Showing papers on "Phenocryst published in 2001"
TL;DR: In this paper, a novel scheme was introduced that enables natural silicic glasses to be projected into the synthetic system Qz-Ab-Or-H2O in order to relate variations in volcanic glass chemistry to changing pressure (P) and temperature (T) conditions in the subvolcanic system.
Abstract: We introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system Qz–Ab–Or–H2O in order to relate variations in volcanic glass chemistry to changing pressure (P) and temperature (T) conditions in the sub-volcanic magma system. By this means an important distinction can be made between ascent-driven and cooling-driven crystallisation under water-saturated or undersaturated conditions. In samples containing feldspar and a silica phase (quartz or tridymite), quantitative P–T estimates of the conditions of last equilibrium between crystals and melt can be made. Formation of highly silicic melts (i.e. >77 wt% SiO2) is a simple consequence of the contraction of the silica phase volume with decreasing pressure, such that high silica glasses can only form by crystallisation at low pressure. Resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation. Groundmass and inclusion glasses in dacites from the 1980–1986 eruption of Mount St Helens volcano (WA) span a wide range in SiO2 (68–80 wt%, anhydrous). The compositions of the least evolved (SiO2-poor) inclusions in amphibole phenocrysts record entrapment of silicic liquids with ≤5.4 wt% water, corresponding to a water saturation pressure of ~200 MPa at 900 °C. The compositions of more evolved (higher SiO2) plagioclase-hosted inclusions and groundmass glasses are consistent with extensive ascent-driven fractional crystallisation of plagioclase, oxide and orthopyroxene phenocrysts and microlites to low pressures. During this polybaric crystallisation, plagioclase phenocrysts trapped melts with a wide range of dissolved water contents (3.5–5.7 wt%). Magmas erupted during the Plinian phase of the 18 May 1980 eruption were derived from a large reservoir at depths of ≥6 km. Subsequent magmas ascended to varying depths within the sub-volcanic system prior to extraction. From glass chemistry and groundmass texture two arrest levels have been identified, at depths of 0.5–1 and 2–4 km. A single dome sample from February 1983 contains groundmass plagioclase, tridymite and quartz, testifying to temperatures of at least 885 °C at 11 MPa. These shallow storage conditions are comparable to those in the cryptodome formed during spring 1980. The corresponding thermal gradient, ≤0.2 °C MPa–1, is consistent with near-adiabatic magma ascent from ~8 km. We argue that the crystallisation history of Mount St Helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation. This challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980–1986 eruption. Because the crystallisation process is both polybaric and fractional, it cannot be simulated directly using isobaric equilibrium crystallisation experiments. However, calculation of the phase proportions in water-saturated 910±15 °C experiments by Rutherford et al. (1985) over the pressure range 220–125 MPa reproduces the crystallisation sequence and phenocryst modes of Mount St Helens dacites from 18 May 1980. By allowing for the effects of fractional versus equilibrium crystallisation, entrained residual source material, and small temperature differences between nature and experiment, phase compositions can also be matched to the natural samples. We conclude that decompression of water-saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres.
438 citations
TL;DR: The Yellowstone Plateau volcanic field is one of the largest centers of zircons on Earth and the residence time of lowO of rhyolitic magmatism on Earth is between a few hundred and a few thousand years as discussed by the authors.
Abstract: The Yellowstone Plateau volcanic field is one of the largest centers exchange time to form zoned zircons is between a few hundred and a few thousand years, which reflects the residence time of lowO of rhyolitic magmatism on Earth. Major caldera-forming eruptions are followed by unusual lowO rhyolites. New oxygen isotope, magmas after formation and before eruption. petrologic and geochemical data from rhyolites belonging to the 2·0 my eruptive history of Yellowstone are presented, with emphasis on the genesis of lowO magmas erupted after the Huckleberry Ridge Tuff (2·0 Ma, 2500 km) and Lava Creek Tuff (0·6 Ma,
255 citations
TL;DR: In this paper, the Pannonian Basin and tensional stress field may have Almandine garnet-bearing andesites and dacites occur frequently in enhanced their fast ascent from lower-crustal depths, allowing the Neogene calc-alkaline volcanic series of the northern Pannonians preservation of early-formed almandine phenocrysts.
