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.     

Mantle thermal structure and active upwelling during continental breakup in the North Atlantic

Seismic reflection and refraction data acquired on four transects spanning the Southeast Greenland rifted margin and Greenland–Iceland Ridge (GIR) provide new constraints on mantle thermal structure and melting processes during continental breakup in the North Atlantic. Maximum igneous crustal thickness varies along the margin from >30 km in the near-hotspot zone (<500 km from the hotspot track) to 18 km in the distal zone (500–1100 km). Magmatic productivity on summed conjugate margins of the North Atlantic decreases through time from 1800±300 to 600±50 km³/km/Ma in the near-hotspot zone and from 700±200 to 300±50 km³/km/Ma in the distal zone. Comparison of our data with the British/Faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist. Joint consideration of crustal thickness and mean crustal seismic velocity suggests that along-margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature. The thermal anomaly (ΔT) at breakup was modest (100–125°C), varied little along the margin, and transient. Data along the GIR indicate that the potential temperature anomaly (125±50°C) and upwelling ratio (4 times passive) of the Iceland hotspot have remained roughly constant since 56 Ma. Our results are consistent with a plume–impact model, in which (1) a plume of radius 300 km and ΔT of 125°C impacted the margin around 61 Ma and delivered warm material to distal portions of the margin; (2) at breakup (56 Ma), the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin; and (3) by 45 Ma, both the remaining plume head and the distal warm layer were exhausted, with excess magmatism thereafter largely confined to a narrow (<200 km radius) zone immediately above the Iceland plume stem. Alternatively, the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been a pre-existing thermal anomaly unrelated to the plume.     

Contrasting rifted margin styles south of Greenland: implications for mantle plume dynamics

We present new and reprocessed seismic reflection data from the area where the southeast and southwest Greenland margins intersected to form a triple junction south of Greenland in the early Tertiary. During breakup at 56 Ma, thick igneous crust was accreted along the entire 1300-km-long southeast Greenland margin from the Greenland Iceland Ridge to, and possibly 100 km beyond, the triple junction into the Labrador Sea. However, highly extended and thin crust 250 km to the west of the triple junction suggests that magmatically starved crustal formation occurred on the southwest Greenland margin at the same time. Thus, a transition from a volcanic to a non-volcanic margin over only 100–200 km is observed. Magmatism related to the impact of the Iceland plume below the North Atlantic around 61 Ma is known from central-west and southeast Greenland. The new seismic data also suggest the presence of a small volcanic plateau of similar age close to the triple junction. The extent of initial plume-related volcanism inferred from these observations is explained by a model of lateral flow of plume material that is guided by relief at the base of the lithosphere. Plume mantle is channelled to great distances provided that significant melting does not take place. Melting causes cooling and dehydration of the plume mantle. The associated viscosity increase acts against lateral flow and restricts plume material to its point of entry into an actively spreading rift. We further suggest that thick Archaean lithosphere blocked direct flow of plume material into the magma-starved southwest Greenland margin while the plume was free to flow into the central west and east Greenland margins. The model is consistent with a plume layer that is only moderately hotter, 100–200°C, than ambient mantle temperature, and has a thickness comparable to lithospheric thickness variations, 50–100 km. Lithospheric architecture, the timing of continental rifting and viscosity changes due to melting of the plume material are therefore critical parameters for understanding the distribution of magmatism.     

Glacier hydrochemistry, solute provenance, and chemical denudation at a surge-type glacier in Kuannersuit Kuussuat, Disko Island, West Greenland

In 1995–1998, Han 11 km terrestrial surge of Kuannersuit Glacier, an outlet glacier of the largest ice cap on Disko Island, West Greenland, affected the catchment dramatically. In order to estimate solute fluxes and provenances, bulk meltwaters were sampled at the main subglacial outlet during the initial part of the quiescent phase. The hydrochemistry is significantly influenced by a subglacial basaltic weathering regime with absence of carbonate minerals. The results show that marine and aerosol derived solutes have minimal contribution to the total ion content, whereas sequestration of atmospheric CO₂ associated with carbonation of Ca-rich feldspar and reactive volcanic glass is more dominant than previously reported from glacierized catchments. Application of a sampling strategy dividing water samples into four groups to determine the content of dissolved HCO₃⁻ and CO₃²⁻ shows that the cationic equivalent weathering rate range is 683–860 Σmeq⁺ m⁻² a⁻¹ and solute flux ranges between 76 and 98 t km⁻² a⁻¹. The crustal denudation rate is estimated to 26 t km⁻² a⁻¹, and the transient CO₂ drawdown amounts to 8500–13700 kg C km⁻² a⁻¹.     

