Icelandic Research Research Articles

The trace element composition of the lavas and dikes from a 3‐km vertical section through the lava pile of eastern Iceland

The Tertiary lava succession of eastern Iceland dips at shallow angles towards and beneath the axial Quaternary volcanic zone. A 3‐km vertical section through part of this Tertiary pile, studied during the Iceland Research Drilling Project, contains flows ranging in composition from basalt to icelandite. This succession can be divided into three distinct stratigraphic groups: upper and lower groups, 1.7 and 0.6 km thick with Zr/Y ratios of about 4.5, and a 0.7‐km‐thick middle group with a Zr/Y ratio of about 6.5. The middle group also has a significantly higher Ce/Yb ratio and higher elemental Sr abundances. The data on within‐group variation are equally compatible with high‐level fractionation of a plagioclase‐olivine‐clinopyroxene assemblage with accessory apatite and opaque minerals or with open system fractionation models. Dikes occur throughout the 3‐km section and are comparable in trace element composition to the lavas of the upper group. The dikes are part of a N‐S trending swarm originating from the Breiddalur volcanic center some 15 km to the south. These dikes were injected laterally, northward into the Reydarfjordur lava pile at the time of the eruption of the upper group lavas. The lower two major lava groups may be related to similar, but unidentified, discrete volcanic centers perhaps buried downdip. The new data on this deeper section corroborate earlier studies which indicated that on a gross scale, the eastern Iceland lava pile becomes less mafic with depth. This vertical change occurs because each major volcanic center erupted more silicic lavas close to the center and more magnesium‐rich compositions toward the periphery. The formation of the lava pile occurred by tilting toward the spreading center and overlapping repetition of such major units which resulted in the more mafic edges of the lava groups forming the topographically higher parts of the crustal section.

Detailed susceptibility log of Iceland Research Drilling Project drill core, Reydarfjordur, eastern Iceland

A detailed log of initial susceptibility, based on 6135 measurements made on drill core with a hand held meter, is presented for the 1919‐m‐long lower part of the IRDP Reydarfjordur crustal section. The log shows typical strong internal variation, with a central maximum, in flow susceptibility while dikes show little internal variation. Differences between dikes and flows and internal variations are explained in terms of variation in degree of deuteric and hydrothermal alteration, grain size, and abundance of iron‐titanium oxides. General decrease of flow susceptibility with depth is related to depth variation in hydrothermal alteration. This has the effect of removing iron from oxides probably for incorporation in nonmagnetic silicates. This effect is intense toward the bottom of the drill core. Linear extrapolation of the decrease of flow susceptibility with depth indicates a zero value at about 4 km beneath the original surface of the crustal section. Whether or not magnetization of the crust as a whole reaches zero level will depend on the dike density at this depth.

Primary and secondary alkali and halogen element distribution in Iceland Research Drilling Project basalts from eastern Iceland

The degree of oxidation and hydration, and the variation of Fe, Mg, Li, Na, K, Rb, F, Cl, and I are reported for hydrothermally altered basaltic lavas and dikes intersected in the Iceland Research Drilling Project (IRDP) borehole sequence from eastern Iceland. The concentrations of water‐soluble Na, Cl, and I have also been determined and represent approximately 1%, 40%, and 53%, respectively, of the total content of each element. Hydration and oxidation in both lavas and dikes generally increase downward in the total lava pile to maxima toward the base of the drilled section. Three lava groups and two dike groups are recognized at different depths on the basis of the relative distribution of Zr and Y. Each group exhibits a trend toward more primitive compositions upward in the pile. Within group and vertical chemical variations reflect both (1) primary chemical features such as parental magma composition and degree of crystal fractionation and (2) secondary mobilization during hydrothermal alteration. The latter caused variable alkali and halogen element dispersion, but initial differences between the chemical groups appear still to be discernible. Mobility sequences were Rb > K > Li > Na and Cl > F > I. Although all the analyzed elements were mobilized at the relatively low levels of hydration, few systematic trends with progressive hydration are observed: e.g., K and Rb were leached and Li was sometimes enriched. The main effect of alteration was a localized redistribution of mobile elements into stable secondary phases. Apparent depletions of mobile elements at the base of the sequence reflect primary variations rather than extensive leaching.

