No abstract

No abstract

In order to obtain precise rare earth element (REE) compositions of sedimentary reference rocks issued by the Geological Survey of Japan (JSd-1, -2, JSl-1, -2, JLk-1), we analyzed their REEs by inductively-coupled plasma mass spectrometry (ICP-MS) using methods employing both acid digestion (HF-HClO 4 ) and carbonate fusion (Na 2 CO 3 -H 3 BO 3 ). Analytical precision of these methods was estimated by analyzing JB-1a and BCR-1 with the same procedure. Analyses of these two reference rocks show good reproducibility and close agreement with compilation values, except for Ho in JB-1a and Tm in BCR-1. CI-normalized REE patterns of JB-1a and BCR-1 compilation values show deviations for Ho and Tm, respectively, from the smooth patterns. Therefore, the compilation values of those elements may be somewhat in error.After acid digestion of JSd-1, -2, JSl-1, -2 and JLk-1, 2-40% of total REEs were remained in the residues, i.e., secondary fluorides and/or undigested minerals. However, subsequent carbonate fusion effectively recovered the remained REEs. Analytical results by this study show relatively good agreement with compilation values, except for Pr, Ho, Er and Tm, and smooth REE patterns normalized to CI-chondrite. The REE patterns of compilation values show anomalous deviations of Pr, Ho Er, and Tm, suggesting that compilation values of these elements may be in errorneous. This study emphasizes that combined method of acid digestion and alkali fusion is an effective method for recovering REEs in sedimentary rocks for bulk REE determination.

The geochemical characteristics of Archean unusual siliceous rocks at Mt. Goldsworthy region in the Pilbara Craton were studied. The siliceous rocks have been assigned to the uppermost Warrawoona Group mafic volcanic rocks, and are overlain by quartz-rich sandstone units of the Strelley Pool Chert that probably represent continental margin sedimentation. The Warrwoona rocks have been heavily altered and are now composed dominantly of microcrystalline quartz, with subordinate mica, Fe-Ti oxides and unidentified silicates; original magmatic textures are only locally preserved. The complex alteration is assumed to result from multiple events including weathering during subaerial exposure, circulation of hydrothermal fluids, and metasomatic silicification. During alteration, Al, Ti, Zr, Th, Cr and Sc remained immobile, although their concentrations were lowered by a substantial increase in silica. Mutual ratios of these immobile elements such as Al 2 O 3 /TiO 2 Cr/Th, Th/Sc, Cr/Al 2 O 3 and Zr/TiO 2 and comparison with the least-altered contemporaneous maficultramafic rocks in the Pilbara Craton show that the altered rocks originated from high-MgO rocks such as komatiite and high-MgO basalt, possibly of Al-depleted type. The Mt. Goldsworthy rocks have significantly higher Th/Sc (0.024-0.1) values compared with primitive mantle values (0.005) and komatiite (0.01). This feature is interpreted as a result of crustal contamination, which is consistent with the early evolution of continental crust.