The impact of land use on stream water quality in Slovakia

In the first part of this paper, the impact of forestry, agriculture and urban activities on the quality of surface water is analysed. Daily data from 15 forest and agricultural experimental catchments of the Institute of Hydrology, Slovak Academy of Sciences are used. It is shown, that the nitrate concentrations in surface water have decreased in Slovakia since 1989 as a result of decreased use of inorganic nitrogen fertilisers (lower intensity of agricultural production in Slovakia owing to recent economic changes). The annual nitrate specific load varies from 5.90 to 110 kg ha⁻¹ year⁻¹, the annual sulphate load varied from 29.16 to 509.60 kg ha⁻¹ year⁻¹ and the annual phosphate load varied from 0.0098 to 0.0224 kg ha⁻¹ year⁻¹ during 1990–1992.

In the second part, a two-step method of three-component hydrograph separation of rain-, soil- and groundwater is proposed. The method is used in the Manelo-Gribov microbasin (O.95 km²) in Eastern Slovakia. The annual contribution of surface runoff in total runoff volume was 57.5%, the contribution of interflow runoff was 21.1%, and the contribution of groundwater was 21.4%, during the period from 1 August to 31 July 1992. A deterministic regression model for predicting daily nitrate concentrations from values of stream daily discharge and flow component data was developed. A set of 1421 modelled NO₃⁻¹ data was compared with the set of measured data.     

Refining the noble gas record of the Réunion mantle plume source: Implications on mantle geochemistry

We report isotope analyses of helium, neon, argon, and xenon using different extraction techniques such as stepwise dynamic and static crushing, and high-resolution stepwise heating of three mantle xenoliths from Réunion Island. He and Ne isotopic compositions were similar to previously reported Réunion data, yielding a more radiogenic composition when compared to the Hawaiian or Icelandic mantle plume sources. We furthermore observed correlated ¹²⁹Xe/¹³⁰Xe and ¹³⁶Xe/¹³⁰Xe ratios following the mantle trend with maximum values of 6.93 ± 0.14 and 2.36 ± 0.06, respectively. High-resolution argon analyses resulted in maximum ⁴⁰Ar/³⁶Ar ratios of 9000–11,000, in agreement with maximum values obtained in previous studies. We observed a well-defined hyperbolic mixing curve between an atmospheric and a mantle component in a diagram of ⁴⁰Ar/³⁶Ar vs. ²⁰Ne/²²Ne. Using a mantle ²⁰Ne/²²Ne of 12.5 (Ne–B) a consistent ⁴⁰Ar/³⁶Ar value of 11,053 ± 220 in sample ILR 84-4 was obtained, whereas extrapolations to a higher mantle ²⁰Ne/²²Ne ratio of 13.8 (solar wind) would lead to a much higher ⁴⁰Ar/³⁶Ar ratio of 75,000, far above observed maximum values. This favours a mantle ²⁰Ne/²²Ne of about 12.5 considered to be equivalent to Ne–B. Extrapolated and estimated ⁴⁰Ar/³⁶Ar ratios of the Réunion, Iceland, Loihi, and MORB mantle sources, respectively, tend to be linearly correlated with air corrected ²¹Ne/²²Ne and show the same systematic sequence of increasing relative contributions in radiogenic isotopes (Iceland–Loihi–Réunion–MORB) as observed for ⁴He/³He. In general, He–Ne–Ar isotope systematics of the oceanic mantle can be explained by following processes: (i) different degree of mixing between pure radiogenic and pure primordial isotopes generating the MORB and primitive plume (Loihi-type) endmembers; (ii) relatively recent fractionation of He relative to Ne and Ar, in one or both endmembers; (iii) after the primary fractionation event, different degrees of mixing between melts or fluids of MORB and primitive plume affinity generate the variety of observed OIB data, also on a local scale; (iv) very late-stage secondary fractionation during magma ascent and magma degassing leads to further strong variation in He/Ne and He/Ar ratios.     

