Geochemistry and petrology of altered peridotite overlain by Maastrichtian to Miocene sediments in the SE Samail Ophiolite, Oman

Abstract:

In the southeastern Oman Mountains the mantle section of the Samail ophiolite is unconformably capped by large units of Maastrichtian to Miocene, shallow marine limestones. Oceanic crust and upper mantle were thrust onto the Arabian continental margin from ca 96 to 80 Ma. Subaerial erosion locally removed the crustal section and exposed peridotite to weathering. A Maastrichtian transgression led to deposition of Late Cretaceous to Miocene, shallow marine limestones and shales above the unconformity.

Near the town of Fins, a deep canyon exposes the unconformity and underlying, altered mantle peridotite. A striking series of clastic dikes of grey, micritic limestone extends across the unconformity, up to 10 meters down into the peridotite. Tips of these dikes are choked with angular peridotite blocks. Deeper, the peridotite is cut by a spectacular grid of carbonate-serpentine veins with abundant vugs. The peridotite matrix has high concentrations of calcium and small enrichments of silica compared to the Oman protolith, resulting from reaction with a hydrous fluid derived from seawater equilibrated with the overlying sediments. δ13C from -1.3-0.61 per mil (VPDB) and δ18O between 22.2 to 28 per mil (SMOW) are similar to values in the overlying sediments (Schluter et al., Facies 2008).

Clumped isotope thermometry on calcite crystals in veins cutting the peridotite outcrops indicate crystallization at 25-60 °C. Sediment thickness reconstructions place the pressure of alteration between 300-600 bars. 87Sr/86Sr values of 0.7078 and 0.7079 correspond to seawater Sr isotope ratios at the Cretaceous-Tertiary boundary, which is present in the overlying limestones about 50 meters above the unconformity. This suggests that the veins in the peridotite formed at about 60 Ma, or that their parental fluid was in Sr isotope exchange equilibrium with 60 Ma sediments.

Reaction path modeling of carbonated saturated seawater and peridotite reproduces the observed vein mineral assemblage composed of carbonate and serpentine at high water/rock ratios, with carbon and silica added to the rock by the reacting fluid.