A complete oxygen isotope profile through the lower oceanic crust, ODP Hole 735B

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ODP Hole 735B located on the Southwest Indian Ridge at 57°E is an in situ sampled long, continuous section of lower oceanic crust. Oxygen isotope compositions of constituent minerals of Leg 176 gabbros have been measured by UV-laser oxygen isotope microprobe. Together with existing data from Leg 118, a complete oxygen isotope profile through the lower oceanic crust has been obtained. Most clinopyroxenes and olivines have normal mantle values of ∼ 5.5‰ and ∼ 5.2‰, respectively, while plagioclases show slight δ18O enrichment relative to its mantle value of 6.1‰. Down-hole variations of Hole 735B gabbro indicate a downward decreasing δ18O profile, with a kink at a depth of about 800 m below sea floor. Above this depth, gabbros are depleted in 18O relative to unaltered basalts, while below ∼ 800 m they show nearly unmodified δ18O values. Abundant seawater penetration appears to be limited to the upper part of the lower crust at ODP site 735 (∼ 800 m into the gabbroic layer and ∼ 2-2.5 km into the oceanic crust from the top of pillow basalts). Mass balance calculations show that the lower crust formed under this ultra-slow-spreading ridge has an average δ18O value of 5.5‰. The whole crust at Site 735 has an overall 18O enrichment with δ18O values of 6.0‰ to 7.8‰, depending on the possible variation of the δ18O values of the upper pillow basalts and sheeted dykes. The apparent difference in oxygen isotope compositions of ocean crusts formed with different spreading rates has important implications on the buffering of ocean water over geological time, as well as on the oxygen recycling between crust and mantle through subduction. The difference of seawater penetration between fast- and slow-spreading ridges could be related to their particular magmatic-tectonic history during the formation and aging of the crust. However, more analyses on continuous sections through oceanic and ophiolitic crust in different tectonic settings are required to derive any predictive models. © 2006 Elsevier B.V. All rights reserved.

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Chemical Geology

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