the lower eastern layered series of rhum the lower eastern layered series lels comprises approximately units 1 5 of previous workers remapping has revealed considerable along strike lithological variation in the units of the lels it is suggested on the basis of field and geochemical evidence that two layers formerly regarded as conformable intrusive sheets of fine grained olivine gabbro may be evolved alii valite layers rather than later intrusions xenolith suites in these layers and elsewhere indicate a component derived from the roof or walls of the magma chamber cryptic variation is more extensive in the lels than in other parts of rhum olivine forsterite content varies from 85 6 to 70 and clinopyroxene mg 100 mg fe varies from 88 to 74 postcumulus effects and subsolidus re equilibration have altered the initial compositions of the mineral phases the data are consistent with a model of repeated replenishment by picritic magma although the replenishing liquids may have been slightly less magnesian than those subsequently available during the formation of the upper els

the skaergaard layered series part ii magmatic flow and dynamic layering re examination of textural and structural features of the skaergaard intrusion has clarified the process responsible for various forms of layering two general types are distinguished one produced by magmatic flow and another by non dynamic processes such as compaction and varied nucleation and growth of crystals our concern here is mainly with layering of the first type magmatic flow results in contrasting layers and strong foliation accompanied by a linear orientation of elongated minerals it is associated with slumping and cross bedding in a zone near the margins of the intrusion rocks in the interior of the layered series are foliated and commonly layered but have little or no lineation their layering cannot be due solely to deposition from currents the principal structural elements produced by dynamic processes in the magmatic state foliation lineation shear zones folds and current structures define parallel belts mainly around the western and northern margins their distribution suggests that the strongest component of flow was for the western side and that its strength decreased toward the east shear flow trajectories deduced from lineations and kinematic indicators confirm this asymmetry and point to mass movement toward the southeastern part of the floor

the combined effects of continents and the 660 km depth endothermic phase boundary on the thermal regime in the mantle abstract we explore a wide range of parameter space to investigate the dynamical interaction between effects due to surface boundary conditions representing continental and oceanic lithosphere and the endothermic phase boundary at 660 km depth in simple two dimensional convection calculations phase boundary induced mantle layering is strongly affected by the wavelength of convective flows and mixed surface boundary conditions strongly increase the horizontal wavelength of convection our study shows that for mixed cases the effects of the surface boundary conditions dominate the effects of the phase boundary we show that the calculations with complete continental coverage have the most significantly decoupled lower and upper mantle flows and substantial thermal and mechanical layering unlike the free slip case where the surface heat flow decreases substantially with increasing magnitude of the clapeyron slope surface heat flow is shown to be almost independent of the clapeyron slope for mixed boundary condition cases although very different when not layered models with free and mixed surfaces have very similar planforms with very large aspect ratio flows when run with large magnitudes of the clapeyron slope we also calculate the critical boundary layer rayleigh number as a measure of the thermal resistance of the surface boundary layer our results show that the thermal resistance in the oceanic and the continental regions of the mixed cases are similar to fully free and no slip cases respectively we find that even for purely basally heated models the mantle becomes significantly subadiabatic in the presence of partial continental coverage this is due to the significant horizontal advection of heat that occurs with very large aspect ratio convection cells

thermal hydrous and mechanical states of the mantle transition zone beneath southern africa abstract observations of p to s conversions from the seismic discontinuities near 410 and 660 km depth reveal the shoaling of the 410 km discontinuity and the deepening of the 660 km discontinuity beneath the archaean cratons in southern africa consequently the mantle transition zone is 20 km thicker than beneath post archaean regions and the global average the discontinuity structure combined with tomographically imaged seismic velocities suggests a relatively cold t 100 k and water enriched oh 20 35 saturation transition zone beneath the cratons the thicker than normal transition zone correlates with surface geology and does not follow the track of african plate motion as numerical simulations of small scale convection imply the observations suggest that cold thermal downwelling is not the primary cause of the anomalously thick transition zone beneath the cratons although it may exist in a confined area near the boundary between the kheis thrust belt and the namaqua natal belt if thermal downwelling exists its temperature anomaly at the 660 km discontinuity must be below the levels detectable by the method used in this study t 60 k the spatial correlation between surface geology high velocities beneath the cratons and the transition zone structure indicates that the cratonic keels extend to the base of the upper mantle substantially deeper than most estimates our results demonstrate the need to closely consider the effect of water content on seismic velocities beneath deep continental roots

disequilibrium dihedral angles in layered intrusions a microstructural record of fractionation the dihedral angle formed at junctions between two plagioclase grains and a grain of augite is only very rarely in textural equilibrium in gabbros from kilometre scale crustal layered intrusions the median of a population of these disequilibrium angles cpp varies systematically within a single layered intrusion remaining constant over large stretches of stratigraphy with significant increases and decreases associated with the addition or reduction respectively of the number of phases on the liquidus of the bulk magma the stepwise changes in cpp are present in the upper zone of the bushveld complex the megacyclic unit i of the sept iles intrusion and the layered series of the skaergaard intrusion the plagioclase bearing cumulates of rum have a bimodal distribution of cpp dependent on whether the cumulus assemblage includes clinopyroxene the presence of the stepwise changes is independent of the order of arrival of cumulus phases and of the composition of either the cumulus phases or the inferred composition of the interstitial liquid the only parameter that behaves in an exactly analogous manner to cpp is the rate of change in enthalpy with temperature h t during crystallization both h t and cpp increase with the addition of a liquidus phase and decrease with the removal of a liquidus phase the replacement of one phase by another has little effect on h t and no discernible effect on cpp an increase of h t results in an increase in the fraction of the total enthalpy budget that is the latent heat of crystallization the fractional latent heat it also increases the mass crystallized in each incremental temperature drop the crystal productivity these increases of both fractional latent heat and crystal productivity are likely to cause an increase in the time taken to form three grain junctions in the mush via thermal buffering of a thickened mushy layer we suggest these are the underlying causes of stepwise increases in cpp stepwise changes in the geometry of three grain junctions in fully solidified gabbros thus provide a clear microstructural marker for the progress of fractionation down the liquid line of descent in layered intrusions

mantle discontinuities and temperature under the north american continental keel a ubiquitous feature of upper mantle seismic velocity models has been the presence of high velocity keels beneath stable continental interiors1 2 3 4 5 uncertainty remains however regarding the maximum depth to which continental keels extend the degree to which they have cooled the mantle that surrounds them and their role in mantle flow here we investigate thermal anomalies across the eastern margin of the north american continental keel by imaging the seismic discontinuities at depths of 410 and 660 km with compressional to shear converted waves recorded by a 1 500 km long seismometer deployment in the eastern united states the thickness of the transition zone the region nominally between depths of 410 and 660 km and the depth to the 410 km discontinuity indicate that cold keel material and sub keel downwellings must be largely confined to the upper mantle and may impinge on the transition zone only in localized regions and with thermal anomalies of less than 150 k a 20 km depression of the 660 km discontinuity to the south of the westernmost stations coincides with a region of fast velocity in the deep transition zone2 and may be associated with the remnants of the subducted farallon plate1 2 4

mantle dynamics and the heat flow into the earth s continents reducedcontinental heat flow defined as the measured heat flow minus that due to radiogenic sources in the upper crust initially decreases with time after the last thermotectonic event in a region1 but then tends to stabilize at a roughly constant value which is higher than would be predicted by a conductive cooling model fig 1 here we argue that this behaviour results from the influence that continents have on mantle heat loss the finite thermal conductivity of continental crust and the fact that it is not recycled into the mantle on a large scale as is oceanic crust allows stable continental blocks to limit the local heat flux out of the mantle numerical models that allow blocks of crust to form over a mantle layer demonstrate how thermal perturbations associated with mantle convection penetrate into continents causing lateral temperature variations at their base that parallel those in the mantle just below this tends to establish a constant flux of heat from the mantle into the continents which accounts for the trend in reduced heat flow

