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Communication Dans Un Congrès Année : 2013

Pb Isotopic Composition of Himalayan Sediments

Résumé

Pb- and Nd- isotopic time-series from the authigenic fraction of Central Indian Ocean sediments have been interpreted as responding to changes in the relative amount of Himalayan weathering during the Neogene [1-3]. Such interpretation relies on the lack of change in the isotopic signature of the weathering in the Himalaya. Based on detrital records from the Bengal deep-sea fan, the Nd isotopic composition of the eroded rocks remained nearly constant for the last 20My [4, 5]. However, the associated variations in the Pb-isotopes are not known, and a more precise reconstruction is hampered by the lack of information about temporal changes in the isotopic composition of dissolved Pb and Nd carried by rivers draining the Himalaya. We present Pb- and Nd-isotope time series, from the bulk detrital and silt-sized fractions as well as the authigenic fraction of deep-sea sediment from Ocean Drilling Program Sites 717 and 718 in the Bengal fan, along with Pb- and Nd- isotopic compositions of Himalayan river bedloads. The oldest ODP samples (7-17My) have relatively uniform Pb- and Nd- isotopic compositions, characteristic of a stable input from the High Himalaya Series. The youngest samples (<1Ma) are also fairly uniform with a shift towards more radiogenic values, implying a greater contribution of the Lesser Himalaya Series. During the Pliocene (1-7Ma) the detrital fraction have more variable Pb- and Nd-isotopic composition associated with a decoupling between the bulk and silt-sized fractions. In detail, Pb-isotopic composition of the detritial is controlled by 1) the source (and correlated with 'Nd), 2) the sorting and the mineralogy as witnessed by relationship with Zr, and 3) weathering and preferential loss of radiogenic Pb. Our results suggest a strong variability in the erosion and weathering regime of the Himalaya over the Neogene with implication on the interpretation of the authigenic records in the signature of deep water of the Central Indian Ocean. [1] Frank et O’Nions (1998) Earth. Planet. Sc. Lett., 158, 121- 130 ; [2] Gourlan et al (2008), Earth. Planet. Sc. Lett., 267, 353-364 ; [3] Gourlan et al (2010) Quaternary Sci., 29, 2484- 2498 ; [4] France-Lanord, Derry & Michard (1993) Geol. Soc. Spec. Pub., 74, 603-621 ; [5] Galy et al (2010) Earth Planet Sc Lett, 290, 474-480

Dates et versions

hal-01813430 , version 1 (12-06-2018)

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Citer

Albert Galy, Alexander M. Piotrowski, Martin Frank, Julie Gattacceca. Pb Isotopic Composition of Himalayan Sediments. 23rd Goldschmidt Conference 2013, Aug 2013, Florence, Italy. pp.1135, ⟨10.1180/minmag.2013.077.5.7⟩. ⟨hal-01813430⟩
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