Decompression History Research Articles

The role of decompression history in gas bubble formation in crystal-rich silicic magma: Gas retention versus segregation

In crystal-rich magma, gas bubble formation causes magma lubrication by loosening the connected crystal framework. In contrast, if the crystal framework behaves as rigid, the gas bubbles segregate. In this study, we experimentally investigated the bifurcation between retention and segregation of gas bubbles in crystal-rich magmas under decompression. In particular, we focused on the history of decompression such as continuous, single-step, and multi-step decompressions with various degrees of each decompression step to consider the processes in the natural system. For this, the decompression experiments for hydrous rhyolitic melts with 50 vol% crystal were conducted with average rates of 10 and 80 MPa h−1 at a temperature of 800 °C. Bubble and crystal microstructures were analyzed using micro X-ray CT. Our experiments observed both retention and expansion of gas bubbles in the crystal-rich zone and permeable gas segregation. At an average rate of 80 MPa h−1 under multistep decompression, the boundary between the retention and segregation was the stepwise decompression of 5 MPa. On the other hand, the gas bubbles are retained when the degree of the stepwise decompression is larger than 20 MPa under an average decompression rate of 10 MPa h−1, resulting in the collapse of the framework. At the decompression degree <10 MPa at the rate of 10 MPa h−1, permeable gas segregation becomes dominant. Based on the experimental results, we established a regime map that predicts gas bubble retention and segregation during decompression. By applying this map to decompression rates and numbers of caldera collapses (twice) during the 1912 Katmai and 1991 Pinatubo eruptions, we infer that the magma during these eruptions experienced gas bubble formation (retention) without permeable segregation.

Thermal and decompression history of the Lanzo Massif, northern Italy: Implications for the thermal structure near the lithosphere–asthenosphere boundary

The lithosphere–asthenosphere boundary (LAB) is a zone where thermal, mechanical, and material interactions take place between the conductive mantle and the underlying convective mantle, and it plays an important role in plate tectonics. In this paper we focus on the thermal aspects of the LAB zone, based on a petrological study of a large peridotite complex that experienced exhumation in the solid state, for the most part. The complex of interest is the Lanzo Massif in the western Alps, northern Italy, where we were able to clarify its thermal and exhumation history and estimate the original thermal structure before exhumation. We examined plagioclase-bearing lherzolites collected from 16 localities, covering the entire massif. All the constituent minerals show compositional heterogeneities on the grain scale. The patterns of Ca, Al, and Cr zoning in pyroxene, the fluorescence-corrected Ca zoning of olivine adjacent to pyroxenes, the Cr and Al zoning of spinel adjacent to plagioclase, and the Ca and Na zoning of plagioclase suggest that an early, nearly isothermal decompression of the Lanzo Massif partly crossing the dry solidus was followed by monotonous cooling through the plagioclase-facies peridotite field. Various deformation microstructures allowed us to specify the timing of the deformation in the framework of the decompression history by carefully observing their relationships with the compositional zoning of the minerals. We show that the deformation took place mainly when effective cooling had started following a period of nearly isothermal exhumation. By applying several geothermometers and evaluating the compositional zoning of the minerals, we were able to quantify the spatial variations in the thermal and decompression history of the Lanzo Massif and constrain the timescales of decompression and cooling. All the estimated temperatures decrease from the southern body towards the northern body. The grain-scale patterns of zoning indicate that the temperatures recorded by the cores of orthopyroxene (1000–1200 °C) indicate a long period of residence in the mantle, whereas those recorded by the rims of pyroxenes and the cores and rims of olivine (600–1100 °C) represent closure temperatures at various times during the decompression. All the closure temperatures decrease from south to north, while the temperature differences between the cores and rims of orthopyroxene increase. This suggests that the cooler and probably shallower northern body cooled at a relatively slow rate than the hotter and probably deeper southern body. The decrease in temperature of ~60 K from south to north, calculated from orthopyroxene cores, may represent the geotherm near the LAB zone. A thermal gradient of ~10 K/km is indicated, which is significantly greater than that estimated for deep subcontinental lithosphere in a steady thermal state. Such a high geotherm might have been caused by thermal perturbation from the underlying hotter asthenospheric mantle.

