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Papers by Renat Almeev

Geochemistry, Geophysics, Geosystems

Geochemistry, Geophysics, Geosystems

Earth and Planetary Science Letters

Proceedings of the European Microscopy Congress 2020

Earth and Planetary Science Letters

American Mineralogist

We present a new calibration for the determination of the iron oxidation state in silicate glasse... more We present a new calibration for the determination of the iron oxidation state in silicate glasses by electron probe micro-analysis (EPMA) with the "flank method". This method is based on the changes in both intensity and wavelength of the FeLα and FeLβ X-ray emission lines with iron oxidation state. The flank method utilizes the maximum difference for the FeLα and FeLβ spectra observed at the peak flanks between different standard materials, which quantitatively correlates with the Fe 2+ content. Provided that this correlation is calibrated on reference materials, the Fe 2+ /ΣFe ratio can be determined for samples with known total Fe content. Two synthetic Ferich ferric and ferrous garnet endmembers, i.e. andradite and almandine, were used to identify the FeLα and FeLβ flank method measuring positions that were then applied to the measurement of a variety of silicate glasses with known Fe 2+ /ΣFe ratio (ranging from 0.2 to 1.0). The measured intensity ratio of FeLβ over FeLα at these flank positions (Lβ/Lα) is a linear function of the Fe 2+ content (in wt%). A single linear trend can be established for both garnets and silicate This is a preprint, the final version is subject to change, of the American Mineralogist (MSA) Cite as Authors (Year) Title. American Mineralogist, in press.

Geochemistry, Geophysics, Geosystems

The Izu-Bonin-Mariana (IBM) fore arc preserves igneous rock assemblages that formed during subduc... more The Izu-Bonin-Mariana (IBM) fore arc preserves igneous rock assemblages that formed during subduction initiation circa 52 Ma. International Ocean Discovery Program (IODP) Expedition 352 cored four sites in the fore arc near the Ogasawara Plateau in order to document the magmatic response to subduction initiation and the physical, petrologic, and chemical stratigraphy of a nascent subduction zone. Two of these sites (U1440 and U1441) are underlain by fore-arc basalt (FAB). FABs have mid-ocean ridge basalt (MORB)-like compositions, however, FAB are consistently lower in the high-field strength elements (TiO 2 , P 2 O 5 , Zr) and Ni compared to MORB, with Na 2 O at the low end of the MORB field and FeO* at the high end. Almost all FABs are light rare earth element depleted, with low total REE, and have low ratios of highly incompatible to less incompatible elements (Ti/V, Zr/Y, Ce/Yb, and Zr/Sm) relative to MORB. Chemostratigraphic trends in Hole U1440B are consistent with the uppermost lavas forming off axis, whereas the lower lavas formed beneath a spreading center axis. Axial magma of U1440B becomes more fractionated upsection; overlying off-axis magmas return to more primitive compositions. Melt models require a two-stage process, with early garnet field melts extracted prior to later spinel field melts, with up to 23% melting to form the most depleted compositions. Mantle equilibration temperatures are higher than normal MORB (1,400°C-1,480°C) at relatively low pressures (1-2 GPa), which may reflect an influence of the Manus plume during subduction initiation. Our data support previous models of FAB origin by decompression melting but imply a source more depleted than normal MORB source mantle. Plain Language Summary This projects looks at how subduction zones form and evolve before island arc volcanism becomes established. Subduction zones are important because they are the primary sites for recycling chemically enriched crustal materials and because they form some of Earth's most important ore deposits. We drilled two deep core holes on the inner trench wall of the Izu-Bonin subduction zone to recover samples from its earliest history, before formation of Izu-Bonin island arc volcanoes. Samples from these cores were analyzed chemically to establish how lava compositions varied through time and to understand the processes that control their chemistry. We found a diverse set of lavas, with chemical compositions that are low in elements that are normally enriched in arc lavas. Calculations also show that these lavas were hotter than normal mid-ocean ridge lavas. We found that the first lavas in a new subduction zone form by the upwelling of hot material from deeper in the Earth, which partially melts as it SHERVAIS ET AL.

