The differentiation of the Skaergaard intrusion

Conclusions We find no support for the claim that the Skaergaard magma followed the trend of common tholeiitic volcanic magmas, such as those of Iceland and the Scottish Tertiary. The end product of differentiation was not a large mass of rhyolite but an iron-rich, silica-poor liquid not unlike that deduced by Wager in 1960.The proposal that a large mass of rhyolitic liquid occupied the upper levels of the intrusion finds no support in the field. The thick series of ferrogabbos, which became richer in iron and poorer in silica until they reached a field of immiscibility cannot be reconciled with crystallization of a large mass of felsic magma. Mass-balance calculations that indicate otherwise are invalid, because they fail to take into account large volumes of rocks that differ in composition from those assumed in the calculations.While ignoring the existence of major units of the intrusion, Hunter and Sparks propose that lavas in Scotland and Iceland are more relevant to the liquid compositions than rocks that are intimately associated with the intrusion. Their argument that the Skaergaard Intrusion followed a trend of silica enrichment that is universal to tholeiitic magmas is based on an incomplete knowledge of the rocks and faulty calculations of mass-balance relations.We agree that much remains to be learned about the Skaergaard Intrusion and the basic mechanisms of magmatic differentiation. In this case, however, we are ready to hang our case on well-established field relations and a mass of laboratory data for what must be the most intensely studied body of rock on Earth.     

Atmospheric and juvenile noble gases in the Skaergaard layered igneous intrusion

Gabbro and diorite from the Skaergaard layered igneous intrusion contain noble gases which are mixtures of atmospheric and juvenile components. Atmospheric noble gases predominate in samples that have undergone extensive oxygen isotope exchange with meteoric-hydrothermal water. The source of the atmospheric noble gas component is inferred to be the hydrothermal circulation system. A juvenile component with ${}^{40}\text{Ar}{}^{36}\text{Ar}\text{≥ 6100}$ and containing fission xenon is also present This component predominates in samples showing unaltered magmatic oxygen isotope compositions. Neon of atmospheric isotopic composition is associated with the juvenile radiogenic ⁴⁰Ar and fission xenon. The source of this second noble gas component may be either the crustal country rock or the upper mantle. If the neon is juvenile primordial neon from a mantle source region, terrestrial primordial ${}^{20}\text{Ne}{}^{22}\text{Ne}$ is the same as atmospheric to within 4%. However, subduction of atmospheric noble gases into the upper mantle may provide an alternate source of neon and other noble gases in the mantle.     

Iron in plagioclase as a monitor of the differentiation of the Skaergaard intrusion

Several recent publications suggest that the appearance of Fe-Ti oxides terminates iron enrichment and starts pronounced silica enrichment (the Bowen trend) during the differentiation of tholeiitic basalt. However, this does not appear to hold for the Skaergaard intrusion. New data from a ∼950 m long drill core (90–22) through its Upper Zone reveal that: (1) iron in plagioclase increases from ∼0.25 to ∼0.45 wt% FeO_T with fractionation of evolved oxide ferrodiorites (An_46-32) and (2) the evolving liquid, which is modelled by incremental bulk-rock summation, increased its iron content from 20.1 to 26.5 wt% FeO_T and its silica content from 47.4 to 49.6 wt% SiO₂ with fractional crystallisation (the Fenner trend). Positive correlation between modelled iron-content of the magmas, and measured iron-content of plagioclase, confirms that iron enrichment is petrologically feasible even with Fe-Ti oxides in the fractionating assemblage. As suggested by previous authors, fractional crystallisation closed to oxygen exchange is the likely reason why some layered intrusions diverge from the Bowen mechanism of differentiation. It is emphasised that both trends seem to exist in nature. Received:13 May 1996 / Accepted:5 January 1997     

The Skaergaard intrusion: from icon to precious metal deposit

Ever since Wager and Deer's classic memoir in 1939 the Skaergaard intrusion has been (or should have been) a central part of every student's education in igneous petrology. Seventy years after the initial fieldwork it is still a major target for academic research, seeking to understand the processes of magmatic diversification. Consequently, a huge literature has accumulated on the subject. Recently, attention has turned to the Skaergaard as a precious metal exploration target.     

