Showing papers on "Phenocryst published in 2012"

High Sr/Y Magmas Reflect Arc Maturity, High Magmatic Water Content, and Porphyry Cu ± Mo ± Au Potential: Examples from the Tethyan Arcs of Central and Eastern Iran and Western Pakistan

Abstract: The temporal and geochemical evolution of arc magmatism that culminated in porphyry Cu ± Mo ± Au deposit formation has been studied in three separate Neo-Tethyan arc systems in central and eastern Iran, and western Pakistan. Porphyry Cu-Au deposits in the Lut block of eastern Iran formed in the middle Eocene at the end of a period of extensive Paleocene-Eocene volcanism; porphyry Cu-Mo deposits in the Kerman belt of central Iran formed in the middle Miocene at the end of a period of voluminous Eocene-Oligocene volcanism; and porphyry Cu-Au deposits in the Chagai belt of western Pakistan formed in four pulses during the Eocene, early Miocene, middle-late Miocene, and late Miocene-Pliocene, after a prolonged period of arc magmatism that began in the Late Cretaceous (and is still active). In each region, the late porphyry-related magmas are more geochemically evolved and more hydrous (as evidenced by the presence of hornblende phenocrysts) than the preceding volcanic rocks. We suggest that this reflects maturation of the arc magmatic system over a period of tens of millions of years, leading to the generation of more evolved, volatile-rich magmas at later stages of the arc’s history. High magmatic water contents are a prerequisite for the shallow crustal emplacement of arc magmas and the subsequent generation of potentially ore-forming subvolcanic magmatic-hydrothermal systems. It is thus suggested that the fertility of arc magmas within a given arc terrane can be assessed by observing the relative timing of plutonic suites (later suites are more prospective), noting the common presence of hornblende or biotite phenocrysts (indicating high magmatic water contents), and through lithogeochemical fingerprinting of magmatic fractionation processes (relatively high Sr/Y and La/Yb ratios, and Eun/Eu* ratios ≈1, indicating abundant early hornblende fractionation and suppression of plagioclase crystallization in hydrous magmas).

Picrites from the Emeishan Large Igneous Province, SW China: a Compositional Continuum in Primitive Magmas and their Respective Mantle Sources

Abstract: Flood basalts are one of the remaining enigmas in global mantle petrology. They come in enormous quantities of up to 10. 6 km. 3 of mantle-derived melt, and they erupt in rather short time intervals of only a few million years. Throughout geological history, all continents have been periodically flooded by dominantly basaltic and rare picritic magmas that can differ widely within the same province in terms of their major element (e.g. high- and low-Ti series), trace element, and radiogenic isotope compositions, suggesting significant compositional heterogeneity within the mantle source regions tapped. In this study of the Late Permian Emeishan large igneous province (ELIP) in SW China picrite lavas from thick stratigraphic successions at Binchuan and Yongsheng represent the low-Ti and high-Ti 'classic' end-members of continental flood basalt magmatism, respectively. This study focuses on the petrochemical variability of the Emeishan magmas, and the genetic links between the suites and their respective mantle sources, based upon estimates of the chemical compositions of the primary melts represented by homogenized melt inclusions hosted by exceptionally primitive olivine (up to 92 mol % Fo in both suites) and Cr-spinel (Cr# 64-72 mol % in Binchuan and 65-80 mol % in Yongsheng) phenocrysts. The average compositions of the melt inclusions and their overall chemical variability, together with the presence of picrites in the province (e.g. Lijiang and Dali localities) with compositions intermediate between the low- and high-Ti end-members, suggest that numerous parental magma batches contributed to a diverse spectrum of more differentiated basaltic magmas within the ELIP. The end-member and intermediate magma compositions are confirmed by the compositions of phenocrysts (Ni and Mn abundances in olivine, Ti abundances in Cr-spinel and clinopyroxene, and trace element abundances in clinopyroxene). The end-member melt and phenocryst compositions (e.g. Gd/Yb in bulk-rocks, melt inclusions and clinopyroxene and Ni-Mn systematics in olivine) suggest a peridotite and garnet pyroxenite mantle source for the low- and high-Ti end-members, respectively. The Sr and Nd isotopic compositions of the two end-member magmas are similar [. 87Sr/. 86Sr. i ∼0·7045; e{open}Nd(t) ∼ +1·7] and are considered to reflect a source in the subcontinental lithospheric mantle rather than the convective asthenosphere or a deep mantle 'plume'. © The Author 2012. Published by Oxford University Press. All rights reserved.

