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Appears in Collections:Biological and Environmental Sciences Journal Articles
Peer Review Status: Refereed
Title: Jarosite in a Pleistocene East African saline-alkaline paleolacustrine deposit: Implications for Mars aqueous geochemistry
Author(s): McHenry, Lindsay J
Chevrier, Vincent
Schröder, Christian
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Keywords: jarosite
Olduvai Gorge
Issue Date: 6-Apr-2011
Date Deposited: 25-Oct-2013
Citation: McHenry LJ, Chevrier V & Schröder C (2011) Jarosite in a Pleistocene East African saline-alkaline paleolacustrine deposit: Implications for Mars aqueous geochemistry. Journal of Geophysical Research: Planets, 116 (E4), Art. No.: E04002.
Abstract: Jarosite occurs within altered tephra from the saline‐alkaline paleolake deposits of Pliocene‐Pleistocene Olduvai Gorge, Tanzania. Zeolites (mainly phillipsite), authigenic K‐feldspar, and Mg/Fe‐smectites dominate the mineral assemblage, indicating salinealkaline diagenetic conditions (pH > 9). As jarosite is ordinarily an indicator of acidic conditions on Earth and Mars, its association with such undisputed high‐pH indicators is unexpected. Of 55 altered tephra samples collected from the paleolake basin and margin deposits, eleven contained jarosite detectable by X‐ray Diffraction (XRD) (>0.15%). Mössbauer spectroscopy, Fourier Transform Infrared Reflectance (FTIR), Electron Probe Microanalysis (EPMA), X‐ray Fluorescence (XRF), and Scanning Electron Microscopy (SEM) analyses confirm the presence and nature of the jarosite. This paper documents this occurrence and presents mechanisms that could produce this unusual and contradictory mineral assemblage. We favor a mechanism by which jarosite formed recently, perhaps as modern ground and meteoric water interacted with and oxidized paleolacustrine pyrite, providing local and temporary acidic conditions. However, local groundwater (at modern springs) has a pH > 9. In recent studies of Mars, the presence of jarosite or other Fe or Mg sulfates is often used to indicate dominantly acidic conditions. Regardless, the current study shows that jarosite can form in sediments dominated by alkaline minerals and solutions. Its coexistence with Mg/Fe smectites in particular makes it relevant to recent observations of Martian paleolakes.
DOI Link: 10.1029/2010JE003680
Rights: Copyright 2011 by the American Geophysical Union. AGU allows authors to deposit their journal articles if the version is the final published citable version of record, the AGU copyright statement is clearly visible on the posting, and the posting is made 6 months after official publication by the AGU.

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