In-situ measurements of U-series nuclides by electron microprobe on zircons and monazites from Gandak river sediments

Abstract:

Determination of sediment transfer time during transport in the alluvial plains is a critical issue to correctly understand the relationship between climate, tectonics and Earth surface evolution. The residence time of river sediments may be constrained by analyzing the U series nuclides fractionations (e.g. [1] and [2]), which are created during water rock interactions by the ejection of the daughter nuclides of the grain (α-recoil) and the preferential mobilization of nuclides in decay damaged crystal structure.

However, recent studies on sediments from the Gandak river, one of the main Ganga tributary, highlighted the difficulties to obtain reproducible data on bulk sediments, due to the nuggets distribution of U-Th enriched minor minerals in the samples (Bosia et al., unpublished data).

We therefore decided to analyze the U and Th isotopic systematic at a grain-scale for Himalayan sediments from the Gandak river. This has been tested by performing in situ depth profiles of ²³⁸U-²³⁴U-²³⁰Th and ²³²Th on zircons and monazites (50-250 μm) by Secondary Ion Mass Spectrometry (SIMS) at the CRPG, Nancy, France. The first results point the occurrence of ²³⁸U-²³⁴U-²³⁰Th disequilibria in the outermost parts of both monazite and zircon minerals with a return to the equilibrium state in the core of the grains. The relative U and Th enrichment is however slightly different depending on considered minerals, suggesting possible adsorption processes of 230-Th.

Coupled to a simple model of U and Th mobility during water-mineral interactions, these data should help to constrain the origin of ²³⁸U-²³⁴U-²³⁰Th disequilibria in these minerals. Moreover, the results of the study should be relevant to discuss the potential of this approach to constrain the residence time of zircons and monazites in the Gandak alluvial plain.

[1] Chabaux et al., 2012, C. R. Geoscience, 344 (11-12): 688-703; [2] Granet et al., 2007, Earth and Planet. Sci. Lett., 261 (3-4): 389-406.