Crystal-bearing Magmas Research Articles

Plug flow and brecciation in volcanic conduits can emerge from shear-induced crystal migration in otherwise Newtonian magmas

In the Earth, the flow of crystal-bearing magma is thought to be non-Newtonian and shear thinning, but the physical origin for this is poorly understood. We use hydro-granular theory to show that the decoupled migration of crystals toward conduit cores during magma ascent is a tenable microphysical mechanism for plug flow, emergent in an otherwise purely Newtonian crystal-bearing magma. We use a numerical conduit model to define the flow development length beyond which crystal migration dominates and strain localises near conduit margins. Applied to magma ascent scenarios, we show that this crystal-migration strain localisation only develops in high crystallinity magmas or magmas ascending in very narrow cracks/conduits. In all other scenarios, crystals do not contribute to non-Newtonian behaviour and such magmas are usually strictly Newtonian. The ascent of very crystal-rich dome magma could be associated with strain localisation and crystal depletion at the conduit margins, lubricating ascent through the crust.

Transcrustal magmatic processes of the Tianshuigou dolerite sill from the ca. 1.32 Ga Yanliao large igneous province: Insights from zoned plagioclase macrocrysts

Transcrustal magmatic processes within the magmatic plumbing systems of large igneous provinces (LIPs) remain poorly constrained. Here we present petrographic textures, whole-rock and mineral geochemistry, and in situ Sr isotopes of several types of zoned plagioclase macrocrysts identified as antecrysts in the shallowly emplaced Tianshuigou dolerite sill from the Yanliao LIP. The goal is to reveal the transcrustal magmatic processes from middle to shallow crust. Our results characterize three zones in the plagioclase macrocrysts: core, mantle, and rim, and we determine that these zones crystallized in different parts of the LIP plumbing system under distinct P-T conditions. The cores of plagioclase macrocrysts originated from a mid-crustal (ca. 17.4 km deep) magma reservoir consisting of crystal mush. The mantle zone of plagioclase macrocrysts records the magma ascent and/or convection processes in a shallower chamber at a depth of ca. 6 km. The rim and groundmass plagioclase crystallized initially in a shallower chamber at a depth of ca. 6 km, and further crystallization of the rim and groundmass occurred during final ascent of the crystal-bearing magma and intrusion into the Xiamaling Formation as dolerite sills. Geothermometers show that the core crystallized at ∼1214 ± 36 °C, the mantle crystallized at ∼1169 ± 36 °C, and the rim and groundmass plagioclase crystallized over a range from ∼1154 ± 36 to 1108 ± 14 °C. The hygrometer data suggest the magma plumbing system is relatively dry (H2O contents <1 wt%). The plagioclase macrocrysts generally show normal zonation from core to rim reflecting overall evolution of magma composition in the plumbing system. Geochemical data suggest that magmas were contaminated by the mafic lower crust prior to crystallization of the plagioclase macrocryst cores in the middle crustal magma reservoir, but that little or no contamination occurred during the magma storage and the ascent in the middle-upper crust. Our results suggest that the zoned plagioclase macrocrysts in mafic sill swarms may record the magmatic processes from the middle to the shallow crust in LIP plumbing systems. The processes that occurred in magma plumbing systems in middle crust play a vital role in the compositions of shallowly emplaced intrusions in LIPs.

Shear thinning and brittle failure in crystal-bearing magmas arise from local non-Newtonian effects in the melt

Crystal-bearing magmas are often assumed to be intrinsically non-Newtonian, but the mechanism underlying suspension shear thinning remains unconstrained. Here, we test the hypothesis that shear thinning in these suspensions arises solely from unrelaxed shear thinning in the melt phase or from viscous heating or both. We compile existing data for the rheology of high-temperature crystal-bearing silicate magmas across a wide range of conditions. In order to test this, we define a ‘lever’ function L(ϕ) which scales the strain rate in the melt phase between crystals relative to the bulk strain rate of the suspension as a function of crystal volume fraction ϕ. We show that, in general, the parameterisation of the amplification of strain rates via the L(ϕ) factor coupled with a shear-thinning law for the melt phase fully accounts for the non-Newtonian behaviour of the suspension. Next, we use the Brinkman number Br to demonstrate that some existing data derive from experiments that were subject to viscous heating, resulting in a further manifestation of apparent shear thinning. Taken together, our results provide a theoretical framework that predicts microphysical regimes for shear thinning. This implies that crystal-bearing magmas are Newtonian except where the scaled conditions for viscous heating are met, or when the strain rates in the melt phase approach the inverse of the structural relaxation timescale. This shear thinning due to non-Newtonian behaviour of the melt between crystals occurs very close to the brittle threshold, yielding an extension of the brittle failure criterion to crystal-bearing magmas, with direct implications for volcanic eruption dynamics.

