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Volcanic dome-building episodes commonly exhibit acceleration in both effusive discharge rate and seismicity before explosive eruptions (1). This should enable the application of material failure forecasting methods to eruption forecasting [...]

The densities of 8 Zn-bearing silicate melts have been determined, in equilibrium with air, in the temperature range of 1363 to 1850 K. The compositional joins investigated [sodium disilicate (NS2)-ZnO; anorthite-diopside 1 bar eutectic (AnDi)-ZnO; and diopside-petedunnite] were chosen based on the pre-existing experimental density data set, on their petrological relevance, and in order to provide a test for significant compositionally induced variations in the structural role of ZnO. The ZnO concentrations investigated range up to 25 tool% for sodium disilicate, 20 tool% for the anorthite-diopside 1 atm eutectic, and 25 tool% for petedunnite. Molar volumes and expansivities have been derived for all melts. The molar volumes of the present liquids decrease with increasing ZnO content. The partial molar volume of ZnO derived here from the volumetric measurements for each binary system is the same within error. A multicomponent fit to the volumetric data for all compositions yields a value of 13.59(0.55) [cm.sup.3]/mol at 1500 K. We find, herewith, no volumetric evidence for compositionally induced coordination number variations for ZnO in alkali-bearing vs. alkali-free silicate melts nor for Al-free vs. Al-bearing silicate melts. Keywords: Melt properties, partial molar volume of ZnO, experimental petrology, igneous petrology, trace elements and REE, high-temperature studies

Fragmentation experiments and detailed textural studies were performed on Vulcanian pyroclasts of variable porosity from Soufriere Hills volcano, Montserrat. We use textural differences to identify two types of pyroclasts with distinct eruption histories. Our experiments show that high-porosity Type 1 pumice fragments rapidly, and that low-porosity Type 2 fragments relatively slowly. Both types of pyroclast contain fractured crystals containing burst melt inclusions. The orientations of the fractures relative to the crystal alignment indicate that Type 1 decompressed vertically and Type 2 decompressed laterally. We interpret that Type 1 clasts were derived from the conduit center, whereas Type 2 clasts were derived from the implosion of the conduit lining. We propose a conceptual model of rapid fragmentation of pumice in the conduit center, and a slower fragmentation of a pressurized conduit wail, vulnerable to implosion. Conduit implosion can explain observed widening of volcanic vents during eruptions. We also hypothesize that the fragmentation front is concave; this has important implications for modeling of explosive eruptions. Keywords: conduit, fragmentation, explosion, melt inclusion, Montserrat, porosity.

The prediction of viscosity in silicate melts, over the range of conditions found in nature, remains one of the most challenging and elusive goals in Earth Sciences. We present a strategy for fitting non-Arrhenian models [e.g., Vogel-Tammann-Fulcher (VTF) or Adam-Gibbs (AG)] to viscosity data that can be employed toward a full multicomponent model for melt viscosity. Our postulate is that the high-T viscosities of silicate melts converge to a common value. The implications are twofold. First, the number of composition-dependent parameters is reduced by a third. Second, our optimization constrains the experimentally inaccessible, high-T properties of silicate melts. The high-T limits to melt viscosity are constrained by the VTF and AG models to between [10.sup.-4.3 [+ or -] 0.74] and [10.sup.[-3.2 [+ or -] 0.66] Pa*s, respectively, and overlap in the interval [10.sup.-3.86] to [10.sup.-3.56] Pa*s.

The shear viscosities of melts in the system Na-Fe-Si-O-F-Cl were determined over a wide range of temperatures (400-1200 [degrees] C) at 1 atm pressure in air. The compositions are based on the addition of [Fe.sub.2][O.sub.3], Fe[Cl.sub.3], and Fe[F.sub.3] to a base melt composition corresponding to sodium disilicate ([Na.sub.2][Si.sub.2][O.sub.5]). Viscosities were determined using concentric cylinder and micropenetration methods and measurements span the range of [10.sup.0.5] to [10.sup.11] Pa[center dot]s. The chemical compositions of these melts were analyzed after the viscometry determinations. The iron is fully oxidized under the conditions of the viscometry. Although F and especially Cl are volatile elements in silicate melts, levels of Cl and F up to over 3 and 4 wt%, respectively, were stabilized in these melts, assisted presumably by the presence of [Fe.sup.3+]. Although some volatilization occurred during the original synthesis of these samples, none occurred during viscometry. The anionic substitutions [Cl.sub.2][O.sub.-1] and [F.sub.2][O.sub.-1] have very different influences on the viscosity. The [F.sub.2][O.sub.-1] substitution causes a drastic decrease in viscosity over the entire investigated range whereas the [Cl.sub.2][O.sub.-1] substitution causes a much smaller decrease in viscosity in the high viscosity range and a slight increase in viscosity in the low viscosity range. As a consequence, minor to major element abundance of Cl in strongly peralkaline undersaturated volcanic rocks are not likely to significantly influence melt viscosity.

