Illite K-Ar Dating of Fault Breccia Samples from ONKALO … · 2008-12-12 · ILLITE K-Ar DATING OF FAULT BRECCIA SAMPLES FROM ONKALO UNDER-GROUND RESEARCH FACILITY, OLKILUOTO, EURAJOKI,

P O S I V A O Y

FI -27160 OLKILUOTO, F INLAND

Tel +358-2-8372 31

Fax +358-2-8372 3709

I rme l i Mänt tä r i

Juss i Matt i l a

Hors t Zwingmann

Andrew J . Todd

August 2007

Work ing Repor t 2007 -67

Illite K-Ar Dating of Fault Breccia Samplesfrom ONKALO Underground Research Facility,

Olkiluoto, Eurajoki, SW Finland

August 2007

Working Reports contain information on work in progress

or pending completion.

Work ing Report 2007 -67

Illite K-Ar Dating of Fault Breccia Samples

from ONKALO Underground Research Facility,Olkiluoto, Eurajoki, SW Finland

I rme l i Mänttär i

Geo log ica l Su rvey o f F in l and

Juss i Matt i l a

Pos iva Oy

Horst Zwingmann , Andrew J . Todd

CSIRO Pe t ro l eum, Ben t l ey , Aus t ra l i a

ILLITE K-Ar DATING OF FAULT BRECCIA SAMPLES FROM ONKALO UNDER-GROUND RESEARCH FACILITY, OLKILUOTO, EURAJOKI, SW FINLAND

ABSTRACT

Illite K-Ar age determinations were done on five fault breccia samples from the ONKALO un-derground research facility, Olkiluoto, Eurajoki, S-W Finland. The XRD, SEM, and TEM stud-ies and K-Ar analyses were done in John deLaeter Center in Mass Spectrometry at Curtin Uni-versity, Perth, Western Australia.

The <2 micron grain size fractions contain illite, chlorite, dickite, and quartz. All fractions had minor contamination phases comprising mainly of quartz but traces of K-feldspar contamination could be identified in all samples. The authigenic illite shows variable K concentrations. The illite contents of the ONK-PL68 and ONK-PL87 samples are the smallest.

The K-Ar ages for the <2 micron fractions vary from ~0.55 Ga to 1.38 Ga. The sample ONK-PL68 yields a K-Ar age of 912 ± 18 Ma corresponding to a Neoproterozoic-Tonian age. This age can be roughly temporally linked with late events related to Sveconorwegian orogeny. Sam-ple ONK-PL87 has a K-Ar age of 550 ± 11 Ma corresponding to a Neoproterozoic – Lower Cambrian age. The samples ONK-PL522 and ONK-PL901 sampled from the storage hall fault show identical K-Ar ages of 1385 ± 27 Ma and 1373 ± 27 Ma, respectively. These correspond to a Mesoproterozoic-Ectasian age related to Subjotnian or Postjotnian events. ONK-PL960 yields a K-Ar age of 1225 ± 24 Ma corresponding to a Mesoproterozoic-Ectasian age. This age agrees well with the ages from Postjotnian diabase dykes in W Finland.

The 2-3 % detrital K-feldspar contamination in clay fractions increases the age. Especially for the youngest sample ONK-PL87, the effect may be geologically meaningful as after the correc-tion the age clearly indicates Caledonian events. Moreover, the age for the low K sample ONK-PL901 shifts to indicate Postjotnian diabase age.

Keywords: fault breccia, clay minerals, K-Ar age, Olkiluoto, Finland

EURAJOEN OLKILUODON ONKALON SIIRROSBREKSIASAVIEN ILLIITTIEN K-Ar IÄT

TIIVISTELMÄ

Eurajoen Olkiluodon ONKALON siirrosbreksioihin liittyvistä autigeenisistä illiiteistä on tehty viisi K-Ar –ikämääritystä. Savifraktioiden XRD, SEM ja TEM tutkimukset ja K-Ar –ikämääritykset tehtiin Länsi-Australian Perthissä Curtinin yliopistossa (John de-Laeter Center in Mass Spectrometry).

Näytteiden alle 2 mikronin raekokofraktiot koostuvat pääosin illiitistä, kloriitista, dickiitistä ja kvartsista. Kontaminaatiofaasina kaikissa fraktioissa on lähinnä vähäisiä määriä kvartsia mutta niistä löytyi myös pienen pieniä määriä detritaalista K-maasälpää. Autigeenisen illiitin K pitoisuudet vaihtelevat näytteiden välillä. Näytteet ONK-PL68 ja ONK-PL87 sisältävät vain vähän autigeenistä illiittiä.

