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Laser ablation multi-collector mass spectrometry (LA-MC-ICP-MS) has emerged as the technique of choice for in situ measurements of Sr isotopes in geological minerals. However, the method poses analytical challenges and there is no widely adopted standardised approach to collecting these data or correcting the numerous potential isobaric inferences. Here, we outline practical analytical procedures and data reduction strategies to help establish a consistent framework for collecting and correcting Sr isotope measurements in geological materials by LA-MC-ICP-MS. We characterise a new set of plagioclase reference materials, which are available for distribution to the community, and present a new data reduction scheme for the Iolite software package to correct isobaric interferences for different materials and analytical conditions. Our tests show that a combination of Kr-baseline subtraction, Rb-peak-stripping using βRb derived from a bracketing glass reference material, and a CaCa or CaAr correction for plagioclase and CaCa or CaAr + REE 2+ correction for rock glasses, yields the most accurate and precise 87Sr/ 86Sr measurements for these materials. Using the analytical and correction procedures outlined herein, spot analyses using a beam diameter of 100 μm or rastering with a 50–65 μm diameter beam can readily achieve 87Sr/ 86Sr measurements for materials with -1 Sr.

The end-Triassic mass extinction (ETME; c. 201.6 Ma), one of the so-called “big-five” mass extinctions in the Phanerozoic era, is associated with widespread marine anoxia, ocean acidification, global warming, carbon cycle perturbations (δ13C) and an extinction of diverse marine and terrestrial groups. This extinction is frequently linked to the volcanic activity in the Central Atlantic Magmatic Province (CAMP) which is often cited to explain e.g., the correlative negative carbon excursions across many sections, mutagenesis of land plants by Hg-toxicity, and enrichment of Hg/TOC. Despite this, the exact identification of a volcanic signal in many of these sections is not well constrained. In this study we present high-precision platinum group element (PGE: Ir, Ru, Pt, Pd) and Re data for the Triassic-Jurassic boundary succession at the Kuhjoch section (Austria). These are the first results from our new analytical setup using high-pressure asher digestion, isotope dilution and multi-collector inductively-coupled plasma mass spectrometry for precise determination in low concentration (e.g. ppt) samples such as sediments. The PGE and Re concentrations and patterns vary significantly with stratigraphy. The c. 13 m of clayey sediments above the onset of the extinction (marked by the c. 16 cm thick T-bed) show pronounced enrichment in Pt, Pd and Ir concentrations relative to the under- and overlying carbonate dominated stratigraphy. Their PGE patterns are non-chondritic with Pd/Ir and Pt/Ir similar to CAMP basalts. Normalised for lithology (Al2O3), however, there are no significant variations in Pt, Pd and Ir values below, within and above the clayey sediments. Re and Ru are, however, depleted compared to the other PGEs in the clayey interval, something also observed in some CAMP basalts. One possibility is therefore to interpret the PGE-rich, clayey sediments, including the main extinction interval in the basal portion, as recording increased weathering of CAMP basalts. As the PGE enrichment increases up through the T-bed, this could show that the onset of CAMP weathering and mass extinction would have therefore coincided. However, further work is needed to identify the relative role of CAMP volatile emission during volcanic activity versus post-eruption weathering of basalts.

There is a temporal correlation between the peak activity of the North Atlantic Igneous Province (NAIP) and the Paleocene–Eocene Thermal Maximum (PETM), suggesting that the NAIP may have initiated and/or prolonged this extreme warming event. However, corroborating a causal relationship is hampered by a scarcity of expanded sedimentary records that contain both climatic and volcanic proxies. One locality hosting such a record is Fur Island in Denmark, where an expanded pre- to post-PETM succession containing hundreds of NAIP ash layers is exceptionally well preserved. We compiled a range of environmental proxies, including mercury (Hg) anomalies, paleotemperature proxies, and lithium (Li) and osmium (Os) isotopes, to trace NAIP activity, hydrological changes, weathering, and seawater connectivity across this interval. Volcanic proxies suggest that NAIP activity was elevated before the PETM and appears to have peaked during the body of the δ13C excursion, but decreased considerably during the PETM recovery. This suggests that the acme in NAIP activity, dominated by flood basalt volcanism and thermogenic degassing from contact metamorphism, was likely confined to just ~200 kyr (ca. 56.0–55.8 Ma). The hundreds of thick basaltic ashes in the post-PETM strata likely represent a change from effusive to explosive activity, rather than an increase in NAIP activity. Detrital δ7Li values and clay abundances suggest that volcanic ash production increased basaltic reactive surface area, likely enhancing silicate weathering and atmospheric carbon sequestration in the early Eocene. Signals in lipid biomarkers and Os isotopes, traditionally used to trace paleotemperature and weathering changes, are used here to track seaway connectivity. These proxies indicate that the North Sea was rapidly cut off from the North Atlantic in under 12 kyr during the PETM recovery due to NAIP thermal uplift. Our findings reinforce the hypothesis that the emplacement of the NAIP had a profound and complex impact on Paleocene–Eocene climate, both directly through volcanic and thermogenic degassing, and indirectly by driving regional uplift and changing seaway connectivity.

Large-scale volcanic activity during the formation of large igneous provinces (LIPs) were contemporaneous with almost every mass extinction event in Earth’s history, and LIP activity is believed to have caused or contributed to at least three, if not all, Big Five mass extinctions. However, compared to the marine fossil record, the effects of the volcanism on the terrestrial plant record is still poorly understood. Extinctions in the animal record during major biotic crises in Earth history are not mirrored by comparable major changes in land plants. Despite being sedentary organisms land plants have evolved adaptations to cope with adverse changes in the environment which may provide autecological advantages compared to animals. Despite their remarkable resilience, land plant communities were still affected in multiple ways during LIP-induced extinction events. During the end-Triassic mass extinction (201.56–201.36 million years ago) emissions of greenhouse gases, sulfur dioxide and aerosols, halocarbons, polycyclic aromatic hydrocarbons, Hg and heavy metals from magmatic activity, as well as sea-level changes, during the emplacement of the Central Atlantic Magmatic Province (CAMP) are considered to have severely stressed land plants. This is exemplified by major changes in ecosystem structure in palynological records, a rise in microscopic charcoal abundance indicating increased wildfire activity, enhanced reworking of palynomorphs indicating increased soil erosion, acid rain damages on macroplant leaves, and increased abundances of abnormal spores and pollen indicating mutagenesis from Hg-toxicity and/or ozone layer depletion. Several of these land plant responses have also been observed during other extinction events contemporaneous to LIP activity. Here, we compare and discuss some of the changes in common between different biotic crises to evaluate whether there is a common pattern or not.

