In Situ-Dating of Saprolites in South East Sweden Using Rb/Sr by LA

UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre

In situ-dating of saprolites

in south east Sweden

using Rb/Sr by LA-ICP-MS

Ellinor Wessel

ISSN 1400-3821 B1047 Bachelor of Science thesis Göteborg 2019

Mailing address Address Telephone Geovetarcentrum Geovetarcentrum Geovetarcentrum 031-786 19 56 Göteborg University S 405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN Abstract

The Precambrian basement of the South Sweden constitute of three main erosion surfaces – The sub-Cambrian peneplain, the sub-Cretaceous peneplain and the South Småland peneplain, formed by episodes of burial and re-exposure of the basement during the Phanerozoicum exposing it to weathering at specific times. Episodes of deep weathering, together with slight Quarternary glacial and glaciofluvial reshaping are the major agents that has formed the relief that is today. Saprolites are remnants of these weathering-episodes and by exploring them we can obtain a better understanding of the time and environment they were formed under. In south of Sweden several sites of deep weathering have been observed and several of these are of gravelly saprolites. These gravelly saprolites have never been dated by radiometric methods but are only stratigraphically interpreted as Plio-Pleistocene of age. From two sites of these gravelly saprolites, Knasekärret and Duvedal, authigenic illite are dated by the Rb/Sr-system in this study in order to see if the stratigraphic constraints of the weathering event can be affirmed. The Rb/Sr-dating is preformed using Laser Ablation (LA)- ICP-MS, a method never before used on weathering material. Illite from the kaolinitic saprolite at Ivö, previously dated by K/Ar, are also dated using the same technique with the aim to confirm LA-ICP-MS as a usable method to date palaeosoils and to strengthen or question the certainty of previously obtained ages. From the gravelly saprolites at Duvedal and Knasekärret large spread of ages with a mean of Neoproterozoic ages were obtained suggesting an illite formation not connected to the episode of deep weathering. The results from Ivö correlate with previous studies and give a strong indication that LA-ICP-MS is a valid method to date palaeosoils and the saprolite being of Mid-Triassic age.

Content

1. Introduction – The South Swedish Dome ...... 1 1.1 Deep weathering and palaeosoils ...... 4 1.2 Illite dating ...... 6 1.3 Aim of the study ...... 6 1.4 Sites ...... 7 1.4.1 Knasekärret ...... 7 1.4.2 Duvedal ...... 8 1.4.3 Ivö ...... 8 2. Methods ...... 9 2.1 Scanning electron microscope ...... 10 2.2 LA-ICP-MS ...... 10 2.2.1 Calibration standards ...... 10 3. Results ...... 12 3.1 Knasekärret ...... 12 3.1.1 Rb/Sr-dating ...... 14 3.2 Duvedal ...... 15 3.2.1 Rb/Sr-dating ...... 17 3.3 Ivö ...... 19 3.3.1 Rb/Sr-dating ...... 21 4. Discussion ...... 24 4.1 Gravelly saprolites ...... 24 4.2 Ivö saprolite ...... 25 4.3 Rb/Sr-dating by LA-ICP-MS ...... 25 5. Conclusions ...... 26

Acknowledgements ...... 27 References ...... 28

1. Introduction – The South Swedish Dome

The Precambrian basement in south Sweden constitute of a low dome, the South Swedish Dome which consists mainly of granites and gneisses. The dome is characterised by its erosion surfaces, or, peneplains (Lidmar-Bergström, et al., 1997), formed by episodes of burial and denudation during the Phanerozoic. The three major peneplains are the inclined re- exposed sub-Cambrian peneplain, the re-exposed hilly sub-Cretaceous peneplain (or etch surface) and the epigene South Småland peneplain, a horizontal plain with residual hills (fig. 1; Lidmar-Bergström, 1996; Lidmar-Bergström et al., 2017).

fig 1: South Swedish Dome with its peneplains presented. The sub-Cambrian peneplain (blue), the sub-Cretaceaous peneplain (green) and the South Småland stepped peneplain (yellow, orange, brown and red). (Lidmar-Bergström et al., 2017)

