Convecting Magma Chamber Research Articles

Crystal retention, fractionation and crustal assimilation in a convecting magma chamber, Nisyros Volcano, Greece

Crystal retention, fractionation and crustal assimilation in a convecting magma chamber, Nisyros Volcano, Greece

Petrogenesis of the St David's Head Layered Intrusion, Wales: a complex history of multiple magma injection andin situcrystallisation

AbstractThe St David's Head Intrusion, exposed in North Pembrokeshire, is a tholeiitic sill up to 570 m thick, comprising principally mafic gabbros, which are in part layered. Layering varies from the centimetre to the metre scale. A range of gabbroic compositions is present, defining seven major petrological types, which are cut by thin silicic (aplitic) veins.Log–log plots of incompatible elements from the various lithological units indicate that all of the rocks in the intrusion are petrogenetically linked, although a variety of processes has been operative. Roach (1969) considered the quartz gabbros and dolerites, which form an envelope around the other units, to relate most closely to the parental composition. However, the least evolved compositions in the intrusion are from the xenolithic laminated olivine gabbros, although these mafic compositions are due primarily to the presence of abundant, mafic, cognate xenoliths. These xenoliths are thought to relate to an earlier episode of crystal accumulation in a high-level magma chamber. The various laminated gabbros reflect crystal accumulationin situafter magma emplacement, leading in certain layers to extreme enrichments in Fe, Ti, and V, related to high modal proportions of cumulus ilmenite. Furtherin situcrystallisation led to differentiation of the residual liquid, producing more silicic gabbros with well-developed granophyric textures, the granophyre reflecting the silicic residuum. Extreme differentiation, possibly combined with expulsion of silicic residual liquid during crystal accumulation and compaction, resulted in the cross-cutting aplite veins.Three different types of layering are present in the intrusion. Firstly, preferred orientation of tabular minerals in the laminated gabbro units is thought to result from discrete sedimentation episodes from a convecting magma chamber. Secondly, macrorhythmic modal layering up to 1 m thick consists of an alternation of relatively ilmenite-rich and ilmenite-poor layers in the laminated gabbro units, although the reason for the modal variation is not certain. Thirdly, a centimetre-scale felsic–mafic microrhythmic layering is present in the envelope quartz gabbros and dolerites, which is similar to the inch-scale layering in the Stillwater Igneous Complex. This layering is thought to relate to metasomatic reaction in the gabbro in the presence of water at a late magmatic stage. Overall, these various lithological units themselves define a large-scale layering in the intrusion.Combined, the petrological and geochemical data suggest that the St David's Head Intrusion was not emplaced in a single event. Rather, a series of magma pulses, of contrasting compositions but petrogenetically linked, was intruded. Some of the chemical variations now seen existed prior to emplacement, indicating the former presence of high-level crustal magma chambers, while other variations developed as a result ofin situcrystallisation processes and related chemical differentiation.

An experimental study on the effects of phenocrysts on convection in magmas

The effect of phenocrysts on convection in magma chambers is investigated experimentally using small heavy particles in convecting fluids. The particles are initially uniformly distributed in a fluid which is either heated from below or cooled from above. The system is allowed to evolve, and temperature and particle concentration profiles are measured as functions of time. When the concentration of particles is sufficiently small, convection is basically unaffected by their presence. When the concentration is above a critical value, however, the convective motion is considerably altered. The effect of particles on the subsequent fluid behaviour is different in the cases of heating from below and cooling from above. In the former case, there are strong convective motions confined to a sedimentary layer of decreasing thickness beneath a clear layer which displays rather weak convective motions. With time, the destabilizing increase of temperature in the lower layer overcomes the stabilizing contribution to the bulk density due to the particles and the layer overturns quite suddenly. In the situation of cooling from above, a critical condition separates a case of continual overturn from a case of no overturn at all, with the sedimentary layer falling unimpeded to the bottom. Theoretical analysis suggests that the critical value is determined primarily by the ratio of the contribution to the bulk density of the suspension due to particles to the change in fluid density due to the thermal effect. The size distribution of the particles can also modify the fluid behaviour. Applying our general results to geological situations, we suggest that the presence of relatively small concentrations of phenocrysts can critically influence the mode of convection in magmas.

