CARBONATE EVOLUTION OF A NATURAL ULTRAMAFIC CO2 SEQUESTRATION SYSTEM
We use the elemental (Mg/Ca, Sr/Ca, Ba/Ca) and stable isotopic (87/86Sr, 18/16O, 13/12C) composition of a well-stratified meter-thick carbonate vein and modern proximal spring waters to reconstruct the history of carbonate precipitation and evolution of the groundwater system within the DPO.
Elemental and stable isotopic signatures within the lower and upper quarters of the carbonate vein yield a generally symmetrical pattern, with Mg/Ca and 87/86Sr decreasing toward the center of the vein and Ba/Ca and Sr/Ca ratios increasing. This chemostratigrahic symmetry suggests that carbonate precipitation initially proceeded coevally on the upper and lower surfaces of the cavity. The observation that Mg/Ca decreased and that Ba/Ca and Sr/Ca increased with time across these initial layers is consistent with a gradual cooling of the formation waters. These elemental trends, along with the decline in 87/86Sr with time, is also consistent with an initial deep, peridotitic influence on the formation waters (Mg-rich, more radiogenic Sr), which is increasingly influenced by meteoric waters whose composition is controlled by shallower crustal weathering (Ba- and Sr-rich, less radiogenic Sr). Sequential closed-system Rayleigh fractionation of the formation waters can be ruled out as the driver of the observed chemostraigraphic trends, as the partition coefficients for Mg/Ca, Sr/Ca, and Ba/Ca are each < 1, yet Mg/Ca varies inversely with Sr/Ca and Ba/Ca.
The chemostratigraphic symmetry found in approximately the upper and lower quarters of the carbonate vein gives way to shorter-lived and less ordered trends, suggesting that the flux and/or ionic composition of the formation water decreased with time, possibly as it was diluted by an increasing meteoric component. Regardless of the precise cause, it appears that a significant change occurred within the groundwater system in the DPO when these central units were deposited.