Showing papers on "Phenocryst published in 2008"
TL;DR: Watson et al. as mentioned in this paper measured the titanium concentration of 484 zircons with U-Pb ages of ∼1 Ma to 4.4 Ga and calculated the apparent temperatures for zircon crystallization using the Ti-in-zircon thermometer.
Abstract: The titanium concentrations of 484 zircons with U-Pb ages of ∼1 Ma to 4.4 Ga were measured by ion microprobe. Samples come from 45 different igneous rocks (365 zircons), as well as zircon megacrysts (84) from kimberlite, Early Archean detrital zircons (32), and zircon reference materials (3). Samples were chosen to represent a large range of igneous rock compositions. Most of the zircons contain less than 20 ppm Ti. Apparent temperatures for zircon crystallization were calculated using the Ti-in-zircon thermometer (Watson et al. 2006, Contrib Mineral Petrol 151:413–433) without making corrections for reduced oxide activities (e.g., TiO2 or SiO2), or variable pressure. Average apparent Ti-in-zircon temperatures range from 500° to 850°C, and are lower than either zircon saturation temperatures (for granitic rocks) or predicted crystallization temperatures of evolved melts (∼15% melt residue for mafic rocks). Temperatures average: 653 ± 124°C (2 standard deviations, 60 zircons) for felsic to intermediate igneous rocks, 758 ± 111°C (261 zircons) for mafic rocks, and 758 ± 98°C (84 zircons) for mantle megacrysts from kimberlite. Individually, the effects of reduced \( a_{{\rm TiO}_{2}}\) or \( a_{{\rm SiO}_{2}}\), variable pressure, deviations from Henry’s Law, and subsolidus Ti exchange are insufficient to explain the seemingly low temperatures for zircon crystallization in igneous rocks. MELTs calculations show that mafic magmas can evolve to hydrous melts with significantly lower crystallization temperature for the last 10–15% melt residue than that of the main rock. While some magmatic zircons surely form in such late hydrous melts, low apparent temperatures are found in zircons that are included within phenocrysts or glass showing that those zircons are not from evolved residue melts. Intracrystalline variability in Ti concentration, in excess of analytical precision, is observed for nearly all zircons that were analyzed more than once. However, there is no systematic change in Ti content from core to rim, or correlation with zoning, age, U content, Th/U ratio, or concordance in U-Pb age. Thus, it is likely that other variables, in addition to temperature and \( a_{{\rm TiO}_{2}}\), are important in controlling the Ti content of zircon. The Ti contents of igneous zircons from different rock types worldwide overlap significantly. However, on a more restricted regional scale, apparent Ti-in-zircon temperatures correlate with whole-rock SiO2 and HfO2 for plutonic rocks of the Sierra Nevada batholith, averaging 750°C at 50 wt.% SiO2 and 600°C at 75 wt.%. Among felsic plutons in the Sierra, peraluminous granites average 610 ± 88°C, while metaluminous rocks average 694 ± 94°C. Detrital zircons from the Jack Hills, Western Australia with ages from 4.4 to 4.0 Ga have apparent temperatures of 717 ± 108°C, which are intermediate between values for felsic rocks and those for mafic rocks. Although some mafic zircons have higher Ti content, values for Early Archean detrital zircons from a proposed granitic provenance are similar to zircons from many mafic rocks, including anorthosites from the Adirondack Mts (709 ± 76°C). Furthermore, the Jack Hills zircon apparent Ti-temperatures are significantly higher than measured values for peraluminous granites (610 ± 88°C). Thus the Ti concentration in detrital zircons and apparent Ti-in-zircon temperatures are not sufficient to independently identify parent melt composition.
387 citations
TL;DR: Results demonstrate that iron isotopes fractionate during magmatic differentiation at both whole-rock and crystal scales, which makes iron a potential tool for studying planetary differentiation.
Abstract: Magmatic differentiation helps produce the chemical and petrographic diversity of terrestrial rocks. The extent to which magmatic differentiation fractionates nonradiogenic isotopes is uncertain for some elements. We report analyses of iron isotopes in basalts from Kilauea Iki lava lake, Hawaii. The iron isotopic compositions (56Fe/54Fe) of late-stagemeltveins are 0.2 permil (per thousand) greater than values for olivine cumulates. Olivine phenocrysts are up to 1.2 per thousand lighter than those of whole rocks. These results demonstrate that iron isotopes fractionate during magmatic differentiation at both whole-rock and crystal scales. This characteristic of iron relative to the characteristics of magnesium and lithium, for which no fractionation has been found, may be related to its complex redox chemistry in magmatic systems and makes iron a potential tool for studying planetary differentiation.
