VALENCE AND STRUCTURAL STATE OF IRON IN BASALTIC MELTS AT PRESSURES UP TO 5 KBAR

Abstract

The effect of pressure on the valence and structural state of iron atoms was experimentally studied at pressures up to 5 kbar in natural basaltic melts under closed system conditions. Basaltic glasses were produced by quenching melts at high pressures in an internally heated gas pressure apparatus and studied by microchemical, Mossbauer, and IR spectroscopic methods. It was shown that, under closed conditions, pressure variations up to 5 kbar (T = 1350°C) have no significant effect on the valence and structural state of iron atoms in basaltic melts independent of their initial Fe3+/SigmaFe ratio. The partial reduction of Fe3+ to Fe2+ that was observed in long experiments (similar to30% of Fe3+ at 5 kbar and similar to2 h) with highly oxidized melts (Fe3+/SigmaFe = 0.8) resulted from hydrogen ingress through the walls of platinum capsules from the enclosing gas medium. This is suggested by elevated H2O concentrations (up to 0.5 wt %) in the experimental melts (quenched glasses). These results cast doubts on the previous experimental results that demonstrated a significant pressure impact on Fe3+/SigmaFe in highly oxidized silicate melts under closed conditions at pressures up to 5-10 kbar. The analysis of hyperfine interaction parameters of partial Mossbauer spectra for Fe3+ and Fe2+ in quenched glasses [average chemical shift, (δ) over bar; quadrupole splitting, (ε) over bar; and their distribution functions, p(sigma) and p(epsilon)] suggests that each iron species is characterized by a single predominant structural state, which can be characterized by its effective coordination number (e.c.n.). The coordination of Fe3+ and Fe2+ in basaltic melts (glasses) is essentially pressure-invatiant up to 5 kbar but depends significantly on Fe3+/SigmaFe. In highly oxidized basaltic glasses (Fe3+/SigmaFe = 0.8), the average values of chemical shift ((δ) over bar) for Fe3+ correspond most closely to a five-fold coordination (e.c.n. 5), and the average (δ) over bar for Fe2+ suggests a predominant tetrahedral anion surrounding (e.c.n. = 4). The values of 8 for Fe3+ and Fe2+ increase gradually with decreasing Fe3+/SigmaFe, and in strongly reduced glasses (Fe3+/SigmaFe < 0.25), they attain values typical of octahedral sites (e.c.n. = 6). The negligible pressure influence on the valence and structural state of iron in basaltic melts justifies the use of the pressure dependency proposed by Kress and Carmichael (1991) for the determination of f(O2) in igneous melts at high pressure (up to 20-30 kbar). According to this dependency, in a closed isothermal system, pressure-induced changes in f(O2) of a basaltic melt with a given Fe3+/Fe2+ approximately parallel the f(O2) path for the FMQ buffer equilibrium.