White Matter Status of Participants in Altitude Chamber Research and Training.

Magnetic resonance imaging (MRI) brain scans of U.S. Air Force (USAF) altitude workers show increased white matter hyperintensities (WMH) that appear related to decompression stress. Relevant exposure thresholds are unknown. This MRI survey compares the white matter status of UK participants (UKP) in altitude chamber research and training with USAF cohorts having background and increased WMH. UKP (N = 20) comprised 13 research subjects and 7 military altitude chamber instructors ages 33 to 50 yr (16 men, 4 women), encompassing 1417 decompressions over a 15,000-ft (4572 m) pressure altitude (range 11-189; median 50). High resolution MRI reproduced USAF sequences and data were analyzed at the University of Maryland to validate comparison with age-matched USAF control (DOC; N = 85) and aerospace operational physiologist (PHY; N = 55) cohorts. UKP data are dichotomous: 17 subjects (85%) had normal scans (total 19 WMH) and three outliers had excess (>15) WMH (total of 83 lesions). WMH were not associated with metrics of decompression history (total exposures, rapid decompression, pressure breathing, hypoxia familiarization, decompression sickness, or exposure intensity). Ranked data indicate that UKP have fewer WMH than PHY but not DOC. UKP outliers' excess WMH are attributable to past mild traumatic brain injury. WMH in UKP are unrelated to subjects' low intensity (brief, infrequent) experience of altitude chamber decompression, encompassing occasional hypobaric hypoxia and mild decompression sickness, even with cumulative experience over many years. Such low intensity hypobaric exposure appears 'subthreshold' for promotion of WMH.Connolly DM, Lee VM, Hodkinson PD. White matter status of participants in altitude chamber research and training. Aerosp Med Hum Perform. 2018; 89(9):777-786.

The effect of nonlinear decompression history on H2O/CO2 vesiculation in rhyolitic magmas

AbstractMagma ascent rate is one of the key parameters that control volcanic eruption style, tephra dispersion, and volcanic atmospheric impact. Many methods have been employed to investigate the magma ascent rate in volcanic eruptions, and most rely on equilibrium thermodynamics. Combining the mixed H2O‐CO2 solubility model with the diffusivities of both H2O and CO2 for normal rhyolitic melt, we model the kinetics of H2O and CO2 in rhyolitic eruptions that involve nonlinear decompression rates. Our study focuses on the effects of the total magma ascent time, the nonlinearity of decompression paths, and the influence of different initial CO2/H2O content on the posteruptive H2O and CO2 concentration profiles around bubbles within the melt. Our results show that, under most circumstances, volatile diffusion profiles do not constrain a unique solution for the decompression rate of magmas during an eruption, but, instead, provide a family of decompression paths with a well‐defined trade‐off between ascent time and nonlinearity. An important consequence of our analysis is that the common assumption of a constant decompression rate (averaged value) tends to underestimate the actual magma ascent time.

Formation of spinel-cordierite-plagioclase symplectites replacing andalusite in metapelitic migmatites of the Alvand aureole, Iran

AbstractSpinel–cordierite–plagioclase symplectites partially replacing andalusite occur in the metapelitic migmatite rocks of the Alvand aureole within the Sanandaj–Sirjan metamorphic belt, Hamadan, Iran. The presence of melt shows that corona development occurred under partial melting conditions. Spinel is predicted to grow with cordierite at around 700°C. Exhaustion of the available SiO2 and/or separation of sillimanite/andalusite from SiO2-rich matrix domains by cordierite resulted in the formation of localized low-silica activity domains and thus triggered the growth of spinel in the rim of andalusite, the reaction Sil/And + Bt = Crd + Spl + Kfs + melt, as the most common reaction for the development of coronas in the metapelitic of Alvand aureole. The breakdown of garnet to plagioclase + sillimanite, dehydration melting and the formation of spinel–plagioclase symplectite could occur during heating or decompression; these textures are limited to the contact aureole in the studied area, so heating is perhaps the more likely explanation for formation of the symplectites in the metapelitic rocks of the Alvand aureole. The P–T diagram, inferred paths and zoning profiles of garnet do not account for the decompression history of the terrane.