Saadut illiittien K-Ar –iät <2 mikronin raekokofraktioista vaihtelevat ~0.55 Ga ja 1.38 Ga välillä. Näyte ONK-PL68 antaa Neoproterotsooisen 912 ± 18 Ma iän. Tämä ikä voidaan mahdollisesti ajallisesti linkittää Svekonorjalaisen orogeniaan liittyviin myöhäisiin tapahtumiin. Näytteen ONK-PL87 K-Ar ikä on 550 ± 11 Ma ja asettuu siten Neoproterotsooisen ajan ja Kambrikauden rajalle. Varastohallin siirrosvyöhykkeen näytteiden ONK-PL522 and ONK-PL901 savifraktioiden K-Ar -iät ovat samat eli 1385 ± 27 Ma and 1373 ± 27 Ma. Nämä Mesoproterotsooiset iät liittynevät Subjotunisiin tai Postjotunisiin tapahtumiin. Mesoproterotsooinen 1225 ± 24 Ma ikä näytteestä ONK-PL960 on sopupoinnussta Länsi-Suomen postjotunisten diabaasijuonien ikien kanssa.

Savifraktiossa kontaminaationa oleva detritaalinen kalimaasälpä vanhentaa saatuja ikiä. Laskemalla mahdollisen 2-3% kontaminaation vaikutus, nuorimman näytteen ONK-PL87 ikä asettuu selkeästi Kaledonisin vuorijonopoimutuksen ikähaarukkaan. Geologisesti merkittävä vaikutus ikään nähdään myös alhaisen K–pitoisuuden näytteessä ONK-PL901. Korjattu ikä vastaa tässä tapauksessa postjotunisten diabaasien ikää.

Asiasanat: siirrosbreksia, savimineraali, K-Ar ikä, Olkiluoto, Suomi

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TABLE OF CONTENTS

ABSTRACT

TIIVISTELMÄ

1 INTRODUCTION...................................................................................................... 2

2 SAMPLES FOR K-Ar DATING................................................................................. 3

2.1 ONK-PL68 / ONK-BFI-6560-6575 ..................................................................... 3 2.2 ONK-PL87 / ONK-BFI-7000-7190 ..................................................................... 3 2.3 Samples from the storage hall fault ................................................................... 5

2.3.1 ONK-PL522 / ONK-BFI-52150-52300......................................................... 5 2.3.2 ONK-PL901 / ONK-BFI-90020-90640......................................................... 5

2.4 ONK-PL960 / ONK-BFI-93190-96300 ............................................................... 8

3 THEORY AND METHODS....................................................................................... 9

3.1 K-Ar dating......................................................................................................... 9 3.2 Sample preparation ........................................................................................... 9 3.3 SEM, XRD, and TEM......................................................................................... 9 3.4 K-Ar analytical procedures .............................................................................. 10

4 RESULTS OF MINERALOGICAL STUDIES ......................................................... 11

4.1 SEM................................................................................................................. 11 4.1.1 ONK-PL68................................................................................................. 11 4.1.2 ONK-PL87................................................................................................. 13 4.1.3 ONK-PL522............................................................................................... 15 4.1.4 ONK-PL901............................................................................................... 17 4.1.5 ONK-PL960............................................................................................... 19

4.2 XRD ................................................................................................................. 21 4.3 TEM ................................................................................................................. 24

4.3.1 ONK-PL68................................................................................................. 24 4.3.2 ONK-PL87................................................................................................. 24 4.3.3 ONK-PL522............................................................................................... 24 4.3.4 ONK-PL901............................................................................................... 24 4.3.5 ONK-PL960............................................................................................... 24

5 K-Ar AGE RESULTS.............................................................................................. 30

6 SUMMARY AND DISCUSSION............................................................................. 33

ACKNOWLEDEGEMENTS .......................................................................................... 36

REFERENCES ............................................................................................................. 37

2

1 INTRODUCTION

Near-surface deformation is accommodated by brittle faults. Displacement on these dis-crete fault planes often results in the development of fault gouge, composed of crushed rock fragments and authigenic clay minerals, in particular illite or sericite, formed by retrograde hydration reactions. Early studies by Lyons and Snellenberg (1971) high-lighted the potential for determining the absolute timing of brittle fault history using iso-topic dating techniques and this has been confirmed and applied in subsequent studies using K-Ar (e.g. Vrolijk and van der Pluijm, 1999; Choo and Chang, 2000), 40Ar-39Ar (van der Pluijm et al., 2001) and Rb-Sr (Kralik et al., 1987). Dating of fault gouges has important implications for general studies. Fault gouge zones are not only the most visi-ble record of near surface brittle deformation, they also exert an important control on fluid flow and fluid-rock interaction. Thus, such zones can either focus fluid flow or act as permeability barriers, restricting fluid flow and acting as a seal. The understanding of fault processes and the timing and extent of clay-rich fault gouge formation is important also in assessing the suitability of sites for waste storage including nuclear waste (Zwingmann and Mancktelow, 2004).

Authigenic illite or sericite formed during faulting contains potassium and is therefore suitable for age determination using the potassium–argon (K/Ar) geochronometer. Comprehensive summaries concerning illite K-Ar dating can be found in Clauer and Chaudhuri (1995), Hamilton et al. (1989), Worden and Morad (2003), and Meunier and Velde (2004).

