THE INDUS RIVER SYSTEM (INDIA-PAKISTAN) - MAJOR-ION CHEMISTRY, URANIUM AND STRONTIUM ISOTOPES

The Indus River is one of the large river systems draining the Himalay a. We report in this paper the major-ion chemistry, Sr and U isotope s ystematics of the Indus system, particularly its headwaters. The resul ts show that: (1) on an average about a third of the cations in the wa ters can be from silicate weathering; however, most of the (Ca + Mg) i s likely to be from the weathering of carbonates and evaporites; (2) t he Sr-87/Sr-86 of the waters ranges between 0.7085 and 0.7595, the hig her values (> 0.72) are typical of the tributaries draining the Precam brian granite/gneissic terrains. The Sr-87/Sr-86 of the Indus main cha nnel, throughout its entire stretch, shows only minor variations, 0.71 04-0.7116. The Indus transports approximately 8.8.10(6) mol Sr to the Arabian Sea annually with a Sr-87/Sr-86 of 0.7111 if our results of Sr concentration and Sr-87/Sr-86 measured at Thatta can be considered ty pical of the Indus throughout the year; and (3) the U concentration in the Indus and its tributaries is generally high, 0.37-10.3 mug 1(-1). The source for the high uranium can be the weathering of granites, zo nes of uranium mineralisation and black shales. The Indus results when compared with our earlier data on the Ganga-Brahmaputra show that in all these three river systems carbonate weathering is the dominant sou rce of (Ca + Mg) and HCO3 and that U concentrations are high. In the c ase of Sr-87/Sr-86, the Indus waters are less radiogenic; however, its tributaries draining the Precambrian granites/gneises have Sr-87/Sr-8 6 in excess of 0.72, similiar to that in the rivers of the Ganga syste m. The low Sr-87/Sr-86 of the Indus, 0.7111, suggests that its contrib ution to the Sr isotope evolution of oceans since the Cenozoic is less significant than that of the Ganga-Brahmaputra.