Crystal-rich Mush Research Articles

Lubrication effects on magmatic mush dynamics

Silicic magma bodies are formed by repeated injections of mobile magma and reside as a crystal-rich mush. Numerical studies of open-system events have revealed the complexity of mixing and rheological behavior. This is associated with the dilation of the crystal network and the possible occurrence of a lubricated regime. Lubrication forces are hydrodynamic interactions occurring when neighboring crystals have relative motion. The effect of such dissipative forces has not yet been explored in the case of magmatic mush. Here, we investigate the effects of lubrication on mush dynamics and on magma transport. First, we propose scaling relationships to assess the relative importance of the forces controlling the motion of one crystal within a mush by adding lubrication terms into the Basset-Boussinesq-Oseen equation that describes crystal motion in a viscous melt. We then investigate lubrication effects at the macroscopic scale with computational fluid dynamics with discrete element modeling (CFD-DEM) simulations that include these forces. We explore two cases: crystal mush sedimentation and the injection of a crystal-free magma inside a mush. We perform all simulations twice, with and without lubrication forces, and compare the results. At the grain scale, we show that three dimensionless numbers and the crystal content can describe the competition between viscous drag, buoyancy, and lubrication. Two of these numbers (Stokes and Froude numbers) have been previously employed in the context of dilute suspensions. The third is a new form of the Sommerfeld number that measures the importance of lubrication. At the macroscopic scale, simulation pairs (with and without lubrication forces) exhibit very similar behavior when in steady state. The duration of the transient regime preceding steady state, however, is increased when lubrication forces are included. Lubrication causes an apparent bulk strain hardening followed by softening at the initiation of the mush motion. Our results show that lubrication opposes dilation and the initiation of motion within the magmatic mush during this transient phase. Our results highlight the control that the crystal network exerts on magma transport and provide a novel way to evaluate when lubrication matters.

Petrochronologic perspective on rhyolite volcano unrest at Laguna del Maule, Chile

Rhyolitic magmas have rarely erupted during historical times, thus we have a poor record of the signals of unrest that precede them. The Laguna del Maule volcanic field (LdM), Chile, is in the midst of a decade-long episode of unrest including surface inflation at more than 200 mm/yr. Geomorphic observations indicate that many similar deformation episodes occurred during the late Pleistocene and Holocene. During this time, approximately 40 km3 of rhyolite has erupted effusively and explosively from at least 24 vents distributed around a 300 km2 lake basin. The large volume, protracted eruptive history, and ongoing unrest of LdM offer an unusual opportunity to integrate petrologic reconstructions of recent rhyolite generation with geophysical and geodetic observations associated with an active, growing magma reservoir. New petrochronologic data shows that the most recent rhyolites, erupted during the last 3200 yr, each resided in the shallow crust for only decades following extraction from an underlying reservoir. The rhyolites contain only limited, cryptic evidence for magma replenishment and reheating in the form of Ba concentration spikes in plagioclase, which suggest biotite breakdown in a crystal-rich mush. The absence of evidence for substantial reheating or mixing with intruding magma preceding the rhyolitic eruptions indicates that they must have been triggered by another process. We propose the accumulation of fluids derived from the deeper degassing of mafic melts is capable of pressurizing eruptible magma bodies of low density rhyolite. This process likely continues to this day and is consistent with the best-fit models of the ongoing unrest. The striking absence of visible surface degassing accompanying the unrest at LdM suggests fluids are trapped beneath an impermeable carapace and could catalyze a future explosive eruption.

Nature of the magma storage system beneath the Damavand volcano (N. Iran): An integrated study

