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The Taishanmiao granitic batholith, located in the Eastern Qinling Orogen in Henan Province, China, contains numerous small (mostly tens of centimeters in maximum dimension) bodies exhibiting textures and mineralogy characteristics of simple quartz and alkali feldspar pegmatites. Analysis of melt inclusions (MI) and fluid inclusions (FI) in pegmatitic quartz, combined with Rhyolite-MELTS modeling of the crystallization of the granite, have been applied to develop a conceptual model of the physical and geochemical processes associated with the formation of the pegmatites. These results allow us to consider the formation of the Taishanmiao pegmatites within the context of varios models that have been proposed for pegmatite formation. Field observations and geochemical data indicate that the pegmatites represent the latest stage in the crystallization of the Taishanmiao granite and occupy ≤4 vol% of the syenogranite phase of the batholith. Results of Rhyolite-MELTS modeling suggest that the pegmatite-forming melts can be produced through continuous fractional crystallization of the Taishanmiao granitic magma, consistent with the designation of the pegmatites as a miarolitic class, segregation-type pegmatites rather than the more common intrusive-type of pegmatite. The mineral assemblage predicted by Rhyolite-MELTS after ~96% of the original granite-forming melt had crystallized consists of ~51 vol% alkali feldspar, 34 vol% quartz, 14 vol% plagioclase, 0.1 vol% biotite, and 1 vol% magnetite, similar to the alkali feldspar + quartz dominated mineralogy of the pegmatites. Moreover, the modeled residual melt composition following crystallization of ~96% of the original melt is similar to the composition of homogenized MI in quartz within the pegmatite. Rhyolite-MELTS predicts that the granite-forming melt remained volatile-undersaturated during crystallization of the batholith and contained ~6.3 wt% H[sub 2]O and ~500 ppm CO[sub 2] after ~96% crystallization when the pegmatites began to develop. The Rhyolite-MELTS prediction that the melt was volatile-undersaturated at the time the pegmatites began to form, but became volatile-saturated during the early stages of pegmatite formation, is consistent with the presence of some inclusion assemblages consisting of only MI, while others contain co-existing MI and FI. The relationship between halogen (F and Cl) and Na abundances in MI is also consistent with the interpretation that the very earliest stages of pegmatite formation occurred in the presence of a volatile-undersaturated melt and that the melt became volatile saturated as crystallization progressed. We propose a closed system, isochoric model for the formation of the pegmatites. Accordingly, the Taishanmiao granite crystallized isobarically at ~3.3 kbar, and the pegmatites began to form at ~734 °C and ~ 3.3 kbar, after ~96% of the original granitic melt had crystallized. During the final stages of crystallization of the granite, small pockets of the remaining residual melt became isolated within the enclosing granite and evolved as constant mass (closed), constant volume (isochoric) systems, similar to the manner in which volatile-rich melt inclusions in igneous phenocrysts evolve during post-entrapment crystallization under isochoric conditions. As a result of the negative volume change associated with crystallization, pressure in the pegmatite initially decreases as crystals form, and this leads to volatile exsolution from the melt phase. The changing PTX conditions produce a pressure-induced "liquidus deficit" that is analogous to liquidus undercooling and results in crystal growth as required to return the system to equilibrium PTX conditions. Owing to the complex closed system, isochoric PVTX evolution of the melt-crystal-volatile system, the pressure does not decrease rapidly or monotonically during pegmatite formation but, rather, gradually fluctuates such that at some stages in the evolution of the pegmatite the pressure is decreasing while at other times the pressure increases as the system cools to maintain mass and volume balance. This behavior, in turn, leads to alternating episodes of precipitation and dissolution that serve to coarsen (ripen) the crystals to produce the pegmatitic texture. The evolution of the pegmatitic melt described here is analogous to that which has been well-documented to occur in volatile-rich MI that undergo closed system, isochoric, post-entrapment crystallization. [ABSTRACT FROM AUTHOR]