Abstract: of the Pannonian Basin and the tensional stress field may have Almandine garnet-bearing andesites and dacites occur frequently in enhanced their fast ascent from lower-crustal depths, allowing the Neogene calc-alkaline volcanic series of the northern Pannonian preservation of early-formed almandine phenocrysts. Basin (Hungary and Slovakia). They were erupted during the early stage of volcanism and occur along major tectonic lineaments. On the basis of petrographic and geochemical characteristics, garnets from these rock types are classified into (1) primary phases,
163 citations
TL;DR: The Emeishan continental flood basalts (ECFB) as discussed by the authors have variable SiO2, ranging from 43.6 to 52.1 wt, and total alkali (K2O + Na2O) from 0.7 to 6.5 wt%.
Abstract: The widespread Emeishan igneous province in southwestern China comprises the Emeishan continental flood basalts (ECFB) and associated mafie-ultramafic intrusions. The ECFB have variable SiO2, ranging from 43.6 to 52.1 wt%, Al2O3 from 5.0 to 12.6 wt%, and total alkali (K2O + Na2O) from 0.7 to 6.5 wt%. These oxides exhibit negative correlations with MgO (5.4 - 23.1 wt%), implying fractional crystallization of olivine and clinopyroxene, which occur as phenocrysts in the rocks. Linear correlations between Zr, Nb, and La suggest that crustal contamination is not important. The primitive-mantle-normalized trace-element patterns show that the ECFB are enriched in high-field-strength trace elements, large-ion-lithophile elements, and light-rare-earth elements, similar to ocean-island basalt. Incompatible element ratios of the ECFB, such as Zr/Nb (7-10), Th/La (0.1-0.15), and Rb/Nb (0.9-1.7), differ from those of primitive mantle, N-MORB, and continental crust, but are similar to ocean-island basalts from an enric...
158 citations
TL;DR: In this paper, the compositions and liquidus temperatures of the magmas crystallizing the most magnesian phenocrysts can be estimated and the authors find that anhydrous liquidus temperature (at 1 bar pressure) of Hawaiian tholeiitic picrites average 1365°C, for E-MOR picrites, for Mid-Ocean Ridge picrites averaged 1355°C.
Abstract: The composition of olivine phenocrysts in Hawaiian tholeiitic picrites and in Mid-Ocean Ridge picrites vary up to Mg#91.3 and Mg#92.1 respectively. The compositions and liquidus temperatures of the magmas crystallizing the most magnesian phenocrysts can be estimated and we find that anhydrous liquidus temperatures (at 1 bar pressure) of Hawaiian tholeiitic picrites average 1365°C, for E-MOR picrites average 1355°C, and for N-MOR picrites average 1335°C. Water contents of the magmas decrease in the order Hawaiian picrites, E-MOR picrites to N-MOR picrites, and consideration of liquidus depression by these water contents leads to the conclusion that magma temperatures for all types were approximately 1325°C at ∼ 1 bar. The data from parental or primary magmas suggests that the temperature contrast between ‘Hot-Spot and MOR magmas is ≤ 20°C. Application of information from partial melting studies of lherzolites and liquidus studies of the Hot-Spot and MOR picrites leads to the conclusion that both ‘Hot-Spot’ and MOR primary basalts are derived from mantle with potential temperature Tp ∼ 1430°C. Insofar as primitive magmas may be used to infer the potential temperature of their sources, there is no evidence for a temperature contrast of Δ Tp = 100–250°C between ‘Hot-Spot’ or ‘Deep Mantle Plume’ sources and ambient (MOR source) asthenospheric mantle.
Although magma temperatures are similar, the residual mantle compositions for Hawaiian picrites are refractory harzburgites, more refractory (including Cr/Cr+Al ratio) than the lherzolite to harzburgite residue from MOR picrite extraction. It is argued that the buoyancy plume and geophysically anomalous mantle beneath the Hawaiian Arch is due to compositional and not temperature contrasts in the upper mantle. The four-component mixing identified in the Hawaiian source is attributed to interaction between old subducted lithospheric slabs, buoyant or suspended in the upper mantle, and surrounding ambient mantle at Tp = 1430°C.