Damavand intraplate stratovolcano constructed upon a moderately thick crust (58–67km) over the last 2Ma. The erupted products are dominantly trachyandesite-trachyte (TT) lavas and pyroclasts, with minor mafic magmas including tephrite-basanite-trachybasalt and alkali olivine basalts emplaced as cinder cones at the base of the stratovolcano. The TT products are characterized by a mineral assemblage of clinopyroxene (diopside-augite), orthopyroxene (clinoenstatite), feldspar (An2–58, Ab6–69, Or2–56), high Ti phlogopite, F-apatite, FeTi oxides, and minor amounts of olivine (Fo73–80), amphibole and zircon, whereas olivine (Fo78–88), high Mg# (80–89) diopside, feldspar, apatite and FeTi oxide occur in the mafic magmas. The presence of hydrous and anhydrous minerals, normal zonings, mafic cumulates, and the composition of magmatic inclusions in the TT products suggest evolutionary processes in polybaric conditions. In the same way, disequilibrium textures - including orthopyroxene mantled with clinopyroxene, reaction rim of phlogopite and amphibole, the coexistence of olivine and orthopyroxene, reverse, oscillatory and complex zonings of pyroxene and feldspar crystals - suggest magmatic evolutions in open systems with a varying temperature, oxygen fugacity, water as well as pressure and, to a lesser extent, melt chemistry. Mineral assemblages are used to model the physicochemical conditions and assess default parameters for the thermodynamic simulation of crystallization using MELTS software to track the P-T-H2O-ƒO2 evolution of the magma plumbing system. Thermobarometry and MELTS models estimated the initial nucleation depth at 16–17kb (56–60km) for olivine (Fo89) and high Al diopside crystals occurring in the mafic primary magma; it then stopped and underwent fractionation between 8 and 10kb (28–35km), corresponding with Moho depth, and continued to differentiate in the lower crust, in agreement with the geophysical models. The mafic rocks were formed by crystal fractionation from the reconstructed primary magma (13wt% MgO) with the minor role of recharge and crustal assimilation. Phenocrysts in TT lavas recorded a wide range of temperature and pressure of crystallization; at least three main levels of magma storage can be recognized according to the statistical analysis of the models, at 6–8kb (22–28km), 4–6kb (15–22km), and 0.6–3kb (2–11km), respectively. The temperature of crystallization ranged from 1430 to 1180°C for primary mafic magma to alkali olivine basalts and 1180–800°C for TT suite. According to the current geophysical models, the present structure of the magma storage system in the crust beneath the Damavand volcano consists of three major accumulation zones located at 20km, 6–8km and 3–4.5km depth. Our data enlarge this scenario, suggesting a more complex magma storage system strongly controlled by the transpressional tectonic regime. Multi-depth magma reservoirs may account for the local thickening of crust below the volcano. The polybaric fractionation model, using the MELTS algorithm, reproduces mineralogy and chemical variations of minerals and whole rock of the Damavand TT lavas. However, some discrepancies between major elements of models and trends of data can be ascribed to the recharge of more mafic magma, minor crustal assimilation, disaggregation of crystal-rich mushes and uptake of magmatic inclusions, as well as crystals from different crustal levels. The polybaric differentiation as the fractionation and/or accumulation of crystals was the probable mechanism for explaining the scarcity of mafic volcanic rocks at the Damavand volcano.

Water transfer during magma mixing events: Insights into crystal mush rejuvenation and melt extraction processes

Many plutons preserve evidence of magma mixing between hydrous mafic magmas and resident felsic crystal-rich mushes. To investigate water transfer processes in such systems following thermal equilibration, we conducted 24 h experiments to establish the petrological evolution of a water-undersaturated (4 wt% H2O in the interstitial melt) quartz-bearing dacite crystal mush (0.5−0.8 in crystal fraction) intruded by a water-saturated (≥6 wt% H2O), initially crystal-free, andesite magma at 950 °C and 4 kbar (12 km depth). Our results show isothermal undercooling resulting from a change in liquidus temperatures of the interacting magmas due to their changing water content. Specifically, mafic samples dramatically crystallize during water escape into the felsic end-members and consequent increase in liquidus temperature. Conversely, the addition of water to the felsic mush reduces the liquidus temperature, leading to an increase in melt fraction. The experiments provide insights into how volatiles contribute to crystal mush rejuvenation (i.e., increase of melt fraction). However, H2O diffusion alone is not sufficient to promote melt extraction from short- and long-lived mushes in the Earth’s crust.

Dynamic Magma Systems: Implications for Forecasting Volcanic Activity

Magma systems that supply volcanoes can extend throughout the crust and consist of mush (melt within a crystalline framework) together with ephemeral magma accumulations. Within a crystal-rich mush, slow processes of melt segregation and heat loss alternate with fast processes of destablisation and magma transport. Magma chambers form by two mechanisms: incremental magma intrusion into sub-solidus rocks or the segregation and rapid merging of melt-rich layers within mush regions. Three volcanic states reflect alternations of slow and fast processes: dormancy, unrest and eruption. Monitoring needs to detect processes of melt and fluid movements in the lower and middle crust during destabilisation to improve forecasting.

