Oxygen isotope evolution of the lower ocean crust: an in-situ study by UV-laser-ablation oxygen isotope microprobe

Systematic analysis of mineral oxygen isotope compositions in gabbro has the potential to constrain the effects of cooling processes in the lower ocean crust. For this reason we measured three representative samples of gabbro from Hole 735B of Leg 176 by UV-laser microprobe for the oxygen isotopic compositions of their constituent minerals. Sample 28, from 514.79 mbsf, is located between two cataclastic faults, with both high and low temperature alteration; sample 1 (880.31 mbsf) is within a high-temperature deformation zone, and sample 16 (1402.16 mbsf) is in the lower part of the core, which is far away from both the crystal-plastic shear zone or from any fault. The measured results show that respective minerals in the three samples have very similar δ ¹⁸O values, although their geological settings and whole rock δ ¹⁸O values are quite different. Clinopyroxene and olivine have normal values for NMORB, +5.5+/-0.4‰ and +5.2+/-0.3‰ respectively, while plagioclases have heavier δ ¹⁸O values (+6.6+/-0.3‰ ) than that of plagioclase in NMORB (+6.0‰ ). This suggests that all three gabbros have encountered similar cooling histories from one source magma by a single intrusion. The large inter-crystal δ ¹⁸O difference in sample 28 between the unaltered plagioclase with δ ¹⁸O values about +6.0‰ and the altered ones with δ ¹⁸O values about +7.6‰ is most probably due to low temperature alteration by seawater. Considering the unaltered constant δ ¹⁸O values of olivine and clinopyroxene, the modification of the O-isotope composition of plagioclase must occur near the time of crystallization and the temperature should be higher or equal to the closure temperature for O-isotopes in plagioclase but lower than that for olivine and clinopyroxene.