254 citations
TL;DR: Melt inclusions are small parcels of melt trapped in crystals within magmatic systems, and are analogous to fluid inclusions formed by trapping of hydrothermal and other fluids during mineral growth in fluid-mineral systems as mentioned in this paper.
Abstract: Melt inclusions are small parcels of melt trapped in crystals within magmatic systems, and are analogous to fluid inclusions formed by trapping of hydrothermal and other fluids during mineral growth in fluid-mineral systems (Sorby 1858; Roedder 1979, 1984). After trapping, melt inclusions are potentially isolated from external melt and thus provide a way to investigate melts trapped during magmatic evolution—driven by processes such as crystal-liquid separation, vapor saturation and degassing, magma mixing and assimilation—which can dramatically alter the compositions of the eventual erupted (or intruded) magmatic end products. Melt inclusions are a powerful tool for the study of basaltic magma systems and their mantle source regions, and are widely used to study the origin and evolution of mantle-derived magmas. Melt inclusions have specific uses in the study of volatile elements (see chapters by Metrich and Wallace 2008, Moore 2008, and Blundy and Cashman 2008), but also provide unique information about the range of melt compositions present within basaltic magmatic systems, and how these reflect mantle sources and the processes that occur during melt generation, evolution, transport and eruption. This review outlines techniques used to obtain chemical and other information from melt inclusions, discusses the processes which lead to melt inclusion trapping in phenocryst minerals, examines the possible means by which melt inclusion compositions might be fractionated during trapping or during subsequent re-equilibration with the host mineral or external melt, and discusses some implications of melt inclusion compositions for the nature of basaltic melt generation and transport systems.
This review is largely restricted in scope to studies of volcanic rocks of basaltic and related composition. This refers to rocks erupted as lavas or tephra with broadly basaltic compositions: SiO2 ~45–52 wt%, relatively high MgO and FeO, and typically containing one or more of the following minerals …
229 citations
TL;DR: In this paper, the authors demonstrate that nearly dry melt inclusions from Galapagos Plateau basalt can gain up to 2.5 wt.% of water if they are placed for 2 days in a water-bearing melt at 200 MPa and 1140 °C.
228 citations
TL;DR: In this article, it is shown that there are no hypabyssal kimberlites which are representatives of the initial primary magma; all kimbers are hybrid and contaminated magmas which have undergone crystallization in the mantle prior to crystallization of the groundmass-mesostasis assemblages.
214 citations
TL;DR: In this paper, P2O5 contents of olivines vary from below the detection limit (≤0.01 wt%) to 0.2-0.4 wt% over a few microns, with no correlated variations in Fo content.
Abstract: We describe P zoning in olivines from terrestrial basalts, andesites, dacites, and komatiites and from a martian meteorite. P2O5 contents of olivines vary from below the detection limit (≤0.01 wt%) to 0.2–0.4 wt% over a few microns, with no correlated variations in Fo content. Zoning patterns include P-rich crystal cores with skeletal, hopper, or euhedral shapes; oscillatory zoning; structures suggesting replacement of P-rich zones by P-poor olivine; and sector zoning. Melt inclusions in olivines are usually located near P-rich regions but in direct contact with low-P olivine. Crystallization experiments on basaltic compositions at constant cooling rates (15–30°C/h) reproduce many of these features. We infer that P-rich zones in experimental and natural olivines reflect incorporation of P in excess of equilibrium partitioning during rapid growth, and zoning patterns primarily record crystal-growth-rate variations. Occurrences of high-P phenocryst cores may reflect pulses of rapid crystal growth following delayed nucleation due to undercooling. Most cases of oscillatory zoning in P likely reflect internal factors whereby oscillating growth rates occur without external forcings, but some P zoning in natural olivines may reflect external forcings (e.g., magma mixing events, eruption) that result in variable crystal growth rates and/or P contents in the magma. In experimental and some natural olivines, Al, Cr, and P concentrations are roughly linearly and positively correlated, suggesting coupled substitutions, but in natural phenocrysts, Cr zoning is usually less intense than P zoning, and Al zoning weak to absent. We propose that olivines grow from basic and ultrabasic magmas with correlated zoning in P, Cr, and Al superimposed on normal zoning in Fe/Mg; rapidly diffusing divalent cations homogenize during residence in hot magma; Al and Cr only partially homogenize; and delicate P zoning is preserved because P diffuses very slowly. This interpretation is consistent with the fact that zoning is largely preserved not only in P but also in Al, Cr, and divalent cations in olivines with short residence times at high temperature (e.g., experimentally grown olivines, komatiitic olivines, groundmass olivines, and the rims of olivine phenocrysts grown during eruption). P zoning is widespread in magmatic olivine, revealing details of crystal growth and intra-crystal stratigraphy in what otherwise appear to be relatively featureless crystals. Since it is preserved in early-formed olivines with prolonged residence times in magmas at high temperatures, P zoning has promise as an archive of information about an otherwise largely inaccessible stage of a magma’s history. Study of such features should be a valuable supplement to routine petrographic investigations of basic and ultrabasic rocks, especially because these features can be observed with standard electron microprobe techniques.
