ALBERT

Notes: Abstract Thermodynamic calculations, modified after Nicholls et al. (1971), which relate the activity of silica in a lava to the temperature and pressure conditions at which the lava could be in equilibrium with a mantle mineral assemblage, have been extended to H2O-bearing magmas by using published experimental data to derive the dependence of $$a_{{\text{SiO}}_{\text{2}} }$$ on the weight fraction of H2O dissolved in a magma. A petrogenetic grid has been calculated which gives the P-T conditions under which a magma with a given $$a_{{\text{SiO}}_{\text{2}} }$$ at its liquidus at 1 atm could equilibrate with a mantle mineral assemblage containing olivine (ol) and orthopyroxene (opx) for different amounts of H2O in the magma at its source. This grid is in good agreement with the results of experimental studies as summarized by Green (1971) and Brey and Green (1975). The results show that the pressure at which a given magma composition can equilibrate with ol + opx increases for increasing amounts of H2O dissolved in the magma at depth. In addition, experimental data have been used to calculate the effect of olivine crystallization and removal on the $$a_{{\text{SiO}}_{\text{2}} }$$ in the residual liquid to assess the effect of low-pressure differentiation on $$a_{{\text{SiO}}_{\text{2}} }$$ . The results show that if 20 % olivine is added to a basalt magma, its calculated pressure of equilibration with ol+opx increases by ∼4–5 kbar for a given temperature. The calculated effects of olivine removal and H2O addition on $$a_{{\text{SiO}}_{\text{2}} }$$ are reasonably consistent with the silicate mixing model of Burnham (1975). Thermodynamic calculations of this type may be useful for assessing the internal consistency of certain experimental data, and in extrapolating the results to other magma compositions. The application of these calculations to determining the possible depth of origin of natural lavas appears to be limited primarily by the difficulty in determining $$_{{\text{SiO}}_{\text{2}} }$$ in a lava at its liquidus temperature.

Notes: Abstract The initial Sr isotopic compositions of glass and mineral separates from the 0.74 Ma Bishop Tuff ashflow in eastern California were determined to investigate the time scales of magmatic processes in a large silicic system. It was found that there is substantial isotopic heterogeneity, both between eruptive units and between glass and phenocryst phases of individual units. The frist-erupted, lower temperature units generally have higher initial 87Sr/86Sr than later crupted, higher temperature units. Within each unit, feldspar phenocrysts have the lowest 87Sr/86Sr, associated glass has higher 87Sr/86Sr, and biotite phenocrysts have the highest 87Sr/86Sr. These isotopic differences were produced by processes in the magma chamber and not by post-eruptive alteration. Two samples were similar Fe−Ti oxide temperatures but from widely separated localities have nearly identifical Sr isotopic characteristics, indicating the existence of compositionally uniform layers of substantial volume within the chamber. Trace element data indicate that the feldspars crystallized from a liquid represented by the associated glass, and that the feldspar-glass pairs are not accidental. The rhyolitic liquids of the Bishoptuff magma chamber apparently experienced increasing 87Sr/86Sr at a rate too fast for feldspar phenocrysts to remain in isotopic equilibrium. The increasing 87Sr/86Sr is caused primarily by radioactive decay of 87Rb in the high-Rb/Sr liquids and not primarily by assimilation of radiogenic wall-rock material. A self-consistent model can be constructed to account for all of the isotopic data except for those on biotite phenocrysts. The time scale for evolution of the system is bounded on the high side at about 500 ky by observations made on precaldera lavas, and on the low side at approximately 300 ky by the time necessary to establish homogeneous layers in an actively differentiating chamber. The deduced time scale is consistent with model Rb−Sr ages, which date the differentiation of low temperature liquids from higher temperature liquids, and is compatible with the observed isotopic disequilibrium between feldspars and glass because of the low diffusivity of Sr in fieldspars (〈10-16 cm2/s). The prolonged (about 500 ky) evolution of the Bishop Tuff system was facilitated by a large influx of basaltic material (about 10-2 km3/y) to the base of the system, which compensated for diffusive heat loss from the top and allowed large volumes of magma to maintain low crystal contents for 〉3x105 years. The silicic-magma production rate within the Bishop Tuff magma chamber is estimated to be 10-3km3/y. The growth rate of alkali feldspar is estimated to be about 10-14 cm/s based on the Sr isotopic difference between sanidine and glass of the lower Bishop Tuff. The biotite population is inferred to be partially (〉50 ppm) xenocrystic, the xenocrysts being introduced to the chamber less than one year prior to eruption.