Crystal-melt Mush Research Articles

Incremental Growth of Layered Mafic-Ultramafic Intrusions Through Melt Replenishment Into a Crystal Mush Zone Traced by Fe-Hf Isotope Systematics

Layered mafic-ultramafic intrusions are among the largest igneous bodies on Earth, and represent aggregations of large volumes of mantle- and some crustal-derived melts. Melts are emplaced over time-intervals of less than one million years, predominantly through multiple pulses of injections into pre-existing melt-crystal slurries. The dynamic interaction of physical processes, including density-driven separation and mixing of different components, within a solidifying magma chamber leads to such extreme chemical diversity between cumulate rock units, that no unified model currently explains all aspects of the genesis of these intrusions. Here we present whole-rock stable Fe isotope data (expressed in ‰ variations as 57Fe relative to IRMM-014) for samples of drill core taken from the stratified paleo-magma chamber of the Upper Zone of the late-Archean Windimurra Igneous Complex, Western Australia. Variations from near chondritic (57Fe ~ +0 ‰) to heavy (57Fe~ +0.2 ‰) values show a co-variation with initial radiogenic Hf isotope data that is unique to the Windimurra Upper Zone. The systematic isotopic variations from the roof to the base of the Upper Zone are best explained by an intricate sequence of events that included fractional crystallisation and physical mixing. We propose that melt freshly sourced from the mantle was injected into and inflated a pre-existing crystal-melt mush comprising the upper Middle Zone. Re-establishment of crystal layering after replenishment introduced a chemical stratification with the formation of what became the Upper Zone and a crystal-interstitial melt ratio decreasing from roof to base. Basal, vanadiferous magnetitite horizons crystallised through enhanced fO2 during liquid replenishment. Variable degrees of perturbation and chaotic stirring of crystals with imperfect mixing of new and old components was followed by rapid crystal settling and subsequent cumulate stratification. Such melt rejuvenation, proposed here to be the cause for a newly established Upper Zone, leaves no unique petrologic fingerprint but can explain not only the observed coupled Fe-Hf isotope systematics but also mineral disequilibria and cryptic layering in layered intrusions.

Imaging crustal melt beneath northeast Japan with Ps receiver functions

To investigate crustal structures related to melt storage, Ps converted waves were used to image crustal velocity discontinuities across the Tohoku region of northeast Japan. Ps receiver functions from 127 permanent seismic stations were migrated to depth, accounting for variations in crustal velocity structure. In the 5-15 km depth range, negative Ps phases indicative of velocity decreases with depth are prevalent in central and western Tohoku, where Holocene volcanoes occur and surface wave tomography indicates low crustal velocities. Large negative Ps phases are largely absent in the crust of the old mountain terranes in eastern Tohoku where crustal velocities are higher. In central and western Tohoku, velocity gradients inferred from Ps phases and shear velocity anomalies are consistent with models for crustal melt storage that involve trans-crustal zones of crystal-melt mush where the highest concentrations of melt are localized at the top of the mush column. Negative Ps phases are concentrated at depths of 5-10 km, and many correlate with the upper margins of localized low velocity zones beneath volcanoes. These correlated features are consistent with the roofs of high melt fraction layers. At depths of 10-30 km, positive Ps phases are consistent with gradual velocity positive gradients that correspond to decreasing melt fraction with depth in the mush column, although some groups of deeper negative Ps phases at depths of 10-20 km may reflect local maxima in melt fraction. The amplitudes of the negative Ps phases at 5-10 km depth vary significantly. In a higher amplitude example, waveform modeling of Ps receiver functions from a station near Hijori volcano indicates a 16% ± 5% Vs drop at 10 km depth, implying melt fractions of 10% ± 3% if Gassmann's equations are assumed, above a velocity increase from 10 to 20 km that indicates decreasing melt fractions with depth. However, modeling of lower amplitude Ps phases in the 5-10 km depth range indicates a 7% ± 3% velocity drop and a maximum melt fraction of 5% ± 2%. The geographic distribution of apparent melt fraction at 5-10 km depth suggests that in some cases the same upper crustal source can supply multiple volcanoes, and in others high melt fraction zones are significantly laterally offset from the nearest volcano.