The Crustal Magma Storage System of Volcán Quizapu, Chile, and the Effects of Magma Mixing on Magma Diversity

Abstract: Crystal zoning as well as temperature and pressure estimates from phenocryst phase equilibria are used to constrain the architecture of the intermediate-sized magmatic system (some tens of km 3 ) of Volcan Quizapu, Chile, and to document the textural and compositional effects of magma mixing. In contrast to most arc magma systems, where multiple episodes of open-system behavior obscure the evidence of major magma chamber events (e.g. melt extraction, magma mixing), the Quizapu magma system shows limited petrographic complexity in two large historical eruptions (1846–1847 and 1932) that have contrasting eruptive styles. Quizapu magmas and peripheral mafic magmas exhibit a simple binary mixing relationship. At the mafic end, basaltic andesite to andesite recharge magmas complement the record from peripheral cones and show the same limited range of compositions. The silicic end-member composition is almost identical in both eruptions of Quizapu. The effusive 1846–1847 eruption records significant mixing between the mafic and silicic end-members, resulting in hybridized andesites and mingled dacites. These two compositionally simple eruptions at Volcan Quizapu present a rare opportunity to isolate particular aspects of magma evolution—formation of homogeneous dacite magma and late-stage magma mixing—from other magma chamber processes. Crystal zoning, trace element compositions, and crystal-size distributions provide evidence for spatial separation of the mafic and silicic magmas. Dacite-derived plagioclase phenocrysts (i.e. An 25 – 40 ) show a narrow range in composition and limited zonation, suggesting growth from a compositionally restricted melt. Dacite-derived amphibole phenocrysts show similar restricted compositions and furthermore constrain, together with more mafic amphibole phenocrysts, the architecture of the magmatic system at Volcan Quizapu to be compositionally and thermally zoned, in which an andesitic mush is overlain by a homogeneous dacitic magma that is the source for most of the 1846–1847 and 1932 erupted magmas. Dacite formation is best explained by mineral–melt separation (crystal fractionation) from an andesitic mush, which is inferred to have thermally and compositionally buffered the dacite magma thereby keeping it at relatively low crystallinity (<30 vol. %). The dominant cause of compositional diversity is melt separation. Back-mixing of mush (i.e. crystals with signatures of growth both in the andesitic mush and in the dacite magma) into the overlying dacite magma is rarely observed. Recharge events that increase crystal and magma diversity in the dacite magma are limited to an episode of mafic recharge and mixing just prior to the 1846–1847 eruption, where evidence for magma mixing is present on all scales. Chamber-wide mixing was incomplete (mixing efficiency of ∼0·53–0·85) as flow lobes vary significantly in composition along the proposed mixing array. Estimates of viscosity variations during the course of magma mixing suggest that mixing dynamics and the degree of magma interaction on all scales were established at the beginning of the recharge event.

Peralkaline magma evolution and the tephra record in the Ethiopian Rift

Abstract: The 3.119 ± 0.010 Ma Chefe Donsa phreatomagmatic deposits on the shoulder of the Ethiopian Rift mark the northern termination of the Silti-Debre Zeyit Fault Zone, a linear zone of focused extension within the modern Ethiopian Rift. These peralkaline pumice fragments and glass shards span a wide range of glass compositions but have a restricted phenocryst assemblage dominated by unzoned sanidine. Glass shards found within the ash occupy a far more limited compositional range (75–76 wt% SiO2) in comparison with the pumice (64–75 wt% SiO2), which is rarely mingled. Thermodynamic modeling shows that liquids broadly similar to the least evolved glass composition can be achieved with 50–60 % fractionation of moderately crustally contaminated basalt. Inconsistencies between modeled solutions and the observed values of CaO and P2O5 highlight the important role of fluorine in stabilizing fluor-apatite and the limitations of current thermodynamic models largely resulting from the scarce experimental data available for the role of fluorine in igneous phase stability. On the basis of limited feldspar heterogeneity and crystal content of pumice at Chefe Donsa, and the difficulties of extracting small volumes of Si-rich melt in classical fractional crystallization models, we suggest a two-step polybaric process: (1) basaltic magma ponds at mid-upper-crustal depths and fractionates to form a crystal/magma mush. Once this mush has reached 50–60 % crystallinity, the interstitial liquid may be extracted from the rigid crystal framework. The trachytic magma extracted at this step is equivalent to the most primitive pumice analyzed at Chefe Donsa. (2) The extracted trachytic liquid will rise and continue to crystallize, generating a second mush zone from which rhyolite liquids may be extracted. Some of the compositional range observed in the Chefe Donsa deposits may result from the fresh intrusion of trachyte magma, which may also provide an eruption trigger. This model may have wider application in understanding the origin of the Daly Gap in Ethiopian magmas—intermediate liquids may not be extracted from crystal-liquid mushes due to insufficient crystallization to yield a rigid framework. The wide range of glass compositions characteristic of the proximal Chefe Donsa deposits is not recorded in temporally equivalent tephra deposits located in regional depocenters. Our results show that glass shards, which represent the material most likely transported to distal depocenters, occupy a limited compositional range at high SiO2 values and overlap some distal tephra deposits. These results suggest that distal tephra deposits may not faithfully record the potentially wide range in magma compositions present in a magmatic system just prior to eruption and that robust distal–proximal tephra correlations must include a careful analysis of the full range of materials in the proximal deposit.