A comprehensive database of crystal-bearing magmas for the calibration of a rheological model

In this work, we present a comprehensive rheological database including most of the existing data relevant for crystal-bearing magmas collected from the scientific literature, covering the entire range of natural volcanic conditions, in terms of crystal content (1–80%), crystal shape (aspect ratio R from 1 to 13), and strain rate (between 10−7 and 102 s−1). Datasets were collected and discerned as a function of the information which we considered necessary for building a general systematic model describing relative viscosity of crystal-bearing magmas, such as the apparent and melt viscosity, the crystal concentration, crystal shape, and the strain rate. The selected dataset was then used for modelling the relative viscosity of a liquid-solid mixture having different concentrations of particles with different R, subjected to different strain rates. The proposed model allows us to quantitatively describe the rheological behaviour of crystal-bearing magmatic systems.

Modelling the Formation of Linear Geochemical Trends Using the Magma Chamber Simulator: A Case Study of the Jindabyne Granitoids, Lachlan Fold Belt, Australia

Abstract Understanding the origins of major and trace element variations and the isotopic character of granite samples in terms of sources and magmatic processes is, arguably, the core of granite petrology. It is central to attempts to place these rocks in the context of broader geologic processes and continent evolution. For the granites of the Lachlan and New England Fold Belts (LFB and NEFB) of Australia there has been great debate between competing petrogenetic models. The open-system view is that the isotopic variability and within-suite compositional trends can be accounted for by magma mixing, assimilation and fractional crystallisation (FC). In contrast, the restite unmixing model views the isotope compositions of diverse granites as a feature inherited from individual protoliths that underwent partial melting to produce magmas entraining varying proportions of residual material in a felsic melt. Reconciling all aspects of the geochemical data in a mixing model is contingent on a plausible fractionation regime to produce the observed consistently linear (or near-linear) trends on Harker diagrams; however, published FC models lack phase equilibria constraints on the liquidus assemblage and do not account for the likely changes in trace element partitioning across the modelled compositional range. The Magma Chamber Simulator (MCS) can be used to model fractional crystallisation alone (FC) or with assimilation (AFC), constraining phase equilibria and accounting for the thermal budget. Here, this tool was used to conduct a case study of the I-type Jindabyne Suite of granites from the LFB, testing whether thermodynamically feasible geochemical trends matching the observed linear variations can arise through FC (with or without assimilation of supracrustal material). The results of 112 MCS models show: (1) that major element liquid lines of descent (LLDs) may be sensibly linear over limited compositional ranges, (2) that the involvement of assimilation extends the range in which trends are relatively simple and near-linear, and (3) that, despite these observations, neither FC nor AFC are able to correctly reproduce the geochemical evolution of the I-type Jindabyne Suite granitoids as an LLD (contrary to existing models)—instead, these processes persistently produce curved and kinked trends. The output of these simulations were further refined to explore models in which: (1) crystal-bearing magmas evolve via FC or AFC (with chemical isolation assumed to be achieved through crystal zoning) and undergo varying degrees of melt-crystal segregation at different stages to produce the sample compositions, and (2) in situ crystallisation occurs via FC within the crystallisation zone, driving the evolution of a liquid resident magma, which the samples represent. These models are able to reproduce the Jindabyne Suite trends reasonably well. The modelling implies that FC, or some variant thereof, is a viable explanation for the linear trends in Jindabyne; however, tendency for grossly non-linear LLDs highlights that it should not be assumed that FC can generally explain linear trends in granites without careful modelling such as shown here.