Inclusions of basaltic melt trapped inside of olivine phenocrysts during igneous crystallization provide a rich, crystal-scale record of magmatic processes ranging from mantle melting to ascent, eruption, and quenching of magma during volcanic eruptions. Melt inclusions are particularly valuable for retaining information on volatiles such as H2O and CO2 that are normally lost by vesiculation and degassing as magma ascends and erupts. However, the record preserved in melt inclusions can be variably obscured by postentrapment processes, and thus melt inclusion research requires careful evaluation of the effects of such processes. Here we review processes by which melt inclusions are trapped and modified after trapping, describe new opportunities for studying the rates of magmatic and volcanic processes over a range of timescales using the kinetics of post-trapping processes, and describe recent developments in the use of volatile contents of melt inclusions to improve our understanding of how volcanoes work. Inclusions of silicate melt (magma) trapped inside of crystals formed by magma crystallization provide a rich, detailed record of what happens beneath volcanoes. These inclusions record information ranging from how magma forms deep inside Earth to its final hours as it ascends to the surface and erupts. The melt inclusion record, however, is complex and hazy because of many processes that modify the inclusions after they become trapped in crystals. Melt inclusions provide a primary archive of dissolved gases in magma, which are the key ingredients that make volcanoes erupt explosively. [ABSTRACT FROM AUTHOR]

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 CO 2 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 CO 2 contents of melt inclusions are in agreement (~3600–4000 ppm), regardless of the method used to correct for CO 2 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract One of the main goals of studying melt inclusions (MI) is to constrain the pre-eruptive physical and chemical processes that have occurred in a magma reservoir at the micro-scale. Recently, several studies that focused on magmatic differentiation of volcanic systems produced detailed interpretations based on data from MI trapped at different times and locations in the plumbing system. Ideally, MI data should be collected and tested following the melt inclusion assemblage (MIA) protocol that consists of studying and analyzing groups of MI that were trapped at the same time, and, thus, at the same chemical and physical conditions. However, the rarity of MIA in juvenile volcanic phenocrysts precludes this methodology in many cases, leading to uncertainty concerning the validity of the MI as recorders of pre-eruptive conditions. In this study, we focused on MI from the Campi Flegrei (CF) and the Island of Procida (IP) volcanic systems in southern Italy, including data from this study and data from the literature. The database included MI hosted in sanidine, clinopyroxene, plagioclase, biotite and olivine, and, thus, represents melts trapped at various stages in the overall differentiation process. We developed a protocol to select the most reliable MI from a dataset associated with a single magmatic system. As a first step we compare MI data with bulk rock data for the same magmatic system. This comparison reveals that most MI show major element compositions that fall within or close to the range for bulk rocks – these MI are considered to be "normal". Some MI show anomalous compositions and are not representative of the melt in equilibrium with the phenocryst host and were excluded from the data set. In the second step we selected only bubble-free MI from the previously identified "normal" MI to interpret the volatile evolution. In the third step we compare compositions of the "normal" bubble-free MI to compositions predicted by rhyolite-MELTS simulations, assuming a variety of initial conditions. Comparison of data obtained from basaltic-trachybasaltic MI with rhyolite-MELTS predictions indicates that one group of MI records the geochemical evolution of a volatile-saturated magma differentiating by polybaric fractional crystallization from ≥200 MPa (≥7.5 km) to 30 MPa (~1 km). Another group of MI records recharge of the magma chamber by a primitive basaltic magma that mixes with the preexisting primitive trachybasaltic magma before eruption. Extensive isobaric crystallization of the trachybasaltic magmas at ~7.5 km beneath CF may have generated trachytic-phonolitic magmas, such as those associated with the Neapolitan Yellow Tuff (NYT) that is characterized by a relatively high H 2 O content. These volatile-saturated trachytic-phonolitic magmas likely trigger high-magnitude eruptions during their ascent to the surface. [ABSTRACT FROM AUTHOR]