Melt Inclusions In Plagioclase Research Articles

Formation Conditions of Ignimbrites of the Khangar Volcano (Kamchatka)

Abstract —The study of minerals, melt inclusions, as well as natural glasses showed that two different melts contributed to the formation of ignimbrites of the Khangar Volcano. The first, providing the information on melt inclusions in plagioclase and quartz phenocrysts, represents the state of magma in a deep source. The other type of melt is responsible for the formation of glasses and microcrystals of feldspars in fiamme. Experimental and analytical studies of melt inclusions showed that crystallization of most plagioclase and quartz phenocrysts from ignimbrites of the Khangar Volcano occurred at temperatures of 840–960 °C and pressures up to 1.1 kbar, from the melt with water contents up to 3.23 wt.%, under the conditions of magma chamber. The presence of syngenetic primary melt and fluid inclusions in plagioclase and quartz phenocrysts from ignimbrites of the Khangar Volcano indicates phase separation (“boiling”) of the melt with mass formation of СО2 microbubbles in magma. The other type of melt is secondary relative to magmatic systems of the Khangar Volcano and is formed by sintering and melting of tuffogenic volcanoclastic material. This melt contributed to the formation of fiamme in the examined ignimbrites. Based on the study of glasses and microcrystals of feldspars in fiamme, it was found that crystallization of oligoclase occurred at temperatures of 770–840 °C in the melt between the spherules (with water content up to 2.91 wt.%). Sanidine crystals grew over spherules at lower temperatures, 680–760 °C.

FORMATION OF TRACHYTES FROM WANGTIAN’E VOLCANIC NECK OF THE CHANGBAISHAN AREA (NORTHEAST CHINA) AS A RESULT OF THOLEIITIC MAGMA CRYSTAL FRACTIONATION

Tholeiitic basalts and trachytes of Wangtian’e bimodal association are studied. It is shown that trachytes were formed under the leading role of crystal fractionation of the initial tholeiitic magma. Based on the method of melt and fluid inclusion studies, it was determined that hedenbergite phenocrysts from Wangtian’e volcanic neck crystallized in the temperature range of 1080–1100°С, and plagioclase phenocrysts crystallized at temperatures of 1050–1060°С. After thermometric experiments with melt inclusions in plagioclase from trachytes, in some cases relics of carbonates and carbon dioxide were observed in the inclusions. Trachyte phenocrysts contain hydrous Fe-rich silicate globules and CO2 inclusions containing carbonate phases. Fe-rich silicate globules are often intergrown with titanomagnetite and covered with films of amorphous carbon. A model the appearance of hydrous Fe-rich globules and CO2 inclusions with carbonate relics is proposed. It results due to the silicate-silicate and silicate-carbonate liquid immiscibility processes, that are caused by the evolution of the initial basaltic melt. As the trachyte melt has been risen to the surface, the ferrocarbonate liquid decomposed into magnetite, carbon, and carbon dioxide.

Formation of Trachytes from Wangtian’e Volcanic Neck of the Changbaishan Area (Northeast China) as a Result of Tholeiitic Magma Crystal Fractionation

Tholeiitic basalts and trachytes of the bimodal association of Wangtian’e volcano are studied. It is shown that trachytes formed with the leading role of crystal fractionation of the parental tholeiitic magma. It has been identified from the melt inclusion study that hedenbergite and plagioclase phenocrysts of trachytes from Wangtian'e volcanic neck crystallized in the temperature range of 1080–1100 and 1050–1060°C, respectively. After heating experiments with melt inclusions in plagioclase of trachytes, in some cases relics of carbonates and CO2 were observed in the inclusions. The trachyte phenocrysts host water-bearing Fe-rich globules and CO2 inclusions with carbonate phases. The Fe-rich silicate globules are often intergrown with titanomagnetite and are covered by amorphous carbon films. A model is suggested explaining the formation of water-bearing Fe-rich globules and CO2 inclusions with carbonate relics as a result of silicate–silicate and silicate–carbonate liquid immiscibility, that was caused by the evolution of the parental basaltic melt. Upon the trachytic melt ascending to the surface, the ferrocarbonate liquid was decomposed on magnetite, carbon, and carbon dioxide.