The Iceland Research Drilling Project: Synthesis of results and implications for the nature of Icelandic and oceanic crust

Study of a 3‐km‐thick section of Tertiary basalts in eastern Iceland has resulted in the formulation of a model for crustal construction based on accumulation of lava flows on the flanks of large central volcanoes and has led to the development of alteration and physical property profiles in a crustal segment formed at the Mid‐Atlantic Ridge. Deep drilling at Reydarfjordur revealed the existence of subaerial lava flows and volcaniclastic rocks to a crustal depth of 3.5 km. Three major stratigraphic units composed largely of basalts are recognized, each characterized by distinctive major and trace element patterns. Each of these units is believed to be the product of a separate episode of mantle melting and fractionation and probably reflects accumulation on the flanks of a central volcano. Extensive fractionation in shallow magma chambers led to intermediate and silicic lavas, which mark the cores of the central volcanic complexes. Dike density does not increase downward below about 1.5‐km crustal depth, and the sampled dikes appear to have been intruded laterally along rifts on the flanks of the central volcanoes. Secondary mineral assemblages are dominated by smectite and zeolites near the top of the section and by epidote and quartz near the base. In general, the temperature of alteration increases downward, but the distribution of individual minerals is irregular. Locally, earlier secondary mineral assemblages are overprinted by contact metamorphism along dike margins. The upper surface of greenschist facies metamorphism crops out at the surface in Thingmuli volcano but it lies at an estimated crustal depth of about 4 km in the Reydarfjordur borehole, located only 3 km east of the edge of Thingmuli. Thus, the metamorphic zones clearly crosscut the lava stratigraphy. Physical property profiles correlate well with variations in secondary mineralogy, and the top of layer 3 velocities corresponds closely with the projected upper boundary of amphibolite facies rocks at about 6 km crustal depth. Magnetic susceptibility decreases with depth and is extrapolated to zero at about 4.3‐km crustal depth close to the estimated upper limit of greenschist facies metabasalts. A much greater crustal thickness, leading to the emplacement of high level magma chambers within the extrusive part of the crust, is thought to account for many of the differences in crustal structure and evolution between Iceland and the rest of the Mid‐Atlantic Ridge.

A gravity survey in the Reydarfjördur area, eastern Iceland, with interpretation

A total of 63 new gravity stations was established in the Reydarfjördur area and around the Iceland Research Drilling Project (IRDP) drill site. Both previous gravity stations and the new ones have been incorporated into a new Bouguer anomaly map. The main gravity features of the area are a strong W‐E gradient in the vicinity of the IRDP drill site, an extended gravity minimum a few kilometers east of Reydarfjördur village, and a gravity maximum at the Reydarfjördur volcanic center. These features correlate with dike density distribution. Dikes, however, can explain only 10% of the observed gravity amplitudes. Relief on a zone of density gradient observed in flows from the IRDP core and extrapolated to about 1 km below the bottom of the hole has been modeled by assuming constant density sublayers. Relief amplitudes of the order of 1 km were necessary to explain the anomalies. We conclude that the main source of the gravity anomaly, at least in the vicinity of the drill site, lies above the depth of the layer 2/3 boundary or transition zone indicated for the general area by refraction seismics.

Electron microprobe studies of Iceland Research Drilling Project high‐temperature hydrothermal mineral geochemistry

The geochemistry of epidote‐ and chlorite‐dominated hydrothermal assemblages in the Iceland Research Drilling Project drill core has been studied by electron microprobe. The association of epidote, andradite, and Cu‐Fe sulphides suggests maximum basalt alteration temperatures of 350°–300°C. Near‐pure andradite implies a fluid XCO2 <0.05. The sequence of successively lower temperature minerals (epidote, prehnite, laumontite) in veins and vesicles suggests precipitation during rapid cooling from >300°C to <200°C and together with zonation in garnet and epidote implies fluid depletion in Fe3+ with time. The eradication of probable early zeolite facies assemblages and this cooling sequence suggest that temperatures rose to >300°C during burial of the lower part of the IRDP sequence before vigorous fluid circulation, producing rapid cooling and intense alteration, was established. Transition metals (Fe, Mn, Cu) were released from primary assemblages but were incorporated in epidote, chlorite, and sulphide. Epidote contains average SrO = 0.23%, but intersample variations in trace element contents of secondary minerals are complex, and there are no correlations with depth. It is concluded that the effects on whole rock chemistry (including Sr) and secondary mineral chemistry were largely controlled by kinetic factors such as (1) differing responses to alteration by primary assemblages and (2) permeability variations.

