Iron isotopic and chemical tracing of basalt alteration and hematite spherule formation in Hawaii: A prospective study for Mars

Abstract

Hematite spherules from Mauna Kea volcano (Hawaii) resemble Martian hematite spherules at Meridiani Planum in morphology, mineralogy, and geochemistry, and they both formed in association with acid-sulfate alteration of basalt. To gain insights into the processes involved in the formation of these spherules, we have measured the Fe isotopic compositions of unaltered basalts and tephras, as well as a variety of weathering/alteration products including altered basalts, hematite-rich breccia and individual hematite spherules, jarosite-bearing tephras, palagonitic tephras, calcined tephras (high-T dry oxidation) and steam-vent tephras. Compared to other alteration processes that induce little Fe mobility, acid-sulfate alteration produces large Fe isotopic variations (δ⁵⁶Fe values ranging from −0.15 to +0.94‰). We used rock slabs from a variably altered basaltic rock to constrain the Fe isotope fractionation factor during acid-sulfate alteration of basaltic rocks. We found that the process followed a Rayleigh distillation with an instantaneous fractionation factor of −0.24‰ between the fluid and the residue for δ⁵⁶Fe, resulting in a heavy Fe isotope enrichment in the alteration residue. However, the constant Mn/Fe ratios of the slabs suggest that acid-sulfate leaching does not significantly fractionate Mn/Fe ratios in basalts, which is consistent with the mobilization of iron as Fe(II). We also measured the Fe isotopic compositions and Mn/Fe ratios (using NanoSIMS) of individual hematite spherules from a spherule-rich breccia HWMK745R. The spherules show heavy Fe isotopic enrichments and low Mn/Fe ratios compared to unaltered samples, which is similar to the fractionations encountered in iron formations. A Rayleigh fractionation model suggests that both Fe isotopic compositions and Mn/Fe ratios of the spherules are consistent with ∼80% Fe oxidation and precipitation from an acid-sulfate fluid. Therefore, direct Fe precipitation from a fluid can readily explain the hematite spherule formation, and a more complex scenario involving jarosite precipitation followed by conversion to hematite is not needed. Combined thermodynamic and kinetic constraints also suggest that precipitation of jarosite would be hindered. The study presents a roadmap for studying future returned Martian samples in the laboratory.