Pre-eruptive Volatile Contents Research Articles

Constraining the volatile evolution of mafic melts at Mt. Somma–Vesuvius, Italy, based on the composition of reheated melt inclusions and their olivine hosts

Abstract. Mount Somma–Vesuvius is a stratovolcano that represents a geological hazard to the population of the city of Naples and surrounding towns in southern Italy. Historically, volcanic eruptions at Mt. Somma–Vesuvius (SV) include high-magnitude Plinian eruptions, such as the infamous 79 CE eruption that occurred after 295 years of quiescence and killed thousands of people in Pompeii and surrounding towns and villages. The last eruption at SV was in 1944 and showed a Volcanic Explosivity Index (VEI) of 3 (0.01 km3 of volcanic material erupted). Following the 1944 eruption, SV has been dormant for the past nearly 79 years, with only minor fumarolic and seismic activity. During its long history, centuries of dormancy at SV have ended with Plinian eruptions (VEI 6) that signal the beginning of a new cycle of eruptive activity. Thus, the current dormancy stage demands a need to better understand the mechanism involved in high-magnitude eruptions in order to better predict future eruption magnitude and style. Despite centuries of research on the SV volcanic system, many questions remain, including the evolution of magmatic volatiles from deep primitive magmas to shallower more evolved magmas. Developing a better understanding of the physical and chemical processes associated with volatile evolution at SV can provide insights into magma dynamics and the mechanisms that trigger highly explosive eruptions at SV. In this study, we present new data for the pre-eruptive volatile contents of magmas associated with four Plinian and two inter-Plinian eruptions at SV based on analyses of reheated melt inclusions (MIs) hosted in olivine. We correct the volatile contents of bubble-bearing MIs by taking into account the volatile contents of bubbles in the MIs. We recognize two groups of MIs: one group hosted in high-Fo olivine (Fo85–90) and relatively rich in volatiles and the other group hosted in low-Fo olivine (Fo70–69) and relatively depleted in volatiles. The correlation between volatile contents and compositions of host olivines suggests that magma fractionation took place under volatile-saturated conditions and that more differentiated magmas reside at shallower levels relative to less evolved/quasi-primitive magmas. Using the CO2 contents of corrected MIs hosted in Fo90 olivine from SV, we estimate that 347 to 686 t d−1 of magmatic CO2 exsolved from SV magmas during the last 3 centuries (38–75 Mt in total) of volcanic activity. Although this study is limited to only few SV magmas, we suggest that further study applying similar methods could shed light on the apparent lack of correlation between the volatile contents of MIs and the style and age of eruptions. Further, such studies could provide additional constraints on the origin of CO2 and the interaction between the carbonate platform and ascending magmas below SV.

Apatite chemistry in shoshonitic magmas: Insights into the volatile evolution at La Fossa volcano (Vulcano Island, Aeolian Arc, Italy)

Apatite, which has demonstrated to be useful for tracking volatile evolution in magmatic systems, is a common accessory in La Fossa active volcano (Vulcano island, Italy) products. We performed in situ geochemical (major and trace elements, Cl, F, S) analyses on apatite of La Fossa, and used the data to track the pre-eruptive magmatic volatile evolution for the La Fossa shoshonitic suite, to provide insights on magma evolution in the plumbing system and to investigate volatile fluxing. Our results confirm that apatite volatile chemistry is a valuable tool to give constraints on pre-eruptive volatile contents of magmas. For specific compositions, however, we obtained an underestimation of the water contents with respect to those estimated with plagioclase-liquid hygrometer in previous works. Vulcano apatite halogen composition also indicates that water saturation and pre-eruptive exsolution of a hypersaline aqueous fluid phase occurred when the magma reached rhyolitic composition. The high As, Bi and Cd contents found in apatite crystals belonging to shoshonite and high-K trachyte suggest that the magma reservoirs feeding these eruptions were affected by a deep volatile fluxing which was able to carry these trace elements up to shallow levels. This is in agreement with the occurrence of the deep degassing already demonstrated at La Fossa volcano on the basis of petrological and geochemical data, and supports the apatite geochemical approach to track deep volatile fluxing.

Role of volatiles in highly explosive basaltic eruptions

Water and carbon dioxide are the most abundant volatile components in terrestrial magmas. As they exsolve into magmatic vapour, they promote magma buoyancy, accelerating ascent and modulating eruptive dynamics. It is commonly thought that an increase in pre-eruptive volatile content produces an increase in eruption intensity. Using a conduit model for basaltic eruptions, covering the upper 6 km of conduit, we show that for the same chamber conditions mass eruption rate is not affected by CO2 content, whereas an increase in H2O up to 10 wt.% produces an increase in eruption rate of an order of magnitude. It is only when CO2 is injected in the magma reservoir from an external source that the resulting pressurisation will generate a strong increase in eruption rate. Results also show that ascent velocity and fragmentation depth are strongly affected by pre-eruptive volatile contents demonstrating a link between volatile content and eruptive style.

