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

通过对地幔岩的研究，我们可以了解地幔的组成，而地幔的组成是隐藏在载体矿物或整个岩石中的。早期结晶的橄榄石中被困的熔体具有显着不同的和更可变的化学成分比熔体中的平衡与散装岩石。在拟议的项目期间，将分析玄武岩熔岩中原始硅酸盐（橄榄石、辉石）中的矿物隔离物的化学和同位素组成。近年来对熔融包裹体同位素组成的研究表明，熔融包裹体的同位素范围超过喷发玄武岩的同位素范围近一个数量级，即地幔中同位素不均一性的程度比以前认为的要大得多。然而，迄今为止，没有一致的主要，微量元素和同位素数据集，从个别熔融包裹体。熔融包裹体和喷发的玄武岩的组合物的比较，但是，提供了重要的见解，在地幔中的实际异质性，以及如何部分熔融和随后的熔体演化影响喷发的熔体的组成。本项目所选的样品来自与羽流有关的海洋岛屿，如冰岛、夏威夷、特里斯坦-达库尼亚，但也有法罗群岛、亚速尔群岛、圣赫勒拿岛、高夫岛和库克群岛，它们代表了几乎所有已知的同位素谱。迄今为止，熔体包裹体同位素组成的测定都是采用原位方法;通过西姆斯或LA-ICP-MS。由于通常10-100微米大熔体包裹体的低离子产额和相当大的等压干扰校正，原位技术的测量误差通常比化学分离的样品溶液的TIMS和MC-ICP-MS测量的误差大10-100倍，其不需要（或需要最小的）同量异位素干扰校正。例如，Sobolev等人（2011）报告的误差为0.0005（S.D.），也就是说，此外，最新的多接收器质谱仪，如Thermo Neptune Plus MC-ICP-MS，提供高测量灵敏度（2000 V/ ppm Sr），可对几纳克材料进行高精度分析，这是典型熔融包裹体中Sr含量所需的。利用高灵敏度，小于5 ng的量就足以进行ppm级的Sr同位素分析，这比原位方法提高了一个数量级以上。MI同位素测量精度的提高将提供地幔（同位素）不均匀性的高分辨率图像。结合主要元素和微量元素等化学信息，将获得对熔融和结晶过程的新见解，这对于更好地了解地球地幔组成至关重要。

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.