The work presented here reports findings from a detailed microscopic study of symplectites in lunar samples, specifically in basalts from the Apollo 15 mission and meteorites collected from Antarctica in 2018-2019. The symplectites studied are consistently composed of the same three minerals, olivine (olv), clinopyroxene (cpx), and silica (si). They represent the products of breakdown, likely from a previously metastable phase. Through study of their occurrence and composition, and via comparison with other extraterrestrial samples (e.g., Mars), the geological conditions under which they form are being explored. The studied symplectites were imaged using a Zeiss Supra 35 Variable Pressure Field Emission Gun Scanning Electron Microscope at Miami University’s Center for Advanced Microscopy and Imaging. In addition, the chemical composition of olivine and clinopyroxene in one lunar meteorite was quantified via electron probe microanalysis at Louisiana State University. The olivine present across all studied symplectites was found to be Fe-rich and is classified as fayalitic (up to Fa98). The clinopyroxene, based on Ca, Mg, and Fe abundances, is specifically augite. These observations are consistent with the occurrence of 3-phase symplectites in Martian meteorites which also contain olv-cpx-si. This suggests that similar geological environments may have existed on both planetary objects. Interestingly, these types of symplectites (to the authors knowledge) have not yet been documented on Earth. Previous work has proposed an origin associated with the breakdown of the metastable phase pyroxferroite, in addition to an origin via impact. Future work will continue to document the occurrence of these features throughout lunar samples, in addition to other meteorite types.
Author(s): Katie Caudill, Geology Major
Advisor(s): Claire McLeod, Department of Geology & Environmental Earth Science
Aleks Gawronska, Department of Geology & Environmental Earth Science
Alex Schweitzer, Department of Geology & Environmental Earth Science
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