Ian S. E. Carmichael

The compressibility of silicate liquids containing Fe2O3 and the effect of composition, temperature, oxygen fugacity and pressure on their redox states

Ultrasonic longitudinal acoustic velocities in oxidized silicate liquids indicate that the pressure derivative of the partial-molar volume of Fe2O3 is the same in iron-rich alkali-, alkaline earth- and natural silicate melt compositions at 1 bar. The dV/dP for multicomponent silicate liquids can be expressed as a linear combination of partial-molar constants plus a positive excess term for Na2O−Al2O3 mixing. Partial-molar properties for FeO and Fe2O3 components allow extension of the empirical expression of Sack et al. (1980) to permit the calculation of Fe-redox equilibrium in a natural silicate liquid as a function of composition, temperature, fo2 and pressure; a more formal thermodynamic expression is presented in the Appendix. The predicted equilibrium fo2 of natural silicate melts, of fixed oxygen content, closely parallels that defined by the metastable assemblage fayalite+magnetite+β-quartz (FMQ), in pressure-temperature space. A silicate melt initially equilibrated at 3 GPa and FMQ, will remain within approximately 0.5 log10 units of FMQ during its closed-system ascent. Thus, for magmas closed to oxygen, iron-redox equilibrium in crystal-poor pristine glassy lavas represents an excellent probe of the relative oxidation state of their source regions.

The iron-titanium oxides of salic volcanic rocks and their associated ferromagnesian silicates

The co-existing microphenocrysts of magnetite and ilmenite together with the ferromagnesian silicates in salic volcanic rocks have been analysed with the electron microprobe. The temperatures and oxygen fugacities of the oxide equilibration have been estimated from the curves of Buddington and Lindsley (1965). The co-existing ferromagnesian silicate phenocrysts are either iron-rich olivine, or orthopyroxene or biotite and amphibole; for each of these groups of phenocrysts, the oxide equilibration data are specific and fall on three distinct curves, parallel to experimental oxygen buffer curves. Many of the investigated rhyolites were quenched at temperatures near 900°C, which may represent liquidus temperatures for those with sparse phenocrysts, and also the intrusion temperature of water-undersaturated granites. The composition of the biotite phenocrysts, which are Al-poor and Ti-rich, taken in conjunction with the oxide data, suggest that two Lassen dacites precipitated biotite at a water fugacity of approximately 400 bars. The composition of the later crystallizing ferromagnesian silicates, particularly the pyroxenes which show a wide range in Fe/Mg ratio, is strongly influenced by the prior crystallization of the oxide phases. If the biotite phenocrysts are typical of acid liquids, then they are incapable of generating by fractionation a peraluminous residual liquid; rather they would tend to make a liquid peralkaline.

Densities of Na2O-K2O-CaO-MgO-FeO-Fe2O3-Al2O3-TiO2-SiO2 liquids: New measurements and derived partial molar properties

The densities of 27 liquids in the system Na2O-K2O-MgO-MgO-FeO-Fe2O3-Al2O3-TiO2-SiO2 have been measured using the double-bob Archimedean technique. These results indicate that multicomponent silicate liquid volumes have a linear dependence on composition with the exception of the TiO2 component. The equation: V(T) = ∑Xi(T)Vi(T) + XNa2OXTiO2VNa2O-TiO2 was used to derive values of the oxide partial molar volumes (Vi) by the method of least squares. Regressions were made separately at 1573, 1673, 1773 and 1873 K (as liquid ferric-ferrous ratios change with temperature) with relative standard errors for each fit of 0.38%, 0.32%, 0.30% and 0.32%, respectively. Derived dVi/dT values by separate least squares regression for each oxide component reproduce the measured dV/dT of the experimental liquids by 20.21% on average. The effects of iron redox state on the density of a variety of natural liquids are demonstrated and at most amount to a variation of 1%. These new data on silicate liquid volumes were used to re-derive oxide partial molar compressibilities, dVi,T/dP and \gbiT, at 1 bar from ultrasonic velocity and calorimetric data from the literature. The fits for dVi,T/dP and \gbi,T at 1673 K have relative standard errors of 3.9% and 1.8%, respectively, which represent substantial improvements over previous fits. Two applications of these volume data are given: firstly, the fusion curve of diopside is calculated up to 95 kbr using an equation of state for liquid volume expressed as: V(T, P) = V(T) exp ∝ -β(T)(1 − bP+cP2)dP (where β = K−1 and therefore dK/dP = K′ = −β−2dβ/dP); secondly, using a range of K′ appropriate to a komatiite liquid, the pressure-temperature conditions where the density of the liquid equals that of its olivine phenocrysts are calculated.

