ALKALI EXCHANGE EQUILIBRIA BETWEEN A SILICATE MELT AND COEXISTING MAGMATIC VOLATILE PHASE: AN EXPERIMENTAL STUDY AT 800°C AND 100 MPA

Abstract

Many experimental studies have been performed to evaluate the composition of coexisting silicate melts and magmatic volatile phases (MVP). However, few studies have attempted to define the relationship between melt chemistry and the acidity of a chloride-bearing fluid. Here we report data on melt composition as a function of the HCl concentration of coexisting brines. We performed 35 experimental runs with a NaCl-KCl-HCl-H2O brine (70 wt% NaCl [equivalent])-silicate melt (starting composition of Qtz0.38Ab0.33Or0.29, anhydrous) assemblage at 800°C and 100 MPa. We determined an apparent equilibrium constant K'meas (K, Na)=(CNamxCKClb)/(CNaClbxCKm)for the equilibrium NaClb+ΣKm=ΣNam+KClb,(where CKClb, CNaClb, CKm, and CNam are total concentrations of potassium and sodium chloride in the brine, and potassium and sodium in the melt, respectively) as a function of the HCl concentration in the brine (CHClb). Although K'meas (K, Na) was not affected by variations in KCl/NaCl of the brine, it did vary inversely with CHClb. The relationship is given by K'meas (K, Na)=K'ex (K, Na)+aCHClb[where CHClb is in wt% and a = 0.03; K'ex (K, Na) = 0.40 +/- 0.03 (1σ) and represents the exchange of model sodium and potassium between chloride components in the brine and the aluminate components (NaAlO2 and KAlO2) in the melt. This empirical result will be discussed in light of a structural hypothesis; however, validation of the model awaits determinations based on spectroscopy or transport properties-thermodynamic relations alone cannot be used as evidence of structure. The form of this equation is consistent with a model wherein sodium is present in the melt as both sodium aluminate and sodium hydroxide components, and HCl reacts with the NaOH component in the melt to produce NaCl and H2O.The correlation between fugacity of H2O (fH2Osys), model NaOHm/ΣNam, aluminum saturation index (ASI), and the ratio (HCl/NaCl)b of an exsolving MVP is complex. fH2Osys and the ASI are the main controls on model NaOHm/ΣNam in the system, with model NaOHm/ΣNam increasing with increasing fH2Osys. This relationship can be used to estimate the CHClb in subaluminous systems, an improvement over previous models. Data for metal partitioning between a volatile phase and melt are commonly presented in the literature as metal-sodium exchange equilibria (i.e., KCu,Na for the exchange of copper and sodium). However, the variation in K'meas (K, Na) observed in this study implies that the treatment of metal partitioning between a volatile phase and melt as metal-alkali exchange equilibria is complex because alkali partitioning is not constant and suggests that experimental partitioning studies need to carefully control the HCl/NaCl in experimental vapors and brines. This effect may explain discrepancies in metal-alkali exchange equilibria presented in the literature. Therefore, metal-alkali exchange cannot be described fully by a single metal-alkali equilibrium but must be examined by multiple equilibria.