The importance of understanding the weathering processes of volcanic rocks has been shown by several authors (e.g. Meybeck, 1987; Gislason et al., 1994)). As the primary rocks of the Early Earth were volcanogenic, then understanding the processes that control alteration of volcanic terrains can potentially provide valuable information on the formation and evolution of the continental crust.
This work is undertaken within the context of a larger study of global geochemical systematics of large scale erosion by rivers (e.g. in the Congo basin (Gaillardet et al., 1995)) in order to assess the erosional processes of basaltic watersheds in relation with climate.
We discuss here the processes of chemical weathering by rivers on Reunion Island and, in particular, the influence of hydrothermal springs on the chemical compositions of those rivers. We then calculate the erosion rates and the consumption of atmospheric CO2 by chemical weathering of basalt. Finally, we attempt to assess the "stream equilibrium" using simple mass balance between dissolved and particulate loads of the rivers.
Thirteen rivers were sampled on Reunion Island during the rain season, in February 1995, for dissolved and particulate loads and sands from the river bedloads. Chemical compositions of those three phases were analysed for major and trace elements by HPLC and ICP-MS and stontium isotopic compositions were also measured.
For the most soluble elements (Na, Ca, Mg, K, Sr, Rb, Ba and U) it is apparent that the chemical compositions of Reunion rivers are the mixing products of both atmospheric inputs (mainly of oceanic composition), hydrothermal spring supply and of basalt weathering. We have estimated the different contribution of each source using an inversion method based on ionic ratios. Results show that the contribution of atmospheric inputs decreases with distance from the ocean. Hydrothermal supply is dominant for the three rivers draining caldeiras of ancient volcano of Piton des Neiges and are of particular importance for elements such as K, Ba and Rb. The corrected basaltic contribution to the rivers has the following order of concentrations: Ca>MgNa>K>Sr>Rb>BaU, and concentrations ratios Ca/Na, Mg/Na, K/Na and Sr/Na of 1.2, 1.0, 0.1 and 0.002, respectively, which are in agreement with ratios measured for french basaltic rivers (Meybeck, 1986).
As noted by Gaillardet and co-authors (1995) for the Congo basin, we observe a simple relationship between element concentrations of particulate and soluble phases of the Reunion rivers, the most soluble elements being enriched in the dissolved phase and depleted in the particulates. Given this, a mass budget model allows us to calculate the suspended sediment concentrations and to compare them with those measured in the rivers.
With total dissolved loads for the basaltic part of the rivers ranging from 40 to 300 mg/l, catchment surfaces of 35 to 100 km2, and mean annual discharges of 2.5 to 6 m3/s, we can estimate chemical erosion rates of 100 to 400 t/km2/yr. Associated consumption of atmospheric CO2 varies between 1000 and 4000 103 mol/km2/yr. Similarly we have estimated mecanical erosion rates of about 100 mm/kyr. Such high erosion rates are consistent with the high annual rainfall and small watersheds.
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