Advective fluid transport during charnockite formation; an example from southern India

Abstract

Isotopic studies of incipient charnockite formation in southern India suggest that isotopic shifts in δ 13C do not coincide with the dehydration reaction front. The relative velocities of the dehydration reaction front and the carbon isotopic front may be calculated assuming that their propagation is due to advection of an infiltrating CO 2-rich fluid. The velocity of the reaction front is a function of the fluid flux rate, the difference in CO 2 content of the fluid phase across the front and the volume rate of water production by the dehydration reaction. The velocity of the carbon isotopic front is also a function of the fluid flux rate, as well as the CO 2 content of the fluid and the graphite content of the gneiss. Using modal and mineral composition data from a graphite-bearing incipient charnockite in southern India we calculate that the carbon isotope front has propagated at about twice the velocity as that of the reaction front. Separation between reaction and carbon fronts constrains the distance over which propagation has occurred; preliminary data suggest that in some charnockite localities from southern India infiltration occurred over a limited distance ( < 2 m), and the minimum fluid/rock volume ratio required for advective propagation of the carbon front in graphite-bearing localities is about 0.1.