tomography based characterization of ice air interface dynamics of temperature gradient snow metamorphism under advective conditions snow at or close to the surface commonly undergoes temperature gradient metamorphism under advective flow which alters its microstructure and physical properties a functional understanding of this process is essential for many disciplines from modeling the effects of snow on regional and global climate to assessing avalanche formation time lapse x ray microtomography was applied to investigate the structural dynamics of temperature gradient snow metamorphism exposed to an advective airflow in controlled laboratory conditions experiments specifically analyzed sublimation and deposition of water vapor on the ice structure in addition an analysis of the ice air interface dynamics was carried out using a macroscopic equivalent model of heat and water vapor transport through a snow layer the results indicate that sublimation of the ice matrix dominated for flow rates 10 6 m3 s 1 while during increased mass flow rates the water vapor deposition supplied by the advective flow counteracted sublimation a flow rate dependence of water vapor deposition at the ice interface was observed asymptotically approaching an average estimated maximum deposition rate on the whole sample of 1 05 10 4 kg m 3 s 1 the growth of microsized whisker like crystals on larger ice crystals was detected on microscope photographs leading to an increase of the specific surface area and thus suggest a change of the physical and optical properties of the snow the estimated values of the curvature effect of the ice crystals and the interface kinetic coefficient are in good agreement with previously published values

sill intrusion into pyroxenitic mush and the development of the lower upper critical zone boundary of the bushveld complex implications for the origin of stratiform anorthosites and chromitites in layered intrusions

implications of a simple mantle transition zone beneath cratonic north america many areas of old continental interiors have thick roots that extend to depths of 250 km or more in contrast to the oceans and younger continents whose lithospheric thickness is less than 100 km these cratonic roots might perturb temperatures in their surrounding mantle though the net result could be either cooling or heating both of which may lead to small scale convective flow around the root we show here using new data from a study of the seismic structure of the canadian shield that the relative positions of the 410 km and 660 km seismic discontinuities are unperturbed beneath one of the deepest and broadest cratonic roots on the earth differential arrival times and internal discontinuity structure are remarkably uniform and simple varying by little more than 0 5 s over the root s 3500 km lateral extent this implies that the root has no significant thermal effect on the underlying mantle 50 k and any small scale convection or cold mantle downwelling associated with the large free air gravity anomaly beneath the shield must be confined to the upper mantle our observations are also consistent with phase changes solely in the olivine system mg fe 2sio4 with the caveat that our method is dominantly sensitive to s wave velocity jumps alone

the interaction of plumes with the transition zone under continents and oceans abstract the dynamics of mantle flow with phase transitions can be influenced by the presence of a thick continental lithosphere especially for the 400 km phase transition we have studied the effects of the continental lithosphere on the mantle convection with the two major phase transitions at 400 km and 660 km a 2 d model with an aspect ratio 8 box was used in which the surface was divided into two domains oceanic and continental the continental lithosphere is modeled by a highly viscous 200 km thick lid which is 100 times stiffer than the oceanic part we also consider the consequences of plate reconfiguration by switching instantaneously the underlying rheological structure of the oceanic and continental domains a depth dependent viscosity with an additional jump at 660 km depth was employed other depth dependent properties employed are the thermal expansivity and the thermal diffusivity both internal and basal heating configurations were studied our results show that there are distinct differences in the mode of interaction of plumes with the transition zone under continents and oceans much larger stationary plumes are developed in the lower mantle under continents and erupt from time to time into the upper mantle creating a very hot continental upper mantle plumes under oceans are weaker but produce fast horizontal jets when the plumes impinge upon the surface very large plumes are formed in the internally heated cases because of the presence of depth dependent properties in the lower mantle creating a thick bottom boundary layer this thick thermal boundary layer is swept upward under the continents by a large scale flow in the lower mantle these results can be important for the interpretation of seismic tomography under africa and the central pacific

isotopic and geochemical investigation of unit 10 from the eastern layered series of the rhum intrusion northwest scotland sr and nd isotopic compositions together with whole rock sr and nd abundances and olivine compositions are presented for cumulates from unit 10 of the eastern layered series of the rhum intrusion northwest scotland the results show an nd sr isotopic correlation consistent with contamination by high 87 sr 86 sr low 143 nd 144 nd crust there is a strong correlation between the degree of contamination sr and nd whole rock abundance and fe enrichment in cumulus olivine the allivalites are the most contaminated while the basal peridotites are uncontaminated this suggests that the cumulates crystallized from a primitive uncontaminated magma and the allivalites from an evolved contaminated magma subsequently movement of contaminated feldspathic intercumulus liquid has perturbed the isotopic trace element and olivine composition of the upper peridotites

srndpb isotopic evidence for crustal contamination in the rhum intrusion abstract sr nd and pb isotope ratios measured on cumulates from the eastern layered series els of the rhum intrusion indicate that the rhum magma chamber was contaminated by upper crustal amphibolite facies lewisian gneiss this is in marked contrast to the skye main lava series of skye which shows extensive contamination by lower crustal granulite facies lewisian gneiss 3 the amount of contamination involved was about 7 cryptic sr isotope profiles through the els show that contamination was extremely variable and may be controlled by mixing between fractionated contaminated magma and periodic injections of primitive magma into the chamber

rhythmic layering formed by deposition of plagioclase phenocrysts from influxes of porphyritic magma in the cuillin centre isle of skye the outer bytownite gabbros of the cuillin centre on the isle of skye include a 175 m rhythmically layered sequence consisting of alternating layers of coarse grained massive gabbro and finer grained laminated gabbro we infer that the massive layers formed from repeated influxes of plagioclase phyric magmas which flowed into an existing magma chamber and deposited their entrained phenocrysts on the chamber floor the phenocrysts accumulated rapidly trapping large amounts of pore liquid in the cumulus pile during the intervals between influxes laminated gabbro layers formed by crystallization of the resident magma these layers accumulated more slowly by crystal settling or in situ growth allowing sufficient time for their pore liquids to be expelled by compaction or adcumulus crystallization the resulting fractions of trapped pore liquid were510 in laminated gabbros and 20 40 in massive gabbros high porosity in the massive layers is attributable to rapid accumulation of a lowdensity cumulus assemblage with trapping of pore liquids facilitated by the confining effects of impermeable laminated cumulates above and below among layers whole rock sr and nd isotope ratios reveal variable degrees of contamination by the archean crust beneath skye in the lower part of the section influxes of plagioclase phyric magma are associated with a shift to more primitive mineral compositions lower sr sr and higher nd nd indicating that the replenishing magmas were initially more primitive and less contaminated than those already residing in the chamber in accordance with this interpretation massive layers have lower sr sr than laminated layers consistent with derivation of the plagioclase phenocrysts from a less contaminated source in the upper part of the section the shift toward more primitive and less contaminated compositions is reversed suggesting that over time the replenishing magmas underwent increasing degrees of fractionation and contamination within the magmatic plumbing system that fed the intrusion these observations and inferences coupled with published evidence for other isotopic reversals support a model for growth of the cuillin centre by frequent replenishment of a small volume magma chamber with pulses of magma that underwent complex histories of fractionation and contamination during their transit through the archean crust periods dominated by assimilation and fractional crystallization within the feeder system alternated with periods of vigorous recharge by relatively primitive and less contaminated magmas these cycles were superimposed on an overall trend toward less contamination through time consistent with increasing dominance of the system by uncontaminated tholeiitic magmas as the hebridean magmatic province matured

