Journal of Petrology Advance Access published online on October 29, 2008
Journal of Petrology, doi:10.1093/petrology/egn051
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The Biotite to Phengite Reaction and Mica-dominated Melting in Fluid + Carbonate-saturated Pelites at High Pressures
Institute for Mineralogy and Petrology, Eth Zürich, Clasiusstrasse 25, CH-8092 Zürich, Switzerland
Received November 27, 2006; Revised typescript accepted September 19, 2008
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Abstract |
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Subsolidus and melting experiments were performed at 2·0–3·7 GPa and 750–1300°C on a carbonated pelite in the model system K2O–CaO–MgO–Al2O3–SiO2–H2O–CO2 to define stabilities of potassic micas and fluid-present melting reactions. The biotite to phengite reaction occurs at pressures between 2· 4 and 2·6 GPa for temperatures of 750–850°C, and the amphibole to clinopyroxene reaction from 2·0 GPa, 875°C to 2·5 GPa, 740°C. Dolomite is the carbonate phase stable at subsolidus conditions. The biotite to phengite reaction preserves K2O, but is not H2O conservative, as a fluid is produced from the decomposition of zoisite. Phengite + quartz control fluid-saturated melting at a pressure (P) >2·6 GPa, whereas biotite + quartz dominate at P <2· 4 GPa. Incongruent melting occurs through the reactions phengite or biotite + zoisite + quartz/coesite + fluid = silicate melt + clinopyroxene + kyanite. Overstepping of the solidus, located at 850–950°C, results in 7–24 wt % metaluminous K-rich granitic melts. The experiments define the melting surface of the model system, projected from kyanite + quartz/coesite + fluid onto the K2O–CaO–MgO plane. The solidus melts in the studied system occur at a peritectic point consuming mica + zoisite and forming clinopyroxene. With increasing temperature (T), carbonated pelites then evolve along a peritectic curve along which further clinopyroxene is produced until zoisite is exhausted. This is then followed by a peritectic curve consuming clinopyroxene and producing garnet. A comparison of CO2-bearing with CO2-free experiments from the literature suggests that the main effect of adding calcite to a continental sediment is not the minor shift of typically 20–30°C of reactions involving fluid, but the change in bulk Ca/(Mg + Fe) ratio stabilizing calcic phases at the expense of ferromagnesian phases. The experiments suggest that in most subduction zones, CO2, H2O and K2O will be carried to depths in excess of 120–150 km through carbonates and K-micas, as partial melting occurs only at temperatures at the uppermost end of thermal models of subduction zones. Nevertheless, the release of fluid through P-induced decomposition of amphibole and zoisite provides some H2O for arc magma formation. Melting at higher temperatures (e.g. resulting from slower burial rates or from incorporation of subducted crust into the mantle) will produce potassic granitic melts and provide a substantial volatile and K source for the formation of arc magmas.
KEY WORDS: biotite to phengite reaction; carbonated pelite; high-pressure experiments; KCMASH–CO2; subsolidus and melting reactions; H2O, K2O and CO2 in subduction zones
*Corresponding author. Present address: Institute of Geological Sciences, University of Bern, CH-3012 Bern, Switzerland. Telephone: +41 (0)31 631 87 61. E-mail: thomsen{at}geo.unibe.ch