Two-tiered semi-empirical model for the viscosity of crystal-bearing magmas

Two-tiered semi-empirical model for the viscosity of crystal-bearing magmas

Subliquidus rheology of basalt from the 2018 Lower East Rift Zone Kilauea eruption: isothermal vs. dynamic expression

The dynamic effects of temperature and strain rate on rheology of crystal-bearing magma are investigated. We conducted high-temperature rheometry experiments in both isothermal and dynamic crystallization regimes and recovered textural data for the isothermal runs. We propose a framework for the parameterization of magma rheology, via an equation describing how the rheological cutoff temperature (the temperature at which magma stops flowing) varies as a function of cooling rate and strain rate. This equation may be used to inform rapid response in effusive crises. Cooling rate has the larger effect, with higher cooling rates yielding lower cutoff temperatures; higher strain rates yield higher cutoff temperatures. Textural analyses reveal differences in crystal aspect ratios, such that higher cooling rates produce only subequant crystals, whereas lower cooling rates also produce a second, higher aspect ratio crystal population. We identify this textural variation as the physical cause for the dependence of cutoff temperature on cooling rate. Plain language summaryAs lava cools, it crystallizes. Eventually, this crystallinity becomes so high that the lava can no longer advance. The temperature at which the crystals “lock up” the lava is called the “rheological cutoff temperature.” This depends, in principle, on the crystallization pathway, which is influenced by both the cooling rate and the strain rate of the lava flow. We conducted rheological experiments on 2018 Kilauea lavas along different crystallization pathways. We determined that higher cooling rates (5 °C/min) yield cooler cutoffs (983–1058 °C), and higher strain rates (8 s−1) yield hotter cutoffs (1058–1093 °C). Moreover, the cooling rate affects the cutoff temperature more than the strain rate. Through a complementary set of experiments, we found that the physical cause for the dependence of the cutoff temperature on the cooling rate is crystal aspect ratio (length/width). At any given crystallinity, crystals with a higher aspect ratio interact more, and lock at higher temperatures. Higher cooling rates produce only crystals with an aspect ratio of 1, whereas lower cooling rates produce higher aspect ratio crystals as well. Therefore, lava which has cooled more slowly, which crystallizes higher aspect ratio crystals, has a higher rheological cutoff temperature.

Rapid alteration of fractured volcanic conduits beneath Mt Unzen

The nature of sub-volcanic alteration is usually only observable after erosion and exhumation at old inactive volcanoes, via geochemical changes in hydrothermal fluids sampled at the surface, via relatively low-resolution geophysical methods or can be inferred from erupted products. These methods are spatially or temporally removed from the real subsurface and thus provide only indirect information. In contrast, the ICDP deep drilling of the Mt Unzen volcano subsurface affords a snapshot into the in situ interaction between the dacitic dykes that fed dome-forming eruptions and the sub-volcanic hydrothermal system, where the most recent lava dome eruption occurred between 1990 and 1995. Here, we analyse drill core samples from hole USDP-4, constraining their degree and type of alteration. We identify and characterize two clay alteration stages: (1) an unusual argillic alteration infill of fractured or partially dissolved plagioclase and hornblende phenocryst domains with kaolinite and Reichweite 1 illite (70)-smectite and (2) propylitic alteration of amphibole and biotite phenocrysts with the fracture-hosted precipitation of chlorite, sulfide and carbonate minerals. These observations imply that the early clay-forming fluid was acidic and probably had a magmatic component, which is indicated for the fluids related to the second chlorite-carbonate stage by our stable carbon and oxygen isotope data. The porosity in the dyke samples is dominantly fracture-hosted, and fracture-filling mineralization is common, suggesting that the dykes were fractured during magma transport, emplacement and cooling, and that subsequent permeable circulation of hydrothermal fluids led to pore clogging and potential partial sealing of the pore network on a timescale of ~ 9 years from cessation of the last eruption. These observations, in concert with evidence that intermediate, crystal-bearing magmas are susceptible to fracturing during ascent and emplacement, lead us to suggest that arc volcanoes enclosed in highly fractured country rock are susceptible to rapid hydrothermal circulation and alteration, with implications for the development of fluid flow, mineralization, stress regime and volcanic edifice structural stability. We explore these possibilities in the context of alteration at other similar volcanoes.

