the nature and genesis of marginal cu pge au sulphide mineralisation in paleogene macrodykes of the kangerlussuaq region east greenland the kangerlussuaq region of east greenland hosts a variety of early tertiary extrusive and intrusive igneous rocks related to continental break up and the passage of the ancestral iceland plume these intrusive bodies include a number of gabbroic macrodykes two of which the miki fjord macrodyke and the newly discovered togeda macrodyke contain cu pge au sulphide mineralisation along their margins sulphides occur as disseminated interstitial blebs and rounded globules of chalcopyrite and pyrrhotite with some fe ti oxides and platinum group minerals comprising largely pd bismuthides and tellurides the globules are interpreted to have formed from fractionation of trapped droplets of an immiscible cu and pd rich sulphide melt and show geopetal indicators sulphur isotopes imply a local crustal source of s in these from pyritic sediments of the kangerlussuaq basin thus generation of these sulphide occurrences was controlled by local country rock type low ni cu and pt pd ratios also present in the platinova reefs in the skaergaard intrusion indicate that early fractionation of olivine may have depleted the magma of ni and suggest the likely presence of a large magma chamber at depth xenoliths of ni rich olivine cumulates in the miki fjord macrodyke may have been sourced from such a body the location of thus far unidentified conduit or feeder zones to the macrodykes beneath the present day surface may represent potential targets for more massive sulphide orebodies

magmatic fractionation by compositional convection in a sheet like magma body constraints from the nosappumisaki intrusion northern japan the mechanism of fractionation in a sheet like magma body was investigated based on observations from the nosappumisaki intrusion northern japan this is one of a number of late cretaceous shoshonite sills in the nemuro peninsula and has a compound structure comprising a bottom cumulate zone containing 50 vol cumulus crystals and a overlying middle zone free of such cumulus crystals these zones are sandwiched between porphyritic rocks of upper and lower chilled and marginal zones it is inferred that massive crystal settling or flotation from a magma initially laden with 20 vol of crystals primocrysts in a sheet like magma body formed a cumulus pile and an overlying crystal free melt layer after the formation of the compound structure with partial freezing of the upper and lower marginal zones the internal part of the intrusion slowly solidified during this period the melt in the central part of the intrusive sheet underwent fractional crystallization this is shown by patterns of vertical variation in the composition of the interstitial melt the overall patterns are inverted mirrorimage s shaped and s shaped for incompatible and compatible elements respectively the incompatible element abundances show maxima near the bottom of the middle zone which corresponds to the final solidification horizon they show minima in the cumulate zone which suggests discharge of an evolved melt from the crystal pile fractionation is inferred to have taken place via compositional convection without settling of crystals grown after the intrusion event or collapse of the upper boundary layer the melt in the central melt layer descended into the permeable crystal pile counterbalancing discharge of buoyant evolved melt formed by crystallization of the pore melt in the crystal pile this conclusion is based on the following observations 1 there is no accumulation of crystals grown after the intrusion of the magma above the cumulate zone 2 leucocratic pipe like structures which represent relict pathways for the evolved melt are developed in the cumulate zone these structures increase in diameter decrease in number density and increase in abundance of incompatible elements with height 3 cumulus augite in the cumulate zone has ca rich and al poor dissolved rims the extent of dissolution increases with stratigraphic height suggesting downwelling of an h2o rich melt from the overlying melt layer 4 there is no evidence for extensive compaction accompanying pore melt crystallization in the main part of the cumulate zone 5 the initial melt composition estimated from the groundmass of the chilled margin rocks and the average composition of the material interstitial to the primocrysts for the whole sill are statistically identical a mass balance model for compositional convection was constructed to quantify the melt transportation processes model parameters were optimized by fitting the observed vertical variation of the interstitial melt compositions the optimized results suggest that the evolved melts discharged from the crystal pile and rose through the central melt layer as plumes with minor mingling with the surrounding melt the initially formed compound structure is a highly preferential environment for compositional convection

the low δ18o late stage ferrodiorite magmas in the skaergaard intrusion result of liquid immiscibility thermal metamorphism or meteoric water incorporation into magma abstractwe report new laser fluorination oxygen isotope analyses of selected samples throughout the skaergaard intrusion in east greenland particularly relying on 1 mg separates of the refractory alteration resistant minerals zircon sphene olivine and ferroamphibole we also reexamine published oxygen isotope data on bulk mineral separates of plagioclase and clinopyroxene our results show that the latest stage strongly differentiated magmas represented by 3 to 6 km3 of ferrodiorites around the sandwich horizon sh where the upper and lower solidification fronts met became depleted in 18o by about 1 5 2 relative to the original skaergaard magma and the normal mantle derived mid ocean ridge basalt earlier studies did not recognize these low 18o ferrodiorite magmas 18o 3 4 because after the intrusion solidified much of the intrusion and its overlying roof rocks were heavily overprinted by low 18o meteoric hydrothermal fluids we consider three possible ways of producing these low

the kraemer island macrodyke east greenland solidification of a flood basalt conduit the kraemer island macrodyke that is exclusively exposed on kraemer island about 7 km west of the skaergaard intrusion belongs to a regional dyke swarm termed the skaergaard like dykes or fg 1 dykes weakly modally layered olivine gabbros dominate the exposed parts of the intrusion that has a width of 650 m to 1000 m plagioclase core an 68 2 and ca rich pyroxene core mg no 79 1 grains are normally zoned whereas olivine grains fo50 65 are homogeneous calculated mineral magma equilibria based on experimentally determined mg fe magma olivine and magma clinopyroxene partition coeffi cients suggest that the observed olivine and clinopyroxene compositions in the gabbros cannot have formed from a common parental magma the unzoned nature of olivine grains and their iron rich com positions relative to clinopyroxene suggest post cumulus mg fe exchange between olivine and interstitial melt a gabbroic pegmatite is developed in the centre of the intrusion along its entire exposed 5 km strike length here mineral zonation is limited and compositions are similar to rims of cumulus minerals in the enveloping olivine gabbros the pegmatite could therefore represent interstitial melt mobilized from gab broic cumulates that later accumulated and crystallized at its present stratigraphic location cumulus olivines in the gabbros are close to equilibrium with ca rich pyroxene in the pegmatite this is interpreted as reflecting interstitial melt mobility during the late stages of solidification of the macrodyke chilled margins are well preserved at the intrusion margins and are rich in fe 14 15 3 feo tot ti 3 3 3 tio2 and light rare earth elements la sm n 1 2 1 3 similar to magmas well represented in the overly ing sequence of contemporaneous high ti series flood basalts it is therefore likely that extensive wall rock melting adjacent to the macrodyke reflects continuous feeding of the overlying flood basalts through the kraemer island macrodyke

