Geochemical data indicate highly similar sediment compositions for the Grasberg and Burns formations on Meridiani Planum, Mars

Abstract

The hematite-bearing, sulfate-rich sandstones of the Burns formation at Meridiani Planum are underlain by a thin stratigraphic unit referred to as the Grasberg formation. The sulfate-bearing Grasberg rocks are fine-grained and lack bedding structures, and were previously interpreted to be a distinct lithologic unit based on morphological and chemical differences from the overlying Burns formation. However, reanalysis of the data indicates that, except for variable amounts of Mg, Ni, SO3 and Mn, the chemical compositions of the Grasberg and Burns rocks are very similar. The relatively low levels of Mg, Ni, and SO3 in the Grasberg rocks indicates that they have experienced diagenetic loss of Mg sulfates similar to that observed in a subset of eleven Burns formation rocks depleted in the same elements, including two Burns rocks immediately above the Grasberg contact. The Grasberg formation and Burns rocks near the contact have also evidently lost Mn during diagenesis. When compensated for diagenetic losses, rocks from the Grasberg and Burns formations are found to have nearly identical chemical compositions, albeit Grasberg rocks contained a few wt.% less SO3. These observations suggest that the sediment sources for the Grasberg and Burns formations are genetically related, and that both formations experienced some of the same diagenetic processes after deposition. Furthermore, the apparent loss of Mg, Ni, SO3, and Mn from the Grasberg formation and immediately overlying Burns rocks is mirrored by enrichments of these same elements in fractures within the underlying Shoemaker formation, suggesting downward movement of fluids during some diagenetic events.