American Mineralogist

The iron oxidation state in silicate melts is important for understanding their physical properti... more The iron oxidation state in silicate melts is important for understanding their physical properties, although it is most often used to estimate the oxygen fugacity of magmatic systems. Often high spatial resolution analyses are required, yet the available techniques, such as μXANES and μMössbauer, require synchrotron access. The flank method is an electron probe technique with the potential to measure Fe oxidation state at high spatial resolution but requires careful method development to reduce errors related to sample damage, especially for hydrous glasses. The intensity ratios derived from measurements on the flanks of FeLα and FeLβ X-rays (FeLβ f /FeLα f) over a time interval (time-dependent ratio flank method) can be extrapolated to their initial values at the onset of analysis. We have developed and calibrated this new method using silicate glasses with a wide range of compositions (43-78 wt% SiO 2 , 0-10 wt% H 2 O, and 2-18 wt% FeO T , which is all Fe reported as FeO), including 68 glasses with known Fe oxidation state. The Fe oxidation state (Fe 2+ /Fe T) of hydrous (0-4 wt% H 2 O) basaltic (43-56 wt% SiO 2) and peralkaline (70-76 wt% SiO 2) glasses with FeO T > 5 wt% can be quantified with a precision of ±0.03 (10 wt% FeO T and 0.5 Fe 2+ /Fe T) and accuracy of ±0.1. We find basaltic and peralkaline glasses each require a different calibration curve and analysis at different spatial resolutions (~20 and ~60 μm diameter regions, respectively). A further 49 synthetic glasses were used to investigate the compositional controls on redox changes during electron beam irradiation, where we found that the direction of redox change is sensitive to glass composition. Anhydrous alkali-poor glasses become reduced during analysis, while hydrous and/or alkali-rich glasses become oxidized by the formation of magnetite nanolites identified using Raman spectroscopy. The rate of reduction is controlled by the initial oxidation state, whereas the rate of oxidation is controlled by SiO 2 , Fe, and H 2 O content.

Contributions to Mineralogy and Petrology

Lithos

Hydrothermal veins and dykelets that cross-cut layered olivine gabbros deep in the plutonic secti... more Hydrothermal veins and dykelets that cross-cut layered olivine gabbros deep in the plutonic section of the Samail Ophiolite, Sultanate of Oman, point towards the occurrence of hydrothermal circulation in the deep oceanic crust, and these features record interactions between rock and high temperature seawater-derived fluids or brines. Deep penetration of seawater-derived fluids down to 100 m above the Moho transition zone and the consequent interactions with the host rock lead to hydrothermal alteration from granulite facies grading down to greenschist facies conditions. Here we present a study of veins and dykelets formed by hydrothermal interaction cutting layered gabbro in the Wadi Wariyah, using petrographic, microanalytical, isotopic, and structural methods. We focus on amphiboles, which show a conspicuous compositional variation from high-Ti magnesiohastingsite and pargasite via magnesiohornblende and edenite, to Cl-rich ferropargasite and hastingsite (up to 1.5 a.p.f.u. Cl) and actinolite. These minerals record a wide range of formation conditions from magmatic to hydrothermal, and reveal a complex history of interactions between rock and hydrothermal fluid or brine in a lower oceanic crustal setting. Large variations in Cl content and cation configurations in amphibole suggest formation in equilibrium with fluids of different salinities at variable fluid/rock ratios. The presence of subsolidus amphibole extremely enriched in chlorine implies phase separation and brine/rock interactions. 87 Sr /86 Sr values of 0.7031 to 0.7039 and stable δ 18 O isotopic compositions of 4.1 to 5.6 ‰ of the different amphibole types suggest a rock-dominated environment, i.e. with low fluid/rock ratios. However, the slight departure from mean Oman isotope values may indicate there was some influence of seawater in the aforementioned fluid-rock interactions. Our study provides new petrological data for the subsolidus evolution of gabbro-hosted amphibole-rich veins in the presence of a seawater-derived fluid.

International Geology Review

International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of v... more International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading and ophiolite genesis.

