ALBERT

Description: The Skaergaard intrusion, Greenland, is the type locality for Skaergaard-type mineralizations. Mineralization levels are perfectly concordant with igneous layering, up to 5 m thick, internally fractionated, and contain crystallized sulphide droplets and precious metal alloys, sulphides, arsenides and telluride. Immiscible Cu-rich sulphide droplets, formed in a mush zone below the roof, scavenged precious metals. They were subsequently dissolved and transported to the floor in late-formed, immiscible, Fe-rich mush melts. Mineralized stratigraphic intervals of floor gabbro formed in ‘proto-macrolayers‘, owing to local sulphide saturation in melt concentrated between floating plagioclase and sinking clinopyroxene. The floor mineralization is divided into four stratigraphic sections. Formation of the Lower Platinum Group Element Mineralization (LPGEM) involved: (1) crystallization of the bulk liquid liquidus paragenesis and in situ fractionation; (2) sulphide saturation and formation of sulphide droplets in melt in the upper part of ‘proto-macrolayers‘. After further in situ fractionation, the following steps occurred: (3) the onset of silicate–silicate immiscibility and the consequent loss of buoyant and immiscible Si-rich melt; (4) dissolution of unprotected droplets of sulphide melt present in the Fe-rich mush melt; (5) compaction-driven upwards loss of residual mush melt enriched in, for example, Au. The LPGEM preserves upward increasing bulk Pd/Pt (~6–13) owing to a continued supply of PGE and Au, with high Pd/Pt. The further development of the LPGEM ceased as the supply of precious metals to the floor waned. The Upper PGE Mineralization (UPGEM) subsequently formed from precious metals recycled in the floor. The UPGEM is characterized by increasing Au substitution in PGE phases, and a decrease in total PGE and Pd/Pt owing to upward fractionation in migrating mush melts and exhaustion of Pd and Pt. An upper Au-rich mineralization level (UAuM) was caused by late remobilization of Au and deposition on grain boundaries in fully crystallized gabbro. Cu concentrations (~150 ppm) are not correlated with PGE and Au. Repeated Cu mineralization levels (CuM), attaining 〉1000 ppm, occur above the Au levels, caused by local mush layer sulphide saturation. PGE, Au and Cu distributions in the floor mineralization reflect sub-liquidus, but supra-solidus, processes and reactions in mushes at the roof, wall and floor. Constraints provided by a new model for the mineralization provide the basis for re-evaluation of the solidification processes in the Skaergaard intrusion. We have identified the importance of extensive in situ fractionation and intrusion-wide elemental redistributions in immiscible Fe- and Si-rich silicate melts. Our model characterizes the floor cumulates as bulk liquid orthocumulates containing an upwards-increasing proportion crystallized from Fe-rich, immiscible mush melt. The roof-rocks are complementary to the floor, with downwards increasing proportions crystallized from the conjugate Si-rich melt. Petrographic observations and the relative timing of crystallization support the hypothesis that crystallization was restricted to marginal mush zones. Bulk melt remaining in the magma chamber evolved not, as generally assumed, as a result of loss of crystals grown from the bulk melt, but as the consequence of mixing with recycled and evolved melt expelled from the mush by compaction. Redistribution of Fe in immiscible melts may be common to mafic intrusions and puts into question the validity of petrogenetic modelling of bulk liquids in mafic intrusions based only on consideration of floor cumulates.

