Post-entrapment Crystallization Research Articles

Ca/Al of plagioclase-hosted melt inclusions as an indicator for post-entrapment processes at mid-ocean ridges?

The composition of melt inclusions in basalts erupted at mid-ocean ridges may be modified by post-entrapment processes, so the present composition of melt inclusions may not represent their original composition at the time of entrapment. By combining the melt inclusion composition in samples from the South Mid-Atlantic Ridge at 19oS analyzed in this study, and from the Petrological Database, we found that post-entrapment crystallization processes resulted in higher Ca/Al, Mg#[100°—atomic Mg 2+ /(Mg 2+ +Fe 2+ )], MgO and FeO contents, and lower CaO and Al 2 O 3 contents of plagioclase-hosted melt inclusions relative to those hosted in olivine. In addition, melt inclusions hosted in plagioclase with anorthite content larger than 80mol.% had been modified more readily than others. By discussing the relationships between Ca/Al and fractional crystallization, post-entrapment crystallization, and the original melt composition, we propose that Ca/Al can be regarded as an indicator of the effect of post-entrapment processes on melt inclusion composition. Specifically, i) when Ca/Al 1.0, the compositions of melt inclusions do not reflect the original melt composition nor preserve information about the mantle source. According to these criteria, plagioclase-hosted melt inclusions with Ca/Al>1.0 in basalts from the South Mid-Atlantic Ridge at 19oS cannot represent the composition of the melt at the moment of their entrapment.

Understanding a volcano through a droplet: A melt inclusion approach

This review paper is intended to be a guideline to novices on how to conduct research on silicate melt inclusions in volcanic environments, which analytical techniques are more suitable to gather the desired data and the major pitfalls scientist may encounter. Silicate melt inclusions (SMIs) are small quantities of silicate melt that are trapped in minerals during their growth or crystallization. They contain liquids formed in equilibrium with their host minerals and therefore record the liquid line of descent of magmatic systems. Upon trapping, SMIs become ideally closed to the surrounding environment, and behave as time capsules, giving important information about processes that originated magmas and the nature of their mantle source. A melt inclusions investigation is composed of several steps: (1) a detailed petrographic study to characterize and select representative SMIs, with the aim of identifying Melt Inclusions Assemblages (MIA), the only reliable tool to assess that SMIs obey Roedder's rules and have not re-equilibrated following entrapment; (2) a careful preparation of samples for re-heating experiments and microanalysis; (3) high temperature studies in order to homogenize the SMIs and be able to perform chemical analysis; (4) interpretation of SMIs data, which should always be compared with bulk rock composition and relevant experimentally derived liquid compositions. We suggest that the search for good SMI candidates for study will be achieved by good petrographic analysis of SMIs and detailed petrographic information (size, shape, appearance, position relative to the host and other SMI within the same host). If the goal of a SMIs study is to determine volatile concentrations, assessing the relative time of trapping among SMIs is of paramount importance, as it allows understanding if the variation in volatile concentrations is consistent with a certain magmatic physical–chemical process. Researchers that choose to work with SMIs face several challenges. SMIs are not always hosted in all samples or they can be difficult to recognize (mostly due to their small size). Additionally, due several processes, such as post-entrapment crystallization, boundary layer and sampling melt heterogeneity at the micron scale, they may not be representative of the original trapped melt composition.

Quantification of the CO2 budget and H2O–CO2 systematics in subduction-zone magmas through the experimental hydration of melt inclusions in olivine at high H2O pressure

Reliable evaluation of CO2 contents in parental arc magmas, which can be preserved in melt inclusions in phenocrysts, is required to verify the proposed efficiency of CO2 recycling at convergent margins. Quantification of bulk CO2 concentration in melt inclusions requires their complete homogenization. Using samples from lavas from the Bulochka vent of Klyuchevskoy Volcano (Kamchatka), we applied a novel experimental approach to homogenize and re-equilibrate naturally dehydrated (<1 wt.% H2O) melt inclusions from high-Fo (85–91 mol.%) olivine. The experiments were performed at temperatures of 1150–1400 °C, pressures of up to 500 MPa, under dry to H2O-saturated conditions and with oxygen fugacity ranging from CCO to QFM+3.3. No homogenization was achieved at dry conditions. Complete dissolution of fluid bubbles (homogenization) in the melt inclusions was achieved at H2O pressures of 500 MPa and temperature of 1150 °C, when water content in the melt inclusions reached 4–5 wt.% H2O. The CO2 content in the homogenized inclusions is 3800±140 ppm and CO2/Nb = 3000 ± 420, which are the highest values reported so far for the typical middle-K primitive arc melts and fall within the range of values inferred from the magmatic flux and volcanic gas data for primary arc magma compositions. About 83% of the CO2 in Klyuchevskoy magmas is likely to be derived from the subducting slab and can be attributed to flux melting with a fluid having a CO2/H2O ratio of ∼0.06. The H2O and CO2 contents in the melt inclusions after hydrous experiments were found to correlate positively with each other and negatively with the volume of fluid bubble, reflecting increasing internal pressure in melt inclusions with increasing melt hydration. Therefore, similar trends observed in some natural sets of melt inclusions can be attributed to a partial dehydration of melts after entrapment, operating simultaneously with or following post-entrapment crystallization. Our study implies that the process of post-entrapment dehydration can be completely reversed under high pressure experimental conditions. If temperature, redox conditions and pressure of melt inclusion entrapment can be independently estimated, then our novel experimental approach (homogenization at high H2O pressure) can be used to reconstruct the initial CO2 content and also the entire composition of melt inclusions in olivine, including their initial H2O content, from any type of volcanic rock. With this approach volatiles in ancient lavas can also be determined, expanding our knowledge of volatile recycling further back in Earth history.

