THE SOLIDUS OF HARZBURGITE TO 3 GPA PRESSURE: THE COMPOSITIONS OF PRIMARY ABYSSAL THOLEIITE

Abstract

Abyssal tholeiite is generated by partial melting of the upper mantle. The mantle sections of ophiolite complexes show that partial melting leaves mantle residua that are lherzolitic or harzburgitic, harzburgite being the more depleted residuum. It is shown that some of the interstitial melt of partially molten harzburgite approximates a tholeiitic composition. Therefore, the solidus of harzburgite constitutes the maximum possible temperature for the generation of primary tholeiite. The MgO contents of primary tholeiites in equilibrium with harzburgite are determined to be about 21 and 23 wt.% MgO at 2.0 and 3.0 GPa, respectively. The possible MgO contents of primary abyssal tholeiite is suggested to range between 10 and 23 wt.% MgO, with the MgO content increasing with increasing pressure.