IN SITU, MULTIPLE-MULTIPLIER, LASER ABLATION ICP-MS MEASUREMENT OF BORON ISOTOPIC COMPOSITION (δ11B) AT THE NANOGRAM LEVEL

Abstract

We present a new in situ, multiple electron-multiplier LA-ICP-MS technique for the analysis of boron isotopes (δ11B) at <1%% 2σ precision and at the nanogram level. Sample materials analyzed were all natural and synthetic glasses, spanning a range of boron concentrations from 0.39 to 30.2 ppm and 11B/10B values from 4.0254 to 4.0799 (δ11B -6.08%% to +7.35%%). The in situ analyses were duplicated at higher concentrations by solution analyses of chemically separated boron using conventional nebulization on the same ICP-MS system.Sample-standard switching between glass chips on an approximate 5-min cycle is used to monitor and correct for instrumental fractionation and drift. Instrumental fractionation of more than 100%% in measured 11B/10B ratios is successfully corrected and the final 11B/10B values are up to 100-fold more accurate than the measured ratios. Reproducibility of <1%% at 2σ are routinely produced at natural sample boron concentrations as low as 0.39 ppm.The amount of boron measured to produce these data is dependent on the boron concentration of the sample, and ranges from 0.014 μg at 30.2 ppm boron to 0.0008 μg at 0.39 ppm boron. These nanogram amounts of boron measured are approximately two orders of magnitude lower than conventional P-TIMS and solution multiple faraday ICP-MS methods which typically require 0.3-3.3 μg of boron. The advantages of this technique are the elimination of procedures for the chemical separation for boron, low instrument memory and background (<300 cps compared to>100 000 cps for solutions), high sensitivity, small sample size, and spatial control on material analyzed. These characteristics will permit boron isotopic analysis on virtually any glass collection.The boron isotope compositions of four fresh MORB glass samples from the northern EPR (15-18°N) were determined using the laser ablation technique presented here. The results indicate a narrow range of boron compositions for these samples (δ11B -6.92%% to -7.68%%), which overlap at the 2σ level. The light δ11B composition of these samples precludes substantial seawater (δ11B +39.5%%) contamination of their pre-eruptive MORB magmas.