Ar/Ar illite dating of Late Caledonian (Acadian) metamorphism and cooling of K-bentonites and slates from the Welsh Basin, U.K.

The vacuum-encapsulation laser ⁴⁰Ar³⁹Ar technique allows extremely small (10⁻⁶ g) samples of fine-grained materials such as diagenetic clays to be dated. Here we show that the method can be extended to higher-grade clay minerals. The integration of transmission electron microscopic (TEM) characterization with ⁴⁰Ar³⁹Ar dating of vacuum encapsulated samples permits the resolution of the timing of metamorphic growth/cooling from the time of diagenesis. We have applied this technique to well characterized Lower Paleozoic slates and K-bentonites from the Welsh Basin, which span the transition from anchizonal to epizonal grade, which had been previously studied using RbSr and SmNd dating.

TEM observations of epizonal K-bentonites and slate showed that illite in these samples is of 2M₁ polytype, of muscovite-like composition, and oriented parallel to cleavage, suggesting that they are of metamorphic origin. Total gas ages (equivalent to conventional KAr ages) for encapsulated epizonal K-bentonites and slate (340–408 Ma) are considerably variable. The Ar retention ages (calculated from ³⁹Ar and ⁴⁰Ar atoms retained in the sample after irradiation) are more consistent (383–411 Ma). The ³⁹Ar recoil losses are minor for illites from whole rock samples of epizonal K-bentonites but very significant for clay separates of epizonal slate. Plateaus in age spectra were observed in epizonal K-bentonites and slate. The plateau ages (414–421 Ma) and retention ages (383–411 Ma) can be correlated with the onset of Acadian metamorphism and culmination of uplift and inversion of the Welsh Basin, respectively. These ages are significantly younger than the 450 Ma ages previously reported for diagenetic clays using the same method, suggesting that diagenetic history has been lost in these epizonal K-bentonites and slate.

TEM observations of anchizonal slates showed that there are two modes of illite. The first mode is similar to that observed in epizonal samples, suggesting a metamorphic origin. The second mode consists of the 1M_d polytype, has typical diagenetic illite composition, and is oriented parallel to bedding, suggesting a diagenetic origin. Total gas ages for encapsulated anchizonal slates vary considerably (361–422 Ma). The retention ages are more consistent (413–432 Ma) than the total gas ages. The ³⁹Ar recoil losses are more significant than those for epizonal K-bentonites and slate. Plateaus in age spectra are generally not observed. However, the consistent retention ages for the anchizonal slates correspond to the plateau ages for the epizonal samples, and are inferred to represent the onset of Acadian metamorphism.

These data, when combined with our previously published results for diagenetic shales, suggest that thermal conditions near the boundary of anchizonal and epizonal grades are necessary to completely reset Ar systems in shales and slates.     

Ar/Ar ages and paleomagnetism of São Miguel lavas, Azores

We present new ⁴⁰Ar/³⁹Ar ages and paleomagnetic data for São Miguel island, Azores. Paleomagnetic samples were obtained for 34 flows and one dike; successful mean paleomagnetic directions were obtained for 28 of these 35 sites. ⁴⁰Ar/³⁹Ar age determinations on 12 flows from the Nordeste complex were attempted successfully: ages obtained are between 0.78 Ma and 0.88 Ma, in contrast to published K–Ar ages of 1 Ma to 4 Ma. Our radiometric ages are consistent with the reverse polarity paleomagnetic field directions, and indicate that the entire exposed part of the Nordeste complex is of a late Matuyama age. The duration of volcanism across São Miguel is significantly less than previously believed, which has important implications for regional melt generation processes, and temporal sampling of the geomagnetic field. Observed stable isotope and trace element trends across the island can be explained, at least in part, by communication between different magma source regions at depth. The ⁴⁰Ar/³⁹Ar ages indicate that our normal polarity paleomagnetic data sample at least 0.1 Myr (0–0.1 Ma) and up to 0.78 Myr (0–0.78 Ma) of paleosecular variation and our reverse polarity data sample approximately 0.1 Myr (0.78–0.88 Ma) of paleosecular variation. Our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long-term geomagnetic field behavior. Negative inclination anomalies are observed for both the normal and reverse polarity time-averaged field. Within the data uncertainties, normal and reverse polarity field directions are antipodal, but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction.     

