2005 年 39 巻 1 号 p. 27-46
In order to understand various magma processes occurring in the terrestrial body, highly precise isotope analyses of U, Th and Ra have been developed. In the first, an effective silicate rock decomposing method was established. Conventional acid digestion of mafic silicate rocks resulted in the precipitation of insoluble fluorides and very poor recovery yields of some trace elements. In contrast, almost 100% of the trace elements were recovered using larger amounts of HClO4 than was conventionally used and evaporating the sample to dryness in a step-wise fashion. Then, new chemical separation methods of U, Th, and Ra were developed by employing some novel extraction chromatographic resins. For U isotope analysis by TIMS, a new activator, silisic-acid and phosphoric acid mix solution was very effective to produce stable and strong UO2+ beam, resulted in excellent improvement for both precision and reproducibility. For Ra isotope analysis, a new, precise and accurate analytical method was developed by employing total evaporation TIMS technique. These new methods have been applied for Miyakejima volcano, Izu arc, Japan. 238U-230Th-226Ra disequilibria observed in lavas with large 238U and 226Ra excesses imply metasomatism of depleted mantle by fluid related processes. In the equiline diagram, the trends for two magmatic stages (Stage 1 and 2) are regarded as two different isochrons with a common initial (230Th/232Th) ratio, although the trend for Stages 3 and 4 is a mixing line. The age difference in the equiline diagram corresponds to the interval of individual fluid-release events. Thus, fluid release from the slab and subsequent magma generation occur as episodic events on a several-kyr timescale. The model calculations show a very rapid ascent time of the slab components in the mantle wedge (<7 kyr), which can be explained by nearly instantaneous material transport in the mantle wedge.