The formation of the very flat sub-Cambrian peneplain, situated over large parts of Sweden, Norway and Finland, began sometime during the Neoproterozoic. Prior to the Cambrian transgression the peneplain was fully developed (fig. 2a) and was covered by early Palaeozoic sediments (fig. 2b) (Lidmar-Bergström 1996; Japsen et al., 2015). The Sub- Cambrian peneplain is well preserved in the eastern part of the South Swedish Dome (fig. 1b and fig 2h) and is inclined to the east. The eastern border of it is overlapped by Palaeozoic sedimentary rocks with a prominent residual hill of the sub-Cambrian peneplain, the Jungfrun island, protruding through the cover (Lidmar-Bergström, et al., 2017). Some parts of the peneplain has not been re-exposed until recent geological times while other parts of the surface were exposed from cover rocks during Late-Palaeozoic - Mesozoic time and were, as a result of the warm and humid climate during the period, subject to extensive deep- weathering (Lidmar-Bergström, 1982). One episode during Late-Perm – Late-Triassic (fig. 2c), forming the smaller Sub-Triassic peneplain and another episode of even heavier weathering during Late-Jurassic to mid Cretaceous (fig. 2e; Japsen et al., 2015). Deep weathering exploited fractures of the basement and subsequent erosion resulted in the hilly relief forming the re-exposed Sub-Cretaceous peneplain. The hilly relief along the west coast are interpreted as the re-exposed sub-Cretaceous peneplain and constitutes both the undulating hilly relief in Halland and the joint aligned valley landscape in Bohuslän, with peaks reaching close to the Sub-Cambrian peneplain (Lidmar-Bergström et. al. 2017). In connection with the Late-Cretaceous transgression the basement was buried by Late-Cretaceous-Paleogene sediments (Japsen et al., 2015). These sediments were eroded due to Miocene uplift leading to re-exposure and denudation of the basement (fig. 2g) assembling another erosion surface, the South Småland peneplain, a stepped erosion sequence, which is truncating both the hilly Sub-Cretaceous peneplain and the Sub-Cambrian peneplain (fig. 2h) (Japsen et al., 2015 and Lidmar-Bergström, et al. 1999).

fig 2: From Japsen et al., 2015 Based on AFTA (Apatite fission track analysis) A) Denudation of the basement during Late Neoproterozoic forming the sub-Cambrian peneplain.B) Palaeozoic sedimentary rocks deposited on the sub-Cambrian peneplain during Middle Triassics. C) Exhumation and destruction of the sub-Cambrian peneplain as a result of Middle Triassic uplift and erosion of Palaeozoic cover rocks. Warm and humid climate in Late Perm - Late Triassics resulted in formation of kaolinitic clays. D) The Triassic peneplain and the remaining Palaeozoic cover was overlain by Upper Triassic to Lower Jurassic sediments during Mid-Jurassic. E) During Middle Jurassic to earliest Cretaceous the final removal of the Palaeozoic cover along the south-western margin of the Baltic Shield occured, this lead to heavy kaolinitic weathering of the sub-Cambrian Peneplain and dissection of parts of the Triassic peneplain forming the hilly relief of the sub-Cretaceous peneplain. F) Upper Cretaceous to Oligocene deposits accumulated over the sub-Cretaceous peneplain and what is left of the Phanerozoic cover. G) Formation of the South Småland Peneplain occurred due to early Miocene uplift. H) Present, Exhumation of the sub-Cretaceous hilly relief along the coasts occurred as a result of early Pliocene uplift.