We performed 27 viscosity determinations on dry and water-bearing peraluminous haplogranitic melts. The dry melt compositions cover the range of normative corundum to be expected in peraluminous granitic melts in nature. The compositions are based on addition of [Al.sub.2][O.sub.3] to a haplogranitic melt (HPG8) whose composition is near that of the projection of the 2 kbar [H.sub.2]O-saturated minimum melt composition into the system NaAl[Si.sub.3][O.sub.8]KA][Si.sub.3][O.sub.8]-Si[O.sub.2]. The [H.sub.2]O contents of the hydrous melts were analyzed using Karl Fischer titration ranging from 1 to 3 wt%. The viscosity determinations were performed using a modified micropenetration method in the viscosity range of [10.sup.10] to [10.sup.11] Pa[center dot]s, at 1 atm pressure, and in the temperature ranges of 880-940 [degrees] C and 470-640 [degrees] C for the dry and wet melts, respectively. For the dry peraluminous melts in this high viscosity range, addition of the first few percent of normative corundum to a metaluminous granitic melt increases the viscosity, which remains nearly constant despite further addition of [Al.sub.2][O.sub.3]. Thus a viscosity maximum is inferred for dry slightly peraluminous granitic melts. The hydrous melt viscosity data were compared with the recent calculational model of Hess and Dingwell (1996), which was based on and designed for metaluminous melt viscosities. That model is capable of describing the viscosities of hydrous peraluminous granitic melts within the uncertainties stated for its application in metaluminous melts.

Water content has a strong effect on the viscosity of hydrous silicic melts. Viscometric studies on synthesized hydrous haplogranitic melts serving as a model for calcalkaline rhyolites show that viscosity drops nonlinearly with the addition of water. The results of the study are used to reveal the similarity in the effects of alkali oxides on the viscosity of silica melt and water on that of a rhyolite melt. The consequences for degassing subvolcanic silicic melts are detailed. Significance of the prevailing pressure-temperature-composition conditions during volcanism is emphasized.

The composition-dependence of the solubility of [H.sub.2]O in silicate melts along the binary joins NaAl[Si.sub.3][O.sub.8]-KAl[Si.sub.3][O.sub.8], NaAl[Si.sub.3][O.sub.8]-LiAl[Si.sub.3][O.sub.8], and KAl[Si.sub.3][O.sub.8]-LiAl[Si.sub.3][O.sub.8] has been determined at 2 kbar and 1040 [degrees] C. The study involved 1 atm dry oxide fusion, hydrothermal saturation, isobaric rapid quench and macroscopic analysis for [H.sub.2]O using Karl Fischer titration (KFT) and microscopic investigation of homogeneity using infrared absorption spectroscopy, respectively. The solubility of [H.sub.2]O in these melts increases in the order KAl[Si.sub.3][O.sub.8] (5.12 wt%) < NaAl[Si.sub.3][O.sub.8] (6.03 wt%) < LiAl[Si.sub.3][O.sub.8] (7.32 wt%). The total relative weight percent variation in solubility is 43%, which corresponds to a relative mole percent variation of 23%. Along the joins, small but systematic deviations from additivity are observed at the 1[Sigma] uncertainty level. These deviations are positive on the NaAl[Si.sub.3][O.sub.8]-KAl[Si.sub.3][O.sub.8] join but negative on the LiAl[Si.sub.3][O.sub.8]-bearing joins. The present results confirm the early suggestion of Voigt et al. (1981) that solubility variation along the NaAl[Si.sub.3][O.sub.8]-KAl[Si.sub.3][O.sub.8] join is nonlinear and disagree in detail with the equimolal approximation of Burnham (1975, 1981) and Burnham and Davis (1974). Models of [H.sub.2]O solubility must take into account not only the nonequimolal solubility of [H.sub.2]O in feldspathic melts but also nonlinear binary variations as well. We interpret the relative solubilities of the end-member compositions in terms of the relative stability of the tectosilicate melt structure and the nonlinearities along the joins in terms of the next-nearest neighbor distributions of Al and Si in these melts.