The High Arctic Large Igneous Province (HALIP) represents extensive Cretaceous magmatism throughout the circum-Arctic borderlands and within the Arctic Ocean (e.g., the Alpha-Mendeleev Ridge). Recent aeromagnetic data shows anomalies that extend from the Alpha Ridge onto the northern coast of Ellesmere Island, Nunavut, Canada. To test this linkage we present new bulk rock major and trace element geochemistry, and mineral compositions for clinopyroxene, plagioclase, and olivine of basaltic dykes and sheets and rhyolitic lavas for the stratotype section at Hansen Point, which coincides geographically with the magnetic anomaly at northern Ellesmere Island. New U-Pb chronology is also presented.The basaltic and basaltic-andesite dykes and sheets at Hansen Point are all evolved with 5.5–2.5 wt% MgO, 48.3–57.0 wt% SiO2, and have light rare-earth element enriched patterns. They classify as tholeiites and in Th/Yb vs. Nb/Yb space they define a trend extending from the mantle array toward upper continental crust. This trend, also including a rhyolite lava, can be modeled successfully by assimilation and fractional crystallization. The U-Pb data for a dacite sample, that is cut by basaltic dykes at Hansen Point, yields a crystallization age of 95.5 ± 1.0 Ma, and also shows crustal inheritance. The chronology and the geochemistry of the Hansen Point samples are correlative with the basaltic lavas, sills, and dykes of the Strand Fiord Formation on Axel Heiberg Island, Nunavut, Canada. In contrast, a new U-Pb age for an alkaline syenite at Audhild Bay is significantly younger at 79.5 ± 0.5 Ma, and correlative to alkaline basalts and rhyo- lites from other locations of northern Ellesmere Island (Audhild Bay, Philips Inlet, and Yelverton Bay West; 83–73 Ma). We propose these volcanic occurrences be referred to collectively as the Audhild Bay alkaline suite (ABAS). In this revised nomenclature, the rocks of Hansen Point stratotype and other tholeiitic rocks are ascribed to the Hansen Point tholeiitic suite (HPTS) that was emplaced at 97–93 Ma. We suggest this subdivision into suites replace the collective term Hansen Point volcanic complex.The few dredge samples of alkali basalt available from the top of the Alpha Ridge are akin to ABAS in terms of geochemistry. Our revised dates also suggest that the HPTS and Strand Fiord Formation volcanic rocks may be the hypothesized subaerial large igneous province eruption that drove the Cretaceous Ocean Anoxic Event 2.

We use new remote sensing data, historical reports, petrology and estimates of viscosity based on geochemical data to illuminate the lava emplacement flow-lines and vent structure changes of the summit ridge of Hekla during the large eruptions of 1845–46 and 1766–68. Based on the planimetric method we estimate the bulk volumes of these eruptions close to 0.4 km3 and 0.7 km3, respectively. However, comparison with volume estimates from the well-recorded 1947–48 eruption, indicates that the planimetric method appears to underestimate the lava bulk volumes by 40–60%. Hence, the true bulk volumes are more likely 0.5–0.6 km3 and 1.0–1.2 km3, respectively. Estimated melt viscosity averages for the 1766–68 eruption amount to 2.5 × 102 Pa s (pre-eruptive) and 2.5×103 Pa s (degassed), and for the 1845–46 eruption 2.2×102 Pa s (pre-eruptive) and 1.9×103 Pa s (degassed). Pre-eruptive magmas are about one order of magnitude more fluid than degassed magmas. In the 1845–46 and 1947–48 eruptions, SiO2 decreased from 58–57 to 55–54 wt% agreeing with a conventional model that Hekla erupts from a large, layered magma chamber with the most evolved (silicarich) magmas at the top. In contrast, the lava-flows from 1766–68 reveal a more complicated SiO2 trend. The lava fields emplaced in 1766 to the south have SiO2 values 54.9–56.5%, while the Hringlandahraun lava-flow that erupted from younger vents on the NE end of the Hekla ridge in March 1767 has higher SiO2 of 57.8%. This shows that the layered magma chamber model is not suitable for all lava-flows emplaced during Hekla eruptions.

The Bushveld Large Igneous Province (BLIP) was emplaced in three stages (I, II, and III) over some 19 m.y. (c. 2061–2042 Ma). The Bushveld Complex is the main constituent of the BLIP and hosts the world’s largest ferroan granite (Lebowa Granite Suite) and several granophyres (Rashoop Granophyre Suite) directly overlying the world’s largest layered mafic intrusion (Rustenburg Layered Suite). New zircon U–Pb ages (measured by ID-TIMS) for the Lebowa Granite Suite and a textural variant termed Zwartbank Pseudogranophyre yield identical crystallization ages ranging from 2054.3 ± 0.7 to 2056.5 ± 0.9 Ma. Zircons from the uppermost Rustenburg Layered Suite (Upper Zone) yield an indistinguishable age of 2055.1 ± 2.1 Ma. These ages demonstrate that the Bushveld granitoids, together with the Rustenburg Layered Suite belong to BLIP Stage II. Additionally, most Lebowa Granite Suite samples contain zircons with primary growth zonation that yield ages up to 2064 Ma. We interpret these as antecrysts originating from BLIP Stage I. Isotope compositions of Hf in zircon (measured by LA-MC-ICPMS in situ and on solutions) are essentially uniform at εHf (2055 Ma) = − 7.2 to − 5.6, and indistinguishable from published data for the uppermost Rustenburg Layered Suite. We interpret the indistinguishable ages and similar isotopic compositions of the Lebowa Granite Suite and the Rustenburg Layered Suite as support for direct petrogenetic linkages. The Hf isotopic composition of the Rustenburg Layered Suite has been attributed to enriched mantle sources, but we show that crustal assimilation by mantle-derived magmas can also explain these compositions for both the Lebowa Granite Suite and Rustenburg Layered Suite. We propose that the Lebowa Granite Suite formed during Stage II, but zircon antecrysts indicate that some remobilization of granitic material formed during Stage I was also involved.

We present a systematic examination of the volatile (Cl, F, Br, S) and trace element (Na, Mg, V, Mn, Fe, Sr, Y, Zr, Ba, REEs, Th, U) abundances in apatite from 29 samples from base to top of the Layered Series in the Skaergaard intrusion, East Greenland. Apatite occurs as an interstitial phase in the lower zones, but joins the liquidus assemblage late in the crystallisation history (c. 90% crystallised), defining the base of Upper Zone b. The apatite can be classified as fluorapatite with low Cl/F ratios (< 0.4), confirming previous result. The REE content of interstitial apatite decreases from the base up to the middle part of the intrusion, followed by an increase towards the base of Upper Zone b. When apatite becomes a primocryst (liquidus phase) it shows significant difference in composition, including elements such as REEs + Y, Cl, Sr, and Eu compared to interstitial apatite. Whilst Sr and Eu are enriched in primocryst, the other REEs + Y and Cl are depleted compared to interstitial grains. Whilst the REE composition of primocryst apatite can be explained by fractional crystallisation of the Skaergaard parental magma, the peculiar trend and enrichment of REEs in interstitial apatite can be reproduced using a simple mass balance model (i.e. a simple mass balance equilibrium model where the bulk concentration of an element is evenly distributed between all phases in the final cumulate rock according to the respective partitioning coefficients). In this regard, the difference in composition between interstitial and primocryst apatite is explained by different formation environments (i.e. crystallising (late) from trapped liquid vs. main magma). Compared to primocryst apatite, interstitial apatite in Skaergaard is more REE (and Cl) rich due to crystallisation and continued reaction with a highly fractionated trapped interstitial melt. During crystallisation and continued reaction with the interstitial melt and sub-liquidus equilibration, Sr and Eu partition into plagioclase, depleting interstitial apatite in Eu and Sr compared to primocryst apatite. The mass balance model demonstrates that the REE content of interstitial apatite can be attributed to partitioning competition within the present mineral assemblage during crystallisation of the interstitial melt and diffusive re-equilibration. In this regard, the proportion of trapped interstitial melt relative to the proportion of different primocryst minerals is crucial. It is shown that especially the modal abundance of clinopyroxene exerts strong control on the distribution of REE in interstitial apatite.