Evidence of crystallization in residual, Cl–F-rich, agpaitic, trachyphonolitic magmas and primitive Mg-rich basalt–trachyphonolite interaction in the lava domes of the Phlegrean Fields (Italy)

Abstract: The lava domes in the northwestern (Cuma), northern (Punta Marmolite) and central (Accademia) parts of the Phlegrean Fields are the subject of this study. The Cuma and Punta Marmolite trachyphonolitic lava domes are among the oldest Phlegrean products cropping out. The Cuma rocks have an agpaitic groundmass, with early alkali feldspar, Fe-rich clinopyroxene, Fe-edenite and sodalite and late rosenbuschite, fluorite, baddeleyite, pyrochlore, britholite, monazite, aegirine (often Zr-rich) and exceptionally Fe–Mn-rich olivine. The bulk-rock compositions at Cuma have some of the highest concentrations of Zn, Mn, Zr, Nb, Th, U and lanthanides among the Phlegrean Fields rocks, and some of the lowest MgO, P2O5, Sr, Eu and Ba. The Punta Marmolite dome is chemically less evolved, and lacks characteristic agpaitic minerals, but features alkali feldspar, sodalite, nepheline and relatively Na-poor, Fe-rich hedenbergite, with rare Ca-rich plagioclase xenocryst cores. The Accademia dome, belonging to the recent activity, is latitic to trachytic in composition, has highly forsteritic olivine (with chromiferous spinel inclusions), calcic plagioclase and Mg-rich diopside (± phlogopite) xenocrysts in an evolved host rock (with phenocrysts and microlites of alkali feldspar, Fe-rich clinopyroxene, Fe-rich amphibole, magnetite, Fe-rich olivine and accessory baddeleyite, zirconolite and fluorite). There is clear evidence of open-system magma crystallization in the form of interaction between a crystallizing, primitive shoshonitic basalt in a reservoir already filled by rather evolved trachytic magma. The magmatic evolution towards the evolved compositions is dominated by crystallization of more and more Na-rich alkali feldspar in a Cl-, F-rich and relatively H2O-poor environment. Input of mafic magma is evident in many trachytic eruptions of the Phlegrean Fields and even in the products of the Campanian Ignimbrite, but eruptions having mineral assemblages rich in xenocryst phases as well as eruptions virtually free of mafic magma input are also frequently observed throughout the history. This suggests a variable pattern of open- and closed-system crystallization, which may or may not be linked to explosive activity, and that can be caused by intermittent supply of basaltic magma from depth.

Highly Oxidized Magma and Fluid Evolution of Miocene Qulong Giant Porphyry Cu‐Mo Deposit, Southern Tibet, China

Abstract: The Miocene Qulong porphyry Cu-Mo deposit, which is located at the Gangdese orogenic belt of Southern Tibet, is the largest porphyry-type deposit in China, with confirmed Cu ∼10 Mt and Mo ∼0.5 Mt. It is spatially and temporally associated with multiphase granitic intrusions, which is accompanied by large-scale hydrothermal alteration and mineralization zones, including abundant hydrothermal anhydrite. In addition to hydrothermal anhydrite, magmatic anhydrite is present as inclusions in plagioclase, interstitial minerals between plagioclase and quartz, and phenocrysts in unaltered granodiorite porphyry, usually in association with clusters of sulfur-rich apatite in the Qulong deposit. These observations indicate that the Qulong magma-hydrothermal system was highly oxidized and sulfur-rich. Three main types of fluid inclusions are observed in the quartz phenocrysts and veins in the porphyry: (i) liquid-rich; (ii) polyphase high-salinity; and (iii) vapor-rich inclusions. Homogenization temperatures and salinities of all type inclusions decrease from the quartz phenocrysts in the porphyry to hydrothermal veins (A, B, D veins). Microthermometric study suggests copper-bearing sulfides precipitated at about 320–400°C in A and B veins. Fluid boiling is assumed for the early stage of mineralization, and these fluids may have been trapped at about 35–60 Mpa at 460–510°C and 28–42 Mpa at 400–450°C, corresponding to trapping depths of 1.4–2.4 km and 1.1–1.7 km, respectively.