Methane-bearing,aqueous, saline solutions in the Skaergaard intrusion,east Greenland

Solutions of H₂O–NaCl–CH₄ occur in fluid inclusions enclosed by quartz, apatite and feldspar from gabbroic pegmatitites, anorthositic structures and intercumulus minerals within the Skaergaard intrusion. The majority of the fluid inclusions resemble 10 m diameter sub-to euhedral negative crystals. A vapour phase and a liquid phase are visible at room temperature, solids are normally absent. The salinity of the fluids ranges from 17.5 to 22.8 wt.% NaCl. CH₄, which comprises less than six mole percent of the solution, was detected in the vapour phase of the fluid inclusions with Raman microprobe analysis. Homogenization of the fluid inclusions occurred in the liquid phase in the majority of the fluid inclusions, though 10% of the inclusions homogenized in the gas phase. Thermodynamic consideration of the stability of feldspars + quartz, and the C–O–H system, indicates that the solutions were trapped at temperatures between 655 and 770°C, at oxygen fugacities between 1.5 and 2.0 log units below the QFM oxygen buffer. Textural evidence and the composition of the solutions suggest that the fluids coexisted with late-magmatic intercumulus melts and the melts which formed gabbroic pegmatites. These solutions are thought to have contributed to late-magmatic metasomatism of the primocryst assemblages of the Skaergaard intrusion.     

Electron-probe studies of the earlier pyroxenes and olivines from the Skaergaard intrusion,East Greenland

Pyroxenes and olivines from the earlier stages of fractionation of the Skaergaard intrusion (Wager and Brown, 1968; Brown, 1957) have been studied using the electron microprobe. The subsolidus trend for both Ca-rich and Ca-poor pyroxenes has been established, from the Mg-rich portion of the quadrilateral to the Hed-Fs join, together with the orientations of the tie-lines joining coexisting pyroxenes. For the Mg-rich Ca-poor pyroxenes, Brown's (1957) solidus trend has been modified slightly. From a study of a previously undescribed drill core, reversals in the cryptic layering have been found in the Lower Zone. The reversals are attributed to existence within the convecting magma chamber of local temperature differences. The Skaergaard magma temperatures are postulated to have passed out of the orthopyroxene stability field into the pigeonite stability field at EnFs ratios of 7228, for Ca-free calculated compositions, and specimen 1849, a perpendicular-feldspar rock, is interpreted as straddling the orthopyroxene-pigeonite transition interval. The cessation of crystallisation of Ca-poor pyroxene and the increase in Wo content of the Ca-rich pyroxene trend have been reexamined, and Muir's (1954) peritectic reaction (pigeonite+liquid=augite) has been confirmed. The composition at which Ca-poor pyroxene starts reacting with the liquid is postulated as Wo₁₀ En_36.7Fs_{53 3}. It is suggested that the cessation of crystallisation of Ca-poor pyroxene is sensitive to the amount of plagioclase crystallising from the liquid.A complete series of accurate olivine compositions for the whole Skaergaard sequence is presented for the first time, including the compositions of the Middle Zone olivine reaction rims.     

Major and trace element composition of Skaergaard plagioclase; geochemical evidence for changes in magma dynamics during the final stage of crystallization of the Skaergaard intrusion