Textural and Geochemical Evidence for Multiple, Sheet-like Magma Pulses in the Limeira Intrusion, Paraná Magmatic Province, Brazil

Abstract Quantitative petrographic, structural, and textural parameters are integrated with geological, geochemical, and Sr-isotope data to examine the emplacement, growth processes, and magmatic evolution of the high-Ti tholeiitic Limeira Intrusion, in the Paraná Magmatic Province of SE Brazil. Our data strongly support a multiple-stage evolution, owing to the nested emplacement of distinct crystal-bearing magma pulses that probably evolved independently, except at their boundaries. A stage of cooling and crystallization between magma injections results in a stepwise T–t path, leading to variations in the plagioclase residence times and effective growth rates inwards, also occasioning sudden changes in crystal shape and size at the boundaries of each magma pulse. The time delay between pulses allows preservation of internal ‘chilled margins’ and the development of near-rigid surfaces at their contacts, increasing the alignment and clustering of crystals during magma replenishment. Isotopic and textural data demonstrate a complex assembly history, in which the appearance of mixed plagioclase populations between magma pulses coincides with the onset of initial Sr isotope ratio increase, which can be attributed to a locally enhanced cooling rate, and the extraction of residual melts from the previous crystallizing batches and mixing with the younger pulses. Typical C- and S-shaped MgO (wt%) compositional profiles within individual pulses indicate that the first one probably evolved by in situ fractional crystallization followed by melt migration inward, whereas the younger ones have contributions from both compaction of the lowermost crystallization front and compositional convection. Mafic globular structures are found at the boundaries of magma pulses and constituting the mafic-rich layers in layered rocks. They are interpreted as evidence for chemical disequilibrium, arguably associated with the trigger of silicate liquid immiscibility. The upwards compositional convection of the silica-rich residual liquid and the accumulation of the Fe–Ti–P-rich crystalbearing end-member in the bottom of the latest magma pulses might represent the most significant mechanism of differentiation in the Limeira Intrusion.

In situ observation of the percolation threshold in multiphase magma analogues

Magmas vesiculate during ascent, producing complex interconnected pore networks, which can act as outgassing pathways and then deflate or compact to volcanic plugs. Similarly, in-conduit fragmentation events during dome-forming eruptions create open systems transiently, before welding causes pore sealing. The percolation threshold is the first-order transition between closed- and open-system degassing dynamics. Here, we use time-resolved, synchrotron-source X-ray tomography to image synthetic magmas that go through cycles of opening and closing, to constrain the percolation threshold ΦC at a range of melt crystallinity, viscosity and overpressure pertinent to shallow magma ascent. During vesiculation, we observed different percolative regimes for the same initial bulk crystallinity depending on melt viscosity and gas overpressure. At high viscosity (> 106 Pa s) and high overpressure (~ 1–4 MPa), we found that a brittle-viscous regime dominates in which brittle rupture allows system-spanning coalescence at a low percolation threshold (ΦC~0.17) via the formation of fracture-like bubble chains. Percolation was followed by outgassing and bubble collapse causing densification and isolation of the bubble network, resulting in a hysteresis in the evolution of connectivity with porosity. At low melt viscosity and overpressure, we observed a viscous regime with much higher percolation threshold (ΦC > 0.37) due to spherical bubble growth and lower degree of crystal connection. Finally, our results also show that sintering of crystal-free and crystal-bearing magma analogues is characterised by low percolation thresholds (ΦC = 0.04 – 0.10). We conclude that the presence of crystals lowers the percolation threshold during vesiculation and may promote outgassing in shallow, crystal-rich magma at initial stages of Vulcanian and Strombolian eruptions.

Polybaric/polythermal magma transport and trace element partitioning recorded in single crystals: A case study of a zoned clinopyroxene from Mt. Etna