This paper was presented at the EMPG VIII meeting in Bergamo, Italy (April 2000)
155 citations
TL;DR: It is shown that the compositional change in primary magmas from Mount Etna reflects a progressive transition from a predominantly mantle-plume source to one with a greater contribution from island-arc (subduction-related) basalts, which implies that the volcanism ofMount Etna has become more calc-alkaline, and hence more explosive, during its evolution.
Abstract: Mount Etna lies near the boundary between two regions that exhibit significantly different types of volcanism. To the north, volcanism in the Aeolian island arc is thought to be related to subduction of the Ionian lithosphere1. On Sicily itself, however, no chemical2,3 or seismological4 evidence of subduction-related volcanism exists, and so it is thought that the volcanism—including that on Mount Etna itself—stems from the upwelling of mantle material5, associated with various surface tectonic processes1,6. But the paucity of geological evidence regarding the primary composition of magma from Mount Etna means that its source characteristics remain controversial. Here we characterize the trace-element composition of a series of lavas emitted by Mount Etna over the past 500 kyr and preserved as melt inclusions inside olivine phenocrysts. We show that the compositional change in primary magmas from Mount Etna reflects a progressive transition from a predominantly mantle-plume source to one with a greater contribution from island-arc (subduction-related) basalts. We suggest that this is associated with southward migration of the Ionian slab, which is becoming juxtaposed with a mantle plume beneath Sicily. This implies that the volcanism of Mount Etna has become more calc-alkaline, and hence more explosive, during its evolution.
143 citations
TL;DR: This paper performed detailed textural analyses of the white and gray pumices of the plinian and pyroclastic flow deposits, and found that differences in color and clast density reflect different crystal and vesicle amounts and size distributions.
Abstract: The climactic event of Mount Pinatubo represents one of the most thoroughly studied eruptions of the century and has provided important insights into the dynamics of explosive volcanism. We have performed detailed textural analyses of the white and gray pumices of the plinian and pyroclastic flow deposits, and found that differences in color and clast density reflect different crystal and vesicle amounts and size distributions. White pumice has higher vesicularity, deformed and highly coalesced vesicles with thin walls, euhedral phenocrysts and microlite-free groundmass. Gray pumice shows lower vesicularity, wider ranges in vesicle number density, limited coalescence, vesicles with thick walls that are less deformed, phenocrysts and microphenocrysts with abundant solution pitting, and groundmass containing ubiquitous microlites and crystal fragments. The presence of white and gray pumice varieties and the broad range in vesicularity and vesicle number density that characterizes both of them appear to record the complexities of conduit processes such as magma vesiculation and fragmentation and the development of conduit regions marked by different rheological behaviors. In particular, the results of this study suggest the likely importance of intense shear and viscous dissipation at the conduit walls, a mechanism that may be responsible for the creation and discharge of the gray pumice of this eruption along with the dominant white variety.
137 citations
TL;DR: In this article, the role of partial melting of clinopyroxene-rich lithologies in the genesis of ultra-calcic UCMI is investigated, which suggests that some or all of the silicicUCMI may originate by a process other than simple partial melting.
Abstract: Ultra-calcic melt inclusions (UCMI: CaO>13.5 wt% and/or CaO/Al2O3>0.9) are magnesian and near-primary liquids trapped in volcanic phenocrysts from mid-ocean ridges, arcs, back-arcs, and ocean islands. UCMI can be subdivided into two classes based on tectonic association and degree of silica saturation: those from arcs are nepheline normative and those from all other localities (silicic UCMI) are hypersthene normative. Silicic UCMI share a number of common features, including primitive host minerals, low alkali contents, and variable ratios of K2O/TiO2 ranging to high values. Their compositions are not easily derived by partial melting of mantle lherzolite. Accordingly, we have performed a series of partial melting experiments on three clinopyroxenite compositions at 1.0 to 2.0 GPa to investigate the role of partial melting of clinopyroxene-rich lithologies in silicic UCMI genesis. Estimated solidus temperatures for all three compositions are similar to those of normal peridotites, but 1.0 GPa isobaric melt productivities are higher for clinopyroxenite than for peridotite. High degree partial melts of the clinopyroxenites are ultra-calcic and have similarities to silicic UCMI, but the experiments produce ultra-calcic liquids only at melt fractions greater than 30% and temperatures higher than 1,350 °C at 1.0 GPa. Such temperatures are higher than those likely to be prevailing beneath normal mid-ocean ridges, which suggests that some or all silicic UCMI may originate by a process other than simple partial melting of clinopyroxene-rich lithologies. We consider a possible role for partial melting of depleted harzburgite in the genesis of silicic UCMI.