Top–down control on eruptive style at Masaya volcano inferred from melt composition

Highly explosive Plinian eruptions of basaltic magma are enigmatic because low melt viscosities should inhibit such eruptive style. Masaya volcano, Nicaragua, is a persistently active basaltic system capable of a wide range of eruptive styles, from open-conduit lava lake activity to voluminous Plinian eruptions; it is thus an ideal natural laboratory to constrain potential controls on basaltic eruption style. Here we report the major, trace, and volatile (CO2, H2O, S, Cl, F) element composition of olivine-, plagioclase- and clinopyroxene-hosted melt inclusions as well as matrix glasses from lava lake ejecta and two Plinian tephra deposits—the 2.1 ka Masaya Triple Layer and the 1.8 ka Ticuantepe Lapilli—to test whether pre-eruptive volatile contents and degassing history may be linked to eruptive style. All samples display a relatively narrow and largely overlapping basaltic–basaltic andesitic compositional range (51.7±1.0 wt.% SiO2, 4.8±0.4 wt.% MgO) with similar trace element signatures (e.g., Ce/Y=0.82±0.10, Ba/La=74±11). However, lava lake and Plinian samples show systematic differences in pre-eruptive volatile contents, forming distinct groups with mean H2O contents of 0.6±0.2 wt.% (lava lake), 1.1±0.2 wt.% (Masaya Triple Layer), and 1.9±0.3 wt.% (Ticuantepe Lapilli). Together, these groups generate broad positive correlations between S, Cl and H2O concentrations, with maximum values reaching 920 ppm, 1300 ppm and 2.3 wt.%, respectively, which are low compared to typical Central American arc magmas. Magma temperature estimates overlap and average at 1115±30°C, while volatile saturation pressures are low, mainly <100 MPa, although only lava lake samples record pressures <31 MPa. These observations reiterate the compositionally buffered state of the volcano's magmatic system highlighted by previous work and demonstrate that — regardless of eruption style — all Masaya magmas undergo variable, but extensive, pre-eruptive degassing at low pressure. Geohygrometry, gas emissions, and H2O/Ce–Ba/La systematics suggest initial, undegassed H2O contents on the order of 3.9–5.5 wt.%. Our results imply that pre-eruptive volatile contents are not the culprit for Plinian events at Masaya. Instead, we propose that the volcano's vigorous magma supply is modulated in a top–down manner to produce a wide range of eruptive styles, whereby temporary sealing of the conduit may instigate a transition to explosive behavior. In this model, rapid magma ascent is triggered when the seal eventually breaks from degassing-induced pressurization, yielding high degrees of undercooling, rapid microlite growth, and a dramatic increase in magma viscosity and explosive eruption potential. There may thus be a thin line between open-conduit conditions and Plinian eruptions at Masaya.

The volatile budget of Hawaiian magmatism: Constraints from melt inclusions from Haleakala volcano, Hawaii

Pre-eruptive volatile contents recorded by melt inclusions from ocean island settings such as Hawaii constrain the extent of deep Earth outgassing on geologic timescales by mantle plume activity. However, melt inclusions trapped from a partially degassed magma will not reflect the original volatile content of the primary melt, and relatively silicic (>45 wt% SiO2) shield-stage tholeiites are more likely to have been affected by degassing during fractionation at shallow depths. In contrast, magmas associated with post-shield volcanoes erupt volatile-rich melts that ascend quickly from the source region and crystallize in deep reservoirs. As a result, melt inclusions from post-shield volcanoes may be more likely to preserve undegassed melt compositions that can be used to determine the primary melt and source volatile contents. In this study, we analyzed melt inclusions from Haleakala Volcano (East Maui, Hawaii) to estimate the volatile budget of Hawaiian post-shield magmas. Melt inclusions from Haleakala contain up to 1.3 wt% CO2, 1.2 wt% H2O, and about 2000 ppm S. We calculate that melts from Haleakala were derived from a parental melt with ~0.7 wt% H2O and ~0.7 wt% CO2 and experienced ~15–40% polybaric, fluid-saturated crystallization starting at 5–7 kbar. Post-shield melt inclusions from Haleakala have H2O/Ce and CO2/Ba compositions that are intermediate between shield-stage melt inclusions from Kilauea and the alkaline North Arch Volcanic Field, which is consistent with a scenario in which primary melts from Haleakala are influenced by the volatile-rich auto-metasomatized periphery of the composite plume source.