Origin of modal and rhythmic igneous layering by sedimentation in a convecting magma chamber

EXPERIMENTAL investigations of convecting, particle-laden fluids show two regimes for convection driven by cooling from above1. In very dilute suspensions, convection will maintain a homogeneous distribution of particles throughout the convecting layer provided that particle fall velocities are small compared with turbulent fluid velocities. Above a critical concentration, convection is unable to keep the particles suspended, so the particles settle, leaving behind a layer of convecting fluid virtually free of particles. Here we apply these results to cooling magma chambers, in which crystallization leads to an increase in suspended crystal content with time. Discrete sedimentation events are predicted each time the concentration exceeds the critical value. For common igneous minerals, critical concentrations are very small (typically 0.002–0.03 wt%) and layers of the order of centimetres to a few metres thick will result. Because minerals of different density and size have different critical concentrations and settling velocities, complex fluctuations in sedimentation rate and mineral proportions can occur in a multi-component melt. This may lead to either regular repetitive cycles or more complex fluctuations. The process is confined to low-viscosity magmas, such as basalts, in which the crystals are able to separate from the active thermal boundary layer during convection.

Evolution of double diffusion convection in a felsic magma chamber

The onset of double diffusion convection (DDC) is modeled in a two-dimensional case in respect to magma chambers. The viscosity model for the melt takes into account the effects of temperature and concentration of the dissolved component (H 2O). The upper boundary of the convecting magma chamber is assumed to be anhydrous and at constant temperature, whereas the lower boundary is treated as being hydrous permeable with a temperature greater than that within the upper boundary. The case of positive compositional and thermal buoyancy of melt is studied assuming a H 2O diffusion coefficient small in comparison with thermal diffusivity. The DDC has been modeled using a system of equations solved by the finite difference method on a square grid. The convective pattern evolution has been studied for fixed boundary conditions as well as for cooling and degassing. Due to the higher viscosity in the upper zone, the upper boundary layer is thicker than the lower one. The variation of water concentration in this zone of the convective cell can be significant. In nature, the high gradient of water concentration can be responsible for the observed variations of water content in minerals crystallized from a granite melt (e.g., biotite). Because of a high Lewis number (= 100), temperature variations in the magma chamber decay much faster than the water concentration. In this case the intensive convection can continue at a constant temperature due to the non-zero water content in the chamber. In principle, the effect can be applied to the formation of magmatic bodies. If the cooling and degassing system reaches a uniform temperature distribution prior to the crystallization temperature, water content throughout the body may still remain variable.

A chemical model for generating the sources of mare basalts: Combined equilibrium and fractional crystallization of the lunar magmasphere

It is generally considered that mare basalts were generated by the melting of a cumulate mantle formed in an early Moon-wide magma ocean or magmasphere. However, the nature and chemistry of this cumulate mantle and the logistics of its origin have remained elusive. Extensive studies of terrestrial layered mafic intrusions over the past sixty years have emphasized the imperfection of fractional crystallization and attendant crystal-crystal and crystal-liquid separation in a convecting magma chamber. Crystal-liquid and crystal-crystal separations were similarly inefficient during evolution of the lunar magma ocean (LMO), allowing for the trapping of interstitial liquid and entrainment of a small proportion of less-dense plagioclase into the denser mafic cumulate mush. Indeed, petrography of lunar highlands samples demonstrates this for anorthosites (with 1–10% olivine). The residual liquid after 80–90% crystallization was very evolved (in fact KREEPy) and, even in small proportions (1–5%), would have a noticeable effect on the trace-element chemistry of melts generated from these cumulates. This trapped residual liquid would elevate total REE abundances in the cumulate pile, while synchronously deepening the already negative Eu anomaly. Essentially, this trapped liquid will make the cumulate more fertile for melting to generate both KREEP basalt and mare basalt magmas. Plagioclase entrained in the mafic cumulate pile adds an essential Al component to the high-Ti basalt source and will moderate the requisite negative Eu anomaly in the cumulate. Early in the evolution of the lunar mantle, when the LMO still was largely liquid, it is likely that vigorous convection was an important factor in crystallization. Such convection would allow crystals to remain suspended and in equilibrium with the LMO liquid for relatively long periods of time. This extended period of equilibrium crystallization would then have been followed by fractional crystallization once plagioclase became a liquidus phase and began to float to form the lunar highlands crust. Previous authors have proposed a three-component model for the evolution of high-Ti mare basalt source regions. This model includes KREEP, early (olivine-rich, high Mg#) cumulates, and late (ilmeniterich, low Mg#) cumulates in various proportions. However, we propose a model for high-Ti basalt parent magmas which is in accord with studies of terrestrial layered intrusions. This model for the high-Ti source includes trapped instantaneous residual liquid (TIRL; 1–3%) and entrainment of a small (2–5%) proportion of plagioclase into the late-stage cumulate pile in order to account for both the observed Al compositions and trace-element characteristics of high-Ti mare basalts. Melting of this relatively shallow, ilmenite- and clinopyroxene-bearing, late-stage cumulate can generate high-Ti mare basalt magmas. Furthermore, we are in agreement with other workers that only through a process of nonmodal melting will the high Ti values for the parent magmas be realized. Large-scale convective overturn of the cumulate pile and mixing of KREEP with early- and late-stage cumulates is not required. However, localized overturn of the upper tenth of the cumulate pile is likely and, in fact, required to achieve an appropriate major-element balance for the high-Ti mare basalt source region.

