Combined chemical and high-precision isotope analysis of silicate melt inclusions – Advancing the understanding of Earth’s heterogeneous mantle

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Studies of MI provide insights into the composition of the Earth’s mantle, which are concealed in the carrier minerals or the whole rock. Trapped melts in early-crystallized olivines have significantly different and more variable chemical compositions than melts in equilibrium with the bulk rock. During the proposed project, MI in primitive silicates (olivine, pyroxene) from basaltic lavas will be analysed for their chemical and isotopic composition. Recent studies on the isotopic composition of melt inclusions have shown that the isotopic range of the inclusions exceeds that of the erupted basalts by almost an order of magnitude, i.e. the extent of isotopic heterogeneity in the mantle is much larger than previously thought. Yet, to date no consistent major-, trace element and isotope datasets from individual melt inclusions are available. Comparisons of the composition of melt inclusions and the erupted basalts, however, provide important insights about the actual heterogeneity in the mantle and about how partial melting and subsequent melt evolution influence the composition of the erupted melts. The samples selected for this project are from plume-related ocean islands, such as Iceland, Hawaii, Tristan da Cunha, but also the Faroe Islands, the Azores archipelago, St. Helena, Gough, and the Cook Australs that are represnetative of almost the entire known isotopic spectrum. So far, the isotopic composition of melt inclusions was determined exclusively by means of in-situ methods; either by SIMS or by LA-ICP-MS. Owing to the low ion yield of typically 10-100 micrometre large melt inclusions and the considerable isobaric interference corrections, the measurement errors of the in-situ techniques are often 10-100 times larger than that of TIMS and MC-ICP-MS measurements of chemically separated sample solutions, which require no (or minimal) isobaric interference corrections. For example, Sobolev et al. (2011) report an error of 0.0005 (S.D.), i.e., 700 ppm for Sr isotope analysis in melt inclusions using single collector LA-ICP-MS. Alternatively, the latest multi-collector mass spectrometers such as the Thermo Neptune Plus MC-ICP-MS offer high measurement sensitivity (2000V / ppm Sr) which enables high-precision analyses of a few nanogram material, as is required by the Sr content in typical melt inclusions. Making use of the high sensitivity, quantities of less than 5 ng are sufficient for Sr isotope analyses to be carried out on the ppm level, which is an improvement of more than one order of magnitude compared to in-situ methods. The improved precision of MI isotope measurements will provide a high-resolution image of the (isotopic) heterogeneity of the mantle. In combination with chemical information, such as major and trace elements, new insights into melting and crystallisation processes will be obtained, which are crucial for a better understanding of Earth’s mantle composition.