Plagioclase separates from the Layered Series (LS), Upper Border Series (UBS), and Marginal Border Series (MBS) of the Skaergaard intrusion were analyzed to examine major and trace element variations. In general, plagioclase from the LS, UBS, and MBS show similar trends in major elements vs. crystallization: SiO₂, Na₂O, and K₂O progressively increase, and CaO and MgO progressively decrease with fractionation. No abrupt changes in the trends of major components of Skaergaard plagioclase during the differentiation of the intrusion are observed. Trace elements in plagioclase reflect changes in the Skaergaard magma and changes in plagioclase distribution coefficients with differentiation. Sr, Ga, and probably Ba are included elements in Skaergaard plagioclase, but were excluded from the other cumulus phases, and as a result systematically increased in the magma and plagioclase during differentiation. Be, Cs, Hf, Rb, Ta, U, and Zr, and the transition metals Co, Cr, Cu, Ni, Sc, V, and Zn were excluded elements in Skaergaard plagioclase, and remained low in plagioclase during differentiation. Changes in the abundances of these elements in plagioclase during differentiation reflect changes in their abundance in the magma. With the exception of the lower zone, which is enriched in the light rare earth elements, rare earth elements in LS plagioclase, in general, increase with differentiation of the Skaergaard intrusion, but decrease dramatically at the UZa/UZb boundary where abundant apatite first appears. Rare earth elements in UBS plagioclase followed a similar trend to LS plagioclase, except during the initial and final stages of differentiation. UBS plagioclase is much more enriched in rare earth elements during the final 20% of crystallization, except for Eu, which is similar in plagioclase from the two series. The observed trends suggest that the floor and roof sequences became isolated from each other and that the floor sequence may have been more reducing and the roof sequence more oxidizing during the final 20% of crystallization. As the Skaergaard magma ceased convection, or convected as isolated cells, during the final stages of differentiation, volatile elements may have accumulated in the UBS magma, resulting in an increase in ƒO₂, and a decrease in Eu/Sm in UBS plagioclase. The observed trends of rare earth elements in plagioclase from the LS and UBS fit well with theoretical calculations that assume closed-system crystallization, and would be difficult to reconcile with any model requiring significant discharge of magma from the chamber during the final 20% of crystallization. The enrichment of light rare earth elements in plagioclase, suggests that the lower part of the intrusion re-equilibrated with a late, light rare earth element-rich fluid or melt. The recharge model proposed by earlier workers to explain anomalous Sr and Nd isotopes appears unlikely in light of the two to fourfold enrichment of light rare earth elements in these samples. Received: 1 October 1999 / Accepted: 14 May 2000     

Modeling incompatible trace-element abundances in plagioclase in the Skaergaard intrusion using the trapped liquid shift effect

Incompatible trace-element abundances in minerals and whole rocks from layered intrusions have been used to model the fractionation processes and evolving liquid compositions. Many such models assume that the analyzed concentration in a mineral represents that of the mineral when it first crystallized. However, overgrowth from residual liquid and subsequent diffusive equilibration can result in significant changes to the bulk mineral compositions (the more incompatible the element the more dramatic the subsequent changes). The proportion of that residual liquid relative to the cumulus minerals is the most important parameter in determining the magnitude of this effect (trapped liquid shift effect). Calculations involving Ba and La contents in plagioclase quantitatively demonstrate this effect. For Ba and La (partition coefficients of 0.4 and 0.04), 50% trapped liquid in a sample can result in two and sevenfold increases, respectively, in concentration between original and final bulk mineral compositions. Different cumulus assemblages also have a major effect on final compositions. We use examples of the concentrations of Ba and La in plagioclase from the Skaergaard intrusion from previous publications to illustrate the importance of this effect. Specifically, the La content of bulk plagioclase steadily decreases upward from the Lower Zone to Upper Zone c, and Ba in plagioclase shows no increase from the Lower Zone to the top of the Middle Zone. Such results are not explicable by fractionation processes, but can be modeled by the trapped liquid shift effect, assuming the well-established evidence for upward decrease in trapped liquid proportion through these zones.     

Plagioclase in the Skaergaard intrusion. Part 1: Core and rim compositions in the layered series

The anorthite content of plagioclase grains (X_An) 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 An_32–30 at the top. On the other hand, crystal rims are of approximately constant composition (An_{50 ± 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 X_An ∼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.     

