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Sample records for trachytes

  1. Trace-element partitioning in pantellerites and trachytes

    SciTech Connect

    Mahood, G.A.; Stimac, J.A. )

    1990-08-01

    In order to investigate the effect of increasing melt peralkalinity on partitioning, partition coefficients have been determined using neutron activation analyses of coexisting phenocrysts and glass of five samples from Pantelleria spanning the range trachyte to pantellerite. Alkali feldspar partition coefficients for Fe, Rb, Ba, Sr, and Eu vary with melt peralkalinity due to changes in melt polymerization and to the systematic increase in X{sub or} and decrease in X{sub an} of the feldspar. In going from trachyte to pantellerite, Fe partition coefficients increase from 0.04 to 0.10, presumably because Fe{sup +3} increasingly substitutes in the feldspar tetrahedral site as melt activity of Al declines and Fe concentrations increase. Partition coefficients for trivalent light REEs (rare earth elements) decrease and the partitioning pattern becomes flatter, the most evolved samples having some of the lowest published values for feldspar. The hundredfold decline in Eu partition coefficients (2.5 to 0.024) and the decrease in the size of the positive partitioning anomaly are attributed to increasing Eu{sup 3+}/Eu{sup 2+} in the melt as it becomes more peralkaline, as well as to concomitant decrease in the Ca content of feldspar. As a result, the behavior of Eu during fractional crystallization of peralkaline suites is fundamentally different from that in metaluminous suites; absolute abundances rise and the size of the negative Eu anomaly changes little with fractionation beyond pantelleritic trachyte.

  2. Severe leaching of trachytic glass without devitrification, Terceira, Azores

    SciTech Connect

    Mungall, J.E.; Martin, R.F. )

    1994-01-01

    Pumice produced during the subplinian phase of eruption of a peralkaline trachyte lava on the island of Terceira, Azores, has undergone extensive leaching at ambient conditions by groundwater, while remaining in a glassy state. Over 25% of F, Na, K, Si, Fe, Ti, and Mn originally present has been removed, whereas U, Al, Nb, Ca, Y, and Rb show smaller but significant losses. Thorium , Zr, Hf, Ta, and Lu have remained immobile. Water content (measured as LOI) has increased up to tenfold, whereas the Sr content has risen by up to 80%. The absence of devitrification requires that leached elements escaped by diffusing through the glass; a coefficient of chemical diffusion D[sub Na] between 1.8 X 10[sup [minus]19] cm[sup 2] s[sup [minus]1] and 6.1 X 10[sup [minus]19] cm[sup 2] s[sup [minus]1] has been calculated, in excellent agreement with previously published experimentally derived values for comparably low temperatures. Water has not replaced the leached cations on a charge-equivalent basis, a finding that requires an explanation other than simple binary interdiffusion. The authors suggest that a simultaneous influx of molecular water and cation exchange between Na[sup +] in the glass and H[sup +] in the groundwater opens the glass structure and frees cations, which can diffuse outward by hydrolysis of bridging oxygen atoms. Reported mobilities have important implications for such diverse fields as igneous petrology, nuclear waste management, and agriculture.

  3. Petrogenesis of basalt-trachyte lavas from Olmoti Crater, Tanzania

    NASA Astrophysics Data System (ADS)

    Mollel, Godwin F.; Swisher, Carl C., III; McHenry, Lindsay J.; Feigenson, Mark D.; Carr, Michael J.

    2009-08-01

    Olmoti Crater is part of the Plio-Pleistocene Ngorongoro Volcanic Highland (NVH) in northern Tanzania to the south of Gregory Rift. The Gregory Rift is part of the eastern branch of the East African Rift System (EARS) that stretches some 4000 km from the Read Sea and Gulf of Aden in the north to the Zambezi River in Mozambique. Here, we (1) characterize the chemistry and mineral compositions of lavas from Olmoti Crater, (2) determine the age and duration of Olmoti volcanic activity through 40Ar/ 39Ar dating of Olmoti Crater wall lavas and (3) determine the genesis of Olmoti lavas and the relationship to other NVH and EARS volcanics and (4) their correlation with volcanics in the Olduvai and Laetoli stratigraphic sequences. Olmoti lavas collected from the lower part of the exposed crater wall section (OLS) range from basalt to trachyandesite whereas the upper part of the section (OUS) is trachytic. Petrography and major and trace element data reflect a very low degree partial melt origin for the Olmoti lavas, presumably of peridotite, followed by extensive fractionation. The 87Sr/ 86Sr data overlap whereas Nd and Pb isotope data are distinct between OLS and OUS samples. Interpretation of the isotope data suggests mixing of enriched mantle (EM I) with high-μ-like reservoirs, consistent with the model of Bell and Blenkinsop [Bell, K., Blenkinsop, J., 1987. Nd and Sr isotopic compositions of East African carbonatites: implications for mantle heterogeneity. Geology 5, 99-102] for East African carbonatite lavas. The isotope ratios are within the range of values defined by Oceanic Island Basalt (OIB) globally and moderate normalized Tb/Yb ratios (2.3-1.6) in these lavas suggest melting in the lithospheric mantle consistent with other studies in the region. 40Ar/ 39Ar incremental-heating analyses of matrix and anorthoclase separates from Olmoti OLS and OUS lavas indicate that volcanic activity was short in duration, lasting ˜200 kyr from 2.01 ± 0.03 Ma to 1.80 ± 0

  4. Experimenting with mixing and layered convection in phono-trachytic magmas: Implications on reservoir dynamics

    NASA Astrophysics Data System (ADS)

    de Campos, C. P.; Civetta, L.; Dingwell, D. B.; Perugini, D.; Petrelli, M.; Fehr, T. K.

    2006-12-01

    Abundant geochemical and volcanological data on the Campanian Ignimbrite, (>200 km3, 39 ka) Phlegrean Fields, Italy, support the existence of a layered magmatic reservoir, which evolved via 1) replenishment of the chamber with trachytic magma and 2) short-term pre-eruptive mixing between new trachytic and phono- trachytic resident magmas. We have initiated an experimental program in order to constrain the dynamics of such mingling/mixing events. We used melted natural products from these two magmas of sub-equal but distinct composition, which are thought to have been involved in the origin of this magmatic system as end-members (phono-trachyte = end- member A and trachyte = end-member B). The two were then stirred together and sampled by experiment termination as a time series, ranging from 1-hour up to 1-week. Stirring under constant low flow velocity (0.5 rotations per minute) generated at first homogenization and mixing of the starting compositions. Then separate convection cells and compositional layering for major and minor elements emerged. Calculated density distributions along sections from the experimental glasses, after decoupling, are very similar to density distributions in aqueous systems under double-diffusive convection. In order to test double- diffusive decoupled convection in this system, we performed 87Sr/86Sr-isotopic and Sr- LA-ICP-MS- measurements, using the 25-hour experimental glasses. The effective chemical separation of different convection cells has been confirmed with clearly distinct isotopic signatures for both bottom and top cells. Comparison with natural samples from the Campanian Ignimbrite strengthens the importance of the role of a double-diffusive similar convection as a major differentiation process leading to layering in this system. Our results support the effectiveness of a DDC-driven fractionation for moderately high-silica magmas under high near-liquidus temperatures, before the onset of fractional crystallization.

  5. Assessment for the use of waste of trachyte in the brick production

    NASA Astrophysics Data System (ADS)

    Coletti, Chiara; Cultrone, Giuseppe; Maritan, Lara; Mazzoli, Claudio

    2015-04-01

    The preliminary results of the potential advantage of incorporating wastes from a quarry's trachyte, as temper in brick production is the main aim of this work. Three different mixtures elaborated by adding 5, 10 and 15 wt. % of trachyte were studied. Each mix was fired in a electric oven at the temperatures of 900, 1000 and 1100 °C. The influence of the waste addition was investigated under micro and meso-scale and considering several aspects in order to define aesthetic, mineralogical and physical features of fired samples. The microstructure of the samples was evaluated by X-ray powder diffraction and scanning electron microscopy. The main phases identified were quartz, plagioclase, K-feldspar, biotite, hematite and diopside. Colorimetric study highlighted changes on the chromatic appearance depending on the trachyte contents and the firing temperatures reached. Hydric parameters (UNI EN 772-7, 1998) showed a decreasing absorption behaviour and an overall poor interconnection of pores for samples with the highest content of trachyte (15%). The values of open porosity determined by hydric test were comprised between 35 and 38% and decreased with increasing the firing temperature and the trachyte content. Mercury intrusion porosimetry showed the pore size distribution in the range comprised between 0.001-100 µm. The results of total open porosity obtained (from 19 to 38%) were quite similar to those measured by hydric tests. Physical-mechanical properties and structural anisotropy were checked combining the uniaxial compressive strength (DT) and the ultrasound velocity test (NDT). A rather closed concordance between these techniques was observed when comparing the average values of mechanical stress and the propagation velocities of ultrasound waves. Finally, freeze-thaw (UNI EN 12371) and salt crystallization tests (UNI EN 12370) carried out on bricks proved their high resistance to decay under stressed environmental conditions. The results of the present

  6. Origin of xenoliths in the trachyte at Puu Waawaa, Hualalai Volcano, Hawaii

    USGS Publications Warehouse

    Clague, D.A.; Bohrson, W.A.

    1991-01-01

    Rare dunite and 2-pyroxene gabbro xenoliths occur in banded trachyte at Puu Waawaa on Hualalai Volcano, Hawaii. Mineral compositions suggest that these xenoliths formed as cumulates of tholeiitic basalt at shallow depth in a subcaldera magma reservoir. Subsequently, the minerals in the xenoliths underwent subsolidus reequilibration that particularly affected chromite compositions by decreasing their Mg numbers. In addition, olivine lost CaO and plagioclase lost MgO and Fe2O3 during subsolidus reequilibration. The xenoliths also reacted with the host trachyte to form secondary mica, amphibole, and orthopyroxene, and to further modify the compositions of some olivine, clinopyroxene, and spinel grains. The reaction products indicate that the host trachyte melt was hydrous. Clinopyroxene in one dunite sample and olivine in most dunite samples have undergone partial melting, apparently in response to addition of water to the xenolith. These xenoliths do not contain CO2 fluid inclusions, so common in xenoliths from other localities on Hualalai, which suggests that CO2 was introduced from alkalic basalt magma between the time CO2-inclusion-free xenoliths erupted at 106??6 ka and the time CO2-inclusion-rich xenoliths erupted within the last 15 ka. ?? 1991 Springer-Verlag.

  7. Discovery of a trachyte ignimbrite sequence at Hualālai, Hawaii

    NASA Astrophysics Data System (ADS)

    Shea, Thomas; Owen, Jacqueline

    2016-05-01

    Ignimbrites are common in many intraplate ocean islands but have been missing from the known geological record in Hawaii. During a recent field campaign, the remnants of a trachytic ignimbrite sequence have been discovered at Hualālai volcano, fortuitously preserved from subsequent basaltic lava flow cover. We provide a preliminary description of these deposits, as well as bulk and glass chemical analyses to determine their potential relationship with other nearby trachytes from Pu'u Wa'awa'a (PWW) and Pu'u Anahulu (PA). The results suggest that these ignimbrites are from neither PWW nor PA, but instead may relate to trachytes that are found as maar wallrock blocks some 20 km distant. Therefore, despite being rare overall in Hawaii, the ignimbrites—and more generally trachytes—were probably widespread around Hualālai. Compared to other intraplate ocean islands, the combination of a fast-moving plate, high magma supply, and eruption rates underneath Hawaiian volcanoes may explain the scarcity of ignimbrites preserved at the surface. Their presence at Hualālai could reflect unusual conditions of edifice stress during the transition from shield to post-shield volcanism.

  8. Crystallization kinetics of alkali feldspars in cooling and decompression-induced crystallization experiments in trachytic melt

    NASA Astrophysics Data System (ADS)

    Arzilli, Fabio; Carroll, Michael R.

    2013-10-01

    Cooling and decompression experiments have been carried out on trachytic melts in order to investigate crystallization kinetics of alkali feldspar, the effect of the degree of undercooling ( ΔT = T liquidus - T experimental) and time on nucleation and crystal growth process. This experimental work gives us new data about crystallization kinetics of trachytic melts, and it that will be useful to better understand the natural system of Campi Flegrei volcanoes. Experiments have been conducted using cold seal pressure vessel apparatus, at pressure between 30 and 200 MPa, temperature between 750 and 855 °C, time between 7,200 and 57,600 s and redox condition close to the NNO +0.8 buffer. These conditions are ideal to reproducing pre- and syn-eruptive conditions of the Campi Flegrei volcanoes, where the "conditions" pertain to the complete range of pressures, temperatures and time at which the experiments were performed. Alkali feldspar is the main phase present in this trachyte, and its abundance can strongly vary with small changes in pressure, temperature and water content in the melt, implying appreciable variations in the textures and in the crystallization kinetics. The obtained results show that crystallization kinetics are strictly related to ΔT, time, final pressure, superheating (- ΔT) and water content in the melt. ΔT is the driving force of the crystallization, and it has a strong influence on nucleation and growth processes. In fact, the growth process dominates crystallization at small ΔT, whereas the nucleation dominates crystallization at large ΔT. Time also is an important variable during crystallization process, because long experiment durations involve more nucleation events of alkali feldspar than short experiment durations. This is an important aspect to understand magma evolution in the magma chamber and in the conduit, which in turn has strong effects on magma rheology.

  9. Dredged trachyte and basalt from kodiak seamount and the adjacent aleutian trench, alaska.

    PubMed

    Forbes, R B; Hoskin, C M

    1969-10-24

    Blocky fragments of aegirine-augite trachyte (with accompanying icerafted gravels.) were recovered from the upper slopes of Kodiak Seamount in several dredge hauls. An alkali basalt pillow segment was also dredged from a moatlike depression, at a depth of 5000 meters, near the west base of the seamount. These retrievals confirm the volcanic origin of Kodiak Seamount and further support the view of Engel, Engel, and Havens that the higher elevations of seamounts are composed of alkali basalts or related variants. PMID:17731907

  10. Ba-rich sanidine megacrysts in trachytic rocks of Eslamy volcano, NW Iran

    NASA Astrophysics Data System (ADS)

    Aßbichler, Donjá; Asadpour, Manijeh; Heuss-Aßbichler, Soraya; Kunzmann, Thomas

    2016-04-01

    The Eslamy volcano is located on a peninsula at the eastern coast of Urumieh lake, NW Iran. The complex stratovolcano with gentle slope flanks exposes a collapsed caldera in the central part. Specific features are different sanidine rich rocks that occur in form of ejecta and flows. According to the field observations they are products of one volcanic event. XRF measurements show they all have trachytic compositions. Typical for this locality are the large sanidine phenocrysts. In the trachytic flow the sanidine crystals reach average size of ~4 cm embedded in a greenish-blue matrix consisting mainly of crystallized feldspar and subordinate pyroxen. Occasionally feldspar megacrysts of approx. 10 cm were observed. Na content of the sanidine megacrysts varies between 0.05 - 0.5 pfu with higher concentrations in the cores. Furthermore they show oscillatory zoning patterns caused by variations of Ba content (0-0.04 pfu). The matrix of the trachytic flow consist mainly of interlocking sanidine crystals (0.05-0.45 pfu Na) partly with Ba-rich cores containing up to 0.06 pfu Ba. In contrast to the megacrysts they show slightly higher Fe contents (0.025-0.035 pfu). The volcanic ejecta with bombs of approx. 50 cm in size were found in one distinct layer within a pyroclastic horizon. The average diameter of the feldspar phenocrysts is much smaller (0.5-2 cm). Sanidine is the main phase of these rocks (up to 80 %). As mafic phase up to 30 % pyroxen (mainly diospide) ± biotite can be observed. Accessories are magnetite ± apatite ± titanite ± zircon. In contrast to the flow rocks the main phase of the matrix of the ejecta is always glass with higher Fe2O3 (total) contents (up to 6 wt.-%) indicating a fast cooling of the sample due to ejection. They are completely depleted in Ba. In two samples zoned feldspar relicts enclosed in glass show remolten rims. Similar to flow rocks the feldspar phenocrysts of all ejecta show a complex zoning pattern, e.g. three samples expose high

  11. Textural study of the Puy Chopine trachytic eruption, Chaîne des Puys, France

    NASA Astrophysics Data System (ADS)

    Lit, Catherine; Gurioli, Lucia; van Wyk de Vries, Benjamin

    2016-04-01

    The Puy Chopine volcano (Quaternary Chaîne des Puys of the French Massif Central) has a trachytic spine, 160 m high and 500 m wide, in a crater formed by collapse of a scoria cone (Puy de Gouttes), during an explosive eruption with the same petrographic features as the spine. The proximal and distal pyroclastic deposits contain an array of fragments, both juvenile (vesiculated rhyolite pumice, fresh dense rhyolite fragments) and non juvenile (altered dense rhyolite, black scoria, granite, schist). Its complexity has perplexed early workers such as Scrope (1858). One explanation for the Chopine volcano eruption is (Boivin in 1983) that the intrusion of a trachyte magma underneath the Gouttes created a pheatomagmatic eruption, leaving a large crater or maar. A final spine was protruded. However, Boudon et al (2015) suggested that the Chopine could have formed from superficial dome explosions, formed at the onset of lava dome formation, where the impermeable carapace of an extruding magma built up significant overpressure to produce lateral explosions. In addition, van Wyk de Vries et al (2015) suggested that the Chopine first developed as a cryptodome, deforming the Gouttes as a 'crater of elevation', which collapsed to trigger shallow explosions from the exposed intrusion. We describe the textures of the Chopine dome and its explosive facies. Observations of a sequence deposited one km from the eruptive vent have identified at least six units linked to the Chopine eruptive sequence. The lowest gray layer is ~1 m thick, and is composed of accidental lithics and fresh dome materials. This layer is interpreted as a pyroclastic density current deposit with ballistics from the initial explosion. Juveniles vary from very dense to pumice-like, and can be tuff-like breccias. Most clasts are angular, except the non-juvenile and the breccia facies. Notable textural features are color-banding/lenses in some juvenile dense and vesicular samples. Inclusions of fluidal basalt

  12. Cenozoic intra-plate magmatism in the Darfur volcanic province: mantle source, phonolite-trachyte genesis and relation to other volcanic provinces in NE Africa

    NASA Astrophysics Data System (ADS)

    Lucassen, Friedrich; Pudlo, Dieter; Franz, Gerhard; Romer, Rolf L.; Dulski, Peter

    2013-01-01

    Chemical and Sr, Nd and Pb isotopic compositions of Late Cenozoic to Quaternary small-volume phonolite, trachyte and related mafic rocks from the Darfur volcanic province/NW-Sudan have been investigated. Isotope signatures indicate variable but minor crustal contributions. Some phonolitic and trachytic rocks show the same isotopic composition as their primitive mantle-derived parents, and no crustal contributions are visible in the trace element patterns of these samples. The magmatic evolution of the evolved rocks is dominated by crystal fractionation. The Si-undersaturated strongly alkaline phonolite and the Si-saturated mildly alkaline trachyte can be modelled by fractionation of basanite and basalt, respectively. The suite of basanite-basalt-phonolite-trachyte with characteristic isotope signatures from the Darfur volcanic province fits the compositional features of other Cenozoic intra-plate magmatism scattered in North and Central Africa (e.g., Tibesti, Maghreb, Cameroon line), which evolved on a lithosphere that was reworked or formed during the Neoproterozoic.

  13. Partial melting and fractionation in the Mesa Chivato alkali basalt-trachyte series, Mount Taylor Volcanic Field, New Mexico

    NASA Astrophysics Data System (ADS)

    Schrader, C. M.; Schmidt, M. E.; Crumpler, L. S.; Wolff, J. A.

    2012-12-01

    Mesa Chivato comprises a series of alkaline cones, flows, and domes within the Mount Taylor Volcanic Field (MTVF) in northwest New Mexico. Compositions range from alkali basalt to trachyte. Intermediate magmas are less well represented than mafic and felsic rocks, but benmoreites and transitional benmoreite-trachytes provide a window into the differentiation processes. Major element, trace element, and isotopic data suggest that petrogenesis of benmoreite proceeded by fractional crystallization of mafic liquids and magma mixing with partially melted mafic rocks. Major element mass balance models permit the derivation of transitional benmoreite/trachyte from the benmoreite by 20-25% crystallization of microphenocryst phases (olivine, plagioclase, Ti-magnetite, and apatite) and further fractionation to trachyte by 10-15% crystallization of olivine, plagioclase and alkali feldspar, Fe-Ti oxide, and apatite. These models are supported by SiO2-Sr and -Ba systematics. However, the hawaiite to benmoreite gap cannot be crossed by fractional crystallization alone. While major element models permit the mafic lavas to yield the benmoreite, they require extensive fractionation of clinopyroxene and plagioclase - this is unsupported by petrography (clinopyroxene phenocrysts are rare in the mafic rocks and lacking in the intermediate rocks) and cannot explain the benmoreite's very high Sr contents (>1800 ppm), which would have been depleted by plagioclase fractionation. From LA-ICPMS analysis of plagioclase: 87Sr/86Sr of early alkali basalt (0.70285-0.70300) and late hawaiite (0.70406-0.70421) bracket the 87Sr/86Sr of the benmoreite (0.70361-0.70406). Thus, either could represent the fractionated liquid parental to the benmoreite and the other the partially melted source.

  14. Petrogenesis of trachyte and rhyolite magmas on Ponza Island (Italy) and its relationship to the Campanian magmatism

    NASA Astrophysics Data System (ADS)

    Angelo, Paone

    2013-11-01

    Magmatism on Ponza island (Italy) has been reviewed, considering both new and literature data, and compared with the Campanian Volcanic District (CVD) rocks. Different origins have been inferred for the two main rock types (trachyte and rhyolite). The trachytes were formed by fractional crystallization (FC) processes coupled with minor upper crustal contamination from magmas similar to the least evolved magma erupted into CVD. The rhyolites formed by partial melting of a lower crust component. The two lithotypes evolved through intra-suite FC, and they are correlated with the volcanic rocks of the CVD by major element, trace element and isotope data, extending the Campanian compositional spectrum. To explain the FC and the melting processes, a tectonic model is developed in which most of the FC for the CVD occurred in the lower-intermediate crust where magma rises from the upper mantle and is stored in a process of magma accumulation and fractionation. These processes have produced enough heat to melt the crust and cause several rhyolite episodes in Ponza Island. A subduction-related setting must be inferred to explain the origin of the Ponza trachytes and rhyolites and the rest of the CVD volcanism.

  15. Extensive mixing features at 27-41 Ka postcaldera trachytes at Long Valley caldera, CA: Mixing/mingling of basalt with trachyte and mobilization of young granitic material to form kspar megacrysts

    NASA Astrophysics Data System (ADS)

    Hagmann, I. J.; Mahood, G.

    2014-12-01

    Five small lava domes erupted at the NW margin of Long Valley caldera from 41 to 27 Ka. They range from trachyte (66% SiO2) to trachyandesite (60%), with the youngest lava being the most mafic. Mixing features are pervasive, with enclaves, kspar megacrysts, crystal clots of various grain sizes, compositions, and degrees of resorption indicating multiple episodes of mafic injection, mobilization of young granitic material, and mixing/mingling of alkali basalt with trachyte to alkali rhyolite magmas similar to those at Mammoth Mountain. Enclaves range from 49 to 57% SiO2 and form a mixing line with a felsic end member at 67% SiO2. In order to quantify the distribution of enclaves and large (1-4 cm), resorbed, kspar megacrysts, outcrop-scale point counting was performed at >200 locations on the lavas. Contour maps show that kspar content is highest at the vent, but mafic-intermediate enclaves are irregularly distributed. Fe-Ti oxide temperatures for the host trachytic magmas are 915-1080°C, with the coolest temperatures at flow termini. Enclave temperatures are similar, 950-1120°C, with cooler temperatures in more felsic enclaves that are typically located near flow termini, indicating prolonged thermal and chemical interaction with the host magma. Calculated pressures are 2-4 kbar for host magmas, but some mafic crystal clots yield pressures up to 12 kbar, near the Moho at Long Valley. The kspar megacrysts match the composition of phenocrysts in late-erupted Bishop Tuff, suggesting that the megacrysts originated from solidified equivalents of magma remaining after eruption of the Bishop Tuff at 760 Ka. These data suggest a model in which alkali basalts are generated in the uppermost mantle and, through AFC, evolve into trachytes. Repeated basaltic injections keep the trachyte hot and partially melt young granites, resulting in entrainment of kspar megacrysts. The most mafic enclaves in the NW domes match the alkali basalt compositions of the most mafic enclaves in

  16. Geology of Volcan Las Navajas, a pleistocene trachyte/peralkaline rhyolite volcanic center in Nayarit, Mexico

    SciTech Connect

    Hegre, J.A.; Nelson, S.A.

    1985-01-01

    Volcan Las Navajas, located in the northwestern portion of the Mexican Volcanic Belt has produced a sequence of volcanic rocks with compositions in marked contrast to the predominantly calc-alkaline volcanoes which predominate in this part of Mexico. The oldest exposed lavas consist of trachytes with 63% SiO/sub 2/, 6% FeO*, and 500 ppm Zr along with comenditic rhyolites with 68% SiO/sub 2/, 5% FeO*, 800 ppm Zr, and an agpaitic index of 1.0. These lavas were followed by the eruption of a comenditic ash-flow tuff and the formation of a caldera 2.7 km in diameter. This caldera was subsequently filled by eruptions of pantelleritic rhyolite obsidian lava flows with 72% SiO/sub 2/, 8% FeO*, 1100 ppm Zr, and an agpaitic index of 1.5 to 1.9. A second caldera was then formed which is offset to the south of the main eruptive vents for previous eruptions. This younger caldera has a diameter of about 4.8 km and its southern walls have been covered by calc-alkaline andesitic lavas erupted from nearby Sanganguey volcano. Volcanoclastic sediments in the floor of the younger caldera have been tilted and faulted in a manner suggestive of late stage resurgence. Subsequent eruptions within the caldera, however, have been restricted to calc-alkaline andesites. Tectonically, the area in which this volcano occurs appears to have been undergoing a crustal rifting event since the Pliocene. The occurrence of these peralkaline rocks lends further support to such a hypothesis.

  17. Cooling and crystallization recorded in trachytic enclaves hosted in pantelleritic magmas (Pantelleria, Italy): Implications for pantellerite petrogenesis

    NASA Astrophysics Data System (ADS)

    Landi, Patrizia; Rotolo, Silvio G.

    2015-08-01

    This study focuses on the comenditic trachyte enclaves hosted in pantelleritic lava and pumices emplaced during one of the most recent eruptive events at Pantelleria (~ 6 ka). Enclaves range from mm-sized fragments to dm-sized blocks with spheroidal to amoeboid shapes and characteristic globular surfaces; they are crystal-rich with ~ 30 vol% large anorthoclase, less abundant Fe-rich olivine, clinopyroxene and Fe-Ti oxides. Vesicles ranging from a few mm to 1-2 cm in size are distributed throughout the enclave and are commonly filled with microlite-free vesicular glass. The groundmass presents spectacular textures, including fine to coarse spherulites and hopper and skeletal microlites (mostly anorthoclase) with 10-35 vol% residual glass and 10-15 vol% small vesicles (< 50 μm to 200 μm). Residual glass has a pantelleritic composition with Na + K/Al > 2. Accordingly, the microlite composition is close to that of the host pantellerite. These textures were acquired during a magma mixing event prior to the eruption: (i) a vapor-saturated comenditic trachyte magma intruded the shallower, cooler pantellerite magma body, triggering a first degassing event (first boiling) and rapid crystallization with a high degree of undercooling (ΔT = 100-150 °C); (ii) vapor exsolution induced by rapid crystallization (second boiling) produced the microvesicular groundmass; and (iii) vapor pressure forced the residual pantelleritic liquid to migrate into the large vesicles and/or outside the enclaves. Based on this interpretation, these enclaves can be used as a natural laboratory for identifying the chemical and physical processes driving the evolution of silicic magmas at Pantelleria. In particular, they can be used to assess whether pantellerite melts can be obtained from comenditic trachytes after extensive crystallization followed by gas-driven filter-pressing and consequent (pantellerite) melt segregation.

  18. Volcanic and magmatic evolution of a small trachytic vent complex, north Burro Mesa, Big Bend National Park, Texas

    USGS Publications Warehouse

    Morgan, Lisa A.; Shanks, Pat

    2009-01-01

    Volcanic rocks exposed on the northern end of Burro Mesa in Big Bend National Park portray the evolution of an Oligocene central volcanic vent complex that produced two generations of welded block and ash deposits associated with 1) initial dome collapse and 2) subsequent central spine collapse. Peripheral to the vent complex, isolated breccia deposit exposures overlie ignimbrites, tephras, and lavas. These blocks are a few meters to several hundred meters long and 30 m high and consist of monolithic angular and welded trachytic lava clasts in finer-grained matrix. Rheomorphic structures in the breccia deposit show ductile deformation and suggest it formed while above the glass transition temperature.

  19. The Axum-Adwa basalt-trachyte complex: a late magmatic activity at the periphery of the Afar plume

    NASA Astrophysics Data System (ADS)

    Natali, C.; Beccaluva, L.; Bianchini, G.; Siena, F.

    2013-08-01

    The Axum-Adwa igneous complex consists of a basalt-trachyte (syenite) suite emplaced at the northern periphery of the Ethiopian plateau, after the paroxysmal eruption of the Oligocene (ca 30 Ma) continental flood basalts (CFB), which is related to the Afar plume activity. 40Ar/39Ar and K-Ar ages, carried out for the first time on felsic and basaltic rocks, constrain the magmatic age of the greater part of the complex around Axum to 19-15 Ma, whereas trachytic lavas from volcanic centres NE of Adwa are dated ca 27 Ma. The felsic compositions straddle the critical SiO2-saturation boundary, ranging from normative quartz trachyte lavas east of Adwa to normative (and modal) nepheline syenite subvolcanic domes (the obelisks stones of ancient axumites) around Axum. Petrogenetic modelling based on rock chemical data and phase equilibria calculations by PELE (Boudreau 1999) shows that low-pressure fractional crystallization processes, starting from mildly alkaline- and alkaline basalts comparable to those present in the complex, could generate SiO2-saturated trachytes and SiO2-undersaturated syenites, respectively, which correspond to residual liquid fractions of 17 and 10 %. The observed differentiation processes are consistent with the development of rifting events and formation of shallow magma chambers plausibly located between displaced (tilted) crustal blocks that favoured trapping of basaltic parental magmas and their fractionation to felsic differentiates. In syenitic domes, late- to post-magmatic processes are sometimes evidenced by secondary mineral associations (e.g. Bete Giorgis dome) which overprint the magmatic parageneses, and mainly induce additional nepheline and sodic pyroxene neo-crystallization. These metasomatic reactions were promoted by the circulation of Na-Cl-rich deuteric fluids (600-400 °C), as indicated by mineral and bulk rock chemical budgets as well as by δ18O analyses on mineral separates. The occurrence of this magmatism post-dating the

  20. Remobilization of granitoid rocks through mafic recharge: evidence from basalt-trachyte mingling and hybridization in the Manori-Gorai area, Mumbai, Deccan Traps

    NASA Astrophysics Data System (ADS)

    Zellmer, Georg F.; Sheth, Hetu C.; Iizuka, Yoshiyuki; Lai, Yi-Jen

    2012-01-01

    Products of contrasting mingled magmas are widespread in volcanoes and intrusions. Subvolcanic trachyte intrusions hosting mafic enclaves crop out in the Manori-Gorai area of Mumbai in the Deccan Traps. The petrogenetic processes that produced these rocks are investigated here with field data, petrography, mineral chemistry, and whole rock major, trace, and Pb isotope chemistry. Local hybridization has occurred and has produced intermediate rocks such as a trachyandesitic dyke. Feldspar crystals have complex textures and an unusually wide range in chemical composition. Crystals from the trachytes cover the alkali feldspar compositional range and include plagioclase crystals with anorthite contents up to An47. Crystals from the mafic enclaves are dominated by plagioclase An72-90, but contain inclusions of orthoclase and other feldspars covering the entire compositional range sampled in the trachytes. Feldspars from the hybridized trachyandesitic dyke yield mineral compositions of An80-86, An47-54, Ab94-99, Or45-60, and Or96-98, all sampled within individual phenocrysts. We show that these compositional features are consistent with partial melting of granitoid rocks by influx of mafic magmas, followed by magma mixing and hybridization of the partial melts with the mafic melts, which broadly explains the observed bulk rock major and trace element variations. However, heterogeneities in Pb isotopic compositions of trachytes are observed on the scale of individual outcrops, likely reflecting initial variations in the isotopic compositions of the involved source rocks. The combined data point to one or more shallow-level trachytic magma chambers disturbed by multiple injections of trachytic, porphyritic alkali basaltic, and variably hybridized magmas.

  1. Stratigraphy, chronology, and sedimentology of ignimbrites from the white trachytic tuff, Roccamonfina Volcano, Italy

    NASA Astrophysics Data System (ADS)

    Giannetti, Bernardino; De Casa, Giancarlo

    2000-03-01

    We describe the stratigraphy, chronology, and grain size characteristics of the white trachytic tuff (WTT) of Roccamonfina Volcano (Italy). The pyroclastic rock was emplaced between 317 and 230 Ma BP during seven major eruptive events (units A to G) and three minor events (units BC, CD, and DE). These units are separated by paleosol layers and compositionally well-differentiated pyroclastic successions. Stratigraphic control is favored by the occurrence at the base of major units of marker layers. Four WTT units (1 to 4) occur within the central caldera. These are not positively correlated with specific extracaldera units. The source of most of the WTT units was the central caldera. Units B and C were controlled by the western wall of the caldera, whereas units D and E were able to overcome this barrier, spreading symmetrically along the flanks of MC. The maximum pumice size (MP) of units increases with distance from the caldera, whereas the maximum lithic size (ML) decreases. MP and ML of the marker layer of unit D (MKDa-MKDp) do not show any systematic variations with respect to the central caldera. In contrast, the thickness of surge MKDa decreases with distance from the source, and MKDp accumulates to the north of MC probably controlled, respectively, by mobility-transport power and by wind blowing northwards. The grain size characteristics of the WTT deposits are used for classifying the units. There is no systematic variation of the grain size as a function of stratigraphic height either among units or within single units. Large variation of components in subunit E1, with repetitive alternation of pyroclastic flow to surge through fallout vs. surge deposits, suggests that the process of eruption took place in a complex or piecemeal fashion. Pumice concentration zones (PCZ) occur at all WTT levels on the volcano, but they are much thicker and pumice clasts are much larger within the central caldera. These were probably originated by the disruption of lava

  2. Occurrence of fluororichterite and fluorian biotite in the In Tifar trachyte neck (Tazrouk district, Hoggar volcanic province, Sahara, Algeria)

    NASA Astrophysics Data System (ADS)

    Azzouni-Sekkal, Abla; Bonin, Bernard; Ben El Khaznadji, Riad

    2013-09-01

    The unusual occurrence in the In Tifar trachyte neck (Tazrouk district, Hoggar volcanic province, Sahara, Algeria) of the fluorian biotite-fluororichterite association is presented. The two mineral species were previously unknown in the Hoggar and their association is uncommon worldwide. Ti-rich biotite has 28-40% OH sites occupied by fluorine, hence the use of the modifier "fluorian". Sodic-calcic fluororichterite has more than 55% OH sites filled by fluorine, hence the use of the prefix "fluoro". Well-defined F-Mg affinities are documented in both cases, while Cl remains very low. Temperatures are estimated roughly at 775-700 °C at low pressures. The fluorian biotite → fluororichterite sequence of crystallisation implies increasingly high fH2F2/fH2O ratios in metaluminous H2O-dominated evolving to peralkaline F-enriched fluids.

  3. Petrology and geochemistry of Late Holocene felsic magmas from Rungwe volcano (Tanzania), with implications for trachytic Rungwe Pumice eruption dynamics

    NASA Astrophysics Data System (ADS)

    Fontijn, Karen; Elburg, Marlina A.; Nikogosian, Igor K.; van Bergen, Manfred J.; Ernst, Gerald G. J.

    2013-09-01

    Rungwe in southern Tanzania is an active volcanic centre in the East African Rift System, characterised by Plinian-style explosive eruptions of metaluminous to slightly peralkaline trachytic silica-undersaturated magmas during its late Holocene history. Variations in whole-rock major and trace element compositions of erupted products have been investigated, in combination with electron microprobe data for melt inclusions and phenocrysts comprising sanidine, biotite, clinopyroxene, titanomagnetite, ilmenite, haüyne, titanite, apatite and traces of plagioclase and amphibole. Compositional variations largely reflect fractional crystallisation, with a limited influence of magma mixing. Subtle variations in whole-rock composition and mineralogical characteristics between and within deposits, suggest the existence of a chemically zoned trachytic magma chamber beneath Rungwe. For the two most important studied deposits, the Isongole and Rungwe Pumice, co-existing Fe-Ti oxides constrain pre-eruptive temperature to 915-950 °C and oxygen fugacity to NNO + 0.25-NNO + 0.45. For the Rungwe Pumice, melt inclusions suggest that the melt was water-undersaturated (maximum inferred H2O concentration 5.5 wt.%). In the range of the defined pre-eruptive temperatures, this corresponds to melt viscosities as low as 103.3 Pa · s, i.e. significantly lower than magmas that typically generate highly explosive eruptions. Because no microlites formed in the conduit during ascent, which would have strongly increased the effective magma viscosity, the highly explosive nature of the eruptions may be attributable to a crucial role of exsolved CO2 and S phases, and very high ascent rates.

  4. Spherulites growth in trachytic melts: a textural quantitative study from synchrotron X-ray microtomography and SEM data

    NASA Astrophysics Data System (ADS)

    Arzilli, Fabio; Mancini, Lucia; Giuli, Gabriele; Cicconi, Maria Rita; Voltolini, Marco; Carroll, Michael R.

    2013-04-01

    This study shows the first textural data on synthetic alkali-feldspar spherulites grown in trachytic melts during cooling and decompression experiments with water-saturated conditions. Previous textural studies have shown the shape evolution and the growth process of spherulites as a function of undercooling (T) and water content, although just in basaltic and rhyolitic melts [1-3]. Spherulites are spherical clusters of polycrystalline aggregates that occur commonly in rhyolitic melts under highly non-equilibrium conditions [3-4]. Cooling and decompression experiments have been carried out on trachytic melts in order to investigate crystallization kinetics of alkali feldspars and the implications for magma dynamics during the ascent towards the surface. Experiments have been conducted using cold seal pressure vessel apparatus at pressure range of 30 - 200 MPa, temperature of 750 - 850 °C and time of 2 - 16 hours, thereby reproducing pre- and syn-eruptive conditions of the Campi Flegrei volcanoes. This study presents quantitative data on spherulite morphologies obtained both by scanning electron microscopy (SEM) and synchrotron X-ray microtomography. Size, aspect ratio, number and crystallographic misorientation of alkali feldspar crystals will be measured. Furthermore, experiments performed at different durations could allow us to follow the growth and the evolution of spherulites. The shape of spherulites changes as a function of ΔT and experimental durations. Two kind of spherulites occured during experiments: open spherulites and close spherulites. The open spherulites are characterized by an structure with large (generally rectangular prismatic), widely spaced fibers with main axis converging towards a central nucleus, in agreement with previous observations [5-6]. Instead, the close spherulites consist of acicular and tiny fibers radially aggregated around a nucleus and single crystals are hardly distinguishable. First preliminary results show: a

  5. Deformation structures associated with the Trachyte Mesa intrusion, Henry Mountains, Utah: Implications for sill and laccolith emplacement mechanisms

    NASA Astrophysics Data System (ADS)

    Wilson, Penelope I. R.; McCaffrey, Ken J. W.; Wilson, Robert W.; Jarvis, Ian; Holdsworth, Robert E.

    2016-06-01

    Deformation structures in the wall rocks of igneous intrusions emplaced at shallow crustal depths preserve an important record of how space was created for magma in the host rocks. Trachyte Mesa, a small Oligocene age intrusion in the Henry Mountains, Utah, is composed of a series of stacked tabular, sheet-like intrusions emplaced at 3-3.5 km depth into sandstone-dominated sedimentary sequences of late Palaeozoic-Mesozoic age. New structural analysis of the spatial distribution, geometry, kinematics and relative timings of deformation structures in the host rocks of the intrusion has enabled the recognition of distinct pre-, syn-, and late-stage-emplacement deformation phases. Our observations suggest a two-stage growth mechanism for individual sheets where radial growth of a thin sheet was followed by vertical inflation. Dip-slip faults formed during vertical inflation; they are restricted to the tips of individual sheets due to strain localisation, with magma preferentially exploiting these faults, initiating sill (sheet) climbing. The order in which sheets are stacked impacts on the intrusion geometry and associated deformation of wall rocks. Our results offer new insights into the incremental intrusion geometries of shallow-level magmatic bodies and the potential impact of their emplacement on surrounding host rocks.

  6. Deformation structures associated with the Trachyte Mesa intrusion, Henry Mountains, Utah: Implications for sill and laccolith emplacement mechanisms

    NASA Astrophysics Data System (ADS)

    Wilson, Penelope I. R.; McCaffrey, Ken J. W.; Wilson, Robert W.; Jarvis, Ian; Holdsworth, Robert E.

    2016-06-01

    Deformation structures in the wall rocks of igneous intrusions emplaced at shallow crustal depths preserve an important record of how space was created for magma in the host rocks. Trachyte Mesa, a small Oligocene age intrusion in the Henry Mountains, Utah, is composed of a series of stacked tabular, sheet-like intrusions emplaced at 3-3.5 km depth into sandstone-dominated sedimentary sequences of late Palaeozoic-Mesozoic age. New structural analysis of the spatial distribution, geometry, kinematics and relative timings of deformation structures in the host rocks of the intrusion has enabled the recognition of distinct pre-, syn-, and late-stage-emplacement deformation phases. Our observations suggest a two-stage growth mechanism for individual sheets where radial growth of a thin sheet was followed by vertical inflation. Dip-slip faults formed during vertical inflation; they are restricted to the tips of individual sheets due to strain localisation, with magma preferentially exploiting these faults, initiating sill (sheet) climbing. The order in which sheets are stacked impacts on the intrusion geometry and associated deformation of wall rocks. Our results offer new insights into the incremental intrusion geometries of shallow-level magmatic bodies and the potential impact of their emplacement on surrounding host rocks.

  7. Settling dynamics of basaltic (Etna) and trachytic (Laacher See) ash particles: insight from laboratory high speed imaging

    NASA Astrophysics Data System (ADS)

    Del Bello, E.; Andronico, D.; Vu Duc, A.; Cristaldi, A.; Kueppers, U.; Ricci, T.; Scarlato, P.; Scollo, S.; Taddeucci, J.

    2014-12-01

    Existing experimental and numerical models of ash sedimentation from volcanic plumes consider aerodynamic properties of particles as a function of their shape, density and size. However, rather than individually, particles are often observed to settle through zones of high particle concentration associated with gravitational instabilities (e.g., particle-rich fingers) where sedimentation is controlled by the properties of the bulk down-flow of settling particles. In order to investigate the differences in the aerodynamic behaviour of ash particles when settling individually or in mass, we performed systematic large-scale ash settling experiments. Natural basaltic ash from Etna (Italy, sampled in July 2014) and trachytic, pumiceous ash from Laacher See (Germany, 12.900 y BP) was used as starting material. For Etna, we used particles in the classes 0-125 and 125-500 μm, for Laacher See, we used 40-90 and 500-1000 μm. For each class, we released 40-500 g of sample from heights of 2 to 5 m with different, controlled volumetric flow rates, in an unconstrained open space and at minimal air movement. All experiments were recorded with a high-speed camera at 2000 fps. A vertical laser sheet crossing the flow enhanced visibility of particles. After release, particles were observed to cluster, leading to locally enhanced fall velocities. High-speed imaging, manual and automatic tracking analyses are being used to provide full characterization of particle settling dynamics as a function of particle concentration in the flow, density and particle size. SEM analysis will provide particle shape characterization. The main results are i) measured settling velocities of individual particles increase with increasing particle concentration; ii) particle sorting during sedimentation is observed. This suggests that particle dispersion during fallout may be one reason explaining larger than theoretical depletion rates of fine particles from volcanic ash clouds.

  8. A trachyte-syenite core within a basaltic nest: filtering of primitive injections by a multi-stage magma plumbing system (Oki-Dōzen, south-west Japan)

    NASA Astrophysics Data System (ADS)

    Brenna, Marco; Nakada, Setsuya; Miura, Daisuke; Toshida, Kiyoshi; Ito, Hisatoshi; Hokanishi, Natsumi; Nakai, Shun'ichi

    2015-08-01

    Oki-Dōzen (Japan) is a Late Miocene (7-5 Ma) intraplate alkalic volcano composed of a central trachytic pyroclastic complex surrounded by a ring-shaped succession of basaltic to trachybasaltic lavas and pyroclastic rocks and dispersed trachytic bodies. The central trachytic complex is in contact with a syenite that was intruded into the basement early Miocene volcano-sedimentary succession. In the centre of the system there are no alkalic basaltic rocks that are correlative of the outer flank ring. We present whole-rock major and trace element chemistry, Sr-Nd-Pb isotopic compositions and petrological data from the central trachytic volcanic complex and the intrusive syenite body, as well as from the outer ring basaltic succession. We also present and discuss a new set of zircon U/Pb ages collected from the central trachyte and syenite bodies. All the eruptive products of Oki-Dōzen, as well as the syenite, plot on a single liquid line of descent initiated from a mantle-derived alkalic basaltic parent. A younger (2.8 Ma) basaltic eruption (Uzuka basalt) has isotopic compositions that distinguish it from the rest of the system. Geochemical modelling indicates that magmatic differentiation through crystal fractionation and minor crustal assimilation occurred in crustal and shallow sub-volcanic magma reservoirs. In the central part of the system, a number of vertically spaced reservoirs acted as a filter, capturing basaltic dykes and hindering their ascent. In the outer region, dykes either reached the surface unhindered and erupted to form the basaltic/trachybasaltic succession or stalled at crustal levels and differentiated to trachyte before forming dispersed domes/flows. The central plumbing system "filter" resulted in a nest-shaped volcano, with a trachytic core surrounded by basaltic products, and stopped direct injection of basaltic magmas into the shallow syenitic magma reservoir, likely preventing its destabilization and explosive eruption.

  9. Deformation Structures associated with the emplacement of high level intrusions: A study of Trachyte Mesa Intrusion, Henry Mountains, Utah

    NASA Astrophysics Data System (ADS)

    Wilson, P. I. R.; McCaffrey, K. J. W.; Jarvis, I.; Murphy, P.; Davidson, J. P.

    2012-04-01

    Most studies of sill and laccolith complexes have focused on the internal architecture and thermal effects of these intrusions, while few have looked in detail at host rock deformation structures associated with their emplacement. Various sill and laccolith emplacement mechanisms have been proposed (e.g. radial growth/ bulldozing, and two-stage growth), each with their own distinct deformation style. Compressional structures likely dominate during radial growth (bulldozing) emplacement, while extensional structures are more likely to form during two-stage growth emplacement. In this study we focus on deformation structures (faults, deformation bands and joints) associated with emplacement of Tertiary sills and laccolith intrusions in the Henry Mountains, Utah. Trachyte Mesa, the most distal satellite intrusion to the Mt. Hilliers intrusive centre, is an elongate (NE-SW) laccolith concordant with the Entrada sandstone it intrudes. The intrusion is comprised of multiple, stacked intrusive sheets. Two structural transects across the northwest lateral margin have identified distinct structural domains within the host rock that reflect both temporal and kinematic variations in deformation. Three deformation phases are identified, interpreted to be pre-, syn- and late-emplacement structures. A background set of deformation bands (phase 1), trending oblique to the intrusion margin, is apparent across the entire area. A second set of deformation bands (phase 2) overprint the early phase. These are characterised by conjugate deformation bands that parallel the intrusion margin, and increase in intensity and spacing towards the intrusion. Within this same zone a series of calcite filled normal faults, striking parallel and perpendicular to the intrusion margin, are apparent. Due to their spatial, kinematic and overprinting relationships we interpret these to be linked to the emplacement of the intrusive body. Overprinting all other structures, are two sets of tensile joints

  10. Trachyandesite scoria-flow and associated trachyte pyroclastic flow and surge at Roccamonfina Volcano (Roman Region, Italy)

    NASA Astrophysics Data System (ADS)

    Giannetti, Bernardino; Luongo, Giuseppe

    1994-02-01

    The pyroclastic deposits (6-7 km 2 in area) of Garofali (Roccamonfina Volcano, Italy) consist of the 239 ± 8 ka Grey Trachyandesite Scoriae of Riella (RTS) conformably capped by the Trachyte Tuff of Garofali (GTT), the latter consisting of pyroclastic flows, surges and overlying lapilli tuff. Locally, the uppermost part of the RTS consists of finely bedded surge deposits. Both RTS and GTT contain RTS-derived debris flows. Grain-size features suggest that the RTS can be interpreted as proximal scoria and lithic flow deposits. The flow origin is suggested by absence of bedding, emplacement of scoria clasts in a still plastic state, attitude to excavating channels in underlying pyroclastics, and grain size features. A surge origin for the transitional RTS is suggested by fine layering, presence of dune, pinch-and-swell structures and accretionary lapilli, and grain size. These characteristics become dominant in the surge component of the GTT, where they are associated with chute-and-pool structures. Grain size distributions show that the GTT deposits — both pyroclastic flows and surges — have a sorting similar to the RTS but a smaller median diameter. Gas-escape structures occur largely in the upper and lower levels of RTS and GTT, respectively. They result from strong gas-fluidification of the RTS during deposition of GTT. Isopach and isograde maps locate the source vent of the RTS between Tuorisichi and S. Lorenzo, but indicate that three source vents were active during eruption of the GTT. Measurements of dunes gave good directional vectors for the GTT. Absence of a depositional break between RTS and GTT deposits suggest that these represent two different episodes of the eruption of a compositionally zoned magma source, through one (RTS) and several (GTT) conduits. The eruption of the RTS was driven essentially by magmatic volatiles partially affected by preeruptive involvement of water. Eruption of the transitional RTS-GTT and mainly of GTT deposits developed

  11. Polybaric evolution of phonolite, trachyte, and rhyolite volcanoes in Eastern Marie Byrd Land, Antarctica: Controls on peralkalinity and silica saturation

    USGS Publications Warehouse

    LeMasurier, W.E.; Futa, K.; Hole, M.; Kawachi, Y.

    2003-01-01

    In the Marie Byrd Land volcanic province, peralkaline and metaluminous trachytes, phonolites, and rhyolites occur in 18 large shield volcanoes that are closely associated in time and space. They are arrayed radially across an 800 km wide structural dome, with the oldest at the crest and the youngest around the flanks. Several lines of evidence suggest that these rocks evolved via open-system, polybaric fractionation. We have used mass balance modeling of major elements together with trace-element data and mineral chemistry to help explain the evolution of this diverse suite of felsic rocks, which appear to have been generated coevally in isolated magma chambers, and erupted close to each other in patterns related to tectonic uplift and extension within the West Antarctic rift system. Isotopic and trace-element data indicate that this occurred with only minimal crustal contamination. We focus on volcanoes of the Executive Committee Range and Mount Murphy, where we find good representation of basalts and felsic rocks within a small area. Our results suggest that the felsic rocks were derived from basaltic magmas that differentiated at multiple levels during their passage to the surface: first to ferrogabbroic compositions near the base of the lithosphere, then to intermediate compositions near the base of the crust, and finally to felsic compositions in mid- to upper crustal reservoirs. The high-pressure history has been largely masked by low-pressure processes. The best indications of a high-pressure history are the mineral phases in cumulate nodules and their correlation with modeling results, with REE anomalies, and with the composition of an unusual gabbroic intrusion. Silica saturation characteristics are believed to have originated in magma chambers near the base of the crust, via fractionation of variable proportions of kaersutite and plagioclase. Development of peralkalinity in felsic rocks took place in upper crustal reservoirs by fractionating a high ratio

  12. Mixing of rhyolite, trachyte and basalt magma erupted from a vertically and laterally zoned reservoir, composite flow P1, Gran Canaria

    NASA Astrophysics Data System (ADS)

    Freundt, Armin; Schmincke, Hans-Ulrich

    1992-10-01

    The 14.1 Ma composite welded ignimbrite P1 (45 km3 DRE) on Gran Canaria is compositionally zoned from a felsic lower part to a basaltic top. It is composed of four component magmas mixed in vertically varying proportions: (1) Na-rhyolite (10 km3) zoned from crystal-poor to highly phyric; (2) a continuously zoned, evolved trachyte to sodic trachyandesite magma group (6 km3); (3) a minor fraction of Na-poor trachyandesite (<1 km3); and (4) nearly aphyric basalt (26 km3) zoned from 4.3 to 5.2 wt% MgO. We distinguish three sites and phases of mixing: (a) Mutual mineral inclusions show that mixing between trachytic and rhyolitic magmas occurred during early stages of their intratelluric crystallization, providing evidence for long-term residence in a common reservoir prior to eruption. This first phase of mixing was retarded by increasing viscosity of the rhyolite magma upon massive anorthoclase precipitation and accumulation. (b) All component magmas probably erupted through a ring-fissure from a common upper-crustal reservoir into which the basalt intruded during eruption. The second phase of mixing occurred during simultaneous withdrawal of magmas from the chamber and ascent through the conduit. The overall withdrawal and mixing pattern evolved in response to pre-eruptive chamber zonation and density and viscosity relationships among the magmas. Minor sectorial variations around the caldera reflect both varying configurations at the conduit entrance and unsteady discharge. (c) During each eruptive pulse, fragmentation and particulate transport in the vent and as pyroclastic flows caused additional mixing by reducing the length scale of heterogeneities. Based on considerations of magma density changes during crystallization, magma temperature constraints, and the pattern of withdrawal during eruption, we propose that eruption tapped the P1 magma chamber during a transient state of concentric zonation, which had resulted from destruction of a formerly layered zonation

  13. Tectonic Controls on the Volumes and Petrologic Evolution of Pantellerite-Trachyte-Phonolite Volcanoes in a Continental Rift Setting, Marie Byrd Land, Antarctica

    NASA Astrophysics Data System (ADS)

    Lemasurier, W. E.