Microscale Chemistry: Raman Analysis of Fluid and Melt Inclusions

Raman spectroscopy is a commonly applied nondestructive analytical technique for characterizing fluid and melt inclusions. The exceptional spatial resolution (~1 µm) and excellent spectral resolution (≤1 cm−1) permits the characterization of micrometer-scale phases and allows quantitative analyses based on Raman spectral features. Data provided by Raman analysis of fluid and melt inclusions has significantly advanced our understanding of complex geologic processes, including preeruptive volatile contents of magmas, the nature of fluids in the deep crust and upper mantle, the generation and evolution of methane-bearing fluids in unconventional hydrocarbon reservoirs. Anticipated future advances include the development of Raman mass spectroscopy and the use of Raman to monitor reaction progress in synthetic and natural fluid inclusion microreactors.

Pre- and syn-eruptive conditions of a basaltic Plinian eruption at Masaya Volcano, Nicaragua: The Masaya Triple Layer (2.1 ka)

The Masaya Triple Layer tephra was deposited ~2100 years ago during a basaltic Plinian eruption of Masaya caldera, Nicaragua, and is one of few known examples of this extreme endmember of basaltic explosive volcanism. Masaya caldera is located approximately 25 km from Managua, the capital of Nicaragua, and a Plinian eruption presents a high potential risk to its population of 1 million people. Here we use geochemical and petrological tools to constrain the pre- and syn-eruptive physico-chemical magmatic conditions, in order to understand how low-viscosity basaltic magmas can erupt with Plinian style.By combining thermometric models with Rhyolite-MELTS simulations, we find that the Masaya Triple Layer magma was last stored between 1080 and 1100 °C and 21–42 MPa, with a pre-eruptive volatile content of ~2 wt% H2O. A small phenocryst volume fraction of 0.1 crystallised under water-saturated conditions within a shallow magma reservoir located at 0.8–1.6 km depth. During ascent, rapid microlite crystallisation occurred and lateral velocity gradients across the conduit produced heterogeneous inter-mingling regions of varying crystallinity, between 20 and 50 vol%. Crystal size distribution analysis shows substantial microlite crystallisation over a timescale of 1–5 min, which induced significant changes in viscosity and magma rheology during ascent, increasing the effective magma viscosity from ~10 Pa s to 106 Pa s and approaching the brittle fragmentation threshold. Comparable pre- and syn-eruptive conditions have been deduced for other examples of basaltic Plinian activity, indicating a common eruption mechanism. We find that the common requirements for a basaltic Plinian eruption are a low magmatic H2O concentration of ~2 wt%, storage at moderate temperatures of 1080–1100 °C, and rapid crystallisation during magma ascent. Ultimately, these conditions contributed to the production of this highly hazardous Plinian eruption of Masaya.

Assessing magmatic volatile equilibria through FTIR spectroscopy of unexposed melt inclusions and their host quartz: a new technique and application to the Mesa Falls Tuff, Yellowstone

Fourier transform infrared spectroscopy (FTIR) is a precise, non-destructive, and highly sensitive means of measuring volatile species in melt inclusions. Existing methods of applying FTIR to quartz-hosted melt inclusions, however, require challenging physical manipulation of the host quartz crystals, which can present a significant limitation. Here, we describe a technique for measuring the H2O and CO2 concentrations of melt inclusions fully enclosed in quartz crystals using transmission FTIR, where melt inclusion thickness is calculated using the host quartz silicate overtones. The greater thickness of quartz crystals permitted by this technique allows an additional assessment of volatile equilibrium by measuring structural OH− within the quartz host itself. To demonstrate the advantages of this technique, we applied it to quartz crystals from the Mesa Falls Tuff, Yellowstone. The majority of melt inclusions consist of transparent or brown glassy melt inclusions and record concentrations of 2.9–3.3 wt% H2O and 181–561 ppm CO2, with precision similar to that obtained by standard double-exposed FTIR. Based on volatile saturation models, such concentrations reflect a largely un-degassed magma body equilibrated at pre-eruptive pressures of 100–150 MPa. These H2O concentrations and equilibrium conditions are supported by independent assessments using mineral–mineral and mineral-melt thermohygrometry and thermodynamic modelling using rhyolite-MELTS. The preservation of pre-eruptive volatile concentrations is further corroborated by measurements of OH− in the quartz hosts. A clear decrease in hydroxyl concentrations from core to rim matches closely the zoning in Al, which, combined with the position of quartz infrared bands, most likely reflects variations in Al-moderated OH− solubility rather than diffusive loss. The outermost 200 µm rims, however, show a sharper decline in OH− and corresponding increase in Li+ which is interpreted to record minor diffusive H+ loss and incorporation of Li+ to maintain charge balance. Our FTIR-based technique is simple, effective, and widely applicable to other quartz-bearing magmatic systems, opening up new possibilities to trace pre- syn- and post-eruptive volatile behaviour.