Chemistry,subsolidus relations and electron petrography of pyroxenes from the late ferrodiorites of the Skaergaard intrusion,East Greenland

The ferroaugites, inverted ferrowollastonites and the brown and green ferrohedenbergites from the Upper Zone (UZb and UZc) of the Skaergaard intrusion (Brown and Vincent, 1963) have been studied with the electron microprobe, and where necessary, with the electron microscope. The cloudy “inclusions” in the inverted ferrowollastonite (Wo_ss) of 4471 are established to be strain fields associated with stacking faults, dislocations and sub-grain boundaries. The green pyroxenes of 1881 have undoubtedly inverted from Wo_ss, as both major and minor element chemistry show. The orientation of the tie-line joining coexisting Ca-rich and Ca-poor pyroxenes has also been established for this part of the quadrilateral, together with the Fe-Mg values at which the 4471 inverted Wo_ss would project on to Brown and Vincent's (1963) trend line for Ca-rich pyroxenes. These Fe-Mg values are the same as those of the 1881 brown ferrohedenbergites (Hed_ss). The subsolidus cooling history of the inverted Wo_ss has been examined in the light of the present data. It is proposed that a Wo_ss of solidus composition Wo₃₉ may either (a) react to a two-phase assemblage of Hed_ss (composition Wo_42.5) + metastable clinoferrosilite, or (b) invert metastably to a Hed_ss of the same composition. For specimen 4471, these two types of subsolidus behaviour may occur in different crystals within the same large mosaic-patterned grain. The proposed model is consistent with difficulty in nucleation of clinoferrosilitic lamellae, combined with the sluggishness of reactions at low temperatures for these Fe-rich compositions. In both case (a) and (b), inversion to Hed_ss (with or without the formation of mosaic texture) precedes exsolution of clinoferrosilite. The two final subsolidus compositions for the host are ～Wo₄₆ and ～Wo₄₂, for types (a) and (b) respectively, and the final subsolidus composition of the lamellae is Wo₀-Wo₂. The brown and green pyroxenes of 4330 show distinct differences in chemistry, the green being richer in Si, and depleted in Al and Ti relative to the brown. The 4330 green pyroxenes are poorer in Mn, and richer in Na, compared to the green inverted Wo_ss. The green colour in these UZc pyroxenes may be due to the drop in Ti content relative to brown pyroxenes.     

Parental magma of the Skaergaard intrusion: constraints from melt inclusions in primitive troctolite blocks and FG-1 dykes

Troctolite blocks with compositions akin to the Hidden Zone are exposed in a tholeiitic dyke cutting across the Skaergaard intrusion, East Greenland. Plagioclase in these blocks contains finely crystallised melt inclusions that we have homogenised to constrain the parental magma to 47.4–49.0 wt.% SiO₂, 13.4–14.9 wt.% Al₂O₃ and 10.7–14.1 wt.% FeO^T. These compositions are lower in FeO^T and higher in SiO₂ than previous estimates and have distinct La/Sm_N and Dy/Yb_N ratios that link them to the lowermost volcanic succession (Milne Land Formation) of the regional East Greenland flood basalt province. New major- and trace element compositions for the FG-1 dyke swarm, previously taken to represent Skaergaard magmas, overlap with the entire range of the regional flood basalt succession and do not form a coherent suite of Skaergaard like melts. These dykes are therefore re-interpreted as feeder dykes throughout the main phase of flood basalt volcanism.     

Xenoliths of gneisses and the conformable,clot-like granophyres in the Marginal Border Group,Skaergaard intrusion,East Greenland

Results of this research and the earlier work of Wager and his colleagues indicate that contamination influenced the overall character of the Marginal Border Group of the Skaergaard intrusion. Precambrian gneisses were a major source of contamination and are identifiable as xenoliths in the Marginal Border. A variety of other xenoliths occur with a wide compositional range.The range of K _D values for partitioning of Fe and Mg in coexisting pyroxene pairs in xenoliths from various parts of the Marginal Border, and in hornfelses adjacent to it, is consistent with temperatures that rose steeply inward. Temperatures estimated on the basis of compositions of coexisting pyroxenes are also consistent with dehydration that exceeded the stability of amphibole (850°C). The texturally compatible association of some granophyres with gneissic xenoliths suggests that both formed during melting. These observations suggest that there was a similar range of temperature for formation of xenoliths and granophyres.The xenoliths of gneisses have bulk rock compositional features which indicate that lithophilic constituents were removed, causing an increase in basic constituents upon assimilation of the gneissic precursors. If we assume that granophyres and xenoliths of gneisses represent a consanguineous set formed by a fusion process from gneiss, enrichment and depletion factors for excluded trace elements are complementary and generally less than 10. Allowable enrichment in granophyres by crystal fractionation using the average for a large number of chilled marginal gabbro analyses as an initial composition, is also less than 10 for the same elements. The calculated low factors for enrichment of lithophile elements in granophyres by both mechanisms favor the hypothesis of partial melting of gneiss.     

