New data on sulphur valence and magmatic oxidation state for Central Andean volcanic rocks, in combination with published data for experimental and natural samples, allow derivation of a simple relationship between magma oxidation state and sulphur speciation. For a number of highly oxidized Central Andean volcanic rocks fO2 has been calculated using magnetite-ilmenite or olivine-spinel pairs and the sulphur valence in glasses has been measured using the peak shift of S-Kα radiation relative to a pyrite standard. Previously published experimental and natural data have been incorporated with a wider range in fO2 and S valence. The variation in sulphur speciation (as S2- or SO42-) as a function of log fO2 is described by an empirical polynomial fit which reproduces the data to within ±0.5 log units and allows use of electron microprobe measurements of the S-Kα wavelength shift for estimation of magmatic oxygen fugacities. This approach is applicable for fO2 between FMQ-2 and FMQ+6, encompassing most terrestrial magmas. The method has been used to calculate the in fO2 conditions under which melt inclusions were trapped in andesitic magmas before magma mixing in two Central Andean volcanoes, and to calculate the oxygen fugacity of a slowly-cooled pyroclastic flow in which the Fe-Ti oxide phases have subsequently re-equilibrated. In combination with Fe-Ti oxide data, two distinct trends emerge for Lascar volcano. Basaltic andesite-andesitic magma chambers follow T-fO2 trends which parallel the FMQ buffer curve, indicating ferrous-ferric iron buffering of oxygen fugacity. Dacitic anhydrite-bearing magmas with admixed basaltic andesite and andesite follow trends of increasing fO2 with decreasing temperature, indicative of buffering of fO2 by SO2-H2S in a co-magmatic gas phase. This trend continues into the metamorphic aureole of the magma chamber, resulting in highly oxidized (close to magnetite-hematite) conditions.

The thermodynamics and kinetics of CO2 speciation in silicate melts have been studied by measuring the concentration of CO2mol and carbonate in jadeite glass annealed at 575, 450 and 400°C. Assuming that the reaction is

1

where CO2mol..Obr represents a CO2 molecule weakly bonded to a bridging oxygen in the network and CO3 represents a bridging carbonate group with no net negative charge, ΔH for the reaction is –17 (+4/–8) kJ mol−1 and ΔS is –24 (+6/–9) J K−1 mol−1. The rate of equilibration of the species was measured at each temperature and the rate constants were deduced. The temperature dependence of the rate constants was used to determine the activation energy of the forward and reverse reactions which are 68 (+3/–31) kJ mol−1 and 86 (+1/–69) kJ mol−1 respectively. The data suggest that CO2mol may be much more abundant in silicate melts than previously assumed on the basis of studies of CO2-bearing glasses. Models of solubility, diffusion, and isotope fractionation should take this into account.

Single crystals of magnetite and of hematite have been dissolved at atmospheric pressure in superheated melts in the systems CaO-MgO-Al2O3-SiO2 and CaO-Al2O3-SiO2, and in a basalt. The crystals were suspended in alumina crucibles containing ca 3.5 cm 3 of melt. Quenched run products were examined optically and by electron probe analysis to establish the distribution of Fe in the glassy charges. There is usually a concentration of Fe at the base of a run product, consistent with flow of dissolved matter from the crystal to the floor. One or more columns of brown, Fe-rich glass may extend from the underside of a relic crystal towards the floor. In CMAS run products, such columns typically extend this entire distance, whereas in the CAS and basalt run products, the columns are either detached from the crystal or do not reach the floor. In the CMAS melt (viscosity ~1 poise) there is apparently continuous release of Fe-bearing melt from around a dissolving crystal, whereas in the CAS and basalt melts (viscosities 7000 and 300 poise, respectively) release is intermittent. In CMAS run products the Fe content is usually greatest, in glass, at the base, and declines gradually upwards; in CAS and basalt runs the bottom of a crucible is occupied by discrete, sharply bounded pillows of Fetich glass, with only slight, or no, gradation in composition. Rising gas bubbles can elevate small blobs of the denser, Fe-bearing melt from around a dissolving crystal, and trains of bubbles in the CAS melt may guide this Fe-bearing melt, against gravity, to the surface of the charge. When the bubbles burst at the surface, this dense melt is left in an unstable location and releases diapirs which descend to the bottom of the crucible. In spite of the evidence that convective fractionation occurs in these haplomagmas and in the basalt, it remains to be demonstrated that it will occur during sidewall crystallization or during the growth of minerals in a cumulus mush to cause magmatic differentiation.

The lithospheric mantle beneath the North China Craton changed dramatically in its geophysical and geochemical characteristics from Palaeozoic to Cenozoic times. This study uses samples of Mesozoic basalts and mafic intrusions from the North China Craton to investigate the nature of this mantle in Mesozoic times. Sr–Nd–Pb isotopic data demonstrate that the Late Mesozoic lithospheric mantle was extremely heterogeneous. In the central craton or the Luzhong region, it is slightly Sr–Nd isotopically enriched, beneath the Taihangshan region it has an EM1 character (87Sr/86Sri=0.7050–0.7066; εNd(t)=−17–−10), and beneath the Luxi–Jiaodong region, it possesses EM2-like characteristics (87Sr/86Sri up to 0.7114). Compositional variation with time is also apparent in the Mesozoic lithospheric mantle. Our data suggest that the old lithospheric mantle was modified during Mesozoic times by a silicic melt, where beneath the Luxi–Jiaodong region it was severely modified, but in the Luzhong and Taihangshan regions the effects were much less marked. The silicic melt may have been the product of partial melting of crustal materials brought into the mantle by the subducted slab during the formation of circum-cratonic orogenic belts. This Mesozoic mantle did not survive for a long time, and was replaced by a Cenozoic mantle with depleted geochemical characteristics.

A two-parameter Margules method has been developed for modelling the thermodynamic mixing properties and hypersolidus phase relations of albite-water (ab-w) melts. The method is based largely on phase-equilibrium and calorimetric data that are either currently available or readily obtainable; P-V-T data are not required.

The new modelling method has been applied to calculate: (1) thermodynamic mixing properties for ab-w melts at 2-5 kbar, 815°C; and (2) hypersolidus phase relations for the ab-w system at 2·5 kbar. Results are uniformly positive—activity-composition relations are similar to those predicted by equations based on P-V-T data, excess enthalpies are in good agreement with data obtained from calorimetry, and calculated phase relations are fully compatible with phase-equilibrium data for ab-w melts acquired at 2·5 kbar.

It is concluded that the Margules method for thermodynamic modelling of ab-w melts is a practical alternative to the P-V-T modelling method.