phase equilibria testing of a multiple pulse mechanism for origin of mafic ultramafic intrusions a case example of the shiant isles main sill nw scotland in this paper we examine the role of multiple emplacement of sills into partly solidified rocks an intrusive mechanism liquid into solid as a possible explanation for some textural and compositional anomalies of single cyclic mafic intrusions as a case study we used the shiant isles main sill that is widely regarded as a classical example of a multiple picrite picrodolerite crinanite alkaline sill this sill is currently interpreted as having been formed by several olivine phenocryst rich pulses of magma which were successively emplaced into their almost solidified predecessors such an intrusive mechanism is a random process in which many parameters vary independently and unpredictably among them are the number relative volume and bulk composition of magma pulses and their place sequence and timing of emplacement as well as modal abundance phase composition and distribution of intratelluric phenocrysts in magmas upon emplacement in terms of these variables one can envisage countless different profiles through alkaline sills produced from only three randomly intruded magma pulses of picritic picrodoleritic and crinanitic composition such multiple sills can readily be distinguished from simple ones formed from a single pulse of magma by anomalous compositional profiles with several prominent breaks in crystallization and compositional sequences the compositional profile of the shiant isles main sill is remarkably similar to an m shaped profile expected from fractional crystallization of a single pulse of olivine saturated magma along a crystallization path ol sp l picrite ol pl sp l picrodolerite troctolite ol pl cpx l crinanite the probability of the accidental formation of such a compositional profile by multiple intrusion liquid into solid is exceedingly small even for the single case of the shiant isles main sill the probability approaches zero when considering that exactly the same sequence of intrusive events must have been repeated in about 20 neighbouring alkaline sills with similar compositional profiles this can only be achieved by some universally operating differentiation process the best candidate for this is the classical fractional crystallization of magma constrained by liquidus phase equilibria this suggests that the shiant isles main sill can be best interpreted and modelled as a simple sill that crystallized from one large pulse of magma with possible involvement of minor refilling events further progress in our knowledge of intrachamber magma fractionation processes will probably enable us to interpret many anomalous textural and compositional features of mafic ultramafic intrusions in the frame of a single magma pulse model

on the development of internal chemical zonation in small mafic dykes three dolerite dykes small 3 2 cm middle 13 5 cm thick 50 cm from torsholma island sw finland reveal distinctly different internal zonation that becomes more complicated with increasing thickness of the dykes the small dyke shows a systematic inward increase in normative pl an ab or and a decrease in normative an 100 an an ab whole rock mgo mg number 100 mg mg fetotal tio2 k 2o and zr the middle dyke exhibits the same compositional pattern at the margins while the centre is distinguished by an abrupt increase in normative pl and an whole rock sr and mg number from the margins inwards the thick dyke displays first a compositional pattern identical to that observed in the small dyke and the margins of the middle dyke this is followed by a region where whole rock mgo mg number and normative an start increasing inwards while the transition to the centre of the dyke is characterized by a compositional pattern similar to that in the centre of middle dyke the origin of chemical zonation in these dykes is attributed to the operation of three independent physico chemical processes namely the small dyke formed exclusively by progressive changes in the composition of inflowing magma the middle dyke by changes in composition of inflowing magma margins and concentration of plagioclase and olivine phenocrysts by flow differentiation centre the thick dyke by changes in composition of inflowing magma margins in situ cumulate growth against dyke sidewalls middle and flow differentiation centre systematic changes in these processes and as a result in internal chemical zonation likely take place in response to crystallization of magma under less supercooled conditions with increasing dyke thickness a comprehensive geochemical study of the internal zonation of small mafic dykes worldwide is required to develop a complete understanding of the processes operating in mafic dykes

mechanism of crystal redistribution in a sheet like magma body constraints from the nosappumisaki and other shoshonite intrusions in the nemuro peninsula northern japan processes of crystal separation in a magma heavily laden with crystals without phase change are investigated from observations on frozen magma systems nosappumisaki and other shoshonite intrusions in the nemuro peninsula japan for which the origin of the crystals and the initial conditions are well constrained the nosappumisaki intrusion is 120 m in thickness and extends for more than 1 5 km it exhibits a wide range of lithological variation principally as a result of crystal redistribution after intrusion crystals in each lithology can be clearly divided into two kinds according to their composition and texture those present before the intrusion of the magma phenocrysts and those that crystallized in situ after intrusion from the vertical change in mode and size of phenocrysts it is shown that 1 augite phenocrysts were rapidly deposited with little overgrowth after intrusion by significant coagulation or clustering on a time scale of more than a few years and 2 plagioclase phenocrysts definitely denser than the melt but concentrated in the upper level floated by counter flow of massive deposition of augite phenocrysts these results indicate that in a magma heavily laden with crystals of a few millimeters in size 20 vol crystal crystal and crystal melt interaction play an important role in the separation of crystals from the host melt

crustal assimilation in basalt and jotunite constraints from layered intrusions abstract to constrain the amount and rate of crustal contamination that is possible in basaltic and jotunitic magma and to gain an insight into the physical and thermal processes of assimilation in crustal magma chambers we have modelled published sr and nd isotopic data from three layered intrusions well exposed sequences of cumulates with no evidence of magma recharge provide direct records of concurrent assimilation and fractional crystallization afc the key to the modelling is that f the mass fraction of magma remaining in the chamber can be estimated from the thicknesses of the studied cumulate sequences this allows afc model curves to be fitted to the isotopic data by varying r the ratio of the rate of mass assimilated to the rate of mass crystallized the results of modelling show that r is nearly constant in 800 to 2000 m thick sequences of cumulates displaying up section decreases in anorthite content of plagioclase increases in whole rock sr 0 initial 87 sr 86 sr and decreases in whole rock e nd 0 initial e nd the r values of the layered sequences range from 0 12 in the fongen hyllingen intrusion over 0 20 in the bjerkreim sokndal intrusion to 0 27 in the hasvik intrusion the total amount of assimilation the bulk crust magma ratio reaches values of 0 08 0 19 and 0 28 at the level of the most contaminated samples after 60 to 80 crystallisation whereas the instantaneous crust magma ratio of the most contaminated magmas were respectively 0 14 0 46 and 0 70 for the three intrusions innumerable country rock xenoliths occur in the three layered intrusions and played a crucial role in the assimilation process the xenoliths spalled off the roofs of the magma chambers during magma emplacement and their initial temperature and composition relate to r in the intrusions in the hasvik intrusion r 0 27 the initial temperature of the country rocks was 450 c and the xenoliths were fusible metasediments and therefore produced a high fraction of partial melt that could be assimilated in the bjerkreim sokndal intrusion r 0 20 the country rocks were initially at temperatures of 640 880 c but included both refractory massif type anorthosite and fusible gneisses in the fongen hyllingen intrusion r 0 12 the country rocks were cooler 300 c and the xenoliths include refractory metabasalt dominant and fusible metapelite we argue that the refractory metabasalt and anorthosite xenoliths acted mainly as heat sinks resulting in reduced r values in fongen hyllingen and bjerkreim sokndal intrusions heating of refractory and fusible xenoliths and melting of fusible xenoliths absorbed sensible and latent heat of the magma energy balanced modelling shows that up to 75 of the heat available was absorbed by xenoliths within the magma chambers promoting higher rates of cooling and crystallisation than would have resulted from loss of heat to the envelope of country rocks alone the high r values reflect the amount of heat absorbed by heating and melting country rock within the magma chambers themselves and their constancy reflects the ready availability of fusible xenoliths

