Phenocryst Content Research Articles

Experimental petrology of monotonous intermediate magmas

Abstract Large-volume, high-crystallinity, chemically homogeneous ignimbrites, dubbed ‘monotonous intermediates’, provide a unique opportunity to investigate the evolution of crustal magmatic reservoirs. We present the results of hydrothermal experiments on a dacite from Fish Canyon Tuff (FCT) in Colorado (USA), a classic example of a monotonous intermediate deposit, in order to characterize the variations in chemical and physical properties of hydrous dacite magmas as a function of temperature. The experiments (200 MPa, 720–1100°C) span the inferred pre-eruptive conditions of FCT magmas, and are shown to provide the best match to the chemical and physical properties of the erupted magmas at 790±10°C under conditions at or close to water-saturation. The results show the important effect of water content in controlling the chemical and physical evolution of magma, and the contrasted behaviour of water-saturated v. water-undersaturated magmas. In both cases, however, there is a broad interval of temperature (200°C) over which crystal fraction changes little. By recasting this behaviour in terms of enthalpy, rather than temperature, as the independent variable we show that this interval corresponds to a minimum in the change in crystallinity per unit of energy added or subtracted from the system, such that small perturbations to the heat content of the system (e.g. by cooling or new magma injections) results in very little change in magma properties. The crystal content in this interval is 55–65 wt%, which is close to the phenocryst content (40–55 wt%) of monotonous intermediates. We propose that crystal-rich magmas tend to settle in this ‘petrological trap’, changing little in physical and chemical properties over time as the system grows. Petrological trapping enables very large volumes of intermediate magma to accumulate in the shallow crust until such time as the net buoyancy force of these crystal-rich magma is sufficient to overcome the strength of the roof rocks, leading to a potentially very large eruption.

Fred’s Flow (Canada) and Murphy Well (Australia): thick komatiitic lava flows with contrasting compositions, emplacement mechanisms and water contents

Two Archaean komatiitic flows, Fred’s Flow in Canada and the Murphy Well Flow in Australia, have similar thicknesses (120 and 160 m) but very different compositions and internal structures. Their contrasting differentiation profiles are keys to determine the cooling and crystallization mechanisms that operated during the eruption of Archaean ultramafic lavas. Fred’s Flow is the type example of a thick komatiitic basalt flow. It is strongly differentiated and consists of a succession of layers with contrasting textures and compositions. The layering is readily explained by the accumulation of olivine and pyroxene in a lower cumulate layer and by evolution of the liquid composition during downward growth of spinifex-textured rocks within the upper crust. The magmas that erupted to form Fred’s Flow had variable compositions, ranging from 12 to 20 wt% MgO, and phenocryst contents from 0 to 20 vol%. The flow was emplaced by two pulses. A first ~20-m-thick pulse was followed by another more voluminous but less magnesian pulse that inflated the flow to its present 120 m thickness. Following the second pulse, the flow crystallized in a closed system and differentiated into cumulates containing 30–38 wt% MgO and a residual gabbroic layer with only 6 wt% MgO. The Murphy Well Flow, in contrast, has a remarkably uniform composition throughout. It comprises a 20-m-thick upper layer of fine-grained dendritic olivine and 2–5 vol% amygdales, a 110–120 m intermediate layer of olivine porphyry and a 20–30 m basal layer of olivine orthocumulate. Throughout the flow, MgO contents vary little, from only 30 to 33 wt%, except for the slightly more magnesian basal layer (38–40 wt%). The uniform composition of the flow and dendritic olivine habits in the upper 20 m point to rapid cooling of a highly magnesian liquid with a composition like that of the bulk of the flow. Under equilibrium conditions, this liquid should have crystallized olivine with the composition Fo94.9, but the most magnesian composition measured by electron microprobe in samples from the flow is Fo92.9. To explain these features, we propose that the parental liquid contained around 32 wt% MgO and 3 wt% H2O. This liquid degassed during the eruption, creating a supercooled liquid that solidified quickly and crystallized olivine with non-equilibrium textures and compositions.

