The effect of H2O on the olivine liquidus of basaltic melts: experiments and thermodynamic models

We designed and carried out experiments to investigate the effect of H2O on the liquidus temperature of olivine-saturated primitive melts. The effect of H2O was isolated from other influences by experimentally determining the liquidus temperatures of the same melt composition with various amounts of H2O added. Experimental data indicate that the effect of H2O does not depend on pressure or melt composition in the basaltic compositional range. The influence of H2O on melting point lowering can be described as a polynomial function C H2O in wt% : DT C ð Þ 1⁄4 40:4 C H2O 2:97 C H2O 2 þ 0:0761 C H2O 3 : This expression can be used to account for the effect of H2O on olivine-melt thermometers, and can be incorporated into fractionation models for primitive basalts. The non-linear effect of H2O indicates that incorporation of H2O in silicate melts is non-ideal, and involves interaction between H2O and other melt components. The simple speciation approach that seems to account for the influence of H2O in simple systems (albite-H2O, diopside-H2O) fails to describe the mixing behavior of H2O in multi-component silicate melts. However, a non-ideal solution model that treats the effect of H2O addition as a positive excess free energy can be fitted to describe the effect of melting point lowering. Introduction It has long been understood that water plays an important role in magmatic processes. Water has a large effect on the mantle solidus (Mysen and Boettcher 1975; Grove et al. 2006) and is responsible for the initiation of melting in subduction zones, as well as broadening of the melting zone under mid-ocean ridges (Asimow and Langmuir 2003). Water also drastically changes the liquid line of descent of basaltic magmas: it is responsible for the calcalkaline versus tholeiitic differentiation trend (Grove and Baker 1984), and small amount of H2O have been argued to have an important effect on the crystallization of mid-ocean ridges magmas (Michael and Chase 1987). Significant water contents have been inferred from primitive magmas in subduction-zone settings (e.g., Anderson 1979; Sisson and Grove 1993b; Sisson and Layne 1993; Sobolev and Chaussidon 1996), back-arc basins (Danyushevsky et al. 1993; Stolper and Newman 1994), intraplate settings (Dixon et al. 1997; Spilliaert et al. 2006), and even at some mid-ocean ridges (Danyushevsky 2001). However, even if the qualitative effect of water on magmatic processes has been long understood, quantitative estimates are scarce and often incompatible with each other. This is a consequence of the experimental difficulties inherent in carrying out hydrous experiments, including the ability to maintain a constant H2O content, and to quench water-rich hydrous melts to analyzable glasses. For example, empirical and thermodynamic models of the effect of water on the olivine-melt equilibrium show a very large variability (Fig. 1; Ghiorso and Sack 1995; Falloon and Danyushevsky 2000; Sugawara 2000; Ghiorso et al. 2002; Ariskin and Barmina 2004). The olivine-melt equilibrium is of critical importance for the understanding of Communicated by J. Hoefs. E. Médard (&) T. L. Grove Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Building 54-1212, Cambridge, MA 02139, USA e-mail: [email protected] 123 Contrib Mineral Petrol (2008) 155:417–432 DOI 10.1007/s00410-007-0250-4