Icelandic Basalts Research Articles

The Distribution of Olivine Compositions in Icelandic Basalts and Picrites

A number of recent studies have used the compositional relationship between magmatic crystals and their carrier liquids to understand processes within volcanic plumbing systems. Here, an extensive compilation of electron microprobe data for Icelandic olivine and glass compositions is used to examine the distribution of the forsterite content of olivine macrocrysts within single lava flows, and the relationship of these olivines to their carrier basaltic liquids. A dataset of 7836 olivine and 233 glass point analyses was examined and 11 eruptions were identified where glass data and over 60 crystal core compositions were available. In common with many basaltic suites, single olivine crystals typically have uniform core compositions with narrow normally zoned rims. Accordingly, in 10 of the 11 Icelandic eruptions over 90% of the olivines are too forsteritic to be in equilibrium with their carrier basaltic liquids. The sampling density of the dataset permitted statistical investigation of the distribution of olivine compositions that contain information that can be used to provide new constraints on magmatic processes. The results of both kernel density estimates and Gaussian mixture modelling indicate that each of the 11 eruptions contained at least one robust peak in olivine compositions. Out of these 11 eruptions, eight show unimodal distributions of macrocryst olivine forsterite content, two are bimodal and one is polymodal. An important feature of the relationship between the carrier glass compositions and the distribution of olivine forsterite contents is that, for 10 of the 11 flows, a strong peak in the olivine compositional distribution occurs at forsterite contents that are 2–3 mol % higher than those expected for olivines in equilibrium with the carrier liquid. This offset peak is not predicted for olivines generated by simple equilibrium or fractional crystallization models. Instead, the distribution of olivine compositions and its relationship with the carrier liquids can be accounted for using a three-stage model. In the first stage, fractional crystallization and crystal settling generate a mush pile on the floor of a magma chamber. Compositional stratification is present in this mush, with the olivines at its base being more forsteritic than those at its top, reflecting the evolution of liquid compositions during fractional crystallization. The olivines in the uppermost part of the mush are close to equilibrium with basaltic liquid in the interior of the chamber. In the second stage local diffusion acts to homogenize single crystals in the mush, creating the uniform cores observed upon eruption. Concurrently the chemical gradient across the full thickness of the mush pile is altered by diffusion through the interstitial melt phase, reducing the variance of olivine core compositions in the mush. This process never reaches completion in the Icelandic flows but does generate a single peak in olivine compositions close to the mean forsterite content of the olivines in the crystal pile. Finally, the mush is disaggregated into the carrier liquid of the chamber interior shortly before eruption, creating the diffusional rim overprints. Quantitative models of this process indicate that the observed offset peak in olivine compositions can be generated after 42–8000 years of diffusion in a mush pile, depending on the mush thickness. Key features of the compositional distribution of olivines in basalts can therefore be accounted for using a simple model of mush generation and disaggregation.

Evaluation of the effects of composition on instrumental mass fractionation during SIMS oxygen isotope analyses of glasses

Significant instrumental mass fractionation (IMF) occurs during measurements of oxygen isotope ratios in magmatic glasses by SIMS. In order to characterise and correct for this fractionation, we measured oxygen isotope ratios in a range of international and internal glass standards ranging in composition from basalt (47wt.% SiO2) to rhyolite (72wt.% SiO2) and with known major element compositions. Oxygen isotope ratios were determined by laser fluorination at SUERC, East Kilbride, or taken from previously published values. A total of 1105 δ18O measurements were made over nine sessions on a Cameca IMS-1270 ion microprobe at the University of Edinburgh. SIMS measurements on glass standards had external precision better than ±0.36‰ (1σ), and the reference material analysed alongside the unknown samples, USGS synthetic glass GSA-1G, had an average external precision of ±0.14‰. The selected standards are thus sufficiently homogeneous in δ18O to be suitable calibration standards. In terms of δ18O, the SIMS measurements show that, within a single session, IMF may vary by up to 4.7‰ from one glass standard to another. IMF is strongly correlated with SiO2 and CaO. A least squares regression calculation was used to explore potential univariate and multivariate correction schemes. For each correction scheme, the correction coefficients determined for each session were then used to calculate the IMF and correct the measured isotopic ratio of each glass standard. A univariate correction scheme using only SiO2 to correct for IMF reproduced 75% of the glass standards to within ±0.2‰ of their true δ18O, and 95% to within ±0.4‰. Bivariate correction schemes using SiO2–CaO and FeO–CaO produced similar results, but did not significantly improve on the SiO2 correction. The correction schemes were applied to δ18O measurements made on melt inclusions and glasses from the Askja volcanic system, North Iceland. The uni- and bivariate correction schemes tested produced δ18O values within the published range for Icelandic basalts. We recommend a simple correction scheme based on the SiO2 content of appropriate standards, which should span a suitable compositional range from basalt to rhyolite.

