Browsing by Subject "Geochemistry"

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  • Dey, Sukanta; Pal, Sayantan; Balakrishnan, S.; Halla, Jaana; Kurhila, Matti; Heilimo, Esa (2018)
    Several profound changes, including those involving formation of the continental crust, occurred on Earth during the Neoarchaean Era. However, the tectonic settings associated with Neoarchaean crustal growth are not well understood and vigorously debated. The Neoarchaean Veligallu greenstone belt, eastern Dharwar craton hosts a variety of ultramafic, mafic and felsic volcanic rocks. Whole-rock elemental and Nd isotope data along with zircon U-Pb dating on these rocks provide significant insights into the origin and tectonic setting of Neoarchaean crust formation. The volcanism in the Veligallu belt started with similar to 2.67 Ga tholeiitic basalts derived from shallow melting of a slightly depleted mantle (epsilon(Ndt) = + 0.6 to + 1.1). Moderate negative Nb anomalies, slightly elevated Th/Yb and LREE, and an absence of evidence for crustal contamination are consistent with extraction of these basalts from a mantle source weakly metasomatized by subducted slab-derived fluids in an incipient oceanic arc setting. As the arc matured, clastic sediments started forming with concurrent emplacement of komatiites, komatiitic basalts and ferropicrites showing strong signatures of contamination with continental crust (negative Nb and Ti anomalies, LREE enrichment and negative epsilon(Ndt)). In the final stage (similar to 2.58 Ga), a variety of felsic volcanic rocks (sodic trachyandesite, high Mg# andesite, rhyolite, calc-alkaline andesite) formed. The rock association and distinct geochemical signatures (enrichment of LILE, negative Nb and Ti anomalies, Mesoarchaean Nd model ages and inherited older zircons) suggest a continental margin arc environment which contained older crust. The evolutionary history of the Veligallu belt implies that both the arcand plume-related processes, and their interplay contributed significantly to the growth of Neoarchaean crust.
  • Silventoinen, Saara (Helsingin yliopisto, 2020)
    Chromite (Fe2+, Mg)(Cr, Al)2O4 contains in addition to the main elements a variety of minor and trace elements. It often occurs as an accessory mineral in cumulate rich olivine rocks. The melt composition is recorded into crystallised chromite. Due to these characteristics, the composition of chromite can be used as a proxy for the petrogenesis of the magma, providing vital information on how the sulphur saturation occurred, and, in particular, to record the conditions of the cooling cumulates. The Sakatti Cu-Ni-PGE orthomagmatic deposit is located in northern Finland in the CLGB. The deposit is hosted by an olivine cumulate body (the main body), where both disseminated and massive sulphides occur. In addition to the main body, the smaller satellite bodies, the NE and the SW bodies also host mineralisation. The upper barren olivine peridotite body occurs on top of the main body. Thin sections were selected from each of the olivine peridotite bodies (the main, the upper, the NE and the SW body). The chromites can be divided into two groups; unzoned chromites from the main body and the NE body, and zoned chromites from the upper body and the SW body. The chromite grains occur either as intercumulus positions relative to olivine grains or enclosed by unaltered olivine and pyroxene cumulus grains. The elemental compositions, including SiO2, TiO2, Al2O3, Fe2O3, FeO, V2O3, MnO, MgO, ZnO and NiO, of the chromites were analysed using EPMA. The trace element, Ru, was analysed using the LA-ICP-MS. The Sakatti chromites are characterised by variable content of Mg# (0.18 -0.56), moderately high Cr# (0.56 -0.92) and Fe3+# (0.11 -0.50). In addition to Mg# and Cr#, the Al2O3 and TiO2 compositions and textural features, the Sakatti chromites resemble more of those chromites which saturate from komatiitic magmas than chromites which are associated with picritic or basaltic melts. A subset from those chromites that are spatially associated with massive sulphides display high ZnO (> 0.5 wt.%), low Ni (as low as 225 ppm) and higher TiO2 (up to 3.22 wt.%). The source behind the elevated Zn-levels could be the assimilation of sulphide bearing sediments to the magma. The Ti enrichment could be explained by the magma being contaminated by iron-rich gabbro during its evolutional history. Nickel contents of the Sakatti chromites range from 225 ppm to 1731 ppm. The majority of the analysed chromite grains are Ni-depleted due to the separation of a sulphide liquid during the magmatic evolution of the Sakatti geological environment. Portion of the Sakatti chromites are Fe3+-rich, which is interpreted to reflect their crystallisation from a melt that have potentially assimilated a sulphur-rich evaporitic rocks, anhydrite. Fe3+-rich chromites together with high Zn, high Ti and low Ni contents represent primary magmatic features in the Sakatti olivine cumulate hosted chromite. Ru content in the Sakatti chromites are low (<2 pbb) and therefore, the low Ru values are an indication of mineralised geological environment.