Recycling of crystal mush-derived melts and short magma residence times revealed by U-series disequilibria at Stromboli volcano

The presence of crystal mushes in the feeding system of active volcanoes is generally revealed by antecrysts, representing the crystalline portion of old magmas recycled in the juvenile material, but very little is known about the fate of interstitial liquid hosted within the crystal-rich mush (i.e. antemelt). U-series disequilibria measured in magmas erupted in the past 18 years at Stromboli volcano provide the first geochemical evidence of the involvement of antemelt and help constraining the timescales of the processes occurring in the plumbing system of the volcano. Despite almost constant major and trace element composition, significant variations in isotope ratios are observed. (230Th/232Th) decreases with time, whilst (238U/232Th), (226Ra/230Th) and 87Sr/86Sr are different in the two types of magma erupted. Magma with low phenocryst content (lp) is erupted as pumices during paroxysm and is thought to belong to a deep reservoir. Highly porphyritic magma (hp) is erupted during the normal “Strombolian” activity as scoria and during the effusive events as lavas, and it is considered to derive from the former one within a shallow reservoir through degassing-driven crystallisation, mixing and incorporation of antecrysts. The distinct (238U/232Th) of lp and hp magma requires the involvement of a component with high 87Sr/86Sr and (238U/232Th) deriving from older magmas erupted earlier in the volcano history (up to 2.5 ka). The incompatibility of U and Th in major mineral phases limits the possible effect of antecrysts, hence requiring the involvement of a U- and Th-rich antemelt. The decrease of 226Ra-excess from lp to hp magmas provides further and independent evidence for the involvement of a few thousands years old antemelt.The variation with time of (230Th/232Th) within lp and hp magmas is exploited to constrain the residence time of magmas in the deep and shallow reservoir of the volcano to <55 yrs (inferred reservoir volume <0.5 km3) and 2–10 yrs (inferred reservoir volume 0.02–0.09 km3), respectively. Our results show the occurrence of magmatic processes operating at different timescale within the feeding system of a so-called steady state volcano, such as Stromboli. We show that, while most of the magma is erupted within few years, a portion of it, made of both crystals (antecrysts) and residual liquid (antemelt), can be stored in the plumbing system for thousands of years to be eventually rejuvenated and mixed back into the shallow reservoir. The presence of antemelts may also affect the eruptive mechanism by promoting heat transfer in locked crystal mushes, hence favouring their remobilisation.

Explosive to effusive transition during the largest volcanic eruption of the 20th century (Novarupta 1912, Alaska)

Silicic volcanic eruptions commonly show abrupt shifts between powerful and dangerous (Plinian) explosive episodes and gentle effusion of lava. Whether the onset of magma permeability and ensuing gas loss controls these transitions has been a subject of debate. We measured porosities and permeabilities in samples from the A.D. 1912 eruption of Novarupta volcano, Alaska, and analyzed them within the context of a well-constrained eruptive sequence that encompasses sustained explosive and effusive activity. For the explosive samples, we find that the degree of vesicle interconnectivity, measured as the ratio of connected to total porosity, decreases with phenocryst content and with increasing eruption intensity. Permeabilities of explosive samples show a weak dependence on porosity. Dome samples are not significantly different in permeability, but are of lower porosity, which together with abundant flattened vesicles is consistent with bubble collapse by permeable outgassing. Quantitative analysis indicates that outgassing alone was insufficient to affect the transition to effusive activity. Rather, the change from explosive to effusive activity was probably a consequence of high versus low magma ascent rates.

Magmatic Processes of Ashi Volcano, Western Kunlun Mountains, China

Abstract:The Ashikule volcanic cluster (AVC) in western Kunlun Mountains is located in a graben region at the convergence of the Altun and Kangxiwa fault zones, and consists of more than 10 main volcanoes and dozens of volcanelloes. The Ashi volcano lies in the central part of the volcanic cluster. The lithology, chemical composition and texture of Ashi volcanic rocks were studied in detail, and their implication in magmatic processes was discussed. The phenocrysts in Ashi volcanic rocks consist mainly of plagioclase and pyroxene, and the statistical results of phenocryst contents show that the rocks can be subdivided into two groups. In group A, the content of pyroxene phenocrysts is generally higher than that of plagioclase phenocrysts, but an inverse relation occurs in group B. In TAS diagram, the compositions of both groups fall into the trachyandensite field, but they are obviously concentrated into two clusters. The two clusters exist also in the oxide diagrams. The pyroxene phenocrysts comprise augite, bronzite and hypersthene, and their Mg# histogram shows two peaks. Plagioclase phenocrysts with reaction rim are observed in rocks of both groups. The An values of the core are generally 30–40, and those of the rim are 44–48, which are closer to those of euhedral plagioclases. The bronzites are in equilibrium with the melt, and two sets of magma depths, i.e., 18–25 km and 13–18 km, can be estimated by using thermobarometer proposed by Putirka. The hypersthenes are not in equilibrium with the melt, and can be assigned to xenocrysts. The crystal size distribution (CSD) curves of plagioclase appear as kinked lines indicative of magma mixing. The above analyses show that two magma pockets might exist beneath the Ashi volcano. It is likely that they are connected with each other. The one has more evolved and contains more acidic magma, and the other is a trachyandensite magma pocket characterized by layering. The magma from the upper part of the trachyandensite magma pocket might mix with more acidic magma, resulting in a magma that is more acidic than the magma from the lower part.