Volatile contents of primitive bubble-bearing melt inclusions from Klyuchevskoy volcano, Kamchatka: Comparison of volatile contents determined by mass-balance versus experimental homogenization

Primitive olivine-hosted melt inclusions provide information concerning the pre-eruptive volatile contents of silicate melts, but compositional changes associated with post-entrapment processes (PEP) sometimes complicate their interpretation. In particular, crystallization of the host phase along the wall of the melt inclusion and diffusion of H+ through the host promote CO2 and potentially S or other volatiles to exsolve from the melt into a separate fluid phase. Experimental rehomogenization and analysis of MI, or a combination of Raman spectroscopy, numerical modeling, and mass balance calculations are potentially effective methods to account for PEP and restore the original volatile contents of melt inclusions. In order to compare these different approaches, we studied melt inclusions from a suite of samples from Klyuchevskoy volcano (Kamchatka Arc) for which volatile compositions have been determined using experimental rehydration, Raman spectroscopy, and numerical modeling. The maximum CO2 contents of melt inclusions are in agreement (~3600–4000 ppm), regardless of the method used to correct for CO2 in the bubble, but significantly more uncertainty is observed using mass balance calculations. This uncertainty is largely due to the lack of precision associated with the petrographic method of determining bubble volumes and may also be related to the presence of daughter minerals at the glass-bubble interface.

Heterogeneously entrapped, vapor-rich melt inclusions record pre-eruptive magmatic volatile contents

Silicate melt inclusions (MI) commonly provide the best record of pre-eruptive H2O and CO2 contents of subvolcanic melts, but the concentrations of CO2 and H2O in the melt (glass) phase within MI can be modified by partitioning into a vapor bubble after trapping. Melt inclusions may also enclose vapor bubbles together with the melt (i.e., heterogeneous entrapment), affecting the bulk volatile composition of the MI, and its post-entrapment evolution. In this study, we use numerical modeling to examine the systematics of post-entrapment volatile evolution within MI containing various proportions of trapped vapor from zero to 95 volume percent. Modeling indicates that inclusions that trap only a vapor-saturated melt exhibit significant decrease in CO2 and moderate increase in H2O concentrations in the melt upon nucleation and growth of a vapor bubble. In contrast, inclusions that trap melt plus vapor exhibit subdued CO2 depletion at equivalent conditions. In the extreme case of inclusions that trap mostly the vapor phase (i.e., CO2–H2O fluid inclusions containing trapped melt), degassing of CO2 from the melt is negligible. In the latter scenario, the large fraction of vapor enclosed in the MI during trapping essentially serves as a buffer, preventing post-entrapment modification of volatile concentrations in the melt. Hence, the glass phase within such heterogeneously entrapped, vapor-rich MI records the volatile concentrations of the melt at the time of trapping. These numerical modeling results suggest that heterogeneously entrapped MI containing large vapor bubbles represent amenable samples for constraining pre-eruptive volatile concentrations of subvolcanic melts.

Slab melting and magma formation beneath the southern Cascade arc

Abstract The processes that drive magma formation beneath the Cascade arc and other warm-slab subduction zones have been debated because young oceanic crust is predicted to largely dehydrate beneath the forearc during subduction. In addition, geochemical variability along strike in the Cascades has led to contrasting interpretations about the role of volatiles in magma generation. Here, we focus on the Lassen segment of the Cascade arc, where previous work has demonstrated across-arc geochemical variations related to subduction enrichment, and H-isotope data suggest that H2O in basaltic magmas is derived from the final breakdown of chlorite in the mantle portion of the slab. We use naturally glassy, olivine-hosted melt inclusions (MI) from the tephra deposits of eight primitive ( MgO > 7 wt % ) basaltic cinder cones to quantify the pre-eruptive volatile contents of mantle-derived melts in this region. The melt inclusions have B concentrations and isotope ratios that are similar to mid-ocean ridge basalt (MORB), suggesting extensive dehydration of the downgoing plate prior to reaching sub-arc depths and little input of slab-derived B into the mantle wedge. However, correlations of volatile and trace element ratios (H2O/Ce, Cl/Nb, Sr/Nd) in the melt inclusions demonstrate that geochemical variability is the result of variable addition of a hydrous subduction component to the mantle wedge. Furthermore, correlations between subduction component tracers and radiogenic isotope ratios show that the subduction component has less radiogenic Sr and Pb than the Lassen sub-arc mantle, which can be explained by melting of subducted Gorda MORB beneath the arc. Agreement between pMELTS melting models and melt inclusion volatile, major, and trace element data suggests that hydrous slab melt addition to the mantle wedge can produce the range in primitive compositions erupted in the Lassen region. Our results provide further evidence that chlorite-derived fluids from the mantle portion of the slab ( ∼ 7 – 9 km below the slab top) cause flux melting of the subducted oceanic crust, producing hydrous slab melts that migrate into the overlying mantle, where they react with peridotite to induce further melting.