the cumulus and post cumulus evolution of chrome spinels in ultrabasic layered intrusions evidence from the cuillin igneous complex isle of skye scotland chrome spinels from the layered peridotilte series of the unmetamorphosed anorogenic 60 ma cuillin igneous complex isle of skye display a wide variety of compositions cumulus within seams chrome spinels from the lowest exposed portion of the peridotite series exhibit features indicative of textural equilibrium are rich in al and mg and have low values of the ratio cr cr al cumulus chrome spinels from higher up in the series are different from these particularly textural disequilibrium is evident intercumulus plagioclase and olivine are present and the chrome spinels are rich in cr fe and ti with high values of the ratio cr cr al intercumulus dispersed chrome spinels tend towards anhedral forms and define enrichment trends towards fe both fe2 and fe3 with decreasing mg cr and al and towards al with decreasing fe2 and cr and increasing mg individual crystals are completely homogeneous and are devoid of reaction rims the observed textural characteristics and compositional data of the chrome spinels documented here suggest that the semi quantitative peritectic reaction aluminous chrome spinel melt plagioclase olivine chromian chrome spinel is responsible for the observed parageneses and that both the environment of crystallization eumulus or intercumulus and the role of plagioclase olivine crystallization are critical parameters for this geochemical trend in spinels within upper crustal magmatic systems the effects of pyroxene crystallization on the development of this geochemical trend are also considered this investigation highlights the need to consider the role of post cumulus mineral melt reactions and their influences upon the final compositions of major oxide and silicate phases within layered intrusions

towards a model for the in situ origin of pge reefs in layered intrusions insights from chromitite seams of the rum eastern layered intrusion scotland the current debate on the origin of platinum group element pge reefs in layered intrusions centres mostly on gravity settling of sulphide liquid from overlying magma versus its introduction with interstitial melt fluids migrating upward from the underlying cumulate pile here we show that pge rich chromitite seams of the rum eastern layered intrusion provide evidence for an alternative origin of such deposits in layered intrusions these laterally extensive 2 mm thick chromitite seams occur at the bases of several cyclic mafic ultramafic units and show lithological and textural relationships suggesting in situ growth directly at a crystal liquid interface this follows from chromitite development along the edges of steeply inclined culminations and depressions at unit boundaries even where these are vertically oriented or overhanging high concentrations of pge up to 2 3 ppm pd pt are controlled by fine grained base metal sulphides which are closely associated with chromitite seams the following sequence of events explains the origin of the pge rich chromitite seams a emplacement of picritic magma that caused thermal and mechanical erosion of underlying cumulate followed by in situ growth of chromite against the base b precipitation of sulphide droplets on chromite grains acting as favourable substrate or catalyst for sulphide nucleation c the scavenging of pge by sulphide droplets from fresh magma continuously brought towards the base by convection since the rate of magma convection is 105 107 times higher than that of the solidification km year to km day versus 0 5 1 0 cm year the in situ formed sulphide droplets can equilibrate with picritic magma of thousands to million times their own volume as a result the sulphide bearing rocks are able to reach economic concentrations of pge several ppm we tentatively suggest that the basic principles of our model may be used to explain the origin of pge rich chromitites and classical pge reefs in other layered mafic ultramafic intrusions

a fundamental dispute a discussion of on some fundamentals of igneous petrology by bruce d marsh contributions to mineralogy and petrology 2013 166 665 690 marsh contrib miner petrol 166 665 690 2013 again claims that crystal free basalt magmas are unable to differentiate in crustal magma chambers and regards layered intrusions as primarily due to the repeated emplacement of crystal suspensions he ignores an earlier critique of his unconventional inferences latypov j petrol 50 1047 1069 2009 as well as a wealth of petrographic geochemical and experimental evidence supporting the dominant role of fractional crystallization in the solidification of layered intrusions most tellingly the cryptic variations preserved in the skaergaard and many other basaltic layered intrusions would require an exceedingly implausible sequence of phenocrystic magmas but are wholly consistent with in situ fractional crystallization a major flaw in marsh s hypothesis is that it dismisses progressive fractional crystallization within any magma chamber and hence prohibits the formation of crystal slurries with phenocrysts and melts that change systematically in composition in any feeder system this inherent attribute of the hypothesis excludes the formation of layered intrusions anywhere

differentiation and compaction in the skaergaard intrusion igneous differentiation processes are constrained from bulk compositions densities and mineral modes of 116 cumulate gabbro samples in a new reference profile through the layered series of the skaergaard intrusion east greenland the stratigraphic distribution of p u and rb in cumulates and residual magma modeled by rayleigh fractionation constrains the final porosities or trapped liquid contents to 30 52 in lza troctolites decreasing to 4 12 at the top of lzb olivine gabbros and remaining low 1 13 4 middle dot 6 on average in the oxide gabbros of lzc mz and uz local variations in trapped liquid content are associated with modal layering leucocratic low density rocks have higher proportions of trapped melt than adjacent melanocratic high density rocks these observations are explained by varying degrees of compaction compaction was most important after the onset of fe ti oxide crystallization because of the high densities of the crystal matrix here computed rates of compaction exceed the rate of crystal accumulation in layers that are metres to a few tens of metres thick in the basal section lza the crystal pile was too thin and the density of the crystal matrix too low to drive the rate of compaction above the high rate of crystal accumulation promoted by initial cooling through the intrusion floor in the overlying section lzb the efficiency of compaction gradually increased as a result of thickening of the crystal pile and lowering of the rate of crystal accumulation the modeling constrains the p2o5 content of the residual magma to 1 middle dot 7 wt at the level of apatite in suggesting that the magma contained 49 wt sio2 and followed a trend of iron enrichment compaction of the uppermost metres to tens of metres of crystal mush at the top of the cumulate pile was an efficient means of differentiation and resulted in layers with variable final porosities and trace element contents depending on the mineralogy and density of the crystal matrix

temperature beneath continents as a function of continental cover and convective wavelength geodynamic modeling studies have demonstrated that mantle global warming can occur in response to continental aggregation possibly leading to large scale melting and associated continental breakup such feedback calls for a recipe describing how continents help to regulate the thermal evolution of the mantle here we use spherical mantle convection models with continents to quantify variations in subcontinental temperature as a function of continent size and distribution and convective wavelength through comparison to a simple analytical boundary layer model we show that larger continents beget warming of the underlying mantle with heating sometimes compounded by the formation of broader convection cells associated with the biggest continents our results hold well for purely internally heated and partially core heated models with rayleigh numbers of 10 5 to 10 7 containing continents with sizes ranging from that of antarctica to pangea results from a time dependent model with three mobile continents of various sizes suggests that the tendency for temperatures to rise with continent size persists on average over timescales of billions of years

braided peridotite sills and metasomatism in the rum layered suite scotland the rum eastern layered intrusion eli scotland is an open system layered intrusion constructed of 16 macro rhythmic units each of the macro rhythmic units consists of a peridotite base and a troctolite gabbro top previously attributed to the fractional crystallisation of a single magma batch this classic paradigm has been challenged however with evidence presented for the emplacement of peridotite sills in units 9 10 and 14 such as cross cutting relationships upward oriented apophyses and lateral discontinuities to test whether the other major peridotites within the eli represent sills we have carried out new field petrographic and mineral chemical analyses of the peridotites in units 7 8 and 9 the peridotites display large and small scale cross cutting relationships with the overlying troctolite indicative of an intrusive relationship the peridotites also show large scale coalescence and lateral spatial discontinuities such that the eli unit divisions become arbitrary harrisite layers and cr spinel seams found throughout units 7 8 and 9 suggest the peridotites were constructed incrementally via repeated injections of picritic magma our observations allow for distinct subtypes of peridotite to be defined separated by intrusive contacts allowing for their relative chronology to be determined older poikilitic peridotite rich in clinopyroxene is truncated by younger well layered peridotite containing abundant harrisite layers in addition to the new peridotite subtypes defined in this study we find strong evidence for laterally oriented metasomatism within clinopyroxene rich wehrlites at the top of the unit 8 peridotite the wehrlites and surrounding peridotites record a complex series of metasomatic reactions that transformed thin picrite sills into clinopyroxene rich wehrlites without any evidence for the sort of vertical melt movement typically posited in layered intrusions the observations presented in this study from the eli cannot be reconciled with the classic magma chamber paradigm and are better explained by the emplacement of composite sills into pre existing feldspathic cumulate gabbro or troctolite the evidence for sill emplacement presented here suggests that the layered complex was constructed by a combination of sill emplacement and metasomatism forming many of the unusual often clinopyroxene rich lithologies that surround the sills the broad scale formation of the layered peridotites via incremental sill emplacement suggested by the occurrence of upward oriented apophyses coalescence and lateral discontinuity could be applied to much larger ultramafic intrusions which might have formed by similar mechanisms