Magma mixing and high fountaining during the 1959 Kīlauea Iki eruption, Hawai‘i

The 1959 Kīlauea Iki eruption provides a unique opportunity to investigate the process of shallow magma mixing, its impact on the magmatic volatile budget and its role in triggering and driving episodes of Hawaiian fountaining. Melt inclusions hosted by olivine record a continuous decrease in H2O concentration through the 17 episodes of the eruption, while CO2 concentrations correlate with the degree of post-entrapment crystallization of olivine on the inclusion walls. Geochemical data, when combined with the magma budget and with contemporaneous eruption observations, show complex mixing between episodes involving hot, geochemically heterogeneous melts from depth, likely carrying exsolved vapor, and melts which had erupted at the surface, degassed and drained-back into the vent. The drained-back melts acted as a coolant, inducing rapid cooling of the more primitive melts and their olivines at shallow depths and inducing crystallization and vesiculation and triggering renewed fountaining. A consequence of the mixing is that the melts became vapor-undersaturated, so equilibration pressures cannot be inferred from them using saturation models. After the melt inclusions were trapped, continued growth of vapor bubbles, caused by enhanced post-entrapment crystallization, sequestered a large fraction of CO2 from the melt within the inclusions. This study, while cautioning against accepting melt inclusion CO2 concentrations “as measured” in mixed magmas, also illustrates that careful analysis and interpretation of post-entrapment modifications can turn this apparent challenge into a way to yield novel useful insights into the geochemical controls on eruption intensity.

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.

Zonation of H2O and F Concentrations around Melt Inclusions in Olivines

Studies of both naturally quenched and experimentally reheated melt inclusions have established that they can lose or gain H_(2)O after entrapment in their host mineral, before or during eruption. Here we report nanoSIMS analyses of H2O, Cl and F in olivine around melt inclusions from two natural basaltic samples: one from the Sommata cinder cone on Vulcano Island in the Aeolian arc and the other from the Jorullo cinder cone in the Trans-Mexican Volcanic Belt. Our results constrain olivine/basaltic melt partition coefficients and allow assessment of mechanisms of volatile loss from melt inclusions in natural samples. Cl contents in olivine from both samples are mostly below detection limits (≤0·03 ± 0·01 ppm), with no detectable variation close to the melt inclusions. Assuming a maximum Cl content of 0·03 ppm for all olivines, maximum estimates for Cl partition coefficients between olivine and glass are 0·00002 ± 0·00002. Olivines from the two localities display contrasting H_(2)O and F compositions: Sommata olivines contain 27 ± 11 ppm H_(2)O and 0·28 ± 0·07 ppm F, whereas Jorullo olivines have lower and proportionately more variable H_(2)O and F (11 ± 12 ppm and 0·12 ± 0·09 ppm, respectively; uncertainties are two standard deviations for the entire population). The variations of H_(2)O and F contents in the olivines exhibit clear zonation patterns, increasing with proximity to melt inclusions. This pattern was most probably generated during transfer of volatiles out of the inclusions through the host olivine. H_(2)O concentration gradients surrounding melt inclusions are roughly concentric, but significantly elongated parallel to the crystallographic a-axis of olivine. Because of this preferential crystallographic orientation, this pattern is consistent with H_(2)O loss that is rate-limited by the ‘proton–polaron’ mechanism of H diffusion in olivine. Partition coefficients based on olivine compositions immediately adjacent to melt inclusions are 0·0007 ± 0·0003 for H_(2)O and 0·0005 ± 0·0003 for F. The H_(2)O and F diffusion profiles most probably formed in response to a decrease in the respective fugacities in the external melt, owing to either degassing or mixing with volatile-poor melt. Volatile transport out of inclusions might also have been driven in part by increases in the fugacity within the inclusion owing to post-entrapment crystallization. In the case of F, because of the lack of data on F diffusion in olivine, any interpretation of the measured F gradients is speculative. In the case of H_(2)O, we model the concentration gradients using a numerical model of three-dimensional anisotropic diffusion of H, where initial conditions include both H2O decrease in the external melt and post-entrapment enrichment of H_(2)O in the inclusions. The model confirms that external degassing is the dominant driving force, showing that the orientation of the anisotropy in H diffusion is consistent with the proton–polaron diffusion mechanism in olivine. The model also yields an estimate of the initial H_(2)O content of the Sommata melt inclusions before diffusive loss of 6 wt % H_(2)O. The findings provide new insights on rapid H_(2)O loss during magma ascent and improve our ability to assess the fidelity of the H_(2)O record from melt inclusions.