    2010-12-01

    The 18 alkaline volcanoes in Marie Byrd Land (MBL) are characterized by large volumes of felsic rock and a large range in composition, from trachyte to pantellerite, comendite, and phonolite. These characteristics are controlled largely by mantle plume activity, a stationary plate environment, and lithospheric structure of the West Antarctic rift system in coastal MBL. Felsic rocks occur either as summit sections of basalt volcanoes, or comprising all the rock exposed above ice level. Their exposed volumes range from ~30-780 km3 in individual volcanoes. Seven fall between 200-780 km3. Field observations and one seismic traverse show that felsic sections are underlain by thick (1-5 km) sequences of basalt, dominated by basanite. Thus, in spite of their large volumes, felsic rocks appear to make up only ~10% of all the volcanic rock in the province. In four of these volcanoes, pantellerite, trachyte, and phonolite eruptions alternated with each other, and/or took place coevally from the same edifice, indicating that these magmas were available throughout the ~15 m.y. histories of these volcanoes from isolated, but closely adjacent upper crustal reservoirs. Isotope data record crustal contamination in some felsic rocks, but constrain it to <3%, at most. This, plus the results of major and trace element modeling, imply that pantellerites, trachytes and phonolites all evolved from basanite magma by fractional crystallization (FC). Phonolites could have evolved largely by low-pressure FC of basanite in the upper crust. However, modeling and experimental data suggest that 90-95% of pantellerite evolution took place below the crust, where inclusion of kaersutite among fractionated phases, in a low fO2 environment, were key to developing an FeO-rich and SiO2-rich pantellerite lineage from basanite. The complexity of the felsic suite seems related to the presence of mechanical boundaries at the base of the lithosphere (~50 km) and base of the crust (~25 km), that trapped

  14. Which came first: the pumice or the obsidian? Complex degassing transitions during the 114ka trachytic Pu'u Wa'aWa'a eruption (Hawaii)

    NASA Astrophysics Data System (ADS)

    Hammer, J. E.; Shea, T.; Hellebrand, E.

    2012-12-01

    Fragmental obsidian clasts are highly correlated with coeval pumice in eruptions that produce obsidian (e.g., Lipari, Aeolian Islands; Little Glass Mountain, California; Mono-Inyo chain, California; Taupo, New Zealand), implying that at least some magma is able to degas quiescently prior to or during the explosive stage of an eruption. However, gross stratigraphic relationships reveal a consistent pattern of explosive activity transitioning to effusive activity (e.g., obsidian flows), suggesting subsurface stratification of magmatic volatiles. A prevailing conceptual model of obsidian formation reconciles these observations through (1) formation of dense glassy material by collapse of vesicles in bubbly magma, occuring in the shallow conduit or at the surface, (2) subsequent ascent of gas-rich magma and fragmentation/assimilation of the previously-emplaced obsidian clasts, followed by (3) transition to dominantly effusive eruptive activity. The Pu'u Wa'aWa'a trachytic pumice cone is unique feature in the Hawaii island volcanic landscape, otherwise dominated by basaltic lava. Around 114 ka, a pulsating explosive eruption at Hualalai Volcano expelled trachytic pumice, forming a ~150-200 m high cone. This phase was immediately followed by the outpouring of a large trachyte flow (the most voluminous silicic lava flow identified in Hawaii ~5 km3), identical in bulk composition to the pumice. The tephra deposits of the cone contain abundant obsidian clasts, as well as pyroclasts bearing striking gradual textural transitions and discretely banded pumiceous, scoriaceous and aphanitic material. The intricate variations in glass H2O contents (measured by microRaman), microlite and vesicle abundances (textural analysis), along with the chemical traits (EMPA) displayed by glasses from the diverse textural end-members suggest a complex ascent and eruption history. We test three hypotheses: (a) the obsidian clasts formed during ascent, stalling and outgassing of the magma (i

  15. Stress-induced comenditic trachyte effusion triggered by trachybasalt intrusion: multidisciplinary study of the AD 1761 eruption at Terceira Island (Azores)

    NASA Astrophysics Data System (ADS)

    Pimentel, A.; Zanon, V.; de Groot, L. V.; Hipólito, A.; Di Chiara, A.; Self, S.

    2016-03-01

    The AD 1761 eruption on Terceira was the only historical subaerial event on the island and one of the last recorded in the Azores. The eruption occurred along the fissure zone that crosses the island and produced a trachybasalt lava flow and scoria cones. Small comenditic trachyte lava domes (known as Mistérios Negros) were also thought by some to have formed simultaneously on the eastern flank of Santa Bárbara Volcano. Following a multidisciplinary approach, we combined geological mapping, paleomagnetic, petrographic, mineral and whole-rock geochemical and structural analyses to study this eruption. The paleomagnetic dating method compared geomagnetic vectors (directions and intensities) recorded by both the AD 1761 lava flow and Mistérios Negros domes and revealed that the two events were indeed coeval. Based on new data and interpretation of historical records, we have accordingly reconstructed the AD 1761 eruptive dynamics and distinguished three phases: (1) a precursory phase characterized by decreased degassing in the fumarolic field of Pico Alto Volcano and a gradual increase of seismic activity, which marked the intrusion of trachybasalt magma; (2) a first eruptive phase that started with phreatic explosions on the eastern flank of Santa Bárbara Volcano, followed by the inconspicuous effusion of comenditic trachyte (66 wt% SiO2), forming a WNW-ESE-oriented chain of lava domes; and (3) a second eruptive phase on the central part of the fissure zone, where a Hawaiian to Strombolian-style eruption formed small scoria cones (E-W to ENE-WSW-oriented) and a trachybasalt lava flow (50 wt% SiO2) which buried 27 houses in Biscoitos village. Petrological analyses show that the two batches of magma were emitted independently without evidence of interaction. We envisage that the dome-forming event was triggered by local stress changes induced by intrusion of the trachybasalt dyke along the fissure zone, which created tensile stress conditions that promoted ascent

  16. Origin of a silica-oversaturated quartz trachyte rhyolite suite through combined crustal melting, magma mixing, and fractional crystallization: the Leyva Canyon volcano, Trans-Pecos Magmatic Province, Texas

    NASA Astrophysics Data System (ADS)

    White, John Charles; Urbanczyk, Kevin M.

    2001-11-01

    The Leyva Canyon Member of the Rawls Formation is a sequence of Oligocene (27.3-27.1 Ma) silicic lava, tuff, and volcaniclastic rock that comprise a trachytic shield volcano in the central Bofecillos Mountains of Big Bend Ranch State Park, Texas. This silicic unit developed within a volcanic field that was otherwise dominated by silica-undersaturated, mafic to intermediate lava. Quartz trachyte to low-silica rhyolite of the Leyva Canyon volcano appears to be the result of magma mixing between mantle-derived, alkalic mafic magmas and peraluminous crustal melt (˜40% crustal input), followed by ˜65% fractional crystallization. The parental mafic component was probably similar to silica-undersaturated, mafic lavas of the Rawls Formation. Peraluminous, A-type high-silica rhyolite represents the earliest-erupted lavas of the Leyva Canyon volcano and is unrelated to quartz trachyte and low-silica rhyolite via fractional crystallization. The high-silica rhyolite provides evidence for an episode of crustal melting beneath the Leyva Canyon volcano. Data from the Rawls Formation suggest that silica-undersaturated, alkalic (ne-normative) mafic magmas may evolve via fractional crystallization alone to silica-undersaturated silicic compositions (ne-trachyte), but require magma mixing with crustal-derived silicic magmas coupled with fractional crystallization to produce silica-oversaturated magmas (quartz trachyte-rhyolite). Mixing with a high-Al 2O 3 crustal component results in silica-oversaturated, mildly peraluminous and metaluminous to mildly peralkalic felsic rocks, rather than the strongly peralkalic rocks more commonly associated with alkalic mafic magmas.

  17. Na2O and Trace Elements Behavior in Trachytes and Phonolites at Suswa Volcano, Kenya: the Result of Combined Magma Mixing and Volatile-rich Na-Trace Element Fluids

    NASA Astrophysics Data System (ADS)

    Espejel-Garcia, V. V.; Anthony, E. Y.; Ren, M.; MacDonald, R.; Skilling, I. P.; White, J. C.

    2008-12-01

    The evolution of Suswa, a Quaternary volcano in the Kenya Rift, was dominated by the eruption of two rock suites, separated by a caldera event. Suswa is part of the Central Kenya Peralkaline Province (CKPP), which includes the Greater Olkaria Volcanic Complex (GOVC) and inter-center mafic fields, e.g. Tandamara and Elmenteita, whose compositions range from basalt to basaltic trachy-andesite (BTA). Both suites at Suswa range from trachyte to phonolite, but are distinguished by the amount of SiO2: pre- and syn-caldera rocks have 60-62%, and post-caldera rocks 57-59%. Trachyte to phonolite trends within each suite result from increasing Na2O, which is accompanied by increases in a number of trace elements (Be, Hf, Nb, Rb, Th, Y, Zn, Zr, and REE, except Eu). Magmatic processes included magma mixing, in which BTA magma similar to those of Tandamara and Elmenteita intruded the pre-caldera Suswa trachytic chamber, and fluid complexing, which was responsible for the enrichment in Na2O and trace elements. The importance of magma mixing in the CKPP has been recently documented at the GOVC by Macdonald et al. (2008, J Pet 49, 1515-1547), for which mafic-intermediate magmatic inclusions within comendites and disequilibrium phenocryst assemblages are part of the evidence. Evidence for mixing at Suswa includes: 1) mixed feldspar assemblages, e.g. syn-caldera ignimbrite samples contain both alkali feldspar (An2Ab62Or36), and xenocrystic plagioclase (An45Ab52Or3), and 2) heterogeneous matrix glass compositions. Glass in pre-caldera rocks is trachytic, similar to whole-rock compositions. Syn-caldera rocks have glass compositions both trachytic and intermediate between trachyte and BTA, while Tandamara BTA rocks contain trachytic glass. Glass in post-caldera rocks is mostly phonolitic. Glass inclusions in plagioclase xenocrysts are basaltic, similar to flows in the area. X-Y elemental plots do not show linear trends, as would be predicted from a mixing process. We attribute this to

  18. Volcán Las Navajas, a Pliocene-Pleistocene trachyte/peralkaline rhyolite volcano in the northwestern Mexican volcanic belt

    NASA Astrophysics Data System (ADS)

    Nelson, Stephen A.; Hegre, Joann

    1990-01-01

    Volcán Las Navajas, a Pliocene-Pleistocene volcano located in the northwestern portion of the Mexican volcanic belt, erupted lavas ranging in composition from alkali basalt through peralkaline rhyolite, and is the only volcano in mainland Mexico known to have erupted pantellerites. Las Navajas is located near the northwestern end of the Tepic-Zacoalco rift and covers a 200-m-thick pile of alkaline basaltic lavas, one of which has been dated at 4.3 Ma. The eruptive history of the volcano can be divided into three stages separated by episodes of caldera formation. During the first stage a broad shield volcano made up of alkali basalts, mugearites, benmoreites, trachytes, and peralkaline rhyolites was constructed. Eruption of a chemically zoned ash flow then caused collapse of the structure to form the first caldera. The second stage consisted of eruptions of glassy pantellerite lavas that partially filled the caldera and overflowed its walls. This stage ended about 200 000 years ago with the eruption of pumice falls and ash flows, which led to the collapse of the southern portion of the volcano to form the second caldera. During the third stage, two benmoreite cinder cones and a benmoreite lava flow were emplaced on the northwestern flank of the volcano. Finally, the calc-alkaline volcano Sanganguey was built on the southern flank of Las Lavajas. Alkaline volcanism continued in the area with eruptions of alkali basalt from cinder cones located along NW-trending fractures through the area. Although other mildly peralkaline rhyolites are found in the rift zones of western Mexico, only Las Navajas produced pantellerites. Greater volumes of basic alkaline magma have erupted in the Las Navajas region than in the other areas of peralkaline volcanism in Mexico, a factor which may be necessary to provide the initial volume of material and heat to drive the differentiation process to such extreme peralkaline compositions.

  19. From pumice to obsidian: eruptive behaviors that produce tephra-flow dyads. II- The 114ka trachyte eruption at Pu'u Wa'awa'a (Hawai'i).

    NASA Astrophysics Data System (ADS)

    Shea, T.; Leonhardi, T. C.; Giachetti, T.; Larsen, J. F.; Lindoo, A. N.

    2014-12-01

    Associations of tephra and lava flow/domes produced by eruptions involving evolved magmas are a common occurrence in various types of volcanic settings (e.g. Pu'u Wa'awa'a ~114ka, Hawaii; South Mono ~AD625, California; Newberry Big Obsidian flow ~AD700, Oregon; Big Glass Mountain ~AD1100, California; Inyo ~AD1350, California, Chaitén AD2008-2009, Chile; Cordón Caulle AD2011-2012, Chile), ejecting up to a few cubic km of material (tephra+flow/dome). Most, if not all, of these eruptions have in common the paradoxical coexistence of (1) eruptive styles which are inferred to be sustained in nature (subplinian and plinian), with (2) a pulsatory behavior displayed by the resulting fall deposits, and (3) the coeval ejection of vesicular tephra and pyroclastic obsidian. Through two case studies, we explore this apparent set of paradoxes, and their significance in understanding transitions from explosive to effusive behavior. In this second case study (also cf. Shea et al., same session), we present new field, textural and geochemical data pertaining to the 114ka Pu'u Wa'awa'a trachyte eruption in Hawai'i. This large volume (>5 km3) event produced both a tephra cone (~1.6 km in diameter) and a thick (>250 m) lava flow, which have been largely covered by the more recent basaltic Mauna Loa and Hualalai lava flows. The trachyte tephra contains juvenile material displaying a large textural variety (pumice, scoria, obsidian, microcrystalline trachyte and banded-clasts), which can be linked with the extent of degassing and the formation of feldspar microlites. Notably, the abundance of microlites can be used to reconstruct an ascent and devolatilization history that accounts for all the seemingly contradictory observations.

  20. Geochemical characteristics and petrogenesis of phonolites and trachytic rocks from the České Středohoří Volcanic Complex, the Ohře Rift, Bohemian Massif

    NASA Astrophysics Data System (ADS)

    Ackerman, Lukáš; Ulrych, Jaromír; Řanda, Zdeněk; Erban, Vojtěch; Hegner, Ernst; Magna, Tomáš; Balogh, Kadosa; Frána, Jaroslav; Lang, Miloš; Novák, Jiří K.

    2015-05-01

    Trachyandesites, trachytes and phonolites represent the most evolved rock types within the České Středohoří Volcanic Complex (CSVC) in the Ohře/Eger Rift. The K-Ar ages of the suite range from ~ 33.8 to ~ 25.8 Ma. Major and trace element variation in the basanite - trachybasalt - trachyandesite series can be explained by several stages of modification of parental magmas by assimilation-fractional crystallization (AFC) involving fractionation of olivine, clinopyroxene, apatite, amphibole and Ti-oxide and bulk continental crust (BCC) as an assimilate. Relative to plausible basanitic starting compositions, the trachytes are moderately depleted in Sr, exhibit more pronounced depletions in P and Ti and some of them also show mild MREE depletion. Such composition requires variable amphibole, clinopyroxene, plagioclase ± apatite, titanite and/or Ti-magnetite fractionation and BCC assimilation. Two types of phonolites (type A and B phonolite) can be distinguished on the basis of overall REE patterns, Gd/Gd* ratios and Ba and Sr contents. Type B phonolites are depleted in Ba, Sr and MREE as a result of extensive alkali feldspar, plagioclase and amphibole fractionation. Modelling of trace element distributions implies basanitic magmas as the most likely parental composition of the basanite - trachybasalt - trachyandesite - trachyte - phonolite suite formed through magmatic differentiation. The Sr-Nd isotopic compositions in the samples can be explained with the assimilation of continental crust by such parental magmas. The highly radiogenic 87Sr/86Sr found in some phonolites are contrasted by uniform Nd isotopic signature; this feature may be explained by contamination and/or overprint of source magmas by Na-Rb-rich material with radiogenic Sr signature formed due to high-Rb (> 200 ppm) character of these melts/fluids. The nature of such contaminant is further evidenced by elevated Li (and Cs in some cases) abundances in type B phonolites although at least two

  1. Multiple explosive rhyolite/trachyte eruptions of alkaline-peralkaline Nemrut and dacite/rhyolite eruptions of neighboring subduction zone-related Süphan volcano over 600 000 years: the East Anatolian tephra province

    NASA Astrophysics Data System (ADS)

    Schmincke, H.-U.; Sumita, M.; Paleovan scientific Team

    2012-04-01

    The active Nemrut stratovolcano (2918 m asl) (Eastern Anatolia) is topped by a spectacular caldera and dominates the area west of huge Lake Van that covers its lower flanks. The stratovolcano has been active explosively for at least ca. 600 ka based on drilling evidence (ICDP Paleovan project). We have identified, correlated and compositionally characterized some 40 fallout sheets on land - none previously known - the largest ones probably with magma volumes exceeding 30 km3(DRE). The alkaline to peralkaline tephras are dominated by anorthoclase, Fe-rich clinopyroxene and fayalite with quartz and aenigmatite in some. Large-volume comenditic to pantelleritic rhyolite eruptions occurred in intervals of 20 000 - 40 000 years with smaller volume trachytic tephra deposits in between reflecting overall fairly constant magma transfer rates periodically fractionating to highly evolved rhyolite in larger magma reservoirs. Many of the ca. 10 widespread ignimbrite sheets, nearly all newly recognized, commonly followed on the heels of rhyolitic fallout sheets. They are more mafic than the underlying fallout deposits, magma mixing being common. Widespread spectacular agglutinates represent a late phase of the youngest large-volume fallout/ignimbrite eruption at ca. 30 ka. Active Süphan stratovolcano (4158 m asl), some 50 km NE of Nemrut and bordering Lake Van to the north, is dominated in contrast by subduction-related chemistry and mineralogy, smaller-volume eruptions and more advanced crystallization of magmas prior to eruption. Chief phenocrysts comprise complex disequilibrium assemblages of clinopyroxene, hypersthene, olivine, strongly zoned plagioclase, biotite and/or amphibole and common clots of fractionating phases. Many of the highly viscous and crystal-laden Süphan magmas were emplaced as domes and debris avalanches next to fallout sheets and ignimbrites. The dominant NE direction of fan axes of partial isopach maps of ca. 15 major fallout deposits reflecting

  2. Relationships between pre-eruptive conditions and eruptive styles of phonolite-trachyte magmas

    NASA Astrophysics Data System (ADS)

    Andújar, Joan; Scaillet, Bruno

    2012-11-01

    Phonolitic eruptions can erupt either effusively or explosively, and in some cases develop highly energetic events such as caldera-forming eruptions. However, the mechanisms that control the eruptive behaviour of such compositions are not well understood. By combining pre-eruptive data of well studied phonolitic eruptions we show that the explosive-effusive style of the phonolitic magma is controlled by the amount of volatiles, the degree of water-undersaturation and the depth of magma storage, the explosive character generally increasing with pressure depth and water contents. However, external factors, such as ingestion of external water, or latter processes occurring in the conduit, can modify the starting eruptive dynamic acquired at the levels of magma ponding.

  3. Influence of starting material on the degassing behavior of trachytic and phonolitic melts

    NASA Astrophysics Data System (ADS)

    Preuss, Oliver; Marxer, Holger; Nowak, Marcus

    2015-04-01

    The dynamic magmatic processes beneath volcanic systems, occurring during magma ascent, cannot be observed directly in nature. Simulation of magma ascent in the lab realized by continuous decompression (CD) of a volatile containing melt is essential to understand these processes that may lead to potentially catastrophic eruptions threatening millions of people in highly populated areas like Naples located between the Campi Flegrei Volcanic Field and the Monte Somma-Vesuvio strato-volcano. In this project, experimental simulations of Campanian Ignimbrite (CI) magma ascent will give insight to the mechanisms of the CI super eruption, thus providing tools for volcanic hazard assessment at the high risk Campanian Volcanic District and other comparable volcanic systems. Additionally, comparable experiments with the same conditions using the 'white pumice' composition of the catastrophic Vesuvius AD 79 (VAD79) eruption, have been conducted. So far, the experiments were performed in an internally heated argon pressure vessel coupled with a high-pressure low-flow metering valve and a piezoelectric nano-positioning system using a starting pressure of 200 MPa, H2O content of about 5 wt% and two different decompression rates (0.024 and 0.17 MPa/s) at a superliquidus temperature of 1050 ° C to ensure a crystal free melt and a homogeneous bubble nucleation. Experiments were conducted with both, glass powder and cylinders, subsequently decompressed to 75 and 100 MPa and rapidly quenched. Beside the results that e.g. decompression rate, volatile content, fluid solubility and target pressure affect the degassing behavior of the melt, the influence of the starting material on the degassing processes is significant. Analyses of BSE- and transmitted light microscopy images revealed a different degassing behavior of glass cylinder experiments compared to powders. Nitrogen has a very low solubility in hydrous silicate melts, supporting our suggestion that preexisting nitrogen rich bubbles (from trapped air between the single glass grains) in the melt lead to growth of these preexisting bubbles resulting in near equilibrium degassing where no further nucleation is needed. This results in much higher porosities of the degassed samples compared to those where pure dissolved H2O is present. The same effect was observed by repeating these experiments with a phonolitic VAD79 composition. In ongoing experiments using glass cylinders as starting material, approximately 0.4 wt% chlorine (average Campanian Ignimbrite melt inclusion data [1]) will be added as a volatile component to study the influence on the degassing behavior of hydrous CI melt. [1] Marianelli et al. (2006) Geology 34(11), 937

  4. Evolved Rocks in Ocean Islands Formed by Melting of Metasomatized Mantle

    NASA Astrophysics Data System (ADS)

    Ashwal, L. D.; Torsvik, T. H.; Horvath, P.; Harris, C.; Webb, S. J.; Werner, S. C.; Corfu, F.

    2015-12-01

    Evolved rocks like trachyte occur as minor components of many plume-related basaltic ocean islands (e.g. Hawaii, Gran Canaria, Azores, Réunion), and are typically interpreted as products of extreme fractional crystallization from broadly basaltic magmas. Trachytes from Mauritius (Indian Ocean) suggest otherwise. Here, 6.8 Ma nepheline-bearing trachytes (SiO2 ~63%, Na2O + K2O ~12%) are enriched in all incompatible elements except Ba, Sr and Eu, which show prominent negative anomalies. Initial eNd values cluster at 4.03 ± 0.15 (n = 13), near the lower end of the range for Mauritian basalts (eNd = 3.70 - 5.75), but initial Sr is highly variable (ISr = 0.70408 - 0.71034) suggesting secondary deuteric alteration. Fractional crystallization models starting with a basaltic parent fail, because when plagioclase joins olivine in the crystallizing assemblage, residual liquids become depleted in Al2O3, produce no nepheline, and do not approach trachytic compositions. Mauritian basalts and trachytes do not fall near the ends of known miscibility gaps, eliminating liquid immiscibility processes. Partial melting of extant gabbroic bodies, either from the oceanic crust or from Réunion plume-related magmas should yield quartz-saturated melts different from the critically undersaturated Mauritian trachytes. A remaining possibility is that the trachytes represent direct, small-degree partial melts of fertile, perhaps metasomatized mantle. This is supported by the presence of trachytic glasses in many mantle xenoliths, and experimental results show that low-degree trachytic melts can be produced from mantle peridotites even under anhydrous conditions. If some feldspar is left behind as a residual phase, this would account for the negative Ba, Sr and Eu anomalies observed in Mauritian trachytes. Two trachyte samples that are less depleted in these elements contain xenocrysts of anorthoclase, Al-rich cpx and Cl-rich kaersutite that are out of equilibrium with host trachyte magmas

  5. Oligocene volcanism and multiple caldera formation in the Chinati Mountains, Presidio County, Texas

    SciTech Connect

    Cepeda, J.C.; Henry, C.D.

    1983-01-01

    The Chinati Mountains caldera, which lies in Trans-Pecos Texas in the southern Basin and Range Province, was formed by eruption of the Mitchell Mesa Rhyolite. Volcanism in the Chinati Mountains area began several million years before formation of the Chinati Mountains caldera. Rocks of the Morita Ranch Formation, Infiernito caldera, and Shely Group ring the caldera on the south, east, and north. After its collapse, the caldera was filled by rhyolitic to trachytic lava flows and an ash-flow tuff of the Chinati Mountains Group. These include, from oldest to youngest, the lower trachyte, middle trachyte, lower rhyolite, upper trachyte, and upper rhyolite (ash-flow tuff). The Chinati Mountains Group was then intruded by the West Chinati Stock, the resurgent dome of the caldera. Three cycles of rhyolitic to trachytic magmatism, all derived from a zoned magma chamber, are represented by (1) Mitchell Mesa Rhyolite to lower and middle trachytes, (2) lower rhyolite to upper trachyte, and (3) upper rhyolite to West Chinati Stock. Dominant caldera collapse followed eruption of the Mitchell Mesa Rhyolite, but collapse is also associated with rhyolitic eruptions in the second and third cycles. The entire sequence erupted between 32 and 33 mya. The Chinati Mountains area is the site of one major, inactive silver mine and numerous prospects for silver, lead, zinc, copper, molybdenum, uranium, and fluorite. The Shafter silver district produced 31 million ounces of silver from Permian dolomitic limestones just south of the southern boundary of the caldera. Major prospects are associated with a quartz-monzonite porphyry intrusion (copper-molybdenum) just west of Shafter and with the West Chinati Stock (silver, lead, zinc, copper, and fluorite). All mineralization is probably genetically related to the caldera. 74 references, 15 figures, 3 tables.

  6. Magma evolution at mount vulture (Southern Italy)

    NASA Astrophysics Data System (ADS)

    de Fino, M.; La Volpe, L.; Piccarreta, G.

    1982-06-01

    The Vulture complex is made up of foiditic, tephritic, phonolitic-trachytic and phonolitic products. New rock analyses have been performed in order to ascertain whether the various rock types derive from a unique parental magma and, if so, to define its nature. The data presented support that the Vulture suite originated from a foiditic melt which had differentiated at low pressures. The main process determining the foidite → → tephrite → phonolitic trachyte evolution seems to be the crystal fractionation of mainly clinopyroxenes, and opaques, with the contribution of plagioclases and haüyne too in the tephrite → trachyte evolution. Additionary role must have been played by a mixing of melts at different evolution stages occurred in a shallow seated magma chamber.

  7. Disequilibrium crystal-liquid processes at Hamblin-Cleopatra volcano, Lake Mead area, Nevada

    NASA Astrophysics Data System (ADS)

    Barker, Daniel S.; Thompson, Keith G.; Smith, Eugene I.; McDowell, Fred W.

    2012-09-01

    The 60 km3 Hamblin-Cleopatra stratovolcano produced shoshonite, latite, and trachyte lavas throughout its Miocene eruptive history. Low-silica rhyolite and silica-undersaturated hawaiite erupted before and after lavas of the Hamblin-Cleopatra volcano. Shoshonite, latite, and trachyte resulted from contamination of felsic (trachyte to low-silica rhyolite) anatectic liquids with crystals from hawaiite. Most of the entrained crystals were not in equilibrium with liquid represented by groundmass, but were mingled with liquid shortly before eruption. Crystal aggregates are common inclusions in the lavas, and are sources of the contaminating minerals. The resulting bulk compositions of these porphyritic lavas form a continuum that resembles a liquid line of descent, as dictated by mass balance.

  8. Petrogenesis of Cenozoic volcanic rocks in the NW sector of the Gharyan volcanic field, Libya

    NASA Astrophysics Data System (ADS)

    Lustrino, Michele; Cucciniello, Ciro; Melluso, Leone; Tassinari, Colombo C. G.; dè Gennaro, Roberto; Serracino, Marcello

    2012-12-01

    The north-western sector of the Gharyan volcanic field (northern Libya) consists of trachytic-phonolitic domes emplaced between ~ 41 and 38 Ma, and small-volume mafic alkaline volcanic centres (basanites, tephrites, alkali basalts, hawaiites and rare benmoreites) of Middle Miocene-Pliocene age (~ 12-2 Ma). Two types of trachytes and phonolites have been recognized on the basis of petrography, mineralogy and geochemistry. Type-1 trachytes and phonolites display a smooth spoon-shaped REE pattern without negative Europium anomalies. Type-2 trachytes and phonolites show a remarkable Eu negative anomaly, higher concentration in HFSE (Nbsbnd Tasbnd Zrsbnd Hf), REE and Ti than Type-1 rocks. The origin of Type-1 trachytes and phonolites is compatible with removal of clinopyroxene, plagioclase, alkali feldspar, amphibole, magnetite and titanite starting from benmoreitic magmas, found in the same outcrops. Type-2 trachytes and phonolites could be the result of extensive fractional crystallization starting from mafic alkaline magma, without removal of titanite. In primitive mantle-normalized diagrams, the mafic rocks (Mg# = 62-68, Cr up to 514 ppm, Ni up to 425 ppm) show peaks at Nb and Ta and troughs at K. These characteristics, coupled with low 87Sr/86Sr(i) (0.7033-0.7038) and positive ɛNd (from + 4.2 to + 5.3) features typical of the mafic anorogenic magmas of the northern African plate and of HIMU-OIB-like magma in general. The origin of the mafic rocks is compatible from a derivation from low degree partial melting (3-9%) shallow mantle sources in the spinel/garnet facies, placed just below the rigid plate in the uppermost low-velocity zone. The origin of the igneous activity is considered linked to passive lithospheric thinning related to the development of continental rifts like those of Sicily Channel (e.g., Pantelleria and Linosa) and Sardinia (e.g., Campidano Graben) in the Central-Western Mediterranean Sea.

  9. Temporal evolution of a post-caldera, mildly peralkaline magmatic system: Furnas volcano, São Miguel, Azores

    NASA Astrophysics Data System (ADS)

    Jeffery, A. J.; Gertisser, R.; O'Driscoll, B.; Pacheco, J. M.; Whitley, S.; Pimentel, A.; Self, S.

    2016-05-01

    Furnas is one of three active central volcanoes on São Miguel Island, Azores, and is considered to be one of the most hazardous in the archipelago. In this study, the pre-eruptive magma plumbing system of the 10 young (<5 ka), intra-caldera, sub-Plinian, trachytic eruptions of the Upper Furnas Group (UFG) is investigated via whole rock major and trace element geochemistry, mineral chemistry, thermobarometry, and petrogenetic modelling. The main aim of this work is to elucidate the petrogenesis of the Furnas trachytes, constrain the P-T-fO2 conditions under which they evolve, and investigate the temporal evolution of the magma plumbing system. Results indicate that the trachytes are derived predominantly from extended fractional crystallisation of alkali basalt parental magmas, at depths between ~3 and 4 km. This is considered to take place in a density-stratified reservoir, with alkali basalt magmas at the base and hydrous trachytes forming an upper cap or cupola. The presence of this reservoir at shallow crustal depths beneath the caldera likely inhibits the ascent and subsequent eruption of mafic magmas, generating a compositional Daly Gap. Rare syenitic ejecta represent in situ crystallisation of trachytic magmas in the thermal boundary zone at the top of the reservoir. Trachytic enclaves within these syenites, in addition to banded pumices and ubiquitous clinopyroxene antecrysts in the UFG pumice falls, provide evidence for mingling/mixing processes within the magmatic system. Despite relatively uniform major element compositions, systematic trace element variations within individual eruptions highlight the importance of fractional crystallisation during late-stage evolution of the trachytes. This is facilitated by the accumulation of water and the development of mild peralkalinity, which contribute to low pre-eruptive melt viscosities and efficient crystal settling. Compositional zoning patterns between individual eruptions cannot be accounted for by periodic

  10. A new felsic cone-sheet swarm in the Central Atlantic Islands: The cone-sheet swarm of Boa Vista (Cape Verde)

    NASA Astrophysics Data System (ADS)

    Ancochea, Eumenio; Huertas, María José; Hernán, Francisco; Brändle, José Luis

    2014-03-01

    The island of Boa Vista is one of the oceanic islands with the largest amount of felsic rocks reported in the world (50% of the total outcropping igneous rocks of Boa Vista). The felsic rocks are trachytes and phonolites generated within the second main stage of the volcanic activity that lasted about four million years.

  11. Volcanic geology and eruption frequency, São Miguel, Azores

    USGS Publications Warehouse

    Moore, Richard B.

    1990-01-01

    Six volcanic zones comprise São Miguel, the largest island in the Azores. All are Quaternary in age except the last, which is partly Pliocene. From west to east the zones are (1) the trachyte stratovolcano of Sete Cidades, (2) a field of alkali-basalt cinder cones and lava flows with minor trachyte, (3) the trachyte stratovolcano of Agua de Pau, (4) a field of alkali-basalt cinder cones and lava flows with minor trachyte and tristanite, (5) the trachyte stratovolcano of Furnas, and (6) the Nordeste shield, which includes the Povoação caldera and consists of alkali basalt, tristanite, and trachyte. New radiocarbon and K-Ar ages augment stratigraphic data obtained during recent geologic mapping of the entire island and provide improved data to interpret eruption frequency. Average dormant intervals for the past approximately 3000 years in the areas active during that time are about 400 years for Sete Cidades, 145 for zone 2, 1150 for Agua de Pau, and 370 for Furnas. However, the average dormant interval at Sete Cidades increased from 400 to about 680 years before each of the past two eruptions, and the interval at Furnas decreased from 370 to about 195 years before each of the past four eruptions. Eruptions in zone 4 occurred about once every 1000 years during latest Pleistocene and early Holocene time; none has occurred for about 3000 years. The Povoação caldera truncates part of the Nordeste shield and probably formed during the middle to late Pleistocene. Calderas formed during latest Pleistocene time at the three younger stratovolcanoes in the sequence: outer Agua de Pau (between 46 and 26.5 ka), Sete Cidades (about 22 ka), inner Agua de Pau (15.2 ka), and Furnas (about 12 ka). Normal faults are common, but many are buried by Holocene trachyte pumice. Most faults trend northwest or west-northwest and are related to the Terceira rift, whose most active segment on São Miguel passes through Sete Cidades and zone 2. A major normal fault displaces Nordeste

  12. Tectonic controls on the genesis of ignimbrites from the Campanian Volcanic Zone, southern Italy

    USGS Publications Warehouse

    Rolandi, G.; Bellucci, F.; Heizler, M.T.; Belkin, H.E.; de Vivo, B.

    2003-01-01

    The Campanian Plain is an 80 x 30 km region of southern Italy, bordered by the Apennine Chain, that has experienced subsidence during the Quaternary. This region, volcanologically active in the last 600 ka, has been identified as the Campanian Volcanic Zone (CVZ). The products of three periods of trachytic ignimbrite volcanism (289-246 ka, 157 ka and 106 ka) have been identified in the Apennine area in the last 300 ka. These deposits probably represent distal ash flow units of ignimbrite eruptions which occurred throughout the CVZ. The resulting deposits are interstratified with marine sediments indicating that periods of repeated volcano-tectonic emergence and subsidence may have occurred in the past. The eruption, defined as the Campanian Ignimbrite (CI), with the largest volume (310 km3), occurred in the CVZ 39 ka ago. The products of the CI eruption consist of two units (unit-1 and unit-2) formed from a single compositionally zoned magma body. Slightly different in composition, three trachytic melts constitute the two units. Unit-1 type A is an acid trachyte, type B is a trachyte and type C of unit-2 is a mafic trachyte. The CI, vented from pre-existing neotectonic faults, formed during the Apennine uplift, Initially the venting of volatile-rich type A magma deposited the products to the N-NE of the CVZ. During the eruption, the Acerra graben already affected by a NE-SW fault system, was transected by E-W faults, forming a cross-graben that extended to the gulf of Naples. E-W faults were then further dislocated by NE-SW transcurrent movements. This additional collapse significantly influenced the deposition of the B-type magma of unit-1, and the C-type magma of unit-2 toward the E-SE and S, in the Bay of Naples. The pumice fall deposit underlying the CI deposits, until now thought to be associated with the CI eruption, is not a strict transition from plinian to CI-forming activity. It is derived instead from an independent source probably located near the

  13. Tephra layers in the Byrd Station ice core and the Dome C ice core, Antarctica and their climatic importance

    NASA Astrophysics Data System (ADS)

    Kyle, Philip R.; Jezek, Peter A.; Mosley-Thompson, Ellen; Thompson, Lonnie G.

    1981-08-01

    Volcanic glass shards from tephra layers in the Byrd Station ice core were chemically analyzed by electron microprobe. Tephra in seven layers have similar peralkaline trachyte compositions. The tephra are believed to originate from Mt. Takahe, on the basis of their chemical similarity to analyzed rocks from Mt. Takahe and because dated rock samples from the volcano are younger than 250,000 years old. Glass shards from 726 m deep in the Dome C ice core, which is 2400 km from Byrd Station, are composed of peralkaline trachyte and may have also been derived from Mt. Takahe. The tephra could have resulted from eruptions which were triggered by increased ice loading during the late Wisconsin glaciation. Preliminary grain size data suggest the eruptions were only minor and they were unlikely to have instantaneously altered global climate as have explosive eruptions in the tropics. Nevertheless, the effect of this localized volcanic activity upon the Antarctic energy budget warrants further investigation.

  14. The ultimate summit eruption of Puy de Dôme volcano (Chaîne des Puys, French Massif Central) about 10,700 years ago

    NASA Astrophysics Data System (ADS)

    Miallier, Didier; Boivin, Pierre; Deniel, Catherine; Gourgaud, Alain; Lanos, Philippe; Sforna, Marie; Pilleyre, Thierry

    2010-11-01

    The Puy de Dôme volcano is a trachytic lava dome, about 11,000 y old. New pyroclastic layers originating from the volcano itself were discovered covering the summit and the flanks of the volcano. These pyroclastic layers do not fit with the previous interpretation, assuming a non-explosive dome-forming eruption. The tephra display pyroclastic surge features and exhibit fresh trachytic lapilli, basement lithics, allogeneous basaltic lava and clinker fragments requiring an open vent eruption. This ultimate eruption occurred after a period of rest, long enough for vegetation to develop on the volcano, as evidenced by carbonized plant fragments. Radiocarbon dating of some of these fragments gave an age of c.10,700 y also suggesting a significant rest duration.

  15. Regional and local correlations of feldspar geochemistry of the Peach Spring Tuff, Alvord Mountain, California

    USGS Publications Warehouse

    Buesch, David C.

    2016-01-01

    The chemical composition of feldspar grains in an ignimbrite from the Spanish Canyon Formation in the Alvord Mountain area, California, have been used to confirm similarities in three measured sections locally, and they are similar to exposures of the Peach Spring Tuff (PST) regionally. Feldspar grains were identified on the basis of texture (zoning, as mantled feldspars, or in crystal clusters), whether the grains were attached to glass or were in pumice clasts, or were simply crystal fragments with no textural context. Chemistry was determined by electron microprobe analysis, and each analysis is calculated in terms of the percent endmember and plotted on orthoclase (Or) versus anorthite (An) plots. In general, the PST has sanidine and plagioclase compositions that are consistent with having formed in high-silica rhyolite and trachyte within a zoned magma chamber. Feldspars from the PST in Spanish Canyon area cluster along the rhyolitic trend with no grains along the trachytic trend. Similar clustering of feldspars along the rhyolitic trend with no grains along the trachytic trend also occur in the PST from Granite Spring and Providence Mountains to the east of the Alvord Mountain area, and the ranges in compositions are also similar in these locations. In contrast, the PST in the Kane Wash area of the Newberry Mountains has feldspars only from the rhyolitic trend in the basal deposits, but some grains from the trachytic trend are in the upper part of the deposit, and the range in compositions are greater than in the Spanish Canyon area. The variations in vertical compositional zoning and compositional range in these different deposits suggests there were probably different flow paths (or timing of the delivery) during the eruption and runout of the pyroclastic flow(s) generated from the climactic eruption of the PST magma chamber.

  16. Geochemical and isotopic insights into the assembly, evolution and disruption of a magmatic plumbing system before and after a cataclysmic caldera-collapse eruption at Ischia volcano (Italy)

    NASA Astrophysics Data System (ADS)

    Brown, R. J.; Civetta, L.; Arienzo, I.; D'Antonio, M.; Moretti, R.; Orsi, G.; Tomlinson, E. L.; Albert, P. G.; Menzies, M. A.

    2014-09-01

    New geochemical and isotopic data on volcanic rocks spanning the period ~75-50 ka BP on Ischia volcano, Italy, shed light on the evolution of the magmatic system before and after the catastrophic, caldera-forming Monte Epomeo Green Tuff (MEGT) eruption. Volcanic activity during this period was influenced by a large, composite and differentiating magmatic system, replenished several times with isotopically distinct magmas of deep provenance. Chemical and isotopic variations highlight that the pre-MEGT eruptions were fed by trachytic/phonolitic magmas from an isotopically zoned reservoir that were poorly enriched in radiogenic Sr and became progressively less radiogenic with time. Just prior to the MEGT eruption, the magmatic system was recharged by an isotopically distinct magma, relatively more enriched in radiogenic Sr with respect to the previously erupted magmas. This second magma initially fed several SubPlinian explosive eruptions and later supplied the climactic, phonolitic-to-trachytic MEGT eruption(s). Isotopic data, together with erupted volume estimations obtained for MEGT eruption(s), indicate that >5-10 km3 of this relatively enriched magma had accumulated in the Ischia plumbing system. Geochemical modelling indicates that it accumulated at shallow depths (4-6 km), over a period of ca. 20 ka. After the MEGT eruption, volcanic activity was fed by a new batch of less differentiated (trachyte-latite) magma that was slightly less enriched in radiogenic Sr. The geochemical and Sr-Nd-isotopic variations through time reflect the upward flux of isotopically distinct magma batches, variably contaminated by Hercynian crust at 8-12 km depth. The deep-sourced latitic to trachytic magmas stalled at shallow depths (4-6 km depth), differentiated to phonolite through crystal fractionation and assimilation of a feldspar-rich mush, or ascended directly to the surface and erupted.

  17. Petrogenesis of extension-related alkaline volcanism in Karaburhan (Sivrihisar-Eskisehir), NW Anatolia, Turkey

    NASA Astrophysics Data System (ADS)

    Sarıfakıoğlu, Ender; Özen, Hayrettin; Hall, Chris

    2009-08-01

    Alkaline lavas were erupted as phonolites and trachytes around Karaburhan (Sivrihisar-Eskisehir, NW Anatolia) within the Izmir-Ankara-Erzincan suture zone. These volcanic rocks were emplaced as domes, close and parallel to the ophiolite thrust line. According to 40Ar/ 39Ar geochronological analyses of sanidine crystals from the phonolites, the age of the alkaline volcanics is 25 Ma (Late Oligocene-Early Miocene). The flow-textured phonolites are porphyritic and consist mainly of sanidine, clinopyroxene, and feldspathoid crystals. The clinopyroxenes show compositional zoning, with aegirine (Na 0.82-0.96Fe +30.68-0.83) rims and aegirine-augite cores (containing calcium, magnesium, and Fe +2). Some aegirine-augites are replaced with sodium-, calcium-, and magnesium-rich amphibole (hastingsite). Feldspathoid (hauyne) crystals enriched with elemental Na and Ca have been almost completely altered to zeolite and carbonate minerals. The fine-grained trachytes with a trachytic texture consist of feldspar (oligoclase and sanidine) phenocrystals and clinopyroxene microphenocrystals within a groundmass made up largely of alkali feldspar microlites. Although there are some differences in their element patterns, the phonolites and trachytes exhibit enrichment in LILEs (Sr, K, Rb, Ba, Th) and LREEs (La, Ce, Pr, Nd) and negative anomalies in Nb and Ta. These geochemical characteristics indicate a lithospheric mantle enriched by fluids extracted from the subduction component. In addition, the high 87Sr/ 86Sr (0.706358-0.708052) and low 143Nd/ 144Nd (0.512546-0.512646) isotope concentrations of the alkaline lavas reflect a mantle source that has undergone metasomatism by subduction-derived fluids. Petrogenetic modeling indicates that the alkaline lavas generated from the subduction-modified lithospheric mantle have undergone assimilation, fractional crystallization, and crustal contamination, acquiring high Pb, Ba, Rb, and Sr contents and Pb isotopic compositions during their ascent

  18. The roles of fractional crystallization, magma mixing, crystal mush remobilization and volatile-melt interactions in the genesis of a young basalt-peralkaline rhyolite suite, the greater Olkaria volcanic complex, Kenya Rift valley

    USGS Publications Warehouse

    Macdonald, R.; Belkin, H.E.; Fitton, J.G.; Rogers, N.W.; Nejbert, K.; Tindle, A.G.; Marshall, A.S.

    2008-01-01

    The Greater Olkaria Volcanic Complex is a young (???20 ka) multi-centred lava and dome field dominated by the eruption of peralkaline rhyolites. Basaltic and trachytic magmas have been erupted peripherally to the complex and also form, with mugearites and benmoreites, an extensive suite of magmatic inclusions in the rhyolites. The eruptive rocks commonly represent mixed magmas and the magmatic inclusions are themselves two-, three- or four-component mixes. All rock types may carry xenocrysts of alkali feldspar, and less commonly plagioclase, derived from magma mixing and by remobilization of crystal mushes and/or plutonic rocks. Xenoliths in the range gabbro-syenite are common in the lavas and magmatic inclusions, the more salic varieties sometimes containing silicic glass representing partial melts and ranging in composition from anorthite ?? corundum- to acmite-normative. The peralkaline varieties are broadly similar, in major element terms, to the eruptive peralkaline rhyolites. The basalt-trachyte suite formed by a combination of fractional crystallization, magma mixing and resorption of earlier-formed crystals. Matrix glass in metaluminous trachytes has a peralkaline rhyolitic composition, indicating that the eruptive rhyolites may have formed by fractional crystallization of trachyte. Anomalous trace element enrichments (e.g. ??? 2000 ppm Y in a benmoreite) and negative Ce anomalies may have resulted from various Na- and K-enriched fluids evolving from melts of intermediate composition and either being lost from the system or enriched in other parts of the reservoirs. A small group of nepheline-normative, usually peralkaline, magmatic inclusions was formed by fluid transfer between peralkaline rhyolitic and benmoreitic magmas. The plumbing system of the complex consists of several independent reservoirs and conduits, repeatedly recharged by batches of mafic magma, with ubiquitous magma mixing. ?? The Author 2008. Published by Oxford University Press. All

  19. Eruption Mechanism of the 10th Century Eruption in Baitoushan Volcano, China/North Korea

    NASA Astrophysics Data System (ADS)

    Shimano, T.; Miyamoto, T.; Nakagawa, M.; Ban, M.; Maeno, F.; Nishimoto, J.; Jien, X.; Taniguchi, H.

    2005-12-01

    Baitoushan volcano, China/North Korea, is one of the most active volcanoes in Northeastern Asia, and the 10th century eruption was the most voluminous eruption in the world in recent 2000 years. The sequence of the eruption reconstructed recently consists mainly of 6 units of deposits (Miyamoto et al., 2004); plinian airfall (unit B), large pyroclastic flow (unit C), plinian airfall with some intra- plinian pyroclastic flows (unit D), sub-plinian airfall (unit E), and large pyroclastic flow (unit F) with base surge (unit G) in ascending order. The magma erupted during steady eruption in earlier phase was comendite (unit B-C; Phase 1), whereas the magma during fluctuating eruptions in later phase is characterized by trachyte to trachyandesite with various amount of comendite (unit D-G; Phase 2). The wide variety in composition and occurrence of banded pumices strongly indicate mixing or mingling of the two magmas just prior to or during the eruption. The initial water contents had been determined for comendite by melt inclusion analyses (ca. 5.2 wt.%; Horn and Schmincke, 2000). Although the initial water content of the trachytic magma has not been correctly determined yet, the reported water contents of trachytic melt inclusions are lower (3-4 wt.%) than those of comenditic melt (Horn and Schmincke, 2000). We investigated juvenile materials of the eruption sequentially in terms of vesicularity, H2O content in matrix glass and textural characteristics. The vesicularity of pumices are generally high (>0.75) for all units. The residual water contents of the comenditic pumices during Phase 1 are relatively uniform (1.6 wt.%), whereas those of the trachytic scoria during Phase 2 and gray pumices during Phase 1 are low (ca. 0.7-1.3 wt.%). These facts may indicate that the difference in the initial water content, rather than the ascent mechanism of magma, controls the steadiness or fluctuation in eruption styles and the mass flux during the eruption.

  20. New Constraints on the Geochemistry of the Millennium Eruption of Mount Paektu (Changbaishan), Democratic People's Republic of Korea/China

    NASA Astrophysics Data System (ADS)

    Iacovino, K.; Kim, J. S.; Sisson, T. W.; Lowenstern, J. B.; Jang, J. N.; Song, K. H.; Ham, H. H.; Ri, K. H.; Donovan, A. R.; Oppenheimer, C.; Hammond, J. O. S.; Weber Liu, K.; Ryu, K. R.

    2015-12-01

    Mount Paektu (also known as Changbaishan) is a large caldera located on the border between China and the Democratic People's Republic of Korea. Circa 946 AD, Paektu produced one of the largest volcanic eruptions in recorded history, the so-called Millennium Eruption (ME), whose combined fall and pyroclastic flow deposits total approximately 25 km3 dense rock equivalent (95% commendite, 5% late stage trachyte). Despite its recent and potentially destructive history, the volcano is not well studied due to its relative inaccessibility. A seismic swarm beneath the volcano's summit in 2002-2005 spurred a unique collaboration between scientists from the DPRK, US, and the UK with the goals of characterizing Paektu's eruptive history and assessing its current state of activity. We present new results from this collaboration, including major and trace element (XRF, EMP and SHRIMP-RG) and volatile data (SHRIMP-RG and FTIR) on feldspar-, clinopyroxene-, and olivine-hosted melt inclusions (MI), matrix glasses, and bulk pumices from four ME comendites and one ME trachyte. MI are halogen rich (F≤4000 ppm, Cl≤5000 ppm) with moderate S (≤250 ppm) and H2O (≤4 wt%) and minimal CO2 (≤15 ppm, detection limit ~2 ppm). H2O contents in comendite MI indicate saturation pressures (at 725 °C) of ~150 MPa, corresponding to a magma chamber depth of ~6 km, similar to the depth inferred for the magmatic injection thought to have resulted in the 2002-05 earthquake swarm. ME comendite is consistent with a ca. 25% residual melt by fractional crystallization from an ME trachyte parent. Published U-series zircon ages from ME comendite indicate a magma residence time of 11ky. Thus, the late stage ME trachyte likely represents a mafic recharge event of a melt separate from but geochemically similar to the original ME comendite parent.

  1. Eruptive history and magmatic stability of Erebus volcano, Antarctica: Insights from englacial tephra

    NASA Astrophysics Data System (ADS)

    Iverson, Nels A.; Kyle, Philip R.; Dunbar, Nelia W.; McIntosh, William C.; Pearce, Nicholas J. G.

    2014-11-01

    tephrostratigraphy of the active Antarctic Erebus volcano was determined from englacial tephra on the ice-covered flanks of Erebus and an adjacent volcano. The tephra are used to reconstruct the eruptive history and magmatic evolution of Erebus. More fine-grained and blocky particles define tephra formed in phreatomagmatic eruptions and larger fluidal shards are characteristic of magmatic eruptions and in some cases both eruptive types are identified in a single mixed tephra. The eruptions forming the mixed tephra likely started as phreatomagmatic eruptions which transitioned into Strombolian eruptions as the nonmagmatic water source was exhausted. We reconstructed the eruptive history of Erebus using the tephra layers stratigraphic position, 40Ar/39Ar ages, shard morphology, and grain size. Major and trace element analyses of individual glass shards were measured by electron probe microanalysis and LA-ICP-MS. Trachybasalt, trachyte, and phonolite tephra were identified. All phonolitic tephra are Erebus-derived with compositions similar to volcanic bombs erupted from Erebus over the past 40 years. The tephra show that Erebus magma has not significantly changed for 40 ka. The uniformity of the glass chemical composition implies that the phonolite magma has crystallized in the same manner without change throughout the late Quaternary, suggesting long-term stability of the Erebus magmatic system. Trachyte and trachybasalt tephra were likely erupted from Marie Byrd Land and the McMurdo Sound area, respectively. The trachytic tephra can be regionally correlated and could provide an important time-stratigraphic marker in Antarctic ice cores.

  2. The volcano-pluton interface; The Longonot (Kenya) and Kûngnât (Greenland) peralkaline complexes

    NASA Astrophysics Data System (ADS)

    Macdonald, R.; Bagiński, B.; Upton, B. G. J.

    2014-05-01

    An important step in studying volcanic processes is to consider the coeval processes in the subjacent magma reservoir(s). The trachytic caldera volcano Longonot (Kenya Rift Valley) and the Kûngnât syenite complex (Gardar province, Greenland) are taken to represent complementary magmatic systems, providing evidence of the volcanic and plutonic stages of evolution, respectively. The systems show many features in common; they have similar sizes, experienced two periods of caldera collapse, and were dominated by trachytic magmas, with smaller volumes of basic magma. Magmatic differentiation was dominantly by fractional crystallization of basaltic parents, with minor episodes of magma mixing and, at Kûngnât, some crustal contamination of parental basalts. A model is presented of a single, hypothetical trachytic centre, showing how evidence from one complex can be used to infer processes at the other. For example, an active convective system, with formation of wall and floor syenitic cumulates, can be inferred to exist in the Longonot magma chamber. At Kûngnât, the intermittent development of compositionally zoned caps to the magma chamber is postulated and the nature of syn-caldera eruptive activity is outlined.

