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Published November 2016 | Published + Supplemental Material
Journal Article Open

Postcrystallization metasomatism in shergottites: Evidence from the paired meteorites LAR 06319 and LAR 12011

Abstract

Apatite is the major volatile-bearing phase in Martian meteorites, containing structurally bound fluorine, chlorine, and hydroxyl ions. In apatite, F is more compatible than Cl, which in turn is more compatible than OH. During degassing, Cl strongly partitions into the exsolved phase, whereas F remains in the melt. For these reasons, the volatile concentrations within apatite are predictable during magmatic differentiation and degassing. Here, we present compositional data for apatite and merrillite in the paired enriched, olivine-phyric shergottites LAR 12011 and LAR 06319. In addition, we calculate the relative volatile fugacities of the parental melts at the time of apatite formation. The apatites are dominantly OH-rich (calculated by stoichiometry) with variable yet high Cl contents. Although several other studies have found evidence for degassing in the late-stage mineral assemblage of LAR 06319, the apatite evolutionary trends cannot be reconciled with this interpretation. The variable Cl contents and high OH contents measured in apatites are not consistent with fractionation either. Volatile fugacity calculations indicate that water and fluorine activities remain relatively constant, whereas there is a large variation in the chlorine activity. The Martian crust is Cl-rich indicating that changes in Cl contents in the apatites may be related to an external crustal source. We suggest that the high and variable Cl contents and high OH contents of the apatite are the results of postcrystallization interaction with Cl-rich, and possibly water-rich, crustal fluids circulating in the Martian crust.

Additional Information

© 2016 The Meteoritical Society. Issue online: 8 November 2016; Version of record online: 15 March 2016; Manuscript Accepted: 12 October 2015; Manuscript Received: 22 June 2015. This work was supported by NASA Cosmochemistry grant NNX11AG58G, awarded to LAT. We thank Allen Patchen for assistance with electron microprobe analyses. YL acknowledge the partial support by NASA Cosmochemistry grants NNN13D465T, and support from the Jet Propulsion Laboratory, which is managed by the California Institute of Technology under the contract with NASA. We are extremely grateful to Francis McCubbin and Justin Filiberto for detailed and thorough reviews, which greatly aided the overall content and discussion of this manuscript. We also thank Cyrena Goodrich for editorial handling.

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Published - Howarth_et_al-2016-Meteoritics___Planetary_Science.pdf

Supplemental Material - maps12576-sup-0001-DataS1.xlsx

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August 20, 2023
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