Chapter 18 Characterization of mafic enclaves in the erupted products of Soufrière Hills Volcano, Montserrat, 2009 to 2010

Abstract Lavas from the current eruption of the Soufrière Hills Volcano (SHV), Montserrat exhibit evidence for magma mingling, related to the intrusion of mafic magma at depth. We present detailed field, petrological, textural and geochemical descriptions of mafic enclaves in andesite erupted during 2009–2010, and subdivide the enclaves into three distinct types: type A are mafic, glassy with chilled margins and few inherited phenocrysts; type B are more evolved with high inherited phenocryst content and little glass, and are interpreted as significantly hybridized; type C are composite, with a mafic interior (type A) and a hybrid exterior (type B). All enclaves define tight linear compositional trends, interpreted as mixing between a mafic end member (type A) and host andesite. Enclave glasses are rhyolitic, owing to extensive crystallization during quenching. Type A quench crystallization is driven by rapid thermal equilibration during injection into the andesite. Conversely, type B enclaves form in a hybridized melt layer, which ponded near the base of the chamber and cooled more slowly. Vesiculation near the mafic–silicic interface resulted in disruption of the hybridized layer and the formation of the type B enclaves. The composite enclaves represent an interface between types A and B, suggesting multiple episodes of mafic injection.

The hydrous properties of subcontinental lithospheric mantle: Constraints from water content and hydrogen isotope composition of phenocrysts from Cenozoic continental basalt in North China

To investigate the hydrous properties of subcontinental lithospheric mantle, phenocrysts from Cenozoic continental basalt in North China were analyzed by FTIR for structural hydroxyl content and by TC/EA-MS for total water content and hydrogen isotope composition. The concentration of structural hydroxyl, while not detectable in olivine, ranges from 62 to 913ppm in clinopyroxene phenocrysts. Both olivine and clinopyroxene phenocrysts exhibit high total water contents, indicating the presence of molecular water as nanoscale inclusions. The majority of clinopyroxene phenocrysts contain less total water than olivine phenocrysts, and all the clinopyroxene phenocrysts exhibit higher δD values than olivine phenocrysts, suggesting that the molecular water in olivine is rather D-poor. The phenocryst minerals exhibit δD values of −145‰ to −94‰ that are negatively correlated with their total water contents.The initial basaltic melt is inferred to contain 0.6wt% total water, which increases to 3.5–6.7wt% due to fractional crystallization, rendering the melt water-saturated in the late stage of its evolution. Also during this process, structural hydroxyl in the melt is partly converted to molecular water, leading to the depletion of D in the phenocrysts and the systematic variation of structural hydroxyl content with the Mg number of clinopyroxene. The mantle source of the basaltic melt is inferred to have a structural hydroxyl content of 290ppm, which is slightly higher than that of MORB-type mantle but similar to that of OIB-type mantle. Thus, we suggest that the subcontinental lithospheric mantle of North China is rather hydrous and D-depleted, probably due to the entrainment of recycling subducted crustal material.

Unraveling the solidification path of a pahoehoe “cicirara” lava from Mount Etna volcano

The solidified surface of a lava flow reflects the viscosity of its molten fraction and the crystal content during flow; crystal-poor basaltic lavas produce pahoehoe fields, whereas crystal-rich ones solidify with aa carapaces. At Mount Etna, volcano aa morphologies are very common, whereas pahoehoe lavas are rare. The latter are locally named “cicirara” due to the presence of centimeter-sized plagioclase phenocrysts much more abundant than in aa lavas. The phenocryst content of “cicirara” lavas contrasts with the low viscosity generally associated with pahoehoe morphology. Therefore, to reconcile the discrepancy between textural and volcanic observations, we have studied the most primitive pahoehoe “cicirara” lava sampled until now. Two samples at 0.5 and 1 m from the bottom of the 2-m thick lava flow were investigated on the basis of their mineral compositional variations and textural features, i.e., size frequency and crystal size distribution (CSD). Results coupled with rheological models indicate that only large phenocrysts of plagioclase (>1 mm) and clinopyroxene have grown before eruption. Thermobarometric models and petrological computations based on the composition of plagioclase and clinopyroxene phenocryst cores highlight that only a small amount (10–15 vol.%) of crystals equilibrated at 12 km of depth. Cumulative size frequency and CSD data also indicate that plagioclase and clinopyroxene phenocryst rims grew heterogeneously and coalesced around their cores at depths <1 km, before eruption. In this view, the “cicirara” lava was erupted with a low crystalline content that favoured the formation of its pahoehoe surface; however, crystals with a size <1 mm (~75 vol.%) solidified at post-eruptive conditions. Our findings underline that the emplacement of high-viscosity aa or low-viscosity pahoehoe lavas is driven by the degree of undercooling imposed by the volatile exsolution rate in the shallowest portion of the Etnean plumbing system. A slow magma ascent rate promotes significant intratelluric degassing and widespread nucleation; consequently, the viscosity of the suspension significantly increases leading to an aa morphology. In contrast, pahoehoe “cicirara” lavas are associated with a rapid rise to the surface of poorly degassed, undercooled magmas.