The redox states of basic and silicic magmas: a reflection of their source regions?

At present the best estimates of the oxygen fugacity of spinel-lherzolites that could be the source material of basic magmas is about five log units below the Ni−NiO buffer to one above it. However partially glassy basic lavas, ranging from MORBs to minettes, all with olivine on their liquidus, cover a wider range, and may have oxygen fugacities that extend to four log units above NNO. Surprisingly the range of oxygen fugacities observed in silicic lavas and ashflows with quartz phenocrysts is smaller, despite a crustal dominated evolution. The peralkaline silicic lava type pantellerite is the most reduced, equivalent to MORBs, whereas the large volume ashflows with phenocrysts of hornblende and/or sphene are the most oxidised. As the concentration of water in the basic lavas is correlated with increase in their redox state, it would seem that water could be the agent of this increase. That this is unlikely is seen in the behavior of silicic ashflows and lavas, particularly those of Yellowstone. Here the silicic magmas of the last 2Ma contain about 2 wt% FeO(total), and typically phenocrysts of fayalite, quartz and Fe−Ti oxides. Despite extensive exchange of the 18O of the magma with meteoric water after caldera collapse (Hildreth et al. 1984), there is no displacement of the redox equilibria. Thus the thermal dissociation of molecular H2O to H2, and its subsequent diffusive loss to cause oxidation must have been minimal. This could only be so if the activity of water was small, as it would be if H2O reacted with the silicate liquid to form OH groups (Stolper 1982). The conclusion is that silicic magmas with small amounts of iron and large amounts of water do not have their redox states reset, which in turn presumably reflect their generation. By analogy basic magmas with large amounts of iron and far less water are even less likely to have their redox equilibria disturbed, so that their oxygen fugacities will also reflect their source regions. The effect of pressure on the ferric-ferrous equilibrium in basic magmas can be calculated from experimental measurements of the partial molar volumes of FeO and Fe2O3, and their pressure derivatives ϖV/ϖP, in silicate liquids. The effect of pressure is found to be about the same on the liquid as it is for various solid oxygen buffers. Accordingly there should be mantle source regions covering the same range of oxygen fugacity as that found in basic lavas, but so far samples of spinel-lherzolite of equivalent oxygen fugacity to minettes or other potassic lavas have not been found. Whether or not the redox state of phlogopite-pyroxenites is equivalent to these potassic lavas cannot be established without experiment.

The ferric-ferrous ratio of natural silicate liquids equilibrated in air

AbstractResults of chemical analyses of glasses produced in 46 melting experiments in air at 1,350° C and 1,450° C on rocks ranging in composition from nephelinite to rhyolite have been combined with other published data to obtain an empirical equation relating in

Ferric-ferrous equilibria in natural silicate liquids at 1 bar

(X_{{\text{Fe}}_{\text{2}} {\text{O}}_{\text{3}} }^{{\text{liq}}} /X_{{\text{FeO}}}^{{\text{liq}}} )

The 1982 eruptions of El Chichón Volcano, Chiapas, Mexico: Mineralogy and petrology of the anhydritebearing pumices

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