North Atlantic Volcanic Province Research Articles

Reduced carbon cycle resilience across the Palaeocene–Eocene Thermal Maximum

Abstract. Several past episodes of rapid carbon cycle and climate change are hypothesised to be the result of the Earth system reaching a tipping point beyond which an abrupt transition to a new state occurs. At the Palaeocene–Eocene Thermal Maximum (PETM) at ∼56 Ma and at subsequent hyperthermal events, hypothesised tipping points involve the abrupt transfer of carbon from surface reservoirs to the atmosphere. Theory suggests that tipping points in complex dynamical systems should be preceded by critical slowing down of their dynamics, including increasing temporal autocorrelation and variability. However, reliably detecting these indicators in palaeorecords is challenging, with issues of data quality, false positives, and parameter selection potentially affecting reliability. Here we show that in a sufficiently long, high-resolution palaeorecord there is consistent evidence of destabilisation of the carbon cycle in the ∼1.5 Myr prior to the PETM, elevated carbon cycle and climate instability following both the PETM and Eocene Thermal Maximum 2 (ETM2), and different drivers of carbon cycle dynamics preceding the PETM and ETM2 events. Our results indicate a loss of “resilience” (weakened stabilising negative feedbacks and greater sensitivity to small shocks) in the carbon cycle before the PETM and in the carbon–climate system following it. This pre-PETM carbon cycle destabilisation may reflect gradual forcing by the contemporaneous North Atlantic Volcanic Province eruptions, with volcanism-driven warming potentially weakening the organic carbon burial feedback. Our results are consistent with but cannot prove the existence of a tipping point for abrupt carbon release, e.g. from methane hydrate or terrestrial organic carbon reservoirs, whereas we find no support for a tipping point in deep ocean temperature.

Peraluminous igneous rocks as an indicator of thermogenic methane release from the North Atlantic Volcanic Province at the time of the Paleocene–Eocene Thermal Maximum (PETM)

Unusual cordierite-bearing peraluminous dacites, produced by melting of organic-rich sediments by intrusion of basaltic magma, are found within the North Atlantic Volcanic Province (NAVP). Calculations suggest that formation of the dacites, radiometric dated at 55.9 ± 0.3 Ma and possibly widespread, could have released an average of ∼4,500 Gt (range from 3,000 to 6,000 Gt) of carbon as methane, with a δ13C of about −35‰. Published model results suggest that such a methane release could explain the negative δ13C excursion in the oceans and atmosphere, the extreme global warming, and the marked dissolution of carbonates in the deep oceans that accompanied the concurrent Paleocene–Eocene Thermal Maximum (PETM). Outgassing from melting of sediments and formation of dacites, possibly in conjunction with methane produced in contact metamorphic aureoles and by methane hydrate release, provides a novel way of explaining the PETM and its timing.

Variation of Icelandic and Hawaiian magmatism: evidence for co-pulsation of mantle plumes?

Based on published estimates of areal extent, thickness and dating of igneous rocks related to formation of the North Atlantic Volcanic Province, we calculate the magmatic production since the first reported magmatism at 70 Ma until present. We relate the magmatism to the Icelandic Plume and estimate the total volumetric production to 22.1 × 106 km3. The magmatic production varied significantly with time, with a clear maximum of 55.5 m3/s around continental break-up at ca. 54 Ma. The lowest production is estimated at 4 m3/s, increasing to 7 m3/s at ca. 23 Ma. Two other pulses with increased activity are found around 40 Ma and in the late Miocene (10–5 Ma). The variations in productivity are consistent with a plume pulsing with a periodicity of ca. 15 million years. The same periodicity and relative timing is found for the Hawaiian Plume. If confirmed, these observations suggest that both plumes originate within the thin boundary layer near the Core-Mantle Boundary. This hypothesis may imply periodic heating of the earth’s core with subsequent heat-release to the mantle and increased global plume activity.

