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Mantle plumes provide an attractive mechanism for generating short-duration, voluminous magmas in large igneous provinces (LIPs) while at the same time providing an explanation for the frequently associated break-up of supercontinents. This model has also been invoked for the Ferrar large igneous province (FLIP) in Antarctica, which zircon and baddeleyite U–Pb dating shows was emplaced over a short duration at 182.7 ± 0.5 Ma, contemporaneously with fragmentation of the supercontinent Gondwanaland. Here, we present platinum-group-element (PGE) and Os-isotopic data for the Basement Sill in the McMurdo Dry Valleys – a part of the FLIP – that challenge the plume interpretation. The Basement Sill samples studied are cumulate-textured gabbro to norite, and pyroxenite with minor ferro- or leuco-lithofacies with MgO ranging from 2 to 19 wt%. The 187Os/188Os values range from 0.1609 ± 0.003 (2σ) to 8.100 ± 1.600 (2σ); the minimum value overlaps with a previously published estimated initial 187Os/188Os ratio for Ferrar magmas of 0.145 ± 0.049 (2σ). The PGE abundance patterns for the Basement Sill define positive, convex-shaped slopes between the IPGE (Os, Ir and Ru) and PPGE (Pt, Pd and Rh). The most significant feature of the entire data set is the extreme sub-chondritic Os/Ir ratios (

To better investigate the behavior of titanium (Ti) isotopes during magmatic processes, we report high-precision Ti isotope compositions for 60 terrestrial igneous rocks from different geological settings worldwide. Based on their major element compositions and petrographic descriptions, these samples can be subdivided into two groups: Fe-Ti oxide unsaturated and Fe-Ti oxide saturated. The Fe-Ti oxide unsaturated group samples show a narrow δ49/47Ti (δ49/47Ti = [(49Ti/47Ti)sample/(49Ti/47Ti)OL-Ti] × 1000) range (− 0.036 ± 0.043‰ to 0.082 ± 0.021 ‰), and no correlation between δ49/47Ti and the degree of differentiation is observed. By contrast, Fe-Ti oxide saturated group samples show a remarkable δ49/47Ti variation, ranging from 0.005 ± 0.018‰ to 1.914 ± 0.006 ‰, which are positively correlated with SiO2 contents, and negatively correlated with MgO contents. In particular, multiple SiO2 vs. δ49/47Ti trends are observed in Fe-Ti oxide saturated group, which are controlled by crystal fractionation degrees, magma SiO2 compositions, and Fe-Ti oxide compositions during magma differentiation.

The characteristic size of two types of intrusions identified beneath Kīlauea's East Rift zone are uniquely estimated by combining time constraints from fractional crystallization and the rates of magma solidification during cooling. Some intrusions were rapidly emplaced as dikes, but stalled before reaching the surface, and cooled and crystallized to feed later fractionated eruptions. More specifically, using the observed time interval between initial emplacement and eruption of fractionated lava, whose degree of fractionation is estimated from petrologic mixing calculations, the extent of solidification or cooling needed to produce this amount of fractionation can be directly inferred. And from the known erupted volumes the spatial extent or size of this fractionated volume can be analytically related to the full size of the source body itself. Two examples yield dike widths of 82 and 68 m. Other intrusions remain close to the east rift magma transport path and are observed to last for decades or longer as viable magma bodies that may participate in feeding later eruptions. The thickness of semi-permanent reservoirs near the East Rift Zone magma transport path can be estimated by assuming a resupply rate that is sufficiently frequent to restrict cooling to These calculations show that long-term heating of the wallrock of the magma transport paths serves to slow conduit cooling, which may be partly responsible for sustaining long East Rift Zone eruptions. Adjacent to the vertical transport path beneath Kīlauea's summit, the combined effects of heating and ever-increasing magma supply rate may have forced a commensurate enlarging of the conduit, perhaps explaining the occurrence of a temporary burst of deep (5–15 km) long-period earthquake swarms between 1987 and 1992.

We have developed a novel enhanced geothermal system (EGS) called radiator EGS (RAD-EGS). This system attempts to emulate naturally occurring hydrothermal systems by creating a vertically oriented heat exchanger or vane in the deep subsurface, mimicking a radiator in an internal combustion engine. Water is injected at the bottom of the vane and produced on the top. We propose to build the RAD-EGS in hot sedimentary aquifers (HSAs) with high-permeability vane(s) created in the plane defined by [Formula: see text] and [Formula: see text] (vertical). We have evaluated 3D heat-transfer simulations to better understand the fluid and heat flows that may occur in RAD-EGSs. The simulations account for subsurface heterogeneity including the presence of underlying basement rock, an overlying confining layer, and an ambient hydraulic gradient, which causes background groundwater flow. Our simulations indicate that our induced upward flow in the vane significantly prolongs the lifetime of RAD-EGS when compared with downward flow because hydraulic short circuiting is avoided. Within the vane, convection may occur, and its onset is analyzed in terms of a characteristic Rayleigh number. A critical aspect of RAD-EGS, therefore, is that thermal recharge does not rely solely on heat conduction from the surrounding wall rock, which is typical for EGS built in hot dry rock (HDR). Instead, recharge is also due to heat advection through the surrounding water-saturated aquifer, substantially prolonging the lifetime of the thermal reservoir. Moreover, fluid losses as typical for EGS built in HDR do not occur. It is also possible that cold water injected at the bottom of the vane may sink into deeper rock layers, which displaces hot water from the surrounding aquifer into the RAD-EGS. We suggest that mimicking a natural hydrothermal system is a successful EGS strategy via RAD-EGS.

Jupiter's moon Europa has a relatively young surface (60–90 Myr on average), which may be due in part to cryovolcanic processes. Current models for both effusive and explosive cryovolcanism on Europa may be expanded and enhanced by linking the potential for cryovolcanism at the surface to subsurface cryomagmatism. The success of cryomagma transport through Europa's crust depends critically on the rate of ascent relative to the rate of solidification. The final transport distance of cryomagma is thus governed by initial melt volume, ascent rate, overall ascent distance, transport mechanism (i.e., diapirism, diking, or ascent in cylindrical conduits), and melt temperature and composition. The last two factors are especially critical in determining the budget of expendable energy before complete solidification. Here we use these factors as constraints to explore conditions under which cryomagma may arrive at Europa's surface to facilitate cryovolcanism. We find that 1–5 km radius warm ice diapirs ascending from the base of a 10 km thick stagnant lid can reach the shallow subsurface in a partially molten state. Cryomagma transport may be further facilitated if diapirs travel along pre-heated ascent paths. Under certain conditions, cryolava transported from 10 km depths in tabular dikes or pipe-like conduits may reach the surface at temperatures exceeding 250 K. Ascent rates for these geometries may be high enough that isothermal transport is approached. Cryomagmas containing significant amounts of low eutectic impurities can also be delivered to Europa's surface by propagating dikes or pipe-like conduits.