The timing of compositionally-zoned magma reservoirs and mafic ‘priming’ weeks before the 1912 Novarupta-Katmai rhyolite eruption

The June, 6, 1912 eruption of more than 13 km3 of dense rock equivalent (DRE) magma at Novarupta vent, Alaska was the largest of the 20th century. It ejected >7 km3 of rhyolite, ∼1.3 km3 of andesite and ∼4.6 km3 of dacite. Early ideas about the origin of pyroclastic flows and magmatic differentiation (e.g., compositional zonation of reservoirs) were shaped by this eruption. Despite being well studied, the timing of events that led to the chemically and mineralogically zoned magma reservoir remain poorly known. Here we provide new insights using the textures and chemical compositions of plagioclase and orthopyroxene crystals and by reevaluating previous U–Th isotope data. Compositional zoning of the magma reservoir likely developed a few thousand years before the eruption by several additions of mafic magma below an extant silicic reservoir. Melt compositions calculated from Sr contents in plagioclase fill the compositional gap between 68 and 76% SiO2 in whole pumice clasts, consistent with uninterrupted crystal growth from a continuum of liquids. Thus, our findings support a general model in which large volumes of crystal-poor rhyolite are related to intermediate magmas through gradual separation of melt from crystal-rich mush. The rhyolite is incubated by, but not mixed with, episodic recharge pulses of mafic magma that interact thermochemically with the mush and intermediate magmas. Hot, Mg-, Ca-, and Al-rich mafic magma intruded into, and mixed with, deeper parts of the reservoir (andesite and dacite) multiple times. Modeling the relaxation of the Fe–Mg concentrations in orthopyroxene and Mg in plagioclase rims indicates that the final recharge event occurred just weeks prior to the eruption. Rapid addition of mass, volatiles, and heat from the recharge magma, perhaps aided by partial melting of cumulate mush below the andesite and dacite, pressurized the reservoir and likely propelled a ∼10 km lateral dike that allowed the overlying rhyolite to reach the surface.

Magmatic Processes at Variable Magma Supply along the Galápagos Spreading Center: Constraints from Single Eruptive Units

Petrological study of eruptive units in two locations along the Galápagos Spreading Center provides insight into how the rate of magma supply affects mid-ocean ridge magmatic systems. Study areas with lower magma supply (95°W) and higher magma supply (92°W) have similar spreading rates (53 and 55 mm a–1), but differ by 30% in the time-averaged rate of magma supply (0·3 × 106 and 0·4 × 106 m3 km–1 a–1) as a result of varying proximity to the Galápagos hotspot. We use major and trace element compositions of glass and whole-rock samples, chemistry of mineral phases, and petrography to characterize parental magma variability, fractional crystallization and magma mixing in crustal reservoirs, and timescales of magmatic recharge relative to eruption. At the low magma supply study area, eruptible magma appears to be present only intermittently within the crust; magma recharge is probably infrequent, occurring with a periodicity of several hundred to one thousand years. The shallowest magma body in the crust is thought to be at ∼3 km below the seafloor, and lavas are restricted to a relatively limited compositional range (6·2–9·1 wt % MgO). Magmatic evolution at this location is probably dominated by processes occurring within a crystal-rich mush, with limited subsequent residence in melt-dominated magma reservoirs. Eruptions here appear to be closely coupled to magmatic recharge events; lower MgO lavas have compositional trends controlled by mixing of low- and high-MgO magmas from compositionally distinct parents, and commonly contain both normally and reversely zoned crystals. In contrast, at the high magma supply study area, where a seismically imaged melt lens is located ∼1·7 km below the seafloor, fractional crystallization within a melt-rich magma reservoir results in a larger range in major element compositions of the erupted magmas (2·7–8·2 wt % MgO) with less variation in trace element concentrations or ratios. Temperatures within the melt lens over the last several hundred years have varied by at least 100°C (1070–1170°C); cooling rates within the melt lens are estimated to be greater than 0·5°C per year. Relatively low-MgO lavas have over-enrichments in Cl that are best explained by assimilation of brine associated with hydrothermal circulation within the overlying crust. Between magmatic recharge events, resident magma fractionates and feeds one or more low-volume fissure eruptions. Small bodies of magma may become isolated from the larger magmatic system in the crust, allowing more extreme degrees of fractionation, locally reaching basaltic andesite. This study demonstrates that persistent melt lenses at intermediate rates of magma supply need not be ‘steady state’. The variations in magma composition among eruptive episodes at each location allow us to assess the temporal variability in magma reservoir properties at ridge segments along the Galápagos Spreading Center, in the context of regional variations in magma supply.