high resolution insights into episodes of crystallization hydrothermal alteration and remelting in the skaergaard intrusive complex this paper presents a new high precision zircon u pb geochronological view on the crystallization and assembly process of one of the most important and intensely studied intrusive bodies on earth the skaergaard intrusion in east greenland with analytical uncertainties of a few tens of thousands of years we were able to resolve several important events during cooling of this intrusion initial cooling of the shallowly intruded 300 km3 of tholeiitic basaltic magma from liquidus to zircon saturation at 1000 c is recorded by a precise zircon crystallization age of 55 960 0 018 ma of an intercumulus gabbroic pegmatite in the lower portion of the intrusion based on this zircon crystallization age and a published cooling model we estimate the true age of emplacement to be 56 02 ma the last portions of skaergaard appear to crystallize completely 100 ka after emplacement as recorded by abundant 55 91 55 93 ma zircons in the sandwich horizon sh where lower and upper solidification fronts met intrusion of an isotopically distinct new magma batch the 600 m thick basistoppen sill into the solidified upper portion of skaergaard happened at 55 895 0 018 ma suggesting close timing between crystallization of evolved rocks around the sh and intrusion of the basistoppen sill the novel result of this work is the demonstration that zircons in the sh 100 m below the basistoppen contact have a bimodal age distribution with the youngest population of 55 838 0 019 ma postdating intrusion of the basistoppen sill by 57 37 ka oxygen isotope analyses reveal that sh zircons are low and heterogeneous with respect to 18o these results support the proposed conclusion that the sh crystallized twice it was fully crystalline then hydrothermally altered by low 18o surface waters and subsequently partially remelted triggered by heat of the basistoppen sill the low degree partial melt generated during remelting partially migrated upward by intergranular compaction driven flow explaining the existence of the most incompatible trace element rich horizon 100 m above sh as the skaergaard intrusion is also the most classic example of a shallow meteoric hydrothermal system this work documents the alternating processes in a life of an intrusion with periods of hydrothermal cooling heating by new intrusions and related remelting events which cause the generation of low 18o magmas our precise temporal framework for intrusion crystallization also provides constraints for the timing of coeval flood basalt volcanism and its synchronicity with the paleocene eocene thermal maximum

a laboratory model for melting erosion of a magma chamber roof and the generation of a rhythmic layering 1 a hot magma chamber can ascend by melting the roof rock a process which in turn affects the magma composition the disaggregated mineral particles which consisted the roof rock will descend in the magma chamber to form a sedimentary cumulate however the fluid dynamics leading to the formation of the sediment and how we can decipher them is unknown here we conducted a series of experiments modeling melting erosion of the roof with particle size consisting the roof rock as the parameter we find that there is a critical particle size below which the melting erosion occurs rhythmically melting erosion stops because the disaggregated particles are suspended in the magma chamber and suppress the vertical heat transfer the suspension then separates into an upper clear layer and a lower suspension layer eventually the heated stratified layers become unstable an overturn occurs and melting erosion resumes when the particles consist of two sizes such that at least one of them is smaller than the critical size a rhythmic erosion occurs particles are sorted during each erosion period and a size graded rhythmic layering is spontaneously generated we estimate that rhythmic layering can be generated from melting erosion in a basaltic magma chamber when the grain size of the roof rock is 0 6 mm assuming a vertical temperature difference of 10 c we suggest that rhythmic roof melting coupled with particle settling is one possible mechanism for generating the rhythmic layering which is commonly observed in solidified magma chambers

the skaergaard liquid line of descent revisited there is a fundamental conflict between the suggestion that the iron content of skaergaard liquids increases during fe ti oxide fractionation and the observation that at the same time oxygen fugacity f text o text 2 drops by two log units below the fayalite magnetite quartz oxygen buffer fmq a new petrographic study of average skaergaard gabbros shows that the total modal content of fe ti oxides is about 22 in the early lzc and markedly decreases to below 5 in the uzc forward modeling based on these modal constraints as well as experimental results on skaergaard related dikes predicts that fractionation of troctolitic lza gabbros drives the derivative liquid towards a high iron content strong iron enrichment continues together with a small decline in silica during lzb crystallization due to the appearance of augite as a fractionating phase the fractionation of fe ti oxides in the lzc initially suppresses iron enrichment and reverses the silica trend to one of slight enrichment however continued evolution into the uz produces liquids with maximum uzc feo content of 23 25 wt and sio2 content of 53 wt feo is total iron as feo the maximum in feo is dependent on several factors of which the fe ti oxide mode has the strongest effect the f text o text 2 during crystallization of the lzc is widely thought to have been at or slightly below the fayalite magnetite quartz oxygen buffer fmq under closed system evolution incorporation of ferric iron into augite during formation of the lzb restricts the increase in f text o text 2 to about 0 1 log units above fmq 0 1 fmq likewise crystallization of the lzc through the uza involving fe ti oxide minerals leads to a decline in f text o text 2 of less than 0 1 fmq crystallization of the uzb c gabbros results in oxidation to a maximum of 0 5 fmq this behavior can account for the iron rich character of the uz gabbros as well as the low modal content of fe ti oxides thus evolved skaergaard liquids are high in iron and contain a modest amount of sio2 our modeling result do not account for a strong drop in f text o text 2 through the layered series such a drop would require an unacceptably high proportion of fe ti oxides and high magnetite content in the fractionating assemblage

proterozoic anorthosite granite nain batholith structure and intrusion processes in an active lithosphere scale fault zone northern labrador we present field evidence of the structure and evolution of a typical proterozoic massif type anorthosite the anorthosite forms a batholith composed of numerous plutons and ring complexes intruded episodically in two main sequences each producing anorthositic rocks and monzonite ferrodiorite followed by granite the batholith is located where a lithosphere scale fault zone 50 km wide and extending over 1000 km from greenland to labrador intersected an older continental suture intermittent episodes of mainly sinistral transtension along this gardar voisey s bay fault zone were associated with the generation of a number of pulses of magmatism and controlled the rise of magmas to middle and upper crustal levels between 1363 and 1289 ma both the large and small scale structures and magma intrusion processes are similar to those of many cordilleran batholiths with tabular plutons associated with ring dykes emplaced by cauldron subsidence and basin shaped plutons associated with cone sheets these

geology u pb zircon geochronology and geochemistry of pge bearing neoarchean and paleoproterozoic gabbroic rocks of the peter lake domain southern hearne craton canada the peter lake domain a component of the hearne craton in saskatchewan includes several intrusive complexes ranging in age from 2 70 to 1 83 ga two gabbroic complexes swan river and porter bay together represent one of the largest accumulations of precambrian mafic intrusion both metamorphosed at amphibolite facies sulfide poor pge occurrences are found in both complexes and share many textural and lithological characteristics with magmatic contact type pge ni cu deposits such as the chaotic lac des iles pluton and with layered stratiform deposits of large layered intrusions such as bushveld or stillwater lithologically both complexes are dominated by leucocratic gabbronorite and gabbro locally characterized by cumulate layering cross bedding brecciated and pegmatitic textures u pb zircon crystallization ages between 2562 and 2560 ma were obtained for the swan river complex sulfide formation is interpreted to be of magmatic origin rather than remobilized as had been speculated by explora

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

mineral lamination development in layered gabbros of the british palaeogene igneous province a combined anisotropy of magnetic susceptibility quantitative textural and mineral chemistry study mineral lamination development in layered gabbros of the british palaeogene igneous province a combined anistotropy of magnetic susceptibility textural and mineral chemistry study