Mathematical modeling of conductive heat transfer from a freezing, convecting magma chamber to a single-pass hydrothermal system: implications for seafloor black smokers

Hydrothermal systems subject to a heat flux boundary condition at the base behave differently from systems with an isothermal bottom boundary. Scaling analysis of convection in a uniform porous layer at large Rayleigh number, Ra, is used to illustrate that the core temperature of the system scales as Ra −1/2 and vertical velocity scales as Ra 1/2. A time-dependent numerical model of heat transfer across a thickening conductive boundary layer between a crystallizing magma chamber and a single-pass hydrothermal system in the oceanic crust is then developed. The model shows that a layer of isotropic gabbro at the top of the magma chamber ∼ 75–200 m thick during a period of high-temperature flow. The parameter kd, where k is the bulk permeability and d is the thickness of the chemical reaction-heat uptake region at the base of the hydrothermal system, exerts a strong control on the duration of high-temperature flow. The thickness d is thought to be a few tens of meters. The simulations yield temperature and flow rates that are similar to black smokers under rather restrictive conditions on kd( kd ∼ 5 × 10 −12 m 3) for a time period of ∼ 100 years. But during this time interval, the temperature and flow rate decreases significantly; and these features do not appear to be characteristic of black smokers. Thus although kd exerts some control on whether high-temperature venting occurs, other factors must stabilize the temperature and heat output of black smoker vents. Downward migration of the hydrothermal system into the isotropic gabbro layer could help to stabilize the flow. Two-phase flow is not explicitly treated in this model; but the results suggest that supercritical two-phase flow may occur for tens to hundreds of years.

Gabbroic xenoliths from the Northern Gorda Ridge: Implications for magma chamber processes under slow spreading centers

Abundant gabbroic xenoliths in porphyritic pillow basalt were dredged form the northern Gorda Ridge. The host lava is a moderately fractionated, normal mid‐ocean ridge basalt with a heterogeneous glass rind (Mg numbers 56–60). Other lavas in the vicinity range from near primary (Mg number 69) to fractionated (Mg number 56). On the basis of textures and mineral compositions, the xenoliths are divided into five types: Type A xenoliths consist of large clinopyroxene oikocrysts, enclosing euhedral to subhedral plagioclase. Olivine and glass are present only in trace amounts. Type B xenoliths consist of olivine and plagioclase (±spinel) with abundant intersertal glass. Partially resorbed olivine with deeply embayed margins in places subophitically encloses small plagioclase crystals. Olivine and large plagioclase crystals often contain glass inclusions and/or spinel. Type C xenoliths are a composite of types A and B, with areas of large clinopyroxene oikocrysts adjoining areas of loosely joined clusters of olivine and plagioclase in glass. Type D is represented by a single large xenolith that, except for containing a larger proportion of crystals, appears identical to the host lava. Type E is represented by a single small xenolith that is mineralogically similar to type B but with a distinct, fine‐grained, tightly interlocking texture, formed by small olivine oikocrysts enclosing euhedral plagioclase, in a small amount of intersertal glass. Many single crystals and glomerocrysts in the host lava appear to be xenocrysts. Some forsteritic olivine and anorthitic plagioclase contain glass inclusions more primitive than the host lava, while rare glass inclusions in skeletal olivine suggest entrapment of melt more evolved than the host lava. The overabundance of plagioclase crystals and their textural features suggest accumulation. The xenoliths are not cognate to the host lava, but they are genetically related. Chemistry of mineral phases in conjunction with textural features suggests that the xenoliths formed in different parts of a convecting magma chamber that underwent a period of closed system fractionation. The chamber was filled with a large proportion of crystalline mush when new, more primitive, and less dense magma was injected and mixed incompletely with the contents in the chamber, forming the hybrid host lava. Plagioclase‐rich leucogabbro layers in ophiolites attest to similar processes in magma chambers of past spreading centers. However, xenoliths in ferrobasalt from the southern Juan de Fuca Ridge differ significantly from the Gorda Ridge xenoliths and suggest that the magma chamber at the southern Juan de Fuca Ridge is located at shallower depth and in cooler crust than the one at the northern Gorda Ridge.

