11B/ 10B analysis of geological materials by ICP–MS Plasma 54: Application to the boron fractionation between brachiopod calcite and seawater

Abstract

The use of MC-ICP-MS for B isotopic measurements offers some advantages compared with other techniques such as the absence of interferences with atomic masses 10 and 11 and an enhanced speed of data acquisition. We have developed a fast and simple protocol for the separation by ion-exchange resins and concentration of B from natural waters, carbonates, phosphates, and silicates. Normalization of sample isotopic ratios to the NIST reference NBS 951 leads to an external reproducibility of ±0.3 δ unit at 95% confidence level. The ionization yield is about 2 ions per 10 6 atoms of B in the analyzed solution. At least 2 μg of boron is required to obtain the above good-quality external reproducibility. Replicated analyses of Mediterranean seawater provided a δ 11B of 40.26±0.29 ( n=11). Aragonite-secreting fauna (corals and bivalves) living in tropical seawater have δ 11B close to 25‰ that are much higher than calcite-secreting invertebrates such as brachiopods from tropical to high latitudes with δ 11B ranging from 16.8‰ to 19.6‰. The first application of this method was to reevaluate the boron isotopic fractionation between calcite and water as a function of pH. We analyzed the calcitic shell of present-day brachiopods living in marine waters (surface down to 450 m) with pH varying from 7.6 to 8.4. By combining our data with those obtained by Sanyal et al. [Paleoceanography 11 (1996) 513] on cultured foraminifera, we derived an isotopic fractionation curve ( α calcite–seawater=0.023 X [B(OH) 4 − ]+0.976), resulting in the following relationship between the pH of seawater and the δ 11B values of biogenic carbonates pH=− log 0.023 δ 11 B calcite +1000 δ 11 B seawater +1000 −0.976 −1 +8.9 This equation has a slope equal to that of the experimental clay–water fractionation curve [Palmer et al., 1987, Geochim. Cosmochim. Acta 51 (1987) 2319], which precludes the use of the boron isotope fractionation among calcite–clay pairs as a “paleo-pH meter”.