1.1 Deep weathering and palaeosoils

Remnants from the Late Precambrian weathering episodes forming the Sub-Cambrian peneplain are close to non-existent, yet shallow (~5 m) kaolinization of the basement at the contact to Cambrian cover rocks has been observed in South Sweden (Elvhage and Lidmar- Bergström, 1987). Deep kaolinitic weathering occurred likewise during the formation of the sub-Mesozoic hilly relief and remnants of these saprolites are abundant along the contact between the Mesozoic cover rocks and the Precambrian basement in southern Sweden (Lidmar-Bergström 1982). Illite clay mineralisation in Mesozoic saprolites at Ivö in North- Eastern Skåne have been dated with K/Ar-dating by Fredin, et al., (2017). The study determined the saprolite of Ivö as Mid- to Late-Triassic age, results that correlate well with the AFTA (Apatite fission track analysis) results that Japsen, et al., (2015) (fig 2.) obtained implying re-exposure of the basement during the period of Late-Perm to Late-Triassic. Saprolites associated to formation of the South Småland Peneplain are thought to have been either completely stripped or covered below Quaternary deposits (Lidmar-Bergström et al., 1997). Howeven Danish Palaeogene and Miocene sediments smectites are dominating the clay fractions (Lidmar-Bergström et al., 1997) indicating the character of weathering at the time (table 1).

Table 1: Common authigenic clay minerals in the South Swedish dome and the weathering condition they indicate.

In south-east Sweden several sites with remnants of deep grus (gravelly) weathering has been described (Lidmar-Bergström et al., 1997; Olvmo et al., 2005). This deep weathering differs from the Triassic-Cretaceous saprolites in grain size distribution as well as in clay mineral composition (Lidmar-Bergström et al., 1997). While the Mesozoic saprolites are dominated by kaolinitic clays, the gravelly saprolites constitute chiefly of vermiculite (Svedlund et al., 2006), however, the mineralogy varies significantly between the localities. Since the gravelly saprolites are distributed over the South Swedish Dome (fig. 3) in both the younger relief (South Småland Peneplain) and older relief they are stratigraphically

4 constrained as younger than mid-Miocene (Olvmo, et al., 2005). The sites are found below Weichselian deposits and show a more advanced weathering than the glacial sediments in the area (Lidmar-Bergström et al., 1997). These mentioned arguments are grounds for the proposal of the gravelly saprolites being of Pliocene-Pleistocene age with a presumed outset during late-Miocene (Olvmo et al., 2005; Lidmar-Bergström et al., 1997). The saprolites are rich in corestones which also affirm their relatively young age.

fig 3: Figure from Olvmo et al. (2006) with 26 sites of gravelly saprolites in south Sweden marked.

Gravelly saprolites commonly develop when the rock obtains increased permeability as a response to rapid relief differentiation. Decompression from exhumation leading to rock dilation are causing micro cracks in the rock giving it a higher porosity (Olvmo et al., 2005). This process allows the weathering to go deeper and explain the thickness of the saprolites in fracture zones of the south east Sweden (up to 15 m) while in intact zones of the basement only shallow weathering has occurred (Olvmo et al., 2005). Deep weathering of the basement and subsequent stripping by mainly glacial erosion and glaciofluvial erosion has been stated as the major agents for the present landforms. Rounded erratic boulders are a common feature of the south Swedish landscape and are in fact mostly corestones detached by glacial plucking (Lidmar-Bergström et al., 1997).

1.2 Illite dating

Dating authigenic clays has become an established method to estimate the age of diagenetic events (Fredin et al., 2017, Viola et al., 2016, Middleton et al., 2014, Gorokhov et al., 2001, Ksienzyk, et al., 2016). Illite is an authigenic 2:1 clay mineral with an interlayered large cation, most commonly Potassium (K). Incorporation of K+ is the major factor that makes illite suitable for geochronology by isotopic dating of K/Ar, Ar/Ar and Rb/Sr. Rubidium commonly substitute for Potassium in mineral structures due to its similar properties and identical charge. This makes the K-bearing illite likely to contain Rb of measurable quantities and therefore suitable for Rb/Sr-dating. Illite is formed by diagenetic processes and depending on the formation process it forms polytypes. If it is formed at low temperatures, for example during saprolisation, illite form mainly by alteration of K-feldspar and micas and preferentially crystallise as the 1M (one layer monoclinic) polytype (Fredin et al., 2017; Pevear, 1999). It is frequently abundant in mixed-layer clays with other clay minerals such as smectite and kaolinite depending on maturity of saprolization (Fredin et al., 2017; Ksienzyk et al., 2016). The saprolites of the south Swedish dome has previously been described to contain illite which would make them suitable for isotopic dating by Rb/Sr.