Numerical experiments on the dynamics of channelised lava flows at Mount Cameroon volcano with the FLOWGO thermo-rheological model

As for many other effusive volcanoes, Mount Cameroon (MC) is a volcano for which only limited information exists on lava flow properties and emplacement dynamics for recent eruptions. This study provides new quantitative constraints for rheological and dynamic characteristics of lava flow effusion for the 1982 and 2000 eruptive events, used to calibrate the FLOWGO thermo-rheological model for these lava flows. The FLOWGO 1-D physical model is used to simulate down-flow evolution of the geometry and rheology of channel-contained cooling-limited lava flows. Morphometric data from historical lava flows were acquired from the field, e.g. channel geometry, levee and background slope, in order to estimate lava yield strength, velocity and effusion rate. Lava density and viscosity were also estimated from compositional data and laboratory methods. To account for uncertainty in the input rheological and geometrical data, three end-member scenarios were used to bracket the potential range in lava channel initial dimension, initial lava temperature and phenocryst content. For each of these scenarios, two crustal growth models were used: one assuming strong insulation due to lava flow surface crusting, the other a much lower crusting rate. Twelve numerical simulations were made per flow and the results were compared against the channel geometry, microlite content, yield strength and viscosity estimates from field and laboratory investigations. Best-fit models where obtained for both the 1982 and 2000 lava flows using a low rate of surface crusting, high initial temperature and low phenocryst content. Model-predicted lengths were within 5% of the actual lengths. Modelled mean effusion rates for the 1982 (52–64m3s−1) and 2000 (10m3s−1) flows closely matched field data derived estimations (26–68 and 9.5m3s−1 respectively). FLOWGO model results are highly sensitive to initial channel dimensions, phenocryst content and the FLOWGO model is unable to reproduce the observed rapid near-vent increase in microlite content. Near-vent rapid cooling induced by lava degassing is yet to be fully documented and integrated into numerical lava flow models such as FLOWGO, but obtained results already provide valuable insights for lava flow hazard assessment at Mount Cameroon.

High water content in Mesozoic primitive basalts of the North China Craton and implications on the destruction of cratonic mantle lithosphere

It has been suggested that the longevity of cratons (i.e. ancient and stable cores of continents) is related in part to the low water content of their deep mantle roots; this gives them a higher viscosity than the underlying asthenosphere. Consequently, the removal of cratonic roots is expected to be closely connected to the hydration of the lithospheric mantle, but direct evidence for this speculation has been scarce. The eastern part of the North China Craton (NCC) is a clear example of a “destroyed craton”. In this study the H2O content of clinopyroxene phenocrysts was measured in lithospheric mantle-derived high-magnesium basalts of the Feixian area, in the eastern part of the NCC. These lavas erupted in the early Cretaceous (∼120Ma), which was the peak time of the NCC destruction. Based on these data, it was estimated that the H2O content of the lithospheric mantle source of these basalts consists of more than 1000ppm by weight. This water content is much higher than in the source of mid-ocean-ridge basalts (50–200ppm by weight) and also higher than in the Kaapvaal cratonic mantle in South Africa (∼120ppm by weight); the latter is still stable after >3 billion years. This study argues that a large amount of water was indeed added to the NCC's lithospheric mantle, probably due to the multi-stage subduction of oceanic plates since the early Paleozoic. This high water content significantly reduced the viscosity contrast between the lithospheric mantle and the underlying asthenosphere, and provided a prerequisite for the removal of the cratonic root of the NCC by reducing its strength.

