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The roof-derived autoliths in the floor cumulates of the Skaergaard Intrusion have been argued to have been extensively metasomatised and recrystallised, forming the foundation of the hypothesis that microstructures in plutonic rocks are essentially metamorphic. However, the augite–plagioclase–plagioclase dihedral angles and plagioclase core composition of the autoliths match with those of the roof rocks, demonstrating that they were generally solid on arrival at the floor, with no subsequent microstructural or compositional modification. Many autoliths have mafic rinds, which were used as evidence of metasomatism: these rinds fall into two groups. The rarely developed rind rock of Irvine et al. (1998) is most likely chilled magma infiltrating along fractures in the roof rocks, either associated directly with detachment of roof material, or occurring before final detachment. Thin mafic rims are widespread in LZc and MZ, present at the tops of the more elongate autoliths, with a corresponding felsic rim at the base of the most elongate autoliths. The close correspondence of thin rim development with autolith shape, rather than composition, is argued to be evidence that they formed as a result of differential migration of immiscible conjugate interstitial liquids: the dense Fe-rich liquid flowed downwards and ponded on the tops of impermeable autoliths, whereas its buoyant Si-rich conjugate flowed upwards and was trapped underneath. Any differences in microstructure and bulk composition of the autoliths compared with the remaining exposures of the roof sequence reflect the wider range of lithologies in the now-eroded regions of the roof. [ABSTRACT FROM AUTHOR]

The Skaergaard intrusion is one of the most thoroughly studied layered mafic intrusions on Earth and an exceptional example of (near) closed-system magmatic differentiation. We report new Fe isotope data for whole rocks, and magnetite and ilmenite separates through the layered series (LS) and upper border series (UBS) of the intrusion. δ56Fe values for gabbroic rocks range from 0.033‰ to 0.151‰ with an abrupt step increase at the base of Lower Zone c (LZc) within LS with the appearance of cumulus magnetite and subsequent decline accompanying FeTi oxide fractionation. The lowest δ56Fe values are found near the Upper Zone b (UZb) to c (UZc) boundary followed by a sharp rise across UZc approaching the Sandwich Horizon. Magnetite–ilmenite separates straddle bulk rock compositions with fractionation factors (Δ56Femt-ilm) of 0.081‰ to 0.239‰, consistent with subsolidus equilibration. Granophyric rocks occurring as pods, sheets and wispy layers from the upper zone and UBS equivalents and having unradiogenic Sr similar to gabbroic rocks of Skaergaard, are isotopically heavier than their host ferrodiorites (Δ56Fegranophyre-ferrodiorite ≥ 0.1‰) reaching a maximum δ56Fe of 0.217‰ in UBS. A fused xenolith from UBS has δ56Fe = 0.372‰. This range in δ56Fe spans much of that reported for terrestrial igneous rocks, and like the global dataset, shows a pronounced increase in δ56Fe with inferred silica content of modeled Skaergaard liquids. Forward modeling of closed system fractional solidification was undertaken to account for Fe isotope systematics, first by testing published liquid lines of descent (LLD), and then by exploring improvements and considering the impacts of liquid immiscibility, crustal contamination, fluid exsolution and diffusional processes. Our modeling relies on published Fe+2 and Fe+3 force constants for magmatic minerals and silicate glasses, and the most reliable estimates of the average bulk composition and mass proportions of the well-defined subzones of the intrusion. We show that the increase in δ56Fe across the LZb–LZc boundary is readily explained by the increased incorporation of Fe+3 into the crystallizing solid including magnetite. We further demonstrate that the classic Fenner LLD, involving strong Fe enrichment at nearly constant silica, does not lead to a rise in δ56Fe toward the end stages of evolution, while a Bowen-like LLD, with little Fe enrichment and strong Si enrichment, also underestimates enrichment in heavy Fe isotopes in the ferrodiorites of UZc. A LLD following an intermediate path involving modest Fe and Si enrichment, followed by Fe depletion best explains the observations. We predict ~3.5% (by mass) residual liquid after crystallization of UZc having a composition similar to felsic segregations in pegmatitic bodies found in the intrusion. While liquid immiscibility may have been encountered within fractionating mush at the margins of the intrusion, the Fe isotope systematics do not support liquid phase separation of the bulk magma. Crustal contamination, fluid exsolution, hydrothermal alteration and thermal diffusion are also shown to have no resolvable effect on the Fe isotope composition of the gabbroic and granophyric rocks. We conclude that the Fe isotope systematics documented in the Skaergaard intrusion reflect the dominant role of fractionating Fe-rich minerals from gabbroic through ferrodioritic to rhyolitic liquids. The success of our model to account for the observed Fe isotope systematics for Skaergaard demonstrates the utility of Fe+2 and Fe+3 force constants determined at ambient conditions to model magmatic conditions and gives critical insights into plutonic processes fractionating Fe isotopes complementary to the volcanic record. [ABSTRACT FROM AUTHOR]