Crustal structure near the Iceland Research Drilling Project borehole from a seismic refraction survey

A seismic refraction survey was conducted at Reydarfjördur, Iceland, as part of the Iceland Research Drilling Project in July 1978. An array of 15 seismometers was used to measure apparent velocities from 25 explosions along a 23 km refraction line. The experiment yielded data used to determine the P‐ and S‐wave velocity structures beneath the drill site and to obtain bounds on the depth to seismic ‘layer 3.’ The observed apparent velocities vary continuously as a function of range with a marked increase in velocity at a range of 16 km. This corresponds to an observed cross‐over in the travel time data. The measurement of apparent velocities has yielded data of the form X(p), the distance to a measured ray parameter, and τ(p), the delay time. Joint inversion of the X(p) and τ(p) data has been performed by linear programing. Layers of constant slowness gradient are used as the basis for the inversion process. Estimates of error in the data are used to produce extremal bounds on the family of earth models that satisfy the data. When low velocity zones are assumed to be absent, the resulting envelope of P‐wave velocity models indicates a continuous variation of velocity with depth. Two regions of high velocity gradient are observed, one near the surface and the other at depths between 2.6 and 3.9 km associated with layer 3 in Iceland. These results suggest that modeling of Iceland's upper crust with a few thick homogeneous constant velocity layers is an inadequate representation of the velocity structure. Some features observed in the seismic data suggest the presence of a low velocity zone with a lid between 1.6 and 2.5 km depth. Rough synthetic modeling places this lid at the shallower end of this range.

On inclined features in the Iceland Research Drilling Project core

Precise dip and direction of dip measurements are reported for 148 dike contacts, 126 bedding features in volcaniclastics, and 30 planar, upper surfaces of secondary mineral infillings in vesicles in flows from a 1919‐m drill core from Reydarfjordur, eastern Iceland. The drill core extends from about 1.6 to 3.6 km beneath the original surface of the crustal section. Lava flow dips and directions of dips are reported for a 750‐m exposed section situated directly above and in continuity with the drill core section. While dips and within‐unit differences in dip direction appear to be accurately represented by the measurements, absolute directions are only known inaccurately. This is a consequence of the use of the paleomagnetic method of obtaining dip direction for which both high site latitude and tectonic rotation contribute to poorly constrained results. The main results of the study are the commonplace irregular form of dike contacts, such that the drill hole often entered and exited from the same side of a dike, evidence for the presence of only one population of minor intrusions and the ability to recognize where several sections of intrusives in the drill core belong to the same dike. The irregular form of the dikes prevents a reliable estimate of change of dilation with depth from being obtained. It is clear, however, that a sheeted dike complex does not occur in the upper 4 km of the crust in the area. Measurement of bedding features in the volcaniclastics shows them to be considerably dispersed, particularly in dip direction. Tectonic tilt in the crustal section increases from 11° at sea level to an extrapolated value of close to 17° at 4 km crustal depth. Corresponding lava deposition rates increase from 1620 to 2580 m/m.y., respectively. Tectonic tilting was probably complete by the time of deposition of the last components of the secondary mineralization event in the vesicles of lava flows. The optimum conditions for this type of tectonic investigation where measurements made on vertical drill core are used are high‐speed diamond drilling and original magnetization at middle to low but not equatorial latitudes.

Hydrofracturing stress measurements in the Iceland Research Drilling Project drill hole at Reydarfjordur, Iceland

Two independent suites of hydrofracturing stress measurements were conducted in the top 600 m of the Iceland Research Drilling Project deep hole at Reydarfjordur, east Iceland. As indicated by the continuously extracted drill core, the tested section consists of tertiary subaerial tholeiitic lava flows cut by many basaltic dikes. The density of the basalt was used to estimate the vertical stress σV (in megapascals) at depth D (in meters): σV = 0.027D. In the 33‐ to 270‐m depth range the horizontal principal stresses (σHmax and σHmin) as calculated from hydrofracturing results increase gradually with depth but at a lower rate than the vertical stress: σHmin = 2.1 + 0.016D; σHmax = 3.3 + 0.016D. In the 300‐ to 550‐m depth range a major perturbation occurs with both horizontal stress magnitudes rapidly rising between 300 and 400 m and then uniformly decreasing between 400 and 550 m to a level apparently predictable by the relationships obtained in the top 270 m. Conditions favoring normal faulting (σV σHmax σHmin) were established in the 540‐ to 580‐m range and are conjectured to persist at greater depths since the vertical stress gradient is higher than that of σHmax and σHmin This would be in accord with the predominant fault type in east Iceland. The relative low horizontal stress values and the small difference between them are supportive of ocean floor spreading theories and contradict near‐surface high compressive stresses previously determined by Hast (1973) in east Iceland. The large increase in horizontal stress in the range of 300‐ to 500‐m depth is not understood. A correlation appears to exist between the zone of almost unintruded thick lava flows (200–500 m) and that of high stresses. An intuitive model suggesting ‘locked in’ stresses in this zone is proposed. The hydrofracturing‐determined overall mean direction of σHmax is N50°E, while a more selective sampling of results yields N40°E. The measured stress directions appear to be reasonably compatible with the local structure such as dike direction (NNE) and lava dip (WSW). Significantly, σHmax direction is subparallel to the N20°E trend of the nearest fissure swarms in the axial rift zone. This almost coincidence of directions, like the measured stress magnitudes, is also in agreement with the model of spreading lithospheric plates being pulled apart.

