metasomatic replacement in the skaergaard intrusion east greenland preliminary observations abstract several unusual types of replacement structures in the layered series and marginal border series of the skaergaard intrusion indicate that metasomatic changes have altered the modal proportions and in places the mineral composition of the gabbro the geometrical forms and compositional patterns of these features indicate that they result from reactive fluids moving through the rocks at high temperatures shortly after the magma had reached an advanced stage of crystallization the principal effect of metasomatism has been to produce strongly contrasting mafic and felsic rocks

a double magmatic heat pump at the core mantle boundary gigantic silicate magma chambers have been postulated to account for seismically inferred ultra low velocity zones at the base of the mantle acting to modulate heat transport out of the core and into plumes in the core rising plumes of liquid metal and light elements pump the latent heat outward from the inner core boundary by compositional convection the outer core thus pumps heat to the core mantle boundary cmb and the giant magma chamber pumps heat into the mantle to act as a plume source magmatic action at the cmb depends critically on the balance between melt composition compressibility and temperature a stably resident magma above the cmb must be denser than the mantle crystals mostly silicate perovskite pv and magnesiowustite mw dense melts should form at the cmb where they must be saturated with fe and feo from the core given normal mg fe partitioning mw will be denser than pv and melts from pv mw iron liq should also be intrinsically dense relative to the bulk crystal assemblage such melts may become superheated to overcome the compositional density and rise upward in the magma chamber assisted by turbulent mixing cooled melt sheds dense metal and metal oxide crystals and growth of these crystals releases light solute that quickly transports heat upward toward the chamber roof where it melts the mantle near the roof dense melt grows from the heat transported upward by compositional convection and sinks excess heat and occasionally some melt escapes to feed plumes in the mantle cmb magmas must be among the most fe rich in the earth

a monolith sampler for saltmarsh sediments a method is described for the collection of unconsolidated salt marsh sediment resulting in no compaction and minimal disturbance to the sample the monolith sampler consists of an acrylic casing of optional length with a wide flat base and short right angled sides that is inserted into a cleaned vertical face use of this inexpensive technique allows observation of any structures present through the sediment record and provides sufficient material for various high resolution geochemical and sedimentological analyses

experimental study of a kiglapait marginal rock and implications for trace element partitioning in layered intrusions abstract experiments on a chilled marginal rock from the kiglapait intrusion labrador have been performed in order to constrain the partitioning behaviour of major and trace elements during formation of kiglapait lower zone lz cumulates at 0 1 mpa and f o 2 close to fmq the liquidus is bracketed between 1225 and 1212 c and the only liquidus phases are plagioclase an 73 74 and olivine fo 82 85 in contrast lz troctolites contain fo 67 5 olivine and an 63 4 plagioclase plagioclase melt partition coefficients d s for li ti rb sr y zr ba la and eu determined by ion microprobe analysis of run products are similar to those for other basaltic systems but differ systematically from apparent d s calculated from bulk analyses of lz plagioclases and modelled liquid compositions obtained by summation the discrepancies in d s and phase compositions are inconsistent with diffusion controlled boundary layer growth crystallisation of trapped intercumulus liquid is quantitatively plausible however a the volume of liquid required to reconcile experimental and model d s 20 1 is larger than that estimated from observed proportions of intercumulus phases 11 2 and b this mechanism alone cannot reduce x an in lz plagioclases due to very slow al si interdiffusion the discrepancy between experiment and model is best resolved if the parental melt to lz cumulates was more evolved higher fe mg na ca ree and alkalis lower sr than the summed bulk composition in accord with a recent revision of the kiglapait fe mg based on olivine compositions