Crystallization conditions of plagioclase in volcanic complexes of Sulawesi island, Pacific Ocean

Mineralogical and thermobarogeochemical studies allowed us to reveal the crystallization conditions of plagioclase in volcanic complexes of Sulawesi Island (Pacific Ocean). The composition of plagioclases indicates the presence of three types of melts, which were involved in the formation of plagioclase phenocrysts in basalts, andesites and dacites, as well as crystallites in the groundmass of andesites and dacites. The analysis of glasses of heated melt inclusions in plagioclase indicates the involvement of two contrasting melts (mafic and felsic) in crystallization of phenocrysts. At the final stages of volcanic processes, plagioclase in matrix of andesites and dacites formed from another felsic melts with maximum alkali content. The presence of primary syngenetic melt and fluid inclusions in plagioclase phenocrysts indicates the phase separation of felsic magmas, which led to the crystallization of feldspar phenocrysts of different composition during the formation of dacites. Computational modeling in a program (Putirka, 2008) based on the composition of plagioclases, glasses of heated inclusions and glasses from the groundmass allowed us to estimate the crystallization temperatures of feldspar phenocrysts in basalts (1160–1120 ?С), andesites (930–890 ?С) and dacites (1020–950 ?С), as well as the feldspar microcrystals in the groundmass of andesites (875–810 ?С) and dacites (890–850 ?С). Keywords: crystallization conditions, plagioclase, melt inclusions, Sulawesi Island, Pacific Ocean.

Features of Mineral Crystallization at Different Stages of the Magmatism Evolution of the Gorely Volcano (Kamchatka): Data on Melt and Fluid Inclusions

Abstract ––Studies of melt and fluid inclusions and minerals as well as computational modeling (based on the data on the composition of melt inclusions, clinopyroxenes, and amphiboles) gave an insight into the physicochemical parameters of magmatic systems during the evolution of the precaldera Pra-Gorely Volcano and during the subsequent formation of rock complexes of the Young Gorely Volcano. The estimated temperatures of crystallization of olivine, clinopyroxene, and plagioclase phenocrysts (1115–1260 °С) and amphibole (740–890 °С) are in agreement with the earlier published data on the magmatism of the Gorely Volcano. Computational modeling based on the compositions and homogenization temperatures of melt inclusions showed that the established depth interval of mineral crystallization (21.0–1.5 km) with pressures of 7.0–0.5 kbar can be divided into two ranges, 21–15 km and 9.0–1.5 km. Both the Pra-Gorely and Young Gorely volcanoes have magma chambers in these depth ranges. The Pra-Gorely Volcano is characterized by higher temperatures of mineral crystallization (1240–1190 °С) as compared with the Young Gorely Volcano (1190–1125 °С). The presence of primary fluid inclusions with low-density CO2 and of syngenetic primary melt inclusions in plagioclase of the Pra-Gorely Volcano indicates that the mineral crystallized from a heterophase melt. At the same time, the cores of plagioclase phenocrysts formed from a homogeneous melt. A drastic drop in pressure led to the phase separation of magma throughout the magma column (upper and lower chambers) and to the growth of zones saturated with CO2 fluid inclusions in the plagioclase crystals formed from a two-phase melt. The subsequent closure of the system and the disappearance of CO2 phase resulted in the growth of plagioclase from a homogeneous melt.

Experimentally Induced Volumetric Re‐equilibration of Plagioclase‐Hosted Melt Inclusions

AbstractThe application of melt inclusions (MI) to infer magmatic processes assumes the MI have remained as constant mass, constant volume systems since the time of trapping. Understanding the effects of both compositional and volumetric re‐equilibration is key for the interpretation of MI data. Although the re‐equilibration behavior MI in quartz and olivine has been studied in some detail, the process is less understood for other MI host phases such as plagioclase, a common phase in igneous rocks. A MI can re‐equilibrate when it experiences pressure and temperature (PT) conditions that differ from formation PT conditions. During laboratory heating, irreversible MI expansion may occur. As a result, the internal pressure within the MI decreases, resulting in chemical and structural changes to the MI and host. We present results of heating experiments on plagioclase‐hosted MI designed to induce volumetric re‐equilibration. The experiments consisted of incrementally heating the MI to temperatures above the homogenization temperatures. At ∼40°C above, the temperature at which the daughter minerals melted, irreversible volume expansion lowered the pressure in the MI, and led to exsolution of CO2 into vapor bubbles. With each additional few degrees of heating, additional episodes of CO2 exsolution, bubble nucleation and expansion of the vapor bubbles occurred. Re‐equilibration of MI in plagioclase occurred through a combination of ductile and brittle deformation of the host surrounding the MI, whereas previous studies have shown that MI in olivine re‐equilibrate dominantly through ductile deformation associated with movement along dislocations. This behavior is consistent with the differing rheological properties of these phases.