Prospectivity of volcanic basins: Trap delineation and acreage de-risking

Prospect delineation in volcanic basins is commonly hampered by poor sub-basalt imaging. Recent seismic advances focusing on low frequencies have significantly enhanced structural imaging, but not the seismic resolution. Currently, the sub-basalt play is mainly a structural play. On the North Atlantic margin, success has been booked with the discovery of the sub-basalt anticlinal Corrib gas field, offshore Ireland. However, not all sub-basalt anticlines observed on seismic data are valid prospects because some of them are located directly above large igneous complexes, displaying highly overmature and overcompacted sedimentary sections with little porosity. Use of filtered gravity and magnetic data and potentially seismic velocity analysis can easily delineate and subsequently de-risk these prospects. Sill intrusion can generate subtle anticlinal traps that might be valid prospects because the thermal effect of sills is relatively limited. Thus, there is less chance of generating an overmature prospect, unless the sills occur in high numbers. Filtered gravity and magnetic data sets, in combination with local geology, can be used in the early stages of sub-basalt exploration to low-grade regions with a high amount of large intrusives. This approach is highlighted for the North Atlantic volcanic province and offshore western India. Potentially, the most prospective regions of volcanic basins are where flood basalts simply blanket sedimentary basins without intrusives in the direct subsurface. In this case, basalt thickness estimation and associated burial depth become an important parameter to estimate prospectivity. Seismic modeling suggests that it might be difficult to constrain basalt thickness from seismic sections without reliable calibration. Although an increase in volcanism in sedimentary basins generally tends to increase risks, proper integration of geophysical and geological data should be able to better evaluate volcanic basins.

The water content of olivines from the North Atlantic Volcanic Province

The water content of olivine crystals from picrites and basalts from the North Atlantic Volcanic Province has been estimated using non-polarised Fourier transform infrared spectra. H2O concentrations vary from about 18 ppm by weight to below the detection limit of ca. 0.5 ppm. The most H2O-poor olivines (≤0.5 ppm H2O) probably reflect olivine crystallisation or re-equilibration at shallow crustal levels, possibly after magma degassing. However, some of the Mg-rich olivines (Fo87–91.5) have elevated H2O contents (3–7 ppm) and indicate the presence of mantle source regions with >300 ppm H2O for any reasonable crystallisation depth. The elevated H2O content may be sufficiently high to significantly affect the mantle viscosity and melting behaviour and thus contribute to the simultaneous production of melt over a vast area from Baffin Island to the British Isles at 58–62 Ma.

Evidence from episodic seamount volcanism for pulsing of the Iceland plume in the past 70 Myr

The North Atlantic volcanic province has been attributed to continental rifting about 60 Myr ago over an Iceland plume head with a diameter of 1,000-2,000 km (refs 1, 2). But evidence from a few igneous centres has been used to infer that earlier plume activity occurred in this region. The three seamounts in the Rockall trough off the Atlantic coast of Scotland are among the few accessible remnants of such early plume activity. Here we present 40Ar-39Ar incremental-heating ages of samples from these seamounts, which show that volcanism began there in the late Cretaceous period (70 +/- 1 Myr ago), and then continued for the next 30 Myr in at least four discrete phases: 62, 52, 47 and 42 Myr ago. We relate this activity to pulsing of large masses (approximately 10(8) km3) of hot Iceland plume material on timescales of 5-10 Myr. This significantly extends the time span for Iceland plume activity both backwards and forwards in time, and provides a possible alternative to the 'plume head' models for the formation of continental flood basalts.

Helium isotope composition of the early Iceland mantle plume inferred from the Tertiary picrites of West Greenland

Picrites from the 61 million year old Vaigat Formation of the Nuussuaq Peninsula in West Greenland have 3He/ 4He ratios trapped in olivine phenocrysts which range up to 30 times the atmospheric ratio. These high values, measured during gas extraction by crushing in vacuum, are similar to the highest magmatic 3He/ 4He ratios found in young terrestrial volcanic rocks. By analogy with young basalts, in which crushing selectively extracts magmatic helium, any significant cosmogenic 3He appears to be absent in these picrites. Additional evidence for the absence of cosmogenic helium is provided by fusion results on the crushed olivine powders and by a single stepwise crushing experiment, in which only magmatic and radiogenic helium components are resolvable. The West Greenland picrites have Pb, Nd and Sr isotope compositions which overlap those found in picrites from Iceland and in basalts from Loihi Seamount, localities which today also have high 3He/ 4He ratios. Isotopic variations in He, Pb, Nd and Sr for the West Greenland picrites are interpreted to largely result from interaction of the early Iceland mantle plume with the upper mantle during plume ascent and dispersion beneath the continental lithosphere. The presence of high 3He/ 4He ratios in West Greenland, and the onset of magmatism across the North Atlantic Volcanic Province near 62 Ma, supports the hypothesis for very rapid dispersion (>1 m/year) of mantle plume head material during the earliest stages of plume impact, as predicted in recent numerical simulations of plume behavior during thermal mantle convection with non-Newtonian rheology.

