MASS BIAS CORRECTIONS FOR OXYGEN AND HYDROGEN ISOTOPIC ANALYSES OF TOURMALINES BY SECONDARY ION MASS SPECTROMETRY

The O and H isotopic composition of minerals can provide valuable information on the source of the fluids and temperature of mineral precipitation. To obtain accurate measurements using SIMS, it is important to select chemically similar standards and samples to correct for both matrix effects and instrumental mass fractionation (IMF), collectively termed instrumental mass bias (IMB). For certain mineral groups (e.g. tourmalines), there are large variations in major element composition between end members. The current study utilizes three common tourmaline end members (schorl, dravite, and elbaite) with the goal of constructing calibration curves to correct for IMB during SIMS analysis of O and H isotopes.

Prior to SIMS analyses, all eight samples were analyzed for their “true” O and H isotopic compositions using CF-IRMS. The results are: δ¹⁸O: 9.5‰ and 6.8‰ for the schorls, 13.4‰ for the dravite, and a range between 10.8‰ to 19.7‰ for the elbaite samples. δD values are: -56‰ and -58‰ for the schorls, -36‰ for the dravite and between -98‰ and -34‰ for the range of elbaites. These data were used to calculate IMB.

Data obtained by SIMS from eight tourmaline grains comprising the three tourmaline end members show that six of the eight samples have spot to spot reproducibility of <1‰ for δ¹⁸O values, and the two samples have spot to spot reproducibility of ≥1‰. All eight samples have spot to spot reproducibility of <4‰ for δD values. Therefore, six samples are sufficiently homogenous and can be used as standards. The mass bias for O isotope analysis of two schorl samples varies from -54‰ to -61‰, whereas for dravite the mass bias is -66‰, and five elbaite samples have mass biases from -70‰ to -68‰. The mass biases for the H isotopic analyses are: -635‰ and -675‰ for the schorl samples, -666‰ for the dravite sample, and a range from -664 to -643‰ for the elbaite samples.

Iron concentration positively correlates with mass bias for the O isotope measurements whereas H isotope mass bias correlates with Mg concentration. Therefore, to correct for IMB during O and H isotopic analysis in tourmalines by SIMS, tourmaline standards and samples must be matrix matched for Fe- and Mg-contents. Alternatively, several standards that cover a range of Fe and Mg contents should be analyzed and calibration curves constructed to correct for mass bias.