|Budget Amount *help
¥1,800,000 (Direct Cost : ¥1,800,000)
Fiscal Year 1998 : ¥900,000 (Direct Cost : ¥900,000)
Fiscal Year 1997 : ¥900,000 (Direct Cost : ¥900,000)
1. Distribution of rare earth elements, thorium and uranium
These elements are largely distributed in Ca-phosphate minerals in equilibrated ordinary chondrites. Considering that Ca-phosphates are soluble in acid, chondritic meteorites were separated into acid-soluble and acid-residual phases. Then, rare earth elements, thorium and uranium were precisely determined for the acid-residual phases by inductively-coupled plasma mass spectrometry. The acid-residual phases consist of pyroxene, plagioclase and glass; glass is more abundant than plagioclase in unequilibrated ordinary chondrites. Rare earth contents and their abundance pattern normalized to their cosmic abundance for acid residual phases are correlated with the degree of thermal metamorphism chondrites experienced on the parent body. Similar change was observed for thorium and uranium contents. From these results, it was observed that contents of rare earth elements, thorium and uranium are good parameters for describing the thermal activity chondrites suffered on the parent body(ies). Furthermore, these contents are also correlated with contents of noble gases, especially heavy noble gases, suggesting that these noble gases were released from chondrites and were degassed from the parent body(ies) upon heating due to thermal activity.
2. Refractory siderophile element abundances in metallic phases of ordinary chondrites
Ordinary chondrites mainly consist of silicates, sulfides and Fe-Ni alloy phases. The weight of Fe-Ni phases ranges from several % up to 20% and siderophile elements are quantitatively distributed in metal phases. Precise determination of refractory siderophile elements revealed that their abundances systematically change from group to group of ordinary chondrites. Such a change was suggested to be caused by the difference in accretion temperature of ordinary chondrite parent bodies.