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 layered series part iii non dynamic layering layering in the skaergaard intrusion has been divided into two introduction general types one produced by magmatic flow and another by recent studies of the structural and textural features of processes resulting from variations of rates of nucleation and crysthe skaergaard intrusion mcbirney nicolas 1997 tallization and in the case of the layered series by compactionhave distinguished two broad types of layering one related processes modal variations caused by shifts of cotectic produced by the dynamic effects of magmatic flow and proportions produce thick layers which in the layered and upper another by processes that operate in situ such as varied border series are diffuse and normally lack strong foliation and nucleation and growth of crystals recrystallization or by lineation in the marginal border series the layers are thinner and compaction related mechanisms we refer to layering sharper possibly because the rate of accumulation was slower formed by these latter processes as non dynamic to oscillatory nucleation may have played a role in producing fineemphasize that it is not the result of fluid dynamic scale cyclic layers but it was less important than solution and processes although most layering combines elements of reprecipitation during slow cooling and ostwald ripening evidence more than one process the contribution of each mechfor compaction is found in deformed plagioclase laths and a relative anism can usually be recognized from its distinctive form deficiency of incompatible elements in rocks formed on the floor and setting layering related to compaction becomes sharper with increasing the distinguishing features of dynamic magmatic layheight in the layered series until it suddenly disappears above the ering have been described in part ii of this series mcbirtrough horizon near the base of upper zone b mechanical sorting ney nicolas 1997 they are best seen near the steep during compaction may have produced crude layering but if it did margins of the layered series in what wager brown the evidence has long since been destroyed by the superimposed effects 1968 referred to as the cross bedded zone where the of solution and reprecipitation when interstitial liquid rose through layering is disrupted by slumping and channeling and the overlying crystals and re equilibrated with them numerical the rocks have a marked foliation and lineation simulations illustrate how small differences of surface energy caused layering not associated with magmatic flow takes a by variations of grain size textural dependence of solubility and variety of forms but we can distinguish two general pressure solution can cause segregation of minerals into layers during end members each of which has a distinct origin and solution and reprecipitation characteristics the first results from varied rates of

trapped intercumulus liquid in the main zone of the eastern bushveld complex south africa lateral variations in the amount of trapped intercumulus liquid in the main zone of the eastern bushveld complex are constrained by new sr isotopic whole rock and mineralogical data from three profiles that are separated laterally by ca 100 km and represent thicknesses of 551 1 127 m of main zone gabbronorites below the pyroxenite marker an analysis of the an contents 100 ca ca na of plagioclase cores within the thornhill north roossenekal and stoffberg south profiles show similar systematic up section variations from an66 to an59 in contrast both the an contents of bulk plagioclase separates and the mg numbers 100 mg mg fet of orthopyroxenes show pronounced lateral variations from thornhill an67 61 mg 67 61 through roossenekal an64 58 mg 64 55 to stoffberg an59 55 mg 59 50 these mineralogical variations are interpreted to be the result of reaction between cumulus minerals and an increasing amount of trapped liquid from north to south modelling of the trapped liquid shift of orthopyroxene compositions suggests that the amount of trapped liquid in the cumulates increased from near 0 at thornhill through 10 30 at roossenekal to 30 45 at stoffberg a two to eightfold southward increase in whole rock concentrations of p ti y zr and ba is consistent with the trapped liquid model however the 14 fold increase in rb from thornhill to stoffberg is too great to be accounted for by trapped liquid alone but can be explained by local assimilation of partial melts of the country rock constant initial 87sr 86sr isotopic ratios of main zone plagioclase separates 0 7081 0 7085 in all the three profiles do not preclude assimilation of adjacent basaltic to rhyolitic country rock with initial ratios between 0 6924 0 7096 the southward increase in the amount of trapped liquid is ascribed to an increased cooling rate by enhanced heat loss and partial assimilation of country rock xenoliths in the distal cumulate sequence at stoffberg