Helium isotopes in early Tertiary basalts, northeast Greenland: Evidence for 58 Ma plume activity in the North Atlantic–Iceland volcanic province

Research Article| May 01, 1998 Helium isotopes in early Tertiary basalts, northeast Greenland: Evidence for 58 Ma plume activity in the North Atlantic–Iceland volcanic province Bernard Marty; Bernard Marty 1Centre de Recherches Pétrographiques et Géochimiques, Centre National de la Recherche Scientifique, Rue Notre-Dame des Pauvres, B.P. 20, 54501 Vandoeuvre Cedex, France Search for other works by this author on: GSW Google Scholar Brian G. J. Upton; Brian G. J. Upton 2Grant Institute of Geology, West Mains Road, Edinburgh EH9 3JW, United Kingdom Search for other works by this author on: GSW Google Scholar Rob M. Ellam Rob M. Ellam 3Isotope Geosciences Unit, Scottish Universities Research and Reactor Centre, East Kilbride G75 0QF, United Kingdom Search for other works by this author on: GSW Google Scholar Geology (1998) 26 (5): 407–410. https://doi.org/10.1130/0091-7613(1998)026<0407:HIIETB>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Bernard Marty, Brian G. J. Upton, Rob M. Ellam; Helium isotopes in early Tertiary basalts, northeast Greenland: Evidence for 58 Ma plume activity in the North Atlantic–Iceland volcanic province. Geology 1998;; 26 (5): 407–410. doi: https://doi.org/10.1130/0091-7613(1998)026<0407:HIIETB>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract 3He/4He ratios in olivine phenocrysts have been measured in 58 Ma basaltic lavas from the Hold with Hope region of northeast Greenland. These lavas constitute a part of the North Atlantic volcanic province. The samples exhibit a wide range of 3He/4He ratios between 1.7Ra and 21Ra (Ra =the atmospheric 3He/4He ratio =1.386 × 10−6), which is comparable to the previously recorded range for Quaternary Icelandic basalts. These He isotope variations may partly result from crustal assimilation, but 3He/4He ratios higher than those of mid-ocean-ridge basalts support early participation of plume-head material at the onset of volcanic activity in the North Atlantic volcanic province. As with the Deccan-Réunion plume and the Ethiopia-Afar plume, this long-term contribution of 3He-rich material is fully consistent with models that ascribe a deep (i.e., deeper than the convective mantle that drives plate tectonics) origin for plume-source material, but contradicts those that postulate a shallow-mantle origin for continental flood basalts and large igneous provinces. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Depleted mantle-plume geochemical signatures: No paradox for plume theories

Research Article| September 01, 1995 Depleted mantle-plume geochemical signatures: No paradox for plume theories Andrew C. Kerr; Andrew C. Kerr 1Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom Search for other works by this author on: GSW Google Scholar Andrew D. Saunders; Andrew D. Saunders 1Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom Search for other works by this author on: GSW Google Scholar John Tarney; John Tarney 1Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom Search for other works by this author on: GSW Google Scholar Neil H. Berry; Neil H. Berry 1Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom Search for other works by this author on: GSW Google Scholar Victoria L. Hards Victoria L. Hards 2Department of Geological Sciences, University of Durham, South Road, Durham DH1 3LE, United Kingdom Search for other works by this author on: GSW Google Scholar Geology (1995) 23 (9): 843–846. https://doi.org/10.1130/0091-7613(1995)023<0843:DMPGSN>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Andrew C. Kerr, Andrew D. Saunders, John Tarney, Neil H. Berry, Victoria L. Hards; Depleted mantle-plume geochemical signatures: No paradox for plume theories. Geology 1995;; 23 (9): 843–846. doi: https://doi.org/10.1130/0091-7613(1995)023<0843:DMPGSN>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract High-MgO liquids erupted in ocean-island settings and in some continental flood-basalt provinces commonly preserve a “depleted” composition, in terms of both highly incompatible trace elements and isotope ratios. These observations strongly imply that their source is also compositionally depleted. However, in at least one case (Iceland and the North Atlantic volcanic province), it can be shown that this depleted source is not the same as that feeding the present-day North Atlantic mid-ocean ridge. The depleted source must also have been much hotter than the mid-ocean ridge basalt (MORB) source to account for the volume of melt and primitive composition of some magmas that were generated. This depleted character, then, is an intrinsic component of mantle plumes, originating from the deep mantle. We propose that mantle plumes consist of a mixture of enriched or fusible streaks in a depleted, refractory matrix; preferential extraction of the enriched component occurs close to the plume axis. The depleted residue from this melting remains in the upper mantle and may therefore be a major contributor to the source region of MORB. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Flood basalts versus central volcanoes and the British Tertiary volcanic Province