Source and behavior of pyroclastic density currents generated by Vulcanian-style explosions of Popocatépetl volcano (Mexico) on 22 January 2001

Since 1994 Popocatépetl volcano has extruded 85 lava domes, each of which has been destroyed by Vulcanian-style explosions. The paroxysmal Vulcanian explosion of 22 January 2001 has been unique by generating multiple pyroclastic density currents (PDCs). Scientists and Civil Protection authorities have used those PDCs to remap the high-risk hazard zone of the volcano, although little study has been carried out to understand the generation and evolution of those PDCs. Here, we examine 38 stratigraphic sections and analyze the granulometry, componentry, vesicularity, and petrology (glass, melt inclusions in plagioclase) of collected samples to interpret the origins of the Vulcanian explosions and the PDC behavior. The 22 January event generated two series of dense PDCs that detached from the base of the eruption column. The first set of PDCs traveled northwest across a glacier on the volcano, partially scouring and melting the ice that hours later generated a lahar. Two pulses of these PDCs were confined within a large ravine, surpassed the slope break, and traveled up to 3 km away from the vent. The deposits are matrix-supported throughout their extent. The second set of PDCs traveled to the northeast, mainly as unconfined currents that split into two lobes prior to the slope break. The farthest traveled current followed a western ravine and reached the forest 3.5 km from the summit. The eastern lobe buried a local hiking hut and reached 2.1 km. The deposits from these northeastern currents are massive and block-supported. All deposits (volume of 5.71 × 105 m3) have finger-like morphology with levees and frontal snouts. All PDCs flowed as a dense basal granular current with turbulent upper clouds at minimum speeds of 7–10 m/s.The components of the deposits are, in order of abundance, black scoria (53–62 wt% SiO2), banded scoria, pumice (63 wt% SiO2), dome lithics (60–61 wt% SiO2), accidental lithics, and free crystals. Juvenile pumice and scoria contain phenocrysts of plagioclase (An47±10), clinopyroxene (En48±3), orthopyroxene (En64±1), ilmenite, and titanomagnetite set in a rhyolitic glass (pumice clast; 74.39 wt% SiO2) or microlite-rich matrix (scoria clast). The dome lithics also contain these phenocrysts as well as rare olivine phenocrysts (Fe2+/(Fe2 + Mg) = 0.1). Dissolved H2O-CO2 contents in melt inclusions hosted by pumice-plagioclase phenocrysts suggest the felsic magma was stored at depths of 2–4 km. Thus, the 2001 event was triggered by the injection of mafic magmas into a small volume of felsic magma, probably forming dikes left from the previous eruptive episode. We propose that the Vulcanian-explosions tap a column of magma (~2 km depth) equivalent to the volume that contained the felsic dikes. This mechanism has been acting during the destruction of all summit domes of the ongoing eruption of the volcano.

Silicate Liquid Immiscibility as a Result of Fenner-Type Crystal Fractionation of Wangtian’e Tholeiitic Melts, Northeast China