131 citations
TL;DR: The role played by plume-generated crustal magmatic complexes in the segmentation of volcanic margins is highlighted by a preliminary study of magma flow directions in shallow intrusives from the East Greenland volcanic margin this article.
122 citations
TL;DR: A group of Pleistocene-recent lavas, with 62 to 64% SiO2 and 5.6 to 3.5% MgO, have erupted along a linear fault system near the volcanic front of central Mexico.
109 citations
TL;DR: In this paper, the eastern Manus Basin is represented by two representative east Manus basalts, respectively, with modest (0.2 wt%) and high ( 0.8 wt) potassium contents, which host ubiquitous CO2-bearing vapour bubbles.
TL;DR: In this article, the authors present a new approach for determining minimum magma residence times which involves dating phenocrysts in a magma using 226Ra-230Th disequilibria, and apply this approach to the early phase of the 1955 east rift eruption at Kilauea.
TL;DR: In this article, the growth zones in quartz phenocrysts from the rhyolitic Bishop Tuff have been studied and three recognizable patterns of growth zones have been identified: weakly zoned cores and bright CL rims, weakly zone cores and rims; and no CL intensity difference from core to rim.
Abstract: 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.
TL;DR: The Tsaolingshan lavas from Taiwan's Northern Taiwan Volcanic Zone as mentioned in this paper have been shown to have a similar composition to the Group I ultrapotassic orogenic lamproites from central Italy, Span and Tibet.
TL;DR: The phase relations of three samples of the Laacher See Tephra (LST) have been determined experimentally as a function of temperature (760 to 880 °C), pressure (200, 300 and 400 MPa), water content of the melt and oxygen fugacity (ƒO2) as discussed by the authors.
Abstract: Phase relations of three samples of the Laacher See Tephra (LST) have been determined experimentally as a function of temperature (760 to 880 °C), pressure (200, 300 and 400 MPa), water content of the melt and oxygen fugacity (ƒO2). The crystallization experiments were carried out at ƒO2=NNO buffer and at NNO=+ 2.3 log units. The melt water contents varied between 6 and more than 8 wt% H2O, corresponding to water-undersaturated and water saturated conditions respectively. The synthetic products are compared to the natural phases to constrain pre-eruptive conditions in the Laacher See magma chamber. The major phases occurring in the LST have been reproduced. The stability of hauyne is favoured at high ƒO2 (≈NNO + 2.3). The CaO contents in melt and plagioclase synthesized under water-saturated conditions are significantly higher than in the natural phases, implying that most of the differentiation of the phonolites took place under water-undersaturated conditions. However, this does not exclude the presence of a S-, Cl- and CO2-rich fluid phase in the upper parts of the magma chamber. The phase relationships and the TiO2 contents of melts show that the temperature was lower than 760 °C in the upper part of the magma column (probably down to 720 °C in the most differentiated levels) and that temperatures above 840–860 °C prevailed in the lower part. The variation of the XMg of ferromagnesian minerals observed in both natural and experimental phases reflects the strong variations in ƒO2 in the lower magma chamber just prior to eruption (probably variation of about 2 log units). The most probable explanation for these ƒO2 variations is the injection of an oxidized alkali-rich magma, containing Mg-rich phenocrysts, at the base of a chemically zoned and more reduced magma column prior to eruption. Although the amount of injected magma may not have been very important, it was sufficient to change the ƒO2 conditions locally, explaining the heterogeneous XMg of ferromagnesian minerals and the formation of hauyne at the base of the chamber.