Abstract A controversy in the final decades of the last century developed between Geikie and Judd over the nature of the volcanoes in the British Tertiary Province. Geikie regarded the lavas as in Antrim and Skye as plateau basalts (today called flood basalts) erupted from widely scattered fissures, while Judd regarded the intrusion complexes as in Central Mull and Skye as eroded stumps of major central volcanoes from which the lavas originated. Soon after, instigated by the controversy, British volcanology flowered for several decades in what may be called its ‘Classic Period’. Detailed mapping projects were undertaken, and new concepts were developed that are basic to volcanology. This paper views the controversy and its aftermath, and also briefly reviews recent conceptual developments in the North Atlantic Volcanic Province of which the British Tertiary rocks are a part. The paper discusses the present understanding of flood basalts and central volcanoes, and presents new criteria based on structural features of lava flows (such as the dependence of lava thicknesses, and the occurrence of pipe vesicles, on ground-slope angle) to distinguish between them. Magnetic fabric study of magma-flow directions in intrusions and lava flows, and palaeomagnetic study of post-emplacement tilting of igneous rocks, have great unrealized potential.

Geophysical response of flood basalts from analysis of wire line logs: Ocean Drilling Program Site 642, Vøring volcanic margin

Analysis of wire line logs at Ocean Drilling Program Site 642, the deepest scientific hole yet drilled in an offshore large igneous province (LIP), reveals the main features of the extrusive complex constructed during formation of a volcanic margin. A characteristic cyclic log response images tholeiitic lavas and thin tuffs corresponding to the seaward dipping reflectors in the seismic record. Flows are 0.6–18.5 m thick, while most sediments are &lt;l m thick. Porosity dependent sonic, density, neutron porosity and resistivity logs respond to gently decreasing porosity in upper 5–7 m of the flow and rapid porosity increase near the flow base. Velocity and density top and bottom gradients are largely independent of flow thickness, with velocity gradients of 400 to 600 s−1 and −1000 to −2000 s−1, respectively. A positive correlation between flow thickness and mean velocity simply reflects increased proportion of massive basalt in thicker flows. There is also an overall increase in mean fine‐grained basalt velocity with depth with a gradient of 0.8±0.3 s−1. Flow velocity‐density relations show a downward velocity strengthening related to a decrease in high aspect ratio voids. High velocities near the flow base correlate with high, &gt;2000 Ω‐m, focused resistivity kicks. The large flow‐scale variations in physical properties show that representative core measurements must be densely sampled. Both on flow and sequence scales Site 642 log responses appears typical for the North Atlantic Volcanic Province and other LIPs but differs from logs in deep ocean crustal holes. Principal differences in log responses reflect mode of crustal emplacement and alteration. Hence Site 642 forms a reference hole for extrusive constructions on oceanic LIPs.

