Sven Dahlgren - Academia.edu (original) (raw)

Papers by Sven Dahlgren

Precambrian Research, Dec 1, 1993

The age of the granulite facies metamorphism in the Bamble Belt has long been a matter of debate.... more The age of the granulite facies metamorphism in the Bamble Belt has long been a matter of debate. Samples from four localities in the coastal section of the belt have, therefore, been used to obtain Sm-Nd mineral ages. All gave dates within the range 1070-1100 Ma. Earlier Rb-Sr data of Field et al. had placed the maximum metamorphism of the region at 1540 Ma. These authors deduced that any subsequent metamorphism in the Sveconorwegian (~ 1100 Ma) was low grade since the Rb-Sr system had not been completely reset at this time. New U-Pb data, published elsewhere, indicate two periods of deformation and metamorphism; a pre-Sveconorwegian metamorphism at 1437 + 8 Ma and a Sveconorwegian period at 1100 Ma. Our Sm-Nd data indicate that the Sveconorwegian period lasted for at least 50 Ma, and included granulite facies metamorphism ending at 1095 Ma. We further deduce that Rb-Sr whole-rock systems were not necessarily totally reset during the granulite facies metamorphic event. Our conclusions represent a return to older interpretations concerning the significance of the Sveconorwegian event.

Mineralogy and Petrology, 2021

Carbonatites in the Fen intrusive complex (southern Norway) contain abundant burbankite (confirme... more Carbonatites in the Fen intrusive complex (southern Norway) contain abundant burbankite (confirmed by Raman microspectroscopy) as inclusions in calcite, dolomite and, less commonly, fluorapatite and pyrochlore. Typically the inclusions occur in the core of calcite or dolomite grains relatively unaffected by subsolidus processes, and are associated with Fe-poor dolomite or Sr-rich calcite, respectively. Burbankite does not exceed 30 × 50 μm in size and is characteristically absent from the peripheral areas of carbonate grains affected by recrystallization or interaction with fluids. Compositionally, the mineral falls within the following range: (Na1.51–2.16Ca0.58–1.21)(Sr1.50–2.42Ca0.28–0.57LREE0.05–0.64Ba0.06–0.41)(CO3)5 and contains low Th, but no detectable Mg, Fe or F (LREE = light rare-earth elements: Ce > La > Nd > Pr > Sm). Burbankite inclusions at Fen are interpreted as primary and indicative of Na enrichment in their parental carbonatitic magma. Dissociation of burbankite during subsolidus re-equilibration of its host phases with fluids undoubtedly served as one of the sources of LREE for the development of late-stage mineralization in the Fen complex.

Ore Geology Reviews, 2018

Carbonatites and alkaline-silicate rocks are the most important sources of rare earth elements (R... more Carbonatites and alkaline-silicate rocks are the most important sources of rare earth elements (REE) and niobium (Nb), both of which are metals imperative to technological advancement and associated with high risks of supply interruption. Cooling and crystallizing carbonatitic and alkaline melts expel multiple pulses of alkali-rich aqueous fluids which metasomatize the surrounding country rocks, forming fenites during a process called fenitization. These alkalis and volatiles are original constituents of the magma that are not recorded in the carbonatite rock, and therefore fenites should not be dismissed during the description of a carbonatite system. This paper reviews the existing literature, focusing on 17 worldwide carbonatite complexes whose attributes are used to discuss the main features and processes of fenitization.

Introduction Banalsite was first described from the Benallt manganese mine, L1eyn Peninsula, Wale... more Introduction Banalsite was first described from the Benallt manganese mine, L1eyn Peninsula, Wales by Smith et al. (1944), while stronalsite was described as an alteration mineral in veins cutting mafic metatuff xenoliths at Rendai, Kochi, Japan (Hori et al. 1987). Their general chemical formula is ANa2A14Si40,6, where A is Ba and Sr, respectively. The study by Liferovich et al. (2006b) has confirmed that there is a complete solid-solution between banalsite and stronalsite. A third tectosilicate closely related to the above mentioned species, lisetite, CaNa2A14Si40'6' has been described from the Liset eclogite pod, Selje, Norway (Smith et al. 1986). Liferovich et al. (2006b), however, has shown that lisetite has limited miscibility with banalsite-stronalsite. The crystal structures of stronalsite and banalsite have been determined by Liferovich et al. (2006a).

Extremely high thorium and considerable uranium concentrations are observed in carbonatite rocks ... more Extremely high thorium and considerable uranium concentrations are observed in carbonatite rocks of the Fen Complex—an alkaline intrusive complex in southern Norway. Since uraniumbearing bedrock and its weathering products are responsible for increased radon-222 concentrations in nearby dwellings, knowledge about the uranium concentrations of the individual rock types is important for evaluating the associated health risk. Earlier core-sample and ground-based scintillator measurements were limited in relating geological setting to indoor concentrations of radon-222 in such a region with very small-scale geological variations. We have performed airborne radiometric measurements over the entire Fen Complex and the nearby town of Ulefoss. The processed airborne data show that regions dominated by different carbonatite types vary significantly in mean thorium concentrations, but have similar uranium concentrations. Despite the complexity of the region, the obtained thorium/uranium ratio...

Ore Geology Reviews

Abstract We present detailed textural and chemical analyses of the hydrothermal sulfide- and REE-... more Abstract We present detailed textural and chemical analyses of the hydrothermal sulfide- and REE-Th-Nb-mineralization observed in the Fen complex (southern Norway), which is the biggest carbonatite-related REE and Th deposit in Europe. The alkaline silicate rocks and carbonatites of the Fen complex underwent two hydrothermal alteration events that caused (i) the formation of sulfides and (ii) (re)-mobilization of REE, Fe and Al. This renders the Fen complex an ideal locality to study the genesis of hydrothermal sulfide and REE mineralization in carbonatites. Our observations record a hydrothermal alteration history of the Fen carbonatite complex, which is relevant to many carbonatite complexes worldwide. The first alteration event (pyrite-stage) caused the crystallization of sulfides (mostly pyrite) in all lithologies and was induced by a sulfide-rich fluid 1 that was probably derived from adjacent mafic alkaline silicate rocks. Veins formed during this hydrothermal event show a typical succession from magnetite via pyrite I + hematite + magnetite to pyrite II. A subsequent alteration event is characterized by the interaction of two evolving fluids, namely (1) a REE-rich fluid 2 that was probably derived from the carbonatites (autometasomatic fluid) and (2) an oxidizing meteoric fluid 3 that mainly introduced Si and was in equilibrium with the basement host rocks. The interaction of these two fluids with the carbonatitic rocks resulted in various characteristic types of calcite-hematite rocks locally called rodberg. During their late-stage silicification by fluid 3, P was mobilized from magmatic apatite and reprecipitated in close-by rodberg veins as small fluorapatite-monazite veins. The interaction of the carbonatite-derived fluid 2 and meteoric fluid 3 also resulted in the formation of distinct zones of LREE-phases, notably REE-F-carbonates (bastnasite, parisite, synchysite), monazite and allanite, and HREE-phases (including samarskite, aeschynite and an unnamed Nb-Fe-REE-Th-oxide. The most pronounced HREE-enrichment occurs in domains of strongest hydrothermal alteration, which was probably caused by preferential precipitation of HREE-rich complexes coupled with later partial leaching of LREE by F-rich fluids during silicification. Thorium-rich minerals were precipitated in close association with HREE-rich minerals implying similar hydrothermal behaviour of Th and HREE.