1.3 Aim of the study The aim of this thesis is to date illite from the gravelly saprolites of Knasekärret and Duvedal to examine if the results correlate with previous interpretations of the stratigraphic constraints. This study is performed in collaboration with Liv Edman and her study about dating of separated clay-fractions which is presented in Edman, 2018. This geochronological analysis is done by in-situ Rb/Sr-dating with LA-ICP-MS (Laser ablation inductively coupled plasma mass spectrometry), a method never before used for dating of palaeosoils. The thesis also includes dating of illite from the kaolinitic saprolites at Ivö using the same methods. This analysis does not only investigate the probability of previous geochronological studies, a correlation of ages would also confirm Rb/Sr-dating by LA-ICP-MS as an applicable method for dating palaeosurfaces. Specific questions are:

• What ages do we obtain from illites formed in the saprolites at Knasekärret and Duvedal?

• What ages do we obtain from illites formed in the saprolite at Ivö?

• What genetic processes does the ages correspond and can these be related to the geological development in the area?

• Is in-situ Rb/Sr-dating by LA-ICP-MS an applicable method for palaeosoils?

1.4 Sites The sites for this study (fig. 4) are located in the South Swedish Dome and are characteristic for their presumed periods of weathering. Knasekärret and Duvedal are two of the gravelly saprolites estimated as Pliocene-Pleistocene age by Lidmar-Bergström et al. (1997) and Ivö is, as mentioned, a site of kaolinitic weathering previously dated as Mid- to Late-Triassic by Fredin et al. (2017).

fig 4: Map of the sites for this study, Knasekärret, Duvedal and Ivö in relation to the erosion surfaces of the South Swedish Dome. Locations are approximate, see fig 1 for legend. (Olvmo, 2018)

1.4.1 Knasekärret The site Knasekärret, at Åsby udde by the lake Sommen in Småland, is situated on the 200-metre erosion surface of the South Småland Peneplain, approximately 25 metres below the sub-Cambrian peneplain (Lidmar-Bergström et al., 1997). It is an around 100-metre-long

7 gravel pit of deeply weathered coarse grained augen granite overlain by a thin layer (~0.5 metres) of glacial till. The saprolite is more than 9 metres thick and the gravel were once excavated and used mainly for house holding purposes such as dirt roads in the surrounding area (Svedlund et al., 2006). The site includes a frequent abundance of corestones in different stages of decomposition. The surrounding area has a large number of boulders of the local rock type interpreted to have been detached from the bedrock by glacial erosion (Lidmar- Bergström et al., 1997). The saprolite is described by Svedlund et al. (2006) as a porphyritic red-gray to gray-red monzogranite-quartzmonzonite of the Filipstadstype with a large amount of biotite (10-15%). Their XRD-analysis of the smaller fractions showed that the clay minerals are mainly kaolinite, illite, goethite and mixed layers of smectite and vermiculite.

1.4.2 Duvedal The site Duvedal, south of Ingatorp in the north-east of Småland, is located on the north-west slope of a large hill on the 200-metre elevation surface of the South Småland Peneplain, approximately 60 metres below the sub-Cambrian peneplain (Lidmar-Bergström et al., 1997). The site is a gravel pit of highly weathered quartzmonzodiorite overlain by a thin bed of till (Lidmar-Bergstrom et.al., 1997). The weathering of the rock is reaching a depth of more than 3 metres and is completely fragmented by the stroke of a hammer (Svedlund et al., 2007). A number of corestones have been exposed as the gravel was excavated. Svedlund et al. (2007) published a report of the site by request of SGU (Swedish Geological Survey) and executed an XRD-analysis of the clay minerals which were interpreted as mainly vermiculite but also illite, kaolinite and chlorite.

1.4.3 Ivö At the north end of Ivö Island in Lake Ivösjön, northeast Skåne, a former kaolinite quarry is located. Ivö is dominated by Cretaceous carbonate rocks with the north side of the island, Ivö klack, as an exception as it consists of Precambrian bedrock, granitic gneiss, of the sub-Cretaceous peneplain with a kaolinitic weathered surface (Surlyk and Mehlin Sörensen, 2010). The kaolinite-rich saprolite has a thickness of approximately 30 meters and is directly overlain by Early Campanian sedimentary rocks (Lidmar-Bergström et al., 1997; Fredin et al., 2017). The quarrying has exposed parts of the weathering front and core-stones are common (Lidmar-Bergström et al., 1997).

