The boron isotopic composition of altered oceanic crust

Abstract

Samples of drilled oceanic crust, from DSDP Holes 417A, 417D and 418A and ODP Hole 735B, and oceanic crust from the Oman and Cyprus ophiolites, were analyzed for B contents and δ 11B. Composite samples from DSDP Holes 417A, 417D and 418A were used to represent the upper 550 m of altered oceanic crustal Layer 2A. Whole-rock samples from the Troodos ophiolite, Cyprus, and the Oman ophiolite wer selected to represent crustal Layer 2B dikes. Composite samples from ODP Hole 735B were used to represent crustal Layer 3. The B content of the DSDP composites ranges from 7.2 ppm to 104 ppm and correlates with both δ 18O and K, showing that it is a good indicator of the extent of low temperature alteration. The δ 11B of the DSDP composites varies between −2.5‰ and 5.4‰. The B content of the samples from the Troodos ophiolite ranges from 2.4 ppm to 8.1 ppm; δ 11B varies from −0.9‰ to 7.8‰. The B content of the Oman ophiolite samples ranges from 5.0 ppm to 11.1 ppm; δ 11B varies from −1.6‰ to 16.9‰. The B content of the samples from ODP Hole 735B ranges from 1.1 ppm to 7.1 ppm; δ 11B varies from −4.3‰ to 24.9‰. The general pattern displayed by these samples is one of greatest (and most variable) B enrichment at the top of the crust and least enrichment at the bottom of the section. All of these samples are enriched compared to unaltered MORB, which is believed to have a B content of approximately 0.5 ppm. The δ 11B values of deeper samples, from Layers 2B and 3, are more variable and generally higher than those from Layer 2A. Boron contents and δ 11B are not correlated. The data from the DSDP Site 417 418 composites indicate that the δ 11B of fluid circulating in the upper crust changes only slightly during alteration, increasing by an average of 5.1‰ with an accompanying decrease in B concentration of 7%. Low temperature alteration appears to be a water-dominated process resulting in minor modification of circulating seawater. A minimum water-rock ratio of 400 is calculated for these samples, implying a minimum low-temperature seawater flux through the upper oceanic crust of 3.4 × 10 14 l/y. The average B content of altered oceanic crust, as represented by these samples, is 5.2 ± 1.7 ppm and the average δ 11B is 3.4 ± 1.1‰. This average isotopic composition is measurably different from the apparent average of oceanic sediments, supporting the idea that δ 11B could be useful for identifying the source(s) of B in island arcs.