We present elemental maps and intra-crystal compositional profiles conducted on a representative clinopyroxene phenocryst from the 1974 eccentric lava flows at Mt. Etna volcano (Sicily, Italy). The eruption was fed by deep-seated and primitive magmas ascending through pathways bypassing the central volcanic conduits. These magmas show MgO and Cr contents higher (and REE + Y lower) than those characterizing younger and more evolved eruptions, albeit the bulk rock compositions of both primitive and more evolved products are invariably classified as trachybasalts in the TAS (total alkali vs. silica) diagram. Mafic recharge episodes are recorded by the complex textural features of the clinopyroxene, with a subrounded core enclosed within a concentrically zoned mantle. The core is enriched in Mg + Fe2++Na and depleted in Fe3++Ca relative to the mantle. The jadeite (Jd) component decreases from core to mantle and is counterbalanced by higher Ca-Tschermak (CaTs) contents, as the number of TAl cations in tetrahedral coordination increases. The Jd-rich core incorporates high proportions of rare earth elements and Y (REE + Y) and low concentrations of high field strength elements (HFSE) and transition elements (TE, such as Ni, Cr and Sc), whereas the opposite occurs for the CaTs-rich mantle. The decoupling of REE + Y and HFSE argues against simple changes in melt composition and indicates an additional mechanism driving trace element zonations. Thermobarometric calculations indicate that the early-formed Jd-rich core equilibrated with the host magma at mantle depths (750–950 MPa and 1190–1210 °C), whereas the later CaTs-rich mantle formed at shallower crustal levels (400–700 MPa and 1150–1180 °C) after magma recharge. Quantitative modeling of apparent cation partitioning between clinopyroxene and melt (Di) indicates that DHFSE increase from the Jd-rich core to the CaTs-rich mantle. In contrast, DREE+Y increase up to one order of magnitude at the Jd-rich core due to the enhanced stability of an Na0.5REE + Y0.5MgSiO6 end-member. We infer that compositional changes in clinopyroxene due to the different P-T conditions of the plumbing system may control the concentrations of REE + Y in residual melts derived after partial crystallization and differentiation of primitive magmas, such as those feeding the 1974 eccentric eruption. On this basis, we use DREE+Y measured across the core-mantle interface to constrain the geochemical evolution of recent 2000–2013 magmas at Mt. Etna volcano by Rayleigh fractional crystallization. Results indicate that magma dynamics proceed via a stepwise polybaric-polythermal process accounting for 1) crystallization of Jd-rich clinopyroxenes at high-P, high-T conditions, 2) upward migration of crystal-bearing magmas due to replenishment phenomena with input of fresh magmas and 3) crystallization of CaTs-rich clinopyroxene in low-P, low-T reservoirs. The resulting total amount (~40 vol%) of clinopyroxene fractionated agrees with geophysical data suggesting the presence of highly crystalline magmatic bodies at shallow to intermediate crustal levels below Mt. Etna.

Crystal plasticity as an indicator of the viscous-brittle transition in magmas

Understanding the flow of multi-phase (melt, crystals and bubbles) magmas is of great importance for interpreting eruption dynamics. Here we report the first observation of crystal plasticity, identified using electron backscatter diffraction, in plagioclase in andesite dome lavas from Volcán de Colima, Mexico. The same lavas, deformed experimentally at volcanic conduit temperature and load conditions, exhibit a further, systematic plastic response in the crystalline fraction, observable as a lattice misorientation. At higher stress, and higher crystal fraction, the amount of strain accommodated by crystal plasticity is larger. Crystal plastic distortion is highest in the intact segments of broken crystals, which have exceeded their plastic limit. We infer that crystal plasticity precludes failure and can punctuate the viscous-brittle transition in crystal-bearing magmas at certain shallow magmatic conditions. Since crystal plasticity varies systematically with imposed conditions, this raises the possibility that it may be used as a strain marker in well-constrained systems.

Rheology of crystal-bearing natural magmas: Torsional deformation experiments at 800 °C and 100 MPa

Rheological behavior of crystal-bearing magma is a controlling factor of magma flow dynamics and hence the style of volcanic eruptions. In this study, we performed torsional deformation experiments to determine the viscosity of crystal-bearing rhyolitic and dacitic magmas. The experiments were conducted at a temperature of 800°C and under confining and pore fluid pressures of 100 and 80MPa, respectively, by using an internally heated gas-medium deformation apparatus. To simulate the rheology of natural magma, we deformed volcanic rocks with 16 and 45vol% crystallinities and <4vol% gas bubbles. These rocks have different crystal and bubble shape and size, which appear to influence magma rheology. By using the mechanical data obtained, we calculated flow indices and viscosities for these magmas. For magma with 16vol% crystallinity, the flow index showed good agreement with previous data obtained from experiments on synthetic magma analogues and model predictions. In contrast, the flow index was smaller than those obtained from previous experiments and model predictions at 45vol% crystallinity, which may be explained by considering that natural magma contains crystals with different shapes and sizes. We also found that the apparent viscosity of the magma increased when sample-scale fractures were observed in run products. This means that the heterogeneity of natural magma causes locally stiff regions within the sample owing to crystal interaction.Our data indicate that the viscosity of crystal-rich magma is strongly dependent on the strain rate. This implies that magma ascending in a volcanic conduit shows a plug-type flow because the viscosity decreases near the conduit rim where the strain rate is high. Additionally, if shear localises near the rim, a currently unrecognised mechanism may contribute to outgassing from the central portion of the conduit because outgassing is difficult to facilitate without shear deformation.