Journal of Petrology

The position of the cotectic curve separating quartz and feldspar stability fields in the rhyolit... more The position of the cotectic curve separating quartz and feldspar stability fields in the rhyolite system Qz-Ab-Or(-An-H 2 O) depends on pressure, making it a potential geobarometer applicable to high-silica volcanic products if melt water contents (H 2 O melt) are known. Until recently, the applicability of this geobarometer has been limited because pressure effects can be largely obscured by the effects of nearly ubiquitous normative anorthite (An, CaAl 2 Si 2 O 8) in rhyolitic melts. In this study, we present new phase equilibria data that allow us to constrain the position of thermal minima and quartz-sanidine-plagioclase triple points on the quartz-feldspar cotectic curves at various pressures and melt normative An contents. Data were derived by conducting crystallization experiments to determine phase relations at the following conditions: 200 MPa, 1Á4 wt % H 2 O melt , 3Á5 wt % An; 200 MPa, 1Á3 wt % H 2 O melt , 7 wt % An; 500 MPa, 3 wt % H 2 O melt , 3Á5 wt % An; 500 MPa, 1Á4 wt % H 2 O melt , 3Á5 wt % An; 500 MPa, 1Á3 wt % H 2 O melt , 7 wt % An. Using this dataset with published phase equilibria results, we present a geobarometer based on the main parameters influencing cotectic compositions in the rhyolitic system: pressure, H 2 O melt and melt An content. Our new geobarometer DERP (DEtermining Rhyolite Pressures) is calibrated to calculate pressures of magma storage from cotectic glass compositions with up to 7 wt % normative melt An. DERP is calibrated for any H 2 O melt in the pressure range 50-500 MPa. Its application is restricted to high-silica rhyolitic systems saturated with respect to quartz and feldspar(s). DERP was tested against various independent methods for estimating rhyolite pressures available in the literature (with an overall error of less than 100 MPa). Comparing pressures estimated with DERP and rhyolite-MELTS, which are based on the same approach, suggests that rhyolite-MELTS underestimates the effect of An.

Contributions to Mineralogy and Petrology, 2014

ABSTRACT Arenal volcano is nearly unique among arc volcanoes with its 42 year long (1968–2010) co... more ABSTRACT Arenal volcano is nearly unique among arc volcanoes with its 42 year long (1968–2010) continuous, small-scale activity erupting compositionally monotonous basaltic andesites that also dominate the entire, ~7000 year long, eruptive history. Only mineral zoning records reveal that basaltic andesites are the result of complex, open-system processes deriving minerals from a variety of crystallization environments and including the episodic injections of basalt. The condition of the mafic input as well as the generation of crystal-rich basaltic andesites of the recent, 1968–2010, and earlier eruptions were addressed by an experimental study at 200 MPa, 900–1,050 °C, oxidizing and fluid-saturated conditions with various fluid compositions [H2O/(H2O + CO2) = 0.3–1]. Phase equilibria were determined using a phenocryst-poor (~3 vol%) Arenal-like basalt (50.5−wt% SiO2) from a nearby scoria cone containing olivine (Fo92), plagioclase (An86), clinopyroxene (Mg# = 82) and magnetite (Xulvö = 0.13). Experimental melts generally reproduce observed compositional trends among Arenal samples. Small differences between experimental melts and natural rocks can be explained by open-system processes. At low pressure (200 MPa), the mineral assemblage as well as the mineral compositions of the natural basalt were reproduced at 1,000 °C and high water activity. The residual melt at these conditions is basaltic andesitic (55 wt% SiO2) with 5 wt% H2O. The evolution to more evolved magmas observed at Arenal occurred under fluid-saturated conditions but variable fluid compositions. At 1,000 °C and 200 MPa, a decrease of water content by approximately 1 wt% induces significant changes of the mineral assemblage from olivine + clinopyroxene + plagioclase (5 wt% H2O in the melt) to clinopyroxene + plagioclase + orthopyroxene (4 wt% H2O in the melt). Both assemblages are observed in crystal-rich basalt (15 vol%) and basaltic andesites. Experimental data indicate that the lack of orthopyroxene and the presence of amphibole, also observed in basaltic andesitic tephra units, is due to crystallization at nearly water-saturated conditions and temperatures lower than 950 °C. The enigmatic two compositional groups previously known as low- and high-Al2O3 samples at Arenal volcano may be explained by low- and high-pressure crystallization, respectively. Using high-Al as signal of deeper crystallization, first magmas of the 1968–2010 eruption evolved deep in the crust and ascent was relatively fast leaving little time for significant compositional overprint by shallower level crystallization.