The Cretaceous High Arctic Large Igneous Province (HALIP) in Canada, although dominated by tholeiites (135-90 Ma), contains two main groups of alkaline igneous rocks. The older alkaline rocks (similar to 96 Ma) scatter around major fault and basement structures. They are represented by the newly defined Fulmar Suite alkaline basalt dykes and sills, and include Hassel Formation volcanic rocks. The younger alkaline group is represented by the Wootton Intrusive Complex (92.2-92.7 Ma), and the Audhild Bay Suite (83-73 Ma), both emplaced near the northern coast of Ellesmere Island. Fulmar Suite rocks resemble EM-type ocean island basalts (OIB) and most show limited crustal contamination. The Fulmar Suite shows increases of P2O5 at near-constant Ba-K-Zr-Ti that are nearly orthogonal to predicted fractionation- or melting-related variations, which we interpret as the result of melting composite mantle sources containing a regionally widespread apatite-bearing enriched component (P1). Low-P2O5 Fulmar Suite variants overlap compositionally with enriched HALIP tholeiites, and fall on common garnet Iherzolite trace element melting trajectories, suggesting variable degrees of melting of a geochemically similar source. High-P2O5 Hassel Formation basalts are unusual among Fulmar rocks, because they are strongly contaminated with depleted lower crust; and because they involve a high-P2O5-Ba-Eu mantle component (P2), similar to that seen in alkali basalt dykes from Greenland. The P2 component may have contained Ba-Eu-rich hawthorneite and/or carbonate minerals as well as apatite, and may typify parts of the Greenlandic sub-continental lithospheric mantle (SCLM). Mafic alkaline Audhild Bay Suite (ABS) rocks are volcanic and hypabyssal basanites, alkaline basalts and trachy-andesites, and resemble HIMU ocean island basalts in having high Nb, low Zr/Nb and low Sr-87/Sr-86(i). These mafic alkaline rocks are associated with felsic alkaline lavas and syenitic intrusions, but crustally derived rhyodacites and rhyolites also exist. The Wootton Intrusive Complex (WIC) contains geochemically similar plutonic rocks (alkali gabbros, diorites and anatectic granites), and may represent a more deeply eroded, slightly older equivalent of the ABS. Low-P2O5 ABS and WIC alkaline mafic rocks have flat heavy rare earth element (HREE) profiles suggesting shallow mantle melting; whereas High-P2O5 variants have steep HREE profiles indicating deeper separation from garnet-bearing residues. Some High-P2O5 mafic ABS rocks seem to contain the P1 and P2 components identified in Fulmar-Hassel rocks, whereas other samples trend towards possible High-P2O5 + Zr (PZr) and High-P2O5 + K2O (PK) components. We argue that the strongly alkaline northern Ellesmere Island magmas sampled mineralogically heterogeneous veins or metasomes in Greenlandic-type SCLM, which contained trace phases such as apatite, carbonates, hawthorneite, zircon, mica or richterite. The geographically more widespread apatite-bearing component (P1) could have formed part of a heterogeneous plume or upwelling mantle current that also generated HALIP tholeiites when melted more extensively, but may also have resided in the SCLM as relics of older events. Rare HALIP alkaline rocks with high K-Rb-U-Th fall on mixing paths implying strong local contamination from either Sverdrup Basin sedimentary rocks or granitic upper crust. However, the scarcity of potassic alkaline HALIP facies, together with the other trace element and isotopic signatures, provides little support for a ubiquitous fossil sedimentary subduction-zone component in the HALIP mantle source. Correction in: JOURNAL OF PETROLOGY, Volume 63, Issue 3, Article Number egac020, DOI 10.1093/petrology/egac020

Highlights • Determination of the tectonomagmatic evolution of the NW Atlantic • Mapping of the breakup-related magmatism using the vocanostratigraphy concept • Mapping of the magnetic and gravimetric crustal domain patterns and their boundaries using integrated SGM method • Analyses of seabed and onshore samples and field analogues work • New plate tectonic reconstruction for the NW Atlantic Abstract Passive margins are the locus of tectonic and magmatic processes leading to the formation of highly variable along-strike and conjugate margins structures. Using extensive new seismic, gravity, and magnetic datasets, complemented by seabed samples and field work, we investigate the tectonomagmatic evolution of the northwest (NW) Atlantic where breakup-related igneous rocks were emplaced during several Paleogene events associated with lithospheric stretching, continental breakup, and the formation of new oceanic basins. Interpretational methods include integrated seismic-gravity-magnetic (SGM) interpretation and seismic volcanostratigraphy. In addition, seabed and field samples were collected and analyzed to constrain the basin stratigraphy, hydrocarbon potential, and the geochronology and geochemistry of the volcanic sequences. Offshore, 2D seismic data reveal several Seaward Dipping Reflector (SDR) wedges and escarpments in the Labrador Sea, Davis Strait, and Baffin Bay. Onshore, eastward prograding foreset-bedded hyaloclastite delta deposits and overlying horizontal lava successions outcrop on Nuussuaq. These hyaloclastites and lava successions are world class analogues to the Lava Delta and Landward Flows volcanic seismic facies units identified offshore. Our mapping results document an aerial extent of the Paleogene breakup-related volcanics of 0.3 × 106 km2, with an estimated volume of 0.5–0.6 × 106 km3. Basalt samples recovered by dredging the Upernavik Escarpment have late Paleocene to/early Eocene ages, whereas the sedimentary samples provide an excellent seismic tie with the stratigraphy and the geology in this frontier area. From the integrated SGM interpretation, we identify a rapidly thinning crust and changes in top and intra-basement seismic reflection characteristics in the oceanic domain correlated with transition between different magnetic domains. The mapping results were subsequently integrated in a plate tectonic model. The plate tectonic reconstruction and basalt geochronology suggest that the majority of the volcanism in the NW Atlantic occurred between ~62 and ~58 Ma, associated with an increased spreading rate in the Labrador Sea, starting from the onset of the Selandian (~61.6 Ma). A change in the spreading direction during the Eocene (~56 Ma), synchronously with a shift of volcanic activity from the NW to the NE Atlantic, correspond to a northward drift of Greenland and the initiation of the Eurekan Orogeny. Finally, our interpretations reveal a complex rift configuration along the NW Atlantic conjugate margins both prior to and during breakup.

