CLASSIFICATION OF TERRESTRIAL MANGANESE ENRICHMENTS BY LASER-INDUCED BREAKDOWN SPECTROSCOPY WITH RELEVANCE FOR GALE CRATER, MARS

In terrestrial settings, the accumulation of sedimentary manganese (Mn)-enrichments co-occurs with the oxygenation of Earth’s surface. The Mn(II)/Mn(III/IV) couple has a high redox potential, consistent with oxygen driving precipitation as Mn(III/IV)-oxides. Although sedimentary Mn-enrichments commonly form as Mn-oxides, Mn-carbonates can form after secondary reduction of Mn-oxides, or as primary precipitates in suboxic environments.

NASA’s Curiosity rover, currently traversing through Gale crater on Mars, has the ability to detect elemental abundances through laser-induced breakdown spectroscopy (LIBS). Mn-enrichments up to 50 wt% MnO have been detected in a variety of sediments within Gale. Although Gale may have once hosted an ancient lakebed, without mineralogical verification it is difficult to determine the conditions that gave rise to the Mn enrichments. On Earth, Mn-oxides strongly adsorb trace elements (e.g. Zn, Cu, Ba), while Mn-carbonates should not. We hypothesize that the presence or absence of trace elements in LIBS data can be used to distinguish Mn oxides from carbonates in Gale sediments.

To test this hypothesis, terrestrial lake and marine sediments containing Mn-oxide and Mn-carbonate minerals were analyzed by LIBS under Mars conditions using the ChemCam engineering model at Los Alamos National Laboratory. These included Paleoproterozoic iron formation from the Animike Basin, ferromanganese crusts from Lake Vermillion in MN and Lake Wentworth in NH, and Mn-rich sediments from ferruginous Otter Lake in MI, Brownie Lake in MN, and Lake Malawi in Southwest Africa. Mineralogy was confirmed with X-ray diffraction and bulk chemistry was obtained by X-ray florescence.

The spectral data produced from the terrestrial Mn-oxides and carbonates will be analyzed using a multivariate analysis to determine if characteristic trace elemental peaks within the spectra might distinguish Mn-oxides or Mn-carbonates. This can then be utilized for comparison to the LIBS spectra from Mn-rich samples in Gale to infer the mineralogy associated with Gale Mn-enrichments. Determining the nature of Mn-enrichments in Gale will provide context about the potential environmental conditions needed to enrich Mn in paleolake sediments on Mars.