Controls on the Sr-87/Sr-86 ratio of carbonates in the Garhwal Himalaya, headwaters of the Ganges

Citation
Mj. Bickle et al., Controls on the Sr-87/Sr-86 ratio of carbonates in the Garhwal Himalaya, headwaters of the Ganges, J GEOLOGY, 109(6), 2001, pp. 737-753
Citations number
45
Categorie Soggetti
Earth Sciences
Journal title
JOURNAL OF GEOLOGY
ISSN journal
00221376 → ACNP
Volume
109
Issue
6
Year of publication
2001
Pages
737 - 753
Database
ISI
SICI code
0022-1376(200111)109:6<737:COTSRO>2.0.ZU;2-B
Abstract
The episodic variation of the seawater Sr-87/Sr-86 ratio has been attribute d to either variations in the Sr flux or the Sr-isotopic composition of the riverine-dissolved load derived from weathering of the continental crust. The discovery that Himalayan rivers are characterized by high concentration s of dissolved Sr concentrations with high Sr-87/Sr-86 ratios has raised th e possibility that collisional orogens play a critical role in moderating t he variations in seawater Sr-87/Sr-86 ratios. Here we describe Himalayan ca rbonates and calc-silicates from Garhwal, the headwaters of the Ganges, wit h extreme Sr-87/Sr-86 ratios (>1.0). Elevated Sr-isotope ratios result from exchange with Rb-rich silicate material during both Himalayan and pre-Hima layan metamorphic episodes, and the carbonates contribute a significant fra ction to the Ganges Sr-87 flux. Particularly elevated Sr-87/Sr-86 ratios ar e found in calc-silicates from the Deoban Formation of the Lesser Himalaya. A detailed traverse of shales and calc-silicates from this unit confirms t hat carbonate horizons have increased Sr-87/Sr-86 ratios as a result of iso topic exchange over length scales of 10-30 cm. We conclude that metamorphis m of carbonates may cause elevation of their Sr-87/Sr-86 ratios and that up lift of metamorphosed carbonates may be a consequence of collisional orogen s, which contributes to the elevation of seawater Sr-87/Sr-86 ratios.