Petrological Insights into Shifts in Eruptive Styles at Volcán Llaima (Chile)

Tephra and lava pairs from two summit eruptions (AD 2008 and 1957) and a flank fissure eruption (� AD 1850) are compared in terms of textures, phenocryst contents, and mineral zoning patterns to shed light on processes responsible for the shifts in eruption style during typical eruptive episodes at Volcan Llaima (Andean SouthernVolcanic Zone, Chile).The mineralogy and whole-rock com- positions of tephra and lavas are similar within eruptive episodes, suggesting a common magma reservoir for Strombolian paroxysms and lava effusion. The zoning profiles and textures of plagioclase record successive and discrete intrusions of volatile-rich mafic magma accompanied by mixing of these recharge magmas with the resident basaltic-andesitic crystal mushes that are commonly present at shallow levels in the Llaima system. Each recharge event destabil- izes the plagioclase in equilibrium with the resident crystal mush melt and stabilizes relatively An-rich plagioclase, as is recorded by the numerous resorption zones. Lavas typically have � 15^20 vol. % more phenocrysts than the tephra. Differences in plagioclase and olivine textures and zoning, combined with different phenocryst contents, indicate that a greater volume fraction of recharge magma is present in the explosively erupted magma than in subsequent effu- sively erupted magma. We propose that Strombolian paroxysms at Volcan Llaima are triggered by interactions with large volume frac- tions of recharge magma, which decrease the bulk viscosity and in- crease the volatile contents of the erupted magmas, leading to the conditions required for the fragmentation of basaltic-andesite. Lava effusion ensues from reduced interactions with the recharge magma, after it has partially degassed and crystallized, thereby impeding rapid ascent. This process could be operating at other steady-state basaltic volcanoes, wherein shallow reservoirs are periodically re- filled by fresh, volatile-rich magmas.

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

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.

The Upper and Lower Nisyros Pumices: Revisions to the Mediterranean tephrostratigraphic record based on micron-beam glass geochemistry

The Greek island of Nisyros is the source of tephra found in distal settings throughout the Aegean. In this contribution, we define geochemical fingerprints for the Lower and Upper Pumice eruptions of Nisyros based on micron-beam major (EMPA) and trace (LA-ICP-MS) element data. These two eruptions are compositionally distinct from each other. The high phenocryst content of proximal Lower and Upper Pumice tephra means that the differences between the two magmas are masked in whole-rock analyses. We demonstrate that all previous reports of distal marine and continental tephras correspond to the younger Upper Pumice not the older Lower Pumice. This has important implications for the marine tephrochronology of the Mediterranean and for the age of the Upper and Lower Pumice eruptions. We suggest an age of at least 47ka for the Upper Pumice and an older, unconstrained age for the Lower Pumice.

Recent explosive eruptions and volcano hazards at Soputan volcano—a basalt stratovolcano in north Sulawesi, Indonesia

Soputan is a high-alumina basalt stratovolcano located in the active North Sulawesi-Sangihe Islands magmatic arc. Although immediately adjacent to the still geothermally active Quaternary Tondono Caldera, Soputan’s magmas are geochemically distinct from those of the caldera and from other magmas in the arc. Unusual for a basalt volcano, Soputan produces summit lava domes and explosive eruptions with high-altitude ash plumes and pyroclastic flows—eight explosive eruptions during the period 2003–2011. Our field observations, remote sensing, gas emission, seismic, and petrologic analyses indicate that Soputan is an open-vent-type volcano that taps basalt magma derived from the arc-mantle wedge, accumulated and fractionated in a deep-crustal reservoir and transported slowly or staged at shallow levels prior to eruption. A combination of high phenocryst content, extensive microlite crystallization and separation of a gas phase at shallow levels results in a highly viscous basalt magma and explosive eruptive style. The open-vent structure and frequent eruptions indicate that Soputan will likely erupt again in the next decade, perhaps repeatedly. Explosive eruptions in the Volcano Explosivity Index (VEI) 2–3 range and lava dome growth are most probable, with a small chance of larger VEI 4 eruptions. A rapid ramp up in seismicity preceding the recent eruptions suggests that future eruptions may have no more than a few days of seismic warning. Risk to population in the region is currently greatest for villages located on the southern and western flanks of the volcano where flow deposits are directed by topography. In addition, Soputan’s explosive eruptions produce high-altitude ash clouds that pose a risk to air traffic in the region.

The H2 O content of andesitic magmas from three volcanoes in Japan, inferred from the infrared analysis of clinopyroxene