The dynamics of solidification front growth along the margins of magmas have been widely found to be fundamental in controlling magma transport and emplacement. In this vein, the role of solidification fronts in determining the basic growth mechanics of laccoliths are investigated here in a series of scaled experiments using two contrasting magma analogs, water and molten wax that are injected into a visco-elastic gelatin based crustal analog. In non-solidifying, water-style experiments, intrusion is a relatively simple process. In contrast, wax magma emplacement displays a vast palette of compelling behaviors: propagation can slow, stop, and reactivate, and the directionality of lateral growth becomes much more variable. Small flow deviations in water-based intrusions are likely the product of flow instabilities, the result of injecting a viscous fluid along an interface (similar to Hele-Shaw cell experiments). However, the much more complex emplacement style of the wax experiments is attributed to solidification at the leading edge of the crack. The overall effects of solidification during emplacement can be described by a non-dimensional parameter measuring the relative competition between the rates of crack propagation and solidification at the crack leading edge. In this context, laccolith growth mechanics can be separated into three distinctive characteristic stages. Namely, I: A thin pancake style sill initially emanating radially from a central feeder zone, II: As solidification stalls magma propagation at the leading edges, enhanced thickening begins, forming a true, low aspect ratio laccolith, and III: As stresses accumulate, tears and disruptions readily occur in the solidified margin causing fresh breakouts, thus reactivating lateral growth into new lobes. The competitive combination of these latter stages often leads to a characteristic pulsatile growth. The unexpected richness of these results promises to add fundamentally to the basic understanding of laccolith growth mechanisms, and also adds key observations to growth processes to be sought in field studies that have been hitherto suspected but rarely observed. Despite seemingly simple shapes, the growth history of laccolithic intrusions is likely quite complex.

The time of crystallization of a 100 km thick ocean on Europa is estimated using a Stefan-style solidification solution. This solution is then extended to estimate the present thickness of the ice shell. It is assumed that the shell is initially in a steady-state conductive regime, and the ocean is taken to be an infinite liquid half space cooling from above. We find that in the absence of tidal heating and without the presence of low-eutectic impurities to serve as anti-freezes, a 100 km thick ocean solidifies in about 64 Myr. Conversely, when considering the present thickness of Europa’s ice shell, if tidal heating is included at a global dissipation rate of ∼1 TW, the shell is found to be, on average, approximately 28 km thick. However, if this dissipative heating is solely restricted to the shell, the local rate of heating may vary significantly due to crustal compositional heterogeneities and it is shown that this process may, in turn, produce thermal maxima in the crust, which could lead to local melting and structural instabilities, perhaps associated with the formation of chaos regions. Our approach is also extended to Ganymede and Callisto in order to estimate the time of solidification of their putative subsurface oceans and the current thicknesses of their ice-I shells.

The Beacon Sill, a member of the Jurassic Ferrar group of the Transantarctic Mountains, South Victoria Land, is a 150 m thick tholeiitic diabase intrusion. Uniform on a field and macroscopic scale, it displays chemical and textural variability indicative of a sustained and complex history of emplacement and differentiation. Emplacement of the sill consisted of at least two, and probably four, discrete massive injections of magma, each averaging � 35 m thick, over a time span of � 100 years. The final injection event is marked by a 30 m thick interval of significantly finer-grained rock at the sill center.These fine-grained textures can be successfully reproduced by combining the results of thermal and crystal growth models, but only if the sill was a multiple intrusion. With the exception of a minor reinjection event captured in the chilled margin of the sill, earlier reinjection events are not evident texturally, most probably because of textural overprinting during prolonged cooling after initial crystallization. The dominant process involved in the postemplacement differentiation of the Beacon Sill was compactiondriven redistribution of interstitial liquid. Transfer of residual liquid from the compacting lower solidification front to the dilating upper solidification front resulted in characteristic chemical and mineralogical effects, such as the depletion of the lower half of the sill and the enrichment of the upper half of the sill in incompatible elements (e.g. TiO2, Zr) and modal granophyre. Based on thermodynamic models, most of the compaction occurred at a crystallinity of roughly 33%. Geochemical profiles are distinctly segmented, suggesting that the sill was repeatedly split and reinjected with fresh magma near the center after compaction had redistributed the interstitial liquid within the partially solidified magma. The recognition of evidence for reinjection and compaction in a macroscopically uniform, relatively quickly cooled sill suggests that these processes may be common in the construction of sills and other mesoscale igneous intrusions in general.

The Isle au Haut Igneous Complex provides a unique opportunity to examine in detail the in situ physical and chemical interactions between contemporaneously emplaced mafic and silicic magmas. The complex contains a 600 m thick sequence of 11 alternating layers of gabbro and diorite (typically 15^40 m thick). Purely on the basis of density contrasts (2·65 g cm 3 gabbro vs 2·55 g cm 3 diorite), the entire system should have undergone wholesale instability and mixing; it is instead arrested in a grossly unstable state of interaction while molten. Chilled margins along the lower contacts of the gabbros and structural integrity of the diorite layers indicate that near-liquidus gabbroic magma invaded partly crystalline, cooler diorite. Mineral assemblages, chemical analyses, and phase equilibria calculations indicate initial temperatures during emplacement of 11808C (gabbro) and 10008C (diorite). Conductive thermal models yield solidification timescales of 15^60 years for single gabbro layers and about a thousand years for the entire complex.There is ample evidence for two phases of small-scale interfacial Rayleigh^Taylor type instabilities of dioritic melt into the gabbros. Phase I occurred immediately upon gabbro emplacement whereas evenly spaced, slender more silicic pipes represent a much later stage (Phase II). Pipe geometry and spacing, estimated viscosities of the gabbroic magma and silicic melt, and the sudden increase in silica near the upper contact of the diorite, all indicate a thin ( 18^53 cm) buoyant layer at the upper contact of the diorite as the source of the pipes. Compaction of the diorite produced this layer over a period of about 10 years. Simultaneous solidification along the lower contact of the overlying gabbro, thickening inwards, increased viscosity enough to arrest pipe ascent after a few meters. Crystal size distribution analyses of the gabbro layers yield crystal growth rates [Go1⁄4 (2 4) 10 10 cm s ] and nucleation rates (Jo1⁄410 ^10 6 cm 3 s ) indicative of conductive cooling coupled with some sluggish convective stirring owing to collapse of the roof-ward gabbro solidification fronts. Were the complex larger, with a much longer solidification time, all this evidence would have been lost, thus suggesting that in larger systems similar processes may commonly take place leaving little direct evidence of their operation apart from the ultimate final petrological product.

In many shallow level intrusions, complex crystal layering structures can be preserved due to pulses of intrusion and magma batches that arrive with a significant crystal payload. In this study, we investigate crystal layering within the Dais intrusion of the Basement Sill, Dry Valley, Antarctica. A single three-dimensional (3D) Rhythmic layer is imaged using X-ray computed tomography (CT) to successfully measure the phase distributions of pyroxene and plagioclase through the rock, and a combination of textural quantification in 2D and the 3D high-resolution data allows the texture to be further broken down into the key components: orthopyroxene (OPX), cumulate OPX, clinopyroxene (CPX), and plagioclase (PLAG). The texture was formed by an initial OPX cumulate framework of 37%–47%. The OPX in the cumulate overgrew by an additional 7%–10%; with the concomitant growth of CPX and PLAG in the interstices, a mixture of fine plagioclase microcrystals and melt were expelled into the upper layer. Textural development in the upper layer was arrested by the rapid growth of OPX and CPX dendrites forming oikiocrysts containing the initial microcrystal population of existing PLAG nuclei, along with plagioclase overgrowths in the areas where the pyroxenes are absent. Calculations of liquid and solidification development at the distance the sample is from the sill contact suggests that the textures in this part of the sill developed over ∼140 yr or less.