The differentiation of the Skaergaard Intrusion

Previous interpretations of the Skaergaard Intrusion suggested that differentiation involved extreme iron-enrichment but no silica-enrichment until a very late stage. This model is difficult to reconcile with petrological and geochemical evidence, with the behaviour of tholeiitic volcanic suites and with phase equilibria. We propose that the Skaergaard magma evolved on a trend of pronounced silica-enrichment after cumulus magnetite appeared at the top of the Lower Zone. At that stage, the magma was of ferrobasaltic composition with close to 50% SiO₂. The Middle and Upper Zones of the intrusion dominantly represent crystal accumulation during differentiation from ferrobasalt through iron-rich basaltic andesite and icelandite to rhyolite, a fractionation sequence common in tholeiitic volcanic provinces. This interpretation requires re-appraisal of the physical processes responsible for the differentiation. In particular, residual liquids became lower in density with fractionation and would have caused the Skaergaard magma chamber to have become compositionally zoned.     

Precambrian Sequence Bordering the Skaergaard Intrusion

Archean tonalitic-granodioritic orthogneisses bordering theSkaergaard Intrusion contain widespread boudins and lenses ofgarnet-biotite schist, quartzite, amphibolite, and ultramaficrocks. These rocks are similar to and locally gradational withnarrow intact supracrustal belts in the region. We correlateearliest isoclinal folds in supracrustal belt rocks and in earliesttonalitic-trondhjemitic-granitic (TTG1) orthogneisses with regionallydeveloped (D₂) deformation. We also correlate strong foliation(Ssp1) in the supracrustal rocks and banding (Sbgn1) in earliestorthogneisses with D₂ deformation which followed and overlappedearliest M₁ metamorphism. Ssp1 foliation is in part axial planarwith D₂ isoclinal folds transposing compositional and subparallelmetamorphic banding in the hinge areas. Ssp1 assemblages inmetapelites consist of folia of coexisting sillimanite-biotite-quartzand correspond roughly to metamorphism at the second sillimaniteisograd. We correlate syntectonic emplacement of a later generationof orthogneisses (TTG2) with strong D₃ shearing-cataclasis associatedwith tectonic intercalation of supracrustal rocks and earliestorthogneisses. The latest metamorphic assemblages (M₂) consistof granoblastic and porphyroblastic minerals that overprintSsp1 (M₁) foliation and D₃ fabrics. These assemblages formedduring largely static regional metamorphism about 2900 Ma agoand are locally aligned with fabric elements of D₄ folding. Temperatures during M₂ metamorphism equalled or exceeded thestability of biotite-sillimanite-quartz in metapelites and chlorite-orthopyroxene-olivine-spinel-hornblendein ultramafic rocks. Fe-Mg biotite-garnet exchange, and thepressure-dependent garnet-plagioclase-sillimanite-quartz equilibriumassemblage in metapelites yield temperature and pressure estimatesfor M₂ metamorphism of 650–700?C and 3–4 kb. Thesedata suggest that M₂ assemblages formed as results of dehydrationreactions at water partial pressures that were less than thetotal pressure. The temperature-dependent equilibrium assemblagechlorite-orthopyroxene-olivine-spinel-vapor (+hornblende), adjustedfor observed phase compositions, is consistent with the Fe-Mgbiotitegarnet exchange geothermometer. Rare-earth element, Rb-Sr and Pb-Pb isotopic, and other compositionalcharacteristics of the orthogneisses are generally consistentwith a multiple stage magmatic origin of their protoliths. OlderTTG1 orthogneisses have compositions generally consistent withformation of the magmas parental to their protoliths by partialmelting of garnetiferous source rocks such as eclogite, or lowercrust. Younger TTG2 orthogneisses have compositions that areconsistent with their formation as water-saturated second meltsin equilibrium with a hornblende-rich residuum. Their formationoccurred within a few 10⁷ y after crustal emplacement of TTG1orthogneisses. The source of water for the formation of later(TTG2) melts may have been M₂ dehydration reactions deeper withinthe supracrustal pile.     