Here, we critically examine models for the development of layering in the ultramafic rocks of the Stillwater Complex of Montana, USA. A fine-grained, high-magnesium (high-Mg) dike in the Mountain View area of the complex is a plausible example of the type of magma that was parental to the layered cumulates in the Ultramafic Series. Modeled assimilation and crystallization of a primary komatiitic magma, involving 20% contamination by Archean granodiorite and 1% iron formation in a batch process, produces a liquid and solid assemblage like that in the dike. Equilibrium crystallization during assimilation of the crust produces cumulus olivine (Fo87–85) and chromite (Cr# = 64–55) consistent with the mineral chemistry of cumulus phases in the Peridotite Zone; each internally differentiated ultramafic layer could have formed by injection of a single highly contaminated batch of komatiite magma carrying about 10–20% suspended crystals. Whereas previous models of fractional crystallization to form the limited range of mineral and bulk-rock compositions observed in the Ultramafic Series required that these rocks accumulated from tens to thousands of times greater thicknesses of melt, the present model requires only four times as much melt as cumulates and dramatically diminishes the apparent volume of missing magma. The modeled crystallization of the proposed parental magma also demonstrates that, relative to olivine, only a small amount of cumulus chromite crystallizes in cotectic volume ratios of around 100:1 to 100:2 olivine to chromite. The modal abundance of chromite in the olivine-bearing cumulates from the Mountain View area varies continuously between 0·1 and 80 vol. %, with an average ratio of 100:9·2 olivine to chromite, rarely showing proportions consistent with either chromite-only or chromite + olivine cotectic crystallization. This, in turn, suggests that if olivine and chromite did precipitate in cotectic proportions, the crystals have subsequently been sorted mechanically to variable extents, producing the observed continuous range of modal proportions of chromite in the cumulates, and olivine has been preferentially removed from the system along with considerable amounts of escaped melt. Furthermore, the behavior of crystallizing assemblages at the olivine–orthopyroxene and the chromite–orthopyroxene peritectics forbids the deposition of the commonly observed mineral assemblages olivine + orthopyroxene + chromite or orthopyroxene + chromite by a process of fractional crystallization, but does permit them to form by deposition of mechanically sorted crystals derived from magmas carrying large crystal loads at internal equilibrium. Extremely efficient sorting of crystal-rich magma during lateral transport of crystals may therefore be responsible for the nearly monomineralic chromite seams in the Peridotite Zone. These considerations lead us to question the purported cyclicity of the cumulate rocks in the Peridotite Zone. Markov Chain analysis of the layering in logged drill core at various locations in the complex shows considerable departure from the suggested regular cycles of cumulate layers and provides no support for the classic model that units formed by closed-system equilibrium crystallization from mixed magmas following a succession of discrete magma recharge events. We suggest that the degree of contamination in the supply of crystal-laden contaminated magma transported to the site of deposition may have varied randomly, leading to the formation of layered rocks that do not conform to the previously accepted cyclic unit model of settled or in situ liquidus minerals. The layered rocks observed in the Peridotite Zone may have formed at the base of a magma chamber or they may not have, but their compositions and mineral modes do not convey any information about such a magma body if it was present at the time of their formation, because each layered unit appears to have formed from a completely separate volume of komatiitic magma. [ABSTRACT FROM AUTHOR]

