Melting of fertile peridotite with variable amounts of H2O

Abstract

An empirical model of the conditions of equilibration of anhydrous and hydrous melts with lherzolite has been developed from the available experimental data. Anhydrous melts in equilibrium with olivine, orthopyroxene and clinopyroxene exhibit clear compositional dependences on pressure and total alkali (Na 2 O+K 2 O) content. With increasing pressure and decreasing alkali content, the equilibrium SiO 2 content of the melt decreases while its MgO concentration increases. In the latter case the relationship for natural bulk compositions is described by: MgO(weight%)= (11.14-1.262Alk) + (2.765+0.0945Alk)P Where Alk refers to total (Na 2 O+K 2 O) in the melt and P is in GPa. For a melt of given MgO content this can be rearranged to give a barometer: P = MgO-11.14+1.262Alk/2.765+0.0945Alk Similarly, the temperature of equilibrium can be expressed in terms of pressure and alkali contents: T (°C)= 1230-23.75(Alk)+(98+5.6[Alk])P The equilibrium MgO content of the melt declines by about 0.23% for every 1 weight % H 2 O added. This is equivalent to an apparent shift in pressure of about 0.06 GPa for every 1% H 2 O and is accompanied by a corresponding increase in SiO 2 content of the melt. As H 2 O is added, the equilibrium temperature declines approximately as: Liquidus depression (°C) ≅ 80(H 2 Owt%) 0.4 The above relationships were applied to a number of putative primitive magmas from arc environments, some of which had been studied experimentally to determine conditions of equilibrium with the 1herzolite assemblage. The empirical relationships seem to work well, generally reproducing measured equilibrium conditions to within about 0.3 GPa and 40°C. The compositions of primitive magmas from the Cascade arc indicate equilibrium with the mantle under conditions close to the crust-mantle boundary. High alumina olivine tholeiites, which are essentially anhydrous yield equilibrium conditions of about 0.9 GPa and 1275°C, in good agreement with experimental data. Hydrous basaltic andesites appear to have equilibrated with the mantle at about 1.4 GPa and 1170°C. The high temperatures at relatively shallow depths are consistent with melt-focussing and heat advection towards the wedge comer at the boundary of upper plate and subducting slab.