However, radiometric dating of shallow crustal faulting in brittle regime is a challenging task, as these low temperatures do not favour syntectonic recrystallization of all the material. Consequently, the radiogenic isotope systematics of fault rocks are complex due to the intimate mixture of minerals of different origins such as protoliths, detrital phases as well as authigenic minerals, often making it problematic to provide unambiguous ages. Special sample preparation techniques involving freeze thaw disaggregation to avoid overcrushing and extensive size separation to reduce the amount of detrital phases can address these issues (Liewig et al., 1987). Progressive size reduction increases the proportion of authigenic clay phases in the clay component, minimizes contamination and provides the most reliable isotopic ages for authigenic clay minerals. The validity and importance of the assumptions involved in K/Ar dating of authigenic illite (e.g. contamination, closed system behavior, excess Ar) must be carefully addressed and the sample material characterized using a wide range of tools comprising XRD, SEM, particle granulometry and TEM.

The purpose of this report is to present new K-Ar age data for five fault breccia clay samples form the Olkiluoto ONKALO underground research facility. In addition to these, brief descriptions of the samples are given. Low temperature fracture minerals from Olkiluoto area have previously been described by Gehör et al. (2001).

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2 SAMPLES FOR K-Ar DATING

2.1 ONK-PL68 / ONK-BFI-6560-6575

Sample ONK-PL68 was taken from a subvertical fault zone located approximately in the chainage 65 meters of the ONKALO tunnel (Fig. 2-1). The zone consists of multiple undulating slickensides with a mean dip/dip direction of 87/271. The slickensides are greenish in colour, having chlorite- and graphite-bearing surfaces. Slip-lineations and sense-of-movement indicators could not be observed within the exposed part of the zone. The zone has a clearly definable core with a width of approximately 10 to 15 cm. The core consists of fault breccia, containing more than 30 percent of crushed rock pieces; the pieces are 0.3-3 cm:s in diameter. The core contains also clay or silt fraction, which is mainly greenish in colour. The mineralogy of this gouge fraction could not be identified with naked eye. The fractures within the fault plane have also oxidised, hematite-bearing surfaces.

The fault crosscuts the pervasive foliation and continues across the whole tunnel perimeter, having a visible trace length of approximately 15 meters; the width of the zone varies from 10 to 80 cm. Within the zone, splaying and the occurrence of “lenses” can be observed. The width of these lenses is up to 80 cm.

2.2 ONK-PL87 / ONK-BFI-7000-7190

Sample ONK-PL87 was taken from a low-angle fault zone located approximately at the chainage 70 meters of the ONKALO tunnel (Fig. 2-2). The zone can be traced across the whole tunnel perimeter, having a trace length of more than 40 meters. The fault zone has a mean dip/dip direction of 10/151. The width of the zone is approximately 2 meters. The fault has partly a semi-brittle character as the foliation near the fault shows well-developed deflection, indicating that the hanging wall of the fault has moved towards west (reverse fault). The fault zone crosscuts the pervasive foliation. The fault has a well-developed core with a width of 10-30 cm and which contains fault breccia (brecciated pieces being 0.1-30 mm in diameter) and greenish clay.

The fault has also a “rim” of quite homogenous K-feldspar porphyric tonalite/ granodiorite; the K-feldspar phenocrysts are 5-50 mm in diameter and are both eu- to subhedral. The width of this K-feldspar porhyritic zone is 20 cm:s to 2 meters in the hanging wall and approximately 1 meter in the footwall. The zone has gradational contacts to veined gneiss, which is the main rock type of the fault area. Small-scale F3-folding can be observed in the vicinity of the fault.

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Figure 2-1. Sampling site of the sample ONK-PL68. Brittle fault zone intersection at

chainage 65 meters, ONKALO access tunnel.

5

Figure 2-2. Sampling site of the sample ONK-PL87. Brittle fault zone intersection on

left wall of theONKALO access tunnel at chainage 70 meters.

2.3 Samples from the storage hall fault

Samples ONK-PL522 (Fig. 2-3) and ONK-PL901 (Fig. 2-4) were taken from the “storage hall fault” which has been met at the ground surface in investigation trench OL-TK11 (Mattila, in prep) and in five different parts of the ONKALO tunnel (of which two were sampled). The zone is a sinistral strike-slip fault, indicated by slip lineations and sense-of-movement criterion such as tension veins, stepped slickensides and the geometry of Riedel fractures.

2.3.1 ONK-PL522 / ONK-BFI-52150-52300

The fault zone ONK-BFI-52150-52300 is located approximately at the chainage 522 m of the ONKALO tunnel and is composed of approximately ten slickensides and has a 5-25 cm wide core consisting of fault breccia (containing crushed rock pieces ~4 cm in diameter), and lenses of the surrounding rock that are 5-35 cm wide (Fig. 2-3). The core also contains unidentified clay minerals that are greyish black in colour. The slickensides in the zone have calcite and pyrite fillings and have a mean dip/dip direction of 89/284°. The slip direction measured from these slickensides show the plunge/trend of 14/012°.

