LITHIUM IN THE ROCKS AND SOILS OF GALE CRATER, MARS, AS OBSERVED BY CHEMCAM
The Li abundances in near-surface soils constrain hypotheses on formation mechanisms of the salt component, which was observed by Viking to be enriched in S and Cl. Li is a fluid-mobile element and, if the observed S/Cl ratio in soils was caused by hydrothermal fluids reaching the surface, Li should be enriched on the order of ~100 ppm. The low (<10 ppm) observed abundance of Li in soils is consistent with processes like volcanic aerosol deposition that could not enrich Li into the soils, but which could enrich S and Cl.
The two drill locations in the YKB Sheepbed mudstone unit, and specifically in the drill tailings and drill hole, have very low Li (<5 ppm). The CheMin instrument observed a large smectite (~20%) component in the sampled materials with diffraction patterns consistent with trioctohedral smectite. The low Li abundance indicates the clay fraction did not retain Li during formation or any subsequent alteration.
In contrast, Li abundances up to ~60 ppm have been observed in several rocks analyzed in the Shaler sandstone unit, on the edge of the YKB region. ChemCam analysis point 8 in the Rocknest_3a line scan has ~60 ppm Li and correlations between Li and major elements indicates the Li may be hosted by an aluminosilicate. Bathurst_Inlet shows enrichments in Li at all 5 analysis points (>30 ppm), and 3 points reveal a marked decrease in Li over the 30 shot mini-depth profile. The alkalis (Rb, K, and Na) tend to follow Li, but to a lesser extent. These trends may be related to fluid mobility and thus could indicate Bathurst_Inlet has undergone near-surface aqueous alteration that has drawn the most mobile elements upwards via matrix wicking and surface evaporation.