Malcolm J. Rutherford

Magma ascent rates from amphibole breakdown: An experimental study applied to the 1980–1986 Mount St. Helens eruptions

Recent 1980–1986 Mount St. Helens dacites contain the phenocryst assemblage, plagioclase, amphibole, low-Ca pyroxene, magnetite, ilmenite, and rare high-Ca pyroxene, which indicates that they all originated from an 8 km deep reservoir at 900°±20°C with XH2O= 0.67 in fluid according to experimental data. Iron-titanium oxide phenocryst compositions indicate that all post May 18 dacitic magmas erupted at 900°±20°C except for the final lava extrusion in October 1986; the magma reservoir may have cooled to 866°C by October 1986. Amphiboles in the post May 18, 1980, magma contain one or more amphibole populations characterized by reaction rims of different thicknesses. The development of the amphibole reaction rims in these rocks is a response to water loss from the coexisting melt during an approximately adiabatic ascent from a deep reservoir. Constant P and T and isothermal decompression experiments show that during a 900°C constant rate decompression from 8 km to the surface, no reaction rim develops on amphibole in 4 days, a 10-μm rim develops in 10 days, and a 35-μm rim develops in 20 days. These experimental data and histograms of rim widths in 1980–1986 Mount St. Helens dacites show that post May 18 eruptions are composed in large part of magma represented by a population of thin-rimmed amphiboles, magma which ascended from the deep (8 km) reservoir in 6 to 10 days. The remainder of each sample consists of magma containing amphiboles with reaction rims ranging from 14 to 60 μm, magma which apparently spent from 8 to 25 days along the conduit margins before being mixed thoroughly (millimeter scale) into the erupting magma. The mixing in a viscous, slowly ascending dacite may be enhanced by its flow through partially crystallized magma emplaced earlier and by the evolution and loss of a large vesicle population. The experimental calibration of amphibole reaction rim width versus decompression time yields average ascent velocities for post May 18 dacites of about 15–30 m/h for magma represented by the thick-rimmed amphiboles and from 35 to 50 m/hr for magma represented by the thin-rimmed crystals. An ascent rate of >66 m/h is indicated for the May 18, 1980, eruption, which contains amphiboles with no reaction rims. The volume of endogenous dome growth which preceded extrusion of magma newly derived from the deep source region suggests that the effective conduit volume beneath Mount St. Helens in 1981–1982 was equivalent to a cylinder 8 km long and 8–9 m in radius.

Comparison of microanalytical methods for estimating H 2 O contents of silicic volcanic glasses

Abstract Extrapolation of laboratory measurements of the viscosity (η)of silicate melts is frequently needed in order to analyze petrological and volcanological processes. Therefore a general understanding of silicate melt viscosities is required. In this paper we survey the present state of our knowledge and distinguish three flow regimes for homogeneous silicate liquids: (1) a low-viscosity regime (η < 1 Pa·s), where the viscosity obeys a temperaturedependence power law in accordance with mode coupling theory (these low viscosities are typical for depolymerized melts); (2) an intermediate regime (1 < η< 1012 Pa·s), where silicate melt viscosity is determined by the availability of configurational states (the dependence of the viscosity on the temperature is described aptly by the configurational entropy theory of Adam-Gibbs); and (3) a high-viscosity regime, where the liquid has been transformed into a glass (η> 1012 Pa·s) (this regime is not well known, but available measurements indicate an Adam-Gibbs or an Arrhenian temperature dependence of the glass viscosity). Examples are given of igneous rocks whose geneses were affected by these flow regimes.

Experimental phase equilibria constraints on pre-eruptive storage conditions of the Soufriere Hills magma

New experimental results are used to constrain the P. T, X(H 2 O) conditions of the Soufriere Hills magma prior to ascent and eruption. The experiments were performed on a powdered andesite erupted in January, 1996, at an fO 2 corresponding to ∼NNO+1 with P H2 O and temperatures in the range 50 to 200 MPa and 800 to 940°C. Amphibole is stable at P H2 O >115 MPa and temperatures 72 wt% SiO 2 in residual melt) at P H2 O >115 MPa. Analyses of rhyolitic glass inclusions in quartz and plagioclase from recently erupted samples indicate melt water contents of 4.27±0.54 wt% H 2 O and CO 2 contents <60 ppm. The evolved Soufriere Hills magma would therefore be H 2 O-saturated at pressures <130 MPa. These results suggest that the Soufriere Hills magma containing the stable assemblage amphibole, quartz, plagioclase, orthopyroxene, magnetite and ilmenite was stored at P H2 O of 115-130 MPa, equivalent to a minimum depth for a water-saturated magma chamber of 5-6 km depth. Magma temperatures were initially low (820-840°C). Quartz is believed to have been destabilised by a heating event involving injection of new basaltic magma. The stability field of hornblende provides a useful upper limit (∼880°C) for the extent of this reheating.