The Cambridge Drill Core provides a continuous sample of the lower part of the floor cumulates of the Skaergaard Intrusion, including ~150m of stratigraphy from the unexposed Hidden Zone. Bulkrock geochemistry together with olivine mineral compositions and augite-plagioclase-plagioclase dihedral angles from the drill core are interpreted as a record of the early history of the Skaergaard magma chamber. A detailed geochemical and microstructural study of the mode and morphology of augite reveals no unambiguous markers that can be used to pinpoint the first appearance of cumulus augite in the stratigraphy sampled by the drill core. The early history of the Skaergaard magma chamber involved the arrival of multiple small batches of magma, each with a variable load of olivine and plagioclase phenocrysts. The region of the core between -120 and -108.8m records at least three separate, but closely spaced, influxes of new magma, with more magma added at -85 and -65 m. The last influx of magma is recorded by the cumulates just below the lowest exposed horizons of the Layered Series, involving a large volume of magma that inflated the chamber to its final size. The Skaergaard magma chamber therefore formed by the progressive inflation of what was likely to have originated as a sill intruded at the discontinuity between the Precambrian gneisses and the overlying plateau lavas. The 'Skaergaard parental magma' should therefore be viewed as the integrated and mixed composition of the various magma influxes that filled the chamber. [ABSTRACT FROM AUTHOR]

The dihedral angle formed at junctions between two plagioclase grains and a grain of augite is only very rarely in textural equilibrium in gabbros from kilometre-scale crustal layered intrusions. The median of a population of these disequilibrium angles, Θcpp, varies systematically within a single layered intrusion, remaining constant over large stretches of stratigraphy with significant increases and decreases associated with the addition or reduction respectively of the number of phases on the liquidus of the bulk magma. The stepwise changes in Θcpp are present in the Upper Zone of the Bushveld Complex, the Megacyclic Unit I of the Sept Iles Intrusion, and the Layered Series of the Skaergaard intrusion. The plagioclase-bearing cumulates of Rum have a bimodal distribution of Θcpp, dependent on whether the cumulus assemblage includes clinopyroxene. The presence of the stepwise changes is independent of the order of arrival of cumulus phases and of the composition of either the cumulus phases or the inferred composition of the interstitial liquid. The only parameter that behaves in an exactly analogous manner to Θcpp is the rate of change in enthalpy with temperature (ΔH/ΔT) during crystallization. Both ΔH/ΔT and Θcpp increase with the addition of a liquidus phase, and decrease with the removal of a liquidus phase. The replacement of one phase by another has little effect on ΔH/ΔT and no discernible effect on Θcpp. An increase of ΔH/ΔT results in an increase in the fraction of the total enthalpy budget that is the latent heat of crystallization (the fractional latent heat). It also increases the mass crystallized in each incremental temperature drop (the crystal productivity). These increases of both fractional latent heat and crystal productivity are likely to cause an increase in the time taken to form three-grain junctions in the mush via thermal buffering of a thickened mushy layer. We suggest these are the underlying causes of stepwise increases in Θcpp. Stepwise changes in the geometry of three-grain junctions in fully solidified gabbros thus provide a clear microstructural marker for the progress of fractionation down the liquid line of descent in layered intrusions. [ABSTRACT FROM AUTHOR]

The Isle au Haut Igneous Complex provides a unique opportunity to examine in detail the in situ physical and chemical interactions between contemporaneously emplaced mafic and silicic magmas. The complex contains a 600 m thick sequence of 11 alternating layers of gabbro and diorite (typically 15–40 m thick). Purely on the basis of density contrasts (2·65 g cm−3 gabbro vs 2·55 g cm−3 diorite), the entire system should have undergone wholesale instability and mixing; it is instead arrested in a grossly unstable state of interaction while molten. Chilled margins along the lower contacts of the gabbros and structural integrity of the diorite layers indicate that near-liquidus gabbroic magma invaded partly crystalline, cooler diorite. Mineral assemblages, chemical analyses, and phase equilibria calculations indicate initial temperatures during emplacement of ∼1180°C (gabbro) and ∼1000°C (diorite). Conductive thermal models yield solidification timescales of 15–60 years for single gabbro layers and about a thousand years for the entire complex. There is ample evidence for two phases of small-scale interfacial Rayleigh–Taylor type instabilities of dioritic melt into the gabbros. Phase I occurred immediately upon gabbro emplacement whereas evenly spaced, slender more silicic pipes represent a much later stage (Phase II). Pipe geometry and spacing, estimated viscosities of the gabbroic magma and silicic melt, and the sudden increase in silica near the upper contact of the diorite, all indicate a thin (∼18–53 cm) buoyant layer at the upper contact of the diorite as the source of the pipes. Compaction of the diorite produced this layer over a period of about 10 years. Simultaneous solidification along the lower contact of the overlying gabbro, thickening inwards, increased viscosity enough to arrest pipe ascent after a few meters. Crystal size distribution analyses of the gabbro layers yield crystal growth rates [Go = (2 − 4) × 10−10 cm s−1] and nucleation rates (Jo = 10−5–10−6 cm−3 s−1) indicative of conductive cooling coupled with some sluggish convective stirring owing to collapse of the roof-ward gabbro solidification fronts. Were the complex larger, with a much longer solidification time, all this evidence would have been lost, thus suggesting that in larger systems similar processes may commonly take place leaving little direct evidence of their operation apart from the ultimate final petrological product. [ABSTRACT FROM PUBLISHER]