Journal of Environmental Radioactivity, 2016

Radon ((222)Rn), thoron ((220)Rn) and their decay products may reach high levels in areas of high... more Radon ((222)Rn), thoron ((220)Rn) and their decay products may reach high levels in areas of high natural background radiation, with increased risk associated with mining areas. Historically, the focus has mostly been placed upon radon and progeny (RnP), but recently there have been reports of significant contributions to dose from thoron progeny (TnP). However, few direct measurements of TnP exist under outdoor conditions. Therefore, we assessed the outdoor activity concentrations of radon, thoron and TnP in an area of igneous bedrock with extreme levels of radionuclides in the thorium decay series. The area is characterized by decommissioned mines and waste rock deposits, which provide a large surface area for radon and thoron emanation and high porosity enhancing exhalation. Extreme levels of thorium and thoron have previously been reported from this area and to improve dose rate estimates we also measured TnP using filter sampling and time-integrating alpha track detectors. We found high to extreme levels of thoron and TnP and the associated dose rates relevant for inhalation were up to 8 μSvh(-1) at 100 cm height. Taking gamma irradiation and RnP into account, significant combined doses may result from occupancies in this area. This applies to recreational use of the area and especially previous and planned road-works, which in the worst case could involve doses as large as 23.4 mSv y(-1). However, radon and thoron levels were much more intense on a hot September day than during time-integrated measurements made the subsequent colder and wetter month, especially along the ground. This may be explained by cold air observed flowing out from inside the mines through a drainage pipe adjacent to the measurement stations. During warm periods, activity concentrations may therefore be due to both local exhalation from the ground and air ventilating from the mines. However, a substantially lower level of TnP was measured on the September day using filter sampling, as compared to what was measured with time-integrative alpha track detectors. A possible explanation could be reduced filter efficiency related to the attached progeny of some aerosol sizes, but a more likely cause is an upwards bias on TnP detectors associated with assumed deposition velocity, which may be different in outdoor conditions with wind or a larger fraction of unattached progeny. There is thus a need for better instrumentation when dealing with outdoor TnP.

American Mineralogist, 1997

Shallow-level plutons caused extensive contact metamorphism of Lower Paleozoic shale and carbonat... more Shallow-level plutons caused extensive contact metamorphism of Lower Paleozoic shale and carbonate sequences in the Permian Oslo Rift. A Ͼ500 m long and 100 m wide shalelimestone xenolith embedded within monzonites belonging to the Skrim plutonic complex experienced high-grade contact metamorphism and generation of minerals and mineral assemblages rarely reported from metamorphic rocks. The peak metamorphic (Stage I) assemblages in calcite-saturated rocks include wollastonite, melilites, fassaitic pyroxenes, phlogopite, titanian grossular, kalsilite, nepheline, perovskite, cuspidine, baghdadite, pyrrhotite, and occasional graphite. Mineral reactions involving detrital apatite produced a series of silicate apatites, including the new mineral species Ca 3.5 (Th,U) 1.5 Si 3 O 12 (OH). This assemblage equilibrated at T ϭ 820-870 ЊC with a C-rich, internally buffered pore-fluid (20-40 mol% CO 2 ϩ CH 4). During cooling the shale-limestone xenolith experienced infiltration of C-poor (Ͻ 0.1 mol% CO 2) fluids, triggering the formation of retrograde (Stage II) mineral assemblages comprising monticellite, tilleyite, vesuvianite, grandite garnets, diopside, and occasional hillebrandite. Rare potassium iron sulfides (rasvumite and djerfisherite) formed at the expense of primary pyrrhotite. These assemblages probably formed near 700 ЊC. Formation of diffuse sodalite-bearing veinlets was associated with breakdown of nepheline and the replacement of kalsilite and wollastonite by potassium feldspar. The sodalite-bearing Stage III assemblage formed by the infiltration of saline brines at a maximum temperature of 550 ЊC. Low-temperature (Stage IV) retrogression of the Stage I-III assemblage produced scawtite, giuseppettite, hydrogrossulars, phillipsite, thomsonite, and three hitherto undescribed mineral species.

Geochemistry International, 1994

ABSTRACT Studies have been made of the barophile mineral phases making up mantle xenoliths and re... more ABSTRACT Studies have been made of the barophile mineral phases making up mantle xenoliths and represented by xenocrysts in alkali picrites, darnkjernites, melilitites, nephelinites, and kimberlites in explosion pipes and dykes from the surroundings of the Fen complex (Norway) and in the Kola peninsula. Based on the composition of the minerals and P-T estimates, conclusions are drawn on the heterogeneity of the mantle sources of dyke magmatism during the epoch of Paleozoic tectonic-magmatic reactivation. The alkali picrites and foidites from the regions of the alkali intrusions represent rocks from the shallowest levels of the spinel lherzolite facies; to the second level, corresponding to the garnet lherzolite facies, is related to the formation ofolivine melilitites; and the deepest kimberlite magmas were generated from the level of the diamond-pyrope facies. Arzamastsev A.A., Dahlgren S. 1994. Plutonic mineral assemblages in Paleozoic dikes and explosion pipes of the Alkaline Province of the Baltic Shield. Geochemistry International, vol.31, no.3, p.57-68. (in English).

Larvikite has for more than a hundred years been appreciated as one of the world's most attractiv... more Larvikite has for more than a hundred years been appreciated as one of the world's most attractive dimension stones, and at present, its production and use is more extensive than ever. The main reason for the continuous success of larvikite on the world market is the blue iridescence displayed on polished surfaces, which is caused by optical interference in microscopic lamellae within the ternary feldspars. The larvikite complex consists of different intrusions, defining several ringshaped structures, emplaced during a period of approximately five million years. Following this pattern, several commercial subtypes of larvikite, characterised by their colour and iridescence, have been mapped. Four of these subtypes are being exploited at the present time and define the most important reserves in the short run. Some other subtypes are less attractive in the present market situation, but may provide an interesting potential for the future. However, the industrial value of the larvikite also depends on other geological features, such as various types of dykes, faults and fractures, ductile deformation zones, late-stage magmatic and hydrothermal alteration and deep weathering. When combining the distribution pattern of such features with the map of the larvikite subtypes, it is possible to delineate various types of larvikite deposit that are considered to have commercial value in the short or long term. Finally, reserve estimates for the different types have been made, showing that some of the most attractive types have rather limited known reserves if the present level of production is maintained or increased.

