Volatile Solubility Research Articles

MINERALS, INCLUSIONS AND VOLCANIC PROCESSES: Keith D. Putirka and Frank J. Tepley, Editors. (2008) Reviews in Mineralogy and Geochemistry, vol. 69, 674 pp. ISBN: 978-0-939950-83-6. $40 (http://www.minsocam.org/MSA/Rim/Rim69.html)

Igneous petrology is moving toward more and more powerful techniques applied at smaller and smaller length scales. Minerals and their inclusions record a wealth of information about the compositional, thermal, volatile, and pressure conditions experienced by magmas between their source regions and the surface. A host of different techniques can now provide us with data about pre-eruptive magma storage, mechanisms and rates of magma ascent, the nature and effects of volatile degassing, magma mingling and hybridization, and the time scales over which these processes occur. This RiMG volume is an impressive compilation of papers that review the fundamental tools of the igneous petrologist, from textural studies to radioactive disequilibria and diffusion modeling. The text formed the basis for a MSA Short Course, and the MSA website (http://www.minsocam.org/MSA/RIM/Rim69.html) provides links to the presentation materials from the course, as well as Excel spreadsheets that accompany several of the chapters, addressing topics such as thermobarometry and volatile solubility. Additional resources are available online through the NAGT “On The Cutting Edge” project, including tutorials on analytical techniques and relevant teaching exercises (http://serc.carleton.edu/NAGTWorskshops/petrology). The volume covers four key aspects of petrology: (1) kinetics and thermobarometry, …

Solubility of C-O-H mixturesin natural melts: new experimental data and application range of recent models

The effect of pressure, temperature, and melt composition on CO2 and H2O solubilities in aluminosilicate melts, coexisting with CO2-H2O fluids, is discussed on the basis of previously published and new experimental data. The datasets have been chosen so that CO2 and H2O are the main fluid components and the conclusions are only valid for relatively oxidizing conditions. The most important parameters controlling the solubilities of H2O and CO2 are pressure and composition of melt and fluid. On the other hand, the effect of temperature on volatile solubilities is relatively small. At pressures up to 200 MPa, intermediate compositions such as dacite, in which both molecular CO2 and carbonate species can be dissolved, show higher volatile solubilities than rhyolite and basalt. At higher pressures (0.5 to 1 GPa), basaltic melts can incorporate higher amounts of carbon dioxide (by a factor of 2 to 3) than rhyolitic and dacitic melts. Henrian behavior is observed only for CO2 solubility in equilibrium with H2O-CO2 fluids at pressures <100 MPa, whereas at higher pressures CO2 solubility varies nonlinearly with CO2 fugacity. The positive deviation from linearity with almost constant CO2 solubility at low water activity indicates that dissolved water strongly enhances the solubility of CO2. Water always shows non-Henrian solubility behavior because of its complex dissolution mechanism (incorporation of OH-groups and H2O molecules in the melt). The model of Newman and Lowenstern (2002), in which ideal mixing between volatiles in both fluid and melt phases is assumed, reproduces adequately the experimental data for rhyolitic and basaltic compositions at pressures below 200 MPa but shows noticeable disagreement at higher pressures, especially for basalt. The empirical model of Liu et al. (2004) is applicable to rhyolitic melts in a wide range of pressure (0-500 MPa) and temperature (7001200°C) but cannot be used for other melt compositions. The thermodynamic approach of Papale (1999) allows to calculate the effect of melt composition on volatile solubilities but needs an update to account for more recent experimental data. A disadvantage of this model is that it is not available as a program code. The review indicates a crucial need of new experimental data for scarcely investigated field of pressures and fluid compositions and new models describing evident non-ideality of H-C-O fluid solubility in silicate melts at high pressures.

Amorphous materials: Properties, structure, and durability: Solubility and solution mechanisms of NOH volatiles in silicate melts at high pressure and temperature--amine groups and hydrogen fugacity

The solubility and solution mechanisms of nitrogen in silicate melts have been examined via nitrogen analyses and vibrational spectroscopy (Raman and FTIR). Pressure (P), temperature (T), hydrogen fugacity (fH₂), and silicate melt composition (degree of melt polymerization) were independent variables in experiments in the 1-2.5 GPa pressure and 1300-1500 °C temperature ranges. The fH₂ was controlled at values defined by the magnetite-hematite (MH), Mn3O4-MnO (MM), NiO-Ni (NNO), magnetite-wustite (MW), and iron-wustite (IW) buffers together with H2O.