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

convection and crystallization in magma cooled from above calculations are presented for the cooling from above of melts in the di an system in which the kinetics of crystallization are incorporated and play a dominant role the results indicate that even with no initial superheat whatsoever convection plays an important role in the cooling of magma chambers and allows substantial internal cooling crystallization and differentiation the calculations show in agreement with observations that in magma bodies hundreds of meters thick crystallization occurs predominantly in the interior or at the floor even though heat is lost only from the roof the ratio of the final thickness of the layer formed at the floor to that formed at the roof increases as the overall size of the chamber increases owing to the effects of convection

post emplacement melt flow induced by thermal stresses implications for differentiation in sills abstract we present the first steps of a new explanation model for differentiation in sills using a combination of geochemical data and field observations numerical modeling and dimensional analysis geochemical data from a saucer shaped dolerite sill intruded into the karoo basin south africa reveal a differentiation process which causes d shaped profiles the geometry name is based on the variation in whole rock mg number mg mg mg fe from floor to roof in a sill the d shaped geochemical profiles represent sheet intrusions with the most primitive composition i e high mg in its center and progressively more evolved composition i e low mg towards the upper and lower margins the differentiation is reversed compared to the normal differentiation produced by fractional crystallization c shaped profiles c shaped profiles are believed to be formed by segregation of crystals from the magma we propose that the opposite the d shaped profile may result from melt segregation from the crystal mush this is achieved by porous melt flow through a consolidated crystal network after the main phase of emplacement and before complete solidification we show that a significant flow is feasible under natural occurring conditions an underpressure of magnitude 108 pa develops at the cooling margins due to volume reduction of the crystallizing porous melt the resulting pressure gradient is the driving force for the melt flow towards cooling margins considered in this work as a result the margins will be enriched in the incompatible elements associated with the melt phase while the center will be depleted we show that the amount of flow is primarily a function of viscosity of the melt and permeability of the crystal network which in turn is a transient phenomenon dependent on a number of parameters diagrams have been constructed to evaluate the feasibility of substantial melt extraction in terms of these poorly constrained parameters data from the golden valley sill and many other natural occurrences of d and i shaped geochemical profiles show a reasonable agreement with our predictions of melt flow potential and are thus well explained by the presented model we conclude that in order to fully understand differentiation processes occurring in sheet intrusions we need to account for post emplacement segregation of melt from crystals and not only segregation of crystals from melt

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

orientation of tabular mafic intrusions controls convective vigour and crystallization style the microstructure in basaltic dykes is significantly different from that in sills and lava lakes of the same bulk composition for a given width of intrusion or depth of lava lake vertical tabular bodies are coarser grained than horizontal bodies with an invariant plagioclase shape across the intrusion when comparing samples from sills and dykes for which the average grain size is the same the dyke samples contain fewer small grains and fewer large grains than the sill samples in contrast the variation of median clinopyroxene plagioclase plagioclase dihedral angles in dykes correlates precisely with that observed in sills and is a function of the rate of diffusive heat loss these patterns can be accounted for if the early stages of crystallization in dykes primarily involve the growth of isolated grains suspended in a well mixed convecting magma with the final stage during which dihedral angles form occurring in a crystal rich static magma in which heat loss is primarily diffusive in contrast crystallization in sills occurs predominantly in marginal solidification fronts suggesting that any convective motions are insufficient to entrain crystals from the marginal mushy layers and to keep them suspended while they grow an exception to this general pattern is provided by members of the mull solitdykes which propagated 100 1000 km se from the mull palaeogene igneous centre scotland through the shallow crust these dykes where sampled 100 km from mull have a microstructure indistinguishable from that of a sill of comparable thickness we suggest that sufficient nucleation and crystallization occurred in these dykes to increase the viscosity sufficiently to damp convection once unidirectional flow had ceased

two independent processes responsible for compositional zonation in mafic dykes of the aland aboland dyke swarm kestio island sw finland abstract detailed sampling across three dolerite dykes of different size small 7 cm middle 75 cm and thick 675 cm of the aland aboland dyke swarm has revealed an internal zonation of an anomalous nature the small almost glassy dyke exhibits a systematic inward decrease in whole rock mgo and mg indicating a normal fractionation trend together with a simultaneous increase in normative an and cpx and decrease in whole rock zr y cao tio 2 indicating a reverse fractionation trend the middle dyke shows similar compositional trends across its narrow margins but in the more crystalline interior whole rock mgo and mg gradually but steadily increase inwards as a result normal and reverse fractionation trends of the margins grade to exclusively reverse fractionation trends of the interior the thick almost totally crystalline dyke exhibits an internal zonation similar to that of the middle dyke with fractionation trends becoming only much more pronounced in the centre of the dyke the almost glassy nature of small dyke suggests that its anomalous compositional zonation most likely resulted from temporal changes in the composition of magma as it formed the dyke the mechanism s responsible for such systematic changes in composition of inflowing magma remains however unknown the margins of middle and thick dyke form in a similar way whereas their interiors formed by in situ cumulate growth against dyke sidewalls this process resulted in a gradual inward increase in the proportion of cumulus phases owing to magma crystallization in progressively less supercooled conditions with increasing distance from cold country rocks the compositional zonation of these dolerite dykes is thus produced by two independently operating mechanisms successive changes in composition of inflowing magma an external liquid state process and an in situ cumulate growth on dyke sidewalls an internal crystal liquid process based on the relatively minor development of internal zonation in interiors of middle and thick dykes the former mechanism appears to be several times more effective in causing magma differentiation than the latter it remains to be determined whether these two processes are a general reason for the formation of marginal reversals in mafic ultramafic dykes sills and large layered intrusions

archean layered mafic ultramafic intrusions in the west pilbara craton western australia a synthesis of some of the oldest orthomagmatic mineralizing systems in the world considerable exploration interest has been generated by the platinum group element pge and ni cu potential of the archean layered mafic ultramafic intrusions in the west pilbara craton western australia the munni munni intrusion contains the largest resource of pge associated with a layered intrusion in australia and the radio hill and mount sholl intrusions host significant resources of ni cu co sulfides titaniferous magnetite layers remobilized sulfides and structurally controlled hydrothermal polymetallic deposits have also been a focus for exploration in recent years the ca 2 9 ga munni munni andover radio hill mount sholl and sherlock layered intrusions are a cogenetic suite of high level 5 kbars bodies that represent some of the oldest mineralizing systems of their type in the world although they display similar field relationships and mineralogical geochemical and isotopic features their contrasting chalcophile metal distribution patterns show that the timing and mechanism of the s saturation event were critical for the development of pge enriched sulfide bearing layers and basal segregations of base metal sulfides the intrusions form thick 5 5 km dikelike bodies or relatively thin 0 5 2 km sheets and sills emplaced at different levels along major lithological discontinuities in the upper crust rhythmically layered ultramafic components are generally thinner than and occur along the northern sides of more massive overlying mafic components the ultramafic zones consist of dunite lherzolite wehrlite olivine websterite clinopyroxenite and websterite inverted pigeonite gabbronorite magnetite gabbro olivine gabbro anorthositic gabbro and anorthosite comprise the mafic sequences olivine and clinopyroxene were generally the first minerals to crystallize except in the andover intrusion where orthopyroxene preceded clinopyroxene and possibly reflects greater contamination of the parent magma by felsic crustal material the crystallization of chromite was inhibited in the ultramafic zones by the partitioning of cr into early crystallizing clinopyroxene thus downgrading the potential for pge chromite associations the pge mineralization in the munni munni intrusion occurs in the upper levels of a porphyritic plagioclase websterite orthocumulate layer directly below the ultramafic gabbroic zone contact mineral compositional trends and nd isotope data indicate that a pd pt au enriched s undersaturated magnesian basaltic magma was frequently injected into a small magma chamber during formation of the ultramafic zone a major influx of more fractionated s saturated tholeiitic gabbroic magma related to the resident magnesian magma rapidly inflated the chamber and induced turbulent magma mixing that resulted in the formation of the pge bearing porphyritic websterite layer in contrast the parent magmas that formed the mount sholl radio hill andover and maitland intrusions were saturated in s before they were emplaced into the magma chambers in these intrusions gravitational and structural controls were important for the concentration of pge poor 5 400 ppb pt pd au massive ni cu co sulfides in depressions and structural embayments along the basal contacts beneath the thickest sequences of mafic ultramafic cumulates the parent magmas to the west pilbara intrusions were siliceous high magnesian basalts of al depleted komatiitic affinity barberton type with 9 to 12 percent mgo 15 to 25 ppm sc 12 to 18 ppm y low al2o3 tio2 ca 11 or half chondrite ratios and light rare earth enrichment chondrite normalized la sm 2 7 la lu 9 0 they were generated with garnet in the residual asthenospheric mantle with probable involvement of a pre 3 0 ga subduction modified lithospheric mantle isotopic and geochemical modeling suggests that the magmas were contaminated by ca 3 0 to 3 3 ga archean tonalitic to granodioritic crust before being emplaced into high level magma chambers

