Alf Lindroos

The late Svecofennian granite-migmatite zone of southern Finland—a belt of transpressive deformation and granite emplacement

Abstract The late Svecofennian granite-migmatite (LSGM) zone in southwestern Finland is a ∼ 100 km wide and 500 km long belt transecting the southern Svecofennides from WSW to ENE. It was formed in an area of thin pillow lavas, volcaniclastic sediments and limestones. The area is interpreted as having been an early basin of crustal extension which was the locus of an inherited zone of weakness in the Proterozoic crust. Early recumbent folding was followed by crustal thickening and intrusions of ∼ 1.89-1.88 Ga old plutonics. The LSGM-zone is characterized by 1.84-1.83 Ga old rhomboidal sheets of late Svecofennian microcline granite and is bounded by ductile shears. Amongst the two major phases of deformation defined in the LSGM-zone, the earlier one (D1) affected only the supracrustals and the 1.89-1.88 Ga old early plutonics. In contrast, the later phase (D2) also deformed the late Svecofennian migmatites and granites. D1 represents a complex and long-lasting deformation event which included overturning and thrusting of the Svecofennian strata. D2 comprised ENE-WSW directed drag accompanied by NNW-SSE compression. The Svecofennian crust was thickened further and anatectic microcline granites intruded along thrusts. The rhomboidal outline of the late Svecofennian granite sheets indicates a sense of movement in agreement with measured dextral strike-slip in the shears delimiting the LSGM-zone. Imbricated feldspar megacrysts in the granites indicate thrusting towards the west during the stage of granitic magmatism. The gently dipping early Svecofennian gneisses and the late granite sheets were folded into upright F2 folds with gently plunging axes. Locally, the F2 axial surfaces were intruded by late Svecofennian granite mobilisates.

Mortar Dating Using AMS 14C and Sequential Dissolution: Examples from Medieval, Non-Hydraulic Lime Mortars from the land Islands, SW Finland

Non-hydraulic mortars contain datable binder carbonate with a direct relation to the time when it was used in a building, but they also contain contaminants that disturb radiocarbon dating attempts. The most relevant contaminants either have a geological provenance and age or they can be related to delayed carbonate formation or devitrification and recrystallization of the mortar. We studied the mortars using cathodoluminescence (CL), mass spectrometry (MS), and accelerator mass spectrometry (AMS) in order to identify, characterize, and date different generations of carbonates. The parametersdissolution rate, 13C/12C and 18O/16O ratios, and 14C agewere measured or calculated from experiments where the mortars were dissolved in phosphoric acid and each successive CO2 increment was collected, analyzed, and dated. Consequently, mortar dating comprises a CL characterization of the sample and a CO2 evolution pressure curve, a 14C age, and stable isotope profiles from at least 5 successive dissolution increments representing nearly total dissolution. The data is used for modeling the interfering effects of the different carbonates on the binder carbonate age. The models help us to interpret the 14C age profiles and identify CO2 increments that are as uncontaminated as possible. The dating method was implemented on medieval and younger mortars from churches in the land Archipelago between Finland and Sweden. The results are used to develop the method for a more general and international use.

AMS 14C DATING OF LIME MORTAR

Abstract A method for refining lime mortar samples for 14 C dating has been developed. It includes mechanical and chemical separation of mortar carbonate with optical control of the purity of the samples. The method has been applied to a large series of AMS datings on lime mortar from three medieval churches on the Aland Islands, Finland. The datings show convincing internal consistency and confine the construction time of the churches to AD 1280–1380 with a most probable date just before AD 1300. We have also applied the method to the controversial Newport Tower, Rhode Island, USA. Our mortar datings confine the building to colonial time in the 17th century and thus refute claims of Viking origin of the tower. For the churches, a parallel series of datings of organic (charcoal) inclusions in the mortar show less reliable results than the mortar samples, which is ascribed to poor association with the construction time.

Environmental monitoring of trace elements in bark of Scots pine by thick-target PIXE

Bark samples were taken from Scots pines (Pinus sylvestris L.) from a polluted area near a metal plant and from a relatively non-polluted site. Thick-target particle-induced X-ray emission (PIXE) was used for the analyses after different types of prepreparation of the samples. The bark samples were analysed directly by radially scanning from inner to outer bark in order to study the variability of elemental concentrations in different layers. Some clear differences were found in the chemical composition of the inner and outer bark. The lowest detection limits for the analyses of heavy metal ions were obtained by combining dry ashing at 550 °C with the PIXE method. More than 100 times higher concentrations were found for the heavy metal ions Fe, Ni, Cu, Zn, Ga, As and Pb in the bark samples from a polluted area in comparison to samples from a non-polluted area. The work demonstrated that external-beam thick-target PIXE is a sensitive and reliable method for quantitative determination of heavy metals in tree bark samples.

Successful AMS 14C Dating of Non-Hydraulic Lime Mortars from the Medieval Churches of the Åland Islands, Finland

Fifteen years of research on accelerator mass spectrometry (AMS) radiocarbon dating of non-hydraulic mortar has now led to the establishment of a chronology for the medieval stone churches of the Aland Islands (Finland), where no contemporary written records could shed light on the first building phases. In contrast to other material for dating, well-pre- served mortar is abundantly available from every building stage. We have gathered experience from AMS dating of 150 Aland mortar samples. Approximately half of them have age control from dendrochronology or from 14C analysis of wooden fragments in direct contact with the mortar. Of the samples with age control, 95% of the results agree with the age of the wood. The age control from dendrochronology, petrologic microscopy, chemical testing of the mortars, and mathematical modeling of their behavior during dissolution in acid have helped us to define criteria of reliability to interpret the 14C results when mortar dating is the only possibility to constrain the buildings in time. With these criteria, 80% of all samples reached conclusive results, and we have thus far been able to establish the chro- nology of 12 out of the 14 churches and chapels, while 2 still require complementary analyses.

