TY - JOUR
T1 - Boron and oxygen isotope systematics for a complete section of oceanic crustal rocks in the Oman ophiolite
AU - Yamaoka, Kyoko
AU - Ishikawa, Tsuyoshi
AU - Matsubaya, Osamu
AU - Ishiyama, Daizo
AU - Nagaishi, Kazuya
AU - Hiroyasu, Yuko
AU - Chiba, Hitoshi
AU - Kawahata, Hodaka
N1 - Funding Information:
We would like to express our appreciation to Professor H. Ishizuka (Kochi University) for providing information about mineral assemblages and for helpful discussions. We also thank associate editor J. Ishibashi (Kyusyu University) and the anonymous reviewers for their valuable comments. This study was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science .
Copyright:
Copyright 2012 Elsevier B.V., All rights reserved.
PY - 2012/5/1
Y1 - 2012/5/1
N2 - 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 δ 18O 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 (δ 18O>+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 (δ 18O=+6‰ to +10‰); the lowermost sheeted-dike complex and upper gabbros underwent high-temperature alteration at 300-450°C (δ 18O<+6‰); the lower gabbros were altered at very high temperatures of >450°C (δ 18O<+6‰). Plagiogranites from the uppermost gabbro section show exceptionally high δ 18O 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 δ 11B 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 δ 11B values negatively correlate with the δ 18O values, suggesting that the δ 11B values of altered rocks are essentially controlled by isotopic equilibrium with hydrothermal fluids, and the increase in the δ 11B values is caused by a decrease in the rock-fluid boron isotopic fractionation factor with increasing alteration temperatures. The δ 11B 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 δ 11B 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 δ 11B value of +13‰. This boron-enriched, high-δ 11B lower oceanic crust may impact estimates of the δ 11B value of fluid liberated from the subducted oceanic slab, which is believed to largely control the δ 11B value of arc magma generated in the mantle wedge.
AB - 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 δ 18O 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 (δ 18O>+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 (δ 18O=+6‰ to +10‰); the lowermost sheeted-dike complex and upper gabbros underwent high-temperature alteration at 300-450°C (δ 18O<+6‰); the lower gabbros were altered at very high temperatures of >450°C (δ 18O<+6‰). Plagiogranites from the uppermost gabbro section show exceptionally high δ 18O 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 δ 11B 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 δ 11B values negatively correlate with the δ 18O values, suggesting that the δ 11B values of altered rocks are essentially controlled by isotopic equilibrium with hydrothermal fluids, and the increase in the δ 11B values is caused by a decrease in the rock-fluid boron isotopic fractionation factor with increasing alteration temperatures. The δ 11B 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 δ 11B 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 δ 11B value of +13‰. This boron-enriched, high-δ 11B lower oceanic crust may impact estimates of the δ 11B value of fluid liberated from the subducted oceanic slab, which is believed to largely control the δ 11B value of arc magma generated in the mantle wedge.
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U2 - 10.1016/j.gca.2012.01.043
DO - 10.1016/j.gca.2012.01.043
M3 - Article
AN - SCOPUS:84859266412
VL - 84
SP - 543
EP - 559
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
SN - 0016-7037
ER -