Double spiking is conventionally used to make accurate determinations of natural mass-dependent isotopic fractionations for elements with four or more stable isotopes. Here we document a methodology which extends the effective application of double spiking to three isotope systems. This approach requires making a mixture with isotope ratios that lie on a 'critical curve' where the sample – double-spike mixing line and the tangent to the instrumental mass-bias curve are coincident. Inversion of the mixing equations for such a mixture leads to a solution for the sample fractionation which is independent (to first order) of the uncertainty in the instrumental mass-bias and, hence, independent of any mass-dependent artefacts in the measurement such as those produced by residual matrix not completely removed by prior chemical purification. In practice, mixtures can be made which yield an accuracy conservatively estimated to be ∼ 0.005‰/amu. The precision of the method is explored as a function of double-spike composition for Mg, Si and K isotope systems. We show that for Mg and Si measurement precision is not compromised by the compositions of viable critical mixtures nor by uncertainty magnification during inversion of the equations. Thus, double spiking provides a valuable means to obtain robust, high precision isotopic measurements of Mg and Si. For K, however, the low abundance of 40K in the optimal critical mixture places a significant practical limitation on the application of double spiking to analyses of this element.
Small mass-dependent variations of molybdenum isotope ratios in oceanic and island arc rocks are expected as a result of recycling altered oceanic crust and sediments into the mantle at convergent plate margins over geological timescales.However, the determination of molybdenum isotope data precise and accurate enough to identify these subtle isotopic differences remains challenging.Large sample sizes -in excess of 200mg -need to be chemically processed to isolate enough molybdenum in order to allow sufficiently high-precision isotope analyses using double-spike MC-ICPMS techniques.Established methods are either unable to process such large amounts of silicate material or require several distinct chemical processing steps, making the analyses very time-consuming.Here, we present a new and efficient single-pass chromatographic exchange technique for the chemical isolation of Mo from silicate and metal matrices.To test our new method we analysed USGS reference materials BHVO-2 and BIR-1.Our new data are consistent with those derived from more involved and time-consuming methods for these two reference materials previously published.We also provide the first molybdenum isotope data for USGS reference materials AGV-2, the GSJ reference material JB-2 as well as metal NIST SRM 361.
We determined equilibrium Mg isotope fractionation between olivine and melt (Δ 26/24 Mg Ol/melt ) in five, naturally quenched, olivine-glass pairs that were selected to show clear textural and chemical evidence of equilibration.We employed a high-precision, critical mixture double-spiking approach to obtain a weighted mean of Δ 26/24 Mg Ol/melt = -0.071± 0.010 ‰, for values corrected to a common olivineglass temperature of 1438 K.As function of temperature, the fractionation can be expressed as Δ 26/24 Mg Ol/melt = (-1.46± 0.26) × 10 5 /T 2 .The samples analysed have variable H 2 O content from 0.1 to ∼1.2 wt.%, yet no discernible difference in Δ 26/24 Mg Ol/melt was evident.We have used this Δ 26/24 Mg Ol/melt to revisit the puzzling issue of elevated Mg isotope ratios in arc lavas.In new Mg isotope data on sample suites from the Lesser Antilles and Mariana arcs, we show that primitive samples have MORB-like Mg isotope ratios while the evolved samples tend to have isotopically heavier compositions.The magnitude of this variability is well explained by olivine fractionation during magmatic differentiation as calculated with our new equilibrium Δ 26/24 Mg Ol/melt .
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