The results of a study of mineral-hosted melt inclusions were used to reconstruct the evolution of tholeiitic melts that produced the rocks of the shield edifice of Wangtian’e volcano, Northeast China, including the P–T parameters of their crystallization. Primary melt inclusions were studied in plagioclase of tholeiitic basalts. They contain “dry” Fe-rich silicate glass or a crystallized fine-grained mineral aggregate, feldspar rim, and globules of various compositions: water-bearing Fe-rich globules, water-bearing Si-rich globules and “dry” Si-rich globules. In the groundmass of tholeiitic basalts, “dry” Si-rich and Fe-rich glasses, as well as hydrated Fe-rich glass with an H2O content of up to 10–15 wt.% were identified. The fractional crystallization path of Wangtian’e tholeiitic melts was calculated by the COMAGMAT-5.2.2 software using glass compositions of homogenized melt inclusions in plagioclase from the basalts. It was shown that the calculated temperatures of the beginning of plagioclase crystallization are in good agreement with temperatures obtained in thermometric experiments with melt inclusions in plagioclase, and correspond to 1180–1200°C. Based on our studies, several stages of the melt differentiation during the formation of Wangtian’e volcanic rocks were identified. The first stage includes the Fenner-type crystal fractionation of the melt due to the appearance of plagioclase on the liquidus at the beginning of magma crystallization. The separation of the ferrobasaltic melt into Si-rich and Fe-rich silicate immiscible liquids (“dry” and hydrated) corresponds to the second stage of differentiation. The occurrence of silicate liquid immiscibility follows from the presence of Fe-rich and Si-rich glasses in the intergranular space of the basalts and in the melt inclusions in plagioclase of these rocks.

Sulphide melt and aqueous fluid saturation in the PGE–Au mineralisation of the Skaergaard intrusion: evidence from melt inclusions

The Skaergaard intrusion in East Greenland hosts mineralisation of platinum group elements and gold that is characterised by a remarkably low amount of sulphides (about 0.02 wt% sulphide) and a very high metal/sulphur ratio. Here we present the first quantitative analyses of sulphur (S), copper (Cu), lithium (Li) and other trace elements from melt inclusions in plagioclase to trace their evolution in the magma and to constrain the formation of the mineralisation. The concentration of these elements in the melt inclusions varies considerably within each sample. The S content in melt inclusions ranges from 165 to 320 ppm in the lower part of the intrusion, but increases to 349–512 ppm immediately below the mineralisation interval. In the same interval, the Li content increases from 22–43 to 40–69 ppm and Cu ranges from 118 to 1586 ppm with no systematic variations. Across the mineralisation, the Li content drops an order of magnitude to 4–7 ppm and S drops to 250–326 ppm. We explain these compositional changes as reflecting the magmatic evolution as the result of fractional crystallisation below, and coinciding aqueous fluid exsolution and sulphide saturation across the mineralisation. Normally the S content of basaltic magmas is much higher at sulphide saturation. We propose that sulphide saturation is possible, nevertheless, mostly due to the high Cu content and high Cu/Fe in the evolved ferrobasaltic melt resulting in a lowering of the S concentration required for sulphide saturation. The relationship between exsolutions of aqueous fluids and sulphides remains unknown.

Melt inclusion evidence for mantle heterogeneity and magma degassing in the Deccan large Igneous Province, India

Silicate melt inclusions (MI) trapped in minerals provide direct tools to evaluate source characteristics and magma evolution including magma chamber processes. Here we present results from the study of MIs in basalts from the Western Ghats region of the Deccan Large Igneous Province (LIP) in India. The MIs in plagioclase and clinopyroxene phenocrysts were analyzed. The MIs exhibit post-entrapment modification by in situ crystal fractionation, chemical interaction with the host phenocryst (plagioclase or clinopyroxene), degassing/decrepitation, and crystallization of daughter minerals during cooling. The MIs display variable silica (SiO2 41–68 wt%), low potassium, and high FeTi contents, corresponding to differentiation of basaltic to andesitic magma. Our data indicate H2O content of about 2 wt% which is consistent with recent evidence for elevated primary H2O content in the Deccan and other LIPs, corresponding to elevated H2O in the lithospheric mantle and plumes passing through the mid-mantle inheriting the hydrous nature.

The characteristic of acid melts which formed tephra of pleistocene-holocene eruptions of Ichinsky Volcano, Kamchatka (Results of melt inclusions study)

This paper presents the results of a study of melt inclusions in plagioclase, amphibole and pyroxene from Ichinsky volcano’s tephras of different age. Two types of melts have been identified, distinguished by different concentrations of potassium (K2O). Major and trace elements’ composition of these melts indicates that magma mixing was the dominating process in the Ichinsky magmatic system.