Magma Differentiation in Vertical Conduits Revealed by the Complementary Study of Plutonic and Volcanic Rocks from Fuerteventura (Canary Islands)

Fractional crystallization is a widely accepted mechanism to account for magmatic differentiation, but the location and geometry of the locations where this process takes place are still largely unknown. This is mainly due to the scarcity of occurrences where direct links between deep-seated cumulate rocks and their associated differentiation products are observed. The island of Fuerteventura in the Canary Archipelago offers such an opportunity. A dismembered Miocene volcano reveals a complex plutonic root zone, named PX1, consisting of a network of feeder conduits that can be related to remnants of contemporaneous lava flows. The lavas reveal a typical alkaline differentiation trend, whereas PX1 is composed of vertically layered cumulate lithologies ranging from olivine-rich wehrlites to clinopyroxenites and gabbros. Clinopyroxene phenocrysts in lavas (73·2–86·5 Mg#) exhibit the same zoning features and compositional ranges in major and trace elements as the plutonic clinopyroxenes, suggesting a direct genetic link between plutonic and effusive rocks. We interpret the PX1 rocks as resulting from the accumulation in volcanic conduits of phenocrysts segregated from ascending crystal-bearing magmas. Semi-quantitative modeling shows that fractional crystallization can indeed explain the evolution of the erupted lavas from basalt to basaltic trachyandesite. Progressive fractionation of the mineral assemblages olivine → olivine + minor clinopyroxene → clinopyroxene → clinopyroxene + plagioclase → plagioclase + kaersutite + apatite, with variable amounts of Fe–Ti oxides, can reproduce the major element compositional trend revealed by the Fuerteventura lavas. This calculated sequence coincides with the sequence of crystallization deduced from the textures of the PX1 rocks and could a priori generate the mineral assemblages that constitute the plutonic cumulates. However, complex core–mantle chemical zoning observed in PX1 clinopyroxenes suggests more complicated differentiation processes than simple crystal segregation and accumulation in vertical conduits. Primitive cores in PX1 clinopyroxenes (Mg# 80–88) display resorption features and sieve textures; they are interpreted as pre-existing crystals entrained from deeper levels or crystallized during the early stages of magma evolution. Clinopyroxene mantles display more evolved compositions (Mg# 74–80) with reverse zoning and external resorption features. Asymmetry in chemical zoning suggests crystallization in a confined environment such as a crystal mush. All these observations point to progressive mineral coarsening in the crystal cumulates, induced by the pervasive percolation of uprising melts on their way to the surface. The main factors controlling the variability of the cumulate lithologies are the crystal–melt segregation efficiency, the degree of differentiation and modal proportions of the magmas at their time of intrusion, and the extraction efficiency of the residual melts from the cumulates. The proposed model suggests that a differentiation process could take place in the feeder conduits and does not require storage of significant magma volumes (or magma chambers) in the crust to explain the observed evolution from mafic to intermediate magmas. Nevertheless, the existence of such large magma reservoirs is not excluded, in particular to explain the generation of significant volumes of differentiated lavas, as observed in other Canary Islands. Magma differentiation in vertical conduits opens up new perspectives in the understanding of fractional crystallization processes associated with the evolution of alkaline mafic magmas in ocean islands and continental intraplate settings.

On the slow decompressive response of volatile- and crystal-bearing magmas: An analogue experimental investigation