Rapid hydrothermal cooling above the axial melt lens at fast-spreading mid-ocean ridge.

Axial melt lenses sandwiched between the lower oceanic crust and the sheeted dike sequences at fast-spreading mid-ocean ridges are assumed to be the major magma source of oceanic crust accretion. According to the widely discussed “gabbro glacier” model, the formation of the lower oceanic crust requires efficient cooling of the axial melt lens, leading to partial crystallization and crystal-melt mush subsiding down to lower crust. These processes are believed to be controlled by periodical magma replenishment and hydrothermal circulation above the melt lens. Here we quantify the cooling rate above melt lens using chemical zoning of plagioclase from hornfelsic recrystallized sheeted dikes drilled from the East Pacific at the Integrated Ocean Drilling Program Hole 1256D. We estimate the cooling rate using a forward modelling approach based on CaAl-NaSi interdiffusion in plagioclase. The results show that cooling from the peak thermal overprint at 1000–1050°C to 600°C are yielded within about 10–30 years as a result of hydrothermal circulation above melt lens during magma starvation. The estimated rapid hydrothermal cooling explains how the effective heat extraction from melt lens is achieved at fast-spreading mid-ocean ridges.

Ring schlieren: Description and interpretation of field relations in the Halifax Pluton, South Mountain Batholith, Nova Scotia

A ∼35 km coastal transect of the Halifax Pluton in South Mountain Batholith (SMB) reveals 96 m-scale ring schlieren, clustered in four locations. Ring schlieren are alternating melanocratic gradational to leucocratic bands in granites forming open to closed, nested, circular to elliptical, concentric to eccentric structures with centres aligned along the major axis or minor axis of the outer ellipse, with a cylindrical form in three dimensions, and in which cross-cutting relations suggest a younging direction towards the centre. We use detailed geometric measurements of these ring schlieren to interpret the process(es) of their formation, and to indicate the relative timing of emplacement of the SMB. The dimensions and orientations of the rings are highly variable: major-axis lengths (1.39 ± 1.43 m), minor-axis lengths (1.01 ± 0.79 m), ellipticities (1.34 ± 0.25), and major-axis orientations (008°±54°). Ring schlieren appear to be the youngest structures in this part of the batholith. Modal-abundance and grain-size variation in the rings suggests that they result from shear flow. In three dimensions, the rings are nested vertical cylinders, and probably represent vertical fossil pathways, either of descending xenoliths or of ascending supercritical vapour bubbles. Such processes can produce, and preserve, a track in a crystal-melt mush having a degree of crystallinity between 55 and 75%. Two miarolitic cavities within one multi-ring structure suggest that a rising bubble train may have produced the rings.

Quantification of flow patterns in sheared tonalite crystal-melt mush: Application of fractal-geometry methods

Fractal-geometry-based analysis techniques offer simple and efficient ways for analyzing magmatic fabrics that are otherwise difficult to describe quantitatively. This study shows an application of two different methods on flow patterns observed in a syntectonic magmatic body injected into the lower crust. XZ and YZ rock cuts are scanned, and the scans are automatically transferred to binary patterns of mafic and felsic minerals. These are analyzed by boxcounting as well as the modified Cantor-dust method. Box-counting leads to characterization of the entire patterns, proves their fractality in two different scale ranges, and yields information about magma mingling and grain-aggregate forming processes. The modified Cantor-dust method quantifies the anisotropy of pattern complexity and represents a potentially powerful method for determination of shear sense during magmatic flow. Both methods represent useful tools specifically for analyzing diffuse magmatic fabrics and for connecting field-related studies with analyses on the microscale.