  • Kärenlampi, Kimmo; Heinonen, Jussi S.; Kontinen, Asko; Hanski, Eero; Huhma, Hannu (2021)
    The origin of ferroan A-type granites in anorogenic tectonic settings remains a long-standing petrological puzzle. The proposed models range from extreme fractional crystallization of mantle-derived magmas to partial melting of crustal rocks, or involve combination of both. In this study, we apply whole-rock chemical and Sm-Nd isotopic compositions and thermodynamically constrained modeling (Magma Chamber Simulator, MCS) to decipher the genesis of a suite of A1-type peralkaline to peraluminous granites and associated intermediate rocks (monzodiorite-monzonite, syenite) from the southwestern margin of the Archean Karelia craton, central Finland, Fennoscandian Shield. These plutonic rocks were emplaced at ca. 2.05 Ga during an early stage of the break-up of the Karelia craton along its western margin and show trace element affinities to ocean island basalt-type magmas. The intermediate rocks show positive epsilon Nd(2050 Ma) values (+1.3 to +2.6), which are only slightly lower than the estimated contemporaneous depleted mantle value (+3.4), but much higher than average epsilon Nd(2050 Ma) of Archean TTGs (-10) in the surrounding bedrock, indicating that these rocks were essentially derived from a mantle source. The epsilon Nd(2050 Ma) values of the peralkaline and peraluminous granite samples overlap (-0.9 to +0.6 and -3.2 to +0.9, respectively) and are somewhat lower than those in the intermediate rocks, suggesting that the mafic magmas parental to granite must have assimilated some amount of older Archean continental crust during their fractionation, which is consistent with the continental crust-like trace element signatures of the granite members. The MCS modeling indicates that fractional crystallization of mantle-derived magmas can explain the major element characteristics of the intermediate rocks. The generation of the granites requires further fractional crystallization of these magmas coupled with assimilation of Archean crust. These processes took place in the middle to upper crust (-2-4 kbar, -7-15 km) and involved crystallization of large amounts of clinopyroxene, plagioclase and olivine. Our results highlight the importance of coupled FC-AFC processes in the petrogenesis of A-type magmas and support the general perception that magmas of A-type ferroan granites become more peraluminous by assimilation of crust. They further suggest that variable fractionation paths of the magmas upon the onset of assimilation may explain the broad variety of A-type felsic and intermediate igneous rocks that is often observed emplaced closely in time and space within the same igneous complex.