2.3.2 ONK-PL901 / ONK-BFI-90020-90640

The fault zone ONK-PL901 is located approximately at the chainage 901 m of the ONKALO tunnel and consists of two subvertical main faults, located approximately ~5 m from each other and which crosscut the whole tunnel perimeter, and of many smaller vertical slickensides that combine with the main faults (Fig. 2-4). The faults have 0,5-4 cm thick fillings of pyrite, calcite, clay, graphite, and chlorite. The rock is very densely fractured between the main faults and many of the observed fractures are parallel to the foliation; in addition, randomly orientated fractures are also present.

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Figure 2-3. Sampling site of sample ONK-PL522, storage hall fault.

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Figure 2-4. Sampling site of sample ONK-PL901, storage hall fault.

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2.4 ONK-PL960 / ONK-BFI-93190-96300

Sample ONK-PL960 (Fig. 2-5) was taken from a low-angle fault zone coinciding with a high-grade ductile shear zone (ONK-HGI-93190-96300) located approximately at the chainage 960 m of the ONKALO tunnel. The fault zone contains abundant healed, foliation parallel fractures with sulphide fillings. The zone also contains many long fractures and slickensides parallel to the foliation that crosscut through the whole tinneltunnel. The main fault plane is undulating, slickensided and consists of a 3-40 cm thick core of chlorite, clay, pyrite, calcite, graphite, and kaolinite and in the thickest parts of this zone the core is heavily brecciated.

Figure 2-5. Sampling site of sample ONK-PL960.

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3 THEORY AND METHODS

3.1 K-Ar dating

Detailed accounts of the conventional K-Ar dating technique have been given by Dalrymple and Lanphere (1969), Faure (1986) and Dickin (1995). Briefly, the ages of rocks or minerals can be determined by measuring the amount of the argon isotope, 40Ar, in the rocks or in individual minerals relative to the potassium content. 40Ar is produced by the decay of an isotope of potassium, 40K. The rocks generally contain negligible amounts of argon when they are formed, although small amounts of atmospheric argon may adhere to the samples, which can be corrected for, using the atmospheric 40Ar/36Ar ratio of 295.5. Thus, by measuring the ratio of 40Ar to 40K, and knowing the decay rate of 40K, it is possible to calculate the time since the rock or mineral formed.

3.2 Sample preparation

All samples for illite dating were crushed into chips by hammer and then gently disaggregated by using a repetitive freezing and thawing technique to avoid artificial reduction of rock components and to minimize contamination of finer size fractions with K-bearing minerals such as K-feldspar (Liewig et al., 1987). Grain size fractions <2 micron were separated in distilled water according to Stoke’s law and the efficiency of this separation was monitored by a laser particle sizer. Sample identification details and weights are listed in Table 1.

3.3 SEM, XRD, and TEM

For SEM studies, freshly broken rock surfaces of rock chips were Carbon coated and examined in mixed secondary and backscattered electron mode using a Philips 300 SEM equipped with an energy dispersive system X-ray analyzer (EDS).

The mineralogy of the <2 micron size fractions was determined by X-ray diffraction (XRD) on air-dried samples using a Philips EPD 1700 instrument. Trace interpretation was done by internal CSIRO software.

A JEOL JEM 2010 200KV TEM was used for a detailed grain-by-grain characterization One drop of clay solution was loaded on a grid film and dried under air.

Table 1. Fault breccia sample identification details and weights.

CSIRO ID Sample ID Sample material [g]872 ONK-PL68 125873 ONK-PL87 120874 ONK-PL522 125875 ONK-PL901 150876 ONK-PL960 125

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3.4 K-Ar analytical procedures

Potassium content was determined in duplicate by atomic absorption (Varian Spectra AA 50) using Cs at 1000 ppm concentration for ionisation suppression. Due to low sample amount it was only possible to analyze one sample aliquot for K concentration for each sample. Approximately 100 mg of sample material were dissolved with HF and HNO3 (see for example Heinrichs and Herrmann, 1990). The samples once in solution were diluted to 0.3 to 1.5 ppm K for the atomic absorption analysis. The pooled error of duplicate K determination of all samples and standards is ca. 2 %. The K blank was measured at 0.33 ppm.