the sudbury igneous complex viscous emulsion differentiation of a superheated impact melt sheet the sudbury igneous complex of ontario canada is the remnant of a voluminous melt sheet produced in a few minutes by impact of a massive meteorite into continental crust 1 85 ga ago the transient cavity and melting zone reached the moho and instantly 2 min relaxed to form a more familiar large shallow crater holding a thick superheated 1700 c melt sheet covered by 2 km of breccia there is little about the resulting bimodal igneous complex that resembles crystallization of well known sheet like bodies of similar composition yet the norite and granophyre exhibit a remarkable similarity in isotopic and trace element compositions suggesting an intimate common parentage from the surrounding crust this petrogenetic enigma is explained here as a natural unavoidable consequence of the impact process in the rapid formation of a superheated magmatic emulsion which we take as the high temperature equivalent of breccia a wide spectrum of viscously discrete interdispersed parcels of mafic and felsic liquids reflecting the compositional heterogeneity of the target crustal materials formed the emulsion within days to months the emulsion components separated according to their relative densities into a bimodal norite granophyre assemblage that formed the basic structure of the present sudbury igneous complex there is clear evidence of this emulsion in the earliest dikes i e offsets which likely give the earliest state of the nascent melt sheet immediately following emulsion separation the strongly superheated bimodal melt sheet underwent vigorous thermal convection in each layer these convective motions homogenized and rapidly cooled the magma to the liquidus temperatures whereupon convection ceased the pattern of convection was pinned in place by the embayment topography of the crater floor which in turn played a pivotal role in directing sulfide deposition into the embayments all further cooling was by conduction of heat through the upper and lower boundaries during which time solidification fronts were established and propagated inward from the upper and lower margins there is clear evidence of solidification from the floor upward and the roof downward minimal differentiation and compositional modification took place throughout cooling and solidification nevertheless during the solidification stage granitic rock fragments on the crater floor and rafts of fallback breccia from the thick overlying onaping formation became unstable and entered the melt sheet and the partially melted remnants collected at the interface between the norite and granophyre some interstitial melt from the norite also percolated upward and altogether the blocks and melt produced the distinctive chemical and physical characteristics of the unusual transition zone the sudbury melt sheet is in essence a full scale magmatic experiment the conditions of formation relative to any other large terrestrial magma are precisely known thus the clear lack of any significant modal layering the overall homogeneous nature of each unit and the lack of any significant chemical differentiation through crystal fractionation establish sudbury as a valuable example of what does not happen under the initial conditions long assumed to prevail at the formation of most large magma chambers

subsurface valleys and geoarcheology of the eastern sahara revealed by shuttle radar the shuttle imaging radar sir a carried on the space shuttle columbia in november 1981 penetrated the extremely dry selima sand sheet dunes and drift sand of the eastern sahara revealing previously unknown buried valleys geologic structures and possible stone age occupation sites radar responses from bedrock and gravel surfaces beneath windblown sand several centimeters to possibly meters thick delineate sand and alluvium filled valleys some nearly as wide as the nile valley and perhaps as old as middle tertiary the now vanished major river systems that carved these large valleys probably accomplished most of the erosional stripping of this extraordinarily flat hyperarid region underfit and incised dry wadis many superimposed on the large valleys represent erosion by intermittent running water probably during quaternary pluvials stone age artifacts associated with soils in the alluvium suggest that areas near the wadis may have been sites of early human occupation the presence of old drainage networks beneath the sand sheet provides a geologic explanation for the locations of many playas and present day oases which have been centers of episodic human habitation radar penetration of dry sand and soils varies with the wavelength of the incident signals 24 centimeters for the sir a system incidence angle and the electrical properties of the materials which are largely determined by moisture content the calculated depth of radar penetration of dry sand and granules based on laboratory measurements of the electrical properties of samples from the selima sand sheet is at least 5 meters recent september 1982 field studies in egypt verified sir a signal penetration depths of at least 1 meter in the selima sand sheet and in drift sand and 2 or more meters in sand dunes

crystal capture sorting and retention in convecting magma the mystery of producing strong compositional diversity among suites of comagmatic igneous rocks is investigated by considering the dynamic evolution of basaltic magma in a sheet like chamber a central conclusion is that inward progressing crystallization produces strong viscosity and temperature gradients that promote convection only near the leading edge of the upper thermal boundary layer convection is apparently confined to an essentially isoviscous isothermal region that hugs the downward growing roof zone strong changes in viscosity with crystallization divide the upper and lower thermal boundary layers into regions of decreasing viscosity and crystallinity n called rigid crust n 0 5 mush 0 5 n 0 25 and suspension n 0 25 the strong increase in viscosity near the mush suspension interface acts as a capture front that overtakes and traps slowly settling crystals initial phenocrysts mostly escape capture but crystals nucleated and grown in the suspension zone can escape only if the capture front slows to a critical rate attainable only in bodies thicker than about 100 m escaping crystals are redistributed and sorted by convection driven by the advance of the capture front itself crystal laden plumes traverse the central hot core of the body and deposit partially resorbed and sorted crystals within the lower suspension zone convection is never vigorous but is part of an overall intimate balance between roofward heat loss rigid crust growth crystallization kinetics and transport and sorting of sinking escaped crystals there is a strong similarity between these processes and those producing both varves and saline pan deposits it is clear that lavas lava lakes and sills are indeed examples of true magma chambers strongly exhibiting certain aspects of this over all process these aspects commonly also characterize the large mafic magmatic bodies because strong compositional changes in the residual melt occur largely outward that is at lower temperatures and higher crystallinities of the capture front which is immobile and mostly within rigid crust the possible range in comagmatic compositions available for eruption anywhere within the active magma is very limited this is in broad agreement with the compositional range observed in basaltic lava lakes sills and plutons like skaergaard the tuning of convection crystallization kinetics and phase equilibria in chambers of this type can produce a variety of textures and layering but not a diversity of compositions

assimilation of crustal xenoliths in a basaltic magma chamber sr and nd isotopic constraints from the hasvik layered intrusion norway contaminated layered intrusions known thousands of recrystallized strontium and neodymium isotopic data for mafic cumulates chilled tabular xenoliths of metasedimentary origin enclosed in the cumulates margins and adjacent crustal rocks of the hasvik layered intrusion are thought to represent the remnants of the assimilated material north norwegian caledonides are reported together with new the xenoliths spalled off the roof during magma emplacement and mineralogical and whole rock analytical data to constrain the extent together with the elevated temperatures 400 600 c of the midand effect of the assimilation of crustal xenoliths in a basaltic crustal country rocks led to a high degree of assimilation in the magma chamber initial sr sr 700 ma of 0 7045 and end hasvik magma chamber 700 ma of 3 03 for the chilled margin which has a tholeiitic composition akin to the chilled rocks of the skaergaard intrusion demonstrate that the parental magma was derived from a depleted mantle source the basal cumulates 0 335 m show an up section decrease in sr sr from 0 7045 to 0 7038 and a correlative

igneous stratigraphy and internal structure of the little minch sill complex trotternish peninsula northern skye scotland detailed sampling of the little minch sill complex reveals that it is composed of both single and multiple sills these are formed of three main genetically related units picrite picrodolerite and crinanite which are the result of differentiation of an alkali olivine basalt magma approximately 10 mgo in an upper crustal magma chamber variations in igneous stratigraphy and the presence of internal chills in the trotternish sills suggest that they were emplaced by multiple intrusion and subsequently differentiated in situ changes in petrography adjacent to pegmatite veins and textures within picrite units indicate compaction and filter pressing were important processes after emplacement rhythmic layering 1 cm to 1 m thick is conspicuous in the sills near contacts but does not involve cryptic mineral variation such modal layering may be more common than realised in relatively small scale intrusions and may be modelled in terms of in situ differentiation under conditions of significant undercooling in a changing thermal gradient at the synthetic for sterite diopside anorthite eutectic