Contributions to Mineralogy and Petrology, 2015

ABSTRACT The magma storage conditions of the 6.62 Ma Blacktail Creek Tuff eruption, belonging to ... more ABSTRACT The magma storage conditions of the 6.62 Ma Blacktail Creek Tuff eruption, belonging to the Heise volcanic field (6.62-4.45 Ma old) of the Yellowstone hotspot system, have been investigated by combining thermobarometric and experimental approaches. The results from different geothermometers (e.g., Fe-Ti oxides, feldspar pairs, apatite and zircon solubility, and Ti in quartz) indicate a pre-eruptive temperature in the range 825-875 °C. The temperature estimated using two-pyroxene pairs varies in a range of 810-950 °C, but the pyroxenes are probably not in equilibrium with each other, and the analytical results of melt inclusion in pyroxenes indicate a complex history for clinopyroxene, which hosts two compositionally different inclusion types. One natural Blacktail Creek Tuff rock sample has been used to determine experimentally the equilibrium phase assemblages in the pressure range 100-500 MPa and a water activity range 0.1-1.0. The experiments have been performed at fluid-present conditions, with a fluid phase composed of H2O and CO2, as well as at fluid-absent conditions. The stability of the quartzo-feldspathic phases is similar in both types of experiments, but the presence of mafic minerals such as biotite and clinopyroxene is strongly dependent on the experimental approach. Possible explanations are given for this discrepancy which may have strong impacts on the choice of appropriate experimental approaches for the determination of magma storage conditions. The comparison of the composition of natural phases and of experimentally synthesized phases confirms magma storage temperatures of 845-875 °C. Melt water contents of 1.5-2.5 wt% H2O are required to reproduce the natural Blacktail Creek Tuff mineral assemblage at these temperatures. Using the Ti-in-quartz barometer and the Qz-Ab-Or proportions of natural matrix glasses, coexisting with quartz, plagioclase and sanidine, the depth of magma storage is estimated to be in a pressure range between 130 and 250 MPa.

Geostandards and Geoanalytical Research, 2015

Geochemistry, Geophysics, Geosystems

Geochemistry, Geophysics, Geosystems

Earth and Planetary Science Letters

Proceedings of the European Microscopy Congress 2020

Earth and Planetary Science Letters

American Mineralogist

We present a new calibration for the determination of the iron oxidation state in silicate glasse... more We present a new calibration for the determination of the iron oxidation state in silicate glasses by electron probe micro-analysis (EPMA) with the "flank method". This method is based on the changes in both intensity and wavelength of the FeLα and FeLβ X-ray emission lines with iron oxidation state. The flank method utilizes the maximum difference for the FeLα and FeLβ spectra observed at the peak flanks between different standard materials, which quantitatively correlates with the Fe 2+ content. Provided that this correlation is calibrated on reference materials, the Fe 2+ /ΣFe ratio can be determined for samples with known total Fe content. Two synthetic Ferich ferric and ferrous garnet endmembers, i.e. andradite and almandine, were used to identify the FeLα and FeLβ flank method measuring positions that were then applied to the measurement of a variety of silicate glasses with known Fe 2+ /ΣFe ratio (ranging from 0.2 to 1.0). The measured intensity ratio of FeLβ over FeLα at these flank positions (Lβ/Lα) is a linear function of the Fe 2+ content (in wt%). A single linear trend can be established for both garnets and silicate This is a preprint, the final version is subject to change, of the American Mineralogist (MSA) Cite as Authors (Year) Title. American Mineralogist, in press.