Magma storage and evolution of the most recent effusive and explosive eruptions from Yellowstone Caldera

Between 70 and 175 ka, over 350 km3 of high-silica rhyolite magma erupted both effusively and explosively from within the Yellowstone Caldera. Phenocrysts in all studied lavas and tuffs are remarkably homogenous at the crystal, eruption, and caldera-scale, and yield QUILF temperatures of 750 ± 25 °C. Phase equilibrium experiments replicate the observed phenocryst assemblage at those temperatures and suggest that the magmas were all stored in the upper crust. Quartz-hosted glass inclusions contain 1.0–2.5 % H2O and 50–600 ppm CO2, but some units are relatively rich in CO2 (300–600 ppm) and some are CO2-poor (50–200 ppm). The CO2-rich magmas were stored at 90–150 MPa and contained a fluid that was 60–75 mol% CO2. CO2-poor magmas were stored at 50–70 MPa, with a more H2O-rich fluid ( $$X_{{{\text{CO}}_{2} }}$$ = 40–60 %). Storage pressures and volatiles do not correlate with eruption age, volume, or style. Trace-element contents in glass inclusions and host matrix glass preserve a systematic evolution produced by crystal fractionation, estimated to range from 36 ± 12 to 52 ± 12 wt%. Because the erupted products contain <10 wt% crystals, crystal-poor melts likely separated from evolving crystal-rich mushes prior to eruption. In the Tuffs of Bluff Point and Cold Mountain Creek, matrix glass is less evolved than most inclusions, which may indicate that more primitive rhyolite was injected into the reservoir just before those eruptions. The presence and dissolution of granophyre in one flow may record evidence for heating prior to eruption and also demonstrate that the Yellowstone magmatic system may undergo rapid changes. The variations in depth suggest the magmas were sourced from multiple chambers that follow similar evolutionary paths in the upper crust.

Textural and chemical consequences of interaction between hydrous mafic and felsic magmas: an experimental study

Mantle-derived, hydrous mafic magmas are often invoked as a mechanism to transfer heat, mass and volatiles to felsic plutons in the Earth’s crust. Field observations suggest that mafic, water-rich magmas often intrude viscous felsic crystal-rich mushes. This scenario can advect water from the crystallising mafic magma to the felsic magma, leading to an increase in melt fraction in the felsic mush and subsequent mobilisation, at the same time as the mafic magma becomes quenched through a combination of cooling and water loss. To investigate such a scenario, we conducted experiments on a water-undersaturated (4 wt% H2O in the interstitial melt) dacitic crystal mush (50–80 vol% quartz crystals) subject to volatile supply from a water-saturated (≥6 wt% H2O) andesite magma at 950 °C and 4 kbar. Our experimental run products show unidirectional solidification textures (i.e. comb layering) as crystals nucleate at the mafic–felsic interface and grow into the mafic end-member. This process is driven by isothermal and isobaric undercooling resulting from a change in liquidus temperature as water migrates from the mafic to the felsic magma. We refer to this process as “chemical quenching” and suggest that some textures associated with natural mafic–felsic interactions are not simply cooling-driven in origin, but can be caused by exsolution of volatiles adjacent to an interface, whether a water-undersaturated felsic magma (as in our experiments) or a fracture.