2. Methods

During two days of fieldwork the sites Knasekärret (fig. 5a), Duvedal (fig. 5b) and Ivö (fig. 5c) were visited. Samples of weathering material from three different spots at each site were collected. The samples were then brought to laboratory at the Department of Earth Sciences at the University of Gothenburg where each sample were thoroughly examined for pieces interpreted to contain clay minerals. An archived sample of weathering material from Ivö, stored at the university, were used as well. From this assorted material in-situ polished mounts were made and these were used for further analysis.

fig 5: Sites visited in this study. a) The gravel pit at Knasekärret b) The gravel pit at Duvedal c) The kaolinite quarry at Ivö klack.

2.1 Scanning electron microscope

The polished mounts were first analysed using a Hitachi S-3400N scanning electron microscope (SEM) equipped with Oxford Instruments energy dispersive system. The aim was to find areas, in open fracture surfaces and in rock fragments, containing illite. The collected material from samples interpreted to include illite were also used for separation analysis described in Liv Edmans bachelor thesis as mentioned.

2.2 LA-ICP-MS Laser Ablasion-ICP-MS analysis was performed on samples interpreted to include illite using an ESI 213NWR laser ablation system connected to an Agilent 8800QQQ ICP-MS with and ORS3 (octoplole reaction system) reaction cell installed between two quadrupoles. Laser ablation was performed in areas of the samples interpreted to contain illite with static spot mode and a constant He flow of 800 ml/min. The spot size was set to 60 μm with a frequency 2 of 10 Hz. Energy output was 85% with a fluence of 5.9 J/cm . N2 and Ar were added and reacted with the ablated material before it was transported to the mass spectrometer. To be able to calculate the 87Rb/86Sr and 87Sr/86Sr ratios Rb and Sr need to be separated. This was done by adding N2O gas since Sr reacts with the gas and changes in mass whereas Rb does not. Therefore, the isotopes become distinguishable for the mass spectrometer. The whole procedure of the N2O reaction gas is found in Hogmalm et al. (2017). Between every 10-15 sample spots, data from known standards were collected to allow quantification of the measurements. For the spots analysed, ratios of 87Rb/86Sr respectively 87Sr/86Sr were calculated from raw ratios of mass 85Rb/mass-shifted 86Sr and mass-shifted 87Sr/mass-shifted 86Sr. These were converted by correction factors derived from the standard samples.

The spot data obtained from the LA-ICP-MS analysis were processed in the computer software Glitter where the sample data were reduced and average count rates calculated. From this data isochrones were produced in a software called Isoplot by Ludwig (2008) using the Rb decay constant of 1.3972±0.0045*10-11 from Villa et al., (2015). Since within-run error calculations of the isotopic ratio were not performed and repeatability is not yet assessed only indicative ages were obtained.

2.2.1 Calibration standards Several standards were used for the LA-ICP-MS data analysis. NIST SRM 610 (Jochum et al., 2011) and BCR-2G (Elburg et al., 2005) are representative glass standards used for

87Rb/86Sr and 87Sr/86Sr ratio normalisation of the sample data and were therefore used in this study. The “La Posta”-standard is a sample of granodiorite from the so-called Small Biotite Zone of the La Posta intrusion in California (Walawender et al.,1990) with a weighted mean age of 91.1 ±5.7 used as the primary calibration (Zack and Hogmalm, 2016). A separate of phlogopite, “Mica-Mg-A2” with a mean age of 533±24 Ma, a reference mineral powder from Bekily, Madagascar, pressed to a nano-pellet tablet using the method by Garbe-Schönberg & Müller (2014). Also, a sample from Högsbo, “Hogsbo MS”, with a weighted mean age of 1025.9±8.2 Ma were used as a secondary standard. In Zack and Hogmalm (2016) and Hogmalm et al. (2017) the implementation of using NIST SRM 610 for normalisation of isotopic data from Rb/Sr obtained by in-situ LA-ICP-MS/MS is discussed.