Tracing suspended sediment and particulate phosphorus sources in catchments

Information on suspended sediment and particulate P (PP) sources is an important requirement in many catchment-based diffuse source pollution studies, in order to assist with model validation and to provide information to support the development of effective sediment and phosphorus control strategies. Such information is, however, frequently unavailable or difficult to assemble. In the study reported, source fingerprinting procedures were successfully used to assemble this information for seven sub-catchments in the Hampshire Avon catchment and five sub-catchments in the Middle Herefordshire Wye catchment. The results provide important new information on the relative importance of the contributions from surface and channel/subsurface sources to the suspended sediment and PP fluxes from the catchments. In the Wye sub-catchments channel/subsurface sources contributed 40–55% of the overall suspended sediment flux and 21–43% of the PP flux from the catchments. Equivalent values for the Avon were 1–41% and 1–54%, respectively. Combination of the information on the relative importance of surface and channel/subsurface sources with measured suspended sediment fluxes has provided the first estimates of the specific fluxes of sediment and PP attributable to channel/subsurface sources for UK catchments. The former are as high as 15–20 t km⁻² year⁻¹ in some of the Wye sub-catchments, whereas the latter exceeded 0.1 kgP ha⁻¹ year⁻¹ in the same sub-catchments. The results emphasize the need to take account of potential contributions from channel/subsurface sources when using measured suspended sediment and PP flux data to validate predictions derived from models incorporating only surface contributions.     

Huanghe (Yellow River) and its estuary: Sediment origin, transport and deposition

The upper part of the Huanghe (Yellow River) drainage basin supplies 50–60% of the annual water discharge and only 10% of the total river sediment load, while the middle reaches contribute 30–40% of the water flow and 90% of the annual sediment load, because of severe erosion over the Loess Plateau. Large variations in both annual water discharge and sediment load occur in the Huanghe. Heavy sedimentation in the lower reaches of the channel makes the river bed aggrade several centimetres per year. Of the suspended sediment in the river, 90–95% is deposited in the lower part of the river course and in the coastal shallow water area; less than 5–10% escapes from Laizhou Bay and enters the Central Bohai and/or North Huanghai (Yellow Sea). The active delta complex now propagates seawards at a mean rate of 42 km² year⁻¹.     

Temporary and definitive fixation of atmospheric lead in deep-sea sediments of the Western Mediterranean Sea

Most lead brought to the Mediterranean Sea has an anthropogenic origin and is mainly transported through the atmosphere. Atmospheric Pb was continuously collected at Cap Ferrat in 1986 and 1987. From this study, the estimation of the anthropogenic Pb flux on the whole Western Mediterranean was, averaged on 1986 and 1987 data, 4080 t. Assuming that the atmospheric anthropogenic Pb input varied in this course of time similarly to the consumption of Pb added to gasolines in France, the mean annual flux could be calculated: 3.95 kg km⁻² yr⁻¹, that is an annual input of 3360 t yr⁻¹. Reaching the sea, this metal seems to become rapidly bound to phytoplankton. Grazing by zooplankton leads to the production of faecal pellets which frequently contain rather high metal concentrations. The sinking rate of pellets of various zooplankton species is high; within a few days pellets may reach deep-sea sediments. After deposition, Pb is released from this organic-rich material during early diagenesis. In most cases, it, therefore, returns to the overlaying water body by ascending diffusion. But, in a deep-sea area of approximately 80 000 km² where Mn oxide precipitation occurs in surficial sediments, Pb seems to remain stored by coprecipitation processes. By considering the lead stored in ‘excess’ in the surficial sediment of the deep-sea area, we estimate that a mean annual anthropogenic Pb amount ranging from 800 up to 1080 t was stored every year from 1950. On the same area, taking into account the Pb loss at the straits, the ‘direct’ atmospheric input to the sea bottom is, on average, 184 t yr⁻¹. The remaining part, that is (800–1080)−184=(616–896) t yr⁻¹, corresponds to an additional ‘indirect’ Pb flux in water due to Pb released from sediments of the surrounding areas where it does not remain stored.     