The origin of Large Igneous Provinces (LIPs) associated with continental breakup and the reconstruction of continents older than c. 320 million years (pre-Pangea) are contentious research problems. Here we study the petrology of a 615 - 590 Myr dolerite dyke complex that intruded rift-basins of the magma-rich margin of Baltica and now is exposed in the Scandinavian Caledonides. These dykes are part of the Central Iapetus Magmatic Province (CIMP), a LIP emplaced in Baltica and Laurentia during opening of the Caledonian Wilson Cycle. The >1000 km long dyke complex displays lateral geochemical zonation from enriched to depleted basaltic compositions from south to north. Geochemical modelling of major and trace elements shows these compositions are best explained by melting hot mantle 75-250°C above ambient mantle. Although the trace element modelling solutions are non- unique, the best explanation involves melting a laterally zoned mantle plume with enriched and depleted peridotite lithologies, similar to present-day Iceland and to the North Atlantic Igneous Province. The origin of CIMP appears to have involved several mantle plumes. This is best explained if rifting and breakup magmatism coincided with plume generation zones at the margins of a Large Low Shear-wave Velocity Province (LLSVP) at the core mantle boundary. If the LLSVPs are quasi-stationary back in time as suggested in recent geodynamic models, the CIMP provides a guide for reconstructing the paleogeography of Baltica and Laurentia 615 million years ago to the LLSVP now positioned under the Pacific Ocean. Our results provide a stimulus for using LIPs as piercing points for plate reconstructions.

The Paleocene-Eocene Thermal Maximum (PETM) is recognized as one of the potential analogues in the geological record for present-day global warming. The aim of the International Continental Scientific Drilling Program (ICDP) project PVOLC is to test the hypothesis that voluminous magmatism in sedimentary basins in the NE Atlantic triggered the PETM. Two ICDP boreholes are planned to core the boundary in the Limfjorden area in Denmark in 2022. PVOLC will be conducted in conjunction with IODP Exp 396 on the mid-Norwegian continental margin. The North Atlantic Igneous Province (NAIP) was is a large igneous province (5–10 million km3 magma) that coincided with both the opening of the NE Atlantic Ocean and the greenhouse conditions of the early Paleogene. The close temporal correlations suggest a possible causal relationship between the NAIP and both the climatic and tectonic changes around 56– 54 Ma. In particular, the main acme of NAIP activity occurred across the Paleocene–Eocene Thermal Maximum (PETM), an extreme hyperthermal event that represents the warmest conditions of the last 60 million years. The NAIP is among several proposed candidates for driving global warming through CO2/CH4 emissions, both by magmatic degassing and through contact metamorphism around shallow intrusions in organic rich sedimentary basins. What is needed to refine the role of the NAIP during the PETM are key sedimentary sequences that contain abundant volcanic and climatic proxies in the same section, thereby allowing a precise geochronology of events to be attained. The sediments exposed on the Fur island, Denmark, are a key sequence of PETM and post-PETM strata with little thermal overprint and hundreds of well-preserved volcanic ash layers from the NAIP. The effects of Quaternary glaciotectonism have disturbed this key stratigraphic interval at Fur, but seismic surveys indicate that undisturbed strata are found a few km to the south. The ICDP PVOLC project plan is to drill both the Paleocene-Eocene and the Cretaceous-Paleocene boundaries, hopefully recovering pristine cores suitable for high-resolution geochemical and climatic studies.

The end-Triassic mass extinction (ETME) is thought to have been caused by voluminous, pulsed volcanic activity of the Central Atlantic Magmatic Province (CAMP). Over the last decades, various geochemical signals and proxy records, including δ13C, pCO2, iridium and other platinum-group elements, mercury, polycyclic aromatic hydrocarbons (PAH), charcoal and SO2, have been directly or indirectly attributed to CAMP magmatism. Here, we compile and discuss these various records in a stratigraphic framework to present a cohesive chain of events for the CAMP and the end-Triassic mass extinction. Mercury and iridium anomalies indicate that CAMP activity commenced prior to the onset of the marine extinctions (as marked by the last occurrence of the Triassic ammonoid Choristoceras marshi or closely related species), and a negative δ13C excursion in organic matter (the Marshi CIE). This CIE may be explained by input of light carbon to the atmosphere from CAMP lavas of the Tiourjdal and Prevalent groups. Pedogenic carbonate below and above the Prevalent group in North America indicates a more than twofold increase in atmospheric pCO2. Subsequent n-alkane C-isotopes, and stomatal pCO2 data seem to indicate a temporary cooling after the Marshi CIE, which is consistent with climate models incorporating volcanic emissions of both CO2 and SO2. Records of excess iridium and Hg/TOC indicate intensified magmatism during the extinction interval. Tectonic and perhaps epeirogenic (i.e. doming due to rise of magma) activity is suggested by the occurrence of multiple and widespread seismites in Europe. Atmospheric pCO2 proxies indicate global warming, which culminated contemporaneously with a second negative CIE (the Spelae CIE) at the level of the first occurrence of the ammonoid Psiloceras spelae, the index taxon fot the Triassic−Jurassic boundary (TJB). Global warming at this level is corroborated by increased wildfire activity testified by charcoal and pyrolytic PAH records. Just prior to the increase in pCO2 from stomatal proxy data, fossil plants exhibit SO2-induced damage indicating excess sulfur dioxide deposition priot to and across the TJB. This coincides with increased ratios of heavy molecular PAHs (coronene/benzo(a)pyrene) in sediments, which may suggest metamorphism of organic sediments also occurred across the TJB. This suggests that thermogenic release of light carbon and sulfur from sill intrusions in the Trans-Amazonian basins, where both evaporate- and organic-rich sediments are known to have been intruded, may have played an important role during the course of the ETME. Geochemical traces of magmatism, i.e. Ir and Hg, appear to have gradually disappeared during the Hettangian, suggesting that later phases of CAMP were less voluminous. Stomatal proxy data from Greenland and n-alkane C-isotope data from the UK, together with oxygen isotope data from carbonate fossils in the UK, may indicate that the global warming at the Spelae CIE was succeeded by another short-term cooling event. A gradual decrease in δ13C culminated at the top-Tilmanni CIE, marking the beginning of a long-term steady state with more negative C-isotope values than prior to the ETME. At this time, terrestrial ecosystems appear to have stabilized globally and ammonoids had begun to rediversify.