A low-pressure high-temperature technique for the piston-cylinder

A method for conducting successful low pressure (0.3-0.5 GPa) and high temperature (900-1200 °C) experiments in the 19 mm piston-cylinder is presented. The technique is capable of running high fluid/melt experiments with minimum hydrogen loss, attaining precise, reproducible pressures (±10%), and has fast initial quench rates (&gt;150 °C/s). These abilities are invaluable when conducting low pressure, fluid-saturated experiments such as phase equilibria, volatile solubility, and dynamic degassing experiments that are relevant to sub-volcanic magma chamber processes. A double capsule construction is also described that uses a solid oxygen buffer, and minimizes both contamination of the sample by carbon and the loss of iron in the melt to the capsule walls.

Kimberlite ascent and eruption

Wilson and Head model kimberlite ascent and eruption by considering the propagation of a volatile-rich dyke. Wilson and Head's model has features in common with Sparks et al., but it is inconsistent with geological observations and constraints on volatile solubility. Here we show that this may be due to erroneous physical assumptions.

Mineral solubility and hydrous melting relations in the deep earth: Analysis of some binary A H2O system pressure-temperature-composition topologies

Phase relations involving hydrous melting, volatile and mineral solubility and supercritical fluid phenomena at high pressure for mineral-H 2 O systems are generally not completely constrained by experimental data or adequately treated in thermodynamic models. Here we examine geometric relations in pressure (P)-temperature (T)-composition (X) topologies of simple hydrous A-H 2 O binaries, thus avoiding some of the pitfalls associated with other approaches. The relations between mineral solubility surfaces, wet melting and critical L=V behavior are shown explicitly in a series of PT, TX and isopleth contoured PT projections. Our analysis highlights the significance of Clapeyron slopes of melt- and fluid-solubility isopleths for L+V coexistence, supercritical-fluid phenomena and the geometry of phase-equilibrium boundaries. The results are useful for understanding wet melting, magma degassing and fluid behavior in high-pressure metamorphic and subduction-zone environments. The diagrams illustrate the general pattern of mineral solubility in aqueous fluids and volatile solubility in silicate melts. We discuss the significance of the critical-curve geometry for phase relations and fluid/melt densities. We examine a continuum of phase relation topologies for A-H 2 O, and show that these can result from subtle but important differences in the compositional behavior of melt coexisting with H 2 O-rich fluid. The systems SiO 2 (quartz)-H 2 O and NaAlSi 3 O 8 (albite)-H 2 O are taken as examples for which there is experimental data available to calibrate a complete phase relation topology.

VOLCANISM IN THE CAMPANIA PLAIN: VESUVIUS, CAMPI FLEGREI AND IGNIMBRITES: Edited by B. De Vivo, Elsevier Developments in Volcanology 9, Amsterdam, 324 p. (2006) ISBN 0444521766 ($165)

This book is a collection of edited papers stemming from a workshop on Vesuvius and volcanism of the Campanian Plain. The workshop was organized by Bernardo De Vivo, and the results are an expressed effort by scientists from Italian institutions and foreign collaborators to tell the public exactly what is known and unknown about how a volcano works with the goal of avoiding problems that come about from oversimplifications and convenient but misleading models. De Vivo sets a critical tone in the book’s Preface by stating that the existing emergency evacuation plan is “bizarre.” He emphasizes this criticism despite the years of ground-breaking efforts by other volcanologists to get volcanic hazards and response on the political agenda. Negotiation and compromise is a fundamental of politics, and if not recognized by scientists the results of this fundamental can be viewed as a “selling out.” Unfortunately this dissenting point of view is echoed as an all-to-apparent undertone in many of the chapters of this book, evidenced by an unwilling acknowledgment and in many places a complete disregard of much of the science used to develop the existing emergency plan. Despite this perhaps negative, critical aspect, the optimism and ingenuity of pure science emerges as the book’s 14 chapters (enumerated below) and 128 illustrations progress from discussions of Campi Flegrei and the Campanian Ignimbrite to a focus on Vesuvius (“probably the best-known volcano in the world” in the words of De Vivo). In the following chapters, contributing authors reinterpret some older data and describe new data concerning magma volatile solubility experiments, fluid and melt inclusions, geophysics, geochronology, and tectonism. Some new models based on these data are detailed in order to illustrate a better understanding of the volcanic system behavior. 1. The volcanological history of the volcanoes of Naples: a review ( R. Scandone, L. …

