Michael T. Naney

Crystallization history of Obsidian dome, Inyo domes, California

Samples obtained by U.S. Department of Energy research drilling at the 600-year-old Obsidian Dome volcano provide the rare opportunity to examine the transition from volcanic (dome) to plutonic (intrusion) textures in a silicic magma system. Textures in the lavas from Obsidian Dome record multiple periods of crystallization initiated in response to changes in undercooling (ΔT) related to variable degassing in the mag-ma. Phenocr)ysts formed first at low ΔT. A drastic increase in ΔT, related to loss of a vapor phase during initial stages of eruption, caused nucleation of microlites. All of the lavas thus contain phenocrysts and microlites. Extrusion and subsequent devitrification of the dry (0.1 wt% H2O) magma crystallized spherulites and fine-grained rhyolite at high ΔT. A granophyric texture, representing crystallization at a moderate ΔT, formed in the intrusions beneath Obsidian Dome. Textures in the intrusion apparently represent crystallization of hydrous (1–2 wt% H2O) rhyolitic magma at shallow depths.

The CO2-H2O system: III. A new experimental method for determining liquid-vapor equilibria at high subcritical temperatures

Abstract A highly precise and accurate vibrating U-tube technique was developed to determine the upper baric stabilities of liquid-vapor assemblages in the CO2-H2O system at high subcritical temperatures (∼275-360 °C). The first step is to create an isobaric-isothermal, physically isolated and chemically homogeneous sample of “high-pressure” CO2-H2O fluid of known composition. Fluid pressure (P) is then lowered slowly at constant temperature. Pressure readings and matching values for τ (the period of vibration of the U-tube) are recorded at 0.1 or 0.2 MPa intervals. When the fluid begins to separate into two phases (liquid + vapor), a distinct inflection is observed in the trend of P vs. τ. Performing such experiments for fluid compositions at 0.05 mole fraction CO2 (XCO2) intervals in the range 0.05 ≤ XCO2 ≤ 0.40 at 300 °C produced a complete high-P liquid-vapor boundary curve for the CO2-H2O system at that temperature. Agreement with corresponding curves determined in previous studies ranges from poor to excellent.

Crystallization processes in an artificial magma: variations in crystal shape, growth rate and composition with melt cooling history

A large (4.8 m3, 1.3x107 g) artificial mafic melt with a bulk composition similar to that of a basalt (but with a high CaO content of 17 wt%) was generated during a demonstration of in situ vitrification and was allowed to cool naturally. During the melting process, convection was vigorous, resulting in a chemically and thermally homogeneous melt body. Once heating was complete, the cooling rate was rapid with the temperature dropping from 1500°C to 500°C in ∼6 days within the interior of the 3 m diameter, 1.5 m thick body. A ∼20 h period of constant temperature (1140°C) observed during colling was the result of latent heat released by widespread crystallization. The final crystalline assemblage consists of diopsidic to hedenbergitic pyroxene and anorthitic feldspar, with a subordinate amount of potassic feldspar, plus a small amount of evolved glass. The compositions and proportions of phases agree well with those predicted by the MELTS thermodynamic model. Thermal and textural evidence suggest that convection within the melt ceased coincident with formation of the first crystals. Textural investigation of core samples reveals large (up to 1 cm in length) acicular diopsidic pyroxenes in a matrix of smaller feldspar and zoned pyroxene crystals (∼500 μm in length). Crystal shape and pyroxene composition vary as a function of position within the solidified body, as a function of cooling rate. Both crystal size and degree of crystallinity are highest in the central, most slowly-cooled parts of the rock. Crystal shape is characterized by tabular, equilibrium-growth forms in the slowly-cooled areas, grading to highly skeletal, dendritic forms at the rapidly-cooled edges of the body. The pyroxene crystals are dominantly homogeneous diopside, but crystals are characterized by thin Fe-rich hedenbergitic rims. These rims were deposited when Mg in the melt was depleted by diopside growth, and melt temperature had cooled sufficiently to allow Fe-rich pyroxene growth. Crystal growth rates can be calculated based on thermal behavior of the melt, reinforced by thermodynamic modelling, and are determined to be between 10-7 and 10-8 cm/s in the central part of the melt. These estimates agree well with growth rates in natural systems with similar cooling rates. Pyroxene crystals that formed at a higher cooling rates are characterized by higher Al and lower Mg contents relative to tabular equilibrium crystalline forms, presumably as a result of disequilibrium melt compositions at the crystal-melt interface.

Petrologic and geophysical studies of an artificial magma

In Situ Vitrification (ISV) is one of the most geologically interesting techniques being developed to stabilize radioactive and hazardous waste sites. The ISV process transforms permeable, easily leached, contaminated soils into low-permeability, leach-resistant vitreous and crystalline materials via in situ joule (resistance) heating [Buelt et al., 1987]. Melting is induced by applying electrical power to the ground through four graphite electrodes in a square configuration inserted vertically into the upper portion of the soil to be melted. Melting proceeds downward, producing a roughly hemispheric body that cools to a glassy to partially crystalline material. A hood is placed over the melt to collect gases and particulates released from the melt.

Generation of rhyolitic melt in an artificial magma: Implications for fractional crystallization processes in natural magmas

Abstract A large (1.3 × 10 7 g) artificial mafic melt with a bulk composition similar to an unusually calcic basalt (17 wt.% CaO) was produced by in-situ heating of soil, and subsequently cooled and crystallized. The final crystalline assemblage consisted dominantly of acicular diopsidic to hedenbergitic pyroxene and anorthitic feldspar, with a subordinate amount of potassic feldspar. Electron microprobe analyses reveal the presence of a small amount (∼10 vol.%) of rhyolitic glass (71% SiO 2 ) within the crystalline network. This glass is the residual material left after crystallization of pyroxene and feldspar, analogous to rhyolitic melt which may be generated from fractional crystallization of a basaltic magma. Ion microprobe imaging indicates that the rhyolitic glass is generally present in isolated triangular- to rectangular-shaped interstices left by crystallization of acicular and tabular phases, and that the glass is enriched in trace elements (such as Zr and Cs) which are incompatible within the crystalline phases. No evidence of coalescence or migration of the rhyolitic melt is suggested by glass morphology. Although these observations were made in an artificial magmatic system, analogies to a natural system may be drawn. The isolated nature and low abundance of the rhyolitic glass highlights the difficulty of extracting and segregating evolved melts produced by high degrees of crystallization of a primitive parent, particularly one characterized by elongated, rather than equant, crystals. The distribution of the rhyolitic glass supports the suggestion that extraction of significant amounts of evolved melts from rocks may require repeated partial melting of crystallized material in order to allow the evolved liquid to physically separate from the parent, and also suggests that the geometry of crystals may be an important factor in melt segregation.