Crystallization of microlites during magma ascent : the fluid mechanics of 1980-1986 eruptions at Mount St Helens

Eruptions of Mount St Helens (Washington, USA) decreased in intensity and explosivity after the main May 18, 1980 eruption. As the post-May 18 eruptions progressed, albitic plagioclase microlites began to appear in the matrix glass, although the bulk composition of erupted products, the phenocryst compositions and magmatic temperatures remained fairly constant. Equilibrium experiments on a Mount St Helens white pumice show that at 160 MPa water pressure and 900°C, conditions deduced for the 8 km deep magma storage zone, the stable plagioclase is An47. The microlites in the natural samples, which are more albitic, had to grow at lower water pressures during ascent. Isothermal decompression experiments reported here demonstrate that a decrease in water pressure from 160 to 2 MPa over four to eight days is capable of producing the albitic groundmass plagioclase and evolved melt compositions observed in post-May 18 1980 dacites. Because groundmass crystallization occurs over a period of days during and after decreases in pressure, microlite crystallization in the Mount St Helens dacites must have occurred during the ascent of each magma batch from a deep reservoir rather than continuously in a shallow holding chamber. This is consistent with data on the kinetics of amphibole breakdown, which require that a significant portion of magma vented in each eruption ascended from a depth of at least 6.5 km (∼160 MPa water pressure) in a matter of days. The size and shape of the microlite population have not been studied because of the small size of the experimental samples; it is possible that the texture continues to mature long after chemical equilibrium is approached. As the temperature, composition, crystal content and water content of magma in the deep reservoir remained approximately constant from May 1980 to at least March 1982, the spectacular decrease in eruption intensity during this period cannot be attributed to changes in viscosity or density of the magma. Simple fluld mechanical considerations indicate, however, that the observed changes in mass flux of magma can be modelled by a five-fold decrease in conduit radius from 35 to 7 m, produced perhaps by plating of magma along the conduit walls. The decreased ascent rates which accompanied the decrease in conduit radius can explain the change from closed-system to open-system degassing and the shift from explosive to effusive eruptions during 1980.

Determination of molar absorptivities for infrared absorption bands of H2O in andesitic glasses

Abstract We have determined infrared molar absorptivities for water absorption bands in Fe-bearing and Fe-free andesitic glasses. Water dissolves in andesitic glasses as both hydroxyl groups and molecular water as observed in other silicate glasses. Concentrations of molecular water and hydroxyl species are a strong function of total water content. IR molar absorptivities for Fe-bearing andesite are ε3570 = 62.32 ± 0.42 L/mol·cm, ε4500 = 0.79 ± 0.07 L/mol·cm, ε5200 = 1.07 ± 0.07 L/mol·cm, and ε1630 = 42.34 ± 2.77 L/mol·cm. Molar absorptivities for Fe-free andesite are 69.21 ± 0.52 L/mol·cm for e3570, 0.89 ± 0.07 L/mol·cm for e4500, 1.46 ± 0.07 L/mol·cm for e5200, and 52.05 ± 2.85 L/mol·cm for ε1630. Molar absorptivities show significant compositional dependencies that can be predicted based on tetrahedral cation (Si+4, Al+3)/total cation fraction