Seismic structure of Iceland from Rayleigh wave inversions and geodynamic implications

We have constrained the shear-wave structure of crust and upper mantle beneath Iceland by analyzing fundamental mode Rayleigh waves recorded at the ICEMELT and HOTSPOT seismic stations in Iceland. The crust varies in thickness from 20 to 28 km in western and northern Iceland and from 26 to 34 km in eastern Iceland. The thickest crust of 34–40 km lies in central Iceland, roughly 100 km west to the current location of the Iceland hotspot. The crust at the hotspot is ∼32 km thick and is underlain by low shear-wave velocities of 4.0–4.1 km/s in the uppermost mantle, indicating that the Moho at the hotspot is probably a weak discontinuity. This low velocity anomaly beneath the hotspot could be associated with partial melting and hot temperature. The lithosphere in Iceland is confined above 60 km and a low velocity zone (LVZ) is imaged at depths of 60 to 120 km. Shear wave velocity in the LVZ is up to 10% lower than a global reference model, indicating the influence of the Mid-Atlantic Ridge and the hotspot in Iceland. The lowest velocities in the LVZ are found beneath the rift zones, suggesting that plume material is channeled along the Mid-Atlantic Ridge. At depths of 100 to 200 km, low velocity anomalies appear at the Tjornes fracture zone to the north of Iceland and beneath the western volcanic zone in southwestern Iceland. Interestingly, a relatively fast anomaly is imaged beneath the hotspot with its center at ∼135 km depth, which could be due to radial anisotropy associated with the strong upwelling within the plume stem or an Mg-enriched mantle residual caused by the extensive extraction of melts.     

Primordial helium isotope signature from Plio–Quaternary alkaline basalts in Yemen

Abstract Isotopic compositions of He, Ne and Ar were measured on Plio–Quaternary alkaline basalts of Marib–Sirwah and Shuqra volcanic fields in Yemen, south-western Arabian Peninsula. Very high ³He/⁴He isotope ratios were found in olivine phenocrysts of some Quaternary alkaline basalts in both volcanic fields, located on the margin of the dispersed Afar mantle plume, compared with the Afar–Ethiopian province in the center of the mantle plume. This suggests that the Afar mantle plume source may consist of common component (C or focal zone (FOZO)) with variable primordial ³He/⁴He ratio rather than high μ mantle (HIMU) component. The three component mixing C as the Afar mantle plume, depleted mantle (DM) as upper mantle and lithospheric mantle with a hybrid enriched mantle I–II (EM I–EM II) characteristics may be adequate to explain He–Sr–Nd–Pb isotope variation for the Afar–Arabian Cenozoic volcanics. The occurrence of high ³He/⁴He ratios in the Marib–Sirwah volcanic field appears to show that the primitive basaltic magma, derived from the margin of the dispersed trous-like Afar mantle plume during 15–0 Ma, was not by contamination of lithospheric and upper mantle materials in comparison with that from the center of the Afar mantle plume as a result of relatively low thermal anomaly.     

Subduction influence on magma supply at the East Scotia Ridge

Despite a spreading rate of 65–70 km Ma⁻¹, the East Scotia Ridge has, along most of its length, a form typically associated with slower rates of sea floor spreading. This may be a consequence of cooler than normal mantle upwelling, which could be a feature of back-arc spreading. At the northern end of the ridge, recently acquired sonar data show a complex, rapidly evolving pattern of extension within 100 km of the South Sandwich Trench. New ridge segments appear to be nucleating at or near the boundary between the South American and Scotia Sea plates and propagating southwards, supplanting older segments. The most prominent of these, north of 56°30′S, has been propagating at a rate of approximately 60 km Ma⁻¹ for at least 1 Ma, and displays a morphology unique on this plate boundary. A 40 km long axial high exists at the centre of this segment, forming one of the shallowest sections of the East Scotia Ridge. Beneath it, seismic reflection profiles reveal an axial magma chamber, or AMC, reflector, similar to those observed beneath the East Pacific Rise and Valu Fa Ridge. Simple calculations indicate the existence here of a narrow (<1 km wide) body of melt at a depth of approximately 3 km beneath the sea floor. From the topographic and seismic data, we deduce that a localised mantle melting anomaly lies beneath this segment. Rates of spreading in the east Scotia Sea show little variation along axis. Hence, the changes in melt supply are related to the unique tectonic setting, in which the South American plate is tearing to the east, perhaps allowing mantle flow around the end of the subducting slab. Volatiles released from the torn plate edge and entrained in the flow are a potential cause of the anomalous melting observed. A southward mantle flow may have existed beneath the axis of the East Scotia Ridge throughout its history.     