The degassing kinetics of ascending magma strongly affect eruption dynamics. The kinetics are in turn influenced by magma properties. The investigation of the relationship between magma properties and eruption dynamics is a key element in revealing the processes characterizing magmatic flows within the shallow conduit. To explore the effects of physical properties on degassing in basaltic eruptive systems, we have designed and carried out experiments on the slow decompressive response of analogue magmas, composed of silicone-oil-based suspensions, using a shock-tube apparatus. Four series of experiments were performed: 1) particle-free silicone oils with viscosity ranging from 1 to 1000 Pa s were used to constrain the liquid response; 2) silicone oils with variable proportion of suspended micrometric spherical particles were employed to assess the effect of different crystal fractions; 3) suspensions of elongated particles in silicone oils were used to investigate the role of crystal shape; 4) the effects of saturation time and pressure were examined. The rheology of both spherical- and elongated-particle-bearing suspensions were characterized by concentric cylinder rotational rheometry. The flow dynamics of the bubbly fluid, from the process of bubble nucleation up to the development of a permeable bubble network, were constrained using image analysis. Different fluid regimes were distinguished: (i) nucleation, (ii) foam build-up and (iii) foam oscillation. By comparing results obtained from the different series of experiments, we were able to assess the primary role played by the presence of particles on the evolution of the gas volume fraction within the samples. Particle fraction has a dominant role at high concentration, affecting the motion of the fluid. Finally, particle shape influences the long-term degassing efficiency of the fluid. Using scaling considerations, such observations are applied to mafic to intermediate systems. The results of our experimental investigation contribute to constraining vesiculation processes in magmas of various crystallinities at shallow depths.

Evidence for multiple pulses of crystal-bearing magma during emplacement of the Doros layered intrusion, Namibia

The Doros Complex is a relatively small (maximum 3.5km×7.5km) shallow-level, lopolithic, layered mafic intrusion in the early Cretaceous Paraná–Etendeka Large Igneous Province. The stratigraphy broadly comprises a minor, fine-grained gabbroic sill and a sequence of primitive olivine-cumulate melagabbros, with a basal chilled margin, an intermediate plagioclase-cumulate olivine gabbro, and a sequence of mineralogically and texturally variable, intermediate, strongly foliated, plagioclase-, olivine- or magnetite-cumulate gabbros. An evolved syenitic (bostonite) phase occurs as cross-cutting dykes or as enclaves within the foliated gabbros. Major element modelling of the liquid line of descent shows that the spectrum of rock types, including the bostonite, is consistent with the fractionation of a basaltic parental magma that crystallised olivine, clinopyroxene, plagioclase, magnetite, K-feldspar and apatite. However, the stratigraphic succession does not correspond to a simple progressive differentiation trend but instead shows a series of punctuated trends, each defined by a compositional reversal or hiatus. Incompatible trace element concentrations do not increase upwards though the body of the intrusion. The major units show similar, mildly enriched rare earth element patterns, with minimal Eu anomalies. Back-calculation of the rare earth element concentrations of these cumulate rocks produces relatively evolved original liquid compositions, indicating fractionation of this liquid from a more primitive precursor.Based on combined field, petrographic, geochemical and geophysical evidence, we propose an origin for the Doros Complex by a minimum of seven closely spaced influxes of crystal-bearing magmas, each with phenocryst contents between 5% and 55%. These findings represent a departure from the traditional single-pulse liquid model for the formation of layered mafic intrusions and suggest the presence of an underlying magmatic mush column, i.e., a large, complex system of interconnected conduits and staging chambers that host a range of co-existing magmas of different compositions and crystal contents.

Quantifying magma segregation in dykes

The dynamics of magma flow is highly affected by the presence of a crystalline load. During magma ascent, it has been demonstrated that crystal-melt segregation constitutes a viable mechanism for magmatic differentiation. Moreover, crystal-melt segregation during magma transport has important implications not only in terms of magma rheology, but also in terms of differentiation of the continental crust. However, the influences of the crystal volume percentage (φ), of their geometry, their size and their density on crystal-melt segregation are still not well constrained. To address these issues, we performed a parametric study using 2D direct numerical simulations, which model the ascension of a crystal-bearing magma in a vertical dyke. Using these models, we have characterised the amount of segregation as a function of different physical properties including φ, the density contrast between crystals and the melt phase (Δρ), the size of the crystals (Ac) and their aspect ratio (R). Results show that small values of R do not affect the segregation. In this case, the amount of segregation depends upon four parameters. Segregation is highest when Δρ and Ac are large, and lowest for large pressure gradient (Pd) and/or large values of dyke width (Wd). These four parameters can be combined into a single one, the Snumber, which can be used to quantify the amount of segregation occurring during magma ascent. Based on systematic numerical modelling and dimensional analysis, we provide a first order scaling law which allows quantification of the segregation for an arbitrary Snumber and φ, encompassing a wide range of typical parameters encountered in terrestrial magmatic systems. Although developed in a simplified system, this study has strong implications regarding our understanding of crystal segregation processes during magma transport. Our first order scaling law allows to immediately determine the amount of crystal-melt segregation occurring in any given magmatic dyke system.

