Madeleine C. S. Humphreys

Magma heating by decompression-driven crystallization beneath andesite volcanoes

Explosive volcanic eruptions are driven by exsolution of H2O-rich vapour from silicic magma. Eruption dynamics involve a complex interplay between nucleation and growth of vapour bubbles and crystallization, generating highly nonlinear variation in the physical properties of magma as it ascends beneath a volcano. This makes explosive volcanism difficult to model and, ultimately, to predict. A key unknown is the temperature variation in magma rising through the sub-volcanic system, as it loses gas and crystallizes en route. Thermodynamic modelling of magma that degasses, but does not crystallize, indicates that both cooling and heating are possible. Hitherto it has not been possible to evaluate such alternatives because of the difficulty of tracking temperature variations in moving magma several kilometres below the surface. Here we extend recent work on glassy melt inclusions trapped in plagioclase crystals to develop a method for tracking pressure–temperature–crystallinity paths in magma beneath two active andesite volcanoes. We use dissolved H2O in melt inclusions to constrain the pressure of H2O at the time an inclusion became sealed, incompatible trace element concentrations to calculate the corresponding magma crystallinity and plagioclase–melt geothermometry to determine the temperature. These data are allied to ilmenite–magnetite geothermometry to show that the temperature of ascending magma increases by up to 100 °C, owing to the release of latent heat of crystallization. This heating can account for several common textural features of andesitic magmas, which might otherwise be erroneously attributed to pre-eruptive magma mixing.

Uturuncu volcano, Bolivia: Volcanic unrest due to mid-crustal magma intrusion

Uturuncu volcano, SW Bolivia, is a dormant stratovolcano (∼85 km3) dominated by dacitic lava domes and flows. 39Ar/40Ar ages show that the volcano was active between 890 ka and 271 ka, with the lavas becoming younger and less extensive at higher elevations. There are current signs of unrest. Between 1992 and 2006 geodetic satellite measurements record an ongoing 70 km deformation field with a central uplift rate of 1 to 2 cm/yr. Deformation indicates volume changes of 400 × 108 m3 over 14 years, an average of ∼1 m3/s (10−2 km3/yr). The deformation is attributed to magma intrusion into the Altiplano-Puna regional crustal magma body. Deformation models indicate a source at depths of 17 to 30 km beneath current local relief. In a reconnaissance survey, persistent seismic activity (mean of 2.6 earthquakes per hour with a maximum of 14 per hour) was recorded at about 4 km depth below the center of the uplift, 4 km SW of the volcanos summit. The seismic events have a normal b value (∼1.04) and activity is attributed to brittle deformation in the elastic crust above the active deep magma intrusion. The porphyritic dacite lavas (64−68% SiO2) have a plagioclase-orthopyroxene-biotite-magnetite-ilmenite assemblage and commonly contain juvenile silicic andesite inclusions, cognate norite nodules and crustal xenoliths. Temperature estimates are in the range 805 to 872°C for the dacites and about 980°C for the silicic andesites. The dacite magmas formed by fractional crystallization of andesite forming norite cumulates and involving partial melting of crust. Compositions and zoning patterns of orthopyroxene and plagioclase phenocrysts indicate that compositional variation in the dacites is caused by magma mixing with the silicic andesite. Reversely zoned orthopyroxene phenocrysts in the andesitic end-member are explained by changing oxidation states during crystallization. Fe3+/Fe2+ ratios from orthopyroxene crystals and Fe3+ in plagioclase provide evidence for a relatively reduced melt that subsequently ascended, degassed and became more oxidized as a consequence of degassing. The geophysical and petrological observations suggest that dacite magma is being intruded into the Altiplano-Puna regional crustal magma body at 17 km or more depth, consistent with deformation models. In the Late Pleistocene dacitic and andesitic magmas ascended from the regional crustal magma body to a shallow magma system at a few kilometers depth where they crystallized and mingled together. The current unrest, together with geophysical anomalies and 270 ka of dormancy, indicate that the magmatic system is in a prolonged period of intrusion. Such circumstances might eventually lead to eruption of large volumes of intruded magma with potential for caldera formation.

Excess volatiles supplied by mingling of mafic magma at an andesite arc volcano

We present the results of a study of volcanic gases at Soufriere Hills Volcano, Montserrat, which includes the first spectroscopic measurements of the major gas species CO2 and H2S at this volcano using a Multisensor Gas Analyzer System (MultiGAS) sensor. The fluxes of CO2 and H2S were 640.2750 t/d and 84.266 t/d, respectively, during July 2008, during a prolonged eruptive pause. The flux of CO2 is similar to estimates for the entire arc from previous geochemical studies, while the measured H2S flux significantly alters our interpretation of the sulphur budget for this volcano. The fluxes of both sulphur and carbon show considerable excesses over that which can be supplied by degassing of erupted magma. We demonstrate, using thermodynamic models and published constraints on preeruptive volatile concentrations, that the gas composition and fluxes are best modeled by mixing between (1) gases derived from isobaric quenching of mafic magma against cooler andesite magma at depth and (2) gases derived from shallower rhyolitic interstitial melt within the porpyritic andesite. The escape of deep-derived gases requires pervasive permeability or vapor advection extending to several kilometers depth in the conduit and magma storage system. These results provide more compelling evidence for both the contribution of unerupted mafic magma to the volatile budget of this andesitic arc volcano and the importance of the intruding mafic magma in sustaining the eruption. From a broader perspective, this study illustrates the importance and role of underplating mafic magmas in arc settings. These magmas play an important role in triggering and sustaining eruptions and contribute in a highly significant way to the volatile budget of arc volcanoes. Copyright