Mantle and crustal contribution in the genesis of Recent basalts from off-rift zones in Iceland: Constraints from Th, Sr and O isotopes

Along the two volcanic off-rift zones in Iceland, the Snfellsnes volcanic zone (SNVZ) and the South Iceland volcanic zone (SIVZ), geochemical parameters vary regularly along the strike towards the centre of the island. Recent basalts from the SNVZ change from alkali basalts to tholeiites where the volcanic zone reaches the active rift axis, and their⁸⁷Sr/⁸⁶Sr andTh/U ratios decrease in the same direction. These variations are interpreted as the result of mixing between mantle melts from two distinct reservoirs below Snfellsnes. The mantle melt would be more depleted in incompatible elements, but with^a higher³He/⁴He ratio (R/Ra≈ 20) beneath the centre of Iceland than at the tip of the Snfellsnes volcanic zone (R/Ra≈ 7.5).

From southwest to northeast along the SIVZ, the basalts change from alkali basalts to FeTi basalts and quartz-normative tholeiites. TheTh/U ratio of the Recent basalts increases and both (²³⁰Th/²³²Th) andδ¹⁸O values decrease in the same direction. This reflects an important crustal contamination of the FeTi-rich basalts and the quartz tholeiites. The two types of basalts could be produced through assimilation and fractional crystallization in which primary alkali basaltic and olivine tholeiitic melts ‘erode’ and assimilate the base of the crust. The increasingly tholeiitic character of the basalts towards the centre of Iceland, which reflects a higher degree of partial melting, is qualitatively consistent with increasing geothermal gradient and negative gravity anomaly.

The highest Sr isotope ratio in Recent basalts from Iceland is observed inÖrfajökull volcano, which has a³He/⁴He ratio (R/Ra≈ 7.8) close to the MORB value, and this might represent a mantle source similar to that of Mauna Loa in Hawaii.     

Magmatic underplating in the Canary Archipelago

The location of the Canary Archipelago, at the east edge of the North Atlantic, a few hundred kilometres from the West African coast, has for years posed a serious difficulty in understanding the complex interaction between long-lasting volcanic activity and one of the oldest seafloor basements (>150 Ma) on earth. Many different hypotheses have been proposed in the last two decades to understand the genesis of the Canary Archipelago. There is an increasing acceptance to explain the archipelago as the product of the slow passage of the African plate over a mantle hotspot. In rival tectonic models, stretching and thinning of the lithosphere would determine the islands as independent volcanic blocks. We review recent geophysical and geological evidence supporting a mantle plume origin for the Canary Islands, based on the recognition of crustal thickening by mafic intrusions produced by the interaction between the mantle plume and the old oceanic lithosphere beneath the Canary Archipelago.     

Results from an aeromagnetic investigation of the nares strait region

An analysis of published and recently collected aeromagnetics in the Naires Strait region shows that short wavelength, moderate amplitude magnitic anomalies are coincident with previously mapped Precambrian Shield rocks under southeast Ellesmere Island. Similar magnetic anomalies are also observed in northwestern Greenland across Nares Strait. On Greenland the characteristic magnetic anomalies persist about 105 km beyond and northeast of the exposed Precambrian outcrops. This suggests that the Greenland Pre-Cambrian Shield structures previously shown as only occuring as far north as Inglefield Land actually extend at least 100 km further north. Correlation of the magnetic anomalies with mapped geological units suggest a 105+−10 km left lateral displacement in the boundary between the Lower Paleozoic and Precambrain terrains across Nares Strait. This, we interpret, is due to the relative motion between Greenland and Ellesmere Island during the evolution of the Labrador Sea and Baffin Bay in the Early to Mid Tertiary.     

Crustal and upper mantle structure of the Mediterranean area derived from surface-wave data

A summary of results based mainly on the inversion of available surface-wave dispersion data is given for the Mediterranean area both for crustal and upper mantle structure. The results are presented on maps outlining the regionalization of the crust and the lithosphere-asthenosphere system in the area. It is possible to distinguish several types of crust with average S-wave velocities in the range 2.8–3.8 km s⁻¹ and thickness varying from a minimum of about 10–16 km, in the Western Mediterranean, to a maximum of about 50 km (including a possible transitional layer) beneath the Ionian Sea. The average properties of the crust and of the lithospheric part of the mantle indicate a possible continuous structure extending from North Africa through the Ionian Sea to the Adriatic Sea, characterized by the presence of a transitional layer at the crust-mantle boundary. Strong lateral variations are present in the lithosphere-asthenosphere system both in thickness, from 30 km in the Western Mediterranean, to about 130 km, under the Alps, and in S-wave velocity, from 4.1–4.2 km s⁻¹ up to 4.7 km s⁻¹. The relatively high position of low resistivity material that seems to characterize the Mediterranean area agrees fairly well with the shallower average top of the asthenosphere found in this area from the study of the elastic properties. The usefulness of combining seismological and electromagnetic studies is stressed.     