Magma mixing during the 2001 event at Mount Etna (Italy): Effects on the eruptive dynamics

During the 2001 eruptive episode three different magmas were erupted on the southern flank of Mount Etna volcano from distinct vent systems. Major and minor element chemistry of rocks and minerals shows that mixing occurred, and that the mixed magma was erupted during the last eruptive phases. The space–time integrated analysis of the eruption, supported by geophysical data, together with major and trace element bulk chemistry (XRF, ICP-MS) and major and trace mineral chemistry (EPMA, LAM ICP-MS), support the following model: 1) trachybasaltic magma rises through a NNW–SSE trending structure, connected to the main open conduit system; 2) ascent of an amphibole-bearing trachybasaltic magma from a 6 km deep eccentric reservoir through newly open N–S trending fractures; 3) just a few days following the eruption onset the two tectonic systems intersect at the Laghetto area; 4) at the Laghetto vent a mixed magma is erupted. Mixing occurred between the amphibole-bearing trachybasaltic magma and an inferred deep more basic end-member. The most relevant aspect in the eruptive dynamics is that the eruption of the mixed magma at the Laghetto vent was highly explosive due to volatile content in the magma. The gas phase formed, mainly because of the decreased volatile solubility due to rapid fractures opening and increased T, related to mixing, and partially because of the amphibole breakdown.

Volatiles in Magmatic-Volcanic Systems

The abundance and flux of volatiles are important to the hazards associated with explosive volcanic eruptions. Volatiles in magmas can be determined from investigations of melt and fluid inclusions in combination with volatile solubility data. A comparison of the newly determined solubilities of H2O, SO2, and Cl in a molten Vesuvius phonolite with the compositions of Mt. Somma-Vesuvius melt and fluid inclusions further elucidates degassing and eruptive processes for this volcano.

The exsolution of magmatic hydrosaline chloride liquids

Hydrosaline liquid represents the most Cl-enriched volatile phase that occurs in magmas, and the exsolution of this phase has important consequences for processes of hydrothermal mineralization and for volcanic emission of Cl to the atmosphere. To understand the exsolution of hydrosaline liquids in felsic to mafic magmas, the volatile abundances and (Cl/H 2O) ratios of more than 1000 silicate melt inclusions (MI) have been compared with predicted and experimentally determined solubilities of Cl and H 2O and associated (Cl/H 2O) ratios of silicate melts that were saturated in hydrosaline chloride liquid with or without aqueous vapor in hydrothermal experiments. This approach identifies the minimum volatile contents and the values of (Cl/H 2O) at which a hydrosaline chloride liquid exsolves from any CO 2- or SO 2-poor silicate melt. Chlorine solubility is a strong function of melt composition, so it follows that Cl solubility in magmas varies with melt evolution. Computations show that the (Cl/H 2O) ratio of residual melt in evolving silicate magmas either remains constant or increases to a small extent with fractional crystallization. Consequently, the initial (Cl/H 2O) in melt that is established early during partial melting has important consequences for the exsolution of vapor, vapor plus hydrosaline liquid, or hydrosaline liquid later during the final stages of melt ascent, emplacement, and crystallization or eruption. It is demonstrated that the melt (Cl/H 2O) controls the type of volatile phase that exsolves, whereas the volatile abundances in melt control the relative timing of volatile phase exsolution (i.e., the time of earliest volatile exsolution relative to the rate of magma ascent and crystallization history). Comparing melt inclusion compositions with experimentally determined (Cl/H 2O) ratios and corresponding volatile solubilities of hydrosaline liquid-saturated silicate melts suggests that some fractions of the eruptive, calc-alkaline dacitic magmas of the Bonnin and Izu arcs should have saturated in and exsolved hydrosaline liquid at pressures of 2000 bars. Application of these same melt inclusion data to the predicted volatile solubilities of Cu-, Au-, and Mo-mineralized, calc-alkaline porphyritic magmas suggests that the chemical evolution of dioritic magmas to more-evolved quartz monzonite compositions involves a dramatic reduction in Cl solubility that increases the probability of hydrosaline liquid exsolution. The prediction that quartz monzonite magmas should exsolve a hydrosaline chloride liquid, that is potentially mineralizing, is consistent with the general observation of metal-enriched, hypersaline fluid inclusions in the more felsic plutons of numerous porphyry copper systems. Moreover, comparing the volatile contents of melt inclusions from the potassic, alkaline magmas of Mt. Somma-Vesuvius with the predicted (Cl/H 2O) ratios of hydrosaline liquid-saturated melts having compositions similar to those of the volatile-rich, alkaline magmas associated with the orthomagmatic gold–tellurium deposits of Cripple Creek, Colorado, suggests that hydrosaline chloride liquid should have exsolved at Cripple Creek as the magmas evolved to phonolite compositions. This prediction is consistent with the well-documented role of Cl-enriched, mineralizing hydrothermal fluids at this major gold-mining district.