Geochemistry International

Paper in Russian Chemical composition of xenocrysts and minerals in mantle nodula found in dikes ... more Paper in Russian Chemical composition of xenocrysts and minerals in mantle nodula found in dikes and pipes of the Kola province and Fen area (SE Norway) was investigated. Estimation of PT conditions have proved the heterogeneity of the mantle sources of dike magmatism, during Paleozoic tectonomagmatic activity of the Baltic Shield. It is shown that the initial picritic melts were generated at the depth of spinel lherzolite facies (T = = 1000°C, P = 15 ± 1 kbar), olivine melilitites are connected with magmatic process at the level of garnet - lherzolite facies, and the deepest kimberlite melts were generated from the depth of diamond - pyrope mantle facies (T = 1100°C, P > 40 kbar).

The Fen Central Complex (FCC) and associated satellite dikes of the Fen Province in southern Norw... more The Fen Central Complex (FCC) and associated satellite dikes of the Fen Province in southern Norway record a magnetization dating to 583 Ma. The paleomagnetic pole calculated from these rocks falls at 56°N, 150°E (dp 5 7°, dm 5 10°) and compares favorably with two previous investigations. The mean inclination in our study is slightly steeper than that of one of the earlier studies and we attribute this to the fact that previous investigators inadvertently sampled younger (late Paleozoic/early Mesozoic) dikes in the area. The age of the Fen paleomagnetic pole is constrained by two consistent 40 Ar/ 39 Ar ages from these rocks, which average 583 6 15 Ma, along with previously published ages ranging from 523-601 Ma. The Fen Central Complex, along with associated intrusions, were emplaced during minor extensional activity that occurred after continental separation between Baltica and Laurentia. This interpretation is consistent with our 580 Ma paleoreconstruction, which shows that Baltica had rifted and rotated from its 615 Ma position adjacent to Greenland. We interpret the ages and the tectonics to reflect opening of an ocean basin from north to south during the latest Neoproterozoic to earliest Paleozoic time (615-540 Ma). The polarity option for the Fen pole remains an important and open question due to recent suggestions of a true polar wander event in the mid-Cambrian; however, we consider that the south polarity option offers the simplest tectonic model for the rifting of Rodinia and opening of the Iapetus Ocean between Baltica and Laurentia. Finally, we note the similarity of the Fen paleomagnetic pole to paleomagnetic poles of Permo-Triassic age in Baltica and urge caution in the unequivocal use of the Fen pole as primary until further substantiated by coeval poles from Baltica.

Precambrian Research, 1993

The age of the granulite facies metamorphism in the Bamble Belt has long been a matter of debate.... more The age of the granulite facies metamorphism in the Bamble Belt has long been a matter of debate. Samples from four localities in the coastal section of the belt have, therefore, been used to obtain Sm-Nd mineral ages. All gave dates within the range 1070-1100 Ma. Earlier Rb-Sr data of Field et al. had placed the maximum metamorphism of the region at 1540 Ma. These authors deduced that any subsequent metamorphism in the Sveconorwegian (~ 1100 Ma) was low grade since the Rb-Sr system had not been completely reset at this time. New U-Pb data, published elsewhere, indicate two periods of deformation and metamorphism; a pre-Sveconorwegian metamorphism at 1437 + 8 Ma and a Sveconorwegian period at 1100 Ma. Our Sm-Nd data indicate that the Sveconorwegian period lasted for at least 50 Ma, and included granulite facies metamorphism ending at 1095 Ma. We further deduce that Rb-Sr whole-rock systems were not necessarily totally reset during the granulite facies metamorphic event. Our conclusions represent a return to older interpretations concerning the significance of the Sveconorwegian event.

Lithos, 1994

Ultramafic dikes with carbonatitic affinities ("damtjernites") in southern Norway were generated ... more Ultramafic dikes with carbonatitic affinities ("damtjernites") in southern Norway were generated during two magmatic events separated by about 275 Ma. The older event is late Proterozoic and the younger is mid Carboniferous. More than 50 satellitic damtjernite intrusions occur within a 1500 km 2 large region surrounding the Fen Central Complex. Phlogopite macrocrysts from 10 of these satellites yield a Rb-Sr isochron age of 578 + 24 Ma (2a expanded errors). This demonstrates that the late Proterozoic carbonatitic magmatism centered at the Fen Central Complex occurred on a regional scale. This region is termed "the Fen Province". The emplacement of the magmas in the Fen Province most likely occurred in connection with minor extensional tectonic activity on the Baltic platform during the drift-phase after the Proto-Atlantic opening. Sr-isotopic data also show that a dike mineralogically and chemically similar to the Fen damtjernites was emplaced at 324 + 4 Ma (mid Carboniferous). This dike very likely dates the initiation of magmatism in the Oslo rift. Consequently very similar carbonate-bearing ultramafic magmas were generated within the south Norwegian mantle during the relatively minor Fen event and in the initial extensional period when the magma production in the Oslo rift was still low.

Earth and Planetary Science Letters, 2008

The Permo-Carboniferous Oslo Rift developed in the foreland of the Variscan orogen over a period ... more The Permo-Carboniferous Oslo Rift developed in the foreland of the Variscan orogen over a period of some 50 million years through a process characterized by moderate extension and widespread magmatism. The overall tectonic situation places the Oslo Rift in a postcollisional, dextral transtensional setting related to the convergence between Baltica, Laurentia, Gondwana and Siberia during assembly of Pangea, the location probably reflecting the control by pre-existing lithospheric structures. Although a detailed understanding of these factors and processes relies strongly on having a good age control, the dating of mafic to ultramafic alkalic volcanic units formed during initial rifting has been a very challenging task. In this study we have successfully employed perovskite from melilitic and nephelinitic volcanic rocks, together with magmatic titanite in a more evolved ignimbrite, to obtain ID-TIMS high-precision U-Pb ages. Three samples from various levels of the Brunlanes succession, in the southernmost exposures of the Oslo Graben, yield ages of 300.2 ± 0.9, 300.4 ± 0.7 and 299.9 ± 0.9 Ma. A melililitic tuff at the base of the Skien succession further to the northwest yields a slightly younger age of 298.9 ± 0.7 Ma. The initial Pb compositions derived mainly from coexisting pyroxene, apatite and hornblende are characterized by extremely radiogenic initial 206 Pb/ 204 Pb ratios (up to 21.3) that confirm a provenance of these early alkaline basalts from HIMU-type sources. The U-Pb ages coincide with the Gzhelian age inferred from fossils in the upper part of the basal rift sedimentary fill of the Asker Group, and postdate the underlying basal sedimentary sequences by some 10 million years, pointing to a relatively rapid initiation of the rifting process.