North Atlantic volcanic margins: Dimensions and production rates

Early Tertiary lithospheric breakup between Eurasia and Greenland was accompanied by a transient (∼3 m.y.) igneous event emplacing both the onshore flood basalts of the North Atlantic Volcanic Province (NAVP) and huge extrusive complexes along the continent‐ocean transition on the rifted continental margins. Seismic data show that volcanic margins extend &gt;2600 km along the early Eocene plate boundary, in places underlain by high‐velocity (7.2–7.7 km/s) lower crustal bodies. Quantitative calculations of NAVP dimensions, considered minimum estimates, reveal an areal extent of 1.3×106 km2 and a volume of flood basalts of 1.8×106 km3, yielding a mean eruption rate of 0.6 km3/yr or 2.4 km3/yr if two‐thirds of the basalts were emplaced within 0.5 m.y. The total crustal volume is 6.6×106 km3, resulting in a mean crustal accretion rate of 2.2 km3/yr. Thus NAVP ranks among the world's larger igneous provinces if the volcanic margins are considered. The velocity structure of the expanded crust seaward of the continent‐ocean boundary differs from standard oceanic and continental crustal models. Based on seismic velocities this “volcanic margin” crust can be divided into three units of which the upper unit corresponds to basaltic extrusives. The regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust, including the high‐velocity lower crust which extends some distance landward of the continent‐ocean boundary, was emplaced during and subsequent to breakup. The volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling, probably reflecting the interaction of a mantle plume and a lithosphere already extending.

Large igneous provinces: Crustal structure, dimensions, and external consequences

Large igneous provinces (LIPs) are a continuum of voluminous iron and magnesium rich rock emplacements which include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts. Such provinces do not originate at “normal” seafloor spreading centers. We compile all known in situ LIPs younger than 250 Ma and analyze dimensions, crustal structures, ages, and emplacement rates of representatives of the three major LIP categories: Ontong Java and Kerguelen‐Broken Ridge oceanic plateaus, North Atlantic volcanic passive margins, and Deccan and Columbia River continental flood basalts. Crustal thicknesses range from 20 to 40 km, and the lower crust is characterized by high (7.0–7.6 km s−1) compressional wave velocities. Volumes and emplacement rates derived for the two giant oceanic plateaus, Ontong Java and Kerguelen, reveal short‐lived pulses of increased global production; Ontong Java's rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid‐ocean ridge system. The major part of the North Atlantic volcanic province lies offshore and demonstrates that volcanic passive margins belong in the global LIP inventory. Deep crustal intrusive companions to continental flood volcanism represent volumetrically significant contributions to the crust. We envision a complex mantle circulation which must account for a variety of LIP sizes, the largest originating in the lower mantle and smaller ones developing in the upper mantle. This circulation coexists with convection associated with plate tectonics, a complicated thermal structure, and at least four distinct geochemical/isotopic reservoirs. LIPs episodically alter ocean basin, continental margin, and continental geometries and affect the chemistry and physics of the oceans and atmosphere with enormous potential environmental impact. Despite the importance of LIPs in studies of mantle dynamics and global environment, scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features.

Volcanism and continental break-up: a global compilation of large igneous provinces

Abstract Large igneous provinces (LIPs) include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts. In some cases transient episodes of voluminous magmatism are temporally and spatially related to continental break-up, e.g. North Atlantic Volcanic Province, Deccan Traps, Paraná-Etendeka basalts. In other cases, however, no relationships are apparent, e.g. Siberian flood basalts, Columbia River flood basalts. Herein we review LIPs worldwide in order to better understand their relationship to the break-up and separation of lithospheric plates. The two most voluminous episodes known of basaltic magmatism not associated with the creation of ‘normal’ oceanic crust, the emplacements of the Ontong Java and Kerguelen plateaus, do not appear to be linked to continental break-up. Volcanic passive margins have now been identified on the edges of many continents, and are clearly related to continental break-up and separation. They cannot always, however, be tied to continental flood basalts. Ocean basin flood basalts and seamount groups are not commonly related to continental break-up. In most instances submarine ridges show temporal and spatial relationships with continental flood basalt provinces or oceanic plateaus. Observational data suggest that existing models do not adequately explain all LIPs; we suggest that a thermally and chemically heterogeneous asthenosphere, occasionally penetrated by deep mantle plumes, can account for their origin.