A key target in the study of layered intrusions is to constrain the liquid line of descent of the magma. However, the evolution of the interstitial liquid is rarely considered, and its liquid line of descent is often assumed to be equivalent to that of the bulk magma. Because of extensive sub-solidus and diffusional changes that occur in slowly cooled rocks, clues to the composition of the interstitial liquid can only be obtained using very slowly diffusing trace elements and components. This study uses the Ti concentrations and anorthite contents of interstitial plagioclase to consider the compositional evolution of the interstitial liquid in the Skaergaard Intrusion. Ti–XAn zoning of interstitial plagioclase does not follow the same cryptic variations that develop in plagioclase primocrysts as a function of stratigraphic height, demonstrating that the bulk and interstitial liquid lines of descent are not equivalent. After Fe–Ti oxides start to crystallize, Ti concentrations decrease in both primocryst and interstitial plagioclase as a result of decreasing melt Ti. However, in the interstitial plagioclase within a single thin section, divergent trends develop adjacent to fine-grained interstitial pockets containing diverse mineral assemblages, which are interpreted to represent the crystallized products of late-stage immiscible liquids. These trends vary systematically as a function of stratigraphic height and spatial location within the intrusion. The distribution and compositions of these plagioclase zoning trends are used to comment on the spatial distribution and differential movement of interstitial immiscible liquids within the intrusion. [ABSTRACT FROM PUBLISHER]

• Mafic cumulates in the Lower Banded series of Stillwater Complex are modeled by crystallization of komatiitic liquid contaminated by lower crust. • The olivine-bearing rocks and high tenor sulfides in the J-M Reef Pd-Pt deposit are generated by infiltration into and reaction with mushy gabbronoritic footwall cumulates by incoming contaminated komatiitic melt. The origin and parental magma for layered cumulates in the Lower Banded series (LBS) and the J-M Reef Pd-Pt deposit of the Stillwater Complex remains poorly constrained. We present whole-rock lithogeochemistry and mineral chemistry from LBS rocks collected from drill holes and surface samples from the Mountain View area of the complex that in total span nearly the entirety of the LBS stratigraphy. Excess S, Pt, and Pd in the noritic and gabbronoritic cumulates of the LBS indicate that small amounts of high tenor sulfide liquid generated at very low degrees of sulfide oversaturation were ubiquitous parts of the cumulate assemblage. We show that a simple two-stage thermodynamic model of assimilation-batch crystallization of a komatiitic parental magma in the lower crust, produces a close match to a common suite of fine-grained gabbronorite dikes and sills that intrude both the complex and its footwall. After fractionating ultramafic cumulates in the lower crust, the model contaminated komatiitic liquid produces upper crustal cumulates by batch crystallization en route to or at the level of the intrusion. The modeled rocks have compositions and mineral assemblages closely resembling pyroxenite of the Bronzitite zone and both norite and gabbronorite cumulates in the lower LBS. The trends from the Bronzitite zone through Norite zone I and Gabbronorite zone I can be understood as the result of deposition of crystals from successive batches of the same contaminated parental magma, with an upward trend toward greater amounts of cooling before the separation of crystals from liquid. The olivine-bearing suite of Olivine-bearing zone I, which includes the J-M Reef, can be modeled by partial remelting of the same norite and gabbronorite cumulates due to a temporarily increased flux of hot, moderately less contaminated LBS parental magma that infiltrated partially molten cumulates because its density exceeded that of the interstitial liquid. This model suggests that infiltration of hot Mg-rich parental liquid into moderately PGE-enriched footwall cumulates may be fundamental to the formation of the extremely high tenor sulfide mineralization in the J-M Reef ore zone, and perhaps other reef-type deposits worldwide. The same metal tenors that would require silicate/sulfide mass ratios (i.e., R-factors) of 105 to 106 in a single stage of equilibration would be attained during this second stage of interaction by the incremental infiltration and passage of LBS parental magma through previously sulfide saturated cumulate mush. [ABSTRACT FROM AUTHOR]