Reconstructing CO2 concentrations in basaltic melt inclusions using Raman analysis of vapor bubbles

Melt inclusions record valuable information about pre-eruptive volatile concentrations of melts. However, a vapor bubble commonly forms in inclusions after trapping, and this decreases the dissolved CO2 concentration in the melt (glass) phase in the inclusion. To quantify CO2 loss to vapor bubbles, Raman spectroscopic analysis was used to determine the density of CO2 in bubbles in melt inclusions from two Cascade cinder cones near Mt. Lassen and two Mexican cinder cones (Jorullo, Parícutin). Using analyses of dissolved CO2 and H2O in the glass in the inclusions, the measured CO2 vapor densities were used to reconstruct the original dissolved CO2 contents of the melt inclusions at the time of trapping. Our results show that 30–90% of the CO2 in a melt inclusion is contained in the vapor bubble, values similar to those found in other recent studies. We developed a model for vapor bubble growth to show how post-entrapment bubbles form in melt inclusions as a result of cooling, crystallization, and eruptive quenching. The model allows us to predict the bubble volume fraction as a function of ΔT (the difference between the trapping temperature and eruptive temperature) and the amount of CO2 lost to a bubble. Comparison of the Raman and modeling methods shows highly variable agreement. For 10 of 17 inclusions, the two methods are within ±550ppm CO2 (avg. difference 290ppm), equivalent to ±~300bars uncertainty in estimated trapping pressure for restored inclusions. Discrepancies between the two methods occur for inclusions that have been strongly affected by post-entrapment diffusive H+ loss, because this process enhances bubble formation. For our dataset, restoring the CO2 lost to vapor bubbles increases inferred trapping pressures of the inclusions by 600 to as much as 4000bars, highlighting the importance of accounting for vapor bubble formation in melt inclusion studies.

Using volatiles in magma to decipher subglacial eruption dynamics

Eruptions beneath ice sheets and glaciers can generate hazardous ash plumes and powerful meltwater floods, as demonstrated by the recent Icelandic eruptions at Eyjafjallajokull in 2010 and at Grimsvotn in 2011. A key scientific objective for volcanologists is to better understand the factors controlling subglacial eruptions, but the eruptions are mostly hidden beneath ice that is often hundreds of metres thick, thereby preventing direct observation. New approaches are therefore needed to reconstruct the factors driving explosive activity and the response of the overlying ice. The dissolved volatile content, preserved in glassy volcanic rock, offers a useful means of reconstructing palaeo-ice thicknesses. However, for subglacial rhyolite at least, there seems to be little or no correlation between loading pressure and eruptive style. Instead, there is a strong correlation between the pre-eruptive volatile content, degassing path and eruptive behaviour. It seems that the style of many subglacial eruptions is controlled by the same mechanisms as subaerial eruptions, with explosivity strongly influenced by degassing and magmatic fragmentation.

Bubbles matter: An assessment of the contribution of vapor bubbles to melt inclusion volatile budgets