abundance and distribution of fluoride concentrations in groundwater la ballenera catchment southeast of buenos aires province argentina high concentrations of fluoride have negative effects on human health and it is one of the most widespread contaminants in groundwater having being found in high concentrations in different countries over the entire world fluoride concentrations exceeding the drinking water levels are very frequent in the region known as pampa plain in argentina associated with loess like sediments bearing volcanic shards and calcium carbonate concretions in this study the behavior of fluoride in the catchment of la ballenera creek is performed to contribute to the understanding of the processes controlling its distribution at catchment scale the catchment is around 100 km2 mostly in a flat landscape outflowing into the atlantic ocean the concentration of major ions silica and fluoride was measured on 34 groundwater and 9 surface water samples obtained in october 2013 all the samples both streamwater and groundwater were sodium bicarbonate type with an average ph of 8 and 7 3 respectively the fluoride concentrations in groundwater ranged between 1 1 and 2 5 mg l with an average value of 1 9 mg l fluoride concentrations in streamwater were close to groundwater concentrations indicating the predominance of baseflow on surface water composition the variations in the contents of fluoride are due to two main factors one geological geomorphological and the other geochemical the first one is related to the dominance of loess like sediments formed by minerals bearing fluoride and the second one is related to the control resulting on calcite precipitation adsorbing fluoride ions

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

adcumulate mafic dykes in layered intrusions a case study of a late stage dyke in the bayantsagaan layered intrusion mongolia we have discovered an adcumulate late stage dyke in the bayantsagaan layered intrusion the 11 cm thick dyke is composed of fresh troctolite with no signs of chilling against the host leucotroctolite texturally both the dyke and its host are medium grained plagioclase olivine magnetite cumulates the dyke is however finer grained and contains less interstitial material two geochemical features characterize the dyke it is compositionally more evolved than the host as indicated by its lower real and normative an content in plagioclase whole rock cr and mg number and it is highly depleted in all incompatible components e g k 2 o y and rare earth elements or rees that have much lower concentrations than in the host the depletion in rees is extreme and has not been reported earlier for any troctolitic rocks it has an internal reverse zonation with an inwards increase in compatible mgo and tio 2 and a decrease in all incompatible elements suggesting an inwards decrease in the amount of trapped melt we interpret this late stage dyke as having formed from a residual melt that was channelled along a fissure in solidifying cumulates the flow of the melt resulted in the very efficient removal of a boundary layer of evolved liquid from crystals growing on the walls of the dyke this resulted in an almost perfect adcumulate that is extremely depleted in incompatible components

relationship between electrical conductivity 18o of water and no3 content in different streamflow stages the ballenera creek has 160 km2 being a small catchment in the pampa plain in argentina this area has been deeply modified by human action through agricultural activities from 2013 to 2017 electrical conductivity stable isotopes of water and nitrate concentration monitoring program were conducted the sampling included weekly and bimonthly samples in two sites along the stream several groundwater wells and monthly precipitation chemical and isotopic tracers are used to discriminate the streamflow components and to evaluate their incidence in the nitrate concentration the easiest conceptual model for gaining streams contemplates two main elements direct runoff and groundwater baseflow and pre event water the direct runoff has the lowest electrical conductivity and 18ow variable content the baseflow component is characterized by the highest electrical conductivity and isotope composition quite constant finally pre event water has an intermediate electrical conductivity and isotopic content close to the rainfall weighted average composition the nitrate concentration obtained was in general related to the different stream stages and was a useful indicator to evaluate the fertilization in agricultural zones

petrology and mineral compositions of the middle banded series of the stillwater complex montana the two olivine beating zones of the middle banded series of the stillwater complex are characterized by an increase in the number of cumulus minerals with height in each anorthosite and anorthositic troctolite dominate the lower part whereas olivine gabbro and gabbronorite form much of the upper portions electron microprobe analyses of cumulus minerals indicate little or no variation of average mineral compositions with height in addition no significant lateral variations in cumulus mineral compositions occur along 8 km of section plagioclase from throughout these zones shows the complex reverse and oscillatory zpnation patterns also seen in plagioclase from the thick 500 m anorthosites that sandwich the zones the data suggest that the entire middle banded series is genetically related and therefore requires models for the origin of the thick anorthosites to also explain the olivine bearing rocks between them however textural features such as discordant troctolites pegmatoids slump structures and variably developed mineral laminations and chemical features such as zpnation in clinopyroxene produced by intergranular exchange with orthopyroxene and large within sample variations in the mg number of olivine in lowolivine troctolites indicate that significant modification of these rocks by postcumulus processes has taken place thereby obscuring evidence of their genesis