The likelihood that a nuclear waste repository at Yucca Mountain could be intersected by igneous activity is very low, but its potential consequences are nevertheless important to performance assessments. An ongoing critical area of concern is the nature and magnitude of the thermal interaction of magma with tunnel walls, high-level nuclear waste packages, and waste forms. Previous assessments consider a variety of dynamic scenarios, but large uncertainties remain in understanding the rheological nature of the magma likely to be involved and its behavior within a repository drift. Here we specifically address the issue of magma rheology during degassing, cooling, and solidification as basaltic magma approaches Earth's surface and enters a drift. Magma containing significant amounts (> ~ 2 wt.%) of dissolved water (Wet Magma), as is anticipated for this region, at or near its liquidus temperature and saturated with water at 200 MPa is at a temperature near or below the 1-atm solidus temperature. Isentropic ascent from this near liquidus temperature promotes extensive solidification and/or glassification. Exsolving water with approach to the surface promotes rapid vesiculation leading to fragmentation and tephra production. With continued ascent the still water-saturated magma traverses the solidification phase field and undergoes a combination of rapid crystallization and quenching, becoming a glassy highly viscous (~ 108 Pa s) mass of greatly reduced mobility. This immobility is reflected in the high effective viscosity regulating flows from nearby cinder cones associated with wet basalt. This also matches well with the experimentally determined rheology of dry basalt glass. This rheology greatly restricts the mobility of basalt within repository drifts, amounting to

Numerous, interconnected, granitic dikes (530 cm in width and hundeds of meters in length) cut Ferrar dolerite sills of the McMurdo Dry Valleys, Antarctica. The source of the granitic dikes is partial melting of granitic country rock, which took place in the crust at a depth of about 2^3 km adjacent to contacts with dolerite sills. Sustained flow of doleritic magma through the sill generated a partial melting front that propagated into the granitic country rock. Granitic partial melts segregated and collected at the contact in a melt-rich, nearly crystal-free reservoir adjacent to the initial dolerite chilled margin. This dolerite chilled margin was subsequently fractured open in the fashion of a trapdoor by the granitic melt, evacuating the reservoir to form an extensive complex of granitic dikes within the dolerite sills. At the time of dike injection the dolerite was nearly solidified. Unusually complete exposures allow the full physical and chemical processes of partial melting, segregation, and dike formation to be examined in great detail.The compositions of the granitic dikes and the textures of partially melted granitic wall rock suggest that partial melting was characterized by disequilibrium mineral dissolution of dominantly quartz and alkali feldspar rather than by equilibrium melting. It is also unlikely that melting occurred under water-saturated conditions. The protolith granite contains only � 7 vol.% biotite and estimated contact temperatures of 900^9508C suggest that melting was possible in a dry system. Granite partial melting, under closed conditions, extended tens of meters away from the dolerite sill, yet melt segregation occurred only over less than one-half a meter from the dolerite chilled margin where the degree of partial melting was of the order of 50 vol.%. This segregation distance is consistent with calculated length scales expected in a compaction-driven process. We suggest that the driving force for compaction was differential stress generated by a combination of volume expansion as a result of granite partial melting, contraction during dolerite solidification, and relaxation of the overpressure driving dolerite emplacement. On a purely chemical basis, the extent of melt segregation necessary under fractional and batch melting to match the Rb concentrations between melt and parent rock is a maximum of 48 and 83 vol.% melt, respectively.

That igneous textures can be collectively described, classified, and related to magma composition, style of emplacement, and spatial position speaks deeply to the existence of a specific set of fundamental kinetic processes controlling all magma crystallization. Textures record magma life history, telling the most recent, local conditions of cooling and also where the magma has been. Yet it is largely a mystery how silicate melts crystallize, how they become what they are, and, especially, how the final texture relates to the early transient textures more closely linked to the governing kinetics of nucleation and growth. These rich and intriguing processes can be understood by deciphering textures. This is done by first dismantling and quantifying them, then by rebuilding them and simulating magma crystallization and transport, and last by taking the results to the final court of appeal, the rocks themselves.

The remoteness and inhospitable nature of natural silicate magma make it exceedingly difficult to study in its natural setting deep beneath volcanoes. Although laboratory experiments involving molten rock are routinely performed, it is the style and nature of crystallization under natural conditions that is important to understand. This is where the crystal size distributions (CSD) method becomes fundamentally valuable. Just as chemical thermodynamics offers a quantitative macroscopic means of investigating chemical processes that occur at the atomic level, crystal size distribution theory quantitatively relates the overall observed spectrum of crystal sizes to both the kinetics of crystallization and the physical processes affecting the population of crystals themselves. Petrography, which is the qualitative study of rock textures, is the oldest, most comprehensively developed, and perhaps most beautiful aspect of studying magmatic rocks. It is the ultimate link to the kinetics of crystallization and the integrated space-time history of evolution of every magma. CSD analysis offers a quantitative inroad to unlocking and quantifying the observed textures of magmatic rocks. Perhaps the most stunning feature of crystal-rich magmatic rocks is that the constituent crystal populations show smooth and often quasi-linear log-normal distributions of negative slope when plotted as population density against crystal size. These patterns are decipherable using CSD theory, and this method has proven uniquely valuable in deciphering the kinetics of crystallization of magma. The CSD method has been largely developed in chemical engineering by Randolph and Larson, 1 ' 2 among many others, for use in understanding industrial crystallization processes, and its introduction to natural magmatic systems began in 1988. The CSD approach is particularly valuable in its ease of application to complex systems. It is an aid to classical kinetic theory by being, in its purest form, free of any atomistic assumptions regarding crystal nucleation and growth. Yet the CSD method provides kinetic information valuable to understanding the connection between crystal nucleation and growth and the overall cooling and dynamics of magma. It offers a means of investigating crystallization in dynamic systems, involving both physical and chemical processes, independent of an exact kinetic theory. The CSD method applied to rocks shows a systematic and detailed history of crystal nucleation and growth that forms the foundation of a comprehensive and general model of magma solidification.

An intimate physical and chemical interplay between crystals and melt in magmatic systems gives rise to a vast diversity of igneous rocks and the very structure of terrestrial planets. Yet the actual physical means by which this happens is unclear. The long-standing notion of crystals nucleating, growing, and settling ad infinitum from the interior of large pools of magma to eventually form continental rocks is foundering. Processes operating at the smallest scales within marginal solidification fronts and in mingling crystal slurries throughout highly integrated, vertically extended mush columns give rise to planetary-scale effects.

