Felsic Melt Generation at the MOR Magma Chamber Roof: Trace Element Evidence of Experimental Hydrous Partial Melts for Anatectic Processes at the East-Pacific Rise

Felsic lithologies in oceanic crust environment are volumetrically small but occur frequently. Based on experimental and geochemical studies, different models for their generation are suggested, as fractional crystallization, partial melting of mafic lithologies, and liquid immiscibility. Geochemical studies on felsic lithologies from fast-spreading ridge systems imply that partial melting of previously hydrothermally altered mafic lithologies at the gabbro/dike transitions may play an important role (e.g., Wanless et al., 2010). For a detailed study of this process, we simulated experimentally anatexis at the gabbro/dike transition. In order to evaluate the potential of MORB contamination by anatectic melts, trace elements of the experimental melts were analyzed in-situ by applying secondary ion mass spectrometry (SIMS). As starting material we used rock powder (125-200 μm) of different hydrothermally altered dikes and basaltic hornfelses (so-called granoblastic dikes) from the base of the sheeted dike complex of the IODP site 1256 (East-Pacific Rise, EPR). Such lithologies are assumed to undergo partial melting due to an upward moving of the axial melt lens after replenishment, while granoblastic lithologies are regarded as restitic material of anatectic processes (France et al., 2010). Partial melting experiments under water-saturated conditions were performed in internally heated pressure vessels (IHPV) under conditions similar to those prevailing at the base of the sheeted dike complex (i.e. 100 MPa, 910 to 1030°C, fO2=ΔQFM+1). Our results show that melting of altered basalt with melt fractions less than 20 % (corresponding to temperatures ≤ 970°C) exhibit residual phases perfectly matching those observed in basalts with granoblastic texture (i.e. clinopyroxene, orthopyroxene, plagioclase, magnetite). Anatectic melts of these low degree melting experiments show trace element pattern which are very similar to those of natural dacites from the EPR. Characteristic features of the compositions of the experimental melts are significantly enrichment in Zr-Hf, most distinctive for LREE, and depletion in Sr, Ti and V. Calculated trace element content of the experimental residual is in the same range as those measured in basalts with granoblastic texture. Our results suggest that anatectic melts formed at the gabbro/dike transition have the potential to contaminate primary MORB significantly. These data provide new insight into details of the process of MORB contamination.