In this study, we have determined the H 2 O content of arc andesitic magmas erupted from three volcanoes in Japan [the 1929 A.D. eruption of Hokkaido-Komagatake (Ko-a), Tenmei eruption of Asama (As-A), and Taisho eruption of Sakurajima volcanoes (Sz-Ts/P1)]. The amount of H 2 O dissolved in the melt at magma chamber was estimated by analyzing the H 2 O content of clinopyroxene phenocrysts and using the partition coefficient of H 2 O between melt and clinopyroxene. On the basis of infrared spectroscopic analyses, the H 2 O contents of clinopyroxene were 329 ± 65, 247 ± 72, and 213 ± 36 ppm for Ko-a, As-A, and Sz-Ts/P1, respectively. The partition coefficient was calculated from the H 2 O content of Ko-a clinopyroxene and that of rhyolitic melt inclusions included in clinopyroxene (3.1 wt% H 2 O). Assuming that clinopyroxene was in equilibrium with melt inclusions, the partition coefficient is 0.011 ± 0.003, which results in H 2 O contents of the melts at the magma chamber of 2.3 ± 0.9 and 2.0 ± 0.6 wt% for As-A and Sz-Ts/P1, respectively. The H 2 O content of magma in the magma chamber calculated at saturation on the basis of magma-chamber depth estimated from geophysical observation shows good agreement with the measured value for As-A; those of Ko-a and Sz-Ts/P1 are slightly higher than the measured values. These results indicate that arc andesitic magmas are saturated with H 2 O or are ready to saturate before the eruptions.

Preeruptive magma viscosity: An important measure of magma eruptibility

[1] Using a compilation of melt compositions, meltwater contents, temperatures, and phenocryst contents, the preeruptive viscosities under magma reservoir conditions are calculated for 83 erupted magmas. The basaltic-to-rhyolitic magmas have preeruptive viscosities over the range 101 to 108 Pa s. Although bulk SiO2 content has often been used as a qualitative measure of preeruptive magma viscosity, the bulk SiO2 content shows a weak correlation with magma viscosity (correlation coefficient r = 0.5). Because of a wide range of phenocryst contents from 0 to ∼50 vol %, andesitic magmas have viscosities ranging from 102 to 107 Pa s, which are lower or higher than those of phenocryst-poor rhyolitic magmas with 105 to 106 Pa s. Focusing on andesitic to rhyolitic magmas, the r between bulk SiO2 contents and magma viscosities changes to −0.1. In contrast, the melt-only SiO2 content from a basaltic-to-rhyolitic melt shows a good linear correlation with melt-only viscosity (r = 0.9). Although most of the calculated viscosities of erupted magmas fall below ∼106 Pa s, as consistent with the previous compilation study, this paper describes 20 examples of highly viscous magmas with >106 Pa s, in most cases, composed of mixtures of high-silica rhyolitic melt (75–79 wt % SiO2) and abundant phenocrysts (30–55 vol %). In these highly viscous magmas, 9 examples have erupted following the precursory eruption of less viscous magma, suggesting that precursory dike propagation and conduit formation by the less viscous magma with <106 Pa s induced the following eruption of less eruptible, highly viscous magmas.

The exhumed Eocene Sultepec-Goleta Volcanic Center of southern Mexico: record of partial collapse and ignimbritic volcanism fed by wide pyroclastic dike complexes

The Sultepec-Goleta area in southern Mexico hosts one of the major Eocene silicic volcanic centers that make up the Cenozoic volcanic record of the north-central part of the Sierra Madre del Sur. This center is represented by a partially exhumed NNE-SSW trending ignimbritic field that covers an area of ∼400 km2 with a preserved volume of ∼200 km3. A remarkable feature of volcanic center is the exposure of pyroclastic dike complexes up to 1 km wide that extend almost continuously along the western and southern flanks of the volcanic field. The ignimbritic record comprises four units with different textural features as well as variable proportions of the main components (i.e. phenocrysts, pumice and lithic clasts). The most extensive unit is the basal Goleta Ignimbrite, of which the middle and upper members are richer in phenocrysts than the lower member. It has a thickness that ranges from ∼200 m in the north to at least 600 m in the south of the volcanic center. The overlying ignimbrite units (Cienega, Lobera and Potrero ignimbrites) are phenocryst-poor and are distributed in the northern sector of the volcanic field. The semi-curvilinear trend of the pyroclastic dikes along the southern flank of the Goleta Range, coupled with the greater thickness of the Goleta Ignimbrite encircled by the dikes, is indicative of the development of a partial-collapse caldera. Based on spatial relationships as well as analogies in the nature and abundances of the components, it has been recognized that the central and southern dike complexes fed the relatively phenocryst-rich Goleta Ignimbrite, whereas the phenocryst-poor units of the northern sector were extruded through the pyroclastic conduits distributed around Sultepec. We suggest that the stirring produced by the partial collapse of the magma chamber roof beneath the southern sector of the study area triggered the tapping of phenocryst-rich portions of the zoned magma chamber, resulting in an increase of phenocryst content in the upper members of the Goleta Ignimbrite. The presence of breccias and lithic-rich facies dominated by wallrock fragments at the borders of the pyroclastic dikes, including those of the northern extra-caldera sector, indicate that erosion of the conduit walls above the fragmentation level contributed to widening of the conduits and the construction of wide dike complexes.

