The Skaergaard Layered Series: I. Structure and Average Compositions

Re-examination of the Skaergaard Layered Series in the lightof more extensive field work and sampling shows that the lithologiczones vary laterally as well as vertically, in both their bulkchemical composition and their mineralogical assemblages. Themargins of the zones differ from both the central part of theLayered Series and Marginal Border Series in being richer inFeO^*, TiO₂, K₂, P₂O₂, and most excluded elements. Mafic mineralstend to be more abundant and more iron-rich, plagioclase ismore albitic and more strongly zoned, and apatite and biotiteare more abundant near the margins. When the average compositions of successive zones are compared,the abundances of most excluded components are seen to declineupward as far as Middle Zone then reverse their trends and increasethrough Upper Zone. P₂O₅ and K₂O are negatively correlated inUpper Zones B and C, owing, perhaps, to separation of immisciblefelsic liquids from the iron-rich magma. No evidence has beenfound for introduction of a new batch of less differentiatedmagma. Layered rocks have an average composition that is more maficthan that of homogenous rocks at the same level. Blocks thatfell from the roof have the opposite relation; they are greatlyenriched in felsic components compared to the original compositionsof the Upper Border Series from which they came. Although some of the compositional variations may be consistentwith differing degrees of fractionation of trapped liquids,no consistent relation has been found between the degree offractionation and rates of crystal accumulation or cooling atthe walls. Contamination with the metamorphic wall rocks, eitherby assimilation or by hydrothermal fluids, seems to have hadonly local effects and cannot account for the large-scale variations.At least some of the compositional differences must have resultedfrom late-stage processes that redistributed certain componentsafter the intrusion reached advanced stages of solidification.     

The Chilled Marginal Gabbro and Other Contact Rocks of the Skaergaard Intrusion

Rocks in the outer selvage of the Skaergaard intrusion havea range of textures and compositions, and among these are materialsrepresenting quenched Skaergaard magma. Pristine chilled marginalgabbro (CMG), however, is not ubiquitous at the intrusive contact,because many of the "contact" rocks have been hydrothermallyor metasomatically altered, contaminated with gneiss or olivinexenocrysts, while others contain accumulated minerals. Materialrepresenting quenched magma appears to be restricted to contactrocks that are texturally and mineralogically similar to diabase,and free of accumulated minerals. Where it exists, the CMG isfound within one to three meters of the exposed intrusive contactexcept at the roof of the intrusion where its thickness is greater.CMG was distinguished from the diverse group of contact rocksby petrographic and geochemical screening of over 80 specimens.Samples of CMG from the eastern and western margins and fromthe roof of the intrusion have relatively uniform compositionsimilar to that of ferrobasalt, and are noticeably richer iniron (mg-number=0?51-0?54), TiO₂ K₂O, and P₂O₅ than other unmodifiedcontact rocks. CMG's also have trace element compositions distinctfrom most other rocks in the outer Marginal Border Series (MBS).They have incompatible element contents up to 3–6 timesgreater than in LZa-type cumulates, negligible Eu anomalies,and Ni and Cr contents and Ni/Cr ratios that are among the lowestof rocks in the outer MBS. The results of melting experiments corroborate selection ofthis material as CMG. The composition of glasses obtained frompartial melting experiments of LZa-type cumulates are essentiallyidentical to those of the CMG. The 1-atm. liquidus phase relationsfor one of the CMG samples (KT-39) is largely consistent withthe sequence and composition of cumulus minerals observed withdistance inward through the MBS and upward through the LayeredSeries. Solidification of magma at the outer margin of the intrusionis interpreted to have involved locally efficient quench crystallizationfollowed by initial primocryst growth in an undercooled transitionzone a short distance inward that finally extended into regionsof near equilibrium crystallization. The similarity in composition between samples of chilled marginalgabbro from the exposed roof and sides of the intrusion, andthose of reconstituted trapped liquid from early cumulates inthe outer MBS suggests that a single magma, similar in compositionto ferrobasalt, was parental to the Skaergaard intrusion. Thisinterpretation corroborates geophysical evidence of a significantlysmaller mass for the intrusion than that estimated by Wager,and provides a basis for revision of models of its chemicalevolution. Samples chosen by Wager as chilled marginal gabbrobelong spatially, texturally, and compositionally to the groupof LZa-type cumulates in the MBS, and should no longer be regardedas chilled marginal gabbro.     