Geochemistry, Geophysics, Geosystems

The Izu-Bonin-Mariana (IBM) fore arc preserves igneous rock assemblages that formed during subduc... more The Izu-Bonin-Mariana (IBM) fore arc preserves igneous rock assemblages that formed during subduction initiation circa 52 Ma. International Ocean Discovery Program (IODP) Expedition 352 cored four sites in the fore arc near the Ogasawara Plateau in order to document the magmatic response to subduction initiation and the physical, petrologic, and chemical stratigraphy of a nascent subduction zone. Two of these sites (U1440 and U1441) are underlain by fore-arc basalt (FAB). FABs have mid-ocean ridge basalt (MORB)-like compositions, however, FAB are consistently lower in the high-field strength elements (TiO 2 , P 2 O 5 , Zr) and Ni compared to MORB, with Na 2 O at the low end of the MORB field and FeO* at the high end. Almost all FABs are light rare earth element depleted, with low total REE, and have low ratios of highly incompatible to less incompatible elements (Ti/V, Zr/Y, Ce/Yb, and Zr/Sm) relative to MORB. Chemostratigraphic trends in Hole U1440B are consistent with the uppermost lavas forming off axis, whereas the lower lavas formed beneath a spreading center axis. Axial magma of U1440B becomes more fractionated upsection; overlying off-axis magmas return to more primitive compositions. Melt models require a two-stage process, with early garnet field melts extracted prior to later spinel field melts, with up to 23% melting to form the most depleted compositions. Mantle equilibration temperatures are higher than normal MORB (1,400°C-1,480°C) at relatively low pressures (1-2 GPa), which may reflect an influence of the Manus plume during subduction initiation. Our data support previous models of FAB origin by decompression melting but imply a source more depleted than normal MORB source mantle. Plain Language Summary This projects looks at how subduction zones form and evolve before island arc volcanism becomes established. Subduction zones are important because they are the primary sites for recycling chemically enriched crustal materials and because they form some of Earth's most important ore deposits. We drilled two deep core holes on the inner trench wall of the Izu-Bonin subduction zone to recover samples from its earliest history, before formation of Izu-Bonin island arc volcanoes. Samples from these cores were analyzed chemically to establish how lava compositions varied through time and to understand the processes that control their chemistry. We found a diverse set of lavas, with chemical compositions that are low in elements that are normally enriched in arc lavas. Calculations also show that these lavas were hotter than normal mid-ocean ridge lavas. We found that the first lavas in a new subduction zone form by the upwelling of hot material from deeper in the Earth, which partially melts as it SHERVAIS ET AL.

American Mineralogist

The iron oxidation state in silicate melts is important for understanding their physical properti... more The iron oxidation state in silicate melts is important for understanding their physical properties, although it is most often used to estimate the oxygen fugacity of magmatic systems. Often high spatial resolution analyses are required, yet the available techniques, such as μXANES and μMössbauer, require synchrotron access. The flank method is an electron probe technique with the potential to measure Fe oxidation state at high spatial resolution but requires careful method development to reduce errors related to sample damage, especially for hydrous glasses. The intensity ratios derived from measurements on the flanks of FeLα and FeLβ X-rays (FeLβ f /FeLα f) over a time interval (time-dependent ratio flank method) can be extrapolated to their initial values at the onset of analysis. We have developed and calibrated this new method using silicate glasses with a wide range of compositions (43-78 wt% SiO 2 , 0-10 wt% H 2 O, and 2-18 wt% FeO T , which is all Fe reported as FeO), including 68 glasses with known Fe oxidation state. The Fe oxidation state (Fe 2+ /Fe T) of hydrous (0-4 wt% H 2 O) basaltic (43-56 wt% SiO 2) and peralkaline (70-76 wt% SiO 2) glasses with FeO T > 5 wt% can be quantified with a precision of ±0.03 (10 wt% FeO T and 0.5 Fe 2+ /Fe T) and accuracy of ±0.1. We find basaltic and peralkaline glasses each require a different calibration curve and analysis at different spatial resolutions (~20 and ~60 μm diameter regions, respectively). A further 49 synthetic glasses were used to investigate the compositional controls on redox changes during electron beam irradiation, where we found that the direction of redox change is sensitive to glass composition. Anhydrous alkali-poor glasses become reduced during analysis, while hydrous and/or alkali-rich glasses become oxidized by the formation of magnetite nanolites identified using Raman spectroscopy. The rate of reduction is controlled by the initial oxidation state, whereas the rate of oxidation is controlled by SiO 2 , Fe, and H 2 O content.