Contributions to Mineralogy and Petrology, 1993

Deformed and metamorphosed dolomite marbles in the Kragero area of the Bamble Shear Belt occur as... more Deformed and metamorphosed dolomite marbles in the Kragero area of the Bamble Shear Belt occur as lenses within metasupracrustal sequences, as matrix in bodies resembling magmatic breccias, and as veins/ dykes cutting amphibolites and metagabbros. A common origin is not evident from the field relationships, but is nevertheless probable due to great geochemical similarities between dolomite from the different occurrences. They are characterized by higher REE, Ni, Co, Cr and Sc, and lower Ba and Sr contents relative to metasedimentary marbles occurring nearby. Sm-Nd isotope data shows that the dolomites are of Sveconorwegian age (1175_+37 Ma). The dolomite marbles are very weakly LREE-enriched and display in most cases positive Eu anomalies. Their stable isotope compositions are uniform (61so= +9.6 to +10.7%o; c~13C=-8.5 to-6.2%o), their initial S7Sr/S6Sr ratios are high (0.706-0.709), whereas their age corrected end varies from + 0.7 to-1.5. Geochemically the dolomitic marbles differ considerably from sedimentary or metasedimentary marbles as well as from carbonatites. The Kragero dolomite marbles represent deformed and metamorphosed hydrothermal veins or vein-complexes deposited in tensional fractures in the deep crust. Although the dolomitic marbles regionally are of minor volume, the dolomite deposition represents a specific and important event in the geological evolution of the Bamble Shear Belt. Their geochemical and isotopic homogeneity on a regional scale suggest that the hydrothermal solutions were supplied from a very large, homogeneous reservoir. Trace elements, stable and radiogenic isotopes, and field and isotopic age relationships are consistent with a deposition from hydrothermal solutions which were exsolved from crystallizing charnockitic intrusions and subse

Chemical Geology, 1995

Classical ion-exchange chromatography combined with high-performance ion chromatography has been ... more Classical ion-exchange chromatography combined with high-performance ion chromatography has been used for the separation of Nd and Sm for isotopic analysis of geological samples by mass spectrometry. The procedure has successfully been applied to samples with Nd concentrations as low as 0.2 μg g−1 and high-quality elemental separation was routinely achieved even for “difficult” samples such as low-Nd ultramafic upper-mantle xenoliths and extremely Ba-rich lamproites. The total blank value was <60 pg Nd. The method has several distinct advantages: (1) it is simpler than some of the other techniques available; (2) it can be used for samples with low Nd and Sm concentrations; and (3) Nd and Sm fractions virtually free of interferences from other elements were obtained for a wide range of samples investigated.

Precambrian research, 1993

The age of the granulite facies metamorphism in the Bamble Belt has long been a matter of debate.... more The age of the granulite facies metamorphism in the Bamble Belt has long been a matter of debate. Samples from four localities in the coastal section of the belt have, therefore, been used to obtain Sm-Nd mineral ages. All gave dates within the range 1070-1100 Ma. Earlier Rb-Sr data of Field et al. had placed the maximum metamorphism of the region at 1540 Ma. These authors deduced that any subsequent metamorphism in the Sveconorwegian (~ 1100 Ma) was low grade since the Rb-Sr system had not been completely reset at this time. New U-Pb data, published elsewhere, indicate two periods of deformation and metamorphism; a pre-Sveconorwegian metamorphism at 1437 + 8 Ma and a Sveconorwegian period at 1100 Ma. Our Sm-Nd data indicate that the Sveconorwegian period lasted for at least 50 Ma, and included granulite facies metamorphism ending at 1095 Ma. We further deduce that Rb-Sr whole-rock systems were not necessarily totally reset during the granulite facies metamorphic event. Our conclusions represent a return to older interpretations concerning the significance of the Sveconorwegian event.

American Mineralogist, 1997

Shallow-level plutons caused extensive contact metamorphism of Lower Paleozoic shale and carbonat... more Shallow-level plutons caused extensive contact metamorphism of Lower Paleozoic shale and carbonate sequences in the Permian Oslo Rift. A Ͼ500 m long and 100 m wide shalelimestone xenolith embedded within monzonites belonging to the Skrim plutonic complex experienced high-grade contact metamorphism and generation of minerals and mineral assemblages rarely reported from metamorphic rocks. The peak metamorphic (Stage I) assemblages in calcite-saturated rocks include wollastonite, melilites, fassaitic pyroxenes, phlogopite, titanian grossular, kalsilite, nepheline, perovskite, cuspidine, baghdadite, pyrrhotite, and occasional graphite. Mineral reactions involving detrital apatite produced a series of silicate apatites, including the new mineral species Ca 3.5 (Th,U) 1.5 Si 3 O 12 (OH). This assemblage equilibrated at T ϭ 820-870 ЊC with a C-rich, internally buffered pore-fluid (20-40 mol% CO 2 ϩ CH 4). During cooling the shale-limestone xenolith experienced infiltration of C-poor (Ͻ 0.1 mol% CO 2) fluids, triggering the formation of retrograde (Stage II) mineral assemblages comprising monticellite, tilleyite, vesuvianite, grandite garnets, diopside, and occasional hillebrandite. Rare potassium iron sulfides (rasvumite and djerfisherite) formed at the expense of primary pyrrhotite. These assemblages probably formed near 700 ЊC. Formation of diffuse sodalite-bearing veinlets was associated with breakdown of nepheline and the replacement of kalsilite and wollastonite by potassium feldspar. The sodalite-bearing Stage III assemblage formed by the infiltration of saline brines at a maximum temperature of 550 ЊC. Low-temperature (Stage IV) retrogression of the Stage I-III assemblage produced scawtite, giuseppettite, hydrogrossulars, phillipsite, thomsonite, and three hitherto undescribed mineral species.

Earth and Planetary Science Letters, 2008

The Permo-Carboniferous Oslo Rift developed in the foreland of the Variscan orogen over a period ... more The Permo-Carboniferous Oslo Rift developed in the foreland of the Variscan orogen over a period of some 50 million years through a process characterized by moderate extension and widespread magmatism. The overall tectonic situation places the Oslo Rift in a postcollisional, dextral transtensional setting related to the convergence between Baltica, Laurentia, Gondwana and Siberia during assembly of Pangea, the location probably reflecting the control by pre-existing lithospheric structures. Although a detailed understanding of these factors and processes relies strongly on having a good age control, the dating of mafic to ultramafic alkalic volcanic units formed during initial rifting has been a very challenging task. In this study we have successfully employed perovskite from melilitic and nephelinitic volcanic rocks, together with magmatic titanite in a more evolved ignimbrite, to obtain ID-TIMS high-precision U-Pb ages. Three samples from various levels of the Brunlanes succession, in the southernmost exposures of the Oslo Graben, yield ages of 300.2 ± 0.9, 300.4 ± 0.7 and 299.9 ± 0.9 Ma. A melililitic tuff at the base of the Skien succession further to the northwest yields a slightly younger age of 298.9 ± 0.7 Ma. The initial Pb compositions derived mainly from coexisting pyroxene, apatite and hornblende are characterized by extremely radiogenic initial 206 Pb/ 204 Pb ratios (up to 21.3) that confirm a provenance of these early alkaline basalts from HIMU-type sources. The U-Pb ages coincide with the Gzhelian age inferred from fossils in the upper part of the basal rift sedimentary fill of the Asker Group, and postdate the underlying basal sedimentary sequences by some 10 million years, pointing to a relatively rapid initiation of the rifting process.