Eruptive history and magmatic evolution of the 1.9 kyr Plinian dacitic Chiltepe Tephra from Apoyeque volcano in west-central Nicaragua

The youngest dacitic Plinian eruption in west-central Nicaragua, forming the 18 km3 Chiltepe Tephra (CT), occurred about nineteen hundred years ago at Apoyeque stratovolcano, which dominates the Chiltepe volcanic complex 15 km north of the capital Managua, where the CT is 2 m thick. We have traced the CT from its proximal facies at the crater rim, through the medial facies in the lowlands around Apoyeque, and to the distal facies up to 550 km offshore in the Pacific. While medial and distal facies consist of widespread Plinian fall deposits, the proximal facies reveals the complexity of this eruption, which we divide into four phases (I–IV). Interaction of rising magma with a pre-existing crater lake generated the phreatomagmatic opening phase I of the eruption, which produced ash fall with accretionary lapilli. Phase II marked a rapid change to persistent magmatic activity that yielded several large Plinian eruptions, declining through a period of unstable eruption conditions, followed by a short hiatus. Phase III began with unstable conditions, probably as a result of eastward migration and widening of the vent, leading to a second period of Plinian eruptions with three major events reaching magma discharge rates five times larger than those of phase II. Phase III again declined through unstable eruption conditions before magmatic activity terminated. Numerous explosions in the shallow hydrothermal system during the final phase IV resulted in the formation of a phreatic tuff ring on the rim of Apoyeque crater. The white, highly-vesicular, dacitic CT pumice contains plagioclase (An45–68), orthopyroxene, clinopyroxene, and minor hornblende, apatite and titanomagnetite phenocrysts. A very subordinate fraction of gray pumice has the highest crystal content, the least evolved bulk-rock, but the most evolved matrix-glass composition. The CT dacite has two unusual compositional features: (1) all white dacite has the same melt (matrix-glass) composition such that variations in bulk-rock compositions (64–68 wt% SiO2) simply reflect different phenocryst contents of 10–35%, interpreted as the result of gradual phenocryst settling in the magma chamber. (2) Abundant olivine crystals with a bimodal distribution in Mg# (modes at Mg# = 0.75 and Mg# = 0.8) are dispersed throughout the erupted dacite. These are clearly out of equilibrium with the dacitic melt and are interpreted as xenocrysts derived from the basaltic Nejapa-Miraflores volcanic lineament that intersects the Chiltepe volcanic complex and was contemporaneously active. Thermobarometric estimates place the dacitic CT magma reservoir in the upper crust (<250 MPa), with a temperature of about 890°C and about 5 wt% water dissolved in the melt. Comparing water and chlorine contents with respective solubility models suggests that volatile degassing began in the magma reservoir and triggered the CT eruption. From the vertical compositional variation pattern of the CT we deduce that the conduit tapped the magma chamber not at the top but from the side, at some deeper level, and that subsequent magma withdrawal was governed by both variations in discharge rate and possible upward migration and/or widening of the conduit entrance.

Evolution of Calc-Alkaline Volcanism and Associated Hydrothermal Gold Deposits at Yanacocha, Peru