Gas‐saturated crystallization and degassing in large‐volume, crystal‐rich dacitic magmas from the Altiplano‐Puna, northern Chile

Melt inclusions hosted in quartz from large‐volume ignimbrites and related lava flows from the late Neogene to Pleistocene Altiplano‐Puna Volcanic Complex, northern Chile, record the magmatic volatile evolution and constrain conditions of magma storage. Glasses from pristine and rehomogenized inclusions have high‐Si rhyolitic compositions (average SiO2=77.5 wt%). Their host rocks range from dacite to rhyodacite (SiO2=63.9–72.5 wt%) and have a high abundance of phenocrysts (33–55%). Infrared spectroscopic analysis of inclusions from pumice samples typically yielded H2O contents between 3.0 and 4.0 wt% and relatively low and more variable CO2 contents &lt;400 ppm. Increasing H2O contents were found in a series of successively trapped inclusions, and bubble‐free inclusions tend to have lower CO2 contents with increasing H2O. Inclusions from lava samples have lower but constant H2O contents of 2.0±0.3 wt% and CO2 close to the detection limit (∼10 ppm). Incompatible trace elements with high affinities to partition into a fluid phase (e.g., B, Cl) show minor enrichment in melt inclusions, whereas compatible trace elements (e.g., Sr, Ba) became strongly depleted due to feldspar‐dominated crystallization. Variations in H2O and CO2 contents as well as concordant preeruptive pressures inferred from volatile solubility and Al‐in‐hornblende barometry (150±50 MPa) also indicate volatile saturation and upper crustal magma storage for the ignimbrite magmas. The melt inclusion record is consistent with near‐isobaric cooling from ∼830° to 780°C (inferred from mineral thermometry) under gas‐saturated conditions in shallow (4–6 km deep) reservoirs. Model calculations for overpressures generated by closed‐system crystallization and gas exsolution either result in premature failing of the magma chamber walls or require the presence of large volumes of highly compressible magmatic foam. As an alternative, open‐system degassing of these magmas prior to their explosive and effusive eruption is proposed.

Phase equilibrium constraints on the viscosity of silicic magmas: 1. Volcanic‐plutonic comparison

By using recently determined experimental phase equilibria we show that the viscosity of granitic magmas emplaced at upper crustal levels is approximately constant at ∼104.5 Pa s, irrespective of their temperature and level of emplacement. Magmas crystallizing as granitic plutons are not water‐poor and thus not more viscous than their extrusive equivalents. Instead, comparison between pre‐eruption magma viscosities of extrusive silicic‐intermediate and intrusive granitic magmas shows that the former are on average slightly more viscous. Given the typical strain rates in silicic magma chambers, magma rheological behavior is expected to be dominantly Newtonian, bubbles having a minor rheological influence at depth although exceptions can exist. Thus whether a silicic‐intermediate magma is erupted or frozen at depth depends primarily on the rheological properties of surrounding terranes or on external tectonic factors, but not on the rheology of the magma itself. However, preemptive viscosities of extrusive magmas rarely exceed 106 Pa.s, which suggests that crystal‐melt mushes with higher viscosities cannot leave the magma storage regions beneath volcanoes. The narrow range of viscosities displayed by silicic‐intermediate magmas results from both the strong control that pressure exerts on volatile solubilities in silicate melts and thermal limitations required to produce acid magmas. Considerations of the relationships between magma crystallinities, bulk SiO2, and preemptive melt H2O contents show that the higher the melt H2O content is the higher the maximum crystallinity that a given magma will be while still being potentially erupted. An empirical correlation is proposed that enables us to estimate preemptive melt H2O contents of erupted magmas by knowing their crystallinity and bulk SiO2.