Precambrian Research, Dec 1, 1993

The age of the granulite facies metamorphism in the Bamble Belt has long been a matter of debate.... more The age of the granulite facies metamorphism in the Bamble Belt has long been a matter of debate. Samples from four localities in the coastal section of the belt have, therefore, been used to obtain Sm-Nd mineral ages. All gave dates within the range 1070-1100 Ma. Earlier Rb-Sr data of Field et al. had placed the maximum metamorphism of the region at 1540 Ma. These authors deduced that any subsequent metamorphism in the Sveconorwegian (~ 1100 Ma) was low grade since the Rb-Sr system had not been completely reset at this time. New U-Pb data, published elsewhere, indicate two periods of deformation and metamorphism; a pre-Sveconorwegian metamorphism at 1437 + 8 Ma and a Sveconorwegian period at 1100 Ma. Our Sm-Nd data indicate that the Sveconorwegian period lasted for at least 50 Ma, and included granulite facies metamorphism ending at 1095 Ma. We further deduce that Rb-Sr whole-rock systems were not necessarily totally reset during the granulite facies metamorphic event. Our conclusions represent a return to older interpretations concerning the significance of the Sveconorwegian event.

Mineralogy and Petrology, 2021

Carbonatites in the Fen intrusive complex (southern Norway) contain abundant burbankite (confirme... more Carbonatites in the Fen intrusive complex (southern Norway) contain abundant burbankite (confirmed by Raman microspectroscopy) as inclusions in calcite, dolomite and, less commonly, fluorapatite and pyrochlore. Typically the inclusions occur in the core of calcite or dolomite grains relatively unaffected by subsolidus processes, and are associated with Fe-poor dolomite or Sr-rich calcite, respectively. Burbankite does not exceed 30 × 50 μm in size and is characteristically absent from the peripheral areas of carbonate grains affected by recrystallization or interaction with fluids. Compositionally, the mineral falls within the following range: (Na1.51–2.16Ca0.58–1.21)(Sr1.50–2.42Ca0.28–0.57LREE0.05–0.64Ba0.06–0.41)(CO3)5 and contains low Th, but no detectable Mg, Fe or F (LREE = light rare-earth elements: Ce > La > Nd > Pr > Sm). Burbankite inclusions at Fen are interpreted as primary and indicative of Na enrichment in their parental carbonatitic magma. Dissociation of burbankite during subsolidus re-equilibration of its host phases with fluids undoubtedly served as one of the sources of LREE for the development of late-stage mineralization in the Fen complex.

Ore Geology Reviews, 2018

Carbonatites and alkaline-silicate rocks are the most important sources of rare earth elements (R... more Carbonatites and alkaline-silicate rocks are the most important sources of rare earth elements (REE) and niobium (Nb), both of which are metals imperative to technological advancement and associated with high risks of supply interruption. Cooling and crystallizing carbonatitic and alkaline melts expel multiple pulses of alkali-rich aqueous fluids which metasomatize the surrounding country rocks, forming fenites during a process called fenitization. These alkalis and volatiles are original constituents of the magma that are not recorded in the carbonatite rock, and therefore fenites should not be dismissed during the description of a carbonatite system. This paper reviews the existing literature, focusing on 17 worldwide carbonatite complexes whose attributes are used to discuss the main features and processes of fenitization.

Introduction Banalsite was first described from the Benallt manganese mine, L1eyn Peninsula, Wale... more Introduction Banalsite was first described from the Benallt manganese mine, L1eyn Peninsula, Wales by Smith et al. (1944), while stronalsite was described as an alteration mineral in veins cutting mafic metatuff xenoliths at Rendai, Kochi, Japan (Hori et al. 1987). Their general chemical formula is ANa2A14Si40,6, where A is Ba and Sr, respectively. The study by Liferovich et al. (2006b) has confirmed that there is a complete solid-solution between banalsite and stronalsite. A third tectosilicate closely related to the above mentioned species, lisetite, CaNa2A14Si40'6' has been described from the Liset eclogite pod, Selje, Norway (Smith et al. 1986). Liferovich et al. (2006b), however, has shown that lisetite has limited miscibility with banalsite-stronalsite. The crystal structures of stronalsite and banalsite have been determined by Liferovich et al. (2006a).

Extremely high thorium and considerable uranium concentrations are observed in carbonatite rocks ... more Extremely high thorium and considerable uranium concentrations are observed in carbonatite rocks of the Fen Complex—an alkaline intrusive complex in southern Norway. Since uraniumbearing bedrock and its weathering products are responsible for increased radon-222 concentrations in nearby dwellings, knowledge about the uranium concentrations of the individual rock types is important for evaluating the associated health risk. Earlier core-sample and ground-based scintillator measurements were limited in relating geological setting to indoor concentrations of radon-222 in such a region with very small-scale geological variations. We have performed airborne radiometric measurements over the entire Fen Complex and the nearby town of Ulefoss. The processed airborne data show that regions dominated by different carbonatite types vary significantly in mean thorium concentrations, but have similar uranium concentrations. Despite the complexity of the region, the obtained thorium/uranium ratio...

Ore Geology Reviews

Abstract We present detailed textural and chemical analyses of the hydrothermal sulfide- and REE-... more Abstract We present detailed textural and chemical analyses of the hydrothermal sulfide- and REE-Th-Nb-mineralization observed in the Fen complex (southern Norway), which is the biggest carbonatite-related REE and Th deposit in Europe. The alkaline silicate rocks and carbonatites of the Fen complex underwent two hydrothermal alteration events that caused (i) the formation of sulfides and (ii) (re)-mobilization of REE, Fe and Al. This renders the Fen complex an ideal locality to study the genesis of hydrothermal sulfide and REE mineralization in carbonatites. Our observations record a hydrothermal alteration history of the Fen carbonatite complex, which is relevant to many carbonatite complexes worldwide. The first alteration event (pyrite-stage) caused the crystallization of sulfides (mostly pyrite) in all lithologies and was induced by a sulfide-rich fluid 1 that was probably derived from adjacent mafic alkaline silicate rocks. Veins formed during this hydrothermal event show a typical succession from magnetite via pyrite I + hematite + magnetite to pyrite II. A subsequent alteration event is characterized by the interaction of two evolving fluids, namely (1) a REE-rich fluid 2 that was probably derived from the carbonatites (autometasomatic fluid) and (2) an oxidizing meteoric fluid 3 that mainly introduced Si and was in equilibrium with the basement host rocks. The interaction of these two fluids with the carbonatitic rocks resulted in various characteristic types of calcite-hematite rocks locally called rodberg. During their late-stage silicification by fluid 3, P was mobilized from magmatic apatite and reprecipitated in close-by rodberg veins as small fluorapatite-monazite veins. The interaction of the carbonatite-derived fluid 2 and meteoric fluid 3 also resulted in the formation of distinct zones of LREE-phases, notably REE-F-carbonates (bastnasite, parisite, synchysite), monazite and allanite, and HREE-phases (including samarskite, aeschynite and an unnamed Nb-Fe-REE-Th-oxide. The most pronounced HREE-enrichment occurs in domains of strongest hydrothermal alteration, which was probably caused by preferential precipitation of HREE-rich complexes coupled with later partial leaching of LREE by F-rich fluids during silicification. Thorium-rich minerals were precipitated in close association with HREE-rich minerals implying similar hydrothermal behaviour of Th and HREE.

