The formation of SiO.sub.2-rich melts within the deep oceanic crust by hydrous partial melting of gabbros

Byline: Juergen Koepke (1), Jasper Berndt (2), Sandrin T. Feig (1), Francois Holtz (1) Keywords: Oceanic plagiogranite; Gabbro; Oceanic crust; Partial melting; Plagioclase Abstract: Small amounts of felsic, evolved plutonic rocks, often called oceanic plagiogranites, always occur as veins or small stocks within the gabbroic section of the oceanic crust. Four major models are under debate to explain the formation of these rocks: (1) late-stage differentiation of a parental MORB melt, (2) partial melting of gabbroic rocks, (3) immiscibility in an evolved tholeiitic liquid, and (4) assimilation and partial melting of previously altered dikes. Recent experimental data in hydrous MORB-type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust. Experiments show that TiO.sub.2 is a key parameter for the discrimination between different processes: TiO.sub.2 is relatively low in melts generated by anatexis of gabbros which is a consequence of the low TiO.sub.2 contents of the protolith, due to the depleted nature of typical cumulate gabbros formed in the oceanic crust. On the other hand, TiO.sub.2 is relatively high in those melts generated by MORB differentiation or liquid immiscibility. Since the TiO.sub.2 content of many oceanic plagiogranites is far below that expected in case of a generation by simple MORB differentiation or immiscibility, these rocks may be regarded as products of anatexis. This may indicate that partial melting processes triggered by water-rich fluids are more common in the deep oceanic crust than believed up to now. At slow-spreading ridges, seawater may be transported via high-temperature shear zones deeply into the crust and thus made available for melting processes. Author Affiliation: (1) Institut fuer Mineralogie, Leibniz Universitaet Hannover, Callinstrasse 3, 30167, Hannover, Germany (2) Institut fuer Mineralogie, Westfaelische Wilhelms-Universitaet, Corrensstrasse 24, 48149, Muenster, Germany Article History: Registration Date: 16/08/2006 Received Date: 01/04/2006 Accepted Date: 15/08/2006 Online Date: 16/09/2006 Article note: Communicated by J. Hoefs.