dynamic accretion beneath a slow spreading ridge segment iodp hole 1473a and the atlantis bank oceanic core complex 809 deep iodp hole u1473a at atlantis bank swir is 2 2 km from 1 508 m hole 735b and 1 4 from 158 m hole 1105a with mapping it provides the first 3 d view of the upper levels of a 660 km 2 lower crustal batholith it is laterally and vertically zoned representing a complex interplay of cyclic intrusion and ongoing deformation with kilometer scale upward and lateral migration of interstial melt transform wall dives over the gabbro peridotite contact found only evolved gabbro intruded directly into the mantle near the transform there was no high level melt lens rather the gabbros crystallized at depth and then emplaced into the zone of diking by diapiric rise of a crystal mush followed by crystal plastic deformation and faulting the residues to mass balance the crust to a parent melt composition lie at depth below the center of the massif likely near the crust mantle boundary thus basalts erupted to the seafloor from 1 550 mbsf by contrast the mid atlantic ridge lower crust drilled at 23 n and at atlantis massif experienced little high temperature deformation and limited late stage melt transport they contain primitive cumulates and represent direct intrusion storage and crystallization of parental morb in thinner crust below the dike gabbro transition the strong asymmetric spreading of the swir to the south was due to fault capture with the northern rift valley wall faults cutoff by a detachment fault that extended across most of the zone of intrusion this caused rapid migration of the plate boundary to the north while the large majority of the lower crust to spread south unroofing atlantis bank and uplifting it into the rift mountains

kiglapait mineralogy iii olivine compositions and rayleigh fractionation models olivints present throughout the layered cumulates of the kiglapait intrusion record extreme iron enrichment during fractional crystallization mn is less compatible than fe the exchange coefficient kd for mn fe ol liq has values near 0 95 but they drop to 0 7 near the lower one upper one boundary ca is depleted by more than a factor of 30 relative to experimental values at 5 kbar pressure attempts to find a sink for ca in the lower zone without invoking liquid as a reactant are unconvincing and questions of metastability arise cumulus olivines vary systematically to lower fo contents with stratigraphic height using equilibrium values for the exchange coefficient kd fe mg ol ljq j and the observed mode of olivine and residual porosity a rayleigh fractionation calculation reproduces the observed lower zone trend without recourse to multiple injections of magma an anomalously fe rich region above the main ore band in the upper zone ascribed to ponding of fe rich magma at the floor of the intrusion until later buoyant residual magma entrained the fe rich residua such fe rich ponds probably account for the abnormal fe enrichment shown by some other fenner trend layered intrusions summation over the observed crystal compositions yields values of xjug that do not represent the equilibrium liquid but that reduce to it by a simple multiplier p 0 78 this result means that if ko is known the high temperature liquidus olivine compositions can be retrieved or vice versa because the observed olivine compositions are supported by realistic forward models there is no need to invoke evolved boundary layer liquids to explain the in situ crystallization of kiglapait cumulates classical cumulate theory suffices

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

multiple merensky reef of the bushveld complex south africa the merensky reef mr of the bushveld complex is known for containing the lion s share of the world s reserves of platinum group elements pge it is commonly described as a single relatively thin layer or package of chromite and sulphide bearing orthopyroxenite hosted by almost barren norite and or anorthosite here we present a drill core section that contains however four separate mr orthopyroxenite layers hosted by mottled anorthosites each of these orthopyroxenites is distinguished through occurrence absence of marginal chromitite seams grain size petrographic composition and pge abundance we interpret them as multiple undercutting mr that occurs as sill like apophyses extending laterally from a pothole margin into the footwall the undercutting mr is attributed to in situ crystallization within sheet like melt filled cavities at potholes margins which are produced through thermal and or chemical erosion of the footwall rocks by replenishing magmas this finding indicates that the mr forming event involved several distinct pulses of replenishing magmas whose crystallization products were mostly eroded away in the stratiform portions but remained locally preserved in apophyses within footwall rocks the mineralized mr formed a few meters below a temporary chamber floor is indicative of strong thermal compositional convection in the basal layer of the bushveld s chamber the convection was able to deliver large volumes of fresh magma for the scavenging of pge by sulphide and chromite growing within the sheet like cavities of the mr potholes

