Rosario Esposito

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

Abstract Melt inclusions (MI) are considered the best tool available for determining the pre-eruptive volatile contents of magmas. H2O and CO2 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 H2O and CO2 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 CO2, that were originally dissolved in the melt. In this study, we determined the contribution of CO2 in the vapor bubble to the overall CO2 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 CO2 in the MI. Reconstructing the original CO2 content by adding the CO2 in the bubble back into the melt results in an increase in CO2 concentration by as much as an order of magnitude (thousands of parts per million). Reconstructed CO2 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 CO2 in the bubble. In addition to CO2 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 CO2 when reconstructing the CO2 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.

Application of the Linkam TS1400XY heating stage to melt inclusion studies

Melt inclusions (MI) trapped in igneous phenocrysts provide one of the best tools available for characterizing magmatic processes. Some MI experience post-entrapment modifications, including crystallization of material on the walls, formation of a vapor bubble containing volatiles originally dissolved in the melt, or partial to complete crystallization of the melt. In these cases, laboratory heating may be necessary to return the MI to its original homogeneous melt state, followed by rapid quenching of the melt to produce a homogeneous glass phase, before microanalyses can be undertaken.Here we describe a series of heating experiments that have been performed on crystallized MI hosted in olivine, clinopyroxene and quartz phenocrysts, using the Linkam TS1400XY microscope heating stage. During the experiments, we have recorded the melting behaviors of the MI up to a maximum temperature of 1360°C. In most of the experiments, the MI were homogenized completely (without crystals or bubbles) and remained homogeneous during quenching to room temperature. The resulting single phase MI contained a homogeneous glass phase. These tests demonstrate the applicability of the Linkam TS1400XY microscope heating stage to homogenize and quench MI to produce homogeneous glasses that can be analyzed with various techniques such as Electron Microprobe (EMP), Secondary Ion Mass Spectrometry (SIMS), Laser ablation Inductively Coupled Plasma Mass Spectrometry (LA ICP-MS), Raman spectroscopy, FTIR spectroscopy, etc.During heating experiments, the optical quality varied greatly between samples and was a function of not only the temperature of observation, but also on the amount of matrix glass attached to the phenocryst, the presence of other MI in the sample which are connected to the outside of the crystal, and the existence of mineral inclusions in the host.

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

Abstract 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.

Detection of liquid H2O in vapor bubbles in reheated melt inclusions: Implications for magmatic fluid composition and volatile budgets of magmas?

Abstract Fluids exsolved from mafic melts are thought to be dominantly CO2-H2O ± S fluids. Curiously, although CO2 vapor occurs in bubbles of mafic melt inclusions (MI) at room temperature (T), the expected accompanying vapor and liquid H2O have not been found. We reheated olivine-hosted MI from Mt. Somma-Vesuvius, Italy, and quenched the MI to a bubble-bearing glassy state. Using Raman spectroscopy, we show that the volatiles exsolved after quenching include liquid H2O at room T and vapor H2O at 150 °C. We hypothesize that H2O initially present in the MI bubbles was lost to adjacent glass during local, sub-micrometer-scale devitrification prior to sample collection. During MI heating experiments, the H2O is redissolved into the vapor in the bubble, where it remains after quenching, at least on the relatively short time scales of our observations. These results indicate that (1) a significant amount of H2O may be stored in the vapor bubble of bubble-bearing MI and (2) the composition of magmatic fluids directly exsolving from mafic melts at Mt. Somma-Vesuvius may contain up to 29 wt% H2O.

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.

Constraints on the origin of sub-effusive nodules from the Sarno (Pomici di Base) eruption of Mt. Somma-Vesuvius (Italy) based on compositions of silicate-melt inclusions and clinopyroxene

Abstract Major and trace element and volatile compositions of reheated melt inclusions (RMI) and their clinopyroxene hosts from a selected “sub-effusive” nodule from the uppermost layer of the Sarno (Pomici di Base; PB) plinian eruption of Mt. Somma-Vesuvius (Italy) have been determined. The Sarno eruption occurred during the first magmatic mega-cycle and is one of the oldest documented eruptions at Mt. Somma-Vesuvius. Based on RMI and clinopyroxene composition we constrain processes associated with the origin of the nodule, its formation depth, and hence the depth of the magma chamber associated with the Sarno (PB) eruption. The results contribute to a better understanding of the early stages of evolution of the long-lived Mt. Somma-Vesuvius volcanic complex. The crystallized MI were heated to produce a homogeneous glass phase prior to analysis. MI homogenized between 1202-1256 °C, and three types of RMI were distinguished based on their compositions and behavior during heating. Type I RMI is classified as phono-tephrite-tephri-phonolite-shoshonite, and is the most representative of the melt phase from which the clinopyroxenes crystallized. The second type, referred to as basaltic RMI, have compositions that have been modified by accidentally trapped An-rich feldspar and/or by overheating during homogenization of the MI. The third type, referred to as high-phosphorus (high-P) RMI, is classified as picro-basalt and has high-P content due to accidentally trapped apatite. Type I RMI are more representative of magmas associated with pre-Sarno eruptions than to magma associated with the Sarno (PB) eruption based on published bulk rock compositions for Mt. Somma-Vesuvius. Therefore, it is suggested that the studied nodule formed from a melt compositionally similar to that which was erupted during the early history of Mt. Somma. The clinopyroxene and clinopyroxene-silicate melt thermobarometer models suggest minimum pressures of 400 MPa (~11 km) for nodule formation, which is greater than pressures and depths commonly reported for the magmas associated with younger plinian eruptions of Mt. Somma-Vesuvius. Minimum pressures of formation based on volatile concentrations of MI interpreted using H2O-CO2-silicate melt solubility models indicate formation pressures ≤300 MPa.

Fluid and melt inclusions from subvolcanic to surface environment in the Campi Flegrei (Napoli, Italy) active volcanic system

We present a summary of the results obtained on more than 25 years of research achieved on Campi Flegrei (CF) past eruptions, focusing our attention on the role played by fluids in the magmatic system based on fluid (FI) and melt inclusions (MI).Particularly FI and MI data from subvolcanic igneous systems in the CF area provide valuable information on the nature of fluid and melt phases trapped during the late evolutionary stages of the alkaline magmatic systems. They also document liquid immiscibility at pre-eruptive magma conditions and furnish evidence of high salinity fluids (brines) exsolving directly from magma in the upper part of the plumbing system at the magmatic/hydrothermal transition and playing critical roles in ore metal transport. CF volcanic system can be interpreted as representing a modern analogue of low sulfidation epithermal deposits or a porphyry copper system. This interpretation led to the formulation of a model to explain ground movements (bradyseism) in the CF.Finally new finding for the estimations of pre-eruptive volatile contents in CF through MI are discussed. MI are the only direct samples to measure volatile contents of undegassed melt at subvolcanic conditions. Since the depth of formation of MI is estimated using H2O-CO2-silicate melt solubility models, it is important to emphasize that, to obtain a more accurate measure of the preeruptive CO2 concentration, not only the CO2 in the glass but also in the bubble have to be measured otherwise the minimum CO2 content in MI is obtained.