CO2 fluid inclusions in mantle xenoliths from Lower Silesia (SW Poland): formation conditions and decompression history

Raman spectroscopy has been applied to determine the density and pressure of formation of CO2 fluid inclusions in mantle xenoliths, carried to the surface at Wilcza Gora in southwestern Poland by e ...

Ultrahigh-temperature metamorphism and decompression history of sapphirine granulites from Rajapalaiyam, southern India: implications for the formation of hot orogens during Gondwana assembly

AbstractSapphirine-bearing Mg–Al granulites from Rajapalaiyam in the southern part of the Madurai Block provide critical evidence for Late Neoproterozoic–Cambrian ultrahigh-temperature (UHT) metamorphism in southern India. Poikiloblastic garnet in quartzo-feldspathic and pelitic granulites contain inclusions of fine-grained subidioblastic to xenoblastic sapphirine associated with quartz, suggesting that the rocks underwent T &gt; 1000°C peak metamorphism. Quartz inclusions in spinel within garnet are also regarded as clear evidence for a UHT condition. Inclusions of orthopyroxene within porphyroblastic garnet in the sapphirine-bearing rocks show the highest Al2O3 content of up to 10.3 wt%, suggesting T = 1050–1070°C and P = 8.5–9.5 kbar. Temperatures estimated from ternary feldspar and other geothermometers (T = 950–1000°C) further support extreme thermal metamorphism in this region. Xenoblastic spinel inclusions in sapphirine coexisting with quartz suggest that the spinel + quartz assemblage pre-dates the sapphirine + quartz assemblage, probably implying a cooling from T ~ 1050°C or an anticlockwise P–T path. The FMAS reaction sapphirine + quartz + garnet → orthopyroxene + sillimanite indicates a cooling from the sapphirine + quartz stability field after the peak metamorphism. Corona textures of orthopyroxene + cordierite (± sapphirine), orthopyroxene + sapphirine + cordierite, and cordierite + spinel around garnet suggest subsequent near-isothermal decompression followed by decompressional cooling toward T = 650–750°C and P = 4.5–5.5 kbar. The sapphirine–quartz association and related textures described in this study have an important bearing on the UHT metamorphism and exhumation history of the Madurai Block, as well as on the tectonic evolution of the continental deep crust in southern India. Our study provides a typical example for extreme metamorphism associated with collisional tectonics during the Late Neoproterozoic–Cambrian assembly of the Gondwana supercontinent.

High P–T granulite relicts from the Imjingang belt, South Korea: Tectonic significance

This study presents a detailed description on crustal metamorphic signatures of garnet–clinopyroxene–quartz–rutile-bearing high P– T granulites, Samgot unit, Imjingang belt, northwestern Korean Peninsula that formed during Permo-Triassic regional metamorphism related to the amalgamation of East Asian continental fragments. Lenses and blocks of high P– T granulites and garnet-bearing leucosomes occur within mafic metamorphic rocks (mainly amphibolites). The mafic blocks comprise relicts of granoblastic garnet and clinopyroxene with medium-grained quartz and rutile. These relict mineral assemblages are confined to local micro-domains and constitute remnants of peak metamorphism. Plagioclase and amphibole form only as retrograde phases in medium to coarse-grained moats that rim grain boundaries between relict peak mineral assemblages. This microstructure represents the reaction between garnet, clinopyroxene, quartz and rutile in the presence of melt to form amphibole, plagioclase and titanite with minor biotite. The leucosome domains consist of euhedral garnets within the quartz-K feldspar-plagioclase (granitic) matrix, probably representing peritectic garnet growth along with melting. The rare earth element (REE) composition of minerals also support the peritectic garnet growth with a positive Eu/Eu⁎ (positive Eu anomaly), while the relict garnet shows a slight negative anomaly typical for high-grade granulites. The peak-metamorphic conditions calculated from thermodynamic modeling and compositional isopleths indicate a temperature around c. 900 °C at a pressure around c. 20 kbar. The present P–T path indicates a clear multi-stage decompression history with initial decompression and cooling followed by a stage of decompression during hydration possibly during Late Triassic exhumation. The results from this study together with the presence of eclogites from the Hongsung area suggest that the Imjingang area and the western Gyeonggi massif likely resided at crustal levels deeper than those of the eastern and southern part of the Gyeonggi massif.