The Deep Plumbing System of Ischia: a Physico-chemical Window on the Fluid-saturated and CO2-sustained Neapolitan Volcanism (Southern Italy)

Ischia, a volcanic island located 18 miles SWof Naples (Southern Italy), is a densely populated active caldera that last erupted in AD 1302. Melt inclusions in phenocrysts of the Vateliero and Cava Nocelle shoshonite^latite eruptive products (6th to 4th centuries BC) constrain the structure and nature of the Ischia deep magmatic feeding system.Their geochemical characteristics make Ischia a natural borehole for probing the physico-chemical conditions of magma generation in mantle contaminated by slab-derived fluids or melts, largely dominated by CO2.Volatile concentrations in olivine-hosted melt inclusions require gas^melt equilibria at between 3 and 18 km depth. In agreement with what has already been demonstrated at the other neighboring Neapolitan volcanoes (Procida, Campi Flegrei caldera and Somma^Vesuvius volcanic complex), a major crystallization depth at 8^10 km has been identified.The analyzed melt inclusions provide clear evidence for CO2-dominated gas fluxing and consequent dehydration of magma batches stagnating at crustal discontinuities. Gas fluxing is further supported by selective enrichment in K owing to fluid-transfer during magma differentiation.This takes place under oxidized conditions (Fe3þ/ Fe 0·3) that can be fixed by an equimolar proportion of divalent and trivalent iron in the melt if post-entrapment crystallization of the host olivine is discarded.The melt inclusion data, together with data from the literature for other Neapolitan volcanoes, show that magmatism and volcanism in the Neapolitan area, despite differences in composition and eruption dynamics, are closely linked to supercritical CO2-rich fluids. These fluids are produced by devolatilization of subducting terrigenous^pelagic metasediments and infiltrate the overlying mantle wedge, generate magmas and control their ascent up to eruption. Geochemical characteristics of Ischia and the other Neapolitan volcanoes reveal that the extent of fluid or melt contamination of the pre-subduction asthenospheric mantle wedge was similar among these volcanoes. However, differences in the isotopic compositions of the erupted magmas (more enriched in radiogenic Sr at Ischia, Campi Flegrei and Somma^Vesuvius with respect to Procida) and the amount of H2O in the plumbing system of these volcanoes (almost double at Ischia, Campi Flegrei and Somma^Vesuvius than at Procida) reflect the different flow-rates of deep slab-derived fluids or melts through the mantle wedge, which, in turn, control the amount of generated magma.The high bulk permeability of the lithosphere below Ischia, Campi Flegrei and Somma^Vesuvius, determined by the occurrence of intersecting NW^SE and NE^SW regional fault systems, favours fluid ascent and accumulation at crustal levels, with consequent larger magma production and storage than at Procida, located along the NE^SWsystem.

Volatile loss from melt inclusions in pyroclasts of differing sizes

We have investigated the loss of H2O from olivine-hosted melt inclusions (MIs) by designing an experiment using tephra samples that cooled at different rates owing to their different sizes: ash, lapilli, and bomb samples that were deposited on the same day (10/17/74) of the sub-Plinian eruption of Volcán de Fuego in Guatemala. Ion microprobe, laser ablation-ICPMS, and electron probe analyses show that MIs from ash and lapilli record the highest H2O contents, up to 4.4 wt%. On the other hand, MIs from bombs indicate up to 30 % lower H2O contents (loss of ~1 wt% H2O) and 10 % post-entrapment crystallization of olivine. This evidence is consistent with the longer cooling time available for a bomb-sized clast, up to 10 min for a 3–4-cm radius bomb, assuming conductive cooling and the fastest H diffusivities measured in olivine (D~10−9 to 10−10 m2/s). On the other hand, several lines of evidence point to some water loss prior to eruption, during magma ascent and degassing in the conduit. Thus, results point to both slower post-eruptive cooling and slower magma ascent affecting MIs from bombs, leading to H2O loss over the timescale of minutes to hours. The important implication of this study is that a significant portion of the published data on H2O concentrations in olivine-hosted MIs may reflect unrecognized H2O loss via diffusion. This work highlights the importance of reporting clast and MI sizes in order to assess diffusive effects and the potential benefit of using water loss as a chronometer of magma ascent.

