Lower Ocean Crust beneath Slow-Spreading Ridges: a Combined Oxygen Isotopic and Elemental in-situ Study on Hole 735B Gabbros

Abstract

Chemical and oxygen isotopic composition of the constituent minerals in oceanic gabbros from ODP Hole 735B have been investigated by a combined in-situ analytical study with UV-laser-ablation oxygen isotope microprobe, ion probe, and electron microprobe. The mineral data verify the known feature of Hole 735B that it was formed by in situ crystallization of 4 or 5 distinct olivine gabbro blocks, indicating at least two major cycles of intrusions with general upward fractionation. However, large scatters on plots of chemical correlations among component minerals and pervasively existing mineral zonations are observed. These suggest that after initial rapid cooling and crystallization, significant amounts of late stage melts have migrated and pervasively reacted with host gabbros through solid state diffusion.Trace element zoning in component minerals of gabbro, including plagioclase and clinopyroxene, is well developed and preserved in Hole 735B, which may represent a common feature in the lower ocean crust, at least for those formed beneath slow-spreading ridges. Petrologic modeling shows that the observed mineral zonations were not caused by any primary magmatic growing process such as in-situ crystallization of trapped melt. Pervasively migrating late stage melts along grain boundaries are required to account for the formation of trace element zoning by diffusive exchange between melts and gabbros.Diffusion modeling of zoning profiles provides constraints to experimentally determined diffusion rates. The calculation results show that trace elements diffuse faster in clinopyroxene under natural conditions than under experimental conditions. In contrast to the large difference found in experimental studies, this study shows that rare earth elements have quite similar diffusivities in clinopyroxene.In contrast to locations of fast-spreading centers with about 5 km seawater penetration the oxygen isotope data show that abundant seawater penetration and circulation are limited to the upper part of the lower crust at ODP site 735 (~600 meters into the gabbro 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 (Southwest Indian Ridge) has average δ18O value of 5.5 and the whole crust has overall 18O enrichment with δ18O values of 7.55 to7.85 , depending on the possible variation of the δ18O values of the upper pillow basalts and sheeted dykes. However crust formed under fast-spreading ridges usually has depleted δ18O values caused by significant hot seawater penetrations into greater depth at temperatures above ~250°C. The difference in oxygen isotope composition of ocean crust formed under ocean ridges with different spreading rate has important effect on the buffering of ocean water over geological time, as well as the oxygen recycling between crust and mantle through subduction.