HR: 0800h
AN: V31A-1409    [Abstracts]
TI: Fluid Induced Dehydration of the Mafic Lower Crust from Amphibolite to Granulite Facies: Nature and Experiment
AU: * Harlov, D E
EM: dharlov@gfz-potsdam.de
AF: GeoForschungsZentrum, Telegrafenberg, Potsdam, D-14473 Germany
AB: Natural evidence for the role of low H$_{2}$O activity fluids (CO$_{2}$ or concentrated brines) in the dehydration of H$_{2}$O-rich, mafic amphibolite facies rocks to H$_{2}$O-poor, Opx-bearing granulite facies rocks (700-900 C and 500-1000 MPa) for both highly localised dehydration zones (CO$_{2}$; cm's) as well as regional terranes (brines; km's) (Harlov and F”rster 2002, J Petrol, 43, 769) include the presence of Kfs micro-veins along Qtz-Plg grain boundaries; Plg grains metasomatised in a K-rich fluid; Mnz and/or Xn inclusions in the FAp grains; Bt enriched in Ti, F, and Cl; and FAp enriched in Cl and F. These features are not seen in the "source" amphibolite facies terrane along the same traverse. When log(fHF/fH$_{2}$O) for either Bt or FAp is plotted as a function of the distance from the fluid/heat source, a uniform decrease in log(fHF/fH$_{2}$O) is observed across the granulite to amphibolite facies traverse suggesting the presence of a uniform low H$_{2}$O activity uniform fluid front. Dehydration experiments (900 C; 1000 MPa; 3 weeks; Au capsule; quenched) involving a cylinder of natural tonalitic Bt gneiss (Plg, Qtz, Bt) (220 mg) and a concentrated KCl brine (20-30 % H$_{2}$O; 70-80 % KCl) (8 mg) placed at the base of the cylinder have been conducted in the piston cylinder apparatus (CaF$_{2}$ setup). Micro-veins primarily of Kfs, with some evidence of partial melting, formed along Qtz/Plg grain boundaries though only where Bt and Qtz were in contact. Here the Bt reacted with Qtz to form numerous small Opx and Cpx grains as well as minor Ilm from the 2-3 wt % of TiO$_{2}$ present in the Bt. The two principle reactions responsible for both the formation of the Kfs micro-veins as well as the pyroxenes include: (1) An (in Plg) + Qtz + KCl (in fluid) = Kfs + CaCl$_{2}$ (in fluid) and (2) Bt + Qtz = Opx + Kfs + H$_{2}$O. The same experiment performed under the same P-T conditions involving either a concentrated NaCl brine (20-30 % H$_{2}$O; 70-80 % NaCl) or a CO$_{2}$-rich fluid (80 % CO$_{2}$, 20 % H$_{2}$O) or a fluid absent dry melt resulted in micro-veins approximating a granitic composition along Qtz/Plg grain boundaries with numerous small Opx grains + minor Ilm forming along biotite grain boundaries, again only when the Bt and Qtz were in contact. Due to an absence of KCl in these three cases, only reaction (2) was relevant.
DE: 5114 Permeability and porosity
DE: 5139 Transport properties
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting