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Valence of Ti, V, and Cr in Apollo 14 aluminous basalts 14053 and 14072
Author(s) -
Simon Steven B.,
Sutton Stephen R.
Publication year - 2017
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.12908
Subject(s) - pyroxene , olivine , basalt , geology , valence (chemistry) , xanes , plagioclase , mineralogy , geochemistry , analytical chemistry (journal) , spectroscopy , chemistry , physics , paleontology , quartz , organic chemistry , chromatography , quantum mechanics
The valences of Ti, V, and Cr in olivine and pyroxene, important indicators of the f O 2 of the source region of their host rocks, can be readily measured nondestructively by XANES (X‐ray absorption near edge structure) spectroscopy, but little such work has been done on lunar rocks, and there is some uncertainty regarding the presence of Ti 3+ in lunar silicates and the redox state of the lunar mantle. This is the first study involving direct XANES measurement of valences of multivalent cations in lunar rocks. Because high alumina activity facilitates substitution of Ti cations into octahedral rather than tetrahedral sites in pyroxene and Ti 3+ only enters octahedral sites, two aluminous basalts from Apollo 14, 14053 and 14072, were studied. Most pyroxene contains little or no detectable Ti 3+ , but in both samples relatively early, magnesian pyroxene was found that has Ti valences that are not within error of 4; in 14053, this component has an average Ti valence of 3.81 ± 0.06 (i.e., Ti 3+ /[Ti 3+ + Ti 4+ = 0.19]). This pyroxene has relatively low atomic Ti/Al ratios (<0.4) due to crystallization before plagioclase, contrary to the long‐held belief that lunar pyroxene with Ti/Al > 0.5 contains Ti 3+ and pyroxene with lower ratios does not. Later pyroxene, with lower Mg/Fe and higher Ti/Al ratios, has higher proportions of Ti (all Ti 4+ ) in tetrahedral sites. All pyroxene analyzed contains divalent Cr, ranging from 15 to 30% of the Cr present, and all but one analysis spot contains divalent V, accounting for 0 to 40% (typically 20–30%) of the V present. Three analyses of olivine in 14053 do not show any Ti 3+ , but Ti valences in 14072 olivine range from 4 down to 3.70 ± 0.10. In 14053 olivine, ~50% of the Cr and 60% of the V are divalent. In 14072 olivine, the divalent percentages are ~20% for Cr and 20–60% for V. These results indicate significant proportions of divalent Cr and V and limited amounts of trivalent Ti in the parental melts, especially when crystal/liquid partitioning preferences are taken into account. These features are consistent with an f O 2 closer to IW − 2 than to IW − 1. Apollo 15 basalt 15555, analyzed for comparison with A‐14 materials, has olivine with strongly reduced Cr (Cr 2+ /(Cr 2+ + Cr 3+ ) ~0.9). Basalts from different sites may record redox differences between source regions.