Apxs Chemical Composition of the Kimberley Sandstone in Gale Crater
Abstract:Kimberley was chosen as a major waypoint of the MSL rover Curiosity on its way to Mount Sharp. APXS data before drilling showed interestingly high K, Fe and Zn. This warranted drilling of the fine-grained sandstone for detailed investigations with SAM and Chemin.
With significantly lower Na, Al and higher K, Mg and Fe, the composition of the drill target Windjana is very distinct from the previous ones in the mudstones at Yellowknife Bay. Up to 2000 ppm Br and 4000 ppm Zn post-brush were among the highest measured values in Gale Crater. The excavated fines, stemming from about 6cm, showed lower Br, but even higher Zn.
Preliminary Chemin results indicate K-feldspar and magnetite being major mineral phases in Windjana, which is consistent with the pre drill APXS result and derived CIPW norms.
Inside the accessible work volume of the arm at the drill site ChemCam exposed a greyish, shinier patch of rock underneath the dust, dubbed Stephen. ChemCam sees a high Mn signal in most of the spots. An APXS integration revealed high MnO as well (~4%), in addition to high Mg, Cl,K,Ni,Zn,Br,Cu,Ge and for the first time an APXS detectable amount of ~300 ppm Co.
The surface might reflect a thin surface layer and may underestimate the higher Z elemental concentration since the APXS analysis assumes an infinite sample. Important elemental correlations are likely not impacted. A four spot daytime raster of Stephen before leaving the drill site showed a good correlation of Mn with Zn, Cu and Ni. All spots have 3-3.5% Cl, the highest values measured on Mars so far. While the stratigraphic setting of the Stephen sample is discussed elsewhere, the similarity with Mn deep-sea nodules is striking, e.g. the APXS calibration sample GBW07296. Whatever process formed Stephen, the process of Mn scavenging high Z trace metals from solutions seems to have happened similarly at this site on Mars.