3. Results

3.1 Knasekärret The material collected in Knasekärret (fig. 6) are of a well weathered coarse grained red-brownish granite composition. Large grains of K-feldspar are well preserved while the plagioclase and quartz have experienced a higher grade of decomposition. A small amount, <6 %, of clay mineralisation can be observed, concentrated mainly in joints. SEM-analysis of clay minerals in polished mounts (fig. 7) show a high amount of Fe and traces of Ti indicating biotite alteration. Clay mineralisation are of mainly kaolinite but a few fields interpreted to contain a mixture of clay minerals with illite included are detected in two of the samples, these samples are therefore used for LA-ICP-MS analysis. Spectrum of sample “KK3” is presented in fig. 8.

fig 6. Weathering material collected in Knasekärret. Scale in centimetres.

fig 7: Polished mounts of in-situ weathering material from Knasekärret. Scale in millimetres.

fig 8: Spectrum of in-situ sample “KK3” from Knasekärret interpreted to contain illite.

3.1.1 Rb/Sr-dating The results obtained from Rb/Sr-dating analysis of the weathering material from Knasekärret (table 2) show a wide spread of ratios making it difficult to produce a combined isochron for all three samples. None of the investigated spots obtained an 87Sr/86Sr ratio >2 needed for a reliable spot-age, although fig. 9 is presenting two isochrons with weighted mean ages of 1300±120 Ma, Mesoproterozic, and 1067±220 Ma, Neoproterozoic giving an indication of the age of the saprolisation. Results from Liv Edmans clay-fraction analysis of separated fraction-dating (Edman, 2018) of material from the same site, sample “pelletkk>9.83”, are also presented in table 2 providing more data to this study.

table 2: Results from Rb/Sr-dating of samples from Knasekärret

fig 9: Isochrons from the samples of Knasekärret.

The isochron (fig. 9) formed from the separated sample, ”pelletkk>9.83”, show a larger spread in spot-ages than the results from “KK2”. “Pelletkk>9.83” also have a larger error margin of the Sr/Sr ratio than “KK2” but provide a better fit to the isochron.

3.2 Duvedal

The samples from Duvedal (fig. 10 and fig. 11) are of a highly weathered granitic composition with a yellow-brownish colour. During SEM-analysis very distinct titanite crystals are observed. The rims of these crystals are depleted from Ca, possibly making the surrounding clay minerals being more rich in Ca. Clay constitutes <10% of the sample and the most frequent clay mineral is vermiculite making the matrix around the granitic crystals rich in Mg along with Si and Al. In two of the polished mounts areas containing illite were detected and therefore used for Rb/Sr-dating by LA-ICP-MS. The spectrum from one of them, sample “DD3”, is presented in fig. 12.

fig 10: Collected grus weathering material from Duvedal. Scale in centimetres

fig 11: In-situ polished mounts of weathering material from site Duvedal. Scale in millimetres.

fig 12: Spectrum indicating content of illite in the in-situ sample “DD3” from Duvedal.

3.2.1 Rb/Sr-dating The results from Duvedal (presented in table 4) show a relatively wide range of ratios. Results from Liv Edmans analysis of separated fraction-dating (Edman, 2018) of weathering material from the same site, sample “pelletdd>9.83” and “DD>3.93”, are also presented in table 3 providing more data to this study. None of the observed spots obtained 87Sr/86Sr-ratios >2 which is as, previously mentioned, needed for a reliable spot-age. However, fig. 13 is

17 presenting an isochron based on the results from table 3 giving an indication of the formation of illite at Duvedal to an age of 795 ± 120 Ma which is Late Proterozoic. table 3: Results of the Rb/Sr-dating of the material from site Duvedal

fig 13: Isochron based on results from the Rb/Sr-dating in table 1.