Contributions to Mineralogy and Petrology

Lithos

Hydrothermal veins and dykelets that cross-cut layered olivine gabbros deep in the plutonic secti... more Hydrothermal veins and dykelets that cross-cut layered olivine gabbros deep in the plutonic section of the Samail Ophiolite, Sultanate of Oman, point towards the occurrence of hydrothermal circulation in the deep oceanic crust, and these features record interactions between rock and high temperature seawater-derived fluids or brines. Deep penetration of seawater-derived fluids down to 100 m above the Moho transition zone and the consequent interactions with the host rock lead to hydrothermal alteration from granulite facies grading down to greenschist facies conditions. Here we present a study of veins and dykelets formed by hydrothermal interaction cutting layered gabbro in the Wadi Wariyah, using petrographic, microanalytical, isotopic, and structural methods. We focus on amphiboles, which show a conspicuous compositional variation from high-Ti magnesiohastingsite and pargasite via magnesiohornblende and edenite, to Cl-rich ferropargasite and hastingsite (up to 1.5 a.p.f.u. Cl) and actinolite. These minerals record a wide range of formation conditions from magmatic to hydrothermal, and reveal a complex history of interactions between rock and hydrothermal fluid or brine in a lower oceanic crustal setting. Large variations in Cl content and cation configurations in amphibole suggest formation in equilibrium with fluids of different salinities at variable fluid/rock ratios. The presence of subsolidus amphibole extremely enriched in chlorine implies phase separation and brine/rock interactions. 87 Sr /86 Sr values of 0.7031 to 0.7039 and stable δ 18 O isotopic compositions of 4.1 to 5.6 ‰ of the different amphibole types suggest a rock-dominated environment, i.e. with low fluid/rock ratios. However, the slight departure from mean Oman isotope values may indicate there was some influence of seawater in the aforementioned fluid-rock interactions. Our study provides new petrological data for the subsolidus evolution of gabbro-hosted amphibole-rich veins in the presence of a seawater-derived fluid.

International Geology Review

International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of v... more International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading and ophiolite genesis.

Journal of Petrology

The position of the cotectic curve separating quartz and feldspar stability fields in the rhyolit... more The position of the cotectic curve separating quartz and feldspar stability fields in the rhyolite system Qz-Ab-Or(-An-H 2 O) depends on pressure, making it a potential geobarometer applicable to high-silica volcanic products if melt water contents (H 2 O melt) are known. Until recently, the applicability of this geobarometer has been limited because pressure effects can be largely obscured by the effects of nearly ubiquitous normative anorthite (An, CaAl 2 Si 2 O 8) in rhyolitic melts. In this study, we present new phase equilibria data that allow us to constrain the position of thermal minima and quartz-sanidine-plagioclase triple points on the quartz-feldspar cotectic curves at various pressures and melt normative An contents. Data were derived by conducting crystallization experiments to determine phase relations at the following conditions: 200 MPa, 1Á4 wt % H 2 O melt , 3Á5 wt % An; 200 MPa, 1Á3 wt % H 2 O melt , 7 wt % An; 500 MPa, 3 wt % H 2 O melt , 3Á5 wt % An; 500 MPa, 1Á4 wt % H 2 O melt , 3Á5 wt % An; 500 MPa, 1Á3 wt % H 2 O melt , 7 wt % An. Using this dataset with published phase equilibria results, we present a geobarometer based on the main parameters influencing cotectic compositions in the rhyolitic system: pressure, H 2 O melt and melt An content. Our new geobarometer DERP (DEtermining Rhyolite Pressures) is calibrated to calculate pressures of magma storage from cotectic glass compositions with up to 7 wt % normative melt An. DERP is calibrated for any H 2 O melt in the pressure range 50-500 MPa. Its application is restricted to high-silica rhyolitic systems saturated with respect to quartz and feldspar(s). DERP was tested against various independent methods for estimating rhyolite pressures available in the literature (with an overall error of less than 100 MPa). Comparing pressures estimated with DERP and rhyolite-MELTS, which are based on the same approach, suggests that rhyolite-MELTS underestimates the effect of An.