Laboratory modeling of convection in magma chambers: Crystallization against sloping floors

Crystallization against inclined floors in magma chambers commonly releases a light depleted fluid which drives compositional convection. We describe the fundamental fluid mechanics of this process and derive equations describing the buoyant compositional boundary layer adjacent to the top of the crystal pile. The question of whether magma chambers become zoned as a result of compositional convection from sloping floors, as has been previously suggested, is addressed in a laboratory study. Because crystal sizes in magma chambers are usually smaller than the thickness of the compositional boundary layer, an appropriate laboratory model is thermal convection from a smooth, sloping heat exchanger. We investigate the circumstances under which this configuration leads to thermal stratification and find that only when the experimental tank has an extension with an unheated floor does a filling box process lead to significant stratification. This geometry, however, is geologically unrealistic, requiring that crystallization be confined to only part of the floor. We conclude that compositional convection from inclined floors will not lead to compositional stratification in magma chambers. Experiments investigating the same problem by cooling and crystallizing aqueous solutions are misleading partly because the size of crystals grown from aqueous solutions is extremely large compared with the thickness of the compositional boundary layer, whereas this is not usually true in magma chambers. This difference has a major influence on the nature of convection and we suggest that caution should be applied in extending the results of aqueous solution experiments involving crystallization to magma chambers. The petrological consequences of this work are (1) the fluid mechanics of crystallization at an inclined floor and horizontal floor are similar and suggestions, made elsewhere, concerning the geological implications in the latter case apply equally to the sloping floor case, (2) large volumes of homogeneous magma can exist in a magma chamber when crystallization is taking place against an inclined floor, and (3) models describing the crystallization of layered intrusions which involve the interaction of crystallization against an inclined floor with compositionally stratified magma require the stratification to be established by an external process.

Flow in an experimental micro–magma chamber

The chemical evolution and eruptive behavior of magmas may be controlled largely by convective processes within magma chambers. According to a recent National Research Council Report [Committee on Physics and Chemistry of Earth Materials, 1987], “the style of convection itself, whether it is turbulent, laminar, large‐scale, of multiple scales, tiered, or localized and intermittent, is very much at question.” In the U.S. National Report to the International Union of Geodesy and Geophysics, Marsh [1987] reviewed recent theoretical and experimental developments related to the style of convection in magma chambers, noting both significant quantitative advances and also the many remaining uncertainties. With regard to double‐diffusive convection, he stated “as ever, the critical question concerns whether or not actual magma chambers convect in this style.” Similarly, Spera et al. [1986] , in discussion of double‐diffusive convection, cautioned against “applying results from saltwater tanks to magma chambers.”

Crystal settling in a vigorously convecting magma chamber

There has been much debate concerning the mechanism of fractional crystallization in magma chambers. The traditional hypothesis of crystal settling has been widely replaced by the concept of in situ crystallization coupled with compositional convection. Observations from layered intrusions, however, are equivocal1–4. Doubts have been raised about crystal settling on theoretical grounds because convective velocities in magma chambers are often much greater than the crystal settling velocities predicted by Stokes' law5, but there has been no experimental study of crystal settling in such vigorous convection. Here we present physical considerations and laboratory experiments which show that the phenomenon of particle settling in these conditions can be accounted for by a simple theory. Application of this theory to crystal settling in magma chambers suggests that crystal settling may be an efficient differentiation mechanism, at least in basaltic magma chambers, despite large convective velocities.

