The early Earth geochemical and isotope record of serpentinizing environments from Archean stichtite (Mg6Cr2(OH)16[CO3]·4H2O)

A record of the conditions and duration of Archean serpentinization is preserved as chemical, isotopic, and textural properties of the magnesium-chromium hydroxycarbonate mineral stichtite. This provides a window into the conditions associated with a potentially habitable environment on early Earth and Mars. Stichtite is associated with Archean chromite-rich serpentinite rocks of South Africa, Western Australia and Southern India. Stichtite compositions range within the Cr-Fe-Al hydrotalcite group, with trends unique to each discrete serpentinite host body. Elemental and textural analysis of stichtite-associated chromite indicates that stichtite forms in fore-arc/layered intrusive/boninite setting rocks through interaction of chromite and methane-rich serpentinizing fluids. The degree to which chromite is replaced by stichtite is inferred to correlate with the length of time that the host rocks spent within the “stichtite window.” Carbon stable-isotope values of Archean stichtite suggest carbon sourcing from marine kerogen with minor marine dissolved inorganic carbonate in most samples, as opposed to the strongly kerogen-dominated Phanerozoic stichtite. The carbon and hydrogen stable isotope profile of Archean stichtite ranges from the methane fields of active serpentinizing zones, to geothermal, with trends suggestive of methane oxidation rather than the substrate depletion noted in Phanerozoic stichtite. Thus, isotope values of stichtite appear to record oceanic oxygenation levels. The association of the stichtite 2H polytype (nee barbertonite) with aragonite ± antigorite suggests this is a higher pressure/temperature polytype of stichtite. Reaction textures, isotopic values, and qualitative mineral thermobarometric indicators indicate that stichtite forms during serpentinization in a methane/H2-rich environment within fluid conduits, ranging from near the surface, to depths where pressure is up to 2.8 GPa and temperature is up to ∼300 °C.