Carbonic Fluids in Granulites: Cause or Consequence?

Abstract

From a generally fluid-free deep crust, carbon dioxide influx may occur related to the highest thermal anomaly attending granulite formation. This would manifest itself as high density fluid inclusions captured within different minerals. The transition from H₂O to CO₂ dominated fluids and the concomitant increase in CO₂ densities with metamorphic grade ohserved in regional amphibolite-granulite transitions, and the spectacular increase in the volume of CO₂ recorded from gneiss-incipient charnockite reaction fronts suggest that carbonic fluids cause granulite formation by the dilution of pore fluids and resultant lowering of water activity. Conversely, carbonic inclusions can also result as a consequence of granulite formation. This involves H₂O loss from pre-existing mixed CO₂-H₂O inclusions, by ductile strain-induced leakage during deformation, removal of H₂O to form retrograde hydrous assemblages, or by the absorption of H₂O in local anatectic melt fractions; in all these cases, pure CO₂ "residue" will result.

CO₂ influx need not necessarily manifest itself in charnockite formation alone; the major imprints of this process in aluminous metapelites could be stable isotope alteration. Highest density CO₂ inclusions in a rock need not necessarily represent the peak metamorphic fluid; entrapment and evolution of fluids are dictated by the nature of post-metamorphic P-T-fluid history. Desiccation along structural pathways of enhanced fluid flow is a viable mechanism to destabilize hydrous minerals, but the potential of such a process in generating local melt pockets is a topic which merits further consideration.

Models for the generation of CO₂ are diverse, and can be broadly grouped into internal and external sources. Stable isotope systematics of fluid inclusions and graphites from the southern Indian granulites attest to distinct pulses of external CO₂ often copious and at timescales short of attaining isotopic equilibrium between the influxing fluid and the precipitating graphite. This study proposes a unified model which links Pan-African syenite-chamockite associations and incipient charnockitic alteration of gneisses. Fluid-rich igneous activity of subcrustal origin is envisaged to have contributed the heat and CO₂ necessary to drive structurally-controlled dehydration reactions.