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

igneous rocks formed by hypervelocity impact abstract igneous rocks are the primary building blocks of planetary crusts most igneous rocks originate via decompression melting and or wet melting of protolith lithologies within planetary interiors and their classification and compositional petrographic and textural characteristics are well studied as our exploration of the solar system continues so too does the inventory of intrusive and extrusive igneous rocks settings and processes the results of planetary exploration have also clearly demonstrated that impact cratering is a ubiquitous geological process that has affected and will continue to affect all planetary objects with a solid surface whether that be rock or ice it is now recognized that the production of igneous rocks is a fundamental outcome of hypervelocity impact the goal of this review is to provide an up to date synthesis of our knowledge and understanding of igneous rocks formed by hypervelocity impact following a brief overview of the basics of the impact process we describe how and why melts are generated during impact events and how impact melting differs from endogenic igneous processes while the process may differ we show that the products of hypervelocity impact can share close similarities with volcanic and shallow intrusive igneous rocks of endogenic origin such impact melt rocks as they are termed can display lobate margins and cooling cracks columnar joints and at the hand specimen and microscopic scale such rocks can display mineral textures that are typical of volcanic rocks such as quench crystallites ophitic porphyritic as well as features such as vesicles flow textures and so on historically these similarities led to the misidentification of some igneous rocks now known to be impact melt rocks as being of endogenic origin this raises the question as to how to distinguish between an impact versus an endogenic origin for igneous like rocks on other planetary bodies where fieldwork and sample analysis may not be possible and all that may be available is remote sensing data while the interpretation of some impact melt rocks may be relatively straightforward e g for clast rich varieties and those with clear projectile contamination we conclude that distinguishing between impact and endogenic igneous rocks is a non trivial task that ultimately may require sample investigation and analysis to be conducted caution is therefore urged in the interpretation of igneous rocks on planetary surfaces

shuttle imaging radar physical controls on signal penetration and subsurface scattenng in the eastern sahara sir a signal penetration and subsurface backscatter within the upper meter or so of the sediment blanket in the eastern sahara of southern egypt and northern sudan are enhanced both by radar sensor parameters and by the physical and chemical characteristics of eolian and alluvial materials interpretation of sir a images by mccauley et al 1 2 dramatically changed previous concepts of the role that fluvial processes have played over the past 10000 to 30000000 years in shaping this now extremely flat featureless and hyperarid landscape in the present paper we summarize the near surface stratigraphy the electrical properties of materials and the types of radar interfaces found to be responsible for different classes of sir a tonal response the dominant factors related to efficient microwave signal penetration into the sediment blanket include a favorable distribution of particle sizes b extremely low moisture content and c reduced geometric scattering at the sir a frequency 1 3 ghz the depth of signal penetration that results in a recorded backscatter here called radar imaging depth was documented in the field to be a maximum of 1 5 m or 0 25 of the calculated skin depth for the sediment blanket radar imaging depth is estimated to be between 2 and 3 m for active sand dune materials diverse permittivity interfaces and volume scatterers within the shallow subsurface are responsible for most of the observed backscatter not directly attributable to grazing outcrops

transient magmatic convection prolonged by solidification magma chambers cool from all sides with expected vigorous convection strong changes in viscosity with crystallization however lock up within an inwardly propagating crust a great deal of buoyancy otherwise available to drive convection with sufficiently small degrees of superheat the rayleigh number re is small and convection is transient and never becomes fully developed in order to understand this transient stage of convection we have performed some fluid dynamic experintents on the solidification of a paraffin layer except in the growing crust and its thin forward boundary layer the interior remains isothermal and the temperature decreases uniformly until it is locked at the convective liquidus the crystals are fine and hair like dendrites and no major differentiation of the fluid composition is observed the time scale of the convective stage is predictable and is very short relative to the time of complete solidification we also report measurements over time of crust thickness convective velocity and heat transfer when the interior temperature reaches the convective liquidus convection wanes although the calculated re based on an arbitr ary small temperature contrast of 0 2oc is still large 107 a small 0 2oc uncertainty in at i e 0 at 0 2oc produces a large uncertainty in ra i e 0 ra 107 this uncertainty in nature may preclude an accurate determination of ra in a magma chamber residing at a temperature very close to the convective liquidus htroduction in 1905 l v pirsson described in some detail how crystallization induced convection at the margins of a magma chamber could greatly facilitate crystal settling fractionation and differentiation ordinarily slowly settling crystals could be swiftly carried downward by descending convective currents to be deposited as the current approaches and spreads across the chamber floor later f f grout 1918 again clearly enunciated the process with the additional concept of a strongly variable magmatic viscosity due to large changes in crystallinity at the margins this form of convection and crystal transfer has many times since been invoked to explain magmatic differentiation e g wager and deer 1939 brandeis and jaupart 1986 marsh 1989 these considerations and results often suggest that most any body of basaltic magma will reveal the effects of this process through a systematic variation in composition downward through a sheetlike body the overall process involving actual crystallization and convection in aqueous solutions has been experimentally modelled by kerr et el 1989 they observe strong differentiation and suggest that this might explain the large compositional change observed in sills such as the palisades sill shirley 1987 however there are also numerous thick sills and lava flows that show no or very little effect of internal differentiation e g stott

regional dust deposits on mars physical properties age and history major dust storms on mars play an important role in the deposition and removal of fine dust material thermal radar and visual remote sensing observations provide important constraints on the martian regolith which have been used to determine the location and physical properties of regional dust deposits these deposits are located in three northern equatorial regions tharsis 20 s to 50 n 60 to 190 w arabia 5 s to 30 n 300 to 360 w and elysium 10 to 30 n 210 to 225 w they are covered by fine 2 40 m bright albedo 0 27 particles with fewer exposed rocks and coarse deposits than found elsewhere dust is currently deposited uniformly throughout the equatorial region at a rate of 40 m global storm over geologic time the rate of accumulation may vary from 0 to 250 m yr due to changes in atmospheric conditions produced by orbital variations dust deposited during global storms is subsequently removed only from dark regions resulting in a net accumulation in the low inertia bright regions the thickness of these current dust deposits is 0 1 2 m the thermal inertia places a lower limit of 0 1 m on the thickness of these deposits while the sparse but ubiquitous presence of exposed rocks and the degree of visible mantling indicate that the thickness is less than 5 m dual polarization radar observations of a very rough texture in tharsis are consistent with this model with a 2 m thick dust layer burying most of the surface rocks but permitting radar sampling of the rough subsurface based on their thickness and rate of accumulation the age of these deposits is 105 106 years suggesting a cyclic process of deposition and removal one possible cause may be cyclic variations in the magnitude and location of maximum wind velocities related to variations in mars orbit at present perihelion and maximum wind velocities occur in the south whereas regional dust deposits occur in the north suggesting net transport from south to north orbital parameters oscillate with periods ranging from 5 104 to 106 years the agreement between these periods and the dust deposit age suggests a possible link at different stages in orbit evolution maximum wind velocities will occur in the north with subsequent erosion and redistribution of the accumulated fines based on this model much of the uppermost martian surface is very young and is being continually reworked