chemical evolution of intercumulus liquid as recorded in plagioclase overgrowth rims from the skaergaard intrusion the intercumulus liquid of a crystal mush fills pore spaces and typically solidifies to form overgrowths on cumulus grains and poikilitic post cumulus minerals if the liquid is immobile solidification produces zoned intercumulus minerals as a result of progressive fractionation of the residual liquid convection within the mush results in buffering of the liquid composition and thus limits mineral zonation for fully solidified cumulates fossil changes in liquid composition or porosity are difficult to identify however detailed study of immobile minor components of plagioclase overgrowth rims can provide information about the progressive solidification of intercumulus material ti contents of plagioclase overgrowths in samples from the lowermost parts of the skaergaard intrusion show strong variations with anorthite content with decreasing xan ti concentrations first rise and then fall consistent with changing tio2 contents of the intercumulus liquid during solidification tio2 in plagioclase decreases sharply at an55 reflecting local saturation of fe ti oxides ti in clinopyroxene oikocrysts also falls rimward but zoning in faster diffusing species fe mg is limited other than slight reverse zones that may occur on the plagioclase margins xan falls continuously during crystallization the reverse zoning is interpreted as the result of compaction driven dissolution and reprecipitation of plagioclase the continual decrease in xan is exploited together with back scattered electron images of the cumulates to produce calibrated images showing regions of progressive crystallization this allows the regions crystallizing at each stage of solidification to be visualized these images show that the final remnants of interstitial melt were present in triangular pockets and as thin grain boundary melt films this approach can provide information about the progressive reduction of porosity during cumulate solidification

comment on the stillwater complex integrating zircon geochronological and geochemical constraints on the age emplacement history and crystallization of a large open system layered intrusion by wall et al j petrology 59 153 190 2018

the stillwater complex integrating zircon geochronological and geochemical constraints on the age emplacement history and crystallization of a large open system layered intrusion a reply to the comment by rais latypov on wall et al j petrology 59 153 190 2018

the timescales of plume generation caused by continental aggregation abstract to understand the thermal evolution of the mantle following the aggregation of non subductable thick continental lithosphere we study a numerical model in which a supercontinent simulated by high viscosity raft hvr covers a part of the top surface of a convection layer we model infinite prandtl number convection either in a three dimensional 3d spherical shell 3d rectangular box aspect ratios 8 and 4 or two dimensional 2d rectangular box aspect ratio 8 and except for the hvr we specify a constant viscosity the hvr which has a viscosity higher than that of its surrounding is instantaneously placed on the top surface of a well developed convection layer and its position is fixed our results from 3d spherical shell cases with and without phase transitions show the emergence of a large plume characterized by a long wavelength thermal anomaly a degree one pattern for a pangea like geometry we analyze the volume averaged temperature under the hvr t c the remaining oceanic area t o and total area t m to determine the timescale of plume generation the difference between t c and t o t co and t m show the existence of two characteristic timescales t co exhibits an initial rapid increase and may become constant or continue to gradually increase meanwhile t m shows a similar behavior but with a longer timescale we find that these timescales associated with the increase of t co and t m can be attributed to the formation of large scale flow i e plume and response of the whole system to the emplacement of the hvr respectively for 3d spherical cases we find that the timescale of plume generation is 1 2 gyr if the rayleigh number is 10 6 to determine the effects of the viscosity of the hvr 2d versus 3d modeling and the effects of the internal heating we have also studied 2d and 3d rectangular box cases a factor of about two variation exists in the timescale of plume generation it appears that the timescale becomes greater for a smaller amount of internal heating this may be attributed to the time dependent flow caused by the internal heating for 2d cases we find that the timescale of the high rayleigh number 10 7 case is shortened by a factor of three to five when compared to the ra 10 6 case which is consistent with the simple boundary layer theory this may imply that well developed plumes may arise with the timescale of 0 2 to 0 4 gyr for ra 10 7

the thickness of the crystal mush on the floor of the bushveld magma chamber the thickness of the crystal mush on magma chamber floors can be constrained using the offset between the step change in the median value of dihedral angles formed at the junctions between two grains of plagioclase and a grain of another phase typically clinopyroxene but also orthopyroxene and olivine and the first appearance or disappearance of the liquidus phase associated with the step change in median dihedral angle we determined the mush thickness in the rustenburg layered suite of the bushveld complex at clinopyroxene in in lower main zone and magnetite in in upper zone we also examined an intermittent appearance of cumulus apatite in upper zone using both the appearance and disappearance of cumulus apatite in all cases the mush thickness does not exceed 4 m these values are consistent with field observations of a mechanically rigid mush at the bases of both magnetitite and chromitite layers overlying anorthosite mush thickness of the order of a few metres suggests that neither gravitationally driven compaction nor compositional convection within the mush layer is likely to have been important processes during solidification adcumulates in the bushveld are most likely to have formed at the top of the mush during primary crystallisation similarly it is unlikely either that migration of reactive liquids occurs through large stretches of stratigraphy or that layering is formed by mechanisms other than primary accumulation

thermal evolution of the mantle following continental aggregation in 3d convection models the claim that supercontinents insulate the mantle is largely based on recognition that seismically slow mantle below the atlantic african geoid high coincides with the former location of pangea we investigate the viability of continental insulation by varying both plate geometries and mantle properties in six three dimensional 3d mantle convection models the efficiency of continental insulation is quantified by calculating the rate of change of the average temperature in the subcontinental mantle our findings generally agree with two dimensional 2d modeling results however we conclude that 2d convection models may exaggerate subcontinental heating particularly in largely bottom heated cases and that subcontinental heating results from an absence of subduction in the subcontinental mantle rather than the insulation of active upwellings correspondingly we find that when large degrees of internal heating are present in the mantle hotter than average regions evolve below large oceanic plates

mineral scale sr isotope constraints on magma evolution and chamber dynamics in the rum layered intrusion scotland sr isotopic zoning within single plagioclase crystals from rocks from unit 9 of the rum layered intrusion is used to infer events during crystal growth in a magma undergoing contamination the 87sr 86sr diversity among minerals and between cores and rims of plagioclase crystals increases as the boundary between unit 9 and the overlying unit 10 peridotite is approached models of near solidus interaction of the cumulate with a fluid or melt or large scale textural re equilibration cannot easily account for the systematic differences in 87sr 86sr between small crystals and the rims of larger crystals