173 citations
TL;DR: Uturuncu volcano, SW Bolivia, is a dormant stratovolcano dominated by dacitic lava domes and flows as mentioned in this paper, and the deformation is attributed to magma intrusion into the Altiplano-Puna regional crustal magma body.
Abstract: Uturuncu volcano, SW Bolivia, is a dormant stratovolcano (~85 km3) dominated by dacitic lava domes and flows. 39Ar/40Ar ages show that the volcano was active between 890 ka and 271 ka, with the lavas becoming younger and less extensive at higher elevations. There are current signs of unrest. Between 1992 and 2006 geodetic satellite measurements record an ongoing 70 km deformation field with a central uplift rate of 1 to 2 cm/yr. Deformation indicates volume changes of 400 x 108 m3 over 14 years, an average of ~1 m3/s (10–2 km3/yr). The deformation is attributed to magma intrusion into the Altiplano-Puna regional crustal magma body. Deformation models indicate a source at depths of 17 to 30 km beneath current local relief. In a reconnaissance survey, persistent seismic activity (mean of 2.6 earthquakes per hour with a maximum of 14 per hour) was recorded at about 4 km depth below the center of the uplift, 4 km SW of the volcano's summit. The seismic events have a normal b value (~1.04) and activity is attributed to brittle deformation in the elastic crust above the active deep magma intrusion. The porphyritic dacite lavas (64–68% SiO2) have a plagioclase-orthopyroxene-biotite-magnetite-ilmenite assemblage and commonly contain juvenile silicic andesite inclusions, cognate norite nodules and crustal xenoliths. Temperature estimates are in the range 805 to 872°C for the dacites and about 980°C for the silicic andesites. The dacite magmas formed by fractional crystallization of andesite forming norite cumulates and involving partial melting of crust. Compositions and zoning patterns of orthopyroxene and plagioclase phenocrysts indicate that compositional variation in the dacites is caused by magma mixing with the silicic andesite. Reversely zoned orthopyroxene phenocrysts in the andesitic end-member are explained by changing oxidation states during crystallization. Fe3+/Fe2+ ratios from orthopyroxene crystals and Fe3+ in plagioclase provide evidence for a relatively reduced melt that subsequently ascended, degassed and became more oxidized as a consequence of degassing. The geophysical and petrological observations suggest that dacite magma is being intruded into the Altiplano-Puna regional crustal magma body at 17 km or more depth, consistent with deformation models. In the Late Pleistocene dacitic and andesitic magmas ascended from the regional crustal magma body to a shallow magma system at a few kilometers depth where they crystallized and mingled together. The current unrest, together with geophysical anomalies and 270 ka of dormancy, indicate that the magmatic system is in a prolonged period of intrusion. Such circumstances might eventually lead to eruption of large volumes of intruded magma with potential for caldera formation.
139 citations
TL;DR: Zircon U-Pb dating reveals that the three plutonic rocks were emplaced contemporaneously at ∼132.5 Ma as discussed by the authors, which suggests that the parental magma to the syenites was likely derived from melting of the enriched SCLM and contaminated slightly by old lower continental crustal material during ascent.
114 citations
TL;DR: In this paper, textural analysis techniques are employed to quantify key aspects of the crystal population, including crystal shape, crystal size distributions, spatial distribution patterns and textural modification using dihedral angles.
Abstract: Crystals are rarely composed of a single crystal population that have grown solely from the batch of magma in which they are resident on emplacement, either by eruption or shallow intrusion. Close investigation of the majority of crystal populations reveal that they comprise up to four main components: phenocrysts, crystals co-genetic with their magmatic host; xenocrysts, crystals wholly, or in part, foreign to the magmatic host and magma system; antecrysts, crystals which are recycled one or several times before inclusion in the host magma but have an origin within the magmatic system; and microlites, which represent small co-genetic crystals which nucleate and grow rapidly on decompression and eruption. Textural analysis techniques are employed to quantify key aspects of the crystal population, including crystal shape, crystal size distributions, spatial distribution patterns and textural modification using dihedral angles. Santorini provides a case study of an active volcanic system where a combined textural analysis study has been developed, highlighting how the crystal population is being continuously modified by a series of replenishment and mixing events. Developing textural and microgeochemical techniques provides the next stage in the interrogation of crystal populations, linking textures to isotopic heterogeneities and providing fingerprints of where crystals are sourced and re-cycled.