Emerald dating through Ar/Ar step-heating and laser spot analysis of syngenetic phlogopite

Emerald, occurring in K-metasomatic rocks developed at the contact of the Carnaíba leucogranite with serpentinite (Bahia State, Brazil), has been dated using an original ⁴⁰Ar/³⁹Ar procedure. It combines step heating and spot fusion experiments on two types of phlogopite crystals: (1) bulk samples and individual grains extracted from the enclosing K-metasomatic host rocks; and (2) syngenetic solid inclusions precipitated along growing zones of the emerald host crystals. The second procedure uses in situ laser probe experiments on rock sections. In spite of the huge amounts of excess ⁴⁰Ar detected in adjacent emerald, we could measure reliable ages of 1951 ± 8 Ma and 1934 ± 8 Ma for the Trecho Velho and Braulia occurrences, respectively. Spot fusion data had higher discrepancy than the step heating data, but minute crystals of phlogopite included in emeralds bearing excess argon do not reveal excess argon. A muscovite belonging to the same granite hydrothermal complex gave a plateau age of 1976 ± 8 Ma, which may correspond to a higher closure temperature of the KAr system during the cooling of the whole pluton and associated hydrothermal halo.

These accurate measurements lead to the following conclusions: (1) direct emerald dating is possible; (2) in spite of a polyphase history during the Transamazonian orogenesis (2 Ga), combined step heating and spot fusion experiments give a better precision for granite-related emerald mineralization than the scattered ages obtained by Rb-Sr and K-Ar methods; (3) the late-Transamazonian tectonothermal retrograde event which probably caused the dispersion of previous Rb-Sr and K-Ar data is not revealed by our procedure; (4) the emerald mineralization and K-metamorphism appear to be linked with the thermal history of the leucogranite; (5) in addition to its use in polyphase crustal domains, accurate ⁴⁰Ar/³⁹Ar dating is of major interest in the field of metallogenic models, even, for instance, for mineralizations characterized by disturbed isotopic systems, which record effects as excess argon.     

Helium isotopes in early Iceland plume picrites: Constraints on the composition of high 3He/4He mantle

A detailed study of the geochemistry of a new suite of early Iceland plume picrites shows that extremely high ³He/⁴He ratios (up to 50 R_a) are found in picrites from Baffin Island and West Greenland. High ³He/⁴He picrites display a wide range in ⁸⁷Sr/⁸⁶Sr (0.70288–0.70403), ¹⁴³Nd/¹⁴⁴Nd (0.51288–0.51308) and incompatible trace element ratios (e.g. La/Sm_n = 0.5–1.6). These overlap the complete range of compositions of mid-ocean ridge basalts and most northern hemisphere ocean island basalts, including Iceland. Crustal contamination modelling in which high-grade Proterozoic crustal basement rocks for the region are mixed with a depleted parent cannot account for the trend displayed by the Baffin Island and West Greenland picrites. This rules out the possibility that the incompatible trace element, Sr and Nd isotope range of the high ³He/⁴He picrites is due to crustal contamination. The compositional range at high ³He/⁴He is also inconsistent with derivation from a primordial-He-rich reservoir that is a residue of ancient mantle depletion. This implies that the composition of the high ³He/⁴He mantle cannot be determined simply by extrapolating ocean island basalt He–Sr–Nd–Pb–Os isotope data.The apparent decoupling of He from trace element and lithophile radiogenic isotope tracers is difficult to attain by simple mixing of a high-[He], high ³He/⁴He reservoir with various depleted and enriched He-poor mantle reservoirs. The possibility that primordial He has diffused into a reservoir with a composition typical of convecting upper mantle cannot be ruled out. If so, the process must have occurred after the development of existing mantle heterogeneity, and requires the existence of a deep, primordial He-rich reservoir.