on modeling the thermal evolution of the oceanic upper mantle an assessment of the cooling plate model the plate cooling model for the thermal evolution of the oceanic upper mantle has been widely accepted to explain observed variations of depth to oceanic basement and conductive heat flow with the age of the seafloor several estimates of best fitting plate model parameters derived from depth heat flow and age data have been proposed but the viability of the plate model itself has not been rigorously evaluated we have used published mean depths and depths to basement at deep sea drilling project ocean drilling program dsdp odp drilling sites to test the plate cooling model based on two criteria first viable plate models must have coefficients that are consistent with the slope of the corresponding root t line because the half space or root t subsidence of young seafloor is implicit in the plate model i e the slope of the root t line can be calculated directly from the coefficients of the plate subsidence model second any viable physical model must fit the data with an acceptable degree of systematic misfit large systematic misfits indicate that the model cannot explain the observations fits of half space root t models to depth versus age data for young 80 ma seafloor indicate basal temperatures in the range 1300 to 1370 c based on the age at which the depths deviate from the root t line the minimum plate thickness that is compatible with the best fitting half space models for young seafloor is 120 km in contrast all best fitting plate models yield systematically higher temperatures 1450 to 1470 c and thinner plates 102 118 km the plate model can explain the depth to basement at dsdp odp drill sites with a satisfactory degree of systematic misfit but we find that there is no plate cooling model that can explain the variation of the mean depths derived from the dbdb5 database with age over the entire range of ages 0 to 165 ma models that best fit the entire data set have unacceptably large systematic misfits over the entire range of seafloor age whereas models that minimize the systematic misfit for young seafloor 0 to 81 ma fail for older seafloor the plate model clearly fails to explain the observations the cooling history of old seafloor is not simply an extension of the cooling history of young seafloor according to a simple plate cooling model previous studies have suggested as an alternative to the plate model that observed variations of basement depth with age are best explained by the combined effects of a normal half space cooling process and the dynamic and or thermal effects of hot spots or mantle plumes our results are entirely consistent with the half space cooling model and we find that the best reference model for normal subsidence is d t 2600 20 m 345 3 m m y t we also find that the heat flow predicted by this half space model is consistent with the most reliable average heat flow values from the pacific

basaltic replenishment of the rhum magma chamber evidence from unit 14 a detailed geochemical and lithological profile through unit 14 of the rhum eastern layered series is presented and interpreted to trace the evolution of this cyclic peridotite troctolite unit the lower 15 5 m of the unit are peridotite which has an abrupt contact with the upper 35 5 m of troctolite numerous thin subsidiary peridotites occur within the troctolite variation in the modes whole rock 87 sr 86 sr ratios and ni and fo in olivine provide most information about the evolution of the unit discontinuities in these parameters divide the unit into six subunits i the lower peridotite ii the middle main peridotite iii the upper main peridotite and lowermost troctolite iv the middle troctolite v the upper troctolite and vi the disturbed troctolite these subunits are interpreted as the result of magma replenishment by picritic and by basaltic magmas field evidence suggests that the subsidiary peridotites are later sills intruded into solid or semi consolidated troctolite the olivine fo and ni values are similar to those of surrounding troctolite suggesting either re equilibration with or assimilation of the troctolite comparison with unit 10 shows that the unit 14 resident basaltic liquid was more mafic than that of unit 10 this factor along with the basaltic replenishment can explain many of the differences in geochemical and lithological variation between the two units

convection in spherical fluid shells with an outer crust of variable thickness abstract we investigate linear and nonlinear convection in a thick spherical fluid shell on which a thin spherical layer of a lighter but much more viscous fluid of variable thickness with lower thermal conductivity is superimposed this crust moves horizontally in response to the convective motion in the inner spherical fluid shell through its varying thickness the thin outer spherical layer acts as a thermal blanket of varying impedance and thus provides a strong feedback on convective motions in the lower mantle shell solutions for the thin outer spherical layer are obtained analytically and matched to the numerical solutions for motions in the mantle shell we first perform a linear stability analysis for the onset of time dependent convection the analysis is then extended to nonlinear axisymmetric solutions with or without equatorial symmetry finally fully three dimensional solutions are computed demonstrating that there exists a rich variety of time dependent spherical convection patterns the time dependence originates from the crustal thickness instability which corresponds to a hopf bifurcation even in the absence of convective overturning busse 1978 the results of the analysis show that even a small volume of crustal material may cause a significant temporal and spatial complexity of the convection flow

layering in peralkaline magmas ilimaussaq complex s greenland abstract the peralkaline to agpaitic ilimaussaq complex s greenland displays spectacular macrorhythmic 5 m layering via the kakortokite agpaitic nepheline syenite which outcrops as the lowest exposed rocks in the complex this study applies crystal size distribution csd analyses and eudialyte group mineral chemical compositions to study the marker horizon unit 0 and the contact to the underlying unit 1 unit 0 is the best developed unit in the kakortokites and as such is ideal for gaining insight into processes of crystal formation and growth within the layered kakortokite the findings are consistent with a model whereby the bulk of the black and red layers developed through in situ crystallisation at the crystal mush magma interface whereas the white layer developed through a range of processes operating throughout the magma chamber including density segregation gravitational settling and flotation primary textures were modified through late stage textural coarsening via grain overgrowth an open system model is proposed where varying concentrations of halogens in combination with undercooling controlled crystal nucleation and growth to form unit 0 our observations suggest that the model is applicable more widely to the layering throughout the kakortokite series and potentially other layered peralkaline agpaitic rocks around the world

morphology and microstructure of chromite crystals in chromitites from the merensky reef bushveld complex south africa the merensky reef of the bushveld complex consists of two chromitite layers separated by coarse grained melanorite microstructural analysis of the chromitite layers using electron backscatter diffraction analysis ebsd high resolution x ray microtomography and crystal size distribution analyses distinguished two populations of chromite crystals fine grained idiomorphic and large silicate inclusion bearing crystals the lower chromitite layer contains both populations whereas the upper contains only fine idiomorphic grains most of the inclusion bearing chromites have characteristic amoeboidal shapes that have been previously explained as products of sintering of pre existing smaller idiomorphic crystals two possible mechanisms have been proposed for sintering of chromite crystals 1 amalgamation of a cluster of grains with the same original crystallographic orientation and 2 sintering of randomly orientated crystals followed by annealing into a single grain the ebsd data show no evidence for clusters of similarly oriented grains among the idiomorphic population nor for earlier presence of idiomorphic subgrains spatially related to inclusions and therefore are evidence against both of the proposed sintering mechanisms electron backscatter diffraction analysis maps show deformation related misorientations and curved subgrain boundaries within the large amoeboidal crystals and absence of such features in the fine grained population microstructures observed in the lower chromitite layer are interpreted as the result of deformation during compaction of the orthocumulate layers and constitute evidence for the formation of the amoeboid morphologies at an early stage of consolidation an alternative model is proposed whereby silicate inclusions are incorporated during maturation and recrystallisation of initially dendritic chromite crystals formed as a result of supercooling during emplacement of the lower chromite layer against cooler anorthosite during the magma influx that formed the merensky reef the upper chromite layer formed from a subsequent magma influx and hence lacked a mechanism to form dendritic chromite this accounts for the difference between the two layers

an origin for harrisitic and granular olivine in the rum layered suite nw scotland a crystal size distribution study dendritic crystal morphologies occur in a number of igneous rocks and are thought to originate from the rapid growth of crystals yet many examples of dendritic morphologies are found in plutonic igneous rocks where cooling rates should be low results from crystal size distribution csd measurements on harrisitic olivines from rum scotland combined with estimated olivine growth rates suggest that the characteristic skeletal hopper and branching olivines of harrisitic cumulates that are up to centimetres long may have exceptionally short crystal growth times several hours to several hundreds of days this together with very low calculated nucleation densities for harrisitic olivine supports the interpretation of harrisite being a disequilibrium texture developed in response to supersaturation of the magma in olivine we propose that this supersaturation arose through undercooling of thin picrite sheets emplaced along the rum magma chamber floor beneath cooler resident magma it is envisaged that the picrite sheets were largely free of suspended olivine crystals coupled with the olivine enriched composition of the melt and the increasing cooling rate this allowed homogeneous nucleation of olivine to set in at deeper undercooling and greater olivine supersaturation than if there had been plentiful suspended olivines to act as heterogeneous nuclei the enhanced supersaturation caused rapid growth of olivine once nucleation began with skeletal and dendritic shapes it is suggested that the observed interlayered sequences of harrisite and cumulus peridotite found throughout the rum layered suite are a result of multiple episodes of harrisite crystallization resulting from picrite emplacement that alternated with periods of crystal growth and accumulation in the main body of magma at lesser degrees of undercooling