Experimental investigation of H 2 O and Cl ? solubilities in F-enriched silicate liquids; implications for volatile saturation of topaz rhyolite magmas

To interpret the degassing of F-bearing felsic magmas, the solubilities of H2O, NaCl, and KCl in topaz rhyolite liquids have been investigated experimentally at 2000, 500, and ≈1 bar and 700° to 975 °C. Chloride solubility in these liquids increases with decreasing H2O activity, increasing pressure, increasing F content of the liquid from 0.2 to 1.2 wt% F, and increasing the molar ratio of ((Al + Na + Ca + Mg)/Si). Small quantities of Cl− exert a strong influence on the exsolution of magmatic volatile phases (MVPs) from F-bearing topaz rhyolite melts at shallow crustal pressures. Water- and chloride-bearing volatile phases, such as vapor, brine, or fluid, exsolve from F-enriched silicate liquids containing as little as 1 wt% H2O and 0.2 to 0.6 wt% Cl at 2000 bar compared with 5 to 6 wt% H2O required for volatile phase exsolution in chloride-free liquids. The maximum solubility of Cl− in H2O-poor silicate liquids at 500 and 2000 bar is not related to the maximum solubility of H2O in chloride-poor liquids by simple linear and negative relationships; there are strong positive deviations from ideality in the activities of each volatile in both the silicate liquid and the MVP(s). Plots of H2O versus Cl− in rhyolite liquids, for experiments conducted at 500 bar and 910°–930 °C, show a distinct 90° break-in-slope pattern that is indicative of coexisting vapor and brine under closed-system conditions. The presence of two MVPs buffers the H2O and Cl− concentrations of the silicate liquids. Comparison of these experimentally-determined volatile solubilities with the pre-eruptive H2O and Cl− concentrations of five North American topaz and tin rhyolite melts, determined from melt inclusion compositions, provides evidence for the exsolution of MVPs from felsic magmas. One of these, the Cerro el Lobo magma, appears to have exsolved alkali chloride-bearing vapor plus brine or a single supercritical fluid phase prior to entrapment of the melt inclusions and prior to eruption.

Fluid inclusions in the Harney Peak Granite, Black Hills, South Dakota, USA: Implications for solubility and evolution of magmatic volatiles and crystallization of leucogranite magmas

Experimental and theoretical studies have shown that magmatic fluids have strong influence on phase equilibria of granitic systems. A microthermometric study of fluid inclusions in the Proterozoic Harney Peak Granite, Black Hills, South Dakota, USA, was undertaken to obtain direct evidence for the composition and evolution of magmatic fluids in a leucogranitic system and to evaluate their effect on liquid-lines-of-descent of leucogranite magmas. A previous stable isotopic study has demonstrated that the granite has not interacted with nonmagmatic fluids since its crystallization. Therefore, fluids found in the inclusions are of magmatic origin and present an opportunity to directly sample magmatic fluids trapped during crystallization of a granitic magma and during subsolidus conditions. The Harney Peak Granite was emplaced as multiple sills, dikes, and small intrusions, many of which differentiated into coupled aplite-pegmatite layers. Three types of fluid inclusions are found in the granite. Type 1 consist of variable saline H 2OC0 2 mixtures with minor CH 4 or other gases. Inclusions of this type are found in tourmaline and quartz in aplite layers, are generally isolated, and have negative crystal shapes. They are interpreted to be primary. There is a general increase in salinity and decrease in isochore temperature with decreasing CO 2/H 2O ratio of the inclusions. The trend is ascribed to progressive change in fluid composition with crystallization due to the differential solubility of CO 2 and H 2O in the high-silica magma. Isochores, coupled with independently determined crystallization temperatures, suggest entrapment pressure ∼3.5 kbar, consistent with geobarometry in the wallrocks. Type 2 inclusions are carbonic fluids dominated by CO 2, whereas type 3 inclusions are saline aqueous fluids without a detected carbonic component. Both types are mostly found along healed fractures in quartz. They are interpreted to be secondary inclusions that formed as a result of unmixing of type 1 inclusions at subsolidus conditions during uplift and cooling of the Harney Peak Granite dome. Freezing-point depressions, birefringent daughter crystals, phase-equilibria considerations, and α-track mapping of B and Li distribution in thin sections, all indicate that both type 1 and type 3 inclusions contain solutes such as K, Li, and B in addition to NaCl. Using published solubility models for H 2O and CO 2 in granitic melts and the composition of the most CO 2-rich type 1 inclusions ( X CO 2 = 0.55), the initial H 2O and CO 2 concentrations in the Harney Peak Granite magma are deduced to have been ∼3.5 wt% and 1500 ppm, respectively. The relatively low water concentration is consistent with temperatures of dehydration-melting reactions that produced the magma. It is suggested that the aplite-pegmatite segregations found in parts of the Harney Peak Granite are the result of initial rapid near liquidus crystallization of minerals with high Na/K ratios, including plagioclase and tourmaline, along a wide range of increasing αH 2O. Aplite crystallization was followed by crystallization of relatively potassic residual liquid enriched in fluxing elements under conditions of high αH 2O.