the influence of weathering processes on labile and stable organic matter in mediterranean volcanic soils the relationship and mechanisms among weathering processes cation fluxes clay mineralogy organic matter composition and stability were studied in soils developing on basaltic material in southern italy sicily the soils were transitions between phaeozems and vertisols intense losses of the elements na ca and mg were measured indicating that weathering has occurred over a long period of time the main weathering processes followed the sequence amphibole mica volcanic glass or if ash was the primary source smectite interstratified smectite kaolinite kaolinite kaolinite formation was strongly related to high al mg and na losses the good correlation between oxyhydroxides and kaolinite in the soils suggests that macro aggregates have formed due to physical or electrostatic interactions between the 1 1 clay minerals and oxides the stability of organic matter was investigated with a h2o2 treatment that assumes that chemical oxidation mimics the natural oxidative processes the ratio of c after the h2o2 treatment to the total organic c ranged from 1 28 no correlation between clay content and organic matter labile or stable fraction was found the refractory organic fraction was enriched in aliphatic compounds and did not greatly interact with the kaolinite smectite or poorly crystalline fe or al phases a part of this fraction most probably proteins was bound to crystalline fe oxides in contrast the oxidisable fraction showed a strong relationship with poorly crystalline oxyhydroxides and kaolinite surprisingly smectite did not contribute to the stabilisation of any of the organic c fractions the stabilisation of organic matter in the soils has therefore two main mechanisms 1 the protection of labile oxidisable with h2o2 organic matter including also aromatic rich compounds such as charcoal by the formation of aggregates with oxyhydroxides and kaolinite and 2 the formation of a refractory fraction enriched in aliphatic compounds

convection and crystal settling in sills it has been advocated that convective and crystal settling processes play significant and perhaps crucial roles in magmatic differentiation the fluid dynamics of magma chambers have been extensively studied in recent years both theoretically and experimentally but there is disagreement over the nature and scale of the convection over its bearing on fractionation and possibly over whether it occurs at all the differential distribution of modal olivine with height in differentiated alkaline basic sills provides critical evidence to resolve this controversy at least for small to medium large magma chambers our own and others published data for such sills show that irrespective of overall olivine content modal olivine contents tend to increase in a roughly symmetrical manner inwards from the upper and lower margins of the sill i e the distribution patterns are more often approximately d shaped rather than the classic s shape generally ascribed to gravity settling we concur with the majority of other authors that this is an original feature of the filling process which has survived more or less unchanged since emplacement we therefore conclude that the magmas have not undergone turbulent convection and that gravity settling has usually played only a minor modifying role since the intrusion of these sills we offer a possible explanation for the apparent contradiction between fluid dynamical theory and the petrological evidence by suggesting that such sills rarely fill by the rapid injection of a single pulse of magma rather they form from a series of pulses or a continuous pulsed influx over a protracted interval during which marginal cooling severely limits the potential for thermal convection

disequilibrium dihedral angles in dolerite sills a new proxy for cooling rate the geometry of clinopyroxene plagioclase plagioclase junctions in mafic rocks measured by the median dihedral angle cpp is created during solidification in the solidifying kilauea iki hawaii lava lake the wider junctions between plagioclase grains are the first to be filled by pyroxene followed by the narrower junctions the final cpp attained when all clinopyroxene plagioclase plagioclase junctions are formed is 78 in the upper crust of the lake and 85 in the lower solidification front cpp in the 3 5 m thick traigh bhan na sgurra sill inner hebrides is everywhere 78 in the whin sill northern england 38 m thick and the portal peak sill antarctica 129 m thick cpp varies symmetrically with the lowest values at the margins the 266 m thick basement sill antarctica has asymmetric variation of cpp attributed to a complex filling history the chilled margins of the basement sill are partially texturally equilibrated with high cpp the plagioclase grain size in the two widest sills varies asymmetrically with the coarsest rocks found in the upper third both cpp and average grain size are functions of model crystallization times cpp increases from 78 to a maximum of 100 as the crystallization time increases from 1 to 500 yr because the use of grain size as a measure of crystallization time is dependent on an estimate of crystal growth rates dihedral angles provide a more direct proxy for cooling rates in dolerites

phase equilibria constraints on relations of ore bearing intrusions with flood basalts in the noril sk region russia phase equilibria analysis has been performed to elucidate whether ore bearing noril sk and poorly mineralized lower talnakh type intrusions are co magmatic with the extremely ni cu and pges depleted and not depleted flood basalts respectively the parental magma of the intrusions is classified as silica undersaturated olivine basalt whereas that of the flood basalts is assigned to silica saturated tholeiite the plutonic equivalent of the former parental magma is olivine gabbro or melagabbro whereas the latter is gabbronorite phase equilibria relations clearly show that these two types of magmas cannot be co magmatic as there is no way to derive one liquid from another by fractional crystallization this provides no support for the current model considering the noril sk type intrusions as exit conduits for a great volume of volcanic magma which lost most of its chalcophile elements in the shallow settling chambers on its way to surface the association of a large volume of chalcophile element depleted basalts with the very sulfide rich intrusions in the noril sk region is most likely coincidental and cannot therefore be used as a regional criterion when prospecting for the noril sk type sulfides deposits in other flood basalts provinces

climbing zibars of the algodones abstract zibars are coarse grained low relief slipfaceless eolian bedforms that occur on sand sheets and within interdune corridors of many eolian sand seas zibar surface features consist exclusively of wind ripples and local shadow and shrub coppice dunes associated with vegetation a study of zibars located on the southwestern margin of the algodones dune field california shows that zibars and their associated interzibar areas are migrating features oriented transverse to oblique to the resultant sand transport direction zibars leave deposits formed by deposition on the zibar lee face the zibar shape modifies the near surface airflow to produce a subtle grain size change across the zibar interzibar surface flow expansion not separation leeward of the zibar crest causes the lee face deposits those preserved upon bedform migration to have the finest grain size on the zibar interzibar surface sediment coarser than that deposited on the zibar lee face is transported more slowly and less frequently and thus becomes concentrated on the zibar stoss slope and ultimately becomes interzibar deposits migration of zibars and interzibar areas can produce two characteristic depositional sequences depending upon the angle of climb for low angles of climb a sequence of stacked amalgamated interzibar deposits is preserved zibar deposits are absent amalgamated interzibar deposits consist of low angle usually less than 5 wind ripple laminae of medium to coarse sand given higher angles of climb deposits of both the entire interzibar and zibar lower lee face are preserved low angle interzibar deposits of medium to coarse sand are overlain tangentially by finer grained fine to medium sand wind ripple laminae dipping less than 15 of the zibar lee face lee face wind ripple laminae are sharply truncated and overlain by the succeeding set of interzibar wind ripple deposits this zibar interzibar sequence is distinctive and allows for the detailed interpretation of some ancient sand sheet deposits as left by migrating zibars a possible example of ancient zibar deposits is present in the entrada sandstone jurassic of southwestern colorado

testing the validity of the petrological hypothesis no phenocrysts no post emplacement differentiation an extravagant hypothesis no phenocrysts no post emplacement differentiation has been put forward by marsh in a series of papers for the development of mafic ultramafic intrusions this hypothesis is based on an assertion that the majority of these intrusions are structureless and undifferentiated because they lack residual granitic rocks to explain this the hypothesis postulates that phenocryst free magmas are not able to differentiate in crustal chambers because all evolved interstitial liquid is locked in solidification fronts producing compositionally uniform magmatic bodies layered well differentiated intrusions are attributed to the successive emplacement of magma pulses with phenocrysts of different phase and chemical compositions rather than to slow magma cooling and fractional crystallization as conventional models imply such phenocryst laden magma pulses are supposed to be derived from an underlying magmatic mush column however structureless undifferentiated mafic ultramafic bodies simply do not exist in nature all wellstudied mafic ultramafic bodies with or without residual granitic rocks that crystallized from parental magmas of non eutectic composition tend to reveal clear evidence of internal compositional differentiation in terms of crystallization sequences e g ol opx opx pl opx pl cpx mineral compositions e g an in plag en in cpx and compatible incompatible major and trace element geochemistry e g mg number cr rare earth elements this is especially evident in layered intrusions that represent the key evidence against the hypothesis essentially by denying the ability of magma to differentiate in intrusive bodies the hypothesis forbids magmatic differentiation in any sub chamber related to the entire magmatic mush column as a result magma pulses in which phenocrysts progressively change in composition cannot be derived from the column to form layered intrusions the hypothesis is thus contradictory baseless and fundamentally flawed it should be abandoned in favour of a classical fractional crystallization model based on the pioneering experiments of bowen and amply confirmed over almost a century by subsequent studies of layered intrusions the classical model was is and will most probably remain the best explanation for the origin of differentiated magmatic bodies

