Boron and oxygen isotope systematics for a complete section of oceanic crustal rocks in the Oman ophiolite

Boron content and the isotopic composition of boron and oxygen were determined for a complete 5.3-km-thick oceanic crust in the Wadi Fizh area of the Oman ophiolite, a fragment of Cretaceous oceanic lithosphere obducted onto the Arabian shield. The depth profiles of the δ18O values and the secondary mineral assemblages are consistent with successively higher temperature hydrothermal alterations occurring with increasing depth. The upper pillow basalts underwent low temperature alteration (<60 °C) or seafloor weathering (δ18O > +10‰); the lower pillow basalts and upper sheeted dikes (<2000 m of stratigraphic depth) were altered at 250–350 °C at the spreading axis and subsequently experienced retrograde alteration (<200 °C) in the flank provinces (δ18O = +6‰ to +10‰); the lowermost sheeted-dike complex and upper gabbros underwent high-temperature alteration at 300–450 °C (δ18O < +6‰); the lower gabbros were altered at very high temperatures of >450 °C (δ18O < +6‰). Plagiogranites from the uppermost gabbro section show exceptionally high δ18O values compared with the adjacent rocks, suggesting the production of 18O-enriched melt.The boron content of the rocks in the oceanic crust decreases in both abundance and range with increasing stratigraphic depth; 1.4–29.1 μg/g in pillow lava (7.9 μg/g on average), 1.5–11.6 μg/g in sheeted dike complex (5.3 μg/g on average), 1.6–5.0 μg/g in dolerite dike in gabbro (2.9 μg/g on average), 0.25–3.8 μg/g in gabbro (1.3 μg/g on average). Considering an original boron content of 0.72 ± 0.47 μg/g for basalt and 0.06 ± 0.09 μg/g for gabbro, boron from seawater was incorporated into the rocks through hydrothermal alteration, even at temperatures higher than 300 °C. The δ11B values systematically increase with stratigraphic depth; −1.1‰ to +11.9‰ in pillow lava (+5.5‰ on average), +1.1‰ to +17.5‰ in sheeted dike complex (+6.3‰ on average), +8.3‰ to +18.6‰ in dolerite dike in gabbro (+13.9‰ on average), +7.3‰ to +17.7‰ in gabbro (+12.0‰ on average). The whole-rock δ11B values negatively correlate with the δ18O values, suggesting that the δ11B values of altered rocks are essentially controlled by isotopic equilibrium with hydrothermal fluids, and the increase in the δ11B values is caused by a decrease in the rock-fluid boron isotopic fractionation factor with increasing alteration temperatures. The δ11B values estimated for hydrothermal fluids from rocks completely altered at 300–450 °C range from +28‰ to +33‰, values indistinguishable from those of vent fluids observed at modern mid-ocean ridges.The boron content of the bulk oceanic crust of the Oman ophiolite is estimated to be 3.6 μg/g, and the δ11B value is estimated at +7.9‰. In contrast to previous views, the hydrothermally altered gabbro section is a large boron sink, accounting for ∼30% of the total boron in the oceanic crust with a high δ11B value of +13‰. This boron-enriched, high-δ11B lower oceanic crust may impact estimates of the δ11B value of fluid liberated from the subducted oceanic slab, which is believed to largely control the δ11B value of arc magma generated in the mantle wedge.