Experimental investigation of gabbro partial melting in the presence of NaCl-rich fluid - implications for the genesis of oceanic plagiogranites

We present results of the experimental study designed to assess the role of NaCl-rich hydrous fluids (modeled seawater-derived fluid) on partial melting of gabbroic rocks. Three contrasting compositions, i.e., olivine-bearing gabbro, gabbro-norite and Fe-Ti-gabbro, were investigated experimentally in the presence of the hydrous fluids with and without an excess of NaCl. The experiments were conducted in the range of pressures (100-200 MPa), temperatures (800-1040°C) and redox conditions (FMQ - FMQ+3). Partial melting in the presence of single aqueous fluid of low salinity (< 20 wt %NaCl) does not show any significant differences from the partial melting in the presence of salt-free aqueous fluid. In contrast, the presence of large amounts of NaCl (20-50 wt % in the fluid) and formation of saline-rich liquid (brine) causes the dramatic decrease in silica concentration of the partial melts and thus is not a premise to produce natural plagiogranites at investigated conditions. However the presence of NaCl may have played an important role at lower temperatures above the hydrous (only H2O-bearing fluid) solidus of the system in the presence of more complex fluids (saline and with low aH2O). Recently, J.Brophy proposed to use SiO2-REE diagrams for natural systems to decode the consequences of ideal fractionation of the basaltic melt and batch melting of the gabbro in application to the genesis of plagiogranites in mid-ocean ridge environments (e.g. Brophy, 2009). He argued that, for liquids with SiO2 greater than ~62 wt. %, hydrous melting of gabbroic cumulate should yield a negative correlation between REE abundances and increasing SiO2, while fractional crystallization of mid-ocean ridge basalt should produce a positive correlation. Our new experiments and trace element determinations in runs with large melt pools can be used to test this model. We observed the depletion of HREE (Yb) with increasing SiO2 (> 62 wt%) well predicted by the model of Brophy. In contrast, concentrations of LREEs (La) in our partial melts vary in the range of only few ppm and definitely do not show a strong (5-fold !) enrichment predicted by the model of Brophy. These data may indicate that the process of hydrous partial melting occurring in nature can not be addressed by simple simulations, assuming an ideal batch melting model. From the other hand, the experimental data presented here represent a snapshot of the hydrous partial melting process proceeded in the time interval between 2 days up to 1 week. Such short experimental times were possibly not sufficient to achieve the full re-equilibration of the partial melts with their solid residue at experimental conditions, especially with respect to trace elements.