Mineralogy and chemistry of altered Icelandic basalts: Application to clay mineral detection and understanding aqueous environments on Mars

We used a suite of techniques, including those emulating compositional data sets obtained from Mars orbit and obtainable at the Mars surface, to examine aqueous alteration of basaltic rocks from Iceland as a mineralogic and geochemical analog for Noachian environments on Mars. A sample suite was collected for laboratory measurement of (1) whole‐rock visible/near‐infrared (VNIR) reflectance and thermal infrared (TIR) emission spectra; (2) VNIR and TIR reflectance spectra of particle‐size separates derived from the bulk rock and from materials extracted from fractures/vesicles; (3) X‐ray diffraction (XRD) patterns for determination of quantitative modal mineralogy; (4) major element chemistry using flux fusion of whole‐rock powders; and (5) electron microprobe analyses of minerals in thin sections. Conclusions about aqueous alteration can be influenced by technique. For these basalts, whole‐rock chemical data showed scant evidence for chemical fractionation, but TIR, VNIR, and XRD measurements identified distinctive assemblages of hydrous silicate minerals, differing by sample. XRD provided the most complete and accurate quantitative determination of sample mineralogy. However, VNIR spectroscopy was the technique most useful for determining composition of low‐abundance smectite clays, and TIR spectroscopy was the most useful for recognizing hydrated silicates in thin surface coatings. High spatial resolution mineralogical and chemical data sets were useful for understanding the texture and distribution of alteration products and variations in fluid chemistry. No single approach provides a complete assessment of the environment of alteration, demonstrating the importance of employing multiple, synergistic mineralogical and geochemical techniques and instruments in exploration of rock strata from aqueous paleoenvironments on Mars.

Insights into crustal assimilation by Icelandic basalts from boron isotopes in melt inclusions from the 1783–1784 Lakagígar eruption

The boron isotope system has great potential for tracing alteration and assimilation in basaltic systems due to the very low concentrations of B in mantle-derived melts and the strong isotopic contrast between the mantle and surface materials. However, variability in B concentrations and isotope ratios in basalts can also be interpreted to reflect inputs from enriched regions of the mantle, as the extent of mantle heterogeneity with respect to boron remains poorly delineated. We have determined boron concentrations and isotope ratios in fresh, glassy, plagioclase-hosted melt inclusions and unaltered scoriaceous matrix glasses from four localities associated with the 1783–1784 Lakagígar (Laki Fissure) eruption, Iceland. Boron concentrations range from 0.59 to 1.25ppm in the melt inclusions, and from 1.25 to 1.65ppm in the matrix glasses, while δ11BNBS-951 ranges from −7.8‰ to −16.5‰ in the melt inclusions and −10.5‰ to −16.9‰ in the matrix glasses. In contrast to previous studies of boron in basaltic melt inclusions from other fissure swarms in Iceland (Gurenko and Chaussidon, 1997, Chem. Geol.135, 21–34), the Lakagígar melt inclusions display a significant range of boron concentrations and isotope ratios at constant K2Owt.%, which is more consistent with B addition by assimilation of altered basalt than it is with mixing between depleted and enriched mantle sources. Assimilation of freshwater-altered crustal materials at depth may impart a light δ11B signature such as that observed in the Lakagígar melt inclusions and tephra host glasses. Considering boron concentrations and isotope ratios in the Lakagígar glasses and previously studied altered Icelandic basalts, together with volatile equilibration depths of the Lakagígar melt inclusions, we propose that (a) mantle-derived magmas formed beneath Lakagígar assimilated ∼5–20% altered crust at a depth of ∼3–4km or more, probably during magma accumulation in sills formed at the boundaries of low-density hyaloclastite layers; and (b) the magma subsequently underwent extensive mixing and homogenization prior to eruption, quite possibly within the magma chamber beneath the Grímsvötn central volcano, assimilating an additional ∼10% of altered crust at that time, for a total of up to 30% crustal assimilation. We hypothesize that volatiles including H2O, CO2, S, F, and Cl, which were responsible for the majority of the considerable casualties attributed to the Lakagígar eruption, were added together with isotopically light B by assimilation of hydrothermally altered crustal materials.