Petrology and Fe–Ti oxide reequilibration of the 1991 Mount Unzen mixed magma

A dacitic magma (64.5 wt.% SiO2), a mixture of phenocryst-rich rhyodacite and an aphyric mafic magma, was erupted during the recent 1991–1995 Mount Unzen eruptive cycle. The experimental and analytical results of this study reveal additional details about conditions in the premixing and postmixing magmas, and the nature of the mixing process. The preeruption rhyodacitic magma was at a temperature of 790±20°C according to Fe–Ti oxide phenocryst cores, and at a depth of 6 to 7 km (160 MPa) according to Al-in-hornblende geobarometry. The mafic magma that mixed with the rhyodacite is found as andesitic (54 to 62 wt.% SiO2) enclaves in the erupted magma and was essentially aphyric when intruded. Phase equilibria indicate that an aphyric andesite at 160 MPa is >1030°C (H2O-saturated) and possibly as high as 1130°C (2 wt.% H2O). The composition of the rhyodacite which was mixed with the andesite is estimated to lie between 67 and 69 wt.% SiO2. Using these compositions and temperatures, the temperature of the Unzen magma after mixing is estimated to be at least 850° to 870°C. The groundmass Fe–Ti oxide microphenocrysts and those in pargasite-bearing reaction zones around biotite phenocrysts both give 890±20°C temperatures; the oxide–oxide contacts give temperatures of 910±20°C. The 900±30°C postmixing temperatures are consistent with phase-equilibria experiments which show that the magma was not above 930°C at 160 MPa. Our Fe–Ti oxide reequilibration experiments suggest that the mixing of the two magmas began within a few weeks of the eruption, which is a shorter time than is calculated using available diffusion data. There is also evidence that some mixing took place much closer to the time of extrusion based on the presence of unrimmed biotite phenocrysts in the magma.

Plagiogranites as late-stage immiscible liquids in ophiolite and mid-ocean ridge suites - An experimental study

Chemical studies of two ophiolite suites and of selected mid-oceanic rift (MOR) regions indicate the presence of certain magmatic compositions: basalt, Fe-enriched basalt, and sodium granite (plagiogranite). There is a notable lack of evidence for melts of intermediate composition (i.e. 50–60 wt.% SiO2). To determine possible relationships between basic rocks (basalts and gabbros) and acidic rocks (plagiogranites) a primitive basalt was fractionated at low pressure, under anhydrous conditions, and at different oxygen fugacities near the iron-wustite buffer and slightly above the quartz-fayalite-magnetite buffer. Samples of this basalt were taken to slightly above liquidus temperatures and then cooled at rates ranging from 1 to 2°C/hr. A liquid line of descent characterized by an Fe enrichment was delineated by quenching these experiments from a final temperature in the range of 1200 to 1000°C and analyzing the residual liquid (glass). After 95% crystallization of olivine, plagioclase, calcium pyroxene, and ilmenite, the residual liquid was an Fe-enriched basalt. This Fe-enriched basalt became immiscible at a temperature of about 1010°C. The immiscible phases produced were a more Fe-enriched basaltic liquid and a granitic liquid. The granitic liquid is similar in composition to the naturally occurring plagiogranites found in small volumes in ophiolites and in certain MOR regions. It is therefore concluded that silicate liquid immiscibility could be the petrogenetic process responsible for producing plagiogranite in some MOR regions and in some ophiolites. On the other hand, plagiogranites in ophiolites and MOR rock suites having andesitic and dacitic composition rocks may have evolved under conditions more closely approximating equilibrium crystallization and/or they may have evolved at high water pressures. The available experimental data suggest that amphibole would crystallize early and yield SiO2-enriched liquids at depths greater than 4.5 km for PH2Os in the range 0.6–1.0 Ptotal. The major problem in interpreting any of the natural plagiogranites as products of silicate liquid immiscibility is the fact that neither the Fe-enriched conjugate liquid or its crystalline equivalent has been described in the ophiolite or MOR literature. The identification of this Fe-rich conjugate magma is essential in any rock suite if a completely convincing case for silicate liquid immiscibility is to be made.

Petrologic evidence for pre‐eruptive pressure‐temperature conditions, and recent reheating, of andesitic magma erupting at the Soufriere Hills Volcano, Montserrat, W.I.

The recent eruption of the Soufriere Hills Volcano in Montserrat (July, 1995, to present; September, 1997) has produced an andesitic dome (SiO2 ∼ 59–61 wt.%). The eruption has been caused by invasion of mafic magma into a preexisting andesitic magma storage region (P ∼ 130 MPa; ≥5 km depth). The composition of the andesite has remained essentially constant throughout the eruption, but heating by the mafic magma increased the andesite temperature from ≤830°C to ≤880°C. Prior to being heated, the stable mineral assemblage in the andesite was plagioclase + amphibole + orthopyroxene + titanomagnetite + ilmenite + quartz. The rise in temperature from ≤830°C to ≤880°C (fO2 ∼ 1 log unit above NNO) has caused quartz to become unstable, and has also caused changes in silicate and Fe-Ti oxide mineral compositions. The andesitic magma is likely saturated with an H2O-rich vapor phase in the upper part of the magma storage region. Melt H2O content is ∼4.7 wt.%.

