Modeled Bulk Mineralogy Suggests Limited Alteration in Eridania Basin, Mars

Eridania basin, a putative Noachian-aged paleolake basin system located in the Martian southern highlands, was recently interpreted as a radiogenic hydrothermal system driven by the decay of Th, U and K (Ojha et al., 2021). Spectral evidence for secondary minerals including carbonates, serpentine, and chlorides also support paleo-hydrothermalism; Bouguer gravity anomalies suggest extensive crustal fluid circulation (cooling the crust rapidly and preventing viscoelastic relaxation); and strong remanent magnetism is consistent with hydrothermally produced magnetite (analogous to serpentinization settings). We investigated the regional context for the existence of this ancient hydrothermal system by modeling the bulk regolith mineralogy. We rely on Gamma Ray Spectroscopy (GRS) and Thermal Emission Spectroscopy (TES) data to model the bulk regional mineralogy at decimeter depth scales. We use TES data to constrain secondary mineral abundances and GRS data to constrain primary igneous mineral abundances, recombining these datasets using a modified CIPW (Cross, Iddings, Pearson and Washington) normative approach first developed by McSween and colleagues for in situ modeling of the Meridiani Planum and Gusev Crater regolith compositions. We find that the bulk regolith mineralogy of Eridania is consistent with relatively unaltered mafic basaltic mineralogy (323 wt.% olivine, with most model runs converging on the upper limit of olivine mass fraction, ~6 wt.% diopside, 3040 wt.% low Ca-pyroxene, 30 wt.% plagioclase, 77.5 wt.% sulfates, 1.6 wt.% chlorides, and 0.8 wt.% clays). The bulk regional regolith lacks compelling mineralogical evidence for an ancient, long-lived, regionally widespread hydrothermal system. We consider an alternative interpretation of Eridania as either extensively volcanically resurfaced, obscuring mineralogical evidence for ancient hydrothermal activity, or as inconsistent with long-lived hydrothermalism. It is possible that compositional signatures previously interpreted as consistent with hydrothermalism (e.g., alteration minerals) may reflect incipient or fumarolic alteration, though the regional gravitational and magnetic anomalies are more readily attributable to deep-seated hydrothermalism.