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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
Authors:Hiroshi Shimizu  Hideyuki Sawatari  Yousuke Kawata  Peter N. Dunkley  Akimasa Masuda
Affiliation:(1) Department of Chemistry, Faculty of Science, The University of Tokyo, 113 Hongo, Tokyo, Japan;(2) Geological Survey Division, Honiara, The Solomon Islands;(3) Present address: Department of Applied Chemistry, School of Engineering, Nagoya University, Chikusa, 464-01 Nagoya, Japan;(4) Present address: Geothermal Energy Project, British High Commission, Box 30465, Nairobi, Kenya
Abstract:138Ce/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 epsiCe and epsiNd. It is shown that these epsiCe and epsiCe values can hardly be interpreted by simple mixing between MORB and oceanic or continental crustal rocks; the former have positive epsiNd and negative epsiCe and the latter have negative epsiCe and positive or negative epsiNd. Existence of sources having positive epsiCe and epsiNd 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.
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