Crystal scale anatomy of a dying supervolcano: an isotope and geochronology study of individual phenocrysts from voluminous rhyolites of the Yellowstone caldera

Abstract: A voluminous (>600 km3) and long-lived (~520–75 ka) phase of rhyolitic eruptions followed collapse of the Yellowstone caldera 640 ka. Whether these eruptions represent a dying cycle, or the growth of a new magma chamber, remains an important question. We use new U–Th zircon ages and δ18O values determined by ion microprobe, and sanidine Pb isotope ratios determined by laser ablation, to investigate the genesis of voluminous post-caldera rhyolites. The oldest post-caldera rhyolites, erupted between ~520 and 470 ka, exhibit extreme age and oxygen isotopic heterogeneity, requiring derivation from individual parcels of low-δ18O melts. We find a progressive increase in zircon homogeneity for rhyolite eruptions from ~260 to 75 ka, with homogeneous low-δ18O zircon values of 2.7–2.8‰ that are in equilibrium with low-δ18O host melts for the majority of the youngest eruptions. New sanidine Pb isotope data define separate arrays for post-caldera rhyolites and the caldera-forming tuffs that preceded them, indicating that they were not sourced from a mushy Lava Creek Tuff batholith that remained after caldera collapse. Rather, our new age and isotopic data indicate that the post-caldera rhyolites were generated by remelting of a variety of intracaldera source rocks, consisting of pre-Lava Creek Tuff volcanic and plutonic rocks and earlier erupted post-Lava Creek Tuff rhyolites. Batch assembly of low-δ18O melts starting at ~260 ka resulted in progressive homogenization, followed by differentiation and cooling up until the last rhyolite eruption ~75 ka, a trend that we interpret to be characteristic of a dying magma reservoir beneath the Yellowstone caldera.

The Bishop Tuff giant magma body: An alternative to the Standard Model

Abstract: The Bishop Tuff, one of the most extensively studied high-silica rhyolite bodies in the world, is usually considered as the archetypical example of a deposit formed from a magma body characterized by thermal and compositional vertical stratification—what we call the Standard Model for the Bishop magma body. We present here new geothermometry and geobarometry results derived using a large database of previously published quartz-hosted glass inclusion compositions. Assuming equilibrium between melt and an assemblage composed of quartz, ±plagioclase, ±sanidine, +zircon, ±fluid, we use Zr contents in glass inclusions to derive quartz crystallization temperatures, and we use (1) silica contents in glass, (2) projection of glass compositions onto the haplogranitic (quartz-albite-orthoclase) ternary, and (3) phase equilibria calculations using rhyolite-MELTS, to constrain crystallization pressures. We find crystallization temperatures of ~740–750 °C for all inclusions from both early- and late-erupted pumice. Crystallization pressures for both early- and late-erupted inclusions are also very similar to each other, with averages of ~175–200 MPa. We find no evidence of late-erupted inclusions having been entrapped at higher temperatures or pressures than early-erupted inclusions, as would be expected by the Standard Model. We argue that the thermal gradient inferred from Fe–Ti oxides—the backbone of the Standard Model—does not reflect equilibrium pre-eruptive conditions; we also note that H2O–CO2 systematics of glass inclusions yields overlapping pressure ranges for early- and late-erupted inclusions, similar to the results presented here; and we show that glass inclusion and phenocryst compositions show bimodal distributions, suggestive of compositional separation between early- and late-erupted populations. These findings are inconsistent with the Standard Model. The similarity in crystallization conditions and the compositional separation between early- and late-erupted magmas suggest that two laterally juxtaposed independent magma reservoirs existed in the same region at the same time and co-erupted to form the Long Valley Caldera and the Bishop Tuff. This hypothesis would explain the lack of mixing between early- and late-erupted crystal populations in pumice clasts; it could also explain the inferred eruption pattern—which resulted in early-erupted magmas being deposited only to the south of the caldera—if the early-erupted magma body resided to the south and the late-erupted magma body was located to the north. Our alternative model is consistent with the patchy distribution of thermal anomalies and the inference of co-eruption of distinct magma types in active volcanic areas such as the central Taupo Volcanic Zone.

Chambers: Evidence from the AD 1783 Laki Eruption

Abstract: High-precision compositional analyses of 54 whole-rock samples from the AD 1783 Laki lava flow field show small but statistically significant variations in trace and major element concentrations and ratios. Strong linear correlations exist between major and trace element concentrations, and variations in incompatible element ratios, such as Zr/Y, are modest. Point-counting results indicate that the lava contains an average of 12 vol. % phenocrysts, with plagioclase, clinopyroxene and olivine present in relative volumetric proportions of 57:32:11. Whole-rock compositions vary linearly with the total mass fraction of phenocrysts in the samples, such that samples with the lowest concentrations of incompatible trace elements have the highest proportion of phenocrysts. On first inspection, such correlations might be interpreted to arise from variable crystal accumulation into the carrier liquid within the magma. However, simple models of crystal accumulation fail to match the relationships between whole-rock composition and phenocryst content. Instead, the phenocrysts must have formed the solid part of a magmatic mush, with the mush liquid being more evolved than the carrier liquid. This mush was entrained into the carrier liquid prior to eruption, with incomplete mixing of the mush into the carrier liquid allowing for the preservation of whole-rock compositional variation. A mathematical description of the mass balance involved in mush mixing is developed to constrain the properties of the mush. Although there is a trade-off between estimates of mush liquid composition and mush porosity, independent constraints on mush liquid composition from phenocryst compositions are used to estimate an average mush porosity of 46^65%. The success of the binary mixing fits to whole-rock compositions indicates that the mean compositions of the mush and the carrier liquid cannot have changed substantially during the eruption. However, more detailed observations reveal that on average the mush proportion was higher during the later stages of the eruption, and this coincides with the presence of primitive high Mg# olivine and clinopyroxene and anorthitic plagioclase primocrysts in the mush.The key observations cannot be accounted for by a model of in situ evolution of mush liquid in the cooling margins of a magma chamber. Instead, the juxtaposition of evolved mush liquid with primitive phenocrysts that is required to generate the mush may perhaps occur as a result of compositional convection at the chamber roof, or alternatively by the partitioning of phenocrysts into more viscous magma during the mixing of primitive basalt and evolved melt in the chamber. It is likely that many porphyritic basaltic eruptions carry disaggregated mush and it is straightforward to apply the methods described in this study to other eruptions, allowing for future improvements in the characterization of the properties of mushes in basaltic magma chambers.

