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- Title
Constraining Ancient Magmatic Evolution on Mars Using Crystal Chemistry of Detrital Igneous Minerals in the Sedimentary Bradbury Group, Gale Crater, Mars.
- Authors
Payré, V.; Siebach, K. L.; Dasgupta, R.; Udry, A.; Rampe, E. B.; Morrison, S. M.
- Abstract
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 challenging because of the following: (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 Mars 2020 mission, which will also investigate an ancient volcaniclastic‐sedimentary environment and will include a LIBS instrument. Plain Language Summary: Using instruments on Mars rovers, classification of rocks as igneous or sedimentary can be ambiguous, as many of the sedimentary rocks are made of barely altered igneous minerals. We avoid this issue by using the chemistry of individual igneous minerals found in both sedimentary and igneous rocks to assess possible magmatic processes. We focus on rocks in the Bradbury group observed by the Curiosity rover, which includes river and lake deposits coming from a source area to the north of Gale crater. We report the chemistry of pure igneous minerals hit by the ChemCam laser. Then, using modeling, we assess several scenarios for magmatic processes to identify conditions that could have reproduced the mineral chemistries observed. This new method shows that fractional crystallization of two compositionally distinct parental melts formed during the ascent of a mantle composition can explain the igneous mineral chemistry observed. These results are consistent with previous works in Gale, and are interesting because (1) both alkaline and subalkaline magmas were likely produced very early in Mars history, and (2) this provides a new way to constrain magmatic processes from observations of sedimentary rocks, which will be important for the Mars 2020 mission. Key Points: Detrital feldspar and pyroxene mineral compositions were estimated for the first time from stoichiometric analyses of ChemCam compositionsMELTS modeling was used to constrain possible ancient magmatic contributions to the Martian surface mineral chemistry without igneous outcropsOur method is important for future missions like Mars 2020 where igneous outcrops may be rare but detrital minerals are preserved
- Subjects
STOICHIOMETRY; SEISMIC waves; IGNEOUS rocks; THERMAL fatigue; THERMAL stresses
- Publication
Journal of Geophysical Research. Planets, 2020, Vol 125, Issue 8, p1
- ISSN
2169-9097
- Publication type
Article
- DOI
10.1029/2020JE006467