The Iceland Research Drilling Project

This issue of the Journal of Geophysical Research contains the collected results of the initial studies of a 3‐km vertical section of Icelandic crust. The material described in these papers was largely presented at a meeting of the investigators working in this project held in Reykjavík, Iceland, from May 13 to May 15, 1980Iceland presents a very well exposed example of crust formed at an accretional plate margin, the crest of the Mid‐Atlantic Ridge. The desirability of a detailed study of a long vertical section of Icelandic crust by deep drilling has been expressed in proposals and recommendations of scientists and international scientific committees since the early 1960's. A formal proposal with descriptions of several alternative drill sites was presented in 1975 [Working Group on Deep Crustal Drilling in Iceland, 1975], but difficulties in financing a drill hole that would penetrate into crustal layer 3 (Vp = 6.5 km/s) delayed implementation of the recommendations.

The Iceland Research Drilling Project in relation to the geology of Iceland

The lava pile penetrated by the Iceland Research Drilling Project (IRDP) hole formed in an axial rift zone that remained active for at least the last 13 m.y. The stratigraphic sequence is formed of subaerial volcanics, and it is likely that volcanism was subaerial for several million years or even tens of millions of years prior to the formation of the IRDP sequence. The volcanics in eastern Iceland are tholeiitic, like all Tertiary sequences investigated in Iceland to date, and the lava extrusion rates in eastern Iceland are reported to be similar to those of Tertiary sequences in northern and western Iceland, respectively. The IRDP hole was sited in a dyke swarm extending from the Breiddalur central volcano in the south. The IRDP hole was sited in a regional thermal anomaly with a gradient of about 80°C/km that was found by a series of 100‐m thermal gradient wells in eastern Iceland. Aquifers of 48°C were encountered at about 600‐m depth, below which the thermal gradient lowered considerably. This suggests that the regional thermal anomaly is caused by the flow of warm water at relatively shallow depths.

Microcrack studies of basalts from the Iceland Research Drilling Project

Microcavities in basaltic core from the IRDP borehole in Reydarfjördur, Iceland, can be characterized by using direct observations with the scanning electron microscope (SEM) and inversion of velocity data. In the Iceland basalts, neither method provides an accurate, complete picture of the crack aspect ratio spectrum, but by using both SEM and inversion data a complete spectrum may be obtained. SEM observations provide information on high aspect ratio pores and cracks (>0.005) that cannot be separated by velocity inversion theory, while velocity inversion theory provides information on low aspect ratio cracks (< 0.005) that are too narrow to be seen with the SEM. Alteration rims along cracks, blunt ends, and aspect ratios which imply closure pressures greater than the vertical stress all provide evidence that cracks are open in situ. Cracks coincident with grain boundaries are abundant in the basalt flows and suggest an origin early in the rock's history, probably during cooling. Crack healing and alteration are observed to increase with depth, causing a shift in crack shape distribution toward higher aspect ratios.

Zones of polarity reversal of stable remanent magnetization within some basaltic flows of the Iceland Research Drilling Project core

Magnetic and ore microscopic investigations of flow units 326.2, 320.1, 263.2, and 298.2 reveal that extensive remagnetization of rocks has taken place in the lower portion of the drill hole leading to multiple polarity changes of NRM within single units. After optimum AF and thermal demagnetization a middle portion of stable normal polarity remanence remains with a top and bottom portion of reversed stable magnetization. The reversed direction is interpreted to be of primary origin, whereas the normal direction could arise from a viscous magnetization acquired at elevated temperatures which is stabilized by both grain subdivision during hydrothermally induced oxidation and by subsequent cooling. Microscopic study shows the presence of secondary magnetite in a few samples from flow bottoms of units 298.2, 320.1, and 326.2. For the latter two units, secondary magnetite appears to be of minor importance, whereas secondary magnetite apparently causes very high natural remanence and susceptibility values in samples from unit 298.2.