The influence of source heterogeneity on the U–Th–Pa–Ra disequilibria in post-glacial tholeiites from Iceland

We investigate the relative influence of mantle upwelling velocity and source heterogeneity on the melting rates recorded by 230Th–238U, 231Pa–235U and 226Ra–230Th disequilibria in post-glacial tholeiites from Iceland’s main rift areas. The measured (230Th/238U) ratios range from 1.085 to 1.247, the (231Pa/235U) ratios from 1.333 to 1.925, and the (226Ra/230Th) ratios from 0.801 to 1.218. A general positive correlation between 230Th excesses and distance from the inferred plume centre is consistent with a model of decreasing mantle upwelling velocity with increasing distance from the plume axis. However, the model is not substantiated by the (231Pa/235U) data as the correlation with distance from the plume centre is weak. On the scale of individual eruption centres, the observed U-series are influenced by variations in melt transport time, source porosity, and local variations in mantle upwelling velocity. Broad correlations between (230Th/238U) and (231Pa/235U) and highly incompatible trace element ratios for samples from the Western Volcanic Zone provide, however, evidence for a significant underlying effect of source heterogeneity on the U-series data. Low 230Th and 231Pa excesses in enriched samples from the Western Volcanic Zone with high U/Th, Nb/U and Nb/La indicate that partial melts from an enriched source component, characterised by high melt productivity but low bulk DU/DTh, influence the U-series systematics of the erupted melts. These results re-affirm the presence of comparatively larger abundances of enriched material in the mantle source beneath the South Western Rift of Iceland, which has been suggested based on relationships between highly incompatible element and Pb isotope ratios in Icelandic basalts. Overall, our results highlight the importance of lithological heterogeneity on the melting behaviour of the upper mantle and the composition of oceanic basalts.

Melting of a Two-component Source beneath Iceland

New trace element and Hf^Nd isotope data on post-glacial basalts from Iceland’s main rift zones are used in conjunction with literature data to evaluate the relative importance of source heterogeneity and the melting process for the final melt composition. Correlations between Hf and Nd isotope compositions and trace element ratios indicate that at least two source components are sampled systematically as a function of the degree and pressure of melting beneath Iceland. Strong depletion in Rb, Ba, U and Th and enrichment in Nb and Ta compared with La in the most enriched samples from the Reykjanes Peninsula and Western Rift Zone suggests that the enriched source component is similar to ancient recycled enriched mid-ocean ridge basalt (E-MORB) crust. Highly incompatible trace element ratios such as Nb/La and Nb/U and Pb isotope ratios are variable across Iceland. This observation suggests that either the enriched component is intrinsically heterogeneous, or that there is a larger proportion of the enriched source component beneath the Southwestern Rift Zone compared with the Northern Rift Zone. The relative effect of source heterogeneity and melting on the final melt composition was evaluated with a one-dimensional polybaric melt mixing model in which accumulated melts from a depleted MORB mantle and a recycled E-MORB crust are mixed in different ways. Two styles of melt mixing were simulated: (1) complete mixing of melts with variable proportions of the depleted mantle and recycled E-MORB components; (2) incomplete mixing with a fixed initial proportion of the two source components. Calculated pressure-dependent compositional changes using these simple two-component melting models can explain the observed trends in trace element ratio and isotope ratio diagrams for Icelandic basalts, even in cases where conventional binary mixing models would require more than two source components. The example of Iceland demonstrates that melt mixing during extraction from the mantle is a key process for controlling the trace element and isotope variability observed in basaltic lavas and must be evaluated before inferring the presence of multiple source components.