adcumulus dunite growth in a laboratory thermal gradient laboratory experiments near 1450 c at 1 bar qfm on komatiite bulk composition show olivine and liquid in cumulus textures which evolve with experiment duration orthocumulus texture with settled olivine crystals separated by liquid matrix is developed within a day experiments quenched after a few days to a week show a progression of textures which include development of columns of olivine crystals separated by channels of liquid olivine grain sizes increase with the cube root of time suggesting that dissolution and reprecipitation of olivine may be involved in the organization into columns and channels experiments quenched after two weeks have well developed adcumulus texture the basal polycrystalline granular olivine aggregate forms from the decay of the olivine columns melt expulsion from the aggregate can be virtually complete leaving 1 or less of the melt originally present

layered intrusions and traffic jams a wide range of explanations has been proposed for the origin of repetitive layering in mafic ultramafic and in per alkaline intrusions here we propose that the interaction of mineral grains that sink and float in the crystallizing magma is an alternative mechanism that can explain many of the features of layered intrusions without the need to invoke extrinsic factors similar to traffic jams on a motorway small perturbations in crystal density develop that impede further ascent or descent of buoyant or heavy minerals respectively these traffic jams separate layers of magma from the rest of the magma chamber the magma in the individual layers further evolves as a largely independent subsystem with gravitational sorting organizing the mineral distribution within each layer layering can develop in the intermediate range between full mineral separation in low viscosity or slowly cooling magma chambers and homogeneous crystallization in high viscosity or fast cooling chambers this self organization mechanism provides a novel explanation for the formation of rhythmic layering in low viscosity magmas for example in the ilimaussaq igneous complex in southwest greenland

noble metal enrichment processes in the merensky reef bushveld complex we have analysed sulphides silicates and chromites of the merensky introduction reef for platinum group elements pges re and au using laser the enrichment of noble metals in layered gabbroic ablation inductively coupled plasma mass spectrometry and synthetic intrusions to form stratiform merensky type ore horizons pyrrhotite standards annealed with known quantities of noble metals is an extremely efficient yet poorly understood process os ir and ru reside in solid solution in pyrrhotite and pentlandite metal enrichment must be related to the formation of rh and part of the reef s pd in pentlandite whereas pt au re magmatic layering as the ore horizons form an integral and some pd form discrete phases olivine and chromite often part of the magmatic stratigraphies of their host insuspected to carry os ir and ru are pge free all phases analysed trusions metal concentration is also related to the prescontain noble metals as discrete micro inclusions with diameters ence of magmatic sulphides because the noble metals are either dissolved in base metal sulphides or occur as typically 100 nm inclusions in sulphides commonly have the discrete noble mineral phases intergrown with sulphides element combinations os ir pt and pt pd au inclusions in vermaak hendriks 1976 ballhaus ryan 1995 olivine and chromite are dominated by pt au pd few inclusion ballhaus ulmer 1995 spectra can be related to discrete noble metal phases and few there is considerable controversy as to where the inclusions have formed by sub solidus exsolution rather some metals came from and how metal enrichment worked pge inclusions notably those in olivine and chromite are earlyfor a stratiform platinum group element pge deposit magmatic nuggets trapped when their host phases crystallized we inside the magmatic stratigraphy of a layered intrusion suggest that the silicate melt layer that preceded the merensky reef there are two possibilities either the metals were derived was pge oversaturated at early cumulus times experiments from the melt column above the ore horizon or they came combined with available sulphide silicate partition coefficients sugfrom the magma equivalent to the cumulate package gest that a silicate melt in equilibrium with a sulphide melt underlying the ore owing to the large density contrast containing the pge spectrum of the merensky ore would indeed be between silicate and sulphide melt a popular view is that oversaturated with respect to the least soluble noble metals sulphide the metals were enriched by gravitational settling of melt apparently played little role in enriching the noble metals in sulphide melt i e out of the magma column above the the merensky reef rather its role was to immobilize a pre existing ore campbell et al 1983 barnes naldrett 1985 in situ stratiform pge anomaly in the liquid stratified magma naldrett et al 1987 on the other hand it is well chamber established that gravitational crystal settling plays little role in producing magmatic layering and magmatic stratigraphies mcbirney noyes 1979 morse 1986 campbell 1987 and this cannot remain without consequence for stratiform noble metal deposits a third alternative is noble metal enrichment by late

