The Drawdown of Atmospheric CO2 by Hyperalkaline Spring Waters Emanating from Cascade Spring, Dun Mountain Ophiolite Belt, New Zealand

Abstract:

The Permian Dun Mountain Ophiolite Belt (DMOB) is an important marker terrane in New Zealand geology and is displaced by ~460 km by right lateral offset on the Alpine Fault that forms the Pacific-Australian plate boundary through the South Island. The DMOB contains a number of ultramafic massifs of partially serpentinized mantle peridotite and notwithstanding that much of this terrane is extremely remote, peridotite-hosted hyperalkaline springs are rare. The Cascade Spring issues ~17°C waters at ~13 L/min from the western slope of a steep harzburgite ridge close to the DMOB’s southern intersection with the Alpine Fault. High pH (11.1-11.6), Ca-OH type fluids with low concentrations of HCO₃^-, Mg and SiO₂ continue to form a steep >500 m² patch of hummocky calcium carbonate travertine. The spring fluids flow more than 50 m across the surface of a ~2 m-thick travertine blanket before it abruptly terminates. This gives the opportunity to study the evolution of the fluids and their precipitates as the waters flow down the travertine terrace.

The spring waters have meteoric oxygen and hydrogen isotope ratios similar to local surface waters. ⁸⁷Sr/⁸⁶Sr of vent fluids are ~0.7042, higher than DMOB primary mantle values (0.7030-0.7035), indicating exchange with either hydrothermally altered ocean crustal rocks or mixing with fluids that have interacted with nearby tectonically juxtaposed metasediments. As waters flow over this steep terrace their chemistry changes; pH decreases from 11.5 to 9.7, and Ca concentrations decrease from 20 µg/g to 9.1 µg/g, corresponding to precipitation of 0.271 mmoles/L calcite and dissolution of 0.012 g/L CO₂ across terrace. Considering spring flow rates this equates to precipitation of ~188 kg of calcite and drawdown of 83 kg of atmospheric CO₂ from this vent per year. The fluid chemistry changes most in the first 17 m from the vent where pH decreases from 11.5 to 10.8 and Ca concentration almost halves from 20 to 10.9 µg/g, indicating calcite precipitation and CO₂ uptake are most rapid when Ca concentration and pH are highest. Despite the clearly defined extent of the terrace, waters dripping from its termination remain saturated in calcite and under-saturated CO₂ meaning a further 0.021 mmoles/L of calcite may be deposited beyond the end of the terrace.