Mobility of U-series nuclides during basalt weathering: An example from the Deccan Traps (India)

Abstract

U-series disequilibria have been measured by TIMS and MC-ICP-MS in dissolved phases and suspended sediments of the main basaltic rivers from the Deccan Trap region (India). For dissolved phases ( 234U/ 238U), ( 230Th/ 238U), and ( 226Ra/ 238U) range between 1.11 and 1.28, between 0.03×10 −2 and 1.35×10 −2, and between 0.02 and 0.16 respectively. For suspended particles, ( 234U/ 238U), ( 230Th/ 238U), and ( 226Ra/ 238U) range from 0.95 to 1.1, from 1.23 to 1.59 and from 0.1 to 0.47 respectively. 230Th– 238U and 234U– 238U systems show direct links with indices of weathering. The data also indicate possible redistributions between the solid and aqueous phases and indicate that erosion processes do not presently operate at steady-state because present-day physical erosion rates are higher than expected (100–400 mm/ka versus 40 mm/ka for a steady-state process) for the Narmada and the Tapti basins. Models assuming either discrete or continuous particle leaching yield timescales for chemical erosion of 40–90 ka. In contrast with basins located in Northern latitudes, erosion in the Deccan basins does not appear to have been significantly disturbed during the last glaciation. 234U– 238U disequilibria in the dissolved phase of large rivers world-wide are also explored and two key factors are highlighted in this study: the dissolution of carbonates, releasing dissolved U at secular equilibrium and, the silicate weathering rate estimated at the scale of the watershed. In particular, a positive correlation is found between dissolved ( 234U/ 238U) and basaltic weathering rates for the Deccan that could reflect a key role of physical erosion since, for silicate monolithological basins, chemical erosion rates are primarily controlled by physical erosion rates. For basins with mixed lithology, dissolved organic matter also plays a key role in the preferential release of 234U from silicate minerals. ( 234U/ 238U) ratios in rivers therefore reflect a mixture between U released by carbonate dissolution and 234U preferentially released from silicate minerals located in high physical erosion and peat areas.