Melt Inclusions In Phenocrysts Research Articles

Gamma mark: an ingenuity to ease the aiming of melt inclusions in phenocrysts with NanoSIMS

AbstractThis short report introduces one of the ways to quickly and accurately aim melt inclusions in phenocryst using NanoSIMS. Graphical Abstract

Magma Composition and Formation Conditions of Alkaline-Salic Rocks of the Early Mesozoic Adaatsag Bimodal Association of the Kharkhorin Rift Zone, Central Asia: Data from the Study of Melt Inclusions in Minerals

The composition and formation conditions of magmas of comendites of the Early Mesozoic Adaatsag volcanic association are determined using electron and ion microprobe analysis of melt inclusions, and mechanisms favorable for the accumulation of trace and rare earth elements are identified. The melt and fluid inclusions are found in quartz of comendites sampled in various parts of the section of a volcanic sequence. The melt inclusions consist of glass, a gas bubble, and daughter minerals (fluorite, polylithionite, and K-feldspar). The composition of a gas phase in melt inclusions was studied using Raman spectroscopy. The dominant gas phase components include water and hydrogen. The fluid inclusions are aqueous fluids probably corresponding to KF. The thermometric studies of melt inclusions and the analysis of glasses of homogenized melt inclusions in quartz phenocrysts of comendites showed that magmas crystallized from water-saturated rare metal melts with high Li, Zr, F, Rb, Nb, Y, and Th contents at a temperature of 880–930°C and a pressure of 1000 bar at a depth of ~3.5 km accompanied by degassing. The comparison of our data of the study of melt inclusions in phenocrysts of alkaline salic rocks of the coeval Adaatsag, Dzarta-Khuduk, and Sant volcanic associations, which occur within the Kharkhorin Rift Zone, revealed common principles of magma evolution. This allowed us to propose a similar mechanism of their formation, involving the accumulation of many rare and rare-earth elements, as well as volatile components (F, H2O) in the process of crystallization differentiation. Subsequently, a salt melt rich in Li, F and water could be separated from such comendite magmas. The detection of fluoride aqueous inclusions in quartz allows us to suppose the further evolution of the salt melt leading to the appearance of a concentrated aqueous fluid and the possible participation of the latter in metasomatic processes.

Importance of Long-Term Shallow Degassing of Basaltic Magma on the Genesis of Massive Felsic Magma Reservoirs: a Case Study of Aso Caldera, Kyushu, Japan

AbstractThis paper presents the chemical composition (including H2O and CO2) of matrix glass, melt inclusions in phenocrysts, and their host minerals in eruptive products from Aso caldera. We found a group of melt inclusions with clearly lower potassium (0.6–2 wt % K2O at 50–70 wt % SiO2) than previously reported high-K2O whole-rock compositions (3–5 wt % K2O at 55–70 wt % SiO2). While most of the high-K2O intermediate to felsic melt inclusions are vapor undersaturated and show the features of H2O, CO2, and K2O accumulation, the low-K2O basaltic melt already has higher H2O and CO2. We reconcile this discrepancy with a model in which (1) the volatile-rich basalt magmas degas near the surface, (2) migrate back to depths of $\ge$12 km in the crust, and (3) crystallize feldspar and quartz to produce high-K2O felsic melt, (4) magma mixing among the undegassed, degassed, and evolved magmas. By crystallization, about five times as much low-K2O basaltic magma is required to produce high-K2O felsic magma. The quantity of felsic magma ejected from 270 ka (Aso-1) to 90 ka (Aso-4) requires a basalt magma supply rate of 18 to 31 km3/ka. This magma supply rate is comparable to or less than the present-day production rate of degassed magma (73 km3/ka) at an active center of Aso, Nakadake. These findings suggest volcanic gas flux monitoring has the potential to be a ‘basalt usage meter’ during the dormant period of caldera volcanoes.

