New insights on the Neogene erosion regime of the Himalaya: a Pb-Nd isotopic study of deep-sea fan sediments (ODP Leg 116). Abstract PP23A-1726 - Archive ouverte HAL Accéder directement au contenu
Poster De Conférence Année : 2010

New insights on the Neogene erosion regime of the Himalaya: a Pb-Nd isotopic study of deep-sea fan sediments (ODP Leg 116). Abstract PP23A-1726

Résumé

Pb- and Nd-isotopic time-series from the authigenic fraction of Central Indian Ocean sediments have been interpreted as primarily responding to changes in the relative amount of Himalayan erosion during the Cenozoic [1]. A more precise interpretation is, however, hampered by the lack of information about temporal changes in the isotopic composition of dissolved Pb and Nd carried by rivers draining the Himalaya. One approach has been to study the composition of detrital (silicate) sediment to reconstruct changes in sediment provenance and the timescales of when certain Himalayan lithological units were subjected to weathering. For example, Os-, Nd-, and Sr-isotopes have been studied in the deep-sea fan sediments in the Bay of Bengal and suggest a dominance of the High Himalaya in the source of the sediment for the last 20My [2]. The activation of the Himalayan Main Boundary Trust and the resulting exhumation of the Lesser Himalaya have been dated to have occurred around 10My ago and a small shift (~1ε unit) is observed in the Nd-isotopes of the sediments from the ODP Sites 717 and 718 (Leg 116) located in the Bengal Fan. The associated variations in the Pb-isotopes are, however, not known. Here, we present a Pb- and Nd-isotope study of the detrital fraction of deep-sea sediment cores from the Bay of Bengal (ODP Sites 717, 718) and of characteristic Himalayan river bedload. We report time-series for both the whole rock and the 2-50μm size fractions for the last 20My. The oldest whole-rock samples (>10My) are less radiogenic than the youngest samples (<4My), revealing a shift towards greater contribution of the Lesser Himalaya in the younger sediments. The average shift in εNd amounts to 1 unit and is associated with an average shift of 0.3 in the 206Pb/204Pb. During these two periods, the 2-50μm size fraction has the same pattern as the bulk sediment samples, with a general shift of 0.1 in the 206Pb/204Pb towards less radiogenic values isotopes due to mineralogical sorting. However, during the late Miocene-early Pliocene transition (8 to 4 My), the isotopic composition of the 2-50μm size fraction is less radiogenic and differs from the studied Himalayan riverine sediments. These differences cannot be accounted by an additional contribution of a hemipelagic source that has a Nd isotopic composition too radiogenic to account for the observed Pb-Nd-isotopes compositions of the Messinian and Pliocene sediments. We hypothesize a shift in the Pb isotopic composition of the eroded Himalayan crust toward less radiogenic values related to an intensification of the weathering during most of the Pliocene. Such a change in the weathering regime is supported by changes in the clay mineralogy associated to the intensification of the Indian monsoon and we will discuss the implications of such changes for the interpretation of the authigenic records. [1] Frank, M. et al., Chem. Geol., 226 (2006), 264-279. [2] Galy, V., et al., EPSL, 290 (2010), 474-480.
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hal-01813416 , version 1 (12-06-2018)

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  • HAL Id : hal-01813416 , version 1

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Julie Gattacceca, Albert Galy, Alexander M. Piotrowski, Martin Frank. New insights on the Neogene erosion regime of the Himalaya: a Pb-Nd isotopic study of deep-sea fan sediments (ODP Leg 116). Abstract PP23A-1726. AGU, 2010 Fall Meeting, Dec 2010, San Francisco, United States. pp.PP23A-1726. ⟨hal-01813416⟩
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