Magnesium isotopic composition of igneous rock standards measured by MC-ICP-MS

To provide inter-lab comparison for high-precision Mg isotope analysis, Mg isotope compositions (expressed as delta(26)Mg relative to DSM-3) for commercially accessible peridotite, basalt, andesite, and granite geostandards have been measured by multi-collector inductively coupled mass-spectrometry (Nu-Plasma) using sample-standard bracketing method. There is a large tolerance of matrix cations during the measurement of Mg isotopes, as intensity ratios of (23)Na/(24)Mg and (27)Al/(24)Mg of about 20% only change the delta(26)Mg by less than 0.1 parts per thousand and low (55)Mn/(24)Mg (<0.1) and (58)Ni/(24)Mg (<0.01) do not cause significant mass bias either. Concentration match between samples and standards within 90% is adequate to obtain accurate isotope analysis, which also mitigates the isobaric interference of (12)C(14)N(+) on (26)Mg. Organic matrix from chemical purification can cause significant analytical errors when the mass of Mg processed is small. The long-term reproducibility Of delta(26)Mg(DSM-3) for samples with relatively higher MgO content is about 0.11 parts per thousand (2SD), and granites with lower MgO content is about 0.2 parts per thousand (2SD). Although the standards in this study have wide ranges of major element compositions with SiO(2) from 40 to 70 wt.% and MgO from 0.75 to 49.6 wt.%, they exhibit a variation of Mg isotopic compositions with delta(26)Mg from -0.07 to -0.40 parts per thousand. delta(26)Mg do not correlate with SiO(2) or MgO contents, suggesting homogenous Mg isotope compositions in igneous rocks at the level of current precision, relative to low temperature samples including sediments and riverine and sea waters. Our data do not support a non-chondritic Mg isotope composition of the Earth. (C) 2009 Elsevier B.V. All rights reserved.