the formation of low volume high tenor magmatic pge au sulfide mineralization in closed systems evidence from precious and base metal geochemistry of the platinova reef skaergaard intrusion east greenland the majority of magmatic platinum group element pge deposits in layered mafic ultramafic intrusions are characterized by visible cu ni sulfide mineralization often located in the lower parts of a magmatic stratigraphy that show evidence of multiple magma injections and crustal contamination the platinova reef hosted by the 55 ma skaergaard intrusion east greenland is an example of a rarer type of pge deposit typified by a pd au cu dominant ni pt poor sulfide assemblage present in the upper parts of the host intrusion such deposits are generally considered to form through prolonged fractional crystallization of the magma with magnetite crystallization playing a role in late stage s saturation in the case of the skargaard intrusion the crystallization of the cumulate rocks and the generation of the mineralization hosted within them occurred within a closed system from a single homogenized batch of magma the platinova reef is characterized by offsets in the stratigraphic position of individual metal peaks an anomalously low sulfide content 1 ppm the intermediate zone pd 0 1 1 ppm the au zone au 1 ppm and the upper cu zone cu 200 ppm with precious metals 3 to 10 4 ppm low cu pd ratios and a remarkable upward decrease in pd au ratio from 40 to 1 whereas the sulfides in the cu zone are lower tenor 10 3 ppm pd higher volume and have high cu pd ratios and pd au to explain these features we present a multistage model of preconcentration dissolution upgrading and further metal enrichment this involves initial s saturation of the skaergaard magma at a late stage in the crystallization history the sulfide liquid produced scavenged metals from the entire volume of the chamber as it sank but was then dissolved as it came into contact with hotter fe rich and s undersaturated magma at the bottom of the chamber producing a highly metal rich magma layer at the bottom of the chamber this layer then became s saturated as a result of continued magnetite crystallization with the tiny sufide droplets that segregated into this precious metal enriched magma layer becoming highly enriched in pge themselves these were then trapped in situ by the crystallizing cumulate pile forming the subzone pd zone and intermediate zone sulfides formed in the rest of the chamber which was pge depleted at this stage would have been of low tenor these sulfide droplets grew and settled concentrating any remaining au when they encounter the floor to form the au zone and formed the low tenor sulfides of the cu zone above this we propose that the platinova reef exhibits an unusual example of a style of magmatic precious metal mineralization formed through sulfide dissolution upgrading in the latter stage of crystallization of a closed system the offsets in individual metal peaks mineralization demonstrate the relative partitioning of pd pt au cu into a sulfide liquid from a silicate magma and demonstrates the importance of partitioning behavior in low volume magmatic sulfide melts this feature can lead to the characteristic stratigraphic offsets in peak metal abundances the role of magnetite crystallization appears to be an important control on s saturation in such systems and thus may be an important indicator of the location of mineralized horizons in addition in such deposits indicators of crustal contamination by external s and multiple injections of magma common features of more ni rich pge reefs are less likely to be present

new insights into the origin of ladder dikes implications for punctuated growth and crystal accumulation in the cathedral peak granodiorite abstract ladder dikes are steep tabular bodies typically a meter or less thick composed of moderately dipping concave upward alternating dark i e schlieren and light bands oriented roughly perpendicular to the ladder dike margins these structures occur widely but sparsely in granitic rocks and are found prominently in the cathedral peak granodiorite cpg of the tuolumne intrusive suite previous studies have interpreted that ladder dikes form as a result of processes including the downward flow of crystal mush in cracks within strong crystal mush or by upward flow in steep tubes that migrate within a strong crystal mush our new observations indicate that ladder dikes formed by downward flow of crystal mush in troughs or valleys in a manner potentially comparable to trough bands in mafic layered intrusions extensions of the schlieren outward and upward away from the ladder dike margins into the host granite demonstrate that the host granite was deposited as mounds on both sides at the same time as the ladder dikes ladder dikes therefore record lateral flows of crystal mush on a magma chamber floor vertical exposures suggest these flows are on the order of ten meters thick some steep exposures on granite domes indicate multiple ladder dikes and flows over a stratigraphic height of 80 100 m later stratigraphically higher flows commonly deform and erode the top of an earlier flow and granitic material rich in k feldspar megacrysts has locally engulfed large blocks of ladder dikes demonstrating that the megacrysts were also transported in flows flows in the cpg are directed away from the center of the pluton toward the western and eastern margins and apparently spread along a strong crystal mush floor and into a rheologically complex cpg magma whereas established dynamical models for spreading single phase gravity currents with simple and complex rheologies explain the elongate geometry spacing and orientation of the tabular bodies the origin and character of the downward flows required to explain the trough band schlieren structures is challenging however an intermittent and progressive deposition of trough bands consistent with field observations is potentially explained if the two phase crystals and melt dynamics governing the response of the cpg magma to a new injection are considered

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

elemental distributions and mineral parageneses of the skaergaard pge au mineralization consequences of accumulation redistribution and equilibration in an upward migrating mush zone

magma chamber processes in central volcanic systems of iceland constraints from layered gabbro of the austurhorn intrusive complex new field work and petrological investigations of the largest gabbro outcrop in iceland the hvalnesfjall gabbro of the 6 7 ma austurhorn intrusive complex have established a stratigraphic sequence exceeding 800 m composed of at least 8 macrorhythmic units the bases of the macrorhythmic units are composed of 2 10 m thick melanocratic layers rich in clinopyroxene and sometimes olivine relative to the thicker overlying leucocratic oxide gabbros while the overall compositional variation is limited mg clinopyroxene 72 84 an plagioclase 56 85 the melanocratic bases display spikes in mg and cr2o3 of clinopyroxene and magnetite indicative of magma replenishment some macrorhythmic units show mineral trends indicative of up section fractional crystallisation over up to 100 m whereas others show little variation two populations of plagioclase crystals large an rich and small less an rich indicate that the recharge magma carried plagioclase xenocrysts high an type the lack of evolved gabbros suggests formation in a dynamic magma chamber with frequent recharge tapping and fractionation modelling of these compositional trends shows that the parent magma was similar to known transitional olivine basalts from iceland that had undergone about 20 crystallisation of olivine plagioclase and clinopyroxene and that the macrorhythmic units formed from thin magma layers not exceeding 200 300 m such a mushy magma chamber is akin to volcanic plumbing systems in settings of high magma supply rate including the mid ocean ridges and present day magma chambers over the iceland mantle plume the austurhorn central volcano likely formed in an off rift flank zone proximal to the iceland mantle plume during a major rift relocation

observations on the characteristics of the exchange flow in the dardanelles strait 1 a mooring section at each end of the dardanelles strait was deployed by the naval research laboratory on 28 29 august 2008 and maintained until 11 october 2009 each section was equipped with two bottom mounted barny moorings and a vertical string that together delivered concurrent bottom pressure temperature salinity and almost full water column current velocity observations these observations at the mooring sections indicated that on longer timescales monthly or longer the exchange through the dardanelles strait showed little variability and had a two layer structure with brackish waters moving approximately southward in the upper layer and more saline aegean waters flowing roughly northward underneath on shorter time scales synoptic there were pronounced fluctuations in both layers in the northern section some manifested as almost simultaneous reversals of the flow direction in both layers with lighter upper layer waters backing into the sea of marmara and dense lower layer waters moving toward the aegean sea in the southern section some of the exchange variability consisted of a three layer flow structure with upper brackish and bottom salty layers moving toward the aegean sea and a middle salty layer flowing toward the sea of marmara this three layer exchange persisted from a day to a week synoptic flow variability in the upper layer was partly related to both the along strait wind stress and the bottom pressure anomaly gradient while lower layer current fluctuations were coherent with the bottom pressure anomaly gradient