  3. Deep to shallow crustal differentiation of within-plate alkaline magmatism at Mt. Bambouto volcano, Cameroon Line

    NASA Astrophysics Data System (ADS)

    Marzoli, Andrea; Aka, Festus T.; Merle, Renaud; Callegaro, Sara; N'ni, Jean

    2015-04-01

    At Mt. Bambouto, a continental stratovolcano of the Cameroon Line, magmatic activity lasted for over 20 Ma and was characterized by at least two caldera formation events. Here we present detailed mineral and whole-rock compositions of Mt. Bambouto basanites, hawaiites, trachytes and phonolites, with emphasis on caldera related volcanic rocks. These data show that differentiation took place within a complex magma plumbing system, with magma chambers occurring at different depths within the crust. Though differentiation was chiefly dominated by fractional crystallization, chemical mineral zoning of olivines, clinopyroxenes, and feldspars is also indicative of open-system processes such as magma mixing and magma chamber recharge. Chemical zoning is evident mainly in the outer 100 microns of the analyzed crystals, suggesting that magma mixing occurred shortly before eruption. The last caldera collapse at about 15 Ma also marked a clear change in the magma plumbing system. Before caldera collapse, Mt. Bambouto was characterized by a dominant production of peralkaline quartz trachytic magmas in shallow magma chambers. During this phase, evolved basic magmas (hawaiites) and strongly evolved alkaline magmas were formed in middle and upper crustal magma chambers, respectively. After emptying of the shallow quartz trachytic magma chamber and caldera collapse, magmas from the deep magmatic plumbing system were mobilized and partially mixed. This triggered eruptions of magmas on the caldera rims.

  4. A basic radial dike swarm of Boa Vista (Cape Verde Archipelago); its significance in the evolution of the island

    NASA Astrophysics Data System (ADS)

    Ancochea, Eumenio; Hernán, Francisco; Huertas, María José; Brändle, José Luis

    2012-10-01

    A basic radial dike swarm unrelated to other basic units of Boa Vista (Cape Verde Archipelago) has been localized and characterized in the central sector of the island. According to new radiometric data three main stages in the evolution of Boa Vista are distinguished: the earlier (the Old Volcanic Complex: 17-16 Ma) is equivalent to the shield building stage of Hawaii and the later (the Recent Volcanics (8-4 Ma) is in some aspects comparable to the post-erosional stage. An important intermediate essentially felsic stage (the Trachytic-Phonolitic Complex: 14.3-12.8 Ma) followed the basaltic shield stage. This felsic stage has equivalents in some other oceanic islands as the Canary Islands, specially Gran Canaria, Tenerife, and La Gomera. The central sector of Boa Vista is also occupied by the Felsic Subvolcanic Complex, a unit consisting of phonolitic breccias, syenites and monzonites that represent the Trachytic-Phonolitic Complex hypabyssal roots. The felsic rocks as a whole constitute half of the total amount of igneous rocks on the island making up Boa Vista, the island with the highest percentage of felsic rocks in the Central Atlantic Ocean. More than 200 dikes of the basic radial swarm intruding the Felsic Subvolcanic Complex have been measured. The intensity of the multiple dike injection is sometimes rather high, roughly a dike every 5 m. The individual dikes have an observable mean length of about 300 m. The composition of these dikes is always foiditic (nephelinites, melilitites, and limburgites), slightly different in composition (more alkaline and richer in incompatible elements) to the other basic units of the island (the Old Volcanic Complex and the Recent Volcanics). The radial dikes converge in an area located NW of the geometrical center of Boa Vista, a zone where the hypothetical center of the Old Volcanic Complex and the Trachytic-Phonolitic Complex edifices must also have been situated. The ages obtained from the dikes (between 14.8 and 11.5 Ma

  5. The role of magma mixing and mafic recharge in the evolution of a back-arc quaternary caldera: The case of Payún Matrú, Western Argentina

    NASA Astrophysics Data System (ADS)

    Hernando, Irene Raquel; Petrinovic, Ivan Alejandro; Llambías, Eduardo Jorge; D'Elia, Leandro; González, Pablo Diego; Aragón, Eugenio

    2016-02-01

    The Quaternary Payún Matrú volcano is a long-lived edifice that developed a summit caldera 8 km wide, with abundant pre- and post-caldera volcanic activity. It is the main volcano of the Payún Matrú Volcanic Field, which is located in the back-arc Payenia Basaltic Province, at mid-western Argentina. The composition of Payún Matrú is mainly trachytic, with lesser amounts of trachyandesitic and basaltic trachyandesitic lavas. The Payún Matrú Volcanic Field includes also the Payún Liso stratovolcano and more than 200 monogenetic basaltic cones and associated lava flows that are located east and west of the caldera. The aim of this work is to show the Payún Matrú evolution based on the mineralogical and textural characterization and to make inferences on the trigger mechanism for the explosive eruption which leads to the caldera formation. Some intermediate lavas and trachytes include phenocrysts with contrasting textures and composition, such as inverse zoning in plagioclase and clinopyroxenes, sieved plagioclases with a more calcic rim and calcic plagioclases with a more sodic rim, indicating that they could not have formed together and suggesting magma mixing processes. In addition, a few lavas show mafic enclaves or texturally different groundmasses with fluidal contacts, indicating mingling between two magmas. These lavas are found in the pre-caldera stage and the early post-caldera stage of Payún Matrú. The trachytic pyroclastic deposits related to the caldera collapse do not exhibit evidence of hybridization, although they present a minor proportion of dissolved feldspars and biotite replaced by anhydrous minerals, suggesting that a restricted zone of the chamber was reheated. This may be explained by a mafic recharge event where part of the host trachytic magma was heated, but without a complete homogenization between the host and recharging magmas. These features, in addition to the abundant basaltic volcanism east and west of the caldera, suggest

  6. Ages of zircons from pre-, syn- and post-caldera eruption products of the Changbaishan Volcano, indicating rapid magmatic development

    NASA Astrophysics Data System (ADS)

    Zou, H.; Fan, Q.; Zhang, H.; Schmitt, A. K.

    2013-12-01

    The Millennium Eruption of Changbaishan Volcano with its eponymous 1000 CE eruption age represents one of the two largest volcanic eruptions on Earth in the past 2000 years. In addition to this major caldera-producing eruption of comendite magma, the Changbaishan Volcano also erupted smaller-scale comendite at ~5 ka and trachyte at ~0.3 ka. Here we report U-Th disequilibrium ages of zircons from pumice and lava of the pre-caldera (~5 ka), syn-caldera (the 1 ka Millennium eruption), and post-caldera (~0.3 ka) events. The zircon isochron ages are 12.4+/-1.5 ka (2σ, n=29, MSWD=0.87) for the 5 ka eruption, and 12.2 +/- 1.7 ka (n=16, MSWD=1.0) for the 1 ka eruption which is in agreement within uncertainty with a previously reported isochron age of 10.6+/-1.6 ka (n=11, MSWD=0.61) for a different 1 ka sample (Zou et al., 2010, Lithos). The zircon storage times in the pre-eruptive magma body are thus 7 ka for the pre-caldera eruption and 11 ka for the syn-caldera eruption. Furthermore, identical zircon ages in pre-caldera and syn-caldera rocks suggest that both comenditic eruptions tapped the same magma body. In contrast to the uniform zircon ages for the 5 ka and 1 ka comenditic eruptions, zircon ages for the post-caldera 0.3 ka trachytic eruption define multimodal age populations. The youngest peak for the 0.3 ka eruption is 2.6+/-1.8 ka (n=11, MSWD=0.90), an older peak is 130+/-10 ka (n=13, MSWD=1.7), and the oldest population is ≧230 ka (near U-Th equilibrium). The youngest mode represents zircon microphenocrysts (autocrysts) that crystallized in a trachytic magma chamber in the built-up to the 0.3 ka eruption, whereas the 130 ka and ≧230 ka zircons are interpreted as antecrysts derived from earlier episodes of magmatism. If this interpretation is correct, the zircon storage time in the eruptible magma body for the 0.3 ka eruption is extremely short at 2.3+/-1.8 ka. The distinct multimodal zircon age distributions for the 0.3 ka eruption of trachytic magma

  7. Glass and mineral analyses from first deposits of Peach Spring Supereruption (SW USA) illuminate initial tapping of a zoned magma chamber

    NASA Astrophysics Data System (ADS)

    Mccracken, R. G.; Miller, C. F.; Buesch, D.; Gualda, G. A.; Covey, A.

    2012-12-01

    The Peach Spring supereruption (18.78±0.02 Ma) was sourced from Silver Creek caldera in the southern Black Mountains, Arizona (Ferguson et al. in press). The resulting ignimbrite, the Peach Spring Tuff (PST), blanketed >32,000 km2 of Arizona, California, and Nevada (Buesch, 1993). Underlying the ignimbrite is a thin (≤ 1m thick) basal layered deposit that consists of texturally distinct layers 1a-e (Valentine et al. 1989) and is present up to ~100 km from the source caldera. Basal layered deposits contain the first material erupted during the PST supereruption, preceding the main eruption event. Petrography and geochemistry of minerals and pumice clasts from basal layered deposits collected ~15-100 km from the caldera, combined with a survey of glass and crystal compositions from both outflow and basal deposits, permit (1) comparisons with the overlying ignimbrite, and (2) insights into the initial stages of the supereruption and extraction of magma from the chamber. Pumice clasts from a pumice-rich layer (1a2) of the basal deposit were characterized by LA-ICPMS and SEM. Unaltered glass has a uniform high-Si rhyolite composition (76.7% SiO2, 13.0% Al2O3, 3.6% Na2O, 5.3% K2O, 0.6% FeO, <0.1% MgO, 0.6% CaO, 0.1% TiO2). Mildly altered glass is similar but has lower Na2O and higher K2O. Pumice clasts are relatively crystal poor (<10% phenocrysts) with an assemblage dominated by sanidine (~Or55Ab43An2), with lesser plagioclase (~Ab73An19Or8), minor hornblende and biotite, and accessory magnetite, sphene, zircon, chevkinite, and apatite; no quartz was identified. Initial LA-ICPMS results for glass reveal REE patterns with large negative Gd (0.21: i.e. U-shaped REE pattern) and Eu (0.31) anomalies, very low Ba and Sr (≤10 ppm), and high Rb (~250 ppm). These compositions are essentially identical to those of the most common pumice from distal outflow ignimbrite, but very different from crystal-rich (>30%) trachyte pumice that dominates the intracaldera fill and is

  8. Geochemistry and petrogenesis of the late Cretaceous potassic-alkaline volcanic rocks from the Amasya Region (northern Turkey)

    NASA Astrophysics Data System (ADS)

    Gülmez, Fatma; Genç, Can; Tüysüz, Okan; Karacık, Zekiye; Roden, Mike; Billor, Zeki; Hames, Willis

    2013-04-01

    The Cretaceous Lokman Formation (Alp, 1972) , is a volcano-sedimantary unit that comprises high- to ultra high-K alkaline volcanic rocks in Amasya Region (40°N, 35°E). The volcanic rocks expose as small outcrops and interfingered with pyroclastic and epiclastic rocks, and are classified as leucitite, tephriphonolite (LT), lamprophyres, trachytes and rarely andesites. LT and lamprophyres occur as dikes cutting each other, and rare lava flows. Trachytes are observed as small domes in the field and lots of pebbles and blocks within the clastic deposits derived from the domes. Samples of LT comprise lct+cpx (diopsite)+plg+mag+ap and classified as leucite-basanite mineralogically and tephri-phonolite geochemically. Ar-Ar age dating from leucites show that the leucite-bearing volcanic activity formed 75.6±3.7 Ma. The mineralogic composition of melanokratic lamprophyre dikes are represented by Kfs+cpx+mica+ap+mag. They defined geochemically as phono-tephrite and phonolite. The Ar-Ar plateau ages from the phlogopites from two different outcrops are 76.78 and 77.48 Ma. The main minerals of trachytic rocks are amp + bt + pl + Kfs + spn + ap +opq. They are classified as alkaline trachyandesite, geochemically. Radiometric age data from Kfs minerals reveal that the trachytic volcanism occurred 75.83±0.09 Ma. Except one andesitic sample, lamprophyres and trachytes of the Lokman Formation are the high- and ultra high-K and alkaline rocks. LT and lamprophyres are characterized by relatively high MgO (3.25-7.04 wt.%), K2O (4.34-6.54 wt.%), Na2O (3.42-5.74 wt.%). Total analcimization of leucite minerals let to decreasing its K2O, and increasing the Na2O contents. Therefore, K2O/Na2O values for LT and the lamprophyres (0.92-2.27) are relatively low. Trachytic suite is also high-K and alkaline in nature. On MORB normalized plots, all of the volcanic rocks from Lokman Formation display enrichment of LIL elements significantly relative to HFSE, and depletions of Nb-Ta and Ti

  9. The `Daly Gap' and implications for magma differentiation in composite shield volcanoes: A case study from Akaroa Volcano, New Zealand

    NASA Astrophysics Data System (ADS)

    Hartung, E.; Kennedy, B.; Deering, C. D.; Trent, A.; Gane, J.; Turnbull, R. E.; Brown, S.

    2010-12-01

    The origin of compositional gaps in volcanic deposits that are found worldwide, and in a range of different tectonic settings, has challenged petrologists since Daly’s first observations at mid-ocean ridges. In the shield-forming Akaroa Volcano (9.6 - 8.6 Ma) of Banks Peninsula, New Zealand, a dramatic compositional gap exists in both eruptive and co-genetic intrusive products between basalt and trachyte, and between gabbro and syenite respectively. Rock compositions display mildly alkaline affinities ranging from picritic basalt, olivine alkali basalt and hawaiite to trachyte. Intermediate mugearite and benmoreite (50 - 60 wt. % SiO2) are not exposed or absent. Equivalent plutonic diorite, monzodiorite and monzonite (45 - 65 wt. % SiO2) are also absent. Previously, the formation of the more evolved trachyte (and syenite) has been ascribed to crustal melting. However, our analysis of new major and trace element data from minerals and bulk-rocks of this hy-normative intraplate alkalic suite provide evidence for an alternative model based on crystal fractionation and punctuated melt extraction. Observed major and trace element trends in bulk-rocks can be reproduced by Rayleigh fractional crystallization from dry melts (< 0.5 wt. % H2O) at oxygen fugacities one unit below the quartz-fayalite-magnetite-buffer (QFM -1). The results of our MELTS models are in agreement with experimental studies, and indicate a fractionation-generated compositional gap, where trachytic liquid (62 - 64 wt. % SiO2) has been extracted after the melt has reached a crystallinity of 65 - 70 %. The fractionated assemblage of clinopyroxene, olivine, plagioclase, magnetite and apatite is left in a mafic cumulate residue (44 - 46 wt. % SiO2). Calculated values of specific trace and minor elements (Sr, Cr, P) from a theoretical cumulate are consistent with measured concentrations from cumulate xenoliths. Compositional trends from individual mineral analysis are also supportive of fractional

  10. Volcanic history and 40Ar/39Ar and 14C geochronology of Terceira Island, Azores, Portugal

    USGS Publications Warehouse

    Calvert, A.T.; Moore, R.B.; McGeehin, J.P.; Rodrigues da Silva, A.M.

    2006-01-01

    Seven new 40Ar/39Ar and 23 new radiocarbon ages of eruptive units, in support of new geologic mapping, improve the known chronology of Middle to Late Pleistocene and Holocene volcanic activity on the island of Terceira, Azores and define an east-to-west progression in stratovolcano growth. The argon ages indicate that Cinco Picos Volcano, the oldest on Terceira, completed its main subaerial cone building activity by about 370-380??ka. Collapse of the upper part of the stratovolcanic edifice to form a 7 ?? 9??km caldera occurred some time after 370??ka. Postcaldera eruptions of basalt from cinder cones on and near the caldera floor and trachytic pyroclastic flow and pumice fall deposits from younger volcanoes west of Cinco Picos have refilled much of the caldera. The southern portion of Guilherme Moniz Volcano, in the central part of the island, began erupting prior to 270??ka and produced trachyte domes, flows, and minor pyroclastic deposits until at least 111??ka. The northern part of Guilherme Moniz Caldera is less well exposed than the southern part, but reflects a similar age range. The northwest portion of the caldera was formed sometime after 44??ka. Several well-studied ignimbrites that blanket much of the island likely erupted from Guilherme Moniz Volcano. The Pico Alto Volcanic Center, a tightly spaced cluster of trachyte domes and short flows, is a younger part of Guilherme Moniz Volcano. Stratigraphic studies and our new radiocarbon ages suggest that most of the Pico Alto eruptions occurred during the period from about 9000 to 1000??years BP. Santa Barbara Volcano is the youngest stratovolcano on Terceira, began erupting prior to 29??ka, and has been active historically. ?? 2006.

  11. Mingling of carbonate and silicate magmas under turbulent flow conditions: Evidence from rock textures and mineral chemistry in sub-volcanic carbonatite dykes, Chagatai, Uzbekistan

    NASA Astrophysics Data System (ADS)

    Moore, K. R.; Wall, F.; Divaev, F. K.; Savatenkov, V. M.

    2009-06-01

    The Triassic Chagatai Complex, Uzbekistan, comprises explosive pipes and dykes, dominantly of silicocarbonatite composition, with cross-cutting relationships indicating multi-stage emplacement. Although the dykes have been reported as diamond-bearing, they have not previously undergone detailed investigation in terms of their mineral chemistry or rock texture. The xenolith-rich dykes contain irregularly-shaped microscopic magmatic enclaves of silicate composition within carbonatite magma and corroded microphenocrysts with crystal overgrowths that record synmagmatic geochemical disequilibrium. Quench crystals of apatite and aegirine, and anhedral baryte, which formed after corrosion of apatite and magnetite microphenocrysts but prior to formation of crystal overgrowths and mantles, indicate contemporaneous rapid undercooling. The anhedral baryte formed as a by-product of an oxidising hydrous reaction from Ba-rich biotite and pyrite to chlorite. The rock and microphenocryst textures suggest that mingling between two magmas occurred and a post-mingling mineral assemblage, including baryte, crystallised in a partially hybridised heterogeneous magma. An initial carbonatite mineral assemblage is identified as calcite + magnetite + apatite ± augite ± barium-rich biotite ± melilite ± pyrite. Changes in mineral chemistry of the carbonatite assemblage that are contemporaneous with the disequilibrium reaction textures suggest addition of a hydrous, Na-Si-Al-rich magma, and the mineral assemblage in the magmatic enclaves is similar to that of trachyte dykes in the Chagatai Complex. Using primarily rock textures and mineral chemistry, supported by mass balance calculations and isotope data, the silicate material is interpreted as a hydrous trachyte magma that had assimilated upper crustal material. The trachyte magma was entrained by carbonatite that was rapidly and turbulently ascending through the crust, shortly before emplacement as silicocarbonatite. The interpretation

  12. Volatiles in pantellerite magmas: A case study of the Green Tuff Plinian eruption (Island of Pantelleria, Italy)

    NASA Astrophysics Data System (ADS)

    Lanzo, Giovanni; Landi, Patrizia; Rotolo, Silvio G.

    2013-07-01

    The Green Tuff (GT) Plinian eruption, the largest in magnitude at Pantelleria, erupted 3 to 7 km3 DRE of pantellerite magma and a small volume of trachyte. Fifty-nine anorthoclase-hosted melt inclusions from the two basal pumice members were analyzed by FT-IR spectroscopy in order to assess the pre-eruptive H2O content in the pantellerite melt. Microanalytical methods were used to determine major element, Cl, F and S contents. Melt inclusions and glassy groundmasses have a nearly homogeneous pantelleritic composition (peralkaline index = 1.9-2.2) and variable water contents ranging from 1.4 to as high as 4.2 wt %, i.e. much higher than the 1.4 wt % of earlier published studies. The chlorine content is constant at about 1 wt %. Combined Cl and H2O data were used to estimate a confining pressure of about 50 MPa (depth around 2-3 km) for the GT magma chamber. The chamber was characterized by a compositional zoning with a dominant pantellerite overlying a trachyte magma. Soon after the GT eruption, intra-caldera volcanism was dominated by the eruption of voluminous trachyte lava flows, while pantellerite melt production resumed after about 20 ka with numerous low-volume, mildly explosive (Strombolian) to effusive eruptions. Comparison with data from the literature reveals that, despite the different explosivity, the post-caldera Strombolian eruptions and the GT Plinian eruption were fed by pantelleritic magmas with similar water contents. Chlorine and CO2 contents suggest that the young magma reservoirs feeding the Strombolian to effusive activity were deeper (h ≥ 4.5 km) than the much larger (based on erupted volumes) magma chamber which fed the GT eruption.

  13. Yield strengths of flows on the earth, Mars, and moon. [application of Bingham plastic model to lava flows

    NASA Technical Reports Server (NTRS)

    Moore, H. J.; Arthur, D. W. G.; Schaber, G. G.

    1978-01-01

    Dimensions of flows on the earth, Mars, and moon and their topographic gradients obtained from remote measurements are used to calculate yield strengths with a view to explore the validity of the Bingham plastic model and determine whether there is a relation between yield strengths and silica contents. Other factors are considered such as the vagaries of natural phenomena that might contribute to erroneous interpretations and measurements. Comparison of yield strengths of Martian and lunar flows with terrestrial flows suggests that the Martian and lunar flows are more akin to terrestrial basalts than they are to terrestrial andesites, trachytes, and rhyolites.

  14. Chemical and mineralogic trends within the Timber Mountain--Oasis Valley caldera complex, Nevada: Evidence for Multiple cycles of chemical evolution in a long-lived silicic magma system

    SciTech Connect

    Broxton, D. E.; Warren, R. G.; Byers, F. M.; Scott, R. B.

    1989-05-10

    Rocks of the Miocene and Pliocene Timber Mountain--Oasis Valley(TM-OV) caldera complex in the southwest Nevada volcanic field are dominantlyrhyolites and quartz latites (trachytes). Minor basalticto dacitic rocks were erupted peripherally to the caldera complex and in themoat of the youngest caldera. We divided the petrologic evolution of thecaldera complex into a series of petrochemicalcycles based on systematic changes in rock chemistry, modal petrography, andmineral chemistry as a function of time. Each petrochemical cycle ischaracterized by periods of systematic differentitiation toward more siliceousor rhyolitic compositions. Breaks between cycles are generally abrupt andfollow either times of major ash flow tuff eruption or episodes of relativelymafic (basaltic to dacitic) volcanism.

  15. Intermittent upwelling of asthenosphere beneath the Gregory Rift, Kenya

    SciTech Connect

    Tatsumi, Yoshiyuki Kyoto Univ. ); Kimura, Nobukazu ); Itaya, Tetsumaru ); Koyaguchi, Takehiro ); Suwa, Kanenori )

    1991-06-01

    K-Ar dates and chemical compositions of basalts in the Gregory Rift, Kenya, demonstrate marked secular variation of lava chemistry. Two magmatic cycles characterized by incompatible element relative depletion are recognized; both occurring immediately after the peak of basaltic volcanism and coeval with both trachyte/phonolite volcanism and domal uplift of the region. These cycles may be attributed to increasing degree of partial melting of mantle source material in association with thinning of the lithosphere by thermal erosion through contact with hot upwelling asthenospheric mantle. Cyclic variation in asthenosphere upwelling may be considered an important controlling process in the evolution of the Gregory Rift.

  16. Transition from steady to unsteady Plinian eruption column: the VEI 5, 4.6 ka Fogo A Plinian eruption, São Miguel, Azores

    NASA Astrophysics Data System (ADS)

    Pensa, Alessandra; Cas, Raymond; Giordano, Guido; Porreca, Massimiliano; Wallenstein, Nicolau

    2015-10-01

    The 4.6 ka Fogo A trachytic Plinian eruption on São Miguel Island, Azores, Portugal, is composed by four main phases: 1) Initial phreatomagmatic activity which emplaced thin surge deposits with accretionary lapilli; 2) fully buoyant steady Plinian activity which generated a prominent trachytic basal fall-out deposit; 3) partial collapsing activity, which led to the emplacement of two trachytic-trachydacitic intra-plinian ignimbrites (pink and black ignimbrites) and associated surge deposits, inter-stratified within two fall-out deposits; 4) vent widening and fully collapsing activity which caused the emplacement of the climactic brown trachydacitic ignimbrite. The deposits of these four main phases have been organised into three members FGA1, FGA2 and FGA3, sub-divided into 15 stratigraphic or emplacement units. The fluctuations from a steady to unsteady Plinian eruption column parallel the change in composition of the magma from trachytic to trachydacitic. Although minimal, the change in composition resulted in a marked change in colour of the juvenile clasts from white at the base (first phase) to pink and light-dark grey banded and black (second and third phases) and to dark brown (fourth phase). Density analysis of juvenile clasts revealed that the change in composition did not significantly affect the density of the juvenile pumice pyroclasts, the vesicularity of which increases from 70% to 85% up through the sequence. The pink and the black intra-plinian ignimbrites were emplaced only, within the narrow paleo-valleys of the southern flank with maximum thicknesses of 9 m and 5 m respectively, transitioning onto adjacent topographic highs where thin veneer surge layers were deposited. The fall-out deposits inter-bedded between the ignimbrites lack basal reverse grading, indicating that the eruption column continued to be sustained during the partial collapses generating the intra-plinian ignimbrites. The climactic dark brown ignimbrite was emplaced radially

  17. Geochemistry and mineralogy of the older (> 40 ka) ignimbrites in the Campanian Plain, southern Italy

    NASA Astrophysics Data System (ADS)

    Belkin, Harvey E.; Raia, Federica; Rolandi, Giuseppe; Jackson, John C.; de Vivo, Benedetto

    2010-05-01

    The Campanian Plain in southern Italy has been volcanically active during the last 600 ka. The largest and best known eruption at 39 ka formed the Campanian Ignimbrite (CI), which has the largest volume (~310 km3) and the greatest areal extent. However, significant, but scattered deposits of older ignimbrites underlie the CI and document a long history of trachytic eruptions. We examined the geochemistry and mineralogy of 11 older ignimbrite strata by optical petrography, electron microprobe, scanning electron microscope, X-ray diffraction, and various whole-rock geochemical techniques. Strata at Durazzano (116.1 ka), Moschiano (184.7 ka), Seiano Valley A (245.9 ka), Seiano Valley B (289.6 ka), Taurano 7 (205.6 and 210.4 ka), Taurano 9 (183.8 ka), and Taurano 14 (157.4 ka) have been previously dated by the 40Ar/39Ar technique (Rolandi et al., 2003, Min. & Pet., 79) on hand-picked sanidine. The older ignimbrites are trachytic, but are highly altered with LOI from 8 to 17 wt%. Whole-rock compositions reflect variable element mobility during weathering; TiO2, Al2O3, Fe-oxide, and CaO tend to be enriched relative to average CI composition, whereas Na2O and K2O are depleted. X-ray diffraction identified major chabazite, kaolinite, and illite-smectite alteration products in some samples. The phenocryst mineralogy in all of the strata is typical for trachyte magma and consists of plagioclase (~An80 to ~An40), potassium feldspar (~Or50 to ~Or80), biotite (TiO2 = ~4.6 wt%, BaO = ~0.70 wt%, F = ~0.65 wt%), diopside (~Ca47Mg48Fe5 to ~Ca48Mg34Fe18), titanomagnetite, and uncommon Ca-amphibole. Relatively immobile trace elements Zr, Hf, Nb, and Th display similar abundance, linear trends, and ratios as those measured in the Campanian Ignimbrite: Th/Hf = ~4, Zr/Hf = ~50, and Zr/Nb = ~6. The similarity of trace element systematics and phenocryst mineralogy among the Campanian Ignimbrite and the older ignimbrites suggests that the magmagenesis processes and parental source have

  18. Petrological Features of The Post-Collisional Volcanics on the Izmir-Ankara-Erzincan Suture Zone at Around Almus and Yıldızeli Regions, NE Turkey.

    NASA Astrophysics Data System (ADS)

    Göçmengil, G.; Karacik, Z.; Genç, Ş. C.

    2014-12-01

    Following the closure of the northern Neo-Tethyan ocean, post-collisional magmatism developed both on the Izmir-Ankara-Erzincan suture zone (IAESZ) and the northern (Pontides) and southern (Kırşehir block) continents. Cenozoic volcanics (particularly Middle to Upper Eocene) from the Almus and Yıldızeli regions (NE, Turkey) have poorly known and exposed along the northern and southern part of the IAESZ respectively. In this presentation; we present geological, geochemical and isotopic data of these volcanic rocks to characterize the petrological evolution and petrogenesis on the different tectonic blocks. Almus region, have a composite basement of Mesozoic metamorphic and ophiolitic rocks. Volcanic and sedimentary units of the Middle Eocene covering the basement units have a stratigraphic sequence of: a basal conglomerate; fossiliferous sandstone/shale; epiclastic rocks; auto-brecciated lavas with basalt/basaltic andesite lava flows. All these units are cut by trachyte stocks. Besides, basement of Yıldızeli region consisting of Cretaceous Kırşehir block metamorphics with IAESZ melangé and flysch units. Volcanic and sedimentary units covers the basement are trachyte-trachyandesite lavas; fossiliferous limestone; basalt/basaltic andesite lava flows; pyroclastic units with dacitic lavas. Basalt/basaltic andesites from Almus region are sub-alkaline/mildly alkaline and display middle to high-K character. REE data display relative enrichments in LILE (Sr, K, Rb, Ba, Th) together with negative anomalies of Nb, Ta and P. Trachyte stocks are shoshonitic and display LILE enrichment together with negative patters of Nb, Ta, P and Eu. Besides, trachyte-trachyandesite and basalt/basaltic andesite from Yıldızeli region are sub-alkaline/mildly alkaline with middle-K to shoshonitic character. REE patterns show enrichments in LILE and depletion of Nb, Ta, P and Ti for both rock suites. The 87Sr/86Sr and 143Nd/144Nd ratios of volcanic rocks from both regions, range from 0

  19. Granodiorite and alkaline suite at Gale crater: continental crust on early Mars

    NASA Astrophysics Data System (ADS)

    Sautter, V.; Toplis, M. J.; Cousin, A.; Fabre, C.; Wiens, R. C.; Mangold, N.; Forni, O.; Gasnault, O.; Pinet, P.; Rapin, W.; Fisk, M.; Le Deit, L.; Meslin, P.-Y.; Maurice, S.; Lasue, J.; Stolper, E.; Beck, P.; Wray, J.; Bridges, J. C.; Le Mouelic, S.

    2015-10-01

    The Curiosity rover landed at Gale, an early Hesperian age crater formed within Noachian terrains on Mars. The rover encountered a great variety of felsic igneous float rocks ranging from granodiorite to trachy andesite and trachyte during the first part of the traverse up to sol 550. They are the first in-situ evidence of low density early Noachian crust on Mars, sampled by Peace Vallis river cross-cutting the crater wall over a 2-3km thick vertical section, below the basaltic regolite.

  20. Stable isotope analyses of the peralkaline volcanics Gregory Rift Valley, Kenya

    NASA Technical Reports Server (NTRS)

    Black, S.; Macdonald, R.; Fallick, A. E.; Kelly, M.

    1993-01-01

    Delta O-18 analyses of the Naivasha rhyolites, basalts, Menengai trachytes and the Yatta phonolite are presented together with D/H analyses of the Naivasha rhyolites. Delta O-18 results vary from 5.7 to 8.9 per mill which is within the reported range of delta O-18 analyses for continental volcanics. Closure temperatures calulated from the basalts and rhyolites show equilibration to be at magmatic temperatures. D/H values range from -40 to -148 per mill indicating that the rhyolites have undergone large scale degasssing.

  1. New radiometric age of volcanic rocks in the central Eritrean plateau (from Asmara to Adi Quala): Considerations on stratigraphy and correlations

    NASA Astrophysics Data System (ADS)

    Zanettin, B.; Bellieni, G.; Visentin, E. Justin

    2006-06-01

    New radiometric data have recently been acquired on basalt and rhyolite sampled at various levels of the volcanic sequence occurring in the central Eritrean plateau, confirming the stratigraphic reconstruction suggested in a previous paper [Zanettin, B., Bellieni, G., Justin Visentin, E., Haile, T., 1999. The volcanic rocks of the Eritrean plateau: stratigraphy and evolution. Acta Volcanologica 11(1), 183-193]. New considerations indicate the tholeiitic, not alkaline, nature of the Asmara basalt. Doubts about the relative age of the Aiba/Alaji and Asmara basalts have now been clarified: they are, at least partly, coeval (about 30 Ma old). The Serae rhyolite intercalated in the Adi Ugri basalt turns out to be about 24 Ma old, like the more abundant ignimbrite outcropping in the Senafe area, of which it is the westernmost extension. Its age confirms that it does not correspond to the trachyte intercalated in the Oligocene stratoid basalt of the Adwa-Axum area (where the Adi Ugri basalt probably also occurs, intercalated with the Serae trachyte and rhyolite). The upper part of the Adi Ugri basalt is 22 Ma old (an age consistent with the finding of a Deinotherium tooth). The radiometric age of these rocks also confirms already indicated correlations between Eritrean and Ethiopian volcanic formations.

  2. Age and petrology of the Tertiary As Sarat volcanic field, southwestern Saudi Arabia

    USGS Publications Warehouse

    du Bray, E.A.; Stoeser, D.B.; McKee, E.H.

    1991-01-01

    Harrat As Sarat forms the second smallest and southernmost of the basalt fields of western Saudi Arabia and is part of a voluminous Red Sea rift-related continental alkali basalt province. The rocks of the As Sarat were emplaced during the first stage of Red Sea rifting and represent the northernmost extension of the Tertiary Trap Series volcanics that occur mainly in the Yemen Arab Republic and Ethiopia. The field consists of up to 580 m of basalt flows, that are intruded by basaltic plugs, necks, minor dikes, and highly evolved peralkaline trachyte intrusions. K-Ar ages indicate that the As Sarat field formed between 31 and 22 Ma and contains an eruption hiatus of one million years that began about 25 Ma ago. Pre-hiatus flows are primarily hypersthene normative intersertal subalkaline basalt, whereas the majority of post-hiatus flows are nepheline normative alkali basalt and hawaiite with trachytic textures. Normative compositions of the basalts are consistent with their genesis by partial melting at varying depths. Trace element abundances in the basalt indicate that varying degrees of partial melting and fractional crystallization (or crystal accumulation) had major and minor roles, respectively, in development of compositional variation in these rocks. Modeling indicates that the pre-hiatus subalkaline basalts represent 8-10 percent mantle melting at depths of about 70 km and the post-hiatus alkali basalts represent 4-9 percent mantle melting at depths greater than 70 km. ?? 1991.

  3. Mineralogy, geochemistry and petrogenesis of the recent magmatic formations from Mbengwi, a continental sector of the Cameroon Volcanic Line (CVL), Central Africa

    NASA Astrophysics Data System (ADS)

    Mbassa, Benoît Joseph; Njonfang, Emmanuel; Benoit, Mathieu; Kamgang, Pierre; Grégoire, Michel; Duchene, Stephanie; Brunet, Pierre; Ateba, Bekoa; Tchoua, Félix M.

    2012-11-01

    The Mbengwi recent magmatic formations consist of volcanics and syenites belonging to the same magmatic episode. Lavas form a bimodal basanite-rhyolite alkaline series with a gap between 50 and 62 wt.% SiO2. Mafic lavas (basanite-hawaiite) are sodic while felsic rocks (trachyte-rhyolite-syenites) are sodi-potassic, slightly metaluminous to peralkaline. The geochemical and isotopic characteristics (0.7031 < (87Sr/86Sr)initial < 0.7043; 1.03 < ɛNdi < 5.17) of these rocks are similar to those of other rocks from the CVL. The main differentiation process is fractional crystallization with two trends of fractionation. Their Rb/Sr isochron age of 28.2 Ma, almost similar to 27.40 ± 0.6 Ma K/Ar age obtained in a trachyte from neighboring Bamenda Mountains system, precludes any local age migration of an hypothetic hotspot. Mafic lavas have OIB features displaying an isotopic signature similar to that of HIMU mantle source different from FOZO known as source of most parental magmas along the CVL.

  4. Complex zoning of clinopyroxenes in the lavas of vulsini, latium, Italy: Evidence for magma mixing

    NASA Astrophysics Data System (ADS)

    Barton, Michael; Varekamp, Johan C.; Van Bergen, Manfred J.

    1982-12-01

    Microprobe analyses of pyroxene phenocrysts occurring in two tephritic leucitites, two leucite phonolites and one trachyte from Vulsini are reported. Three compositionally distinct types of pyroxene occur in the tephritic leucitites: (a) salite, forming resorbed cores in pyroxene phenocrysts; (b) diopside, forming euhedral-anhedral cores in pyroxene phenocrysts: and (c) pyroxene of intermediate composition to (a) and (b), which occurs as mantels around the phenocryst cores, as separate phenocrysts and as microphenocrysts. The pyroxenes in the leucite phonolites and in the trachyte do not show discontinuous zoning such as that shown by pyroxenes in the tephritic leucitites. The compositional characteristics of the latter are most easily explained by the mixing of two magmas, both tephritic leucitite in composition, one of which was relatively evolved (Fe-rich) and carried salite phenocrysts, the other relatively primitive (Mg-rich) which carried diopside phenocrysts. A review of petrographic and geochemical evidence indicates that magma mixing may have been an important process, in addition to fractional crystallization, at Vulsini and at other central Italian volcanic centres. The implications for theories about the origin of potassium-rich magmas in western Italy are briefly discussed.

  5. Occurrence of an unknown Atlantic eruption in the Chaîne des Puys volcanic field (Massif Central, France)

    NASA Astrophysics Data System (ADS)

    Jouannic, G.; Walter-Simonnet, A. V.; Bossuet, G.; Cubizolle, H.; Boivin, P.; Devidal, J. L.; Oberlin, C.

    2014-08-01

    A volcanic ash layer, called MF1, was recently identified in Holocene sediments from the Gourgon and Molhiac peat bogs (Monts du Forez, French Massif Central). This ash layer consists of colorless shards with a heterogeneous trachytic to rhyolitic composition. The trace elements analyzed by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) attest to a local origin. Radiocarbon dating of peat samples taken within and below the ash layer indicates the best age at 6339 ± 61 cal yr BP, i.e. an age contemporaneous with the volcanic activity of Montchal, Montcineyre and Pavin volcanoes from the Chaîne des Puys volcanic field. These volcanoes are characterized by basaltic and trachytic products, thus the rhyolitic composition of MF1 tephra suggests that it is likely originated from an unknown eruption. These results again confirm the interest of studying the distal volcanic ash fallouts in order to establish or specify records of past eruptions of volcanic fields. Identification of this new tephra layer also provides an additional tephrochronological marker for Eastern French Massif Central.

  6. Evolution of anorthoclase phonolite, Mt. Erebus, Antarctica

    SciTech Connect

    Moore, J.A.; Kyle, P.R.

    1985-01-01

    Over the last 1 m.y. Mt. Erebus (3794 m) has erupted mainly anorthoclase phonolite (AP) and lesser trachyte, kaersutite phonolite and intermediate differentiates. An active convecting AP lava lake, identical in composition to the older lavas, existed from 1972 until late 1984. Most of the rocks define a strongly undersaturated continuous sodic differentiation series, composed of basanite, Ne-hawaiite, Ne-mugearite, Ne-benmoreite and AP. The main phenocryst phases and their ranges are: olivine (Fo 81-43), clinopyroxene (Wo 50-44, En 42-24, Fs 11-30), opaque oxides (Usp 52-79) and feldspar. Major, trace and REE analyses exhibit smooth trends on variation diagrams. REE are strongly LREE enriched and increase from La/sub N/=220 in the basanites to 400 in AP. There are no significant Eu anomalies. Published isotopic data show derivation of the basanite parental magmas from a depleted (/sup 87/Sr//sup 86/Sr=0.703) heterogeneous mantle source similar to that for oceanic island basalts. Crustal contamination is insignificant except in the trachytes. Evolution of the Erebus lineage by fractional crystallization of the above phases plus apatite is supported by mass balance models. Differentiation probably occurred in larger, hotter and lower P/sub H20/ magma chambers compared to the basanite to kaersutite phonolite DVDP lineage (Kyle, 1981) of the neighboring Hut Point Peninsula. Mt. Erebus may mark the site of a major mantle upwelling.

  7. Geochemical fingerprint of the primary magma composition in the marine tephras originated from the Baegdusan and Ulleung volcanoes

    NASA Astrophysics Data System (ADS)

    Lim, Chungwan; Kim, Seonyoung; Lee, Changyeol

    2014-12-01

    The intraplate Baegdusan (Changbai) and Ulleung volcanoes located on the border of China, North Korea, and East/Japan Sea, respectively, have been explained by appeals to both hotspots and asthenospheric mantle upwelling (wet plume) caused by the stagnant Pacific plate. To understand the origin of the Baegdusan and Ulleung volcanism, we performed geochemical analyses on the tephra deposits in the East/Japan Sea basins originating from the Baegdusan and Ulleung volcanoes. The volcanic glass in the tephra from the Baegdusan and Ulleung volcanoes ranged from alkaline trachyte to peralkaline rhyolite and from phonolite to trachyte, respectively. The tephra from the two intraplate volcanoes showed highly enriched incompatible elements, such as Tb, Nb, Hf, and Ta, distinct from those of the ordinary arc volcanoes of the Japanese islands. The straddle distribution of the Th/Yb and Ta/Yb ratios of the tephra deposits from the Baegdusan volcano may originate from the alkali basaltic magma resulting from mixing between the wet plume from the stagnant Pacific plate in the transition zone and the overlying shallow asthenospheric mantle. In contrast, the deposits from the Ulleung volcano show a minor contribution of the stagnant slab to the basaltic magma, implying either partial melting of a more enriched mantle, smaller degrees of partial melting of a garnet-bearing mantle source, or a combination of both processes as the magma genesis. Our study indicated that the Baegdusan and Ulleung volcanoes have different magma sources and evolutionary histories.

  8. A comparison of binary and multiclass support vector machine models for volcanic lithology estimation using geophysical log data from Liaohe Basin, China

    NASA Astrophysics Data System (ADS)

    Mou, Dan; Wang, Zhu-Wen

    2016-05-01

    Lithology estimation of rocks, especially volcanic lithology, is one of the major goals of geophysical exploration. In this paper, we propose the use of binary and multiclass support vector machine models with geophysical log data to estimate the volcanic lithology of the Liaohe Basin, China. Using neutron (CNL), density (DEN), acoustic (AC), deep lateral resistivity (RLLD), and gamma-ray (GR) log data from 40 wells (a total of 1200 log data points) in the Liaohe Basin, China, we first construct the binary support vector machine model to classify volcanic rock and non-volcanic rock. Then, we expand the binary model to a multiclass model using the approach of directed acyclic graphs, and construct multiclass models to classify six types of volcanic rocks: basalt, non-compacted basalt, trachyte, non-compacted trachyte, gabbro and diabase. To assess the accuracy of these two models, we compare their predictions with core data from four wells (at 800 different depth points in total). Results indicate that the accuracy of the binary and multiclass models are 98.4% and 87%, respectively, demonstrating that binary and multiclass support vector machine models are effective methods for classifying volcanic lithology.

  9. A chemostratigraphic study of the Campanian Ignimbrite eruption (Campi Flegrei, Italy): Insights on magma chamber withdrawal and deposit accumulation as revealed by compositionally zoned stratigraphic and facies framework

    NASA Astrophysics Data System (ADS)

    Fedele, L.; Scarpati, C.; Sparice, D.; Perrotta, A.; Laiena, F.

    2016-09-01

    Petrochemical analyses of juvenile samples from twenty stratigraphic sections of the Campanian Ignimbrite medial deposits, located from 30 to 79 km from the vent, are presented here. Sampling has accurately followed a well-defined stratigraphic framework and the new component facies scheme. The Campanian Ignimbrite succession is formed by a basal plinian pumice fall deposit, overlain by a complex architecture of pyroclastic density current deposits emplaced from a single sustained pyroclastic density current through a mechanism of vertical and lateral accretion. The deposit is broadly zoned, from more evolved trachyte at its base to less evolved trachyte at its top, and is similarly less evolved with increasing distance from the area of emission. Irregular chemical trends are locally observed and interpreted to represent only a limited, "patchy" record of the entire vertical geochemical trend. The petrochemical variation observed horizontally was ascribed to changes in the flow dynamics and interaction between the advancing flow and the underlying topography. The results of this study were used to propose a unified volcanological-petrological model for the Campanian Ignimbrite eruption, taking into account the emplacement of both the proximal (i.e., the "Breccia Museo" formation) and medial deposits.

  10. Petrologic evolution of divergent peralkaline magmas from the Silent Canyon caldera complex, southwestern Nevada volcanic field

    USGS Publications Warehouse

    Sawyer, D.A.; Sargent, K.A.

    1989-01-01

    The Silent Canyon volcanic center consists of a buried Miocene peralkaline caldera complex and outlying peralkaline lava domes. Two widespread ash flow sheets, the Tub Spring and overlying Grouse Canyon members of the Miocene Belted Range Tuff, were erupted from the caldera complex and have volumes of 60-100 km3 and 200 km3, respectively. Eruption of the ash flows was preceded by widespread extrusion of precaldera comendite domes and was followed by extrusion of postcollapse peralkaline lavas and tuffs within and outside the caldera complex. Lava flows and tuffs were also deposited between the two major ash flow sheets. Rocks of the Silent Canyon center vary significantly in silica content and peralkalinity. Weakly peralkaline silicic comendites (PI 1.0-1.1) are the most abundant precaldera lavas. Postcollapse lavas range from trachyte to silicic comendite; some have anomalous light rare earth element (LREE) enrichments. Silent Canyon rocks follow a common petrologic evolution from trachyte to low-silica comendite; above 73% SiO2, compositions of the moderately peralkaline comendites diverge from those of the weakly peralkaline silicic comendites. The development of divergent peralkaline magmas, toward both pantelleritic and weakly peralkaline compositions, is unusual in a single volcanic center. -from Authors

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

    NASA Technical Reports Server (NTRS)

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

    1993-01-01

    Strong mixing trends on a (Th-230/Th-232) versus Th diagram show that the basalts are mixed magmas which have undergone interaction with the crust. Instantaneous Th/U ratios are less than time integrated ones but these exceed the Th/U ratios in the MORB and OIB sources. This indicates that the mantle may have undergone some metasomatic fluxing, crustal contamination of the basalts will also enhance these ratios. Early activity on the Akira plain is represented by early basalts and hawaiites. The early basalt samples are known to predate the earliest comendites. The most recent phase of activity is represented by another cinder cone 40-50 m high being feldspar and clinopyroxene phyric. Inclusions which occur in the comendites vary in size and distribution. The largest and most porphyritic are the trachytes (up to 40 cm) with alkali feldspar phases up to 6 mm and small pyroxenes in the ground mass. The second set of inclusions are smaller (up to 10 cm) and are largely aphyric. The distribution of the inclusions are not uniform, the Broad Acres (C5) lavas contain 2-5 percent. The size of the inclusions decrease from south to north, as does the abundance of the trachytic inclusions. The major element variations in the Naivasha basalts, hawaiites and magmatic inclusions are discussed.

  12. Origin of Large Felsic Rock Volumes in Marie Byrd Land Volcanoes:Possible Influence of Tectonic Environment and Continental Lithospheric Structure

    NASA Astrophysics Data System (ADS)

    Lemasurier, W. E.

    2002-12-01

    The Marie Byrd Land volcanic province, in the West Antarctic rift system, includes several volcanoes with large volumes of peralkaline trachyte, phonolite, and rhyolite. The roughly 780 cu. km. of trachyte at Mt. Takahe volcano, for example, is about twice the volume of Mt. Shasta, the largest Cascade volcano.Geochemical data provide evidence for only minor amounts of crustal contamination, affecting just a small proportion of the felsic rocks, hence crustal assimilation is unlikely to have supplemented the volume of felsic rocks. Most appear to have been derived entirely by fractional crystallization of basaltic magma; but the volumes of felsic rock in Marie Byrd Land volcanoes is much larger than is found in their counterparts in oceanic islands, produced by essentially the same process, suggesting that continental structure and/or tectonic environment may have played a role in felsic rock evolution. The absence of Antarctic plate motion, over at least the past 26 m.y., seems an obvious factor. This has apparently led to repeated replenishment of the same magma chambers, rather than formation of linear chains of volcanoes. In addition, modeling results suggest that a multi-level, polybaric plumbing system within the lithosphere has acted as a filter, delaying the rise of at least some large volume magma batches, thereby prolonging and refining the fractionation process.

  13. Sm-Nd and Rb-Sr isotopic systematics of the Pea Ridge Fe-P deposit and related rocks, southeast Missouri

    SciTech Connect

    Marikos, M.A.; Barton, M.D. . Dept. of Geosciences)

    1993-03-01

    Pea ridge is a discordant Middle Proterozoic Fe-P deposit hosted in rhyolite tuffs and flows of the 1.4--1.5 Ga St. Francois terrane. Host rocks and the deposit are cut by basalt and aplite/pegmatite dikes. The deposit overlies a blind pluton which is partially surrounded by a trachytic ring complex. In the deposit, which is mined for Fe, early Qtz+Amph+Mag+Ap rock is cut by Mag+Ap+Qtz rock. Subsequently, portions of the deposit and host rocks were brecciated, oxidized and silicified to produce a complex suite of rocks enriched in Hem+Qtz+Ksp+Mu. Late breccia pipes/dikes cut the complex and were mineralized with Bar+Ksp+Flu+Chl+Cc+REE-phosphates. Sm/Nd and Rb/Sr isotopic systematics have been studied to: (1) constrain source(s) of igneous rocks and deposit components, (2) refine ages of magmatism, mineralization, and later hydrothermal activity, (3) begin regional comparison of isotopic systematics in SE Missouri Fe deposits, and (4) complement ongoing Missouri DGLS/USGS studies. Fourteen combined Sm-Nd and Rb-Sr analyses were done on materials including two host rhyolites, two nearby trachytes, two gneiss samples representing plausible basement, two intramineral dikes, and six samples of mineralization.

  14. Isotopic evidence for the origin of Cenozoic volcanic rocks in the Pinacate volcanic field, northwestern Mexico

    NASA Astrophysics Data System (ADS)

    Lynch, D. J.; Musselman, T. E.; Gutmann, J. T.; Patchett, P. J.

    1993-02-01

    Six volcanic rocks, reconnaissance samples representing most of the temporal and compositional variation in the Pinacate volcanic field of Sonora and Arizona, are characterized for major element and NdSr isotopic compositions. The samples consist of basanite through trachyte of an early shield volcano, and alkali basalts and a tholeiite from later craters and cinder cones. With the exception of the trachyte sample, which has increased 87Sr/ 86Sr due to crustal effects, all 87Sr/ 86Sr values fall between 0.70312 and 0.70342, while ɛNd values are all between + 5.0 and + 5.7. Clinopyroxene in a rare spinel-lherzolite nodule derived from the uppermost mantle beneath the field has 87Sr/ 86Sr of 0.70320 but ɛNd of + 8.8, three ɛNd units higher than the volcanic rocks. Both the volcanic rocks and the nodule record the presence of asthenospheric, rather than enriched lithospheric mantle beneath Pinacate. This is consistent with one or both of (a) proximity of Pinacate to the Gulf of California spreading center and (b) presence of similar asthenospheric mantle signatures in volcanic rocks over a wide contiguous area of the southwestern USA. We consider the comparison to other southwestern USA magma sources as the more relevant alternative, although a definite conclusion is not possible at this stage.

  15. Radiating columnar joints in Gyeongju, Korea as a educational site

    NASA Astrophysics Data System (ADS)

    Woo, H.; Kim, J. H.; Jang, Y. D.

    2015-12-01

    Gyeongju is located in the central eastern part of South Korea. There are various directional columnar joint sets in Tertiary trachytic basalt formation along the shore. In particular, rare radiating columnar joints occur in this area. Columnar joints are parallel, prismatic columns that are formed as a result of contraction during the rapid cooling of lava flow, forming a three dimensional fracture network. In general, the radius and direction of the rock column represent the cooling rate and surface respectively. Radiating direction of columns here indicates that dome- or lobe-shaped lava was cooled from its surface to the core during the viscous lava flow. The fact that the trachytic textures of plagioclase laths are indistinct suggests that the radiating columnar joints are equivalent to the frontal end of the lava lobes. This area is currently has a shore trail course, which is being developed into a picturesque educational park. There are corresponding information boards on the trail near each type of columnar joints to explain not only the forming process and geological mechanisms but the importance of nature conservation to visitors, especially students. A variety of educational materials and educational programs linked to regular school curriculum are also being developed.

  16. Evolution of melts of the Changbaishan Tianchi volcano (China-North Korea) as a model of ore-magmatic system formation: Data from melt inclusions studies

    NASA Astrophysics Data System (ADS)

    Andreeva, O. A.; Andreeva, I. A.; Yarmolyuk, V. V.; Borisovskiy, S. E.

    2016-01-01

    The composition and evolution of the melts of trachytes from the volcano were studied based on examining the inclusions of mineral-forming media by means of X-ray and ion microanalysis. A correlation was shown between the degree of enrichment of these melts in rare elements and the processes of magmatic differentiation. It was found that trachytes of the volcano were generated in highly differentiated alkaline melts enriched in Hf, Nb, Zr, Ta, U, Th, Rb, Y, and REEs under 1020-1060°C. The evolution of melts was determined by the processes of crystal fractionation. The main volatile components in the melts are water, fluorine, and chlorine with the concentrations of 0.1-0.5, 0.2-0.5, and 0.2-0.3 wt %, respectively. The melt crystallization was accompanied by degassing caused by the decrease in the outer pressure. The low concentrations of water and fluorine represent the melt composition by these components exclusively at the time of the melt movement towards the Earth's surface just before the eruption.

  17. Stratigraphy, geochronology and evolution of the Mt. Melbourne volcanic field (North Victoria Land, Antarctica)

    NASA Astrophysics Data System (ADS)

    Giordano, Guido; Lucci, Federico

    2016-04-01

    Mt. Melbourne (2,732 ma.s.l.) is a large quiescent stratovolcano located in Northern Victoria Land (Antarctica) and is one of a handful of volcanoes on the Antarctic plate with the potential for large-scale explosive eruptions. The early, Lower Pleistocene, volcanism was largely alkali basaltic to hawaiitic in composition and monogenetic in style, producing tens of small scoria cones and lava flows scattered over a wide area across the Transantarctic Mountains (Random Hills Period). During the Middle Pleistocene, volcanic activity focused to the area of the Mt. Melbourne stratovolcano, where several monogenetic centres show the transition from early subglacial/ subaqueous conditions to emergent subaerial conditions (Shield Nunatak Period). The oldest exposed deposit associated with the early activity of the Mt. Melbourne stratovolcano (Mt. Melbourne Period) is a trachytic subaerial ignimbrite dated at 123.6±6.0 ka, which reflects the establishment of a crustal magma chamber. Above the ignimbrite a succession of alkali basaltic, hawaiitic, and subordinate benmoreitic lavas and scoria cones is exposed, dated at 90.7±19.0 ka. The most recent deposits are exposed at the top of Mt. Melbourne, where the crater rim is composed of trachytic to rhyolitic pumice fall deposits, which are also extensively dispersed around the volcano, originated from Plinian-scale eruptions. The most recent explosive deposit proved difficult to date accurately because very low quantities of radiogenic 40Ar were released, resulting in imprecise plateau ages of 50±70 and 35±22 ka.