106 citations
TL;DR: In this article, a physical model that links the rhyolites and parental intermediate intermediate magmas to primitive olivine tholeiite by fractional crystallization is presented, which can explain rhyolite production by 77% low-pressure fractional crystalization of a basaltic trachyandesite parent magma, accompanied by minor assimilation of Archean upper crust.
Abstract: Rhyolites occur as a subordinate component of the basalt-dominated Eastern Snake River Plain volcanic field. The basalt-dominated volcanic field spatially overlaps and post-dates voluminous late Miocene to Pliocene rhyolites of the Yellowstone–Snake River Plain hotspot track. In some areas the basalt lavas are intruded, interlayered or overlain by ~15 km3 of cryptodomes, domes and flows of high-silica rhyolite. These post-hotspot rhyolites have distinctive A-type geochemical signatures including high whole-rock FeOtot/(FeOtot+MgO), high Rb/Sr, low Sr (0.5–10 ppm) and are either aphyric, or contain an anhydrous phenocryst assemblage of sodic sanidine ± plagioclase + quartz > fayalite + ferroaugite > magnetite > ilmenite + accessory zircon + apatite + chevkinite. Nd- and Sr-isotopic compositions overlap with coeval olivine tholeiites (ɛNd = −4 to −6; 87Sr/86Sri = 0.7080–0.7102) and contrast markedly with isotopically evolved Archean country rocks. In at least two cases, the rhyolite lavas occur as cogenetic parts of compositionally zoned (~55–75% SiO2) shield volcanoes. Both consist dominantly of intermediate composition lavas and have cumulative volumes of several 10’s of km3 each. They exhibit two distinct, systematic and continuous types of compositional trends: (1) At Cedar Butte (0.4 Ma) the volcanic rocks are characterized by prominent curvilinear patterns of whole-rock chemical covariation. Whole-rock compositions correlate systematically with changes in phenocryst compositions and assemblages. (2) At Unnamed Butte (1.4 Ma) the lavas are dominated by linear patterns of whole-rock chemical covariation, disequilibrium phenocryst assemblages, and magmatic enclaves. Intermediate compositions in this group resulted from variable amounts of mixing and hybridization of olivine tholeiite and rhyolite parent magmas. Interestingly, models of rhyolite genesis that involve large degrees of melting of Archean crust or previously consolidated mafic or silicic Tertiary intrusions do not produce observed ranges of Nd- and Sr-isotopes, extreme depletions in Sr-concentration, and cogenetic spectra of intermediate rock compositions for both groups. Instead, least-squares mass-balance, energy-constrained assimilation and fractional crystallization modeling, and mineral thermobarometry can explain rhyolite production by 77% low-pressure fractional crystallization of a basaltic trachyandesite parent magma (~55% SiO2), accompanied by minor (0.03–7%) assimilation of Archean upper crust. We present a physical model that links the rhyolites and parental intermediate magmas to primitive olivine tholeiite by fractional crystallization. Assimilation, recharge, mixing and fractional melting occur to limited degrees, but are not essential parts of the rhyolite formation process.
101 citations
TL;DR: In this article, the authors consider petrogenesis of the largest Holocene basaltic fissure eruptions of Iceland and present oxygen isotope laser fluorination analyses of 55 individual and bulk olivine crystals.
TL;DR: Crater Hill is a small volume alkali olivine basalt volcano in the Auckland volcanic field as mentioned in this paper, which is a pyroclastic and effusive volcano with high Mg/Fe ratios and high incompatible elements.
Abstract: Crater Hill is a small volume alkali olivine basalt volcano in the Auckland volcanic field. Crater Hill consists of a sequence of pyroclastic and effusive eruptive units of which the earliest have low silica, ferromagnesian elements and Mg/Fe ratios, high incompatible elements and are more silica undersaturated while the last material to be erupted has higher silica, ferromagnesian elements and Mg/Fe ratios but relatively low incompatible elements. Through the sequence, Mg-number changes from 59 to 67 and LaN/LuN decreases by a factor of 3. This systematic compositional variation is interpreted to be the result of clinopyroxene ± spinel fractionation at pressures of at least 1.4–1.9 GPa, from a primary magma generated by small-degree partial melting in the garnet peridotite stability field (>2.5 GPa). Fractionation occurred where early crystals grew and accumulated along the conduit walls. The rising magma evolved along a polybaric liquid line of descent until it encountered lithosphere cold enough to chill the dike margin. Above this depth, further cooling resulted only in the growth of suspended phenocrysts in a magma separated from the country rock by a chilled margin. This process is observed in the Auckland volcanic field because the rate of magma production is very small allowing compositional features to be preserved that would be overwhelmed in a larger scale magmatic system.
