Classification of Igneous Rocks analyzed by ChemCam at Gale Crater, Mars.

Violaine Sautter, Valérie Payré, Olivier Forni, Nicolas Mangold, Olivier Gasnault, Laetitia Le Deit, Jeff Johnson, Sylvestre Maurice, Mark Salvatore, Roger C Wiens, Patrick Gasda, William Rapin, Agnes Cousin

In Press, Accepted Manuscript, Available online 9 February 2017


• ChemCam onboard Curiosity at Gale crater, Mars, has analyzed 59 igneous rocks during the first 800 sols.
• Igneous rocks encountered at Gale crater show different textures: aphanitic, coarse and fine-grained, and even porphyritic.
• Feldspar-rich rocks are observed mainly close to the Peace Vallis alluvial fan.
• Mafic minerals correspond to Fe-rich pigeonites and Fe-rich augites, as observed in some mars meteorites such as NWA 7034.
• Some of the basalts encountered at Gale show a low Mg# and are therefore different from the basalts observed by the MER.”

“Several recent studies have revealed that Mars is not a simple basalt-covered planet, but has a more complex geological history. In Gale crater on Mars, the Curiosity rover discovered 59 igneous rocks. This paper focuses on their textures (acquired from the cameras such as MAHLI and MastCam) and their geochemical compositions that have been obtained using the ChemCam instrument. Light-toned crystals have been observed in most of the rocks. They correspond to feldspars ranging from andesines/oligoclases to anorthoclases and sanidines in the leucocratic vesiculated rocks. Darker crystals observed in all igneous rocks (except the leucocratic vesiculated ones) were analyzed by LIBS and mainly identified as Fe-rich pigeonites and Fe-augites. Iron oxides have been observed in all groups whereas F-bearing minerals have been detected only in few of them. From their textural analysis and their whole-rock compositions, all these 59 igneous rocks have been classified in five different groups; from primitive rocks i.e. dark aphanitic basalts/basanites, trachybasalts, tephrites and fine/coarse-grained gabbros/norites to more evolved materials i.e. porphyritic trachyandesites, leucocratic trachytes and quartz-diorites. The basalts and gabbros are found all along the traverse of the rover, whereas the felsic rocks are located before the Kimberley formation, i.e. close to the Peace Vallis alluvial fan deposits. This suggests that these alkali rocks have been transported by fluvial activity and could come from the Northern rim of the crater, and may correspond to deeper strata buried under basaltic regolith (Sautter et al., 2015). Some of the basaltic igneous rocks are surprisingly enriched in iron, presenting low Mg# similar to the nakhlite parental melt that cannot be produced by direct melting of the Dreibus and Wanke (1986) martian primitive mantle. The basaltic rocks at Gale are thus different from Gusev basalts. They could originate from different mantle reservoirs, or they could have undergone a more extensive fractional crystallization. Gale basaltic rocks could have been the parental magma of residual liquid extending into alkali field towards trachyte composition as magma fractionated under anhydrous condition on its way to the surface before sub adiabatic ascent.”