The Skaergaard Layered Series. Part VI. Excluded Trace Elements

In contrast to the smooth trends of major elements and mineralcompositions, the excluded trace elements in the SkaergaardLayered Series have an irregular distribution that does notconform to the normal trends of Rayleigh-type fractionation.Their concentrations are about constant or even decline throughthe Lower and Middle Zones before increasing sharply to reachmaximum concentrations 100–200 m above the Sandwich Horizon.As in the case of included elements, the relative concentrationsof excluded elements in coexisting phases deviate widely fromthose predicted by experimentally determined partition coefficientsunder presumed magmatic conditions. This is seen most clearlyin the immiscible melanogranophyres and conjugate ferrogabbros.Although the major elements conform to the experimentally determinedrelations for immiscible liquids, the trace elements do not;they follow a totally independent trend. The abrupt increasein the concentrations of excluded elements in the upper partof the intrusion could plausibly be attributed to an additionof new magma or to a density inversion that resulted in upwardmigration of a late liquid or fluid, but these possibilitiesare inconsistent with the compositional and spatial relationsof the upper parts of the intrusion. Although a late residualliquid certainly migrated upward, the most likely explanationfor the observed distribution of excluded elements is that thepartition coefficients were altered by volatile components,which gradually increased during the early stages of crystallizationthen began to exsolve near the top of the Middle Zone. KEY WORDS: igneous differentiation; Skaergaard intrusion     

The Skaergaard Layered Series, Part VII: Sr and Nd Isotopes

The initial isotopic ratios of strontium and neodymium in theSkaergaard Layered Series vary both vertically and laterally,on every scale from the intrusion as a whole down to coexistingminerals in a single rock. The magma that filled the Skaergaardchamber was contaminated to various degrees with the metamorphicrocks through which it rose and was never completely homogenizedafter being intruded. The contamination was most pronouncedin contact zones and aureoles around rare xenoliths. The greaterconcentrations of lithophile trace elements in the Upper BorderSeries was previously attributed to assimilation of buoyantfragments of gneiss that collected under the roof, but mostof the rocks of the Upper Border Series are isotopically indistinguishablefrom those of the Layered Series. It is doubtful, therefore,that this part of the intrusion assimilated much more of themetamorphic basement than did the rest of the magma. Similarly,the marked increase in the concentrations of excluded elementsin the upper part of the Layered Series is not matched by achange in the isotopic character of the rocks and cannot beattributed to a later influx of new magma. Analyses of mineralsseparated from rocks with exceptionally mafic or felsic modalcompositions revealed marked inhomogeneities in the isotopiccompositions of their constituent minerals. For example, coexistingplagioclase and pyroxene from closely associated anorthositesand pyroxenites have very different initial isotopic ratiosof both strontium and neodymium. The same is true of mafic andfelsic layers in modally graded gabbros. These differences areunrelated to the low-temperature alteration shown by oxygenisotopes. They must have been introduced when the original gabbrowas largely crystallized and underwent local metasomatic replacementby nearly mono-mineralic mafic and felsic assemblages. KEY WORDS: Nd isotopes; Skaergaard; Sr isotopes     

Electrochemical measurements bearing on the oxidation state of the Skaergaard Layered Intrusion