Journal of Environmental Radioactivity, 2016

Radon ((222)Rn), thoron ((220)Rn) and their decay products may reach high levels in areas of high... more Radon ((222)Rn), thoron ((220)Rn) and their decay products may reach high levels in areas of high natural background radiation, with increased risk associated with mining areas. Historically, the focus has mostly been placed upon radon and progeny (RnP), but recently there have been reports of significant contributions to dose from thoron progeny (TnP). However, few direct measurements of TnP exist under outdoor conditions. Therefore, we assessed the outdoor activity concentrations of radon, thoron and TnP in an area of igneous bedrock with extreme levels of radionuclides in the thorium decay series. The area is characterized by decommissioned mines and waste rock deposits, which provide a large surface area for radon and thoron emanation and high porosity enhancing exhalation. Extreme levels of thorium and thoron have previously been reported from this area and to improve dose rate estimates we also measured TnP using filter sampling and time-integrating alpha track detectors. We found high to extreme levels of thoron and TnP and the associated dose rates relevant for inhalation were up to 8 μSvh(-1) at 100 cm height. Taking gamma irradiation and RnP into account, significant combined doses may result from occupancies in this area. This applies to recreational use of the area and especially previous and planned road-works, which in the worst case could involve doses as large as 23.4 mSv y(-1). However, radon and thoron levels were much more intense on a hot September day than during time-integrated measurements made the subsequent colder and wetter month, especially along the ground. This may be explained by cold air observed flowing out from inside the mines through a drainage pipe adjacent to the measurement stations. During warm periods, activity concentrations may therefore be due to both local exhalation from the ground and air ventilating from the mines. However, a substantially lower level of TnP was measured on the September day using filter sampling, as compared to what was measured with time-integrative alpha track detectors. A possible explanation could be reduced filter efficiency related to the attached progeny of some aerosol sizes, but a more likely cause is an upwards bias on TnP detectors associated with assumed deposition velocity, which may be different in outdoor conditions with wind or a larger fraction of unattached progeny. There is thus a need for better instrumentation when dealing with outdoor TnP.

American Mineralogist, 1997

Shallow-level plutons caused extensive contact metamorphism of Lower Paleozoic shale and carbonat... more Shallow-level plutons caused extensive contact metamorphism of Lower Paleozoic shale and carbonate sequences in the Permian Oslo Rift. A Ͼ500 m long and 100 m wide shalelimestone xenolith embedded within monzonites belonging to the Skrim plutonic complex experienced high-grade contact metamorphism and generation of minerals and mineral assemblages rarely reported from metamorphic rocks. The peak metamorphic (Stage I) assemblages in calcite-saturated rocks include wollastonite, melilites, fassaitic pyroxenes, phlogopite, titanian grossular, kalsilite, nepheline, perovskite, cuspidine, baghdadite, pyrrhotite, and occasional graphite. Mineral reactions involving detrital apatite produced a series of silicate apatites, including the new mineral species Ca 3.5 (Th,U) 1.5 Si 3 O 12 (OH). This assemblage equilibrated at T ϭ 820-870 ЊC with a C-rich, internally buffered pore-fluid (20-40 mol% CO 2 ϩ CH 4). During cooling the shale-limestone xenolith experienced infiltration of C-poor (Ͻ 0.1 mol% CO 2) fluids, triggering the formation of retrograde (Stage II) mineral assemblages comprising monticellite, tilleyite, vesuvianite, grandite garnets, diopside, and occasional hillebrandite. Rare potassium iron sulfides (rasvumite and djerfisherite) formed at the expense of primary pyrrhotite. These assemblages probably formed near 700 ЊC. Formation of diffuse sodalite-bearing veinlets was associated with breakdown of nepheline and the replacement of kalsilite and wollastonite by potassium feldspar. The sodalite-bearing Stage III assemblage formed by the infiltration of saline brines at a maximum temperature of 550 ЊC. Low-temperature (Stage IV) retrogression of the Stage I-III assemblage produced scawtite, giuseppettite, hydrogrossulars, phillipsite, thomsonite, and three hitherto undescribed mineral species.

Geochemistry International, 1994

ABSTRACT Studies have been made of the barophile mineral phases making up mantle xenoliths and re... more ABSTRACT Studies have been made of the barophile mineral phases making up mantle xenoliths and represented by xenocrysts in alkali picrites, darnkjernites, melilitites, nephelinites, and kimberlites in explosion pipes and dykes from the surroundings of the Fen complex (Norway) and in the Kola peninsula. Based on the composition of the minerals and P-T estimates, conclusions are drawn on the heterogeneity of the mantle sources of dyke magmatism during the epoch of Paleozoic tectonic-magmatic reactivation. The alkali picrites and foidites from the regions of the alkali intrusions represent rocks from the shallowest levels of the spinel lherzolite facies; to the second level, corresponding to the garnet lherzolite facies, is related to the formation ofolivine melilitites; and the deepest kimberlite magmas were generated from the level of the diamond-pyrope facies. Arzamastsev A.A., Dahlgren S. 1994. Plutonic mineral assemblages in Paleozoic dikes and explosion pipes of the Alkaline Province of the Baltic Shield. Geochemistry International, vol.31, no.3, p.57-68. (in English).

Larvikite has for more than a hundred years been appreciated as one of the world's most attractiv... more Larvikite has for more than a hundred years been appreciated as one of the world's most attractive dimension stones, and at present, its production and use is more extensive than ever. The main reason for the continuous success of larvikite on the world market is the blue iridescence displayed on polished surfaces, which is caused by optical interference in microscopic lamellae within the ternary feldspars. The larvikite complex consists of different intrusions, defining several ringshaped structures, emplaced during a period of approximately five million years. Following this pattern, several commercial subtypes of larvikite, characterised by their colour and iridescence, have been mapped. Four of these subtypes are being exploited at the present time and define the most important reserves in the short run. Some other subtypes are less attractive in the present market situation, but may provide an interesting potential for the future. However, the industrial value of the larvikite also depends on other geological features, such as various types of dykes, faults and fractures, ductile deformation zones, late-stage magmatic and hydrothermal alteration and deep weathering. When combining the distribution pattern of such features with the map of the larvikite subtypes, it is possible to delineate various types of larvikite deposit that are considered to have commercial value in the short or long term. Finally, reserve estimates for the different types have been made, showing that some of the most attractive types have rather limited known reserves if the present level of production is maintained or increased.