During crystallization, a variety of competing kinetic processes determine the evolution of the igneous microstructure, yet, the relative contribution of each process remains elusive. To this end, a stochastic algorithm is developed to yield a detailed spatial representation of the igneous microstructure during progressive crystallization. This algorithm is used to test a variety of kinetic models for nucleation and crystal growth that yield realistic igneous microstructures. The algorithm itself relies on a theoretical model of crystallization (the Avrami method) and can therefore be internally validated in addition to being verified against natural microstructures using crystal size distributions (CSDs). The most realistic simulated igneous microstructure, in which the CSDs remain approximately log-linear throughout the crystallization interval, is produced using a kinetic model involving exponential nucleation rate and constant growth rate. An extension of the algorithm is used to simulate multiply saturated mineral growth, emulating basalt crystallization through simultaneous nucleation and growth of plagioclase and clinopyroxene. A fundamentally important feature of this procedure is the numerical production of detailed 3D representations of the microstructure that can be interrogated to study many physical and chemical processes. For example, the critical crystallinity necessary for the onset of a finite yield strength and the interstitial melt flow within the igneous microstructure depend on the percolation of the solid and melt phases, respectively. Bounds on the percolation thresholds, the critical crystallinity at which the phase of interest is connected or contiguous such that it extends across the full microstructure from one face to the opposite face, are determined for the simulated igneous microstructure and are comparable with previously published results for the percolation threshold of both the melt phase and solid network. The simulated igneous microstructure can also be used as input into other physical models to ascertain the physical properties of partially molten magmas that are otherwise difficult to estimate by experiment. In a real sense, these computed microstructures are equivalent to 3D microtomographic images of partially molten basalt within a solidification front.

A Lattice–Boltzmann method is used to calculate grain-scale, low Reynolds number, single phase, interstitial porous melt flow in two independent igneous microstructures: 1) a microstructure simulated using a stochastic algorithm for progressive crystallization, and 2) a natural microstructure obtained through X-ray Computed Tomography (CT) of a partially melted basalt. In both cases, the error in the calculated flow field due to finite discretization and domain size has been estimated or demonstrated to be insignificant. Visually, the porous melt flow tends to localize into high flux channels, especially with increasing crystallinity, and this impression is quantitatively confirmed by increasing skewness of the velocity distribution in the direction parallel to the imposed pressure gradient driving the flow. A change from uniform to localized melt flow in naturally occurring situations may have a profound effect on the distribution of trace elements and, if the melt is reactive, the chemical and structural evolution of the igneous microstructure through localized phase change. Permeabilities of both microstructures are determined from the calculated steady flow field. The permeabilities are then fitted to two different correlation models which are based on the Rumpf–Gupte and Carman–Kozeny relations. In these models, we express permeability as functions of melt fraction and either the mean crystal length or specific surface area. Extrapolated permeability to higher melt fractions using both correlation models for the partially melted basalt is shown to be within a factor of four of experimentally determined permeability of a similar sample. We determine permeability estimates in the relatively unexplored melt fraction range of 30%–80% and find consistency with previous work for permeability estimates in the melt fraction range of 20%–30%.

The common association of mid-crustal migmatites with an upper-level granite pluton could indicate that the migmatites are a feeder zone for the pluton. If magma from a deeper level pervasively intrudes a high temperature metamorphic complex, most of the intruded magma would not freeze because of the prevailing temperature. The interaction between the magma and country rocks, which could include partial melting and crystallisation of the magma passing through, would modify magma to a more granitic composition, as found in the higher-level pluton.The physical aspect of the magma transport through such a hot feeder zone is modelled by introducing a dimensionless melt transport (MT) number, which is the ratio of the rate of melt movement caused by the bulk flow of the entire mass (melt+solid) to that of porous media flow of melt only through the solid framework. The MT number is strongly dependent on the melt content of the melt-rich zone (MRZ), the diameter of the MRZ and typical particle size in the MRZ.The ∼300-Ma, diatexitic, Lauterbrunnen migmatites (LM) in the Aar massif, Swiss Alps, may be such a feeder zone for the nearby 303-Ma Gastern granite (GG). The chemical and field evidence indicates that the LM formed by an intrusion of intermediate composition magma, which interacted with country rocks to produce a magma of GG composi

The Atka volcanic center, the largest modern, central Aleutian magmatic complex, covers 360 km2 with an estimated eruptive volume of ~200 km3. The oldest exposed rocks, the Old Harbor Series, are a suite of basaltic andesite to dacite flows, pyroclastic rocks and lahars of presumed Tertiary age. The youngest phase of activity on Atka probably began 1–2 Ma ago near the center of the present volcanic field, and consists of basaltic and basaltic andesite flows which produced a large shield volcano, Atka Volcano. Subsequent to this activity, satellite vents formed along the margin of this structure. Collapse of the ancestral Atka Volcano produced a caldera 5 km in diameter and was accompanied by the eruption of a large dacitic flow (Big Pink). After caldera formation, four major volcanic cones (Korovin, Mount Kliuchef, Sarichef and Konia) grew around the margins of Atka caldera. This study focuses on the Atka Volcano, Korovin and Mount Kliuchef volcanic centers. Atka lavas range from basalt through dacite, with all major volcanic vents erupting lavas spanning similar compositional ranges. They are porphyritic with phenocrysts of plagioclase (25–36%), olivine (0.4–3.6%) or orthopyroxene (2.8%), clinopyroxene (1–12%) and a single Fe–Ti oxide (0–2%). Hydrous phases are lacking even in the most evolved lavas. Two generations of plagioclase occur in the Korovin suite but not in lavas from Mount Kliuchef. Korovin plagioclase also displays resorption surfaces and inclusion zones which are either absent in the phenocrysts of Mount Kliuchef lavas or very rare. Similarly, clinopyroxene in Korovin has reaction rims and abundant glass inclusions which are absent in the Mount Kliuchef lavas. Lavas erupted in the Atka volcanic field vary from 47 to 66% SiO2, although the complex is dominated volumetrically by basaltic lavas. On major-element Harker diagrams, mafic and intermediate Atka Volcano lavas form linear trends but the most siliceous lavas do not fall along these trends. By contrast, Mount Kliuchef lavas are compositionally bimodal whereas Korovin samples range from 51 to 62 wt% silica with intermediate lavas clustering around 55% SiO2. Incompatible trace-element abundances of Korovin lavas do not increase with silica content but those of Atka Volcano and Mount Kliuchef do. On incompatible-incompatible element diagrams, the three suites behave very differently. Differences also exist between the REE characteristics of the three Atka suites. The petrographic and geochemical differences between the three suites suggest formation by markedly different crustal magmatic processes. The characteristics of the Atka Volcano suite suggest formation of the basalt to dacite suite by crystal fractionation of an anhydrous phenocryst assemblage. Incompatible-incompatible element trends suggest the mafic Mount Kliuchef lavas were also produced by crystal fractionation, a conclusion supported by quantitative mass balance calculations. The difference in slope between the Atka Volcano and Mount Kliuchef suites indicates, however, that there were significant differences in these fractionation schemes. Modeling as well as geochemical characteristics suggest that the Mount Kliuchef dacites are not the product of crystal fractionation. They may represent (1) crustal melts, (2) complete melts of siliceous lava produced during earlier magmatic stages, or (3) residual liquids from the Atka Volcano magmatic system. In contrast to these suites, the Korovin lavas could not have been formed by crystal fractionation. Rather the andesites are probably the result of mixing basaltic and dacitic magmas. The Atka data suggest: (1) large volcanic centers may be supplied by a series of non-communicating, crustal magma chambers; (2) local factors may be important in determining liquid-lines-of-descent; and (3) very different magmatic processes may occur in adjacent magma chambers. If the complexity of the Atka suite is a common feature of arc magmatic plumbing systems, reconstructing differentiation processes requires analytical sample sets which are well constrained spatially and temporally.

