CHANGES IN TRACE ELEMENT COMPOSITION BETWEEN FLOAT BASALTS AND THE SEDIMENTS OF YELLOWKNIFE BAY, GALE CRATER, MARS

Yellowknife Bay was explored by the Curiosity Rover in the first ~300 sols of its mission; it was determined to be a lacustrine setting that had undergone limited chemical weathering as indicated by the low CIA values at this site, with all chemical analyses displaying values under 50. Within the Yellowknife Bay stratigraphy there are roughly 115 individual ChemCam targets and >1200 individual LIBS points; this coverage provides ample opportunity for in-depth investigation of the trace-element geochemistry at this site that can build on past geochemical and sedimentological research at this site. In addition, ChemCam has sampled 16 float rocks which were observed to be mafic and aphanitic and so were identified as basalts in Cousin et al. (2017). By assuming that these rocks represent the relatively unaltered source of the sediments that make up Yellowknife Bay, we can investigate source-to-sink geochemical changes. This research is primarily interested in the ability of trace elements to complement previous research; especially as ChemCam’s ability to quantify Ba, Li, Rb, and Sr are much more refined than at the time of the original research and could provide new insights.

Compared to the basalts, in the sediments Ba shows no change, Li is slightly depleted, Sr is enriched in the Sheepbed mudstone, but not the other layers. Sheepbed contains calcium sulfate veins, which account for all Sr values over 175 ppm; however, the host rock is still slightly enriched in Sr which is still probably related to calcium sulfate enrichment. Rb displays a bimodal distribution in the basalts while the sediments are consistent with either mainly containing sediment derived from the low concentration source or both sources if the sediments sourced from the high Rb concentration basalts were depleted before deposition.

These preliminary results are consistent with previous work that the Yellowknife Bay sediment underwent relatively minor chemical weathering; the changes from source to sink are either small in scale or easily explainable. Further work will map the distribution of these elements within the stratigraphy in which they are found, and attempt to model the conditions necessary for the trace elements to be deposited in the manner described above.