oxygen isotopic and chemical compositions of rocks of the sudbury basin ontario the whole rock oxygen isotopic composition of the main units of the sudbury irruptive and surrounding rocks has been studied using samples from two traverses of the north range and one of the south range norite has an average 18o of 6 7 about 1 greater than that of fresh oceanic basalts and similar to that of some continental basalts granophyre is slightly richer in 18o 18o 7 3 in neither unit is there significant correlation between 18o and sio2 content or degree of alteration as estimated by water content or microscopic appearance the pervasive hydrous alteration of norite and granophyre apparently occurred in the presence of a small volume of water whose isotopic composition was buffered by the igneous rocks inclusion free devitrified glass melt rock and matrix from the onaping formation have 18o values in the range 6 2 12 1 its average 18o is 8 2 comparable to that of archean gneiss as required by the model for the origin of the formation as a fallback breccia from a me

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

origin of the sudbury complex by meteoritic impact neodymium isotopic evidence samarium neodymium isotopic data on whole rocks and minerals of the sudbury complex in canada gave an igneous crystallization age of 1840 21 x 10 6 years the initial epsilon neodymium values for 15 whole rocks are similar to those for average upper continental crust falling on the crustal trend of neodymium isotopic evolution as defined by shales the rare earth element concentration patterns of sudbury rocks are also similar to upper crustal averages these data suggest that the sudbury complex formed from melts generated in the upper crust and are consistent with a meteoritic impact

contrasting stratified plutons exposed in tilt blocks eldorado mountains colorado river rift nv usa abstract roof to floor exposures of mid miocene plutons in tilt blocks south of las vegas nv reveal distinct but strongly contrasting magma chamber statigraphy the searchlight and aztec wash plutons are well exposed stratified intrusions that show a similar broad range in composition from 45 75 wt sio 2 homogeneous granites that comprise about one third of each intrusion are virtually identical in texture and elemental and isotopic chemistry mafic rocks that are present in both plutons document basaltic input into felsic magma chambers isotopic compositions suggest that mafic magmas were derived from enriched lithospheric mantle with minor crustal contamination whereas more felsic rocks are hybrids that are either juvenile basaltic magma crustal melt mixtures or products of anatexis of ancient crust young mesozoic or miocene mafic intraplate despite general similarities the two plutons differ markedly in dimensions and lithologic stratigraphy the searchlight pluton is much thicker 10 vs 3 km and has thick quartz monzonite zones at its roof and floor that are absent in the aztec wash pluton isotopic and elemental data from searchlight pluton suggest that the upper and lower zones are cogenetic with the granite we interpret the finer grained slightly more felsic upper zone to represent a downward migrating solidification front and the lower zone to be cumulate in contrast the upper part of the aztec wash pluton is granite and a heterogeneous mafic rich injection zone with distinct isotopic chemistry forms the lower two thirds of the intrusion similar mafic rocks are relatively sparse in searchlight pluton and do not appear to have played a central role in construction of the pluton large felsic and composite dikes that attest to repeated recharging and intrachamber magma transfer are common in the aztec wash pluton but absent in the searchlight pluton thus although both intrusions were filled by similar magmas and both developed internal stratification the two intrusions evolved very differently the distinctions may be attributable to scale and resulting longevity and or to subtle differences in tectonic setting

adcumulus growth of anorthosite at the base of the lunar crust unzoned plagioclase crystals imply isothermal adcumulus solidification and cannot result from reactive equilibrium crystallization in dry magma extraction of latent heat upward through lunar crust results only in chilled or cotectic mesocumulate rocks not anorthosite or adcumulates adcumulus growth of anorthosite requires exchange of refractory for incompatible components between the site of crystal growth and nearby fresh magma and the extraction of calories from growing crystals to the flowing supercooled magma supercooling can be acquired at remote sites like upwellings and then be carried by convective flow beneath crust which is accreting plagioclase by flotation or in situ nucleation and growth if transport distances are larger than the scale of rockbergs cotectic norites and troctolites may result impact induced upwellings tend to promote growth of buoyant rather than mafic crust hence impacts are not solely destructive

relationship between the layered series and the overlying evolved rocks in the bjerkreim sokndal intrusion southern norway abstract the bjerkreim sokndal layered intrusion bksk consists of a 7000 m thick layered series comprising anorthosites leuconorites troctolites norites gabbronorites and jotunites hypersthene monzodiorites overlain by an unknown thickness of massive evolved rocks mangerites hypersthene monzonites mg quartz mangerites qmg and charnockites ch the layered series is subdivided into six megacyclic units that represent the crystallisation products of successive major influxes of magma we have studied a ca 2000 m thick section that straddles the sequence from the uppermost part of the layered series to the qmg in the northern part of the intrusion mineral compositions in 37 samples change continuously in the lower part of the sequence up to the middle of the mg unit plagioclase an37 18 olivine fo40 7 ca poor pyroxene mg 57 15 ca rich pyroxene mg 65 21 above this compositions are essentially constant in the upper part of the mg unit and in the qmg an21 13 fo6 4 mg opx17 13 mg cpx25 20 the amount of interstitial quartz and the amount of normative orthoclase however both increase systematically upwards through the qmg unit implying that these rocks are cumulates there is no evidence of a compositional break in the mg qmg sequence that could reflect influx of relatively primitive magma two types of qmg ch are known in the uppermost part of bksk olivine bearing types are comagmatic with the underlying layered series the studied stratigraphic sequence belongs to this suite two pyroxene qmg and amphibole ch define a separate compositional lineage related to jotunites an intrusive unit of dominantly two pyroxene qmg is discordant to the olivine bearing jotunite mg qmg sequence near rapstad confirming the presence of two compositionally distinct suites of qmg and related lithologies in the upper part of bksk a xenolith rich unit near the olivine bearing mg qmg boundary represents a major collapse of the roof of the magma chamber during the final stages of crystallisation

the sudbury igneous complex a differentiated impact melt sheet the sudbury structure ontario is the remnant of a 1 85 ga old impact crater which originally had a diameter of 200 to 250 km the sudbury igneous complex occurs within the sudbury structure the sudbury igneous complex is a 2 5 to 3 0 km thick 60 27 km elliptical igneous rock body which consists of four major lithologies from top to bottom traditionally termed granophyre quartz gabbro norite and contact sublayer sulfide and inclusion bearing noritic rock with the exception of the latter all these lithologies are continuous across the structure modal analyses reveal that following the iugs system of nomenclature quartz gabbro samples are in fact quartz monzogabbros a few of the norite samples are quartz gabbros and most norite samples are quartz monzogabbros in view of these observations and in order to clarify the nomenclature an updated terminology is proposed from top to bottom upper unit middle unit lower unit and contact sublayer the bulk composition of the sudbury igneous complex from north range data is granodioritic continuous and gradational mineralogical and geochemical variations between the lithological units are evidence that the complex behaved as a single melt system all the sudbury igneous complex lithologies have the same light to heavy rare earth element ree ratio and an overall pattern of increased light ree and depleted heavy ree the occurrence of primary hydrous minerals hornblende and biotite deuteric alteration and abundant micrographic and granophyric intergrowths demonstrate that the melt was rich in h2o moreover the granophyric and other far from equilibrium textures are most likely due to rapid crystallization triggered by exsolution of a volatile phase the sudbury igneous complex differs from traditional layered mafic complexes in the following aspects it has an overall intermediate composition a hydrous nature a crustal isotopic signature normative corundum and an unusually large volume of granophyre the sudbury complex differs from known terrestrial impact melt sheets only by its great thickness and the presence of chemical and therefore mineralogical layering reported here for the first time and similar to those found in impact melt rocks elsewhere are the occurrences of plagioclase xenocrysts with complex twinning and zoning patterns and planar deformation features in quartz xenocrysts the well known ore deposits of the sudbury region are directly related to the genesis of the sudbury igneous complex some ores precipitated from the sudbury melt whereas others were concentrated by hydrothermal fluids that percolated through the crystallized complex it is concluded that the sudbury igneous complex is the best exposed and only well documented to date terrestrial impact melt sheet to have differentiated