Clusters of high-sulfidation epithermal deposits containing more than 50 Moz of gold are hosted by advanced argillic-altered Miocene volcanic rocks in the Yanacocha district, northern Peru (lat. 6°59'30 S, long. 78°30'45 W). We describe the nature of the volcanism and its relation to the gold ores on the basis of new district-scale geologic mapping, 69 40Ar/39Ar ages on igneous rocks and hydrothermal alunite, and petrologic and geochemical investigations. Volcanic rocks of the Calipuy Group are the oldest Cenozoic rocks at Yanacocha, and include the Huambo Cancha andesite, andesitic lahars of Tual and Chaupiloma (19.5–15.9 Ma), and the dacitic Cerro Fraile pyroclastics (15.5–15.1 Ma). The younger Yanacocha Volcanics (14.5–8.4 Ma) form a cogenetic series of lavas and pyroclastic rocks with a cumulative volume of ~88 km3 that represents eruption from a moderate-size volcanic center. Early pyroxene>hornblende-bearing lavas of the Atazaico Andesite (14.5–13.3 Ma) erupted from small stratovolcanoes progressively younger from southwest to northeast. Dacitic dikes followed that are spatially associated with gold deposits at Quilish and Cerro Negro (~7 Moz Au) and stage 1 alunite (13.6–12.6 Ma). The Colorado Pyroclastics erupted in the center of the district and include the hornblende- and biotite-bearing andesitic to dacitic Cori Coshpa (12.6 Ma) and Maqui Maqui (12.5–12.4 Ma) ignimbrites. The Colorado Pyroclastics are overlain by hornblende>pyroxene-bearing andesitic to dacitic lavas, flow-domes, and minor pyroclastic rocks of the Azufre Andesite (12.1–11.6 Ma). Widespread stage 2 alunite (11.5 Ma) and minor gold deposition (~0.5 Moz) closely follow. The San Jose Ignimbrite (11.5–11.2 Ma) overlies the Azufre Andesite and stage 2 alunite and includes three members of hornblende-pyroxene (biotite) dacite and andesite that erupted in the center of the district and flowed southward. Mineralogically similar domes were emplaced into the inferred vents. The Coriwachay Dacite (10.8–8.4 Ma) forms the youngest and most silica rich igneous rocks in the district, and includes intrusions and flow domes of dacite to rhyolite at Corimayo (10.8 Ma), Cerro Yanacocha (9.9 Ma), and Yanacocha Lake (8.4 Ma). Most of the gold (>47 Moz) was deposited at Yanacocha during intrusion of the late Coriwachay Dacite. These late dacites are volumetrically smallest (~2% of the total volume of erupted magma) and are temporally associated with stage 3 to 5 alunites. Stage 3 alunite (11.0–10.7 Ma) developed along a northeast trend for 9 km that includes the gold deposits of Corimayo, San Jose, Carachugo, and Maqui Maqui. The deeper Kupfertal Cu-Au porphyry has an age of 10.7 Ma on hydrothermal biotite and underlies zones of stage 4 and 5 quartz-alunite alternation that are 0.8 and 1.5 m.y. younger, respectively. Stage 4 alunite ranges in age from 10.2 to 10.3 Ma at the Tapado and Chaquicocha Sur gold deposits to Cerro Sugares east of the Maqui Maqui deposit. Stage 4 also includes a younger alunite age of 9.9 Ma from the San Jose gold deposit. Stage 5 alunite ranges from ~9.3 to ~8.2 Ma at Cerro Yanacocha, the largest gold deposit in the district. All these deposits contain massive and vuggy quartz, quartz-alunite, and quartz-pyrophyllite alteration associated with pyrite±enargite-tennantite-covellite. Magmatism in the Yanacocha district lasted for ~11 m.y. The Yanacocha Volcanics spanned the last ~6 m.y. of this period and were associated with long-lived magmatic-hydrothermal activity and episodic gold mineralization. The Yanacocha calc-alkaline suite was oxidized, water and sulfate rich, and evolved from early pyroxene>hornblende andesite to late titanite-bearing dacite and minor rhyolite. Several dacites contain populations of both high- and low-aluminum hornblendes that crystallized in the middle and upper crust, respectively. The variation of Mg, Ti, P, Sr, and Ba contents in these rocks is consistent with a complex magmatic origin via both cooling, fractional crystallization, periodic recharge of deeply derived hydrous basaltic or andesitic melts, and mixing with silicic melts derived by crustal melting. Low eruption rates, high phenocryst contents of the volcanic rocks, and widespread hydrothermal alteration are consistent with the hypothesis that most of the magmas at Yanacocha crystallized in shallow chambers as granitoids that passively degassed ore fluids. The compositional diversity of the volcanic rocks together with an extended magmatic-hydrothermal history reflect complex magmatic processes that were optimum for producing the world-class gold deposits at Yanacocha.

Are arc-type rocks the products of magma crystallisation? Observations from a simple oceanic arc volcano: Raoul Island, Kermadec Arc, SW Pacific

Raoul Island is the emergent summit of a large intra-oceanic strato-volcano in what is globally one of the simplest of subduction settings. In this simple setting erupted magmas span the compositional range from basalt to dacite but none have the high Mg-numbers and high Ni and Cr expected of primitive mantle-derived melts. The lavas range from aphyric to highly porphyritic and are characterised by phenocryst assemblages dominated by plagioclase accompanied by clinopyroxene, olivine, orthopyroxene and spinel. Phenocryst core compositions and zoning profiles are remarkably uniform irrespective of total phenocryst content or geochemical composition, indicating a decoupling of melt and crystal components in the system. A consistent model for the Raoul magmatic system is that primitive high-Mg magma generated in a melt column within the underlying mantle wedge is transformed into a series of derivative low-Mg magmas by fractional crystallisation within the lower crust. Low-Mg magma accumulates variable quantities of crystal cargo as it ascends toward the surface through a crystal mush zone. These processes are essentially those that characterise continental subduction-related magmatic systems but differ only in the absence of an evolved crustal component.