Consequences of explosive eruptions on small Solar System bodies: the case of the missing basalts on the aubrite parent body

Aubrite meteorites are composed of constituents which are almost certainly of igneous origin. If they were generated by the melting and fractionation of enstatite chondrite-like parental material, as seems very likely, then plagioclase-rich, basaltic complements to the aubrites should have formed. However, such materials are not known as individual meteorites, and the compositions of two plagioclase-silica clasts and one albite-silica-(diopside-anorthite) clast (probably an impact melt) in the Norton County aubrite breccia suggest that they are not the putative enstatite-plagioclase basalts. We propose a new mechanism that explains the absence of these materials, showing that the expansion of even very small amounts of volatiles present in a melt approaching the surface of a small, low-gravity body will be enough to disrupt the melt into a spray of droplets moving faster than the local escape velocity. This explosive volcanic process of melt removal requires larger melt volatile contents on larger bodies, and data on the solubility of volatiles in basaltic melts suggest that the process was limited to bodies smaller than about 100 km in radius.

Absorption of Citrus Flavor Volatiles by Low Density Polyethylene

ABSTRACTThe ability of low density polyethylene (LDPE) to absorb citrus flavor compounds was examined. The LDPE sample was mounted on an oxygen electrode. As volatiles were absorbed, oxygen permeation through the polymer increased. Limonene, ethyl butyrate, myrcene, and α‐pinene were readily absorbed by LDPE. Octanal, citral, linalool, and α‐terpineol were absorbed at much lower levels. Time‐course changes in readings were used to calculate diffusion coefficients of volatiles in the polymer. Diffusion coefficients were proportional to the volatile's solubility in the polymer.

Degassing of magmas: Constraints from experimental determinations of volatile solubilities and studies of volcanic glasses

Solubilities of CO_2 and H_2O have been determined experimentally for basaltic and rhyolitic melts over a range of pressures. Infrared (IR) spectroscopy was used to measure concentrations of carbonate, hydroxyl, and CO_2 and H_2O molecules in glasses quenched from vapor-saturated melts.

Volatile solubilities in magmas: Transport of volatiles from mantles to planet surfaces

Experimental measurements of the solubility of CO2, CO, CH4, H2, and H2O in silicate melt were made at pressures from 10 to 20 Kb and temperatures of 1200°C. The C‐O‐H fluid phase was buffered by graphite, iron, and wüstite. Detailed measurements were made on melts of NaAlSi3O8 composition and the results confirmed for alkalic and tholeiitic basalt liquids. The melt volatiles have dramatically lower H2/H2O and higher CO/CO2 ratios than the fluid. The result is large differences between the C‐O‐H compositions of the melt and the fluid (fluids about 16 atm % C, 6% O, and 78% H; melts about 4% C, 32% O, and 64% H). The measured solubilities allow calculation of activity coefficients for volatile species in the melt. They are, roughly, CO = 0.1, H2O = 0.6, CO2 = 1, H2 = 3, and CH4 = 60. This two‐order‐of‐magnitude variation in activity coefficients causes the significant volatile fractionation observed between fluid and melt. We argue that the same fractionation will occur in the more usual case of fluid‐absent partial melting of planetary mantles. The fractionation is such that reducing mantles will form magmas that yield volcanic gases about 1.5 orders‐of‐magnitude more oxidizing than the mantle source, while oxidizing mantles will form magmas with more reduced volcanic gases. We conclude that a volcanic gas composition cannot be directly used to estimate either the oxygen fugacity or volatile composition of its magma source region. The results suggest that volcanic gases will normally lie in the “neutral” range of oxygen fugacity, near that of the quartz‐fayalite‐magnetite buffer. These gases are predominately H2O with minor CO2, CO, H2, and CH4.

