Water and magmas; a mixing model

摘要: Abstract A model for the mixing of H2O and silicate melts has been derived from experimentally determined effects on viscosity (fluidity), volumes, electrical conductivities, especially thermodynamic properties hydrous aluminosilicate melts. It involves primarily reaction with those O−2 ions melt that are shared (bridging) between adjacent (Al, Si)O4 tetrahedra to produce OH− ions. However, in contain trivalent tetrahedral coordination, such as Al3+ ion feldspathic melts, further exchange a proton non-tetrahedrally coordinated cation must be present balance net charge AlO4 group. This reaction, which goes essentially completion, results dissociation is limited only by availability number exchangeable cations per mole aluminosilicate. In system NaAlSi3O8-H2O, upon this based, there 1 (Na+) (GFW) NaAlSi3O8, consequently occurs contents up 1:1 ratio (Xmw = fraction 0.5). For fractions greater than 0.5, no can occur additional bridging oxygens produces 2 moles associated dissolved. These reactions lead linear dependence activity (amw) square its (Xmw) values Xmw, 0.5 an exponential Xmw at higher contents. Thus, X m w ↬ k(X ) , where k Henry's law constant dissociated solute. Extension NaAlSi3O8-H2O predict solubilities other behavior compositionally more complex (magmas) requires placing these equimolal basis NaAlSi3O8. readily accomplished using chemical analyses quenched glasses normalizing stoichiometric requirements first terms equal numbers secondly 8 oxygen Chemical three igneous-rock glasses, ranging composition tholeiitic basalt lithium-rich pegmatite, were thus recast experimental computed basis. The resulting all same, within error, solubility NaAlSi3O8 calculated relations. equivalence implies (k), function temperature pressure, independent over wide range. Moreover, consequence Gibbs-Duhem relation exact differentials, it clear components melt, properly defined, mix ideally. relatively simple led quantitative magmas far-reaching consequences igneous petrogenesis.