Evolution of crystal-settling in magma-chamber convection

The motion of negatively buoyant particles embedded within kinematically prescribed convective velocity fields is used to study the evolution of crystal settling in magma chambers. We consider the effects of Stokesian settling velocity, crystal growth, aspect ratio of the convective domain and the site of crystal nucleation on mixing and transport of heavy crystals in this system. The percentage of particles retained in a convecting fluid is very sensitive to the site of nucleation. Particles nucleating along the cold roof or within the wall boundary layers are generally not retained. As a consequence, crystal settling can occur even in rapidly convecting magma chambers. Equilibrium crystal densities are reached relatively quickly, within one overturn, and the distribution of particles deposited on the bottom of the chamber is always asymmetric. This feature may facilitate the formation of primary sedimentary structures in plutons. Laboratory experiments using small, negatively buoyant spheres in a highly viscous fluid with axisymmetric thermal convection confirm the basic predictions of this model.

Evolutionary structures in double‐diffusive convection in magma chambers

Finite‐amplitude solutions to subcritical, time‐dependent, double‐diffusive convection (D.D.C.) applicable for magma chambers are obtained by a two‐dimensional, finite‐element method based on stream‐function, temperature and compositional fields. Grid‐refinement is used for resolving the disparately‐scaled thermal and chemical boundary layers present for large ratios of the thermal to chemical diffusivity (Lewis number) characteristic of magmas. The occurrence of layered convection depends strongly on the initial conditions of the temperature and composition. It is shown that in the infinite Prandtl number limit a local transition from a diffusive to a finger‐like regime can take place for large enough Lewis numbers. This time‐dependent finding suggests that a strict categorization of D.D.C. into the finger and diffusive regimes requires modification in geological applications.

Effects of Earth's rotation on convection in magma chambers

The rotation of the Earth is predicted to have a strong influence on the convective motions in basaltic magmas which cool at high and intermediate latitudes on the Earth's surface. Convection in layers greater than 100 m deep is characterised by large Taylor numbers and small Rossby numbers, for which laboratory experiments provide evidence of strong rotationally-induced flows. In the case of convection driven by either thermal or compositional buoyancy fluxes from horizontal top or bottom boundaries Coriolis forces induced by the Earth's rotation are expected to cause the turbulent convective motions to form into intense vortices whose axes tend to be close to the vertical. These vortices should be tall and thin, be very unsteady, and have rapid vertical motion in their cores. Earth's rotation is likely to have little or no effects on convection in very shallow convecting layers ( < 100 m) of basaltic magmas or in chambers of more viscous (granitic) magmas. When convection is driven by horizontal density differences (such as those produced by cooling or crystallization at sloping or vertical walls or by simple lateral variation of layer depth) in basaltic chambers of order 10 km or greater in width the rotation of the Earth may cause relatively rapid horizontal (geostrophic) circulation over the lateral scales of the chamber. These predictions involve some extrapolation of fluid dynamical principles from laboratory to magma chamber conditions. Speculative comments on some possible petrological implications are included.

Compositional and thermal convection in magma chambers

Magma chambers cool and crystallize at a rate determined by the heat flux from the chamber. The heat is lost predominantly through the roof, whereas crystallization takes place mainly at the floor. Both processes provide destabilizing buoyancy fluxes which drive highly unsteady, chaotic convection in the magma. Even at the lowest cooling rates the thermal Rayleigh number Ra is found to be extremely large for both mafic and granitic magmas. Since the compositional and thermal buoyancy fluxes are directly related it can be shown that the compositional Rayleigh number Rs (and therefore a total Rayleigh number) is very much greater than Ra. In the case of basaltic melt crystallizing olivine Rs is up to 106 times greater than Ra. However compositional and thermal buoyancy fluxes are roughly equal. Therefore thermal and compositional density gradients contribute equally to convection velocities in the interior of the magma. Effects of thermal buoyancy generated by latent heat release at the floor are included. The latent heat boundary layer at the floor of a basaltic chamber is shown to be of the order of 1 m thick with very low thermal gradients whereas the compositional boundary layer is about 1 cm thick with large compositional gradients. As a consequence, the variation in the degree of supercooling in front of the crystal-liquid interface is dominated by compositional effects. The habit and composition of the growing crystals is also controlled by the nature of the compositional boundary layer. Elongate crystals are predicted to form when the thickness of the compositional boundary layer is small compared with the crystal size (as in laboratory experiments with aqueous solutions). In contrast, equant crystals form when the boundary layer is thicker than the crystals (as in most magma chambers). Instability of the boundary layer in the latter case gives rise to zoning within crystals. Diffusion of compatible trace elements through the boundary layer can also explain an inverse correlation, observed in layered intrusions, between Ni concentration in olivine and the proportion of Ni-bearing phases in the crystallizing assemblage.