crystal settling and convection in the shiant isles main sill the 168 m thick shiant isles main sill is a composite body dominated by an early 24 m thick picrite sill formed by the intrusion of a highly olivine phyric magma and a later 135 m thick intrusion of olivine phyric magma that split the earlier picrite into a 22 m thick lower part and a 2 m thick upper part forming the picrodolerite crinanite unit pcu the high crystal load in the early picrite prevented effective settling of the olivine crystals which retain their initial stratigraphic distribution in contrast the position of the most evolved rocks of the pcu at a level 80 of its total height point to significant accumulation of crystals on the floor as evident by the high olivine mode at the base of the pcu crystal accumulation on the pcu floor occurred in two stages during the first most of the crystal load settled to the floor to form a modally and size sorted accumulation dominated by olivine leaving only the very smallest olivine grains still in suspension the second stage is recorded by the coarsening upwards of individual olivine grains in the picrodolerite and their amalgamation into clusters which become both larger and better sintered with increasing stratigraphic height large clusters of olivine are present at the roof forming a foreshortened mirror image of the coarsening upwards component of the floor accumulation the coarsening upwards sequence records the growth of olivine crystals while in suspension in a convecting magma and their aggregation into clusters followed by settling over a prolonged period with limited trapping at the roof as olivine was progressively lost from the convecting magma crystal accumulation on the contemporaneous floor of the pcu was increasingly dominated by plagioclase most likely forming clusters and aggregates with augite and olivine both of which form large poikilitic grains in the crinanite while the pcu is unusual in being underlain by an earlier still hot intrusion that would have enhanced any driving force for convection we conclude from comparison with microstructures in other sills that convection is likely in tabular bodies 100 m thickness

included blocks and blocks within blocks in the skaergaard intrusion geologic relations and the origins of rhythmic modally graded layers the early eocene skaergaard intrusion of greenland includes enormous numbers of rocks of both exotic and cognate origins the lower parts of the marginal border series contain abundant fragments of feldspathic peridotite that are possibly autoliths intermixed with occasional xenoliths of precambrian gneiss and metasomatized cretaceous paleocene sediments derived from adjoining country rocks the upper border series includes one exceptionally large block of gneiss several hundred meters across and numerous smaller fragments these originating from the intrusion9s footwalls plus a few pieces of peridotite the layered series contains countless autoliths of troctolite gabbroic anorthosite and oxide magnetite ilmenite gabbro broken from parts of the upper border series that have otherwise been lost to erosion at the upper midlevel of its western half it contains a few xenoliths of basalt derived probably from the now eroded eocene roof of the intrusion a distinctive postintrusion composite basaltic dike at one place contains 40 or more xenoliths of troctolite olivine gabbro and gabbroic anorthosite that may represent parts of the layered series still hidden at depth the layered series autoliths range from fragments a few centimeters on a side to blocks more than 400 m across and they typically are coarser grained than their host cumulates being in this respect more like upper border series rocks the autoliths are spread stratigraphically through the lower 70 of the exposed 2500 m thickness of the layered series and are generally concentrated in three broad stratigraphic zones their physical relationships to their host rocks particularly the way they indent older layers beneath and are covered by younger layers above provide abundant evidence that there was generally a sharp well defined interface between the top of the cumulate pile and the main body of magma in the intrusion while the layered series was forming the distribution of the autoliths between and through the well known rhythmic thin modally graded layers shows that these layers were spread by magmatic currents and their relations to the more extensive macrorhythmic layering suggest that it too was significantly shaped by currents many of the larger autoliths are crudely layered internally and in places it is evident that their stratification existed before they broke loose therefore it must have formed in the upper border series one particularly large block of oxide gabbro exhibits extraordinarily well developed modal and textural layering and includes small troctolitic autoliths of an earlier generation and it provides evidence that currents also spread crystalline materials across the top of the magma body many of the very small autoliths in the layered series are highly anorthositic in composition apparently because they were leached of mafic minerals and some of the larger blocks show local patchy internal replacement by anorthosite most large blocks show little sign of postaccumulation modification and some have thin fine grained augite rich rims or rinds demonstrating that even though they were out of thermal and chemical equilibrium with their host cumulates they still were effectively armored against extensive chemical change also documented is a large block that was cut by several early basaltic dikes before it broke free from the top of the intrusion these early dikes transgress small anorthositic replacement pipes in the block showing that the replacement process also occurred in the upper border environment two mechanisms are described whereby graded cumulate layers can be sorted and deposited by magmatic crystal liquid suspension currents one involving density surge currents has been advocated previously the other is a new concept based on boundary flow separation and reattachment vortex cells the two mechanisms are used in complementary ways to illustrate the formation of 1 some of the principal skaergaard structures involving blocks and layers 2 modally graded layers in the layered series that rhythmically alternate with uniform layers and 3 modally sorted layers in the upper border series featuring underside draping beneath small included blocks explanations are provided for 1 why plagioclase did not float away from the tops of graded layers even though it was less dense than the liquid and 2 how the liquid part of a current was fractionated away from the crystalline materials modal and grain size data from skaergaard intrusion graded layers are shown to be in excellent accord with characteristics predicted for layers sorted by currents a synthesis diagram is presented illustrating how all the above processes may have functioned in concert in the intrusion

the graveyard point intrusion an example of extreme differentiation of snake river plain basalt in a shallow crustal pluton the graveyard point intrusion is the only known example of a wellexposed differentiated mafic pluton associated with the late miocene pleistocene magmatism of the western snake river plain srp it is exposed in a 6 km by 4 km area adjacent to the oregon idaho border and exposures range in thickness from 20 to 160 m the thicker parts of the intrusion are strongly differentiated and contain a 25 60 m thick section of well laminated cumulus textured gabbros that grade upward into pegmatoidal ferrogabbro evolved liquids formed sheets of fe rich siliceous granophyre at least two injections of magma are indicated by abrupt discontinuities in the rock and mineral compositions and by the lack of mass balance between the bulk intrusion and its chilled borders the laminated gabbros are interpreted to have formed from a tongue of augite and plagioclase crystals that were carried in with the second pulse of magma following the final emplacement of the intrusion in situ differentiation proceeded through a two stage process the ferrogabbros are explained as interstitial liquids forced out of the crystal mush by compaction and the siliceous granophyres are interpreted to be residual liquids that migrated out of the partly crystallized ferrogabbros in response to the exsolution of volatiles because the geochemical trend inferred for the mafic to intermediate composition liquids in the graveyard point intrusion is similar to the trend for many western snake river plain lavas the pluton may be a good model for shallow sub volcanic magma chambers elsewhere in the srp however some western srp lavas contain anomalously high concentrations of p2o5 which are best explained by mixing within the active crustal mush column or with partial melts of previously formed differentiated mafic intrusions