  • Eranti, Olli (Helsingin yliopisto, 2020)
    Geochemistry of stone tools and sources remains largely unknown in Finnish archaeology, but the potential is promising. Most formal Finnish stone tools beside quartz are made from ground metamorphic stone, which often has a specific source of collection. Geochemistry can reveal compositional and trace element links between the sources and tools found in various Stone Age settlement sites. Discoveries about technological properties of the stone types can be a helpful comparison, since many geochemical methods can produce data that has inconsistencies. In this thesis both geochemical and technological aspects of two lithic material sources are examined. Samples were collected from two lithic material sources: Rakkaviita and Rieskapaikka in Tervola, Southern Lapland. The 62 collected samples were measured with a Bruker S1 Titan portable x-ray fluorescence device as a preliminary method. Three samples from Rakkaviita and two samples from Rieskapaikka were chosen for the primary method, which was the PANalytical Axios mAX 4 kW, Wavelength Dispersive X-Ray fluorescence spectrometer in the Department of Geosciences in the University of Helsinki. The data from these measurements is evaluated and plotted to reveal geochemical properties of the stone. In this study, the properties of these stone materials are documented for the first time, so it can also be considered as a mapping study. The measurements revealed differentiation between methods, especially on the SiO2 percentages. The WD-XRF measurements are done without sample specific calibration, which obscures the quantitative proportions of some elements. The content of the stone revealed various components in different proportions. The result of major components was a coarsely qualitative definition of stone from both sources, which can be used in further material studies of stone tools. The trace element comparison between yttrium (Y) and strontium (Sr) revealed clear similarity between sources, excluding one sample that had significantly different tool production properties than others. According to this study, methods that handle trace elements well like ICP-MS are best suited for provenance studies on this type of stone. With trace elements it’s likely that these types of stones can be successfully sourced by geological region. The technological properties of the material are studied to find out the potential of the raw material as a stone to make and use tools with. Differences in the technological properties of the two sources is reflected in the composition and formation differences. Rieskapaikka included more mafic, porphyritic and fine-grained samples while Rakkaviita stones were more foliated and deteriorated.
  • Holmqvist-Sipilä, Elisabeth; Wessman, Anna; Mänttäri, Irmeli; Lahaye, Yann (2019)
    This article presents the results of the first-ever lead isotope (LI) analysis of copper-based archaeological artefacts found in the region of Finland. Eight metal objects recovered from the Iron Age water burial site of Levänluhta in western Finland were analysed via multi collector inductively coupled plasma mass spectrometry (MC-ICP-MS) and portable X-ray fluorescence spectrometry (pXRF) in order to attain geochemical and LI data. The majority of the objects are Merovingian period (ca. 550–800 CE) jewellery, displaying domestic Iron Age artefact styles, and were probably cast by local workshops in Finland. Until recently, the copper exploited in Bronze and Iron Age metallurgy in Finland had been linked to Scandinavian ores. However, this provenance scenario seems implausible in the light of recent LI studies demonstrating that Scandinavian Bronze Age artisans in fact relied on long-distance metal transport. Comparisons between the LI data of the analysed objects and published ore databases exclude the possibility of a domestic or Scandinavian copper source for the metals. Instead, it appears likely that the copper originated from southern European ores. The low arsenic and antimony levels in the copper alloys provide indication of long recycling patterns of the metals used in the Iron Age workshops in Finland. It is possible that the Iron Age artefacts contain recycled copper-alloys already acquired in the Bronze Age. The metals were transported for long distances, and it appears that the pan-European metal circulation network also crossed the Baltic Sea to reach coastal Finland.
  • Takalo, Mimmi (Helsingin yliopisto, 2021)
    In 2011 Anglo American published a promising multi metal ore deposit in Sodankylä, Northern Finland. The ore is named Sakatti, after a small pond in the vicinity of the discovering place. The ore is under the Viiankiaapa mire, which is part of national mire protecting program and Natura 2000 program. Viinkiaapa is at the eastern side of river Kitinen, which is known to have flooded, bringing mineral material to the mire. To prevent the possibly environment effects in the future, it is essential to know present conditions of the mire. The study area is at the southern part of the Viiankiaapa mire and consist of eight sampling sites for peat. The basal sediment of the study area was determined with ground penetrating radar profiles that pass the sampling sites. To study the mineral supply of the mire, nine elements (Na, Mg, Al, S, P, K, Ca, Mn, and Fe) were chosen for geochemical analyses and the ash content of the peat profiles was determined. The basal sediment is highly affected by the vicinity of the river Kitinen. Fluvial channels have eroded till, which was deposited during the last glacial period. At the eastern side of the study area possibly aeolian sand is detected. Depressions eroded by fluvial channels are filled by gyttja, typically below 179 m a.s.l. The geochemistry of the peat indicates that the early phase of the mire was characterized with higher mineral supply. At the eastern part of Viiankiaapa the mineral supply has decreased after the early phase of the mire. The mineral supply has been higher at the middle parts of the mire throughout the Holocene. The floods of the river Kitinen are the main source of the mineral supply. The decrease in the mineral supply indicates that the flooding events have reduced, and the normal floods inundate smaller area than the early floods.