The origin of Large Igneous Provinces (LIPs) associated with continental breakup and the reconstruction of continents older than c. 320 million years (pre-Pangea) are contentious research problems. Here we study the petrology of a 615 - 590 Myr dolerite dyke complex that intruded rift-basins of the magma-rich margin of Baltica and now is exposed in the Scandinavian Caledonides. These dykes are part of the Central Iapetus Magmatic Province (CIMP), a LIP emplaced in Baltica and Laurentia during opening of the Iapetus Ocean within the Caledonian Wilson Cycle. The >1000 km long dyke complex displays lateral geochemical zonation from enriched to depleted basaltic compositions from south to north. Geochemical modelling of major and trace elements shows these compositions are best explained by melting hot mantle 75-250°C above ambient mantle. Although the trace element modelling solutions are non-unique, the best explanation involves melting a laterally zoned mantle plume with enriched and depleted peridotite lithologies, similar to present-day Iceland and to the North Atlantic Igneous Province. The origin of CIMP appears to have involved several mantle plumes. This is best explained if rifting and breakup magmatism coincided with plume generation zones at the margins of a Large Low Shear-wave Velocity Province (LLSVP) at the core mantle boundary. If the LLSVPs are quasi-stationary back in time as suggested in recent geodynamic models, the CIMP provides a guide for reconstructing the paleogeography of Baltica and Laurentia 615 million years ago to the LLSVP now positioned under the Pacific Ocean. Our results provide a stimulus for using LIPs as piercing points for plate reconstructions.

Cretaceous High Arctic large igneous province (HALIP) sub-alkaline magmatic rocks in Canada are mostly evolved (MgO 2-7 wt%), sparsely plagioclase + clinopyroxene +/- olivine-phyric tholeiitic basalts. There were two main HALIP continental flood basalt (CFB) eruption episodes: 135-120 Ma (Isachsen Fm.) and 105-90 Ma (Strand Fiord Fm.), both associated with cogenetic doleritic sills and dykes. Building on a large modern database, 16 HALIP tholeiite types are defined and grouped into genetic series using Ce vs Sm/Yb-NMORB distributions. Comparison with model melting curves implies that higher-Sm/Yb HALIP basalt types record low-degree melting of garnet-bearing mantle sources. More voluminous intermediate- and low-Sm/Yb HALIP basalt types separated from the mantle at shallower levels after further extensive melting in the spinel-peridotite field. Within a given Sm/Yb range, increases in incompatible elements such as Ce are coupled with progressive clockwise rotation of normalized incompatible trace element profiles. Trace element modeling implies this cannot be due to closed-system fractional crystallization but requires progressive and ubiquitous incorporation of a component resembling continental crust. The fractionation models imply that low-Sm/Yb HALIP basalts (similar to 7 wt% MgO) initially crystallized olivine gabbro assemblages, with lower-MgO basalts successively crystallizing gabbro and ilmenite-gabbro assemblages. In contrast, higher-Sm/Yb basalts fractionated more clinopyroxene and ilmenite, but extensive plagioclase fractionation is still required to explain developing negative Sr-Eu anomalies. Backfractionation models require about 40% addition of olivine to bring the most primitive HALIP basalts (similar to 7% MgO) into equilibrium with Fo(89) mantle. Inverse fractionation-assimilation modeling shrinks the CFB signature, making decontaminated model parental melts more similar to enriched mid-ocean ridge basalt. The progressive increase of the contamination signature within each HALIP tholeiitic differentiation series is not consistent with models involving derivation of HALIP basalts from a mantle source previously enriched by subduction. Strong interaction of basalt with Sverdrup Basin sedimentary rocks may cause localized over-enrichment in K-Rb-Th-U, but cannot explain strong Ba enrichment in the absence of concomitant K-Rb-Th-U enrichment. The localized Ba enrichment could reflect either a Ba-rich lithospheric mantle component that is strongly manifested in the coeval HALIP alkaline suites, or syn- to post-emplacement fluid-mediated transfer from Ba-rich host rocks. Correction in: JOURNAL OF PETROLOGY, Volume 63, Issue 3, Article Number egac019, DOI 10.1093/petrology/egac019

In the Bushveld Complex of South Africa, numerous granitic and granophyric rocks termed the Lebowa Granite and Rashoop Granophyre Suites, respectively, overlie the layered ultramafic-mafic rocks of the Rustenburg Layered Suite. Despite their close spatial and temporal association, the granites and granophyres are often interpreted as being unrelated to the Rustenburg Layered Suite. This paper describes the transition from the uppermost Rustenburg Layered Suite into overlying granite and granophyre at three locations in the western (the Bierkraal drill core) and eastern (Diepkloof farm and Stoffberg town) limbs of the Bushveld Complex. In the western limb, a ~60 m thick transition zone bridges the petrological gap between the overlying Nebo Granite (Lebowa Granite Suite) and Upper Zone (Rustenburg Layered Suite). Across the transition zone, the composition of olivine changes from Fo6 to Fo1, clinopyroxene from Mg#25 to Mg#2 and plagioclase from An45 to An16. At Stoffberg in the eastern limb, the Upper Zone dioritic cumulates grade into the overlying monzonitic Roof Zone. Across the Roof Zone, plagioclase compositions change from An42 to An4, clinopyroxene from Mg#30 to Mg#11 and olivine from Fo9 to Fo5. Based on geochemistry and petrography, we correlate the lowermost stratigraphy at the farm Diepkloof with the Stoffberg Roof Zone. At Diepkloof, the Roof Zone grades into the overlying Stavoren Granophyre (Rashoop Granophyre Suite). All units are indistinguishable in terms of bulk rock Nd (εNd = −6.4 to −5.4) and Hf (εHf = −9.2 to −6.6) isotopes (corrected to 2055 Ma). Similarly, the Upper Main Zone, Upper Zone and transition zone are indistinguishable in terms of bulk rock Sr isotopes ((87Sr/86Sr)2055 Ma = 0.7071 to 0.7076, except one transition zone outlier at 0.7058). We submit that the transition zone represents the fossil record of bulk and/or diffusional mixing between coexisting Upper Zone and Nebo Granite magmas. We test this hypothesis by combining field, petrographic and geochemical observations with forward modelling using the Rhyolite-MELTS algorithm. Our work on the Bierkraal core (western limb) shows that at least a portion of the granitic magma was emplaced before the residual liquid of the Upper Zone had solidified. At Stoffberg and Diepkloof in the eastern limb (where the granite is absent), the Roof Zone underwent uninterrupted fractional crystallization.

The end-Triassic mass extinction is considered to have been caused by voluminous and repeated emissions of CO2 and/or methane and other gases from magmatic activity in the Central Atlantic Magmatic Province. Despite improved geochronological dating and correlation between the magmatic activity and the extinctions, exactly how the biotic crisis commenced remains poorly understood. Here, we compile palynological and palaeobotanical data, bulk organic δ13C, biomarkers, mercury and other geochemical proxies, charcoal, and sedimentology, from a Rhaetian terrestrial succession in southern Sweden. Our results provide an insight into the climatic, environmental and ecosystem changes that took place at the onset of the mass extinction event.