Extensive, coarse-grained silicic lenses are common in the upper portions of large diabase sills, lava lakes, and gabbroic intrusions. These lenses, which often contain up to 10 wt% more silica than their host rocks, record a clear process of bimodal differentiation that may play a fundamental role in generating significant quantities of silicic magma in predominantly basaltic systems. The spatial, compositional, textural, and contact relations suggest that these silicic segregations form by infilling of tears formed during gravitational instability of the upper solidification front. This process is investigated in some detail by exploring the thermal and mechanical conditions leading to internal failure of the front and segregation formation. As the upper solidification front grows inward and thickens, the local strength of the developing crystalline network must increase upward through the front sufficiently fast to offset the increasing weight of the front itself. This necessary local variation in strength dictates how crystallinity must vary to prevent failure. But the actual variation in crystallinity is determined by phase equilibria, which in general varies in a fashion that is noncovariant with that required for mechanical stability. It is this mismatch that apparently leads to failure and formation of silicic segregations. The rate of filling of the tears by porous flow of interstitial melt from below regulates the rate of tear opening, and the overall size of the tears is regulated by the local rate of advance of the solidification front. Segregations therefore thicken downward as the growth of the front slows and then die out altogether with approach of the lower solidification front. This process irreversibly introduces siliceous noise into basaltic systems. The silicic signal can be enhanced through subsequent reprocessing of the host rock to form volumetrically important masses of silicic magma. This process may be fundamental to achieving the strong compositional diversity of the igneous rocks, to yielding continental masses, and to the differentiation of Earth itself.

This study demonstrates the value of the combined use of scaling of the chemical evolution of cooling magmas have investigated the detailed relationships between temanalysis and crystal size distribution (CSD) measurements in the interpretation of igneous textures and outlines applications of these perature, phase abundance, and phase composition. The reaction series (Bowen, 1922) and the CIPW norm (Cross methods to other fundamentally important problems. Theoretical calculations and measurements of natural samples are used to et al., 1903) have been supplemented by modern thermodynamic models that predict the compositions and abundcharacterize the relationship between igneous texture and cooling history. The total number (NT) and mean length ( L ) of crystals ances of minerals in equilibrium with melt of a given composition and temperature (e.g. Ariskin et al., 1993; in a sample are correlated through a scaling relationship of the form N T ∝ L. This proportionality depends on the mineralogy Ghiorso & Sack, 1995). Although there are a few areas of uncertainty, the chemistry of igneous petrogenesis is of the rock, and so a modal normalization factor (C) is introduced. The CSD slope, S, and intercept, ln(n°), are uniquely related to fairly well understood. But chemical models cannot address the central probeach other through the equation ln(n°) = 4ln(S) − ln(C), where S = L. This overall relationship allows the texture of a rock lem of igneous crystallization, which is the physical process of generating a set of crystals from magma. Two to be related to the local duration of cooling through an arbitrary crystal growth model. Quantification of the link between the texture magmas with the same composition will normally produce the same minerals in the same proportions. However, of a rock and its cooling history makes it possible to predict spatial variations in texture. We show an example of this method using depending on their thermal histories, these two magmas can solidify into rocks with drastically different physical the Sudbury Igneous Complex, Canada. Additional applications include relating the textures of volcanic rocks to spatial and temporal appearances (e.g. basalt and gabbro). The set of minerals is the same, but what distinguishes them is the abundance variations in the magma chamber and extracting kinetic parameters from suites of comagmatic rocks. and size of the constituent crystals. The fine-grained rocks in the chilled margins of most basic intrusions cooled rapidly and the coarse-grained rocks in the interiors of plutons cooled slowly. This has

Two recent papers by Holness et al. (2007a, 2007b) have introduced an interesting new petrographic technique that could become a valuable tool for judging the textural maturity of mafic igneous rocks. They have shown that variations of the dihedral angles of mineral grains can reflect differing rates of heat loss of large layered intrusions, such as Rum and Skaergaard. Although they say in one paper (Holness et al., 2007b) that these variations record ‘the time-integrated thermal history at sub-solidus temperatures’, in the other paper they attribute all the observed variations to changes in the magmatic regime during active crystallization: ‘the arrival of fresh magma in the chamber, the onset of chamber-wise convection and changes in the liquidus assemblage’ (Holness et al., 2007a). The effects of important sub-solidus processes that are known to have extensively altered the textures, modal proportions, and bulk compositions of these same rocks have been ignored, even though they are conspicuous in the field and have been pointed out repeatedly in the literature (Hunter, 1987; McBirney & Sonnenthal, 1990; Sonnenthal, 1992; McBirney & Hunter, 1995; Sonnenthal & McBirney, 1998; McBirney & Creaser, 2003). For example, a sharp change in the textures found near the middle of the section in the Rum intrusion is said to mark the ‘onset of convection’. This interpretation seems to be based on the idea that a magma can remain static after its initial intrusion and then, at some later time, after crystallizing hundreds of meters of layered rocks, start to convect. This is just backwards. The thermal and compositional gradients that drive convection are most pronounced when the magma is first intruded and decline with time. Similarly, the statement (Holness et al., 2007b) that a textural change might coincide with ‘a significant change in the rate and mode of heat loss from the intrusion as it moves from a convective to a conductive regime’ seems to assume that the rate of heat loss is more rapid if the magma is convecting. As Carrigan (1988) has pointed out, the rate of heat loss is not governed by convective transfer in the interior but by the much more limiting rate of

a buoyant liquid becomes non-turbulent or even ceases upon loss of Evidence from experiments and theoretical modeling suggests that the superheat. Transferring these results to magmatic systems, we systems crystallizing exclusively from the top down and rejecting a suggest that the dynamics inside intrusive bodies upon cooling are buoyant liquid effectively cease convecting once the initial superheat very sensitive to the actual phase diagram, the kinetics of cryshas been lost. We report here on a combined experimental and tallization and the density relation between crystals and melt. theoretical study designed to investigate in some detail the interaction of convection and crystallization in a fluid cooled from above. The experiments are carried out in a small tank where temperature, composition, mush thickness, and convective velocities have been monitored. After an initial period of turbulent convection removing