  18. The VORISA Project: An Integrated Approach to Assessing Volcanic Hazard and Risk in the Kingdom of Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Lindsay, J. M.; Moufti, R.

    2013-12-01

    The Kingdom of Saudi Arabia has numerous large monogenetic volcanic fields, known locally as 'Harrat'. The largest of these, Harrat Rahat, produced a basaltic fissure eruption in 1256 AD with lava flows travelling within 20 km of the Islamic holy city Al-Madinah. With over 900 visible basaltic and trachytic vents and periodic seismic swarms indicating stalled eruptions, an understanding of the risk of future eruptions in this volcanic field is vital. To systematically address this need we developed the Volcanic Risk in Saudi Arabia (VORISA) Project, a 3-year, multi-disciplinary international research collaboration that integrates geological, geophysical, hazard and risk studies. Detailed mapping and geochemical studies are being combined with new and existing age determinations to determine the style and sequence of events during past basaltic and trachytic eruptions. Data from gravity and magnetotelluric surveys are being integrated with microearthquake data from an 8-station borehole seismic research array to geophysically characterise the structure and nature of the crust, and thus constrain possible physical controls on magma propagation. All available data are being synthesised in hazard models to determine patterns in eruption frequency, magnitude, and style of past activity, as well as the probable location and style of a future event. Combined with geospatial vulnerability data, these hazard models, which include a reconstruction of the 1256 AD eruption, enable us to calculate and communicate volcanic risk to the city of Al-Madinah.

  19. Unravelling carbonatite-silicate magma interaction dynamics: A case study from the Velay province (Massif Central, France)

    NASA Astrophysics Data System (ADS)

    Valentini, Luca; Moore, Kathryn Ruth; Chazot, Gilles

    2010-04-01

    Mechanisms of interaction between a carbonatite magma and a silicate magma are inferred on the basis of the textural and geochemical features of a carbonate-bearing tuff from the Massif Central, characterized by the presence of carbonatite globules within a silicate glass matrix. A two-fluid dynamical model is proposed in order to constrain the formation of the carbonatite globules that underwent no geochemical re-equilibration with the trachyte magma due to rapid thermal re-equilibration and crystallization of dolomite. It is suggested that a viscous fingering instability occurred simultaneously with the injection of a carbonatite magma into a chamber filled with a more viscous trachytic magma. Instantaneous fragmentation of the fingers, induced by the action of gradient stresses, resulted in the formation of a magmatic emulsion. The replenishment event triggered immediate evacuation of the magma chamber such that interaction between the two magmas occurred over a short time scale and large-scale mixing was prevented. The exsolution of volatiles, perhaps as a consequence of the recharge event shortly before the eruption, drove a metasomatic event that produced the subtle chemical variations observed at the interface between carbonatite globules and silicate matrix.

  20. Clinopyroxene-liquid thermometers and barometers specific to alkaline differentiated magmas

    NASA Astrophysics Data System (ADS)

    Masotta, M.; Mollo, S.; Freda, C.; Gaeta, M.; Moore, G.

    2013-12-01

    We present new thermometers and barometers based on clinopyroxene-liquid equilibria specific to alkaline differentiated magmas. The new models were calibrated through the regression analyses of experimental datasets obtained by merging phase equilibria experiments from the literature with new experiments performed by using trachytic and phonolitic starting compositions. The regression strategy was twofold: (1) we have tested previous thermometric and barometric equations and recalibrated these models using the new datasets; (2) we have calibrated a new thermometer and a new barometer including only regression parameters that closely describe the compositional variability of the datasets. The new models yield more precise estimates than previous thermometers and barometers when used to predict temperatures and pressures of alkaline differentiated magmas. We have tested the reliability of the new equations by using clinopyroxene-liquid pairs from trachytes and phonolites erupted during major explosive eruptions at the Phlegrean Fields and Mt. Vesuvius (central Italy). The test yielded crystallization conditions comparable to those determined by means of melt and fluid inclusion analyses and phase equilibria studies; this validates the use of the proposed models for precise estimates of crystallization temperatures and pressures in differentiated alkaline magmas. Because these magmas feed some of the most voluminous, explosive, and threatening volcanic eruptions in the world, a better understanding of the environmental conditions of their reservoirs is mandatory and this is now possible with the new models provided here.

  1. Trace element distribution coefficients in alkaline series. [Titanites; bitite

    SciTech Connect

    Lemarchand, F.; Villemant, B.; Calas, G.

    1987-05-01

    Mineral/groundmass partition coefficients for U, Th, Zr, Hf, Ta, Rb, REE, Co and Sc have been systematically measured in olivine, clinopyroxene, amphibole, biotite, Ti-magnetites, titanite, zircon and feldspars, in basaltic to trachytic lavas from alkaline series (Velay, Chaine des Puys: Massif Central, France and Fayal: Azores). Average partition coefficients are defined within the experimental uncertainty for limited compositional ranges (basalt-hawaiite, mugearites, benmoreite-trachyte), and are useful for trace element modelling. The new results for U, Th, Ta, Zr and Hf partition coefficients show contrasting behaviour. They can thus be used as ''key elements'' for identifying fractionating mineral phases in differentiation processes (e.g. Ta and Th for amphibole and mica). Partition coefficient may be calculated using the two-lattice model suggested by NIELSEN (1985). Such values show a considerably reduced chemical dependence in natural systems, relative to weight per cent D values. The residual variations may be accounted for by temperature or volatile influence. This calculation greatly enhances modelling possibilities using trace elements for comparing differentiation series as well as for predicting the behaviour of elements during magmatic differentiation.

  2. Evidence of Middle Jurassic magmatism within the Seychelles microcontinent: Implications for the breakup of Gondwana

    NASA Astrophysics Data System (ADS)

    Shellnutt, J. G.; Lee, T.-Y.; Chiu, H.-Y.; Lee, Y.-H.; Wong, J.

    2015-12-01

    The breakup of East and West Gondwana occurred during the Jurassic, but the exact timing is uncertain due to the limited exposure of rocks suitable for radioisotopic dating. Trachytic rocks from Silhouette Island, Seychelles, yielded a range of zircon ages from Paleoproterozoic to Cenozoic. The 206Pb/238U age of the trachyte is 64.9 ± 1.6 Ma (Danian) but the majority of zircons yielded an age of 163.8 ± 1.8 Ma (Callovian) with a small subset yielding an age of 147.7 ± 4.5 Ma (Tithonian). The Hf isotopes of the Callovian (ɛHf(t) = +4.1 to +13.4) and Danian (ɛHf(t) = +1.9 to +7.1) zircons indicate that they were derived from moderately depleted mantle sources whereas the Tithonian zircons (ɛHf(t) = -7.0 to -7.3) were derived from an enriched source. The identification of middle Jurassic zircons indicates that rifting and magmatism were likely contemporaneous during the initial separation of East and West Gondwana.

  3. Heat capacity, configurational heat capacity and fragility of hydrous magmas

    NASA Astrophysics Data System (ADS)

    Di Genova, D.; Romano, C.; Giordano, D.; Alletti, M.

    2014-10-01

    The glassy and liquid heat capacities of four series of dry and hydrous natural glasses and magma as a function of temperature and water content (up to 19.9 mol%) were investigated using differential scanning calorimetry (DSC). The analyzed compositions are basalt, latite, trachyte and pantellerite. The results of this study indicate that the measured heat capacity of glasses (Cpg) is a linear function of composition and is well reproduced by the empirical model of Richet (1987). For the investigated glasses, the partial molar heat capacity of water can be considered as independent of composition, in agreement with Bouhifd et al. (2006). For hydrous liquids, the heat capacity (Cpliq) decreases nonlinearly with increasing water content. Previously published models, combined with the partial molar heat capacity of water from the literature, are not able to reproduce our experimental data in a satisfactory way. We estimated the partial molar heat capacity of water (CpH2O) in hydrous magma over a broad compositional range. The proposed value is 41 ± 3 J mol-1 K-1. Water strongly affects the configurational heat capacity at the glass transition temperature [Cpconf (Tg)]. An increases of Cpconf (Tg) with water content was measured for the polymerized liquids (trachyte and pantellerite), while the opposite behavior was observed for the most depolymerized liquids (basalt and latite). Structural and rheological implications of this behavior are discussed in light of the presented results.

  4. Geochemistry and Ar/Ar dating of upper pleistocene volcanic rocks from Kerguelen islands (Indian Ocean)

    NASA Astrophysics Data System (ADS)

    Ethien, R.; Feraud, G.; Gerbe, M. C.; Cottin, J. Y.; O'Reilly, S. Y.; Giret, A.

    2003-04-01

    The Kerguelen islands archipelago (6500 Km^2) is the third largest oceanic island in the world, after Island and Hawaï. It is located upon the Kerguelen plateau, which is the second Large Igneous Province (LIP) after Ontong-Java. This oceanic plateau consist of an accumulation of flood basalts, related to the long-lived ˜119 Ma Kerguelen plume. The flood basalts (˜29-24 Ma; Nicolaysen et al., 2000) represent 85% of the rocks of Kerguelen. The Rallier-du-Baty (R.d.B.) peninsula, which forms the southwestern part of the Kerguelen archipelago, is mostly made of alkaline rocks constituting two well-defined ring-complexes. The northern ring-complex consists of a succession of seven discrete syenitic ring-dykes, one later caldera volcano and a more recent volcanic complex. The volcanism is bimodal with trachy-basalts and trachy-andesites, with true scarce basalts constituting the mafic lavas and trachytes and rhyolites constituting the felsic lavas. The felsic magmas were erupted as abundant pyroclastic deposits and lava flows. The mineralogy of those volcanic rocks is typical of an alcaline series, with the presence of K-feldspars (sanidines) in the most differentiated volcanic rocks. The evolution from trachyte to rhyolite seems to be controlled by crystal fractionation, with some trace element distribution and Sr isotopic ratios largely disturbed by open-system processes such as assimilation of hydrothermally altered crust and interaction with seawater. The studies of the oxygen isotopes confirm this hypothesis. Indeed, the high values of δ18O for the rhyolites (δ18O= 10.3 and 12.4) could be interpreted by an alteration by fluids at low temperatures. The Nd isotopic ratio are typical of mantellic values, with no significant variations. Whereas some units of the northern R.d.B. plutonic complex yield a narrow range of K/Ar ages on bulk rocks, from 6.2 ± 0.2 Ma to 4.9 ± 0.2 Ma (Dosso and al., 1979), the formation of a discrete caldera centered on the "Table de l

  5. Geology of the peralkaline volcano at Pantelleria, Strait of Sicily

    USGS Publications Warehouse

    Mahood, G.A.; Hildreth, W.

    1986-01-01

    Situated in a submerged continental rift, Pantelleria is a volcanic island with a subaerial eruptive history longer than 300 Ka. Its eruptive behavior, edifice morphologies, and complex, multiunit geologic history are representative of strongly peralkaline centers. It is dominated by the 6-km-wide Cinque Denti caldera, which formed ca. 45 Ka ago during eruption of the Green Tuff, a strongly rheomorphic unit zoned from pantellerite to trachyte and consisting of falls, surges, and pyroclastic flows. Soon after collapse, trachyte lava flows from an intracaldera central vent built a broad cone that compensated isostatically for the volume of the caldera and nearly filled it. Progressive chemical evolution of the chamber between 45 and 18 Ka ago is recorded in the increasing peralkalinity of the youngest lava of the intracaldera trachyte cone and the few lavas erupted northwest of the caldera. Beginning about 18 Ka ago, inflation of the chamber opened old ring fractures and new radial fractures, along which recently differentiated pantellerite constructed more than 25 pumice cones and shields. Continued uplift raised the northwest half of the intracaldera trachyte cone 275 m, creating the island's present summit, Montagna Grande, by trapdoor uplift. Pantellerite erupted along the trapdoor faults and their hingeline, forming numerous pumice cones and agglutinate sheets as well as five lava domes. Degassing and drawdown of the upper pantelleritic part of a compositionally and thermally stratified magma chamber during this 18-3-Ka episode led to entrainment of subjacent, crystal-rich, pantelleritic trachyte magma as crenulate inclusions. Progressive mixing between host and inclusions resulted in a secular decrease in the degree of evolution of the 0.82 km3 of magma erupted during the episode. The 45-Ka-old caldera is nested within the La Vecchia caldera, which is thought to have formed around 114 Ka ago. This older caldera was filled by three widespread welded units

  6. Petrology of the Guenfalabo ring-complex: An example of a complete series along the Cameroon Volcanic Line (CVL), Cameroon

    NASA Astrophysics Data System (ADS)

    Donald Ngonge, E.; Hollanda, Maria Helena B. M.; Nsifa, E. Nkonguin; Tchoua, Felix M.

    2014-08-01

    In the Guenfalabo ring-complex (GRC), two non-comagmatic rock suites have been identified as a result of two volcanic episodes: Suite 1 (68.8 ± 1.7 Ma by K/Ar on trachyte) of peralkaline trachytes and pantellerites cogenetic with alkaline syenites, granites and rhyolitic flows and tuffs; Suite 2 (62 ± 2 Ma by K/Ar on basalt), a bimodal and complete series of alkali olivine basalts and associated microgabbro dykes, diorites, syenites and granites, cross-cutting the former. Kaersutite in the trachytes of Suite 1 has mantle-derived signatures: TiO2 > 4%, MgO < 15%, FeO > 8%, Ti = 0.63 c.p.f.u. and Al = 2, characteristic of kaersutites of HP and HT origin: 13-23 kbar, 1100-1220 °C. The trachytes are probably products of FC of a basaltic parent that did not attain higher crustal levels. The Suite 1 rocks are enriched in Rb, K, Zr, Nb, LREE, alkalis, and (Ce/Yb)N = 7-15 probably due to some effect of metasomatism during the magma ascension. Fe-Ti enrichment is corroborated by the presence of ferropseudobrookite-ilmenite-ulvospinel in the syenites and ilmenite in the pantellerite. The Rb/Ba > 1 in the trachytes (2.44, 26.7), pantellerite (6.33), alkaline granites (0.63-1.8) and the 87Sr/86Sr in the alkaline granites (=0.74060) depict the role of AFC. The ankaramites of the Suite 2 rocks are olivine-phyric (25%), Fo85-88, have 50-52% clinopyroxene (salite), 5% plagioclase (An55-36) and 7% Fe-Ti oxides. Trace element modeling indicates an origin from a basaltic magma of about 25% PM of spinel lherzolite mixed with a magma from <1% PM of garnet lherzolite (3-4% garnet) in a proportion of 1:4. The cogenetic alkali basalts and the microgabbro-diorite-syenite-granite that constitute the Suite 2 rocks, with a Daly gap of 54% > SiO2 < 58%, result from this Early Cenozoic magmatic event. The basalts have: Zr = 225-253, Nb = 98-111, Y = 33-56, typical of FOZO, a HIMU-type OIB related magmas (Sr/Sri = 0.70202-0.7034; Nd/Nd = 0.51282-0.512545; 206Pb/204Pb = 19.13, 207Pb/204Pb = 15

  7. The Mount Manengouba, a complex volcano of the Cameroon Line: Volcanic history, petrological and geochemical features

    NASA Astrophysics Data System (ADS)

    Pouclet, André; Kagou Dongmo, Armand; Bardintzeff, Jacques-Marie; Wandji, Pierre; Chakam Tagheu, Pulchérie; Nkouathio, David; Bellon, Hervé; Ruffet, Gilles

    2014-09-01

    The volcanic story of Mount Manengouba is related to four chronological stages: (1) forming of the early Manengouba shield volcano between 1.55 and 0.94 Ma, (2) building of the Eboga strato-cone between 0.94 and 0.89 Ma, (3) caldera collapse and silicic extrusions of the Elengoum Complex between 0.89 and 0.70 Ma, and (4) intra-caldera and flank activity between 0.45 and 0.11 Ma. The volume of the volcano is calculated at 320 km3 ± 5%. The volcanic rocks are attributed to two magmatic outputs. The first and main magma generation produced the shield volcano, the strato-cone, and the syn- to post-caldera extrusions, displaying a complete series from basanites to trachytes (magmatic Group 1). The second magma generation is limited to the late and flank activity evolving from basanites to trachy-phonolite (magmatic Group 2). Both magmatic groups belong to the under-saturated alkaline sodic series. Petrological calculations locate the magmatic reservoir between 37 and 39 km in the upper mantle for the Group 1 lavas, and between 42 and 44 km for the Group 2 lavas. Trachytes were generated in a secondary crustal reservoir. Magmatic series evolve with medium to low pressure fractional crystallization of olivine, pyroxene, oxides, feldspar, and apatite. Significant crustal assimilation is evidenced in trachytes. The magma of Group 1 was generated with 3-6% of partial melting of a moderately enriched source containing 3-7% of garnet. Melting took place in the spinel to garnet transition zone located at 70-90 km and around 25 kb. The magma of Group 2 resulted from a slightly higher partial melting from a less garnet-rich source that indicates uprising of the melting column in the upper part of transition zone. Sr, Nd, and Pb isotope data of the Manengouba rocks and neighboring lavas are analyzed and compared with those of the mafic lavas of the CVL. Three source components are distinguished: a depleted component originated from the asthenospheric swell, a radiogenic component

  8. The effect of air pollution on the stone decay of the Cologne Cathedral

    NASA Astrophysics Data System (ADS)

    Graue, B.; Siegesmund, S.; Licha, T.; Simon, K.; Oyhantcabal, P.; Middendorf, B.

    2012-04-01

    Different building stones of the Cologne Cathedral show a large variation of weathering phenomena. The Drachenfels trachyte, which was the construction material for the medieval part of the cathedral, shows significant surface deterioration, back-weathering coexisting with flaking, crumbling or the massive formation of gypsum crusts. Wolff (1992) first mentioned the negative interferences between the Schlaitdorfer sandstone and the Londorfer basalt lava or the Drachenfels trachyte and the Krensheimer muschelkalk. Crust formation on limestone, sandstone, and volcanic rock from the Cologne Cathedral as well as from the Xanten and Altenberg Cathedral are investigated. These three buildings are located in different areas and exposed to varying industrial, urban, and rural environmental situations. The material investigated range from dark grey to black framboidal crusts. This 3 to 10 mm thick cauliflower-like form of gypsum crust incorporates particles from the pollution fluxes. It covers the stone surface and mainly occurs at sites protected from wind and direct rain. Secondly, thin laminar black crusts trace the stone surface and may cover complete sections of the building's structure not necessarily preferring protected sites. This kind of crust seems to have very strong bonds between the thin black crust and the stone surface. Major and trace element distribution show an enrichment of sulfur, indicating the presence of gypsum, lead and other typical pollutants (arsenic, antimony, bismuth, tin etc.), which generally can be linked to traffic and industry. This indicates that even though the SO2 emission has decreased due to i.e. stronger regulations of waste incineration plants and the ban of leaded petrol, the pollutants are still present in the crusts on the building stones. From systematic SEM observations it becomes evident that the total amount of pollution is less pronounced in the Altenberg and Xanten Cathedrals as compared with the Cologne Cathedral. The

  9. Formation of U-depleted rhyolite from a basanite at El Hierro, Canary Islands

    NASA Astrophysics Data System (ADS)

    Sigmarsson, Olgeir; Laporte, Didier; Carpentier, Marion; Devouard, Bertrand; Devidal, Jean-Luc; Marti, Joan

    2013-03-01

    Phonolite and trachyte are the felsic magmas of the alkaline magma suites, which characterize the Canary Islands. The October 2011 submarine eruption off El Hierro, the westernmost island, nevertheless, produced a small volume of rhyolitic magma. The rhyolite occurred as highly vesicular, white coloured pumices enveloped in and mingled with darker coloured basanitic pumice. The basanitic pumice is relatively crystal poor with a few euhedral olivines (mostly Fo77-79), clinopyroxenes and Fe-rich spinels, whereas very rare olivine of same composition is found together with equally rare Fe-sulphide and FeTi-rich oxides in the rhyolite. The Fe-Mg exchange equilibrium in the oxides permits to calculate an equilibrium temperature of 970-890 °C for the rhyolite, in agreement with quartz-melt equilibrium at ca. 930 °C. A striking mineralogical feature of the rhyolite is the presence of rounded to contorted grains of milky quartz, which are xenocrysts incorporated and partly dissolved into the magma. Analyses of residual volatile concentrations in the glasses show that the rhyolite melt was highly degassed, whereas the basanitic glass still has important halogen concentrations. Trace element patterns of the mafic glasses and their elevated incompatible element concentrations are typical of the western Canary Island basanites. In contrast, the trace element composition of the rhyolite shows surprisingly low concentrations for all elements except the most incompatible ones (e.g. Rb, Ba, K and Th). All other measured LILE, HFSE and REE have significantly lower concentration than the basanitic counterpart that can be explained by fractionation of accessory phases (1 % apatite, 1 % sphene and 0.1 % zircon). Surprisingly, low U concentration is presumably related to elevated oxygen fugacity in the rhyolite, causing U to be in a hexavalent state, and fluxing of F-rich gas leading to volatilization of UF6, known to emanate at low temperature. The results suggest that a gas

  10. Geochemistry and age of Shatsky, Hess, and Ojin Rise seamounts: Implications for a connection between the Shatsky and Hess Rises

    NASA Astrophysics Data System (ADS)

    Tejada, Maria Luisa G.; Geldmacher, Jörg; Hauff, Folkmar; Heaton, Daniel; Koppers, Anthony A. P.; Garbe-Schönberg, Dieter; Hoernle, Kaj; Heydolph, Ken; Sager, William W.

    2016-07-01

    Shatsky Rise in the Northwest Pacific is the best example so far of an oceanic plateau with two potential hotspot tracks emanating from it: the linear Papanin volcanic ridge and the seamounts comprising Ojin Rise. Arguably, these hotspot tracks also project toward the direction of Hess Rise, located ∼1200 km away, leading to speculations that the two plateaus are connected. Dredging was conducted on the massifs and seamounts around Shatsky Rise in an effort to understand the relationship between these plateaus and associated seamounts. Here, we present new 40Ar/39Ar ages and trace element and Nd, Pb, and Hf isotopic data for the recovered dredged rocks and new trace elements and isotopic data for a few drill core samples from Hess Rise. Chemically, the samples can be subdivided into plateau basalt-like tholeiites and trachytic to alkalic ocean-island basalt compositions, indicating at least two types of volcanic activity. Tholeiites from the northern Hess Rise (DSDP Site 464) and the trachytes from Toronto Ridge on Shatsky's TAMU massif have isotopic compositions that overlap with those of the drilled Shatsky Rise plateau basalts, suggesting that both Rises formed from the same mantle source. In contrast, trachytes from the southern Hess Rise (DSDP Site 465A) have more radiogenic Pb isotopic ratios that are shifted toward a high time-integrated U/Pb (HIMU-type mantle) composition. The compositions of the dredged seamount samples show two trends relative to Shatsky Rise data: one toward lower 143Nd/144Nd but similar 206Pb/204Pb ratios, the other toward similar 143Nd/144Nd but more radiogenic 206Pb/204Pb ratios. These trends can be attributed to lower degrees of melting either from lower mantle material during hotspot-related transition to plume tail or from less refractory shallow mantle components tapped during intermittent deformation-related volcanism induced by local tectonic extension between and after the main volcanic-edifice building episodes on Shatsky

  11. U-series zircon age constraints on the plumbing system and magma residence times of the Changbai volcano, China/North Korea border

    NASA Astrophysics Data System (ADS)

    Zou, Haibo; Fan, Qicheng; Zhang, Hongfu; Schmitt, Axel K.

    2014-07-01

    The Changbai (or Baitoushan, Paektu) volcano on the China/North Korea border is best known for its climactic caldera-forming eruption of 100 km3 of comendite materials 1000 years (1 ka) ago. The polygenetic Changbai volcano also erupted moderate-volume pre-caldera comendite lava at ~ 4 ka and small-volume post-caldera trachyte ignimbrite at ~ 0.3 ka. Here we report 238U-230Th disequilibrium ages of zircons from lavas and ignimbrites of the pre-caldera (~ 4 ka), syn-caldera (1 ka), and post-caldera (~ 0.3 ka) events. The zircon isochron ages are 12.2 ± 1.1 ka (2σ) for the 4-ka comendite lava and 12.2 ± 1.7 ka for the 1-ka comendite ignimbrite. Zircons from the 0.3-ka trachyte ignimbrite exhibit 3 respective peaks at 2.6 ± 1.8 ka, 130 ± 10 ka and > 230 ka. The indistinguishable zircon ages for the 4-ka pre-caldera eruption of comendite lava and the 1-ka caldera-forming eruption of comendite pumice and ignimbrite suggest that the 4-ka lava provides an early sampling of a much larger magma body at depth and thus serves as a kind of petrologic early-warning signal. In addition, the 4-ka lava may represent the lowest-temperature magma in the roof zone of a thermally zoned magma chamber that usually escapes first. The distinctive multi-modal zircon age distributions of the 0.3-ka trachytic eruption, however, reveal that this post-caldera eruption tapped a different magma body and indicate that Changbai's magmatic plumbing system had changed after the 1-ka caldera-forming climactic eruption. Our results suggest very short zircon and magma residence times for the Changbai volcano (8 kyr for the 4-ka eruption, 11-12 kyr for the 1-ka eruption and 2.3 kyr for the 0.3-ka eruption).

  12. Radiocarbon dates for lava flows and pyroclastic deposits on Sao Miguel, Azores

    USGS Publications Warehouse

    Moore, R.B.; Rubin, M.

    1991-01-01

    We report 63 new radiocarbon analyses of samples from Sao Miguel, the largest island in the Azores archipelago. The samples are mainly carbonized tree roots and other plant material collected from beneath 20 mafic lava flows and spatter deposits and from within and beneath 42 trachytic pyroclastic flow, pyroclastic surge, mudflow, pumice-fall and lacustrine deposits and lava flows. One calcite date is reported. These dates establish ages for 48 previously undated lava flows and pyroclastic deposits, and revise three ages previously reported. These data are critical to deciphering the Holocene and late Pleistocene eruptive history of Sao Miguel and evaluating its potential volcanic hazards. Average dormant intervals during the past 3000 years are about 400 years for Sete Cidades volcano, 145 years for volcanic Zone 2, 1150 years for Agua de Pau volcano and 320 years for Furnas volcano. No known eruptions have occurred in volcanic Zone 4 during the past 3000 years. -from Authors

  13. Geological investigations of volcanic rocks at Mount Discovery, Mount Morning, and Mason Spur, McMurdo Sound

    SciTech Connect

    Wright, A.C.; Kyle, P.R.; More, J.A.; Meeker, K.

    1986-01-01

    This work includes mapping of volcanic geology, description of geologic sections, and collection of samples for geochemical analysis and potassium-argon dating. Reconnaissance mapping of Mount Discovery shows that this 2681-meter-high composite volcano comprises a core of plagioclase-phyric nepheline-benmoreite flows, lahars, and volcanoclastic fluviatile sediments, which are thought to be the unit dated at 5.44 million years. Reconnaissance mapping of Mount Morning shows that this is a young volcano principally composed of kaersutite-bearing phonolite flows erupted both from the summit crater and from small parasitic domes on the upper northern slopes of the mountain. A sample from one of these parasitic domes has been dated at 1.15 million years. Description of five geologic sections at Mason Spur has provided a more detailed understanding of the older trachytic volcanic complex, which is now dated at 11.5 to 12.8 years and is divided into seven mapped units.

  14. Petrology and mineralogy of the La Peña igneous complex, Mendoza, Argentina: An alkaline occurrence in the Miocene magmatism of the Southern Central Andes

    NASA Astrophysics Data System (ADS)

    Pagano, Diego Sebastián; Galliski, Miguel Ángel; Márquez-Zavalía, María Florencia; Colombo, Fernando

    2016-04-01

    The La Peña alkaline igneous complex (LPC) is located in the Precordillera (32°41‧34″ S - 68°59‧48″ W) of Mendoza province, Argentina, above the southern boundary of the present-day flat-slab segment. It is a 19 km2 and 5 km diameter subcircular massif emplaced during the Miocene (19 Ma) in the Silurian-Devonian Villavicencio Fm. The LPC is composed of several plutonic and subvolcanic intrusions represented by: a cumulate of clinopyroxenite intruded by mafic dikes and pegmatitic gabbroic dikes, isolated bodies of malignite, a central intrusive syenite that develops a wide magmatic breccia in the contact with clinopyroxenite, syenitic and trachytic porphyries, a system of radial and ring dikes of different compositions (trachyte, syenite, phonolite, alkaline lamprophyre, tephrite), and late mafic breccias. The main minerals that form the LPC, ordered according to their abundance, are: pyroxene (diopside, hedenbergite), calcium amphibole (pargasite, ferro-pargasite, potassic-ferro-pargasite, potassic-hastingsite, magnesio-hastingsite, hastingsite, potassic-ferro-ferri-sadanagaite), trioctahedral micas (annite-phlogopite series), plagioclase (bytownite to oligoclase), K-feldspar (sanidine and orthoclase), nepheline, sodalite, apatite group minerals (fluorapatite, hydroxylapatite), andradite, titanite, magnetite, spinel, ilmenite, and several Cu-Fe sulfides. Late hydrothermal minerals are represented by zeolites (scolecite, thomsonite-Ca), epidote, calcite and chlorite. The trace element patterns, coupled with published data on Sr-Nd-Pb isotopes, suggest that the primary magma of the LPC was generated in an initially depleted but later enriched lithospheric mantle formed mainly by a metasomatized spinel lherzolite, and that this magmatism has a subduction-related signature. The trace elements pattern of these alkaline rocks is similar to other Miocene calc-alkaline occurrences from the magmatic arc of the Southern Central Andes. Mineral and whole

  15. Isotopic composition of lead and strontium from Ascension and Gough Islands

    USGS Publications Warehouse

    Gast, P.W.; Tilton, G.R.; Hedge, C.

    1964-01-01

    Isotopic composition of lead and strontium has been determined in a series of rock samples from two islands on the Mid-Atlantic Ridge. Both inter-and intra-island variations exist in the abundance of radiogenic isotopes of both elements. Lead from basalt of Ascension Island has a Pb206-Pb 204 ratio of 19.5, while the corresponding ratio at Gough Island is only 18.4. The Pb208-Pb204 ratios from the two islands do not differ. Conversely, strontium from basalt of Ascension Island is less radiogenic than that from Gough Island basalts. The trachytes of both islands have lead and strontium that is more radiogenic than that found in the basalts. The inter-island differences indicate the existence of regional variations in the uranium-lead and rubidium-strontium ratios of the upper mantle source of these rocks and show that isotope compositions are a means for investigating chemical heterogeneities in the mantle.

  16. The Petrochemistry of Jake_M: A Martian Mugearite

    NASA Astrophysics Data System (ADS)

    Stolper, E. M.; Baker, M. B.; Newcombe, M. E.; Schmidt, M. E.; Treiman, A. H.; Cousin, A.; Dyar, M. D.; Fisk, M. R.; Gellert, R.; King, P. L.; Leshin, L.; Maurice, S.; McLennan, S. M.; Minitti, M. E.; Perrett, G.; Rowland, S.; Sautter, V.; Wiens, R. C.; Kemppinen, Osku; Bridges, Nathan; Johnson, Jeffrey R.; Cremers, David; Bell, James F.; Edgar, Lauren; Farmer, Jack; Godber, Austin; Wadhwa, Meenakshi; Wellington, Danika; McEwan, Ian; Newman, Claire; Richardson, Mark; Charpentier, Antoine; Peret, Laurent; Blank, Jennifer; Weigle, Gerald; Li, Shuai; Milliken, Ralph; Robertson, Kevin; Sun, Vivian; Edwards, Christopher; Ehlmann, Bethany; Farley, Kenneth; Griffes, Jennifer; Grotzinger, John; Miller, Hayden; Pilorget, Cedric; Rice, Melissa; Siebach, Kirsten; Stack, Katie; Brunet, Claude; Hipkin, Victoria; Léveillé, Richard; Marchand, Geneviève; Sánchez, Pablo Sobrón; Favot, Laurent; Cody, George; Steele, Andrew; Flückiger, Lorenzo; Lees, David; Nefian, Ara; Martin, Mildred; Gailhanou, Marc; Westall, Frances; Israël, Guy; Agard, Christophe; Baroukh, Julien; Donny, Christophe; Gaboriaud, Alain; Guillemot, Philippe; Lafaille, Vivian; Lorigny, Eric; Paillet, Alexis; Pérez, René; Saccoccio, Muriel; Yana, Charles; Armiens-Aparicio, Carlos; Rodríguez, Javier Caride; Blázquez, Isaías Carrasco; Gómez, Felipe Gómez; Gómez-Elvira, Javier; Hettrich, Sebastian; Malvitte, Alain Lepinette; Jiménez, Mercedes Marín; Martínez-Frías, Jesús; Martín-Soler, Javier; Martín-Torres, F. Javier; Jurado, Antonio Molina; Mora-Sotomayor, Luis; Caro, Guillermo Muñoz; López, Sara Navarro; Peinado-González, Verónica; Pla-García, Jorge; Manfredi, José Antonio Rodriguez; Romeral-Planelló, Julio José; Fuentes, Sara Alejandra Sans; Martinez, Eduardo Sebastian; Redondo, Josefina Torres; Urqui-O'Callaghan, Roser; Mier, María-Paz Zorzano; Chipera, Steve; Lacour, Jean-Luc; Mauchien, Patrick; Sirven, Jean-Baptiste; Manning, Heidi; Fairén, Alberto; Hayes, Alexander; Joseph, Jonathan; Squyres, Steven; Sullivan, Robert; Thomas, Peter; Dupont, Audrey; Lundberg, Angela; Melikechi, Noureddine; Mezzacappa, Alissa; DeMarines, Julia; Grinspoon, David; Reitz, Günther; Prats, Benito; Atlaskin, Evgeny; Genzer, Maria; Harri, Ari-Matti; Haukka, Harri; Kahanpää, Henrik; Kauhanen, Janne; Kemppinen, Osku; Paton, Mark; Polkko, Jouni; Schmidt, Walter; Siili, Tero; Fabre, Cécile; Wray, James; Wilhelm, Mary Beth; Poitrasson, Franck; Patel, Kiran; Gorevan, Stephen; Indyk, Stephen; Paulsen, Gale; Gupta, Sanjeev; Bish, David; Schieber, Juergen; Gondet, Brigitte; Langevin, Yves; Geffroy, Claude; Baratoux, David; Berger, Gilles; Cros, Alain; d'Uston, Claude; Forni, Olivier; Gasnault, Olivier; Lasue, Jérémie; Lee, Qiu-Mei; Meslin, Pierre-Yves; Pallier, Etienne; Parot, Yann; Pinet, Patrick; Schröder, Susanne; Toplis, Mike; Lewin, Éric; Brunner, Will; Heydari, Ezat; Achilles, Cherie; Oehler, Dorothy; Sutter, Brad; Cabane, Michel; Coscia, David; Israël, Guy; Szopa, Cyril; Teinturier, Samuel; Dromart, Gilles; Robert, François; Le Mouélic, Stéphane; Mangold, Nicolas; Nachon, Marion; Buch, Arnaud; Stalport, Fabien; Coll, Patrice; François, Pascaline; Raulin, François; Cameron, James; Clegg, Sam; DeLapp, Dorothea; Dingler, Robert; Jackson, Ryan Steele; Johnstone, Stephen; Lanza, Nina; Little, Cynthia; Nelson, Tony; Williams, Richard B.; Kirkland, Laurel; Baker, Burt; Cantor, Bruce; Caplinger, Michael; Davis, Scott; Duston, Brian; Edgett, Kenneth; Fay, Donald; Hardgrove, Craig; Harker, David; Herrera, Paul; Jensen, Elsa; Kennedy, Megan R.; Krezoski, Gillian; Krysak, Daniel; Lipkaman, Leslie; Malin, Michael; McCartney, Elaina; McNair, Sean; Nixon, Brian; Posiolova, Liliya; Ravine, Michael; Salamon, Andrew; Saper, Lee; Stoiber, Kevin; Supulver, Kimberley; Van Beek, Jason; Van Beek, Tessa; Zimdar, Robert; French, Katherine Louise; Iagnemma, Karl; Miller, Kristen; Summons, Roger; Goesmann, Fred; Goetz, Walter; Hviid, Stubbe; Johnson, Micah; Lefavor, Matthew; Lyness, Eric; Breves, Elly; Fassett, Caleb; Blake, David F.; Bristow, Thomas; DesMarais, David; Edwards, Laurence; Haberle, Robert; Hoehler, Tori; Hollingsworth, Jeff; Kahre, Melinda; Keely, Leslie; McKay, Christopher; Wilhelm, Mary Beth; Bleacher, Lora; Brinckerhoff, William; Choi, David; Conrad, Pamela; Dworkin, Jason P.; Eigenbrode, Jennifer; Floyd, Melissa; Freissinet, Caroline; Garvin, James; Glavin, Daniel; Harpold, Daniel; Mahaffy, Paul; Martin, David K.; McAdam, Amy; Pavlov, Alexander; Raaen, Eric; Smith, Michael D.; Stern, Jennifer; Tan, Florence; Trainer, Melissa; Meyer, Michael; Posner, Arik; Voytek, Mary; Anderson, Robert C.; Aubrey, Andrew; Beegle, Luther W.; Behar, Alberto; Blaney, Diana; Brinza, David; Calef, Fred; Christensen, Lance; Crisp, Joy; DeFlores, Lauren; Ehlmann, Bethany; Feldman, Jason; Feldman, Sabrina; Flesch, Gregory; Hurowitz, Joel; Jun, Insoo; Keymeulen, Didier; Maki, Justin; Mischna, Michael; Morookian, John Michael; Parker, Timothy; Pavri, Betina; Schoppers, Marcel; Sengstacken, Aaron; Simmonds, John J.; Spanovich, Nicole; Juarez, Manuel de la Torre; Vasavada, Ashwin; Webster, Christopher R.; Yen, Albert; Archer, Paul Douglas; Cucinotta, Francis; Jones, John H.; Ming, Douglas; Morris, Richard V.; Niles, Paul; Rampe, Elizabeth; Nolan, Thomas; Radziemski, Leon; Barraclough, Bruce; Bender, Steve; Berman, Daniel; Dobrea, Eldar Noe; Tokar, Robert; Vaniman, David; Williams, Rebecca M. E.; Yingst, Aileen; Lewis, Kevin; Cleghorn, Timothy; Huntress, Wesley; Manhès, Gérard; Hudgins, Judy; Olson, Timothy; Stewart, Noel; Sarrazin, Philippe; Grant, John; Vicenzi, Edward; Wilson, Sharon A.; Bullock, Mark; Ehresmann, Bent; Hamilton, Victoria; Hassler, Donald; Peterson, Joseph; Rafkin, Scot; Zeitlin, Cary; Fedosov, Fedor; Golovin, Dmitry; Karpushkina, Natalya; Kozyrev, Alexander; Litvak, Maxim; Malakhov, Alexey; Mitrofanov, Igor; Mokrousov, Maxim; Nikiforov, Sergey; Prokhorov, Vasily; Sanin, Anton; Tretyakov, Vladislav; Varenikov, Alexey; Vostrukhin, Andrey; Kuzmin, Ruslan; Clark, Benton; Wolff, Michael; Botta, Oliver; Drake, Darrell; Bean, Keri; Lemmon, Mark; Schwenzer, Susanne P.; Anderson, Ryan B.; Herkenhoff, Kenneth; Lee, Ella Mae; Sucharski, Robert; Hernández, Miguel Ángel de Pablo; Ávalos, Juan José Blanco; Ramos, Miguel; Jones, Andrea; Kim, Myung-Hee; Malespin, Charles; Plante, Ianik; Muller, Jan-Peter; Navarro-González, Rafael; Ewing, Ryan; Boynton, William; Downs, Robert; Fitzgibbon, Mike; Harshman, Karl; Morrison, Shaunna; Dietrich, William; Kortmann, Onno; Palucis, Marisa; Sumner, Dawn Y.; Williams, Amy; Lugmair, Günter; Wilson, Michael A.; Rubin, David; Jakosky, Bruce; Balic-Zunic, Tonci; Frydenvang, Jens; Jensen, Jaqueline Kløvgaard; Kinch, Kjartan; Koefoed, Asmus; Madsen, Morten Bo; Stipp, Susan Louise Svane; Boyd, Nick; Campbell, John L.; Pradler, Irina; VanBommel, Scott; Jacob, Samantha; Owen, Tobias; Atlaskin, Evgeny; Savijärvi, Hannu; Boehm, Eckart; Böttcher, Stephan; Burmeister, Sönke; Guo, Jingnan; Köhler, Jan; García, César Martín; Mueller-Mellin, Reinhold; Wimmer-Schweingruber, Robert; Bridges, John C.; McConnochie, Timothy; Benna, Mehdi; Franz, Heather; Bower, Hannah; Brunner, Anna; Blau, Hannah; Boucher, Thomas; Carmosino, Marco; Atreya, Sushil; Elliott, Harvey; Halleaux, Douglas; Rennó, Nilton; Wong, Michael; Pepin, Robert; Elliott, Beverley; Spray, John; Thompson, Lucy; Gordon, Suzanne; Newsom, Horton; Ollila, Ann; Williams, Joshua; Vasconcelos, Paulo; Bentz, Jennifer; Nealson, Kenneth; Popa, Radu; Kah, Linda C.; Moersch, Jeffrey; Tate, Christopher; Day, Mackenzie; Kocurek, Gary; Hallet, Bernard; Sletten, Ronald; Francis, Raymond; McCullough, Emily; Cloutis, Ed; ten Kate, Inge Loes; Kuzmin, Ruslan; Arvidson, Raymond; Fraeman, Abigail; Scholes, Daniel; Slavney, Susan; Stein, Thomas; Ward, Jennifer; Berger, Jeffrey; Moores, John E.

    2013-09-01

    “Jake_M,” the first rock analyzed by the Alpha Particle X-ray Spectrometer instrument on the Curiosity rover, differs substantially in chemical composition from other known martian igneous rocks: It is alkaline (>15% normative nepheline) and relatively fractionated. Jake_M is compositionally similar to terrestrial mugearites, a rock type typically found at ocean islands and continental rifts. By analogy with these comparable terrestrial rocks, Jake_M could have been produced by extensive fractional crystallization of a primary alkaline or transitional magma at elevated pressure, with or without elevated water contents. The discovery of Jake_M suggests that alkaline magmas may be more abundant on Mars than on Earth and that Curiosity could encounter even more fractionated alkaline rocks (for example, phonolites and trachytes).

  17. The Breccia Museo formation, Campi Flegrei, southern Italy: Geochronology, chemostratigraphy and relationship with the Campanian Ignimbrite eruption

    USGS Publications Warehouse

    Fedele, L.; Scarpati, C.; Lanphere, M.; Melluso, L.; Morra, V.; Perrotta, A.; Ricci, G.

    2008-01-01

    The Breccia Museo is one of the most debated volcanic formations of the Campi Flegrei volcanic district. The deposit, made up of six distinctive stratigraphic units, has been interpreted by some as the proximal facies of the major caldera-forming Campanian Ignimbrite eruption, and by others as the product of several, more recent, independent and localized events. New geochemical and chemostratigraphical data and Ar - Ar age determinations for several units of the Breccia Museo deposits (???39 ka), correlate well with the Campanian Ignimbrite-forming eruption. The chemical zoning of the Breccia Museo deposits is interpreted here to be a consequence of a three-stage event that tapped a vertically zoned trachytic magma chamber. ?? Springer-Verlag 2008.

  18. Bottom characterization of Lagoa das Furnas on São Miguel, Azores archipelago

    NASA Astrophysics Data System (ADS)

    Andersson, Tommy; Hermelin, Otto; Skelton, Alasdair; Jakobsson, Martin

    2016-07-01

    Lagoa das Furnas is a crater lake located in an area exposed to geohazards from earthquakes and volcanic activity on the island of São Miguel in the Azores Archipelago. Geophysical mapping of Lagoa das Furnas reveals a previously undiscovered volcanic dome. This dome is comprised largely of subaquatic pyroclastic debris of trachytic composition. Sedimentological, petrological, geochemical and geochronological studies of pyroclastic deposits from the dome link it to the historically documented "Furnas 1630" eruption. The chemistry of glass and crystal fragments sampled from the dome suggests that it is comprised of more evolved magma than that of the main Furnas 1630 dome located 1400 m away. This suggests that the dome was formed during a final phase of the 1630 eruption in the Lagoa das Furnas area.

  19. The petrochemistry of Jake_M: a martian mugearite.

    PubMed

    Stolper, E M; Baker, M B; Newcombe, M E; Schmidt, M E; Treiman, A H; Cousin, A; Dyar, M D; Fisk, M R; Gellert, R; King, P L; Leshin, L; Maurice, S; McLennan, S M; Minitti, M E; Perrett, G; Rowland, S; Sautter, V; Wiens, R C

    2013-09-27

    "Jake_M," the first rock analyzed by the Alpha Particle X-ray Spectrometer instrument on the Curiosity rover, differs substantially in chemical composition from other known martian igneous rocks: It is alkaline (>15% normative nepheline) and relatively fractionated. Jake_M is compositionally similar to terrestrial mugearites, a rock type typically found at ocean islands and continental rifts. By analogy with these comparable terrestrial rocks, Jake_M could have been produced by extensive fractional crystallization of a primary alkaline or transitional magma at elevated pressure, with or without elevated water contents. The discovery of Jake_M suggests that alkaline magmas may be more abundant on Mars than on Earth and that Curiosity could encounter even more fractionated alkaline rocks (for example, phonolites and trachytes). PMID:24072927

  20. Relation of peralkaline magmatism to heterogeneous extension during the Middle Miocene, southeastern Nevada

    USGS Publications Warehouse

    Scott, R.B.

    1995-01-01

    Volcanism migrated southward in the northern Basin and Range province in the Oligocene and early Miocene to produce voluminous calcalkaline silicic ash flow tuffs. Alkaline volcanism became dominant by middle Miocene (17-14 Ma) as smaller volumes of rhyolite-trachyte-basalt suites were erupted from the relatively small Kane Springs Wash caldera complex in southeastern Nevada. Only minor extension affected the Kane Wash area before the end of calcalkaline activity, but extension expressed by rate of progressive stratal tilt peaked (15-13.5 Ma) with peralkaline magmatism (14.7-14.4 Ma). Variations in distribution, degree, style, and timing of deformation demonstrate heterogeneous extension in the Kane Wash area. Only minor extension and tilting persisted post-middle Miocene (<12 Ma). All major eruptive sources overlap domains of rapid extension. -from Authors

  1. Age of Kōko Seamount, Emperor Seamount chain

    USGS Publications Warehouse

    Clague, David A.; Dalrymple, G. Brent

    1973-01-01

    KAr ages obtained by the conventional isotope-dilution and the 40Ar/39Ar techniques on two sanidine trachytes, four basalts, and a phonolite dredged from the top of Ko¯ko Seamount, 300 km north of the Hawaiian-Emperor bend, show that the seamount is 46.4 ± 1.1 my old. These data indicate that the volcanoes in the Hawaiian-Emperor chain continue to increase in age to the west and north beyond Midway Atoll, as predicted by the melting-spot hypothesis for the origin of the chain, and that the rate of volcanic migration along the chain was nonlinear between the time of formation of the island of Hawaii and Ko¯ko Seamount.

  2. Generation of compositionally atypical hydrocarbons in CO[sub 2]-rich geologic environments

    SciTech Connect

    Gize, A ); Macdonald, R. )

    1993-02-01

    Bitumen seepages from a trachyte flow on the Suswa volcano (East African Rift) are anomalously enriched in O and N heteroatomic organic compounds and depleted in aliphatics. Compositional and geologic data suggest that the biogenically derived bitumens were extracted and/or transported by liquid or supercritical CO[sub 2] rather than H[sub 2]O, possibly from caldera-lake sediments. Compositionally atypical hydrocarbons and reduced carbon associated with other alkaline-peralkaline igneous complexes, as well as Hg and Au deposits, may also reflect CO[sub 2] fluids. Hydrocarbon extraction and migration in CO[sub 2]-rich fluids are considered realistic mechanisms in certain geologic environments. 22 refs., 2 figs., 1 tab.

  3. The use of leaves and roots of Laurus novocanariensis as an indicator for soil and rock chemical composition in the environment of a subtropical cloud forest (Tenerife, Canary Islands, Spain).

    NASA Astrophysics Data System (ADS)

    Heidak, M.; Glasmacher, U. A.; Schöler, H. F.; Hernández-Moreno, J. M.

    2012-04-01

    On the Canary Island of Tenerife exists a sensitive and endangered ecosystem called "laurel forest". Laurel forest is an endemic type of a humid subtropical forest and it still covers a terrain of roughly 60 km2 on Tenerife (nearly 7% of the territory) (FERNANDEZ et al. 2001). The existing Laurel forest soils have been developed on different rocks. Corresponding to different moisture regimes, Vertisols, Alfisols, Ultisols, and Inceptisols are developed on basaltic lava flows. Inceptisols, allophanic Andisols, and vitric Andisols are present on pyroclastic rocks (ARNALDS et al., 2007). Three volcanic rock types of the basanite-phonolite assemblage are recognised (Rothe, 2008): Basic (basanites, ankaramites), intermediate (trachybasanites, plagioclase phonolites), and salic (trachyte, trachyphonolite, phonolite). Trachytes (sensu stricto) are comparatively rare. The present study aims to understand the element cycle and feed back mechanism between volcanic rocks, soils, roots, and leaves. Laurus novocanariensis stands as a key example how leaves and roots in a subtropical cloud forest, such as on Canary Islands, can be used as an indicator for soil and rock geochemistry. To obtain a wide spectrum of inorganic elements, we chose for our samples a combination of ICP-OES and ICP-MS. Our results show clearly that certain elements are enriched or depleted in leaves and roots. Other elements mirror the chemical composition of the soils and the volcanic rocks in great detail. This study indicates that Laurus novocanariensis can be used to trace the element distribution of certain elements from volcanic rocks thru soils to roots and leaves without a large disturbance of a sensitive ecosystem.

  4. A new Eemian record of Antarctic tephra layers retrieved from the Talos Dome ice core (Northern Victoria Land)

    NASA Astrophysics Data System (ADS)

    Narcisi, Biancamaria; Petit, Jean Robert; Langone, Antonio; Stenni, Barbara

    2016-02-01

    Polar ice sheets are remarkable repositories of tephra layers. The Talos Dome ice core (72°49‧S, 159°11‧E), drilled at the edge of the East Antarctic Plateau, close to Late Quaternary volcanoes, offers considerable potential to extend the current tephra time-stratigraphic framework. A tephrochronological study was undertaken of the ice core sections related to the Last Interglacial and the transition to the subsequent glacial period. Thirteen macroscopically visible layers, interpreted to be related to primary deposition of fallout tephra, have been analysed for quantitative grain size and glass shard geochemistry. The layers, precisely framed within the climate (δ18O) record for the core, span in age from 111.6 ± 1.9 to 123.3 ± 2.2 ka. Coarse particle size suggests origin from regional sources. Indeed, the vast majority of the samples display an alkaline affinity and trachytic composition that are both typical geochemical features of rifting Antarctic volcanism. Using subtle differences in the geochemical signatures and the comparison with data from previous studies, a few layers are attributed to known coeval Mt. Melbourne eruptions. Another sample subset is consistent with derivation from The Pleiades and Mt. Rittmann volcanoes. One peculiar trachytic glass population appears to be related to activity of the more distant Marie Byrd Land volcanoes. The newly detected tephras provide stratigraphic markers that could facilitate future synchronisation and dating of palaeoclimatic records. The Talos Dome tephra inventory also contributes significantly to the reconstruction of the Northern Victoria Land explosive volcanism, for which chronostratigraphic data for the Last Interglacial temporal segment are poor.

  5. Petrogenesis of coeval silica-saturated and silica-undersaturated alkaline rocks: Mineralogical and geochemical evidence from the Saima alkaline complex, NE China

    NASA Astrophysics Data System (ADS)

    Zhu, Yu-Sheng; Yang, Jin-Hui; Sun, Jin-Feng; Zhang, Ji-Heng; Wu, Fu-Yuan

    2016-03-01

    A combined study of zircon U-Pb ages, mineral chemistry, whole-rock elements and Sr-Nd-Hf isotopes was carried out for the Saima alkaline complex in the northeastern China, in order to investigate the source and petrogenesis of coeval silica-saturated and silica-undersaturated alkaline rocks. The Saima alkaline complex consists of nepheline syenites, quartz-bearing syenites and alkaline volcanic rocks (i.e., phonolite and trachyte), with minor mafic dikes and carbonatitic veins. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and secondary ion mass spectrometry (SIMS) zircon U-Pb dating gives consistent ages of 230-224 Ma for these rocks, suggesting that they are coeval. All alkaline rocks in the Saima complex are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs) with significant negative Nb, Ta and Ti anomalies. Geochemical data and Sr-Nd-Hf isotopic compositions indicate that the various alkaline rocks were all derived from partial melting of an ancient, re-enriched lithospheric mantle in the garnet stability field, but experienced variable siliceous- or carbonate-rich crustal contamination. Based on petrographic evidence, mineral compositions, and whole-rock geochemical data, two distinct magmatic evolutionary trends are proposed to explain the coeval emplacement of the various rock types within the Saima alkaline complex. The silica-undersaturated rocks (nepheline syenites and phonolites) result from alkali feldspar + apatite + titanite crystal fractionation of an alkaline mafic parental melt combined with assimilation of marine carbonate host rocks. In contrast, the generation of silica-saturated rocks (quartz-bearing syenites and trachytes) may be attributed to subsequent and continued clinopyroxene + apatite + biotite crystal fractionation coupled with assimilation of siliceous sediments.