Possible layering of mantle convection at the top of the Iceland Hotspot: a crosscheck between 3-D numerical models and gravimetric, seismic and petrological data

SUMMARY 3-D numerical models describing convection cells that may develop in a buoyant layer over an upper-mantle hotspot are developed. Results are compared with those of an alternative model, which assumes an upper partially molten layer having the same density as the underlying mantle, but of much higher viscosity. Geophysical and geochemical data over Iceland are used to assess the models. For the case of a buoyant upper layer, a major finding of our modelling is the prediction of a cold downward moving sheet in the upper layer situated just above the axis of the hot underlying mantle hotspot. When compared to geophysical data from Iceland, this modelled feature may explain why: (i) along the hotspot axis, seismic tomography reveals a mantle that remains cold down to ∼80 km depth; (ii) the ridge crest systematically avoids the axis of the hotspot and (iii) the ridge axis is displaced by ∼60 km to the east or the west of the hotspot axis, these ridge axes corresponding to the modelled upwellings of hot material in the uppermost layer driven by the cold downwelling axial sheet. Finally, the model also predicts development of gravity swells a few hundred kilometres long and several tens of kilometres wide, with a peak to valley amplitude of 5–10 mGal, lying over ascending sheets of upwelling material that float over the hotspot. Comparison with the residual gravity map over Iceland does indeed show the presence of a long gravity high along the active eastern section of the ridge. Over the AskjaVolcano, the long gravity high intersects an off-axis gravity high oriented northwest, which joins the Kolbeinsey Ridge. It is proposed that this branching may be the cause of the eastward drift of the western section of the Iceland ridge that took place 3 Ma. Further south, parallel to the same ridge-associated positive gravity anomaly, a more recent branching is found with a very long gravity high, oriented in an easterly direction, but that eventually contours the whole eastern coast of Iceland. This latter feature may be the precursor of future eastward drift of the whole eastern section of the ridge. When combined with experimentally derived models of dry peridotite melting, it is found that the temperature field of the models is consistent with the principal constraints derived from compositional characteristics of Icelandic basalts and the 3-D map of the upper mantle below the Icelandic hotspot derived from S-wave seismic tomography. The high viscosity layer model is also relatively successful at explaining geophysical and petrological data, except that it does not predict the development of hot and cold sheets in the partially molten mantle, and it leads to a large and strong negative gravity and bathymetry anomaly over the Iceland Hotspot, at a place where available data show a swell. However, this latter result is found to depend on the lateral boundary conditions used in the numerical modelling. For this reason we conclude that the presence of a very high viscosity layer below the ridge cannot be ruled out on the basis of gravity and bathymetric data alone. On the other hand, the influence of a floating upper layer provides an interesting and plausible alternative to explain available geophysical data, given that experimental data tend to indicate only modest changes in viscosity of the mantle in response to partial melting.

Compositional trends of Icelandic basalts: Implications for short-length scale lithological heterogeneity in mantle plumes

Lithological variations in the mantle source regions under mid-ocean ridges and ocean islands have been proposed to play a key role in controlling melt generation and basalt composition. Here we combine compositional observations from Icelandic basalts and modelling of melting of a bi-lithologic peridotite-pyroxenite mantle to demonstrate that, while short-lengthscale major element variation is present in the mantle under Iceland, source heterogeneity does not make an important contribution to excess melt production. By identifying the major element characteristics of endmember Icelandic melts, we find enriched melts to be characterised by low SiO2 and CaO, but high FeO. We quantitatively compare endmember compositions to experimental partial melts generated from a range of lithologies, pressures and melt fractions. This comparison indicates that a single source composition cannot account for all the major element variation; depleted Icelandic melts can be produced by depleted peridotite melting, but the major element composition of enriched melts is best matched by melting of mantle sources that have been refertilised by the addition of up to 40% mid-ocean ridge basalt. The enriched source beneath Iceland is more fusible than the source of depleted melts, and as such will be over-represented in accumulated melts compared with its abundance in the source. Modelling of peridotite-pyroxenite melting, combined with our observational constraints on the composition of the Icelandic mantle, indicates that crustal thickness variations in the North Atlantic must be primarily due to mantle temperature and flow field variations.