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

the skaergaard trough layering sedimentation in a convecting magma chamber the upper parts of the floor cumulates of the skaergaard intrusion east greenland contain abundant features known as troughs the troughs are gently plunging synformal structures comprising stacks of crescentic modally graded layers with a sharply defined mafic base that grades upward into plagioclase rich material the origin of the troughs and layering is contentious attributed variously to deposition of mineral grains by magmatic currents descending from the nearby walls or to in situ development by localised recrystallisation during gravitationally driven compaction they are characterised by outcrop scale features such as mineral lineations parallel to the trough axis evidence of erosion and layer truncation associated with migration of the trough axis and disruption of layering by syn magmatic slumping a detailed microstructural study of the modal trough layers using electron backscatter diffraction together with geochemical mapping demonstrates that these rocks do not record evidence for deformation by either dislocation creep or dissolution reprecipitation instead the troughs are characterised by the alignment of euhedral plagioclase crystals with unmodified primary igneous compositional zoning we argue that the lineations and foliations are therefore a consequence of grain alignment during magmatic flow post accumulation amplification of the modal layering occurred as a result of differential migration of an unmixed immiscible interstitial liquid with upwards migration of the si rich conjugate into the plagioclase rich upper part of the layers whereas the fe rich immiscible conjugate remained in the mafic base both field and microstructure evidence support the origin of the troughs as the sites of repeated deposition from crystal rich currents descending from the nearby chamber walls

silicate liquid immiscibility within the crystal mush late stage magmatic microstructures in the skaergaard intrusion east greenland late stage microstructures developed during the last stages of solidification of the skaergaard intrusion comprise a wide array of reactive and non reactive intergrowths reactive microstructures ascribed to open system behaviour include serrated grain boundaries between pyroxene and plagioclase primocrysts fish hook pyroxenes and mafic symplectites they form by the addition of fe and ca and removal of alkalis and silica other microstructures include those formed by internally generated redox reactions between olivine fe ti oxides and pyroxene non reactive microstructures include closely spatially associated granophyric intergrowths and previously undescribed ilmenite rich intergrowths that are interpreted as a consequence of crystallization of separated conjugate immiscible liquids the open system reactive microstructures occur predominantly in the cumulates on the chamber floor appearing in lzb and disappearing to be replaced by a granophyre ilmenite rich intergrowth association in the upper zone they are not common in the marginal border series their distribution mirrors that of efficient expulsion of trapped liquid and can be attributed to the gravitationally driven loss of a si rich immiscible component from the interstitial liquid the loss of the si rich component causes the remaining fe rich liquid to react with the primocrysts bulk rock major element data are consistent with little or no preferential loss of the si rich liquid from the mush but the resolution of the available data is not sufficient to assess the effect of this relative movement on the liquid line of descent of the bulk magma the first appearance of the paired conjugate non reactive intergrowths in mz marginal border series points to the early onset of immiscibility in the bulk liquid

the centre 3 layered gabbro intrusion ardnamurchan nw scotland detailed remapping of the palaeogene ardnamurchan centre 3 gabbros nw scotland suggests that this classic sequence of ring intrusions forms a composite layered lopolith the area mapped by previous studies as the great eucrite gabbro intrusion comprises 70 by area of centre 3 field observations suggest that most of the other smaller ring intrusions of centre 3 interior to the great eucrite constitute either distinct petrological facies of the same intrusion or included country rock or peridotite blocks these observations together with syn magmatically deformed inward dipping modal layering are used here to support the interpretation that significant central sagging occurred in the intrusion at a late stage in its crystallization history

plagioclase in the skaergaard intrusion part 1 core and rim compositions in the layered series the anorthite content of plagioclase grains xan in 12 rocks from the layered series of the skaergaard intrusion has been studied by electron microprobe typically 30 core and 70 rim analyses per thin section mean core compositions vary continuously from an66 at the base of the layered series lza to an32 30 at the top on the other hand crystal rims are of approximately constant composition an50 1 from the lza to the lower middle zone mz above the mz core and rim compositions generally overlap profiles across individual plagioclase grains from the lower zone show that most crystals have an external zone buffered at xan 50 1 the simplest explanation for these features is that during postcumulus crystallization in the lower zone interstitial liquids passed through a density maximum this interpretation is consistent with proposed liquid lines of descent that predict silica enrichment of the liquid associated with the appearance of cumulus magnetite

the effect of crystallization time on plagioclase grain shape in dolerites the average aspect ratio of plagioclase measured in thin section varies systematically through dolerite sills with a symmetrical m shaped profile observed in sills thinner than 200 m thicker sills show the same marginal reversal at the base but average aspect ratios appear to continue increasing towards the top creating an s shaped profile a distorted m shaped profile is visible in the stratigraphic variation of plagioclase average aspect ratios in the prehistoric makaopuhi lava lake with the centre of the m in the olivine enriched horizon close to the base of the lake higher overall values of average aspect ratio are observed in thin sills compared to thicker sills demonstrating that the plagioclase in more slowly crystallized bodies is more equant than that in more rapidly crystallized bodies there is a strong correlation between the plagioclase average aspect ratio in the central parts of the sills and the crystallization time calculated using a simple one dimensional thermal model assuming conductive cooling the cause of the marginal reversals in average aspect ratio is not well understood but may result from early grain impingement in the developing crystal mushy layer

role of magma pressure tectonic stress and crystallization progress in the emplacement of syntectonic granites the a type estrela granite complex carajas mineral province brazil the archaean syntectonic a type estrela granite complex carajas mineral province brazil consists of three plutons emplaced in a greenstone sequence under low pressure conditions 180 p 310 mpa it is composed mainly of annite ferropargasite hedenbergite and ilmenite bearing monzogranites the contact aureole is affected by a subvertical penetrative schistosity conformable with the limits of the plutons meso to microstructures and mineral reactions in the granites indicate that deformation occurred in a continuum from above solidus to low t subsolidus conditions two distinct planar structures are observed i a concentrical primary foliation s0 corresponding to rhythmic isomodal phase layering associated with a faint grain shape fabric it is horizontal in the centre and vertical towards the edges of the plutons and ii a steep to subvertical foliation s1 associated with the deformation of s0 and accompanied with emplacement of synplutonic dykes and veins of leucocratic granites and pegmatites emplacement differentiation and consolidation of the estrela granite complex are considered to result from a continuous evolution under decreasing temperatures in a single stage strained crust transpression with two main periods 1 the first period is controlled by body forces and it corresponds to inflation with magma ponding as long as the rheology is melt dominated magma pressure is the critical parameter and almost no strain is recorded with decreasing t magmas crystallize and differentiate leading to a concentrical magmatic phase layering the growing magmatic bodies are mechanically decoupled from the country rocks and their evolution depends on internal magma chamber processes 2 for higher amount of crystallization residual melt fraction f 0 5 the role of magma pressure becomes insignificant establishment of a continuous crystal framework leads to the coupling of plutons with their surroundings and deformation in response to tectonic stress most of the strain is recorded during this period which starts from the rigid percolation threshold and extends to subsolidus low grade conditions this leads to deformation of the partially crystallized volume and redistribution of fluid enriched differentiated melts the amount of crystallization through the rheological thresholds appears as the critical parameter determining the transition from magma controlled processes inflation and differentiation of the magma chamber with development of a phase layering to tectonic controlled processes deformation of the phase layering and redistribution of residual melts this accounts for the fact that syntectonic plutons commonly display intermingled boudinaged layers with distinct modal compositions and in some cases well preserved rhythmic layering

partial melting at the earth s surface implications for assimilation rates and mechanisms in subvolcanic intrusions abstract the relatively recent routine availability of isotopic and elemental analytical results from large sample suites of igneous rocks has revolutionized the study of magmatic processes large uncertainties remain however with regard to the highly variable rates of magma differentiation and ascent through the crust although the repose interval prior to an eruption may be equivalent to magma residence time at a certain level in the crust this cannot be assumed a priori even where this happens to be the case uncertainties in determining such intervals for prehistoric eruptive sequences may be large with respect to process rates consequently independent insights from geologically well constrained situations must be sought this paper first presents documentation of an example wherein fragments of dacite incorporated by a mafic lava flow on the surface were melted to the point that they viscously deformed in response to stresses in this flow the transformation from the solid state to a largely molten state is inferred to have occurred within a few weeks these results are then discussed in the context of examples from the literature that support rapid melting and digestion of xenoliths in magma it is suggested that the rate of crustal assimilation may be sufficiently rapid that it can operate within the durations of extended eruptive episodes years decades which are generally much shorter than typical repose periods at arc volcanoes decades millennia