TL;DR: The first direct determination of the oxygen fugacities (fO2) of potassic igneous rocks from the vicinity of epithermal gold mineralization at Lihir Island, Papua New Guinea, was reported in this article.
TL;DR: In this paper, the authors reported new laser fluorination oxygen isotope data for 60 volcanic rocks and 15 sediments distributed over the whole length of the Banda Arc, eastern Indonesia.
TL;DR: In this article, a 50 km-wide segment of the central part of the Mexican volcanic belt has been mapped, and five types of lava have been found, including shoshonites, quaternary basaltic andesite, high-TiO 2 lavas, and porphyritic dacite.
Abstract: As part of the continuing study of the young Mexican volcanic belt designed to document the ages and types of volcanism from the Gulf of California to the Valley of Mexico, a 50-km-wide segment of the central part of the belt has been mapped, and five types of lava have been found. Pliocene (3.77 Ma) shoshonite lava flows (K 2 O >2 wt%; SiO 2 52–58 wt%) form eroded plateaus more than 50 km behind the present volcanic front, but in the past 1 m.y. shoshonites have erupted closer to the volcanic front (∼300 km from the Middle America Trench). The shoshonite lava type is the most enriched in Ba, Sr, and Zr of the suite, and plagioclase phenocrysts are absent, presumably because of high contents of dissolved water (3–5 wt%). Quaternary shield volcanoes and several cinder cones with small-volume lava flows are composed of high-TiO 2 lavas (>1.2 wt%), which have 51–57 wt% SiO 2 , 9 ppm) and Zr (160–230 ppm). Pliocene high-TiO 2 lava is found within the eroded plateaus located ∼50 km behind the present volcanic front. Quaternary basaltic andesite (52–57 wt% SiO 2 ) with up to ∼10 wt% MgO is found at the volcanic front, along with more siliceous andesite (57–63 wt% SiO 2 ). Representatives of the siliceous andesites (57–63 wt% SiO 2 ) are free of plagioclase phenocrysts, low in Al 2 O 3 (∼15.7 wt%), but rich in MgO (∼5 wt%). Experiments reported elsewhere suggest that the magmas contained 3–7 wt% dissolved water. This andesite erupted along a normal fault between ∼0.3 and 0.005 Ma, and it is associated with dacite, similarly lacking plagioclase phenocrysts, but having comparatively abundant pyroxene. Other dacite in the Zitacuaro area is richly porphyritic, having plagioclase and hornblende; this dacite forms clusters of steep-sided domes or widespread pyroclastic deposits, and the latest eruptions, dated by radiocarbon and K-Ar, occurred between 0.03–0.05 Ma. Estimates of the volume of magma erupted in the Zitacuaro–Valle de Bravo region in the past 1 m.y. (1.8 km 3 · m.y. –1 · km –1 ) show that this area is somewhat less productive per 1 km of arc than those to the west (3.5 km 3 · m.y. –1 · km –1 ) in the Michoacan-Guanajuato Volcanic Field (MGVF). A large volume of dacite has erupted in the Zitacuaro–Valle de Bravo region, which is rare in the MGVF. The cone density in the Zitacuaro–Valle de Bravo region (2.1/100 km 2 ) is only slightly lower than the 2.6 cones/100 km 2 found in the MGVF.
TL;DR: The authors examined the halogen contents of biotite, amphibole, and apatite from four magmatic phases of the Emerald Lake pluton to document the evolution in composition of magmatic volatiles in this system, and to determine if this evolution is consistent with the nature and distribution of weak gold mineralization in the pluton.