Mush Disaggregation in Basaltic Magma Chambers: Evidence from the AD 1783 Laki Eruption

Abstract: High-precision compositional analyses of 54 whole-rock samples from the ad 1783 Laki lava flow field show small but statistically significant variations in trace and major element concentrations and ratios. Strong linear correlations exist between major and trace element concentrations, and variations in incompatible element ratios, such as Zr/Y, are modest. Point-counting results indicate that the lava contains an average of 12 vol. % phenocrysts, with plagioclase, clinopyroxene and olivine present in relative volumetric proportions of 57:32:11. Whole-rock compositions vary linearly with the total mass fraction of phenocrysts in the samples, such that samples with the lowest concentrations of incompatible trace elements have the highest proportion of phenocrysts. On first inspection, such correlations might be interpreted to arise from variable crystal accumulation into the carrier liquid within the magma. However, simple models of crystal accumulation fail to match the relationships between whole-rock composition and phenocryst content. Instead, the phenocrysts must have formed the solid part of a magmatic mush, with the mush liquid being more evolved than the carrier liquid. This mush was entrained into the carrier liquid prior to eruption, with incomplete mixing of the mush into the carrier liquid allowing for the preservation of whole-rock compositional variation. A mathematical description of the mass balance involved in mush mixing is developed to constrain the properties of the mush. Although there is a trade-off between estimates of mush liquid composition and mush porosity, independent constraints on mush liquid composition from phenocryst compositions are used to estimate an average mush porosity of 46–65%. The success of the binary mixing fits to whole-rock compositions indicates that the mean compositions of the mush and the carrier liquid cannot have changed substantially during the eruption. However, more detailed observations reveal that on average the mush proportion was higher during the later stages of the eruption, and this coincides with the presence of primitive high Mg# olivine and clinopyroxene and anorthitic plagioclase primocrysts in the mush. The key observations cannot be accounted for by a model of in situ evolution of mush liquid in the cooling margins of a magma chamber. Instead, the juxtaposition of evolved mush liquid with primitive phenocrysts that is required to generate the mush may perhaps occur as a result of compositional convection at the chamber roof, or alternatively by the partitioning of phenocrysts into more viscous magma during the mixing of primitive basalt and evolved melt in the chamber. It is likely that many porphyritic basaltic eruptions carry disaggregated mush and it is straightforward to apply the methods described in this study to other eruptions, allowing for future improvements in the characterization of the properties of mushes in basaltic magma chambers.

Magma storage conditions beneath Dabbahu Volcano (Ethiopia) constrained by petrology, seismicity and satellite geodesy

Abstract: A variety of methods exist to constrain sub-volcanic storage conditions of magmas. Petrological, seismological and satellite geodetic methods are integrated to determine storage conditions of peralkaline magmas beneath Dabbahu Volcano, Afar, Ethiopia. Secondary ion mass spectrometry (SIMS) analysis of volatile contents in melt inclusions trapped within phenocrysts of alkali feldspar, clinopyroxene and olivine from pantellerite obsidians representing the youngest eruptive phase (<8 ka) show H2O contents ≤5.8 wt.% and CO2 contents generally below 500 ppm, although rarely as high as 1,500 ppm. Volatile saturation pressures (at 679–835°C) are in the range 43–207 MPa, consistent with published experimental data for similar pantellerites, which show that the phenocryst assemblage of alkali feldspar + cpx + aenigmatite ± ilmenite is stable at 100 to 150 MPa. Inferred magma storage depths for these historic eruptions are ~1–5 km below sea-level, consistent with the depths of earthquakes, associated with magma chamber deflation following a dyke intrusion in the period Oct 2005–Apr 2006. Interferometric synthetic aperture radar (InSAR) data for the same period reveal a broad ~20 km diameter area of uplift. Modelling of different geometries reveals that a series of stacked sills over a 1–5 km depth range best matches the InSAR data. The consistency of depth estimates based on petrological study of ancient eruptions and the seismicity, inflation and deflation of Dabbahu observed in relation to the dyking event of 2005, suggest a small but vertically extensive and potentially long-lived magma storage region.