Deformation and magma transport in a crystallizing plutonic complex, Coastal Batholith, central Chile

The Carboniferous–early Permian Santo Domingo complex in coastal Chile (33.5°S) preserves magmatic structures that allowed us to partially reconstruct and compare the deformation histories of two intrusive units within a mid-upper crustal zoned pluton. The oldest history is preserved in the Punta de Tralca tonalite, where microgranitoid enclaves record the emplacement and partial assimilation of mostly mafic magma into an intermediate host. Enclaves record early foliation development by a mechanical sorting and alignment of minerals during hypersolidus flow in melt-rich magma currents, followed by diffusion creep and sliding along melt-coated crystals. Structures in a weaker, tonalitic matrix record compaction, flattening, and near-solidus deformation as porous flow, aided by brittle deformation, drained residual melts. These processes produced penetrative S > L fabrics (i.e., planar more dominant that linear fabric) in an increasingly viscous, crystal-rich mush and promoted folding, fracturing, shearing, and crystal-plastic deformation as the mush approached its solidus. The deformation disrupted igneous layering and helped mobilize and concentrate melt-rich aggregates, forming diffuse patches and dikes that intruded previously deformed enclaves and matrix and aided pluton differentiation. A different deformation history is recorded by the Estero Cordoba dike, which intruded and interacted comagmatically with the Punta de Tralca tonalite. The dike records how magma flow near stiff boundaries resulted in velocity gradients that drove deformation during magma replenishment. This deformation reset inherited enclave fabrics, increased ductile stretching and winnowing, and formed linear (L > S) fabrics. This example illustrates how different styles of deformation assisted magma movement through a mid-upper crustal magma chamber and highlights the diverse origins and significance of structures generated by deformation in magmas of variable crystal-melt ratios.

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.

Zircon textures and composition: refractory recorders of magmatic volatile evolution?

Zircon textures and composition have been used to infer magmatic processes including closed-system fractional crystallization, magma mixing or replenishment, and country-rock assimilation. Here, we propose that zircon textures and composition may also be refractory recorders of magmatic volatile evolution. We present field, whole-rock chemical, textural, mineral chemical, and U–Pb age data from evolved, fine-to-coarse-grained granite intrusions on Melville Peninsula, Nunavut, Canada. Zircon forms two main populations in these granites, Type-1 and Type-2 zircon. Type-1 zircon is present in all samples, but predominant in fine-grained granite. Crystals are euhedral and inclusion-rich and show periodic, fine-scale oscillatory zoning, comparatively low concentrations of U (<2,200 ppm) and Hf (<1.6 wt%), high Zr/Hf (~40–62), and pervasive alteration. Type-2 zircon is predominant in coarse-grained granite. Crystals form overgrowths on Type-1 zircon and individual crystals. They are subhedral and inclusion-poor and show weak, irregular, large-scale oscillatory zoning, high U (up to ~7,250 ppm) and Hf (1.5–2.0 wt%), low Zr/Hf (~37–44), and only local alteration. Compatible trace-element concentrations and Zr/Hf change sharply across the boundary of Type-1 to Type-2 zircon; 207Pb/206Pb ages preclude a significant hiatus between crystallization of the two types. We argue against magmatic versus hydrothermal crystallization, country-rock assimilation, or magma mixing as causes for the crystallization of Type-1 and Type-2 zircon. We propose instead that Type-1 zircon formed from volatile-undersaturated magmas and that Type-2 zircon formed from volatile-saturated magmas. Magmas fractionated by volatile-driven filter pressing into crystal-rich mush and crystal-poor magma. Crystal-rich mush with abundant Type-1 zircon crystallized to fine-grained granite. Volatile-rich magma crystallized to Type-2 zircon and coarse-grained granite. While Type-1 zircon was pervasively altered by exsolving magmatic volatiles, Type-2 zircon was only locally affected by subsolidus hydrothermal alteration.

Multi-batch, incremental assembly of a dynamic magma chamber: the case of the Peninsula pluton granite (Cape Granite Suite, South Africa)

The S-type Peninsula Pluton (South Africa) exhibits substantial compositional variability and hosts a large variety of mafic and felsic magmatic enclaves with contrasting textures and compositions. Moreover, the pluton is characterized by mechanical concentrations of K-feldspar megacrysts, cordierite and biotite, generating a complex array of magmatic structures including schlieren, pipes, and spectacular sheeted structures. Chemical evidence indicates that the pluton is constructed incrementally by rapid emplacement of numerous magma pulses. Field, and textural data suggest that magmatic structures form by local flow at the emplacement level of highly viscous crystal-rich magmas (i.e. crystallinity up to 50 vol.%) through magma mushes assembled from older batches. At the time of arrival of relatively late magma batches, some areas within the pluton had achieved crystal fractions that allowed the material to act as a solid, whilst maintaining enough melt to prevent formation of sharp intrusional contacts. Magmatic structures represent “snapshots” of processes that operate in multiphase crystal-rich mushes and their genesis is due to mechanical and thermal instabilities in the crystal-rich magma chamber that are triggered by the emplacement of pulses of new magma derived from the melting of a compositionally variable metasedimentary source.

