Ilmo T. Kukkonen

Xenolith-controlled geotherm for the central Fennoscandian Shield: implications for lithosphere–asthenosphere relations

Abstract We combine petrological pressure–temperature ( P – T ) data on kimberlite-hosted mantle xenoliths with a 2-dimensional numerical thermal model of the lithosphere to provide a calibrated geotherm for the central Fennoscandian Shield in eastern Finland, where seismic estimates of lithosphere thickness range from 160 km to more than 200 km. The studied xenolith samples can be divided into garnet and garnet–spinel facies peridotites. The results suggest that the P – T data of the samples are in agreement with a conductive geotherm corresponding to a surface heat flow density (HFD) of 36 mW m −2 . Samples with either textures, or P – T results characteristic of sheared high-temperature peridotites often recovered from kimberlites in other shields, were not detected in our samples. Thus, we suggest, that petrologically the lithosphere is at least 240 km thick in the study area. Temperatures in the lower lithosphere are 1200–1400°C at 200–240 km, being close to the solidus of volatile-bearing peridotite, but partial melting textures are not present in the xenoliths. Direct estimates from the thermobarometric P – T data suggest an average temperature gradient of 3.7±0.6 K km −1 and HFD of 11±4 mW m −2 in the mantle for the kimberlite pipe area. According to the calibrated geotherm the rheological thickness of the lithosphere is estimated to be about 130–185 km depending on the applied value of creep strength of mantle rocks. The central part of the shield apparently has a thick root below the rheological lithosphere, and it is transported together with the lithospheric plate as a thermal boundary layer. This layer has remained undisturbed at least for the last 530 Ma (approximate age of the kimberlite emplacement), and possibly since the stabilization of the crust during the Proterozoic. The results are suggestive of the possible absence of or only a very weakly developed partially molten asthenospheric layer under the central parts of the Fennoscandian Shield, but do not exclude the existence of such a layer at depths exceeding 240 km.

Detecting organic chemical contaminants by spectral‐induced polarization method in glacial till environment

We present results that considerably extend the range of applications of spectral induced polarization (IP) measurements in surveying soil contaminated by organic chemicals. Soil/organic mixtures are polarizable even with very low contents of clay minerals, even though the use of spectral IP has so far been restricted to soils consisting predominantly of clay minerals. Laboratory measurements made on contaminated (toluene, heptane, ethylene glycol) and uncontaminated glacial till samples show that organic chemicals have a distinct effect on the phase spectra. Theoretical modeling further indicates that the phenomena measured in the laboratory are also relevant and detectable in field investigations.

Geochemistry and origin of saline groundwaters in the Fennoscandian Shield

Chemical and isotopic analyses of water from drill holes and mines throughout the Fennoscandian Shield show that distinct layers of groundwater are present. An upper layer of fresh groundwater is underlain by several sharply differentiated saline layers, which may differ in salinity, relative abundance of solutes, and O, H, Sr and S isotope signature. Saline groundwater can be classified into four major groups based on geochemistry and presumed origin. Brackish and saline waters from 50–200 m depth in coastal areas around the Baltic Sea exhibit distinct marine chemical and isotopic fingerprints, modified by reactions with host rocks. These waters represent relict Holocene seawater. Inland, three types of saline groundwater are observed: an uppermost layer of brackish and saline water from 300–900 m depth; saline water and brines from 1000–2000 m depth; and superdeep brines which have been observed to a depth of at least 11 km in the drill hole on the Kola Peninsula, U.S.S.R. Electrical and seismic studies in shield areas suggest that such brines are commonly present at even greater depths. The salinity of all inland groundwaters is attributed predominantly to water-rock interaction. The main solutes are Cl, Ca, Na and Mg in varying proportions, depending on the host rock lithology. The abundance of dissolved gases increases with depth but varies from site to site. The main gas components are N2, CH4 (up to 87 vol.%) and locally H2. The δ13C value for methane is highly variable (−25 to −46%), and it is suggested that hydrothermal or metamorphic gases trapped within the surrounding rocks are the most obvious source of CH4. The uppermost saline water has meteoric oxygen-hydrogen isotopic compositions, whereas values from deeper water plot above the meteoric water line, indicating considerably longer mean residence time and effective low temperature equilibration with host rocks. Geochemical and isotopic results from some localities demonstrate that the upper saline water cannot have been formed through simple mixing between fresh water and deep brines but rather is of independent origin. The source of water itself has not been satisfactorily verified although superdeep brines at least may contain a significant proportion of relict Precambrian hydrothermal or metamorphic fluids.