δD and δ18O variations of the magmatic system beneath Deception Island volcano (Antarctica): Implications for magma ascent and eruption forecasting

Geochemistry of volatiles in active volcanoes provides insights into the magmatic processes and evolution at depth, such as magma evolution and degassing, which can be implemented into volcanic hazards assessment. Deception Island is one of the most active volcanoes in Antarctica, with more than twenty explosive eruptions documented over the past two centuries. Hydrogen and oxygen isotopic variations in the volatiles trapped in the Deception Island rocks (glass and melt inclusions in phenocrysts) provide essential information on the mechanisms controlling the eruptive history in this volcanic suite. Thus, understanding the petrological and related isotopic variations in the island, has the potential to foresee the possible occurrence and its main eruptive features of a future eruption.The new isotopic results of this study combined with exhaustive petrologic knowledge reveal in Deception Island (i) fast ascent and quenching of most magmas, preserving pre-eruptive magmatic signal of water contents and isotopic ratios, with local modification by rehydration due to glass exposition to seawater, and by meteoric and fumarolic waters; (ii) a plumbing system(s) variable with time and currently dominated by closed-system degassing leading to explosive eruptions; (iii) control on the interactions of ascending magmas with the surface waters producing hydrovolcanic activity throughout the two main fault systems in Deception Island and at different magma stagnation depths. These results can be considered in further studies of volcanic monitoring to improve the capability to interpret geophysical data and signals recorded during volcanic unrest episodes, and hence, forecast volcanic eruptions and related hazards.

The composition of melt inclusions in phenocrysts in tephra of the Simushir island, Central Kuriles

The composition of melt inclusions in phenocrysts in tephra of the Simushir island, Central Kuriles

Water content of melt inclusions in phenocrysts using secondary ion mass spectrometer

Water content of melt inclusions in phenocrysts using secondary ion mass spectrometer

A43 中部地方の主な火山の斑晶メルト包有物のイオウ濃度 : マフィックマグマによる大量のイオウ供給(火山の岩石学 (2), 日本火山学会2005年秋季大会)

A43 中部地方の主な火山の斑晶メルト包有物のイオウ濃度 : マフィックマグマによる大量のイオウ供給(火山の岩石学 (2), 日本火山学会2005年秋季大会)

Felsic magmas of the caldera-forming eruptions on the Iturup Island: the first results of studies of melt inclusions in phenocrysts from pumices of the Lvinaya Past and Vetrovoy Isthmus calderas

The paper reports the first results of the petrological studies of magmatic melts that formed siliceous pyroclastic deposits related to voluminous eruptions on Iturup Island. The caldera-forming eruptions of the Lvinaya Past and the Vetrovoy Isthmus, having similar features, resulted from the evolution of silicic melts that originated from partial melting of metabasalts. According to the mineral thermometry results, the melt was crystallized at ~800°C. The phenocrysts from the Vetrovoy Isthmus pumices were crystallized at <1 kbar, while those from the Lvinaya Past were formed at higher pressures. The pyroclastic rock compositions in both calderas correspond to moderately aluminous dacite and rhyolitic dacite of the normal series, whose melts likely did not undergo significant crystallization differentiation before the eruptions. The main volatile components of the magma include H2O, CO2, S, F, and Cl. Degassing with emission of water–carbon-dioxide fluid accompanied the early crystallization of plagioclase in the Vetrovoy Isthmus pumice. Evidence of pre-eruption melt degassing in the Lvinaya Past were not found. Water release from the melts may be related to both the early magma degassing and the eruptions. The lack of data evidencing the deep differentiation and mixing of contrasting melts implies a relatively small time period between the acid melt appearance and eruptions.

In situ study of the fractionation of hydrogen isotopes between aluminosilicate melts and coexisting aqueous fluids at high pressure and high temperature – Implications for the δD in magmatic processes

