Mineral chemistry of the Tissint meteorite: Indications of two-stage crystallization in a closed system
Abstract
The Tissint meteorite is a geochemically depleted, olivine-phyric shergottite. Olivine megacrysts contain 300–600 μm cores with uniform Mg# (~80 ± 1) followed by concentric zones of Fe-enrichment toward the rims. We applied a number of tests to distinguish the relationship of these megacrysts to the host rock. Major and trace element compositions of the Mg-rich core in olivine are in equilibrium with the bulk rock, within uncertainty, and rare earth element abundances of melt inclusions in Mg-rich olivines reported in the literature are similar to those of the bulk rock. Moreover, the P Kα intensity maps of two large olivine grains show no resorption between the uniform core and the rim. Taken together, these lines of evidence suggest the olivine megacrysts are phenocrysts. Among depleted olivine-phyric shergottites, Tissint is the first one that acts mostly as a closed system with olivine megacrysts being the phenocrysts. The texture and mineral chemistry of Tissint indicate a crystallization sequence of: olivine (Mg# 80 ± 1) → olivine (Mg# 76) + chromite → olivine (Mg# 74) + Ti-chromite → olivine (Mg# 74–63) + pyroxene (Mg# 76–65) + Cr-ulvöspinel → olivine (Mg# 63–35) + pyroxene (Mg# 65–60) + plagioclase, followed by late-stage ilmenite and phosphate. The crystallization of the Tissint meteorite likely occurred in two stages: uniform olivine cores likely crystallized under equilibrium conditions; and a fractional crystallization sequence that formed the rest of the rock. The two-stage crystallization without crystal settling is simulated using MELTS and the Tissint bulk composition, and can broadly reproduce the crystallization sequence and mineral chemistry measured in the Tissint samples. The transition between equilibrium and fractional crystallization is associated with a dramatic increase in cooling rate and might have been driven by an acceleration in the ascent rate or by encounter with a steep thermal gradient in the Martian crust.
Additional Information
© 2016 Meteoritical Society. Received 04 February 2016; revision accepted 10 July 2016. Version of Record online: 5 Oct 2016. We thank Allan Patchen and Luca Fedele for their assistance with data collection. Comments from the AE (C. Goodrich), T. Usui, C. Herd, and M. McCanta on earlier versions of the manuscript have significantly improved the presentation and are greatly appreciated. We acknowledge partial support by NASA Cosmochemistry grants NNX11AG58G to LAT, NNN13D465T to YL, NSF Grant EAR-1226270 to PDA, and NSF Grant EAR-1019770 to RJB. YL is supported by the Jet Propulsion Laboratory, which is managed by the California Institute of Technology under a contract with NASA.Attached Files
Published - Liu_et_al-2016-Meteoritics___Planetary_Science.pdf
Supplemental Material - maps12726-sup-0001-SupInfo.docx
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Additional details
- Eprint ID
- 70996
- Resolver ID
- CaltechAUTHORS:20161011-120243183
- NNX11AG58G
- NASA
- NNN13D465T
- NASA
- EAR-1226270
- NSF
- EAR-1019770
- NSF
- NASA/JPL/Caltech
- Created
-
2016-10-11Created from EPrint's datestamp field
- Updated
-
2021-11-11Created from EPrint's last_modified field
- Caltech groups
- Division of Geological and Planetary Sciences