Description: 〈span〉〈div〉Abstract〈/div〉We present 〈span〉in situ〈/span〉 laser ablation–multicollector–inductively coupled plasma–mass spectrometry Sr isotope data for plagioclase from a reference stratigraphic profile of the entire Layered Series and Upper Border Series of the Skaergaard intrusion (East Greenland). Plagioclase Sr isotope compositions and anorthite contents vary systematically from the margins of the intrusion inwards. The lowest 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr〈sub〉i〈/sub〉 (calculated at 56 Ma) values (∼0.7041) occur near the base and top of the intrusion and systematically increase over several lithostratigraphic zones to a value of ∼0.7044, which is uniform throughout the middle ∼2000 m of intrusion. Across this same profile, anorthite content of plagioclase varies smoothly from An〈sub〉65–70〈/sub〉 at the base and top to ∼An〈sub〉25〈/sub〉 approaching the purported “Sandwich Horizon.” Plagioclase near the roof and proximal to rafts of partially assimilated basement gneiss are markedly more radiogenic (〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr〈sub〉i〈/sub〉 up to ∼0.7046). We explain the stratigraphic relationships by progressive contamination of the magma during early stages of differentiation by basement gneiss rafts entrained during emplacement and accumulated near the top of the chamber. Contamination was transient, ceasing once the entrained gneiss was consumed or isolated from the main magma reservoir as the solidification front advanced. Modeling of fractionation-assimilation processes accounts for the observed isotopic trends with only a few percent assimilation (relative to the original magma mass). The record of contamination revealed by Sr in plagioclase supports the view that the bulk of the Skaergaard intrusion formed by closed-system differentiation with only minor 〈span〉in situ〈/span〉 contamination and no magma recharge. Comparing plagioclase and bulk-rock Sr suggests that the latter may have witnessed late-stage metasomatic overprinting of phases other than plagioclase.〈/span〉

Description: Understanding the structure of the ocean-continent transition (OCT) in passive margins is greatly enhanced by comparison with onshore analogues. The North Atlantic margins and the "fossil" system in the Scandinavian Caledonides show variations along strike between magma-rich and magma-poor margins, but are different in terms of exposure and degree of maturity. They both display the early stages of the Wilson cycle. Seismic reflection data from the mid-Norwegian margin combined with results from Ocean Drilling Program Leg 104 drill core 642E allow for improved subbasalt imaging of the OCT. Below the Seaward-Dipping Reflector (SDR) sequences, vertical and inclined reflections are interpreted as dike feeder systems. High-amplitude reflections with abrupt termination and saucer-shaped geometries are interpreted as sill intrusions, implying the presence of sediments in the transition zone beneath the volcanic sequences. The transitional crust located below the SDR of the mid-Norwegian margin has a well-exposed analogue in the Seve Nappe Complex (SNC). At Sarek (Sweden), hornfelsed sediments are truncated by mafic dike swarms with densities of 70%–80% or more. The magmatic domain extends for at least 800 km along the Caledonides, and probably reached the size of a large igneous province. It developed at ca. 600 Ma on the margin of the Iapetus Ocean, and was probably linked to the magma-poor hyperextended segment in the southern Scandinavian Caledonides. These parts of the SNC represent an onshore analogue to the deeper level of the mid-Norwegian margin, permitting direct observation and sampling and providing an improved understanding, particularly of the deeper levels, of present-day magma-rich margins.

Description: Multiple levels of earthquake-induced soft-sediment deformations (seismites) are concentrated in the end-Triassic mass extinction interval across Europe. The repetitive nature of the seismites rules out an origin by an extraterrestrial impact. Instead, this intense seismic activity is linked to the formation of the Central Atlantic magmatic province (CAMP). By the earliest Jurassic the seismic activity had ceased, while extrusive volcanism still continued and biotic recovery was on its way. This suggests that magmatic intrusions into sedimentary strata during early stages of CAMP formation caused emission of gases (SO 2 , halocarbons, polycyclic aromatic hydrocarbons) that may have played a major part in the biotic crisis.

Description: The opening of the Arctic Ocean involved multiple stages of continental rifting and intrusion of extensive dyke swarms. To trace tectonomagmatic processes of the High Arctic, we present the first U–Pb ages for alkaline dyke swarms of North Greenland. Concordia ages of 80.8 ± 0.6 and 82.1 ± 1.5 Ma indicate that north–south and east–west dykes are coeval. The north–south dykes reflect initial east–west rifting that led to break-up along the Gakkel Ridge and formation of the Eurasia Basin. The east–west dykes reflect local variations in the stress field associated with reactivated Palaeozoic faults. Supplementary materials: U–Pb data are available at http://www.geolsoc.org.uk/SUP18857 .