The hydrogen isotopic composition of melt inclusions trapped in phenocrysts during their crystallization and growth in a magma may contribute to a better understanding of the water cycle between the atmosphere, the hydrosphere and the lithosphere. Such understanding relies on the knowledge of the hydrogen isotopic fractionation factors between aqueous fluids, silicate melts, and minerals at temperature and pressure conditions relevant to the Earth's interior. Significant D/H fractionation between silicate melts and aqueous fluids was reported at hundreds of MPa and °C by using in situ measurements in hydrothermal diamond anvil cell (HDAC) experiments (Mysen, 2013a, 2013b, Am. Mineral. 98, 376–386 and 1754–1764). However, the available dataset is focused on fluids and melts with D/H ratios close to unity. The relevance of such data for natural processes that involve per mil variations of δD-values may not always be clear. To address such concerns, the effect of the bulk D/H ratio on hydrogen isotope partitioning between water-saturated silicate melts and coexisting silicate-saturated aqueous fluids has been determined in the Na2O–Al2O3–SiO2–H2O–D2O system. To this end, in situ Raman spectroscopic measurements were performed on fluids and melts with bulk D/H ratios from 0.05 to 2.67 by using an externally-heated diamond anvil cell in the 300–800 °C and 200–1500 MPa temperature and pressure range, respectively.In these pressure/temperature ranges, the D/H ratios of fluids in equilibrium with melt barely change with temperature (in average ΔHfluid=0.47±1.15 kJ/mol). In contrast, the D/H ratios of coexisting melts display strong dependence on temperature (average ΔHmelt=7.18±1.27 kJ/mol). The temperature-dependence of the D/H fractionation factor between melt and fluid (αfluid-melt=D/Hfluid/D/Hmelt) is comparable in all the experiments and can be written: 1000⋅ln⁡(αfluid-melt)=263(±26)⋅T−2–126(±48). Therefore, the αfluid-melt is independent of the bulk D/H ratio of the melt + fluid system.Experimentally determined αfluid-melt using D-enriched fluids, therefore, can be applied to natural systems. It follows that for water-saturated magma strong isotopic fractionation of D and H between water dissolved in magmas and deep aqueous fluids may occur. The δD-values in melt inclusions in phenocrysts in such water-saturated magma will reflect such fluid/melt fractionation effects. A likely result is underestimation of the δD isotopic composition of slab fluids based on δD-values in melt inclusions. The temperature-dependent hydrogen isotope fractionation must be taken into account in the modeling of slab fluid–magma interaction in the mantle wedge.

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.

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.

Evidence for extreme fractionation of peralkaline silicic magmas, the Boseti volcanic complex, Main Ethiopian Rift

Matrix glass and melt inclusions in phenocrysts from pantellerite lavas of the Boseti volcanic complex, Ethiopia, record extreme fractionation of peralkaline silicic magma, with Al2O3 contents as low as 2.3 wt.%, FeO* contents up to 17 wt.% and SiO2 contents ~65 wt.%. The new data, and published data for natural and experimental glasses, suggest that the effective minimum composition for peralkaline silicic magmas has ~5 wt.% Al2O3, 13 wt.% FeO* and 66 ± 2 wt.% SiO2. The dominant fractionating assemblage is alkali feldspar + fayalite + hedenbergite + oxides ± quartz. Feldspar – melt relationships indicate that the feldspar is close to the minimum on the albite-orthoclase solid solution loop through the entire crystallization history. There is petrographic, mineralogical and geochemical evidence that magma mixing may have been a common process in the Boseti rhyolites.

Composition and evolution of the melts erupted in 1996 at Karymskoe Lake, Eastern Kamchatka: Evidence from inclusions in minerals

The powerful eruption in the Akademii Nauk caldera on January 2, 1996, marked a new activity phase of Karymsky volcano and became a noticeable event in the history of modern volcanism in Kamchatka. The paper reports data obtained by studying more than 200 glassy melt inclusions in phenocrysts of olivine (Fo82–72), plagioclase (An92–73), and clinopyroxene (Mg# 83–70) in basalts of the 1996 eruption. The data were utilized to estimate the composition of the parental melt and the physicochemical parameters of the magma evolution. According to our data, the parental melt corresponded to low magnesian, highly aluminous basalt (SiO2 = 50.2 wt %, MgO = 5.6 wt %, Al2O3 = 17 wt %) of the mildly potassic type (K2O = 0.56 wt %) and contained much dissolved volatile components (H2O = 2.8 wt %, S = 0.17 wt %, and Cl = 0.11 wt %). Melt inclusions in the minerals are similar in chemical composition, a fact testifying that the minerals crystallized simultaneously with one another. Their crystallization started at a pressure of approximately 1.5 kbar, pro ceeded within a narrow temperature range of 1040 ± 20°C, and continued until a nearsurface pressure of approximately 100 bar was reached. The degree of crystallization of the parental melt during its eruption was close to 55%. Massive crystallization was triggered by H2O degassing under a pressure of less than 1 kbar. Magma degassing in an open system resulted in the escape of 82% H2O, 93% S, and 24% Cl (of their initial contents in the parental melt) to the fluid phase. The release of volatile compounds to the atmosphere during the eruption that lasted for 18 h was estimated at 1.7 × 106 t H2O, 1.4 × 105 t S, and 1.5 × 104 t Cl. The concen trations of most incompatible trace elements in the melt inclusions are close to those in the rocks and to the expected fractional differentiation trend. Melt inclusions in the plagioclase were found to be selectively enriched in Li. The Lienriched plagioclase with melt inclusions thought to originate from cumulate layers in the feeding system beneath Karymsky volcano, in which plagioclase interacted with Lirich melts/brines and was subsequently entrapped and entrained by the magma during the 1996 eruption.

