HR: 17:15h
AN: V12J-06    [PDF]
TI: Hydrous Partial Melting Within the Deep Oceanic Crust
AU: * Koepke, J
EM: koepke@mineralogie.uni-hannover.de
AF: Institut f�r Mineralogie, Universitaet Hannover, Callinstr. 3, Hannover, 30167 Germany
AU: Feig, S T
EM:
AF: Institut f�r Mineralogie, Universitaet Hannover, Callinstr. 3, Hannover, 30167 Germany
AU: Snow, J
EM: jesnow@mpch-mainz.mpg.de
AF: Max-Planck-Institute f�r Chemie, Postfach 3060, Mainz, 55020 Germany
AB: Our knowledge on the structure, composition, and mineralogy of the in-situ lower oceanic crust (layer 3) and the mechanism how it is formed is poor. Petrological models for its generation, seismic and thermal models require an effective cooling of the deep oceanic crust. The current model implies a conductive mechanism for the cooling, and hydrothermal circulation is regarded to play a small role in transport of heat and masses in the deep oceanic crust (Wilcock, 2003). Here we demonstrate that hydrothermal circulation within the gabbroic layer starts at much higher temperatures (900$\deg$ - 1000$\deg$C) as up to now believed. Water-rich fluids causes hydrous partial melting in a large scale, which is manifested by characteristic microtextures visible within many investigated rocks of all reference locations for oceanic gabbros (sampled by the Ocean Drilling Program (ODP Legs 147, 153, 176) and from the Oman ophiolite). The observed process has the potential for significant transfer of heat and masses between the upper and lower oceanic crust. The interpretation of the observed microstructures as products of hydrous partial melting is based on experimental work. Water-saturated melting experiments on a variety of natural gabbros between 900 and 1000$\deg$C at 200 MPa produced silicic melts similar in composition to oceanic plagiogranites (Koepke et al., 2003 in press). The newly formed minerals form a characteristic paragenesis consisting of plagioclase, orthopyroxene and pargasitic amphibole. In all experiments the An content of the new plagioclases is higher compared to that of the protolith, even at the lowest investigated temperature. It can be observed that olivine and clinopyroxene of the protolith react to orthopyroxene and pargasite. Very similar features can be observed in the natural gabbros. The most striking feature are zones within the plagioclase grains showing a strong enrichment in An component, often with An contents which are 20 to 25 mol% higher than those of the host plagioclase. Primary olivines and clinopyroxenes in contact with such zones very often react to orthopyroxene and pargasitic amphibole as in our experiments. These phases rim olivine and clinopyroxene and grows "interstitially", typical petrographic characteristics of a late-stage magmatic phase. Koepke J., Feig S.T., Snow J. and Freise M., 2003 in press. Contrib. Mineral. Petrol. Wilcock W.S.D., 2003. Geophys. Res. Abstr., 5: 13273.
DE: 1020 Composition of the crust
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting