Constraining ancient magmatic evolution on Mars using crystal chemistry of detrital igneous minerals in the sedimentary Bradbury group, Gale crater, Mars

1V. Payré,1K. L. Siebach,1R. Dasgupta,2A. Udry,3E. B. Rampe,4S. M. Morrison
Journal of Geophysical Research (Planets) (In Press) Link to Article [https://doi.org/10.1029/2020JE006467]
1Department of Earth, Environmental and Planetary Sciences, Rice University, Houston, TX, USA
2Department of Geoscience, University of Nevada, Las Vegas, NV, USA
3Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Houston, TX, USA
4Geophysical Laboratory, Carnegie Institution for Science, Washington, D.C., USA
Published by arrangement with John Wiley & Sons

Understanding magmatic processes is critical to understanding Mars as a system, but Curiosity’s investigation of dominantly sedimentary rocks has made it difficult to constrain igneous processes. Igneous classification of float rocks is made difficult by: (1) the possibility that they have been affected by sedimentary processes or weathering, and (2) grain size heterogeneity in the observed rock textures makes the small‐scale compositions measured by rover instruments unreliable for bulk classification We avoid these ambiguities by using detrital igneous mineral chemistry to constrain models of magmatic processes in the source region for the fluvio‐deltaic Bradbury group. Mineral chemistry is obtained from X‐ray diffraction of three collected samples and a new stoichiometric and visual filtering of ~5,000 laser induced breakdown spectroscopy (LIBS) spots to identify compositions of individual igneous minerals. Observed mineral chemistries are compared to those produced by MELTS thermodynamic modeling to constrain possible magmatic conditions. Fractionation of two starting primary melts derived from different extent of adiabatic decompression melting of a primitive mantle composition could result in the crystallization of all minerals observed. Crystal fractionation of a subalkaline and an alkaline magma is required to form the observed minerals. These results are consistent with the collection of alkaline and subalkaline rocks from Gale as well as clasts from the martian meteorite Northwest Africa 7034 and paired stones. This new method for constraining magmatic processes will be of significant interest for the Mars2020 mission, which will also investigate an ancient volcaniclastic‐sedimentary environment and will include a LIBS instrument.

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