USING LASER-INDUCED BREAKDOWN SPECTROSCOPY (LIBS) TO DISTINGUISH TERRESTRIAL MANGANESE-ENRICHMENTS WITH RELEVANCE FOR GALE CRATER, MARS

In terrestrial settings, sedimentary enrichments of manganese (Mn) are thought to be an indicator of Earth’s oxygenation throughout geologic history. Mn(II) precipitates out from water in highly oxic conditions as Mn(III, IV, or III/IV)-oxide/hydroxide minerals due to its high redox potential, or can precipitate as either primary or secondary/primary Mn(II)-carbonate minerals.

Gale crater, Mars, once hosted an ancient lake where NASA’s Curiosity rover is currently traversing. Curiosity can detect elemental abundances of rocks by laser-induced breakdown spectroscopy (LIBS). LIBS has detected Mn-enrichments, as high as 50 wt% MnO, in surface coatings, within fracture patterns, and interbedded in sandstones, far above the average Gale Mn-abundance of 0.6 wt% MnO. Although Gale’s Mn-enrichments are thought to occur as (hydr)oxide minerals, direct mineralogical verification is not possible with Curiosity’s CheMin X-ray diffraction instrument, due to its inability to analyze thin surface layers hosting the Mn-enrichments. We suggest the suite of trace element co-enrichments in the Mn-enriched rocks could help in determining whether the Mn-enrichments in Gale occur within (hydr)oxides or carbonates, as Mn-(hydr)oxides strongly adsorb trace elements.

Ten terrestrial sedimentary Mn-enrichments were analyzed under Mars conditions using the LIBS engineering model at Los Alamos National Laboratory. The terrestrial samples were also analyzed by inductively coupled plasma mass spectrometry (ICP-MS) to quantify elemental abundances. If Mn-oxides and Mn-carbonates can be statistically distinguished by their trace element co-enrichments, it would allow us to use characteristic LIBS peaks to indicate Mn mineral class. If the Mn-enrichments in Gale exist as a Mn-(hydr)oxide it provides evidence of past, highly oxidizing conditions. If they are in a Mn-carbonate it provides further evidence of a redox-stratified conditions, as well as active redox and carbon cycling. Determining the context of Mn-enrichments in Gale provides key information about potential environmental conditions that may have been present in the paleolake.