the unit 12 allivalite eastern layered intrusion isle of rum a textural and geochemical study of an open system magma chamber the eastern layered intrusion of the rum central complex nw scotland comprises 16 macro units broadly defined by a basal peridotite body overlain by gabbros and troctolites the local term for the upper plagioclase bearing cumulates is allivalite the unit 12 allivalite contains five subsidiary peridotite horizons each continuous on the kilometre scale field textural and geochemical evidence points to the unit 12 allivalite forming by progressive accumulation of crystals from up to seven separate magma batches varying from picrite to basalt the thickness of the magma lens in the sill like chamber was as low as 30 m during a significant part of the development of the unit 12 allivalite and evidence supports one eruption from the chamber during its accumulation

lateral variations in the unit 7 8 boundary zone of the rum eastern layered intrusion nw scotland implications for the origin and timing of cr spinel seam formation the rum layered suite nw scotland hosts cr spinel seams at the bases of peridotite troctolite macro rhythmic units in the eastern portion of the intrusion here we present detailed field observations together with microstructural and mineral chemical analyses for the unit 7 8 cr spinel seam and associated cumulates in the eastern layered intrusion detailed mapping and sampling reveal significant lateral variations in the structural characteristics and mineral compositions of the unit 7 8 boundary zone rocks although the cr spinel seam is laterally continuous over 3 km it is absent towards the centre and the margins of the intrusion the compositional characteristics of cr spinel and plagioclase vary systematically along strike exhibiting a chemical evolution towards more differentiated compositions with increasing distance from the main feeder conduit of the rum intrusion the long loch fault on the basis of our combined datasets we propose that the upper part of the troctolite the anorthosite layer underlying the cr spinel seam and the seam itself formed during a multi stage magma replenishment event the stages can be summarised as follows 1 peridotite schlieren and anorthosite autoliths formed following melt infiltration and cumulate assimilation in the crystal mush of the unit 7 troctolite 2 the anorthosite layer then formed from the unit 7 troctolite crystal mush by thermal erosion and dissolution due to infiltrating magma 3 subsequent dissolution of the anorthosite layer by new replenishing magma led to peritectic in situ crystallisation of the unit 7 8 cr spinel seam with 4 continued magma input eventually producing the overlying unit 8 peridotite in the central part of the rum layered suite the aforementioned assimilation of the troctolitic footwall formed the anorthosite layer however the absence of anorthosite in close proximity to the long loch fault can be explained by enhanced thermochemical erosion close to the feeder zone and its absence close to the margins of the intrusion at maximum distance from the long loch fault may be due to cooling of the magma and loss of erosion potential in line with other recent studies on pge bearing chromitites in layered intrusions we highlight the importance of multi stage intrusive magma replenishment to the formation of spatially coupled anorthosite and cr spinel seams as well as the lateral mineral chemical variations observed in the unit 7 8 boundary zone cumulates

infiltration metasomatism of cumulates by intrusive magma replenishment the wavy horizon isle of rum scotland the eastern layered intrusion of the rum layered suite comprises paired peridotite and allivalite troctolite and gabbro layers forming 16 macro rhythmic units whereas the majority of these macro units are believed to have formed by a process of crystal liquid differentiation involving successive accumulation of crystals from multiple picritic replenishments of the chamber the unit 9 peridotite is interpreted as a layer parallel picrite intrusion closely correlated with this discontinuous peridotite body is a distinctive feature generally known as thewavy horizon which divides the overlying allivalite into a lower troctolite and an upper gabbro along a well defined undulating contact we propose that the wavy horizon is a metasomatic feature formed consequent to the removal of clinopyroxene from an original gabbroic mush foundering of the mush into the picritic sill resulted in the replacement of the original interstitial liquid by one saturated only in olivine plagioclase progressive through flow of this liquid resulted in the stripping out of clinopyroxene from the lower parts of the allivalite we interpret thewavy horizon as a reaction front representing the point at which the invading liquid became saturated in clinopyroxene the distinctive pyroxene enriched zone immediately above thewavy horizon could have formed when mixing of the interstitial liquids on either side of the reaction front formed a supercooled liquid oversaturated in pyroxene as a result of the curvature of the olivine plagioclase clinopyroxene cotectic the presence of many such approximately layer parallel features defined by differences in pyroxene content in the eastern layered intrusion of rum suggests that such an infiltration reaction process was not unique to unit 9

trace element stratigraphy of the bellevue core northern bushveld multiple magma injections obscured by diffusive processes the 3 km long bellevue core was recovered from the upper zone uz and main zone mz of the northern limb of the bushveld complex the largest known layered mafic intrusion we present abundance data for 57 trace elements hosted in plagioclase clinopyroxene low ca pyroxene and olivine obtained by laser ablation inductively coupled plasma mass spectrometry these data permit the first comprehensive investigation of trace element distribution in the bushveld complex on a mineral by mineral basis and show that the cr content of pyroxene is the only proxy that records three cryptic magma injections coincident with two pyroxenite horizons and the uz mz boundary on the basis of cr abundance we infer that an anomalous troctolitic horizon at the base of the bellevue core was related to injection of relatively primitive magma analogous to the lower or critical zone magmas in the eastern and western limbs of the bushveld complex this horizon defines the greatest inconsistency of magmatic stratigraphy between the northern limb and the eastern and western limbs of the complex and is prospective for platinum group element exploration however the most significant finding of this study is that plagioclase pyroxene and olivine have undergone extensive diffusive modification the major and trace element geochemistry of these minerals records sub solidus equilibration during the prolonged cooling of the bushveld complex controlled by diffusion rates and local partitioning between adjacent minerals by analogy texturally mature cumulates in any layered intrusion are also likely to be chemically re equilibrated

ocean bathymetry and mantle convection 2 small scale flow numerical models demonstrate that a boundary layer instability of the lower lithosphere does not stop the lithosphere from subsiding and so it cannot explain the so called flattening of old seafloor the slower subsidence of the old seafloor is in any case better seen as a perturbation probably due to plumes furthermore if a boundary layer instability or any mode of upper mantle convection transported most of the heat emerging from old seafloor there would be pervasive large amplitude local bathymetrie gravity and geoid anomalies such anomalies have never been convincingly differentiated from the effects of the cooling lithosphere and hotspot tracks except for some low amplitude linear geoid anomalies near spreading centers the only modes of mantle convection with horizontal scales less than those of plates for which there is reasonably clear evidence are hotspot plumes and some possible near ridge boundary layer instabilities hotspots transport less than about 10 of the earth s heat flux and other small scale modes are probably less important

textural immaturity of cumulates as an indicator of magma chamber processes infiltration and crystal accumulation in the rum eastern layered intrusion textural maturity describes the extent to which rocks have approached textural equilibrium for igneous rocks this is a function of the time integrated thermal history in the sub solidus which for cumulate rocks generally translates to the cooling history via the rate of solidification of the accumulating crystal mush the most straightforward way to quantify the textural maturity of orthocumulates sensu lato is to measure the angle subtended at the corners of interstitial grains texturally mature cumulates have angle populations with median values of c 120 whereas texturally immature cumulates have lower medians inherited by the interstitial phase which infills either an impingement texture in the accumulating mush or a texturally equilibrated pore topology textural observations of allivalites from the rum layered suite demonstrate that stratigraphically localized excursions towards textural immaturity are the result of either transient relatively rapid solidification rates perhaps a result of the arrival of cold crystal laden bodies of fluid on the chamber floor or late stage infiltration of fully solidified cumulates by evolved liquids expelled upwards by compaction of underlying horizons these two processes have distinct textural and compositional signatures

