Low Temperature Serpentinite Replacement by Carbonates during Seawater Influx in the Newfoundland Margin

Abstract

Serpentinite replacement by carbonates in the seafloor is one of the main carbonation processes in nature providing insights into the mechanisms of CO2 sequestration; however, the onset of this process and the conditions for the reaction to occur are not yet fully understood. Preserved serpentine rim with pseudomorphs of carbonate after serpentine and lobate-shaped carbonate grains are key structural features for replacement of serpentinite by carbonates. Cathodoluminescence microscopy reveals that Ca-rich carbonate precipitation in serpentinite is associated with a sequential assimilation of Mn. Homogeneous δ18O values at the µm-scale within grains and host sample indicate low formation temperature (<20 °C) from carbonation initiation, with a high fluid to rock ratio. δ13C (1−3 ± 1‱) sit within the measured values for hydrothermal systems (−3−3‱), with no systematic correlation with the Mn content. δ13C values reflect the inorganic carbon dominance and the seawater source of CO2 for carbonate. Thermodynamic modeling of fluid/rock interaction during seawater transport in serpentine predicts Ca-rich carbonate production, at the expense of serpentine, only at temperatures below 50 °C during seawater influx. Mg-rich carbonates can also be produced when using a model of fluid discharge, but at significantly higher temperatures (150 °C). This has major implications for the setting of carbonation in present-day and in fossil margins.

Keywords

• carbonation
• co₂ sequestration
• replacement process
• low temperature carbonate precipitation
• secondary ion mass spectrometer
• seawater influx
• hydrothermal circulation
• ophicalcite