Determining relative magma and host rock xenolith rheology during magmatic fabric formation in plutons: Examples from the middle and upper crust

Field observations, structural analysis, and analytical calculations are utilized to evaluate the strength of intermediate magmas during crystallization in a regional strain field. Two plutons are examined, the subvolcanic 98 Ma old Jackass Lakes pluton, central Sierra Nevada, California, and the voluminous middle crustal 442 Ma old Andalshatten pluton, central Norway. The Andalshatten example contains millimeter- to kilometer-scale xenoliths that display evidence for synmagmatic deformation, including fold reactivation and boudinage, after being isolated in the magma. Fabrics within the pluton adjacent to the xenoliths are usually magmatic, with only local, discontinuous zones of crystal-plastic deformation <1 m from the xenolith contact. Examination of particularly well exposed mafic metavolcanic xenoliths in the Jackass Lakes pluton indicates that all were strained prior to incorporation and then separated from the remaining host rock by brittle cracking. Once isolated from the host rocks, some of these xenoliths were intruded by veins fed by the in situ draining of melt and magma from the surrounding crystal mush zone. The xenoliths continued to deform ductilely at presumably fast strain rates. Axial-planar magmatic foliations within folded granodioritic dikes within xenoliths are parallel to magmatic foliations throughout the Jackass Lakes pluton and metamorphic foliations within the host rocks, indicating that the xenolith deformation occurred within the regional 98 Ma old strain field that affected the pluton. The behavior of these xenoliths suggests that late in the crystallization history, magmas in both middle crustal and subvolcanic settings behaved as a high-strength crystal-melt mush capable of transmitting deviatoric stresses, which drove both elastic and plastic deformation in the enclosed xenoliths. Simultaneously, intercrystalline melt, and in some cases magma, was drained from the host intrusions into the xenoliths. Rheological modeling based on geochemical data yields an effective viscosity of a crystal-free melt of ~104 Pa s and increased to ~107 Pa s as cooling proceeded to 758 °C and crystal content approached 40% for the Jackass Lakes pluton. Such viscosities are too low to impart or transmit deformation into the xenoliths. The preservation of xenoliths in both plutons is compatible with higher crystallinities and/or magma yield strengths as an explanation to arrest the xenoliths in their final position and allow deformation. Estimated effective viscosities considering magma yield strength and measured density variables (melt and solid) are ~1013 Pa s.

Magma–tectonic interaction and the eruption of silicic batholiths

Due to its unfavorable rheology, magma with crystallinity exceeding about 50 vol.% and effective viscosity > 10 6 Pa s is generally perceived to stall in the Earth's crust rather than to erupt. There is, however, irrefutable evidence for colossal eruption of batholithic magma bodies and here we analyze four examples from Spain, Mexico, USA and the Central Andes. These silicic caldera-forming eruptions generated deposits characterized by i) ignimbrites containing crystal-rich pumice, ii) co-ignimbritic lag breccias and iii) the absence of initial fall-out. The field evidence is inconsistent with most caldera-forming deposits, which are underlain by initial fall-out indicating deposition from a sustained eruption column before the actual collapse sequence. In contrast, the documented examples suggest early deep-level fragmentation at the onset of eruption and repeated column collapse generating eruption volumes on the order of hundreds of cubic kilometers almost exclusively in the form of ignimbrites. These examples challenge our understanding of magma eruptability and eruption initiation processes. In this paper, we present an analysis of eruption promoters from geologic, theoretical and experimental considerations. Assessing relevant dynamics and timescales for failure of crystal-melt mush we propose a framework to explain eruption of batholithic magma bodies that primarily involves an external trigger by near-field seismicity and crustal failure. Strain rate analysis for dynamic and static stressing, chamber roof collapse and rapid decompression indicates that large “solid-like” silicic reservoirs may undergo catastrophic failure leading to deep-level fragmentation of batholithic magma at approximately 2 orders of magnitude lower strain rates than those characteristic for failure of crystal-poor magmas or pure melt. Eruption triggers can thus include either amplified pressure transients in the liquid phase during seismic shaking of a crystal-melt mush, decompression by block subsidence or a combination of both. We find that the window of opportunity for the eruption of large silicic bodies may thus extent to crystallinities beyond 50 vol.% for strain rates on the order of > 10 − 3 to 10 − 4 s − 1 .