Geochemistry International

Paper in Russian Chemical composition of xenocrysts and minerals in mantle nodula found in dikes ... more Paper in Russian Chemical composition of xenocrysts and minerals in mantle nodula found in dikes and pipes of the Kola province and Fen area (SE Norway) was investigated. Estimation of PT conditions have proved the heterogeneity of the mantle sources of dike magmatism, during Paleozoic tectonomagmatic activity of the Baltic Shield. It is shown that the initial picritic melts were generated at the depth of spinel lherzolite facies (T = = 1000°C, P = 15 ± 1 kbar), olivine melilitites are connected with magmatic process at the level of garnet - lherzolite facies, and the deepest kimberlite melts were generated from the depth of diamond - pyrope mantle facies (T = 1100°C, P > 40 kbar).

The Fen Central Complex (FCC) and associated satellite dikes of the Fen Province in southern Norw... more The Fen Central Complex (FCC) and associated satellite dikes of the Fen Province in southern Norway record a magnetization dating to 583 Ma. The paleomagnetic pole calculated from these rocks falls at 56°N, 150°E (dp 5 7°, dm 5 10°) and compares favorably with two previous investigations. The mean inclination in our study is slightly steeper than that of one of the earlier studies and we attribute this to the fact that previous investigators inadvertently sampled younger (late Paleozoic/early Mesozoic) dikes in the area. The age of the Fen paleomagnetic pole is constrained by two consistent 40 Ar/ 39 Ar ages from these rocks, which average 583 6 15 Ma, along with previously published ages ranging from 523-601 Ma. The Fen Central Complex, along with associated intrusions, were emplaced during minor extensional activity that occurred after continental separation between Baltica and Laurentia. This interpretation is consistent with our 580 Ma paleoreconstruction, which shows that Baltica had rifted and rotated from its 615 Ma position adjacent to Greenland. We interpret the ages and the tectonics to reflect opening of an ocean basin from north to south during the latest Neoproterozoic to earliest Paleozoic time (615-540 Ma). The polarity option for the Fen pole remains an important and open question due to recent suggestions of a true polar wander event in the mid-Cambrian; however, we consider that the south polarity option offers the simplest tectonic model for the rifting of Rodinia and opening of the Iapetus Ocean between Baltica and Laurentia. Finally, we note the similarity of the Fen paleomagnetic pole to paleomagnetic poles of Permo-Triassic age in Baltica and urge caution in the unequivocal use of the Fen pole as primary until further substantiated by coeval poles from Baltica.

Precambrian Research, 1993

The age of the granulite facies metamorphism in the Bamble Belt has long been a matter of debate.... more The age of the granulite facies metamorphism in the Bamble Belt has long been a matter of debate. Samples from four localities in the coastal section of the belt have, therefore, been used to obtain Sm-Nd mineral ages. All gave dates within the range 1070-1100 Ma. Earlier Rb-Sr data of Field et al. had placed the maximum metamorphism of the region at 1540 Ma. These authors deduced that any subsequent metamorphism in the Sveconorwegian (~ 1100 Ma) was low grade since the Rb-Sr system had not been completely reset at this time. New U-Pb data, published elsewhere, indicate two periods of deformation and metamorphism; a pre-Sveconorwegian metamorphism at 1437 + 8 Ma and a Sveconorwegian period at 1100 Ma. Our Sm-Nd data indicate that the Sveconorwegian period lasted for at least 50 Ma, and included granulite facies metamorphism ending at 1095 Ma. We further deduce that Rb-Sr whole-rock systems were not necessarily totally reset during the granulite facies metamorphic event. Our conclusions represent a return to older interpretations concerning the significance of the Sveconorwegian event.

Lithos, 1994

Ultramafic dikes with carbonatitic affinities ("damtjernites") in southern Norway were generated ... more Ultramafic dikes with carbonatitic affinities ("damtjernites") in southern Norway were generated during two magmatic events separated by about 275 Ma. The older event is late Proterozoic and the younger is mid Carboniferous. More than 50 satellitic damtjernite intrusions occur within a 1500 km 2 large region surrounding the Fen Central Complex. Phlogopite macrocrysts from 10 of these satellites yield a Rb-Sr isochron age of 578 + 24 Ma (2a expanded errors). This demonstrates that the late Proterozoic carbonatitic magmatism centered at the Fen Central Complex occurred on a regional scale. This region is termed "the Fen Province". The emplacement of the magmas in the Fen Province most likely occurred in connection with minor extensional tectonic activity on the Baltic platform during the drift-phase after the Proto-Atlantic opening. Sr-isotopic data also show that a dike mineralogically and chemically similar to the Fen damtjernites was emplaced at 324 + 4 Ma (mid Carboniferous). This dike very likely dates the initiation of magmatism in the Oslo rift. Consequently very similar carbonate-bearing ultramafic magmas were generated within the south Norwegian mantle during the relatively minor Fen event and in the initial extensional period when the magma production in the Oslo rift was still low.

Earth and Planetary Science Letters, 2008

The Permo-Carboniferous Oslo Rift developed in the foreland of the Variscan orogen over a period ... more The Permo-Carboniferous Oslo Rift developed in the foreland of the Variscan orogen over a period of some 50 million years through a process characterized by moderate extension and widespread magmatism. The overall tectonic situation places the Oslo Rift in a postcollisional, dextral transtensional setting related to the convergence between Baltica, Laurentia, Gondwana and Siberia during assembly of Pangea, the location probably reflecting the control by pre-existing lithospheric structures. Although a detailed understanding of these factors and processes relies strongly on having a good age control, the dating of mafic to ultramafic alkalic volcanic units formed during initial rifting has been a very challenging task. In this study we have successfully employed perovskite from melilitic and nephelinitic volcanic rocks, together with magmatic titanite in a more evolved ignimbrite, to obtain ID-TIMS high-precision U-Pb ages. Three samples from various levels of the Brunlanes succession, in the southernmost exposures of the Oslo Graben, yield ages of 300.2 ± 0.9, 300.4 ± 0.7 and 299.9 ± 0.9 Ma. A melililitic tuff at the base of the Skien succession further to the northwest yields a slightly younger age of 298.9 ± 0.7 Ma. The initial Pb compositions derived mainly from coexisting pyroxene, apatite and hornblende are characterized by extremely radiogenic initial 206 Pb/ 204 Pb ratios (up to 21.3) that confirm a provenance of these early alkaline basalts from HIMU-type sources. The U-Pb ages coincide with the Gzhelian age inferred from fossils in the upper part of the basal rift sedimentary fill of the Asker Group, and postdate the underlying basal sedimentary sequences by some 10 million years, pointing to a relatively rapid initiation of the rifting process.

