Valences of Ti, Cr, and V in Apollo 17 high‐Ti and very low‐Ti basalts and implications for their formation
[1];
- Department of the Geophysical Sciences University of Chicago 5734 S. Ellis Ave Chicago Illinois 60637 USA, The Field Museum of Natural History 1400 South Lake Shore Drive Chicago Illinois 60605 USA, Institute of Meteoritics University of New Mexico Albuquerque New Mexico 87131 USA
- Department of the Geophysical Sciences University of Chicago 5734 S. Ellis Ave Chicago Illinois 60637 USA, Center for Advanced Radiation Sources (CARS) The University of Chicago 5640 S. Ellis Ave Chicago Illinois 60637 USA
Abstract To assess the variability of redox states among mare basalt source regions, investigation of the valence of Ti, Cr, and V and the coordination environment of Ti in pyroxene and olivine in lunar rocks via XANES (X‐ray absorption near‐edge structure) spectroscopy has been extended to Apollo 17 basalts: two high‐Ti (70017 and 74275) hand samples, and three very low‐Ti (70006,371, 70007,289B, and 70007,296) basalt fragments from the Apollo 17 deep drill core. Valences of Ti in pyroxene of both suites range from 3.6 to 4, or from 40% to 0% Ti 3+ , averaging 15–20% Ti 3+ . Assuming Ti 3+ is more compatible in pyroxene than Ti 4+ , then even lower Ti 3+ proportions are indicated for the parental melts. The VLT pyroxene exhibits a slightly wider range of V valences (2.57–2.96) than the high‐Ti pyroxene (2.65–2.86) and a much wider range of Cr valences (2.32–2.80 versus 2.68–2.86); Cr is generally reduced in VLT pyroxene compared to high‐Ti pyroxene. Valences of Ti and Cr in VLT pyroxene become less reduced with increasing FeO contents, possibly indicating change in oxygen fugacity during crystallization. Olivine in all samples has very low (<20%) proportions of Ti 3+ , with no Ti 3+ and higher proportions of Ti in tetrahedral coordination in the VLT s than in the high‐Ti basalts. Olivine in 74275, including that in a dunite clast, has much higher proportions of Cr 2+ than the pyroxene in that sample, consistent with previous studies indicating that the olivine grains in this sample are xenocrysts and possibly indicating oxidation just prior to pyroxene crystallization. Results for this sample, the VLT s, and previously studied Apollo 14 and 15 basalts all indicate that mare magmas were in reducing environments at depth, as recorded in early crystallization products, and that later, presumably shallower environments, were relatively oxidizing; single, characteristic f O 2 s of formation cannot be assigned to these samples. A process likely to account for this feature seen in multiple samples is loss by degassing of a reducing, H‐rich vapor (probably H 2 ) during ascent and/or eruption, causing oxidation of the residual melt, recorded in relatively late‐crystallized pyroxene.
- Sponsoring Organization:
- USDOE
- Grant/Contract Number:
- DE‐FG02‐94‐ER14466
- OSTI ID:
- 1457100
- Journal Information:
- Meteoritics and Planetary Science, Journal Name: Meteoritics and Planetary Science Vol. 53 Journal Issue: 10; ISSN 1086-9379
- Publisher:
- Wiley-BlackwellCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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