  6. A 3D model of crustal magnetization at the Pinacate Volcanic Field, NW Sonora, Mexico

    NASA Astrophysics Data System (ADS)

    García-Abdeslem, Juan; Calmus, Thierry

    2015-08-01

    The Pinacate Volcanic Field (PVF) is located near the western border of the southern Basin and Range province, in the State of Sonora NW Mexico, and within the Gulf of California Extensional Province. This volcanic field contains the shield volcano Santa Clara, which mainly consists of basaltic to trachytic volcanic rocks, and reaches an altitude of ~ 1200 m. The PVF disrupts a series of discontinuous ranges of low topographic relief aligned in a NW direction, which consist mainly of Proterozoic metamorphic rocks and Proterozoic through Paleogene granitoids. The PVF covers an area of approximately 60 by 55 km, and includes more than 400 well-preserved cinder cones and vents and eight maar craters. It was active from about 1.7 Ma until about 13 ka. We have used the ages and magnetic polarities of the volcanic rocks, along with mapped magnetic anomalies and their inverse modeling to determine that the Pinacate Volcanic Field was formed during two volcanic episodes. The oldest one built the Santa Clara shield volcano of basaltic and trachytic composition, and occurred during the geomagnetic Matuyama Chron of reverse polarity, which also includes the normal polarity Jaramillo and Olduvai Subchrons, thus imprinting both normal and reverse magnetization in the volcanic products. The younger Pinacate series of basaltic composition represents monogenetic volcanic activity that extends all around the PVF and occurred during the subsequent geomagnetic Brunhes Chron of normal polarity. Magnetic anomalies toward the north of the Santa Clara volcano are the most intense in the PVF, and their inverse modeling indicates the presence of a large subsurface body magnetized in the present direction of the geomagnetic field. This suggests that the magma chambers at depth cooled below the Curie temperature during the Brunhes Chron.

  7. A volcanological and geochemical investigation of Boa Vista, Cape Verde Islands; 40Ar/ 39Ar geochronology and field constraints

    NASA Astrophysics Data System (ADS)

    Dyhr, Charlotte T.; Holm, Paul M.

    2010-01-01

    Boa Vista, the easternmost island in the Cape Verde archipelago, consists of volcanic products, minor intrusions and a thin partial sedimentary cover. The first 15 age results from 40Ar/ 39Ar incremental heating analysis of groundmass separates from volcanic and plutonic rocks from Boa Vista are presented. The combination of age results and field observations demonstrates that the volcanic activity that formed the island occurred in three main stages: (1) > 16 Ma, (2) 15-12.5 Ma and (3) 9.5-4.5 Ma. The first stage, restricted to the north eastern part of the island, is dominated by ankaramitic lavas. The second stage, consisting of evolved lavas of phonolitic-trachytic compositions and nepheline syenites, makes up large central parts of the island. The large volume of evolved rocks and the extended eruption period of several Ma make stage 2 in Boa Vista unique to Cape Verde. Mainly basanites and nephelinites were erupted during the third stage, initially dominated by eruption of subaerial mafic lavas around 9 Ma. Pillow lavas are erupted around 7 Ma whereupon dominantly subaerial mafic lavas were erupted. Stage 3 saw volcanism in many centres distributed mainly along the present coastline and with activity partly overlapping in time. The volcanic evolution of Boa Vista constrains the initiation of volcanic activity in the Cape Verde archipelago to the eastern islands. Major and trace element geochemistry of 160 volcanic and plutonic rocks representing the entire exposed chronological sequence on Boa Vista is presented, revealing an extremely well developed Daly Gap. Only a little was modified from the mafic magmas that rose in small batches from the mantle. Compositional variation distinguishes each volcanic complex and was to a large extent present in the mantle melts. The highly evolved stage 2 phonolites and trachytes are related through the fractional crystallization of three compositionally distinct magmas. Two of these may have been derived by crystal

  8. Comenditic and pantelleritic ash-flow tuffs from Volcan Las Navajas, Nayarit, Mexico

    SciTech Connect

    Nelson, S.A.; Hebre, J.A.

    1985-01-01

    Two distinctive ash-flow tuffs occur around the base of Volcan Las Navajas, a Pleistocene trachyte - peralkaline rhyolite center located in the northwestern segment of the Mexican Volcanic belt. The lower ash-flow unit is locally up to 65 m thick, is lithic rich and contains pumice blocks of comenditic rhyolite. The unit is not extensively exposed, and thus its areal extent and volume cannot be determined. Its chemical characteristics and stratigraphic relationship to other products erupted from Las Navajas suggest that it is related to the formation of the older of the two calderas which occur on Las Navajas. Unconformably overlying this unwelded ash-flow is a pantelleritic airfall pumice unit which is locally welded. This airfall unit is conformably overlain by a welded as-flow tuff that contains fiamme of pantelleritic composition (72 %SiO/sub 2/, 8% FeO*, 900 ppm Zr, agpaitic index of 1.7) as well as pumice blocks that show evidence of various degrees of mixing between pantellerite and trachyte. This suggests eruption from a chemically zoned magma chamber. This unit is locally up to 20 m thick, although its top has been removed by erosion. It is found on all sides of Las Navajas except on the south where it may be covered by Volcan Sanganguey, a Pleistocene to Recent calc-alkaline volcano. The welded ash-flow has been dated by K - Ar at 0.2+/-0.1 m.y. Stratigraphically and chemically this ash-flow appears to be related to the formation of younger of the two calderas.

  9. Petrology and Geochemistry of Pyroclastic Flow and Phreatomagmatic Deposits in Gölcük Area, Isparta

    NASA Astrophysics Data System (ADS)

    Akin, Lutfiye; Akkaş, Efe; Evren Çubukçu, H.

    2016-04-01

    Alkali potassic-ultra potassic magmatism occurs in Gölcük area, Isparta according to the post collisional extensional tectonic process from late Miocene to Pliocene and Quaternary. The formation of volcanism can be divided three main stages: (1) ancient trachytic lavas/domes, (2) initial pyroclastic flow deposits and tephriphonolite lava flows that located at the rim of the crater (3) pyrolcastic flow deposits, tuff ring deposits, phreatomagmatic deposits and young domes. Pumice samples, collected from pyroclastic flow deposits and phreatomagmatic deposits of Gölcük area, consists of plagioclase (vary from andesine to oligoclase, normally zoned with calcic cores and more sodic rims), clinopyroxene (vary from diopside to augite and commonly zoned), amphibole (magnesian hastingsite), biotite majorly. Oxides and apatites are the accessory phases. Pyroclastic flow deposits and phreatomagmatic deposits exhibit alkaline major oxide trend with composition range in trachyte-trachyandesite (SiO2=60-70%). The geochemical data show that all the samples are potassium rich. These samples are also contain high amount of LILE and LREE relative to HREE. The abundances of LREE and HREE are variable (LaN =400-700 and YbN=8-11). LREE/HREE ratios (LaN/YbN=50-65) imply that they can be derived from a source which is enriched in LREE. The MORB normalized patterns have shown that samples are enriched in Ba, Sr, K and Rb relative to Nb and Ti. This is the significant characteristics of island-arc magmas (Ba/Nb >30). Keywords: Gölcük, pyroclastic deposits, geochemistry, pumice

  10. Shield volcanoes of Marie Byrd Land, West Antarctic rift: oceanic island similarities, continental signature, and tectonic controls

    NASA Astrophysics Data System (ADS)

    LeMasurier, Wesley

    2013-06-01

    The Marie Byrd Land volcanic province is largely defined by 18 large (up to ~1,800 km3) alkaline shield volcanoes, each surmounted by a summit section of varied felsic rocks dominated by trachytic flows. They are distributed over a 500 × 800-km block-faulted dome within the West Antarctic rift. The basement contact of volcanic sections is ~500 masl at one site and 3,000 mbsl at another, 70 km away, which illustrates the scale of block faulting but complicates an understanding of volcanic structure. Furthermore, the continental ice sheet buries 16 volcanoes to progressively greater heights inland. However, five are sufficiently exposed to allow meaningful comparisons with alkaline oceanic island volcanoes; these comparisons are used as a guide to estimate the structure of Marie Byrd Land volcanoes. The type example for this study is Mt. Murphy, the most completely exposed volcano. It consists of a 1,400-m section of alkaline basalt overlain by trachyte and benmoreite flows that make up ~7-13 % of the volcano volume. In gross structure and composition, Mt. Murphy is similar to Gran Canaria volcano, Canary Islands, but the percent of felsic rock may be three times that of Gran Canaria, if the estimate is approximately correct. Departures from the oceanic island example are believed to represent the imprint of the Marie Byrd Land lithosphere and tectonic environment on volcano evolution. These include a lack of order in the sequence of felsic rock types, lack of progression toward more silica undersaturated compositions with time, absence of a highly undersaturated mafic resurgent stage, and perhaps, a relatively large volume of felsic rock.

  11. Stratigraphy, geochronology and evolution of the Mt. Melbourne volcanic field (North Victoria Land, Antarctica)

    NASA Astrophysics Data System (ADS)

    Giordano, Guido; Lucci, Federico; Phillips, David; Cozzupoli, Domenico; Runci, Valentina

    2012-11-01

    Mt. Melbourne (2,732 m a.s.l.) is a large quiescent stratovolcano located in Northern Victoria Land (Antarctica) and is one of a handful of volcanoes on the Antarctic plate with the potential for large-scale explosive eruptions. During the XVIII Italian Expedition in 2002-2003, the Mt. Melbourne volcanic succession was studied in terms of stratigraphy and sampled for 40Ar/39Ar age determinations and geochemistry. The early, Lower Pleistocene, volcanism was largely alkali basaltic to hawaiitic in composition and monogenetic in style, producing tens of small scoria cones and lava flows scattered over a wide area across the Transantarctic Mountains (Random Hills Period). During the Middle Pleistocene, volcanic activity focused to the area of the Mt. Melbourne stratovolcano, where several monogenetic centres show the transition from early sub-glacial/subaqueous conditions to emergent subaerial conditions (Shield Nunatak Period). The oldest exposed deposit associated with the early activity of the Mt. Melbourne stratovolcano (Mt. Melbourne Period) is a trachytic subaerial ignimbrite dated at 123.6 ± 6.0 ka, which reflects the establishment of a crustal magma chamber. Above the ignimbrite a succession of alkali basaltic, hawaiitic, and subordinate benmoreitic lavas and scoria cones is exposed, dated at 90.7 ± 19.0 ka. The Holocene deposits are exposed at the top of Mt. Melbourne, where the crater rim is composed of trachytic to rhyolitic pumice fall deposits, which are also extensively dispersed around the volcano, likely originated from Plinian-scale eruptions. The most recent explosive deposit proved difficult to date accurately because very low quantities of radiogenic 40Ar were released, resulting in imprecise plateau ages of 50 ± 70 and 35 ± 22 ka.

  12. Geochemistry of subalkaline and alkaline extrusives from the Kermanshah ophiolite, Zagros Suture Zone, Western Iran: implications for Tethyan plate tectonics

    NASA Astrophysics Data System (ADS)

    Ghazi, A. Mohamad; Hassanipak, A. A.

    1999-06-01

    The Kermanshah ophiolite is a highly dismembered ophiolite complex that is located in western Iran and belongs to the Zagros orogenic system. The igneous rocks of this complex consist of both mantle and crustal suites and include peridotites (dunite and harzburgite), cumulate gabbros, diorites, and a volcanic sequence that exhibits a wide range in composition from subalkaline basalts to alkaline basalts to trachytes. The associated sedimentary rocks include a variety of Upper Triassic to Lower Cretaceous deep- and shallow-water sedimentary rocks (e.g., dolomite, limestone, and pelagic sediments, including umber). Also present are extensive units of radiolarian chert. The geochemical data clearly identifies some of the volcanic rocks to have formed from two distinct types of basaltic melts: (i) those of the subalkaline suite, which formed from an initial melt with a light rare earth elements (LREE) enriched signature and incompatible trace element patterns that suggest an island arc affinity; and (ii) those of the alkaline suite with LREE-enriched signature and incompatible trace element patterns that are virtually identical to typical oceanic island basalt (OIB) pattern. The data also suggests that the trachytes were derived from the alkaline source, with fractionation controlled by extensive removal of plagioclase and to a lesser extent clinopyroxene. The presence of compositionally diverse volcanics together with the occurrence of a variety of Triassic-Cretaceous sedimentary rocks and radiolarian chert indicate that the studied volcanic rocks from the Kermanshah ophiolite represent off-axis volcanic units that were formed in intraplate oceanic island and island arc environments in an oceanic basin. They were located on the eastern and northern flanks of one of the spreading centers of a ridge-transform fault system that connected Troodos to Oman prior to its subduction under the Eurasian plate.

  13. The Chaîne des Puys: how complicated can monogentic get?

    NASA Astrophysics Data System (ADS)

    Van Wyk de Vries, B.; Grosse, P.; Marquez, A.; Petronis, M. S.; Kervyn, M.; Delcamp, A.; Mossoux, S.; Troll, V. R.

    2012-12-01

    The Chaîne des Puys (Massif Centrale of France) is part of a prospective volcano-tectonic UNESCO World Heritage site including the Limagne Rift fault (http://www.chainedespuys-failledelimagne.com/). The strategy is to present the monogentic field that is as representative of other such fields, but which itself is uniquely special. Effectively, the Chaîne des Puys would become a sort of ambassador for monogentic volcanism, raising the profile of all other sites. Here, I want to go through some recent work on the chain, looking at morphology, morphometry, structure, lithology and petrology and show some of the intriguing complexities of this classic highly variable monogentic alignment. Also, I want to build on the historical development of ideas that can be traced back to characters such as Montlosier, Humphrey Davey, Faraday, Lyell, and Von Humbolt... and many more. I focus, first, on the central the Puy de Dôme. This classic trachyte dome has been known for some time to be the product of two eruptions, and recently we have found that it is related to a number of cryptodome intrusions that have created flanking bulges, fed at least two major explosive eruptions, and extensively modified the topography of the field. Strangely, Von Humbolt's concept of 'craters of elevation' rises up in a reanalysis of these structures. Secondly, I visit Lemptégy (www.auvergne-volcan.com/), that in 1857, Scrope called 'an insignificant cone grazed by sheep', but now quarried out and showing the internal structure expected for the Puy de Dôme bulges, as well as illustrating that a seemingly simple scoria cone plumbing can be infernally complicated. Thirdly, I consider the Beaunit, a bucolic village in a maar, where the process of crustal ingestion suggests an intimate relationship between eruptive dynamics and assimilation. With these three examples I show some of the complications and interactions of monogentic basaltic to trachytic volcanism typified by the Chaîne des Puys.

  14. The ˜1000-years BP explosive eruption of Caldeira Volcano (Faial, Azores): the first stage of incremental caldera formation

    NASA Astrophysics Data System (ADS)

    Pimentel, Adriano; Pacheco, José; Self, Stephen

    2015-05-01

    The ˜1000-years BP eruption of Caldeira Volcano (Faial Island) was one of the last major explosive events recorded in the Azores. It produced a complex succession of pyroclastic deposits, known as the C11, divided into three members. At the base is the Brejo Member, a sequence of fine- to coarse-grained parallel-bedded ash layers found in the NW sector of the island. The middle part corresponds to the Inverno Member, a coarse-grained massive pumice fall deposit, restricted to the north flank of Caldeira Volcano. The top is dominated by the Cedros Member which includes massive to diffuse-stratified lapilli-ash and lithic breccias, exposed along the north and east flanks of the volcano. A minimum bulk volume of at least 0.22 km3 (>0.1 km3 dense rock equivalent (DRE)) is estimated for the C11 eruption, although a large portion may have been deposited offshore. The juvenile products are trachytic (59 wt% SiO2) with a homogenous whole-rock composition and mineral assemblage throughout the pyroclastic succession. However, petrographic and groundmass glass analyses indicate magma mingling/mixing processes between two trachytic batches. The C11 eruption history is divided into three phases (following the member division) with distinct eruptive styles: (1) an initial phreatomagmatic phase caused by rising magma (˜950 °C) encountering a crater pond or aquifer, (2) a fall-dominated phase which established a sub-Plinian column up to 14 km high (mass eruption rate (MER) of 1.2 × 107 kg/s) and (3) prolonged pyroclastic fountaining and sustained quasi-steady pyroclastic density current generation followed by summit collapse. The C11 eruption is interpreted as the first stage in the formation of an incremental caldera. This study provides valuable insights for a better understanding of small but complex explosive eruptions and their impact on ocean islands.

  15. Petrogenesis of the Miocene felsic volcanism from the south of Izmir (Western Turkey) and its regional tectono-magmatic implications

    NASA Astrophysics Data System (ADS)

    Karacik, Z.; Genç, C.

    2013-12-01

    Miocene volcanism, mainly intermediate and rarely mafic and felsic in composition, was produced under the effects of the extensional tectonic regime in western Turkey. The Karaburun and Cumaovasi volcanics are the cases for understanding the magma source(s) and petrological processes, producing the extension-related mafic and felsic bimodal volcanism. The Karaburun volcanics (KV) are mainly distributed north to south direction within the Karaburun peninsula and span a wide compositional spectrum from basalt (20 Ma) to rhyolite (16 Ma), and younger trachyte and trachydacites (13 Ma). The products of the subaerial felsic volcanism (the Cumaovasi volcanics, CV; 17 Ma) which are represented by cluster of topaz bearing rhyolite domes, related pyroclastic rocks occur within the NE-SW trending Cubukludag graben. The intermediate and mafic volcanic rocks are lack in the volcanic succession of CV. The lavas of the Cumaovasi volcanics are silicic which are calc alkaline, peralumious and enriched significantly in LILE. Extremely low Sr, Ba values, Eu depletions and very low LaN/YbN ratios are typical for rhyolites. The Karaburun volcanics, with the exception of the minor alkaline basaltic and trachytic lavas, are mainly calc alkaline and metaluminous intermediate lavas. The petrological data revealed that the KV and CV were formed in extensional tectonic setting, but evolved by different petrological processes in different magma chambers. The Cumaovasi lavas have a unique chemical composition, and closely similar to the extension related topaz bearing rhyolites formed from small magma bodies. Our data reveal that extension related mafic injections caused crustal anatectic melting and produced felsic melts that rapidly ascended into the upper crust.

  16. Petrological and geochemical comparition between the upper and lower rhyolite of the Binchuan basaltic profile,Emeishan LIP succession

    NASA Astrophysics Data System (ADS)

    Huixin, H.; Yu, W.

    2013-12-01

    Emeishan basalt is well known worldwide,and it has been well tested from the geochemistry and petrology.However,the eruptional rock sequences is rare reported. Some former work reported that on the top of Emeishan basalt,there is always sit with rhyolite(or felsic-composition,mainly are rhyolite and trachyte ).This work is focusing on newly found rhyolite and trachyte succession at the bottom of Binchuan basalt pofile,and the comparition between the bottom and top felsic-composition of the Binchuan pofile basaltic related succession from petrological and geochemistry points. The bottom rhyolite is lack of phenocryst,and the filling of blowhole is observed quartz only. Otherwise,the phenocryst of the top rhyolite is mainly alkaline-feldspar. The upper layered rhyolite is less sillical rich than the lower part with the content of SiO2 65-70,68-74 respectively. Additionally,the content of TiO2 (0.82-0.87,0.57-0.70),Total Fe2O3(5.15-5.87,2.89-4.88),MgO(2.13-2.64,0.19-0.48),CaO(1.18-1.49,0.13-0.42), P2O5(0.18-0.25,0.02-0.25) of the upper layered rhyolite is more abundant than the lower layer. However,the amount of the Na2O,K2O of both upper and lower rhyolite can not be distinguished clearly as weathering effect suspectively. This may note that the two kinds of rhyolite are formed from different geological process. The upper may due to the crystallization differentiation of the mafic magmas ,while the lower is formed in the result of crust remelting.

  17. Time Scale of Gas Accumulation before the 2010 Eyjafjallajökull Eruption (Iceland) from 210Po Radioactive Excess

    NASA Astrophysics Data System (ADS)

    Sigmarsson, O.

    2015-12-01

    The 2010 mildly explosive eruption at Eyjafjallajökull, Iceland, produced mingled tephra of benmoreitic and trachytic composition whereas alkali basalt was emitted during preceding flank eruption. Tephra of the first explosive phase are composed of three glass types, alkaline rhyolite, mixed benmoreite, and basalt. The early formed benmoreitic tephra (erupted on 15 and 17 April 2010) had large 210Po in excess of 210Pb [(210Po/210Pb)0= 1.88)] at the time of eruption, and possibly a small 210Pb excess over its parent 226Ra. In contrast, the preceding flank eruption produced basalt with (210Po)0 = 0, upon eruption, and the final trachyte (end of May 2010) had lost most of its 210Po during open-system degassing. The 210Po excess in the first erupted benmoreites is interpreted to result from 210Po degassing of basaltic magma and the accumulation of 210Po-enriched gas, either in the upper part of the basaltic intrusion, below the rhyolite-basalt interface, on in the pre-existing residual rhyolitic magma chamber. A model of radon and polonium accumulation in the rhyolitic reservoir, allows calculating the ratio of mass of basalt magma degassing over mass of magma accumulating excess gas. This ratio decreased from 20 to 15 over 2 days, implying zoned magma reservoir, with the uppermost and gas-richest part erupted first. The duration of pre-eruptive gas accumulation in this model is approximately one year. This corresponds closely to the initiation of a seismic swarm beneath Eyjafjallajökull, early June 2009, which was the first pre-eruptive signal detected. The coincidence between initiation of gas accumulation at relatively shallow depth and deeper seismicity strongly suggests that the excess gas phase originated from a basalt magma batch intruded at depth, and that this gas phase reached the surface approximately a year later.

  18. Chemically diverse, episodic volcanism offshore southern and peninsular California

    SciTech Connect

    Davis, A.S.; Gunn, S.H. ); Bohrson, W.A. . Dept. Earth and Space Sciences)

    1993-04-01

    Volcanic rocks recovered from eight seamounts offshore southern and peninsular California are chemically diverse. Compositions of lavas from seven small to moderately sized seamounts between 30.5[degree] and 34.0 N latitudes include low-K[sub 2]O tholeiitic, transitional, and mildly to moderately alkalic basalt. Volcanic rocks from the upper part of the much larger and morphologically complex edifice of Rocas Alijos, offshore central Baja California at about 25.0 N latitude, are highly differentiated trachyandesite and trachyte. The low-K[sub 2]O basalts are MORB-like with low abundances of incompatible elements and lower [sup 87]Sr/[sup 86]Sr and higher [sup 143]Nd/[sup 144]Nd ratios than MORB from the East Pacific Rise. The alkalic compositions have higher abundances of incompatible elements and isotopic data indicate more variably enriched mantle sources than those of seamounts near the East Pacific Rise, but the compositions of all samples are within the mantle array defined by other ocean-island basalts. [sup 40]Ar/[sup 39]Ar laser fusion ages for the seamounts span a large range. MORB-like lava from one of the northern edifices is as old as the underlying oceanic crust ([approximately]23 Ma), indicating that it originated at a spreading center. Other seamount lava ages are much younger (16.6--9.1 Ma) than the underlying oceanic crust. The trachytes from Rocas Alijos are less than 300,000 years old, indicating that the last volcanism on this large edifice occurred recently. The region offshore southern and peninsular California is tectonically complex and has many volcanic edifices of varying sizes, shapes, and orientations. The data available for volcanic rocks from this region suggest that the seamounts formed from multiple episodes of chemically diverse volcanism occurring sporadically from early Miocene to Recent.

  19. A-type,'' flood rhyolites of Trans-Pecos Texas: A major crustal melting event at 36. 8 Ma

    SciTech Connect

    Henry, C.D.; James, E.W. . Bureau of Economic Geology)

    1993-04-01

    The Bracks Rhyolite, Star Mountain Formation, and Crossen Trachyte are related, voluminous, high-temperature ([ge]900 C) silicic lavas that constitute an abrupt, major eruptive event in Trans-Pecos Texas. They were emplaced rapidly at 36.8 Ma, at the onset of the main phase of subduction-related volcanism and immediately preceding a major basalt outpouring. Individual flows are as much as 55 km long, cover 1,000 km[sup 2], and have volumes of as much as 75 km[sup 3]. Cumulatively, they cover 10,000[sup 2] km and comprise 1,000 km[sup 3]. Despite areal extents comparable to those of ash-flow tuffs, outcrop and petrographic features clearly demonstrate that these rocks were emplaced as lavas. All the silicic lavas are mildly peralkaline quartz trachytes to low-SiO[sub 2] rhyolites (67%--72% SiO[sub 2]). They have mineralogic and chemical characteristics of A-type'' granites, including Fe-rich pyroxene, fayalite, and sodic amphibole; high Na[sub 2]O + K[sub 2]O, Ga/Al, Zr, Nb, and Y; and low CaO, MgO, and Sr. Most elements do not correlate with SiO[sub 2], indicating that they are not differentiation suites. Individual flows are strikingly homogeneous. Pb isotopic compositions show a narrow range that indicate either a homogeneous source with Th/U [approx] 6 or thorough mixing. These characteristics are consistent with magma generation by high degrees of partial melting of depleted, anhydrous crust. Trace-element models support crustal melting over AFC processes. Heat was supplied by major infusion of basalt that coincided with a regional flare-up in magmatism in the southern Cordillera. Thus, the lavas indicate both introduction of new crust and recycling of old crust.

  20. Tectonic significance of Neoproterozoic magmatism of Nakora area, Malani igneous suite, Western Rajasthan, India

    NASA Astrophysics Data System (ADS)

    Kumar, Naresh; Vallinayagam, G.

    2014-05-01

    Three magmatic phases are distinguished in the Neoproterozoic Nakora Ring Complex (NRC) of Malani Igneous Suite (MIS), namely (a) Extrusive (b) Intrusive and (c) Dyke phase. Magmatism at NRC initiated with minor amount of (basic) basalt flows and followed by the extensive/voluminous acid (rhyolites-trachytes) flows. The ripple marks are observed at the Dadawari area of NRC in tuffaceous rhyolite flow which suggests the aqueous condition of flows deposition. The emplacement of the magma appears to have been controlled by a well defined NE-SW tectonic lineament and cut by radial pattern of dykes. These NE-SW tectonic lineaments are the linear zones of crustal weakness and high heat flow. The spheroidal and rapakivi structures in the Nakora acid volcanics indicate the relationship between genetic link and magma mixing. Basalt-trachyte-rhyolite association suggests that the large amount of heat is supplied to the crust from the magma chamber before the eruption. The field (elliptical/ring structures), mineralogical and geochemical characteristics of Nakora granites attest an alkaline character in their evolution and consistent with within plate tectonic setting. The emplacement of these granites and associated volcanics is controlled by ring structures, a manifestation of plume activity and cauldron subsidence, an evidence of extensional tectonic environment. NRC granites are the product of partial melting of rocks similar to banded gneiss from Kolar Schist Belt of India. The present investigations suggest that the magmatic suites of NRC rocks are derived from a crustal source and the required heat supplied from a mantle plume.

  1. Volcanic unrest in Kenya: geological history from a satellite perspective

    NASA Astrophysics Data System (ADS)

    Robertson, E.; Biggs, J.; Edmonds, M.; Vye-Brown, C.

    2013-12-01

    The East African Rift (EAR) system is a 5,000 km long series of fault bounded depressions that run from Djibouti to Mozambique. In the Kenyan Rift, fourteen Quaternary volcanoes lie along the central rift axis. These volcanoes are principally composed of trachyte pyroclastics and trachyte and basaltic lavas forming low-angle multi-vent edifices. Between 1997 and 2008, geodetic activity has been observed at five Kenyan volcanoes, all of which have undergone periods of caldera collapse and explosive activity. We present a remote-sensing study to investigate the temporal and spatial development of volcanic activity at Longonot volcano. High-resolution mapping using ArcGIS and an immersive 3D visualisation suite (GeovisionaryTM) has been used with imagery derived from ASTER, SPOT5 and GDEM data to identify boundaries of eruptive units and establish relative age in order to add further detail to Longonot's recent eruptive history. Mapping of the deposits at Longonot is key to understand the recent geological history and forms the basis for future volcanic hazard research to inform risk assessments and mitigation programs in Kenya. Calderas at Kenyan volcanoes are elliptical in plan view and we use high-resolution imagery to investigate the regional stresses and structural control leading to the formation of these elliptical calderas. We find that volcanoes in the central and northern segments of the Kenyan rift are elongated nearly parallel to the direction of least horizontal compressive stress, likely as a reflection of the direction of the plate motion vector at the time of caldera collapse. The southern volcanoes however are elongated at an acute angle to the plate motion vector, most likely as a result of oblique opening of the Kenyan rift in this region.

  2. Exceptional Volumes of Rejuvenated Volcanism in Samoa

    NASA Astrophysics Data System (ADS)

    Konter, J. G.; Jackson, M.; Storm, L.

    2010-12-01

    The internal structure of within-plate volcanoes is typically compared to the stages of volcanic evolution in Hawaii. In Samoa, these stages show some differences with the Hawaiian model, in terms of the duration, volume and geochemical composition of the stages. Particularly, the rejuvenated stage of volcanism in Samoa is significantly more voluminous, with increasing geographic coverage with age, completely repaving the island of Savai’i. This unusual outpouring of rejuvenated lavas has previously been proposed to be related to the tectonic setting, near the northern terminus of the Tonga Trench. Therefore, Samoan volcanism might be caused by lithospheric fracturing, a mantle plume, or potentially a combination of the two. We collected new samples from a deeply eroded canyon on Savai’i to determine a time evolution of the transition from shield to eventual rejuvenated lavas. The canyon exposes several hundred meters of lavas, and we collected samples about 200m vertically down into the canyon. These samples are dominantly olivine basalts, and their Pb isotope compositions fall within the compositional field of young rejuvenated lavas on Savai’i and Upolu. This canyon section, therefore, represents a minimum thickness for the rejuvenated lavas of 200m. Assuming eruption of rejuvenated lavas only occurred subaerially, with a universal thickness of 200m, the new data suggest more than one percent of the volume of Savai’i consists of rejuvenated lavas. This is an order of magnitude greater than the largest relative volumes in Hawaii (Kauai), and implies a different cause for rejuvenated volcanism in Samoa. Another feature that suggests different processes may be important is the transition between the shield and rejuvenated stage. Although Samoan volcanoes do not seem to erupt exactly the same rock types as characteristic Hawaiian post-shield stage lavas, there is a definite shift to more evolved compositions (including trachytes) during the later stages of

  3. Hazards associated with alkaline glaciovolcanism at Hoodoo Mountain and Mt. Edziza, western Canada: comparisons to the 2010 Eyjafjallajokull eruption

    NASA Astrophysics Data System (ADS)

    Edwards, B. R.

    2010-12-01

    The hazards associated with 2010 eruption from Eyjafjallajokull were well documented, and included flooding, pyroclastic activity, and local/regional ash and aerosol dispersal (e.g., Gudmundsson et al, 2010, Session V27). At least two ice-capped, alkaline volcanoes in northwestern British Columbia could produce similar styles of eruptive activity with associated local and regional hazards: Hoodoo Mountain and the Mt. Edziza volcanic complex. Similar to Eyjafjallajokull, both of the Canada volcanoes have likely had multiple Holocene eruptions of lava flows from beneath snow/ice cover, both eruption mildly alkaline basaltic to trachytic lavas, and both also have a history of explosive eruptions. Hoodoo Mountain volcano, which is approximately 17 cubic kilometers in volume, erupts dominantly trachyte-phonolite composition lavas, although it also has closely associated basaltic centers. Most of its history has been dominated by effusive eruptions, but at least one thick sequence of eutaxitic pyroclastic materials has been erupted in the past 50 ka (Edwards et al, 2002). It is presently covered by snow and a 3 km diameter ice cap that feeds a few small, flank alpine glaciers. The Mt. Edziza volcanic complex is much larger, with an estimated total eruptive volume of approximately 650 cubic km; it has had an extended eruptive history during the Plio-Pleistocene (Souther et al, 1984) including basaltic and trachytic eruptions. It presently hosts an ice-filled summit caldera approximately 2.8 by 2 km, which feeds several alpine glaciers radiating outwards in all directions. Edziza has several striking morphological similarities to Eyjafjallajokull, including the similar-sized summit ice cap dominated by silicic eruption products, an overall elongate morphology, and flanking fields of basaltic lava flows. Although Hoodoo and Edziza volcanoes are located in relatively remote parts of British Columbia, eruptions from either would likely partly melt existing snow and ice cover

  4. Crystallization kinetics in magmas during decompression

    NASA Astrophysics Data System (ADS)

    Arzilli, Fabio; Burton, Mike; Carroll, Michael R.

    2016-04-01

    Many variables play a role during magma crystallization at depth or in a volcanic conduit, and through experimentally derived constraints we can better understand pre- and syn-eruptive magma crystallization behavior. The thermodynamic properties of magmas have been extensively investigated as a function of T, P, fO2 and magma composition [1], and this allows estimation of the stability of equilibrium phases and physical parameters (e.g., density, viscosity). However, many natural igneous rocks contain geochemical, mineralogical and textural evidence of disequilibrium, suggesting that magmas frequently follow non-equilibrium, time-dependent pathways that are recorded in the geochemical and petrographic characteristics of the rocks. There are currently no suitable theoretical models capable of calculating nucleation and growth rates in disequilibrium conditions without experimental constraints. The aim of this contribution is provide quantitative data on growth and nucleation rates of feldspar crystals in silicate melts obtained through decompression experiments, in order to determine the magma evolution in pre- and sin-eruptive conditions. Decompression is one of the main processes that induce the crystallization of feldspar during the magma ascent in the volcanic conduit. Decompression experiments have been carried out on trachytic and basaltic melts to investigate crystallization kinetics of feldspar as a function of the effect of the degassing, undercooling and time on nucleation and crystal growth process [2; 3]. Furthermore, feldspar is the main crystals phase present in magmas, and its abundance can strongly vary with small changes in pressure, temperature and water content in the melt, implying appreciable variations in the textures and in the crystallization kinetics. Crystallization kinetics of trachytic melts show that long experiment durations involve more nucleation events of alkali feldspar than short experiment durations [2]. This is an important

  5. High to ultrahigh potassic alkaline volcanic belt along the Ankara-Erzincan suture (northern Turkey): new geochemical and Ar-Ar data constraining petrogenesis with implications for the late Cretaceous subduction of the Neotethys Ocean

    NASA Astrophysics Data System (ADS)

    Genc, S. Can; Gulmez, Fatma; Tuysuz, Okan; Karacik, Zekiye; Roden, Michael F.; Zeki Billor, M.; Hames, Willis E.

    2013-04-01

    Remnants of some high- to ultrahigh-K alkaline volcanic rocks crop out as isolated small and discontinuous bodies along the Ankara-Erzincan suture belt in northern Turkey. These rocks are represented by leucite-bearing lavas (LB), basaltic andesites, trachytes, monzonite/syenites) and lamprophyres. Leucite-bearing rocks are small stocks, dikes and lava flows. Pebbles and blocks of the LB are found in the coeval volcanic debris avalanche deposits and volcanoclastic breccias. Leucite-bearing rocks are mainly phonotephrite, tephriphonolite, trachyandesite and basaltic trachyandesites (shoshonite) and have mineral assemblages of lct + cpx + ol + pl + Kfs + mag+ ap. Leucites were almost totally analcimized. Trachytes and monzonite/syenites, which are seen as small stocks and dikes, are characterized by amp + bt + pl + Kfs + spn + ap + opq paragenesis. Lamprophyres are mica-rich melanocratic dikes, and include cpx + mica (phlogopitic) + Kfs + ap + opq. Rarely leucite, olivine and plagioclase are also present. Ar-Ar data reveal that this volcanic activity occurred between 73.6±0.18 and 76.78±0.19 Ma, corresponding to latest Cretaceous. All the samples from the high- and ultrahigh-K volcanic belt are alkaline in nature. Leucite-bearing lavas are characterized by their MgO (2.70-5.81, av. 4.58 wt.%), K2O (0.79-4.81, av. 2.35 wt.%), Na2O (4.86-7.48, av. 3.58 wt.%) and K2O/Na2O (0.13-0.92, av. 0.42 wt.%). The low K2O and K2O/Na2O contents of these rocks are due to extensive analcimization of the leucites. Major oxide contents in lamprophyric rocks are 3.25-7.48 (MgO), 1.35-7.76 (K2O), 1.77-4.00 (Na2O) and 0.31-2.69 (K2O/Na2O). The silica content of these rocks are variable and range from 47.18-50.26 (wt.%) (LB) to 39.14-53.28 (lamprophyres). Based on their major element contents, these rocks are classified as plagioleucitites or ultrapotassic rocks of the active orogenic zones (Foley, 1992). Leucite-bearing rocks, lamprophyres and the trachytes (with their hypabyssal

  6. Incision of the Colorado River in southern Utah - insights from channel profiles, local incision rates, and modeling of lithologic controls

    NASA Astrophysics Data System (ADS)

    Cook, K.; Whipple, K.; Heimsath, A.

    2009-04-01

    The Colorado River and its tributaries in southern Utah and northern Arizona provide an opportunity to study the propagation of bedrock incision through a large heterogeneous fluvial network, as the system is continuing to adjust to the baselevel fall responsible for the Grand Canyon. Although the carving of the Grand Canyon was largely complete by ~1 Ma, the canyon ends at Lee's Ferry and the incision history of Colorado River system upstream of the associated large knickpoint has been the subject of debate. In conjunction with existing incision rate estimates based on the dating of strath terraces, we use longitudinal profiles of the Colorado and tributaries between Marble Canyon and Cataract Canyon to investigate the incision history of the Colorado in this region. We find that all but two of the tributaries in this region steepen as they enter the Colorado River. The consistent presence of oversteepened reaches with similar elevation drops in the lower section of these channels, and their coincidence within a corridor of high local relief along the Colorado, suggest that the tributaries are steepening in response to an episode of increased incision rate on the Colorado River. This analysis is supported by available incision rate data, as the spatial distribution of incision rates predicted by the channel profiles is consistent with existing rate estimates. The two analyzed tributaries that show no evidence for this incision pulse, Trachyte Creek and Bullfrog Creek, have smoothly concave profiles and do not contain knickpoints. In order to evaluate the significance of these anomalous channel profiles, we measure in situ 10Be concentrations on four gravel-covered strath surfaces elevated from 1 m to 110 m above Trachyte Creek. The surfaces yield exposure ages that range from approximately 2.5 ka to 267 ka and suggest incision rates that vary between 350 and 600 m/my. These incision rates are similar to other rates determined within the high-relief corridor, and

  7. Pyroclastic flows and lavas of the Mogan and Fataga formations, Tejeda Volcano, Gran Canaria, Canary Islands: mineral chemistry, intensive parameters, and magma chamber evolution

    NASA Astrophysics Data System (ADS)

    Crisp, Joy A.; Spera, Frank J.

    1987-12-01

    The Mogan and Fataga formations on the island of Gran Canaria, Canary Islands, represent a sequence of approximately 30 intercalated pyroclastic and lava flows (total volume about 500 km3 dense-rock equivalent) including subalkaline rhyolitic, peralkaline rhyolitic and trachytic pyroclastic flows, nepheline trachyte lavas and a small volume of alkali basaltic lavas and tephra deposits. The eruption of the intermediate to silicic rocks of the Mogan and Fataga formations follows the roughly 4 Ma duration of basaltic shield volcanism. The most common assemblage in the evolved (Mogan and Fataga) rocks is anorthoclase+ edenitic amphibole+ilmenite+magnetite±augite±hypersthene +apatite+pyrrhotite. A few flows also contain plagioclase, biotite, or sphene. Coexisting Fe-Ti oxides yield equilibrium temperatures between 835 and 930° C and logf_{O_2 } between -11.2 and -12.6. The lowermost pyroclastic flow of the Mogan formation is zoned from a rhyolitic base (848° C) to a basaltic top (931° C). Unit P1 has an oxygen isotope feldspar-magnetite temperature (850° C) very close to its Fe-Ti oxide temperature. One of the youngest Mogan flows is zoned from a comendite (836° C) at the base to a comenditic trachyte (899° C) at the top. The Fataga formation pyroclastic flows show only slight compositional zonation, and one flow has the same Fe-Ti oxide compositions at top and base. Calculations using the reaction 1/3 magnetite+SiO2 (melt)=ferrosilite+1/6 O2 indicate total pressures of 1 4 (±3) kb for six of the Mogan flows and one of the Fataga flows. For four of the pyroclastic flows, equilibria involving tremolite-SiO2-diopside-enstatite-H2O and phlogopite-SiO2-sanidine-enstatite-H2O imply water contents of 0.9 to 2.6 (±0.5) wt% andf_{H_2 O} between 80 and 610 bars, which indicates that magma within the Tejeda reservoir was H2O-undersaturated throughout the entire history of Mogan to Fataga volcanism. The fluorine contents of amphibole, biotite, and apatite, and chlorine

  8. Peralkaline magma evolution and the tephra record in the Ethiopian Rift

    NASA Astrophysics Data System (ADS)

    Rooney, Tyrone O.; Hart, William K.; Hall, Chris M.; Ayalew, Dereje; Ghiorso, Mark S.; Hidalgo, Paulo; Yirgu, Gezahegn

    2012-09-01

    The 3.119 ± 0.010 Ma Chefe Donsa phreatomagmatic deposits on the shoulder of the Ethiopian Rift mark the northern termination of the Silti-Debre Zeyit Fault Zone, a linear zone of focused extension within the modern Ethiopian Rift. These peralkaline pumice fragments and glass shards span a wide range of glass compositions but have a restricted phenocryst assemblage dominated by unzoned sanidine. Glass shards found within the ash occupy a far more limited compositional range (75-76 wt% SiO2) in comparison with the pumice (64-75 wt% SiO2), which is rarely mingled. Thermodynamic modeling shows that liquids broadly similar to the least evolved glass composition can be achieved with 50-60 % fractionation of moderately crustally contaminated basalt. Inconsistencies between modeled solutions and the observed values of CaO and P2O5 highlight the important role of fluorine in stabilizing fluor-apatite and the limitations of current thermodynamic models largely resulting from the scarce experimental data available for the role of fluorine in igneous phase stability. On the basis of limited feldspar heterogeneity and crystal content of pumice at Chefe Donsa, and the difficulties of extracting small volumes of Si-rich melt in classical fractional crystallization models, we suggest a two-step polybaric process: (1) basaltic magma ponds at mid-upper-crustal depths and fractionates to form a crystal/magma mush. Once this mush has reached 50-60 % crystallinity, the interstitial liquid may be extracted from the rigid crystal framework. The trachytic magma extracted at this step is equivalent to the most primitive pumice analyzed at Chefe Donsa. (2) The extracted trachytic liquid will rise and continue to crystallize, generating a second mush zone from which rhyolite liquids may be extracted. Some of the compositional range observed in the Chefe Donsa deposits may result from the fresh intrusion of trachyte magma, which may also provide an eruption trigger. This model may have

  9. U-Pb Geochronology of Hydrous Silica (Siebengebirge, Germany)

    NASA Astrophysics Data System (ADS)

    Tomaschek, Frank; Nemchin, Alexander; Geisler, Thorsten; Heuser, Alexander; Merle, Renaud

    2015-04-01

    Low-temperature, hydrous weathering eventually leads to characteristic products such as silica indurations. Elevated U concentrations and the ability of silica to maintain a closed system permits silica to be dated by the U-Pb method, which, in turn, will potentially allow constraining the timing of near-surface processes. To test the feasibility of silica U-Pb geochronology, we sampled opal and chalcedony from the Siebengebirge, Germany. This study area is situated at the terminus of the Cenozoic Lower Rhine Basin on the Rhenish Massif. The investigated samples include silicified gravels from the Mittelbachtal locality, renowned for the embedded wood opal. Structural characterization of the silica phases (Raman spectroscopy) was combined with in situ isotopic analyses, using ion microprobe and LA-ICPMS techniques. In the Siebengebirge area fluviatile sediments of Upper Oligocene age were covered by an extended trachyte tuff at around 25 Ma. Silica is known to indurate some domains within the tuff and, in particular, certain horizons within the subjacent fluviatile sediments ('Tertiärquarzite'). Cementation of the gravels occurred during at least three successive growth stages: early paracrystalline silica (opal-CT), fibrous chalcedony, and late microcrystalline quartz. It has traditionally been assumed that this silica induration reflects intense weathering, more or less synchronous with the deposition of the volcanic ashes. Results from U-Pb geochronology returned a range of discrete 206Pb-238U ages, recording a protracted silicification history. For instance, we obtained 22 ± 1 Ma for opal-CT cement from a silicified tuff, 16.6 ± 0.5 Ma for silicified wood and opal-CT cement in the fluviatile gravels, as well as 11 ± 1 Ma for texturally late chalcedony. While silicification of the sampled tuff might be contemporaneous with late-stage basalts, opaline silicification of the subjacent sediments and their wood in the Mittelbachtal clearly postdates active

  10. The proximal marine record of the Marsili Seamount in the last 7 ka (Southern Tyrrhenian Sea, Italy): Implications for the active processes in the Tyrrhenian Sea back-arc

    NASA Astrophysics Data System (ADS)

    Tamburrino, S.; Vallefuoco, M.; Ventura, G.; Insinga, D. D.; Sprovieri, M.; Tiepolo, M.; Passaro, S.

    2015-10-01

    The volcanism of back-arc basins resembles that of oceanic spreading centers, rifts, and, in vanishing stages, extensional arcs, depending on the amount and rate of the dynamic processes associated to the subduction. Marsili Seamount (MS) represents the axial ridge of the Southern Tyrrhenian Sea back-arc basin, which is connected to the slab roll-backing processes affecting the Calabrian Arc (Italy). The Southern Tyrrhenian Sea back-arc is characterized by a significant decline in the spreading rate with time (2.8-3.1 mm/a to less than 1.8 mm/a in the last 0.78-1 Ma). MS develops between about 1 Ma and 3 ka and mainly consists of lava flows erupted from central and fissural vents. The MS products belong to the calcalkaline association and range in composition from basalts to trachytes. We present new stratigraphic, geochronological, and geochemical data (glass shards and minerals) of tephra from a 2.35 m long gravity core (Marsili1 core) recovered on MS at 943 m b.s.l. We recognize five tephras [M1 (top of the core) to M5 (bottom)] represented by poorly to highly vesiculated ashes. The lowermost tephra M5 emplaced between ca. 7 and 26 ka B.P.; it represents the less evolved distal counterpart of the Unit D related to the Biancavilla-Montalto products of Mount Etna (Sicily). The M1 to M4 tephras emplaced between 2.1 and 7.2 ka B.P. and are related to strombolian-like submarine eruptions of NNE-SSW aligned MS vents. The composition of the M1-M4 glasses ranges from basaltic trachyandesites to andesites and trachytes. The M1 to M4 magmas mainly originated by crystal fractionation from a heterogeneous mantle source with varying LILE enrichments by subduction-related fluids. The degree of evolution of the MS magmas increases with decreasing time. The formation of vertically stacked magma storage zones at the crust/mantle interface and within MS is related to the vanishing Southern Tyrrhenian Sea opening, which implies the rapid (< 1 Ma) evolution from a slow spreading

  11. The campi flegrei (Italy) geothermal system: A fluid inclusion study of the mofete and San Vito fields

    USGS Publications Warehouse

    de, Vivo B.; Belkin, H.E.; Barbieri, M.; Chelini, W.; Lattanzi, P.; Lima, A.; Tolomeo, L.

    1989-01-01

    Vito 3 cores show an approach to fluid/rock Sr equilibrium with a fluid similar to modern ocean water in 87Sr/86Sr ratio. The Campi Flegrei volcanic system has evolved undersaturated products, mostly trachyte, and defines a large (??? 12 km) caldera. The hydrothermal system developed in this location can be used as an analog for fossil systems in similar trachytic environments. The potential for ore mineralization is expressed by the recognition, from fluid inclusion and drilling data, of ore-forming environments such as boiling and brine stratification. ?? 1989.

  12. Spatio-temporal evolution of a dispersed magmatic system and its implications for volcano growth, Jeju Island Volcanic Field, Korea

    NASA Astrophysics Data System (ADS)

    Brenna, Marco; Cronin, Shane J.; Smith, Ian E. M.; Sohn, Young Kwan; Maas, Roland

    2012-09-01

    Jeju Island is the emergent portion of a basaltic volcanic field developed over the last c. 1.8 Ma on continental crust. Initial volcanism comprised dispersed, small-volume (< 0.01 km3) alkali basaltic eruptions that incrementally constructed a tuff pile. Lavas and scoria from continuing small-scaled monogenetic volcanism capped this foundation. From c. 0.4 Ma large-volume (> 1 km3) eruptions began, with lavas building a composite shield. Three magma suites can be recognized: Early Pleistocene high-Al alkali (HAA), and Late Pleistocene to Holocene low-Al alkali (LAA) and subalkali (SA). The chemical similarity between small-volume and primitive large-volume eruptions suggests analogous parent magmas and fractionation histories that are independent of erupted volumes. The large-volume magmas evolved to trachyte, which erupted in two distinct episodes: the HAA Sanbangsan suite at c. 750 ka and the LAA Hallasan suite at c. 25 ka. Sr and Nd isotopes indicate that the early trachytes were contaminated by upper crustal material, whereas the later magmas were not. Both suites bear a Nd isotope signature indicative of lower crustal interaction. Sub-suites transitional between HAA and LAA, and between LAA and SA, indicate that melting occurred in discrete, but adjacent, mantle domains. Throughout the evolution of this volcano, each magma batch erupted separately, and a centralized plumbing system was never created. The Island's central peak (Mt. Halla 1950 m a.s.l.) is therefore not a sensu stricto stratovolcano, but marks the point of peak magma output in a distributed magmatic system. Jeju's shape and topography thus represent the spatial variation of fertility of the mantle below it. An increase in melt production in the Late Pleistocene was related to a deepening of the melting zone due to regional tectonic rearrangements. Temporal coincidences between magmatic pulses on Jeju and large-scale caldera eruptive events along the nearest subduction system in Kyushu, Japan

  13. Carbonatites and primary carbonates in the Rio Apa and Amambay regions, NE Paraguay

    NASA Astrophysics Data System (ADS)

    Comin-Chiaramonti, Piero; De Min, Angelo; Girardi, Vicente A. V.; Gomes, Celso B.

    2014-02-01

    In the Rio Apa and Amambay regions, north-eastern Paraguay (NPAA), potassic, alkaline-carbonatitic rocks (138-139 Ma) predate the eruption of tholeiitic flood basalts (133 ± 1 Ma). These rocks, mainly outcropping as dykes or ring-like complexes, intrude a Cambro-Ordovician carbonate platform and Precambrian metamorphic rocks along with their Silurian and Permo-Carboniferous sediments. The main rock-types range from basanite to trachyte and trachyphonolite (and intrusive equivalents) to carbonatite, in addition to glimmeritic and pyroxenitic veins. Geological and geophysical evidence indicate that the NPAA magmatism is related to extensional tectonics, like the Early Cretaceous alkaline-carbonatitic complexes from central-eastern Paraguay (127 ± 1 Ma) and the eastern magmatic occurrences of the Paraná Basin. Oxygen and carbon isotope compositions (whole rocks and carbonates) vary from values close to the field of continental lithospheric mantle, or that of primary carbonatites, up to values typical of a hydrothermal environment. An isotope exchange model implies that the main isotope variations, ranging from a magmatic (e.g., 1200 °C) to a low temperature (< 400 °C) environment, involved fluids with CO2/H2O ratios between 0.8 and 1.0. In particular, the Osbnd C isotopic variations, in combination with the La vs La/Yb ratios, suggest an increasingly higher level of carbonate in the silicate liquids, with increasing evolution, i.e., basanite → phonotephrite → trachyphonolite-phonolite (trachyte) → carbonatite. Srsbnd Nd isotopes show that the carbonatites are mantle derived without significant crustal contamination and that they can be related to isotopically enriched sources where newly formed veins (enriched component) and peridotite matrix (depleted component) underwent differing isotopic evolution. TDM model ages for NPAA range from 1.2 to 2.3 Ga. Considering that in the whole Paraná Basin isotopically distinct K-alkaline and tholeiitic magmas were

  14. Rifting, landsliding and magmatic variability in the Canary Islands

    NASA Astrophysics Data System (ADS)

    Carracedo, J. C.; Troll, V. R.; Guillou, H.; Badiola, E. R.; Pérez-Torrado, F. J.; Wiesmaier, S.; Delcamp, A.; Gonzalez, A. R.

    2009-04-01

    Rifts, probably the most influential structures in the geology of the Canary Islands, may also be responsible for the development of central felsic volcanoes, which are consistently nested in the collapse basins of the massive lateral collapses found in the Canaries. Three main types of post-collapse volcanism have been observed, particularly in the western Canaries: 1. Collapses followed by relatively scant, non-differentiated volcanism inside the collapse depression (El Golfo, El Hierro; La Orotava and Güímar, Tenerife), 2. those with important, although short-lasting (tens of thousands of years), post-collapse activity including felsic (phonolitic, trachytic) central volcanism (Bejenado, La Palma; Vallehermoso, La Gomera), and 3. those with very important, long-lasting (>100 kyr) post-collapse activity, evolving from primitive to felsic magmatism, eventually resulting in very high stratovolcanoes (Teide, Tenerife). Three consecutive sector collapses (Micheque, Güímar and La Orotava) mass-wasted the flanks of in the NE rift of Tenerife after intense and concentrated eruptive activity, particularly from about 1.10 Ma to 0.96 Ma, with periods of growth up to 15-25 m/kyr. Volcanic activity completely filled the Micheque collapse, evolving from basaltic to differentiated trachytic eruptions. Conversely, nested volcanism was less abundant in the Güímar and La Orotava collapses. This requires two fundamentally different scenarios which may be a function of active versus passive flank collapse trigger mechanisms: 1. The collapse occurs as a result of one of these short but intense intrusive-eruptive periods and probably triggered by concurring extensional stresses at the rifts (rift push), or 2. the giant landslide is derived only from gravitational instability. In the first scenario, the collapse of the flank of the rift may disrupt an established fissural feeding system that rapidly fills the collapse basin. Due to its disruption and the progressive new

  15. A Gradual Compositional Change from Samoan Shield to Rejuvenated Lavas?

    NASA Astrophysics Data System (ADS)

    Konter, J. G.; Jackson, M. G.; Koppers, A. A.