seismogenic magma intrusion before the 2010 eruption of eyjafjallajokull volcano iceland we present relatively relocated earthquake hypocentres for 1000 microearthquakes ml 3 that occurred during the 2 weeks immediately prior to the 2010 march 20 fissure eruption at fimmvor uhals on the flank of eyjafjallajokull volcano in iceland our hypocentre locations lie predominantly in horizontally separated clusters spread over an area of 10 km2 and approximately 4 km below sea level 5 km below the surface seismic activity in the final 4 d preceding the eruption extended to shallower levels 2 km below sea level and propagated to the surface at the fimmvor uhals eruption site on the day the eruption started we demonstrate using synthetic data that the observed apparent 1 km vertical elongation of seismic clusters is predominantly an artefact caused by only small errors 0 01 0 02 s in arrival time data where the signal to noise ratio was sufficiently good to make subsample arrival time picks by cross correlation of both p and s wave arrivals the mean depth of 103 events in an individual cluster were constrained to 3 84 0 06 km epicentral locations are significantly less vulnerable to arrival time errors than are depths for the seismic monitoring network we used within clusters of typically 100 recorded earthquakes most of the arrivals exhibit similar waveforms and identical patterns of p wave first motion polarities across the entire monitoring network the clusters of similar events comprise repetitive sources in the same location with the same orientations of failure probably on the same rupture plane the epicentral clustering and similarity of source mechanisms suggest that much of the seismicity was generated at approximately static constrictions to magma flow in an inflating sill complex these constrictions may act as a form of valve in the country rock which ruptures when the melt pressure exceeds a critical level then reseals after a pulse of melt has passed through this would generate recurring similar source mechanisms on the same weak fault plane as the connection between segments of the sill system is repeatedly refractured in the same location we infer that the magmatic intrusion causing most of the seismicity was likely to be a laterally inflating complex of sills at about 4 km depth with seismogenic pinch points occurring between aseismic compartments of the sills or between adjacent magma lobes as they inflated during the final 4 d preceding the eruption onset between 22 30 and 23 30 utc on 2010 march 20 the seismicity suggests that melt progressed upwards to a depth of 2 km this seismicity was probably caused by fracturing of the country rock at the margins of the propagating dyke subsequently on the morning of the eruption a dyke propagated eastward from the region of precursory seismic activity to the fimmvor uhals eruption site

strontium isotope systematics for plagioclase of the skaergaard intrusion east greenland a window to crustal assimilation differentiation and magma dynamics author s hagen peter g tegner c lesher ce abstract 2019 geological society of america we present in situ laser ablation multicollector inductively coupled plasma mass spectrometry sr isotope data for plagioclase from a reference stratigraphic profile of the entire layered series and upper border series of the skaergaard intrusion east greenland plagioclase sr isotope compositions and anorthite contents vary systematically from the margins of the intrusion inwards the lowest 87sr 86sri calculated at 56 ma values 0 7041 occur near the base and top of the intrusion and systematically increase over several lithostratigraphic zones to a value of 0 7044 which is uniform throughout the middle 2000 m of intrusion across this same profile anorthite content of plagioclase varies smoothly from an65 70 at the base and top to an25 approaching the purported sandwich horizon plagioclase near the roof and proximal to rafts of partially assimilated basement gneiss are markedly more radiogenic 87sr 86sri up to 0 7046 we explain the stratigraphic relationships by progressive contamination of the magma during early stages of differentiation by basement gneiss rafts entrained during emplacement and accumulated near the top of the chamber contamination was transient ceasing once the entrained gneiss was consumed or isolated from the main magma reservoir as the solidification front advanced modeling of fractionation assimilation processes accounts for the observed isotopic trends with only a few percent assimilation relative to the original magma mass the record of contamination revealed by sr in plagioclase supports the view that the bulk of the skaergaard intrusion formed by closed system differentiation with only minor in situ contamination and no magma recharge comparing plagioclase and bulk rock sr suggests that the latter may have witnessed late stage metasomatic overprinting of phases other than plagioclase

paleomagnetism of the duke island alaska ultramafic complex revisited the duke island ultramafic intrusion was emplaced into the alexander terrane immediately preceding development of a regional mid cretaceous thrust belt paleomagnetic samples were collected from exposures of ultramafic rock with cumulate layering northwest of judd harbor and northwest of hall cove thermal demagnetization results were analyzed using principal component analysis to isolate the characteristic remanent magnetization site mean characteristic directions determined from 16 sites fail the fold test at 95 confidence indicating that cumulate layering attitudes were highly contorted at the time of magnetization at least on a scale of tens of meters variations in cumulate layering attitudes probably resulted from the combined effects of thermal convection phenomena during crystallization and deformation following crystallization but prior to magnetization analysis of cumulate layering over larger structural domains indicates that kilometer scale deformation produced southwest plunging folds within the hall cove and judd harbor bodies bogue et al 1995 proposed that a compound structural correction involving unplunging of fold axes followed by unfolding of average cumulate layering could restore cumulate layering to horizontal however using the full set of 21 site mean paleomagnetic directions from duke island 16 from the current study and 5 from bogue et al 1995 the compound structural correction yields mean paleomagnetic directions from the judd harbor and hall cove areas that are statistically distinguishable at 99 confidence this result indicates that even on the kilometer scale cumulate layering within the duke island ultramafic intrusion was neither coplanar nor horizontal at the time of magnetization observations of cumulate layering in other ultramafic intrusive rocks indicate that this layering can significantly depart from horizontal by 10 20 even on the kilometer scale therefore use of cumulate layering of ultramafic rocks as a proxy for paleohorizontal is not justified and paleomagnetic directions from the duke island ultramafic intrusion cannot be used to infer the cretaceous paleolatitude of the insular superterrane

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

chemistry morphology and origin of magmatic reaction chromite stringers associated with anorthosite in the upper critical zone at winnaarshoek eastern limb of the bushveld complex

a review of mesoscopic magmatic structures and their potential for evaluating the hypersolidus evolution of intrusive complexes abstract the perception that plutonic bodies are structurally simple is disappearing with the recognition of an array of magmatic structures useful for constraining hypersolidus temporal histories evolving rheologies strain fields flow directions growth and cooling patterns tilting and syn emplacement tectonism these histories provide a powerful means of testing an array of growth emplacement chamber evolution and tectonic models important points include that 1 many structures form in hydrogranular or congested magma slurries during magma mush avalanching local convection and late hypersolidus strain challenging the notion that magmas must have 55 crystals to convect fractionate and form compositional diversity in upper crustal magma chambers 2 magmatic fabrics reflect transient strain in these slurries rather than flow directly the latter must be inferred by linking geometries with temporal and kinematic information 3 caution is needed when using anisotropy of magnetic susceptibility and similar quantitative tools that characterize through a single ellipsoid preferred orientations of mineral grains given the increasing recognition of multiple fabrics in plutons and 4 that future studies are particularly needed in plutons focusing on the distribution and styles of compositionally defined structures magmatic folding shear zones and faults multiple fabrics and of the physical chemical behaviors in hydrogranular slurries

response to comment by mcbirney boudreau and marsh the concept of variable textural maturity at grain junctions in igneous rocks was first introduced by holness et al 2005 the papers of holness et al 2007a 2007b demonstrate how this concept can be applied to cumulates and present a hypothesis to explain the observation of excursions in textural maturity either localized or stepwise within a cumulate pile the comments by mcbirney et al refer to difficulties they have in accepting our hypothesis and we welcome the opportunity to clarify some confusions in holness et al 2007a 2007b here we identify the issues to be addressed with separate headings