The close temporal correlation between the emplacement of large igneous provinces and environmental crises in the geological record suggests a causal relationship. One such example is the emplacement of the North Atlantic Igneous Province (NAIP) and the Paleocene-Eocene Thermal Maximum (PETM), an extreme climate change event that occurred ~56 Ma. The main pulse of activity from the NAIP is around this time, but current radioisotopic ages are too low-resolution to constrain whether this activity was before, during and/or after the PETM. An ideal locality for understanding the initiation and development of the PETM is the island of Fur, northwest Denmark. The sedimentary sequence consists of clays and diatomites deposited in an epicontinental, shallow marine sea. The high sedimentation rates and close proximity to the NAIP means there are numerous volcanic and climatic proxies in the strata that can be used to provide high-resolution records constraining the relative and absolute timings of these events.Here we present the findings of the project ‘Ashlantic’, which focuses on pre- to post-PETM strata. We adopt a multiproxy approach using volcanic tracers, including tephra horizons, Hg anomalies, and Os isotopes, to infer the intensity and timing of NAIP activity. Volcanic glass morphology and chemistry suggests a hydromagmatic origin for key tephra intervals, while U-Pb dating of magmatic zircon constrains the timing of NAIP activity and the development of the PETM. Detailed chemostratigraphic logs and datasets (e.g. δ13C analyses) define the onset and duration of the PETM, while clay chemistry, Li isotopes, total organic carbon (TOC), and the paleothermometer TEX86 are used to assess the climate response to global warming during the PETM. In concert, our results suggest that the NAIP was active just before, during, and after the PETM, but the relationship between the NAIP and the marine and terrestrial environments is complex. These findings call for further work, such as ICDP and/or IODP drilling of North Sea sediments.

Apatite geochronology is a versatile method for providing medium temperature history constraints of magmatic and metamorphic rocks. The LA-ICP-MS technique is widely applied to U/Pb geochronology using various minerals. Apatite U/Pb geochronology, in contrast to e.g., zircon, is compromised by variable amounts of common Pb incorporated into the crystal during growth. Magmatic apatite often shows a sufficient spread in data to obtain a precise and accurate lower intercept age. If this is not the case, the initial Pb isotopic composition needs to be estimated to obtain accurate and precise age information from apatite. Two approaches are common, one being the estimation of common Pb from a Pb evolution model and the other being the measurement of a coexisting mineral phase that tends to incorporate Pb but not U, e.g., feldspar. Most recent studies applying LA-ICP-MS to the analysis of Pb isotopes in feldspar utilize either multicollector or magnetic sector mass spectrometers. In this study we first evaluate the application of quadrupole mass spectrometry for apatite U/Pb geochronology combined with Pb isotopic measurements in feldspar and compare the results with modeled initial Pb isotopic compositions. The resulting age information is accurate and precise despite using plagioclase rather than K-feldspar, as is normally used, to define initial Pb isotope compositions. We apply this method to apatite-bearing gabbroic rocks from layered intrusions (Bushveld, Bjerkreim-Sokndal, Hasvik, and Skaergaard) ranging in age from ca. 2 Ga to ca. 55 Ma and generate metamorphic/cooling ages generally consistent with the known geologic history of these intrusions.

Plagioclase ultraphyric basalt lava with high fraction of solids have a mode of emplacement that is poorly understood. In this study we conduct detailed mapping of a PUB group in eastern Iceland, namely the Grænavatn group, and assess the group architecture, flow morphology and internal structure with additional constraints from petrography, petrology and crystal size distribution, to derive information on emplacement dynamics of plagioclase ultraphyric basalts. We also derive information on the plumbing system of the group with reference to the source of the macrocysts. The group is exposed in steep glacially carved fjords and can be traced for more than 70km along strike. The flows have mixed architecture of simple and compound flows. Individual flow lobes have thicknesses in the range of 1-24m and many reach widths and lengths exceeding 1000m. The flows vary from rubbly to slabby pahoehoe, but are predominantly of pahoehoe type. The aspect ratio of the group and the nature of the flows indicate fissure-fed eruptions. The plagioclase macrocrysts (5-30mm) are An-rich, exhibit bimodal size distribution and the modal proportions within the group varies from 15-40%. Clinopyroxene macrocrysts are also present ranging from 1-6%. The lowermost flow is thickest and carries the greatest crystal cargo load. The morphology of the lava flows suggests low viscous behavior, at odds with the high crystal content. The very calcic plagioclase macrocrysts (An80-85) are in disequilibrium with the groundmass and plagioclase microlaths therein (An50-70), meaning that the crystal-laden magmas quickly ascended from deeper crustal levels to the surface. The flows with highest crystal content may have maintained high temperatures by heat exchange with the primitive macrocrysts in the flows and developed non-Newtonian behavior such as shear thinning. Such conditions would have enabled the flows to advance rapidly during episodes with high effusion rates forming the simple flows, and subsequently maintained by insulated lobe-by-lobe emplacement. Occasionally surges disrupted the crust to form rubbly and slabby pahoehoe. A Herschel-Bulkley model is applied to the flows yielding effusion rates in the order of 103-104 m 3/s for the largest eruptions. Variation in crystal proportions within the group and individual flows is considered to be the result of magmas penetrating a crystal mush, followed by disaggregation and mobilization, sorting by flotation, mixing of magma batches and transportation of heterogeneous crystal-laden magmas to the surface with opportunities for staging en route.

The Skaergaard intrusion in East Greenland hosts mineralisation of platinum group elements and gold that is characterised by a remarkably low amount of sulphides (about 0.02 wt% sulphide) and a very high metal/sulphur ratio. Here we present the first quantitative analyses of sulphur (S), copper (Cu), lithium (Li) and other trace elements from melt inclusions in plagioclase to trace their evolution in the magma and to constrain the formation of the mineralisation. The concentration of these elements in the melt inclusions varies considerably within each sample. The S content in melt inclusions ranges from 165 to 320 ppm in the lower part of the intrusion, but increases to 349–512 ppm immediately below the mineralisation interval. In the same interval, the Li content increases from 22–43 to 40–69 ppm and Cu ranges from 118 to 1586 ppm with no systematic variations. Across the mineralisation, the Li content drops an order of magnitude to 4–7 ppm and S drops to 250–326 ppm. We explain these compositional changes as reflecting the magmatic evolution as the result of fractional crystallisation below, and coinciding aqueous fluid exsolution and sulphide saturation across the mineralisation. Normally the S content of basaltic magmas is much higher at sulphide saturation. We propose that sulphide saturation is possible, nevertheless, mostly due to the high Cu content and high Cu/Fe in the evolved ferrobasaltic melt resulting in a lowering of the S concentration required for sulphide saturation. The relationship between exsolutions of aqueous fluids and sulphides remains unknown.

