The isotopic composition of Ce in seawater has been measured for the first time, together with that of Nd. In spite of rather great uncertainties of measurement, we can say that the extent of 138Ce/142Ce variation appears considerably large, and that the ratio can be used as a geochemical indicator characteristic of water mass. According to our data, 138Ce/142Ce ratio appears to be related with Ce content or Ce anomaly, reflecting sample locality. 143Nd/144Nd ratio also decreases with Nd content, but their relationship is less localized than that observed for 138Ce/142Ce ratio.
Nitrogen is considered to have been a major component of terrestrial atmosphere over the long periods of geological time. The isotopic composition of ancient atmosphere, if obtained, may provide rigid constraints on models of atmospheric evolution. However there are few data available in literature. Chert is a candidate for this purpose possibly to trap some aliquot of paleoatmosphere. We report here argon and nitrogen abundances and nitrogen isotope ratios in six chert samples with various formation ages using stepped heating and static mass spectrometry. Results indicate that the argon in the samples has multi-component signatures. Nitrogen in the lower temperature release with the maximum argon release gives relatively constant δ15N values of around 0‰ while that of the highest step shows variable δ15N values from about 0 to + 12‰. The N2/Ar ratios of the lower step are significantly constant compared with those of the highest step. If the lower step nitrogen is trapping and reflecting the ancient atmosphere, the atmospheric δ15N value has been constant over the period of 3 b.y.
Sr, Nd, Ce and O isotopic ratios and trace element abundances were measured for Cenozoic SiO2- and (Na2O + K2O)-rich volcanic rocks from Yutian of Xinjiang, Yumen of Gansu and Tengchong of Yunnan, Western China. All samples investigated, especially the samples from Yutian of Xinjiang, are highly enriched in LREE and other incompatible elements, and are characterized by higher 87Sr/86Sr (0.706-0.709) and lower 143Nd/144Nd (0.5121-0.5126) ratios as compared with MORB and OIB. High 138Ce/142Ce ratios (0.0225767-0.0225810) were also found for the volcanic rocks from Yutian of Xinjiang and Yumen of Gansu provinces. The volcanic rocks from the three areas are concluded to have been derived from LILE (large ion lithophile element)-enriched mantle source regions without significant crustal contamination. Their high δ18O, and low HFSE (high field strength element)/LILE ratio characters imply a crust-like component existing in their mantle source regions, and hence the isotope variations of the volcanic rocks are modeled with mixing between two end members; depleted mantle and crust-like component. The mantle source region below western China, which indicates integration of subduction-related mantle, is disclosed to be clearly different from that below eastern China.
Four CM2 chondrites from Antarctica were analyzed for nucleic acid bases. The Yamato-74662 and Yamato-791198 chondrites gave positive evidence for the presence of guanine, tentative evidence for xanthine, and possibly hypoxanthine. Their individual abundances were in the range of 420 to 30 ng/g in the two chondrites, while no pyrimidines were found. No purines and pyrimidines were found in the Yamato-793321 and Belgica-7904 chondrites.