抄録
Incompatible elements are geochemically very important elements. For the determination of these elements, inductively coupled plasma mass spectrometry (ICP-MS) is very effective, because (1) multi-element analysis is available, (2) the sensitivity for the every incompatible element is high (<1 ng mL-1), and (3) the interference spectra are very simple compared to the spectrometry, such as the inductively coupled plasma atomic emission spectrometry (ICP-AES) or the X-ray fluorescene analysis (XRF). A new analytical technique using ICP-MS was developed for the determination of the incompatible elements in rocks. The sensitivity of every incompatible element in rocks was 10 ng g-1 level, and sufficient for the trace element geochemistry in most cases. This technique was applied to characterize the elemental behavior at the subduction zone, which is one of the most important place where material recycling between mantle and crust takes place. First, the elemental behavior during subduction-related high-pressure metamorphism was studied. In this study, not only rare-earth elements (REE) but also high field strength elements (HFSE) such as Zr, Nb, Hf or Ta were found to be mobile during metamorphism. This result indicates that more careful interpretation is required in the trace element geochemistry of metamorphic rocks, especially when we interpret a tectonic setting and magmatic evolution in the Archean. Second, in order to characterize slab-derived aqueous fluid, the analytical technique was combined with a high pressure-temperature experiment. In the experiment, a model slab material, metabasite was dehydrated under a high P-T condition, and the fluid was trapped in a model mantle material, dry olivine. The "squeezed" slab material was analyzed by ICP-MS, and it was found that LREE was depleted, while HREE remained constant. This suggests that the slab-derived fluid is LREE enriched, and the subducted slab has lower LREE/HREE ratio. Such interdisciplinary studies will be more and more important in the earth science in the forthcoming century. For this purpose, I am organizing "Comprehensive Elemental Analysis Project (CEAP)", which is briefly introduced in this paper.