Contributions to Mineralogy and Petrology, 1993

Deformed and metamorphosed dolomite marbles in the Kragero area of the Bamble Shear Belt occur as... more Deformed and metamorphosed dolomite marbles in the Kragero area of the Bamble Shear Belt occur as lenses within metasupracrustal sequences, as matrix in bodies resembling magmatic breccias, and as veins/ dykes cutting amphibolites and metagabbros. A common origin is not evident from the field relationships, but is nevertheless probable due to great geochemical similarities between dolomite from the different occurrences. They are characterized by higher REE, Ni, Co, Cr and Sc, and lower Ba and Sr contents relative to metasedimentary marbles occurring nearby. Sm-Nd isotope data shows that the dolomites are of Sveconorwegian age (1175_+37 Ma). The dolomite marbles are very weakly LREE-enriched and display in most cases positive Eu anomalies. Their stable isotope compositions are uniform (61so= +9.6 to +10.7%o; c~13C=-8.5 to-6.2%o), their initial S7Sr/S6Sr ratios are high (0.706-0.709), whereas their age corrected end varies from + 0.7 to-1.5. Geochemically the dolomitic marbles differ considerably from sedimentary or metasedimentary marbles as well as from carbonatites. The Kragero dolomite marbles represent deformed and metamorphosed hydrothermal veins or vein-complexes deposited in tensional fractures in the deep crust. Although the dolomitic marbles regionally are of minor volume, the dolomite deposition represents a specific and important event in the geological evolution of the Bamble Shear Belt. Their geochemical and isotopic homogeneity on a regional scale suggest that the hydrothermal solutions were supplied from a very large, homogeneous reservoir. Trace elements, stable and radiogenic isotopes, and field and isotopic age relationships are consistent with a deposition from hydrothermal solutions which were exsolved from crystallizing charnockitic intrusions and subse

Chemical Geology, 1995

Classical ion-exchange chromatography combined with high-performance ion chromatography has been ... more Classical ion-exchange chromatography combined with high-performance ion chromatography has been used for the separation of Nd and Sm for isotopic analysis of geological samples by mass spectrometry. The procedure has successfully been applied to samples with Nd concentrations as low as 0.2 μg g−1 and high-quality elemental separation was routinely achieved even for “difficult” samples such as low-Nd ultramafic upper-mantle xenoliths and extremely Ba-rich lamproites. The total blank value was <60 pg Nd. The method has several distinct advantages: (1) it is simpler than some of the other techniques available; (2) it can be used for samples with low Nd and Sm concentrations; and (3) Nd and Sm fractions virtually free of interferences from other elements were obtained for a wide range of samples investigated.

Precambrian research, 1993

The age of the granulite facies metamorphism in the Bamble Belt has long been a matter of debate.... more The age of the granulite facies metamorphism in the Bamble Belt has long been a matter of debate. Samples from four localities in the coastal section of the belt have, therefore, been used to obtain Sm-Nd mineral ages. All gave dates within the range 1070-1100 Ma. Earlier Rb-Sr data of Field et al. had placed the maximum metamorphism of the region at 1540 Ma. These authors deduced that any subsequent metamorphism in the Sveconorwegian (~ 1100 Ma) was low grade since the Rb-Sr system had not been completely reset at this time. New U-Pb data, published elsewhere, indicate two periods of deformation and metamorphism; a pre-Sveconorwegian metamorphism at 1437 + 8 Ma and a Sveconorwegian period at 1100 Ma. Our Sm-Nd data indicate that the Sveconorwegian period lasted for at least 50 Ma, and included granulite facies metamorphism ending at 1095 Ma. We further deduce that Rb-Sr whole-rock systems were not necessarily totally reset during the granulite facies metamorphic event. Our conclusions represent a return to older interpretations concerning the significance of the Sveconorwegian event.

American Mineralogist, 1997

Shallow-level plutons caused extensive contact metamorphism of Lower Paleozoic shale and carbonat... more Shallow-level plutons caused extensive contact metamorphism of Lower Paleozoic shale and carbonate sequences in the Permian Oslo Rift. A Ͼ500 m long and 100 m wide shalelimestone xenolith embedded within monzonites belonging to the Skrim plutonic complex experienced high-grade contact metamorphism and generation of minerals and mineral assemblages rarely reported from metamorphic rocks. The peak metamorphic (Stage I) assemblages in calcite-saturated rocks include wollastonite, melilites, fassaitic pyroxenes, phlogopite, titanian grossular, kalsilite, nepheline, perovskite, cuspidine, baghdadite, pyrrhotite, and occasional graphite. Mineral reactions involving detrital apatite produced a series of silicate apatites, including the new mineral species Ca 3.5 (Th,U) 1.5 Si 3 O 12 (OH). This assemblage equilibrated at T ϭ 820-870 ЊC with a C-rich, internally buffered pore-fluid (20-40 mol% CO 2 ϩ CH 4). During cooling the shale-limestone xenolith experienced infiltration of C-poor (Ͻ 0.1 mol% CO 2) fluids, triggering the formation of retrograde (Stage II) mineral assemblages comprising monticellite, tilleyite, vesuvianite, grandite garnets, diopside, and occasional hillebrandite. Rare potassium iron sulfides (rasvumite and djerfisherite) formed at the expense of primary pyrrhotite. These assemblages probably formed near 700 ЊC. Formation of diffuse sodalite-bearing veinlets was associated with breakdown of nepheline and the replacement of kalsilite and wollastonite by potassium feldspar. The sodalite-bearing Stage III assemblage formed by the infiltration of saline brines at a maximum temperature of 550 ЊC. Low-temperature (Stage IV) retrogression of the Stage I-III assemblage produced scawtite, giuseppettite, hydrogrossulars, phillipsite, thomsonite, and three hitherto undescribed mineral species.

Earth and Planetary Science Letters, 2008

The Permo-Carboniferous Oslo Rift developed in the foreland of the Variscan orogen over a period ... more The Permo-Carboniferous Oslo Rift developed in the foreland of the Variscan orogen over a period of some 50 million years through a process characterized by moderate extension and widespread magmatism. The overall tectonic situation places the Oslo Rift in a postcollisional, dextral transtensional setting related to the convergence between Baltica, Laurentia, Gondwana and Siberia during assembly of Pangea, the location probably reflecting the control by pre-existing lithospheric structures. Although a detailed understanding of these factors and processes relies strongly on having a good age control, the dating of mafic to ultramafic alkalic volcanic units formed during initial rifting has been a very challenging task. In this study we have successfully employed perovskite from melilitic and nephelinitic volcanic rocks, together with magmatic titanite in a more evolved ignimbrite, to obtain ID-TIMS high-precision U-Pb ages. Three samples from various levels of the Brunlanes succession, in the southernmost exposures of the Oslo Graben, yield ages of 300.2 ± 0.9, 300.4 ± 0.7 and 299.9 ± 0.9 Ma. A melililitic tuff at the base of the Skien succession further to the northwest yields a slightly younger age of 298.9 ± 0.7 Ma. The initial Pb compositions derived mainly from coexisting pyroxene, apatite and hornblende are characterized by extremely radiogenic initial 206 Pb/ 204 Pb ratios (up to 21.3) that confirm a provenance of these early alkaline basalts from HIMU-type sources. The U-Pb ages coincide with the Gzhelian age inferred from fossils in the upper part of the basal rift sedimentary fill of the Asker Group, and postdate the underlying basal sedimentary sequences by some 10 million years, pointing to a relatively rapid initiation of the rifting process.