The natural magnetization of a 3‐kilometer section of Icelandic crust

The magnetization and magnetic properties of samples from a 3 km vertical section of Icelandic crust, the Iceland Research Drilling Project section, are described. The section is located at the head of Reydarfjordur, eastern Iceland at 65°01′N, 14°21′W and extends vertically from about 0.5 km to 3.5 km depth beneath the original surface. The section consists of 1 km of exposed flows with minor dikes and volcaniclastics underlain by a 2‐km drill core component consisting of 60% flows and minor volcaniclastics and 40% dikes. The section is largely normally magnetized, corresponding in age and polarity to seafloor spreading anomaly 5 (geomagnetic polarity epoch 9). Flows which accumulated during anomaly 5 time extend vertically for 2145 m in contrast with 1000‐m‐thick equivalent sections which are exposed updip. This great increase in downdip thickening is consistent with observations elsewhere in eastern Iceland and models for Icelandic crustal formation involving the combination of volcanic activity, subsidence, and spreading. A combination of paleomagnetic and structural evidence is used to provide an age calibration for the section and to predict the local presence of magnetic overprinting in the lower part of the section. This overprinting is probably in part due to the production of secondary magnetite and is developed to a varying extent in epidote bearing flows at below 2.8‐km crustal (original) depth. Where moderate secondary magnetite development has occurred, flows frequently show both normal and reverse stable polarities, with the latter being the original. Secondary magnetite development is high where dike density in the epidote bearing flows is also high. Here very strong, well defined, pervasive normal magnetization characterizes the flows, this also being the dominant polarity of the dikes. Magnetic overprinting of this type, imposed shortly after crustal formation, could occur at 800‐ to 1200‐m depth in typical ocean crust and may provide a contribution to the source for the linear magnetic anomaly patterns of the ocean basins. The initial susceptibility of the flows shows the best‐defined depth trends over the section. Susceptibility increases with depth to about 2‐km crustal depth and then decreases systematically to the bottom of the section. The reason for the initial increase is not known, but the decrease at depth can be explained by transfer of iron from primary oxides to secondary oxides and silicates. The decrease is sufficiently linear to warrant extrapolation to a zero value at 4.3‐km crustal depth, or just below the estimated level of onset of greenschist facies metamorphic conditions. In typical oceanic crust the equivalent level is estimated to be at 1.2‐km depth. The net magnetization of the section, the combined remanent and induced contributions, when expressed as average values for 500‐m depth intervals, is everywhere positive, that is, is in the sense of the present field. Variation in net magnetization is related to the proportions of the normal and reversely magnetized units in each interval.

Drilling plans

The management group of the International Crustal Research Drilling Group (ICRDG) (formally Iceland Research Drilling Project) met on December 7, 1979, at the Royal Society in London. They agreed to undertake two international major crustal drilling investigations during the first half of the decade 1980–1989. Both relate to a major interest of the group in the nature of and physical conditions of formation of oceanic crust. Important specific aspects of both projects are the thermal state of newly formed crust (and thence geothermal energy) and in the conditions of formation of base metal sulfide deposits on the ocean floor.

Geodynamical Research in Iceland by USSR

Geodynamical Research in Iceland by USSR

Regulations concerning research and travel in northern Canada, Greenland, and Iceland

Regulations concerning research and travel in northern Canada, Greenland, and Iceland

The Fecundity of Icelandic Plaice

This paper is one of a series on the fecundity of the plaice Pleuronectes platessa L. over a wide area of its geographical distribution and is concerned with the number of ripening eggs in the roes of female Icelandic plaice prior to spawning (see Bagenal, 1962, for references to other areas).Thirty-eight plaice were caught by the Icelandic research vessel ‘Maria Julia’ on 23 and 24 March 1961 in the Midnessjor area. They were measured and weighed and the otoliths were extracted. The ovaries were preserved in modified Gilson's fluid (Simpson, 1951). I am very grateful to Mr Jon Jonsson who kindly made all the arrangements and Mr Gunnar Joakimsson who prepared the material in Iceland. The fish were very ripe and a few had developed translucent eggs, but spawning had not begun. The subsampling and statistical treatment of the results were the same as before (Bagenal, 1962). The otoliths, after being kept dry, were very difficult to read.The Icelandic plaice are an isolated population, separated by deep water from the rest of Europe, and there are several spawning areas. currents move clockwise round Iceland and will carry the fertilized eggs and young larvae from Midnessjor and the S.W. coast spawning areas (Thomsen, 1948) to the good nursery grounds of Faxa Bay. Tåning (1929, 1948) has given accounts of the Faxa Bay plaice biology and suggests that ‘the eggs and tiny fry, which are found in the Bay in the spring months, seem mainly to come from the outside, for the greater part most probably from S.W. Iceland’. The eggs from the population offish considered in this paper, from Midnessjór, would drift into Faxa Bay which forms a good nursery ground.