Occurrence of Jarosite In a Saline-Alkaline Paleo-Lake Deposit: Implications for Mars Aqueous Geochemistry

Jarosite occurs in trachytic to phonolitic tephra altered under predominantly saline-alkaline conditions in the Plio-Pleistocene lacustrine deposits of Olduvai Gorge, Tanzania. Authigenic K-feldspar and the zeolites analcite and phillipsite dominate the assemblage, indicating alkaline conditions during diagenesis. Jarosite is typically formed only under acidic conditions.

The Olduvai Gorge deposits include saline-alkaline lacustrine sediments containing altered tephra. The distribution of authigenic minerals follows a typical “bulls-eye” pattern with clays dominating further out, zeolites at the margin, and zeolites and K-feldspar at the center. Jarosite is observed only in a few samples from the paleolake center. Jarosite was first detected by XRD and confirmed by SEM-EDS. Qualitative EDS data indicate K-rich jarosite with significant Al and Fe, but the XRD pattern is consistent with jarosite, not alunite. Of over 30 altered tephra samples from the lake margin and basin, only four contained XRD-detectable jarosite. These samples were also analyzed by FTIR.

The overall zeolite-rich assemblage excludes the possibility of dominant acidic conditions. Previous studies also reveal a highly alkaline groundwater environment, leading to salt efflorescences of sodium carbonate/ bicarbonate consistent with a pH of 9.5-10. Pyrite and altered pyrite occur within the lake sediments, and their oxidation most likely resulted in the localized acidic conditions needed for jarosite stability. Its persistence indicates water-limited conditions or very recent pyrite oxidation. Locally acidic conditions have not persisted long enough to destroy the saline-alkaline assemblage.

Jarosite is currently used as an indicator of dominantly saline-acidic weathering conditions at the Opportunity site at Meridiani Planum, Mars. However, the occurrence of jarosite in a terrestrial saline-alkaline lacustrine deposit suggests that jarosite can form in localized acidic environments and does not require widespread acidic conditions. Localized oxidation of pyrite or other sulfides could lead to acidic conditions in otherwise neutral or even highly alkaline environments.