3.3 Ivö The weathering material collected at Ivö (fig. 14, fig. 15 and fig. 16) are of a highly weathered coarse-grained granite with a large amount of biotite. SEM-analysis show a relatively large content of clay (30-50%) which is determined as mainly kaolinite (>90%). Crystals of granite composition are observed in the clayey areas of the samples. A few smaller areas interpreted to contain mixtures of illite are found in two of the samples. Spectrum of sample “Ivö 1.2” is presented in fig. 17.

fig 14: Weathering material collected in Ivö. Finger for scale.

fig 15: Polished mounts of in-situ weathering material from Ivö collected for this study. Scale in millimetres

fig 16: Polished mounts of in-situ weathering material from Ivö of the archived sample.

fig 17: Spectrum from in-situ sample “Ivö 1.2” showing an area interpreted to contain illite.

3.3.1 Rb/Sr-dating In table 4 results from the Rb/Sr-dating analysis are presented. Results from Liv Edmans analysis of separated fraction-dating of weathering material (Edman, 2018), sample “pelletivö>3.93”, from the same site are also presented in table 4 providing more data to this study. The ratios obtained from the archived in-situ sample “ivöarkiv” were all >2 giving reliable spot-ages, from these ratios the isochron in fig. 18 was produced. The results suggest two populations of illite formation. One episode 256.7 ± 4.0 Ma and another episode 220.2 ± 6.1 Ma, indicating Late Permian and Mid-Triassic origin.

Table 4: Results from Rb/Sr dating analysis of the Ivö samples.

fig 18: Graph showing two isochrons based on the results from table 3 indicating two populations of illite formation.

4. Discussion

4.1 Gravelly saprolites The results obtained from Knasekärret samples show low ratios of 87Sr/86Sr which give the produced isochrons, showing Meso- and Neoproerozoic ages, low reliability. Na- and Ca content in sample “KK3” imply that mixed-layer clays occur in the sample. The results could assemble a correct age, although, sparseness of illite in the samples was confirmed during sample preparation and SEM-analysis and is also reflected in the low measurements of initial Rubidium. This indicate a high influence of protolithic material in the measured spots and increase the uncertainty of interpreting the period of illite formation.

The results obtained from Duvedal show a weighted mean age of 795 ±120 Ma. The ratios of 87Sr/86Sr are also in these samples unfavourably low and are giving uncertain spot-ages. The isochron indicate the time of illite formation as Neoproterozoic.

The ages obtained from Knasekärret and Duvedal are unlikely to reflect the event of saprolite formation. Though it is easy to think that this could mark a Proterozoic denudation and weathering event. It is known that the Svecokarelian rocks (latest ductile deformation 1.8- 1.4 Ga) was denuded to a surface of low relief onto which an at least 1000-metre thick sequence of Jotnian (1.69-1.25 Ga) sedimentary rocks were deposited (Stephens et al., 1997; Lidmar-Bergström, 1996; Rohde, 1987). During the Sveconorwegian orogeny (intrusion of the Bohus Granite 920 Ma during the final stage (Eliasson and Schöberg, 1991)) this sedimentary sequence was slightly deformed and downfaulted (Rohde, 1987). After that the basement was again denuded to a flat surface and covered by Early Cambrian sediments (Lidmar-Bergström, 1996). This was what is today known as the Sub-Cambrian Peneplain. This mean that there is a vast period of time between the end of the Sweconorwegian orogeny and formation of the Sub-Cambrian peneplain. However, there are two reasons speaking against this. Firstly, weathering residuals connected to the Sub-Cambrian peneplain are clayey and kaolin dominated (Lidmar-Bergström et al., 1997; Lidmar-Bergström et al., 1999). Secondly, since the grus weathering appear ca 60 metres below the Subcambrian peneplain and occur in valleys related to the 200-metre level of the South Småland peneplain it is more likely of post-Miocene age. An explanation of the ages found in this study would be that the illite formation probably is not connected to an episode of denudation but from a hydrothermal event. Different polytypes of illite are formed by different diagenetic processes.

In the development of this research, XRD-analysis of the weathering products could show the polytype of illite present in this saprolite and would provide more information about the type of event forming the mineral.