Melt inclusions (MI) are considered the best tool available for determining the pre-eruptive volatile contents of magmas. H 2 O and CO 2 concentrations of the glass phase in MI are commonly used both as a barometer and to track magma degassing behavior during ascent due to the strong pressure dependence of H 2 O and CO 2 solubilities in silicate melts. The often unstated and sometimes overlooked requirement for this method to be valid is that the glass phase in the MI must represent the composition of the melt that was trapped at depth in the volcanic plumbing system. However, melt inclusions commonly contain a vapor bubble that formed after trapping owing to differential shrinkage of the melt compared to the host crystal, and/or crystallization at the inclusion-host interface. Such bubbles may contain a substantial portion of volatiles, such as CO 2 , that were originally dissolved in the melt. In this study, we determined the contribution of CO 2 in the vapor bubble to the overall CO 2 content of MI based on quantitative Raman analysis of the vapor bubbles in MI from the 1959 Kilauea Iki (Hawaii), 1960 Kapoho (Hawaii), 1974 Fuego volcano (Guatemala), and 1977 Seguam Island (Alaska) eruptions. We found that the bubbles typically contain 40 to 90% of the total CO 2 in the MI. Reconstructing the original CO 2 content by adding the CO 2 in the bubble back into the melt results in an increase in CO 2 concentration by as much as an order of magnitude (thousands of parts per million). Reconstructed CO 2 concentrations correspond to trapping pressures that are significantly greater than one would predict based on analysis of the volatiles in the glass alone. Trapping depths can be as much as 10 km deeper than estimates that ignore the CO 2 in the bubble. In addition to CO 2 in the vapor bubbles, many MI showed the presence of a carbonate mineral phase. Failure to recognize the carbonate during petrographic examination or analysis of the glass and to include its contained CO 2 when reconstructing the CO 2 content of the originally trapped melt will introduce additional errors into the calculated volatile budget. Our results emphasize that accurate determination of the pre-eruptive volatile content of melts based on analysis of melt inclusions must consider the volatiles contained in the bubble (and carbonates, if present). This can be accomplished either by analysis of the bubble and the glass followed by mass-balance reconstruction of the original volatile content of the melt, or by re-homogenization of the MI prior to conducting microanalysis of the quenched, glassy MI.

An assessment of the reliability of melt inclusions as recorders of the pre-eruptive volatile content of magmas

Many studies have used melt inclusions (MI) to track the pre-eruptive volatile history of magmas. Often, the volatile contents of the MI show wide variability, even for MI hosted in the same phenocryst. This variability is usually interpreted to represent trapping of a volatile-saturated melt over some range of pressures (depths) and these data are in turn used to define a magma degassing path. In this study, groups of MI that were all trapped at the same time (referred to as a melt inclusion assemblage or MIA) based on petrographic evidence, were analyzed to test the consistency of the volatile contents of MI that were all trapped simultaneously from the same melt. MIA hosted in phenocrysts from White Island (New Zealand) and from the Solchiaro eruption on the Island of Procida (Italy) were analyzed by secondary ion mass spectrometry (SIMS). In most MIA, H2O, F, and Cl abundances for all MI within the MIA are consistent (relative standard errors <27%, with the exception of two MIA), indicating that the MI all trapped a melt with the same H2O, F, and Cl concentrations and that the composition was maintained during storage in the magma as well as during and following eruption. In several MIA, S abundances are consistent (relative standard errors <33%, with the exception of five out of 28 MIA). Conversely, CO2 (White Island and Solchiaro MIA) showed wide variability in several MIA. The result is that some MIA display a wide range in CO2 content at approximately constant H2O. Similar trends have previously been interpreted to represent degassing paths, produced as volatile-saturated melts are trapped over some significant pressure (depth) range in an ascending (or convecting) magma body. However, the CO2 vs. H2O trends obtained in this study cannot represent degassing paths because the MI were all trapped at the same time (same MIA). This requires that all of the MI within the MIA trapped a melt of the same composition (including volatile content) and at the same temperature and pressure (depth). The cause of the variable concentration of CO2 within some MIA is unknown, but may reflect micrometer-scale heterogeneities within the melt during trapping, heterogeneities within individual MI, post-entrapment crystallization within the MI, or C-contamination during sample preparation. These results suggest that trends showing variable CO2 and relatively uniform H2O obtained from MI may not represent trapping of volatile-saturated melts over a range of pressure, and care must be taken when interpreting volatile contents of MI to infer magma degassing paths. Results of this study have been used to estimate the uncertainties in volatile concentrations of MI determined by SIMS analysis. The H2O, F, and Cl contents have an average estimated uncertainty of 11, 9, and 12%, respectively, which is consistent with the SIMS analytical error. In contrast, the S and CO2 contents have an average estimated uncertainty of 24 and 69%, respectively, which is considerably larger than the SIMS analytical error.

Water in the martian interior: Evidence for terrestrial MORB mantle-like volatile contents from hydroxyl-rich apatite in olivine–phyric shergottite NWA 6234

Understanding the distribution and behavior of volatiles in martian magmas and the corresponding mantle provides crucial constraints on planetary properties such as mantle melting temperatures and melt production, as well as the availability of magmatic fluids for any potential occurrences of life. Current evaluations of the pre-eruptive volatile concentrations of martian magmas are based on analyses of hydrous minerals in martian meteorites such as apatite, because they record the volatile content (OH, F, Cl) and the halogen and water fugacities of their parental magmas. Estimates for the pre-eruptive volatile contents of martian basaltic magmas range from nearly anhydrous to 2wt% H2O. However, these magmas may have potentially degassed upon eruption and/or emplacement losing their magmatic water while retaining some halogens. Here we report apatite compositional data from the martian meteorite NWA 6234, an intermediate olivine–phyric shergottite, and calculate F2, Cl2, H2O fugacity ratios for its parental magma. Apatites in NWA 6234 contain the highest observed OH–F content among martian meteorites and importantly have volatile fugacity ratios similar to terrestrial basalts. This suggests that the actual content of fluorine, chlorine and water of the martian mantle, parental to SNC meteorites, are higher than previously thought and are similar to the volatile content of the terrestrial mid-ocean ridge basalt source region.

