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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.

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.

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

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

Most models of high-alumina arc basalt petrogenesis rely heavily on the supposition that the abundances of certain trace elements, in particular the relatively unfractionated Rare Earth Element (REE) patterns and the unusually high concentrations of K, Rb, Sr, and Ba are incompatible with a garnet-bearing subducted oceanic crustal (quartz eclogite) source rock. We have carefully examined this apparently unequivocal evidence in light of recent progress on the physics of melt extraction and the heat transfer and mechanics of magma ascent. The weakest element of all trace element models involving a quartz eclogite source is the assumption that the element concentrations are fixed at the source and only later modified in the near-surface environment. We expand on such models by monitoring the concentrations of R EE and major and trace elements during initial melting, ascent, and extraction of magma. This is done by combining calculated cooling curves for ascending magmatic bodies with high pressure phase equilibria. The amount that each phase contributes to the melt is monitored along with the composition of the melt and residual solids. With quartz eclogite, initial melting initiates gravitational instability of the entire source material (melt plus solids) before melt extraction can occur. During ascent of this mush, melting increases until the solids can be repacked to free the melt. This extraction takes place some 15-20 km above the slab, after about 50 per cent melting, at which point the melt has a pattern of REE and other trace element concentrations almost identical to those observed in high-alumina arc basalts, assuming an initial composition equivalent to altered oceanic crust plus 5 per cent pelagic sediment. Sr abundances are the only ones which are not well-matched by this process. The major element concentrations of the extracted melt also closely match those of high-alumina arc basalt. A similar, but less detailed evaluation of both fertile and depleted peridotite source rocks yields good agreement for the REE and other trace element concentrations assuming a LREE-enriched source rock strongly enriched in K, Rb, Sr, and Ba. Ni, Cr, and Co abundances are satisfied only through substantial low pressure fractionation of mafic phases, in particular olivine. Though not rigorously tested, such a process may be compatible with the observed major element concentrations of high-alumina basalt. However, the experimentally verified fact that high-alumina basalts could never have been in equilibrium with either an olivine-beari ng magma or source rock eliminates this possibility altogether. Thus, the simultaneous consideration of the mechanics of ascent and melt extraction along with phase equilibria clearly shows that partial melting of quartz eclogite best satisfies the chemical constraints of major, trace, and REE characteristics of high-alumina arc basalts.

The Edgecumbe volcanics, which range from basalt through rhyodacite, have Sr contents between 125 and 370 p.p.m., Rb contents of 1 to 70 p.p.m., Ba ranging from 50-550 p.p.m. and initial l7Sr/**Sr ratios between 0-70291 and 0-70404. No simple correlation exists between these components and silica. The highest "Sr/^Sr values occur in a group of intermediate lavas (55-60 wt. per cent SiO2) while the rhyodacites have initial ratios between 0-7035 and 0-7038. With increasing silica, Sr increases to a maximum in the andesites and then steadily decreases; Ba and Rb increase over the same compositional range. The highest l7Sr/86Sr lavas have major and trace element concentrations which depart from trends defined by most of the lavas. The variation in strontium isotopic compositions suggests interaction between parental basaltic magma and crustal material. Attempts to model the assimilation process using fixed end-member assimilation and assimilation-fractional crystallization models have failed to produce the observed chemical trends. Because the parental basaltic liquid underwent little fractionation, the variability in hybrid lavas is attributed to variation in contaminant composition. Initial melts were low in CaO, Al2Oj, MgO and Sr and enriched in SiO2. K2O, Na2O, Rb and Ba. As melting progressed, melts became enriched in the more refractory components. Because hybrid strontium isotopic composition is a function of Sr concentration as well as isotopic compositions, the Sr content of the assimilant strongly influences resultant isotopic systematics. The development of the assimilants suggests plagioclase was a residual phase during early melting. This model of crustal assimilation represents one end-member in the spectrum of processes responsible for the generation of continental volcanic suites.