ALBERT

Notes: Abstract Densities of 21 silicate liquids have been determined from 1,000 ° to 1,600 ° C. The compositions studied contain from two to eight oxide components and have the following ranges in composition (mole %): SiO2, 35–79%; TiO2, 4–36%; Al2O3, 5–25%; FeO, 11–41%; MgO, 7–28%; CaO, 7–35%; Na2O, 5–50%; and K2O, 4–20%. The compositions thus cover the upper range observed in magmas for each oxide. Precision for each determination of liquid density is always better than ±1%. Volumes/gfw (gram formula weight) calculated from the density measurements and the chemical compositions of the analyzed liquids have been combined with data on 96 silicate liquids reported in the literature. From this data set we derive, by using multiple linear regression, partial molar volumes of the components SiO2, TiO2, A12O3, FeO, MgO, CaO, Na2O, and K2O at five temperatures. The standard deviation of the multiple regression is 1.8% of the molar volumes, which is considered about equal to the total errors due to compositional and instrumental uncertainties. These derived partial molar volumes have been used to calculate volumes/gfw of natural silicate liquids which are found to agree within 1% of the measured values. No compositional dependence of the partial molar volumes can be detected within the error considered to be typical of the measurements. This is further supported by the close agreement between the calculated volumes of CaMgSi2O6 and Fe2SiO2 liquids derived from the initial slopes of their fusion curves and their heats of fusion, and the volumes obtained by summing the respective partial molar volumes. The experimental data indicate that silicate liquids mix ideally with respect to volume, over the temperature and composition range of this data set.

Notes: Abstract The voluminous Pleistocene—Recent Taupo rhyolites typically contain phenocrysts of plagioclase (oligoclase-labradorite), quartz, titanomagnetite, ilmenite, and ferromagnesian silicates. Ferromagnesian assemblages correlate with well defined Fe-Ti oxide equilibration temperature ranges and allow the rhyolites to be subdivided as follows: (1) Cummingtonite (c)—calcic hornblende (hb)—orthopyroxene (opx); 725–755°C, (2) Hb-opx, 750–825°C, (3) Biotite-hb-(c-opx), 720–765°C, (4) Opx-clinopyroxene (cpx), 860–915°C, (5) Fe olivine-opx-cpx, one sample with temperature of 900°C. Plagioclase and orthopyroxene phenocryst compositions typically exhibit a range of composition up to ∼20 mol.%. Calculated average phenocryst equilibration pressures (P total) range between 0.5–4.9 kb, and average 2.2 kb (∼7–8 km depth), indicating upper crustal crystallization. These calculations are very sensitive to variations in phenocryst composition. Calculated $$/_{{\text{H}}_2 {\text{O}}} $$ for the amphibole and biotite-bearing rhyolites indicate phenocryst equilibration under $$P_{{\text{H}}_2 {\text{O}}} \simeq P_{{\text{total}}} $$ , with $$X_{{\text{H}}_2 {\text{O}}} $$ ∼0.17–0.24 (5–8 wt. %). The precipitation of cummingtonite is thus temperature dependent, the upper limit being close to 760°C. Eruptive mechanisms of the lavas, pumices, and ash-flow deposits are evidently not primarily controlled by temperature, P total, $$P_{{\text{H}}_2 {\text{O}}} $$ , or crystal content of the magmas, and explanations must lie in kinetic and fluid dynamic behavior of the magmas. For the Taupo rhyolites, it is suggested that the critical size of a magma body (i.e. Rayleigh number) is a controlling factor in that it will influence the convective regime; fully turbulent convection is deduced to have occurred within the larger magma bodies. One consequence is intense vesiculation, prior to eruption, within the uppermost zones of these magma chambers, and the voluminous pumice deposits are believed to emanate from such chambers. Oscillatory compositional zoning within pyroxene phenocrysts is consistent with magma convection.