The viscous-brittle transition of crystal-bearing silicic melt: Direct observation of magma rupture and healing

Magmas may fl ow or break depending on their deformation rate. The transition between such viscous and brittle behavior controls the style of volcanic eruptions. While the brittle failure of silicate melts is reasonably well characterized, the effect of crystals on the viscous-brittle transition has not yet been constrained. Here we examine the effect of suspended crystals on the mechanical failure of magmas using torsion experiments performed at temperatures (600‐ 900 °C), strain rates (10 ‐4 ‐10 ‐1 s ‐1 ), and confi ning pressures (200‐300 MPa) relevant for volcanic systems. We present a relationship that predicts the critical stress and associated strain rate at which magmas fail as a function of crystal fraction. Furthermore, the results demonstrate that the viscous to brittle transition occurs at lower stresses and strain rates when crystals are present. The fractures formed during brittle failure of crystal-bearing magma originate in the melt phase, which enables gas to escape, and hence to reduce gas overpressure. These degassing pathways heal on relatively short time scales owing to the high confi ning pressure at depth, highlighting the possibility that coherent lavas may actually be the healed remains of partially degassed magma parcels that have undergone many cycles of fracturing and healing.

The effect of particle shape on suspension viscosity and implications for magmatic flows

[1] The rheology of crystal-bearing magma and lava depends on both the shape and volume fraction of the suspended crystals. We present the results of analogue rheometric experiments on monodisperse suspensions of solid particles in a Newtonian liquid, in which particle volume fraction ϕ and aspect ratio rp are varied systematically. We find that the effect of ϕ on viscosity is well captured by the Maron–Pierce model, and that this model is valid across the range of particle aspect ratios investigated (0.04 ≤ rp ≤ 22, i.e., strongly-oblate to strongly-prolate) when the maximum packing fraction ϕm is treated as a fitting parameter. The value of ϕm derived from fitting to our experimental data depends strongly and systematically on particle aspect ratio; hence, ϕm represents an effective proxy for the influence of particle shape on suspension rheology. We present a simple relationship for ϕm (rp) which allows the viscosity of a suspension to be calculated as a function of ϕ and rp only. We investigate the impact of accounting for crystal shape when modelling volcanic flows by simulating the eruption of magma carrying crystals of different aspect ratio, and conclude that the effect of crystal shape should not be neglected.

Geochemistry of the Permian Kalatongke Mafic Intrusions, Northern Xinjiang, Northwest China: Implications for the Genesis of Magmatic Ni-Cu Sulfide Deposits

The Kalatongke Cu-Ni sulfide deposit in northern Xinjiang, northwest China, is located on the southern side of the regional Irtysh fault zone, which is the boundary between the early Paleozoic Altai orogenic belt to the north and the late Paleozoic Junggar terrane to the south. In the Kalatongke region there are 11 mafic intrusions which were emplaced in Lower Carboniferous strata. Economic Ni-Cu sulfide ores are found within three of these intrusions, which are well differentiated, and compositionally zoned. Four rock types are recognized which, from top to base of the three mineralized intrusions, include biotite diorite, biotite-hornblende norite, biotite-hornblende-olivine norite, and biotite hornblende-dolerite. The biotite-hornblende olivine norites and biotite-hornblende norites are the most favorable host rocks for Ni-Cu mineralization. The initial (87Sr/86Sr)t (t = 280 Ma) ratios of the intrusions vary from 0.70375 to 0.70504, and eNd(t) from 6.3 to 8.2, implying that the magmas originated from depleted asthenospheric mantle. However, the strong enrichment of lithophile elements such as K, Rb, Th, U, and LREE, the negative Nb and Ta anomalies, and the high δ18O values of whole rocks (5.4‐10.6‰) suggest significant crustal contamination. These patterns are present in almost all samples from the intrusions, indicating that the contamination took place before the magma was emplaced at its present level in the crust. Crustal contamination is interpreted to have driven the magma to S-saturation and brought orthopyroxene onto the liquidus. Convection, or perhaps flow differentiation of the crystal-bearing magma during ascent, caused the dense sulfide melt with entrained olivine and orthopyroxene crystals to become concentrated in the center of the intrusions, whereas the fractionated magmas formed other, less mafic intrusions, which intrude the nearby strata.