    2013-12-01

    trachytes erupted. A similar slope break is found in the canyon sampled on Savaii, and trachytes have previously been reported as cobbles in the draining river. Therefore, we infer that early shield volcanism in Savaii erupted extremely radiogenic Sr isotope compositions, and that by the end of shield building, compositions had changed to a composition similar to Upolu's Fagaloa. Some of the samples have Pb-Sr isotope compositions between Upolu and rejuvenated lavas, while their trace element compositions correspond to those of rejuvenated lavas. Thus, it appears that during the final stages of shield building, a shift to rejuvenated composition takes place. This contrasts with the definition of rejuvenated volcanism in Samoa, based on erosional contacts, and suggests rejuvenated source material may be sampled before the volcanic rejuvenation really occurs.

  16. Early Miocene Kirka-Phrigian caldera, western Anatolia - an example of large volume silicic magma generation in extensional setting

    NASA Astrophysics Data System (ADS)

    Seghedi, Ioan; Helvacı, Cahit

    2014-05-01

    Large rhyolitic ignimbrite occurrences are close connected to the Early Miocene initiation of extensional processes in the central-west Anatolia along Taşvanlı-Afyon zones. Field correlations, petrographical, geochemical and geochronological data lead to a substantial reinterpretation of the ignimbrite surrounding Kırka area, known from its world-class borate deposits, as representing the climatic event of a caldera collapse, unknown up to now and newly named "Kırka-Phrigian caldera". The caldera, which is roughly oval (24 km x 15km) in shape, one of the largest in Turkey, is supposed to have been formed in a single stage collapse event, at ~19 Ma that generated huge volume extracaldera outflow ignimbrites. Transtensive/distensive tectonic stresses since 25 Ma ago resulted in the NNW-SSE elongation of the magma chamber and influenced the roughly elliptical shape of the subsided block (caldera floor) belonging to the apex of Eskişehir-Afyon-Isparta volcanic area. Intracaldera post-collapse sedimentation and volcanism (at ~ 18 Ma) was controlled through subsidence-related faults with generation of a series of volcanic structures (mainly domes) showing a large compositional range from saturated silicic rhyolites and crystal-rich trachytes to undersaturated lamproites. Such volcanic rock association is typical for lithospheric extension. In this scenario, enriched mantle components within the subcontinental lithospheric mantle will begin to melt via decompression melting during the initiation of extension. Interaction of these melts with crustal rocks, fractionation processes and crustal anatexis driven by the heat contained in the ascending mantle melts produced the silicic compositions in a large crustal reservoir. Such silicic melts generated the initial eruptions of Kırka-Phrigian caldera ignimbrites. The rock volume and geochemical evidence suggests that silicic volcanic rocks come from a long-lived magma chamber that evolved episodically; after caldera

  17. Geochemical and isotopic constraints on the petrogenesis of the Puesto La Peña undersaturated potassic complex, Mendoza province, Argentina: Geodynamic implications

    NASA Astrophysics Data System (ADS)

    Zappettini, Eduardo O.; Villar, Luisa M.; Hernández, Laura B.; Santos, João O.

    2013-03-01

    Major and trace element and radiogenic and stable isotope data are reported for the Miocene Puesto La Peña undersaturated potassic complex, located in the Argentine Precordillera. The complex is composed of a pyroxenite core surrounded by a malignite-borolanite association, followed by radial and ring dikes composed of alkaline lamprophyres, tephrite to phonolite and intermediate varieties, porphyritic microledmorite, benmoreite, alkaline trachyte and a swarm of ultrabasic ouachitite dikes. Late trachytic to phonolitic volcanic necks and associated dikes crop out surrounding the malignitic-borolanitic massif. Major and trace element data are consistent with their derivation by partial melting from an enriched, probably lithospheric, metasomatized heterogeneous mantle involving spinel- and garnet-facies mantle sources. The trace element and isotope data indicate that all rock types are derived from a common parental magma and are thus cogenetic. Sr, Nd and Pb variations in the different studied lithologies are restricted, except for the evidence of Sr decoupling in the ouachitite samples. Their Sr-Nd isotope compositions follow the "mantle array" defined by oceanic basalts, within the OIB field, consistent with an intraplate depleted source. The genesis of the ouachitite is interpreted to be related to mixing between asthenospheric magma and melts from the lower lithosphere consisting of K-rich metasomatic layers. The Dupal like Pb isotopes signature suggests a mantle modification by introduction of continental crust material in the upper mantle; this is consistent with the Sr-Nd isotope data from the late silica-undersaturated felsic dikes and volcanic necks. Multistage mantle extraction would have occurred by the end of Neoproterozoic to lower Paleozoic times as indicated by the isotopic data. Partial melting was initiated by mantle upwelling decompression during lithosphere extension. Back-arc extensional conditions during the latest Early Miocene (19 Ma

  18. Electrical conductivity and streaming potential coefficient in a moderately alkaline lava series

    NASA Astrophysics Data System (ADS)

    Perrier, Frédéric; Froidefond, Thierry

    2003-05-01

    Coupled hydraulic and electrical problems are being investigated with renewed interest in geophysical applications. In this study, electrical conductivity and streaming potential coefficient (SPC) have been measured in the laboratory for crushed oceanite, hawaite, and trachyte samples from an almost complete moderately alkaline lava series from the Mururoa atoll (French Polynesia). For pH varying from 7 to 9.5 and ionic strength varying from 0.1 to 100 mmol/l, despite the closely related mineralogy of the rocks, contrasted results are obtained for the surface conductivity as defined by Revil and Glover: 0.10±0.07 mS/m, 8.3±0.2 mS/m and 1.05±0.16 mS/m for oceanite, hawaite, and trachyte, respectively, and for the inferred ζ potential: -6.6±0.3 mV, -26.3±0.5 mV, and -14.6±0.3 mV, respectively. The higher values obtained with hawaite suggest that electrical properties are controlled by secondary and accessory minerals such as clay minerals and zeolites resulting from low-temperature seawater alteration, rather than by the magmatic differentiation. Furthermore, a synthesis of results obtained with 20 rock samples indicates that signatures of the alteration process may emerge from the study of the ζ potential as a function of surface conductivity. The results with the lava series can be used to derive estimates of the scaling of basalt resistivity and associated SPC as a function of permeability. For permeability values smaller than 10 -15 m 2, the value of the SPC can be dramatically affected by the value of the surface conductivity. At least in the brittle crust, a better knowledge of the contributions of clay minerals and zeolites to surface conduction is therefore needed, both for the interpretation of electrical conductivity profiles and for the estimation of electrical variations associated with groundwater flow, as could be produced for example during volcanic or tectonic cycles.

  19. 40Ar/39Ar dating of the eruptive history of Mount Erebus, Antarctica: volcano evolution

    NASA Astrophysics Data System (ADS)

    Esser, Richard P.; Kyle, Philip R.; McIntosh, William C.

    2004-12-01

    Mt. Erebus, a 3,794-meter-high active polygenetic stratovolcano, is composed of voluminous anorthoclase-phyric tephriphonolite and phonolite lavas overlying unknown volumes of poorly exposed, less differentiated lavas. The older basanite to phonotephrite lavas crop out on Fang Ridge, an eroded remnant of a proto-Erebus volcano and at other isolated locations on the flanks of the Mt. Erebus edifice. Anorthoclase feldspars in the phonolitic lavas are large (~10 cm), abundant (~30 40%) and contain numerous melt inclusions. Although excess argon is known to exist within the melt inclusions, rigorous sample preparation was used to remove the majority of the contaminant. Twenty-five sample sites were dated by the 40Ar/39Ar method (using 20 anorthoclase, 5 plagioclase and 9 groundmass concentrates) to examine the eruptive history of the volcano. Cape Barne, the oldest site, is 1,311±16 ka and represents the first of three stages of eruptive activity on the Mt. Erebus edifice. It shows a transition from sub-aqueous to sub-aerial volcanism that may mark the initiation of proto-Erebus eruptive activity. It is inferred that a further ~300 ky of basanitic/phonotephritic volcanism built a low, broad platform shield volcano. Cessation of the shield-building phase is marked by eruptions at Fang Ridge at ~1,000 ka. The termination of proto-Erebus eruptive activity is marked by the stratigraphically highest flow at Fang Ridge (758±20 ka). Younger lavas (~550 250 ka) on a modern-Erebus edifice are characterized by phonotephrites, tephriphonolites and trachytes. Plagioclase-phyric phonotephrite from coastal and flank flows yield ages between 531±38 and 368±18 ka. The initiation of anorthoclase tephriphonolite occurred in the southwest sector of the volcano at and around Turks Head (243±10 ka). A short pulse of effusive activity marked by crustal contamination occurred ~160 ka as indicated by at least two trachytic flows (157±6 and 166±10 ka). Most

  20. Topographic Attributes of Three Hawaiian Lava Flows: Implications for Evaluation of Lava Flow Emplacement on Mars

    NASA Astrophysics Data System (ADS)

    Zimbelman, J. R.

    2004-12-01

    Differential Global Positioning System surveys were carried out recently across portions of three lava flows on the Big Island of Hawaii. Transects crossed an entire flow in several cases, and in other cases provided detailed information about selected flow margins. The 1907 basalt (a'a) flow from the southwestern rift zone of Mauna Loa has easy access at several points via the Ocean View Estates road system; flow thickness ranges from about 1 m near the middle of the eastern flow lobe to more than 10 m toward the distal end of this flow. Several components of a benmoreite (alkali-rich basaltic andesite) flow complex from Mauna Kea were examined near the small community of Mana (with permission of the Parker Ranch management), on the western flank of the volcano. The flows are more than 14,000 years old and completely covered with soil more than a meter thick, but flow morphology at the decameter scale remains very evident in aerial photographs; some benmoreite flows have up to 30 m of relief along their middle reaches. A trachyte flow more than 100,000 years old extends down slope from Puu Waawaa, on the northern flank of Hualalai; Puu Anahulu represents a very advanced stage of magmatic differentiation that resulted in a flow complex with more than 120 m of relief at its southern margin. Width/thickness represents a good discriminator between these three Hawaiian lava flows. Unfortunately, width is often the most difficult parameter to measure remotely for flows on other planets. Recent imaging data from the Thermal Emission Imaging System on the Mars Odyssey spacecraft reveal important new details of lava flows in the Tharsis region of Mars, some of which can be combined with elevation information from the Mars Orbiter Laser Altimeter. The precise topographic characteristics of diverse Hawaiian lava flows provide a new tool for evaluating the potential emplacement conditions for some Martian lava flows, which appear to be more consistent with the basalt to

  1. Megacrystic Clinopyroxene Basalts Sample A Deep Crustal Underplate To The Mount Taylor Volcanic Field, New Mexico

    NASA Astrophysics Data System (ADS)

    Schmidt, M. E.; Schrader, C. M.; Crumpler, L. S.; Wolff, J. A.

    2012-12-01

    The alkaline and compositionally diverse (basanite to high-Si rhyolite) Mount Taylor Volcanic Field (MTVF), New Mexico comprises 4 regions that cover ~75 x 40 km2: (1) Mount Taylor, a large composite volcano and a surrounding field of basaltic vents; (2) Grants Ridge, constructed of topaz rhyolitic ignimbrite and coulees; (3) Mesa Chivato, a plateau of alkali basalts and mugearitic to trachytic domes; and (4) the Rio Puero basaltic necks. Distributed throughout its history (~3.6 to 1.26 Ma; Crumpler and Goff, 2012) and area (excepting Rio Puerco Necks) is a texturally distinct family of differentiated basalts (Mg# 43.2-53.4). These basalts contain resorbed and moth-eaten megacrysts (up to 2 cm) of plagioclase, clinopyroxene, and olivine ±Ti-magnetite ±ilmenite ±rare orthopyroxene. Some megacrystic lava flows have gabbroic cumulate inclusions with mineral compositions similar to the megacrysts, suggesting a common origin. For instance, gabbroic and megacrystic clinopyroxenes form linear positive arrays in TiO2 (0.2-2.3 wt%) with respect to Al2O3 (0.7-9.3 wt%). The lowest Al clinopyroxenes are found in a gabbroic inclusion and are associated with partially melted intercumulus orthopyroxene. Megacrystic and gabbroic plagioclase (An 41-80) in 4 representative thin sections were analyzed for 87Sr/86Sr by Laser Ablation ICP-MS. 87Sr/86Sr values for the suite range from 0.7036 to 0.7047. The low 87Sr/86Sr plagioclases (0.7036 to 0.7037) are associated with high Ti-Al clinopyroxenes. Likewise, the higher 87Sr/86Sr plagioclases (0.7043 to 0.7047) are associated with the low-Al clinopyroxenes. Taken together, these megacrysts track the differentiation of an intrusive body (or related bodies) from alkaline to Si-saturated conditions by fractional crystallization and crustal assimilation. The intrusive body likely underplates portions of the MTVF that have generated silicic magmas (Mount Taylor, Grants Ridge, Mesa Chivato). Although disequilibrium is implied by resorbed

  2. Petrochemical features of Miocene volcanism around the Çubukludağ graben and Karaburun peninsula, western Turkey: Implications for crustal melting related silicic volcanism

    NASA Astrophysics Data System (ADS)

    Karacık, Z.; Genç, Ş. C.; Gülmez, F.

    2013-09-01

    Widespread Neogene volcanism, mainly intermediate and rarely mafic and felsic in composition, was controlled by the extensional tectonic regime in western Turkey. The Karaburun and Cumaovası volcanics are the cases for understanding the magma source(s) and petrological processes, producing the extension-related mafic and felsic volcanism. The Karaburun volcanics (KV) are mainly oriented north to south in the Karaburun peninsula and span a wide spectrum from basalt (20 Ma) to rhyolite (16 Ma), and younger trachyte and trachydacites (13 Ma). The products of the subaerial silicic volcanism (the Cumaovası volcanics, CV; 17 Ma) which are represented by cluster of rhyolite domes, related pyroclastics occur within the NE-SW trending Çubukludağ graben, and intermediate and mafic volcanic rocks are lack in this area. The lavas of the Cumaovası volcanics are high silica rhyolites and rare dacites which are calc alkaline, peralumious and enriched significantly in LILE. Extremely low Sr, Ba values, extremely Eu depletions and very low LaN/YbN ratios are typical for the rhyolites of CV, similar to the topaz rhyolites. The Karaburun volcanics, with the exception of the minor alkaline basaltic and trachytic lavas, are mainly calc alkaline and metaluminous intermediate lavas. 87Sr/86Sr ratios of the KV and dacitic samples of CV are close to each other and range from 0.708 to 0.709; while Sr isotopic ratios of the rhyolites are significantly high and variable (0.724-0.786). 143Nd/144Nd ratios of the CV and KV, except for the alkaline samples, are similar for both sequences vary from 0.51230 to 0.51242. Geological, geochemical, isotopic and radiochronologic data reveal that the KV and CV were formed in extensional tectonic setting, but evolved by different petrological processes in different magma chambers. During the Neogene, underplated mafic magma was injected into the crust and hybridized by mantle and crustal derived materials. Geochemical features and trace element

  3. Evidence of a Pre-eruptive Fluid Phase for the Millennium Eruption, Paektu Volcano, North Korea

    NASA Astrophysics Data System (ADS)

    Iacovino, K.; Sisson, T. W.; Lowenstern, J. B.

    2014-12-01

    We present initial results of a study of comenditic to trachytic melt inclusions from the Millennium Eruption (ME) of Paektu volcano (AD 946; VEI≥7; 25 km3 DRE). Paektu volcano (aka Changbaishan) is a remote and poorly studied intraplate stratovolcano whose 37 km2 caldera is bisected by the political border between North Korea and China, limiting studies of its proximal deposits. ME magmas are predominantly phenocryst-poor (≤3 vol%) comendites plus a volumetrically minor late-stage, more phenocryst-rich (10-20 vol%) trachyte. Sizeable (100-500 µm diameter) glassy but bubble-bearing melt inclusions are widespread in anorthoclase and hedenbergite phenocrysts, as well as in rarer quartz and fayalite phenocrysts. Comparing the relative enrichments of incompatible volatile and non-volatile elements in melt inclusions along a liquid line of descent shows decreasing volatile/Zr ratios suggesting the partitioning of volatiles into a fluid phase. This suggests that current gas-yield estimates (Horn & Schminke, 2000) for the ME, based on the petrologic method (difference in volatiles between melt inclusions and matrix glass), could be severe underestimates. Establishing the composition and quantity of a pre-eruptive fluid phase is the primary goal of this study and has implications for eruption triggering and for modeling the climatic effects of one of the largest eruptions in the last 10,000 years. Including results from Horne and Schminke (2000), melt inclusions from within a single pumice fall unit show a wide range in dissolved volatile contents and magma chemistries. Concentrations of H2O are moderate (2-3.5 wt% via FTIR), with Cl and F ranging from 500-4600 ppm and 1100-4700 ppm (via EPMA). CO2 is below the detection limit of 2 ppm (FTIR with N2 purge) in bubble-bearing melt inclusions, but is detectable (≤56 ppm) in melt inclusions homogenized at 100 MPa and 850-900 °C for ~30 min (conditions also leading to reduction of dissolved H2O to 0.6-2 wt

  4. 40Ar/(39)Ar dating of the Kapthurin Formation, Baringo, Kenya.

    PubMed

    Deino, Alan L; McBrearty, Sally

    2002-01-01

    The(40)Ar/(39)Ar radiometric dating technique has been applied to tuffs and lavas of the Kapthurin Formation in the Tugen Hills, Kenya Rift Valley. Two variants of the(40)Ar/(39)Ar technique, single-crystal total fusion (SCTF) and laser incremental heating (LIH) have been employed to date five marker horizons within the formation: near the base, the Kasurein Basalt at 0.61+/-0.04 Ma; the Pumice Tuff at 0.543+/-0.004 Ma; the Upper Kasurein Basalt at 0.552+/-0.015 Ma; the Grey Tuff at 0.509+/-0.009 Ma; and within the upper part of the formation, the Bedded Tuff at 0.284+/-0.012 Ma. The new, precise radiometric age determination for the Pumice Tuff also provides an age for the widespread Lake Baringo Trachyte, since the Pumice Tuff is the early pyroclastic phase of this voluminous trachyte eruption. These results establish the age of fossil hominids KNM-BK 63-67 and KNM-BK 8518 at approximately 0.510-0.512 Ma, a significant finding given that few Middle Pleistocene hominids are radiometrically dated. The Kapthurin hominids are thus the near contemporaries of those from Bodo, Ethiopia and Tanzania. A flake and core industry from lacustrine sediments in the lower part of the formation is constrained by new dates of 0.55-0.52 Ma, a period during which the Acheulian industry, characterized by handaxes, is known throughout East Africa. Points, typical of the Middle Stone Age (MSA), are found in Kapthurin Formation sediments now shown to date to between 0.509+/-0.009 Ma and 0.284+/-0.012 Ma. This date exceeds previous estimates for the age of the MSA elsewhere in East Africa by 49 ka, and establishes the age of Acheulian to MSA transition for the region. Evidence of the use of the Levallois technique for the manufacture of both small flakes and biface preforms, the systematic production of blades, and the use and processing of red ochre also occurs in this interval. The presence of blades and red ochre at this depth is important as blades signify a high degree of technical

  5. Petrogenesis of the Sabongari alkaline complex, cameroon line (central Africa): Preliminary petrological and geochemical constraints

    NASA Astrophysics Data System (ADS)

    Njonfang, Emmanuel; Tchoneng, Gilbert Tchuenté; Cozzupoli, Domenico; Lucci, Federico

    2013-07-01

    The petrography, mineral chemistry and geochemical features of the Sabongari alkaline complex are presented and discussed in this paper with the aim of constraining its petrogenesis and comparing it with other alkaline complexes of the Cameroon Line. The complex is mainly made up of felsic rocks: (i) granites predominate and include pyroxene-amphibole (the most abundant), amphibole-biotite, biotite and pyroxene types; (ii) syenites are subordinate and comprise amphibole-pyroxene and amphibole-biotite quartz syenites; (iii) pyroxene-amphibole-biotite trachyte and (iv) relatively abundant rhyolite. The minor basic and intermediate terms associated with felsic rocks consist of basanites, microdiorite and monzodioites. Two groups of pyroxene bearing rocks are distinguished: a basanite-trachyte-granite (Group 1) bimodal series (SiO2 gap: 44 and 63 wt.%) and a basanite-microdiorite-monzodiorite-syenite-granite (Group 2) less pronounced bimodal series (reduced SiO2 gap: 56-67 wt.%). Both are metaluminous to peralkaline whereas felsic rocks bare of pyroxene (Group 3) are metaluminous to peraluminous. The Group 1 basanite is SiO2-undersaturated (modal analcite in the groundmass and 11.04 wt.% normative nepheline); its Ni (240 ppm) and Cr (450 ppm) contents, near mantle values, indicate its most primitive character. The Group 2 basanite is rather slightly SiO2-saturated (1.56 wt.% normative hypersthene), a marker of its high crustal contamination (low Nb/Y-high Rb/Y). The La/Yb and Gd/Yb values of both basanites (1: 19.47 and 2.92; 2: 9.09 and 2.23) suggest their common parental magma composition, and their crystallization through two episodes of partial melting (2% and 3% respectively) of a lherzolite mantle source with <4% residual garnet. The effects of crustal contamination were selectively felt in the values of HFSE/LREE, LREE/LILE and LREE/HFSE ratios, known as indicators. Similar features have been recently obtained in the felsic lavas of the Cameroon Volcanic Line.

  6. Fluid-melt partitioning of sulfur in differentiated arc magmas and the sulfur yield of explosive volcanic eruptions

    NASA Astrophysics Data System (ADS)

    Masotta, M.; Keppler, H.; Chaudhari, A.

    2016-03-01

    The fluid-melt partitioning of sulfur (DSfluid/melt) in differentiated arc magmas has been experimentally investigated under oxidizing conditions (Re-ReO2 buffer) from 800 to 950 °C at 200 MPa. The starting glasses ranged in composition from trachyte to rhyolite and were synthesized targeting the composition of the residual melt formed after 10-60% crystallization of originally trachy-andesitic, dacitic and rhyodacitic magmas (Masotta and Keppler, 2015). Fluid compositions were determined both by mass balance and by Raman spectroscopy of fluid inclusions. DSfluid/melt increases exponentially with increasing melt differentiation, ranging from 2 to 15 in the trachytic melt, from 20 to 100 in the dacitic and rhyodacitic melts and from 100 to 120 in the rhyolitic melt. The variation of the DSfluid/melt is entirely controlled by the compositional variation of the silicate melt, with temperature having at most a minor effect within the range investigated. Experiments from this study were used together with data from the literature to calibrate the following model that allows predicting DSfluid/melt for oxidized arc magmas: where nbo/t is the non-bridging oxygen atoms per tetrahedron, ASI is the alumina saturation index, Al# and Ca# are two empirical compositional parameters calculated in molar units (Al # = XAl2O3/XSiO2 +XTiO2 +XAl2O3 and Ca # = XCaO/XNa2O +XK2O). The interplay between fluid-melt partitioning and anhydrite solubility determines the sulfur distribution among anhydrite, melt and fluid. At increasing melt polymerization, the exponential increase of the partition coefficient and the decrease of anhydrite solubility favor the accumulation of sulfur either in the fluid phase or as anhydrite. On the other hand, the higher anhydrite solubility and lower partition coefficient for less polymerized melts favor the retention of sulfur in the melt. At equilibrium conditions, these effects yield a maximum of the sulfur fraction in the fluid phase for slightly

  7. Excess 210Po in 2010 Eyjafjallajökull tephra (Iceland): Evidence for pre-eruptive gas accumulation

    NASA Astrophysics Data System (ADS)

    Sigmarsson, Olgeir; Condomines, Michel; Gauthier, Pierre-Jean

    2015-10-01

    Excess gas phase in magmas erupting explosively is well known worldwide. However, the origin of this gas phase, in excess of what can be dissolved in the erupting magma at depth, and the rate of gas accumulation is less well defined. The 2010 mildly explosive eruption at Eyjafjallajökull, Iceland, produced mingled tephra of benmoreitic and trachytic composition whereas alkali basalt was emitted during preceding flank eruption. Tephra of the first explosive phase are composed of three glass types, alkaline rhyolite, mixed benmoreite, and basalt, which suggests that the basaltic magma intruded a pre-existing rhyolitic magma chamber, and ultimately triggered the eruption. The mixed benmoreitic tephra (erupted on 15 and 17 April 2010) had large 210Po in excess of 210Pb [(210Po /210Pb) 0 = 1.88 ] at the time of eruption, and possibly a small 210Pb excess over its parent 226Ra. In contrast, the preceding flank eruption produced basalt with (210Po) 0 = 0, upon eruption, and the final trachyte had lost most of its 210Po during open-system degassing. The 210Po excess in the first erupted benmoreites is interpreted to result from 210Po degassing of basaltic magma and the accumulation of 210Po-enriched gas, either in the upper part of the basaltic intrusion, below the rhyolite-basalt interface, or in the pre-existing residual rhyolitic magma chamber. From a simple model of radon and polonium accumulation in the rhyolitic reservoir, the ratio of the mass of basalt magma degassing over the mass of magma accumulating the excess gas decreased from 20 to 15 over 2 days, implying zoned magma reservoir, with the uppermost and gas-richest part erupting first. The duration of pre-eruptive gas accumulation in this model is approximately one year. This corresponds closely to the initiation of a seismic swarm beneath Eyjafjallajökull, early June 2009, which was the first pre-eruptive signal detected. The coincidence between initiation of gas accumulation at relatively shallow depth and

  8. Mafic and felsic igneous rocks at Gale crater

    NASA Astrophysics Data System (ADS)

    Sautter, Violaine; Cousin, Agnès; Mangold, Nicolas; Toplis, Michael; Fabre, Cécile; Forni, Olivier; Payré, Valérie; Gasnault, Olivier; Ollila, Anne; Rapin, William; Fisk, Martin; Meslin, Pierre-Yves; Wiens, Roger; Maurice, Sylvestre; Lasue, Jérémie; Newsom, Horton; Lanza, Nina

    2015-04-01

    The Curiosity rover landed at Gale, an early Hesperian age crater formed within Noachian terrains on Mars. The rover encountered a great variety of igneous rocks to the west of the Yellow Knife Bay sedimentary unit (from sol 13 to 800) which are float rocks or clasts in conglomerates. Textural and compositional analyses using MastCam and ChemCam Remote micro Imager (RMI) and Laser Induced Breakdown Spectroscopy (LIBS) with a ˜300-500 µm laser spot lead to the recognition of 53 massive (non layered) igneous targets, both intrusive and effusive, ranging from mafic rocks where feldspars form less than 50% of the rock to felsic samples where feldspar is the dominant mineral. From morphology, color, grain size, patina and chemistry, at least 5 different groups of rocks have been identified: (1) a basaltic class with shiny aspect, conchoidal frature, no visible grains (less than 0.2mm) in a dark matrix with a few mm sized light-toned crystals (21 targets) (2) a porphyritic trachyandesite class with light-toned, bladed and polygonal crystals 1-20 mm in length set in a dark gray mesostasis (11 targets); (3) light toned trachytes with no visible grains sometimes vesiculated or forming flat targets (6 targets); (4) microgabbro-norite (grain size < 1mm) and gabbro-norite (grain size >1 mm) showing dark and light toned crystals in similar proportion ( 8 targets); (5) light-toned diorite/granodiorite showing coarse granular (>4 mm) texture either pristine or blocky, strongly weathered rocks (9 rock targets). Overall, these rocks comprise 2 distinct geochemical series: (i) an alkali-suite: basanite, gabbro trachy-andesite and trachyte) including porphyritic and aphyric members; (ii) quartz-normative intrusives close to granodioritic composition. The former looks like felsic clasts recently described in two SNC meteorites (NWA 7034 and 7533), the first Noachian breccia sampling the martian regolith. It is geochemically consistent with differentiation of liquids produced by low

  9. Monogenetic volcanic fields and their geoheritage values of western Saudi Arabia and their implication to holistic geoeducation projects locally and globally (Invited)

    NASA Astrophysics Data System (ADS)

    Nemeth, K.; Moufti, R.

    2013-12-01

    Monogeneitc volcanic fields are the most common manifestation of volcanism on Earth and other planets. They composed of small volume and short lived volcanoes each of them with a relatively simple eruption history. In spite of recent researches demonstrated complex, repeated and geochemically distinct eruption histories commonly associated with te formation of small-volume volcanoes, they are still considerred as volcanoes that are in human-scale and therefore ideal to use them as educational tools or part of volcanic geoheritage projects including geopark developments. In the western margin of the Kingdom of Saudi Arabia there are at least 9 intracontinental volcanic fields subparalell with the Red Sea Rift ranging from alkaline basaltic to basalt-trachyte bimodal dispersed volcanic systems. Among these volcanic fields the geoheritage value of three fields were recently evaluated and proposed that they are suitable for further development to establish the first volcanic geoparks in the Arabian Peninsula in the area of 1) Al Madinah (AMVF) 2) Kishb (KVF) and 3) Hutaymah Volcanic Fields (HVF). The AMVF offers a natural concept based on specific volcanic precinct ordering of its volcanic geoheritages from the most accessable and most common volcanism that is historically significant (eg. scoria and lava spatter cones with extensive lava fields) toward a more adventure geotourism style approach in remote, less common but more destructive type of volcanism (eg. trachytic explosion craters). In the contrary, the KVF is a perfect site where phreatomagmatic volcanism and their consequences were identified as a major driving force for further geopark developments. The HVF with its rich archaeological and cultural sites and superbly exposed variously eroded tuff rings and maars offer a good location to develop geoeducation programs to highlight short- and long-term climatic and hydrologic changes in an area a volcanic field evolved. The three Saudi projects also demonstrate

  10. Improved Constraints on the Eruptive History of Northern Harrat Rahat Volcanic Field, Kingdom of Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Stelten, M. E.; Downs, D. T.; Calvert, A. T.; Sherrod, D. R.; Hassan, K. H.; Muquyyim, F. A.; Ashur, M. S.

    2015-12-01

    Harrat Rahat is a large (~20,000 km2) alkalic volcanic field located in central western Saudi Arabia. A variety of eruptive products ranging from alkali basalt to trachyte have erupted at Harrat Rahat over the past ~10 m.y., with the most recent eruptions occurring at 641 CE (uncertain) and 1256 CE in the northern part of the volcanic field. Despite the field's young age and its close proximity to two major city centers, the eruptive history of Harrat Rahat remains poorly constrained. Previous researchers grouped the volcanic strata of northern Harrat Rahat into seven subunits based on limited K-Ar and 40Ar/39Ar dating, and on the degree of erosion displayed by the eruptive products. The youngest eruptive products (subunits Qm7 - Qm4) are thought to be ≤600 ka, whereas the older lavas (Qm3 - Qm1) are thought to be >600 ka. However, due to the sparse geochronologic control on the ages of the eruptive units, it remains unclear if the currently defined subunits accurately reflect the age distribution of lavas in northern Harrat Rahat. Additionally, the temporal relation between basaltic magmatism and the more evolved eruptive products has yet to be examined. To better constrain the eruptive history of Harrat Rahat we measured >50 new 40Ar/39Ar eruption ages for Qm1 through Qm5 lavas in northern Harrat Rahat. These new 40Ar/39Ar ages suggest that the majority of volcanism in the region occurred ≤400 ka and is significantly younger than previously thought, indicating that the magmatic system at Harrat Rahat has been more active over the past 400 kyr then previously recognized. Additionally, these new age data suggest that nearly all trachytic magmatism occurred <125 ka and was preceded by a pulse of more mafic magmatism. It is likely the magmatic system at Harrat Rahat reached an evolved state late in the history of the volcanic field due to increased and/or prolonged input of basaltic magmas into the crust.

  11. Assessment of the atmospheric impact of volcanic eruptions

    NASA Technical Reports Server (NTRS)

    Sigurdsson, H.

    1988-01-01

    The dominant global impact of volcanic activity is likely to be related to the effects of volcanic gases on the Earth's atmosphere. Volcanic gas emissions from individual volcanic arc eruptions are likely to cause increases in the stratospheric optical depth that result in surface landmass temperature decline of 2 to 3 K for less than a decade. Trachytic and intermediate magmas are much more effective in this regard than high-silica magmas, and may also lead to extensive ozone depletion due to effect of halogens and magmatic water. Given the assumed relationship between arc volcanism and subduction rate, and the relatively small variation in global spreading rates in the geologic record, it is unlikely that the rates of arc volcanism have varied greatly during the Cenozoic. Hotspot related basaltic fissure eruptions in the subaerial environment have a higher mass yield of sulfur, but lofting of the valcanic aerosol to levels above the tropopause is required for a climate impact. High-latitude events, such as the Laki 1783 eruption can easily penetrate the tropopause and enter the stratosphere, but formation of a stratospheric volcanic aerosol form low-latitude effusive basaltic eruptions is problematical, due to the elevated low-latitude tropopause. Due to the high sulfur content of hotspot-derived basaltic magmas, their very high mass eruption rates and the episodic behavior, hotspots must be regarded as potentially major modifiers of Earth's climate through the action of their volcanic volatiles on the chemistry and physics of the atmosphere.

  12. Volcanology, geochemistry and age of the Lausitz Volcanic Field

    NASA Astrophysics Data System (ADS)

    Büchner, J.; Tietz, O.; Viereck, L.; Suhr, P.; Abratis, M.

    2015-11-01

    The Lausitz (Lusatia) Volcanic Field is part of the Central European Volcanic Province, and its magmas represent an alkaline trend from olivine nephelinites and basanites to trachytes and phonolites, typical for intraplate settings. Neighbouring volcanic fields are the České Středohoří Mountains to the south-west and the Fore-Sudetic Basin in Lower Silesia to the east. More than 1000 volcanic structures associated with approximately 500 vents have been located within this volcanic field. Residuals of scoria cones, lava lakes, lava flows and maar-diatreme in filling occur in situ near the level of the original syn-volcanic terrain. In more deeply eroded structures, volcanic relicts outcrop as plugs or feeders. Evolved rocks occur as monogenetic domes or intrusions in diatremes, while their volcaniclastic equivalents are rare. Twenty-three localities were dated using the 40Ar/39Ar method. The ages range from 35 to 27 Ma, with a focus around 32-29 Ma, indicating Late Eocene and mainly Oligocene volcanism for the LVF. Differentiated rocks appear to be slightly younger than less differentiated. No geographical age clusters are apparent.

  13. Geochemical trends through time and lateral variability of diatom floras in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley

    NASA Astrophysics Data System (ADS)

    Owen, R. B.; Renaut, R. W.; Potts, R.; Behrensmeyer, A. K.

    The Olorgesailie Formation (1.2-0.49 Ma) consists of fluvial and lacustrine rift sediments that have yielded abundant Acheulean artifacts and a fossil hominin ( Homo cf. erectus). In testing prior understandings of the paleoenvironmental context, we define nine new geochemical zones. A Chemical Index of Alteration suggests increased catchment weathering during deposition of Members 1, 2, 7, 11, and 13. Biophile elements (Br, S) peak in M8-9 and lower M13 possibly reflecting increased input from soil erosion. REE data show that the Magadi Trachytes supplied most siliciclastic grains. Sixteen diatom stages indicate conductivities of 200-16,000 μS cm - 1 and pH of 7.5-9.5 for five deep-water lakes, ten shallow lakes and sixteen wetlands. These results are compared with diatom data from other sections in the basin and show aquatic spatial variability over km-scale distances. Similar floras are traceable over several kilometers for M2, M3 and M9, indicating broadly homogeneous lacustrine conditions during these times, but diatoms in other members imply variable conditions, some related to local tectonic controls. This lateral and temporal variability emphasizes the importance of carrying out stratigraphic sampling at multiple sites within a basin in efforts to define the environmental context relevant to human evolution.

  14. Experimentally observed iron redox kinetics in silicic liquids: Implications for Fe/sup 3 +//Fe/sup 2 +/ variations in rhyolite lava

    SciTech Connect

    Naney, M.T.; Swanson, S.E.

    1989-01-01

    Iron redox kinetics in silicate liquids were investigated by melting 100 mg pellets of compacted rhyolite, pantellerite, pantelleritic trachyte, and andesite rock powders at 1243 and 1343/degree/C in a moderately reducing furnace atmosphere (log fO/sub 2/ = /minus/7.83) for periods of 1 to 4320 minutes. The redox state of glasses produced by quenching these liquids was determined by colorimetric analysis of the ferrous iron and total iron content. Redox equilibrium, indicated by the attainment of a constant FeO/FeO/sub tot/ ratio, was observed for all temperature-composition conditions studied, except for 1243/degree/C experiments with USGS rhyolite standard RGM-1. This is consistent with the low diffusivity of reacting components in high viscosity rhyolite liquids. In the 1243/degree/C experiments with RGM-1, no change in the FeO/FeO/sub tot/ ratio was observed after 4320 minutes. This implies that redox equilibrium is not maintained in natural rhyolite lavas which erupt as significantly lower temperatures (720--850/degree/C). We conclude that sluggish redox kinetics precludes major changes in the oxidation state of a rhyolite magma during the eruption process. If this is true, then the quenched magma, represented by glassy rhyolites, preserves the pre-eruption redox signature of the magma. 2 refs.

  15. Crystal chemistry of diagenetic zeolites in volcanoclastic deposits of Italy

    NASA Astrophysics Data System (ADS)

    Passaglia, Elio; Vezzalini, Giovanna

    1985-07-01

    Zeolites from the most important volcanoclastic deposits of Italy include: (1) phillipsite and heulandite from the cinerite of the central northern Apennines; (2) chabazite and phillipsite from the phonolitic tephritic ignimbrite with black pumices; (3) phillipsite from the “tufo lionato” of Vulcano Laziale; (4) chabazite and phillipsite from the Campanian ignimbrite; (5) phillipsite from the Neapolitan yellow tuff; and (6) chabazite and phillipsite from the pyroclastics of Monte Vulture. Compared with sedimentary phillipsites and chabazites described in the literature, the chabazites and phillipsites studied here have lower Si/Al ratios and higher K contents. These chemical peculiarities are correlated with both the K-rich vesuvitic-leucititic, latitic-phonolitic, and potassic alkali-trachytic chemistry of the ash from which they were derived and, very likely, with the character of the hydrologically open system environment in which they formed. The zeolite of the heulandite-clinoptilolite group from the cinerite of the central northern Apennines is classified as a true heulandite on the basis of its chemical composition and thermal behavior.

  16. Review of eruptive activity at Tianchi volcano, Changbaishan, northeast China: implications for possible future eruptions

    NASA Astrophysics Data System (ADS)

    Wei, Haiquan; Liu, Guoming; Gill, James

    2013-04-01

    One of the largest explosive eruptions in the past several thousand years occurred at Tianchi volcano, also known as Changbaishan, on the China-North Korea border. This historically active polygenetic central volcano consists of three parts: a lower basaltic shield, an upper trachytic composite cone, and young comendite ash flows. The Millennium Eruption occurred between 938 and 946 ad, and was preceded by two smaller and chemically different rhyolitic pumice deposits. There has been at least one additional, small eruption in the last three centuries. From 2002 to 2005, seismicity, deformation, and the helium and hydrogen gas contents of spring waters all increased markedly, causing regional concern. We attribute this event to magma recharge or volatile exhalation or both at depth, followed by two episodes of addition of magmatic fluids into the overlying aquifer without a phreatic eruption. The estimated present magma accumulation rate is too low by itself to account for the 2002-2005 unrest. The most serious volcanic hazards are ash eruption and flows, and lahars. The available geological information and volcano monitoring data provide a baseline for comprehensive assessment of future episodes of unrest and possible eruptive activity.

  17. Eruptive viscosity and volcano morphology

    NASA Technical Reports Server (NTRS)

    Posin, Seth B.; Greeley, Ronald

    1988-01-01

    Terrestrial central volcanoes formed predominantly from lava flows were classified as shields, stratovolcanoes, and domes. Shield volcanoes tend to be large in areal extent, have convex slopes, and are characterized by their resemblance to inverted hellenic war shields. Stratovolcanoes have concave slopes, whereas domes are smaller and have gentle convex slopes near the vent that increase near the perimeter. In addition to these differences in morphology, several other variations were observed. The most important is composition: shield volcanoes tend to be basaltic, stratovolcanoes tend to be andesitic, and domes tend to be dacitic. However, important exceptions include Fuji, Pico, Mayon, Izalco, and Fuego which have stratovolcano morphologies but are composed of basaltic lavas. Similarly, Ribkwo is a Kenyan shield volcano composed of trachyte and Suswa and Kilombe are shields composed of phonolite. These exceptions indicate that eruptive conditions, rather than composition, may be the primary factors that determine volcano morphology. The objective of this study is to determine the relationships, if any, between eruptive conditions (viscosity, erupted volume, and effusion rate) and effusive volcano morphology. Moreover, it is the goal of this study to incorporate these relationships into a model to predict the eruptive conditions of extraterrestrial (Martian) volcanoes based on their morphology.

  18. the sub-Plinian Greenish Pumice eruption (19,065±105 yr cal BP) of Mount Somma - Vesuvius. Geochemical and textural constrains.

    NASA Astrophysics Data System (ADS)

    Zdanowicz, Géraldine; Boudon, Georges; Balcone-Boissard, Hélène; Cioni, Raffaello; Mundula, Filippo; Orsi, Giovanni; Civetta, Lucia

    2016-04-01

    Researches are currently focused on large intensity and stable eruptive columns as for Plinian event. But the large variability in deposits issued from sub-Plinian eruptions needs more observations, theoretical and experimental investigations to be better described and enhances criteria of classification and the knowledge on processes at the origin of this unsteadiness of various timescales. Here, we focus on the well-known example of sub-Plinian eruption exhibiting by Mount Somma-Vesuvius: the Greenish Pumice eruption (GP). On the basis of coupled geochemical and textural analyses we investigate the volatile behavior (H2O, CO2 and halogen (F, Cl)) to better constrain (1) the magma reservoir location and pre-eruptive state and (2) the sub-Plinian eruptive style through a detailed study of the degassing processes in relation with the dynamic of the eruptive column. Results evidence that Cl act as a geobarometer for the trachytic-phonolitic melt involved during the eruption indicating that magma reservoir was at 100 MPa (Cl buffer value: 5300 ±130 ppm) and wholly H2O-saturated (pre-eruptive H2O content between 3.8 and 5.2 wt%). The eruption dynamic is clearly explained by open-system degassing processes responsible of the eruptive column instability, correlated to textural heterogeneities of the eruptive products reflecting conduit heterogeneity (smaller diameter and higher horizontal gradient in magma ascent velocity).

  19. Geochemical composition, petrography and 40Ar/39Ar age of the Heldburg phonolite: implications on magma mixing and mingling

    NASA Astrophysics Data System (ADS)

    Abratis, Michael; Viereck, Lothar; Pfänder, Jörg A.; Hentschel, Roland

    2015-11-01

    Differentiated magmatic rocks such as trachyte and phonolite are volumetrically subordinate to mafic volcanic rocks within the Cenozoic Central European Volcanic Province (exceptions are the East Eifel and the Rhön volcanic fields). Within the volcanic field of the "Heldburg dike swarm" (Heldburger Gangschar), the phonolite of the Burgberg near Heldburg represents the only known occurrence of differentiated magmatic rocks. However, the Heldburg phonolite is famous foremost for containing mantle xenoliths (spinel lherzolite). Former studies proposing a cogenetic relationship between the phonolite and the peridotites concluded that the phonolite magma must have evolved under upper mantle conditions. Herewith, we present petrographic and geochemical evidence for magma mixing and mingling in the Heldburg phonolite melt due to the intrusion of mantle-derived basanitic magma, which is exposed today as dikes at the foot of the Heldburg Burgberg. During this process, the mantle xenoliths were introduced into the phonolite melt as they all contain rims of basanitic magma. Extensive mingling features (e.g., schlieren layers, load casts, flame structures, mafic enclaves) are developed, indicating that the basanite and the zoned phonolitic body were melts at the time of mixing. These petrographic and geochemical indications of two coeval melts of different composition are substantiated by 40Ar/39Ar dating, revealing identical ages of ca. 15 Ma.

  20. Chemistry and chronology of magmatic processes, Central Kenya Peralkaline province, East African Rift

    NASA Astrophysics Data System (ADS)

    Anthony, E.; Deino, A. L.; White, J. C.; Omenda, P. A.

    2014-12-01

    We report here a synthesis of the geochemistry of magma evolution correlated with 40Ar/39Ar, 14 C, and U-series chronology for volcanoes in the Central Kenya Peralkaline Province (CKPP). The volcanic centers - Menengai, Eburru, Olkaria, Longonot, and Suswa - are at the apex of the Kenya Dome, and consist of trachyte, phonolite, comendite, and pantellerite. These volcanic centers are within the graben of the EARS and are characterized by a shield-building phase followed by caldera collapse and subsequent post-caldera eruptions. Geochemical modeling demonstrates that the magmas are the result of fractional crystallization of alkali basaltic magmas and magma mixing. Longonot and Suswa have the most chronologic data -14 C, Ar/Ar and U-series - and they show that the youngest eruptions have 230 Th/232Th of 0.8, which was inherited from the magma system prior to eruption. Subsequent changes in 230 Th/232 Th are due to post-eruptive decay of 230 Th and correlate well with 14 C and Ar/Ar.

  1. Magma mixing in a zoned alkalic intrusion

    SciTech Connect

    Price, J.G.; Henry, C.D.; Barker, D.S.; Rubin, J.N.

    1985-01-01

    The Marble Canyon stock is unique among the alkalic intrusions of the Trans-Pecos magmatic province in being zoned from a critically silica-undersaturated rim of alkali gabbro (AG) to a silica-oversaturated core of quartz syenite (QS). Hybrid rocks of intermediate chemical and mineralogical compositions occur between the rim and core. Nepheline-syenite dikes occur only within the AG. Silica-rich dikes of quartz trachyte, pegmatite, and aplite cut the AG, QS, and hybrid rocks. Thermodynamic calculations of silica activity in the magmas illustrate the presence of two trends with decreasing temperature: a silica-poor trend from AG to nepheline syenite and a silica-rich trend from hybrid rocks to QS. Least-square modeling of rock and mineral compositions suggests 1) the nepheline syenites were derived by crystal-liquid fractionation from nearly solidified AG at the rim of the stock, 2) AG magma farther from the rim mixed with a small proportion of granitic magma, and 3) the mixture then differentiated to produce the hybrid rocks and QS. Zirconium dioxide inclusions in plagioclase crystals of the hybrid rocks and QS indicate that the AG magma contained some crystals before it mixed with the granitic magma. Two origins for the granitic magma are possible: 1) a late-stage differentiate of a mantle-derived hypersthene-normative magma and 2) melting of crustal material by the AG magma. Recognition of magma mixing might not have been possible if the AG had been hypersthene-normative.

  2. Trace element evaluation of a suite of rocks from Reunion Island, Indian Ocean

    USGS Publications Warehouse

    Zielinski, R.A.

    1975-01-01

    Reunion Island consists of an olivine-basalt shield capped by a series of flows and intrusives ranging from hawaiite through trachyte. Eleven rocks representing the total compositional sequence have been analyzed for U, Th and REE. Eight of the rocks (group 1) have positive-slope, parallel, chondrite-normalized REE fractionation patterns. Using a computer model, the major element compositions of group 1 whole rocks and observed phenocrysts were used to predict the crystallization histories of increasingly residual liquids, and allowed semi-quantitative verification of origin by fractional crystallization of the olivine-basalt parent magma. Results were combined with mineral-liquid distribution coefficient data to predict trace element abundances, and existing data on Cr, Ni, Sr and Ba were also successfully incorporated in the model. The remaining three rocks (group 2) have nonuniform positive-slope REE fractionation patterns not parallel to group 1 patterns. Rare earth fractionation in a syenite is explained by partial melting of a source rich in clinopyroxene and/or hornblende. The other two rocks of group 2 are explained as hybrids resulting from mixing of syenite and magmas of group 1. ?? 1975.

  3. Ash generation and distribution from the April-May 2010 eruption of Eyjafjallajökull, Iceland

    PubMed Central

    Gudmundsson, Magnús T.; Thordarson, Thorvaldur; Höskuldsson, Ármann; Larsen, Gudrún; Björnsson, Halldór; Prata, Fred J.; Oddsson, Björn; Magnússon, Eyjólfur; Högnadóttir, Thórdís; Petersen, Guðrún Nína; Hayward, Chris L.; Stevenson, John A.; Jónsdóttir, Ingibjörg

    2012-01-01

    The 39-day long eruption at the summit of Eyjafjallajökull volcano in April–May 2010 was of modest size but ash was widely dispersed. By combining data from ground surveys and remote sensing we show that the erupted material was 4.8±1.2·1011 kg (benmoreite and trachyte, dense rock equivalent volume 0.18±0.05 km3). About 20% was lava and water-transported tephra, 80% was airborne tephra (bulk volume 0.27 km3) transported by 3–10 km high plumes. The airborne tephra was mostly fine ash (diameter <1000 µm). At least 7·1010 kg (70 Tg) was very fine ash (<28 µm), several times more than previously estimated via satellite retrievals. About 50% of the tephra fell in Iceland with the remainder carried towards south and east, detected over ~7 million km2 in Europe and the North Atlantic. Of order 1010 kg (2%) are considered to have been transported longer than 600–700 km with <108 kg (<0.02%) reaching mainland Europe. PMID:22893851

  4. Enhanced crystal fabric analysis of a lava flow sample by neutron texture diffraction: A case study from the Castello d'Ischia dome

    NASA Astrophysics Data System (ADS)

    Walter, Jens M.; Iezzi, Gianluca; Albertini, Gianni; Gunter, Mickey E.; Piochi, Monica; Ventura, Guido; Jansen, Ekkehard; Fiori, Fabrizio

    2013-01-01

    The crystal fabric of a lava has been analyzed for the first time by neutron texture diffraction. In this study we quantitatively investigate the crystallographic preferred orientation of feldspars in the Castello d'Ischia (Ischia Island, Italy) trachytic exogenous dome. The crystallographic preferred orientation was measured with the monochromatic neutron texture diffractometer SV7 at the Forschungszentrum Jülich in Germany and a Rietveld refinement was applied to the sum diffraction pattern. The complementary thin section analysis showed that the three-dimensional crystal shape and the corresponding shape preferred orientation are in agreement with the quantitative orientation distributions of the neutron texture data. The (0k0) crystallographic planes of the feldspars are roughly parallel to the local flow bands, whereas the other corresponding pole figures show that a pivotal rotation of the anorthoclase and sanidine crystals was active during the emplacement of this lava dome. In combination with scanning electron microscopy investigations, electron probe microanalysis, XRF, and X-ray diffraction, the Rietveld refinement of the neutron diffraction data indicates a slow cooling dynamic on the order of several months during their crystallization under subaerial conditions. Results attained here demonstrate that neutron texture diffraction is a powerful tool that can be applied to lava flows.

  5. Silurian magmatism in eastern Senegal and its significance for the Paleozoic evolution of NW-Gondwana

    NASA Astrophysics Data System (ADS)

    Fullgraf, Thomas; Ndiaye, Papa Moussa; Blein, Olivier; Buscail, François; Lahondère, Didier; Le Métour, Joël; Sergeev, Sergey; Tegyey, Monique

    2013-02-01

    Submarine basalt and trachyte of the Nandoumba group occur in eastern Senegal within the Bassarides branch of the Mauritanides orogen. The unit forms part of the parautochthonous domain which is stacked between underlying Neoproterozoic to Paleozoic foreland and overlying Variscan nappes. The crystallisation age of the volcanic to subvolcanic rocks has been determined by U-Pb single zircon SHRIMP method at 428 ± 5.2 Ma whereas zircon xenocryst ages vary from 500 to 2200 Ma. The shape of the xenocryst grains document proximal Neo- and Paleoproterozoic and distal Mesoproterozoic provenance areas for assimilated sediments. This is compatible with the Paleoproterozoic Birimian basement and Neoproterozoic cover rocks nearby whereas an origin from the Amazonian craton could be assumed for distal Mesoproterozoic zircons. Geochemical and Sm-Nd isotope whole rock analysis show that basalts of the Nandoumba group are similar to modern transitional to alkaline volcanic lavas in intraplate settings. Those basalts have a deep mantle source with a great contribution of a recycled mantle component such as EM1 and/or EM2. The basalts resemble in their composition those from the Meguma terrane of Nova Scotia which are of similar age suggesting a common source and therefore connection of Meguma with Gondwana during this period. Review of circum-Atlantic Silurian magmatism indicates ongoing fragmentation of NW-Gondwana that started in Cambro/Ordovician times.

  6. Gels composed of sodium-aluminum silicate, Lake Magadi, Kenya

    USGS Publications Warehouse

    Eugster, H.P.; Jones, B.F.

    1968-01-01

    Sodium-aluminum silicate gels are found in surftcial deposits as thick as 5 centimeters in the Magadi area of Kenya. Chemical data indicate they are formed by the interaction of hot alkaline springwaters (67?? to 82??C; pH, about 9) with alkali trachyte flows and their detritus, rather than by direct precipitation. In the process, Na2O is added from and silica is released to the saline waters of the springs. Algal mats protect the gels from erosion and act as thermal insulators. The gels are probably yearly accumulates that are washed into the lakes during floods. Crystallization of these gels in the laboratory yields analcite; this fact suggests that some analcite beds in lacustrine deposits may have formed from gels. Textural evidence indicates that cherts of rocks of the Pleistocene chert series in the Magadi area may have formed from soft sodium silicate gels. Similar gels may have acted as substrates for the accumulation and preservation of prebiological organic matter during the Precambrian.

  7. The Sagatu Ridge dike swarm, Ethiopian rift margin. [tectonic evolution

    NASA Technical Reports Server (NTRS)

    Mohr, P. A.; Potter, E. C.

    1976-01-01

    A swarm of dikes forms the core of the Sagatu Ridge, a 70-km-long topographic feature elevated to more than 4000 m above sea level and 1500 m above the level of the Eastern (Somalian) plateau. The ridge trends NNE and lies about 50 km east of the northeasterly trending rift-valley margin. Intrusion of the dikes and buildup of the flood-lava pile, largely hawaiitic but with trachyte preponderant in the final stages, occurred during the late Pliocene-early Pleistocene and may have been contemporaneous with downwarping of the protorift trough to the west. The ensuing faulting that formed the present rift margin, however, bypassed the ridge. The peculiar situation and orientation of the Sagatu Ridge, and its temporary existence as a line of crustal extension and voluminous magmatism, are considered related to a powerful structural control by a major line of Precambrian crustal weakness, well exposed further south. Transverse rift structures of unknown type appear to have limited the development of the ridge to the north and south.