Crystallization and Layering of the Skaergaard Intrusion

Solidification of large slowly cooled intrusions is a complexprocess entailing progressive changes of rheological propertiesas the crystallizing magma passes through successive stagesbetween a viscous Newtonian fluid and a brittle solid rock.Studies of this transition in the Skaergaard intrusion indicatethat most crystallization took place in an advancing front ofsolidification against the floor, walls, and roof where crystalsnucleated and grew in a static boundary layer, much in the mannerproposed by Jackson in 1961. The non-Newtonian properties ofthe crystallizing magma account for the fact that plagioclase,which was lighter than the liquid, is a major component of rockson the floor, while mafic minerals that were heavier than theliquid accumulated under the roof. Crystals that nucleated andgrew in these zones were trapped by an increasingly rigid zonethat advanced more rapidly than the crystals sank or floated.If any crystals escaped entrapment, they were those of the largestsize and density contrast. The rates of accumulation in different parts of the intrusionwere not governed by rates of gravitational accumulation somuch as by the nature of convection and heat transfer. Cumulatetextures, preferred orientations of crystals, and layering,all of which have been taken as evidence of sedimentation, canbe explained in terms of in situ crystallization. Layering cannothave been caused by density currents sweeping across the floor;it is well developed on the walls and under the roof, lacksthe size and density grading and mineralogical compositionsthat would be expected, and shows no evidence of having beenaffected by obstructions in the paths of the currents. We propose an alternative origin of layering that is based onprocesses governed by the relative rates of chemical and thermaldiffusion during cooling. Intermittent layering resulted fromgravitational stratification of the liquid, and cyclic layeringwas produced by an oscillatory process of nucleation and crystalgrowth. The effects of differentiation during in situ crystallizationare strongly dependent on relative rates of diffusion of individualcomponents, and some of the compositional variations in differentparts of the intrusion can be explained in terms of these differences.     

Exsolution in augite from the Skaergaard Intrusion

The exsolution phenomena of augite from Ferrogabbro 4430 of the Skaergaard Intrusion were examined in detail by single crystal X-ray diffraction and heating experiments to study the stepwise exsolution process. In the augite crystals, five different phases were detected: pigeonite (001), pigeonite (100), orthopyroxene (a), orthopyroxene (p) and a small amount of clinoamphibole. The two different pigeonites nearly share the corresponding (001) and (100) planes with the host. Orthopyroxene (a) and orthopyroxene (p) have (100) in common with the host and with exsolved pigeonite (001), respectively. Clinoamphibole was observed in the form of rather weak reflections in many crystals. It has (010) in common with the host.A large number of augite crystals exhibited a pigeonite (001) phase with curved, rotated reflections and diffuse streaks along the a* direction in (h0l) precession photographs. It appears that these streaks are related to orthopyroxene (p). Orthopyroxene (p) seems to be crystallized from pigeonite (001) by nucleation at (100) stacking fault planes (inverted pigeonite). Pigeonite (100) may be formed at growth ledges between augite host and exsolved orthopyroxene (a) at a later stage of exsolution to stabilize the boundaries.From the X-ray diffraction profiles and the results of the heating experiments, a possible exsolution sequence is suggested. Clinoamphibole appears to be a product of alteration at the latest stage of the exsolution process. It seems to be related to particular conditions of partial water pressure.     