The influence of magmatic differentiation on the oxidation state of Fe in a basaltic arc magma

Subduction zone basalts are more oxidized than basalts from other tectonic settings (e.g., higher Fe3+/∑Fe), and this contrast may play a central role in the unique geochemical processes that generate arc and continental crust. The processes generating oxidized arc magmas, however, are poorly constrained, although they appear inherently linked to subduction. Near-surface differentiation processes unique to arc settings might drive oxidation of magmas that originate in equilibrium with a relatively reduced mantle source. Alternatively, arc magmas could record the oxidation conditions of a relatively oxidized mantle source. Here, we present new measurements of olivine-hosted melt inclusions from a single eruption of Agrigan volcano, Marianas, in order to test the influence of differentiation processes vs. source conditions on the Fe3+/∑Fe ratio, a proxy for system oxygen fugacity (fO2). We determined Fe3+/∑Fe ratios in glass inclusions using μ-XANES and couple these data with major elements, dissolved volatiles, and trace elements. After correcting for post-entrapment crystallization, Fe3+/∑Fe ratios in the Agrigan melt inclusions (0.219 to 0.282), and their modeled fO2s (ΔQFM +1.0 to +1.8), are uniformly more oxidized than MORB, and preserve a portion of the evolution of this magma from 5.7 to 3.2wt.% MgO. Fractionation of olivine±clinopyroxene±plagioclase should increase Fe3+/∑Fe as MgO decreases in the melt, but the data show Fe3+/∑Fe ratios decreasing as MgO decreases below 5wt.% MgO. The major element trajectories, taken in combination with this strong reduction trend, are inconsistent with crystallization of common ferromagnesian phases found in the bulk Agrigan sample, including magnetite. Rather, decreasing Fe3+/∑Fe ratios correlate with decreasing S concentrations, suggesting that electronic exchanges associated with SO2 degassing may dominate Fe3+/∑Fe variations in the melt during differentiation. In the case of this magma, the dominant effect of differentiation on magmatic fO2 is reduction rather than oxidation. Tracing back Agrigan melts with MgO>5wt.% (i.e., minimally degassed for S) along a modeled olivine fractionation trend to primary melts in equilibrium with Fo90 olivine reveals melts in equilibrium with the mantle beneath Agrigan at fO2s of ΔQFM +1 to +1.6, significantly more oxidized than current constraints for the mantle beneath mid-ocean ridges.

Thermodynamic Model for the Effect of Post-entrapment Crystallization on the H2O-CO2 Systematics of Vapor-saturated, Silicate Melt Inclusions

Melt inclusions (MI) represent the best source of information concerning the pre-eruptive volatile contents of magmas. If the trapped melt is enriched in volatile species, following trapping the MI may generate a vapor bubble containing volatiles that have exsolved from the melt.Thermodynamic modeling of vapor-saturated albitic composition (NaAlSi3O8) MI shows that the CO2 content of the melt phase in the MI is sensitive to small amounts of post-entrapment crystallization (PEC), whereas the H2O content of the melt is less sensitive to PEC. During PEC, CO2 is transferred from the melt to the vapor phase and the vapor bubble may contain a significant amount, if not most, of the CO2 in the MI.The contrasting behaviors of H2O and CO2 during PEC lead to H2O^CO2 trends that are similar to those predicted for open-system degassing during magma ascent and decompression.Thus, similar H2O^CO2 trends may be produced if (1) vapor-saturated MI are trapped at various depths along a magmatic ascent path, or (2) MI having the same volatile content are all trapped at the same depth, but undergo different amounts of PEC following trapping. It is not possible to distinguish between these two contrasting interpretations based on MI volatile data alone. However, by examining the volatile trends within the context of other geochemical monitors of crystallization or magma evolution progress, it may be possible to determine whether the volatile trends were generated along a degassing path or if they reflect various amounts of PEC in an originally homogeneous melt inclusion assemblage. The volatile trends resulting from PEC of MI described in this study are directly applicable to silica-rich (granitic) MI trapped in non-ferromagnesian host phases, and are only qualitatively applicable to more mafic melt compositions and/or host phases owing to modifications resulting from Fe exchange with the host and to post-entrapment re-equilibration processes.

Volatile Evolution of Magma Associated with the Solchiaro Eruption in the Phlegrean Volcanic District (Italy)

More than 1·5 million people live in or near the Phlegrean Volcanic District (PVD) in southern Italy, which represents one of the most carefully studied volcanic hazard areas in the world. Throughout its history, the style of volcanic activity has varied greatly, from relatively quiescent lava flows to explosive phreatomagmatic eruptions. The goal of this study is to develop a more detailed understanding of the physical and chemical processes associated with the Solchiaro eruption in the PVD. The PVD includes three volcanic fields: the Campi Flegrei (CF) caldera and the volcanic islands of Ischia and Procida. The Solchiaro eruption on the island of Procida is one of the few primitive (less evolved) eruptions in the PVD and can provide information on the source of the more evolved magmas associated with this volcanic system. One of the more important chemical parameters that determine the style of volcanic eruptions is the volatile budget of the magma before and during eruption. Melt inclusions (MI) provide the most direct information on the volatile contents of the pre-eruptive melt in the source region for the PVD. The composition of the melt phase before eruption was determined by analyzing the major, minor and trace element and volatile contents of 109 MI in olivine from four samples of the Solchiaro eruption, representing different stratigraphic heights in the deposits and, therefore, different relative times of eruption. Olivine compositions vary from Fo82 to Fo88, with one maximum value of Fo90. The compositions of the MI in olivine were corrected for post-entrapment crystallization (PEC) and for Fe loss by diffusion. Most (97 out of 109) of the MI studied are classified as ‘normal’ MI because they show chemical evolution trends consistent with that of bulk-rocks from the PVD. Two types of anomalous MI were also recognized based on their major and trace element compositions: (1) Sr-rich MI, and (2) enriched MI that are variably enriched in TiO2, K2O, P2O5, large ion lithophile elements, high field strength elements and rare earth elements relative to ‘normal’ MI. These MI probably originated from dissolution–reaction–mixing processes in the mush zone of the magma body. ‘Normal’ MI include both bubble-bearing and bubble-free (containing only glass ± trapped chromite) types. Bubble-free MI most closely record the pre-eruptive volatile content of the melt over a range of temporal and spatial conditions. The observed trends in CO2 contents of MI versus crystallization indicators (e.g. Al2O3/CaO) support the interpretation that variations in the volatile contents of bubble-free MI reflect real variations in the volatile budget of the melt during the evolution of the magma. The correlation between CO2 contents of MI and the relative stratigraphic position of each sample is consistent with eruption of a volatile-saturated magma that initially ascended through the crust from an original depth of at least 8 km. The magma ponded at 4–2 km depth prior to eruption and crystallization and the concomitant volatile exsolution from the saturated melt in the shallow chamber triggered the Solchiaro eruption. As the eruption proceeded, the Solchiaro magma continued to ascend through the crust to a final storage depth of about 1 km.

