Recent discovery of multiphase solid inclusions (MSI) in peak minerals from ultra-high pressure (UHP) terranes opened up new prospects for understanding the fluid-rock interaction during deep subduction in both crust and mantle. The first report on MSI in UHP rocks was from Dora-Maira (DM; Case Parigi; western Alps) whiteschists, more than ten years ago (Philippot et al, 1995, CMP, 121, 29-44). Nevertheless, the nature of such a fluid, and its role on the origin of the unusual composition of these rocks is still matter of debate. We report data on inclusions in DM UHP pyropes and HP prograde kyanite, part of them from a new sampling site (SSW Case Parigi). Primary MSI (30 micron) are present only in small UHP pyropes (1 - 6 cm) and often show post-entrapment decrepitation. Each MSI contains Mg-chlorite, Na-phlogopite, minor Cl-rich apatite, talc, pyrite, magnesite, Ca-rich chlorides +/- liquid water. Maps of total water concentrations collected in MSI-rich pyropes by infrared synchrotron radiation show gradients that suggest considerable H diffusion from inclusions into the host garnet (Frezzotti et al, 2007, abstract ECROFI XIX). In prograde kyanite, rare fluid inclusions are high salinity brines, containing different salts. Present data indicate that at HP conditions brines were present in the rocks and that at UHP peak aqueous fluids were enriched in Si, Al, Mg, Na, Ca, but still containing significant amounts of Cl, P, S, C. DM whiteschists are commonly considered metasomatic rocks from a granitic protolith. Our data on MSI in UHP pyrope and on rare brines in prograde HP kyanite strongly support metasomatism by external high-Ca-Mg fluids, probably evolved during serpentinite dehydration as proposed by Sharp and Barnes (2004, EPSL, 226, 243-254). Present data support the model of Compagnoni and Hirajima (2001, Lithos, 57, 219-236), who proposed that metasomatic fluids were introduced into the system during prograde metamorphism, channelled along shear zones cutting across the precursor granitoid/orthogneiss.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 3600 MINERALOGY AND PETROLOGY;
- 3613 Subduction zone processes (1031;