Contributions to Mineralogy and Petrology, 2014

ABSTRACT Arenal volcano is nearly unique among arc volcanoes with its 42 year long (1968–2010) co... more ABSTRACT Arenal volcano is nearly unique among arc volcanoes with its 42 year long (1968–2010) continuous, small-scale activity erupting compositionally monotonous basaltic andesites that also dominate the entire, ~7000 year long, eruptive history. Only mineral zoning records reveal that basaltic andesites are the result of complex, open-system processes deriving minerals from a variety of crystallization environments and including the episodic injections of basalt. The condition of the mafic input as well as the generation of crystal-rich basaltic andesites of the recent, 1968–2010, and earlier eruptions were addressed by an experimental study at 200 MPa, 900–1,050 °C, oxidizing and fluid-saturated conditions with various fluid compositions [H2O/(H2O + CO2) = 0.3–1]. Phase equilibria were determined using a phenocryst-poor (~3 vol%) Arenal-like basalt (50.5−wt% SiO2) from a nearby scoria cone containing olivine (Fo92), plagioclase (An86), clinopyroxene (Mg# = 82) and magnetite (Xulvö = 0.13). Experimental melts generally reproduce observed compositional trends among Arenal samples. Small differences between experimental melts and natural rocks can be explained by open-system processes. At low pressure (200 MPa), the mineral assemblage as well as the mineral compositions of the natural basalt were reproduced at 1,000 °C and high water activity. The residual melt at these conditions is basaltic andesitic (55 wt% SiO2) with 5 wt% H2O. The evolution to more evolved magmas observed at Arenal occurred under fluid-saturated conditions but variable fluid compositions. At 1,000 °C and 200 MPa, a decrease of water content by approximately 1 wt% induces significant changes of the mineral assemblage from olivine + clinopyroxene + plagioclase (5 wt% H2O in the melt) to clinopyroxene + plagioclase + orthopyroxene (4 wt% H2O in the melt). Both assemblages are observed in crystal-rich basalt (15 vol%) and basaltic andesites. Experimental data indicate that the lack of orthopyroxene and the presence of amphibole, also observed in basaltic andesitic tephra units, is due to crystallization at nearly water-saturated conditions and temperatures lower than 950 °C. The enigmatic two compositional groups previously known as low- and high-Al2O3 samples at Arenal volcano may be explained by low- and high-pressure crystallization, respectively. Using high-Al as signal of deeper crystallization, first magmas of the 1968–2010 eruption evolved deep in the crust and ascent was relatively fast leaving little time for significant compositional overprint by shallower level crystallization.

Contributions to Mineralogy and Petrology, 2015

ABSTRACT The magma storage conditions of the 6.62 Ma Blacktail Creek Tuff eruption, belonging to ... more ABSTRACT The magma storage conditions of the 6.62 Ma Blacktail Creek Tuff eruption, belonging to the Heise volcanic field (6.62-4.45 Ma old) of the Yellowstone hotspot system, have been investigated by combining thermobarometric and experimental approaches. The results from different geothermometers (e.g., Fe-Ti oxides, feldspar pairs, apatite and zircon solubility, and Ti in quartz) indicate a pre-eruptive temperature in the range 825-875 °C. The temperature estimated using two-pyroxene pairs varies in a range of 810-950 °C, but the pyroxenes are probably not in equilibrium with each other, and the analytical results of melt inclusion in pyroxenes indicate a complex history for clinopyroxene, which hosts two compositionally different inclusion types. One natural Blacktail Creek Tuff rock sample has been used to determine experimentally the equilibrium phase assemblages in the pressure range 100-500 MPa and a water activity range 0.1-1.0. The experiments have been performed at fluid-present conditions, with a fluid phase composed of H2O and CO2, as well as at fluid-absent conditions. The stability of the quartzo-feldspathic phases is similar in both types of experiments, but the presence of mafic minerals such as biotite and clinopyroxene is strongly dependent on the experimental approach. Possible explanations are given for this discrepancy which may have strong impacts on the choice of appropriate experimental approaches for the determination of magma storage conditions. The comparison of the composition of natural phases and of experimentally synthesized phases confirms magma storage temperatures of 845-875 °C. Melt water contents of 1.5-2.5 wt% H2O are required to reproduce the natural Blacktail Creek Tuff mineral assemblage at these temperatures. Using the Ti-in-quartz barometer and the Qz-Ab-Or proportions of natural matrix glasses, coexisting with quartz, plagioclase and sanidine, the depth of magma storage is estimated to be in a pressure range between 130 and 250 MPa.

Geostandards and Geoanalytical Research, 2015