The Torfajokull central volcano in south-central Iceland contains the largest volume of exposed silicic extrusives in Iceland (∼225 km^3). Within SW-Torfajokull, postglacial mildly alkalic to peralkalic silicic lavas and lava domes (67–74 wt.% SiO_2) have erupted from a family of fissures 1–2.5 km apart within or just outside a large caldera (12×18 km). The silicic lavas show a fissure-dependent variation in composition, and form five chemically distinct units. The lavas are of low crystallinity (0–7 vol.%) and contain phenocrysts in the following order of decreasing abundance: plagioclase (An_(10-40)), Na-rich anorthoclase (

holocrystalline under a wide spectrum of cooling regimes implies batch system. Instead, the CSDs of each system reflect a combination that cooling and crystallization can be uncoupled and considered of kinetic and dynamic influences on crystallization. Heterogeneous separately. This is tantamount to realizing that the Avrami number nucleation and annexation of small crystals by larger ones, enis large in most igneous systems. Crystallization automatically trainment of earlier grown and ripened crystals, rate of solidification adjusts through nucleation and growth to the cooling regime, and front advance, and protracted transit of a well-established mush all aspects of the ensuing crystal population reflect the relative roles column are some of the eVects revealed in the observed CSDs. There of nucleation and growth, which reflect the cooling regime. The may be an overall CSD evolution, reflecting the maturity of characteristic scales of crystal size, crystal number, and crys- the magmatic system, from simple straight nonkinked CSDs in tallization time are intimately tied to the characteristic rates of monogenetic systems to multiply kinked, piecewise continuous CSDs nucleation and growth, but it is the crystal size distributions (CSDs) in well-established systems such as Hawaii and Mount Etna. This that provide fundamental insight on the time variations of nucleation is not unlike the evolution of CSDs in some industrial systems. and growth and also on the dynamics of magmatic systems. Crystal Finally, the fact that comagmatic CSDs are not often captured size distributions for batch systems are calculated by employing evolving systematically through large changes in nucleation rates, the Johnson‐Mehl‐Avrami equation for crystallinity related to even in low crystallinity systems, may suggest that magma is always exponential variations in time of both nucleation and growth. The laced with high population densities of nuclei, supernuclei, and slope of the CSD is set by the diVerence a ‐ b, where a and b crystallites or clusters that together set the initial CSD at high are exponential constants describing, respectively, nucleation and characteristic population densities. Further evolution of the CSD growth. The batch CSD has constant slope and systematically occurs through sustained heterogeneous nucleation and rapid anmigrates to larger crystal size (L) with increasing crystallinity. The nealing at all crystallinities beginning at the liquidus itself and diminution in nucleation with loss of melt is reflected in the CSD operating under more or less steady (not exponentially increasing) at late times by a strong decrease in population density at small rates of nucleation. crystal sizes, which is rarely seen in igneous rocks themselves. Observed CSDs suggest that a ‐ b ~6‐10 and that b ~0. That is, growth rate is approximately constant and nucleation rate apparently increases exponentially with time. Correlations among CSD slope, intercept, and maximum crystal size for both batch and open systems suggest that certain diagnostic relations may be useful in interpreting the CSD of comagmatic sequences. These systematics are explored heuristically and through the detailed

From G. F. Becker's and L. V. Pirsson's early enunciations linking the dynamics of magma chambers to the rock records of sills and plutons to this day, two features stand at the centre of nearly every magmatic process: solidification fronts and phenocrysts. The structure and behaviour of the envisioned solidification front, however, has been mostly that akin to non-silicate, non-multiply-saturated systems, which has led to confusion in appreciating its role in magmatic evolution. The common habit of intruding magmas to carry significant amounts of phenocrysts, which can lead to efficient fractionation, layering, and interstitial melt flow within extensive mush piles, when coupled with solidification fronts, allows a broad understanding of the processes leading to the rock records of sills and lava lakes. These same processes are fundamental to understanding all magmas.The spatial manifestation of the liquidus and solidus is the Solidification Front (SF); all magmas, stationary or in transit, are encased by SFs. In the ideal case of an initially crystal-free, cooling magma, crystallinity increases from nucleation on the leading liquidus edge to a holocrystalline rock at the trailing solidus. The package of SF isotherms advances inward, thickening with time and, depending on location — roof, floor, or walls — and the initial crystallinity of the magma, is instrumental in controlling magmatic evolution. Bimodal volcanism as well as much of the structure of the oceanic crust may arise from the behaviour of SFs.In mafic magmas, somewhere near a crystallinity (N) of 55% (vol), depending on the phase assemblage, the SF changes from a viscous fluid (suspension (0Differentiation of initially crystal-free, stationary magmas is limited to processes occurringwithinSFs, which operate in competition with the rate of inward advancement of solidification. Local processes operating on characteristic time scales longer than the time for the SF to advance a distance equal to its own thickness are suppressed. Enormous increases in viscosity outward within the viscous, leading portion of the SF efficiently partition the distribution of melt accessible to eruption. Eruptible melts lie essentially inward of the SF and are thus severely restricted in silica enrichment. The silica-enriched SFI melts are thus generally inaccessible to collection and eviction unless the host SF is reprocessed or “burned back” through, respectively, later regional magmatism or massive, late-stage re-injection. And because of large viscosity contrasts between SFI melts and host basalts, once freed, SFI melts are literally impossible to homogenize back into the system and may collect and compact against the roof to form large silicic masses. Unusually voluminous, bulbous masses of silicic granophyre present along, and sometimes warping, the roofs of large diabase sills may reflect collections of remobilized blobs of SFI melts. These bulbous masses may be later added to the continental crust through solid state creep.In sheets made of phenocryst-rich, singly saturated magma, most phenocrysts are able through settling or floating to avoid capture by the advancing SFs. Significant differentiation is possible through extensive settling of initial phenocrysts and upward leakage of interstitial residual melt from the associated cumulate pile, which over-thickens the lower SF, greatly tipping the competitive edge against suppression of melt leakage by advancing solidification. Dense interstitial melts may similarly drain from roofward cumulates of light phenocrysts. The variation in crystal size and modal abundance in these cumulate piles are intimate records of prior crystallization, transport, and filling.Magmas in transit erode SFs and thoroughly charge the magma with crystals, facilitating fractionation and differentiation, especially if the body occasionally comes to rest. The key to protracted differentiation through fractional crystallization is not crystallization in stationary, closed chambers, but the repeated transport and chambering of magma or the periodic resupply to chambers of phenocryst-rich magma. This is punctuated differentiation, which may be the general case. Close corollaries are that thick, closed sheets of initially crystal-free, multiply-saturated magma undergo precious little overall differentiation, and that deciphering the sequence and crystallinity, including in transit phenocryst entrainment, growth, and sorting, of the filling events is central to unravelling intrusive history.Variations in temperature, whether on phase diagrams or in actual magmas, are intrinsically linked to commensurate variations in space and time in magmatic systems. The spectrum of all physical and chemical processes associated with magma is accordingly strongly partitioned in space and time.The idea of a magma chamber as a vat of low crystallinity melt crystallizing everywhere within and differentiating through crystal settling is unrealistic. A magma chamber formed of any number of crystal-laden inputs, encased by inward-propagating, dynamic solidification fronts, and where significant differentiation is tied to the dynamics of late-stage, interstitial melt within extensive mush piles is more in accord with the rock record.