Fluoride and chloride melts included in phenocrysts of agpaitic acid volcanic rocks from Pantelleria Island

978 Many deposits of rare elements (Be, Zr, Nb, REE, Y, and others) are genetically related to acid agpaitic (with the mole ratio (Na2O + K2O)/Al2O3 > 1) mag� mas [1, 2]. The concentration of these elements most likely results from crystallization differentiation lead� ing to their accumulation in the residual melts at the expense of low coefficients of minor element parti� tioning between silicate minerals and melts [3, 4]. Gradual accumulation of incoherent minor elements in melts may be violated by separation of saline melts from magma. According to the experimental data [5], minor elements are most effectively extracted by fluo� ride melts. Because of this, silicate–saline liquid immiscibility is an important factor of geochemical and metallogenic evolution of magmas. The formation of chloride melts during the evolu� tion of magmatic systems was established for some objects [6 and others]. The presence of fluoride melts in acid magmatic systems was described in individual cases [7, 8], and each new finding is of intense interest. In this paper we discuss the results of the study of melt inclusions in phenocrysts from magmatic rocks of Pantelleria Island (Central Mediterranean, Italy) [9], in which liquid immiscibility of alkaline acid magmas with the formation of saline melt was observed. Chlo� ride melts in inclusions and glasses from rocks of Pan� telleria Island were first discovered in 1987 [10] and hereafter were described in glasses of the rock ground� mass [11]. The melt and fluid inclusions were studied in phe� nocrysts from ignimbrite and pantellerite collected in different parts of Pantelleria Island and related to the precaldera and postcaldera stages of volcanism evolu� tion.

Pressure estimated from syngenetic melt and fluid inclusions: Probable reasons for discrepancies

Estimation of fluid pressure during crystallization of natural water-saturated magmas is important to resolve a number of problems related to petrogenesis and ore formation. Therefore, the Gudzhir granite porphyry pluton, with which Mo and W deposits of the Dzhida ore field in Transbaikalia are related, has been studied. This intrusion was formed as a result of successive emplacement of three magma portions from the same deep-seated source and in situ crystallized vein aplite that is a product of crystallization differentiation. Melt inclusions (MIs) in quartz phenocrysts and fine-grained mesostasis contain both crystalline aggregate and gas bubbles and an insignificant aqueous solution; the gas bubbles and the aqueous solution homogenize into liquid at 230–270°e . Melt inclusions less than 3 µ m in size, on being heated higher this temperature, collapse. Therefore, the temperature of their total homogenization was determined accurate to ± 15°e under restricting pressure of argon up to 5 kbar. The content of c 2 e in MI ( ) was calculated within ± 15% (1 σ ) from thermal and volumetric data [1] and taking into account the water content in daughter muscovite and the volume of quartz, which turned to melt from inclusion walls at the moment of homogenization of MIs [2, 3]. Most likely, MIs in phenocrysts characterize the early stage of magma crystallization during the preintrusion period, whereas MIs in quartz of mesostasis of granite porphyry and vein aplite fit the late stage of crystallization. The former differs from the latter in the higher (790–760°e against 750–740°e ) and wider range of (7.3‐4.1 wt % against 4.5 ± 0.5 wt %). Along with MI, syngenetic primary inclusions of magmatic fluid (IMF) homogenized at 300–290°e , less

The deep magmatic system of the Campi Flegrei caldera (Italy)

