Other
Scientific paper
Aug 1975
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975gecoa..39.1077b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 39, Issue 8, pp.1077-1084
Other
69
Scientific paper
A model for the mixing of H 2 O and silicate melts has been derived from the experimentally determined effects of H 2 O on the viscosity (fluidity), volumes, electrical conductivities, and especially the thermodynamic properties of hydrous aluminosilicate melts. It involves primarily the reaction of H 2 O with those O -2 ions of the melt that are shared (bridging) between adjacent (Al, Si)O 4 tetrahedra to produce OH - ions. However, in those melts that contain trivalent ions in tetrahedral coordination, such as the Al 3+ ion in feldspathic melts, the model further involves exchange of a proton from H 2 O with a non-tetrahedrally coordinated cation that must be present to balance the net charge on the AlO 4 group. This cation exchange reaction, which goes essentially to completion, results in dissociation of the H 2 O and is limited only by the availability of H 2 O and the number of exchangeable cations per mole of aluminosilicate. In the system NaAlSi 3 O 8 -H 2 O, upon which this thermodynamic model is based, there is 1 mole of exchangeable cations (Na + ) per mole (GFW) of NaAlSi 3 O 8 , consequently ion exchange occurs for H 2 O contents up to a 1:1 mole ratio ( X m w = mole fraction H 2 O = 0.5). For mole fractions of H 2 O greater than 0.5, no further exchange can occur and the reaction with additional bridging oxygens of the melt produces 2 moles of associated OH - ions per mole of H 2 O dissolved. These reactions lead to a linear dependence of the thermodynamic activity of H 2 O ( a m w ) on the square of its mole fraction ( X m w ) for values of X m w , up to 0.5 and an exponential dependence on X m w at higher H 2 O contents. Thus, for values of X m w 0.5, a m w = k ( X m w ) 2 , where k is a Henry's law constant for the dissociated solute. Extension of the thermodynamic model for NaAlSi 3 O 8 -H 2 O to predict H 2 O solubilities and other behavior of compositionally more complex aluminosilicate melts (magmas) requires placing these melts on an equimolal basis with NaAlSi 3 O 8 . This is readily accomplished using chemical analyses of quenched glasses by normalizing to the stoichiometric requirements of NaAlSi 3 O 8 , first in terms of equal numbers of exchangeable cations for mole fractions of H 2 O up to 0.5 and secondly in terms of 8 moles of oxygen for higher H 2 O contents. Chemical analyses of three igneous-rock glasses, ranging in composition from tholeiitic basalt to lithium-rich pegmatite, were thus recast and the experimental H 2 O solubilities were computed on this equimolal basis. The resulting equimolal solubilities are all the same, within experimental error, as the solubility of H 2 O in NaAlSi 3 O 8 melt calculated from the thermodynamic relations. The equivalence of equimolal solubilities implies that the Henry's law constant ( k ), which is a function of temperature and pressure, is independent of aluminosilicate composition over a wide range. Moreover, as a consequence of the Gibbs-Duhem relation and the properties of exact differentials, it is clear that the silicate components of the melt, properly defined, mix ideally. Thus, a relatively simple mixing model for H 2 O in silicate melts has led to a quantitative thermodynamic model for magmas that has far-reaching consequences in igneous petrogenesis.
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