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Abstract Whole rock compositions at Buldir Volcano, western Aleutian arc, record a strong, continuous trend of iron depletion with decreasing MgO, classically interpreted as a calc-alkaline liquid line of descent. In contrast, olivine-hosted melt inclusions have higher total iron (FeO*) than whole rocks and show little change in FeO* with decreasing MgO. To investigate this discrepancy and determine the conditions required for strong iron depletion, we conducted oxygen fugacity (ƒO2) buffered, water-saturated crystallization experiments at 800 MPa and ƒO2 = QFM + 1.6 ± 0.4 (1$$\sigma$$) (where QFM refers to the quartz-fayalite-magnetite buffer) on a high-Al, basaltic starting material modeled after a Buldir lava. Experimental conditions were informed by olivine-hosted melt inclusions that record minimum entrapment pressures as high as 570 MPa, >6 wt % H2O, and ƒO2 of QFM + 1.4 (±0.2), making Buldir one of the most oxidized and wettest arc volcanoes documented globally. The experiments produce melts with Si-enrichment and Fe-depletion signatures characteristic of evolved, calc-alkaline magmas at the lowest MgO, although FeO* remains roughly constant over most of the experimental temperature range. Experiments saturate CrAl-spinel and olivine at 1160°C, followed by clinopyroxene and Al-spinel at 1085°C, hornblende at 1060°C, and, finally, plagioclase and magnetite between 1040°C and 960°C. Hornblende crystallization, not magnetite, generates the largest increase in SiO2 and largest decrease in FeO* in coexisting melts. Compositions of melt inclusions are consistent with experimental melts and reflect crystallization of a basaltic parent magma at high PH2O. In contrast, the whole rock compositional trends are influenced by magma mixing and phenocryst redistribution and accumulation. The crystallization experiments and natural liquids (melt inclusions and groundmass glass) from Buldir suggest that for an oxidized, hydrous primary basalt starting composition, significant Fe depletion from the melt will not occur until intermediate to late stages of magma crystallization (< ~4.5 wt % MgO). We conclude that the Buldir whole rock trend cannot be reproduced by crystallization at arc-relevant oxygen fugacities and is not a true liquid line of descent, warranting caution when interpreting volcanic trends globally.
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High-temperature Stability of an Iron-rich Smectite: Implications for Smectite Formation on Mars
Iron-rich phyllosilicates on Mars comprise nearly 90% of the H2O- and OH-bearing phases observed directly by rovers and remotely by orbiters (Chemtob et al., 2017, JGR). Theories concerning the possible origin of Fe-rich smectite during Mars’ earliest history (phyllosian) are hard to test because of limited knowledge of the upper-thermal stability of Fe-rich phyllosilicates. In this study we present data on the upper-thermal stability of a pure-iron smectite to put some minimum constraints on its possible high-temperature origin early in Mars history either from a primordial atmosphere or by hydrothermal activity. Smectite coexisting with quartz and magnetite was synthesized from the oxides in the system Na2O-FeO-Fe2O3-Al2O3-SiO2-H2O at 500°C and 2 kbar and fO2 near FMQ. Reversal experiments involved mixtures with equal portions of the smectite-synthesis and breakdown products (quartz, fayalite, albite, magnetite (mt) treated in the presence of about 10 wt% H2O over the range of 1-3 kbar and 530-640°C. The average composition (electron microprobe) of smectite formed both in synthesis and in reversal experiments was Na0.35(Fe2+2.28Fe3+0.31Al0.41)(Al1.07Si2.93)O10(OH)2·nH2O, where ferric iron was calculated by summing the octahedral cations to 3.0. Reversals for the reaction smec+mt1 = fayalite+albite+mt2+quartz+H2O were obtained at 538±8, 590±10, and 610±10°C at 1, 2, and 3 kbar, respectively, where mt1 and mt2 have the approximate compositions Fe2.8Si0.2O4 and Fe2.8Al0.1O4, respectively, with all other phases being pure. This smectite has up to 2 interlayer H2O at 25°C (and high humidity), losing 1 H2O at <50°C, and the second at 125 ± 25°C. Thermodynamic modeling of this reaction was used to extrapolate the upper-thermal stability of this Fe-smectite down to 10 bars and approximately 239°C. Applications of these results indicate the maximum temperature for forming Fe-smectite from a dense primordial atmosphere of 100 bars is 390 ± 25°C. Crustal storage of water in Fe-smectite ranges up to a maximum of 10.7 wt% at ~2 km and 40°C, 7.4 wt% at 6 km and 120°C, and 3.8 wt% H2O at 32 km and 625°C for a Noachian geotherm of 20°C/km. Plain language summary: This study presents experimental limits on the temperatures at which the clay mineral smectite might form on Mars, either from a dense primordial atmosphere (390°C at 100 bars) or by high-temperature hydrothermal activity (625°C at 32 km). Because this study deals with iron end-member clay, these are minimum temperatures; any solid solution with magnesium will increase these temperatures.
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- Award ID(s):
- 1725053
- PAR ID:
- 10280286
- Date Published:
- Journal Name:
- American Geophysical Union meeting, Fall 2020 Meeting
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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