The Dome Mountain ( DM) lavas, located in the moat of the Timber Mountain caldera of the SW Nevada volcanic field, cap a thick sequence of welded tuffs, massive and bedded tuffs, and mafic and rhyolitic lava flows. These 10.9-m.y.-old flows comprise a pile of some twenty flows with a total thickness of about 300 m. Individual flows range in thickness from 3 to 20 m and are characterized by a basal vesicular zone, a massive central region and an upper vesicular region containing, in most cases, cooling fractures. The flows dip gently away from the summit of DM, a prominent topographic feature of the region with an elevation of 2065 m. Silica contents range from 48.8 wt.% at the base of the pile to 59.4 wt.% at the top, although most of the flows are andesitic. Crystal size distributions (CSDs) for plagioclase in these comagmatic lavas have been determined to provide information on magma storage times. These times, when coupled with phase equilibria, can be used to infer cooling rates, which are indicative of the relative dimensions of magmatic bodies and the vigor of various magmatic cooling processes. Most of the CSDs show no sign of crystal fractionation or accumulation. Assuming a growth rate of 10 −10 cm/s, calculated mean crystal residence times for most lavas cluster in the range of 1.5 to 4 years. Two of the lower lavas have residence times in the range of 4 to 9 years. The residence times correlate inversely with nucleation density and broadly correlate inversely with silica content: high-residence-time, low-silica lavas occur at the base of the pile, whereas low-residence-time, high-silica lavas occur at the top of the pile. One interpretation of the residence time data is that the lavas are from an open system of more or less constant residence time but varying spatially in system locations; e.g., a magma chamber in which steady-state recharge and eruption may have been reached. Alternatively, these lavas may represent small samples, closely spaced in time, of a slowly cooling, large batch or reservoir of magma. The greater residence times associated with the lower flows may indicate the increased amount of time necessary to form the conduit of the magma-volcano system by the oldest magma or simply that part of the system with the highest liquidus temperature. A minimum volume of 1.4 to 2.5 km 3 for the magma chamber is estimated for the DM system using the estimated residence times in a simplified cooling model of a sheet of aspect ratio R . The actual system volume depends on R , which is strictly unknown, and thus the actual system volume may be much larger (e.g., ~ 200 km 3 ). A mean paleoeffusion rate of 0.05 km 3 /yr is obtained for the lavas by assuming that the residence time of each lava corresponds to the amount of time elapsed since the eruption of the underlying flow.

Oscillatory or competitive nucleation about a binary (or perhaps pseudo binary) eutectic and ensuing crystal growth and settling is a commonly suggested means of producing layering in magmatic systems. A quantitative model is presented of this, outwardly, relatively simple process of crystal nucleation, growth, and settling in an otherwise initially crystal-free magma. Avrami-style kinetics of crystallization in an always wellmixed body, buried in conductive wall rock, are coupled to a Stokes-like formulation of crystal settling in magma whose viscosity depends on temperature and crystallinity. Two dimensionless numbers (Se, the settling number and Av, the Avrami or kinetic number) govern all the results. Av and Se measure the relative importance of crystallization time and settling time, respectively, relative to the overall cooling time. For any value of Av, which increases strongly with the maximum nucleation and growth rates and cooling time, layering is possible only over a range or window of values of Se. Both above and below this window a single layer (crystalline below, vitric above) forms, and within this window the number of layers increases systematically with increasing Av and Se. Grain size within any single layer generally coarsens upward. Because the characteristic settling and cooling times both depend on body thickness, the lower limit of the settling window is also dependent on sheet thickness. Within the confines of this model and for nucleation and growth rates set by those observed in natural systems, layering is unlikely in sheet-like magmas thinner than about 100 m. When the body is not always well-mixed and crystallization is within inward-propagating solidification fronts, it is expected that this minimum body thickness will increase.

The emplacement history of a magma, whether it becomes a pluton, dome, or lava, results from its path-dependent cooling during transport. Transport is coupled to the cooling rate through the production of solids (crystals) which greatly affects magma viscosity. Thus, the temperature/H 2 O history of a magma, measured by its near-surface phase diagram, may strongly influence the style of emplacement or eruption. This idea has been tested against textural and stratigraphical data collected from the Tatoosh igneous complex. Mount Rainier National Park, WA. Detailed mapping suggests a system evolving from extrusive lavas, through dome-building plugs and flows to a shallow sill-like pluton. Explosive volcanism preceded emplacement of the pluton. Chemical and mineralogical data point to a magma that equilibrated at about 2 kbar pressure with a low (about 1.5 wt.%) water content, 10–15 vol.% crystallinity (plag. mag. opx. cpx Fol. hrn). Continued crystallization built up a volatile-rich (about 4.5% H 2 O) cap, which eventually erupted as a low-energy, “over-spilling cloud” pyroclastic flow. Maximum crystallinity in these rocks was about 27% in rocks containing a cryptocrystalline groundmass. Textural distinction can be drawn between dome-filling, cryptocrystalline, subvolcanic plugs and dome-centered, granophyric, shallow plutons. In all cases, water acts to buffer the mass of groundmass crystallinity, and this effect increases with increasing initial water content. Low water contents allow the melt to reach more easily the conditions necessary to produce granophyric and granitic textures. High water contents give rise to coarsening of pre-existing phenocrysts and cryptocrystalline groundmass as temperature is controlled by a dynamic, water-buffered liquidus. A simple mass balance model of these ideas suggests that faster cooling is favored by melts deficient in water. High crystallinities of dome rocks (> 25%) are probably attained through explosive, in situ degassing and textural coarsening of magma that was emplaced at a much lower crystallinities.

Basal cumulate zones are common in sills formed of magma-carrying phenocrysts upon emplacement, and the process leading to these cumulates and the corresponding roofward depleted zones are well understood. Many large sills which seem to have been emplaced with phenocryst-free magma, are nearly uniform in composition and show little tendency to differentiate by crystal settling. We examine whether, for an initially crystal-free sill solidifying inwardly from top and bottom, crystals nucleated and grown along the roof can escape incorporation, survive resorption in settling through the hotter central region, and reach the basal solidification front to form a cumulate. We call this process solidification front fractionation and build a relatively simple model to examine the rheologic, dynamic, and crystal growth rate conditions leading to the downward escape of newly grown, single crystals from the roof of a sheet-like body of mafic magma cooling by conduction. Our results suggest that, whereas a thin micro-...

Fluid dynamic experiments have been conducted on the solidification of a paraffin layer, in order to elucidate the transient stage of convection created in cooling magma by the fact that strong changes in viscosity with crystallization lock up within an inwardly propagating crust much buoyancy that would otherwise be available to drive convection. The interior of the magma remains isothermal, and the temperature decreases uniformly until it is locked at the convective liquidus; the crystals are fine hairlike dendrites without major compositional differentiations. Measurements over time are presented of crust thickness, convective velocity, and heat transfer.

[1] Yucca Mountain is a potential repository site for high-level radioactive waste. We examine claims that the frequency of future basaltic dike intersection could be 1 × 10−6/yr or higher (i.e., on average, one penetration every million yrs). These claims fail simple tests at four time scales. Spatial-temporal models that predict intrusion frequencies >2 × 10−7/yr are overly pessimistic because no dikes have been found in the 13 Myr-old potential repository footprint. More realistic models are developed by considering this non-detection of dikes and by applying probabilistic hazard assessments based on known patterns of Pleistocene volcanism. Using a hazard assessment code, we estimate the frequency of dike intersection at 5.4 × 10−8/yr with an upper 95% confidence bound of 9.8 × 10−8/yr.