Ar isotopic determinations were performed using a procedure similar to that described by Bonhomme et al. (1975). Samples were pre-heated under vacuum at 80°C for several hours to reduce the amount of atmospheric Ar adsorbed onto the mineral surfaces during sample handling. Argon was extracted from the separated mineral fractions by fusing samples within a vacuum line serviced by an on-line 38Ar spike pipette. The isotopic composition of the spiked Ar was measured with a high sensitivity on-line VG3600 mass spectrometer. The 38Ar spike was calibrated against international standard biotite GA1550 (McDougall and Roksandic, 1974). After fusion of the sample in a low blank Heine resistance furnace, the released gases were subjected to a two-stage purification procedure with a CuO getter for the first step, two Ti getters and a SORB-AC getter for the second step. Blanks for the extraction line and mass spectrometer were systematically determined and the mass discrimination factor was determined periodically by airshots. Ca. 10 mg of sample material was required for Argon analyse. During the course of the study two international standards (1 HD-B1 and 1 GLO) and two airshots were analysed. The results are summarised in Table 2. The error for Argon analyses is below 1.00 % and the 40Ar/36Ar value of the airshots yields 296.59 ± 0.20. The K-Ar ages were calculated using 40K abundance and decay constants recommended by Steiger and Jäger (1977). The age uncertainties take into account the errors during sample weighing, 38Ar/36Ar and 40Ar/38Ar measurements and K analysis. K-Ar age errors are within 2 sigma uncertainty.

The XRD, SEM, and TEM studies and K-Ar analyses were done by H. Zwingmann and A.J. Todd in John deLaeter Center in Mass Spectrometry at Curtin University, Perth, Western Australia.

Table 2. K-Ar dating standard, and air-shot data. International standard references

HD-B1 (Hess and Lippolt, 1994) and GLO (Odin et al., 1982). Recommended 40

Ar/36

Ar

value is 294.5 (Steiger and Jäger, 1977).

CSIRO ID K Rad. 40Ar Rad. 40Ar Age Error % difference from

standard [%] [mol/g] [%] [Ma] [Ma]recommendedreference age

HD-B1-70 7.96 3.3749E-10 92.87 24.29 0.36 +0.33 GLO-97 6.55 1.1067E-09 94.52 94.88 1.44 -0.16

Airshot ID 40Ar/36Ar +/-

AS66-AirS-2 296.64 0.23 AS66-AirS-3 296.54 0.17

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4 RESULTS OF MINERALOGICAL STUDIES

4.1 SEM

SEM images of whole-rock chips from samples ONK-PL68, ONK-PL87, ONK-PL522, ONK-PL901, and ONK-PL960 are illustrated in Figures 4-1, 4-3, 4-5, 4-7, and 4-9, respectively. Moreover, representative EDS spectra of each sample are illustrated on Figures 4-2, 4-4, 4-6, 4-8, and 4-10. SEM images indicate abundant occurrence of authigenic illite for all samples. The EDS spectra show the high Si and Al content of the analyzed authigenic illite, and variable K content.

4.1.1 ONK-PL68

SEM images of sample ONK-PL68 are illustrated in Fig. 4-1. Image A illustrates the typical sample surface within a 50-micron overview. Large clusters of synkinematic minerals comprising illite can be identified. The images B to D show orientated illite anastomosing around angular fragments. Illite/sericite was confirmed by EDS analysis (see Fig. 4-2). Authigenic mineral components are platy and fibrous illite material. Authigenic illite fibres extent from platy particles (D) and can extend up to 10 microns.

Figure 4-1. SEM images of sample ONK-PL68.

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Figure 4-2. EDS spectra of sample ONK-PL68.

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4.1.2 ONK-PL87

SEM images of sample ONK-PL87 are illustrated in Fig. 4-3. Image A illustrates the typical sample surface within a 50-micron overview. Large clusters of synkinematic minerals comprising illite can be identified. The images B to D show orientated illite anastomosing around angular fragments. This sample is less compacted and brittle. Illite was confirmed by EDS analysis (see Fig. 4-4). Authigenic mineral components consist of platy and fibrous illite material. Authigenic illite fibres extent from platy particles (image D) and can extend up to several microns. Illite clusters are characterized by a dense meshwork of platy and mainly fine grained authigenic fibrous clay material in the 2 micron range.


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15

4.1.3 ONK-PL522

SEM images of sample ONK-PL522 are illustrated in Fig. 4-5. Image A illustrates the typical sample surface within a 50-micron overview. Large clusters of synkinematic minerals comprising illite can be identified. The images B to D showing orientated illite anastomosing around angular fragments. This sample is highly compacted and less brit-tle compared to samples ONK-PL 68 and ONK-PL87. Illite was confirmed by EDS analysis (see Fig. 4-6). Authigenic mineral components are mainly of platy origin and only minor amounts of fibrous illite material could be identified.


16


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4.1.4 ONK-PL901

This sample looks similar to sample ONK-PL522. SEM images of the sample are illus-trated in Fig. 4-7. Image A illustrates the typical sample surface within a 50-micron overview. Large clusters of synkinematic minerals comprising illite can be identified. The images B to D show orientated illite anastomosing around angular fragments. This sample is again strongly compacted and less brittle compared to samples ONK-PL 68 and 87. Illite was confirmed by EDS analysis (see Fig. 4-8). Authigenic mineral compo-nents are mainly of platy origin and only minor amounts of fibrous illite material could be identified.