The oxygen fugacities (fO₂'s) of magnetically-concentrated fractions (MCF) of three rock samples from the Skaergaard Layered Intrusion were measured between 800–1150° C using oxygen-specific, solid zirconia electrolytes at atmospheric pressure. Two of the bulk rock samples (an oxide cumulate and an oxide-bearing gabbro) are from the Middle Zone (MZ) and the other (an olivine plagioclase orthocumulate) is from the Lower Zone (LZ). All MCF define fO₂ versus T arrays that lie 1.5–0.5 log units above the fayalite-magnetite-quartz (FMQ) buffer. Experiments with different cell-imposed initial redox states (one from a reduced direction and one from an oxidized direction) were run on each sample in an attempt to achieve experimental reversibility. This was accomplished by imposing a known redox memory on the galvanic cell prior to loading each sample. Reversibility for each sample agreed to better than 0.2 of a log unit. Irreversible autoreduction of 0.2 of a log unit was observed on the two MZ samples at temperatures exceeding 1065° C. Scanning electron microscope and electron microprobe study of pre- and post-run products shows that reaction and textural re-equilibration occurred among the oxide phase assemblages under the experimental conditions employed. Careful characterization of pre- and post-run assemblages is clearly necessary before adequate interpretation of the experimental results can be made in these types of electrochemical studies. Different approaches to investigations of the fO₂ of the Skaergaard Intrusion, be it thermodynamic calculations or experimental methods, should yield concordant results or at least understandable discrepancies. Calculated fO₂'s using thermobarometry applied to the ilmenite-magnetite pairs in the post-experimental assemblages agree with the experimentally determined fO₂'s to within one log unit at a given temperature. These results are also consistent with previously calculated fO₂ values (Buddington and Lindsley 1964; Morse et al. 1980), but are considerably more oxidized than a previous electrolyte-based fO₂ study of a different sample suite from the Skaergaard (Sato and Valenza 1980) that include values close to the iron-wustite (IW) buffer from both MZ and LZ oxide separates. Differences between this electrochemical study and that of Sato and Valenza (1980) may be due to variations in the level of indigenous (or curatorially-introduced) carbon in the samples studied. Despite a number of experimental difficulties, electrochemical cells can provide an accurate and precise method of determining the oxygen fugacity of naturally occurring, complex oxide assemblages. Tight experimental reversals and reproducible values obtained in heating and cooling cycles are an indication of the precision and accuracy of the data recoverable with electrochemical cells.     

Upscaling of Hooke''s Law for Imperfectly Layered Rocks

In this paper, we consider the upscaling of Hooke's law and its parameters on the fine scale, to a similar law with upscaled parameters on a larger scale. It is assumed that the fine scale material properties of the rock are imperfectly layered. In the governing equations, the deviations from perfect layering introduce a small parameter that can be used in perturbation series expansions for the stress, the strain, and the displacement. In the approximation of order zero the upscaled compliance matrix contains the well-known Backus parameters; this approximation holds exactly for a perfect layering. However, many natural rock types are imperfectly layered and in that case the approximation of order zero may not be sufficiently accurate. Therefore, we consider also the first order corrections. The derivation and results are presented both for the most general case and for the much simpler case in which the fine scale Poisson ratio may be assumed constant. From thermodynamic principles, it follows that the compliance tensor is symmetric on the fine scale. However, it is shown that the argument for symmetry cannot be extended to upscaled rigidities. One of the most important conclusions is that upscaled compliance tensors are nonsymmetric when there are trends in the deviations from perfect layering.     

Upscaling of Hooke's Law for Imperfectly Layered Rocks

In this paper, we consider the upscaling of Hooke's law and its parameters on the fine scale, to a similar law with upscaled parameters on a larger scale. It is assumed that the fine scale material properties of the rock are imperfectly layered. In the governing equations, the deviations from perfect layering introduce a small parameter that can be used in perturbation series expansions for the stress, the strain, and the displacement. In the approximation of order zero the upscaled compliance matrix contains the well-known Backus parameters; this approximation holds exactly for a perfect layering. However, many natural rock types are imperfectly layered and in that case the approximation of order zero may not be sufficiently accurate. Therefore, we consider also the first order corrections. The derivation and results are presented both for the most general case and for the much simpler case in which the fine scale Poisson ratio may be assumed constant. From thermodynamic principles, it follows that the compliance tensor is symmetric on the fine scale. However, it is shown that the argument for symmetry cannot be extended to upscaled rigidities. One of the most important conclusions is that upscaled compliance tensors are nonsymmetric when there are trends in the deviations from perfect layering.     