Characterization of bacterial diversity to a depth of 1500 m in the Outokumpu deep borehole, Fennoscandian Shield

This paper demonstrates the first microbiological sampling of the Outokumpu deep borehole (2516 m deep) aiming at characterizing the bacterial community composition and diversity of sulphate-reducing bacteria (SRB) in Finnish crystalline bedrock aquifers. Sampling was performed using a 1500-m-long pressure-tight tube that provided 15 subsamples, each corresponding to a 100-m section down the borehole. Microbial density measurements, as well as community fingerprinting with 16S rRNA gene-based denaturing gradient gel electrophoresis, demonstrated that microbial communities in the borehole water varied as a function of sampling depth. In the upper part of the borehole, bacteria affiliated to the family Comamonadaceae dominated the bacterial community. Further down the borehole, bacteria affiliated to the class Firmicutes became more prominent and, according to 16S rRNA gene clone libraries, dominated the bacterial community at 1400-1500 m. In addition, the largest number of bacterial classes was observed at 1400-1500 m. The dsrB genes detected in the upper part of the borehole were more similar to the dsrB genes of cultured SRBs, such as the genus Desulfotomaculum, whereas in the deeper parts of the borehole, the dsrB genes were more closely related to the uncultured bacteria that have been detected earlier in deep earth crust aquifers.

Taxonomically and functionally diverse microbial communities in deep crystalline rocks of the Fennoscandian shield.

Microbial life in the nutrient-limited and low-permeability continental crystalline crust is abundant but remains relatively unexplored. Using high-throughput sequencing to assess the 16S rRNA gene diversity, we found diverse bacterial and archaeal communities along a 2516-m-deep drill hole in continental crystalline crust in Outokumpu, Finland. These communities varied at different sampling depths in response to prevailing lithology and hydrogeochemistry. Further analysis by shotgun metagenomic sequencing revealed variable carbon and nutrient utilization strategies as well as specific functional and physiological adaptations uniquely associated with specific environmental conditions. Altogether, our results show that predominant geological and hydrogeochemical conditions, including the existence and connectivity of fracture systems and the low amounts of available energy, have a key role in controlling microbial ecology and evolution in the nutrient and energy-poor deep crustal biosphere.

Low geothermal heat flow of the Urals fold belt — implication of low heat production, fluid circulation or palaeoclimate?

Abstract The Urals are characterized by extremely low heat flow density (HFD). We present a discussion on the relevant factors which may contribute to the observed distribution of heat flow values. The available heat flow data in the Urals and surrounding East European and West Siberian platforms are based on borehole measurements at about 300 sites which range from the Arctic Sea coast to the Kazakshtan Plain. Along the Urals fold belt heat flow density is 30 mW m−2 or less, whereas the platform areas are characterized by 20–40 mW m−2 higher values. The low heat flow density zone is 50–100 km wide. Its extension to the north is not exactly known, but the minimum extends at least to the latitude 61°N. We present new results of heat flow and heat production measurements, Peclet number analyses on advective heat transfer by groundwater flow, as well as numerical conductive models of heat transfer in the lithosphere in the Troitsk DSS transect. The most important factor contributing to the low heat flow density in the Urals seems to be the low level of radiogenic heat production in the crust in the Tagil-Magnitogorsk Zone. The minimum is apparently enhanced by the palaeoclimatically induced vertical variation in HFD produced by the periglacial climatic conditions during the latest glaciation epoch 70,000–10,000 years B.P. The boreholes used for HFD measurements are shallower (500–1500 m) in the Tagil-Magnitogorsk Zone than in the adjoining areas in the west (1000–3000 m) or in the east (300–3000 m), and therefore the palaeoclimatic disturbance is more pronounced in these boreholes.

GEOTHERMAL MODELLING OF THE LITHOSPHERE IN THE CENTRAL BALTIC SHIELD AND ITS SOUTHERN SLOPE

Lithospheric temperature and heat flow density (HFD) were studied in the central Baltic (Fennoscandian) Shield and its subsurface continuation to the south, along a transect trending from eastern Finland to southern Estonia. The transect represents an example of a low HFD (≤ 30 mW m−2) Archaean craton on a thick (150–190 km) lithosphere surrounded by Early and Middle Proterozoic mobile belts on a thinner (110–150 km) lithosphere with slightly elevated HFD (35–55 mW m−2). Numerical 2-D conductive models were constructed in which peridotite solidus temperatures were assigned to those depths which correspond to the seismically determined lithosphere/asthenosphere boundary. This technique was found to reduce the effect of uncertainties in heat production and thermal conductivity values on the simulation results. Upper crustal heat production values for the Finnish terrain were taken from published geochemical analyses of outcropping rocks. For the Estonian terrain new heat production values were measured from core samples representing nineteen deep boreholes. Middle and lower crustal lithologies were estimated with the aid of the deep seismic VPVS data, and corresponding heat production values were adapted from global xenolith averages and from data for granulites cropping out in other Precambrian areas. The results of the modelling suggest that the lithosphere and Moho depth variations are only weakly reflected in the measured surface heat flow data, which are mainly controlled by heat sources in the upper crust. The simulated heat flow densities at 50 km depth (approximately at the Moho) are relatively low and range from 12 mW m−2 at the Archaean northeastern end to 19 mW m−2 on the Proterozoic southwestern end of the transect. Simulated temperatures at 50 km depth increase from northeast to southwest, ranging from 450–550°C in eastern Finland to about 650°C in Estonia. Sensitivity of the simulations to parameter changes was studied by varying the heat production and thermal conductivity values. The extreme values for the Moho temperature estimates thus obtained may be about 50 K lower or 100 K higher than the values above. The corresponding sensitivity of the Moho HFD is about ±6 mW m−2 and of the surface HFD ±5–20 mW m−2, respectively.