Internal Evolution of Miarolitic Granitic Pegmatites at the Little Three Mine, Ramona, California, USA

Abstract: Miarolitic granitic pegmatites at the Little Three mine property near Ramona, San Diego County, California, USA, possess a bulk composition that closely matches that of a hydrous peraluminous granitic liquid that is saturated with respect to tourmaline at a temperature of ∼450°C. As such, the pegmatites appear to represent the compositions of the silicate liquid from which they crystallized, which contained 2 O 3 and minor amounts of Li and F. They do not appear to have contained phenocrysts upon emplacement, and they are neither partially cumulate nor hydrothermal in nature. The Little Three pegmatites are, however, sharply zoned. Chemical zonation across one small dike matches the patterns expected from crystallization of undercooled granitic liquids, in which the far-field diffusion of alkalis and local constitutional zone refining of fluxing and incompatible elements contribute to the chemical and textural changes from the margins inward. Feldspar thermometry records nearly isothermal crystallization at ∼420°–430°C for dikes from 1 to 2.5 m in thickness. Temperatures recorded by feldspars fall toward the miarolitic center of the thicker (Main) dike, but increase to ∼500°C in the thinner (Swamp) dike. Fluid inclusions within quartz and topaz from miarolitic cavities of the Main dike contain cryolite, arsenides, arsenates, pollucite, and borates, including Cs borate, the latter of which indicates a higher degree of chemical fractionation than is present in the minerals that line the cavities. Extrapolation along isochores based on the homogenization (200°–225°C) of the low-salinity aqueous fluid (

Heterogeneous mantle source and magma differentiation of quaternary arc-like volcanic rocks from Tengchong, SE margin of the Tibetan Plateau

Abstract: The Tengchong volcanic field north of the Burma arc comprises numerous Quaternary volcanoes in the southeastern margin of the Tibetan Plateau. The volcanic rocks are grouped into four units (1–4) from the oldest to youngest. Units 1, 3 and 4 are composed of olivine trachybasalt, basaltic trachyandesite and trachyandesite, and Unit 2 consists of hornblende dacite. The rocks of Units 1, 3, and 4 form a generally alkaline suite in which the rocks plot along generally linear trends on Harker diagrams with only slight offset from unit to unit. They contain olivine phenocrysts with Fo values ranging from 65 to 85 mol% and have Cr-spinel with Cr# ranging from 23 to 35. All the rocks have chondrite-normalized REE patterns enriched in LREE and primitive mantle-normalized trace element patterns depleted in Ti, Nb and Ta, but they are rich in Th, Ti and P relative to typical arc volcanics. Despite minor crustal contamination, 87Sr/86Sr ratios (0.706–0.709), eNd values (−3.2 to −8.7), and eHf values (+4.8 to −6.4) indicate a highly heterogeneous mantle source. The Pb isotopic ratios of the lavas (206Pb/204Pb = 18.02–18.30) clearly show an EMI-type mantle source. The underlying mantle source was previously modified by subduction of the Neo-Tethyan oceanic and Indian continental lithosphere. The present heterogeneous mantle source is interpreted to have formed by variable additions of fluids and sediments derived from the subducted Indian Oceanic lithosphere, probably the Ninety East Ridge. Magma generation and emplacement was facilitated by transtensional NS-trending strike-slip faulting.

Melt inclusion constraints on the magma source of Eyjafjallajökull 2010 flank eruption

Abstract: [1] The 2010 eruptive activity at the Eyjafjallajokull volcanic system began 20 March with a basaltic flank eruption on a 300 m long fissure on the Fimmvorðuhals Pass, in between Eyjafjallajokull and Mýrdalsjokull volcanoes. The magma expelled from the fissure is olivine- and plagioclase-bearing mildly alkali basalt that exhibits uniform and rather primitive whole-rock composition. This event provides a rare opportunity to assess deep magmatic processes in Iceland. Melt inclusions (MIs) hosted in olivine phenocrysts were analyzed for their major, trace and volatile element concentrations to enable identification of magmatic source(s) for Eyjafjallajokull volcano and to better constrain processes occurring at depth. The MIs, in particular those in Mg-rich olivines, record primary magma composition before homogenisation and differentiation during magma ascent. The olivine phenocrysts hosting the MIs have a large compositional range, extending from Fo73 to Fo87, reflecting changes in the magma characteristics from the source to the surface. The MI compositions exhibit significant variations with MgO ranging from 5.2 to 7.2 wt%. This compositional range was caused by a binary mixing of two basaltic end-members followed by fractional crystallization process. The sources of these end-members are identical to those of Katla and Surtsey basalts, with a dominant role of the Katla source. Trace element characteristics of the Fimmvorðuhals MIs suggest important proportions of recycled oceanic crust in their mantle sources.