A rapid mechanism to remobilize and homogenize highly crystalline magma bodies

The largest products of magmatic activity on Earth, the great bodies of granite and their corresponding large eruptions, have a dual nature: homogeneity at the large scale and spatial and temporal heterogeneity at the small scale. This duality calls for a mechanism that selectively removes the large-scale heterogeneities associated with the incremental assembly of these magmatic systems and yet occurs rapidly despite crystal-rich, viscous conditions seemingly resistant to mixing. Here we show that a simple dynamic template can unify a wide range of apparently contradictory observations from both large plutonic bodies and volcanic systems by a mechanism of rapid remobilization (unzipping) of highly viscous crystal-rich mushes. We demonstrate that this remobilization can lead to rapid overturn and produce the observed juxtaposition of magmatic materials with very disparate ages and complex chemical zoning. What distinguishes our model is the recognition that the process has two stages. Initially, a stiff mushy magma is reheated from below, producing a reduction in crystallinity that leads to the growth of a subjacent buoyant mobile layer. When the thickening mobile layer becomes sufficiently buoyant, it penetrates the overlying viscous mushy magma. This second stage rapidly exports homogenized material from the lower mobile layer to the top of the system, and leads to partial overturn within the viscous mush itself as an additional mechanism of mixing. Model outputs illustrate that unzipping can rapidly produce large amounts of mobile magma available for eruption. The agreement between calculated and observed unzipping rates for historical eruptions at Pinatubo and at Montserrat demonstrates the general applicability of the model. This mechanism furthers our understanding of both the formation of periodically homogenized plutons (crust building) and of ignimbrites by large eruptions.

PETROLOGY, GEOCHEMISTRY, AND STRUCTURE OF THE CHUGWATER ANORTHOSITE, LARAMIE ANORTHOSITE COMPLEX, SOUTHEASTERN WYOMING

The Chugwater Anorthosite is one of several 1.43 Ga intrusions that make up the Laramie Anorthosite Complex in the Laramie Mountains, Wyoming, USA. The southwestern portion of the Chugwater Anorthosite has a mappable magmatic stratigraphy totaling at least 8000 meters, and probably 10,000 m or more. It consists of three major units, each of which comprises a lower, dominantly anorthosite portion (>90 vol.% plagioclase) and an upper, dominantly gabbroic anorthosite section (80–90 vol.% plagioclase). Each anorthosite portion contains layers of gabbroic anorthosite and vice versa , on a variety of scales ranging down to centimeters. Plagioclase is dominantly An 50–55 , although higher and lower values are also found. This “main series” of the Chugwater Anorthosite mainly lacks modal and normative olivine. Oxygen fugacity ranged from FMQ to FMQ + 0.5. We interpret that the “main series” was produced by at least three major injections of mildly hyperfeldspathic magma containing approximately 40% entrained plagioclase megacrysts (tabular crystals at least 5 cm across). The Ti contents of the megacrysts (0.3–0.4 wt% TiO 2 ) suggest that these had crystallized at pressures near 10 kbar, consistent with initial formation in a magma chamber at or near the base of the crust. Emplacement pressure is poorly constrained, but appears to have been near 3.5–4 kbar. However, before the main series had completely solidified, it was repeatedly intruded by at least two (and probably more) leucotroctolitic magmas. Contacts of leucotroctolite are sharp against anorthosite, but commonly are diffuse against gabbroic anorthosite, suggesting that sufficient residual melt remained in the latter to permit local mixing with leucotroctolite. We estimate that 80–85% of the Chugwater Anorthosite is main-series, 10–15% is mixed rock, and 5% is leucotroctolite. Prior to final solidification, the entire Chugwater Anorthosite was domed, probably as a result of gravitational instability of the relatively buoyant plagioclase-rich material. The emplacement of the Chugwater Anorthosite as a series of crystal-rich magmas, followed by continued fractionation in a magma chamber at a mid-crustal level and subsequent doming, are characteristics that make the Chugwater Anorthosite intermediate between the Poe Mountain Anorthosite to the north ( in situ fractionation in a magma chamber) and the classic diapiric emplacement of a crystal-rich mush.

