Bernard J. Wood

Garnet-Orthopyroxene and Orthopyroxene-Clinopyroxene Relationships in Simple and Complex Systems

Use of simple mixing models of orthopyroxene and garnet solid solutions enables extrapolation of experimentally determined equilibria in the MgSiO3-Al2O3 system to uninvestigated parts of pressure-temperature-composition space. Apparent discrepancies in the experimental data for simple and multicomponent systems may be explained by considering the effect of CaO and FeO on reducing pyrope activity in the garnet solid solutions. Equilibration pressures of natural garnet-orthopyroxene assemblages may be calculated, provided temperatures are known, from a combination of the experimental data on the MgSiO3-Al2O3 system and analyses of coexisting natural phases.Despite the presence of a compositional gap in the system, the solubility of enstatite in diopside coexisting with orthopyroxene can also be approximately treated by an ideal solution model. An empirical approach has been developed to take account of Fe2+ on the orthopyroxene-clinopyroxene miscibility gap in natural systems in order to calculate equilibration temperatures of 2-pyroxene assemblages. The model presented reproduces almost all of the available experimental data for multicomponent systems to within 60° C.

An experimental study of Fe-Mg partitioning between garnet and olivine and its calibration as a geothermometer

The partitioning of Fe and Mg between coexisting garnet and olivine has been studied at 30 kb pressure and temperatures of 900 ° to 1,400 °C. The results of both synthesis and reversal experiments demonstrate that KD (= (Fe/Mg)gt/(Fe/Mg)OI) is strongly dependent on Fe/Mg ratio and on the calcium content of the garnet. For example, at 1,000 °C/30 kb, KD varies from about 1.2 in very iron-rich compositions to 1.9 at the magnesium end of the series. Increasing the mole fraction of calcium in the garnet from 0 to 0.3 at 1,000 ° C increases KD in magnesian compositions from 1.9 to about 2.5.The observed temperature and composition dependence of KD has been formulated into an equation suitable for geothermometry by considering the solid solution properties of the olivine and garnet phases. It was found that, within experimental error, the simplest kind of nonideal solution model (Regular Solution) fits the experimental data adequately. The use of more complex models did not markedly improve the fit to the data, so the model with the least number of variables was adopted.Multiple linear regression of the experimental data (72 points) yielded, for the exchange reaction: 3Fe2SiO4+2Mg3Al2Si3O12 olivine garnet ⇌ 2Fe2Al2Si3O12+3Mg2SiO4 garnet olivine ΔH ° (30kb) of −10,750 cal and ΔS ° of −4.26 cal deg−1 mol−1. Absolute magnitudes of interaction parameters (Wij) derived from the regression are subject to considerable uncertainty. The partition coefficient is, however, strongly dependent on the following differences between solution parameters and these differences are fairly well constrained: WFeMgol-WFeMggt≃ 800 cal WCaMggt-WCaFegt≃ 2,670 cal.The geothermometer is most sensitive in the temperature and composition regions where KDis substantially greater than 1. Thus, for example, peridotitic compositions at temperatures less than about 1,300 ° C should yield calculated temperatures within 60 °C of the true value. Iron rich compositions (at any temperature) and magnesian compositions at temperatures well above 1,300 °C could not be expected to yield accurate calculated temperatures.For a fixed KDthe influence of pressure is to raise the calculated temperature by between 3 and 6 °C per kbar.

The solubility of alumina in orthopyroxene coexisting with garnet

The solubility of alumina in complex orthopyroxenes crystallised in equilibrium with garnet has been determined over the pressure-temperature range 8–30 Kb and 800–1250° C. The results are in good agreement with predictions made using the simple thermodynamic model of Wood and Banno (1973). The model has been refined using a combination of the new data on orthopyroxenes of about En60Fs40 (with variable calcium content) and previously published data on more magnesian systems. Given analyses of coexisting Orthopyroxene and garnet in natural assemblages it is possible to calculate a P-T curve for the rock which should, in most cases, be accurate to within 2 or 3 kbar.