crystallization sequence of the upper border series of the skaergaard intrusion revised subdivision and implications for chamber scale magma homogeneity although it is one of the best studied layered mafic intrusions in the world the crystallization sequence of the skaergaard intrusion east greenland remains in debate in particular it has been argued that the crystallization sequence in the upper border series which crystallized downwards from the roof of the magma chamber differs from that in the layered series formed at the floor the proposed deviation would require chemical stratification of the magma and a reexamination of the crystallization sequence therefore has important implications for understanding the dynamics of the system here we examine a new sample set from the upper border series combining field observations petrography and anorthite contents of plagioclase with bulk rock ti v p cu and mn concentrations we demonstrate that the first phases on the liquidus were plagioclase and olivine followed by augite then ilmenite and magnetite simultaneously sulfides apatite and finally ferrobustamite now inverted to hedenbergite this crystallization sequence represents extreme differentiation along the tholeiitic trend and it mirrors those at the floor layered series and walls marginal border series we therefore propose a revised subdivision of the upper border series into equivalents of the subzones in the layered series denoted by apostrophes lza lzb etc moreover the first appearance of each of the cumulus phases occurs at similar plagioclase core anorthite contents the mirror images of the crystallization sequences and the anorthite contents of plagioclase cores in the three series imply that the skaergaard magma chamber solidified by in situ crystallization along the floor walls and roof from one largely homogenous convecting magma body

the plagioclase magma density paradox re examined and the crystallization of proterozoic anorthosites intermediate composition plagioclase an40 60 is typically less dense than the relatively evolved basaltic magmas from which it crystallizes and the crystallization of plagioclase produces a dens residual liquid thus plagioclase should have a tendency to float in these magmatic systems there is however little direct evidence for plagioclase flotation cumulates either in layered intrusions or in proterozoic anorthosite complexes the layered series of the poe mountain anorthosite southeast wyoming contains numerous anorthosite leucogabbro blocks that constrain density relations during differentiation all blocks are more mafic than their hosting anorthositic cumulates their plagioclase compositions are more calcic and each block is in strong sr isotopic disequilibrium with its host cumulate associated structures disrupted and deformed layering indicate that 1 a floor was present during crystallization and that plagioclase was accumulating and or crystallizing on the floor 2 compositional layering and plagioclase lamination formed directly at the magma crystal pile interface and 3 the upper portions of the crystal pile contained significant amounts of interstitial melt liquid densities are calculated for proposed high al olivine gabbroic parental magmas and fe enriched ferrodioritic and monzodioritic residual magmas of the anorthosites taking into account pressure oxygen fugacity p2o5 estimated volatile contents and variable temperatures of crystallization for all reasonable conditions calculated block densities are greater than those of the associated melt the liquid densities however are greater than those for an40 60 plagioclase which cannot have settled to the floor plagioclase must either have been carried to the floor in relatively dense packets of cooled liquid plus crystals or have crystallized in situ a sloping floor possibly produced by diapiric ascent of relatively light plagioclase rich cumulates is required to allow for draining and removal of the dense interstitial liquid produced in the crystal pile and may be a characteristic feature during the crystallization of many proterozoic anorthosites and layered intrusions

plagioclase an range and residual porosity in igneous cumulates of the kiglapait intrusion the range in an content of plagioclase in grain mounts of igneous cumulates provides a measure of diversity that is uniquely preserved in plagioclase because of its well known refractory nature to a first approximation such data provide when calibrated an estimate of the residual porosity or fraction of trapped liquid in each specimen the ensemble of specimens then provides a model for the stratigraphic variation of residual porosity the raw data however include pre cumulus zoning that can be isolated from in situ zoning by textural analysis in thin section the baseline of residual porosity determinations was earlier determined for the lower zone of the kiglapait intrusion from the content of excluded components in the solid rock compared with their content in the melt as calculated by summation and rayleigh fractionation the baseline equation was then used to calibrate the residual porosity obtained from the an range in grain mounts this calibration is now extended to the remainder of the intrusion the an range and the calculated residual porosity decrease to zero at 99 solidified pcs and then rise to the end of crystallization the data suggest initial porosities smaller than 0 35 allowing for pre cumulus zoning the data suggest a dominance of adcumulates in the intrusion and these impermeable barriers occupy 75 of the rocks in the lower zone they occur at intervals of 1 to rarely 15 m and thereby restrict the likelihood of compaction over thick mushy zones variations in the fo range of olivine are also observed in grain mounts and they follow those in plagioclase however they are in part due to subsolidus equilibration with fe ti oxides and augite the new calibration is successfully applied to the skaergaard intrusion to supplement the published results from excluded components with some interesting contradictions

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

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

origin of extremely pge rich mafic magma system an example from the jinbaoshan ultramafic sill emeishan large igneous province sw china abstract the 260 ma jinbaoshan sill is part of the permian emeishan large igneous province in sw china and is mainly composed of wehrlite and hosts a pge deposit containing 45 tonnes of pt and pd with ore grades ranging from 1 to 5 ppm pt pd the sill is composed of several pge rich horizons at different stratigraphic heights a 5 m thick pge rich horizon at the base of the sill which is underlain by a chromite rich pge poor layer accounts for 44 of the total ore reserve the jinbaoshan sill is rich in pges with ir ranging from 1 7 to 138 ppb ru from 0 9 to 39 ppb rh from 3 1 to 133 ppb and a total pt and pd from 76 to 5371 ppb all the rocks from the sill display primitive mantle normalized chacophile element patterns enriched in pges relative to ni and cu the low cu pd ratios 20 to 1938 of the rocks relative to the primitive mantle cu pd 7000 imply that there was no sulfide fractionation of the magmas the pge enrichment of the sill indicates that it formed from extremely pge rich magmas modeling results show that such pge rich magmas likely resulted from a multistage dissolution upgrading process in an open magma conduit system large amounts of chromite crystallization triggered sulfide saturation when pge rich magmas emplaced into the upper magma chamber of the system settling and sorting of chromite and most sulfide liquids along with olivine resulted in the formation of the largest pge rich horizon at the base of the sill the rest minor sulfide liquid accumulated to form pge rich horizons in the middle part of the sill aided by turbulence and convection within the magma chamber

magmatic lineations inferred from anisotropy of magnetic susceptibility fabrics in units 8 9 and 10 of the rum eastern layered series nw scotland the eastern layered series of the rum layered suite nw scotland comprises a sequence of sixteen 30 150 m thick cyclic units the upper troctolite olivine gabbro parts of each of these units exhibit small scale modal layering and a pervasive layerparallel mineral lamination that is often associated with soft sediment deformation structures a sporadic macroscopic magmatic lineation measurable on mineral lamination surfaces is also observed in places anisotropy of magnetic susceptibility ams fabrics were studied in three of these cyclic units 8 9 and 10 in the northern part of the eastern layered series magnetic fabrics measured in the troctolites and gabbros yield one dominant trend in which magnetic foliations parallel magmatic layering and magnetic lineations trend nw se and plunge gently magnetic fabrics measured for two detailed traverses through unit 10 on the northern side of hallival also yield one dominant trend similar to that measured elsewhere in the eastern layered series however toward the centre of unit 10 in each traverse magnetic lineations sometimes plunge approximately downdip sw on the magnetic foliation planes the implications of these results are discussed with reference to previous textural and fabric observations on rum a model is suggested in which weak linear arrangements of cumulus olivine and plagioclase crystals are developed due to slumping and soft sediment deformation of unconsolidated crystal mushes during central sagging of the rum layered suite 2007 elsevier b v all rights reserved

magma chamber processes in the formation of the low sulphide magmatic au pge mineralization of the platinova reef in the skaergaard intrusion east greenland

