Clumped isotope disequilibrium during rapid CO2 uptake and carbonate precipitation in subaerial alkaline springs associated with ongoing serpentinization

Abstract:

Ongoing serpentinization in tectonically exposed ultramafic bodies is manifested at the surface in alkaline springs (pH >11). Where these high-pH waters come in contact with CO₂ at the surface, rapid calcite precipitation forms extensive travertines. We study natural travertine samples from Oman and synthetic witherite (BaCO₃) from high-pH experiments to identify disequilibrium signals in δ¹⁸O, δ¹³C and clumped isotopes (measured as Δ47) that characterize rapid uptake of atmospheric CO₂ and carbonate precipitation from high pH fluids. Kinetic effects preclude the use of clumped or oxygen isotopes for carbonate thermometry in these environments, but trends in δ¹⁸O, δ¹³C and Δ47 could help identify extinct alkaline systems or distinguish CO₂ sources.

Oman travertines formed at peridotite-hosted alkaline springs have long been known to exhibit a large range of kinetically depleted δ¹⁸O and δ¹³C values. We find fresh carbonate precipitated at these alkaline springs also exhibit large enrichments in Δ47 that covary with the depletions in δ¹⁸O and δ¹³C, thought to arise during hydroxylation of CO₂ in high-pH fluids. Witherite precipitated during rapid CO₂ uptake and carbonate precipitation in high pH experiments also exhibits disequilibrium values in δ¹⁸O, δ¹³C and Δ47, with the Δ47 of carbonate precipitates strongly affected by the Δ47 the reactant CO₂.

δ¹⁸O, δ¹³C and Δ47 trends could serve as a marker for carbonates formed in subaerial alkaline environments and track carbon sources in these systems. For example, the δ¹⁸O-δ¹³C slope in carbonates from Martian meteorites is similar to that observed in carbonates from terrestrial alkaline springs, so if corresponding enrichments in Δ47 could be identified in Martian carbonates, it could suggest that alkaline springs were present on the surface of Mars. Clumped isotope signals could also help distinguish carbon sources: kinetic enrichments in Δ47 would be absent or diminished in high-pH carbonates formed via mixing of alkaline fluids with fluids bearing dissolved CO₂ (e.g., bedrock-derived CO₂) versus direct uptake of atmospheric CO₂. Incorporation of older sediment or later precipitation in pore space may be difficult to detect in ¹⁴C, δ¹⁸O or δ¹³C, but the very non-linear nature of clumped isotope mixing may allow identification of mixed carbonates.