Boninites formed in deep hydrothermal fault zones at mid-ocean ridges: experimental evidence

Water-saturated partial melting experiments were carried out using a refractory harzburgite from the Oman Ophiolite as starting material. The experiments were performed at pressures of 100, 200 and 500 MPa using both reducing (corresponding to the FMQ buffer) and oxidizing (FMQ+3) conditions. Specially designed internally heated pressure vessels were used to control oxygen fugacity and allow rapid quenching. Temperatures varied between 980 and 1220°C, and run durations were up to 82 hours.  The solidus and clinopyroxene-out curve show significant variation with pressure.  As expected, the melts produced were generally SiO₂-rich, with SiO₂ concentrations ranging between 55 and 65 wt%. These melts exhibit boninitic characteristics. Due to the refractory character of the starting material, the experimental melts are highly depleted in incompatible trace elements, showing chondrite-normalized REE patterns with a characteristic concave-upward shape. Calcium and sodium in the system are mainly derived from the clinopyroxene in the starting harzburgite, resulting in extremely high Ca/Na ratios in the experimental melts. At temperatures above the clinopyroxene breakdown, the residual mineral paragenesis exhibits characteristics similar to extremely refractory harzburgites, with Cr# in Cr-spinel (Cr2O3 /(Al2O3 + Cr2O3), molar) reaching up to 86, reminiscent of ophiolites formed under supra-subduction zone conditions.

The melts produced have compositions of high-Mg andesite and boninite. Our experimental results show that the formation of distinct rock types within the paleocrust of the Oman Ophiolite such as high-Ca boninites, high-Si boninites, high-Mg andesites, depleted gabbronorite cumulate rocks, and extremely refractory harzburgites containing Cr-spinel with Cr# > 80, could, in principle, be attributed to a single process of fluid-induced partial melting of harzburgite below the crust/mantle boundary of the Oman paleocrust. The temperatures for the heating process (> 1040°C) for such a model, could be provided by ascending MORB magmas. The presence of water-rich fluids at the crust/mantle boundary or within the uppermost mantle which are necessary for such a model, could be derived from seawater via deep hydrothermal fault zones. We present amphibole data from deep hydrothermal fault zones in the lowermost gabbros of the Oman Ophiolite, which provide evidence that temperatures of deep hydrothermal fault zones are high enough to trigger the melting of hydrated harzburgites.