Fundamentals haunt me. Certain words ignite unavoidable trains of thought, trains that begin in a cascade, unexpectedly leaping chasm after chasm, rushing from single words to whole paragraphs to full books to men's lives. So it is with me with seeing the word “fundamental” in print. I cannot evade the euphoric excitement of thinking that someone has found something terribly original and simple, understandable by every journeyman, explaining everything.

The mystery of producing strong compositional diversity among suites of comagmatic igneous rocks is investigated by considering the dynamic evolution of basaltic magma in a sheet-like chamber. A central conclusion is that inward-progressing crystallization produces strong viscosity and temperature gradients that promote convection only near the leading edge of the upper thermal boundary layer. Convection is apparently confined to an essentially isoviscous, isothermal region that hugs the downward-growing roof zone. Strong changes in viscosity with crystallization divide the upper and lower thermal boundary layers into regions of decreasing viscosity and crystallinity (N) called "rigid crust" (N ≥ 0.5), "mush" (0.5 ≥ N ≥ 0.25), and "suspension" (N ≤ 0.25). The strong increase in viscosity near the mush-suspension interface acts as a capture front that overtakes and traps slowly settling crystals. Initial phenocrysts mostly escape capture, but crystals nucleated and grown in the suspension zone can escape only if the capture front slows to a critical rate attainable only in bodies thicker than about 100 m. Escaping crystals are redistributed and sorted by convection driven by the advance of the capture front itself. Crystal-laden plumes traverse the central, hot core of the body and deposit partially resorbed and sorted crystals within the lower suspension zone. Convection is never vigorous but is part of an overall intimate balance between roofward heat loss, rigid-crust growth, crystallization kinetics, and transport and sorting of sinking escaped crystals. There is a strong similarity between these processes and those producing both varves and saline pan deposits. It is clear that lavas, lava lakes, and sills are indeed examples of true magma chambers strongly exhibiting certain aspects of this over-all process. These aspects commonly also characterize the large mafic magmatic bodies. Because strong compositional changes in the residual melt occur largely outward (that is, at lower temperatures and higher crystallinities) of the capture front, which is immobile and mostly within rigid crust, the possible range in comagmatic compositions available for eruption anywhere within the active magma is very limited. This is in broad agreement with the compositional range observed in basaltic lava lakes, sills, and plutons like Skaergaard. The tuning of convection, crystallization kinetics, and phase equilibria in chambers of this type can produce a variety of textures and layering but not a diversity of compositions.

Earth's geologic past has been punctuated by episodes of voluminous outpouring of lava that flooded large areas of continents and oceans, giving rise to what are commonly called flood basalt provinces. These flood basalt eruptions often coincided with major stratigraphic boundaries and mass extinction events. The Deccan Traps of India are a muchstudied example of a flood basalt province that erupted at or near the Cretaceous/Tertiary (K/T) boundary 65 million years ago. These lavas reach a maximum thickness of 2.7 kilometers at the Western Ghats of India.

The Sudbury Igneous Complex of Ontario, Canada, is the remnant of a voluminous melt sheet produced in a few minutes by impact of a massive meteorite into continental crust 1.85 Ga ago. The transient cavity and melting zone reached the Moho and instantly (∼2 min) relaxed to form a more familiar large, shallow crater holding a thick, superheated (∼1700 °C) melt sheet covered by ∼2 km of breccia. There is little about the resulting bimodal igneous complex that resembles crystallization of well-known sheet-like bodies of similar composition. Yet, the norite and granophyre exhibit a remarkable similarity in isotopic and trace element compositions, suggesting an intimate common parentage from the surrounding crust. This petrogenetic enigma is explained here as a natural, unavoidable consequence of the impact process in the rapid formation of a superheated magmatic emulsion, which we take as the high-temperature equivalent of breccia. A wide spectrum of viscously discrete, interdispersed parcels of mafic and felsic liquids, reflecting the compositional heterogeneity of the target crustal materials, formed the emulsion. Within days to months, the emulsion components separated according to their relative densities into a bimodal norite-granophyre assemblage that formed the basic structure of the present Sudbury Igneous Complex. There is clear evidence of this emulsion in the earliest dikes (i.e., offsets), which likely give the earliest state of the nascent melt sheet. Immediately following emulsion separation, the strongly superheated bimodal melt sheet underwent vigorous thermal convection in each layer. These convective motions homogenized and rapidly cooled the magma to the liquidus temperatures, whereupon convection ceased. The pattern of convection was pinned in place by the embayment topography of the crater floor, which in turn played a pivotal role in directing sulfide deposition into the embayments. All further cooling was by conduction of heat through the upper and lower boundaries during which time solidification fronts were established and propagated inward from the upper and lower margins. There is clear evidence of solidification from the floor upward and the roof downward. Minimal differentiation and compositional modification took place throughout cooling and solidification. Nevertheless, during the solidification stage, granitic rock fragments on the crater floor and rafts of fallback breccia from the thick overlying Onaping Formation became unstable and entered the melt sheet, and the partially melted remnants collected at the interface between the norite and granophyre. Some interstitial melt from the norite also percolated upward, and, altogether, the blocks and melt produced the distinctive chemical and physical characteristics of the unusual Transition Zone. The Sudbury melt sheet is, in essence, a full-scale magmatic experiment. The conditions of formation, relative to any other large terrestrial magma, are “precisely” known. Thus the clear lack of any significant modal layering, the overall homogeneous nature of each unit, and the lack of any significant chemical differentiation through crystal fractionation establish Sudbury as a valuable example of what does not happen under the initial conditions long assumed to prevail at the formation of most large magma chambers.

A long-running mystery of Earth science concerns the physical and dynamic connection between volcanism, plutonism, magma chambers, layered intrusions, and the production from mantle material of oceanic and continental crust. The basic chemical connection of fractionating melt from crystals and resorting crystals again and again is clear, but volcanologists and plutonists covet almost mutually exclusive, process-oriented sciences to explain the compositional sequences and textures found in stacks of lavas and expanses of plutons. Lavas present an extensive time series of quenched aliquots of magma without any clear measure of the evolutionary spatial context of sampling within the magmatic system. Plutons offer extensive spatial context without a clear physical connection to the greater active magmatic system and without a sequential capturing of time; critical original textural details are erased under long cooldown times. A deadly middle ground must be crossed to meld the timescales and textures captured by volcanism with the spatial scales and textural processes of plutonism into a conceptual representation of an integrated working magmatic system.

Frankly, I was offended at the outset by two things about this book: the title and the blurb about the author. It is one thing for the Chicago Press to put out a book on the technical details of style. After all, someone has to layout more than anyone would ever possibly need to know on the exactitudes of how to quote, reference, and punctuate. But what on Earth could Chicago ever tell us about how to write, review, and illustrate a scientific paper, proposal, or technical report? And, even more, what do they know about giving a riveting talk? Would this be the Midwestern version of how to present science? Will rival accounts follow from California, Massachusetts, and England? Who should we take seriously about something at which we already take ourselves to be masterful?

The results and conclusions in the recent paper by [Pan (2001)][1] are based on an inadequate understanding of the concept and measurement of population density and the fundamentals of the method of crystal size distributions. Not only does Pan perform his example calculations incorrectly, which