18


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4.1.5 ONK-PL960

This sample looks similar to sample ONK-PL 87. SEM images of the sample are illus-trated in Fig. 4-9. Image A illustrates the typical sample surface within a 50-micron overview. Large clusters of synkinematic minerals comprising illite can be identified. The images B to D showing orientated illite anastomosing around angular fragments This sample is less compacted and brittle. Illite was confirmed by EDS analysis (see Fig. 4-10). Authigenic illite fibres extent from platy particles (image D) and can extend up to several microns. Illite clusters are characterized by a dense meshwork of platy and mainly fine grained authigenic fibrous clay material in the 2 micron range.


20


21

4.2 XRD

The XRD results of the <2 micron grain size fractions are summarized on figures 4-11 to 4-15. XRD analyses indicate illite, chlorite, dickite and quartz as the major mineral phases. Samples ONK-PL68 and ONK-PL87 contain only low amounts of illite. Traces of K-feldspar contamination could be identified in all samples. Except for sample ONK-PL68 all < 2 micron fractions of the investigated samples contain a variable amount of montmorillonite.

Figure 4-11. XRD spectrum of <2 micron grain size fraction, sample ONK-PL68.

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Figure 4-15. XRD spectra of <2 micron grain size fraction, sample ONK-PL960.

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4.3 TEM

TEM was used for detailed grain-by-grain characterization of clay mineral particles and corresponding morphologies as well as to monitor of the grain-size distribution within the fractions. Mineralogy of individual particles was investigated by an attached EDS system. Figures 4-16 to 4-20 illustrate typical clay morphologies in the separated clay fractions. The fractions illustrate aggregates of clay mineral particles. All fractions had minor contamination phases comprising mainly of quartz. Examples of semi-quantitative TEM-EDS spectra for typical sample grain analyses are included in each figure.

4.3.1 ONK-PL68

TEM images of the < 2 micron clay fraction are summarized on Fig. 4-16. This fraction contains mainly well-crystallized authigenic platy and fibrous illite/sericite material with elongated, sharp edges and minor quartz contamination. Electron-dense dark parti-cles were identified by EDS as Fe-rich accessory minerals.

4.3.2 ONK-PL87

TEM images of the < 2 micron clay fraction are summarized on Fig. 4-17. The fraction contains mainly well-crystallized authigenic fibrous illite material. The particles are ar-ranged in clusters and illustrate euhedral, elongated, sharp edges as morphological fea-tures, which are typical for an in-situ authigenic origin. The fraction contains minor quartz amounts. Some dark electron-dense particles were identified by EDS as Fe-rich accessory minerals.

4.3.3 ONK-PL522

TEM images of the < 2 micron clay fraction are summarized on Fig. 4-18. This fraction contains mainly well-crystallized authigenic platy and fibrous illite/sericite material with elongated, sharp edges and minor quartz contamination. The particles are arranged in clusters and K concentration is low. Electron-dense dark particles were identified by EDS as Fe-rich accessory minerals.

4.3.4 ONK-PL901

TEM images the < 2 micron clay fraction are summarized on Fig. 4-19. This fraction contains well-crystallized authigenic fibrous and platy illite and less chlorite clay mate-rial. The particles are arranged in clusters and illustrate euhedral, elongated, sharp edges as morphological features. The fraction contains minor quartz amounts and some dark electron-dense particles, which are Fe-rich accessory minerals as identified by EDS.

4.3.5 ONK-PL960

TEM images of the < 2 micron clay fraction are summarized on Fig. 4-20. The fraction contains mainly well-crystallized authigenic fibrous illite material. The particles are ar-ranged in clusters and illustrate euhedral, elongated, sharp edges as morphological fea-tures, which are typical for an in-situ authigenic origin. The fraction contains minor

25

quartz amounts. Some dark electron-dense particles were identified by EDS as Fe-rich accessory minerals.

Figure 4-16. TEM images and EDS spectra of sample ONK-PL68.

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5 K-Ar AGE RESULTS

The obtained <2 micron ages range from 550 ± 11 Ma for sample ONK-PL 87 to 1385 ± 27 Ma for sample ONK-PL 522. The K-Ar dating results are summarised in Table 3 and Figure 5-1. Figure 5-2 illustrates an age vs. K concentration plot and figure 5-3 summarizes a K concentration vs. radiogenic 40Ar concentration plot. Radiogenic 40Ar is ranging from 87.27 to 99.30% indicating negligible atmospheric Ar contamination and reliable analytical conditions for all analyses. K content is ranging from a low 1.55 for sample ONK-PL522 to 3.96 % for sample ONK-PL87 for the obtained <2 micron frac-tions.

Table 3. Illite K-Ar dating results of the fault breccia samples, ONKALO underground

research facility, Olkiluoto.