Mineralogy and Mineral Chemistry of the Cretaceous Duolong Gold-Rich Porphyry Copper Deposit in the Bangongco Arc, Northern Tibet

Abstract: The Early Cretaceous Duolong gold-rich porphyry copper deposit is a newly discovered deposit with proven 5.38 Mt Cu resources of 0.72% Cu and 41 t gold of 0.23 g t−1 in northern Tibet. Granodiorite porphyry and quartz diorite porphyrite are the main ore-bearing porphyries. A wide range of hydrothermal alteration associated with these porphyries is divided into potassic, argillic and propylitic zones from the ore-bearing porphyry center outward and upward. In the hydrothermal alteration zones, secondary albite (91.5–99.7% Ab) occurs along the rim of plagioclase phenocryst and fissures. Secondary K-feldspar (75.1–96.9% Or) replaces plagioclase phenocryst and matrix or occurs in veinlets. Biotite occurs mainly as matrix and veinlet in addition to phenocryst in the potassic zone. The biotite are Mg-rich and formed under a highly oxidized condition at temperatures ranging from 400°C to 430°C. All the biotites are absent in F, and have high Cl content (0.19–0.26%), with log (XCl/XOH) values of −2.74 to −2.88 and IV (Cl) values of −3.48 to −3.35, suggesting a significant role of chloride complexes (CuCl2- and AuCl2-) in transporting and precipitating copper and gold. Chlorites are present in all alteration zones and correspond mainly to pycnochlorite. They have similar Fe/(Fe+Mg), Mn/(Mn+Mg) ratios, and a formation temperature range of 280–360°C. However, the formation temperature of chlorite in the quartz-gypsum-carbonate-chlorite vein is between 190°C and 220°C, indicating that it may have resulted from a later stage of hydrothermal activity. Fe3+/Fe2+ ratios of chlorites have negative correlation with AlIV, suggesting oxygen fugacity of fluids increases with decreasing temperature. Apatite mineral inclusions in the biotite phenocrysts show high SO3 content (0.44–0.82%) and high Cl content (1–1.37%), indicating the host magma had a high oxidation state and was enriched in S and Cl. The highest Cl content of apatite in the propylitic zone may have resulted from pressure decrease, and the lowest Cl content of apatite in the argillic zone may have been caused by a low Cl content in the fluids. The low concentration of SO3 content in the hydrothermal apatite compared to the magmatic one may have resulted from the decrease of oxygen fugacity and S content in the hydrothermal fluid, which are caused by the abundant precipitation of magnetite.

Implications of a Newly Dated ca. 1000-Ma Rhyolitic Tuff in the Indravati Basin, Bastar Craton, India

Abstract: A rhyolitic tuff with a minimum thickness of about 2.5 m occurs at the top of the sedimentary succession of the Indravati Group in south-central India. The tuff is a high-K, low-Na rhyolite with dominantly K-feldspar phenocrysts. Cathodoluminescence color of the quartz phenocrysts is blue. U-Pb isotopic analyses (LA MC-ICPMS) of magmatic zircons separated from the tuff give a weighted-mean average 207Pb/206Pb age of Ma that we interpret as the age of crystallization. We conclude that the closure age of the Indravati succession is ca. 1000 Ma, very similar to that of the Chhattisgarh and Vindhyan successions. The age of this tuff is identical, within analytical errors, to those of the Sukhda and Dhamda Tuffs in the Chhattisgarh Basin some 350 km away. We suggest that these three tuffs represent a ca. 1000-Ma rhyolitic flare-up, possibly related to the assembly of Rodinia and docking of India and East Antarctica.

Crystallization Stages of the Bishop Tuff Magma Body Recorded in Crystal Textures in Pumice Clasts

Abstract: The Bishop Tuff is a giant silicic ignimbrite erupted at 0.76 Ma in eastern California, USA. Five pumice clasts from the late-erupted Bishop Tuff (Aeolian Buttes) were studied in an effort to better understand the pre- and syn-eruptive history of the Bishop magma body and place constraints on the timescales of its existence. This study complements and expands on a previous study that focused on early-erupted Bishop Tuff pumice clasts. Bulk densities of pumice clasts were measured using an immersion method, and phenocryst crystal contents were determined using a sieving and winnowing procedure. X-ray tomography was used to obtain qualitative and quantitative textural information, particularly crystal size distributions (CSDs). We have determined CSDs for crystals ranging in size from {approx}10 to {approx}1000 {micro}m for three groups of mineral phases: magnetite ({+-}ilmenite), pyroxene + biotite, quartz + feldspar. Similar to early-erupted pumice, late-erupted pumice bulk density and crystal contents are positively correlated, and comparison of crystal fraction vs size trends suggests that the proportion of large crystals is the primary control on crystallinity. Porosity is negatively correlated with crystal content, which is difficult to reconcile with closed-system crystallization. Magnetite and pyroxene + biotite size distributions are fractal in nature, often attributedmore » to fragmentation; however, crystals are mostly whole and euhedral, such that an alternative mechanism is necessary to explain these distributions. Quartz + feldspar size distributions are kinked, with a shallow-sloped log-linear section describing large crystals (> 140 {micro}m) and a steep-sloped log-linear section describing small crystals (< 140 {micro}m). We interpret these two crystal populations as resulting from a shift in crystallization regime. We suggest that the shallow-sloped section describes a pre-eruptive quartz + feldspar growth-dominated regime, whereas the steep-sloped section represents a population that grew during a nucleation-dominated regime that began as a result of decompression at the onset of eruption. Timescales of quartz growth calculated from the slopes of these two segments of the size distributions indicate that the pre-eruptive crystal population grew on timescales on the order of millennia and may describe the timescale of crystallization of the Bishop magma body. The syn-eruptive population gives timescales of < 1-2 years (but possibly much less) and probably marks the onset of eruptive decompression.« less

