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 language | English |
|---|---|
| Pages (from-to) | 543-559 |
| Number of pages | 17 |
| Journal | Geochimica et Cosmochimica Acta |
| Volume | 84 |
| DOIs | |
| Publication status | Published - May 1 2012 |
Fingerprint
ASJC Scopus subject areas
- Geochemistry and Petrology
Cite this
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 journal › Article
}
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
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 (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.
UR - http://www.scopus.com/inward/record.url?scp=84859266412&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=84859266412&partnerID=8YFLogxK
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 -