01 Jan 2008
TL;DR: In this paper, a steel box dredge was used to sample the interior and carapaces of the seven spines of the new lava dome of Mt. St. Helens.
Abstract: Eighteen years after dome-forming eruptions ended in 1986, and with little warning, Mount St. Helens began to erupt again in October 2004. During the ensuing two years, the volcano extruded more than 80×10 6 m 3 of gas-poor, crystal-rich dacite lava. The 2004-6 dacite is remarkably uniform in bulk-rock composition and, at 65 percent SiO 2 , among the richest in silica and most depleted in incompatible elements of the magmas erupted at Mount St. Helens during the past 500 years. Since shortly after the first spine of lava appeared, samples have been collected using a steel box dredge ("Jaws") suspended 20-35 m below a helicopter and, occasionally, by hand sampling. As of the spring of 2006, 25 age-controlled samples have been collected from the seven spines of the new lava dome. Samples were obtained from both the interiors of spines and from their carapaces, which are composed of fault gouge and cataclasite 1-2 m thick. The dacite lava is crystal rich, with 40-50 percent phenocrysts. The groundmass is extensively crystallized to a cotectic assemblage of quartz, tridymite, and Na- and K-rich feldspar microlites, raising the total crystal content to more than 80 percent on a vesicle-free basis in all but the earliest erupted.
TL;DR: In this article, an Al IV-dependent partitioning relationship was used to calculate magmatic H 2 O from direct measurements of H 2O in clinopyroxene phenocrysts.
Abstract: Water is fundamental to magma genesis, evolution, and eruption. Few direct measurements of magmatic H 2 O exist, however, because rocks found at the surface have extensively degassed upon eruption. Olivine-hosted melt inclusions provide a standard approach to measur ing volatiles in undegassed magma, but many volcanic deposits do not contain melt inclusions large enough for analysis (>30 μm), or olivine at all. Here we use an Al IV -dependent partitioning relationship to calculate magmatic H 2 O from direct measurements of H 2 O in clinopyroxene phenocrysts. We test this approach using phenocrysts from four arc volcanoes (Galunggung, Irazu, Arenal, and Augustine) that span the global range in H 2 O contents as measured in olivine-hosted melt inclusions (from 0.1 to 7 wt% H 2 O). The average and maximum magmatic H 2 O contents calculated from the clinopyroxene measurements agree within 15% of the melt inclusion values for most of the samples. The evolutionary paths recorded in H 2 O-Mg# variations overlap in some clinopyroxene and olivine-hosted melt inclusion populations, and in others, the clinopyroxenes record a larger portion of the liquid line of descent or a different portion of the magma system. Thus, the use of phenocrysts to estimate magmatic H 2 O contents creates a new and powerful tool in igneous petrology and volcanology.
TL;DR: This paper analyzed glass and mineral compositions of anorthoclase phonolite lava from the Erebus volcano in Antarctica and found that the composition of the lake was invariant to minor variations in eruptive activity from 1972 to 2004.
TL;DR: In this paper, 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.
Abstract: [1] 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.
01 Jan 2008
TL;DR: In this article, major element, trace element, and volatile concentrations in 187 glassy melt inclusions and 25 groundmass glasses from the 1980-86 eruption of Mount St. Helens are presented, together with 103 analyses of touching Fe-Ti oxide pairs from the same samples.
