Dissolution processes of elements from subducting sediments into fluids: Evidence from the chemical composition of the Sanbagawa pelitic schists

抄録

In order to evaluate the dissolution processes of elements from subducting sediments into fluids during early stages of metamorphism (up to oligoclase–biotite zone at about 30 km depth), the chemical composition of Sanbagawa pelitic schists, Sanbagawa Metamorphic Belt, Japan was studied. Samples from different metamorphic grades show similarities in their major element and Rb compositions, suggesting that the Sanbagawa schists experienced insignificant dissolution of these elements. Arsenic, N (as ammonium, NH₄⁺) and Cs contents decrease with increasing metamorphic grade, demonstrating that their dissolution is enhanced under respective metamorphic stages (As: upper–garnet zone, NH₄⁺: chlorite ~ oligoclase–biotite zone, Cs: garnet ~ albite–biotite zone). Dissolution mechanisms proposed are as follows. The NH₄⁺ dissolution accompanies dehydration of the pelitic schists, which may produce fluids with a high potential to dissolve or oxidize NH₄⁺. The thermal structure of the subducted slab is likely to influence the depth of the NH₄⁺ dissolution. The Cs dissolution is caused mainly by fluid flow from underlying meta-mafic rocks and the amount of fluids possibly controls the degree of the Cs dissolution. Heterogeneous dissolution of Cs suggests that the fluid flow from the underlying meta-mafic rocks has a channelized structure. The mechanism of As dissolution is not clearly understood; however, it may be strongly related to graphitization of kerogens in the pelitic schists. Differences in Cs dissolution were observed between the Sanbagawa pelitic schists and other well-studied meta-sedimentary sequences, particularly the Catalina Schist, California and New Caledonia Schistes Lustrés. In the case of the Catalina Schists, Cs was dissolved into fluids more effectively than the Sanbagawa pelitic schists. In contrast, meta-sedimentary rocks of the Schistes Lustrés nappe did not experience any Cs dissolution, although rocks subducted deeper than their dehydration depth. One of the reasons for these differences may be variations in the amount of channelized structures among subduction zones.