The Mammoth Peak sheeted complex, Tuolumne batholith, Sierra Nevada, California: a record of initial growth or late thermal contraction in a magma chamber?

The Mammoth Peak sheeted intrusive complex formed in the interior of a ~7–10 km deep magma chamber, specifically in the Half Dome granodiorite of the Tuolumne batholith, central Sierra Nevada, CA (USA). The sheets consist of fractionated melts with accumulated hornblende, biotite, magnetite, titanite, apatite, and zircon. The accumulation, especially of titanite, had a profound effect on minor and trace elements (Nb, Ta, Ti, REE, U, Th, P, Zr, Hf, etc.), increasing their contents up to five to six times. Our thermal–mechanical modeling using the finite element method shows that cooling-generated tensile stresses resulted in the inward propagation of two perpendicular sets of dilational cracks in the host granodiorite. We interpret the sheeted complex to have formed by a crack-seal mechanism in a high strength, crystal-rich mush, whereby outward younging pulses of fractionated magma were injected into these syn-magmatic cracks at the margin of an active magma chamber. Thermal–mechanical instabilities developed after the assembly of the sheeted complex, which was then overprinted by late ~NW–SE magmatic foliation. This case example provides a cautionary note regarding the interpretation that sheeted zones in large granitoid plutons imply a diking mechanism of growth because the sheeted/dike complexes in plutons (1) may display inverse growth directions from the growth of the overall intrusive sequence; (2) need not record initial chamber construction and instead may reflect late pulsing of magma within an already constructed magma chamber; (3) have an overprinting magmatic fabric indicating the continued presence of melt after construction of sheeted complexes and thus a prolonged thermal history as compared to dikes; and (4) because the scale of the observed sheeted complexes may be small (<1%) in comparison to large homogenous parts of plutons, in which there is no evidence for sheeting or diking. Thus, where extensive dike complexes in plutons are absent, such as in much of the Tuolumne batholith, the application of an incremental diking model to explain chamber construction is at best speculative.

Magmatic structures in the Krkonoše–Jizera Plutonic Complex, Bohemian Massif: evidence for localized multiphase flow and small-scale thermal–mechanical instabilities in a granitic magma chamber

The present paper examines magmatic structures in the Jizera and Liberec granites of the Krkonoše–Jizera Plutonic Complex, Bohemian Massif. The magmatic structures are here interpreted to preserve direct field evidence for highly localized magma flow and other processes in crystal-rich mushes, and to capture the evolution of physical processes in an ancient granitic magma chamber. We propose that after chamber-wide mixing and hybridization, as suggested by recent petrological studies, laminar magma flow became highly localized to weaker channel-like domains within the higher-strength crystal framework. Mafic schlieren formed at flow rims, and their formation presumably involved gravitational settling and velocity gradient flow sorting coupled with interstitial melt escape. Local thermal or compositional convection may have resulted in the formation of vertical schlieren tubes and ladder dikes whereas subhorizontal tubes or channels formed during flow driven by lateral gradients in magma pressure. After the cessation or deceleration of channel flow, gravity-driven processes (settling of crystals and enclaves, gravitational differentiation, development of downward dripping instabilities), accompanied by compaction, filter pressing and melt segregation, dominated in the crystal mush within the flow channels. Subsequently, magmatic folds developed in schlieren layers and the magma chamber recorded complex, late magmatic strains at high magma crystallinities. Late-stage magma pulsing into localized submagmatic cracks represents the latest events of magmatic history of the chamber prior to its final crystallization. We emphasize that the most favorable environments for the formation and preservation of magmatic structures, such as those hosted in the Jizera and Liberec granites, are slowly cooling crystal-rich mushes. Therefore, where preserved in plutons, these structures may lend strong support for a “mush model” of magmatic systems.