Chassigny petrogenesis - Melt compositions, intensive parameters, and water contents of Martian (?) magmas

Abstract The SNC meteorites are a class of eight basaltic achondrites that have extremely young crystallization ages (≤ 1.3 Ga). Chassigny, a unique SNC meteorite, is a dunite containing Fo68 olivines and rare poikilitic Ca-pyroxenes. It is one of the most primitive SNC meteorites and thus is most likely to reveal information about the SNC basalt source region. This study presents a detailed examination of partially crystallized melt inclusions in cumulus olivine grains in Chassigny. These trapped melts are argued to be representative samples of the melt that existed when Chassigny crystallized. This melt has been modified, however, by interaction with the host olivine and by closed-system crystallization. The phases inside the melt inclusions include hydrous Ti-rich amphibole (kaersutite), biotite, two pyroxenes, and rhyolitic and alkali feldspar glasses. All phases have been extensively analyzed with an electron microprobe. The new mineralogical information is combined with a complementary experimental study of kaersutite/melt equilibria. This combined data set can be used to formulate a system of linear equations which can then be solved to determine the composition of the originally trapped melt. This calculation reveals that the trapped melt was an FeO-rich and Al2O3-poor basalt. The melt composition is shown to be consistent with the crystallization sequence both inside the melt inclusions and in the rock matrix. The major element chemistry of this liquid closely resembles terrestrial boninite lavas. The new data also allow the intensive conditions (temperature, total pressure and water fugacity) of Chassigny crystallization to be estimated. Two-pyroxene geothermometry indicates that equilibration temperatures were ∼1000 ± 50°C, although amphibole does not coexist with melt until T = 960°C. The trapped liquid initially contained 1.5 wt% dissolved water. Crystallization of anhydrous phases caused water to buildup in the trapped melt. Prior to kaersutite crystallization, the melt must have contained at least 4 wt% dissolved water, suggesting that a minimum of 1.5 kbar total pressure is required. A maximum total pressure cannot be inferred, but all available data are consistent with low pressure (⪡5 kbar) crystallization. Finally, X(H2O) in the fluid in the melt inclusion is estimated to have been at least 0.8 in order to stabilize amphibole without plagioclase, implying that the water fugacity was ∼ 1480 bars. The existence of hydrous melts and conditions appropriate for amphibole crystallization suggest that evolved, SiO2-rich lavas exist on the Chassigny parent body (Mars).

Experimental constraints on pre-eruptive water contents and changing magma storage prior to explosive eruptions of Mount St Helens volcano

Compositionally diverse dacitic magmas have erupted from Mount St Helens over the last 4000 years. Phase assemblages and their compositions in these dacites provide information about the composition of the pre-eruptive melt, the phases in equilibrium with that melt and the magmatic temperature. From this information pre-eruptive pressures and water fugacities of many of the dacites have been inferred. This was done by conducting hydrothermal experiments at 850°C and a range of pressures and water fugacities and combining the results with those from experiments at temperatures of 780 and 920°C, to cover the likely range in equilibration conditions of the dacites. Natural phase assemblages and compositions were compared with the experimental results to infer the most likely conditions for the magmas prior to eruption. Water contents disolved in the melts of the dacites were then estimated from the inferred conditions. Water contents in the dacites have varied greatly, from 3.7 to 6.5 wt.%, in the last 4000 years. Between 4000 and about 3000 years ago the dacites tended to be water saturated and contained 5.5 to 6.5 wt.% water. Since then, however, the dacites have been significantly water-undersaturated and contained less than 5.0 wt.% water. These dacites have tended to be hotter and more mafic, and andesitic and basaltic magmas have erupted. These changes can be explained by variable amounts of mixing between felsic dacite and basalt, to produce hotter, drier and more mafic dacites and andesites. The magma storage region of the dacitic magmas has also varied significantly during the 4000 years, with shifts to shallower levels in the crust occurring within very short time periods, possibly even two years. These shifts may be related to fracturing of overlying roof rock as a result of magma with-drawal during larger volume eruptions.