Vesiculation and crystallization under instantaneous decompression: Numerical study and comparison with laboratory experiments

Vesiculation and crystallization in ascending magmas are key processes that control the eruption behavior, and they interplay each other through the water exsolution process. We conducted a numerical study in order to quantitatively understand the water exsolution and crystallization processes in natural eruptions (decompression history is unknown) and in laboratory experiments (the amount of decompression is constant with time). The numerical results, which take into account homogeneous or heterogeneous nucleation and growth of bubbles with varying diffusivity of water, viscosity, and the amount of decompression, provide a quantitative understanding of their control on bubble formation and water exsolution in the constant amount of decompression. The bubble nucleation in the homogeneous nucleation can be divided into two regimes – the diffusion control regime and viscosity control regime – depending on the modified Peclet number and the effective supersaturation. In the cases of both homogeneous and heterogeneous nucleations, the bubble growth is controlled by diffusion or viscosity, depending on the modified Peclet number and bubble number density. The water exsolution rate, which is controlled by the modified Peclet number in the viscosity control regime and by the bubble number density and diffusive driving force in the diffusion control regime, acts as an effective cooling rate in a decompression-induced crystallization process. A comparison of the numerical results with the results of laboratory experiments suggests that water exsolution proceeds by the diffusion-limited growth of bubbles under disequilibrium vesiculation through the heterogeneous nucleation of bubbles, and this in turn controls the crystallization kinetics of microlite with the homogeneous nucleation of microlite and the diffusion-limited growth of crystal. The several orders of variation of microlite number density with the amount of decompression in laboratory experiments can be interpreted as the effect of the amount of decompression on the driving force for the diffusive bubble growth that controls the water exsolution rate.

Isothermal decompression history in the “Western Granulite” terrain, central Tanzania: Evidence from reaction textures and trapped fluids in metapelites

The Mozambique Belt (MB) of the East Africa Orogen contains large areas of granulite-facies migmatitic gneisses with Archaean and Palaeoproterozoic protolith ages and that were recycled during the Neoproterozoic Pan-African orogeny. The study area is situated along the Great Ruaha River and within the Mikumi National Park in central Tanzania where migmatitic gneisses and mafic to intermediate granulites are interlayered with Neoproterozoic granulite-facies migmatitic metapelites. Mineral textures suggest isothermal decompression, with the peak mineral assemblage comprising Grt–Bt–Ky–Kfs–Pl–Qtz ± Phn ± Ti-Oxide ± melt and amphibolite-facies retrograde assemblage Grt–Bt–Sil–Ms–Kfs–Pl–Qtz ± Fe–Ti-Oxide. The near isothermal retrograde overprint is seen in well-developed formation of pseudomorphs after garnet. The HP granulite-facies assemblages record P– T conditions of 13–14 kbar at 760–800 °C. Retrogression and the release of fluids from crystallizing melts occurred at ∼7 kbar and 650–700 °C. A fluid inclusion study shows three types of fluid inclusion consisting of nearly pure CO 2, as well as H 2O–NaCl and H 2O–CO 2 mixtures. We suggest that a immiscible CO 2-bearing brine represents the fluid composition during high-grade peak metamorphism, and that the fluid inclusions containing H 2O–NaCl or nearly pure CO 2 represent trapped fluids from in situ crystallised melt. The results suggest strong isothermal decompression, which is probably related to a fast exhumation after crustal thickening in the central part of the Mozambique Belt in Tanzania.

