It seems to be generally accepted that addition of water to mantle compositions has a dramatic effect on the compositions of the silicate melts produced. Thus, for example, experiments at close to H2O saturation in the 70's demonstrated expansion of the liquidus field of olivine and implied that liquids of "andesitic" composition could be generated by partial melting of water-saturated peridotite. At that, time, however, experimental difficulties cast doubt on some of the conclusions. As part of a study of high Mg-andesites from arc settings, we have re-visited the question of the effects of P, T, H2O (and other minor components) on the compositions of mantle melts. We began with dry lherzolite melting in the systems CMAS, NCMAS and natural bulk compositions. Simple and systematic variations in SiO2 and MgO contents of lherzolite-saturated melts with pressure and alkali content can be observed. Specifically, SiO2 concentration increases with decreasing pressure and increasing total alkalis while MgO shows exactly opposite behavior. The effects of H2O, at least up to ~10 weight per cent are much less obvious. There is a slight depression of MgO content and, surprisingly, little systematic effect on SiO2. It appears, therefore that, in the concentration range of relevance to arc settings, H2O may not be particularly important in mantle melting except for its effect on liquidus temperature. We then turned to the question of how to generate high Mg-andesites which precipitate olivines of Fo90 composition if water isn't important. Harzburgitic rather than lherzolitic residue is the most obvious answer. Loss of clinopyroxene from the residue means that Ca and Al contents of the melts decrease while Si and Mg increase. In terms of oxides we find that MgO and SiO2 increase in the approximate ratio 3:1 after cpx is lost. Thus a "typical" high Mg-andesite with 55 SiO2, 10 MgO and 3.5 per cent total alkali oxide could be generated at around 1 GPa without water in equilibrium with a harzburgite residue.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 3619 Magma genesis and partial melting (1037);
- 3630 Experimental mineralogy and petrology