鄂尔多斯盆地奥陶系不同组构碳酸盐岩埋藏溶蚀实验

鄂尔多斯盆地奥陶系碳酸盐岩储层受埋藏溶蚀作用控制明显,而地层深部复杂的水-岩反应造成埋藏溶蚀研究难度较大,并进而影响了储层的评价与预测。分别利用CO₂溶液和乙酸溶液为流体介质进行溶蚀模拟实验,探讨埋藏条件下温度、压力、流体等因素对不同矿物及组构碳酸盐岩溶蚀作用的影响。结果表明:1）随着温度与压力升高,碳酸盐岩样品在乙酸溶液中的溶解速率均相应提高,在CO₂溶液中的溶解速率则先增加后减小,且在110℃~130℃区间内溶蚀速率最大;深埋藏环境下,各岩类溶蚀速率差异减小,并趋于一致;2）岩石矿物成分和组构,原岩初始孔隙度的大小及其连通关系,以及晶体的产状对成岩后期的埋藏溶蚀作用也具有重要的影响。不溶组分含量低、颗粒/灰泥比高、矿物成分复杂的碳酸盐岩由于组构选择性溶蚀作用而更易被溶蚀;碳酸盐岩的溶蚀速率随方解石含量的增加而增加,但深埋藏环境下,矿物成分含量差异对溶蚀速率的影响作用减弱;硬石膏与白云岩相伴生时,可优先溶蚀形成膏模孔,并促进白云石的溶解,改善储层效果。不同岩性,总体上灰岩较白云岩及过渡岩类更易发生埋藏溶蚀作用。结合研究区实际地质条件分析,砂屑灰岩、膏质白云岩等埋藏溶蚀强度较大,通过对原岩早期组构选择性溶蚀形成孔隙的继承和调整,叠加埋藏期岩溶作用后,可形成规模优质储层。     

An Orthotropic Cosserat Elasto-Plastic Model for Layered Rocks

Summary Modelling the behaviour of rock masses consisting of a large number of layers is often necessary in mining applications (e.g. coal mining). Such a modelling can be carried out in a discontinuum manner by explicit introduction of joints. When the number of rock layers is large, it is advantageous to devise a continuum-based model in which case the joints are considered to be virtually smeared across the mass. In this study, a fully elasto-plastic equivalent continuum model suitable for describing the behaviour of such layered rock masses is considered. The model is based on the Cosserat continuum theory and incorporates the moment stresses in its formulation. In contrast to the earlier Cosserat models, the possibility of rock layer plasticity is considered. The accuracy of the developed Cosserat model is verified against analytical and experimental results. Received October 15, 2000; accepted July 30, 2001 Published online August 2, 2002     

O18/O16 Ratios in Rocks and Coexisting Minerals of the Skaergaard Intrusion, East Greenland

The Skaergaard intrusion of East Greenland is a gravitationallystratified gabbroic mass that has undergone extreme fractionalcrystallization. Oxygen-isotopic analyses have been obtainedfor the various rock types of this intrusion and for severalcoexisting minerals of these rocks. The general relationshipsamong the O¹⁸/O¹⁶ ratios of the minerals are the same as havebeen found for other igneous rocks, but the isotopic fracticnationsare smaller, probably as a result of the higher temperatureof formation of the Skaergaard rocks. The later differentiatesare progressively depleted in O¹⁸ to a marked degree relativeto the earlier-formed portions of the layered series; the late-stagegranophyres are 4–5 per mil lower in O¹⁸/O¹⁶ than thelayered Lower Zone gabbros, and are 7–9 per mil lowerthan normal granitic rocks from other localities. This progressivedepletion in O¹⁸ is a result of crystallization and settlingout of minerals that are, on the whole, about 1 per mil higherin O¹⁸/O¹⁶ than the magma liquid. Calculations based on a simplecrystallization model are in agreement with the experimentalresults.     

The Significance of the Mesostasis of Basic Layered Igneous Rocks

Many layered igneous rocks consist of two main parts: the cumulus(plus adcumulus and heteradcumulus) material and the pore material,or mesostasis, produced by the crystallization of the trappedliquid. Knowledge of the chemistry of the mesostasis will beuseful in deducing magma fractionation trends and in the determinationof solid/liquid trace element partition coefficients, whichin turn will be significant in helping to elucidate the petrogeneticrelationship between certain basalts and layered gabbros. Anew geochemical method for the determination of both the compositionand amount of mesostasis in layered rocks is proposed. Thismethod uses elements of contrasting behaviour and is best appliedto rhythmically layered rocks rich in mafic or felsic minerals.Data on strontium, cobalt, and uranium are used to apply themethod to two rocks from the Skaergaard intrusion. The resultsobtained are in agreement with published data.     