near surface and deep groundwaters abstract groundwater occurs in all climatic zones and differs in hydrodynamic isotopic and chemical terms according to its activity within the water cycle and its interaction with rocks among the many chemical components in groundwaters tritium concentrations allow differentiation between groundwater of high and low mobility and thus permit the distinction between shallow and deep groundwaters in germany shallow groundwaters range in depth between 10 and 100 m and account for more than 85 of groundwater recharge deep groundwaters in contrast participate in the water cycle only at intervals of hundreds or thousands of years and reach depths of several hundred metres both types of groundwater overlie connate waters that have been isolated from the water cycle since the accumulation of the sediments in which they occur the exploitation of deep groundwaters mostly causes long term non steady state conditions large scale changes of flow patterns of the groundwaters and hydraulic short circuits to shallow aquifers thus persistent contaminants may penetrate extraction wells in deep groundwaters even if the wells are believed to be well protected against pollution in deep groundwaters it is difficult to apply the conventional use of groundwater protection zones to prevent this kind of intrusion of contaminants

a three increase model for the origin of the marginal reversal of the koitelainen layered intrusion finland to gain insights into the long standing problem of marginal reversals we have studied in detail the 200 m thick marginal zone of the 3 km thick mafic koitelainen layered intrusion kli this marginal zone consists of pigeonite gabbros that are chilled against gneisses of the underlying basement but reveals a non chilled upper contact with orthopyroxenites of the overlying layered series there is a sharp textural and compositional break between the rocks of the marginal zone and the layered series of the kli the marginal zone is remarkable in showing systematic reverse fractionation trends from the bottom to the very top these are exemplified by a significant upward increase in whole rock and pyroxene mg number along with normative and actual an content of plagioclase as well as by an upward decrease in all incompatible components e g tio2 zr y ree this is accompanied by a systematic upward decrease in ratios of highly incompatible elements such as zr yand la yb and in initial whole rock end the uppermost rocks of the marginal zone are very fine grained and show well developed plagioclase lamination and ultra depletion in all incompatible elements e g zr1 4 0 5 ppm ce1 4 0 11ppm the marginal zone is interpreted as having developed by in situ crystallization of magma in a slowly opening fissure the origin of reverse compositional trends is attributed to three principal factors which we refer to as the threeincrease model 1 an increase in compositional primitivity of the magma that gradually filled the chamber 2 an increase in the degree of chemical equilibrium among phases associated with increasing distance from the cold country rocks 3 an increase in the proportion of cumulus minerals in response to more effective removal of evolved liquid from in situ growing crystals with distance from the intrusive contact the marginal reversal of the kli is distinct from typical reversals in layered intrusions in that its development was interrupted by the emplacement of a large volume of new magma that was parental to the overlying layered series this new hot magma caused significant textural and compositional reconstitution of the uppermost gabbroic rocks of the marginal reversal by their partial melting compaction and recrystallization this gave rise to a considerable decrease in grain size appearance of plagioclase lamination and ultra depletion in highly incompatible elements the anomalous nature of these rocks is thus a manifestation of their restitic rather than primary magmatic origin the proposed three increase model may represent a general explanation for the origin of marginal reversals in many mafic sills and layered intrusions