Elevated concentrations of iridium (Ir) and other platinum-group elements (PGE) have been reported in both terrestrial and marine sediments associated with the end-Triassic mass extinction (ETE) c. 201.5 million years ago. The source of the PGEs has been attributed to condensed vapor and melt from an extraterrestrial impactor or to volcanism. Here we report new PGE data for volcanic rocks of the Central Atlantic Magmatic Province (CAMP) in Morocco and show that their Pd/Ir, Pt/Ir and Pt/Rh ratios are similar to marine and terrestrial sediments at the ETE, and very different from potential impactors. Hence, we propose the PGEs provide a new temporal correlation of CAMP volcanism to the ETE, corroborating the view that mass extinctions may be caused by volcanism.

Mantle melting dynamics of the Central Atlantic Magmatic Province (CAMP) is constrained from new platinum group element (PGE), gold (Au), rare earth element (REE), and high field strength element (HFSE) data and geochemical modelling of flood basalts in Morocco. The PGE are enriched similarly to flood basalts of other large igneous provinces. The magmas did not experience sulphide saturation during fractionation and were therefore fertile. The CAMP is thus prospective for PGE and gold mineralization. The Pt/Pd ratio of the Moroccan lavas indicates that they originated by partial melting of the asthenospheric mantle, not the subcontinental lithospheric mantle. Mantle melting modelling of PGE, REE and HFSE suggests the following: (1) the mantle source for all the lavas was dominated by primitive mantle and invariably included a small proportion of recycled continental crust (

The Central Atlantic Magmatic Province (CAMP) is a large igneous province (LIP) composed of basic dykes, sills, layered intrusions and lava flows emplaced before Pangea break-up and currently distributed on the four continents surrounding the Atlantic Ocean. One of the oldest, best preserved and most complete sub-provinces of the CAMP is located in Morocco. Geochemical, geochronologic, petrographic and magnetostratigraphic data obtained in previous studies allowed identification of four strato-chemical magmatic units, i.e. the Lower, Intermediate, Upper and Recurrent units. For this study, we completed a detailed sampling of the CAMP in Morocco, from the Anti Atlas in the south to the Meseta in the north. We provide a complete mineralogical, petrologic (major and trace elements on whole-rocks and minerals), geochronologic (40Ar/39Ar and U–Pb ages) and geochemical set of data (including Sr–Nd–Pb–Os isotope systematics) for basaltic and basaltic–andesitic lava flow piles and for their presumed feeder dykes and sills. Combined with field observations, these data suggest a very rapid (

Many layered intrusions are considered to have been repeatedly inflated by magma additions, but rates of magma mixing relative to rates of layer accumulation are difficult to model. The nature of magma recharge through the interval including the Pyroxenite Marker (PM), Main Zone, Bushveld Complex, South Africa, is examined with regard to such processes. The plagioclase compositions (An value) in five previously published and three new profiles (presented here and focusing on the core compositions) that are at least 600 m in vertical extent and spread along a strike length of 110 km are evaluated. The compilation of the eight profiles shows the following trends. Upward reversals in compositions show considerable lateral as well as vertical variations. Lateral variations show a range in: (1) the minimum An value reached in each profile prior to the onset of magma recharge (An51 to An59); (2) the depth below the PM at which the minimum value is observed (50 to 575 m); (3) the An value close to the PM (An54 to An75); (4) the maximum value recorded above the PM (An63 to An76); (5) the height above the PM at which this maximum value is reached (0 to 300 m) – in all cases, the highest values of An occur at the northern end of the studied sections; and (6) the vertical extents over which the reversals occur range from 150 to over 600 m indicating very protracted magma additions and/or slow mixing. The PM terminates toward the south, and close to this termination the immediate footwall rocks to the PM change from north to south from gabbronorite to magnetite gabbronorite. A cross-section through these profiles defines two basins, with an intervening structural upwarp. The magma pulses that were added to produce very gradual and protracted reversals in mineral compositions through the PM interval ponded initially at the base of the northern basin, and did not homogenize the entire magma column. These added magmas did not overflow and have an effect on mineral compositions in the southern basin until after considerable replenishment and crystallization (including the PM) had taken place in the northern basin. We emphasize the prolonged period(s) of magma input and slow rate of vertical homogenization of the magma column during the formation of this sequence of as much as 400 m of the Main Zone.

We present in situ 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 87 Sr/ 86 Sr i (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 65-70 at the base and top to ~An 25 approaching the purported "Sandwich Horizon." Plagioclase near the roof and proximal to rafts of partially assimilated basement gneiss are markedly more radiogenic ( 87 Sr/ 86 Sri 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 in situ 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.

During the terminal stages of Wilson cycles, remnants of magma-poor margins may be incorporated into the orogens, whereas the magma-rich margins often are lost in subduction due to low buoyancy. The understanding of magma-rich margins is therefore mostly based on drill holes and geophysical observations. In this contribution, we explore the temporal evolution and the ambient conditions of a magma-rich rifted margin preserved within the Scandinavian Caledonides. The Scandinavian Dike Complex was emplaced into a sedimentary basin during the initial break-up and opening of the Iapetus Ocean 615 to 590 million years ago. The dike complex constitutes 70-90% of the magma-rich, syn-rift basins and is locally well-preserved despite the complex Caledonian history. This contribution provides new observations about the geometry, relative timing, and development of the margin. Jadeite-in-clinopyroxene geothermobarometry, titanium-in-biotite geothermometry, and garnet isopleth modeling show that the ambient pressure and temperature conditions were similar for the entire dike complex at 0.25 to 0.45 GPa, with contact metamorphic temperatures up to c. 700°C. In the northernmost part of the study area U-Pb dating of magmatic zircon shows that partial melting of the sedimentary host-rock, at relatively shallow levels, occurred at 612 Ma. This shows that the crust was molten already 6 million years before the northernmost dike swarm was emplaced at 605.7 ± 1.8 Ma. We propose that the locally pervasive partial melting occurred due to high geothermal gradients and introduction of mafic melt in the lower crust. These processes significantly reduced the strength of the crust, eventually facilitating continental break-up. ©2019. American Geophysical Union. All Rights Reserved.

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.