  8. Insights into the Structure and Surface Geology of Isla Socorro, Mexico, from Airborne Magnetic and Gamma-Ray Surveys

    NASA Astrophysics Data System (ADS)

    Paoletti, V.; Gruber, S.; Varley, N.; D'Antonio, M.; Supper, R.; Motschka, K.

    2016-05-01

    The island of Socorro is located in the eastern Pacific Ocean, 650 km off the coast of Mexico. It is a rare example of an oceanic volcanic island whose above sea level volume is made up mostly of peralkaline trachytes and rhyolites, with subordinate mafic rocks. Subaerial volcanism started several hundred thousand years ago and continues until recent times. We present an investigation of surface and subsurface geology of the island, based on the first detailed extensive geophysical survey on the island. Acquired airborne magnetic and gamma-ray data were compared to existing geological information and supplemented with field investigations and satellite imagery. Magnetic data show a wide minimum in the central part of the island, possibly connected to a high-temperature zone in the deeper central portion of the volcano, likely to be due to a still hot magma body. The data also depict two parallel edges possibly suggesting the existence of a nested caldera. Analysis on upward continued magnetic data by recent imaging techniques highlighted two deep sources located around 5 km b.s.l., interpreted as feeding structures that are now filled with crystalline rocks. Gamma-ray data have been interpreted through integration with the geological survey results. Several previously known volcanic deposits have been identified based on radioelement distribution, and others have been redefined based on field evidence. A new succession of volcanic members is proposed, to be verified through more detailed geological mapping, geochemical analyses of rock samples and radiometric dating.

  9. Calibration of the Mars Science Laboratory Alpha Particle X-ray Spectrometer

    NASA Astrophysics Data System (ADS)

    Campbell, John L.; Perrett, Glynis M.; Gellert, Ralf; Andrushenko, Stefan M.; Boyd, Nicholas I.; Maxwell, John A.; King, Penelope L.; Schofield, Céleste D. M.

    2012-09-01

    The alpha-particle X-ray spectrometer (APXS) for the Mars Science Laboratory (MSL) mission was calibrated for routine analysis of: Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, Cr, Mn, Fe, Ni, Zn, Br, Rb, Sr, and Y. The following elements were also calibrated, but may be too low to be measured (10s-100s ppm) for their usual abundance on Mars: V, Cu, Ga, As, Se and W. An extensive suite of geological reference materials, supplemented by pure chemical elements and compounds was used. Special attention was paid to include phyllosilicates, sulfates and a broad selection of basalts as these are predicted minerals and rocks at the Gale Crater landing site. The calibration approach is from first principles, using fundamental physics parameters and an assumed homogeneous sample matrix to calculate expected elemental signals for a given instrument setup and sample composition. Resulting concentrations for most elements accord with expected values. Deviations in elements of lower atomic number (Na, Mg, Al) indicate significant influences of mineral phases, especially in basalts, ultramafic rocks and trachytes. The systematics of these deviations help us to derive empirical, iterative corrections for different rock groups, based on a preliminary APXS analysis which assumes a homogeneous sample. These corrections have the potential to significantly improve the accuracy of APXS analyses, especially when other MSL instrument results, such as the X-ray diffraction data from CheMin, are included in the overall analysis process.

  10. Spectral analysis of the Namarunu volcanic complex in the Northern Kenya Rift

    NASA Astrophysics Data System (ADS)

    Riedl, S.; Trauth, M. H.

    2009-04-01

    The Namarunu volcanic complex, situated on the western side of Suguta Valley in the northern part of the Kenya Rift, is dominated by trachytic and basaltic volcanics with a Pliocene to Holocene age range. The analysis of ASTER satellite imagery with special focus on the VNIR and SWIR bands covering a wavelength from 0.5µm to 2.4µm provides the possibility to distinguish these different volcanic rock types by means of spectral characteristics. The visualisation of calculated ratio bands also shows a distinct gradient within alluvial fans and scree surrounding Namarunu, indicating varying source areas. Based on this satellite information, samples both from in-place volcanics and from the enclosing fans were taken for additional spectral analysis. With hyperspectral lab measurements, high resolution spectra of the rock samples were acquired. These spectral signals allow establishing a basic provenance analysis of the fans. Due to limitations of the spectral characteristics of volcanics, the spectral rock classification additionally depends on alteration patterns. As Namarunu itself is active since at least 0.87Ma, its volcanic rocks comprise the latest history of the Rift Valley, including rift tectonics and the influence of lacustrine environment, thus the superimposed climate fluctuations; the analysis of the fans delivers an further insight into the volcanic evolution in Suguta Valley. Presented are the possibilities and limitations of the technique that uses the connection of remote sensing data and field samples, as well as the approach to comprehend the volcanic history of Namarunu with the help of spectral analysis.

  11. Active synchronous counterclockwise rotation and northwards translation of Africa toward Eurasia during the Late Cretaceous: A paleomagnetic study on the Alkaline volcanic field of Wadi Natash (ca. 100-86Ma), South Eastern Desert, Egypt

    NASA Astrophysics Data System (ADS)

    Lotfy, H.

    2009-04-01

    In order to shed light on the paleo-tectonic movement of Africa during the Late Cretaceous, the two end members of the alkaline volcanic field of Wadi Natash (ca. 100-86Ma) in the South Eastern Desert of Egypt were studied paleomagnetically. The Wadi Natash volcanic field (24.5°N-34.25°E) is made up of a succession of differentiated flows grading from alkali olivine basalt [AOB] to trachyte-phonolite [Tr/Ph]. The oldest flows of the AOB (104±7 Ma) and the youngest Tr/Ph plugs and ring dykes (86Ma) as well as the interflows sandstones [ previously know as Nubian sandstone were sampled allover the field > 400km2. The isothermal remanent magnetization [IRM] study revealed that the remanence in Wadi Natash volcanics reside mainly in magnetite with some subsidiary goethite/hematite sites. On the other hand, goethite/hematite are the sole remanence carriers in the Nubian-type interflow sandstone. After the progressive stepwise thermal demagnetization of all samples, the visual isolation and subsequent calculation of the best-fit line of the characteristic remanence [ChRM] direction of each sample, followed by the calculation of the site and rock-unit means revealed that: 1- In the tilt-corrected coordinates, the mean ChRM of the oldest AOB flows [N=12 sites

  12. Distribution, geochemistry and age of the Millennium eruptives of Changbaishan volcano, Northeast China -- A review

    NASA Astrophysics Data System (ADS)

    Sun, Chunqing; You, Haitao; Liu, Jiaqi; Li, Xin; Gao, Jinliang; Chen, Shuangshuang

    2014-04-01

    Large explosive volcanic eruptions generate extensive regional tephra deposits that provide favorable conditions for identifying the source of volcanoes, comparing the sedimentary strata of a region and determining their ages. The tephra layer, referred to as BTm, generated by the Millennium eruption of Changbaishan volcano, is widely distributed in Northeast China, Japan, D.P.R. Korea, and the nearby coastal area of Russia. It forms part of the widespread northeast Asian strata and is significant for establishing an isochronal stratigraphic framework. However, research on the temporal characterization and stratigraphic correlation of associated strata using this tephra layer is mainly concentrated in and near Japan. In northeastern China, this tephra layer is seldom seen and its application in stratigraphic correlations is even rarer. More importantly, the determination of accurate ages for both distal and proximal tephras has been debated, leading to controversy in discussions of its environmental impacts. Stratigraphic records from both distal and proximal Changbaishan ash show that this eruption generally occurred between 1,012 and 1,004 cal yr BP. Geochemical comparison between Changbaishan ash and the Quaternary widespread ash around Japan illustrates that Changbaishan ash is a continuous composition from rhyolitic to trachytic and its ratio of FeOT to CaO is usually greater than 4, which can be used as a distinguishing identifier among worldwide contemporary eruptions.

  13. Distribution, geochemistry and age of the Millennium eruptives of Changbaishan volcano, Northeast China — A review

    NASA Astrophysics Data System (ADS)

    Sun, Chunqing; You, Haitao; Liu, Jiaqi; Li, Xin; Gao, Jinliang; Chen, Shuangshuang

    2014-06-01

    Large explosive volcanic eruptions generate extensive regional tephra deposits that provide favorable conditions for identifying the source of volcanoes, comparing the sedimentary strata of a region and determining their ages. The tephra layer, referred to as B-Tm, generated by the Millennium eruption of Changbaishan volcano, is widely distributed in Northeast China, Japan, D.P.R. Korea, and the nearby coastal area of Russia. It forms part of the widespread northeast Asian strata and is significant for establishing an isochronal stratigraphic framework. However, research on the temporal characterization and stratigraphic correlation of associated strata using this tephra layer is mainly concentrated in and near Japan. In northeastern China, this tephra layer is seldom seen and its application in stratigraphic correlations is even rarer. More importantly, the determination of accurate ages for both distal and proximal tephras has been debated, leading to controversy in discussions of its environmental impacts. Stratigraphic records from both distal and proximal Changbaishan ash show that this eruption generally occurred between 1,012 and 1,004 cal yr BP. Geochemical comparison between Changbaishan ash and the Quaternary widespread ash around Japan illustrates that Changbaishan ash is a continuous composition from rhyolitic to trachytic and its ratio of FeOT to CaO is usually greater than 4, which can be used as a distinguishing identifier among worldwide contemporary eruptions.

  14. Italian zeolitized rocks of technological interest

    NASA Astrophysics Data System (ADS)

    de'Gennaro, M.; Langella, A.

    1996-09-01

    Large areas of Italian territory are covered by thick and widespread deposits of zeolite-bearing volcaniclastic products. The main zeolites are phillipsite and chabazite spread over the whole peninsula, and clinoptilolite recorded only in Sardinia. A trachytic to phonolitic glassy precursor accounts for the formation of the former zeolites characterized by low Si/Al ratios (?3.00), while clinoptilolite is related to more acidic volcanism. The genesis of most of these zeolitized deposits is linked to pyroclastic flow emplacement mechanisms characterized by quite high temperatures and by the presence of abundant fluids. The main utilization of these materials has been and still is as dimension stones in the building industry. Currently, limited amounts are also employed in animal farming (dietary supplement, pet litter and manure deodorizer) and in agriculture as soil improvement and slow-release fertilizers. New fields of application have been proposed for these products on account of their easy availability, very low cost, their high-grade zeolites (50 70%), and good technological features such as high cation exchange capacities and adsorption properties.

  15. Age discrimination among eruptives of Menengai Caldera, Kenya, using vegetation parameters from satellite imagery

    NASA Technical Reports Server (NTRS)

    Blodget, Herbert W.; Heirtzler, James R.

    1993-01-01

    Results are presented of an investigation to determine the degree to which digitally processed Landsat TM imagery can be used to discriminate among vegetated lava flows of different ages in the Menengai Caldera, Kenya. A selective series of five images, consisting of a color-coded Landsat 5 classification and four color composites, are compared with geologic maps. The most recent of more than 70 postcaldera flows within the caldera are trachytes, which are variably covered by shrubs and subsidiary grasses. Soil development evolves as a function of time, and as such supports a changing plant community. Progressively older flows exhibit the increasing dominance of grasses over bushes. The Landsat images correlated well with geologic maps, but the two mapped age classes could be further subdivided on the basis of different vegetation communities. It is concluded that field maps can be modified, and in some cases corrected by use of such imagery, and that digitally enhanced Landsat imagery can be a useful aid to field mapping in similar terrains.

  16. Ash generation and distribution from the April-May 2010 eruption of Eyjafjallajökull, Iceland.

    PubMed

    Gudmundsson, Magnús T; Thordarson, Thorvaldur; Höskuldsson, Armann; Larsen, Gudrún; Björnsson, Halldór; Prata, Fred J; Oddsson, Björn; Magnússon, Eyjólfur; Högnadóttir, Thórdís; Petersen, Guðrún Nína; Hayward, Chris L; Stevenson, John A; Jónsdóttir, Ingibjörg

    2012-01-01

    The 39-day long eruption at the summit of Eyjafjallajökull volcano in April-May 2010 was of modest size but ash was widely dispersed. By combining data from ground surveys and remote sensing we show that the erupted material was 4.8±1.2·10¹¹ kg (benmoreite and trachyte, dense rock equivalent volume 0.18±0.05 km³). About 20% was lava and water-transported tephra, 80% was airborne tephra (bulk volume 0.27 km³) transported by 3-10 km high plumes. The airborne tephra was mostly fine ash (diameter <1000 µm). At least 7·10¹⁰ kg (70 Tg) was very fine ash (<28 µm), several times more than previously estimated via satellite retrievals. About 50% of the tephra fell in Iceland with the remainder carried towards south and east, detected over ~7 million km² in Europe and the North Atlantic. Of order 10¹⁰ kg (2%) are considered to have been transported longer than 600-700 km with <10⁸ kg (<0.02%) reaching mainland Europe. PMID:22893851

  17. Effects of crystallization and bubble nucleation on the elastic properties of magmas

    NASA Astrophysics Data System (ADS)

    Tripoli, B. A.; Ulmer, P.; Eric, R.; Cordonnier, B.; Burg, J.

    2012-12-01

    Seismic tomography of potentially hazardous volcanoes is a prime tool to assess the physical state of magma reservoirs. Processes occurring in the conduit or in the chamber, such as crystallization and bubble exsolution, control the magma rheology, hence the style of volcanic eruption. Elastic parameters of vapor-saturated, partially molten systems are thus providing fundamental information for the identification of such reservoirs under active and seemingly dormant volcanoes. This knowledge will potentially serve to assess their risk. We present preliminary data on compression and shear wave propagation velocities of a chemically simplified melt analogous to andesite and trachyte, in the system CaO-Na2O-Al2O3-SiO2-H2O-CO2. These ultrasonic velocities are measured simultaneously in a Paterson-type internally-heated gas pressure apparatus at confining pressures up to 300 MPa and temperatures up to 1000°C. Using the pulse transmission technique, the experiments are performed at frequencies ranging from 0.1 to 3 MHz. Variations in the elastic parameters induced by the presence of bubbles or dissolved water in glassy samples are discussed for various pressures and temperatures. As the investigated melt undergoes plagioclase crystallization, a thermal plateau is maintained over specific time duration in order to measure the changes in seismic properties of in-situ crystallizing magmas. This maintained temperature varies between 800° and 1000°C depending on the amount of dissolved water in the system.

  18. 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.

  19. Effect of water on the heat capacity of polymerized aluminosilicate glasses and melts

    NASA Astrophysics Data System (ADS)

    Bouhifd, M. Ali; Whittington, Alan; Roux, Jacques; Richet, Pascal

    2006-02-01

    The effect of water on heat capacity has been determined for four series of hydrated synthetic aluminosilicate glasses and supercooled liquids close to albite, phonolite, trachyte, and leucogranite compositions. Heat capacities were measured at atmospheric pressure by differential scanning calorimetry for water contents between 0 and 4.9 wt % from 300 K to about 100 K above the glass transition temperature ( Tg). The partial molar heat capacity of water in polymerized aluminosilicate glasses, which can be considered as independent of composition, is =-122.319+341.631×10-3T+63.4426×105/T2 (J/mol K). In liquids containing at least 1 wt % H 2O, the partial molar heat capacity of water is about 85 J/mol K. From speciation data, the effects of water as hydroxyl groups and as molecular water have tentatively been estimated, with partial molar heat capacities of 153 ± 18 and 41 ± 14 J/mol K, respectively. In all cases, water strongly increases the configurational heat capacity at Tg and exerts a marked depressing effect on Tg, in close agreement with the results of viscosity experiments on the same series of glasses. Consistent with the Adam and Gibbs theory of relaxation processes, the departure of the viscosity of hydrous melts from Arrhenian variations correlates with the magnitude of configurational heat capacities.

  20. Upper mantle magma storage and transport under a Canarian shield-volcano, Teno, Tenerife (Spain)

    NASA Astrophysics Data System (ADS)

    Longpré, Marc-Antoine; Troll, Valentin R.; Hansteen, Thor H.

    2008-08-01

    We use clinopyroxene-liquid thermobarometry, aided by petrography and mineral major element chemistry, to reconstruct the magma plumbing system of the late Miocene, largely mafic Teno shield-volcano on the island of Tenerife. Outer rims of clinopyroxene and olivine phenocrysts show patterns best explained by decompression-induced crystallization upon rapid ascent of magmas from depth. The last equilibrium crystallization of clinopyroxene occurred in the uppermost mantle, from ˜20 to 45 km depth. We propose that flexural stresses or, alternatively, thermomechanical contrasts create a magma trap that largely confines magma storage to an interval roughly coinciding with the Moho at ˜15 km and the base of the long-term elastic lithosphere at ˜40 km below sea level. Evidence for shallow magma storage is restricted to the occurrence of a thick vitric tuff of trachytic composition emplaced before the Teno shield-volcano suffered large-scale flank collapses. The scenario developed in this study may help shed light on some unresolved issues of magma supply to intraplate oceanic volcanoes characterized by relatively low magma fluxes, such as those of the Canary, Madeira and Cape Verde archipelagoes, as well as Hawaiian volcanoes in their postshield stage. The data presented also support the importance of progressive magmatic underplating in the Canary Islands.

  1. A 90,000 200,000 yrs marine tephra record of Italian volcanic activity in the Central Mediterranean Sea

    NASA Astrophysics Data System (ADS)

    Paterne, M.; Guichard, F.; Duplessy, J. C.; Siani, G.; Sulpizio, R.; Labeyrie, J.

    2008-10-01

    A detailed tephrochronological study was undertaken in three deep-sea cores collected in the Tyrrhenian and Ionian Seas. The age and the origin of the marine tephra were inferred from oxygen isotope records of foraminifera and from major element compositions of glass-shards. Seventy-one eruptions were detected in the time interval 90,000-200,000 yrs during which the volcanoes of the Roman and Campanian regions and of the southern Italy were in activity. This is attested by the consistency of the geochemical compositions of both marine and terrestrial deposits. Most of the marine tephra consisted in trachytes and phonolites characterizing a Roman and Campanian origin. Several tephra were proposed as key-horizons for proximal and distal sediments. Among them, one tephra originating from Mount Etna (149,300 yrs) and five tephra from Pantelleria island (130,000 yrs, 163,600 yrs, 192,500 yrs, 197,400 yrs and 198,400 yrs) were northerly dispersed. Several other key horizons originated from the Campanian or Roman provinces were detected as far as 1000 km from the vents.

  2. Geochemical "fingerprints" for Olduvai Gorge Bed II tuffs and implications for the Oldowan-Acheulean transition

    NASA Astrophysics Data System (ADS)

    McHenry, Lindsay J.; Njau, Jackson K.; de la Torre, Ignacio; Pante, Michael C.

    2016-01-01

    Bed II is a critical part of early Pleistocene Olduvai Gorge, Tanzania. Its deposits include transitions from humid to more arid conditions (with associated faunal changes), from Homo habilis to erectus, and from Oldowan to Acheulean technology. Bed II (~ 1.8-1.2 Ma) is stratigraphically and environmentally complex, with facies changes, faulting, and unconformities, making site-to-site correlation over the ~ 20 km of exposure difficult. Bed II tuffs are thinner, less evenly preserved, and more reworked than those of Bed I. Five marker tuffs (Tuffs IIA-IID, Bird Print Tuff (BPT)), plus local tephra, were collected from multiple sites and characterized using stratigraphic position, mineral assemblage, and electron probe microanalysis of phenocryst (feldspar, hornblende, augite, titanomagnetite) and glass (where available) composition. Lowermost Bed II tuffs are dominantly nephelinitic, Middle Bed II tuffs (BPT, Tuff IIC) have basaltic components, and upper Bed II Tuff IID is trachytic. The BPT and Tuff IID are identified widely using phenocryst compositions (high-Ca plagioclase and high-Ti hornblende, respectively), though IID was originally (Hay, 1976) misidentified as Tuff IIC at Loc 91 (SHK Annexe) in the Side Gorge. This work helps establish a high-resolution basin-wide paleolandscape context for the Oldowan-Acheulean transition and helps link hominin, faunal and archaeological records.

  3. Evaluation of the Strength of Railway Ballast Using Point Load Test for Various Size Fractions and Particle Shapes

    NASA Astrophysics Data System (ADS)

    Koohmishi, Mehdi; Palassi, Massoud

    2016-07-01

    The ballast layer is one of the most important components of the railway track superstructure in which angular aggregates of high strength rocks are used. Ballast degradation is one of the main sources of railway problems in which the ballast aggregates are gradually degraded due to the abrasion of the sharp corners of the angular particles and splitting each individual particle into two or several small pieces under loading. In this paper, the effects of rock type, aggregate size and particle shape on the strength of the single ballast particles are investigated. For this purpose, point load test is carried out on ballast aggregates of four rock types including basalt, marl, dolomite and trachyte. According to the obtained results, as the size of the aggregates increases, the point load strength index decreases. The influence of size on the strength is more noticeable for ballasts obtained from higher strength rocks. It is also found that the shape of ballast particles has no major effect on its strength. Furthermore, our findings show that the failure pattern for ballasts of higher strength is so that each particle commonly splits into three pieces; while the dominant failure pattern for ballast particles with less strength is breaking the particle into two pieces.

  4. 40Ar/(39)Ar geochronology and paleomagnetic stratigraphy of the Lukeino and lower Chemeron Formations at Tabarin and Kapcheberek, Tugen Hills, Kenya.

    PubMed

    Deino, Alan L; Tauxe, Lisa; Monaghan, Marc; Hill, Andrew

    2002-01-01

    (40)Ar/(39)Ar single-crystal laser-fusion dating, K-Ar dating, and paleomagnetic reversal stratigraphy have been used to determine the chronostratigraphy of the Kabarnet Trachyte, Lukeino Formation, Kaparaina Basalt Formation, and Chemeron Formation at the sites of Kapcheberek (BPRP#77) and Tabarin (BPRP#77) in the Tugen Hills, Kenya. The succession ranges in age from 6.56-3.8 Ma. The upper Lukeino Formation at Kapcherberek, including the fauna from the site BPRP#76, was deposited during chron C3r and can be constrained to the interval 5.88-5.72 Ma. The Chemeron Formation at Tabarin includes at the base an ignimbrite and associated basal air-fall tuff with a combined age of 5.31+/-0.03 Ma. Sedimentary and volcaniclastic rocks of the Chemeron Formation which unconformably overlie the ignimbrite record chrons C3n.2n through C2Ar. The combined(40)Ar/(39)Ar and paleomagnetic data constrain the age of this sequence to 4.63-3.837 Ma. The age of the Tabarin mandible fragment (KNM-TH 13150) and associated fauna at site BPRP#77 in the Chemeron Formation is 4.48-4.41 Ma, marginally older than similar early hominids from Aramis, Ethiopia. Basin subsidence appears to be defining an overall accumulation rate of about 17 cm/ka over the 2.7 Ma represented at Tabarin and Kapcheberek, despite episodes of rapid accumulation and hiatuses. PMID:11795971

  5. Quantifying the Reduction Intensity of Handaxes with 3D Technology: A Pilot Study on Handaxes in the Danjiangkou Reservoir Region, Central China

    PubMed Central

    Li, Hao; Kuman, Kathleen; Li, Chaorong

    2015-01-01

    This paper presents an approach to analyzing the reduction intensity of handaxes with the aid of 3D scanning technology. Two quantitative reduction indices, the Scar Density Index (SDI) and the Flaked Area Index (FAI), are applied to handaxes from the third terrace of the Danjiangkou Reservoir Region (DRR), central China, dated to the Middle Pleistocene. The results show that most of the DRR handaxes in this sample show moderate reduction, which also reflects a least-effort reduction strategy and a generally short use-life for these tools. Detailed examination of the DRR handaxes by sector reveals that the tips generally show the most reduction, while the bases show the least shaping, with cortex often preserved on the base to facilitate handling. While western Acheulean assemblages in this regard are variable, there are many examples of handaxes of varying age with trimming of the bases. We also found no significant differences in the levels of reduction between the two main raw materials, quartz phyllite and trachyte. However, the type of blank used (large flakes versus cobbles) and the type of shaping (bifacial, partly bifacial and unifacial) do play a significant role in the reduction intensity of the DRR handaxes. Finally, a small number of handaxes from the younger (the early Late Pleistocene) second terrace of the DRR was compared with those from the third terrace. The results indicate that there is no technological change in the reduction intensity through time in these two DRR terraces. PMID:26331954

  6. Nature of basalt-deep crust interaction in the petrogenesis of a potassium-rich, silicic-dominated eruptive system, Davis Mountain volcanic field, west Texas

    SciTech Connect

    Ward, R.L.; Walker, J.A. . Dept. of Geology)

    1993-04-01

    The Davis Mountain volcanic field (DMVF) is one of several silicic-dominated eruptive centers that constitute the bulk of the Trans Pecos volcanic province (TPVP). New major-, trace element, and Pb-O isotope data on local granulite-facies xenoliths and the DMVF are used in evaluating the extent of basalt-deep crust interaction to produce voluminous silicic lavas and -ignimbrites. The DMVF (39.3--35.4 Ma) is a high-K, alkali basalt-potassic trachybasalt-shoshonite-latite-trachyte-rhyolite volcanoplutonic series with the evolved members being silica-saturated. DMF silicic rocks are characterized by high concentrations of Rb, Th, U, and K, low-[sup 18]O and have a broad range in Pb isotopes. These characteristics are inconsistent with an origin by partial melting of a Rb-Th-U depleted, unradiogenic Pb granulitic deep crust. However, distinctly different Pb isotope compositions between mafic and silicic rocks preclude an origin by fractional crystallization alone. Multistage-AFC involving a mantle-source, various proportions of OL-CPX-PLAG-KSPAR-MAG-AP-BIO-QTZ-aenigmatite-ZR differentiation, limited (<10%) amounts of deep and upper crustal contamination, and mixing between mafic and silicic magmas can satisfactorily account for the observed chemical and isotopic variation in the DMVF.

  7. Fault-related carbonate breccia dykes in the La Chilca area, Eastern Precordillera, San Juan, Argentina

    NASA Astrophysics Data System (ADS)

    Castro de Machuca, Brígida; Perucca, Laura P.

    2015-03-01

    Carbonate fault breccia dykes in the Cerro La Chilca area, Eastern Precordillera, west-central Argentina, provide clues on the probable mechanism of both fault movement and dyke injection. Breccia dykes intrude Upper Carboniferous sedimentary rocks and Triassic La Flecha Trachyte Formation. The timing of breccia dyke emplacement is constrained by cross cutting relationships with the uppermost Triassic unit and conformable contacts with the Early Miocene sedimentary rocks. This study supports a tectonic-hydrothermal origin for these breccia dykes; fragmentation and subsequent hydraulic injection of fluidized breccia are the more important processes in the breccia dyke development. Brecciation can be triggered by seismic activity which acts as a catalyst. The escape of fluidized material can be attributed to hydrostatic pressure and the direction of movement of the material establishes the direction of least pressure. Previous studies have shown that cross-strike structures have had an important role in the evolution of this Andean segment since at least Triassic times. These structures represent pre-existing crustal fabrics that could have controlled the emplacement of the dykes. The dykes, which are composed mostly of carbonate fault breccia, were injected upward along WNW fractures.

  8. Quantifying the Reduction Intensity of Handaxes with 3D Technology: A Pilot Study on Handaxes in the Danjiangkou Reservoir Region, Central China.

    PubMed

    Li, Hao; Kuman, Kathleen; Li, Chaorong

    2015-01-01

    This paper presents an approach to analyzing the reduction intensity of handaxes with the aid of 3D scanning technology. Two quantitative reduction indices, the Scar Density Index (SDI) and the Flaked Area Index (FAI), are applied to handaxes from the third terrace of the Danjiangkou Reservoir Region (DRR), central China, dated to the Middle Pleistocene. The results show that most of the DRR handaxes in this sample show moderate reduction, which also reflects a least-effort reduction strategy and a generally short use-life for these tools. Detailed examination of the DRR handaxes by sector reveals that the tips generally show the most reduction, while the bases show the least shaping, with cortex often preserved on the base to facilitate handling. While western Acheulean assemblages in this regard are variable, there are many examples of handaxes of varying age with trimming of the bases. We also found no significant differences in the levels of reduction between the two main raw materials, quartz phyllite and trachyte. However, the type of blank used (large flakes versus cobbles) and the type of shaping (bifacial, partly bifacial and unifacial) do play a significant role in the reduction intensity of the DRR handaxes. Finally, a small number of handaxes from the younger (the early Late Pleistocene) second terrace of the DRR was compared with those from the third terrace. The results indicate that there is no technological change in the reduction intensity through time in these two DRR terraces. PMID:26331954

  9. Gels composed of sodium-aluminium silicate, lake magadi, kenya.

    PubMed

    Eugster, H P; Jones, B F

    1968-07-12

    Sodium-aluminum silicate gels are found in surficial deposits as thick as 5 centimeters in the Magadi area of Kenya. Chemical data indicate they are formed by the interaction of hot alkaline springwaters (67 degrees to 82 degrees C; pH, about 9) with alkali trachyte flows and their detritus, rather than by direct precipitation. In the process, Na(2)O is added from and silica is released to the saline waters of the springs. Algal mats protect the gels from erosion and act as thermal insulators. The gels are probably yearly accumulates that are washed into the lakes during floods. Crystallization of these gels in the laboratory yields analcite; this fact suggests that some analcite beds in lacustrine deposits may have formed from gels. Textural evidence indicates that cherts of rocks of the Pleistocene chert series in the Magadi area may have formed from soft sodium silicate gels. Similar gels may have acted as substrates for the accumulation and preservation of prebiological organic matter during the Precambrian. PMID:17770594

  10. Textural and Mineralogical Analysis of Volcanic Rocks by µ-XRF Mapping.

    PubMed

    Germinario, Luigi; Cossio, Roberto; Maritan, Lara; Borghi, Alessandro; Mazzoli, Claudio

    2016-06-01

    In this study, µ-XRF was applied as a novel surface technique for quick acquisition of elemental X-ray maps of rocks, image analysis of which provides quantitative information on texture and rock-forming minerals. Bench-top µ-XRF is cost-effective, fast, and non-destructive, can be applied to both large (up to a few tens of cm) and fragile samples, and yields major and trace element analysis with good sensitivity. Here, X-ray mapping was performed with a resolution of 103.5 µm and spot size of 30 µm over sample areas of about 5×4 cm of Euganean trachyte, a volcanic porphyritic rock from the Euganean Hills (NE Italy) traditionally used in cultural heritage. The relative abundance of phenocrysts and groundmass, as well as the size and shape of the various mineral phases, were obtained from image analysis of the elemental maps. The quantified petrographic features allowed identification of various extraction sites, revealing an objective method for archaeometric provenance studies exploiting µ-XRF imaging. PMID:27160144

  11. Geochemistry of high-potassium rocks from the mid-Tertiary Guffey volcanic center, Thirtynine Mile volcanic field, central Colorado

    SciTech Connect

    Wobus, R.A.; Mochel, D.W. ); Mertzman, S.A.; Eide, E.A.; Rothwarf, M.T. ); Loeffler, B.M.; Johnson, D.A. ); Keating, G.N.; Sultz, K. ); Benjamin, A.E. ); Venzke, E.A. ); Filson, T. )

    1990-07-01

    The Guffey volcanic center is the largest within the 2000 km{sup 2} mid-Tertiary Thirtynine Mile volcanic field of central Colorado. This study is the first to provide extensive chemical data for these alkalic volcanic and subvolcanic rocks, which present the eroded remnants of a large stratovolcano of Oligocene age. Formation of early domes and flows of latite and trachyte within the Guffey center was followed by extrusion of a thick series of basalt, trachybasalt, and shoshonite flows and lahars. Plugs, dikes, and vents ranging from basalt to rhyolite cut the thick mafic deposits, and felsic tuffs breccias chemically identical to the small rhyolitic plutons are locally preserved. Whole-rack major and trace element analyses of 80 samples, ranging almost continuously from 47% to 78% SiO{sub 2}, indicate that the rocks of the Guffey center are among the most highly enriched in K{sub 2}O (up to 6%) and rare earth elements (typically 200-300 ppm) of any volcanic rocks in Colorado. These observations, along with the relatively high concentrations of Ba and Rb and the depletion of Cr and Ni, suggest an appreciable contribution of lower crustal material to the magmas that produced the Thirtynine Mile volcanic rocks.

  12. Dyella jiangningensis sp. nov., a γ-proteobacterium isolated from the surface of potassium-bearing rock.

    PubMed

    Zhao, Fei; Guo, Xin-qi; Wang, Peng; He, Lin-yan; Huang, Zhi; Sheng, Xia-fang

    2013-09-01

    A Gram-stain-negative, aerobic, motile with one polar flagellum γ-proteobacterium, designated strain SBZ3-12(T), was isolated from surfaces of weathered potassic trachyte. Phylogenetic analysis of this strain based on 16S rRNA gene sequences showed that it was most closely related to Dyella japonica XD53(T) (97.9% 16S rRNA gene sequence similarity), Dyella terrae JS14-6(T) (97.7%), Dyella soli JS12-10(T) (97.5%) and Dyella koreensis BB4(T) (97.0%). The DNA G+C content of strain SBZ3-12(T) was 64.0 mol%. In addition, iso-C(17:1)ω9c, iso-C(15:0) and iso-C(16:0) were the major cellular fatty acids and ubiquinone Q-8 was the predominant respiratory quinone. The low DNA-DNA relatedness values between strain SBZ3-12(T) and recognized species of the genus Dyella and the many phenotypic properties supported the classification of strain SBZ3-12(T) as a representative of a novel species of the genus Dyella, for which the name Dyella jiangningensis sp. nov. is proposed. The type strain is SBZ3-12(T) ( =CCTCC AB 2012160(T) =KACC 16539(T) =DSM 26119(T)). PMID:23435246

  13. Igneous evolution of a complex laccolith-caldera, the Solitario, Trans-Pecos Texas: Implications for calderas and subjacent plutons

    USGS Publications Warehouse

    Henry, C.D.; Kunk, M.J.; Muehlberger, W.R.; McIntosh, W.C.

    1997-01-01

    The Solitario is a large, combination laccolith and caldera (herein termed "laccocaldera"), with a 16-km-diameter dome over which developed a 6 x 2 km caldera. This laccocaldera underwent a complex sequence of predoming sill, laccolith, and dike intrusion and concurrent volcanism; doming with emplacement of a main laccolith; ash-flow eruption and caldera collapse; intracaldera sedimentation and volcanism; and late intrusion. Detailed geologic mapping and 40Ar/39Ar dating reveal that the Solitario evolved over an interval of approximately 1 m.y. in three distinct pulses at 36.0, 35.4, and 35.0 Ma. The size, duration, and episodicity of Solitario magmatism are more typical of large ash-flow calderas than of most previously described laccoliths. Small volumes of magma intruded as abundant rhyolitic to trachytic sills and small laccoliths and extruded as lavas and tuffs during the first pulse at 36.0 Ma. Emplacement of the main laccolith, doming, ash-flow eruption, and caldera collapse occurred at 35.4 Ma during the most voluminous pulse. A complex sequence of debris-flow and debris-avalanche deposits, megabreccia, trachyte lava, and minor ash-flow tuff subsequently filled the caldera. The final magmatic pulse at 35.0 Ma consisted of several small laccoliths or stocks and numerous dikes in caldera fill and along the ring fracture. Solitario rocks appear to be part of a broadly cogenetic, metaluminous suite. Peralkaline rhyolite lava domes were emplaced north and west of the Solitario at approximately 35.4 Ma, contemporaneous with laccolith emplacement and the main pulse in the Solitario. The spatial and temporal relation along with sparse geochemical data suggest that the peralkaline rhyolites are crustal melts related to the magmatic-thermal flux represented by the main pulse of Solitario magmatism. Current models of laccolith emplacement and evolution suggest a continuum from initial sill emplacement through growth of the main laccolith. Although the Solitario

  14. Nd, Sr, and O isotopic variations in metaluminous ash-flow tuffs and related volcanic rocks at the Timber Mountain/Oasis Valley Caldera, Complex, SW Nevada: implications for the origin and evolution of large-volume silicic magma bodies

    USGS Publications Warehouse

    Farmer, G.L.; Broxton, D.E.; Warren, R.G.; Pickthorn, W.

    1991-01-01

    Nd, Sr and O isotopic data were obtained from silicic ash-flow tuffs and lavas at the Tertiary age (16-9 Ma) Timber (Mountain/Oasis Valley volcanic center (TMOV) in southern Nevada, to assess models for the origin and evolution of the large-volume silicic magma bodies generated in this region. The large-volume (>900 km3), chemically-zoned, Topopah Spring (TS) and Tiva Canyon (TC) members of the Paintbrush Tuff, and the Rainier Mesa (RM) and Ammonia Tanks (AT) members of the younger Timber Mountain Tuff all have internal Nd and Sr isotopic zonations. In each tuff, high-silica rhyolites have lower initial e{open}Nd values (???1 e{open}Nd unit), higher87Sr/86Sr, and lower Nd and Sr contents, than cocrupted trachytes. The TS, TC, and RM members have similar e{open}Nd values for high-silica rhyolites (-11.7 to -11.2) and trachytes (-10.5 to -10.7), but the younger AT member has a higher e{open}Nd for both compositional types (-10.3 and -9.4). Oxygen isotope data confirm that the TC and AT members were derived from low e{open}Nd magmas. The internal Sr and Nd isotopic variations in each tuff are interpreted to be the result of the incorporation of 20-40% (by mass) wall-rock into magmas that were injected into the upper crust. The low e{open}Nd magmas most likely formed via the incorporation of low ??18O, hydrothermally-altered, wall-rock. Small-volume rhyolite lavas and ash-flow tuffs have similar isotopic characteristics to the large-volume ash-flow tuffs, but lavas erupted from extracaldera vents may have interacted with higher ??18O crustal rocks peripheral to the main magma chamber(s). Andesitic lavas from the 13-14 Ma Wahmonie/Salyer volcanic center southeast of the TMOV have low e{open}Nd (-13.2 to -13.8) and are considered on the basis of textural evidence to be mixtures of basaltic composition magmas and large proportions (70-80%) of anatectic crustal melts. A similar process may have occurred early in the magmatic history of the TMOV. The large-volume rhyolites

  15. Understanding the Peach Spring supereruption through its basal layer deposits (Southwestern USA)

    NASA Astrophysics Data System (ADS)

    McCracken, R. G.; Miller, C. F.; Dufek, J.; Gualda, G. A.; Buesch, D.; Brooks, C. E.

    2011-12-01

    , chevkinite, apatite, and fluorite; quartz is minimal or absent. Phenocrysts in the pumice are mostly fragments, suggesting syn-eruption fragmentation within magma chamber and/or conduit. Preliminary SEM analysis of the glass indicates dominantly high-silica rhyolite compositions (≥77% wt SiO_2) consistent with previous electron microprobe values. The siliceous glass composition and low phenocryst abundance are similar to typical pumice clasts in the outflow sheet. These pumice clasts are much more evolved than crystal-rich, trachytic pumice clasts that form a minor component in the basal layer unit, are more common in the overlying ignimbrite, and dominate the intracaldera fill. Trachytic magma is interpreted as a mush from the base of the chamber that was heated and remobilized by a recharging episode prior to the climactic eruption. Basal layer pumice compositions suggest the initial magma was drawn primarily from the chamber's top with minor contribution from near the bottom and that extent of entrainment of deep-level magma increased over the course of the eruption.

  16. Recent explosive eruptions in the Rungwe Volcanic Province, Tanzania

    NASA Astrophysics Data System (ADS)

    Fontijn, K.; Ernst, G. G.; Elburg, M. A.; Williamson, D.; Jacobs, P.

    2009-12-01

    The fundamental base of volcanic hazard assessment on any volcano is the study of its most recent eruptive history. Although the presence of extensive surficial pumice deposits was long known in the Rungwe Volcanic Province (RVP, SW Tanzania, East African Rift), the recent eruptive history was never studied in detail and is presented here for the first time. The RVP had several Plinian-style explosive eruptions in its Holocene history, originating from the two main volcanoes, Rungwe and Ngozi. Field observations are combined with whole-rock major (ICP-OES) and trace (ICP-MS) element analyses as well as major element analyses (EMPA) on glass. 14C ages of paleosols constrain all recognized deposits to <10 ka. Trace element data, e.g. Zr/Y ratios, allow discriminating between Ngozi and Rungwe as deposit source. All studied samples are trachyte to phonolitic trachyte. A ~30 m long sediment core in the Masoko maar lake (26 and 42 km SSE of Rungwe and Ngozi resp.) reveals >60 tephra layers deposited during the last 50 ky. Its Holocene record shows 7 tephra layers of which 2 (10.2 and 4.35 ka calBP) contain abundant pumice lapilli. Based on chemical constraints, the oldest of these pumice layers is believed to correspond to the Kitulo Pumice, the oldest on-land deposit found, originating from Ngozi. This eruption likely formed the 3 x 3 km Ngozi caldera. The 4.35 ka calBP pumice layer in the Masoko core was correlated with a Plinian pumice fallout deposit from Rungwe, the Rungwe Pumice, based on its appearance and paleosol 14C dating. It was traced over an area of ~1,500 km2 and probably extends even further. The Rungwe Pumice postdates a debris avalanche that was generated by a flank collapse of the volcano. This collapse left an amphitheatre-shaped depression on the summit that is now filled with domes, cones and explosion craters produced by effusive and explosive eruptions. A second large explosive eruption from Rungwe, the Isongole Pumice, is underlain by a 2.0 ± 0

  17. Volcanic stratigraphy and geochemical variations in Miocene-age rocks in western and southeastern Fort Irwin, California

    NASA Astrophysics Data System (ADS)

    Buesch, D.

    2015-12-01

    Lava flows and tuffaceous deposits ranging in composition from basalt to rhyolite, including basaltic trachyandesite to trachyte, are exposed in 800 km2 of western Fort Irwin area, California, and form the eastern edge of the Eagle Crags volcanic field (ECVF). The main ECVF has 40Ar/39Ar ages from ~18.7-12.4 Ma (mostly 18.7-18.5 Ma; Sabin et al. 1994), and on Fort Irwin, the ages are from 21.0-15.8 Ma (mostly 18.6-15.8 Ma; Schermer et al. 1996). 68 samples (56 lava flow, 4 dome-collapse breccia, 3 ignimbrite, and 5 fallout tephra) were analyzed for major, minor, and trace elements. Typically, stratigraphic sequences dip <30° (mostly <15°) except near faults, with local buttress unconfomities and no large unconfomities. Compositions are moderate-to-high-K type, and similar to Na2O+K2O from Sabin et al. (1994) but with slightly smaller ranges. The generalized stratigraphic sequence is rhyolite (R), dacite (D), or trachyte (T) that form domes, lava flows (up to 3.5 km long), dome-collapse deposits, or pyroclastic deposits, overlain by andesite (A), trachyandesite (TA), basaltic andesite (BA), basaltic trachyandesite (BT), or basalt (B) lava flows (up to 7 km long), and minor cinder cones. A general upward felsic to mafic compositional sequence occurs throughout the area, but is not continuous as B is locally in a R-D sequence and B is at the base of and interstratified with a BA-A sequence. Also, there are compositional variations at different locations along the edges of the field. In the Goldstone Mesa, Pink Canyon, and Stone Ridge areas (~70 km2), B-BA forms the youngest lava flows, but ~21 km to the north in the Garry Owen area (~25 km2), BTA forms the youngest lava flows. Compared to the Stone Ridge area with a D-A-TA-BA trend, ~6 km west in the Pioneer Plateau area is R-TA-D, ~3 km south in the Pink Canyon area is R-B-BA-A, and ~8 km east at Dacite Dome is D only (all areas have slightly different Na2O+K2O in each rock type). A non-ECVF, 5.6 Ma BA flow in SE

  18. Some New Constraints On The Stratigraphic And Structural Setting Of The Soda Lake Geothermal Field, Churchill County, Nevada - McLACHLAN, Holly S. and FAULDS, James E., Nevada Bureau of Mines and Geology, University of Nevada, Reno, NV 89557

    NASA Astrophysics Data System (ADS)

    McLachlan, H. S.

    2012-12-01

    Our research group is currently conducting a regional survey to identify favorable structural settings of producing and prospective geothermal fields in the Great Basin. The Soda Lake geothermal field - one of the oldest consistently producing fields in this study region - is located in west-central Nevada near the heart of the Carson Sink. Producing and prospective geothermal fields in the surrounding highlands are hosted in 1) fault termination zones (Desert Queen), 2) accommodation zones (Brady's Hot Springs) and 3) fault step-overs (Desert Peak). However, the structural setting is challenging to identify at the Soda Lake field, because it lies in the central part of a large basin with no nearby bedrock exposures. The well field at Soda Lake is centered ~3.5 km NNE of the Holocene Soda Lake maar, from which it takes its name. The geothermal field was identified serendipitously during the drilling of an irrigation survey well in the early 20th century. Modern exploratory drilling at the field began in the mid-1970s and has continued sporadically to the present. There are currently more than 28 500+ m wells at and near the production site. The exceptional drilling density at Soda Lake allows for comparatively reliable correlation of stratigraphy in the subsurface below the feature-poor Carson Sink. Stratigraphy in the Soda Lake geothermal area is relatively "layer cake" at the scale of the well field. Unconsolidated sediments extend more than 1000 m below surface. The upper few hundred meters are composed of fluvial and lacustrine sediments derived from Sierran batholith source rocks. The deeper basin fill derives from more proximal mafic to felsic Miocene volcanic rocks along the basin margins. At ~450-650 m depth, basin sediments are interrupted by a 5.11 Ma trachytic basalt of restricted lateral extent and variable thickness. Most wells intercept ~50-250 m of fine lacustrine sediments below this basalt body before intercepting the basin floor. Basin floor rocks

  19. Geochronology and geochemistry of Eocene-aged volcanic rocks around the Bafra (Samsun, N Turkey) area: Constraints for the interaction of lithospheric mantle and crustal melts

    NASA Astrophysics Data System (ADS)

    Temizel, İrfan; Arslan, Mehmet; Yücel, Cem; Abdioğlu, Emel; Ruffet, Gilles

    2016-08-01

    40Ar-39Ar age, whole-rock chemical, and Sr-Nd isotope data are presented for the post-collisional, Eocene (51.3-44.1 Ma)-aged volcanic rocks from the Bafra (Samsun) area in the western part of the Eastern Pontides (N Turkey) aiming to unravel their sources and evolutionary history. The studied Eocene volcanic rocks can be divided into two groups: analcime-bearing (tephritic lava flows and dykes) and analcime-free (basaltic to trachytic lava flows and basaltic dykes). The analcime-bearing volcanic rocks have a fine-grained porphyritic texture with clinopyroxene phenocrysts, whereas analcime-free volcanic rocks show a variety of textures including hyalo-microlitic microgranular porphyritic, intersertal, trachytic, fluidal, and glomeroporphyritic. The volcanic rocks also show evidence of mineral-melt disequilibrium textures such as sieved, rounded, and corroded plagioclases, partially melted and dissolved clinopyroxenes and poikilitic texture. Petrochemically, the parental magmas of the volcanic rocks evolved from alkaline to calc-alkaline lava suites and include high-K and shoshonitic compositions. They display enrichments in light rare earth and large ion lithophile elements such as Sr, K, and Rb, as well as depletions in high field strength elements such as Nb, Ta, Zr, and Ti, resembling subduction-related magmas. The analcime-bearing and -free volcanic rocks share similar incompatible element ratios and chondrite-normalised rare rearth element patterns, indicating that they originated from similar sources. They also have relatively low to moderate initial 87Sr/86Sr (0.7042-0.7051), high positive εNd(t) values (+ 0.20 to + 3.32), and depleted mantle Nd model ages (TDM1 = 0.63-0.93 Ga, TDM2 = 0.58-0.84 Ga). The bulk-rock chemical and Sr-Nd isotope features as well as the high Rb/Y and Th/Zr, but low Nb/Zr and Nb/Y ratios, indicate that the volcanic rocks were derived from a lithospheric mantle source that had been metasomatised by slab-derived fluids. Trace element

  20. Edaphics, active tectonics and animal movements in the Kenyan Rift - implications for early human evolution and dispersal

    NASA Astrophysics Data System (ADS)

    Kübler, Simon; Owenga, Peter; Rucina, Stephen; King, Geoffrey C. P.

    2014-05-01

    The quality of soils (edaphics) and the associated vegetation strongly controls the health of grazing animals. Until now, this has hardly been appreciated by paleo-anthropologists who only take into account the availability of water and vegetation in landscape reconstruction attempts. A lack of understanding the importance of the edaphics of a region greatly limits interpretations of the relation between our ancestors and animals over the last few million years. If a region lacks vital trace elements then wild grazing and browsing animals will avoid it and go to considerable length and take major risks to seek out better pasture. As a consequence animals must move around the landscape at different times of the year. In complex landscapes, such as tectonically active rifts, hominins can use advanced group behaviour to gain strategic advantage for hunting. Our study in the southern Kenya rift in the Lake Magadi region shows that the edaphics and active rift structures play a key role in present day animal movements as well as the for the location of an early hominin site at Mt. Olorgesailie. We carried out field analysis based on studying the relationship between the geology and soil development as well as the tectonic geomorphology to identify 'good' and 'bad' regions both in terms of edaphics and accessibility for grazing animals. We further sampled different soils that developed on the volcanic bedrock and sediment sources of the region and interviewed the local Maasai shepherds to learn about present-day good and bad grazing sites. At the Olorgesailie site the rift valley floor is covered with flood trachytes; basalts only occur at Mt. Olorgesailie and farther east up the rift flank. The hominin site is located in lacustrine sediments at the southern edge of a playa that extends north and northwest of Mt. Olorgesailie. The lakebeds are now tilted and eroded by motion on two north-south striking faults. The lake was trapped by basalt flows from Mt. Olorgesailie

  1. REE variation in alkaline mafic lavas across the North Tanzanian Divergence zone, a possible indicator of varying lithospheric thickness

    NASA Astrophysics Data System (ADS)

    Mana, S.; Carr, M. J.; Feigenson, M.; Furman, T.; Swisher, C. C.

    2012-12-01

    The magmatic evolution of the North Tanzanian Divergence zone (NTD), the southern termination of the eastern branch of the East African Rift (EAR), is of interest as a currently active magmatic-rich continental rift. In fact, the role of magmatism in continental rift initiation and evolution is of much debate and contrasts are currently drawn between amagmatic and magmatic-rich rift systems. The NTD possesses a wide array of pre-rift Miocene volcanoes to currently active volcanoes broadly distributed across the valley floor to the adjacent rift margins and characterized by having very heterogeneous chemical compositions. A highly diverse array of magmas from basalt to rhyolite, trachyte, phonolite and carbonatite occur at various volcanic centers, some of which have erupted more then one magma type. We analyzed 11 samples from Ketumbeine volcano for whole rock major and trace element abundances, Sr-Nd-Pb isotopic signatures and Ar/Ar ages. Two alkaline rock series are distinguished: a basanite to tephra-phonolite magma series and an alkali basalt to basaltic trachy-andesite magma series. Interestingly, the two magma series represent distinct ages of volcanism: the alkali basalt series is older with ages ranging between 2.2 and 1.9 Ma, while the basanite series erupted at about 1.2 Ma. The temporal separation of the two magma series occurs on a regional scale as well with the basanite to phonolite lavas erupted during the entire period of the NTD volcanism (5.8 Ma to Present), whereas the basalt to trachyte magma series is restricted to two brief intervals: the first between 2.4 and 1.6 Ma, immediately before the major rifting event dated by MacIntyre et al. (1974) at 1.2 Ma, and the second at about 0.5 Ma at Kibo and Mawenzi (Kilimanjaro). We model the source of the NTD as being derived from a metasomatized lithospheric mantle including variable amounts of garnet, amphibole and phlogopite. The most mafic and presumably less contaminated NTD volcanics have

  2. Geochemistry of Natural Components in the Near-Field Environment, Yucca Mountain, Nevada

    SciTech Connect

    Z.E. Peterman; T.A. Oliver

    2006-06-19

    The natural near-field environment in and around the emplacement drifts of the proposed nuclear waste repository at Yucca Mountain, Nevada, includes the host rock, dust, seepage water, and pore water. The chemical compositions of these components have been analyzed to provide a basis for assessing possible chemical and mineralogical reactions that may occur in and around the emplacement drifts during the heating and cooling cycle. The crystal-poor rhyolite of the Topopah Spring Tuff of Miocene age with an average silica (SiO{sub 2}) content of 76 percent will host the proposed repository. Samples of the rhyolite are relatively uniform in chemical composition as shown by an average coefficient of variation (CV) of 8.6 percent for major elements. The major component of underground dust is comminuted tuff generated during construction of the tunnel. Average CVs for major elements of dust samples collected from the main tunnel (Exploratory Studies Facility, ESF) and a cross drift (Enhanced Characterization of the Repository Block, ECRB) are 25 and 28 percent, respectively. This increased variability is due to a variable amount of dust derived from trachyte with SiO{sub 2} contents as low as 66 percent (from overlying crystal-rich members) and from surface dust with an even lower average SiO{sub 2} content of 60 percent (from the abundance of trachyte in outcrop and carbonate dust derived from nearby ranges). The composition of the water-soluble fraction of dust is of interest with regard to possible salt deliquescence on waste canisters. The nitrate-to-chloride (NO{sub 3}{sup -}/Cl{sup -}) ratio (weight) is used to assess the potential corrosive nature of the salts because an excess of NO{sub 3}{sup -} over Cl{sup -} may inhibit the formation of the more corrosive calcium chloride brines in deliquescing salts. The soluble fractions of dust samples typically have NO{sub 3}{sup -}/Cl{sup -} ratios between 1 and 10. About 30 samples of seepage into the south ramp of the

  3. Genesis and evolution of mafic and felsic magmas at Quaternary volcanoes within the Main Ethiopian Rift: Insights from Gedemsa and Fanta 'Ale complexes

    NASA Astrophysics Data System (ADS)

    Giordano, F.; D'Antonio, M.; Civetta, L.; Tonarini, S.; Orsi, G.; Ayalew, D.; Yirgu, G.; Dell'Erba, F.; Di Vito, M. A.; Isaia, R.

