Fluid Enhanced Mass Transport in Deep Crust and its Influence on Element Abundances and Isotope Systems

Abstract

We report here the results of two studies designed to establish the chemical and isotopic compositional makeup of fluids in deep crustal settings. One study concentrates on a small shear zone where fluids associated with retrogression of granulites were probably aqueous, were CI-and N-rich and had significant sulfate and potassium contents. REE abundances in the shear zones are fractionated (La/Yb ~160), relative to the country rock (La/Yb ~22), with a 50% increase in the Nd/Sm ratio. The second study, dealing with granulite facies rocks in a large (> 40 km wide) shear zone, demonstrates that Rb-Sr systems are reset to varying degrees, on the scale of tens to thousands of sq meters. In some cases, complete resetting occurs of Archaean gneisses to Proterozoic ages with low Sr87/86 initial ratios. Other areas of the same region experience only weak disturbance of the original Archaean Rb-Sr systematics. The degrees of resetting are correlated with the degree of mineral homogenization, and suggest that fluid volume varies widely from region to region, although no clear lithologic characteristics delineate the different regions. Theoretical consideration of the chemical characteristics of the fluids which modified these two rock systems demonstrates 1) nitrogen may significantly modify C02/H20 ratios, 2) geothermal gradients strongly influence equilibrium fluid composition and solvent properties of the fluid, and 3) the relative proportions of the ionic species in the fluid are strongly dependent on fluid pH.