Petrology of Late Albian tuffites from Bakhchysarai district (Southwestern Crimea)

We present first data on the composition of melt inclusions in plagioclase and quartz crustal clasts and detailed study of plagioclase chemical zoning and mineral compositions from Bakhchysarai Late Albian tuffites. All magmatic melt compositions fall into field of high-K rhyolites. Measured water content in melt inclusions is up to 4 wt.%. Phenocrysts crystallized at the temperature within range 820-860 °C and pressure not less than 1 kbar. Bakhchysarai tuffites were formed in active continental margin geotectonical settings.

Controls on long-term low explosivity at andesitic arc volcanoes: Insights from Mount Hood, Oregon

The factors that control the explosivity of silicic volcanoes are critical for hazard assessment, but are often poorly constrained for specific volcanic systems. Mount Hood, Oregon, is a somewhat atypical arc volcano in that it is characterized by a lack of large explosive eruptions over the entire lifetime of the current edifice (~500,000years). Erupted Mount Hood lavas are also compositionally homogeneous, with ~95% having SiO2 contents between 58 and 66wt.%. The last three eruptive periods in particular have produced compositionally homogeneous andesite–dacite lava domes and flows.In this paper we report major element and volatile (H2O, CO2, Cl, S, F) contents of melt inclusions and selected phenocrysts from these three most recent eruptive phases, and use these and other data to consider possible origins for the low explosivity of Mount Hood. Measured volatile concentrations of melt inclusions in plagioclase, pyroxene, and amphibole from pumice indicate that the volatile contents of Mount Hood magmas are comparable to those in more explosive silicic arc volcanoes, including Mount St. Helens, Mount Mazama, and others, suggesting that the lack of explosive activity is unlikely to result solely from low intrinsic volatile concentrations or from substantial degassing prior to magma ascent and eruption. We instead argue that an important control over explosivity is the increased temperature and decreased magma viscosity that results from mafic recharge and magma mixing prior to eruption, similar to a model recently proposed by Ruprecht and Bachmann (2010). Erupted Mount Hood magmas show extensive evidence for mixing between magmas of broadly basaltic and dacitic–rhyolitic compositions, and mineral zoning studies show that mixing occurred immediately prior to eruption. Amphibole chemistry and thermobarometry also reveal the presence of multiple amphibole populations and indicate that the mixed andesites and dacites are at least 100°C hotter than the high-SiO2 resident magma prior to mixing. Viscosity models suggest that recharge by hot, mafic magma prior to eruption can lower magmatic viscosity by at least a factor of four. Lower viscosity during ascent delays fragmentation and allows volatile escape through degassing, thus lowering the potential for explosive eruptions. These results suggest that low explosivity should be more common in volcanoes where intermediate magmas are produced through mixing of mafic and silicic magmas shortly before eruption.

PROPENSITY FOR MINERALIZATION IN VOLCANOES — EVIDENCE FROM MELT INCLUSIONS

Rocks and melt inclusions (MI) from 11 volcanic centers of the Kermadec-Tofua arc, in the South West Pacific, were petrographically studied prior to chemical analysis under the ion beam. The abundance of MI with daughter minerals are volcano-specific with the most abundant found in "U", Putoto, and Hinepuia volcanoes where >50% of MI contain daughter minerals. The B , Li , Cl and S contents in MI generally increase with the silica content of the rock. Fe , Ni , Mn , Cu and Zn are common in MI of all rock types but Mo , Hg and Cu have the highest concentrations in dacite-rhyodacites. The highest concentrations of B , Ti , V , Fe , Co and Mo occur in plagioclase MI; S , Ni , Ge and Hg in pyroxene MI; Cl and Li in quartz MI; and Cu , Zn and M in hornblende MI. Different ore-forming components in volcanic rocks can be correlated with rock composition, Cl/S and B/S of the melts, the presence and abundance of mineral sinks for various elements and the occurrence of hydrothermally altered rock at depth and on the seafloor.

