Paper No. 262-3
Presentation Time: 8:00 AM-5:30 PM
NEAR-SURFACE AQUEOUS CONDITIONS INFERRED FROM THE COMPOSITION OF MAGNESITES FROM CENTRAL QUEENSLAND, AUSTRALIA: A JEZERO CRATER ANALOG
SWINDLE, Carl1, VASCONCELOS, Paulo M.2, DIMARCO, Zoe3, DALLESKA, Nathan4, CARDARELLI, Emily5, BHATTACHARJEE, Surjyendu4, FARLEY, Kenneth A.4 and PRESENT, Theodore6, (1)Department of Materials Science and Engineering, University of California, Los Angeles, Garrick Institute for the Risk Sciences, Los Angeles, CA 90095, (2)Division of Geological and Planetary Sciences, California Institute of Technology, MC170-25, Pasadena, CA 91125, (3)School of Earth and Environmental Sciences, The University of Queensland, St Lucia, QLD 4067, Australia, (4)Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, (5)Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095, (6)Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125
The Marlborough Terrane in QLD, AUS contains ultramafic serpentinite highlands surrounding low-lying sedimentary basins that both contain magnesite. Near surface aqueous alteration of the deposits is broadly analogous to carbonated ultramafic highlands and sedimentary lowlands in Jezero Crater, Mars. This study develops methods to determine aqueous alteration conditions that produce magnesite using trace element compositions. Pit mines near Marlborough provide exposures of serpentinite- and sedimentary-associated magnesite. In the serpentinite highlands, the Gumigil mine contains weathered serpentinite with syn-tectonic metasomatic magnesite veins. In the sedimentary lowlands, the Kunwarara and Yaamba mines contain magnesite nodules forming within unlithified alluvial sediments by reaction of magnesium-rich groundwater with arkosic sands and soils.
An ICP-MS protocol is developed to measure trace elements in Mg-carbonate samples, while minimizing influences of non-carbonate phases and adsorbed ions. Bulk rock compositions of serpentinite and sedimentary matrix materials were characterized by traditional methods.
Composition of Gumigil magnesite veins retain geochemical signatures typical of depleted mantle rocks, but Ce and Y anomalies record surficial alteration. In sedimentary deposits at Yaamba and Kunwarara, magnesite compositions are influenced by aqueous interactions with arkosic sediments and chemically evolved during multiple phases of diagenetic alteration by groundwater and soil waters. Zoned magnesite nodules at Yaamba contain Ce anomalies that correlate with increasing Mn from core to rim. Zoning captures redox shifts in fluids where magnesite nodules formed. At Kunwarara, Ce anomalies in magnesite diminish with depth coincident with decreasing abundance of authigenic Fe/Mn-oxides/hydroxides in the host sediments.
Results reveal that Mg-carbonates may replace host sediments in the subsurface along redox gradients in the capillary zone near the water table. These findings imply that Ce anomalies in Mg-carbonates can be used to trace paleoredox conditions of the ancient Martian surface. Ce anomalies calculated from published trace element data collected on Mg-carbonates in Martian meteorite ALH84001 suggest that Noachian Martian atmosphere could have been more oxidizing than the early Earth.
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