The reaction mechanism of fluid-induced partial melting of gabbro in the oceanic crust

Anatexis at the roof of an oceanic magma chamber at IODP Site 1256 (equatorial Pacific): an experimental study

“…Thus, a starting material grain size of 125 to 200 µm was used here in order to obtain larger melt pools in low-temperature experiments, under virtually equilibrium conditions. Additionally, for low-temperature experiments close to the solidus we used "micro-rocks" as starting material (mm-sized fragments of the whole rocks; see Wolff et al 2013) in order to obtain melt pools large enough for analysis. To ensure comparability among starting compositions, deionized water was added to the majority of the experiments in order to guarantee water-saturated conditions.…”

“…One study that included experiments at low pressure (100 MPa) was performed by Beard and Lofgren (1991) who systematically investigated the effects of pressure and water on different metamorphosed protoliths. For simulating anatectic processes operating in the deep oceanic crust (e.g., Koepke et al 2005a,b), partial melting experiments were performed using oceanic gabbro as starting material (Koepke et al 2004;Wolff et al 2013;Koepke et al 2014). However, these studies used fresh, rather primitive cumulate gabbro as starting material, and did not address the special compositional nature of the AML roof rocks which consist of a rock spectrum ranging from hydrated, altered basalts to dry granoblastic hornfels.…”

“…Thus, the possibility of disequilibrium melting and the subsequent effect on interpretation of results should be considered. Wolff et al (2013) addressed the potentially unsystematic effects caused by unreacted starting material cores which do not take part in the general melting reaction on both phase relations and phase compositions. Because the effect is strongest at low temperatures, this should be considered when comparing low-temperature runs with those performed at higher temperature.…”

Zircon record of fractionation, hydrous partial melting and thermal gradients at different depths in oceanic crust (ODP Site 735B, South-West Indian Ocean)

“…Both processes produce high Si melts, but partial melting produces melts with lower Ti (Koepke et al 2007) and REE contents (Brophy 2009). Also, microstructural record in gabbros from Site 735 drilled during IODP Leg 176, particularly the formation of plagioclase strongly enriched in anorthite content at grain boundaries, indicates that felsic melts were produced during the late stage evolution by hydrous partial melting of just frozen cumulate rocks (Koepke et al 2004;Wolff et al 2013). Experimental results, simulating hydrous gabbro melting show that extreme modal changes occur over the first 10% of melting with plagioclase abundance decreasing by over 20% and that of amphibole increasing from 0 to 50% (Koepke et al 2004).…”

“…In particular, REE concentrations should increase in silicic melts formed by fractional crystallization as compared to the melts formed by gabbro melting (Brophy 2009). However, recent experiments of Wolff et al (2013) produced small amounts of unusually high P, Zr and Ti melt, immiscible from the silicic melt. Such melt is expected to be rich both in incompatible elements and Nb and Ta, and therefore, crystallization of the Group 3 zircon in such a melt cannot be excluded.…”