    2014-02-01

    This paper presents the results of an investigation carried out on young volcanic rocks from the Gedemsa and Fanta 'Ale complexes, located in the Main Ethiopian Rift, the site of an intense magmatism since Eocene-Oligocene. The earlier NW-SE direction of extension of the Rift, which generated NE-SW trending faults, rotated around E-W in Quaternary times, and produced the still active N to N-NE Wonji Fault System. The Gedemsa volcano is located in the central part of the Ethiopian Rift, about 100 km SE of Addis Ababa. It is characterized by a wide central caldera, about 8 km in diameter. The general stratigraphic sequence in the area includes, from base upwards, rift-floor ignimbrites, pantelleritic and subordinate trachytic pyroclastic deposits and lava flows and domes, and widespread basaltic deposits. The Fanta 'Ale volcanic complex is located in the northern part of the Main Ethiopian Rift, where the Afar depression begins. It is characterized by a summit caldera of which the diameter is about 4 km. This volcano erupted trachytic and rhyolitic lavas, whereas the most diffuse unit is an ignimbrite related to the caldera collapse. Explosive activity has occurred inside and outside the caldera, forming tuff cones and thick pumice-fallout deposits. The only mafic unit is represented by a basaltic eruption that occurred in 1870 AD. Historical eruptions and intense fumarolic activity are evidence for the persistence activity of the Fanta 'Ale in this part of the Main Ethiopian Rift. New geochemical and Sr-Nd-Pb isotope data on representative samples from Gedemsa and Fanta 'Ale volcanoes are presented and discussed in order to shed light on the genesis of mafic and felsic magmas, the genetic link between them, and their possible interaction with the local crust. Volcanic rocks show a typical mafic-felsic bi-modal distribution with few intermediate terms (Daly Gap), as observed at regional scale along the Main Ethiopian Rift as well as on the plateau. Geochemical data

  4. Geochemical and B-Sr-Nd isotopic evidence for mingling and mixing processes in the magmatic system that fed the Astroni volcano (4.1-3.8 ka) within the Campi Flegrei caldera (southern Italy)

    NASA Astrophysics Data System (ADS)

    Tonarini, Sonia; D'Antonio, Massimo; Di Vito, Mauro Antonio; Orsi, Giovanni; Carandente, Antonio

    2009-02-01

    The Astroni volcano was built through seven eruptions that generated pyroclastic deposits and lava domes within the Campi Flegrei caldera (southern Italy) 4.1-3.8 ka BP. Whole-rock geochemical and B-Sr-Nd isotopic investigations were carried out on representative samples of all seven eruptions. The products vary from tephriphonolites to phonolites, and from latites to trachytes. They show textural, mineralogical and isotopic evidence of disequilibrium, including distinct clinopyroxene populations, rounded and/or resorbed plagioclase and alkali-feldspar, and reverse-zoned phenocrysts of all these mineral phases. The Sr, Nd and B isotopic composition of whole rocks is variable and correlated with the degree of chemical evolution, suggesting open-system processes in addition to fractional crystallisation. Moreover, significant Sr-isotopic disequilibrium between the phenocrysts and glass has been documented for one sample. The chemostratigraphy of the products indicates that Astroni eruptions 1 through 5 were fed by magmas of trachytic to phonolitic composition that were less enriched in radiogenic Sr and 11B up-section. This variability has been interpreted as the result of mingling between at least two distinct magmatic end-members, one more evolved and the other less evolved. Another heterogeneous batch of magma, resulting from almost complete mixing between the same two end-members, was drained during eruptions 6 and 7. The more evolved end-member, characterised by 87Sr/ 86Sr ≥ 0.7075, 143Nd/ 144Nd ≤ 0.51247 and δ11B ≥ - 8‰, was very similar to the magma that fed the final phases of the Agnano-Monte Spina eruption, which occurred a few centuries earlier in the Astroni vent area. The less evolved end-member had 87Sr/ 86Sr ≤ 0.70726, 143Nd/ 144Nd ≥ 0.51251 and δ11B ≤ 10‰, and was likely derived by fractional crystallisation of a mantle-derived magma. An abrupt decrease in both the Sr isotope ratio and the Th content, detected at the transition

  5. Reviving Moribund Intrusive Complexes: Mafic Thermal Input, the Accessory Mineral Record, and the Pluton-Volcano Connection

    NASA Astrophysics Data System (ADS)

    Miller, C. F.; Gualda, G. A.; Padilla, A. J.; Pamukcu, A. S.; Claiborne, L. L.; Carley, T. L.; Flanagan, D. M.

    2011-12-01

    Intrusive and extrusive silicic systems spanning subduction zone, oceanic, and intracontinental settings and ranging from small, persistent eruptive centers to supereruptions show evidence for periodic thermal input. We present examples demonstrating that this input is responsible for remobilizing stagnant intrusions and apparently triggering eruptions. Mafic recharge is the evident heat source in some and implicated indirectly in others. Field relations, textures, accessory mineral zoning, and U-Pb and U-Th geochronology provide evidence for the inferred thermal reinvigoration of these systems: --Mount St. Helens, WA (active subduction zone volcano): Combined U-Th dating and elemental analysis demonstrates that zircon grew from residual silicic melt in relatively cool storage zones,10's to 100's of ky prior to eruption, and that its growth was episodic over the history of the system. Zircon crystals are thus samples of stagnant intrusive ("plutonic") parts of the St. Helens system, entrained in hotter ascending magmas during replenishment and local rejuvenation. --Highland Range volcanics & Searchlight pluton, NV (Miocene volcanic-plutonic system, incipient crustal extension): Evidence for late-stage mingling between intermediate and highly evolved, crystal-rich magma is preserved in the pluton, large dikes, a near-surface plug, and the final erupted products of the system, which were accompanied and followed by andesite lavas. Resorption and reaction rimming of sphene and large feldspar and quartz crystals in the dikes, plug, and volcanic rocks attest to heating. We infer that voluminous andesite invaded nearly-solid leucogranite, remobilized it, and triggered final eruptions. --Peach Spring Tuff, AZ, CA, NV (Miocene supereruption, incipient crustal extension): In contrast to phenocryst-poor rhyolitic outflow, intracaldera tuff is crystal-rich trachyte. Feldspar and sphene phenocrysts are extensively resorbed and rims of zircon crystals record a >100 degree C

  6. U-Pb geochronology of the Kap Washington Volcanic Province, North Greenland: Constraints on the timing of continental rifting and implications for the development of the Arctic Basin

    NASA Astrophysics Data System (ADS)

    Thorarinsson, S. B.; Holm, P. M.; Tappe, S.; Heaman, L.; Tegner, C.

    2009-12-01

    The Kap Washington volcanic sequence at the north coast of Greenland is bimodal with alkaline basalts, trachytic to rhyolitic lavas, tuffs and ignimbrites predominating. In terms of geochemistry and distribution of rock types, the sequence bears resemblance to presently active continental rift systems, e.g. the Main Ethiopian Rift. Associated with the volcanics is a swarm of coast-normal alkaline basaltic dykes which intensifies towards the outer coast. The volcanics are believed to be linked to rifting in the Arctic Basin and have featured prominently in geotectonic reconstructions of the Arctic region (e.g. Batten et al. 1981). Here we report the first U-Pb zircon ages from silicic lavas and intrusions of the Kap Washington sequence. A total of ten samples have been dated and the duration of magmatism is constrained at present to ca. 10 million years - from 71 to 61 Ma (based on 206Pb/238U ages of concordant analyses). Three age ‘groups’ have been identified: 71-69 Ma (n = 6); 68-65 Ma (n = 2); and 64-61 Ma (n = 2). The oldest group comprises trachytic and rhyolitic lava flows from Kap Kane and a rhyolitic sill from the Kap Washington peninsula. These ages agree well with new 40Ar/39Ar ages obtained on amphiboles from benmoreitic tuffs exposed on Kap Kane (Holm et al., this session) and suggest that most of the ~1.5 km thick Kap Kane sequence was extruded within a period of 1-2 million years. The two younger groups comprise silicic lavas exposed on Lockwood Island. The exposed sequence on Lockwood Island is estimated to be 3-4 km thick and was previously thought to be the oldest part of the succession (Brown et al. 1987). The large scatter in ages on Lockwood Island indicates that magmatism was episodic rather than continuous. The new age data from the Kap Washington volcanics together with 40Ar/39Ar ages for the associated dyke swarm (Kontak et al. 2001) suggest that continental extension and magmatism occurred in the area between ca. 82 and 61 Ma. This age

  7. Geochemistry and petrology of the Early Miocene lamproites and related volcanic rocks in the Thrace Basin, NW Anatolia

    NASA Astrophysics Data System (ADS)

    Ersoy, Yalçın E.; Palmer, Martin R.; Uysal, İbrahim; Gündoğan, İbrahim

    2014-08-01

    The extensional Thrace basin (NW Anatolia) contains an association of early Miocene diopside-leucite-phlogopite (Doğanca) and diopside-phlogopite (Korucuköy) lamproites with Oligocene medium-K calc-alkaline andesites (Keşan volcanics), early Miocene shoshonitic rocks (Altınyazı trachyte) and middle Miocene Na-alkaline basalts (Beğendik basalts). The Doğanca lamproite (K2O = 5.1-5.5 wt.%; K/Na = 2.78-2.89; MgO = 11.4-11.8 wt.%) consists of olivine (Fo71-86), diopside (Al2O3 = 1.0-5.0, Na2O = 0.2-0.6), phlogopite (TiO2 = 1.1-9.4, Al2O3 = 11.1-13.9), spinel (Mg# = 22.9-32.6; Cr# = 64-83.4), leucite, apatite, zircon, Fe-Ti-oxides and magnetite in a poikilitic sanidine matrix. The potassic volcanic units (lamproites and trachytes) in the region have similarly high Sr and low Nd isotopic compositions (87Sr/86Sr(i) = 0.70835-0.70873 and 143Nd/144Nd(i) = 0.51227-0.51232). The major and trace element compositions and Sr-Nd-Pb isotopic ratios of the shoshonitic, ultrapotassic and lamproitic units closely resemble those of other Mediterranean ultrapotassic lamproites (i.e., orogenic lamproites) from Italia, Serbia, Macedonia and western Anatolia. The Beğendik basalts show intraplate geochemical signatures with an Na-alkaline composition, an absence of Nb negative anomalies on primitive mantle-normalized multi-element diagrams, as well as low Sr (~ 0.70416) and high Nd (0.51293) isotopic ratios; and include olivine (Fo72-84), diopside, spinel, Fe-Ti-oxides and magnetite. The Oligocene Keşan volcanics were emplaced in the earlier stages of extension in Thrace, and represent the typical volcanic products of post-collisional volcanism. The continental crust-like trace element abundances and isotopic compositions of the most primitive early Miocene ultrapotassic rocks (Mg# up to 74) indicate that their mantle sources were intensely contaminated by the continental material. By considering the geodynamic evolution of the region, including oceanic subduction, crustal

  8. Two mantle sources, two plumbing systems: Tholeiitic and alkaline magmatism of the Maymecha River basin, Siberian flood volcanic province

    USGS Publications Warehouse

    Arndt, N.; Chauvel, C.; Czamanske, G.; Fedorenko, V.

    1998-01-01

    Rocks of two distinctly different magma series are found in a ???4000-m-thick sequence of lavas and tuffs in the Maymecha River basin which is part of the Siberian flood-volcanic province. The tholeiites are typical low-Ti continental flood basalts with remarkably restricted, petrologically evolved compositions. They have basaltic MgO contents, moderate concentrations of incompatible trace elements, moderate fractionation of incompatible from compatible elements, distinct negative Ta(Nb) anomalies, and ??Nd values of 0 to + 2. The primary magmas were derived from a relatively shallow mantle source, and evolved in large crustal magma chambers where they acquired their relatively uniform compositions and became contaminated with continental crust. An alkaline series, in contrast, contains a wide range of rock types, from meymechite and picrite to trachytes, with a wide range of compositions (MgO from 0.7 to 38 wt%, SiO2 from 40 to 69 wt%, Ce from 14 to 320 ppm), high concentrations of incompatible elements and extreme fractionation of incompatible from compatible elements (Al2O3/TiO2 ??? 1; Sm/Yb up to 11). These rocks lack Ta(Nb) anomalies and have a broad range of ??Nd values, from -2 to +5. The parental magmas are believed to have formed by low-degree melting at extreme mantle depths (>200 km). They bypassed the large crustal magma chambers and ascended rapidly to the surface, a consequence, perhaps, of high volatile contents in the primary magmas. The tholeiitic series dominates the lower part of the sequence and the alkaline series the upper part; at the interface, the two types are interlayered. The succession thus provides evidence of a radical change in the site of mantle melting, and the simultaneous operation of two very different crustal plumbing systems, during the evolution of this flood-volcanic province. ?? Springer-Verlag 1998.

  9. Role of crustal assimilation and basement compositions in the petrogenesis of differentiated intraplate volcanic rocks: a case study from the Siebengebirge Volcanic Field, Germany

    NASA Astrophysics Data System (ADS)

    Schneider, K. P.; Kirchenbaur, M.; Fonseca, R. O. C.; Kasper, H. U.; Münker, C.; Froitzheim, N.

    2016-06-01

    The Siebengebirge Volcanic Field (SVF) in western Germany is part of the Cenozoic Central European Volcanic Province. Amongst these volcanic fields, the relatively small SVF comprises the entire range from silica-undersaturated mafic lavas to both silica-undersaturated and silica-saturated differentiated lavas. Owing to this circumstance, the SVF represents a valuable study area representative of intraplate volcanism in Europe. Compositions of the felsic lavas can shed some new light on differentiation of intraplate magmas and on the extent and composition of potential crustal assimilation processes. In this study, we provide detailed petrographic and geochemical data for various differentiated SVF lavas, including major and trace element concentrations as well as Sr-Nd-Hf-Pb isotope compositions. Samples include tephriphonolites, latites, and trachytes with SiO2 contents ranging between 53 and 66 wt%. If compared to previously published compositions of mafic SVF lavas, relatively unradiogenic 143Nd/144Nd and 176Hf/177Hf coupled with radiogenic 87Sr/86Sr and 207Pb/204Pb lead to the interpretation that the differentiated volcanic rocks have assimilated significant amounts of lower crustal mafic granulites like the ones found as xenoliths in the nearby Eifel volcanic field. These crustal contaminants should possess unradiogenic 143Nd/144Nd and 176Hf/177Hf, radiogenic 87Sr/86Sr, and highly radiogenic 207Pb/204Pb compositions requiring the presence of ancient components in the central European lower crust that are not sampled on the surface. Using energy-constrained assimilation-fractional crystallisation (EC-AFC) model calculations, differentiation of the SVF lithologies can be modelled by approximately 39-47 % fractional crystallisation and 6-15 % crustal assimilation. Notably, the transition from silica-undersaturated to silica-saturated compositions of many felsic lavas in the SVF that is difficult to account for in closed-system models is also well explained by

  10. New constraints on the pyroclastic eruptive history of the Campanian volcanic Plain (Italy)

    USGS Publications Warehouse

    de Vivo, B.; Rolandi, G.; Gans, P.B.; Calvert, A.; Bohrson, W.A.; Spera, F.J.; Belkin, H.E.

    2001-01-01

    The ???150 km3 (DRE) trachytic Campanian Ignimbrite, which is situated north-west of Naples, Italy, is one of the largest eruptions in the Mediterranean region in the last 200 ky. Despite centuries of investigation, the age and eruptive history of the Campanian Ignimbrite is still debated, as is the chronology of other significant volcanic events of the Campanian Plain within the last 200-300 ky. New 40Ar/39Ar geochronology defines the age of the Campanian Ignimbrite at 39.28 ?? 0.11 ka, about 2 ky older than the previous best estimate. Based on the distribution of the Campanian Ignimbrite and associated uppermost proximal lithic and polyclastic breccias, we suggest that the Campanian Ignimbrite magma was emitted from fissures activated along neotectonic Apennine faults rather than from ring fractures defining a Campi Flegrei caldera. Significantly, new volcanological, geochronological, and geochemical data distinguish previously unrecognized ignimbrite deposits in the Campanian Plain, accurately dated between 157 and 205 ka. These ages, coupled with a xenocrystic sanidine component >315 ka, extend the volcanic history of this region by over 200 ky. Recent work also identifies a pyroclastic deposit, dated at 18.0 ka, outside of the topographic Campi Flegrei basin, expanding the spatial distribution of post-Campanian Ignimbrite deposits. These new discoveries emphasize the importance of continued investigation of the ages, distribution, volumes, and eruption dynamics of volcanic events associated with the Campanian Plain. Such information is critical for accurate assessment of the volcanic hazards associated with potentially large-volume explosive eruptions in close proximity to the densely populated Neapolitan region.

  11. The Olorgesailie Drilling Project (ODP): a high-resolution drill core record from a hominin site in the East African Rift Valley

    NASA Astrophysics Data System (ADS)

    Dommain, R.; Potts, R.; Behrensmeyer, A. K.; Deino, A. L.

    2014-12-01

    The East African rift valley contains an outstanding record of hominin fossils that document human evolution over the Plio-Pleistocene when the global and regional climate and the rift valley itself changed markedly. The sediments of fossil localities typically provide, however, only short time windows into past climatic and environmental conditions. Continuous, long-term terrestrial records are now becoming available through core drilling to help elucidate the paleoenvironmental context of human evolution. Here we present a 500,000 year long high-resolution drill core record obtained from a key fossil and archeological site - the Olorgesailie Basin in the southern Kenya Rift Valley, well known for its sequence of archeological and faunal sites for the past 1.2 million years. In 2012 two drill cores (54 and 166 m long) were collected in the Koora Plain just south of Mt. Olorgesailie as part of the Olorgesailie Drilling Project (ODP) to establish a detailed climate and ecological record associated with the last evidence of Homo erectus in Africa, the oldest transition of Acheulean to Middle Stone Age technology, and large mammal species turnover, all of which are documented in the Olorgesailie excavations. The cores were sampled at the National Lacustrine Core Facility. More than 140 samples of tephra and trachytic basement lavas have led to high-precision 40Ar/39Ar dating. The cores are being analyzed for a suite of paleoclimatic and paleoecological proxies such as diatoms, pollen, fungal spores, phytoliths, ostracodes, carbonate isotopes, leaf wax biomarkers, charcoal, and clay mineralogy. Sedimentological analyses, including lithological descriptions, microscopic smear slide analysis (242 samples), and grain-size analysis, reveal a highly variable sedimentary sequence of deep lake phases with laminated sediments, diatomites, shallow lake and near shore phases, fluvial deposits, paleosols, interspersed carbonate layers, and abundant volcanic ash deposits. Magnetic

  12. Igneous Rocks of the East Pacific Rise: The alkali volcanic suite appear to be differentiated from a tholeiitic basalt extruded from the mantle.

    PubMed

    Engel, A E; Engel, C G

    1964-10-23

    The apical parts of large volcanoes along the East Pacific Rise (islands and seamounts) are encrusted with rocks of the alkali volcanic suite (alkali basalt, andesine- and oligoclase-andesite, and trachyte). In contrast, the more submerged parts of the Rise are largely composed of a tholeiitic basalt which has low concentrations of K, P, U, Th, Pb, and Ti. This tholeiitic basalt is either the predominant or the only magma generated in the earth's mantle under oceanic ridges and rises. It is at least 1000-fold more abundant than the alkali suite, which is probably derived from tholeiitic basalt by magmatic differentiation in and immediately below the larger volcanoes. Distinction of oceanic tholeiites from almost all continental tholeiites is possible on the simple basis of total potassium content, with the discontinuity at 0.3 to 0.5 percent K(2)O by weight. Oceanic tholeiites also are readily distinguished from some 19 out of 20 basalts of oceanic islands and seamount cappings by having less than 0.3 percent K(2)O by weight and more than 48 percent SiO(2). Deep drilling into oceanic volcanoes should, however, core basalts transitional between the oceanic tholeiites and the presumed derivative alkali basalts. The composition of the oceanic tholeiites suggests that the mantle under the East Pacific Rise contains less than 0.10 percent potassium oxide by weight; 0.1 part per million of uranium and 0.4 part of thorium; a potassium:rubidium ratio of about 1200 and a potassium: uranium ratio of about 10(4). PMID:17806796

  13. The volcanic-subvolcanic rocks of the fernando de noronha archipelago, southern atlantic ocean: Mineral chemistry

    NASA Astrophysics Data System (ADS)

    Lopes, Rosana Peporine; Ulbrich, Mabel N. Costas; Ulbrich, Horstpeter

    2014-12-01

    Fernando de Noronha archipelago presents an older Remédios Formation with subvolcanic intrusions, belonging to two different alkaline series, the sodic (undersaturated: basanites, tephrites, essexites, tephriphonolites, phonolites), and potassic ones (mildly undersaturated to silicic, with alkali basalts, basaltic trachyandesites, trachyandesites, trachytes), and lamprophyres. The upper Quixaba Formation presents nephelinite flows and basanites. A third minor unit, São José, is constituted by basanites carrying mantle xenoliths. Magnesian olivines occur in the Remédios basanites and alkali basalts, and in nephelinites. Melilites are present as groundmass grains in melilite melanephelinites (MEM). Clinopyroxenes (cpx) are mostly salites to titaniferous salites (Remédios sodic series), grading into aegirines in the differentiated aphyric phonolites. Cpx in the lamprophyres show disequilibrium textures. In the Quixaba flows, cpx are salites, enriched in Mg (especially in MEM). Amphiboles, remarkably, are common in tephriphonolites and phonolites and in basaltic trachyandesites, sometimes with disequilibrum zoning textures, and a conspicuous phase in lamprophyres. Dark micas are present as groundmass plates in MEM, OLM and PYM (olivine and pyroxene melanephelinites), with compositional variety (enriched in Ti, Ba, Sr) depending on the composition of the parent rock; BaO can be as high as 16-19%. Feldspars crystallize as calcic plagioclases, sanidines and anorthoclases, depending on the rock types, as phenocrysts and in groundmass, both in Quixaba and Remédios rocks; they are absent in nephelinites. Nephelines are found in Remédios sodic series types and Quixaba rocks. Haüyne and noseane are rarely observed in Remédios rocks.

  14. Petrogenesis of Monte Vulture volcano (Italy): inferences from mineral chemistry, major and trace element data

    NASA Astrophysics Data System (ADS)

    de Fino, M.; La Volpe, L.; Peccerillo, A.; Piccarreta, G.; Poli, G.

    1986-02-01

    The paper presents major and trace element data and mineral compositions for a series of foiditic-tephritic to phonolitic rocks coming from Monte Vulture, Southern Italy, and investigates their origin, evolution and relationship with the other centres of the Roman province. Major and trace element variation in the foiditic to tephritic suite agrees with a hypothesis of evolution by simple crystal/liquid fractionation, whereas the early erupted phonolitic trachytes and phonolites have geochemical characteristics which do not support their derivation from tephritic magma by crystal fractionation. Foiditic and phonolitic rocks have mineral compositions which are interpreted as indicating magma mixing. However geochemical evidence shows that this process did not play an important role during the magma evolution. The Vulture rocks have compositional peculiarities such as high abundance of Na2O, CaO, Cl and S, when compared with other Roman volcanics. Instead, the distribution of incompatible elements is similar to those of Roman rocks, except for a lower content of Rb and K, higher P and lower Th/Ta and Th/Nb ratios which are still close to the values of arc volcanics. The high contents of Na, Ca and of volatile components are tentatively attributed to the interaction of magma with aqueous solutions, rich in calcium sulphate and sodium chloride, related to the Miocene or Triassic evaporites occurring within the sedimentary sequence underlying the volcano. The distribution pattern of the incompatible elements is interpreted as indicative of magma-forming in a subduction modified upper mantle and of the peculiar location of M. Vulture.

  15. Geochemical study of laterites of the Jamnagar district, Gujarat, India: Implications on parent rock, mineralogy and tectonics

    NASA Astrophysics Data System (ADS)

    Meshram, R. R.; Randive, K. R.

    2011-11-01

    The laterite deposits occur in a linear stretch along the northern Arabian Sea coast in the Jamnagar and Porbandar districts of, Gujarat state, India. These deposits are characterised by presence of gibbsite, kaoline, calcite, quartz, anatase, natroalunite, goethite and hematite, and relicts of mafic minerals and plagioclase. On the basis of petro-mineralogy and geochemistry, these deposits are grouped as aluminous laterites (Fe 2O 3 - 1.45-3.84%, Av. 3.13, Al 2O 3 - 39.31-57.24, Av. 45.80) and laterites (Fe 2O 3 - 9.84-32.21, Av. 25.13%, Al 2O 3 - 34.74-49.59, Av. 41.27). The major, trace and REE characteristics of laterites indicate that these were formed in situ by the alteration of parent rocks of trachytic/andesitic composition, and the process of bauxitisation followed the path of destruction of kaolinite and deferruginisation. The correlation patterns of several trace and rare earth elements and their preferential enrichment have indicated that there is an influence of precursor rock on the distribution of trace elements. The Jamnagar laterite deposits occur as capping over the Deccan Trap basaltic lava flows and pyroclasic deposits. Lateritisation prevailed during Palaeocene age when India was separated from the Seychelles and passing over the equator. During this time climate, morphology and drainage conditions were favourable for lateritisation that result in the formation of Jamnagar and other laterite deposits within the Deccan Province. Flood basaltic provinces of Deccan, Columbia, North Australia and Hawaii appear good location for hosting laterite deposits due to their wide areal extent, small geological time span and uniform chemical composition. However, comparison of the major flood basaltic provinces of the world has indicated that their palaeopositions along with palaeoclimate, morphology and drainage are equally important factors for facilitating lateritisation.

  16. Authigenic Mineral Cycling in Roman Seawater Concrete with Campi Flegrei Pumiceous Ash Pozzolan

    NASA Astrophysics Data System (ADS)

    Jackson, M. D.; Mulcahy, S. R.; Chen, H.; Li, Q.; Cappelletti, P.; Carraro, C.; Wenk, H. R.

    2015-12-01

    Alteration of Campi Flegrei pumiceous ash in Roman concrete harbor structures along the central Italian coast produced zeolite and Ca-silicate minerals that have reinforced cementitious fabrics for >2000 years. X-ray microdiffraction experiments and electron microprobe analyses show that diverse alteration paths produced authigenic phillipsite and Al-tobermorite in the pyroclasts, pores, and cementing matrix of mortars in Romacons drill cores from Portus Cosanus, Portus Neronis, and Baianus Sinus. These minerals have cation exchange capabilities for some radionuclides and heavy metal cations and are candidate sorbents for concrete waste encapsulations. Compositions of phillipsite in certain Portus Cosanus and Portus Neronis pumice clasts are similar to those in the Neapolitan Yellow Tuff. Dissolution of this phillipsite and alkali feldspar produced new, authigenic phillipsite with less Si, greater Al and Ca, Al-tobermorite, and poorly-crystalline binder in pumice vesicles. Conversely, alteration of trachytic glass to clay mineral (nontronite) in a Baianus Sinus tuff clast is associated with new, authigenic phillipsite and Al-tobermorite in the tuff and cementing matrix. The Al-tobermorite has lower Al/(Si+Al) and Ca/(Si+Al) compared to Al-tobermorite in relict lime clasts. These more siliceous crystals, similar to those in hydrothermally-altered basalt, have 11.3 Å d-spacing in [001]. Raman spectra show symmetrical bending of Si-O-Si and Si-O-Al linkages, Si-O and Si-Al symmetrical stretching, and possible Q3 Si and Al tetrahedral peaks that suggest cross-linking of silicate chains-an important factor in cation exchange. The authigenic crystals refine pore space, contribute to binding in interfacial zones, and obstruct microcrack propagation. The well-constrained history of temperature variations and seawater immersion could provide further information for understanding alteration in volcanoclastic deposits and predicting regenerative processes in high performance

  17. Geology and geochemistry of the Mount Riley-Mount Cox pluton, Dona Ana County, New Mexico

    SciTech Connect

    Zimbelman, D.R.; Siems, D.F.; Kilburn, J.E.; Hubert, A.E.

    1985-01-01

    The Mount Riley-Mount Cox area is comprised of a relatively homogeneous pluton of rhyodacite rising some 1600 feet above the La Mesa surface. The pluton, of apparent Tertiary age, intrudes Cretaceous sedimentary rocks and Tertiary ( ) latite and tuff. The rhyodacite is holocrystalline, light gray to pinkish gray, porphyritic to microporphyritic, and locally banded. Phenocrysts include hornblende, quartz, biotite, and calcite. The phenocrysts range in size from 0.2 to 2 mm and make up one to fifteen percent of the rock. The phenocrysts often display a glomerophyric texture within a trachytic groundmass. The groundmass ranges from cryptocrystalline to very fine grained and is composed of plagioclase, quartz, potassium feldspar, hornblende/biotite, and iron-oxide material. Locally, the rhyodacite displays millimeter-scale banding and a poikilitic texture consisting of quartz oikiocrysts and plagioclase chadocrysts. The rhyodacite averages 68.74%, SiO/sub 2/, 0.39% TiO/sub 2/, 16.40% Al/sub 2/O/sub 3/, 2.87% Fe/sub t/, 0.10% MnO, 1.21% MgO, 2.56% CaO, 3.79% Na/sub 2/O, and 3.96% K/sub 2/O. The rhyodacite is cut by veins and veinlets of brown to white calcite. The veins attain a maximum thickness of one meter, are locally bordered by calcite-cemented breccia zones, and locally include pyrite. The veins trend north or northwest, consistent with regional trends for the Rio Grande rift and the Texas Lineament, respectively. Sixty-five samples of rhyodacite, breccia, and vein were analyzed for 31 elements by emission-spectrographic methods. Trace-element data suggestive of hydrothermal mineralization was not recognized.

  18. Zircon evidence for a ~200 k.y. supereruption-related thermal flare-up in the Miocene southern Black Mountains, western Arizona, USA

    NASA Astrophysics Data System (ADS)

    McDowell, Susanne M.; Miller, Calvin F.; Mundil, Roland; Ferguson, Charles A.; Wooden, Joseph L.

    2014-07-01

    The Silver Creek caldera (southern Black Mountains, western Arizona) is the source of the 18.8 Ma, >700 km3 Peach Spring Tuff (PST) supereruption, the largest eruption generated in the Colorado River Extensional Corridor (CREC) of the southwestern United States. Within and immediately surrounding the caldera is a sequence of volcanics and intrusions ranging in age from ~19 to 17 Ma. These units offer a record of magmatic processes prior to, during, and immediately following the PST eruption. To investigate the thermal evolution of the magmatic center that produced the PST, we applied a combination of Ti-in-zircon thermometry, zircon saturation thermometry, and high-precision U-Pb CA-TIMS zircon dating to representative pre- and post-supereruption volcanic and intrusive units from the caldera and its environs. Similar to intracaldera PST zircons, zircons from a pre-PST trachytic lava (19 Ma) and a post-PST caldera intrusion (18.8 Ma) yield exceptionally high-Ti concentrations (most >20 ppm, some up to nearly 60 ppm), corresponding to calculated temperatures that exceed 900 °C. In these units, Ti-in-zircon temperatures typically surpass zircon saturation temperatures (ZSTs), suggesting the entrainment of zircon that had grown in hotter environments within the magmatic system. Titanium concentrations in younger volcanic and intrusive units (~18.7-17.5 Ma) decline through time, corresponding to an average cooling rate of 10-3.5 °C/year. The ~200 k.y. thermal peak evident at Silver Creek caldera is spatially limited: elsewhere in the Miocene record of the northern CREC, Ti-in-zircon concentrations and ZSTs are much lower, suggesting that felsic magmas were generally substantially cooler.

  19. Late Ordovician volcanism in Korea constrains the timing for breakup of Sino-Korean Craton from Gondwana

    NASA Astrophysics Data System (ADS)

    Cho, Deung-Lyong; Lee, Seung Ryeol; Koh, Hee Jae; Park, Jun-Beom; Armstrong, Richard; Choi, Duck K.

    2014-12-01

    In the early Paleozoic the Sino-Korean Craton (SKC) and South China Craton (SCC) were situated along the margin of east Gondwana. The SKC was connected to core Gondwana by an epeiric sea which was the site for deposition of lower Paleozoic sequences of SKC. The SKC and SCC may have drifted away from core Gondwana sometime during the mid-Paleozoic and would have been outboard microcontinents in the late Paleozoic, until they collided to form the East Asian continent in the Triassic. The breakup of SCC from Gondwana was suggested to have taken place at ∼380 Ma, while no reliable suggestions have hitherto been made for breakup of SKC from Gondwana. This study presents a convincing evidence for breakup of SKC from Gondwana, based on the recognition of Late Ordovician volcanism in Korea. New SHRIMP U-Pb zircon ages, 445.0 ± 3.7 Ma and 452.5 ± 3.2 Ma, are obtained from trachytic rocks of the Ongnyeobong Formation of Taebaeksan Basin in Korea which occupied the marginal part of the SKC in the early Paleozoic. This Late Ordovician volcanism along with previous records of Ordovician volcanic activities along the western margin of the SKC is interpreted indicating the development of an incipient oceanic ridge. The oceanic ridge uplifted the SKC including the epeiric sea, which subsequently resulted in terminating the early Paleozoic sedimentation of the epeiric sea. The paucity of lower Paleozoic volcanic rocks across much of the SKC however suggests that the oceanic ridge did not extend into the epeiric sea. Instead, spreading of oceanic ridge entailed dextral movement of associated transform faults, which may have played a major role in breakup of SKC from mainland Gondwana by the end of Ordovician.

  20. Petrogenesis of coexisting SiO 2-undersaturated to SiO 2-oversaturated felsic igneous rocks: The alkaline complex of Itatiaia, southeastern Brazil

    NASA Astrophysics Data System (ADS)

    Brotzu, P.; Gomes, C. B.; Melluso, L.; Morbidelli, L.; Morra, V.; Ruberti, E.

    1997-07-01

    The Itatiaia alkaline complex is a Late Cretaceous intrusion (72 Myr) made up of felsic differentiates, with syenitic rocks dominant throughout and with presence of both nepheline- and quartz-rich varieties. Dykes with phonolitic or trachytic composition cross-cut the coarse-grained facies. The rocks are arranged concentrically, with the core of the complex being formed by SiO 2-oversaturated syenites (with a small outcrop of granites), and are radially displaced by faults related to regional tectonic lineaments. The minerals show gradual but significant changes in composition (salitic and augitic to aegirine-rich pyroxenes, hastingsite and actinolite to richterite and arfvedsonite amphiboles, sodic plagioclase to orthoclase feldspars and so on) and the whole-rock trends are broadly consistent with fractional crystallization processes dominated by alkali feldspar removal. Sr-isotopic data indicate more radiogenic ratios for the SiO 2-oversaturated rocks (0.7062-0.7067 against 0.7048-0.7054 for the SiO 2-undersaturated syenites), consistent with small amounts of crustal input. The favored hypothesis for the petrogenesis of the different syenitic groups is the prolonged differentiation starting from differently SiO 2-undersaturated mafic parental magmas (potassic alkali basalts to ankaratrites, present in the Late Cretaceous dyke swarms of the area), accompanied by variable crustal contamination prior to the final emplacement. The lack of carbonatite as a significant lithotype, the potassic affinity of the Itatiaia complex, and the relatively high Sr-isotopic ratios match the characteristics of the other complexes of the Rio de Janeiro-Sa˜o Paulo states coastline and confirm the ultimate derivation of these differentiated rocks from an enriched lithospheric mantle source.

  1. Phonolites and peralkaline rhyolites from a single magma source in the mantle : A new look at some Black Hills rocks

    SciTech Connect

    Kirchner, J.G. . Dept. of Geography-Geology)

    1993-03-01

    A re-evaluation of existing data from the Deer Mountain-Terry Peak-Sugarloaf Mountain area of the Black Hills, plus some new data, suggests the real possibility that both silica-undersaturated and silica-oversaturated alkaline-peralkaline rocks evolved from the same mantle-derived parent magma. Mineralogically, aegirine rhyolites, phonolites, a minette and the mantle are linked by an association of Mg-rich olivine-phlogopite structures, zenocrystic phlogopite and diopside-cored pyroxene phenocrysts. Trends of silica vs. major elements, trace elements (V,Sc,Ni) and MgO/FeOt are continuous and preclude being fortuitous. Peralkalinity also increases with silica in a well-defined trend. Increasing ferric oxide to total iron oxide indicates increasing oxygen fugacity with silica saturation. A mantle origin for the phonolites is supported by Sr-isotope data of Beintema (1986) and Beintema and Montgomery (1986). Higher Sr-isotope ratios for the aegirine rhyolites, suggesting a lower crustal origin, actually may result from magmatic processes, as shown by others for ocean island basalt-phonolite-comendite associations. Early fractionation of mafic phases drives trends away from the Ne-minimum on the residua diagram, indicating that magma evolution took place above residua temperatures, thus avoiding the thermal divide. Later fractionation of alkali feldspars accounts for variation in the aegirine rhyolites. Rising alkalies and oxygen explain variations in peralkalinity and ferric iron content. Pressure-dependent immiscibility possibly may be the cause of a silica gap in rock types, as rocks with low quartz contents are not found, except as phaneritic inclusions. A model is suggested in which either a fractionating minette or trachyte magma could yield the series of rocks under study.

  2. The Ngorongoro Volcanic Highland and its relationships to volcanic deposits at Olduvai Gorge and East African Rift volcanism.

    PubMed

    Mollel, Godwin F; Swisher, Carl C

    2012-08-01

    The Ngorongoro Volcanic Highland (NVH), situated adjacent and to the east of Olduvai Gorge in northern Tanzania, is the source of the immense quantities of lava, ignimbrite, air fall ash, and volcaniclastic debris that occur interbedded in the Plio-Pleistocene sedimentary deposits in the Laetoli and Olduvai areas. These volcanics have proven crucial to unraveling stratigraphic correlations, the age of these successions, the archaeological and paleontological remains, as well as the source materials from which the bulk of the stone tools were manufactured. The NVH towers some 2,000 m above the Olduvai and Laetoli landscapes, affecting local climate, run-off, and providing varying elevation - climate controlled ecosystem, habitats, and riparian corridors extending into the Olduvai and Laetoli lowlands. The NVH also plays a crucial role in addressing the genesis and history of East African Rift (EAR) magmatism in northern Tanzania. In this contribution, we provide age and petrochemical compositions of the major NVH centers: Lemagurut, basalt to benmorite, 2.4-2.2 Ma; Satiman, tephrite to phonolite, 4.6-3.5 Ma; Oldeani, basalt to trachyandesite, 1.6-1.5 Ma; Ngorongoro, basalt to rhyolite, 2.3-2.0 Ma; Olmoti, basalt to trachyte, 2.0-1.8 Ma; Embagai, nephelinite to phonolite, 1.2-0.6 Ma; and Engelosin, phonolite, 3-2.7 Ma. We then discuss how these correlate in time and composition with volcanics preserved at Olduvai Gorge. Finally, we place this into context with our current understanding as to the eruptive history of the NVH and relationship to East African Rift volcanism. PMID:22404967

  3. On the anatomy of magma chamber and caldera collapse: The example of trachy-phonolitic explosive eruptions of the Roman Province (central Italy)

    NASA Astrophysics Data System (ADS)

    Palladino, Danilo M.; Gaeta, Mario; Giaccio, Biagio; Sottili, Gianluca

    2014-06-01

    Textural and compositional features of pyroclastic products erupted during caldera-forming events often reveal the tapping of different portions of variably zoned magma chambers due to changing geometries of the conduit/vent systems. Here we report on ultrapotassic trachytic-phonolitic explosive eruptions of the Roman Province (central Italy), which show remarkable changes of textural features and glass compositions in the juvenile material, even if the bulk chemical composition is essentially constant. In each example, the lower eruption sequence contains whitish, crystal-poor (leucite-free), highly vesicular pumice, emplaced by early Plinian fallout and/or pyroclastic currents; upsection, the eruption sequence contains black, low porphyritic (sanidine + leucite-bearing), moderately vesicular, scoria or spatter, emplaced by major pyroclastic flows (red tuff with black scoria) and associated co-ignimbrite, coarse lithic-rich breccias. This suggests a shift from a central feeder conduit, tapping the central part of the magma chamber, to a ring fracture vent system, tapping the peripheral portions of the magma chamber, during caldera collapse. Key features of these evacuating magma chambers are the thermal and volatile concentration (Xvol) gradients that produce the observed textural and compositional spectrum of trachy-phonolitic rock types. In particular, the degrees of freedom during the crystallization of these ultrapotassic magmas are increased by the variation of the leucite stability field at different PH2O conditions. Both leucite-free and leucite-bearing differentiated ultrapotassic rock types can be produced in the course of individual eruptions, as a result of pre-eruptive conditions in the feeder magma, with no need to invoke different differentiation suites related to mantle source heterogeneities of parental magmas.

  4. Rb-Sr and oxygen isotopic study of alkalic rocks from the Trans-Pecos magmatic province, Texas: Implications for the petrogenesis and hydrothermal alteration of continental alkalic rocks

    SciTech Connect

    Lambert, D.D.; Malek, D.J.; Dahl, D.A. )

    1988-10-01

    Rb-Sr and O isotopic data for mid-Tertiary alkalic rocks from the Trans-Pecos magmatic province of west Texas demonstrate that hydrothermal alteration and fluid/rock (cation exchange) interactions have affected the isotope geochemistry of these rocks. Strontium and O isotopic data for late-stage minerals in an alkali basalt (hawaiite) still record two episodes of fluid/rock interactions. These data suggest that later meteoric fluids introduced Sr with a Cretaceous marine {sup 87}Sr/{sup 86}Sr ratio into minerals with significant cation exchange capacity. Dilute HCl leaching experiments demonstrate the removal of this labile or exchangeable Sr from the alkali basalt. Rb-Sr isotopic data for the leached alkali basalt and handpicked calcite record a crystallization age of 42 Ma, consistent with K-Ar data for an unaltered alkali basalt (hawaiite) dike from the same area (42.6 {plus minus} 1.3 Ma). Leaching experiments on one phonolite suggest the Sr isotopic variability in unleached phonolite and nepheline trachyte samples may be attributed to Sr in secondary calcite and zeolites, which have an upper Cretaceous marine {sup 87}Sr/{sup 86}Sr ratio. Rb-Sr isotopic data for leached phonolite and sanidine separate yield an age of 36.5 {plus minus} 0.8 Ma, within analytical uncertainty of a K-Ar biotite age (36.0 {plus minus} 1.1 Ma) of another phonolite. These leaching experiments demonstrate that the Rb-Sr isotopic systematics of hydrothermally-altered continental alkalic rocks may be significantly improved, providing more reliable geochronologic and isotopic tracer information necessary in constructing precise models of mantle sources.

  5. Kizilcaören ore-bearing complex with carbonatites (northwestern Anatolia, Turkey): Formation time and mineralogy of rocks

    NASA Astrophysics Data System (ADS)

    Nikiforov, A. V.; Öztürk, H.; Altuncu, S.; Lebedev, V. A.

    2014-02-01

    The results of isotope-geochronological and mineralogical studies of the rocks making up the Kizilcaören fluorite-barite-REE deposit, northwestern Anatolia, Turkey are discussed in the paper. The ore is a constituent of the subvolcanic complex localized in a large fault zone. The complex combines (from earlier to later rocks): (1) phonolite and trachyte stocks, (2) carbonatite and carbonate-silicate dikelike bodies; and (3) fluorite-barite-bastnaesite ore in the form of thick homogeneous veins and cement in breccia. The K-Ar dating of silicate igneous rocks and carbonatites shows that they were formed in the Chattian Age of the Oligocene 25-24 Ma ago. Mineralogical observations show that the ore is the youngest constituent in the rock complex. Supergene alteration deeply transformed ore-bearing rocks, in particular, resulting in leaching of primary minerals, presumably Ca-Mn-Fe carbonates, and in cementation of the residual bastnaesitefluorite framework by Fe and Mn hydroxides. Most of the studied rocks contain pyrochlore, LREE fluorocarbonates, Nb-bearing rutile, Fe-Mg micas, and K-feldspar. The genetic features of the deposit have been considered. In general, the ore-bearing rock complex is compared in the set of rocks and their mineralogy and geochemistry with deposits of the Gallinas Mountains in the United States, the Arshan and Khalyuta deposits in the western Transbaikalia region, and Mushugai-Khuduk deposit in Mongolia. The Kizilcaören deposit represents a variant of postmagmatic mineralization closely related to carbonatite magmatism associated with alkaline and subalkaline intermediate rocks.

  6. Post 19 ka B.P. eruptive history of Ulleung Island, Korea, inferred from an intra-caldera pyroclastic sequence

    NASA Astrophysics Data System (ADS)

    Kim, G. B.; Cronin, S. J.; Yoon, W. S.; Sohn, Y. K.

    2014-04-01

    Ulleung Island is a Quaternary volcanic island located in the mid-western part of the East Sea (Sea of Japan) back-arc basin, which has erupted from the Pliocene until the late Holocene. This study focuses on reconstructing the latest eruptive history of the island by describing the sedimentological and stratigraphic characteristics of the most recent, trachytic/phonolitic pyroclastic sequence, named the Nari Tephra Formation. This formation is preserved as a succession of unwelded pyroclastic and epiclastic deposits within an embayed margin of the Nari Caldera. The embayment acted as a topographic trap for proximal pyroclastic deposits, and contains a complete record of the past 19,000 years of eruption history. The formation includes evidence for five separate eruptive episodes (Member N-1 to N-5), with intervening weathered and/or soil horizons indicating hundreds to thousands of years of repose between each eruption. Eruption styles and depositional mechanisms varied between and during individual episodes, reflecting changing dynamics of the magma plumbing system, magmatic gas coupling, and a variable role of external water. Extra-caldera sequences show that only a few of these eruptions generated sustained eruption columns or pyroclastic density currents (PDCs) large enough to overtop the caldera wall. Thus tephra sequences outside the caldera provide an underestimate of eruption frequency, and care needs to be taken in the interpretation and correlation to distal tephra sequences recognized in marine and terrestrial records. In addition, topographic effects of caldera structures should be considered for the assessment of PDC-related hazards in such moderately sized pyroclastic eruptions.

  7. Megacrystic pyroxene basalts sample deep crustal gabbroic cumulates beneath the Mount Taylor volcanic field, New Mexico

    NASA Astrophysics Data System (ADS)

    Schmidt, Mariek E.; Schrader, Christian M.; Crumpler, Larry S.; Rowe, Michael C.; Wolff, John A.; Boroughs, Scott P.

    2016-04-01

    Distributed over the ~ 2.3 m.y. history of the alkaline and compositionally diverse Mount Taylor Volcanic Field (MTVF), New Mexico is a widespread texturally distinct family of differentiated basalts that contain resorbed megacrysts (up to 3 cm) of plagioclase, clinopyroxene, and olivine ± Ti-magnetite ± ilmenite ± orthopyroxene. These lavas have gabbroic cumulate inclusions with mineral compositions similar to the megacrysts, suggesting a common origin. Gabbroic and megacrystic clinopyroxenes form positive linear arrays in TiO2 (0.2-2.3 wt.%) with respect to Al2O3 (0.7-9.3 wt.%). Plagioclase (An41-80) from representative thin sections analyzed for 87Sr/86Sr by laser ablation ICP-MS range from 0.7036 to 0.7048. The low 87Sr/86Sr plagioclases (0.7036 to 0.7037) are associated with high Ti-Al clinopyroxenes. Likewise, the higher 87Sr/86Sr plagioclases (0.7043 to 0.7047) are associated with the low-Al clinopyroxenes. Taken together, the pyroxene and plagioclase megacrysts appear to track the differentiation of a gabbroic pluton (or related plutons) from alkaline to Si-saturated conditions by fractional crystallization and crustal assimilation. Clinopyroxene-liquid geobarometry calculations suggest that crystallization occurred near the crust-mantle transition at an average of ~ 1200 °C and 12-13 kbar. The distribution of the megacrystic pyroxene basalts suggests that a gabbroic intrusive body underlies subregions of the MTVF that have generated silicic magmas. The gabbro is interpreted to be a significant heat and mass input into the lower crust that is capable of driving the petrogenesis of diverse silicic compositions (through fractionation and crustal assimilation), including mugearites, trachytes, trachy-andesites and dacites, high-Si rhyolites, and topaz rhyolites of the MTVF.

  8. Mineralogy, geochemistry and petrology of the phonolitic to nephelinitic Sadiman volcano, Crater Highlands, Tanzania

    NASA Astrophysics Data System (ADS)

    Zaitsev, A. N.; Marks, M. A. W.; Wenzel, T.; Spratt, J.; Sharygin, V. V.; Strekopytov, S.; Markl, G.

    2012-11-01

    Sadiman volcano is located in the Crater Highlands area of northern Tanzania, which lies next to the western escarpment of the Gregory rift—a part of the eastern branch of the East African Rift system. It consists of interlayered phonolitic tuffs, tuff breccias (with blocks of nephelinites) and nephelinitic lava flows. Rare xenoliths of phonolite lava and ijolite were observed within the nephelinite lavas with ijolite blocks occurring in phonolitic tuffs. No evidence for the presence of melilite-bearing and/or carbonatitic rocks was found during this study. On the basis of petrography, mineralogy and geochemistry the nephelinites are divided into highly porphyritic nephelinite, wollastonite nephelinite and phonolitic nephelinite, the latter of which is the dominant variety at Sadiman. Nepheline + clinopyroxene + titanite ± perovskite ± andradite-schorlomite ± wollastonite ± sanidine ± sodalite are the principle pheno- and microphenocryst phases. The nephelinites are highly evolved (Mg# = 0.17-0.26) alkaline to peralkaline (AI = 0.88-1.21) rocks enriched in incompatible elements such as Rb, Ba, Th, U, Nb, Pb, Ta, Sr and light REEs, and strongly depleted in P and Ti. This suggests derivation from an enriched mantle source and fractionation of apatite and Ti-rich mineral(s). Primary melt inclusions in nepheline phenocrysts (Thomogenization = 860-1100 °C) indicate enrichment of volatile components in the melts, particularly of fluorine (up to 1.8 wt.% in silicate glass) resulting in the formation of daughter fluorite in partly and complete crystallized inclusions. The Sadiman nephelinites crystallized under relatively oxidizing conditions (above the FMQ buffer), which differ from the reducing conditions reported for trachytic and pantelleritic rocks from other parts of the Gregory rift. Similar rock types and relatively oxidizing conditions are known from Oldoinyo Lengai and other localities, all of which are closely associated with carbonatites. By analogy, we

  9. Thermochronological evolution of an intra-plate magmatic event inferred from an integrated modeling approach: A case study in the Westerwald, Germany

    NASA Astrophysics Data System (ADS)

    Tirone, M.; Rokitta, K.; Schreiber, U.

    2016-09-01

    A lava sample from the Tertiary Westerwald volcanic field was selected for a detailed study using various analytical techniques in combination with petrological, thermodynamic and diffusion modeling to extract information related to the thermochronological evolution of a magmatic event before eruption. The lava sample contains large olivine phenocrysts which are compositionally zoned and two coexisting but chemically distinct melts, a host melt with basaltic composition and small spherical pockets of a less abundant trachytic melt (globules). The sample was analyzed by electron microprobe, x-ray fluorescence (XRF) X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The primary melt of the host lava was determined using the program PRIMELT2.XLS. Partial fractional crystallization of olivine was modeled using the program alphaMELTS. Timescale and cooling rate were retrieved by fitting the measured Fe-Mg zoning along two directions in four olivine grains from the host lava using a 3-D numerical diffusion model. The measured variation of Ca is also consistent with a chemical diffusion process, while a numerical growth model applied to the same olivines does not appear to explain the Fe-Mg zoning. Chemical zoning of major elements in the melt globules were reproduced with a multicomponent diffusion model. The results of this study show that the host magma fractionated about 9% of olivine in a first stage, then the crystallization proceeded without further separation of mineral phases. Modeling of diffusion in the olivine crystals suggests that this second stage lasted at least ~ 5 yrs and the temperature of the melt decreased from 1120-1150 °C to 1090 °C during this time. According to the results of the multicomponent diffusion model applied to the melt globules, the coexistence of the two melts was extremely short (less than few hours), possibly recording the assimilation of the globules during eruption or cooling of the whole system on the surface.

  10. The double solid reactant method: II. An application to the shallow groundwaters of the Porto Plain, Vulcano Island (Italy)

    NASA Astrophysics Data System (ADS)

    Lelli, Matteo; Cioni, Roberto; Marini, Luigi

    2008-11-01

    This paper presents an example of application of the double solid reactant method (DSRM) of Accornero and Marini (Environmental Geology, 2007a), an effective way for modeling the fate of several dissolved trace elements during water-rock interaction. The EQ3/6 software package was used for simulating the irreversible water-rock mass transfer accompanying the generation of the groundwaters of the Porto Plain shallow aquifer, starting from a degassed diluted crateric steam condensate. Reaction path modeling was performed in reaction progress mode and under closed-system conditions. The simulations assumed: (1) bulk dissolution (i.e., without any constraint on the kinetics of dissolution/precipitation reactions) of a single solid phase, a leucite-latitic glass, and (2) precipitation of amorphous silica, barite, alunite, jarosite, anhydrite, kaolinite, a solid mixture of smectites, fluorite, a solid mixture of hydroxides, illite-K, a solid mixture of saponites, a solid mixture of trigonal carbonates and a solid mixture of orthorhombic carbonates. Analytical concentrations of major chemical elements and several trace elements (Cr, Mn, Fe, Ni, Cu, Zn, As, Sr and Ba) in groundwaters were satisfactorily reproduced. In addition to these simulations, similar runs for a rhyolite, a latite and a trachyte permitted to calculate major oxide contents for the authigenic paragenesis which are comparable, to a first approximation, with the corresponding data measured for local altered rocks belonging to the silicic, advanced argillic and intermediate argillic alteration facies. The important role played by both the solid mixture of trigonal carbonates as sequestrator of Mn, Zn, Cu and Ni and the solid mixture of orthorhombic carbonates as scavenger of Sr and Ba is emphasized.