Hideyuki Sawatari

Ce and Nd isotope geochemistry on island arc volcanic rocks with negative Ce anomaly: existence of sources with concave REE patterns in the mantle beneath the Solomon and Bonin island arcs

Abstract138Ce/142Ce isotope ratios in Cenozoic island arc volcanic rocks are reported for the first time, together with isotope ratios of Nd and Sr and abundances of REE, Ba and Sr. The island arc volcanics studies here are boninites from Chichijima, the Bonin Islands, and basalts and andesites from the Solomon Islands. REE patterns of the island arc volcanic rocks from the Solmon Islands and the Bonin Islands are confirmed to have negative Ce anomalies. It is also disclosed that the majority of these island arc volcanic rocks show mainly positive values for both εCe and εNd. It is shown that these εCe and εCe values can hardly be interpreted by simple mixing between MORB and oceanic or continental crustal rocks; the former have positive εNd and negative εCe and the latter have negative εCe and positive or negative εNd. Existence of sources having positive εCe and εNd values is strongly suggested. If the sources are assumed to have been fractionated from CHUR (chondritic uniform reservoir) at the early or middle Precambrian era, the sources from which the volcanics were derived are concluded to have kept concave REE patterns with larger (La/Ce)N and smaller (Nd/Sm)N ratios than chondritic values over a substantial period of time, until the time of Cenozoic magmatism forming island arc volcanic rocks in question. During the periods of the Cenozoic magmatic activities and their related events, Ce anomalies are considered to have been created. From Ce and Nd isotope ratios, however, it is difficult to determine which of the following processes was responsible for the Ce anomaly; the incorporation process of subducted oceanic crust into magma at the mantle or the slab dehydration and metasomatism process. Nevertheless, so far as Ce and Nd isotopic ratios are concerned, incorporation of oceanic sediments did not take place to any clearly detectable degree.

Multielement profiling analysis of geochemical and environmental samples by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and inductively coupled plasma-mass spectrometry (ICP-MS)

Simultaneous multielement analysis of geochemical and environmental samples such as rock, sediment, seawater, and lake water have been carried out by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS). In these experiments, some instrumental improvement and sample pretreatment techniques were required to fulfil accurate multielement determination of 30-50 elements for each sample described above. Consequently, it becomes possible to characterize the samples by comparing the profiles of their multielement concentration distributions. Such multielement profiling analysis of various samples may be useful in the research fields such as geochemistry, environmental chemistry, material science and so forth.<<ETX>>