the fractional latent heat of crystallizing magmas the fractional latent heat of a crystallizing system is simply the latent heat of fusion divided by the total enthalpy when plotted against temperature this function displays robust pulses with the successive saturation of each incoming crystal phase each pulse endures for tens of degrees corresponding to tens to hundreds of thousands of years in large magma bodies from an original 1963 development by p j wyllie using a synthetic system i show that the liquidus slopes degrees per gram of solid produced decrease discontinuously at the arrival of a new phase and their inverse the crystal productivity undergoes sharp upward pulses at the same time the overall liquidus slope increases continuously interrupted by small downward jumps with the evolution of the multicomponent melt the crystal productivity pulses feed the fractional latent heat pulses which dominate the crystallization history of a melt these elementary relationships govern the near solidus growth of dihedral angles in cumulates as they relate to the liquidus events the latent heat ordinarily approximates to about 80 of the total enthalpy budget but jumps to 100 by definition when solidification occurs by isothermal adcumulus growth and approaches 100 when mafic phases such as augite and fe ti oxides are over produced relative to their equilibrium saturation in layered intrusions the feedback of latent heat to a self regulating cooling history of large magma bodies is deduced here in principle the overall results help to clarify the crystallization history of mafic magmas in particular they support the solidification of floor cumulates by interchange with parent magma and without the help of compaction

evidence for basal marine ice in the filchner ronne ice shelf the filchner ronne ice shelf which drains most of the marine based portions of the west antarctic ice sheet is the largest ice shelf on earth by volume the origin and properties of the ice that constitutes this shelf are poorly understood because a strong reflecting interface within the ice and the diffuse nature of the ice ocean interface make seismic and radio echo sounding data difficult to interpret1 2 ice in the upper part of the shelf is of meteoric origin but it has been proposed2 5 that a basal layer of saline ice accumulates from below here we present the results of an analysis of the physical and chemical characteristics of an ice core drilled almost to the bottom of the ronne ice shelf we observe a change in ice properties at about 150 m depth which we ascribe to a change from meteoric ice to basal marine ice the basal ice is very different from sea ice formed at the ocean surface and we propose a formation mechanism in which ice platelets in the water column accrete to the bottom of the ice shelf

formation of igneous layering in granodiorite by gravity flow a field microstructure and geochemical study of the tuolumne intrusive suite at sawmill canyon california layered granites and granodiorites occur in many plutons they occur together with non layered granodiorites at the margin of the tuolumne intrusive suite one of the largest late cretaceous intrusions in the sierra nevada results from field observations quantitative microstructural analysis and whole rock compositions from the non layered kuna crest half dome and cathedral peak granodiorites and layered granodiorites from the sawmill canyon area are compared to determine which processes contributed to the formation of the layering the non layered granodiorites have foliated but homogeneous microstructures which are attributed to the orientation of crystals with high aspect ratios when the magma flowed before solidification their compositions are consistent with magma mixing and fractional crystallization the microstructure in the layered granodiorite is heterogeneous and shows normal gradation of the dense primocryst minerals magnetite hornblende and titanite at the base and inverse size gradation with large crystals of the less dense primocryst phases plagioclase and biotite at the top and a systematic change in the preferred orientation of minerals from base to top macroscopic structures in the layers indicate erosion and the action of traction currents when the layers formed and the sinking of the dense minerals and rise of interstitial melt immediately afterwards the layered rocks have similar grain size and mineral compositions to the non layered granodiorites and thus could be derived from the same magmas as the kuna crest half dome and cathedral peak non layered granodiorites the major and trace element variations in the layered rocks are very different from those in the non layered rocks and indicate a strong segregation of crystals during formation of the layers however the separation process did not fractionate the main rare earth element bearing phases from one another the macrostructure and microstructural features in the layers resemble those from hyper concentrated sediment gravity flows the layering is interpreted to have formed from gravity flows of melt and crystals initiated by syn magmatic tectonic deformation and seismic shaking two types of layering are identified one called normal layering represents material derived from a single granodiorite magma and deposited from a relatively low energy gravity flow whereas the other called melanocratic layering incorporated mafic minerals eroded from the substrate had the felsic tops of layers eroded off by subsequent flows incorporated crystals from more than one magma source and formed from gravity flows that had higher energy

dynamics of evolving magma chambers textural and chemical evolution of cumulates at the arrival of new liquidus phases abstract basaltic magma chambers are best exemplified by layered intrusions fossilized natural laboratories that historically constrain most fundamental principles of igneous petrology progressive fractional crystallization of basaltic melts in layered intrusions results in successive appearance of 5 10 liquidus minerals most minerals appear in the stratigraphy of layered intrusions via two distinct stages they first emerge as oikocrysts large irregularly shaped and interstitial crystals that form in a crystal liquid framework and then as primocrysts relatively small idiomorphic and cumulus crystals that crystallize from a main magma body the transition from oikocrysts to primocrysts is commonly marked by the following textural and chemical features a a similarity in chemical composition of the last forming oikocrysts and the first forming primocrysts b slow arrival modal overabundance and finer than normal size of the first primocrysts c a low abundance of trapped liquid in rocks with the first forming primocrysts d a step like increase in mineral dihedral angle in rocks slightly below the level of the primocryst arrival and e the rims on plagioclase primocrysts that are identical in composition to cores of plagioclase primocrysts at the first appearance of new liquidus phases higher up in the section the phase equilibria control on the cumulus stratigraphy of layered intrusions require large long lived and largely molten magma chambers and therefore their origin is not compatible with several recent concepts that deny the existence of such reservoirs in the earth s crust we therefore suggest that the systematic changes associated with the arrival of cumulus phases in layered intrusions are best explained by a traditional concept that considers layered intrusions as large initially crystal free melt bodies that gradually lose heat and crystallize from margins inwards through thin solidification fronts the entire magma body during its internal evolution is thought to be kinetically supercooled i e kept slightly below its liquidus temperature with respect to cumulus phases that crystallize at the margins each new cumulus phase arrives on the liquidus with some kinetic delay and nucleates heterogeneously against pre existing crystals at the floor roof and sidewalls of basaltic magma chambers this implies that internal differentiation of basaltic magma chambers mostly occurs through convective separation of evolved liquid from in situ growing crystals and its mixing with the main magma body

proterozoic massif type anorthosites as the archetypes of long lived 100 myr magmatic systems new evidence from the kunene anorthosite complex angola abstract proterozoic massif type anorthosites are voluminous plagioclase dominated plutonic suites with characteristic intermediate compositions an50 10 that represent mantle derived magmas ponded at moho depths and crystallized polybarically until emplacement at mid crustal levels we present new id tims and la icpms u pb ages as well as detailed field observations from the kunene anorthosite complex kac of sw angola our main objective in this work is to contextualize a variety of important petrographic and geochronological features of the kac that have implications for proterozoic anorthosite petrogenesis and the geological evolution of the sw congo craton high precision u pb zircon and baddeleyite ages for anorthositic rocks from across the complex n 11 range between 1376 and 1438 ma a period of 60 myr showing that such systems may be remarkably long lived dating of magmatic zircons from a comagmatic pegmatoidal enclave in the kac which also contains high al orthopyroxene megacrysts haom gives an age of 1500 ma indicating as much as 120 myr between some of the earliest and latest crystallization products of the system these enclaves provide a snapshot of the liquid line of descent of the anorthosites and are compelling evidence for the comagmatic nature of the haom field evidence for block structure magma mingling and extreme textural heterogeneity in the kac supports emplacement as multiple influxes of magma of various volumes carrying variable proportions of crystals and autoliths from below we infer that the kac represents a long lived mushy magmatic system that was likely emplaced in distinct pulses these observations refine our understanding of proterozoic anorthosite petrogenesis they also provide evidence for an array of magmatic processes operating at various levels of crystallinity that may create textural and chemical heterogeneity in evolving magma products at lower to mid crustal levels

