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Hydrogen Incorporation in Plagioclase.

ABSTRACT: We conducted experiments at high pressure (P) and temperature (T) to measure hydrogen solubility in plagioclase (Pl) with a range of compositions (An15 to An94). Experiments were run at 700-850 °C, 0.5 GPa, and f O 2 close to either the Ni-NiO (NNO) or iron-wüstite (IW) oxygen buffers. Experiments at 700 °C on An15 (containing 0.03 wt% FeO) reveal no dependence of H solubility on f O 2 between IW and NNO, but experiments at 800-850 °C on other compositions (with 0.3-0.5 wt% FeO) demonstrate that H solubility is enhanced by a factor of ~2 to 3 at IW compared to NNO, consistent with previous experiments by Yang (2012a) on An58. By analogy with synthetic hydrogen feldspar (HAlSi3O8), we infer that the predominant mechanism for H incorporation in Pl is through bonding to O atoms adjacent to M-site vacancies, and we propose likely O sites for H incorporation based on M-O bond lengths in anhydrous Pl structures. Increased uptake of structurally bound H at low f O 2 is explained by the formation of defect associates resulting from the reduction of Fe3+ in tetrahedral sites to Fe2+, allowing additional H to be incorporated in adjacent M-site vacancies. This mechanism counteracts the expected effect of water fugacity on H solubility. We also speculate on possible substitutions of H on tetrahedral vacancies, as well as coupled H-F substitution. Enhanced incorporation of H in Pl at low f O 2 may have implications for estimating the water content of the lunar magma ocean. However, mechanisms unrelated to low f O 2 are needed to explain high H contents in terrestrial Pl xenocrysts, such as those found in basalts from the Basin and Range.

SUBMITTER: Mosenfelder JL

PROVIDER: S-EPMC7325727 | biostudies-literature | 2020 May

REPOSITORIES: biostudies-literature

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Hydrogen Incorporation in Plagioclase.

Mosenfelder Jed L JL Andrys Janine L JL VON DER Handt Anette A Kohlstedt David L DL Hirschmann Marc M MM

Geochimica et cosmochimica acta 20200318

We conducted experiments at high pressure (P) and temperature (T) to measure hydrogen solubility in plagioclase (Pl) with a range of compositions (An15 to An94). Experiments were run at 700-850 °C, 0.5 GPa, and f O 2 close to either the Ni-NiO (NNO) or iron-wüstite (IW) oxygen buffers. Experiments at 700 °C on An15 (containing 0.03 wt% FeO) reveal no dependence of H solubility on f O 2</sub ...[more]

PMID: 32606478

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Project description:Textures and compositions of minerals can be used to infer the physiochemical conditions present within magmatic systems. Given that plagioclase is an abundant phase in many magmatic systems, understanding the link between texture and process is vital. Here, we present a database of textural and compositional data for?>?1800 plagioclase crystals in mid-ocean ridge basalt from the Gakkel Ridge (Arctic Ocean) to investigate the physiochemical conditions and processes that govern the formation of plagioclase textures and compositions. The Gakkel basalts have high modal crystal contents (up to 50%). The crystal cargo is complex, with both individual plagioclase and glomerocrysts showing large variations in crystal habit, zoning and resorption. The most common types of zoning are reverse and patchy; we attribute patchy zoning to infilling following either skeletal growth or resorption. Resorption is abundant, with multiple resorption events commonly present in a single crystal, and results from both magmatic recharge and decompression. Periods of strong undercooling, distinct to quench crystallisation, are indicated by matured skeletal crystals and thin normally zoned melt inclusion-rich bands following resorption. Individual samples often contain diverse textural and compositional plagioclase groups. Furthermore, most plagioclase is not in equilibrium with its host melt. Finally, the porous open structures of some glomerocrysts suggest that they represent pieces of entrained disaggregated mush. We interpret this to indicate that the crystal cargo is not generally phenocrystic in origin. Instead, plagioclase crystals that formed in different parts of a mush-dominated plumbing system were entrained into ascending melts. The textures of individual crystals are a function of their respective histories of (under)cooling, magma mixing and decompression. The morphologies of melt inclusion trapped in the plagioclase crystals are associated with specific host crystal textures, suggesting a link between plagioclase crystallisation processes and melt inclusion entrapment. The database of plagioclase presented herein may serve as a template for the interpretation of plagioclase textures in magmatic systems elsewhere.

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Hydrogen Incorporation in Plagioclase.

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Hydrogen Incorporation in Plagioclase.

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