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Highly differentiated lavas examined by M2020 PIXL in Jezero Crater

Presentation #503.03 in the session Mars Surface Mapping and Geology.

Published onOct 20, 2022
Highly differentiated lavas examined by M2020 PIXL in Jezero Crater

Textural, bulk chemical, and mineralogical data collected by PIXL (Planetary Instrument for X-ray Lithochemistry) indicate that the first crater floor rock unit (dubbed Máaz formation) examined by the M2020 Perseverance rover in Jezero crater consists of basaltic lava flows. PIXL has produced four X-ray maps of abraded patches (Guillaumes, sol 167; Bellegarde, sol 187; Montpezat, sol 347; and Alfalfa, sol 369). Bulk rock compositions of low alteration regions of Máaz formation rocks range from basalt/trachy-basalt to trachy-andesite. Low abundances of igneous compatible elements (Mg, Ni) and high FeOT and Al2O3 are consistent with fractional crystallization of basaltic magma. The most primitive rock target examined by PIXL is Montpezat (Mg#, molar MgO/(MgO+FeO)*100=34.5, while the most evolved is Alfalfa (Mg#=10.7). CIPW norms range from olivine to quartz normative. The norms lack Al-rich minerals (e.g., corundum) that would (for a basaltic composition) imply significant chemical weathering. PIXL element maps for three of the targets show micro-gabbroic textures of interlocking, mm-scale grains of plagioclase feldspar, Fe-rich augite, and Fe-silicate. Phosphate and Fe-Ti oxide minerals occur at interstices between pyroxene and feldspar, which is consistent with a late-stage formation. Alfalfa contains similar minerals, but has a porphyritic texture with larger laths of plagioclase feldspar (up to 3 mm) surrounded by fine-grained, K-Si-rich groundmass. With decreasing bulk Mg#, modal abundances of feldspar increase, while modal augite and Mg/Fe in pyroxene decrease. The augites’ Fe contents are notably high and overlap the range of last crystallizing pyroxenes of the evolved shergottite meteorite Los Angeles. The high degree of fractionation implied by the Máaz bulk compositions and mineral assemblages implies the existence of complementary high-Mg crystalline lithologies.

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