Re-Os Isotopic Characteristics of Himalayan River Sediments and Source Rocks

Abstract

The 187Os/188Os ratio of the oceans has increased markedly during the Cenozoic Era, particularly during the past 15 My (Pegram et al., 1992; Peucker-Ehrenbrink et al., 1995). Some authors attribute this rise to increased weathering caused by the Himalayan collision, in analogy with similar models proposed to explain the seawater Sr isotopic record. In order to test this hypothesis we analysed sediment samples from major rivers and several tributaries in Central Nepal and Bangladesh, as well as potential source rocks in the various formations of the Himalayan Range. The Re-Os extraction techniques described by Birck et al. (1997) were used for bulk sample analyses, while leachates were prepared using the procedure described by Pegram et al. (1992). The Himalayan range can be divided into three main units. These are, from N to S: (1) the Tethyan Sedimentary Series (TSS), composed primarily of sediments of Cambian to Eocene age; (2) the High Himalaya Crystalline series (HHC), comprising mostly highly metamorphosed paragneisses and leucogranites; and (3) the Lesser Himalaya (LH), composed mainly of less strongly metamorphosed metasediments with Sr and Nd ratios indicative of very long crustal residence ages. In all three units, several sediment samples were collected in small river basins confined to single lithologic terrains. In the LH, sediments were also collected from larger rivers and thus include contributions from the lithologic units (TSS + HHC) further upstream. The 187Os/lSSOs ratios (Fig. 1) of the analysed river sediments and rocks are quite variable. Nevertheless, we can distinguish several systematic features. (1) The TSS samples are rather unradiogenic (187Os/188Os between 0.9 and 1.I for the sediments, 0.7 and 1.4 for the source rocks) and have concentrations varying between 20 and 106 ppt. (2) The HHC rocks have 187Os/188Os ratios between 0.9 and 2.6 and HHC sediments, with ratios of about 1.7, indicate a rather radiogenic average composition. Os concentrations of both rocks and sediments are quite low, between 7 and 50 ppt. (3) The LH is very heterogeneous. 187Os/188Os varies from 0.7 to 3.0 among the sediments derived solely from this formation and from 1.4 to 7.8 among the rocks. The concentrations are also extremely variable, ranging from less than 10 to 500 ppt. Notably, the source rock samples that have the highest Os isotopic ratios, the organic-rich metasediments, also have the highest Os concentrations. (4) River bedload from major rivers draining the whole system have 187Os/188Os ratios between 2.0 and 4.0 and their composition appears to be essentially buffered by the contribution of the Lesser Himalaya erosion. (5) Three sediments from the Indo-Gangetic Plain were analysed. The single sample from the Ganges has a 187Os/188Os ratio (2.60) consistent with those of sediments collected in one of the major Ganges tributaries at the outflow of the Himalayan range, as well as with the Ganges leachate Os isotopic ratio measured by Pegram et al. (1994). In contrast, 2 samples from the Brahmaputra have Os isotopic ratios of about 0.6, which is lower than any value measured among the Central Nepal samples. The reason for the non-radiogenic nature of the Brahmaputra samples (loss of radiogenic Os, mineralogical sorting, or change in the composition of the Himalayan source rocks from west to east along the chain) remains to be clarified. The highly radiogenic character of many river sediments, including those at the outflow of the range, can be explained by the addition of a small fraction (< 5%) of organic-rich sedimentary rocks with high Os concentrations and strongly radiogenic Os isotopic ratios to less radiogenic sediments derived from the HHC or the LH formations. Sr and Nd isotopic data indicate that the LH formation does not contribute more than 35% of the sediment bedload at the outflow of the range. Despite the limited volumetric input from this formation, LH black shales may have a disproportionately large