Notes: Abstract Fifty-two samples of inverted high-temperature quartz from volcanic rocks were investigated by Guinier-Jago powder diffractometry and differential scanning calorimetry (DSC). Quartz megacrysts from Clear Lake and Cinder Cone, California show a variability of ≃2.5 ° K in their α-β transition temperature (T α-β). Quartz phenocrysts and quartz from crystalline rocks give a range of 0.5 ° K in T α-β. Neutron activation analysis of single crystals demonstrates that Al is the principal impurity (17–380 ppm). Its concentration is inversely correlated with T α-β. A very small variation was found in the a and c lattice parameters among the specimens of volcanic quartz studied. This variation does not correlate with Al content or transition temperature. Mean values at 22 ° C (a=4.1934±0.0004 Å, c=5.4046±0.0006 Å) are similar to those of quartz grown at low temperatures. Enthalpy of the α-β transition (ΔH α-β), obtained over 9.0 ° from DSC runs, is dependent upon sample grain size and for a crushed powder with zero hysteresis (T α-β on heating=T α-β on cooling) is 92.0 ±1.4 cal/mol. In contrast, a single piece of quartz requires ΔH α-β be 107.7±1.4 cal/mol and has a T α-β hysteresis of 1.1 ° K. Regression of published data provides equations for the variation of the molar volume (cc/mol) of quartz with v. These equations imply a ΔV α-β of 0.205±0.031 cc/- mol. Expressions are also provided for the temperature dependence of the thermal coefficient of expansion, α, the compressibility, β, and (∂/gb/∂T)p (which is identically -(∂α/∂P) T ). DSC heat capacity measurements over the range 400 to 900 ° K were fitted to extended Maier-Kelley type expressions to give: $$\begin{gathered} C_P = 10.31 + 9.116 \times 10^{ - 3} T - \frac{{1.812 \times 10^5 }}{{T^2 }} \hfill \\ - {\text{5}}{\text{.630}} \times 10^{ - 2} {\text{ }}\frac{T}{{(T - 848)}} - 0.3553\frac{T}{{(T - 848)^2 }} \hfill \\ - 0.9011\frac{T}{{\left( {T - 848} \right)^3 }} \hfill \\ (400{\text{ to 842}}^ \circ {\text{K), and}} \hfill \\ C_P = - 318.8 + 0.2532T \hfill \\ {\text{ + }}\frac{{8.687 \times 10^7 }}{{T^2 }} + 0.1603\frac{T}{{\left( {T - 848} \right)^4 }} \hfill \\ \end{gathered} $$ (851 to 900 ° K), which together with the values of ΔH α−β measured over the range 842–851° K give 7875.3 cal/mol for H900-H400. The behavior of α, β, and C p as a function of T emphasizes that structural changes which occur at the α−β transition do so over a broad temperature interval.

Notes: Abstract There is a positive correlation between the concentration of P2O5 in basic magmas and the concentration of the REE and also between their light: heavy ratio, represented by both Ce/Yb and Nd/Sm. This suggests that a phosphate mineral, such as apatite or whitlockite, both of which can contain high concentrations of REE, is present in the magma source regions. Thermodynamic calculations indicate that fluorapatite is stable over the whole P-T range of magma generation, but at the greatest depths it may co-exist with whitlockite in the presence of fluorphlogopite. Using published REE crystal-liquid distribution coefficients, it is evident that for P2O5-rich basic magmas with 700–1000 times chondritic abundances of LREE, garnet need not have contributed significantly to their composition. The most convincing match of hypothetical liquid with actual basic magma is for the derivation of mid-ocean ridge (MOR) basalts from plagioclase- or spinel-lherzolite containing 3 times chondritic REE by ∼5% partial melting. The more P2O5-rich, and hence REE-rich, basic magmas are apparently derived from crystalline sources which are progressively impoverished in garnet and clinopyroxene, or in other words, the greater the REE concentration of basic magma, the more refractory is the mineral assemblage of the source. There is some evidence for a compositional dependence of radiogenic neodymium and lead in basic magma, and one way that this can be reconciled with mantle source-region evolution is to postulate that fusion is not always accompanied by isotopic equilibrium.