Osmium isotopes and Fe/Mn ratios in Ti-rich picritic basalts from the Ethiopian flood basalt province: No evidence for core contribution to the Afar plume

Variations in the Fe/Mn ratio and Osmium isotopes in magnesium-rich mafic rocks from plume-related volcanic provinces have been exploited to imply the entrainment of core material in mantle plumes and the involvement of ancient recycled oceanic lithosphere. Here we present new major and trace element, 187Os/ 188Os ratios and precise Fe/Mn ratios on a suite of MgO-rich basalts, picritic basalts and ankaramites from the 30 Ma Ethiopian flood basalt province that shed new light on these arguments. The lavas show a range of compositions with MgO varying from 5 to 20 wt.% although the primary magma is inferred to have an MgO content of 15–16 wt.%. The lavas are also characterised by low Al 2O 3 contents (7–9 wt.% at 15 wt.% MgO), implying an origin from ∼ 150 km depth at a temperature in excess of 1600 °C, consistent with an origin in the early phases of Afar mantle plume activity. Osmium isotopes in samples with > 10 wt.% MgO are unradiogenic with 187Os/ 188Os < 0.127 while those with lower MgO define a positive correlation with Re/Os and appear to have been contaminated by crustal material. Fe/Mn ratios determined by ICP-MS vary from 65.4 to 78.5 in rocks with > 10 wt.% MgO and show greater variation to both higher and lower values in less magnesian samples. These values are high compared with MORB and Icelandic basalts, comparable with the Fe/Mn ratios of Hawaiian and other selected ocean island basalts, and are a characteristic of the primary magma. There is no evidence to suggest that Fe/Mn is fractionated during peridotite melting at low pressures less than 5 GPa, implying in agreement with previous studies that high Fe/Mn ratios are a compositional feature of the magma source region. The lack of association of radiogenic 187Os/ 188Os with the high Fe/Mn ratios of the Ethiopian picritic basalts calls into question the link to possible entrainment of core material in the source of the Afar mantle plume. Similarly, the unradiogenic 187Os/ 188Os ratios preclude a significant contribution from ancient recycled oceanic lithosphere. An alternative model is suggested in which melts generated at high pressures (> 7 GPa) during the initial turbulent ascent of the Afar plume head form pyroxene rich veins with high Fe/Mn ratios and high incompatible element contents in a peridotite matrix. These highly fertile source regions contribute significantly to melt compositions during the early phases of plume emplacement.

10.1007/s11486-008-1010-5

Results of microprobe analysis of titanomagnetites and hemoilmenites from Cenozoic basalts of Mongolia and Iceland, Eastern Siberia Permo-Triassic traps (the Norilsk borehole), and Proterozoic and Paleozoic dike basaltoids on the White Sea coast of the Kandalaksha Gulf (Por’ya Bay), as well as data on titanomagnetites of Primorski Krai basalts [Shcheka et al., 1980], are used to examine specific features of the implantation of impurity atoms into solid solutions with various concentrations of the ulvospinel component.

Frequency and fluid effects on elastic properties of basalt: Experimental investigations

In order to investigate the effects of fluid and frequency on the elastic properties, we performed hydrostatic experiments on an Icelandic basalt specimen under both dry and saturated conditions. This basalt is characterized by a bimodal porosity, i.e., cracks and equant pores. The elastic properties ‐bulk moduli in our case‐ were investigated under high pressure through two experimental methods: (1) a classical one using ultrasonic P‐ and S‐waves velocities (frequency 106 Hz), (2) and a new one, using oscillation tests (frequency 10−2 Hz). In dry condition, experimental data show no significant difference between high (HF) and low (LF) frequency bulk moduli. However, in saturated conditions, two effects are highlighted: a physico‐chemical effect emphasized by a difference between drained and dry moduli, and a squirt‐flow effect evidenced by a difference between HF and LF undrained moduli.