4.2 Ivö saprolite The results from Ivö suggest two populations of illite formation. One episode during the Late Permian, 256.7 ±4.0 Ma, and another episode during the Mid-Triassic, 220.2 ±6.1 Ma. The sample used from the archived collection, “IvöArkiv”, show a large content of Rb making it highly suitable for this method of dating and are making the obtained age more reliable compared to the results from the gravelly saprolites. The ages agree with the K/Ar- dating results of 221.3 ± 7 Ma that Fredin et al. (2017) obtained. They also correlate well with the AFTA-results performed by Japsen et al. (2015) suggesting cooling, uplift and denudation of the basement during this period. The results contradict the theory of the saprolite being of Cretaceous age based on stratigraphy relying on the fact that the saprolite is directly overlain by Late Cretaceous (Campanian) sediments (Lidmar-Bergström, 1989). Even though the age results in this study are estimates, the Triassic age of the illite at the Ivö site is now obtained by two different radiometric techniques and can therefore be considered constrained. This means that the tectonic development of this area during the Mesozoic is more well understood. AFTA-data suggests a heating event in at 175 Ma indicating a thick Late Jurassic cover before Mid-Cretaceous cooling (100 Ma) and subsequent Late-Cretaceous covering (Japsen et al. 2016). This means that the saprolite was formed in the Triassic, later on protected by a Jurassic cover, partly eroded during the Cretaceous before burial again in the Campanian.

4.3 Rb/Sr-dating by LA-ICP-MS Correlation of the Ivö dating results with previous studies are confirming Rb/Sr-dating by LA-ICP-MS as an applicable method for dating weathering material. Though it is clear from this study that high potassium bearing clay content is a requirement of high priority to be able to perform the analysis. Though, a higher clay content, such as in the Ivö samples, did cause a problem during in-situ sample preparation as the samples tended to fall apart in the epoxy mounts. Due to that the mounts needed to be polished very sparsely and carefully. Despite the difficulties during sample preparation, a more mature saprolisation obtained better results. Less developed saprolisation contain less authigenic clay minerals making the early

25 stage work of the study more important to preform thoroughly. This is since more effort needs to be put in finding measurable zones of illite mineralisation in the samples.

Dating of separated clay fractions has, by previous studies (Fredin et al., 2017; Villa et. al. 2015; Ksienzyk et. al., 2016), shown to be an effective method to date weathering clays and could diminish the risk of dating bedrock material which is impendent during this type of in- situ dating, especially in combination with low clay content. However, in this study the results from the in-situ samples provided more useful information than the separated samples. This could be due to that all the separated samples were of silt-sized fractions, 3.93 microns and 9.83 microns, and are likely to contain weathered protolithic material, such as micas. Further discussion about the clay fraction analysis is found in Edman (2018).

5. Conclusions

• Estimate ages of the kaolinitic saprolite from Ivö indicate illite formation during Late-Perm and Mid-Triassic which is consistent with previous studies.

• Ages obtained from Ivö samples are matching results from previous studies and therefore confirm Rb/Sr-dating by LA-ICP-MS as an applicable method for dating palaeosoils.

• Illite formation in the gravelly saprolites of Duvedal and Knasekärret is unlikely to be connected to the episode of grus weathering.

• Content of illite in the samples from Duvedal and Knasekärret were too low to obtain a solid result.

• The samples from Ivö contained more K-bearing illite and a higher measure Rb to Sr which made the results more reliable and suitable for this study.

• XRD – analysis to obtain more information of the illite properties and formation would bring more certainties to this study.

Acknowledgements

I would first and foremost like to thank my friend and co-worker Liv Edman for another well working cooperation. Many hours have been spent together and in each of them we complemented each other well.

Secondly I would like to thank my supervisors: Mikael Tillberg, for many occasions of practical and theoretical guidance, for his patience and ability to be reached whenever his expertise is needed, and Mats Olvmo for sharing his inspiration, knowledge and experience of the geomorphological subject and his invaluable guidance during interpretation of the geological context from the results and during the writing process of this thesis.

I would also like to thank Thomas Zack for inputs to the methods, cheerful and helpful feedback and for examining this project.

Tanks to everyone who has, in any way, helped and supported me and Liv during our projects!

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