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 (<2000m 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; 1.4–29.1μg/g in pillow lava (7.9μg/g on average), 1.5–11.6μg/g in sheeted dike complex (5.3μg/g on average), 1.6–5.0μg/g in dolerite dike in gabbro (2.9μg/g on average), 0.25–3.8μg/g in gabbro (1.3μg/g on average). Considering an original boron content of 0.72±0.47μg/g for basalt and 0.06±0.09μg/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 3.6μg/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.