Description: The Platinova Reef comprises a series of platinum group element (PGE)- and Au-rich layers that contain precious metals intimately associated with magmatic Cu-rich sulphides. This study presents new PGE, Au, S, Se and Te data for samples collected along a stratigraphic reference section from the base of the Lower Zone up to the Sandwich Horizon of the Skaergaard intrusion, and seeks to address the magma chamber processes that led to the formation of the Platinova Reef. The majority of the Skaergaard rocks have low S contents (〈500 ppm), with the S contents of those within and below the Platinova Reef being especially low (〈100 ppm). The very low S contents of these rocks are due in large part to the low S content of the initial Skaergaard magma; there is no evidence for any post-magmatic S loss in this part of the stratigraphy. Rayleigh fractionation modelling of the variations in metal concentrations of samples from this stratigraphic reference section indicates that the initial Skaergaard magma contained 240 ppm Cu, 89 ppm S, 4·0 ppb Au, 18·7 ppb Pd, 9 ppb Pt, 90 ppb Se and 5·7 ppb Te. The high Pd/Pt ratio of the Skaergaard magma indicates that it had undergone a considerable amount of differentiation prior to its entry into the Skaergaard magma chamber. Precious metal enrichment commenced at a stratigraphic level ~300 m below the Platinova Reef owing to saturation of the magma in Au–PGE-rich Cu sulphides. Although only tiny amounts of sulphides were initially formed, precious metal enrichment increased rapidly upwards to culminate in the formation of the Platinova Reef. Sulphide saturation of the magma was initially restricted to the boundary layer between the magma and the crystal mush where cumulus Fe–Ti oxides were forming. Although only very small amounts of sulphides were initially formed, the rate of sulphide production increased with time, leading to the entire residual magma in the chamber becoming sulphide saturated immediately after the formation of the Platinova Reef. Sulphide saturation and the eventual formation of the Au–PGE mineralization in the Reef was the product of a number of important factors. These include prolonged fractionation of the magma, which produced a residual melt that was enriched in Au, Pd, Cu, S and FeO and led to the build-up and eventual saturation of the magma in Fe–Ti oxides and Cu sulphides. The formation of cumulus ilmenite and magnetite slowed down the rate of build-up of FeO in the magma and also removed O 2 and caused some of the SO 4 2– in the residual magma to be converted to S 2– , whereas the high Cu content (621 ppm calculated from Rayleigh fractionation modelling) of the residual magma at this stage drove the magma to sulphide saturation, forming very small amounts of immiscible PGE–Au-rich Cu sulphides. Other factors that contributed to sulphide saturation of the magma were decreases in temperature and f O 2 of the magma, which were accompanied by an increase in the SiO 2 concentrations of the residual melt.

Description: The presence of Fe- and Si-rich liquids found as melt inclusions in apatite and olivine in the Upper Zone of the Skaergaard intrusion, East Greenland, demonstrates the occurrence of liquid immiscibility in the late-stage evolution of tholeiitic magmas in a plutonic setting. However, it remains unclear at which stage of crystallization unmixing began. To constrain the onset and the petrological importance of liquid immiscibility in the Skaergaard and tholeiitic magmas in general, we have studied crystallized melt inclusions entrapped in early primocryst plagioclase. Such melt inclusions become abundant from the top of the Lower Zone and upwards in the Layered Series, in primocryst plagioclase of composition An 54–26 . The daughter phase assemblage is the same in all the inclusions, although the modal proportions of the daughter phases are highly variable: plagioclase (42–59%), clinopyroxene (28–41%), ilmenite (4–9%), magnetite (3–10%), apatite (1–9%) and accessory phases (〈 1%). Accordingly, the bulk compositions of reheated and homogenized melt inclusions show large variations in SiO 2 (40–54 wt %), FeO t (7–23 wt %), P 2 O 5 (0–1·9 wt %) and K 2 O (0–2·8 wt %), and have variable CaO/Al 2 O 3 ratios. These variations are best explained by trapping of varying proportions of immiscible iron- and silica-rich melts and demonstrate that liquid immiscibility started in the upper part of the Lower Zone. We conclude that a significant part of the Skaergaard intrusion crystallized from an emulsion of Fe- and Si-rich immiscible melts. The heterogeneous trapping of a mixture of Fe- and Si-rich immiscible liquids in primocryst plagioclase indicates that the dispersed droplets in the Lower and Middle Zones were smaller than the size of the inclusion (〈 500 µm). In the Upper Zone, most of the inclusions in apatite are composed of the conjugate end-member liquids, indicating a larger size for the dispersed droplets. Metre-sized pods and layers of melanogranophyre in the upper part of the intrusion are believed to represent pooled bodies of the immiscible Si-rich liquid. Differentiation of an emulsified magma must be considered in petrogenetic models for the Skaergaard intrusion.