Abstract: Major element, trace element, and volatile concentrations in 187 glassy melt inclusions and 25 groundmass glasses from the 1980-86 eruption of Mount St. Helens are presented, together with 103 analyses of touching Fe-Ti oxide pairs from the same samples. These data are used to evaluate the temporal evolution of the magmatic plumbing system beneath the volcano during 1980-86 and so provide a framework in which to interpret analyses of melt inclusions from the current (2004-2006) eruption. Major and trace element concentrations of all melt inclusions lie at the high-SiO 2 end of the data array defined by eruptive products of late Quaternary age from Mount St. Helens. For several major and trace elements, the glasses define a trend that is oblique to the whole-rock trend, indicating that different mineral assemblages were responsible for the two trends. The whole-rock trend can be ascribed to differentiation of hydrous basaltic parents in a deep-seated magma reservoir, probably at depths great enough to stabilize garnet. In contrast, the glass trends were generated by closed-system crystallization of the phenocryst and microlite mineral assemblages at low pressures. The dissolved H 2 O content of the melt inclusions from 1980-86, as measured by ion microprobe, ranges from 0 to 6.7 wt. percent, with the highest values obtained from the plinian phase of May 18, 1980. Water contents decrease with increasing SiO 2 , consistent with decompression-driven crystallization. Preliminary data for dissolved CO 2 in melt inclusions from the May 18 plinian phase and from August 7, 1980, indicate that X H2O in the vapor phase was approximately constant at 0.80, irrespective of H 2 O content, suggestive of closed-system degassing with a high bubble fraction or gas streaming through the subvolcanic system. Temperature and f O2 estimates for touching Fe-Ti oxides show evidence for heating during crystallization owing to release of latent heat. Consequently, magmas with the highest microlite crystallinities record the highest temperatures. Magmas also become progressively reduced during ascent and degassing, probably as a result of redox equilibria between exsolving S-bearing gases and magmas. The lowest temperature oxides have f O2 ≈ NNO, similar to high-temperature fumarole gases from the volcano. The temperature and f O2 of the magma tapped by the plinian phase of May 18, 1980, are 870-875°C and NNO+0.8, respectively. The dissolved volatile contents of the melt inclusions have been used to calculate sealing pressures; that is, the pressure at which chemical exchange between inclusion and matrix melt ceased. These are greatest for the May 18 plinian magma (120 to 320 MPa); lower pressures are recorded by samples of the preplinian cryptodome and by all post-May 18 magmas. Magma crystallinity, calculated from melt-inclusion Rb contents, is negatively correlated with sealing pressure, consistent with decompression crystallization. Elevated contents of Li in melt inclusions from the cryptodome and post-May 18 samples are consistent with transfer of Li in a magmatic vapor phase from deeper parts of the magma system to magma stored at shallower levels. The Li enrichment attains its maximum extent at ~150 MPa, which is ascribed to separation of a single vapor phase into H 2 O-rich gas and dense Li-rich brine at the top of the magma column. There are striking correlations between melt-inclusion chemistry and monitoring data for the 1980-86 eruption.
TL;DR: Compositional and isotopic zoning patterns in plagioclase and amphibole phenocrysts from El Chichon eruptions reveal multiple cycles of country rock assimilation, magma injection, hybridization, and mixing.
TL;DR: The authors measured growth zoning in apatite phenocrysts from the Cerro Galan ignimbrite to study the volatile history of a magma chamber in the days to months prior to a supereruption.
Abstract: Measurements of OH and Cl growth zoning in apatite phenocrysts from the Cerro Galan ignimbrite describe the tumultuous volatile history of a magma chamber in the days to months prior to a supereruption. The chemical stratigraphy in these crystals provides a unique relative time scale not otherwise available when studying pre-eruptive volatiles in ancient magma chambers, and it allows us to temporally correlate the data within each crystal, resulting in a “volatile history.” The five crystals studied show broadly similar volatile histories, but distinctive differences between the various crystals suggest that each recorded a specific perspective on the magma chamber prior to eruption, perhaps correlating to different depths or laterally separated regions within the >1000 km 3 magma chamber. All crystals are consistent with the interpretation of a multistage magmatic history, including a period of residence in a quiescent magma, and a period of rapid growth under strongly varying conditions. Several apparent “events” during this period of growth, consistent with two competing processes of H 2 O degassing and recharge, are recorded by nearly all of these crystals. Such zoning could not have survived for more than ∼400 d prior to eruption, suggesting that during the days to months leading up to a supereruption, apatite records key aspects of the processes that directly lead to and influence these catastrophic events.
TL;DR: The Thjorsa lava in South Iceland is an early Holocene fissure lava within the Bardarbunga volcanic system as mentioned in this paper, and the groundmass was analyzed for major and trace elements and isotopic ratios of Sr, Nd and Pb.
TL;DR: In this article, a nearly 100m-thick series of high-Mg lava flows is found in a Devonian arc in the Junggar terrane, and they are classified into two classes: picrites and ankaramites.
TL;DR: In this paper, the growth history of zoned K-feldspar phenocrysts in microgranular magmatic enclaves in the Karkonosze granite (SW Poland) is investigated using cathodoluminescence and profiling by Electron Microprobe and Laser Ablation ICP-MS.
TL;DR: The results of as discussed by the authors highlight the integrated use of crystal size distribution, textural, and in situ chemical data in identifying distinct crystal populations and linking these populations to the thermal and chemical characteristics of complex magma bodies.
