 50 to 5 MPa at water saturated conditions. Moreover, these experiments show that anorthite (An) content decreases with decreasing PO, reaching the same composition as the natural plagioclase in Stromboli scoria at pressure below ˜20 MPa and that the plagioclase crystal fraction increases as the experimental conditions tend to lower final pressure. Plagioclase growth rate (GL) is observed to increase with undercooling for the Pf investigated during decompression experiments, except for the 75 MPa Pf serie that only has two samples with the presence of plagioclase crystals. The values of GL vary from 10-7 to 10-8 cm/s for Pf from 75 to 25 MPa, while at Pf from 10 to 5 MPa growth rates are approximately of 10-6 cm/s. A series of dissolution experiments at atmospheric pressure and over a range of temperature has been done for plagioclase (T range of 1220-1240 °C). Dissolution rate (G-) for plagioclase (10-7 cm/s) tends to be slightly higher at higher temperature in the range of 1220-1240 °C and appears to be time independent for the experimental durations investigated (10-30 h). These trends could be related to development of a diffusion-limited boundary layer adjacent to the dissolving crystal. By comparison of the experimental data on plagioclase composition, growth rates and dissolution in Stromboli basalt, it is possible to