Pre-eruptive volatile content, degassing paths and depressurisation explaining the transition in style at the subglacial rhyolitic eruption of Dalakvísl, South Iceland

Many volcanoes express a variety of contrasting eruptive styles with behavioural transitions between explosive and effusive activities. Understanding eruptive mechanisms is of key importance but we currently have little insight into the behavioural controls for rhyolitic eruptions beneath ice. This paper addresses a subglacial rhyolitic eruption at Dalakvísl in South Iceland that involved both explosive and effusive activities. We present measurements of pre- and post-eruptive magmatic volatile contents that we use to track magma degassing from the chamber to the surface and reconstruct pressure conditions during the eruption. Explosive and effusive phases involve significant differences in: (1) pre-eruptive volatile contents; (2) the ‘openness’ of magma degassing; and (3) inferred confining pressure. This final factor is potentially associated with a jökulhlaup-induced decompression. Due to the combination of these factors Dalakvísl experienced a change from effusive to explosive behaviour, although we cannot currently resolve cause from effect. Our work points towards complex interactions between the dynamics of magma ascent and the response of overlying ice and meltwater during rhyolitic eruptions beneath ice.

Explosive subglacial rhyolitic eruptions in Iceland are fuelled by high magmatic H2O and closed-system degassing

Research Article| February 01, 2013 Explosive subglacial rhyolitic eruptions in Iceland are fuelled by high magmatic H2O and closed-system degassing Jacqueline Owen; Jacqueline Owen 1Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK Search for other works by this author on: GSW Google Scholar Hugh Tuffen; Hugh Tuffen 1Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK Search for other works by this author on: GSW Google Scholar David W. McGarvie David W. McGarvie 2Department of Environment, Earth and Ecosystems, The Open University, Milton Keynes MK7 6AA, UK Search for other works by this author on: GSW Google Scholar Author and Article Information Jacqueline Owen 1Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK Hugh Tuffen 1Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK David W. McGarvie 2Department of Environment, Earth and Ecosystems, The Open University, Milton Keynes MK7 6AA, UK Publisher: Geological Society of America Received: 18 May 2012 Revision Received: 13 Aug 2012 Accepted: 12 Sep 2012 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2013 Geological Society of America Geology (2013) 41 (2): 251–254. https://doi.org/10.1130/G33647.1 Article history Received: 18 May 2012 Revision Received: 13 Aug 2012 Accepted: 12 Sep 2012 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jacqueline Owen, Hugh Tuffen, David W. McGarvie; Explosive subglacial rhyolitic eruptions in Iceland are fuelled by high magmatic H2O and closed-system degassing. Geology 2013;; 41 (2): 251–254. doi: https://doi.org/10.1130/G33647.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Rhyolitic eruptions beneath Icelandic glaciers can be highly explosive, as demonstrated by Quaternary tephra layers dispersed throughout northern Europe. However, they can also be small and effusive. A subglacial rhyolitic eruption has never been observed, so behavioral controls remain poorly understood and the influence of pre-eruptive volatile contents is unknown. We have therefore used secondary ion mass spectrometry to characterize pre-eruptive volatile contents and degassing paths for five subglacial rhyolitic edifices within the Torfajökull central volcano, formed in contrasting styles of eruption under ice ∼400 m thick. This includes the products of the largest known eruption of Icelandic subglacial rhyolite of ∼16 km3 at ca. 70 ka. We find pre-eruptive water contents in melt inclusions (H2OMI) of up to 4.8 wt%, which indicates that Icelandic rhyolite can be significantly more volatile-rich than previously thought. Our results indicate that explosive subglacial rhyolite eruptions correspond with high H2OMI, closed-system degassing, and rapid magma ascent, whereas their effusive equivalents have lower H2OMI and show open-system degassing and more sluggish ascent rates. Volatile controls on eruption style thus appear similar to those for subaerial eruptions, suggesting that ice plays a subsidiary role in influencing the behavior of subglacial rhyolitic eruptions. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Sixty thousand years of magmatic volatile history before the caldera-forming eruption of Mount Mazama, Crater Lake, Oregon