Radiometric ages for undated parts of the volcanic succession and intrusions in West Greenland were obtained by the40Ar–39Ar incremental heating method. Acceptable crystallization ages were obtained for 27 samples. Combined with published results the new data provide a volcanic stratigraphy correlatable throughout the Nuussuaq Basin. The thick onshore volcanic pile consists of four widespread formations: 62.5–61 Ma picrites (Vaigat Formation), 61–60 Ma depleted basalts (Maligât Formation and the Hellefisk-1 well), 60–58 Ma less-depleted basalts (Svartenhuk Formation) and 56–54 Ma enriched basalts (Naqerloq Formation). Two local successions comprise 53.5 Ma alkali basalts (Erqua Formation) and 38.7 Ma transitional basalts (Talerua Member). A central volcano developed on Ubekendt Ejland, leading to the Sarqâta qáqâ gabbro-granophyre intrusion at 57–55 Ma. Pre-break-up volcanism took place further south as early as 64–63 Ma. The offshore volcanic succession most probably comprises the known onshore succession plus some younger lavas. The change in spreading direction near the Paleocene–Eocene boundary took place west of the Nuussuaq Basinc.56.2 Ma. Some tectonomagmatic events are correlatable across the entire North Atlantic Igneous Province. A quiescent 58–56 Ma period correlates with similar periods in East Greenland and the Faroes, and the Naqerloq Formation is coeval with the Eocene basalts in East Greenland. The Paleocene and Eocene tholeiitic basalts are distinguishable chemically; in early Eocene time, mantle typical of the Iceland plume seems to have extended beneath the whole West Greenland margin as well as the central East Greenland margin.

The Seiland Igneous Province (SIP), Northern Norway, contains > 5000 km2 of mafic and ultramafic intrusions with minor alkaline, carbonatite and felsic rocks that were intruded into the lower continental crust at a depth of 25 to as much as 35 km. The SIP can be geochemically and temporally correlated to numerous dyke swarms throughout Scandinavia at 560–610 Ma, and is linked to magmatic provinces in W-Greenland and NE-America that are collectively known as the Central Iapetus Magmatic Province (CIMP). Revised mapping show that the SIP exposes 85–90% layered tholeiitic- alkaline- and syeno-gabbros, 8–10% peridotitic complexes, 2-5% carbonatite, syenite and diorite that formed within a narrow (< 10 Ma) time frame in the Ediacaran (560–570 Ma). Large peridotite complexes were emplaced into the still hot and unconsolidated gabbro (no dating available) and are regarded as the main-conduit systems. Gravimetric data implies an average thickness of igneous rocks of 4–5 km and also features six deep lithospheric roots of ultramafic rocks extending min 9 km into the crust. Together, the root structures represent the main volcanic conduits conveying thousands of km3 of mafic-ultramafic melts from the asthenosphere to the lithosphere. The ultramafic complexes were predominantly emplaced into the layered gabbros at four major igneous centres, respectively, Nordre Brumandsfjord, Melkvann, Kvalfjord and Reinfjord. All complexes are situated in a right-way-up position and are steep sided forming large plugs. A marginal hybrid zone forms at the contact with country-rock and transitions gradually from olivine-mela-gabbro over pyroxenites that grades in to an olivine-clinopyroxenite zone, which is followed by a wehrlite zone and, finally, the centre of the complexes comprises pure dunite. From pyroxenite to dunite, olivine changes from Fo72 to Fo85 and clinopyroxene from Di80 to Di92 i.e. the complexes observe a reverse fractional crystallisation sequence with time. Parental melt compositions modelled from early dykes indicate komatiitic to picritic melts with 16–22 wt% MgO, Cr of 1594 ppm and Ni of 611 ppm, which were emplaced at 1450–1500 °C. Melt compositions calculated from clinopyroxene compositions from Reinfjord are OIB-like with LREE enriched over HREE. The high abundance of carbonatites and lamproites demonstrates the volatile-rich nature of the mantle source region and is further corroborated by the unusually high abundance of magmatic sulphides (0.5–1%) and carbonated and hydrous assemblages (c. 1%) throughout the region. In Reinfjord, they are also closely associated with PGE-Cu-Ni reef deposits. Essentially, the ultramafic complexes in the SIP comprises deep-seated transient magma chambers that facilitated mixing and homogenisation of a rich diversity of fertile asthenospheric melts en route to the upper parts of the continental crust.

We use new models of crustal structure and the depth of the lithosphere–asthenosphere boundary to calculate the geopotential energy and its corresponding geopotential stress field for the High Arctic. Palaeostress indicators such as dykes and rifts of known age are used to compare the present day and palaeostress fields. When both stress fields coincide, a minimum age for the configuration of the lithospheric stress field may be defined. We identify three regions in which this is observed. In north Greenland and the eastern Amerasia Basin, the stress field is probably the same as that present during the Late Cretaceous. In western Siberia, the stress field is similar to that in the Triassic. The stress directions on the eastern Russian Arctic Shelf and the Amerasia Basin are similar to that in the Cretaceous. The persistent misfit of the present stress field and Early Cretaceous dyke swarms associated with the High Arctic Large Igneous Province indicates a short-lived transient change in the stress field at the time of dyke emplacement. Most Early Cretaceous rifts in the Amerasia Basin coincide with the stress field, suggesting that dyking and rifting were unrelated. We present new evidence for dykes and a graben structure of Early Cretaceous age on Bennett Island.

The Skaergaard Intrusion of East Greenland is the quintessential example of low-pressure closed-system fractionation of basaltic magma. Field evidence of extensive layering and associated quasi-sedimentary structures, and the resultant 'cumulate' paradigm of crystal settling in magma chambers, has led to many long-standing controversies. Of particular significance is the lack of consensus about the microstructural record and the mechanisms by which interstitial liquid is expelled from solidifying crystal mushy zones. Skaergaard remains a cradle for new insights into igneous processes, with recent work highlighting the importance of separation of immiscible liquids on magma evolution.

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.

Late-stage microstructures developed during the last stages of solidification of the Skaergaard intrusion comprise a wide array of reactive and non-reactive intergrowths. Reactive microstructures ascribed to open-system behaviour include serrated grain boundaries between pyroxene and plagioclase primocrysts, fish-hook pyroxenes and mafic symplectites. They form by the addition of Fe and Ca and removal of alkalis and silica. Other microstructures include those formed by (internally generated) redox reactions between olivine, Fe-Ti oxides and pyroxene. Non-reactive microstructures include closely spatially associated granophyric intergrowths and previously undescribed ilmenite-rich intergrowths that are interpreted as a consequence of crystallization of separated conjugate immiscible liquids. The open-system reactive microstructures occur predominantly in the cumulates on the chamber floor, appearing in LZb and disappearing (to be replaced by a granophyre-ilmenite-rich intergrowth association) in the Upper Zone. They are not common in the Marginal Border Series. Their distribution mirrors that of efficient expulsion of trapped liquid and can be attributed to the gravitationally driven loss of a Si-rich immiscible component from the interstitial liquid. The loss of the Si-rich component causes the remaining Fe-rich liquid to react with the primocrysts. Bulk-rock major element data are consistent with little or no preferential loss of the Si-rich liquid from the mush, but the resolution of the available data is not sufficient to assess the effect of this relative movement on the liquid line of descent of the bulk magma. The first appearance of the paired conjugate non-reactive intergrowths in MZ* (Marginal Border Series) points to the early onset of immiscibility in the bulk liquid. © The Author 2010. Published by Oxford University Press. All rights reserved.

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.