Stratigraphy and geochemistry in the proterozoic mafic volcanic rocks of the Nagu-Korpo area, SW Finland

The volcanic rocks of the Nagu-Korpo area in SW Finland form a thin (< 1000 m) formation that is folded into narrow synforms separated by migmatite-filled antiforms. In Nagu the volcanic formation comprises three units: a lower unit of subvolcanic banded gabbro sills, a middle unit of amphibolitic volcanic rocks and an upper unit consisting of a thin ultramafic volcanic rock layer. The volcanic formation is interlayered within a thicker sequence of metasedimentary gneisses. Geochemically the metavolcanic rocks are tholeiitic and plots based on Ti, Y, Zr and other trace elements show that they resemble recent within-plate lavas. MORB- and mantle-normalised trace element plots show patterns strongly differing from those of recent arc lavas, indicating a tectonic environment of (plate boundary related?) initial rifting. The upper ultramafic lavas are enriched in LREE relative to the lavas of the much thicker amphibolitic middle unit below. A model for the initial formation of the volcanic rocks of the Svecofennian fold belt must acknowledge both the episode of initial rifting and the later evolution of an island arc-like environment.

Study of seasonal variations of trace-element concentrations within tree rings by thick-target PIXE analyses

Abstract Thick-target PIXE has been used for the quantitative determination of trace elements in annual growth rings of different tree species. A scanning device was developed for the remote control of the sample and a video-camera system for the exact monitoring of the spot to be analyzed. The samples were analyzed in steps of 1 mm. The widths of the tree rings studied were in the range 2.5–8.0 mm. Samples of Norway spruce and Scots pine were collected from Harjavalta, a polluted area in southwestern Finland. The elements studied were S, Cl, Br, K, Ca, Mn, Fe, Zn, Cu, Ni, Pb, Rb and Sr. Large variations were observed in elemental concentrations within individual tree rings. The highest concentrations for most elements were obtained for earlywood in the beginning of the growth season and the lowest values for latewood thus reflecting the biological activity. The method was calibrated using international standard reference materials. For most elements the matrix effects were found to be negligible. The detection limits for most metals studied were in the range 1–5 ppm.

Late-Svecofennian transpressive deformation in SW Finland—evidence from late-stage D3 structures

Abstract Late-Svecofennian deformation was investigated in the Bjensböle area of SW Finland. The deformation history was revealed by following marker horizons of amphibolite, marble and black schist, in addition to microscopic studies and U-Pb monazite geochronology. The D1 structures are transposed and overprinted by D2 structures. The mesoscopic F2 folds are tight to isoclinal with fold axes plunging 20-40[ddot] NE-ENE. Regional large-scale open F3 folds formed during the main D3 phase, and the macroscopic structure in Bjensböle is an F2-F3 interference pattern formed by N-S compression. The area consist of low P/high T amphibolite facies sillimanite gneiss and low P/low T amphibolite facies quartz schist, separated by a low P/low T amphibolite horizon in the central part of the area. The sillimanite gneiss yielded a U-Pb monazite age of 1824±5 Ma. This age gives a timing for the peak metamorphism and the D3 folding on the Kemiö Island. After the peak metamorphism some deformational structures developed. A NE-SW directed crenulation cleavage in the quartz schist and the amphibolite, small open folds in the marble, a foliation in the sillimanite gneiss and a muscovite orientation in the quartz schist all have the same direction. These structures are interpreted as late-stage D3 (D3L). The angle between the NE-SW directed D3L structures and the locally predominant S0 + S1 + S2 schistosity is ca. 25[ddot]. The structural evidence related to the D3L point to a dextral sense of shearing along roughly E-W directed zones with a NW-SE compression. This is related to a regional ductile shearing signifying the vaning stages of the late-Svecofennian transpressive deformation in SW Finland.

Preconcentration of trace elements in biological materials by dry ashing for TTPIXE analysis A study of matrix effects

Abstract Trace elements in biological materials were preconcentrated by dry ashing at 550°C in order to improve the detection limits for thick-target PIXE analyses. The analytical procedure was calibrated with equally prepared ashes of certified biological standard reference materials (SRMs). The matrix effects were studied both theoretically and experimentally. A drawback of the ashing procedure is that volatile elements such as halogenides and sulfur can be partly lost depending on the composition of the material studied. Thick-target PIXE combined with dry ashing is a sensitive and reliable technique for the determination of elements with atomic number > 20 in biological materials with a low ash content.

Early Proterozoic Svecofennian volcanism and associated plutonism in Enklinge, SW Finland

Abstract The c. 1.9 Ga old Enklinge volcano-plutonic centre is part of the Svecofennian province of SW Finland. The rocks of the area consist of an early subaquatic extrusion of mafic/intermediate to rhyolitic volcanics underlain by marbles, graywackes and quartz-feldspar-rich volcanoclastic sediments. The volcanic sequence is deformed and syntectonically intruded by a biotite tonalite accompanied and mixed with coeval basaltic dykes. Dacitic quartz porphyry dykes and small breccia pipes occur along the margins of the tonalite intrusion. Contrary to the Bergslagen area in Sweden no large co-magmatic granites can be found in the volcano-plutonic complex and the rhyolites are much less voluminous and only slightly mineralized. The rocks of the Enklinge area are strongly deformed and the tectonic environment differs strongly from that of the almost nonorogenic and more terrestrial Bergslagen area W. of Enklinge.