Investigation of Li/Ca variations in aragonitic shells of the ocean quahog Arctica islandica, northeast Iceland

Interannual and intra‐annual variations in lithium‐to‐calcium ratio were investigated with high temporal resolution in the aragonitic outer shell layer of juvenile Arctica islandica (Mollusca; Bivalvia) collected alive in 2006 off northeast Iceland. Li/Cashell ranged between 7.00 and 11.12 μmol mol−1 and presented well‐marked seasonal cycles with minimum values recorded at the annual growth lines; a general pattern was a progressive increase in Li/Cashell from March to May, followed by a plateau in June and a decrease down to minimum values in July–August. Li/Cashell was correlated with δ18Oshell‐derived temperature, but the strength of this relationship was weak (r2 &lt; 0.25 and p &lt; 0.05). It covaried significantly with microgrowth increment width and with the discharge from one of the closest rivers. Seasonal variations of Li/Cashell in A. islandica may most likely be explained (1) by calcification rate and/or (2) by significant river inputs of Li‐rich silicate particles flowing to the sea as soon as snow melts. In the first case, Li/Cashell may be a useful proxy for addressing seasonal variations of growth rate in bivalves that lack discernable microgrowth patterns. Abrupt decreases of Li/Cashell may, in turn, help identify growth retardations due to harsh environmental conditions. Alternatively, if Li/Cashell variations are linked to particulate Li inputs by rivers, this could be a new proxy for the intensity of mechanical weathering of Icelandic basalts, with interesting perspectives for the reconstruction of frequency and intensity of past jökulhlaups (subglacial outburst floods). Further works, including experimental studies, are needed to test these hypotheses.

Processes controlling Sr in surface and ground waters of Tertiary tholeiitic flood basalts in Northern Iceland

Strontium concentrations of 253 natural water samples from Skagafjördur, a Tertiary tholeiitic flood basalt region in northern Iceland range between 0.10 and 28 ppb. Surface environments (rivers, lakes, and peat soil waters) include the whole range of observed Sr concentrations whereas the Sr concentrations of ground waters are, in most cases, <3.5 ppb. Concentrations of Sr derived from basalt dissolution (i.e., rock-derived Sr) in waters of rivers and lakes exhibit a near linear correlation with the concentration of rock-derived Ca with a median molar Ca/Sr ratio of 1350. This systematic correlation suggests that Ca and Sr concentrations are controlled by weathering processes, i.e., the extent of dissolution of the basalt. The relative mobility of Sr during weathering in Skagafjördur is approximately half that of Ca, which is consistent with observed relative mobilities of these elements elsewhere in Iceland and in other basaltic regions. Peat soil waters commonly have lower concentrations of Sr and higher Ca concentrations than rivers and lakes, and molar ratios of rock-derived Ca to Sr in peat soil waters exhibit no systematic pattern. In several cases calculated concentrations of rock-derived Sr in peat soil waters yield negative values, suggesting a mineralogic sink for Sr in these waters. The low Sr concentrations in cold and thermal ground waters (<3.5 ppb) suggest mineralogic control over Sr in the ground water systems. Precipitation of secondary Sr minerals such as strontianite and celestite is ruled out as the ground waters are understaturated with respect to these minerals. Ground waters are characterized by high Ca/Sr molar ratios (∼5000 compared to bedrock Ca/Sr ratio of 730) suggesting that Sr is being preferentially incorporated (relative to Ca) into secondary minerals. The secondary minerals present in the bedrock in Skagafjördur that can preferentially incorporate Sr include zeolites, such as heulandite, chabazite, and thomsonite, and smectite. Ion-exchange calculations demonstrate that activities of Sr 2+ and Ca 2+ in ground water solutions in Skagafjördur are consistent with ion-exchange equilibria between these waters and heulandite from other Tertiary basalts in Iceland suggesting that this mineral may play an important role in controlling the concentration of Sr in the Skagafjördur ground waters. Incorporation of Sr into calcite cannot explain the observed high Ca/Sr ratios of the Skagafjördur ground waters because calcite, when precipitating, only admits limited amounts of Sr. Aragonite is not considered a likely candidate either because it has only very slight preference for Sr over Ca and ground waters above 40 °C are undersaturated with respect to this phase. However, predicted Sr content of calcite in equilibrium with the Skagafjördur ground waters (0.5–83 ppm Sr) is in good agreement with measured Sr content of this mineral in Tertiary basalts elsewhere in Iceland (<0.1–63 ppm), suggesting that the Skagafjördur ground waters can be used as analogues for Tertiary crustal solutions involved in the zeolite facies metamorphism of the Icelandic crust.