Magmatic degassing of ore-metals at the Menez Gwen: Input from the Azores plume into an active Mid-Atlantic Ridge seafloor hydrothermal system

Menez Gwen is an active seafloor hydrothermal system located on the Mid-Atlantic Ridge at 37°50′N under the Azores plume influence and within enriched MORB type rocks. Two distinct highly plagioclase phyric (HPP) lavas occur at Menez Gwen, type I and type II. Type I HPP lavas are vesicular and highly enriched in incompatible trace elements; type II HPP lavas are less vesicular and less enriched. Melt inclusions in plagioclase and olivine phenocrysts in these HPP basalt lavas were studied by laser-ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). The LA-ICP-MS data support previous findings that mixing of melts occurred below Menez Gwen. Olivine crystals enclosing melt inclusions having compositions of both type I and type II magmas occur in type I HPP basalts. All plagioclase hosted melt inclusions have type II signatures regardless of their host rock character. The melt inclusions of the less incompatible element enriched type II lavas are more enriched in Cu than type I, an observation that is relevant to the origin of metals characteristic of these particular basalt-hosted seafloor hydrothermal systems. Furthermore, petrographic and analytical observations of melt inclusions indicate exsolution from the magma of an immiscible volatile phase carrying Cu and S. Subsequent injection into the magma chamber of the more enriched type I melt coupled with extensive pre-eruptive degassing of CO 2 and S likely triggered the mobilization of the exsolved ore-metals. High volatile contents and extensive magmatic degassing in a shallow magma chamber and mixing of enriched melts were contributors to the seafloor hydrothermal activity at Menez Gwen.

Crystallization of the Skaergaard Intrusion from an Emulsion of Immiscible Iron- and Silica-rich Liquids: Evidence from Melt Inclusions in Plagioclase

The presence of Fe- and Si-rich liquids found as melt inclusions in apatite and olivine in the Upper Zone of the Skaergaard intrusion, East Greenland, demonstrates the occurrence of liquid immiscibility in the late-stage evolution of tholeiitic magmas in a plutonic setting. However, it remains unclear at which stage of crystallization unmixing began. To constrain the onset and the petrological importance of liquid immiscibility in the Skaergaard and tholeiitic magmas in general, we have studied crystallized melt inclusions entrapped in early primocryst plagioclase. Such melt inclusions become abundant from the top of the Lower Zone and upwards in the Layered Series, in primocryst plagioclase of composition An54^26. The daughter phase assemblage is the same in all the inclusions, although the modal proportions of the daughter phases are highly variable: plagioclase (42^59%), clinopyroxene (28^41%), ilmenite (4^9%), magnetite (3^10%), apatite (1^9%) and accessory phases (51%). Accordingly, the bulk compositions of reheated and homogenized melt inclusions show large variations in SiO2 (40^54 wt %), FeO t (7^23 wt %), P 2O5 (0^1 ·9 wt %) and K2 O( 0^2·8 wt %), and have variable CaO/Al2O3 ratios.These variations are best explained by trapping of varying proportions of immiscible iron- and silica-rich melts and demonstrate that liquid immiscibility started in the upper part of the Lower Zone. We conclude that a significant part of the Skaergaard intrusion crystallized from an emulsion of Fe- and Si-rich immiscible melts.The heterogeneous trapping of a mixture of Fe- and Si-rich immiscible liquids in primocryst plagioclase indicates that the dispersed droplets in the Lower and Middle Zones were smaller than the size of the inclusion (5500mm). In the Upper Zone, most of the inclusions in apatite are composed of the conjugate end-member liquids, indicating a larger size for the dispersed droplets. Metre-sized pods and layers of melanogranophyre in the upper part of the intrusion are believed to represent pooled bodies of the immiscible Si-rich liquid. Differentiation of an emulsified magma must be considered in petrogenetic models for the Skaergaard intrusion.