Modern Igneous Petrology

Mohan K. Sood prefaces Modern Igneous Petrology by stating that his objective is to facilitate the understanding of the application of phase equilibrium data to the crystallization and evolution of magmas. He states that the book may be valuable as either a reference or source text or as a text for courses in igneous petrology for undergraduate and graduate students. A review of recent literature and extensive references are promised the reader.There are eight chapters in the book. The presentation is theme‐oriented and progresses from discussion of phase equilibrium in simple binary systems to those in complex synthetic and natural systems. The Introduction is intended to be ‘an overview of the igneous process and the state of the art of experimental petrology.’ Chapters 2 and 3, comprising in excess of one half of the book in extent, detail anhydrous silicate systems at 1‐atm bearing on mafic magmas and residual alkaline magmas, respectively. The following four chapters deal in a perfunctory manner with silicate systems containing volatiles, volatile solubilities in silicate melts, melting relations of rocks, and the genesis of magmas. Chapter 8 summarizes the conclusions.

Investigation of volcanic gas analyses and magma outgassing from Erta' Ale lava lake, Afar, Ethiopia

The analyses of 18 volcanic gas samples collected over a two-hour period at 1075°C from Erta' Ale lava lake in December 1971 and of 18 samples taken over a half-hour period at 1125–1135°C in 1974 display moderately to intensely variable compositions. These variations result from imposed modifications caused by (1) atmospheric contamination and oxidation, (2) condensation and re-evaporation of water during collection, (3) analytical errors, and (4) chemical reactions between the erupted gases and a steel lead-in tube. Detailed examinations of the analyses indicate the erupted gases were at chemical equilibrium before collection. This condition was partially destroyed by the imposed modifications. High-temperature reaction equilibria were more completely preserved in the 1974 samples. Numerical procedures based on thermodynamic calculations have been used to restore each analysis to a composition representative of the erupted gases. These procedures have also been used to restore the anhydrous mean compositions reported for two series of collections taken at the lava lake in January 1973. The corrected analyses for 1971 and 1973 have similar compositions (69.6–71.3% H 2O, 1.6–2.4% H 2, 17.8–19.4% CO 2, 0.8–1.6% CO, 4.9–8.8% SO 2, 0.2–0.5% S 2, and 0.4–1.0% H 2S); those for 1974 were somewhat different (77.1% H 2O, 1.6% H 2, 11.7% CO 2, 0.5% CO, 7.4% SO 2, 0.3% S 2, 0.9% H 2S and 0.4% HCl). The O 2 and S 2 fugacities of all restored analyses are similar when compared at the same temperatures. O 2 fugacities are close to those of the quartz-magnetite-fayalite buffer. The restored analyses show no evidence of significant short-term (minutes, hours) variations in the compositions of the gases released from the lava lake, and evidence of long-term variations is limited. The restored analyses indicate the O 2 and S 2 potentials of the lava lake remained nearly constant from 1971 to 1974. However, there is a relative decrease in CO 2 between the 1973 and 1974 corrected compositions. This change is qualitatively compatible with magma outgassing controlled by solubility differences and could reflect a longer term trend. The chemistry of the Erta' Ale gases is consistent with models invoking the generation of basalts resembling mid-ocean ridge tholeiites along axial zones of crustal spreading in the Afar Triangle. The CO 2-rich vesicle gases of submarine basalts and the relatively H 2O-rich volcanic gases of Erta'Ale appear to be end-member compositions of an outgassing trend controlled largely by the effect of pressure on volatile solubilities in tholeiitic basalts from spreading zones. By analogy with submarine basalts H 2O and sulfur in the Erta' Ale gases are mainly contributed by magma outgassing down to shallow depths (50–250 m); CO 2 contributions probably come from larger volumes of magma down to depths of at least 1000 m.