Mantle xenoliths and thick lithosphere in the Fennoscandian Shield

We applied data on kimberlite-hosted mantle xenoliths from Lahtojoki, Kaavi, in eastern Finland for thermal, rheological and seismic velocity modeling of the lithospheric mantle in the central part of the Fennoscandian Shield. We also report petrographic evidence for decrepitated fluid inclusions indicating a presence of fluids in the upper mantle. Our data and models suggest that the thermal and rheological lithosphere is very thick (230–250 km), contains small amounts of fluids, and follows wet (but not fluid-saturated) olivine rheology. The lithospheric part of the mantle where heat transfer is mainly conductive and which does not participate in convection, changes into a mechanical asthenosphere in a solid state. Mantle viscosity shows a weak minimum at the mechanical lithosphere–asthenosphere boundary. The lithosphere–asthenosphere transition does not require partial melting, and very probably there is no partial melt-bearing asthenosphere beneath the lithosphere at all. This also precludes an electrical asthenosphere in the sense of high conductivity due to partial melting. Seismic velocity models calculated using data on mineral composition of the xenoliths and mineral elastic parameters indicate no low-velocity layer at the lithosphere–asthenosphere boundary, instead only a downward increase in the vertical velocity gradients. The velocity model indicates velocities higher than iasp91 (up to 2%) at depths above 250 km, and it is in good agreement with seismic body-wave tomography results.

Composition of the Uralide crust from seismic velocity (Vp, Vs), heat flow, gravity, and magnetic data

Abstract P-wave velocity (Vp), S-wave velocity (Vs), Poisson’s ratio (σ), heat flow, potential field, and surface geological data are integrated to constrain a model for the composition of the Uralide crust along the URSEIS transect. The model is constructed using published laboratory measurements of Vp, Vs, σ and density for a variety of crustal rock types. These laboratory data have been corrected for depth (pressure) and the Uralides temperature–depth function. The model shows clear differences between the composition of the old continental crustal nucleus of the East European Craton and the newly added crust of the accreted arc terranes to the east. The crust of the East European Craton is more felsic than that of the Magnitogorsk and East Uralian zones, and the latter two have a lowermost crust whose characteristics indicate a high garnet content (mafic garnet granulite) and/or the presence of hornblendite. The overall composition of the arc terranes is basaltic. Physical properties suggest that eclogite is not present in the arc terranes, or if present it exists in small amounts that are below the resolution of the data set. The lack of eclogite in the lower crust favours an intracrustal differentiation model for the evolution of the bulk composition of the continental crust. Nevertheless, the absence of surface uplift, the lack of metamorphism and late orogenic mantle melts, and the current crustal thickness indicate that crustal thinning did not affect the bulk composition of the Uralide crust.

Anomalously low heat flow density in eastern Karelia, Baltic Shield: a possible palaeoclimatic signature

Abstract We report new heat flow density (HFD) values in seven drill holes in the Kamennye Lakes area in eastern Karelia, Russia, approximately at latitude 63°15′N, longitude 36°10′E. The investigated holes are 250–750 m deep and they intersect Archaean ultrabasic serpentinites and talc-carbonate rocks. Measured gradients range from 0.8 to 3.7 mK m−1 and the apparent HFD values from 2.4 to 11.6 mW m−2. The holes are not technically disturbed by fluid flow or any drilling effects. Average heat production of the rocks as analysed in the core samples of the deepest measured hole is 0.25 μ W m−3, but the low heat production is not a critical factor in producing the low HFD values. This is due to refraction of heat as shown with 2-D conductive simulations of heat transfer in a low heat-production formation surrounded by higher heat production. Hydrogeological disturbances can be ruled out by the presence of saline groundwater in the sections deeper than 150–400 m, and low topographic variation in the area, as well as Peclet number estimates, which suggest negligible convective heat transfer in the bedrock. All the temperature profiles are curved indicating recent palaeoclimatic disturbances. Inversion studies with singular value decomposition techniques yielded a climatic warming of about 1.0–1.5 K which started 150–200 years ago and was preceded by a cool period which lasted about 100 years. Nevertheless, recent climatic changes cannot explain the very low apparent HFD values, but long-period effects of the Weichselian glaciation are sufficient to decrease the HFD values to the levels measured. These effects were investigated with forward simulations and suggest that present temperature gradients in the range of 1–4 mK m−1 in the uppermost 1 km can be created by a very cold ground temperature (−10 to −15°C) during the glaciation time (60-11 ka ago).