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

Kyoko Yamaoka, Tsuyoshi Ishikawa, Osamu Matsubaya, Daizo Ishiyama, Kazuya Nagaishi, Yuko Hiroyasu, Hitoshi Chiba, Hodaka Kawahata

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Abstract

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 (18O>+10‰); the lower pillow basalts and upper sheeted dikes (18O=+6‰ to +10‰); the lowermost sheeted-dike complex and upper gabbros underwent high-temperature alteration at 300-450°C (δ 18O450°C (δ 18O18O 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.

Original languageEnglish
Pages (from-to)543-559
Number of pages17
JournalGeochimica et Cosmochimica Acta
Volume84
DOIs
Publication statusPublished - May 1 2012

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Oxygen Isotopes
boron isotope
Boron
boron
ophiolite
oxygen isotope
Rocks
Levees
gabbro
dike
rock
oceanic crust
Fluids
pillow lava
basalt
diabase
hydrothermal alteration
hydrothermal fluid
fluid
Temperature

ASJC Scopus subject areas

  • Geochemistry and Petrology

Cite this

Yamaoka, K., Ishikawa, T., Matsubaya, O., Ishiyama, D., Nagaishi, K., Hiroyasu, Y., ... Kawahata, H. (2012). Boron and oxygen isotope systematics for a complete section of oceanic crustal rocks in the Oman ophiolite. Geochimica et Cosmochimica Acta, 84, 543-559. https://doi.org/10.1016/j.gca.2012.01.043

Boron and oxygen isotope systematics for a complete section of oceanic crustal rocks in the Oman ophiolite. / Yamaoka, Kyoko; Ishikawa, Tsuyoshi; Matsubaya, Osamu; Ishiyama, Daizo; Nagaishi, Kazuya; Hiroyasu, Yuko; Chiba, Hitoshi; Kawahata, Hodaka.

In: Geochimica et Cosmochimica Acta, Vol. 84, 01.05.2012, p. 543-559.

Research output: Contribution to journalArticle

Yamaoka, K, Ishikawa, T, Matsubaya, O, Ishiyama, D, Nagaishi, K, Hiroyasu, Y, Chiba, H & Kawahata, H 2012, 'Boron and oxygen isotope systematics for a complete section of oceanic crustal rocks in the Oman ophiolite', Geochimica et Cosmochimica Acta, vol. 84, pp. 543-559. https://doi.org/10.1016/j.gca.2012.01.043
Yamaoka, Kyoko ; Ishikawa, Tsuyoshi ; Matsubaya, Osamu ; Ishiyama, Daizo ; Nagaishi, Kazuya ; Hiroyasu, Yuko ; Chiba, Hitoshi ; Kawahata, Hodaka. / Boron and oxygen isotope systematics for a complete section of oceanic crustal rocks in the Oman ophiolite. In: Geochimica et Cosmochimica Acta. 2012 ; Vol. 84. pp. 543-559.
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abstract = "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 (18O>+10‰); the lower pillow basalts and upper sheeted dikes (18O=+6‰ to +10‰); the lowermost sheeted-dike complex and upper gabbros underwent high-temperature alteration at 300-450°C (δ 18O450°C (δ 18O18O 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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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

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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 (18O>+10‰); the lower pillow basalts and upper sheeted dikes (18O=+6‰ to +10‰); the lowermost sheeted-dike complex and upper gabbros underwent high-temperature alteration at 300-450°C (δ 18O450°C (δ 18O18O 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 (18O>+10‰); the lower pillow basalts and upper sheeted dikes (18O=+6‰ to +10‰); the lowermost sheeted-dike complex and upper gabbros underwent high-temperature alteration at 300-450°C (δ 18O450°C (δ 18O18O 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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