Petrologic constraints on the decompression history of magma prior to Vulcanian explosions at the Soufrière Hills volcano, Montserrat

A series of 88 Vulcanian explosions occurred at the Soufrière Hills volcano, Montserrat, between August and October, 1997. Conduit conditions conducive to creating these and other Vulcanian explosions were explored via analysis of eruptive products and one-dimensional numerical modeling of magma ascent through a cylindrical conduit. The number densities and textures of plagioclase microlites were documented for twenty-three samples from the events. The natural samples all show very high number densities of microlites, and > 50% by number of microlites have areas < 20 μm 2. Pre-explosion conduit conditions and decompression history have been inferred from these data by comparison with experimental decompressions of similar groundmass compositions. Our comparisons suggest quench pressures < 30 MPa (origin depths < 2 km) and multiple rapid decompressions of > 13.75 MPa each during ascent from chamber to surface. Values are consistent with field studies of the same events and statistical analysis of explosion time-series data. The microlite volume number density trend with depth reveals an apparent transition from growth-dominated crystallization to nucleation-dominated crystallization at pressures of ∼ 7 MPa and lower. A concurrent sharp increase in bulk density marks the onset of significant open-system degassing, apparently due to a large increase in system permeability above ∼ 70% vesicularity. This open-system degassing results in a dense plug which eventually seals the conduit and forms conditions favorable to Vulcanian explosions. The corresponding inferred depth of overpressure at 250–700 m, near the base of the dense plug, is consistent with depth to center of pressure estimated from deformation measurements. Here we also illustrate that one-dimensional models representing ascent of a degassing, crystal-rich magma are broadly consistent with conduit profiles constructed via our petrologic analysis. The comparison between models and petrologic data suggests that the dense conduit plug forms as a result of high overpressure and open-system degassing through conduit walls.

The Nagercoil Granulite Block, southern India: petrology, fluid inclusions and exhumation history

The Nagercoil Granulite Block (NGB) constitutes the southernmost domain in the high-grade metamorphic terrain of southern India and dominantly comprises orthopyroxene–K-feldspar–plagioclase–quartz–biotite–ilmenite bearing massive charnockites. Where they are mixed with aluminous supracrustals, the charnockites contain garnet, often-displaying typical decompression textures with coronal orthopyroxene and plagioclase surrounding relict garnet grains. Orthopyroxene is preserved in near-pristine state in the charnockite suggesting low water activity. Garnet–orthopyroxene thermometry of NGB charnockites yields temperatures of 691–934 °C. Garnet–biotite thermometry yield values that broadly overlap (538–864 °C) with the above estimate, although some lower values indicative of retrograde Fe–Mg exchange are also obtained. The presence of mesoperthite in the rock and high fluorine content in biotites also suggest high metamorphic temperatures. Charnockite barometry using garnet–orthopyroxene–plagioclase–quartz assemblage yields 4.0–6.3 kbar. An overall drop in pressure by over 2 kbar correlate with the isothermal decompression history inferred from textural criteria. Detailed fluid inclusion petrographic, microthermometric and laser Raman spectroscopic data presented here allow an evaluation of the fluid evolution history of NGB charnockites. We distinguish four groups of inclusions, with the earliest generation characterised by CO 2 with variable but minor proportion of H 2O. Subsequent generations are dominated by CO 2. Locally, CH 4 and N 2 occur as trace components. CO 2-rich inclusions dominate in all minerals: garnet, K-feldspar, plagioclase and apatite. Homogenisation temperatures assign densities of 0.92–0.93 g/cm 3 for the CO 2 involved in charnockite formation. A close correspondence of the isochores for CO 2+H 2O and CO 2 inclusions with P–T data derived from mineral phase equilibria suggests fluid entrapment at peak metamorphic conditions. The metamorphic evolution of NGB and its exhumation path characterised by isothermal decompression are comparable with those of charnockites from Pan-African terrains elsewhere in East Gondwana.