effects of mantle heat source distribution on supercontinent stability a two dimensional cartesian geometry numerical model is used to study the influence of internal heating on the ability of mantle convection to rift and disperse an assembled supercontinent our wide aspect ratio models incorporate migrating plate boundaries and finite thickness rigid mobile oceanic and continental plates 12 000 km across we examine models characterized by both whole mantle and upper mantle convection parameters specifying different degrees of internal heating in the mantle we compare the patterns of convection that develop below stationary continental plates that form as a result of the aggregation of smaller mobile continents following continental collisions we find that a reorganization of mantle flow may result in the development of deviatoric stresses at the base of the continental lithosphere which are capable of initiating continental breakup our results suggest that continental basal stresses are largely controlled by the wavelength of the subcontinental convection pattern and that the inclusion of internal heating in the mantle generally increases this wavelength for models with 80 internal heating in which active upwellings are intrinsically absent basal stresses arise primarily from subduction at the continental margins and result in a geologically reasonable duration for our model supercontinent assemblages 200 myr for models with 40 or less internal heating continental breakup is controlled by mantle upwellings and our supercontinents remain intact much longer 600 700 myr

spatial constraints on magma chamber replenishment events from textural observations of cumulates the rum layered intrusion scotland the eastern layered series of the rum layered suite is formed of 16 macro units each comprising a lower peridotite and an overlying feldspar rich layer the local term is allivalite the origin of the peridotite layers is unresolved with two contrasting models the earlier of the two is based on repeated replenishment of an opensystem magma chamber with deposition of fractionated material on the chamber floor the second is based on the early formation of a troctolitic complex which is then repeatedly intruded by sills of replenishing picritic magma to form the peridotite horizons the lack of resolution of this fundamental problem is a consequence of the reliance of previously published studies on field observations i present evidence to show that the clinopyroxene in the allivalites preserves information about the distribution of the last melt to solidify permitting determination of not only the extent of supersolidus textural equilibration but also the sub solidus history of the allivalite horizons comparison of profiles of clinopyroxene plagioclase plagioclase median dihedral angle across allivalite units demonstrates that it is possible to distinguish between those that were intruded by later picrite sills and those adjacent to peridotite horizons formed by replenishment and subsequent deposition of fractionated crystals above the pre existing pile in the region studied only the main peridotite body of unit 9 was intruded into a pre existing allivalitic mush

effect of chromium on phase relations in the join forsterite anorthite diopside in air at 1 atm the effect of cr on the silicate system has been studied in air at 1 atm by adding a small amount of mgcr2o4 0 2 0 5 wt to the join mg2sio4 forsterite caal2si2o8 anorthite camgsi2o6 diopside which has been considered to form a thermal divide in the system cao mgo al2o3 sio2 the spinel primary field is enlarged compared with that in the cr free join at the expense of the anorthite primary field the piercing points forsterite anorthite diopside liquid and forsterite anorthite spinel liquid approach each other with increasing mgcr2o4 meet at the join with 0 25 wt mgcr2o4 0 20 wt cr2o3 to form the isobaric quaternary invariant point forsterite anorthite diopside spinel liquid and then separate again as new piercing points of diopside spinel anorthite liquid and forsterite diopside spinel liquid this process indicates that the join mg2sio4 caal2si2o8 camgsi2o6 containing more than 0 2 wt cr2o3 cannot be a thermal divide in the basalt tetrahedron the results of the present study show that the presence of a minor amount of cr causes a significant effect on the phase relations and therefore the role of cr must be taken into account in the formulation of a petrologic model

fluid dynamic and geochemical evolution of cyclic unit 10 rhum eastern layered series lithological major element trace element and sr isotope data from cyclic unit 10 of the rhum eastern layered series are presented the lower 65 metres of the unit are peridotite subdivided on textural and geochemical grounds into a lower homogeneous portion approximately 50 metres thick and an upper heterogeneous portion approximately 15 metres thick the uppermost 16 5 metres of the unit are allivalite there are steep geochemical gradients across the peridotite allivalite boundary in ni content of olivine and whole rock sr isotope composition calculations are presented on the geochemical evolution of a rhum picritic liquid undergoing olivine precipitation both when the olivines remain suspended in the residual liquid as they precipitate and when they are continuously fractionated quenched groundmass and olivine compositions from the rhum dykes and the unit 10 peridotite olivines show good agreement with the suspension model but are inconsistent with the fractionation model the rhum chamber is thought to have been replenished with a picritic liquid from which olivine crystallized while held in suspension however replenishment by a highly olivine phyric basalt is also possible the peridotite probably accumulated rapidly as olivines were dumped out of suspension onto the chamber floor the lower part of the peridotite is a poikilitic adcumulate it is suggested that this formed by convective circulation of melt in the pores of the pile of cumulus olivines in the latter stages of adcumulus growth more fe rich and isotopically contaminated magma entered the top of the cumulus pile causing cumulus olivines to re equilibrate and giving the intercumulus plagioclase a higher sr isotope ratio than lower down the olivines in the allivalite show steep stratigraphic gradients in major element composition but not in their ni content they also show substantial variation in major element composition laterally within the allivalite it is suggested that these features are a consequence of postcumulus re equilibration of olivine with migrating intercumulus magma

vertically extensive and unstable magmatic systems a unified view of igneous processes volcanoes are an expression of their underlying magmatic systems over the past three decades the classical focus on upper crustal magma chambers has expanded to consider magmatic processes throughout the crust a transcrustal perspective must balance slow plate tectonic rates of melt generation and segregation in the lower crust with new evidence for rapid melt accumulation in the upper crust before many volcanic eruptions reconciling these observations is engendering active debate about the physical state spatial distribution and longevity of melt in the crust here we review evidence for transcrustal magmatic systems and highlight physical processes that might affect the growth and stability of melt rich layers focusing particularly on conditions that cause them to destabilize ascend and accumulate in voluminous but ephemeral shallow magma chambers

the contact zone of the rhum ultrabasic intrusion evidence of peridotite formation from magnesian magmas the eastern layered series of the rhum ultrabasic complex formed in situ and was not tectonically emplaced as previously suggested field relations along the contact show no evidence for the existence of a late marginal gabbro instead peridotite and allivalite layers can be traced to within 2 m of the surrounding partially melted country rocks local preservation of a chilled margin to one peridotite layer indicates that the parental magma consisted of an olivine tholeiite liquid carrying approximately 19 vol olivine crystals in suspension

successive episodes of reactive liquid flow through a layered intrusion unit 9 rum eastern layered intrusion scotland we present a detailed microstructural and geochemical study of reactive liquid flow in unit 9 of the rum eastern layered intrusion scotland unit 9 comprises an underlying lens like body of peridotite overlain by a sequence of troctolite and gabbro termed allivalite with some local and minor anorthosite the troctolite is separated from the overlying gabbro by a distinct sub horizontal undulose horizon the major wavy horizon higher in the stratigraphy is another similar horizon the minor wavy horizon that separates relatively clinopyroxene poor gabbro from an overlying gabbro to the north of the peridotite lens both troctolite and gabbro grade into poikilitic gabbro clinopyroxene habit in the allivalite varies from thin rims around olivine in troctolite to equigranular crystals in gabbro and to oikocrysts in poikilitic gabbro the poikilitic gabbros contain multiple generations of clinopyroxene with cr rich 1 1 wt cr2o3 anhedral cores with moderate ree concentrations core1 overgrown by an anhedral ree depleted second generation with moderate cr 0 7 wt cr2o3 core2 these composite cores are rimmed by cr poor 0 2 wt cr2o3 and ree poor to moderate clinopyroxene we interpret these microstructures as a consequence of two separate episodes of partial melting triggered by the intrusion of hot olivine phyric picrite to form the discontinuous lenses that comprise the unit 9 peridotite loss of clinopyroxene saturated partial melt from the lower part of the allivalite immediately following the early stages of sill intrusion resulted in the formation of clinopyroxene poor gabbro the spatial extent of clinopyroxene loss is marked by the minor wavy horizon a second partial melting event stripped out almost all clinopyroxene from the lowest allivalite to form a troctolite with the major wavy horizon marking the extent of melting during this episode the poikilitic gabbro formed from clinopyroxene saturated melt moving upwards and laterally through the remobilized cumulate pile and precipitating clinopyroxene en route this process called reactive liquid flow is potentially important in open magma chambers