Double‐diffusive convection in magma chambers: Single or multiple layers?

An approximate set of equations applicable to double‐diffusive convection in magma chambers has been solved in the steady state for a range of geologically relevant parameters. No evidence for the steady state existence of layered convection for conditions appropriate to magma chambers has been found. At large ratios of the thermal to chemical diffusivity (Lewis number, Le) the heat transport is independent of the Lewis number and equal to that found for thermal convection in a single component fluid. The vertical upward transport of light component varies directly with both the Rayleigh and Lewis numbers (Nuc ∼ Ra0.23 Le0.37).

Double-diffusive convection in partially molten silicate systems: Its role during magma production and in magma chambers

In this paper, two problems involving double-diffusive convection in partially molten, silicate systems are investigated mathematically. The first problem concerns the role of double-diffusive convection during magma production. The systems Di-An and Di-An-Ab are used as examples. The calculations show that, in a growing layer of partial melt, heated from below, the thickness of the layer at which convection initiates depends strongly upon the bulk composition. For example, assuming reasonable values of the physical parameters, in a Di-rich system, convection will occur when the layer is ∼ 800 m thick, whereas in an An-rich system convection, driven by the destabilizing compositional gradient, will begin in a layer only centimeters thick. A qualitative investigation of the Di-An-Ab system suggests similar results, depending upon whether the bulk composition lies in the diopside or the plagioclase field, respectively. Moreover, if one considers a two-component, one-dimensional diapir of Di-An, an examination of the phase diagram indicates that vigorous compositional convection will occur in the partially molten diapir as it ascends, regardless of whether the bulk composition is Di-rich or An-rich. These results suggest that during magma production, convective processes will tend to homogenize the melt before it segregates from the source zone; however, the vigor of mixing is dependent upon the bulk composition of the source, among several other factors. Models of melt segregation should be modified to include double-diffusive processes. The second problem concerns the structure of a porous boundary layer that forms as a result of side-wall crystallization in a convecting magma chamber. An examination of the steady-state boundary layer equations for flow through a porous medium shows that the boundary layer structure may be of two types. If the residual melt fraction, upon crystallization at the wall, has negative compositional buoyancy, or if the negative thermal buoyancy at the cold wall exceeds the positive compositional buoyancy of the residual melt, the flow across the entire boundary layer will be downward. If the residual melt fraction has negative compositional buoyancy, the magma chamber will become stratified as a result of accumulation of a layer of dense, cold liquid on the floor; whereas if the melt fraction has positive compositional buoyancy, the boundary layer fluid will tend to be remixed into the interior of the magma chamber. If, however, the positive compositional buoyancy exceeds the negative thermal buoyancy, counterflowing boundary layers will occur; and the compositionally buoyant liquid will tend to be fractionated towards the top of the magma chamber. An examination of the Di-An and Di-An-Ab systems suggests that both types of boundary layer structures may occur in magma chambers. An approximate calculation of the rate of fractionation suggests that, for a range of parameters that may be representative of basaltic magmas, the flow in the compositional boundary layer may transport ∼ 10 −3 km 3/yr of low-density liquid to the top of the magma chamber.

Co-mingling of acid and basic magma with implications for the origin of mafic I-type xenoliths: Field and petrochemical relations of an unusual dike complex at eagle lake, Sequoia National Park, California, U.S.A.