The effects of re-homogenization on plagioclase hosted melt inclusions

Melt inclusions trapped in phenocrysts provide a unique picture of magma systems prior to modification by crustal processes. However, post-entrapment crystallization complicates their interpretation. Re-heating the phenocryst to the temperature of entrapment is a commonly applied method to recover the original melt composition. To understand the effects of re-homogenization, we compared the composition of re-heated and naturally quenched melt inclusions and inclusion compositions that had been subjected to over-heating and under-heating to examine the degree to which anomalous compositions were produced. Our results on plagioclase hosted inclusions from ocean floor basalts indicate that the general patterns represented by naturally quenched inclusions are the same as observed for rehomogenized inclusions. Most important, the range of minor elements described for plagioclase hosted inclusions from basalts is found in naturally quenched inclusions, and is therefore not a consequence of the re-homogenization process.

The November 2002 eruption of Piton de la Fournaise, Réunion: tracking the pre-eruptive thermal evolution of magma using melt inclusions

The November 2002 eruption of Piton de la Fournaise in the Indian Ocean was typical of the activity of the volcano from 1999 to 2006 in terms of duration and volume of magma ejected. The first magma erupted was a basaltic liquid with a small proportion of olivine phenocrysts (Fo81) that contain small numbers of melt inclusions. In subsequent flows, olivine crystals were more abundant and richer in Mg (Fo83–84). These crystals contain numerous melt and fluid inclusions, healed fractures, and dislocation features such as kink bands. The major element composition of melt inclusions in this later olivine (Fo83–84) is out of equilibrium with that of its host as a result of extensive post-entrapment crystallization and Fe2+ loss by diffusion during cooling. Melt inclusions in Fo81 olivine are also chemically out of equilibrium with their hosts but to a lesser degree. Using olivine–melt geothermometry, we determined that melt inclusions in Fo81 olivine were trapped at lower temperature (1,182 ± 1°C) than inclusions in Fo83–84 olivine (1,199–1,227°C). This methodology was also used to estimate eruption temperatures. The November 2002 melt inclusion compositions suggest that they were at temperatures between 1,070°C and 1,133°C immediately before eruption and quenching. This relatively wide temperature range may reflect the fact that most of the melt inclusions were from olivine in lava samples and therefore likely underwent minor but variable amounts of post-eruptive crystallization and Fe2+ loss by diffusion due to their relatively slow cooling on the surface. In contrast, melt inclusions in tephra samples from past major eruptions yielded a narrower range of higher eruption temperatures (1,163–1,181°C). The melt inclusion data presented here and in earlier publications are consistent with a model of magma recharge from depth during major eruptions, followed by storage, cooling, and crystallization at shallow levels prior to expulsion during events similar in magnitude to the relatively small November 2002 eruption.

Deep Fractionation of Clinopyroxene in the East Pacific Rise 13°N: Evidence from High MgO MORB and Melt Inclusions

Abstract:Mid‐ocean ridge basalts (MORBs) from East Pacific Rise (EPR) 13°N are analysed for major and trace elements, both of which show a continuous evolving trend. Positive MgO–Al2O3 and negative MgO–Sc relationships manifest the cotectic crystallization of plagioclase and olivine, which exist with the presence of plagioclase and olivine phenocrysts and the absence of clinopyroxene phenocrysts. However, the fractionation of clinopyroxene is proven by the positive correlation of MgO and CaO. Thus, MORB samples are believed to show a “clinopyroxene paradox”. The highest magnesium‐bearing MORB sample E13–3B (MgO = 9.52%) is modelled for isobaric crystallization with COMAGMAT at different pressures. Observed CaO/Al2O3 ratios can be derived from E13–3B only by fractional crystallization at pressure &gt;4 ±1 kbar, which necessitates clinopyroxene crystallization and is not consistent with cotectic crystallization of olivine plus plagioclase in the magma chamber (at pressure ∼1 kbar). The initial compositions of the melt inclusions, which could represent potential parental magmas, are reconstructed by correcting for post‐entrapment crystallization (PEC). The simulated crystallization of initial melt inclusions also produce observed CaO/Al2O3 ratios only at &gt;4±1 kbar, in which clinopyroxene takes part in crystallization. It is suggested that MORB magmas have experienced clinopyroxene fractionation in the lower crust, in and below the Moho transition zone. The MORB magmas have experienced transition from clinopyroxene+plagioclase+olivine crystallization at &gt;4±1 kbar to mainly olivine+plagioclase crystallization at &lt;1 kbar, which contributes to the explanation of the “clinopyroxene paradox”.