on some fundamentals of igneous petrology the age old process of crystal fractionation leading to the diversity of the igneous rocks and earth itself is an exceedingly well understood chemical process in magmatism and physical chemistry but the broader physical aspects of this and related processes have proven elusive on many fronts especially in its relation to the spatial variations in rock composition texture and macroscopic features like layering magmatic systems be they volcanic dikes sills or plutons are generally analyzed with a problem at hand and an end result in mind the processes invoked to solve these problems which are most often purely chemical are often unique to each problem with few if any general principles emerging that are central to understanding the wider perspective of magmatic processes and problems an attempt is made at the outset to provide a list of inviolate magmatic first principles that are relevant to analyzing most magmatic problems these involve initial conditions critical crystallinity solidification fronts transport and emplacement fluxes phenocrysts xenocrysts primocrysts crystal size layering and crystal sorting thermal convection magmatic processes are physical along with these principles two reference magmatic systems are suggested where the initial conditions and outcome are unequivocal the sudbury impact melt sheet and the hawaiian lava lakes sudbury formed in 5 min by superheated magma crystallized to a near uniform sequence while the tiny lava lakes formed of crystal laden slurries form a highly differentiated layered sequence the major difference is in the initial conditions of formation especially the nature of the input materials the challenge is to construct and analyze magmatic systems i e magma chambers sills dikes and lavas using these reference end members and the suggested principles the hawaiian 500 000 year volcanic record exhibits what can be expected as input materials namely a highly varied output of magma of an overall composition reflecting the abundance of entrained olivine primocrysts the provenance of these crystals is varied and within any single sample the population may be highly heterogeneous in composition from crystal to crystal yet the overall pattern of chemical fractionation is exceedingly regular and well defined if similar inputs go to form large intrusions these systems will undoubtedly be dominated by crystal rich slurries which provide a vast set of physical processes promoting exotic layering and at the same time given the effects of annealing and continued crystal growth a final chemical record adhering to all the time honored effects of crystal fractionation the long assumed initial condition of instantaneously emplaced crystal free magmas cannot reasonably produce the observed rock records

the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber platinum group element pge deposits in the bushveld complex and other layered intrusions form when large incompletely solidified magma chambers undergo central subsidence in response to crustal loading resulting in slumping of semi consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides oxides olivine and pyroxene and less dense layers enriched in plagioclase the most economic pge cr and v reefs form in large multiple replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification the depth of emplacement has to be relatively shallow as otherwise ductile crust would not be able to flex and collapse in smaller intrusions cooling rates are faster subsidence is less pronounced and where it occurs the cumulate may be largely solidified resulting in insignificant mush mobility and mineral sorting layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower but the reefs can be relatively thicker an additional factor controlling the pge cr and v prospectivity of intrusions is their location within cratons intra cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large layered igneous bodies furthermore intrusions sited within cratons are more readily preserved because cratons are underlain by thick buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust

the influence of continental surface area on the assembly time for supercontinents 1 we employ a kinematic numerical model of circular continental blocks moving about randomly on a two dimensional periodic model of the earth s surface and monitor the time required for all of the blocks to assemble into a single model supercontinent we examine the sensitivity of the assembly times of our model supercontinents to the size of individual continental blocks the fraction of the model earth s surface covered by the continents and the degree of continental deformation that occurs upon collision assembly times are found to be most sensitive to the total continental surface area we show that in the absence of any dynamic effects due to deep mantle flow continents of present day scale moving randomly about the earth s surface would naturally aggregate into a single mass in approximately 0 4 gy

the earliest history of the skaergaard magma chamber a textural and geochemical study of the cambridge drill core ct and mh were supported by a royal society international joint project grant and by the carlsberg foundation ct was also supported by the danish council of independent research and the danish national research foundation the project was supported by the natural environment research council grant number ne f020325 1 on acknowledges support from magdalene college university of cambridge and the von humboldt foundation germany university of hannover

emplacement and crystallization time for the bushveld complex the bushveld complex formed by the crystallization of successive basaltic magmatism may be of the order of several million years for example the columbia river basalts hooper injections of magma which were sufficiently closely spaced in time that each previous magma had not cooled and differentiated 1988 were erupted in the period 17 12 ma with minor eruptions for a further 5 my although most outpouring significantly before the addition of the next one to constrain the emplacement and crystallization times a thermal model is presented occurred within the first 2 my it is now recognized that large intrusions were not emplaced in a single pulse but which permits the investigation of the rate of cooling of magma in an intrusion repeatedly subjected to magma addition and subresult from multiple magma injection the question is how rapidly were magma chambers such as the bc traction such modelling indicates that magmas injected into the bushveld complex were emplaced within 75 000 years at that filled and how much magma was involved answers are relevant to the dynamics of melt production storage and time injection into the complex ceased the volume of rock in the eastern and western limbs is 370 000 600 000 km however transport in the mantle and crust this paper describes a thermal modelling technique not previously applied a quantitative evaluation of the cr budget in the formation of chromitite layers indicates that large volumes of magma cannot be to magma chambers which can be used to analyse this process and to obtain an estimate of the emplacement accounted for in the preserved rock sequence similarly an evaluation of the incompatible trace element abundances such as those for zr time to present this model it is necessary to discuss the stratigraphy size and connectivity of the different limbs and k suggests that the chamber was open and that large volumes of differentiated magma escaped the volume of magma therefore of the bushveld complex and to consider the extent of tapping of the magma chamber as well as its filling greatly exceeded the preserved volume of cumulate rocks giving an estimated magma volume of over 1 10 km an average the term bushveld complex has been given several meanings in the literature and according to the south emplacement rate of 13 km year is indicated by these calculations african commission on stratigraphy 1980 includes not only the ultramafic mafic layered rocks but also the sills beneath the intrusion volcanic rocks which pre date the

peridotite sills and metasomatic gabbros in the eastern layered series of the rhum complex mapping of the eastern layered series els of the rhum ultrabasic complex on the northern flank of hallival shows that peridotite and allivalite troctolite or gabbro layers are laterally discontinuous and vary both in thickness and lithology peridotite generally has sharp upper and lower contacts against the allivalites which sometimes cut across the layering in the allivalite reaction dissolution and hybridization effects between peridotite and allivalite are developed locally some troctolite layers terminate as isolated fingered blocks in peridotite there are many small peridotite bodies which are clearly intrusive into allivalite and have previously been identified as distinct peridotite sheets and plugs they are petrographically almost identical to the major stratiform peridotites and in some cases are apophyses from them we propose that many of the peridotite layers in the els formed as thick sills of picritic magma emplaced into a partly solidified layered troctolite complex the stratiform gabbros of the els are heterogeneous layered rocks that commonly contain relicts of troctolite and anorthosite wavy metre scale contacts between gabbro and troctolite cut across pre existing grain size modal and rhythmic layering with little disruption these metasomatic gabbros mimic the textures grain size and rhythmic layering of their troctolitic protoliths we propose that many of the els gabbros formed as a result of interaction between porous troctolites and a low temperature basaltic melt residual basaltic melt segregated from solidifying peridotite may have caused this metasomatism