SIMS investigation of electron-beam damage to hydrous, rhyolitic glasses: Implications for melt inclusion analysis

Abstract Electron-beam irradiation causes permanent damage to hydrous, silica-rich glasses. The extent of electron-beam damage is quantified using data generated by SIMS analysis of points subjected to previous electron microprobe analysis (EPMA). Even optimum EPMA conditions cause damage to the glass, manifest as a marked depletion in alkali ions at the surface of an irradiated sample. Deeper in the sample, an enrichment in alkali ions to above-baseline levels is followed by a decay back to baseline. The depth of the final decay correlates with species diffusivity and increases in the order K-Li-Na. H-bearing species are also affected by electron beam irradiation, but in the opposite sense to the alkalis, i.e., they are enriched at the surface. Migration of alkaline earth cations is not observed because of their low diffusivities. Ion depletion or enrichment results from simple migration of ions toward or away from electrons implanted by the beam. Migration depth depends on species diffusivity and heating caused by the electron beam, and therefore increases with increasing electron beam current. Because of the reverse behavior of H, the mobile hydrous species in the presence of an electric field is probably OH-. The extent of electron beam damage to glasses may increase with total water content. Critically, SIMS measurements of H, Li, Na, D/H, and 6Li/7Li after electron-probe analysis are compromised by the damage. Despite the damage caused by the electron beam, use of appropriate electron-beam conditions (e.g., 2 nA, 15 kV) gives volatiles by difference accurate to ~0.6 wt%.

Volcanic gas emissions from Soufrière Hills Volcano, Montserrat 1995–2009, with implications for mafic magma supply and degassing

Volcanic gas emissions illustrate a complex volatile budget for Soufriere Hills Volcano, Montserrat. Fluxes of sulphur dioxide, carbon dioxide, hydrogen sulphide and hydrogen chloride (and probably water) are substantial from this arc volcano. Additional sources of volatiles in addition to the erupting andesite are required to satisfy the mass balance defined by gas emissions and petrological constraints. Mafic magma is intruded at depths of >10 km, supplying volatiles by quenching, crystallising and vesiculating at the andesite-mafic magma interface. Sulphur, carbon dioxide and other volatiles supplied to the system migrate to the surface and their fluxes remain high while mafic recharge at depth is sustained. Decompression degassing of porphyritic andesite supplies chlorine and water to the gas plume. Chlorine partitions into vapour from the melt during periods of active extrusion, making the HCl/SO2 ratio of volcanic gases a reliable indicator of the onset of eruptive activity. Studies of this volatile budget highlight the role of deep degassing and vapour production at arc volcanoes, which are important mechanisms for driving the eruption due to its effects on magma buoyancy and the advection of heat through the system.

Magma hybridisation and diffusive exchange recorded in heterogeneous glasses from Soufrière Hills Volcano, Montserrat

Arc volcanoes commonly show evidence of mixing between mafic and silicic magma. Melt inclusions and matrix glasses in andesite erupted from Soufriere Hills Volcano, Montserrat, include an anomalously K2O-rich population which shows close compositional overlap with residual glass from mafic inclusions. We suggest that these glasses represent the effects of physical mixing with mafic magma, both during ascent and by diffusive exchange during the formation of mafic inclusions. Many glasses are enriched only in K2O, suggesting diffusive contamination by high-K mafic inclusion glass; others are also enriched in TiO2, suggesting physical mixing of remnant glass. Some mafic inclusion glasses have lost K2O. The preservation of this K-rich melt component in the andesite suggests short timescales between mixing and ascent. Diffusive timescales are consistent with independent petrological estimates of magma ascent time.

New constraints on electron-beam induced halogen migration in apatite.

Abstract Fluorine and chlorine X‑ray count rates are known to vary significantly during electron probe microanalysis (EPMA) of apatite. Since the rate, timing, and magnitude of this variation are a function of apatite orientation and composition, as well as EPMA operating conditions, this represents a significant problem for volatile element analysis in apatite. Although the effect is thought to be an intrinsic crystallographic response to electron-beam exposure, the mechanisms and causes of the count rate variability remain unclear. We tackle this by examining directly the effects of electron-beam exposure on apatite, by performing secondary ion mass spectrometry (SIMS) depth profiles of points previously subject to electron-beam irradiation. During irradiation of fluorapatite, oriented with the c-axis parallel to the electron beam, halogens become progressively concentrated at the sample surface, even under a relatively low power (15 nA, 10-15 kV) beam. This surface enrichment corresponds to an observed increase in EPMA FKa X‑ray count rates. After prolonged irradiation, the surface region starts to lose halogens and becomes progressively depleted, corresponding with a drop in EPMA count rates. Under normal EPMA operating conditions there is no halogen redistribution in fluorapatite oriented with the c-axis perpendicular to the electron beam, or in chlorapatite. We infer that anionic enrichment results from the migration of halogens away from a center of charge build-up caused by the implantation of electrons from the EPMA beam, assisted by the thermal gradient induced by electron-matter interactions. The process of surface enrichment is best explained by halogen migration through interstitial crystallographic sites in the c-axis channel. This suggests that once the thermal and electric fields are removed, halogens may relax back to their original positions on very long timescales or with sample heating.

