An experimental investigation of F, Cl and H2O mineral-melt partitioning in a reduced, model lunar system

Nicola J Potts; Geoffrey Bromiley; Richard A Brooker

doi:10.1016/j.gca.2020.12.003

An experimental investigation of F, Cl and H2O mineral-melt partitioning in a reduced, model lunar system

Nicola J Potts, Geoffrey Bromiley^*, Richard A Brooker

^*Corresponding author for this work

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Abstract

It is believed that the Moon formed following collision of a large planetesimal with the early Earth. Over the ~4 Gyr since this event the Moon has been considerably less processed by geological activity than the Earth, and may provide a better record of processes and conditions in the early Earth-Moon system. There have been many studies of magmatic volatiles such as H, F, Cl, S and C in lunar materials. However, our ability to interpret variable volatile contents in the lunar sample suite is dependent on our understanding of volatile behaviour in lunar systems. This is currently constrained by limited experimental data. Here, we present the first experimental mineral-melt partitioning coefficients for F, Cl and H2O in a model lunar system under appropriately reduced conditions (log fO2 to IW-2.1, i.e. oxygen fugacity down to 2.1 log units below the Fe-FeO buffer). Data are consistent with structural incorporation of F, Cl and OH- in silicate melt, olivine and pyroxene under conditions of the lunar mantle. Oxygen fugacity has a limited effect on H2O speciation, and partitioning of H2O, F and Cl is instead largely dependent on mineral chemistry and melt structure. Partition coefficients are broadly consistent with a mantle source region for lunar volcanic products that is significantly depleted in F, Cl and H2O, and depleted in Cl relative to F and H2O, compared to the terrestrial mantle. Partitioning data are also used to model volatile redistribution during lunar magma ocean (LMO) crystallisation. The volatile content of lunar mantle cumulates is dependent upon proportion of trapped liquid during LMO solidification. However, differences in mineral-melt partitioning during LMO solidification can result in significant enrichment on F relative to Cl, and F relative to H2O, in cumulate phases relative to original LMO composition. As such, Cl depletion in lunar volcanic products may in part be a result of LMO solidification.

Original language	English
Pages (from-to)	232-254
Number of pages	23
Journal	Geochimica et Cosmochimica Acta
Volume	294
Early online date	15 Dec 2020
DOIs	https://doi.org/10.1016/j.gca.2020.12.003
Publication status	Published - 1 Feb 2021

Bibliographical note

Funding Information:
Work was supported by the UK National Environmental Research Council via NE/M000346/1 (to Bromiley) and award of instrument time at the Edinburgh Ion Microprobe facility, IMF597/0516. Brooker was funded by the NERC Thematic Grant consortium NE/M000419/1. Dr Amrei Baasner is thanked for preparing starting mixes, and Dr Chris Hayward and Dr Richard Hinton for assistance in conducted EMPA and SIMS analyses, respectively. The authors thank Prof. Youxue Zhang and two anonymous reviewers whose comments and suggestions improved this manuscript considerably.

Publisher Copyright:
© 2020

Keywords

Water
Fluorine
Chlorine
Partitioning
Magma
Mineral
Moon

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10.1016/j.gca.2020.12.003

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abstract = "It is believed that the Moon formed following collision of a large planetesimal with the early Earth. Over the ~4 Gyr since this event the Moon has been considerably less processed by geological activity than the Earth, and may provide a better record of processes and conditions in the early Earth-Moon system. There have been many studies of magmatic volatiles such as H, F, Cl, S and C in lunar materials. However, our ability to interpret variable volatile contents in the lunar sample suite is dependent on our understanding of volatile behaviour in lunar systems. This is currently constrained by limited experimental data. Here, we present the first experimental mineral-melt partitioning coefficients for F, Cl and H2O in a model lunar system under appropriately reduced conditions (log fO2 to IW-2.1, i.e. oxygen fugacity down to 2.1 log units below the Fe-FeO buffer). Data are consistent with structural incorporation of F, Cl and OH- in silicate melt, olivine and pyroxene under conditions of the lunar mantle. Oxygen fugacity has a limited effect on H2O speciation, and partitioning of H2O, F and Cl is instead largely dependent on mineral chemistry and melt structure. Partition coefficients are broadly consistent with a mantle source region for lunar volcanic products that is significantly depleted in F, Cl and H2O, and depleted in Cl relative to F and H2O, compared to the terrestrial mantle. Partitioning data are also used to model volatile redistribution during lunar magma ocean (LMO) crystallisation. The volatile content of lunar mantle cumulates is dependent upon proportion of trapped liquid during LMO solidification. However, differences in mineral-melt partitioning during LMO solidification can result in significant enrichment on F relative to Cl, and F relative to H2O, in cumulate phases relative to original LMO composition. As such, Cl depletion in lunar volcanic products may in part be a result of LMO solidification.",

keywords = "Water, Fluorine, Chlorine, Partitioning, Magma, Mineral, Moon",

author = "Potts, {Nicola J} and Geoffrey Bromiley and Brooker, {Richard A}",

note = "Funding Information: Work was supported by the UK National Environmental Research Council via NE/M000346/1 (to Bromiley) and award of instrument time at the Edinburgh Ion Microprobe facility, IMF597/0516. Brooker was funded by the NERC Thematic Grant consortium NE/M000419/1. Dr Amrei Baasner is thanked for preparing starting mixes, and Dr Chris Hayward and Dr Richard Hinton for assistance in conducted EMPA and SIMS analyses, respectively. The authors thank Prof. Youxue Zhang and two anonymous reviewers whose comments and suggestions improved this manuscript considerably. Publisher Copyright: {\textcopyright} 2020",

