大野 剛

Abstract We determined the local structure of magnesium in biogenic aragonite using X-ray absorption fine structure spectroscopy. It was revealed for the first time that local structure of magnesium in biogenic aragonite is identical to that in synthesized aragonite by using adequate standard material. The incorporated magnesium is six-fold coordination and does not isomorphously substitute for the nine-fold calcium ion in aragonite. This suggests that magnesium either adopts distorted coordination in the aragonite or is incorporated into an amorphous calcium carbonate.

ABSTRACT Owing to high resistance to alteration, detrital zircons retain information about their formation ages and parental magmas for a long period of time. Many geochemical researchers have proposed various indicators for zircon to constrain tectonic settings and to identify source rock. Because most detrital zircons analyzed by geochronologic studies are derived primarily from granitoids, we focus on the classification of zircon within granitoids. In the style of alphabetical classification scheme (Igneous, I; Sedimentary, S; and Alkaline, A types), some discrimination diagrams have been proposed. To improve the database and enhance discriminating studies, we examined trace‐element compositions of zircons extracted from some Cenozoic granitoids exposed in the Japan Islands. The zircons showed systematic differences in Nb, Ta, Ce, and P contents. Zircons in Oceanic Arc I‐type granite are poor in Nb and Ta, and these signatures clearly reflect those elements in their parental bodies. Despite their low abundance at the whole‐rock level, zircons in Oceanic Arc I‐type granite are characterized by high Ce content. This is attributable to the relatively oxidizing conditions of Oceanic Arc I‐type magma. Zircons in S‐type granite are characterized by high P and low Ce contents. The former can be explained by high apatite solubility in Al‐rich magma, whereas the reducing environment of S‐type magma is accountable for the latter. The zircon crystallized at the later stage during S‐type granite solidification is slightly depleted in Nb and Ta. This is attributable to the depletion of these elements in the magma by Ti‐bearing minerals such as ilmenite prior to zircon crystallization. In analogy with whole‐rock composition, zircons in transitional I‐A‐type granite have intermediate composition between I‐type and A‐type zircons. On the basis of the updated database, we demonstrated that the Nb/P–Ce/P or Ta/P–Ce/P crossplots are the most useful for discriminating zircons in Oceanic Arc I‐type, I‐type, S‐type, and A‐type granites.