Crust–primitive magma interaction beneath neovolcanic rift zone of Iceland recorded in gabbro xenoliths from Midfell, SW Iceland

We studied glass-bearing gabbro xenoliths from picritic pillow lavas erupted in the Midfell area, SW Iceland. The aim of this study is to investigate the processes affecting chemical composition of primitive Icelandic magmas at crustal depths. The xenoliths formed at 340±160 MPa (2σ) lithostatic pressure are cumulates of earlier crystallized magmas. Later interaction of the xenoliths with the transporting magmas occurred at 1,190–1,230°C and shallower depths (40±120 MPa). The xenoliths consist of reversely zoned, slightly resorbed, subhedral plagioclase (PL, An81-90, 50–60%), anhedral, strongly resorbed clinopyroxene (CPX, mg-number=83–90, 25–32%), trace amounts (<1%) of small, up to 500 μm, euhedral olivine (OL, Fo85-89) and spinel (SP, mg-number=66–73, cr-number=29–51), and interstitial glass (9–22%). No significant difference in chemical composition between surrounding host pillow rim glasses (K2O/TiO2=0.02−0.04, CaO/Al2O3=0.93−1.04, [La/Sm] n =0.38–0.54, where subscript n points on element concentrations normalized to those in chondrite) and interstitial glasses (K2O/TiO2=0.03–0.06, CaO/Al2O3=1.00–1.07, [La/Sm] n =0.41–0.74) was recognized. Deep glassy embayments in CPX (K2O/TiO2=0.02–0.06, CaO/Al2O3=1.00–1.06, [La/Sm] n =0.66–0.99) have also chemical compositions very close to those of interstitial and host glasses. Glass embayments in PL (K2O/TiO2=0.03–0.11, CaO/Al2O3=0.87–1.06, [La/Sm] n =0.70–0.97) vary more significantly in CaO/Al2O3 ratios than those in CPX likely due to their stronger modification by post-entrapment crystallization. All glass types, regardless of whether they are enclosed in minerals or represent interstitial or host matrix glasses, are characterized by significant excess of Sr ([Sr/Sr*] n =1.7–2.7) but not of Ba relative to Nb ([Ba/Nb] n =0.45–0.96). Partial melting of a gabbro with [La/Sm] n =0.54, [Sr/Sr*] n =10.5 and [Ba/Nb] n =4.5 inferred from the composition of the xenoliths studied cannot account for the observed trace element concentrations and ratios in the associated glasses. In fact, the compositions of melts calculated for 1–40% of partial melting match neither compositions of the interstitial glasses nor CPX- and PL-hosted glassy embayments with respect to [Ba/Nb]n ratios. Mixing of these partial melts with presumably uncontaminated magma composition does not explain chemical variations observed in the glasses as well. In contrast, the model of melt percolation through gabbroic matrix reproduces in a satisfactory way the compositions of glasses. We believe that the pronounced enrichment in Sr relative to other trace elements of similar incompatibility observed in many Icelandic tholeiites and probably in many MORB-type magmas worldwide could result from such interaction of the ascending magmas with gabbroic cumulates at crustal depths. We emphasize that the percolation model discussed here results in hybrid, Sr-enriched magmas which are saturated with CPX and PL. The enrichment in Sr of PL-undersaturated melts (so-called “ghost plagioclase signature‘’; Sobolev et al. in Nature 404:986–990, 2000) could not be produced by such percolation process.

Evidence of primitive melt heterogeneities preserved in plagioclase-hosted melt inclusions of South Atlantic MORB

Melt inclusions contained in plagioclase and olivine phenocrysts have been studied in tholeiitic basalts from the Shona ridge-centered hot spot region, South Atlantic Ocean. Two types of primitive melt inclusions exist within a hand sample, Normal-Mid Ocean Ridge Basalt (N-MORB) and Enriched-Mid Ocean Ridge Basalt (E-MORB) as defined by trace elements. The N-MORB melt inclusions have low (La/Sm)n (0.54-0.72), high Zr/Nb (22-44) and depleted Light Rare Earth Element (LREE) patterns, whereas the E-MORB melt inclusions have high (La/Sm)n (0.79-1.09), low Zr/Nb (12-21) and flat LREE. The composition of melt inclusions has been modified by post-entrapment crystallization of the host phase, but this effect is considered relatively small and does not affect the incompatible trace element ratios. The matrix glasses have mildly E-MORB compositions that lie between the two different types of melt inclusions, suggesting that they are produced by mixing of primary magmas of E-MORB and N-MORB type.The N-MORB primitive melt inclusions are geochemically similar to Indian and other South Atlantic N-MORB in having high Ba/Nb ratios (4-10), but for the first time much more extreme values are found (8-18). The reason for this anomalous enrichment can be related to the presence of old pelagic sediments in a regionally depleted asthenosphere.The melt inclusions of this study display positive anomalies of Sr and Eu, the first time that have been recorded in the South Atlantic. Their characteristics could be explained by partial melting of mantle containing recycled oceanic crust.