Magmatic tectonic evolution of a volcanic rifted margin

Many North Atlantic continental margins are underlain by huge volcanic edifices near the continent-ocean boundary. A deep crustal hole drilled on the Vøring Plateau (ODP Leg 104) has provided important constraints on the initial breakup history and the subsequent margin evolution by penetrating more than 900 m of igneous rocks and interbedded sediments below a post-early Eocene cover. The rocks constitute two distinct, different volcanic series. The upper series, comprising a seaward dipping reflector wedge, consists of transitional mid-oceanic tholeiitic lava flows and thin volcaniclastic sediments. Dacitic flows, some dikes and thicker interbedded sediments constitute the lower series. The margin evolved by Paleocene crustal extension accompanied by uplift and pervasive intrusion of the rift zone. Just prior to breakup, magma from shallow crustal melts produced the lower series. The upper series was constructed during an intense, rapidly waning, subaerial volcanic surge following breakup near the Paleocene-Eocene transition. It covers both the new oceanic crust and large areas of the continental crust. The dipping wedge was formed by subsidence due to loading and thermal contraction, possibly amplified by tectonism. When the surge abated, the injection center rapidly subsided and normal oceanic crust was formed. There is a direct temporal and compositional relationship between the onshore North Atlantic Volcanic Province and the margin volcanism. Compared to the transverse part of the province including the Iceland Hotspot, which has been active over 60 m.y., the volcanic margins reflect a 2500 km long transient segment related to continental breakup, lasting only about 3 m.y. The volcanic surge and uplift at breakup are related to a modest regional increased temperature at the base of the lithosphere. This mode of margin formation has had important paleoenvironmental implications.

Geology and petrology of the Lower Tertiary plateau basalts of the Scoresby Sund region, East Greenland

Geology and petrology of the Lower Tertiary plateau basalts of the Scoresby Sund region, East Greenland

Early Cenozoic crust at the Norwegian continental margin and the conjugate Jan Mayen Ridge

Multichannel seismic profiles at the Vøring Plateau Margin off Norway and the northern Jan Mayen Ridge have provided a framework for the early Tertiary evolutionary history of these areas which were adjacent prior to the opening of the Norwegian Sea. We propose that their Paleogene evolution is quite similar and that the two acoustic basement reflectors, EE and JO, were formed in the early Eocene by extrusion of flow basalts. From the time of opening to about anomaly 23 time, oceanic seafloor was generated by Icelandic‐type spreading. During that time, the Central Jan Mayen Fracture was the main transform. The seaward dipping reflector sequences are flow basalts generated by the Icelandic spreading. The existence of less well developed dipping sequences seaward of the primary ones probably reflects that these regions submerged later because of higher initial elevation. During the early Eocene subaerial spreading episode, which is related to the North Atlantic Volcanic Province, basalt flows also covered the adjacent thinned and heavily intruded continental crust. It is proposed that the continent‐ocean boundary is located just landward of anomaly 24B, a short distance seaward of the Vøring Plateau Escarpment. Reflector K is only recognized landward of this boundary in a region of “transitional crust.” The escarpment experienced syndepositional faulting, and the most elevated parts of the Vøring Marginal High did not subside below shallow water depths before Oligocene/Miocene time. We suggest that the Vøring Margin and the Jan Mayen Ridge represent “volcanic” passive margins. The evolution of this margin type may relate to initial uplift due to rifting in previously thinned crust.

Evidence from mantle xenoliths for an enriched lithospheric keel under the Outer Hebrides

Several independent lines of evidence suggest the presence of old regions in the Earth's upper mantle, particularly beneath stable cratons, which are enriched in incompatible elements such as barium and the light rare earth elements (LREE). Such 'lithospheric keels' are well documented beneath the Archaean of South Africa1–4 and North America5,6. Recent studies7 have shown the Lewisian of Scotland to be underlain by heterogeneous mantle, fragments of which are entrained by magmas that later intrude the crustal rocks of north-west Scotland. Here we present chemical and isotopic data from megacryst suites in these magmas, which point to niineralbgical complexity in the lower crust and upper mantle. Moreover, variable trace-element ratios in the xenoliths are now recorded as extreme isotopic variations. The Sr and Fb isotopic compositions are similar to those of lamprbites6 and micaceous kimberlites8 respectively. In contrast, the Nd isotopic ratios are highly variable (eNd = −3.4 to −32.6) and overlap with those observed in garnet inclusions9 in diamonds (South Africa) and Iamproitic6–potassic5 volcanic rocks (USA). We suggest that the Hebridean lithospheric mantle has evolved in a manner totally unlike that of the mantle source regions tapped by Tertiary volcanic rocks erupted in the North Atlantic volcanic province.