Injection of vesicular magma into an andesitic dome at the effusive–explosive transition

A single, vesicular pyroclast collected from a major (11×106m3) dome collapse eruption of the Soufrière Hills volcano, Montserrat (21st September 1997) was found to contain textural and Raman evidence for multiple decompression events and magma injection into the dome just prior to the onset of dome collapse. Quartz, plagioclase and hornblende phenocrysts contain closed and burst melt inclusions (MIs), the latter in the form of either foam or cylindrical melt extrusions from cavities on fracture surfaces. Fractures within ruptured quartz phenocrysts were infilled with glass, whereas those within plagioclase are entirely free of glass, indicating relatively early fracturing (decompression) of the quartz. This interpretation is supported by slightly higher measured H2OT in burst MIs in quartz (0.44wt.%±0.07, 1s.d.) compared with plagioclase (0.31wt.%±0.05, 1s.d.), indicating equilibration on rupturing of the quartz at higher PH2O. H2OT levels in glass from burst MIs in plagioclase are consistent with plagioclase fragmentation at H2O saturation pressures <5MPa, at an estimated depth of <210m, within the dome. The fragmentation occurred whilst the groundmass glass was still plastic, as evident from the presence of stretched vesicles in glass at fracture terminations. The fractures in plagioclase are empty of vapour phase precipitates (mainly cristobalite that infills matrix vesicles) which constrains the fracturing of plagioclase to the last depressurisation event recorded in the pyroclast, which was the dome collapse which led to the pyroclast being ejected from the dome. We suggest that our study provides the first direct (i.e. non seismic) evidence for the injection of gas-charged magma into the dome at the onset of dome collapse, marking the transition from effusive to explosive eruption, and that the magma injection could have provided the triggering mechanism for dome collapse.

10.1007/s11495-008-1001-5

Melt inclusions were examined in phenocrysts in basalt, andesite, dacite, and rhyodacite from the Karymskii volcanic center in Kamchatka and dacite form Golovnina volcano in Kunashir Island, Kuriles. The inclusions were examined by homogenization and by analyzing glasses in more than 80 inclusions on an electron microscope and ion microprobe. The SiO2 concentrations in the melt inclusions in plagioclase phenocrysts from basalts from the Karymskii volcanic center vary from 47.4 to 57.1 wt %, these values for inclusions in plagioclase phenocrysts from andesites are 55.7–67.1 wt %, in plagioclase phenocrysts from the dacites and rhyodacites are 65.9–73.1 wt %, and those in quartz in the rhyodacites are 72.2–75.7 wt %. The SiO2 concentrations in melt inclusions in quartz from dacites from Golovnina volcano range from 70.2 to 77.0 wt %. The basaltic melts are characterized by usual concentrations of major components (wt %): TiO2 = 0.7–1.3, FeO = 6.8–11.4, MgO = 2.3–6.1, CaO = 6.7–10.8, and K2O = 0.4–1.7; but these rocks are notably enriched in Na2O (2.9–7.4 wt % at an average of 5.1 wt %, with the highest Na2O concentration detected in the most basic melts: SiO2 = 47.4–52.0 wt %. The concentrations of volatiles in the basic melts are 1.6 wt % for H2O, 0.14 wt % for S, 0.09 wt % for Cl, and 50 ppm for F. The andesite melts are characterized by high concentrations (wt %) of FeO (6.5 on average), CaO (5.2), and Cl (0.26) at usual concentrations of Na2O (4.5), K2O (2.1), and S (0.07). High water concentrations were determined in the dacite and rhyodacite melts: from 0.9 to 7.3 wt % (average of 15 analyses equals 4.5 wt %). The Cl concentration in these melts is 0.15 wt %, and those of F and S are 0.06 and 0.01 wt %, respectively. Melt inclusions in quartz from the dacites of Golovnina volcano are also rich in water: they contain from 5.0 to 6.7 wt % (average 5.6 wt %). The comparison of melt compositions from the Karymskii volcanic center and previously studied melts from Bezymyannyi and Shiveluch volcanoes revealed their significant differences. The former are more basic, are enriched in Ti, Fe, Mg, Ca, Na, and P but significantly depleted in K. The melts of the Karymskii volcanic center are most probably less differentiated than the melts of Bezymyannyi and Shiveluch volcanoes. The concentrations of water and 20 trace elements were measured in the glasses of 22 melt inclusions in plagioclase and quartz from our samples. Unusually high values were obtained for Li concentrations (along with high Na concentrations) in the basaltic melts from the Karymskii volcanic center: from 118 to 1750 ppm, whereas the dacite and rhyolite melts contain 25 ppm Li on average. The rhyolite melts of Golovnina volcano are much poorer in Li: 1.4 ppm on average. The melts of the Karymskii volcanic center are characterized by relative minima at Nb and Ti and maxima at B and K, as is typical of arc magmas.

