Evaluation and application of the ion microprobe in the strontium isotope geochemistry of carbonates

Abstract

A modified AEI-IM20 ion microprobe has been used to measure 87Sr/ 86Sr ratios in carbonates. A suite of carbonates with varying major elements (Ca, Mg, Fe, Mn) was studied at low and high ( M/ΔM ⋍ 3000) mass resolution to determine the types and intensities of molecular species isobaric with Sr peaks; Sr data collected at low mass resolution must be corrected for Ca 2 and CaMgO species. Rb/Sr ratios are extremely low, and correction for 87Rb is not required (< 0.1‰ of 87Sr). Usable Sr isotopic data may be obtained from calcite given Sr≥ 400ppm, and for Sr > 5000 ppm a precision of ∼ ± 1‰ (± 0.0007) in 87Sr/ 86Sr (2σ mean) can be achieved under optimum conditions. The corrections for Ca 2 and CaMgO are smaller than the within-run precision in calcite, but in dolomite the correction for CaMgO is + 1.5%. Mass fractionation corrections to 87Sr/ 86Sr (based on 86Sr/ 88Sr= 0.1194) are typically +8 to +10‰. Good agreement between ion probe and solid source mass spectrometer results was found for calcites of known Sr isotopic composition: ST4a (∼ 400ppm Sr), average ion probe 87Sr/ 86Sr= 0.7267 ± 0.0015, solid source 87Sr/ 86Sr= 0.72680 ± 0.00008 [14]; JCG36 (∼ 6000ppm Sr), average ion probe 87Sr/ 86Sr= 0.7056 ± 0.0009, solid source 87Sr/ 86Sr= 0.70588 ± 0.00009 [16]; JCG44 (∼ 6000ppm Sr), average ion probe 87Sr/ 86Sr= 0.7057 ± 0.0006, solid source 87Sr/ 86Sr= 0.70540 ± 0.00008 [16]. The ability of the ion microprobe to measure 87Sr/ 86Sr for 10-μm spots in calcite was used: (1) to measure variation in 87Sr/ 86Sr of ∼ 0.01 on a centimetre scale in a hydrothermally altered basalt from the Isle of Skye, northwestern Scotland; and (2) to determine the Sr isotopic composition of tiny (< 35 μm) calcite grains in a veined mantle lherzolite from Bultfontein, South Africa. Because of calcite's ubiquitous occurrence in many parageneses this technique may offer many opportunities for the measurement of fine scale heterogeneities in 87Sr/ 86Sr.