Closed-system geochemical recycling of crustal materials in alpine-type peridotite

We examined aluminous mafic rock (with or without corundum or sapphirine) alternating with peridotite from the Ronda peridotite massif, southern Spain. On the basis of petrographic characteristics, these mafic rocks show a decompression history from high pressure (P > 1.5 GPa), but on the basis of their geochemical characteristics, they are crystal accumulates of plagioclase, clinopyroxene, and olivine formed within the lower crust (P < 1 GPa). A complex evolution history, including higher-pressure recrystallization after initial formation as cumulate gabbros at lower-pressure conditions, is proposed. The aluminous mafic rocks and their peridotite hosts are inferred to be recycled crustal materials now observed as centimeter-scale layered components in alpine-type peridotite. The rocks retained their original cumulate compositions; that is, their compositions were not affected by melting and metasomatic modifications during subduction, intense deformation within the upper mantle, and upwelling to the surface.

Petrology and P–T evolution of garnet peridotites from central Sulawesi, Indonesia

Alpine‐type orogenic garnet‐bearing peridotites, associated with quartzo‐feldspathic gneisses of a 140–115 Ma high‐pressure/ultra‐high‐pressure metamorphic (HP‐UHPM) terrane, occur in two regions of the Indonesian island of Sulawesi. Both exposures are located within NW–SE‐trending strike–slip fault zones. Garnet lherzolite occurs as &lt;10 m wide fault slices juxtaposed against Miocene granite in the left‐lateral Palu‐Koro (P‐K) fault valley, and as 10–30 m wide, fault‐bounded outcrops juxtaposed against gabbros and peridotites of the East Sulawesi ophiolite within the right‐lateral Ampana fault in the Bongka river (BR) valley. Six evolutionary stages of recrystallization can be recognized in the peridotites from both localities. Stage I, the precursor spinel lherzolite assemblage, is characterized by Ol+Cpx+Opx±Prg‐Amp ± Spl±Rt±Phl, as inclusions within garnet cores. Stage II, the main garnet lherzolite assemblage, consists of coarse‐grained Ol+Opx+Cpx+Grt; whereas finer‐grained, neoblastic Ol+Opx+Grt+Cpx±Spl±Prg‐Amp±Phl constitutes stage III. Stages IV and V are manifest as kelyphites of fibrous Opx+Cpx+Spl in inner coronas, and Opx+Spl+Prg‐Amp±Ep in outer coronas around garnet, respectively. The final (greenschist facies) retrogressive stage VI is accompanied by recrystallization of Serp+Chl±Mag±Tr±Ni sulphides±Tlc±Cal. P–T conditions of the hydrated precursor spinel lherzolite stage I were probably about 750 °C at 15–20 kbar. P–T determinations of the peak stage IIc (from core compositions) display considerable variation for samples derived from different outcrops, with clustering at 26–38 kbar, 1025–1210 °C (P‐K &amp; BR); 19–21 kbar, 1070–1090 °C (P‐K), and 40–48 kbar, 1205–1290 °C (BR). Stage IIr (derived from rim compositions) generally records decompression of around 4–12 kbar accompanied by cooling of 50–240 °C from the IIc peak stage. Stage III, which post‐dates a phase of ductile deformation, yielded 22±2 kbar at 750±25 °C (P‐K) and 16±2 kbar at 730±40 °C (BR). The granulite–amphibolite–greenschist decompression sequence reflects uplift to upper crustal levels from conditions of 647–862 °C at P=15 kbar (stage IV), through 580–635 °C at P=10–12 kbar (stage V) to 350–400 °C at P=4–7 kbar (stage VI), respectively, and is identical to the sequence recorded in associated granulite, gneiss and eclogite. Sulawesi garnet peridotites are interpreted to represent minor components of the extensive HP‐UHP (peak P &gt;28 kbar, peak T of c. 760 °C) metamorphic basement terrane, which was recrystallized and uplifted in a N‐dipping continental collision zone at the southern Sundaland margin in the mid‐Cretaceous. The low‐T , low‐P and metasomatized spinel lherzolite precursor to the garnet lherzolite probably represents mantle wedge rocks that were dragged down parallel to the slab–wedge interface in a subduction/collision zone by induced corner flow. Ductile tectonic incorporation into the underthrust continental crust from various depths along the interface probably occurred during the exhumation stage, and the garnet peridotites were subsequently uplifted within the HP‐UHPM nappe, suffering a similar decompression history to that experienced by the regional schists and gneisses. Final exhumation from upper crustal levels was clearly facilitated by entrainment in Neogene granitic plutons, and/or Oligocene trans‐tension in deep‐seated strike–slip fault zones.