compaction of igneous cumulates part ii compaction and the development of igneous foliations quantitative petrofabric analysis of 66 samples from olivine bearing zones iii and iv of the middle banded series of the stillwater complex montana was conducted to allow direct comparison among textural and chemical variations systematic variations in foliations with stratigraphic height are observed in both zones statistical analysis of the orientations of inequant grains was used to calculate an alignment factor af corresponding to the extent of the development of the foliation both petrofabric and petrographic evidence indicates that the foliation is best explained by compaction of the crystal pile comparison of afs with estimates of the amount of compaction based on whole rock compositions shows approximate agreement variations in mole fraction cl of interstitial apatite mimic the observed trends in the af indicating that local concentrations of fluids exsolved from interstitial liquid were important in the development of strong foliations we conclude that 1 differing amounts of co

deflection of sand movement on a sinuous longitudinal seif dune use of fluorescent dye as tracer abstract sand was marked by fluorescent dye in order to trace sand movement and deposition on a longitudinal seif sand dune in the sinai desert the wind regime was monitored simultaneously tracing the dyed sand was possible after light to moderate sand storms and was graphically represented on maps the dune was subjected to a seasonally bidirectional wind regime with the wind hitting the dune obliquely on either side on the windward flank the sand was transported parallel to the wind direction on the lee flank sand movement was deflected towards parallelism with the crest line sand movement was deflected if the dune had a sharp profile which favored separation of wind flow on the lee flank the deflection depended on the angle of incidence between the wind and the crest line when the angle of incidence was

the origin of basic ultrabasic sills with s d and i shaped compositional profiles by in situ crystallization of a single input of phenocryst poor parental magma an attempt is made to develop an in situ crystallization model based on the concept of soret fractionation to explain the origin of commonly observed s d and i shaped compositional profiles in sills formed from a single pulse of phenocryst poor parental magma the model envisages that the various compositional profiles observed in sills can be interpreted in terms of different combinations of four principal units basal zone and layered series forming the floor sequence and top zone and upper border series constituting the roof sequence the basal and top zones represent mirror images of the layered and upper border series respectively and therefore are referred to as basal and top reversals it is proposed that the formation of basal and top reversals takes place through the non equilibrium soret differentiation of liquid boundary layers which form as a consequence of the temperature gradient imposed by the cold country rock in contrast the layered and upper border series originate predominantly through the crystal liquid boundary layers developing in equilibrium conditions the model permits the production of s d and i shaped compositional profiles from the same magma composition all that is necessary to produce a specific shape of compositional profile is an appropriate temperature gradient imposed by the cold country rock on the liquid boundary layers of a parental magma of a given composition

crystallization dynamics of granite magma chambers in the absence of regional stress multiphysics modeling with natural examples numerical models built linking an internally consistent rheological dataset for a cooling granite magma with equations of heat transfer and fluid motion for geometrically different magma chambers cooling at various crustal depths reveal that granite magmas first undergo a short period of chaotic convection during which wall rock contamination and magma mixing are possible followed by a long period of no convective cooling during which melt segregation occurs convection is driven by the negative density gradient generated in the upper cooling zone by melt to solid phase transformation convection breaks the upper mushy zone and drags the fragments downwards with descending rayleigh taylor fingers such fragments can be preserved as microgranular enclaves the descending rayleigh taylor fingers split low aspect ratio sill like magma chambers into nearly isolated convection cells if the magma is initially heterogeneous this effect divides the chamber into contiguous homogeneous zones with distinct trace element and isotope ratios and finally results in a pluton with marked lateral compositional variations easily misinterpreted as different intrusive batches convective heat loss quickly leads most of the magma chamber to critical crystallinity independently of the vertical coordinate so that a chamber wide three dimensional skeleton of crystals with uniform initial porosity c 0 4 0 5 is formed this configuration is gravitationally unstable therefore it spontaneously compacts towards an equilibrium vertical variation of porosity approaching atty s law in the absence of regional stress the upwards migration of the inter crystalline melts as a result of compaction is the most effective way of melt solid segregation and causes vertically zoned plutons with an upper layer of felsic segregates granite magma chambers fractionated by these mechanisms will produce short range differentiation series from a composition slightly less silicic than the initial magma to high silica segregates in the presence of regional stress tectonic squeezing and shearing during the post convective stage can expel residual fluid more efficiently and lead to wide range granite differentiation series from rocks notably less silicic than the initial magma to high silica leucogranites

surface geology from viking landers on mars a second look aside from reddish dust products of martian weathering are not abundantly obvious on viking lander images surface stones appear sound spalls and disintegration fragments are largely lacking pitting of stones is probably not due to differential weathering and reddish stone colors more likely result from adhered dust than from patination judging from wind scoured drifts deflated ground rippled bed forms lee side tails basal scours fretted rocks and ubiquitous dust wind is currently the most effective transportive and erosive agent the dogma that martian eolian material is almost exclusively of 10 100 size is challenged by granule ripples and the possibility that wind drifted deposits consist largely of sand sized particles fretting is more common than faceting on martian ventifacts crusts on martian surface fines are more akin to case hardened rinds than to true duricrust crusting a possibly continuing process may be the product of atmospheric breathing of the regolith pits on surfaces of many martian stones are probably vesicles modified by eolian erosion pits on nonvesicular rocks may be solely the product of wind blasting vesicular lava dense lava and breccias are the most likely rocks among surface stones around the landers the bedrock outcrops near vl 1 may be large boulders embedded in a breccia substrate pyroclastics seem a likely constituent of surface deposits

on bimodal differentiation by solidification front instability in basaltic magmas part 1 basic mechanics extensive coarse grained silicic lenses are common in the upper portions of large diabase sills lava lakes and gabbroic intrusions these lenses which often contain up to 10 wt more silica than their host rocks record a clear process of bimodal differentiation that may play a fundamental role in generating significant quantities of silicic magma in predominantly basaltic systems the spatial compositional textural and contact relations suggest that these silicic segregations form by infilling of tears formed during gravitational instability of the upper solidification front this process is investigated in some detail by exploring the thermal and mechanical conditions leading to internal failure of the front and segregation formation as the upper solidification front grows inward and thickens the local strength of the developing crystalline network must increase upward through the front sufficiently fast to offset the increasing weight of the front itself this necessary local variation in strength dictates how crystallinity must vary to prevent failure but the actual variation in crystallinity is determined by phase equilibria which in general varies in a fashion that is noncovariant with that required for mechanical stability it is this mismatch that apparently leads to failure and formation of silicic segregations the rate of filling of the tears by porous flow of interstitial melt from below regulates the rate of tear opening and the overall size of the tears is regulated by the local rate of advance of the solidification front segregations therefore thicken downward as the growth of the front slows and then die out altogether with approach of the lower solidification front this process irreversibly introduces siliceous noise into basaltic systems the silicic signal can be enhanced through subsequent reprocessing of the host rock to form volumetrically important masses of silicic magma this process may be fundamental to achieving the strong compositional diversity of the igneous rocks to yielding continental masses and to the differentiation of earth itself

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

fine scale chemical zonation in small mafic dykes kestio island sw finland detailed centimetre scale sampling across two small dolerite dykes 7 and 21 cm wide of kestio island sw finland has revealed a well developed internal zonation with surprisingly systematic compositional variations from the margins inwards the dykes exhibit a steady decrease in whole rock mgo mg number 100mg mg fe total and normative opx indicating a normal fractionation trend with simultaneous increase in normative an 100an an ab and decrease in incompatible zr y tio 2 and p 2 o 5 indicating a reverse fractionation trend in addition marginal rocks of dykes contain normative corundum that is apparently associated with their significant depletion in whole rock cao the extent of margin to centre differentiation of dykes in terms of most components is slight to modest although in some petrochemical parameters it is quite high e g 15 mol of normative an the dykes are almost glassy and uncontaminated by host rocks suggesting that their compositional profiles are primary and most likely reflect temporal changes in composition of magma filling the dykes a mechanism responsible for the systematic changes in composition of inflowing magma remains elusive however since no known processes are able to force magma to evolve simultaneously along both normal and reverse fractionation trends the study thus appears to indicate some not yet specified process of magma differentiation