Accretion and core formation: constraints from metal–silicate partitioning

Experimental metal-silicate partitioning data for Ni, Co, V, Cr, Nb, Mn, Si and W were used to investigate the geochemical consequences of a range of models for accretion and core formation on Earth. The starting assumptions were chondritic ratios of refractory elements in the Earth and the segregation of metal at the bottom of a magma ocean, which deepened as the planet grew and which had, at its base, a temperature close to the liquidus of the silicate. The models examined were as follows. (i) Continuous segregation from a mantle which is chemically homogeneous and which has a fixed oxidation state, corresponding to 6.26 per cent oxidized Fe. Although Ni, Co and W partitioning is consistent with chondritic ratios, the current V content of the silicate Earth cannot be reconciled with core segregation under these conditions of fixed oxidation state. (ii) Continuous segregation from a mantle which is chemically homogeneous but in which the Earth became more oxidized as it grew. In this case, the Ni, Co, W, V, Cr and Nb contents of core and mantle are easily matched to those calculated from the chondritic ratios of refractory elements. The magma ocean is calculated to maintain a thickness approximately 35 per cent of the depth to the core-mantle boundary in the accreting Earth, yielding a maximum pressure of 44GPa. This model yields a Si content of the core of 5.7 per cent, in good agreement with cosmochemical estimates and with recent isotopic data. (iii) Continuous segregation from a mantle which is not homogeneous and in which the core equilibrates with a restricted volume of mantle at the base of the magma ocean. This is found to increase depth of the magma ocean by approximately 50 per cent. All of the other elements (except Mn) have partitioning consistent with chondritic abundances in the Earth, provided the Earth became, as before, progressively oxidized during accretion. (iv) Continuous segregation of metal from a crystal-melt mush. In this case, pressures decrease to a maximum of 31GPa and it is extremely difficult to match the calculated mantle contents of the highly incompatible elements Nb and W to those observed. Progressive oxidation is required to fit the observed mantle contents of vanadium. All of the scenarios discussed above point to progressive oxidation having occurred as the Earth grew. The Earth appears to be depleted in Mn relative to the chondritic reference.

Emplacement kinematics of nepheline syenites from the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt, west of Khariar, NW Orissa: Evidence from meso- and microstructures

Nepheline syenite plutons emplaced within the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt west of Khariar in northwestern Orissa are marked by a well-developed magmatic fabric including magmatic foliation, mineral lineations, folds and S-C fabrics. The minerals in the plutons, namely microcline, orthoclase, albite, nepheline, hornblende, biotite and aegirine show, by and large, well-developed crystal faces and lack undulose extinction and dynamic recrystallization, suggesting a magmatic origin. The magmatic fabric of the plutons is concordant with a solid-state strain fabric of the surrounding mylonites that developed due to noncoaxial strain along the Terrane Boundary Shear Zone during thrusting of the Eastern Ghats Mobile Belt over the Bastar Craton. However, a small fraction of the minerals, more commonly from the periphery of the plutons, is overprinted by a solid state strain fabric similar to that of the host rock. This fabric is manifested by discrete shear fractures, along which the feldspars are deformed into ribbons, have undergone dynamic recrystallization and show undulose extinction and myrmekitic growth. The shear fractures and the magmatic foliations are mutually parallel to the C-fabric of the host mylonites. Coexistence of concordant solid state strain fabric and magmatic fabric has been interpreted as a transitional feature from magmatic state to subsolidus deformation of the plutons, while the nepheline syenite magma was solidifying from a crystal-melt mush state under a noncoaxial strain. This suggests the emplacement of the plutons synkinematic to thrusting along the Terrane Boundary Shear Zone. The isotopic data by earlier workers suggest emplacement of nepheline syenite at 1500 +3/−4Ma, lending support for thrusting of the mobile belt over the craton around that time.