Quantitative Simulation of the Hydrothermal Systems of Crystallizing Magmas on the Basis of Transport Theory and Oxygen Isotope Data: An analysis of the Skaergaard Intrusion

Application of the principles of transport theory to studiesof magma-hydrothermal systems permits quantitative predictionsto be made of the consequences of magma intruding into permeablerocks. Transport processes which redistribute energy, mass,and momentum in these environments can be represented by a setof partial differential equations involving the rate of changeof extensive properties in the system. Numerical approximationand computer evaluation of the transport equations effectivelysimulates the crystallization of magma, cooling of the igneousrocks, advection of chemical components, and chemical and isotopicmass transfer between minerals and aqueous solution. Numerical modeling of the deep portions of the Skaergaard magma-hydrothermalsystem has produced detailed maps of the temperature, pressure,fluid velocity, integrated fluid flux, ¹⁸O-values in rock andfluid, and extent of nonequilibrium exchange reactions betweenfluid and rock as a function of time for a two-dimensional cross-sectionthrough the pluton. An excellent match was made between calculated¹⁸O-values and the measured ¹⁸O-values in the three principalrock units, basalt, gabbro, and gneiss, as well as in xenolithsof roof rocks that are now embedded in Layered Series; the latterwere evidently depleted in ¹⁸O early in the system's coolinghistory, prior to falling to the bottom of the magma chamber.The best match was realized for a system in which the bulk rockpermeabilities were 10^–13 cm² for the intrusion, 10^–11cm² for basalt, and 10^–16 cm² for gneiss; reaction domainsizes were 0.2 cm in the intrusion and gneiss and 0.01 cm inthe basalts, and activation energy for the isotope exchangereaction between fluid and plagioclase was 30 kcal/mole. The calculated thermal history of the Skaergaard system wascharacterized by extensive fluid circulation that was largelyrestricted to the permeable basalts and to regions of the plutonstratigraphically above the basalt-gneiss unconformity. Althoughfluids circulated all around the crystallizing magma, fluidflow paths were deflected around the magma sheet during theinitial 130,000 years. At that time, crystallization of thefinal sheet of magma and fracture of the rock shifted the circulationsystem toward the center of the intrusion, thereby minimizingthe extent of isotope exchange between rocks near the marginof the intrusion at this level. For comparison, similar calculationswere also made for pure conductive cooling; it was found thatthe rate of crystallization of the magma body was not changed.The solidified pluton cooled by a factor of about 2 faster inthe presence of a hydrothermal system. Transport rates of thermal energy out of the intrusion and oflow-¹⁸O fluids into the intrusion controlled the overall isotopeexchange process. During the initial 150,000 years, temperatureswere high and reaction rates were fast; thus, fluids flowinginto the intrusion quickly equilibrated with plagioclase. However,the temperature decreased between 120,000 and 175,000 yearsand caused a decrease in reaction rates and an increase in theequilibrium fractionation factor between plagioclase and fluid.Consequently, during this time period fluids in the intrusiontended to be out of equilibrium with plagioclase. After 175,000years temperatures had decreased sufficiently that reactionrates became insignificant, but convection rates were largeenough to redistribute fluid and enlarge the regions where fluidand plagioclase were out of equilibrium. By 400,000 years, thepluton had cooled to approximately ambient temperatures, andthe final ¹⁸O values were ‘frozen in’. Reactionsbetween hydrothermal fluid and the intrusion occurred over abroad range in temperature, 1000-200 °C, but 75 per centof the fluid circulated through the intrusion while its averagetemperature was >480 °C. This relatively high temperatureis consistent with the observation that only minor amounts ofhydrothermal alteration products were formed in the naturalsystem, even where several per mil shifts in ¹⁸O were detected. The relative quantities of fluid to rock integrated over theentire cooling history were 0.52 for the upper part of intrusion,0.88 for the basalt, 0.003 for the gneiss, and 0.41 for theentire domain. Almost all of the fluid flowed into the intrusionfrom the basalt host rocks that occur adjacent to the side contactsof the intrusion. Convection transferred about 20 per cent ofthe total heat contained in the gabbro upward into the overlyingbasalts; the remaining 80 per cent of the heat was transferredby conduction.     