High-temperature properties of silicate liquids: applications to the equilibration and ascent of basic magma

High-temperature heat content measurements have been made on a series of silicate liquids, which in conjunction with published data, are used to derive partial molar heat capacities of SiO2, TiO2, Al2O3, Fe2O3, FeO, MgO, CaO, Na2O and K2O in the temperature range 1200-1650 K. Only Fe2O3 appears to be compositionally dependent, and the best evidence suggests that there is no excess heat capacity (CPi i = C°). In combination with calorimetric data and the effect of pressure on the fusion temperature of solid compounds, a consistent set of enthalpy, entropy and volume data have been derived for the liquid compounds CaMgSi2O6, NaAlSi3O8, KAlSi3O8, Fe2SiO4 and TiO2. By using activities (relative to a liquid standard state) calculated at 1 bar for a range of lavas, the equilibration pressures and temperatures of lavas with a lherzolitic source material are calculated, and for basanites indicate 22-26 kbar and 1310-1360 °G. The regular solution formulation used in these calculations gives an estimated error of 40 °G and 5.7 kbar when compared to experimental equilibria. It is suggested that one of the thermal responses of ascending alkali basalt magma to engulfing cooler lherzolitic nodules could be the precipitation of megacrysts, and the calculated equilibration pressures and temperatures of the megacryst assemblage (16-20 kbar, 1220-1240 °G) is in accord with this. The importance of viewing volcanic eruptions as the last stage in a sequence of chemical and thermomechanical instabilities is pointed out. Equations expressing the conservation of energy, mass and momentum on a macroscopic scale are given. The high Rayleigh numbers appropriate for even the relatively small magma volumes of erupted alkali basalts indicate turbulent flow-regimes with characteristic thermal convection velocities of the same order as nodule settling velocities. There is a significant partial melting effect in the mantle surrounding an ascending diapir if buoyancy is a significant force acting to drive the magma upwards. The effect of latent heat and convective heat losses on the thermal budget of a rising diapir has been calculated and shows the assumption of adiabaticity is often unwarranted - even for rapidly ascending magma. Finally, mass transfer rates due to convective diffusion have been calculated for all the major components in a basic silicate liquid. Integral mass exchange depends inversely on the ascent rate and is quite small for the rapidly ascending alkalic basalts.

Thermodynamics of water in cordierite and some petrologic consequences of cordierite as a hydrous phase

AbstractThe H2O content of cordierite is often regarded as incidental to its stability, probably because cordierite has substantial fields of stability at low pressures both in wet and dry experimental systems. In this paper we show that, in contrast, the molecular water content of cordierite has profound effects on many equilibria involving this phase.Mg-cordierite has been modelled as an ideal solid solution of the hydrous and anhydrous end-members Mg2Al4Si5O18·1.2H2O and Mg2Al4Si5O18 respectively. The H2O-solubility data of Mirwald and Schreyer (1977) fit this model within experimental uncertainty and yield 1 bar enthalpy and entropy changes for the reaction:

The influence of pressure, temperature and bulk composition on the appearance of garnet in orthogneisses — an example from South Harris, Scotland

\begin{gathered} Mg_2 Al_4 Si_5 O_{18} + 1.2H_2 O = Mg_2 Al_4 Si_5 O_{18} \cdot 1.2H_2 O \hfill \\ cordierite fluid codierite \hfill \\ \end{gathered}

The activities of components in clinopyroxene and garnet solid solutions and their application to rocks

of −12,300 cal and −32.87 cal/K. This implies that the partial molal entropy of H2O in cordierite at 298 K/l bar is almost exactly the same as the molar entropy of liquid water (16.9 cal/K as opposed to 16.7 cal/K) and that the interaction energy of liquid water with cordierite is only of the order of a few hundred calories per mole.Application of the model to the hydrous experiments of Fawcett and Yoder (1966) and Chernosky (1974) yields a value for ΔGf,2980of anhydrous Mgcordierite of between −2,062.71 and −2,074.21 Kcal per mole. This in in good agreement with the calorimetric data of Charlu, Newton and Kleppa (1975) which yield ΔGf,2980of −2,067.03±1.18 Kcal.Water pressure has a considerable influence on the (Mg, Fe) isopleths of coexisting cordierite and garnet, and hence, their use as geobarometric curves. Pressures estimated from the Mg/Fe ratios in the high-Mg range can vary by two kilobars or more, depending on the assumed