Abstract: We examine the halogen contents of biotite, amphibole, and apatite from four magmatic phases of the Emerald Lake pluton to document the evolution in composition of magmatic volatiles in this system, and to determine if this evolution is consistent with the nature and distribution of weak gold mineralization in the pluton. In general, the variations in halogen compositions of hydrous minerals are greater between magmatic phases than within single magma batches, and the abundance of chlorine decreases and fluorine increases in hydrous minerals during the evolution of the Emerald Lake pluton. By applying the internally consistent thermodynamic database and mineral activity–composition relationships that account for coupled site-substitution, we further employ biotite and apatite compositions as independent monitors of fluid composition. Most analyzed biotite exhibits textural and/or compositional features characteristic of late, subsolidus halogen exchange with fluid. However, we have identified biotite that appears to retain halogen contents indicative of magmatic conditions. Fluorine contents of these biotites range from around 1.5 (syenite and monzonite) to greater than 2.2 wt% (in a late-stage pegmatite dike). Chlorine, by contrast, shows a more restricted range of values, and typically falls between 1.0 (syenite and monzonite) and 0.6 wt% (pegmatite). Apatite inclusions within anhydrous phenocrysts have fluorine contents of around 3.3 (syenite) to greater than 3.7 wt% (granite), while chlorine contents fall from 0.5 to 0.1 wt% with evolution from syenite to granite phases of the pluton. The fluid aHCl/aHF conditions recorded by biotite and apatite are in close agreement. These two independent measures of HCl and HF both show a modest decrease in HCl/HF as the system evolves. The highest log aHCl/aHF values (0.6 to 0.3) are associated with magmatic volatiles of syenite, the first phase to intrude at Emerald Lake. Activity ratios recorded from the second phase (monzonite) show a slight fall to ~0.4 to 0.1. A similar drop is recorded in the transition to hydrothermal activity recorded in biotite from a late pegmatite dike (~0.1 to –0.1). A decrease in log aHCl/aHF is consistent with a loss of chlorine and residual enrichment of fluorine in the magma, due to progressive release of a chlorine-bearing (metalliferous?) fluid. The relatively small variation in inferred fluid HCl and HF activity suggests that the magmatic volatiles present during crystallization were relatively homogeneous, and argues against both the incursion of external fluids, and of substantial temporal variation in fluid composition from the magma source. Furthermore, if magmatic volatiles are the source of the gold mineralization found across all phases of the Emerald Lake pluton, as advocated by other workers, our data would suggest that the mineralizing fluids were remarkably homogeneous and distributed throughout the pluton. The disseminated and low-grade nature of mineralization may be, at least in part, a consequence.
TL;DR: In this article, the conditions under which the rhyolitic part of the erupted magma last resided in the crust prior to the 1912 Novarupta, Alaska eruption were investigated.
Abstract: Recent studies have proposed contrasting models for the plumbing system that fed the 1912 eruption of Novarupta, Alaska. Here, we investigate the conditions under which the rhyolitic part of the erupted magma last resided in the crust prior to eruption. Geothermometry suggests that the rhyolite was held at ∼800–850 °C, and analyses of melt inclusions suggest that it was fluid saturated and contained ∼4 wt% water. Hydrothermal, water-saturated experiments on rhyolite pumice reveal that at those temperatures the rhyolite was stable between 40 and 100 MPa, or a depth of 1.8–4.4 km. These results suggest that pre-eruptive storage and crystal growth of the rhyolite were shallow; if the rhyolite ascended from greater depths, it did so slowly enough for unzoned phenocrysts to grow as it passed through the shallow crust.
TL;DR: In this article, tephra lapilli from six explosive eruptions between April 1996 and February 1998 at Popocatepetl volcano in central Mexico have been studied to investigate the causes of magma diversification in thick-crusted volcanic arcs.