Sample CSIRO K Rad. 40

Ar Rad. 40

Ar Age Error

ID ID [%] [mol/g] [%] [Ma] [Ma]

*)Era-Period (Precambrian)

*)Period-Epoch (Phanerozoic)

ONK-PL68 872 <2µm 3.89 8.004E-09 96.60 912 18 Neoproterozoic-Tonian

ONK-PL87 873 <2µm 3.96 4.4144E-09 87.27 550 11 Neoproterozoic - Lower Cambrian

ONK-PL522 874 <2µm 1.55 5.6003E-09 99.30 1385 27 Mesoproterozoic-Ectasian



*) Geologic Time Scale 2004 (Gradstein et al., 2004)

Figure 5-1. Diagram showing the illite K-Ar dating results of the fault breccia samples, ONKALO underground research facility,

Olkiluoto.

500

600

700

800

900

1000

1100

1200

1300

1400

1500

ONK-PL68 ONK-PL87 ONK-PL522 ONK-PL901 ONK-PL960

Sample ID [<2 micron fraction]

Ag

e [

Ma

]

Neoproterozoic -Sveconorvegian?

Neoproterozoic - lower Cambrian

Mesoproterozoic -Subjotnian/Postjotnian?

Mesoproterozoic -Postjotnian

31

32

Figure 5-2. K concentration vs. age plot.

Figure 5-3. K concentration vs. radiogenic 40

Ar (mol/g) plot.

500

600

700

800

900

1000

1100

1200

1300

1400

1500

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00

K conc [%]

ONK-PL522 ONK-PL901

ONK-PL960

ONK-PL68

ONK-PL87

0.0000E+00

2.0000E-09

4.0000E-09

6.0000E-09

8.0000E-09

1.0000E-08

1.2000E-08

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00

K conc [%]

ONK-PL87

ONK-PL901

ONK-PL960

ONK-PL522

ONK-PL68

33

6 SUMMARY AND DISCUSSION

In this study, only < 2 micron clay fractions were dated as an relatively fast and non-expensive overview. The <2 micron fractions of samples from ONKALO underground research facility, Olkiluoto, Eurajoki, SW Finland yield ages between 550 Ma to 1.4 Ga. These ages should only be taken as age approximates because possible detrital phase contamination (too old ages), recrystallization (younger ages), open system behavior (meaningless younger ages/mixed), and/or excess Ar (too old ages) may have biased the ages. Most reliable K-Ar/Ar-Ar isotopic ages for authigenic minerals can be reached by progressive size reduction down to submicron (<0.2 micron) size fractions as it in-creases the proportion of authigenic phases and minimizes contamination in the clay component. Furthermore, 40Ar/39Ar dating of fine-grained illite fractions using micro-encapsulation to address 39Ar recoil can be applied to work with smaller sample sizes.

Sample ONK-PL68 yields a K-Ar age of 912 ± 18 Ma corresponding to a Neoprotero-zoic-Tonian age (Geologic Time Scale 2004 by Gradstein et al., 2004). This may be temporally linked with the speculative Sveconorwegian foreland basin stage at ~950 Ma (Tullborg et al., 1996; Larson et al., 1999) and the beginning of the Neoproterozoic ex-humation stage at ~900-600 Ma (Kohonen and Rämö, 2005). Magmatism approxi-mately coinciding with the Sveconorwegian orogeny ~1.2-0.9 Ga (Johansson et al., 1991) include, for example the ca. 1.0 Ga Laanila-Ristijärvi diabase dykes in northern Finland as well as extensive dolerite dike swarms in Kautokeino (Mertanen et al., 1996), southern Sweden (Pisarevsky and Bylund, 1998), and southern Norway (Stearn and Piper, 1984).

Sample ONK-PL87 has a K-Ar age of 550 ± 11 Ma corresponding to a Neoproterozoic – Lower Cambrian age. According to Kohonen and Rämö (2004), a platform sedimenta-tion stage existed in Finland at 600-420 Ma, the younger end coinciding temporally with Caledonian events. In Finland, the previous age data reveal uraninite mobilation ages of ca. 0.45 Ga from East-Uusimaa (Vaasjoki et al., 2002), kimberlite magmatism at ~0.6 Ga in eastern Finland (O’Brien et al., 2005), and Phanerozoic low-temperature fluorite-calcite-galena dykes occurring along the margins of the Fennoscandian shield at ~0.42 Ga (Alm et al., 2005). The U-Pb age of Halti-Ridnishokka intrusion from the Finnish Caledonian has an age of 0.44 Ga (Vaasjoki and Sipilä, 2001).

ONK-PL522 and ONK-PL901 sampled from the storage hall fault show identical K-Ar ages of 1385 ± 27 Ma and 1373 ± 27 Ma, respectively. These correspond to a Mesopro-terozoic-Ectasian age (Geologic Time Scale 2004 by Gradstein et al., 2004) related to Subjotnian or Postjotnian events. Ages closest to these are the Postjotnian diabase dykes ca. 1270 Ma (Suominen, 1991) and Mesoproterozoic age of 1.46 Ga for the CFB mag-matism on the Valaam Island (Rämö et al., 2005).

