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ABSTRACT

Syntectonic carbonate veins were found along the décollement zone between the Atlantic and Caribbean plates and in two sites 12 and 17 km landward of the deformation front of the Barbados accretionary prism (ODP Leg 110). Both calcite and lesser amounts of rhodochrosite (MnCO3) occur in microcrystalline, fibrous, and blocky mosaic forms in veins deposited along scaly shear zones. Descriptions of the occurrences, textures, mineralogy, and stable isotope geochemistry of these veins presented herein help trace the hydrological evolution of the Barbados accretionary prism, which has seen the impact of more deeply derived fluids on elevated sediment temperatures and altered geochemistry of interstitial fluids. At one site, for example, vein-forming fluids have been as much as 10°C warmer and 1‰ more enriched in 18O than expected.

Carbon isotope ratios of veins indicate a mixture of carbon derived from seawater and from oxidized organic matter during sulphate reduction, although dissolution of calcareous skeletons may be an additional component. The presence of methane is recorded in one vein sample in the accretionary prism, indicating that either the plumbing of the prism has changed with time, allowing more fluid to leak into the prism in the past, or that materials currently 1 km above the décollement zone were once deformed in the décollement.

Using oxygen isotope analyses of rhodochrosite samples from two sites in conjunction with measurements of fluid temperature and δ18O values of interstitial fluids, it is proposed that the fractionation of oxygen isotopes between rhodochrosite and water (1000 1nα)∼38‰ at 25°C, 10‰ greater than the fractionation between calcite and water. However, it appears that substitution of 10-20 mol.% Ca into rhodochrosite reduces the fractionation factor by 2-9‰ so impure diagenetic rhodochrosite samples from the deep ocean will rarely be this enriched in 18O.