Chapter 16 Pre-eruptive vapour and its role in controlling eruption style and longevity at Soufrière Hills Volcano

Abstract We use volatiles in melt inclusions and nominally anhydrous phenocrysts, with volcanic gas flux and composition, and textural analysis of mafic inclusions to estimate the mass of exsolved vapour prior to eruption at Soufrière Hills Volcano (SHV). Pre-eruptive andesite coexists with exsolved vapour comprising 1.6–2.4 wt% of the bulk magma. The water content of orthopyroxenes indicates a zone of magma storage at pressures of approximately 200–300 MPa, whereas melt inclusions have equilibrated at shallower pressures. Inclusions containing >3 wt% H2O are enriched in CO2, suggesting flushing with CO2-rich gases. Intruding mafic magma contains >8 wt% H2O at 200–300 MPa. Rapid quenching is accompanied by crystallization and vesiculation. Upon entrainment into the andesite, mafic inclusions may undergo disaggregation, where expansion of volatiles in the interior overcomes the strength of the crystal frameworks, thereby recharging the vapour content of the andesite. Exsolved vapour may amount to 4.3–8.2 vol% at 300 MPa, with implications for eruption longevity and volume; we estimate the magma reservoir volume to be 60–200 km3. Exsolved vapour may account for the small volume change at depth during eruptions from geodetic models, and has implications for magma flow: exsolution is likely to be in equilibrium during rapid magma ascent, with little nucleation of new bubbles.

Toward an understanding of disequilibrium dihedral angles in mafic rocks

The median dihedral angle at clinopyroxene-plagioclase-plagioclase junctions in mafic rocks, Θcpp, is generally lower than equilibrium (109˚ {plus minus} 2˚). Observation of a wide range of mafic bodies demonstrates that previous work on systematic variations of Θcpp is incorrect in several important respects. Firstly, the spatial distribution of plagioclase compositional zoning demonstrates that the final geometry of three-grain junctions, and hence Θcpp, is formed during solidification (the igneous process): sub-solidus textural modification in most dolerites and gabbros, previously thought to be the dominant control on Θcpp, is insignificant. Θcpp is governed by mass transport constraints, the inhibiting effects of small pore size on crystallization, and variation in relative growth rates of pyroxene and plagioclase. During rapid cooling, pyroxene preferentially fills wider pores while the narrower pores remain melt-filled, resulting in an initial value of Θcpp of 78˚, rather than 60˚ which would be expected if all melt-filled pores were filled with pyroxene. Lower cooling rates create a higher initial Θcpp due to changes in relative growth rates of the two minerals at the nascent three-grain junction. Low Θcpp (associated with cuspate clinopyroxene grains at triple junctions) can also be diagnostic of infiltration of previously melt-free rocks by late-stage evolved liquids (the metasomatic process). Modification of Θcpp by sub-solidus textural equilibration (the metamorphic process) is only important for fine-grained mafic rocks such as chilled margins and intra-plutonic chill zones. In coarse-grained gabbros from shallow crustal intrusions the metamorphic process occurs only in the centres of oikocrysts, associated with rounding of chadacrysts.

Chapter 18 Characterization of mafic enclaves in the erupted products of Soufrière Hills Volcano, Montserrat, 2009 to 2010

Abstract Lavas from the current eruption of the Soufrière Hills Volcano (SHV), Montserrat exhibit evidence for magma mingling, related to the intrusion of mafic magma at depth. We present detailed field, petrological, textural and geochemical descriptions of mafic enclaves in andesite erupted during 2009–2010, and subdivide the enclaves into three distinct types: type A are mafic, glassy with chilled margins and few inherited phenocrysts; type B are more evolved with high inherited phenocryst content and little glass, and are interpreted as significantly hybridized; type C are composite, with a mafic interior (type A) and a hybrid exterior (type B). All enclaves define tight linear compositional trends, interpreted as mixing between a mafic end member (type A) and host andesite. Enclave glasses are rhyolitic, owing to extensive crystallization during quenching. Type A quench crystallization is driven by rapid thermal equilibration during injection into the andesite. Conversely, type B enclaves form in a hybridized melt layer, which ponded near the base of the chamber and cooled more slowly. Vesiculation near the mafic–silicic interface resulted in disruption of the hybridized layer and the formation of the type B enclaves. The composite enclaves represent an interface between types A and B, suggesting multiple episodes of mafic injection.