Melt inclusions in phenocrysts of the Minopoli2 shoshonite and Fondo Riccio latite eruptive products (Campi Flegrei caldera; 10.2 to 9.5 ka BP) constrain the nature of deep Phlegraean magmatic fluids, their role in volcanism and give new insights into the magmatic plumbing system. The analyzed melt inclusions show that CO2 is an abundant gaseous species, confirming the results of geochemical studies on the magmatic gas fraction involved in present‐day fumarolic emissions. Volatile concentrations within the melt inclusions require gas‐melt equilibria between 8 and 9 km depth, and closed‐system degassing in presence of ascending CO2‐rich fluids of deep provenance. Magmas later re‐equilibrate at depths up to 3–2 km, i.e. above the top of the carbonate basement. These observations correlate well with independent geophysical and geochemical evidence and are useful constraints for volcanic hazards assessment at Campi Flegrei.

Storage conditions and evolution of andesitic magma prior to the 1991–95 eruption of Unzen volcano: Constraints from natural samples and phase equilibria experiments

The compositions of homogenized melt inclusions trapped in plagioclase (Pl) microphenocrysts (40–200 μm length) from mafic enclaves within dacitic rocks erupted at Unzen volcano in 1991–95 were investigated. The SiO 2 contents of the melt inclusions vary from 58 to 70 wt.% and Pl anorthite content is An 50–70. The stability fields (in terms of temperature and water activity) of natural Pl and coexisting melts from the melt inclusions were estimated from data of phase equilibria experiments performed with a synthetic andesite composition at T = 900–1140 °C, P = 200 MPa, log fO 2 = NNO−2–NNO, and water activity of about 0–1. This composition is representative of the average composition of mafic enclaves from the 1991–95 eruption and nearly identical to the composition of andesitic lavas from 1663 Unzen eruption. The temperatures and H 2O melt concentrations, calculated using the compositions of coexisting Pl and melt inclusions, provide an estimation of the conditions of andesitic magma evolution within the mafic enclaves prior to eruption. The formation of melt inclusions in plagioclase microphenocrysts occurred at a maximum temperature of ~ 1010 ± 35 °C for a melt containing 2 wt.% H 2O and a minimum temperature of ∼ 945 °C ± 30 °C for a melt with ∼ 4 wt.% dissolved H 2O. The compositional range of the melt inclusions indicates that the composition of the mafic enclave was not significantly contaminated by the host magma when inclusions were formed. The difference between concentrations of dissolved S (up to 0.06 wt.%) and Cl (up to 0.05 wt.%) in melt inclusions in Pl of mafic enclaves and concentrations of S (< 0.005 wt.%) and Cl (0.05–0.11 wt.%) in melt inclusions in phenocrysts of the dacitic magma clearly implies that two distinct sources for S and Cl in the 1991–95 eruption of Unzen volcano need to be considered. Sulfur degassing was generated by a release of fluids from the high-temperature andesitic magma whereas Cl was degassed from the low-temperature silicic magma. The combination of data from melt inclusions and phase equilibria experiments indicates that the mafic end-member magma at Unzen was already partially crystallized and contained significant proportions (20 to 40 wt.%) of Pl and orthopyroxene (Opx) when melt inclusions started to form. Since clinopyroxene (Cpx) and magnetite (Mt) crystallize after Pl and Opx, the temperatures derived for the mafic end-member magma from coexisting Opx–Cpx and Ilm–Mt pairs do not represent temperatures near to the liquidus. Assuming that the injected mafic magma was nearly aphyric, its initial temperature might have been higher than estimated in previous studies.

Effect of Miocene basaltic volcanism in Shanwang (Shandong Province, China) on environmental changes

