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Graduate School of Natural Science and Technology, Kanazawa University, Department of Earth Sciences, Kanazawa University

Department of Earth Sciences, Faculty of Science, Kanazawa University, The solution introduction ICP-MS technique for trace element analysis of rock samples was constructed at Kanazawa University. Trace element concentrations of geological standards (JB-2, JP-1, JA-1, and JGB-2) and Avacha peridotite xenoliths, Russia, which are strongly depleted in major elements and the hardest matter for analysis, were determined by the ICP-MS equipment. The values of concentration of middle to heavy masses (Rb-U) in geological standards coincide with those reported by other studies with ICP-MS techniques. The reproducibility is within 5% of differences of values of Rb to Hf in JB-2 in response to the changes in dates, dilutions and calibration methods. In the case of peridotites (low trace elements concentrations in the order of ppb to sub-ppb), relative standard deviations and relative differences among the runs under the various conditions are larger than in basalt with trace element concentrations in the order of ppm. Especially, large relative standard deviations (>10s%) for alkaline elements (Li, Rb, Cs), HFSE (Zr, Nb, Ta), Pb and Bi are shown in the refractory Avacha peridotite xenoliths with trace element concentrations in the order of sub-ppb. The solution introduction ICP-MS technique is a powerful tool for a precise insight into trace element characteristics of rocks of wide chemical compositions.

Department of Earth Sciences, Faculty of Science, Kanazawa University:Graduate School of Natural Science and Technology, Kanazawa University, In-situ trace-element analyses on minerals are especially useful for characterization of geological materials. We explored optimistic conditions of a inductively coupled plasma mass spectroscopy with laser-ablation sample introduction method (LA-ICP-MS) for in-situ quantitative trace-element analysis of geological materials on thin section. The sampling with an excimer laser with energy density of 8 J/cm^2 is appropriate for in-situ analysis of minerals on thin section based on ablation hole morphology. The rate of material removal is estimated from deep holes on a glass after 500 shots with energy on sample surface of 8 J/cm^2 to be approximately 0.2ƒﾊm/pulse. 150-200 shots might be available for a sample prepared as normal thin section (30ƒﾊm in thickness). All data were obtained by ablating in He gas prior to combination with the dominant Ar carrier flow because ablation in He minimized post-ablation surface condensation, resulting in high sensitivity. We examined the relationship between the size of laser-ablation hole and sensitivity of analysis in conducting in-situ simultaneous quantitative LA-ICP-MS analysis of 37 elements including rare earth elements (REEs). The diameter of laser beam can be as small as 30ƒﾊm to obtain sufficiently good dataset for trace-element concentrations of minerals from thin section.

Petological constitution of the upper mantle beneath the ocean floor has been poorly known except for oceanic fracture zones of slow-spreading ridges. Information from ophiolites may supplement the paucity of data to some extent ; however, the ophiolites should be treated carefully because of their polygenetic nature. The abyssal peridotite varies from lherzolite with Cr# of spinel of 0.1 to harzburgite with Cr# of spinel of 0.6. Dunite is relatively rare from the ocean floor. An exotic lherzolite with continental mantle signatures appears in midoceanic areas. The refractoriness of the abyssal peridotie has been proposed to correlate with the spreading rate of the ridge system, but this is false. The upper mantle beneath the ocean floor changes downwards from dunite to lherzolite via harzburgite, being independent of spreading rate. The lithological change is more abrupt in a slowspreading system than in a fast-spreading one, so it is around ridge segment boundaries rather than around the segment center on the same spreading ridge. The thin harzburite layer in slow-spreading ridges has resulted in its rarity there, and the deep seat of lherzolite in fast-spreading ridges has caused its apparent absence. The primitive MORB can be in equilibrium with dunite, which is formed along the melt conduit beneath the ridge via peridotie/melt reaction, and the dunite part is laid down by the corner flow of the mantle just below the lowermost gabbro layer as it leaves the ridge axis. We proposed the following deep ocean-floor drilling to explore scientific proglems concerning the abyssal upper mantle : (1) non-riser drilling on the “continental peridotite” to know the relationship with abyssal peridotite, and (2) non-riser or riser drilling on the ocean floor where deep-seated rocks have already been exposed to examine the deep constitution of the upper mantle. The “21st Century Mohole” drilling through the oceanic Moho should primarily be directed to the segment center of a fast-spreading ridge system. The back-arc basin such as the Sea of Japan will be the alternate for Mohole drilling because we have had relatively little information on the petrological nature of the back-arc basin lithosphere despite its importance. We can solve the “ophiolite problem” simultaneously if we are careful in choosing the drilling sites. We also propose a close linkage between the ophiolite study and ocean drilling in the coming IODP.

金沢大学大学院自然科学研究科自然計測