Greenstones in the Mino Paleozoic‐Mesozoic Terrane of the East Takayama Area, Central Japan: Evidence for Magmatism Evolution from Normal Ridge to Plume Volcanism

Greenstones of early Permian age occur extensively in the Mino terrane of the east Takayama area. Greenstones are mainly tholeiitic basalt and subordinately alkalic basalt. Tholeiitic basalt comprises a lava sequence, about 1000 m thick, and tectonically contacts alkalic basalt. Fusuline fossils in associated limestones show that alkalic basalt is slightly younger than tholeiitic basalt. Chrome spinel occurs locally in tholeiitic and alkalic basalts. It is occasionally aluminous and similar to spinel in tholeiitic and alkalic basalts of oceanic ridge regions. Basalts in the lower portion of the tholeiite sequence are depleted in TiO2, Zr, and Nb and are characterized by extremely low Nb/Y ratios; the lower basalts appear to be normal mid‐oceanic ridge basalt (N‐MORB). The middle‐upper basalts are enriched in TiO2, Zr, and Nb and show Nb/Y ratios that are high relative to Zr/Y; they are similar to Icelandic tholeiite. Basalt, chemically transitional from N‐MORB to Icelandic basalt, occurs in the lower to middle portion. The rock type in the greenstone body, including the tholeiite and alkalic basalt units, suggests that greenstones originally occupied part of a ridge‐seamount sequence. The variation in rock type may be explained by magmatism evolution of normal ridge volcanism, followed by plume‐influenced ridge volcanism and plume igneous activity.

Continental geochemical signatures in dacites from Iceland and implications for models of early Archaean crust formation

Whether early Archaean felsic crust was formed by processes related to plate subduction or melting of thick basaltic plateaus is vividly debated. Ultimately, the discussion hinges on the question of how Archaean felsic crust has obtained its distinct chemical traits. Here we report chemical and isotopic data for a suite of Cenozoic felsic volcanic rocks from Iceland. The samples exhibit the key-chemical characteristics of early Archaean felsic continental crust such as calc-alkaline composition, strong enrichment in Na relative to K, high Pb/Ce, La/Nb, and Ta/Nb ratios. Involvement of pre-existing continental lithosphere in the petrogenesis of the samples can be excluded, because their 207Pb/ 204Pb and 206Pb/ 204Pb ratios plot well within the range of Iceland basalts. Model calculations suggest that the chemical characteristics were produced by high-pressure partial melting of basaltic lower crust followed by fractional crystallisation of amphibole, plagioclase, and ilmenite. These findings demonstrate that plate subduction and melting of subduction-modified mantle or lithosphere are not necessarily required to produce the key-chemical signatures of continental crust. Hence, the calc-alkaline dacites provide intriguing support for early Archaean continental crust formation by melting of thick mafic plateaus.

Hydrogeology in the Nordic Countries

Hydrogeology in the Nordic Countries is characterized by many types of aquifers and great differences in groundwater recharge. Fracture aquifers in crystalline, hard rocks are the most common type of aquifer with, in general, low fracture porosity and low well yields. The most productive crystalline rocks are the basalts in Iceland and the rapakivi granite in Finland. The fracture and fault zones have mostly high conductivity. Porous aquifers are found in various types of geology. The most porous ones are the lava-fields and the pyroclastic rocks in the active volcanic zone of Iceland, but glaciofluvial deposits such as eskers and deltas in Finland, Norway and Sweden, outwash plains in SW Denmark and sandurs in Iceland are also very porous and good aquifers, as are some sedimentary rocks in Denmark. The glacial till has, in general, low conductivity. Fractured porous aquifers in consolidated limestones and sandstones have high well yields in relatively young formations in Denmark and Scania in southern Sweden, but medium or low yields in older strata. Karst aquifers have limited extension, but there are some well developed ones in the Caledonian mountain range in Norway and Sweden. Geothermal water with a lot of springs and even geysers are common in Iceland and of great economic importance. The groundwater chemistry of the crystalline, hard rocks is notable for bacteria, brines and mixed water at great depths (more than 500 m) as well as high contents of arsenic, fluoride and radon in drilled wells in certain regions.