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AU - Potts, Nicola J

AU - Bromiley, Geoffrey

AU - Brooker, Richard A

N1 - Funding Information: Work was supported by the UK National Environmental Research Council via NE/M000346/1 (to Bromiley) and award of instrument time at the Edinburgh Ion Microprobe facility, IMF597/0516. Brooker was funded by the NERC Thematic Grant consortium NE/M000419/1. Dr Amrei Baasner is thanked for preparing starting mixes, and Dr Chris Hayward and Dr Richard Hinton for assistance in conducted EMPA and SIMS analyses, respectively. The authors thank Prof. Youxue Zhang and two anonymous reviewers whose comments and suggestions improved this manuscript considerably. Publisher Copyright: © 2020

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N2 - It is believed that the Moon formed following collision of a large planetesimal with the early Earth. Over the ~4 Gyr since this event the Moon has been considerably less processed by geological activity than the Earth, and may provide a better record of processes and conditions in the early Earth-Moon system. There have been many studies of magmatic volatiles such as H, F, Cl, S and C in lunar materials. However, our ability to interpret variable volatile contents in the lunar sample suite is dependent on our understanding of volatile behaviour in lunar systems. This is currently constrained by limited experimental data. Here, we present the first experimental mineral-melt partitioning coefficients for F, Cl and H2O in a model lunar system under appropriately reduced conditions (log fO2 to IW-2.1, i.e. oxygen fugacity down to 2.1 log units below the Fe-FeO buffer). Data are consistent with structural incorporation of F, Cl and OH- in silicate melt, olivine and pyroxene under conditions of the lunar mantle. Oxygen fugacity has a limited effect on H2O speciation, and partitioning of H2O, F and Cl is instead largely dependent on mineral chemistry and melt structure. Partition coefficients are broadly consistent with a mantle source region for lunar volcanic products that is significantly depleted in F, Cl and H2O, and depleted in Cl relative to F and H2O, compared to the terrestrial mantle. Partitioning data are also used to model volatile redistribution during lunar magma ocean (LMO) crystallisation. The volatile content of lunar mantle cumulates is dependent upon proportion of trapped liquid during LMO solidification. However, differences in mineral-melt partitioning during LMO solidification can result in significant enrichment on F relative to Cl, and F relative to H2O, in cumulate phases relative to original LMO composition. As such, Cl depletion in lunar volcanic products may in part be a result of LMO solidification.

AB - It is believed that the Moon formed following collision of a large planetesimal with the early Earth. Over the ~4 Gyr since this event the Moon has been considerably less processed by geological activity than the Earth, and may provide a better record of processes and conditions in the early Earth-Moon system. There have been many studies of magmatic volatiles such as H, F, Cl, S and C in lunar materials. However, our ability to interpret variable volatile contents in the lunar sample suite is dependent on our understanding of volatile behaviour in lunar systems. This is currently constrained by limited experimental data. Here, we present the first experimental mineral-melt partitioning coefficients for F, Cl and H2O in a model lunar system under appropriately reduced conditions (log fO2 to IW-2.1, i.e. oxygen fugacity down to 2.1 log units below the Fe-FeO buffer). Data are consistent with structural incorporation of F, Cl and OH- in silicate melt, olivine and pyroxene under conditions of the lunar mantle. Oxygen fugacity has a limited effect on H2O speciation, and partitioning of H2O, F and Cl is instead largely dependent on mineral chemistry and melt structure. Partition coefficients are broadly consistent with a mantle source region for lunar volcanic products that is significantly depleted in F, Cl and H2O, and depleted in Cl relative to F and H2O, compared to the terrestrial mantle. Partitioning data are also used to model volatile redistribution during lunar magma ocean (LMO) crystallisation. The volatile content of lunar mantle cumulates is dependent upon proportion of trapped liquid during LMO solidification. However, differences in mineral-melt partitioning during LMO solidification can result in significant enrichment on F relative to Cl, and F relative to H2O, in cumulate phases relative to original LMO composition. As such, Cl depletion in lunar volcanic products may in part be a result of LMO solidification.

KW - Water

KW - Fluorine

KW - Chlorine

KW - Partitioning

KW - Magma

KW - Mineral

KW - Moon

U2 - 10.1016/j.gca.2020.12.003

DO - 10.1016/j.gca.2020.12.003

M3 - Article (Academic Journal)

SN - 0016-7037

VL - 294

SP - 232

EP - 254

JO - Geochimica et Cosmochimica Acta

JF - Geochimica et Cosmochimica Acta

ER -

An experimental investigation of F, Cl and H2O mineral-melt partitioning in a reduced, model lunar system

Abstract

Bibliographical note

Keywords

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