We describe an analytical methodology to quantify 238 Ue 234 U and 230 Th using isotope-dilution systematics and inductively coupled plasma-quadrupole mass spectrometry (ICP-QMS) for geochronological purposes. This required characterization of the mass spectrometer detector performance over a large dynamic range, including a careful calibration between pulse and analogue ion-counting modes. Our methodology gives an analytical precision for 234 U/ 238 U and 230 Th/ 238 U of 4& and 7& (2 Â standard error), respectively, and yields 230 Th-ages with 4e5% uncertainty from 500 ng of 238 U. We tested our methodology by analyzing carbonate samples (corals and speleothems) which were previously dated using MC-ICPMS; excellent agreement between both techniques was obtained. Our results indicate that ICP-QMS is suitable to obtain valuable meaningful geochronological data, but it is limited to dating samples older than 1 ka, and younger than 400 ka. Our methodology thus represents an attractive alternative to obtain geochonological data to select the most interesting samples prior to high-precision analysis. Moreover, such data would also valuable to build preliminary age-models, which can be further complemented with high-precision analyses.
The Mahanadi river basin (MRB) is one of the eight major fluvial basins of India. This paper presents a very first systematic study of geochemical characteristics, including rare earth elements of surface sediments collected from the riverbed at different parts of this basin to infer the chemical weathering, provenance, and tectonic settings. Lower mean values of major elements (except for SiO2 and K2O) are observed compared with that of the upper continental crust (UCC). It is attributed to the dominance of quartz and orthoclase in sediments and removal of mobile elements from the sediment during the transportation. The concentrations of Rb, Ba, Cu, Th, and Pb are elevated with respect to the UCC. The higher concentration of K2O and Rb, the ratio of Al2O3 to TiO2, and other bivariant plots indicate that the sediments are derived mostly from acidic to intermediate igneous rock. Majority of the sediments have geochemical similarity with the arkose and litharenite sandstones. The chemical index of alteration (CIA) values of the MRB sediments varies between 52 and 69 with most of the samples having CIA < 60 suggesting the lower degree of weathering which is further supported by the mineralogy. The eroded sediments have compositional maturity comparable to sediments deposited under semi‐arid climatic conditions. Sediments show both positive and negative Eu anomalies based on the source rock. Based on the commonly used discrimination diagrams, it is concluded that the MRB has a tectonic setting similar to the passive continental margin.
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