Silicate melt inclusions in the phenocrysts of the Szomolya Ignimbrite, Bükkalja Volcanic Field (Northern Hungary): Implications for magma chamber processes

Silicate melt inclusions provide important information on the magma evolution processes and therefore they are the subjects of rapidly growing geochemical studies. However, the original geochemical fingerprint of the trapped melts could be often masked by post-entrapment processes. In this paper, we investigate silicate melt inclusions hosted by various minerals (plagioclase, orthopyroxene, clinopyroxene, quartz) of a silicic ignimbrite and the enclosed igneous lithic clasts in order to reconstruct the magma chamber processes. The Szomolya Ignimbrite was formed at the initiation of the widespread Neogene volcanism of the Pannonian Basin, eastern–central Europe. It is composed of fresh rhyolitic pyroclastics and contains abundant andesitic lithic clasts. The melt inclusions in the phenocrysts are composed mostly of glass with or without shrinkage bubble suggesting rapid cooling. They all have rhyolitic composition, but show significant chemical variability. The melt inclusions in quartz and plagioclase of the ignimbrite have similar compositions as the glass shards. They represent a strongly silicic and potassic (SiO 2 = 76.5 to 79 wt.%; K 2O / Na 2O = > 2) melt resided at the top of the magma chamber at about 2–3 km depth. The melt inclusions in the phenocrysts of the lithic clast show larger chemical variation. It is remarkable that the plagioclase-hosted melt inclusions differ significantly from all the other glass data. This can be explained by the influence of significant post-entrapment crystallization (PEC), i.e. chemical modifications between the entrapment and quenching. Based on major and trace element mass balance modeling, we show that about 10% PEC took place in the plagioclase-hosted melt inclusions, whereas PEC was less in the pyroxene-hosted melt inclusions (about 3%). After correcting this modification, the melt inclusions from the pyroxenes still show wide chemical variation forming linear trends in the Harker diagrams overlapping partly the compositional trend of the groundmass glass. We interpret this as incorporation of heterogeneous melt batches during the mineral growth at the solidification front of the magma chamber at about 5–6 km depth. We assume that there is a genetic relationship between the andesitic lithic clasts and the host ignimbrite. The former ones could reflect crystallization at the solidification front at greater depth, whereas the latter one could represent the silicic residual melt accumulating at the top of the magma chamber. Difference in trace element compositions can be explained by late stage removal of accessory minerals such as zircon and allanite, which strongly controlled the trace element budget of the most differentiated melt. Withdrawal of the rhyolitic magma could enhance the disruption of the deeper solidification front. Its mineral assemblage (clinopyroxene, orthopyroxene and plagioclase) came in contact with a slightly more evolved melt, which quenched suddenly due to the eruption of the silicic magma.

Accurate quantification of melt inclusion chemistry by LA-ICPMS: a comparison with EMP and SIMS and advantages and possible limitations of these methods

Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) has recently emerged as a powerful in situ microanalytical technique for major to trace elements in heterogeneous samples such as fluid and melt inclusions. Here, a rigorous comparison of melt inclusion (MI) data acquired by electron microprobe (EMP), ion microprobe (the secondary ion mass spectrometry, SIMS) and LA-ICPMS is used to evaluate the applicability and advantages/drawbacks of these approaches. We are specifically interested in determining if LA-ICPMS data on entire, unexposed, crystallized MI that cannot be homogenized in the lab are accurate and of a useful precision. Quantification of LA-ICPMS MI signals requires the use of an internal standard, i.e., the concentration of one element, or an element ratio, at the time of MI entrapment must be known independently, in order to derive the pure MI composition from the MI plus host mixed signal. Analysis of plagioclase-hosted glassy MI of a mid-ocean ridge basalt (MORB) sample from the East Pacific Rise illustrates that melt inclusion chemistry can be accurately quantified by LA-ICPMS, including the correction for postentrapment sidewall crystallisation of the host mineral without prior reheating in the lab. The LA-ICPMS data obtained on crystallized MI demonstrate agreement with the EMP and SIMS data on exposed glassy MI at the 1 standard deviation uncertainty level except for a few elements close to their limits of detection. LA-ICPMS data reduction schemes include the quantification of analytical uncertainty on each element of single MI. Therefore, weighted average element concentrations can be obtained for MI assemblages, at precisions that compare well with those of average element concentrations obtained by EMP and SIMS. Simple sample preparation minimizing inclusion loss through polishing combined with the analytical efficiency of 50 inclusions plus neighbouring host mineral at up to 40 elements per day enable the collection of statistically relevant datasets by LA-ICPMS. These allow to recognize nonrepresentative MI (e.g., heterogeneous entrapment). Application to individual clinopyroxene crystals from the AD79 pumice horizon of Mt. Somma-Vesuvius reveals chemical variability that exceeds the analytical precision on single melt inclusions. This variability was not obvious from the limited data set obtained by SIMS and EMP. The largest source of nonquantifiable error for EMP and SIMS data stems from the requirement of reheating the melt inclusions in the lab in order to reverse postentrapment crystallisation onto inclusion walls or growth of crystallites. For LA-ICPMS analysis of unexposed MI, the reliability with which the internal standard (IS) element concentration is known determines the quality of the data. LA-ICPMS, however, cannot analyse H 2O, F, S and Cl reliably, has higher limits of detection (LODs) than SIMS for some elements for MI below ∼25 μm, has lower spatial resolution than both EMP and SIMS and consumes much more sample per analysis. Therefore, EMP, SIMS and LA-ICPMS are complementary in MI research, and the type of application will determine the analytical method or methods of choice.