Miocene (16–10 Ma) basalts, together with significantly well-preserved fossils (including animal and plant fossils) in the contemporaneously tephra-rich Maar sediments, are located in Shanwang volcanic region, Shandong Province, China. Distribution area of the basaltic eruption products is about 240 km2. Detailed field observations indicate that most of basaltic rocks are fissure eruptive products and some are central eruptives constrained by linear faults. The well-preserved fossils in the lacustrine deposits have been considered to be a result of mass mortalities. Based on physically volcanologic modeling results, eruption column of the basaltic fissure activities in the Shanwang volcanic region is estimated to have entered the stratosphere. Petrographic observations indicate that the basalts have porphyritic textures with phenocrysts of olivine, pyroxene, plagioclase feldspar and alkali feldspar setting in groundmass of plagioclase feldspar, alkali feldspar, quartz, apatite and glass. Based on observations of tephra, tuff and tuffites collected in the Maar sediments of the Shanwang area, we determined major element oxide concentrations and volatile composition of melt inclusions in phenocrysts and matrix glasses by electron microprobe analysis. Volatile (including S, Cl, F and H2O) concentrations erupted into the stratosphere were estimated by comparing pre-and post-eruptive volatile concentrations. Our determination results show that contents of S, Cl, F and H2O emitted into the stratosphere were 0.18%–0.24%, 0.03%–0.05%, 0.03%–0.05% and 0.4%–0.6%, respectively, which was characterized by high-S contents erupted. Amounts of volatiles emitted in the Shanwang volcanic region are much higher than those in eruptions which had a substantial effect on climate and environment. According to the compositions and amounts of the volatiles erupted from the Miocene basaltic volcanism in Shanwang, we propose a hypothesis that volatile-rich basaltic volcanism could result in the mass mortalities by injecting volatiles (e.g., SO2, H2S, HCl, HF and H2O) into the stratosphere that would have triggered abrupt environmental changes (including formation of acid rain, temperature decline, ozone depletion, etc.) and altered lake chemistry, and subsequently volcanic ash fall buried and covered the dead animals and plants, forming well-preserved fossils in Shanwang Maar sediments.

Melt inclusions from Volcán Popocatépetl and Volcán de Colima, Mexico: Melt evolution due to vapor-saturated crystallization during ascent

Melt inclusions in phenocrysts from Volcán Popocatépetl and Volcán de Colima within the Trans Mexican Volcanic Belt (TMVB) are dacitic to rhyolitic. Trends in melt inclusion major element and water concentrations form the evolved extension of other Mexican volcanics including those presumed to be derived directly from primitive melts. Water concentrations in Popocatépetl and Colima melt inclusions are similar (0.3 to 3.4 wt.% H 2O). Melt-vapor equilibration pressures calculated from dissolved H 2O and CO 2 (Popocatépetl) or H 2O (Colima) in melt inclusions correspond to depths of entrapment of 12 km or less. Water and carbon dioxide concentrations correlate negatively with SiO 2 and potassium. Normalized olivine–augite–quartz compositions are consistent with near cotectic crystallization under vapor-saturated conditions at pressures of 1.5 kb or less. Our results show that Popocatépetl and Colima magmas have undergone vapor-saturated crystallization during ascent in conjunction with varying degrees of mixing between degassed rhyodacitic and less degassed, mafic melts in the upper portions of the crust. These data suggest melt evolution occurred in conduits or inter-fingered dikes rather than a large stratified magma chamber.

Towards a reconstruction of the magmatic feeding system of the 1944 eruption of Mt Vesuvius

Geochemistry and mineralogy of both juvenile clasts and xenoliths ejected during the 1944 eruption of Mt Vesuvius provide major contraints on the magmatic feeding system. Melt inclusions in phenocrysts of juvenile scoriae highlight that the magmas feeding the eruption underwent differentiation at different pressures. A K-tephritic volatile-rich melt evolved to reach K-phonotephritic composition at pressures higher than 300 MPa before being fed into a very shallow reservoir (P<100 MPa) in which it mixed with the low-volatile resident K-phonotephritic magma. The newly arrived magma forced the transition from the effusive to the lava fountain phase of the 1944 eruption. The outer portion of the shallow reservoir is formed by a crystallizing margin (glass-bearing fergusites). From this, the transition to the carbonate country rocks occurs throughout a front of infiltration of magmatic melts in porous decarbonating host rocks. Magmatic melts are contaminated by the addition of Ca and Mg deriving from decarbonation reactions and/or melting of host rocks. These modified melts metasomatize the carbonates inducing skarn reactions and forming an endoskarn shell. Isotope (Sr, O) composition of juvenile products and xenoliths marks the amount of magma–carbonate interaction. Isotopic data point out that the crystallizing margin of the chamber may present a very limited contamination by carbonates. This suggests that the main volume of magma hosted in the magma chamber did not suffer any mass exchange with the wall rocks.