On Estimating the Magnitude of the Hidden Zone and the Compositions of the Residual Liquids of the Skaergaard Layered Series

Wager's more general assumptions about the relations betweenthe layered series of Skaergaard and its parent magma form thebasis for a simple procedure by which, from the compositionof the proposed initial magma and the compositions and amountsof the exposed zones, the amount and composition of the hiddenzone and the compositions of liquids remaining after the consolidationof each zone are readily computed. Application of the procedure to the Skaergaard data—moreproperly, to the relevant summary statistics presented by Wager(1960) and by Wager & Brown (1968)—suggests that currentlyaccepted graphical solutions materially overestimate the extentof end-stage alkali enrichment and may materially understatethe magnitude of the hidden zone; the data are quite as compatiblewith numerical solutions yielding a considerably larger hiddenzone and virtually no perceptible end-stage alkali enrichment.Differences between numerical and graphical solutions seem tobe generated primarily by the zone weights used in the latter.     

The Major Element Variation of the Layered Series of the Skaergaard Intrusion and a Re-estimation of the Average Composition of the Hidden Layered Series and of the Successive Residual Magmas

Using variation diagrams for the major elements in the layeredrocks, estimates are made of the average amounts of the variouselements in the total rock separating at successive stages.From the analyses of the chilled marginal gabbro, taken to representthe composition of the initial magma, and with the further likelyassumption that the Skaergaard intrusion is a closed system,at any rate for most of the elements, various hypotheses onthe relative volumes of the different parts of the intrusionare tested to find the one best fitting the known distributionof the elements in the observable rocks. Estimates are thenmade of (1) the overall composition of the hidden part of theintrusion by subtracting the amounts of an element in the observedrocks from the total in the initial magma, and (2) the compositionof the successive residual magmas formed as a result of thecrystal fractionation.     

The Major Element Variation of the Layered Series of the Skaergaard Intrusion and a Re-estimation of the Average Composition of the Hidden Layered Series and of the Successive Residual Magmas

Using variation diagrams for the major elements in the layeredrocks, estimates are made of the average amounts of the variouselements in the total rock separating at successive stages.From the analyses of the chilled marginal gabbro, taken to representthe composition of the initial magma, and with the further likelyassumption that the Skaergaard intrusion is a closed system,at any rate for most of the elements, various hypotheses onthe relative volumes of the different parts of the intrusionare tested to find the one best fitting the known distributionof the elements in the observable rocks. Estimates are thenmade of (1) the overall composition of the hidden part of theintrusion by subtracting the amounts of an element in the observedrocks from the total in the initial magma, and (2) the compositionof the successive residual magmas formed as a result of thecrystal fractionation.     

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.     

Hydrothermal clinopyroxenes of the Skaergaard intrusion

Magmatic augites reacted with high temperature aqueous solutions to form secondary calcic pyroxenes during the subsolidus cooling of the Skaergaard intrusion. Secondary, hydrothermal clinopyroxenes replace wall rock igneous augites at the margins of veins filled with calcic amphibole. These veins are up to several millimeters wide and tens of meters in length. Hydrothermal clinopyroxenes are a ubiquitous and characteristic phase in the earliest veins throughout the Layered Series of the intrusion, and occur rarely in late veins that, in some places, crosscut the early veins. Associated secondary phases in early veins include amphiboles ranging in composition from actinolite to hornblende, together with biotite, Fe-Ti oxides and calcic plagioclase. Hydrothermal clinopyroxenes in late veins may be associated with actinolite, hornblende, biotite, magnetite and albite.Hydrothermal clinopyroxenes are depleted in Fe, Mg and minor elements, and enriched in Ca and Si relative to igneous augites in the Layered Series gabbros. Secondary vein pyroxenes are similar in composition to calcic pyroxenes from amphibolite facies metamorphic rocks. Clinopyroxene solvus thermometry suggests minimum temperatures of equilibration of between 500° and 750° C. These temperatures, combined with numerical transport models of the intrusion, suggest that vein clinopyroxenes could have formed during 20,000 to 60,000 year time intervals associated with a maximum in the fluid flux through fractures in the Layered Series.