Abstract:
Carbonate veins recovered from the mafic/ultramafic footwall of an oceanic detachment fault on
the Mid-Atlantic Ridge record multiple episodes of fluid movement through the detachment and secondary
faults. High-temperature ( ~75–175° C) calcite veins with elevated REE contents and strong positive Euanomalies
record the mixing of up-welling hydrothermal fluids with infiltrating seawater. Carbonate precipitation
is most prominent in olivine-rich troctolite, which also display a much higher degree of greenschist and
sub-greenschist alteration relative to gabbro and diabase. Low-temperature calcite and aragonite veins likely
precipitated from oxidizing seawater that infiltrated the detachment fault and/or within secondary faults late
or post footwall denudation. Oxygen and carbon isotopes lie on a mixing line between seawater and
Logatchev-like hydrothermal fluids, but precipitation temperatures are cooler than would be expected for
isenthalpic mixing, suggesting conductive cooling during upward flow. There is no depth dependence of vein
precipitation temperature, indicating effective cooling of the footwall via seawater infiltration through fault
zones. One sample contains textural evidence of low-temperature, seawater-signature veins being cut by
high-temperature, hydrothermal-signature veins. This indicates temporal variability in the fluid mixing, possibly
caused by deformation-induced porosity changes or dike intrusion. The strong correlation between carbonate
precipitation and olivine-rich troctolites suggests that the presence of unaltered olivine is a key
requirement for carbonate precipitation from seawater and hydrothermal fluids. Our results also suggest that
calcite-talc alteration of troctolites may be a more efficient CO2 trap than serpentinized peridotite.