Abstract: Tephra lapilli from six explosive eruptions between April 1996 and February 1998 at Popocatepetl volcano (=Popo) in central Mexico have been studied to investigate the causes of magma diversification in thick-crusted volcanic arcs. The tephra particles are sparsely porphyritic (≈5 vol%) magnesian andesites (SiO2=58–65 wt%; MgO=2.6–5.9 wt%) that contain phenocrysts of NiO-rich (up to 0.67 wt% NiO) magnesian olivine (Fo89–91 cores) with inclusions of Cr-spinel (cr#=59–70), orthopyroxene (mg#=63–76), clinopyroxene (mg#=68–86), intermediate to sodic plagioclase (An33–66), and traces of amphibole. Major and trace element systematics indicate magma mixing. The liquid mg#melt ratios inferred from the ferromagnesian phenocrysts suggest the existence of a mafic (mg#melt ≈ 72–76) and an evolved component magma (mg#melt ≈ 35–40). These component magmas form a hybrid magnesian andesite with an intermediate range of mg#melt=50–72. The mafic end member (mg#melt ≈ 72–75) is saturated with olivine and spinel and crystallizes at temperatures ≈1170–1085 °C with oxygen fugacities close to the fayalite–magnetite–quartz buffer and elevated water contents of several wt% H2O. A likely location of crystallization is at lower crustal levels, possibly at the Moho. Olivine is followed by high-mg# clinopyroxene which could start to crystallize during magma ascent. At depths of ≈4 to 13 km, the mafic magma mixes with an evolved composition containing low-mg# clino- and orthopyroxene and plagioclase at a temperature of ≈950 °C. The repetitive ascent of batches of mafic magmas spaced days to weeks apart implies multiple episodes of crystallization and magma mixing. The tephra is similar to the Popo magnesian andesites, suggesting similar generic processes for the common lavas of the volcano. The advantage of the tephra is that it can be used to reconstruct the composition of the mafic magma. Building on the elemental systematics of the tephra and a comparison to the near-primary basalts from the surrounding monogenetic fields, we infer that the Popo mafic end member is a magnesian andesite with variable, but high SiO2 contents of ≈55–62 wt% and near-primary characteristics, such as high-mg#melt of 72–75, FeO*/MgO ratios <1 (if extrapolated to an mg#melt of 72–75), and high Ni contents (=200 ppm Ni). This model implies that the typical elemental signature of the Popo andesites, such as the low CaO, Al2O3, FeO*, high Na2O contents, and the depletion in high-field strength elements (e.g., P, Zr, Ti), are mantle source phenomena. Thus, determining the elemental budget of the magnesian andesite, as it is prior to the modifications by crustal differentiation, is central to quantifying the subcrustal mass fluxes beneath Popo.
TL;DR: In the case of Galunggung as mentioned in this paper, the inclusions in olivine phenocrysts from magma erupted during the Strombolian phase contain up to ∼2000 ppm sulfur, but concentrations decrease rapidly with increasing SiO2 down to matrix glass values (50-100ppm).
TL;DR: In this article, the authors show that plagioclase phenocrysts display sieved and normal type populations, a wide range in rim compositions, and oscillatory zoning in the Saraykent dacites.
TL;DR: Recently erupted rocks from Montserrat, Lesser Antilles, were found to be coarsely porphyritic including plagioclase, magnesiohornblende, orthopyroxene, titano-magnetite, and rare quartz phenocrysts as mentioned in this paper.
TL;DR: The melt inclusions from large-volume ignimbrites and related lava flows from the late Neogene to Pleistocene Altiplano-Puna Volcanic Complex, northern Chile, record the magmatic volatile evolution and constrain conditions of magma storage.
Abstract: Melt inclusions hosted in quartz from large-volume ignimbrites and related lava flows from the late Neogene to Pleistocene Altiplano-Puna Volcanic Complex, northern Chile, record the magmatic volatile evolution and constrain conditions of magma storage. Glasses from pristine and rehomogenized inclusions have high-Si rhyolitic compositions (average SiO2=77.5 wt%). Their host rocks range from dacite to rhyodacite (SiO2=63.9–72.5 wt%) and have a high abundance of phenocrysts (33–55%). Infrared spectroscopic analysis of inclusions from pumice samples typically yielded H2O contents between 3.0 and 4.0 wt% and relatively low and more variable CO2 contents <400 ppm. Increasing H2O contents were found in a series of successively trapped inclusions, and bubble-free inclusions tend to have lower CO2 contents with increasing H2O. Inclusions from lava samples have lower but constant H2O contents of 2.0±0.3 wt% and CO2 close to the detection limit (∼10 ppm). Incompatible trace elements with high affinities to partition into a fluid phase (e.g., B, Cl) show minor enrichment in melt inclusions, whereas compatible trace elements (e.g., Sr, Ba) became strongly depleted due to feldspar-dominated crystallization. Variations in H2O and CO2 contents as well as concordant preeruptive pressures inferred from volatile solubility and Al-in-hornblende barometry (150±50 MPa) also indicate volatile saturation and upper crustal magma storage for the ignimbrite magmas. The melt inclusion record is consistent with near-isobaric cooling from ∼830° to 780°C (inferred from mineral thermometry) under gas-saturated conditions in shallow (4–6 km deep) reservoirs. Model calculations for overpressures generated by closed-system crystallization and gas exsolution either result in premature failing of the magma chamber walls or require the presence of large volumes of highly compressible magmatic foam. As an alternative, open-system degassing of these magmas prior to their explosive and effusive eruption is proposed.