Dynamics and energetics of bubble growth in magmas: Analytical formulation and numerical modeling

We have developed a model of diffusive and decompressive growth of a bubble in a finite region of melt which accounts for the energetics of volatile degassing and melt deformation as well as the interactions between magmatic system parameters such as viscosity, volatile concentration, and diffusivity. On the basis of our formulation we constructed a numerical model of bubble growth in volcanic systems. We conducted a parametric study in which a saturated magma is instantaneously decompressed to one bar and the sensitivity of the system to variations in various parameters is examined. Variations of each of seven parameters over practical ranges of magmatic conditions can change bubble growth rates by 2–4 orders of magnitude. Our numerical formulation allows determination of the relative importance of each parameter controlling bubble growth for a given or evolving set of magmatic conditions. An analysis of the modeling results reveals that the commonly invoked parabolic law for bubble growth dynamics R∼t1/2 is not applicable to magma degassing at low pressures or high water oversaturation but that a logarithmic relationship R∼log(t) is more appropriate during active bubble growth under certain conditions. A second aspect of our study involved a constant decompression bubble growth model in which an initially saturated magma was subjected to a constant rate of decompression. Model results for degassing of initially water‐saturated rhyolitic magma with a constant decompression rate show that oversaturation at the vent depends on the initial depth of magma ascent. On the basis of decompression history, explosive eruptions of silicic magmas are expected for magmas rising from chambers deeper than 2 km for ascent rates &gt;1–5 m s−1.

Thermal effects of magma degassing

The thermodynamics of diffusive bubble growth is dominated by latent heat of water exsolution (vaporization) and the work of gas expansion ( P d V work). A numerical assessment of the effects of these on cooling of bubbly magma (water in albite) indicates that a magma can exsolve volatiles at equilibrium, or with varying degrees of oversaturation, depending on decompression history and degassing kinetics. Heat of water exsolution (vaporization) from magmatic melts has not previously been experimentally or analytically determined, and we have evaluated it from others thermodynamic functions known for the albite-water system. Heat of exsolution is small for pressures over 100 MPa, but can reach 20 kJ/mole at 1–2 MPa (10–20 bar). Oversaturation degassing at 10–20 bar can cause cooling of albite by 8 K/wt.% of exsolved water. The thermal effect of equilibrium degassing depends on the starting pressure of decompression because it follows the solubility curve. For a saturated melt at 100 MPa (4 wt.% water), it can cool a magma at least by 35 K before it disrupts into spray as gas volume fraction exceeds 0.8. Although we do not specifically consider the dynamics of bubble interactions in this study, our results suggest that the dynamic cooling rate could be significant at the vent of an erupting magma column, and under extreme eruption conditions, can lead to glass formation around fast growing bubbles of magmatic foam. This could cause fragmentation into fine ash by brittle failure. Conversely, bubble wall cooling (even before glass formation) can serve to reduce diffusive volatile flux into bubbles, decreasing the overall cooling of the system, and lead to solidification of oversaturated magmas, as occasionally observed in eruption products.

P-T history of a mantle diapir: the Horoman peridotite complex, Hokkaido, northern Japan

