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Lithium isotopes and light lithophile element abundances in shergottites: Evidence for both magmatic degassing and subsolidus diffusion
Author(s) -
Udry Arya,
McSween Harry Y.,
Hervig Richard L.,
Taylor Lawrence A.
Publication year - 2016
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.12582
Subject(s) - lithophile , geology , isotope fractionation , isotope , meteorite , basalt , achondrite , equilibrium fractionation , pyroxene , geochemistry , olivine , pigeonite , fractionation , diffusion , mineralogy , chemistry , chondrite , plagioclase , astrobiology , partial melting , quartz , augite , paleontology , quantum mechanics , thermodynamics , physics , organic chemistry
Degassed magmatic water was potentially the major source of surficial water on Mars. We measured Li, B, and Be abundances and Li isotope profiles in pyroxenes, olivines, and maskelynite from four compositionally different shergottites—Shergotty, QUE 94201, LAR 06319, and Tissint—using secondary ion mass spectrometry ( SIMS ). All three light lithophile elements ( LLE ) are incompatible: Li and B are soluble in H 2 O‐rich fluids, whereas Be is insoluble. In the analyzed shergottites, Li concentration decreases and Be concentration increases from cores to rims in pyroxenes. However, B concentrations do not vary consistently with Li and Be abundances, except in QUE 94201 pyroxenes. Additionally, abundances of these three elements in olivines show a normal igneous‐fractionation trend consistent with the crystallization of olivine before magma ascent and degassing. We expect that kinetic effects would lead to fractionation of 6 Li in the vapor phase compared to 7 Li during degassing. The Li isotope profiles, with increasing δ 7 Li from cores to rims, as well as Li and B profiles indicate possible degassing of hydrous fluids only for the depleted shergottite QUE 94201, as also supported by degassing models. Conversely, Shergotty, LAR 06319, and Tissint appear to have been affected by postcrystallization diffusion, based on their LLE and Li isotope profiles, accompanied by diffusion models. This process may represent an overlay on a degassing pattern. The LLE profiles and isotope profiles in QUE 94201 support the hypothesis that degassing of some basaltic shergottite magmas provided water to the Martian surface, although evidence may be obscured by subsolidus diffusion processes.