The dynamics of xenolith assimilation

Xenolith assimilation has been simulated with experiments involving melting wax spheres into hot water and aqueous solutions and characterised by a theoretical analysis. Both the cases of neutrally buoyant stationary spheres and spheres sinking through the hot host fluid are examined. Melt generated on a sphere's surface flows (compositional convection) in two regimes; as a sheet over one hemisphere and then detaching as finger plumes from the other. Positional variations in the melting rate are dominantly controlled by differences in the melt layer thickness which influence the thermal gradient and heat flux across the layer. The theoretical model predicts melt layer thicknesses and the heat flux from the surrounding fluid. Calculated melting rates agree well with experimental measurements. Partial melting of non-eutectic compositions produces a layer of crystal-melt mush at the xenolith's surface. The theoretical analysis is extended to account for variation in rheological properties across the mush layer. When stoped into typical magmas, xenoliths of common continental crust lithologies are predicted to melt at rates in the order of 2 mm/hour. Thicknesses of the mobile mush layer around xenoliths are predicted to be typically a few centimetres. Relatively mafic lithologies can melt quicker than silicic compositions because, although they are typically more refractory, their lower melt viscosities result in thinner mush layers and so higher heat fluxes. Especially quick melting of water-saturated lithologies occurs as a consequence of both the reduction in melting temperatures and melt viscosities. Due to hot ambient conditions in the lower continental crust the assimilation of xenoliths into underplated basaltic magma can be very rapid. For granulites and mafic-granulites the predicted melting rates are up to 17 mm/hour. Fast rates of melting and efficient mixing of melt into the host magma indicate that assimilation of xenoliths will have a significant influence on the compositional and thermal evolution of magmas.

High level and low level plagiogranites from the nicoya ophiolite complex, Costa Rica, Central America

Two suites of plagiogranitic rocks within the oceanic crust of the Jurassic-Cretaceous ophiolitic Nicoya complex are distinguished. High level plagiogranites (HLP) crop out at the top of the cumulate sequence in the northwestern part of the Nicoya peninsula. They are associated with iron-rich dolerites and noncumulus gabbros. Based on field relationships and geochemical characteristics (e.g. enrichment in REE and HFS elements) the origin of these rocks is explained in terms of a two stage model of fractional crystallization and filter pressing. The first stage comprises the formation of tholeiitic Fe-rich melt and a highly differentiated crystal-melt mush. The second stage is characterized by filter pressing of plagiogranitic interstitial magma. Fe-dolerites form the residual rocks. Crystal fractionation of this plagiogranitic magma is due to the formation of a rather complete series of siliceous rocks.

Mechanics of Flow Differentiation in Ultramafic and Mafic Sills

Experimental model studies support the view that the mechanical process of magmatic flow differentiation may be a quantitatively important factor in causing a fractionation of rock types, mineralogical zoning, and chemical differentiation in nature. Scale models were constructed using homogeneous mixtures of solids in suspension and partly differentiated solids in solid-fluid mixtures in order to study the process of magmatic flow differentiation in ultramafic and mafic sills with thicknesses of 5-2,000 feet. Structures geometrically similar to some of the sills of north Skye and Soay, Scotland; the Leopard rock sills of Laborador trough; and the Palisades, New Jersey, were produced in quasi-scale models. Study of these models yields a more satisfactory explanation for many of the observed features than do other hypotheses. Conditions of magma flow in the presence of a discontinuity or magma floor and formation of sill by the intrusion of fluid between two parallel surfaces were simulated. In models, agglomeration of crystals and shear rotation appeared to produce structures geometrically similar to olivine nodules and mottled plagioclase. From experimentation, the volume concentration of intratelluric crystals in the crystal-melt mush during flow and the velocity-pressure relationship of the flowing melt have been found to be critical in determining the nature of differentiation producing chemical and crystal fractionation in nature. The wall effect and fluid slippage along the walls due to the continuity of flow imposed by the presence of a discontinuity or magma floor are considered significant in producing crystal-free or non-porphyritic margins and inward migration of intratelluric crystals to form central plug of phenocrysts.