Boron isotopes in DSDP cherts: Fractionation and diagenesis

Abstract

Twenty samples of cherts from the Central Pacific (18 of them from DSDP, Leg17, Hole 167) were analyzed for B content and δ 11 B by ion-probe. In all samples δ 18 O, 87 Sr/P 86 Sr and Ge have been previously measured. δ 11 B of DSDP cherts varies between — 9.3 and + 8%, indicating a “bulk” fractionation of 32–45% with respect to present day seawater B (39.5%), the highest fractionation of any marine phase. This fractionation is similar to the previously observed values in opal-A radiolarians (having δ 11 B of +2 to + 4.5%, Earth Planet. Sci. Lett. 117 , 1993, 567–580). Siliceous sediments are a significant sink of boron in the ocean, an even more significant sink of 10 B. Assuming no secular variation in δ 11 B sw our results can be the reflection of a three-stage precipitation-dissolution process of DSDP cherts: the first leading to opal-A and involving a large fractionation of B isotopes between tetrahedral boron in solution and opal-A, the second and third resulting in quartz via opal-CT. The dissolution-reprecipitation occurred at variable temperatures and water/rock ratios. The deepest cherts underwent diagenesis at higher temperature (low δ 18 O values) and in closed system (low δ 11 B values). Boron isotopic composition of marine cherts cannot be a clue to the paleochemistry (specifically paleo-pH) of seawater but indicates that some mineral phases might fractionate B isotopes differently than carbonates and thus be used in conjunction with carbonates to reconstruct paleo-pH and δ 11 B values of seawater.