ONK-PL960 yields a K-Ar age of 1225 ± 24 Ma corresponding to a Mesoproterozoic-Ectasian age. This age agrees quite well with the ages from Postjotnian diabase dykes in W Finland (Suominen, 1991). These dykes may manifest initial extension related to the beginning of the Sveconorwegian orogeny (Rämö, 1990; Kohonen and Rämö, 2005).

34

The effect of detrital K feldspar and other K-bearing minerals (micas) contamination is one major problem with clay fraction K-Ar ages. The contamination is a serious prob-lem especially if samples are geologically young and are contaminated by Precambrian K-feldspars. However, the closure temperature for radiogenic argon of detrital K-feldspars can be as low as ca. 150 C (see McDougall and Harrison, 1999, p. 22; Foland, 1974; Harrison and McDougall, 1982).

From the ONKALO fault breccia samples, the XRD investigations indicate traces of K-felspar, but no major K-feldspar contamination was confirmed by TEM investigations within the clay fractions. The K-feldspar traces are noisy and are below 2-3 %. Horst Zwingmann calculated the effect of the pure K-feldspar contamination on the obtained clay ages at CSIRO. The data assume that the obtained <2 micron K-Ar age has con-taminated by 1, 2 or 3 weight % of a K-feldspar with 10 % K concentration for a poten-tial 1880 Ma and 1600 Ma event (Figs. 6-1 and 6-2). One % K-feldspar contamination yields ages identical within error and reducing 2 and 3 % contamination phase would yield younger ages. Geologically meaningful effect of the K-feldspar contamination is evident on the youngest (550 Ma) illite sample ONK-PL87 as the age after correction would be clearly Caledonian. Furthermore, the two samples ONK-PL522 and ONK-PL901 collected from storage hall fault show ages of Postjotnian diabases if possible K-feldspar contamination is taken into account. This is more evident with sample ONK-PL522 which K concentration is low (see Fig. 5-3). The calculated K-feldspar contami-nation would maximally decrease the age over 100 Ma in the ca. 900 Ma ONK-PL68 and 1.2 Ga ONK-PL960 samples. For the ONK-PL68 geologically reasonable explana-tion for the younger age is difficult to constrain but in the case of sample ONK-PL960, the corrected age would correspond to age of Sveconorwegian dyke magmatism.

Recrystallization of authigenic clay phases is another age disturbing factor. If re-crystallization occurs this could have an influence on the ages depending on the tem-perature history of the study area. According to Hunziker et al (1986) illite required temperatures greater than 250 C for a significant time to transform them to the 2M1 polytype and to loose much of their pre-existing radiogenic argon and therefore reduce the age. Complete resetting of the clay mineral K/Ar system occurs only subsequent to the reconstitution to muscovite within the fine-grained fractions (see McDougall and Harrison, 1999, p. 36). Closure temperature for radiogenic argon of muscovite is ca. 350 C (see McDougall and Harrison, 1999, p. 26 and references therein). Different genera-

tions of closely related minerals commonly occur together and therefore the isotopic signatures may be complex.

From the ONKALO fault breccia samples, no clay polytype investigations were per-formed (see e.g. Solum et al, 2005) and therefore it is difficult to judge if re-crystallization has occurred within the breccia samples.

If more detailed age data is needed, it is suggested to date clay samples from submicron size fractions.

35

1880 Ma K-feldspar 10% K wt contamination

0 200 400 600 800 1000 1200 1400 1600

ONK-PL68

ONK-PL87

ONK-PL522

ONK-PL901

ONK-PL960

Sa

mp

le ID

[<

2 m

icro

n]

Age [Ma]

3 wt % KF

2 wt % KF

1 wt % KF

age [Ma]

Figure 6-1. Histogram showing the effect of pure K-feldspar contamination on the ob-

tained clay ages. The data assume that the obtained <2 micron age is contaminated by

1, 2 or 3 weight % of a 1.88 Ga K-feldspar with 10 % K concentration.

1600 Ma K-feldspar 10% K wt contamination

0 200 400 600 800 1000 1200 1400 1600

ONK-PL68

ONK-PL87

ONK-PL522

ONK-PL901

ONK-PL960

Sam

ple

ID [

<2 m

icro

n]

Age [Ma]

3 w t % KF

2 w t % KF

1 w t % KF

age [Ma]

Figure 6-2. Histogram showing the effect of pure K-feldspar contamination on the ob-

tained clay ages. The data assume that the obtained <2 micron age is contaminated by

1, 2 or 3 weight % of a 1.60 Ga K-feldspar with 10 % K concentration.

36

ACKNOWLEDGEMENTS

We appreciate Ismo Aaltonen and the "Geological mapping team" for sampling and fault zone descriptions. Moreover, Hannu Huhma from the Geological Survey of Finland is thanked for reading the manuscript.

37

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Illite K-Ar Dating of Fault Breccia Samples from ONKALO … · 2008-12-12 · ILLITE K-Ar DATING OF FAULT BRECCIA SAMPLES FROM ONKALO UNDER-GROUND RESEARCH FACILITY, OLKILUOTO, EURAJOKI,