The Fe/Mn ratio in MORB and OIB determined by ICP-MS

Variations in the abundance of iron in the mantle may have important consequences for mantle dynamics and geochemistry. The abundance of iron in lavas derived from mantle source regions varies during partial melting and subsequent fractionation, so that source heterogeneities are not easily resolved in iron abundances alone. However, manganese is a geochemically similar element, so that the planetary Fe/Mn ratio is approximately constant. Here, we report new Fe/Mn results for mid-oceanic ridge basalts (MORBs), oceanic island basalts (OIBs) and komatiites using a precise inductively coupled plasma mass spectrometric (ICP-MS) method to measure Fe/Mn to better than 0.5% (2 σ). As a measure of reproducibility of Fe/Mn, five olivine and five orthopyroxene grains from a Kilbourne Hole peridotite xenolith yielded Fe/Mn 69.8 ± 0.4 and 44.3 ± 0.2, respectively. To avoid ubiquitous secondary Fe–Mn oxides, Fe/Mn ratios in Pacific, Atlantic, and Indian MORBs were determined by Laser Ablation ICP-MS. MORB Fe/Mn (53–56) corrected for crystal fractionation yielded a value of 54.0 ± 1.2 (1 σ). Icelandic basalts and picrites (MgO 10–29%) had Fe/Mn ratios 56–61, with a single exception. Six relatively fresh komatiites from Belingwe (MgO 20–29%) yielded Fe/Mn values of 58.3 ± 0.2 (1 σ). Basalts from Tahiti and Reunion exhibited high Fe/Mn (>65), like Hawaii. This implies that the mantle source regions of Tahiti and Reunion lavas may have been enriched in Fe relative to other mantle reservoirs (e.g., MORBs, Iceland, Belingwe). Combined with previous results for Hawaii, we now find that Fe/Mn > 65 is characteristic of at least two plumes from the Pacific Superswell. It is conceivable that this is evidence for excess Fe due to core–mantle interaction in these mantle plumes, although partial melting of secondary pyroxenites may cause similar variations in Fe/Mn. Heterogeneity of Fe/Mn in mantle-derived lavas is now clearly documented.

Characteristic features of the implantation of Al, Mg, and Mn impurity atoms into the titanomagnetite structure

Results of microprobe analysis of titanomagnetites and hemoilmenites from Cenozoic basalts of Mongolia and Iceland, Eastern Siberia Permo-Triassic traps (the Norilsk borehole), and Proterozoic and Paleozoic dike basaltoids on the White Sea coast of the Kandalaksha Gulf (Por’ya Bay), as well as data on titanomagnetites of Primorski Krai basalts [Shcheka et al., 1980], are used to examine specific features of the implantation of impurity atoms into solid solutions with various concentrations of the ulvospinel component.

Rates of carbon ingrowth and nutrient release from young Icelandic basalts

Rates of carbon ingrowth and nutrient release from young Icelandic basalts

Temporal variations in crustal assimilation of magma suites in the East Greenland flood basalt province: Tracking the evolution of magmatic plumbing systems

We review published radiogenic isotope data (> 350 samples in total) on various suites of magmatic rocks within the Palaeogene central East Greenland flood basalt province to evaluate the types of crustal assimilants and the extent of crustal assimilation involved in each suite. We use these observations to build a regional picture of how magmatic plumbing systems changed with time and location during the sequential development of the province as magmatism responded to the development of a volcanic rifted margin and eventual plate separation. The earliest phase of magmatic activity (c. 62–57 Ma) is characterised by highly contaminated magmas that show a temporal change in assimilant type from amphibolite to granulite. This transition has been linked to the effects of an increasing magma supply rate which allows the more refractory granulite lithologies to be melted. The voluminous break-up phase of magmatism (c. 56–54 Ma) saw a significant decrease in the extent of assimilation because of the decreasing availability of assimilant material in the mature feeder systems, and many samples have Sr–Nd–Pb isotope compositions that overlap with those of asthenospheric melts (as represented by recent Icelandic basalts and North Atlantic MORB). Detailed study has allowed us to recognise packets of lavas that ponded at different levels in the crust and assimilated material of different compositions. The later stages of break-up magmatism show more diverse and more contaminated compositions that indicate a shift from a few large robust feeder systems to numerous small new conduits as the rifting continued. The post-break-up magmatism (c. 54–13 Ma) is characterised by a return to more highly contaminated magmas, which reflects a change in the style of magmatism: the eruption of small-volume alkalic lava flows from newly established conduits through the thicker inland crust, and the intrusion of mafic and silicic alkalic magmas at shallow levels in the Archaean basement along the present coast.