Abstract: Products of the 1915 Lassen Peak eruption reveal evidence for a magma recharge^magma mixing event that may have catalyzed the eruption and from which four compositional members were identified: light dacite, black dacite, andesitic inclusion, and dark andesite. Crystal size distribution, textural, and in situ chemical (major and trace element and Sr isotope) data for plagioclase from these compositional products define three crystal populations that have distinct origins: phenocrysts (long axis40!5 mm) that typically have core An contents between 34 and 36 mol %, microphenocrysts (long axis between 0!1 and 0!5 mm) that have core An contents of 66^69, and microlites (long axis50!1mm) with variable An core contents from 64 to 52. Phenocrysts are interpreted to form in an isolated dacitic magma chamber that experienced slow cooling. Based on textural, compositional, and isotopic data for the magma represented by the dacitic component, magma recharge was not an important process until just prior to the 1915 eruption. Average residence times for phenocrysts are in the range of centuries to millennia. Microphenocrysts formed in a hybrid layer that resulted from mixing between endmember reservoir dacite and recharge magma of basaltic andesite composition. High thermal contrast between the two end-member magmas led to relatively high degrees of undercooling, which resulted in faster crystal growth rates and acicular and swallowtail crystal habits. Some plagioclase phenocrysts from the dacitic chamber were incorporated into the hybrid layer and underwent dissolution^precipitation, seen in both crystal textures and rim compositions. Average microphenocryst residence times are of the order of months. Microlites may have formed in response to decompression and/or syn-eruptive degassing as magma ascended from the chamber through the volcanic conduit. Chemical distinctions in plagioclase microlite An contents reveal that melt of the dark andesite was more mafic than the melt of the other three compositions.We suggest that mixing of an intruding basaltic andesite and reservoir dacite before magma began ascending in the conduit allowed formation of a compositionally distinct microlite population. Melt in the other three products was more evolved because it had undergone differentiation during the months following initial mixing; as a consequence, melt and microlites among these three products have similar compositions.The results of this study highlight the integrated use of crystal size distribution, textural, and in situ chemical data in identifying distinct crystal populations and linking these populations to the thermal and chemical characteristics of complex magma bodies.
TL;DR: In this article, the authors proposed several strategies to account for the presence of radiogenic 4He in olivine and pyroxene phenocrysts and provided a simple expression for a correction factor (R) that can be applied in this case.
TL;DR: In this paper, the authors focused on lavas and tephras of the last two regional eruptions in 871 and 1477 AD, using samples that approximate mafic and felsic endmember compositions relative to regional mixing trends in literature data from the area.
TL;DR: In this paper, two distinctive differentiation trends, tholeiitic and calc-alkalic, are recognized in Zao volcano, which is located immediately behind the volcanic front of the NE Japan arc.
Abstract: Two distinctive differentiation trends, tholeiitic and calc-alkalic, are recognized in Zao volcano, which is located immediately behind the volcanic front of the NE Japan arc. The genetic relationship between these two magma series is critical for a better understanding of andesite genesis, because they often coexist in close spatial and temporal proximity in arc volcanoes. Petrographic features indicative of ‘disequilibrium’ , such as reversely zoned pyroxene phenocrysts, the wide and bimodal compositional distribution in Ca/(CaþNa) of plagioclase phenocrysts, honeycomb textures and dusty zones that these plagioclase phenocrysts often exhibit, and the presence of olivine^pyroxene pairs with different Mg/Fe, are observed exclusively in calc-alkalic rocks. In tholeiitic rocks the Sr isotopic ratios of plagioclase phenocrysts, determined by both micromilling combined with thermal ionization mass spectrometry, and laser-ablation inductively coupled plasma mass spectrometry techniques, are constant at 0 7042^0 7044. On the other hand, those in calc-alkalic rocks (0 7033^0 7042) show more complex characteristics, which can be best understood if at least three end-member components, a calc-alkalic basaltic melt, a tholeiitic basaltic melt and a tholeiitic felsic melt, contribute to the production of mixed calc-alkalic magmas.The Sr/Sr and trace element compositions of the leastdifferentiated basalt magmas, which are inferred from the composition of the calcic plagioclase [Ca/(CaþNa)40 9], suggest that two types of basaltic magma, calc-alkalic and tholeiitic, exist beneath the volcano. The tholeiitic basalt magma has a higher Sr/Sr than the calc-alkalic magma (0 7042 vs 0 7038) and a characteristic trace element signature consistent with the presence of plagioclase and amphibole as melting residues.This suggests that the tholeiitic magmas are produced via anatexis of amphibolitic crust caused by underplating and/or intrusion of mantle-derived calcalkalic basalt magmas into the sub-Zao crust. The mantle-derived calc-alkalic basalt magma mixes with crust-derived tholeiitic melts to form calc-alkalic andesite magmas.The hypothesis proposed here requires revision (or even abandonment) of the general consensus that calc-alkalic magmas have greater contributions of a crustal component than tholeiitic magmas.
01 Jan 2008
TL;DR: The 2004-6 eruption of Mount St. Helens produced dacite that contains 40-50 volume percent phenocrysts of plagioclase, amphibole, low-Ca pyroxene, magnetite, and ilmenite in a groundmass that is nearly totally crystallized.