An evaluation of the effect of degassing on the oxidation state of hydrous andesite and dacite magmas: a comparison of pre- and post-eruptive Fe 2+ concentrations

Abstract: The bulk (post-eruptive) wt% FeO concentration in each of 11 phenocryst-poor (<5%) andesite and dacite (60–69 wt% SiO2) lavas from different monogenetic vents in the Mexican arc has been measured by titration, in duplicate. The results match, within analytical error, the wt% FeO content of the magmas during phenocryst growth (pre-euptive), which were calculated on the basis of oxygen fugacity and temperature results from Fe–Ti two-oxide oxygen barometry. The average deviation between the pre- and post-eruptive FeO concentrations is ±0.15 wt%. Application of the plagioclase-liquid hygrometer shows that at the time of phenocryst growth, these 11 magmas contained from ~3–8 wt% H2O, which was extensively degassed upon eruption. There is no evidence that degassing of ≤8 wt% H2O changed the oxidation state of these magmas. Calculations of pre-eruptive and post-eruptive oxygen fugacity values relative to the Ni-NiO buffer (in terms of log10 units) for the 11 samples span a similar range; pre-eruptive ∆NNO = −0.9 to +0.7 and post-eruptive ∆NNO = −0.4 to +0.8. The data further show that extensive groundmass (closed-system) crystallization had no affect on bulk Fe3+/Fe2+ ratios. Finally, there is no systematic variation in the range of pre-eruptive Fe3+/FeT values of the samples as a function of SiO2 concentration (i.e., differentiation). Therefore, the results of this study indicate that the elevated Fe3+/FeT ratios of arc andesites and dacites, compared with magmas erupted in other tectonic settings, cannot be attributed to the effects of (1) degassing of H2O, (2) closed-system crystallization, and/or (3) differentiation effects, but instead must be inherited from their parental source rocks (i.e., mantle-derived arc basalts).

Magma Evolution in the Primitive, Intra-oceanic Tonga Arc: Petrogenesis of Basaltic Andesites at Tofua Volcano

Abstract: Tofua volcano is situated midway along the Tonga oceanic arc and has undergone two phases of ignimbrite-forming activity. The eruptive products are almost entirely basaltic andesites (52·5-57 wt % SiO ) with the exception of a volumetrically minor pre-caldera dacite. The suite displays a strong tholeiitic trend with K O <1 wt %. Phenocryst assemblages typically comprise plagioclase + clinopyroxene ± orthopyroxene with microlites of Ti-magnetite. Olivine (Fo ) is rare and believed to be dominantly antecrystic. An increase in the extent and frequency of reverse zoning in phenocrysts, sieve-textured plagioclase and the occurrence of antecrystic phases in post-caldera lavas record a shift to dynamic conditions, allowing the interaction of magma batches that were previously distinct. Pyroxene thermobarometry suggests crystallization at 950-1200°C and 0·8-1·8 kbar. Volatile measurements of glassy melt inclusions indicate a maximum H O content of 4·16 wt % H O, and CO -H O saturation curves indicate that crystallization occurred at two levels, at depths of 4-5·5 km and 1·5-2·5 km. Major and trace element models suggest that the compositions of the majority of the samples represent a differentiation trend whereby the dacite was produced by 65% fractional crystallization of the most primitive basaltic andesite. Trace element models suggest that the sub-arc mantle source is the residuum of depleted Indian mid-ocean ridge basalt mantle (IDMM-1% melt), whereas radiogenic isotope data imply addition of 0·2% average Tongan sediment melt and a fluid component derived from the subducted altered Pacific oceanic crust. A horizontal array on the U-Th equiline diagram and Ra excesses of up to 500% suggest fluid addition to the mantle wedge within the last few thousand years. Time-integrated ( Ra/ Th) vs Sr/Th and Ba/Th fractionation models imply differentiation timescales of up to 4500 years for the dacitic magma compositions at Tofua.