Partial melting of hydrated harzburgite at shallow pressures: An option to produce melts with boninitic characteristics

Abstract

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 h. 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. Fractionation of such melts could potentially yield highly depleted gabbronorites as cumulate rocks, with anorthite (An) contents of plagioclase generally in excess of 90 mol%. 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.

While most of the melts produced have compositions of high-Mg andesite, only a few have compositions of true boninites. It is clear, therefore, that the partial melting of hydrous harzburgite at shallow pressure is not a general model for the formation of typical boninites in subduction zone initiation environments. However, 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. In support of this contention, we present new amphibole data from cores drilled in the lower gabbros of the Oman Ophiolite by the International Continental Scientific Drilling Program (ICDP; Oman Drilling Program), which provide evidence that temperatures of deep hydrothermal fault zones are high enough to trigger the melting of hydrated harzburgites.