THE AMPHIBOLITE-ECLOGITE TRANSFORMATION - AN EXPERIMENTAL-STUDY ON BASALT

Phase relationships and phase compositions through the transformation from feldspar-amphibolite to eclogite have been experimentally investigated in a basaltic system, with specific attention given to amphibole crystal-chemistry as a monitor of P-T conditions. A crystal-glass mixture corresponding to an olivine basalt (ol = 4 percent; hy = 20 percent), in the system Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O has been investigated between 0.8 and 2.6 GPa, at 650 degrees C, and 0.8, 1.2, and 1.4 GPa at 550 degrees C. Runs in a piston cylinder apparatus were up to 860 hrs in duration, were water saturated, and were buffered at fO(2) equal to NNO. At 650 degrees C, the assemblage amphibole + plagioclase + epidote + quartz is stable at 0.8 GPa. Garnet appears at 1.0 GPa and coexists with albite up to 1.4 GPa. From 1.6 GPa omphacite replaces albite to give the association amphibole + Na-clinopyroxene + garnet + epidote + quartz. Paragonite crystallizes as a minor phase on albite seeds from 1.7 to 2.2 GPa. Amphibole breaks down above 2.3 GPa to give a fine intergrowth of omphacite and Mg-rich chloritoid. The stable assemblage at 2.6 GPa 650 degrees C is garnet (27 percent), omphacite (52 percent), chloritoid (7 percent), epidote (4 percent), quartz (10 percent). This represents the first experimental, analogue of natural chloritoid bearing metagabbros. Microprobe investigations on synthetic mineral phases have shown: 1. Amphibole: tschermak exchange (Al-VI) increases with pressure when amphibole coexists with garnet, plagioclase, epidote, ana quartz; conversely a decrease of tschermak with pressure is observed in the presence of clinopyroxene and garnet. The maximum Al-VI found is about 1.3 atoms per formula units (a.p.fu.) at plagioclase-out. Ca continuously decreases from 1.8 a.p.f.u. at 0.8 GPa down to 0.7 at 2.2 GPa at 650 degrees C. Using a recalculation scheme with Fe3+/Fe-tot = 0.2, Na-M4 increases with P up to 1.7 GPa, where paragonite stabilizes, and then it remains constant, suggesting a significant cummingtonite substitution with increasing pressure. Fe, Mg, and Fe/Mg are strongly dependent on pressure and mineral assemblage. Present data also suggest a moderate dependence of Fe/Mg on temperature. No systematic variation of Na in the amphibole A-site with pressure was found. 2. Clinopyroxene: a uniform jadeite fraction (0.45-0.50) characterizes clinopyroxene solid solution from 1.6 up to 2.6 GPa at 650 degrees C. Lower X(jd) (0.28) is obtained at 1.4 GPa and 550 degrees C. 3. Garnet: an increase in pyrope and grossular components is observed from 2.0 GPa (X(py) = 0.15, X(gr) = 0.20) to 2.6 GPa (X(py) = 0.25, X(gr) = 0.25). Garnet-pyroxene thermometry testifies to attainment of equilibrium. 4. Chloritoid: X(Mg) increases from 0.38 at 2.4 to 0.45 at 2.6 Gpa confirming; a major pressure effect on Mg-chloritoid stability. Modeling of amphibolite-eclogite transformation by mass-balance, including a reaction space calibration, and by a series of multivariant reactions shows a complex isopleth topology for amphibole cation substitutions in P-T projections. The notion of a dehydration front in the subducting slab is not viable if progessive dehydration occurs through continuous reactions involving chlorite, amphibole, paragonite, epidote, chloritoid. The beginning of eclogite hydration during the exhumation of a thickened crust may be monitored by aluminium behavior in amphibole at eclogite facies conditions.