Open-system magmatic processes result in a continuum of disequilibrium features, from hybrid magmas to lamprophyre dikes. Through application of recent theoretical work on magma chamber convection we quantify thermal and mechanical parameters for a single, somewhat unique, plutonic system and propose a general scheme for interpreting magma mixing based on petrographic, dynamical and heat-transfer considerations. At Eagle Lake, intrusion of nearly aphyric basaltic melt into mostly crystallized acidic host has created a 2–4-m-wide dike-like train of ellipsoidal mafic inclusions set in a matrix of unfoliated leucocratic material. The host magma (∼65% SiO 2) is estimated to have been 70–90% crystallized at the time of mafic melt injection; mafic magma (∼51% SiO 2) was intruded near its liquidus, at 1050–1150°C. Formation of a hybrid compositional product is not observed. Upon intrusion, the mafic magma formed ellipsoidal pillows of average areal size 50 cm 2, many are encased by fine-grained margins interpreted as remnant quench rims. The host acted initially as a brittle solid; the absence of evidence for creep or brittle-ductile transitional flow implies strain rates around 10 −2 s −1 within the zone of mafic melt injection. A thermal model predicts that the heat released during thermal equilibration was sufficient to raise the temperature of the host to above its rheological locking point in the region adjacent to the mafic conduit; lack of foliation in the leuco-cratic material in this region supports the model. Schlieren zones developed along dike complex margins, where strain gradients were maximized. The schlieren are chemically linked to the mafic melt; mineral deformation features indicate strain rates ∼10 −5 s −1 at 700–1000°C. A general model of mixing is proposed in which the relative volumes, temperatures and viscosities of the distinct magmas play a pivotal role in determining the spatial scale of the resultant compositional heterogeneity. For example, evidence for intrusion of mafic melt 10 3 years after silicic pluton emplacement will not be visible on outcrop scale; vigorous convection will have eradicated all but xenolithic and xenocrystic traces. We believe that many I-type mafic “xenoliths” in Sierran plutons are the result of this style of magma mixing. Alternatively, if intrusion occurs 10 6 years following pluton emplacement, map-scale features such as lamprophyre dikes and large inclusion swarms are evident. In the context of a general model, the Eagle Lake quartz monzodiorite pluton preserves the earliest stages of formation of individual mafic inclusions.

The fluid dynamics of evolving magma chambers

Recent developments in petrology indicate that fluid dynamic effects are of fundamental importance in controlling magma genesis. The forms of convection in magma chambers arise from compositional variations caused by processes such as fractional crystallization, partial melting and contamination, as well as from thermal effects. These processes, together with phase changes such as volatile exsolution, generally cause much larger density changes in magmas than the thermal effects arising from associated temperature changes. Magmas exhibit a wide range of convective phenomena not encountered in one-component fluids that are due to these compositional changes and to the differences between the diffusivities of chemical components and heat. When crystallization occurs in such multi-component systems, fluid immediately adjacent to the growing crystals is generally either depleted or enriched in heavy components and can convect away from its point of origin. Experimental studies of convection in crystallizing systems together with theoretical analyses suggest that convective separation of liquid from crystals is the dominant process of fractionation in magmas. This paper provides a synopsis of these new ideas on convection in magmas and their application to the interpretation of igneous rocks. Crystal settling is shown to be an inadequate and, in many situations, improbable mechanism for fractional crystallization. The convective motions in chambers are usually sufficiently vigorous to keep crystals in suspension, although settling can occur from thin fluid layers and within the boundary layers at the margins of a magma chamber. We propose that convective fraction , a term introduced to embrace a wide variety of convective phenomena caused by crystallization, is the dominant mechanism for crystal fractionation. The process enables compositional and thermal gradients to be formed in magma chambers both by closed-system crystallization and by repeated replenishment in open systems. During crystallization along the margins of a chamber, highly fractionated magmas can be generated without requiring large amounts of crystallization, because the removal and concentration of chemical components affects only a small fraction of the total magma. These convective effects also give insights into many features observed in layered intrusions, including the various types of layering and the formation of different kinds of cumulate rock.

Stagnant layers at the bottom of convecting magma chambers

The evolution and crystallization of igneous complexes has received much attention from petrologists1–3 and more recently from physicists4–10. Current models emphasize the role of compositional effects. We present here a different viewpoint. Because compositional effects are due to crystallization, they depend on the thermal structure and regime of cold boundary layers in convecting magma chambers, particularly of the bottom layer where the thickest rock sequences form. We have studied purely thermal convection in a large aspect ratio magma chamber which is cooled through both its upper and lower boundaries. We made laboratory fluid dynamical experiments in turbulent and transient conditions and show that a stagnant layer develops at the bottom, isolated from the convective part of the chamber. The essential features of this layer are that it is not affected by mixing and that a significant thermal gradient is maintained across it. These imply peculiar crystallization conditions.