T15 Impact Cratering Across the Solar System

Sunday, 15 October 2023
8:00 AM - 12:00 PM
323

MANGANESE-IRON PHOSPHATE NODULES AT THE GROKEN SITE, GALE CRATER, MARS

Paper number: T15-4
Program: Technical Sessions
Time: 8:35 AM – 8:50 AM

Authors

Abstract

The MSL Curiosity rover investigated dark, Mn-Fe-P-rich nodules in shallow fluvio-lacustrine sediments at the Groken site in Glen Torridon, Gale Crater, Mars. The nodules have flat faces and linear boundaries, and stand above their siltstone matrix. Chemical analyses by ChemCam LIBS (laser-induced breakdown spectrometry) show that the nodules have abundances of Si and Al like those of the siltstone, and are enriched in Mn and Fe. The deconvolution of APXS (fluoresced X-ray) analyses of nodule-bearing targets, interpreted here as representing the nodules’ non-silicate components, shows high concentrations of Mn, P, and Fe, and a molar ratio P/Mn = 2. Visible to near-infrared reflectance spectra of the nodules (by ChemCam passive and Mastcam multispectral) are dark and relatively flat, consistent with a mixture of host siltstone, hematite, and a dark spectrally bland material. The Groken drill sample contains minimal nodule material, meaning that analyses CheMin and SAM data on the drill material do not constrain the nodules’ mineralogy or composition.

The nodules’ molar P/Mn = 2, a small integer ratio, suggests that the nodules contained a stoichiometric Mn-phosphate mineral, in which the abundant Fe could not substitute for Mn. The most likely minerals are laueite and strunzite, Mn2+Fe3+2(PO4)2(OH)2·8H2O and -·6H2O, which form as alteration products of other Mn-bearing phosphates including vivianite, (Fe2+,Mn2+,Mg)3(PO4)2·8H2O. Vivianite is a common precipitate from low-oxygen, near-neutral pH, P-enriched waters. Calculated phase equilibria show that Mn-bearing vivianite could be replaced by laueite/strunzite and then by hematite plus pyrolusite (MnO2) as the system became more oxidizing and acidic. These data suggest that the nodules originated as euhedral crystals of vivianite in the sediment, enclosing its grains as they grew. After formation, the nodules were oxidized – first to laueite/strunzite, and then to hematite plus a Mn oxy-hydroxide (like pyrolusite). The restricted occurrence of these Mn-Fe-P nodules, both in space and time (i.e., stratigraphic position) suggests a local control on their origin. By terrestrial analogies, the nodules might have precipitated near a spring or seep of Mn-rich water, which could have been generated during alteration of olivine in the underlying sediments.

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-389279

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