The well-documented eruptive history of Mount Mazama, Oregon, provides an excellent opportunity to use pre-eruptive volatile concentrations to study the growth of an explosive silicic magmatic system. Melt inclusions (MI) hosted in pyroxene and plagioclase crystals from eight dacitic–rhyodacitic eruptive deposits (71–7.7 ka) were analyzed to determine variations in volatile-element concentrations and changes in magma storage conditions leading up to and including the climactic eruption of Crater Lake caldera. Temperatures (Fe–Ti oxides) increased through the series of dacites, then decreased, and increased again through the rhyodacites (918–968 to ~950 to 845–895 °C). Oxygen fugacity began at nickel–nickel-oxide buffer (NNO) +0.8 (71 ka), dropped slightly to NNO +0.3, and then climbed to its highest value with the climactic eruption (7.7 ka) at NNO +1.1 log units. In parallel with oxidation state, maximum MI sulfur concentrations were high early in the eruptive sequence (~500 ppm), decreased (to ~200 ppm), and then increased again with the climactic eruption (~500 ppm). Maximum MI sulfur correlates with the Sr content (as a proxy for LREE, Ba, Rb, P2O5) of recharge magmas, represented by basaltic andesitic to andesitic enclaves and similar-aged lavas. These results suggest that oxidized Sr-rich recharge magmas dominated early and late in the development of the pre-climactic dacite–rhyodacite system. Dissolved H2O concentrations in MI do not, however, correlate with these changes in dominant recharge magma, instead recording vapor solubility relations in the developing shallow magma storage and conduit region. Dissolved H2O concentrations form two populations through time: the first at 3–4.6 wt% (with a few extreme values up to 6.1 wt%) and the second at ≤2.4 wt%. CO2 concentrations measured in a subset of these inclusions reach up to 240 ppm in early-erupted deposits (71 ka) and are below detection in climactic deposits (7.7 ka). Combined H2O and CO2 concentrations and solubility models indicate a dominant storage region at 4–7 km (up to 12 km), with drier inclusions that diffusively re-equilibrated and/or were trapped at shallower depths. Boron and Cl (except in the climactic deposit) largely remained in the melt, suggesting vapor–melt partition coefficients and gas fractions were low. Modeled Li, F, and S vapor–melt partition coefficients are higher than those of B and Cl. The decrease in maximum MI CO2 concentration following the earliest dacitic eruptions is interpreted to result from a broadening of the shallow storage region to greater than the diameter of subjacent feeders, so that greater proportions of reservoir magma were to the side of CO2-bearing vapor bubbles ascending vertically from the locus of recharge magma injection, thereby escaping recarbonation by streaming vapor bubbles. The Mazama melt inclusions provide a picture of a growing magma storage region, where chemical variations in melt and magma occur due to changes in the nature and supply rate of magma recharge, the timing of degassing, and the possible degree of equilibration with gases from below.

Experimental Constraints on Parameters Controlling the Difference in the Eruptive Dynamics of Phonolitic Magmas: the Case of Tenerife (Canary Islands)

We have performed phase equilibrium experiments to determine the pre-eruptive conditions of the explosive eruption of Montana Blanca (2020 BP) that occurred from a satellite vent located in the east flank of Teide volcano (Tenerife Island). Crystallization experiments were performed using a phonolitic obsidian from the fall out deposit that contains 5 wt% anorthoclase, diopside and magnetite with minor amounts of biotite and ilmenite, set in a glassy matrix that contains microlites of Ca-rich alkali feldspars. Temperature was varied between 850oC and 800oC, and pressure between 200MPa to 50 MPa. The oxygen fugacity (fO2) was varied between NNO+0.2 (0.2 log units above to the Ni-NiO solid buffer) to NNO-2, whilst dissolved water contents varied from 7 to 1.5 wt%. Comparison between natural and experimental phase proportions and compositions indicates that the main body of phonolite magma was stored at 850±15oC, 50±20MPa, 2.5±0.5wt% H2O at an fO2 around NNO-0.5 prior to eruption, equivalent to depths between 1 to 2 km below the surface. Some clinopyroxene hosting H2O-rich melt inclusions possibly originates from intermittent supply of phonolitic magma stored at somewhat deeper levels (100 MPa). The Ca and Fe-rich composition of alkali feldspar phenocrysts rims and microlites attests to the intrusion of a mafic magma in the reservoir just prior to eruption, as borne out by banded pumices appearing in the later products of the eruptive sequence. The comparison with other phonolitic magmas from Tenerife and elsewhere (e.g., Vesuvius, Laacher See) shows that differences in the eruptive dynamic of phonolitic magmas can be correlated to differences in storage depths, along with variation in pre-eruptive volatile contents.