TL;DR: In this article, a study of dacite lavas of three volcanoes in Japan (Unzen, Yakedake and Daisen) has been carried out, showing that the shape of these pores ranges from irregular to planar, and that they are the result of failure of the magma by flow during ductile-brittle transition.
TL;DR: In this paper, ion microprobe analyses of S and O isotopes in glass inclusions and their host clinopyroxenes (Cpx) from Miocene basaltic hyaloclastites were performed during Ocean Drilling Program (ODP) Leg 157.
TL;DR: The Streltsovka caldera contains the largest uranium field associated with volcanics in the world as mentioned in this paper, and the presence of daughter minerals such as fluorite, F-rich mica and REE fluocarbonate, and the composition of the silicate melt after homogenization, indicate that the Streltska rhyolites correspond to highly fractionated mildly peralkaline melts.
TL;DR: In this article, the authors investigated the tapping process of the phenocryst-rich andesite chamber and estimated the viscosity and density of the end-member and mixed magmas with constraints drawn from petrography.
Abstract: During the 1929 activity of Hokkaido-Komagatake volcano, the Plinian eruption of a phenocryst-rich andesite was preceded by a small eruption of more mafic magma formed by magma mixing. A similar eruption sequence has been reported for some other eruptions (Pallister et al. 1996; Venezky and Rutherford 1997), suggesting that eruption of a mixed magma is a precursor of phenocryst-rich magmas. For the purpose of understanding the tapping processes of the phenocryst-rich magma chamber, we investigated the temporal variation in the erupted magma and estimated the viscosity and density of the end-member and mixed magmas with constraints drawn from petrography. For the precursory mixed magma we estimate 33±4 vol.% phenocrysts, andesitic–dacitic melt composition, 3 wt.% H2O content, and temperature of 1040°C. In comparison, for the climactic, silicic end-member magma we estimate 48±3 vol.% phenocryst, high-silica rhyolitic melt, 3 wt.% H2O, and temperature of 950°C, respectively. The mafic end-member magma, which was not erupted, is thought to be an almost aphyric basaltic–andesitic magma, based on mass balance calculation of the phenocryst content. The proportion of the mafic end-member magma component in the mixed magma was calculated to be 20–40 wt.%. On the basis of these data, we estimate magma viscosities of 103.9, 106.9, and 102.0 Pa s for the mixed, silicic end-member, and mafic end-member magmas, respectively. The calculated density differences among these magmas are inconsequential when possible errors are considered. We calculate the minimum excess pressure required for dike propagation to be 31 MPa for the silicic end-member magma and 8 MPa for the mixed magma, using the estimated viscosity and dike propagation model of Rubin (1995). If we assume that excess pressure is limited by the wall rock strength of the magma chamber, excess pressure retainable in the magma chamber is less than ca. 20 MPa. This suggests that the mixed magma was able to ascend to the surface without freezing, whereas the viscous silicic end-member magma could not. The formation and precursory eruption of the mixed magma are, therefore, effective and necessary initiation processes for the phenocryst-rich, viscous magma eruption.
TL;DR: The Monte Arcuentu volcanic complex is a typical product of subduction-related volcanic activity as mentioned in this paper, and in terms of K 2 O−SiO 2 relations they resemble calc-alkaline lavas of the Aeolian and Aegean arcs.