XANES and Mg isotopic analyses of spinels in Ca-Al-rich inclusions: Evidence for formation under oxidizing conditions
Abstract
Ti valence measurements in MgAl_2O_4 spinel from calcium-aluminum-rich inclusions (CAIs) by X-ray absorption near-edge structure (XANES) spectroscopy show that many spinels have predominantly tetravalent Ti, regardless of host phases. The average spinel in Allende type B1 inclusion TS34 has 87% Ti^(+4). Most spinels in fluffy type A (FTA) inclusions also have high Ti valence. In contrast, the rims of some spinels in TS34 and spinel grain cores in two Vigarano type B inclusions have larger amounts of trivalent titanium. Spinels from TS34 have approximately equal amounts of divalent and trivalent vanadium. Based on experiments conducted on CAI-like compositions over a range of redox conditions, both clinopyroxene and spinel should be Ti^(+3)-rich if they equilibrated with CAI liquids under near-solar oxygen fugacities. In igneous inclusions, the seeming paradox of high-valence spinels coexisting with low-valence clinopyroxene can be explained either by transient oxidizing conditions accompanying low-pressure evaporation or by equilibration of spinel with relict Ti^(+4)-rich phases (e.g., perovskite) prior to or during melting. Ion probe analyses of large spinel grains in TS34 show that they are enriched in heavy Mg, with an average Δ^(25)Mg of 4.25 ± 0.028‰, consistent with formation of the spinel from an evaporating liquid. Δ^(25)Mg shows small, but significant, variation, both within individual spinels and between spinel and adjacent melilite hosts. The Δ^(25)Mg data are most simply explained by the low-pressure evaporation model, but this model has difficulty explaining the high Ti^(+4) concentrations in spinel.
Additional Information
© 2013 The Meteoritical Society. Received 11 March 2013; revision accepted 12 September 2013. Article first published online: 24 Oct. 2013. This work was supported by NASA grants NNX09AC35G (D. S. Burnett, PI); NNX09AG40G (E. Stolper, PI); NNX08AE06G (L. Grossman, PI) and by a Monka-sho grant (H. Yurimoto, PI). We thank Jérôme Aléon and an anonymous reviewer for their constructive comments. Editorial Handling—Dr. Christine FlossAttached Files
Published - maps12216.pdf
Supplemental Material - maps12216-sup-0001-AppendixA-D.docx
Files
Name | Size | Download all |
---|---|---|
md5:9efe307839d8f6d614fc3fcd81ff4e03
|
3.2 MB | Download |
md5:58253f5ace5e44f05ac72d5ab22d2f65
|
880.8 kB | Preview Download |
Additional details
- Eprint ID
- 43366
- Resolver ID
- CaltechAUTHORS:20140114-141328012
- NNX09AC35G
- NASA
- NNX09AG40G
- NASA
- NNX08AE06G
- NASA
- Monka-sho grant
- Created
-
2014-01-14Created from EPrint's datestamp field
- Updated
-
2021-11-10Created from EPrint's last_modified field
- Caltech groups
- Division of Geological and Planetary Sciences