Thermodynamic modeling of post-entrapment crystallization in igneous phases

Inclusions of quenched silicate liquid in igneous phenocryst phases represent important windows into the pre-eruption chemistry of volcanic rocks. Melt inclusions are subject to a variety of potential modifications after entrapment, which obscure the connection between final inclusion composition, and entrapment conditions. We concentrate on the effects of post-entrapment crystallization (PEC) in the cooling inclusion. PEC is neither an isobaric nor an isochoric process. Pressure decreases between 2 and 27 bars per degree of cooling, depending on the chemistry of melt and host and on the degree of PEC. In the equilibrium case, between about 50% and 65% of this pressure effect is due to thermal expansivity of the liquid, 10–35% from thermal expansivity of the host, and 5–40% from mass transfer between the inclusion and host. This complicates the application of simple element-partitioning schemes for back-calculating the effects of post-entrapment crystallization except in the simplest cases. We present a thermodynamic algorithm for PEC correction. This method is based on the self-consistent thermodynamic model set used in the MELTS software package. The algorithm moves backward through the PEC process, incrementally adding equilibrium crystal composition to the liquid while accounting for consequent variations in pressure and oxygen fugacity. Entrapment conditions are assumed to have been reached when the instantaneous liquidus solid composition most closely matches that of the bulk host crystal. Besides giving information on the degree of PEC and initial inclusion composition, the proposed algorithm can provide constraints on the pressure, temperature and oxygen fugacity at the time of entrapment. Olivine- and orthopyroxene-hosted inclusions from Popocatépetl, Mexico help constrain pre-eruption conditions for mixed magmas from recent eruptive products. Feldspar-hosted inclusions from Satsuma-Iwojima, Japan suggest that these magmas were substantially undersaturated with respect to supercritical vapor phase at the time of entrapment and underwent on the order of 29% post-entrapment crystallization. Quartz-hosted inclusions can potentially be employed in more silicic compositions, but this will require refinement of existing thermodynamic models.

Zoned quartz phenocrysts from the rhyolitic Bishop Tuff

Cathodoluminescence (CL) reveals growth zones in quartz phenocrysts from the rhyolitic Bishop Tuff. Melt inclusions occur in various zones and record the evolving melt composition during zonal growth. The zones form an oscillatory pattern between bright and dark CL quartz. There are three recognizable patterns of CL zoning in these crystals: (1) weakly zoned cores and bright CL rims; (2) weakly zoned cores and dark CL rims; and (3) no CL intensity difference from core to rim. Dark CL quartz generally occurs at crystal edges, contains most of the melt inclusions and is interpreted as fast-growing. Zones that occur along recognizable crystal edges (edge zones) are thicker than the same zone on adjacent faces, consistent with relatively fast growth of these zones. In each successive zone, these edge zones decrease in size toward the rim, while the zones along the crystal faces increase. Some of the melt inclusions have bright CL quartz locally associated with them. This is interpreted as the postentrapment crystallization of slow-growing quartz in the melt inclusions. Many crystals display zone discordance from the weakly zoned interiors to the rims. Most of the discordant surfaces are rational and probably are primary growth features. Pumice clasts from the southern vents are largely compositionally and texturally distinct from those from the northern vents, and this distinction is also evident in the quartz CL. The crystals that have bright CL rims are all associated with the late-erupted northern part of the Bishop Tuff. Melt inclusion compositions and CL zoning patterns suggest a common origin for early and middle-erupted quartz and the interior zones of late-erupted quartz; however, the bright CL rim on the late-erupted quartz indicates an additional stage of crystallization in late-erupted magma. Melt inclusions in individual early erupted crystals have small variations in Ba whereas inclusions in late-erupted crystals markedly increase in Ba toward the rim, which is opposite to the normal zoning of sequentially trapped melts expected during closed system crystallization differentiation. The quartz zoning features are consistent with the hypothesis of crystal settling in evolving magma that erupted late from northern vents.