  • Kylander, Malin E.; Plikk, Anna; Rydberg, Johan; Löwemark, Ludvig; Salonen, J. Sakari; Fernandez-Fernandez, Maria; Helmens, Karin (2018)
    Biological proxies from the Sokli Eemian (Marine Isotope Stage 5e) paleolake sequence from northeast Finland have previously shown that, unlike many postglacial records from boreal sites, the lake becomes increasingly eutrophic over time. Here, principal components (PC) were extracted from a high resolution multi-element XRF core scanning dataset to describe minerogenic input from the wider catchment (PC1), the input of S, Fe, Mn, and Ca-rich detrital material from the surrounding Sokli Carbonatite Massif (PC2), and chemical weathering (PC3). Minerogenic inputs to the lake were elevated early in the record and during two abrupt cooling events when soils and vegetation in the catchment were poor. Chemical weathering in the catchment generally increased over time, coinciding with higher air temperatures, catchment productivity, and the presence of acidic conifer species. Abiotic edaphic processes play a key role in lake ontogeny at this site stemming from the base cation- and nutrient-rich bedrock, which supports lake alkalinity and productivity. The climate history at this site, and its integrated effects on the lake system, appear to override development processes and alters its long-term trajectory.
  • Pietilä, Maija (Helsingin yliopisto, 2020)
    Geological Survey of Finland conducted bedrock mapping in the eastern parts of Central Finland Granitoid Complex (CFGC) and the area next to the Archean craton in the 1990s. The area consists mainly of Paleo-proterozoic paragneisses, with minor volcanic rocks present. The granitoids belonging to the Central Finland Granitoid Complex make up part of the bedrock in the area. The granitoids of CFGC are divided into a 1.89-1.88 Ga syn-kinematic group, and a crosscutting, 1.88-1.87 Ga post-kinematic group. In this Master’s thesis, three post-kinematic granitoid intrusions of Löytölamminvuori, Sorsakoski and Karvalevä are studied, covering their lithological, petrographical and geochemical features. The intrusions are non-foliated, porphyritic granites and quartz-monzonites, with a minor mafic phase of mostly dioritic composition in the Karvalevä intrusion. The main mafic silicates in the granite phase are biotite and hornblende, in the quartz-monzonite and mafic phases also clino- and orthopyroxene are present. Resembling the other post-kinematic plutons of the CFGC, the studied intrusions are geochemically high in Al2O3, FeO and K2O, and low in MgO, CaO and Sr. One U-Pb age of 1876+6 Ma has been measured for the Löytölamminvuori intrusion, which places the intrusion at the same time frame as the other post-kinematic plutons. Geochemically the intrusions show A-type affinity and close similarities to the post-kinematic pluton Types 2 and 3, fitting best with the Type 3a, which is transitional between the two. The magmas forming Löytölamminvuori, Sorsakoski and Karvalevä were derived from partial melting of mantle derived basalts, which underwent crustal contamination by partial melts from the lower crust. Slight deviation in composition from the strictly A-type magma and the volcanic arc affinity can be explained by the crustal component. The mafic phases show more primitive geochemistry, and thus present the mantle-derived source component with less crustal assimilation in the source. The intrusions show signs of bimodal mafic-felsic magmatism, the dioritic phases in Karvalevä intrusion and one syn-plutonic dyke in Sorsakoski intrusion representing the mafic component. The mafic magmatism was cogenetic with the felsic phases, but not comagmatic, the diorites intruding simultaneously but slightly after the felsic phases. The mafic phases show a continuum in chemical composition to the granites and quartz-monzonites, but with a slight compositional gap.