Melt inclusion analysis of the Unzen 1991–1995 dacite: implications for crystallization processes of dacite magma

Dacitic magma, a mixture of high-temperature (T) aphyric magma and low-T crystal-rich magma, was erupted during the 1991–1995 Mount Unzen eruptive cycle. Here, the crystallization processes of the low-T magma were examined on the basis of melt inclusion analysis and phase relationships. Variation in water content of the melt inclusions (5.1–7.2 wt% H2O) reflected the degassing history of the low-T magma ascending from deeper levels (∼250 MPa) to a shallow magma chamber (∼140 MPa). The ascent rate of the low-T magma decreased markedly towards the emplacement level as crystal content increased. Cooling of magma as well as degassing-induced undercooling drove crystallization. With the decreasing ascent rate, degassing-induced undercooling decreased in importance, and cooling became more instrumental in crystallization, causing local and rapid crystallization along the margin of the magma body. Some crystals contain scores of melt inclusions, whereas there are some crystals without any inclusions. This heterogeneous distribution suggests the variation in the crystallization rate within the magma body; it also suggests that cooling was dominant cause for melt entrapment. Numerical calculations of the cooling magma body suggest that cooling caused rapid crystal growth and enhanced melt entrapment once the magma became a crystal-rich mush with evolved interstitial melt. The rhyolitic composition of melt inclusions is consistent with this model.

Progressive or continual exsolution in pyroxenes: an indicator of polybaric igneous crystallization for the Perinthatta anorthositic gabbro, northern Kerala, southwestern India

Clinopyroxene and orthopyroxene from the Perinthatta anorthositic gabbro in southwestern India exhibit exsolution intergrowths of pyroxenes (occurring in five stages involving pigeonite, orthopyroxene and/or augite) and Fe–Ti oxides. The thicker irregular pyroxene lamellae appear to be the result of heterogeneous nucleation, indicating that this was the first unmixing process to begin. This was followed by homogeneous nucleation of regularly spaced, quite uniform pyroxene lamellae and defect-free fine pyroxene platelets between early-formed exsolution lamellae, suggesting progressive or continual nature of exsolution during slow cooling. Pyroxene thermometric calculations indicate that subcalcic augite and pigeonite equilibrated at ∼935 °C, with the different lamellar generations formed between ∼914 and ∼748 °C. Pre-metamorphic anorthositic gabbro crystallization is considered to have taken place at deep crustal levels (∼30 km; ∼9–10 kbar) and later emplacement at shallower crustal levels (∼15 km; ∼5–6 kbar) suggesting polybaric conditions. The apparent difference between the pressure of crystallization and the emplacement of the Perinthatta anorthositic gabbro supports the common Proterozoic anorthosite petrogenetic model, involving initial crystallization at depth, followed by intrusion as a crystal-rich mush. Following emplacement, the igneous assemblage was affected by granulite facies metamorphism on cooling, as indicated by smaller granulite minerals along grain boundaries and the partial re-equilibration of exsolved phases.

Basaltic pillow mounds in the Vinalhaven intrusion, Maine

The Vinalhaven intrusion is a dominantly granitic pluton of probable Devonian age, located on Vinalhaven Island and adjacent islands, Maine. It consists of four main units: coarse-grained granite, fine-grained granite, a gabbro-diorite unit consisting of interlayered mafic, hybrid and granitic rocks, and a heterogeneous granitic unit. The gabbro-diorite unit occurs along the south and east coast of the island as a sheet-like body, hundreds of meters to more than 1km thick, that dips beneath the central granitic units and rests on heterogeneous granitic rocks that form the base of the intrusion and are exposed on islands to the southeast. Load-cast and pipe structures at the bases of mafic sheets indicate that the gabbro-diorite unit represents a sequence of basaltic injections that ponded on crystal-rich mush at the base of a silicic magma chamber and variably interacted with overlying crystal-poor granitic magma. The pluton, therefore, represents a fossilized silicic magma chamber that was periodically replenished by basaltic magma. Near the base of the gabbro-diorite unit, some basaltic injections produced large mounds up to more than 10m high and 100m wide of tightly packed, meter-scale chilled basaltic pillows, tubes and sheets in a granitic matrix. The mounds appear to represent flow fronts of basaltic injections that entered and ponded on the floor of a silicic magma chamber. Although physical conditions differ significantly, these plutonic pillow mounds appear to share many characteristics with submarine pillow basalts and lava flows.