Modally-Graded Rhythmic Layering in the Skaergaard Intrusion

The origin of igneous layering in the Skaergaard and other intrusionsis the subject of much debate. Currently held theories for itsorigin include gravitational sorting, rhythmic nucleation, andchemical diffusion models. This paper presents data on small-scale,modally-graded, rhythmic layers, one of several distinct typesof layering found in the Skaergaard. These layers occur onlyin the Layered Series of the intrusion, in which they are awidespread, almost ubiquitous feature. They are characterizedby strong variations in modal composition, with magnetite, ilmenite,and olivine concentrated in the lower part of the layers, pyroxenein the middle of the layers, and plagioclase in the upper partof the layers. In addition, the layers are size graded, withthe largest grain sizes of the different minerals generallyoccurring where the mineral is most abundant. The chemical compositionof both plagioclase and Ca-rich pyroxene do not vary in anyregular manner within the layers studied. The forsterite compositionof olivine is consistently higher in the lower part of the layers.This trend, however, correlates very strongly with the modalvariations of olivine and can be explained by minor amountsof subsolidus reequilibration between cumulus and adcumulusolivine, suggesting that the original composition of cumulusolivine within individual layers was constant. These results are consistent with layers formed by some gravitationalsorting process and, in several cases, are at odds with othertheories that have been proposed for the origin of igneous layers.Based on these data and the widespread evidence for currentinteraction (crossbeds, trough-shaped layers, and internallygraded layers), we propose that these layers result from periodicdisruption of a static zone of crystal growth on the floor ofthe magma chamber by convection or density currents. This mechanism,however, is almost certainly not responsible for the formationof all of the types of layering found in the intrusion.     

Petrology of the Marginal Border Series of the Skaergaard Intrusion

The Marginal Border Series (MBS) of the Skaergaard intrusionconsists of rocks formed by in situ crystallization againstthe walls of the intrusion. Most of these rocks are productsof fractional crystallization, though samples believed to representchilled liquid occur locally at the intrusive contact. The MBScomprises only 5% of the exposed volume of the intrusion, butwithin its thickness, the order of crystallization and the compositionsof fractionated rocks and minerals vary systematically withdistance inward from the intrusive contact in largely the samemanner as rocks and minerals upward through the Layered Series(LS). Earliest differentiates are cumulates of olivine and plagioclase.The most basic compositions of cumulus plagioclase (An₇₂) andolivine (Fo₈₄) in these rocks indicate that the amount of fractionationpreceding formation of the exposed LS was substantially lessthat previously believed. Field and compositional data indicatethat picritic blocks are xenoliths rather than cumulates ofthe Skaergaard magma. Xenoliths of gneiss in all stages of reactionare locally abundant; however, there is no evidence that uppercrustal material contaminated the magma from which the MBS cumulatesformed. Compositions of cumulus minerals in the MBS differ fromthose in comparable LS rocks. Cumulates in the lower marginscontain more calcic plagioclase, more magnesian augite in allbut the late differentiates, and more iron-rich olivine. Thecompositions of cumulus olivine and to a lesser degree thoseof other mafic silicates, were modified to more iron-rich compositionsby re-equilibration with relatively large amounts of interstitialliquid. The lower MBS and LS crystallized from the same magma, but fractionationoccurred at different rates on the walls and floor of the intrusion.The upper margin may have crystallized from a magma of modifiedcomposition and fractionated at rates different from that inthe lower margin and Upper Border Series (UBS). Crystals onthe floor and roof of the intrusion accumulated faster or moreefficiently than on the walls. At any given stage of fractionation,crystals also accumulated against all sides of the magma chamberat about the same rate. Either the rates of cooling, crystallization,and crystal retention affected accumulation rates locally asfunctions of rock type and geometry of the walls, or these rateswere largely independent of wall rock owing to buffering ofconductive heat loss possibly to an envelope of hydrothermalfluid circulating around the crystallizing magma. The appearanceor disappearance of cumulus minerals in the lower MBS occursat higher structural levels than in the LS and at lower structurallevels than in the UBS. These relationships together with cumulusmineral compositions indicate that magma at the margins wasalways somewhat less fractionated than that at the floor androof of the chamber. It is proposed that these relationshipsreflect the combined effects of liquid and crystal fractionationof the magma within largely independent convection systems inthe lower and upper parts of the chamber.