The Horoman peridotite complex, Hokkaido, Japan is divided into Lower and Upper zones on the basis of contrasting geological features. The complex recorded a consecutive decompression history in chemical zoning of pyroxenes and plagioclase in plagioclase lherzolite, which is interpreted to have been derived from garnet lherzolite by subsolidus decompression reactions. In the Lower Zone, and earlier decompression history is clearly preserved in large pyroxene porphyroclasts, which show marked M-shaped Al zoning characterized by low Al concentration at the core (Al=0.12/6 oxygens), gradual increase toward the marginal region, and rapid decrease toward the rim. The Ca content in the core is nearly constant (Ca=0.03/6 oxygens) with slight increase toward the margin followed by abrupt decrease toward the rim. The Al and Ca contents in the core of orthopyroxene in plagioclase lherzolite from the Upper Zone (Al=0.22, Ca=0.055/6 oxygens) are much higher than those for the Lower Zone, and the Al content typically decreases monotonously from the core to the rim with several exceptions that show poorly developed M-shaped zoning profiles. The earliest P-T conditions, inferable from the core compositions of pyroxenes are 900–950°C and ∼20 kbar for the Lower Zone and 1100–1150°C and ∼20 kbar for the Upper Zone. The increase of Al from the core to the margin is inferred to have resulted from nearly adiabatic decompression from these conditions into spinel peridotite facies. The complex experienced further decompression from the spinel stability field into the plagioclase stability field, which is inferred from plagioclase zoning in fine-grained aggregates composed mostly of plagioclase, chromite spinel, and olivine with minor pyroxenes. The Na-Ca ratio of each plagioclase grain decreases from the core to the rim, suggesting continuous decompression reaction producing olivine and plagioclase from pyroxenes and spinel. The sharp increase in Ca content toward the rim indicates that fairly rapid cooling associated with decompression is necessary to form and preserve the marked zoning. The sharp decrease in Al and Ca contents toward the rim of orthopyroxene was also formed during this final ascent of the complex. The systematic changes of the mineralogic and petrographic features that are gradational between the Lower and Upper zones suggest that the Horoman complex retains a temperature variation from the upper mantle. The Upper Zone is interpreted to have followed a higher temperature decompression path than the Lower Zone and probably represents a relatively hotter portion of a mantle diapir ascending from a depth greater than 60 km in the upper mantle.

High-pressure behavior in alpha -AlPO4: Amorphization and the memory-glass effect.

Previous studies have shown that \ensuremath{\alpha}-${\mathrm{AlPO}}_{4}$ (berlinite) undergoes a crystalline to amorphous transition at 15\ifmmode\pm\else\textpm\fi{}3 GPa. Moreover, it has been proposed that the high-pressure amorphous phase recrystallizes upon decompression, conserving both the structure and the crystallographic orientation of the original crystal. We show in this study that this so-called memory-glass effect is instead due to a polymorphic crystal-crystal phase transition occurring between 14 and 15 GPa on compression. This transition is fully reversible, with a hysteresis of 6--7 GPa, when hydrostatic media are used for compression. When quasihydrostatic pressure media are used, the decompression history is more complex. The samples develop a rim of back-transformed material, but the interior of the grains remains in the high-pressure phase. On recovery, this interior consists of amorphous material, coexisting with the back-transformed \ensuremath{\alpha}-${\mathrm{AlPO}}_{4}$ phase. Quasihydrostatic runs above 18 GPa show evidence for amorphization during pressurization. In some highly nonhydrostatic runs, the recovered sample contains both amorphous material, and a crystalline phase of ${\mathrm{AlPO}}_{4}$. This could correspond to a rutile-structured phase, or to a phase with the ${\mathrm{InPO}}_{4}$ structure, formed during decompression in the presence of shear stresses.

New data on magmatic H2O contents of pantellerites, with implications for petrogenesis and eruptive dynamics at Pantelleria

Infrared spectroscopic analyses of melt inclusions in quartz phenocrysts from pantellerites erupted at Pantelleria, Italy, show that the magmas contained moderate pre-eruptive H2O contents, ranging from 1.4 to 2.1 wt.%. Melt H2O concentrations increase linearly with incompatible elements, demonstrating that H2O contents were not buffered significantly during fractionation by any crystalline or vapor phase. The relatively low H2O contents of pantellerites are consistent with an origin by partial melting of alkali gabbros rather than fractional crystallization of basalt. Preeruptive H2O concentrations do not correlate with the volume or explosivity of pantellerite eruptions; decompression history is critical in determining the style of pantellerite (and other) eruptions.