pressure conditions for the solidification of the skaergaard intrusion eruption of east greenland flood basalts in less than 300 000 years abstract primary granophyres are differentiated from olivine tholeiitic magma and occur interstitially throughout the cumulus stratigraphy of the skaergaard intrusion east greenland samples from the lower zones a c lza c the middle zone mz and the sandwich horizon sh are included in the present study together with granophyric accumulations in gabbroic pegmatite from lza c fluid inclusions in quartz and the mineral assemblage in the granophyres record the pressure under which the skaergaard intrusion crystallised pegmatitic granophyre from lza c consists mainly of quartz plagioclase an 4 7 and alkali feldspar or 40 80 enclosing an earlier formed assemblage of ferrohastingsitic and ferroedenitic hornblende fayalite fo 4 5 titanite biotite and fluor apatite granophyric quartz albite and alkali feldspar crystallised from water saturated granitic melts near eutectic minimum conditions between 680 and 660 c the pressure of granophyre crystallisation was modelled by the intercept between fluid inclusion isochores and the minimum melt solidus for granitic compositions pressures recalculated to the roof pendant of the intrusion are 0 7 0 5 for lza 2 0 0 2 for lzb c 2 3 0 8 for mz the triple group level and 3 3 1 3 kb for sh amphibole geobarometry independently confirm the pressure estimates for pegmatitic granophyres in lza c the granophyres formed as the intrusion cooled through the minimum melt solidus in lza lzb lzc mz and sh respectively the pressure increase from lza to sh granophyres is explained by progressive burial during cooling of the intrusion and contemporaneous outpouring of 5 3 6 3 2 7 km of flood basalts during the initial opening of the northeast atlantic ocean accordingly the skaergaard intrusion evolved from a subvolcanic magma chamber at emplacement to a more deep seated igneous system during terminal crystallization when the majority of the intercumulus phases formed the present cooling history suggests the intrusion cooled through the temperature of granophyre formation in less than 300 000 years the increase in the pressure of granophyre formation therefore implies that 5 3 6 3 2 7 km of flood basalts were emplaced within this time window

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

the skaergaard pge and gold deposit the result of in situ fractionation sulphide saturation and magma chamber scale precious metal redistribution by immiscible fe rich melt the skaergaard intrusion greenland is the type locality for skaergaard type mineralizations mineralization levels are perfectly concordant with igneous layering up to 5 m thick internally fractionated and contain crystallized sulphide droplets and precious metal alloys sulphides arsenides and telluride immiscible cu rich sulphide droplets formed in a mush zone below the roof scavenged precious metals they were subsequently dissolved and transported to the floor in late formed immiscible fe rich mush melts mineralized stratigraphic intervals of floor gabbro formed in proto macrolayers owing to local sulphide saturation in melt concentrated between floating plagioclase and sinking clinopyroxene the floor mineralization is divided into four stratigraphic sections formation of the lower platinum group element mineralization lpgem involved 1 crystallization of the bulk liquid liquidus paragenesis and in situ fractionation 2 sulphide saturation and formation of sulphide droplets in melt in the upper part of proto macrolayers after further in situ fractionation the following steps occurred 3 the onset of silicate silicate immiscibility and the consequent loss of buoyant and immiscible si rich melt 4 dissolution of unprotected droplets of sulphide melt present in the fe rich mush melt 5 compaction driven upwards loss of residual mush melt enriched in for example au the lpgem preserves upward increasing bulk pd pt 6 13 owing to a continued supply of pge and au with high pd pt the further development of the lpgem ceased as the supply of precious metals to the floor waned the upper pge mineralization upgem subsequently formed from precious metals recycled in the floor the upgem is characterized by increasing au substitution in pge phases and a decrease in total pge and pd pt owing to upward fractionation in migrating mush melts and exhaustion of pd and pt an upper au rich mineralization level uaum was caused by late remobilization of au and deposition on grain boundaries in fully crystallized gabbro cu concentrations 150 ppm are not correlated with pge and au repeated cu mineralization levels cum attaining 1000 ppm occur above the au levels caused by local mush layer sulphide saturation pge au and cu distributions in the floor mineralization reflect sub liquidus but supra solidus processes and reactions in mushes at the roof wall and floor constraints provided by a new model for the mineralization provide the basis for re evaluation of the solidification processes in the skaergaard intrusion we have identified the importance of extensive in situ fractionation and intrusion wide elemental redistributions in immiscible fe and si rich silicate melts our model characterizes the floor cumulates as bulk liquid orthocumulates containing an upwards increasing proportion crystallized from fe rich immiscible mush melt the roof rocks are complementary to the floor with downwards increasing proportions crystallized from the conjugate si rich melt petrographic observations and the relative timing of crystallization support the hypothesis that crystallization was restricted to marginal mush zones bulk melt remaining in the magma chamber evolved not as generally assumed as a result of loss of crystals grown from the bulk melt but as the consequence of mixing with recycled and evolved melt expelled from the mush by compaction redistribution of fe in immiscible melts may be common to mafic intrusions and puts into question the validity of petrogenetic modelling of bulk liquids in mafic intrusions based only on consideration of floor cumulates

postmagmatic sulphur loss in the skaergaard intrusion implications for the formation of the platinova reef abstract the stratabound platinova au pd reef in the skaergaard intrusion is intimately linked to cu fe sulphide minerals and occurs at a level that shows changes in the sulphur concentration and the cu s ratio these features suggest that the reef is related to silicate sulphide liquid immiscibility in the skaergaard magma however although the evidence is strong present day sulphur concentrations are too low to support such a model the detailed knowledge of the shape volume and compositional relations of the skaergaard lithologies allows for numerical modeling of sulphur and copper concentrations during fractionation of the magma these data can be evaluated with the current models for sulphur saturation in iron rich mafic magmas to assess the original sulphur concentrations the extent of fractionation the composition of sulphide liquids and the extent of postmagmatic sulphur loss mass balance modeling yields initial copper and sulphur concentrations of 250 and 894 ppm for the parental magma to the lower zone a prior to sulphur saturation cu behaves as an incompatible element with a bulk partition coefficient in the cumulus material dcumuluscu of 0 13 indicating some incorporation of cu into mafic silicates probably augite sulphur can be assumed to be perfectly incompatible in the cumulus minerals although it will be included as a component of the interstitial liquid these values yield sulphur saturation at around 635 ppm cu and 2607 ppm s assuming the sulphur to originally be present as a mixture of monosulphide mss and intermediate solid solution sulphide iss the primary sulphides in the cumulates would amount to 0 07 modal through lower and middle zones and 0 53 modal on average through upper zone the expected weight proportion of primary chalcopyrite pyrrhotite evolves from 30 70 at the base of lza through 26 74 at the level of the platinova reef to 18 82 at the sandwich horizon the present day bornite magnetite mineral assemblage is likely to have been derived from a primary pyrrhotite chalcopyrite assemblage through reequilibration and postmagmatic oxidation significant sulphur appears to have been lost from the lower and middle parts of the layered series lower zone a to upper zone b whereas the sulphides of the upper zone c can be explained in the context of a sulphur gain local scale reequilibration of cu from mafic silicates into sulphides may have contributed to the sulphide mineralogy of the lower and middle zones the structure of the platinova reef probably formed from an original single stratabound reef sulphur loss through oxidation in upward migrating deuteric or hydrothermal fluids could have caused dissolution and reprecipitation of pd and au in a cumulate with several stratiform redox barriers this process can thus explain the successive pd peaks of the reef as well as the variable stratigraphic offset of pd and au across the intrusion