Abstract: The 2004-6 eruption of Mount St. Helens produced dacite that contains 40-50 volume percent phenocrysts of plagioclase, amphibole, low-Ca pyroxene, magnetite, and ilmenite in a groundmass that is nearly totally crystallized. Phenocrysts of amphibole and pyroxene range from 3 to 5 mm long and are cyclically zoned, with one to three alternations of Fe- and Al-rich to Mg- and Si-rich layers showing little indication of phenocryst dissolution between zones. Similar-size plagioclase phenocrysts also contain several cyclic zones ranging between ~An 68 and An 45-35 . Textural evidence indicates that amphibole, pyroxene, and ilmenite began to crystallize before the most An-rich plagioclase. Magnetite and ilmenite phenocrysts are small (less than 100 μm), vary somewhat in composition from grain to grain, and are sporadically zoned. Magnetite-ilmenite pairs yield temperatures of equilibration ranging from 820°C to 890°C and f O2 values of NNO +1 log unit. Magnetite compositions suggest that the 2004-6 magma was formed by mingling of magmas less than 5-8 weeks before eruption and that the magma last equilibrated within this temperature range. The amphibole phenocryst zoning involves approximately equal amounts of a pressure-sensitive Al-Tschermak molecular substitution and a temperature-sensitive edenite substitution in one cycle of growth. Hydrothermal experiments done on the natural dacite show that crystallization of the Fe- and Al-rich amphibole end member requires pressures of 200-300 MPa at temperatures of 900°C, conditions approaching the upper temperature limit of amphibole stability. The dacitic magma crystallizes the An 68 plagioclase when the pressure drops to 200 MPa at 900°C. The magma must cool at this depth to produce a complete An 68 -An 40 plagioclase zone and a Mg-rich layer on the amphiboles before the magma is cycled back to a high pressure, when a new layer of Fe-rich amphibole is acquired. The amphibole crystallizing in the dacite experiments at less than 200 MPa is lower in aluminum than any compositions in the natural cyclically zoned phenocrysts. The outer rim on some 2004-6 amphibole phenocrysts appears to have formed in the 100-200 MPa range, as do some phenocrysts in the May 1980 dacite pumice. Plagioclase rims of An 35 in the 2004-6 magmas indicate that phenocryst growth continued until the pressure decreased to 130 MPa and that ascent was slow until this depth. Magma then entered the conduit for a relatively rapid ascent to the surface as indicated by the very thin (less than 5 μm) decompression-induced rims on the amphibole phenocrysts.
TL;DR: In this paper, phase equilibrium experiments were performed to determine the pre-eruptive conditions of the largest caldera-forming eruption (∼20km 3 of DRE) that has occurred on Tenerife (Canary Islands).
TL;DR: The 3He/4He of Pantelleria, Etna, Iblei, Ustica, Alicudi, and Filicudi basalts (7.0 ± 0.6 Ra) are consistent with a HIMU-type mantle that either is younger than the Cook-Austral island end-member or has a lower 238U/204Pb as mentioned in this paper.
Abstract: [1] Helium isotope ratios of olivine and pyroxene phenocrysts from Plio-Quaternary volcanic rocks from southern Italy (seven Aeolian Islands, Mt. Vulture, Etna, Ustica, and Pantelleria) range from 2.3 to 7.1 Ra. Importantly, the phenocryst 3He/4He correlate well with whole rock Sr isotopic composition (0.70309–0.70711), reflecting the mixing of two sources. A significant contribution of He from crustal contamination is recorded only occasionally (e.g., pyroxenes from Vulcano). When merged with data from the Roman Comagmatic Province, a remarkably strong near-linear He-Sr isotope correlation is apparent. The general northward decrease in 3He/4He corresponds to an increase in 87Sr/86Sr (and a decrease in 143Nd/144Nd and 206Pb/204Pb) that is due to increasing metasomatic enrichment of the mantle wedge via subduction of the Ionian-Adriatic plate. Calculations based on the ingrowth of 4He in the wedge and on the 4He content of the subducting crust show that mechanisms of enrichment in radiogenic He are effective only if the wedge is strongly depleted in He relative to best estimates of the depleted mantle. This can be accommodated if the process of metasomatism by the subduction fluids depletes the mantle wedge. The 3He/4He of Pantelleria, Etna, Iblei, Ustica, Alicudi, and Filicudi basalts (7.0 ± 0.6 Ra) define the mantle composition least affected by subduction-related metasomatism. Although these volcanoes are from a variety of tectonic regimes (subduction-related, intraplate, rifting), their similarities suggest a common origin of geochemical features. Their characteristics are consistent with a HIMU-type mantle that either is younger than the Cook-Austral island end-member or has a lower 238U/204Pb.