APXS CHEMISTRY OF RECENT FLOAT ROCKS AND THEIR POSSIBLE RELATION TO MAJOR FORMATIONS AT GALE CRATER

Abstract:

Chemistry is one of the tools the Mars Science Lab (MSL) Curiosity rover applies to unravel the stratigraphy in Gale Crater on its traverse up Mount Sharp. Complimentary to mineralogy and imagery it is important to assess sedimentary processes, relate the extended bedrock formations and to imply the chemistry of the fluids involved in the formation. Here we discuss several encountered out of place float rocks and their possible relation to the bedrock, which possibly allows to predict major uphill layers. The elemental patterns of the main formations allows pointing out possible relations of the floats with major units.

The main units found at Gale Crater are (1) Murray fm with low Mg, Ca, Cr, elevated and variable Fe/Mn>50, elevated K, Ni, Zn and very diagnostic ~100 ppm Ge and (2) Stimson fm with average Mars like composition as encountered in the basaltic soils in Gale and other landing sites. Samples on the Greenheugh pediment are generally similar to Stimson, but elevated in K, about twice of usual Stimson.

Just below the pediment, three floats were investigated that possibly rolled down the steep slopes. HeinrichWaenke and LomondHills are enriched beyond the typical elevated K that was found in the pediment outcrops and Murray. Their Fe/Mn is ~ 50 and they don’t have the lower Mg and Ca, while having Ge>100ppm and ~1500 ppm ZN. This could indicate a K-rich source rock uphill, that interacted with Ge and Zn-enriched fluids that laid down the Murray fm, possibly related to the Windjana area found downhill. Blackwaterfoot on the other hand has low Ge, Ni, Zn and higher Cr, while otherwise in family with the other two, indicating less interaction with the fluids. None of the floats is a good match to the pediment, however, the fluid chemistry signature is consistent with the Murray fm. GretnaGreen is a nearby 5cm pebble that could be a stony meteorite fragment with very high Ni or an ultramafic rock fragment from uphill. Similar floats were found far below the current elevation, for example in the Bimbe area with an estonishing variety of floats in a small area.

So far, no S rich floats were found, but two bedrock samples, ClachGlas and HuttonsSection, have elevated ~ 15% SO₃ without clearly related cation and low Ge, which is similar to the massive Burns fm at Meridiani Planum.