TY - JOUR
T1 - The biotite to Phengite reaction and mica-dominated melting in fluid + carbonate-saturated pelites at high pressures
AU - Thomsen, Tonny B.
AU - Schmidt, Max W.
N1 - Funding Information:
This work is part of the Ph.D. project of T. B. Thomsen, funded by ETH grant TH 38/02-1. We are grateful to J. A. D. Connolly, P. Ulmer and E. Reusser for discussions that improved the manuscript, and to J. A. D. Connolly and M. Caddick for help with the calculation of the pseu-dosection. Thanks go to R. Ru« tti, B. Kuhn and L. Caricchi for assistance with the microprobe, to K. Kunze for help with SEM, and to Signe for her support and patience. Reviews by K. Okamoto and two anonymous reviewers improved significantly a first version of the manuscript, and those by M. Hirschmann, J. Hermann and an anonymous reviewer the final version of the manuscript. Further, we thank J. Hermann for insisting on a comparison of our results with the petrogenetic grid of Hermann (2002), which was not contained in the first versions of the manuscript.
PY - 2008/10
Y1 - 2008/10
N2 - 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.
AB - 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.
KW - Biotite to phengite reaction
KW - Carbonated pelite
KW - HO, KO and CO in subduction zones
KW - High-pressure experiments
KW - KCMASH-CO
KW - Subsolidus and melting reactions
KW - Electron microprobe
KW - Experimental Petrology
KW - Piston cylinder
KW - Multi-anvil
UR - http://www.scopus.com/inward/record.url?scp=56849121265&partnerID=8YFLogxK
U2 - 10.1093/petrology/egn051
DO - 10.1093/petrology/egn051
M3 - Article
AN - SCOPUS:56849121265
VL - 49
SP - 1889
EP - 1914
JO - Journal of Petrology
JF - Journal of Petrology
SN - 0022-3530
IS - 10
ER -