Petrology and geochemistry of ca. 2100–1000 a.B.P. magmas of Augustine volcano, Alaska, based on analysis of prehistoric pumiceous tephra

Geochemical and textural features of whole-rock samples, phenocrysts, matrix glasses, and silicate melt inclusions from five prehistoric pumiceous tephra units of Augustine volcano, Alaska, were investigated to interpret processes of magma storage and evolution. The bulk-rock compositions of the tephra (designated G, erupted ca. 2100 a.B.P.; I ca. 1700 a.B.P.; H ca. 1400 a.B.P.; and C1 and C2 ca. 1000 a.B.P.) are silicic andesite; they contain rhyolitic matrix glasses and silicate melt inclusions with 74–79 wt.% SiO 2. The rocks are comprised of microlite-bearing matrix glass and phenocrysts of plagioclase, orthopyroxene, clinopyroxene, magnesio–hornblende, titanomagnetite, and ilmenite ± Al-rich amphibole with minor to trace apatite and rare sulfides and quartz. The felsic melt inclusions in plagioclase, pyroxenes, and amphibole are variably enriched in volatile components and contain 1.6–8.0 wt.% H 2O, 2100–5400 ppm Cl, < 40–1330 ppm CO 2, and 30–390 ppm S. Constraints from Fe–Ti oxides imply that magma evolution occurred at 796 ± 6 °C to 896 ± 8 °C and log ƒ O2 of NNO + 2.2 to + 2.6. This is consistent with conditions recorded for 1976, 1986, and 2006 eruptive materials and implies that magmatic and eruptive processes have varied little during the past 2100 years. Prehistoric Augustine magmas represented by these silicic andesites evolved via fractional crystallization, magma mingling and mixing, and/or chemical contamination due to magma–volcanic rock interaction. The occurrence of fractional crystallization is supported by the abundance of normally zoned phenocrysts, the presence of felsic matrix glass and melt inclusions within andesitic rock samples, trace-element data, and by geochemical modeling. The modeling constrains the influence of crystal fractionation on melt differentiation and is consistent with the evolution of the melt phase from felsic andesite to rhyodacite compositions. Magma mixing, mingling, and/or contamination by magma–volcanic rock interaction are indicated by abundant reversely zoned phenocrysts, rare mixed pumice-bearing rock samples, and abundant resorption–growth features in plagioclase.

The fidelity of melt inclusions as records of melt composition

A series of experiments created melt inclusions in plagioclase and pyroxene crystals grown from a basaltic melt at 1,150°C, 1.0 GPa to investigate diffusive fractionation during melt inclusion formation; additionally, P diffusion in a basaltic melt was measured at 1.0 GPa. Melt inclusions and melts within a few 100 microns of plagioclase–melt interfaces were analyzed for comparison with melt compositions far from the crystals. Melt inclusions and melt compositions in the boundary layer close to the crystal–melt interface were similar, but both differ significantly in incompatible element concentrations from melt found greater than approximately 200 microns away from the crystals. The compositional profiles of S, Cl, P, Fe, and Al in the boundary layers were successfully reproduced by a two-step model of rapid crystal growth followed by diffusive relaxation toward equilibrium after termination of crystal growth. Applying this model to investigate possible incompatible element enrichment in natural melt inclusions demonstrated that at growth rates high enough to create the conditions for melt inclusion formation, ∼10−9–10−8 m s−1, the concentration of water in the boundary layer near the crystal was similar to that of the bulk melt because of its high diffusion coefficient, but sulfur, with a diffusivity similar to major elements and CO2, was somewhat enriched in the boundary layer melt, and phosphorus, with its low diffusion coefficient similar to other high-field strength elements and rare earth elements, was significantly enriched. Thus, the concentrations of sulfur and phosphorus in melt inclusions may over-estimate their values in the bulk melt, and other elements with similar diffusion coefficients may also be enriched in melt inclusions relative to the bulk melt.