Abstract
The amount of aqueous fluids circulating into the oceanic crust can be estimated using mass balance equations based on stable isotope exchange between rock and water. Unlike oxygen and strontium, isotopic exchange of trace elements (such as B or Li) between fluids and rocks, operates along with a chemical evolution of the rocks (e.g. a large enrichment of B or Li) that must be integrated into any model of water-rock interaction. We propose a general dimensionless mass balance equation for single-pass open systems that describes the equilibrium elemental distribution and the isotopic composition of reacting rocks as a function of the amount of circulating water. Water-rock ratios calculated from B compositions of hydrothermally-altered basalts range from 8 to 100. They are lower than those previously published (most W/R > 300) but comparable to those inferred from Sr isotope ratios measured in the same samples (3 < W/R < 30). Similar low water-rock ratios from 2 to 20 are calculated from Li isotope compositions of altered basalts and serpentinized peridotites.
Highlights
The interactions between fluids and rocks in geological settings are commonly quantified using the principle of stable isotope exchange at equilibrium for calculating water-rock ratios (W/R)
The use of stable isotope ratios of trace elements needs to take into account the variation of the elemental ratio between fresh and altered rocks as a function of the amount of water reacting with rocks
High variations are reported for boron (~ 0.5 ppm in fresh MORB, and up to 100 ppm for altered oceanic basalts; Smith et al, 1995) and for lithium (~ 4 ppm in fresh MORB and up to 75 ppm for low temperature altered basalts; Chan et al, 1992) with increasing rates of alteration of the oceanic crust
Summary
The interactions between fluids and rocks in geological settings are commonly quantified using the principle of stable isotope exchange at equilibrium for calculating water-rock ratios (W/R). The main applications have involved the 18O/16O (Taylor, 1977; Gregory and Taylor, 1981), D/H (Taylor, 1978; Sakai et al, 1991) and 87Sr/86Sr ratios measured in altered and fresh rocks (Albarède et al, 1981; 1995) Such techniques provide minimal values of water-rock ratios depending on both isotope partitioning and kinetics of exchange between rock and water. The use of stable isotope ratios of trace elements needs to take into account the variation of the elemental ratio between fresh and altered rocks as a function of the amount of water reacting with rocks. This parameter may be neglected when modeling water-rock interactions with oxygen and strontium since their concentrations in rocks and reacting water remain quite constant as reactions progress. We propose to formulate a mass balance equation for open systems that could be applied to trace element stable isotope ratios
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