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Investigation of Rock Matrix Porosity in Alteration Profiles. Development Of F 980009 GEOLOGIAN TUTKIMUSKESKUS GEOLOGICAL SURVEY OF FINLAND Ydinjatteiden sijoitustutkimukset Nuclear Waste Disposal Research Tiedonanto YST-97 Report YST-97 INVESTIGATION OF ROCK MATRIX POROSITY IN ALTERATION PROFILES Development of site characterization methodology Marja Siitari-Kauppi 1, Antero Lindberg2, Karl-Heinz Hellmuth3 and Sakari Lukkarinen4 1 Laboratory of Radiochemistry, 00014 University of Helsinki 2 Geological Survey ofFinland 3 Finnish Centre for Radiation and Nuclear Safety 4 Helsinki University of Technology Espoo 1997 TIIVISTELMA Peruskallioon sijoitetuista ydinpolttoaineen Ioppusijoitusvarastoista mahdollisesti vapautuvat radionuklidit kulkeutuvat paaasiassa kalliorakoja pitkin virtaavan veden mukana. Veteen liuenneiden radionuklidien kulkeutumista hidastaa niiden pidattyminen kalliorakojen pintaan ja rakotaytteisiin seka diffuusio kiven mikrorakoihin ja huokoisiin mineraaleihin. Kulkeutumiseen vaikuttavien erilaisten fysikaalisten ja kemiallisten ilmioiden vaikutusten, kuten myos kivimatriisin ominaisuuksien tunteminen on oleellista ennustettaessa ydinjatenuklidien kulkeutumista ja pidattymista. Tassa tyossa maaritettiin yksityiskohtaisesti kivimatriisin ominaisuudet pohjavetta johtavien kalliorakojen laheisyydessa. Huokoisuusprofiilit mitattiin 14C-polymetyylimetakrylaatti (14C- PMMA) -menetelmalla kolmelle Syyryn alueen kivilajille: tonaliitille, kiillegneissille ja vulkaniitille. Tutkimuksessa kaytetyt 20 - 30 cm:n pituiset kairansydannaytteet impregnoitiin (imeytettiin) 14C-leimatulla metyylimetakrylaatilla (MMA) tyhjiokuivauksen jalkeen. Monomeeri polymeroitiin Co-60-sateilylahteen kentassa. Taman jalkeen naytteet sahattiin ja sahatut pinnat tutkittiin autoradiografisesti. Huokoisuusprofiilit ja huokoisuuden jakaumat maaritettiin autoradiogrammeista digitaalisen kuvienkasittelyn avulla. Naytteiden mineraalikoostumukset ja rakenteet tutkittiin mikroskooppisesti autoradiogrammeissa havaittujen piirteiden selvittamiseksi. Mineraalien muuntumisaste maaritettiin elektronimikroskooppisesti energiadispersiivisella alkuaineanalyysilla (SEM/EDS). Kaytetylla 14C-PMMA metelmalla saadaan uutta kvalitatiivista ja kvantitatiivista tietoa diffuusiolle kaytettavissa olevan huokostilan leveydesta vetta johtavien ja pidattavien (varastoivien) rakojen laheisyydessa. Huokoisuutta tarkasteltiin kolmena eri alueena: muuntumattoman matriisin (tausta-) huokoisuutena, raon taytteen huokoisuutena ja raon vaikutuspiirissa olevan muuntumisvyohykkeen huokoisuutena. Matriisien taustahuokoisuus oli alhainen ja vaihteli vain vahan (0.05 - 0.2%), raontaytteiden huokoisuus puolestaan vaihteli valilla 1 - 5%. Taytteellisten ja vetta pidattavien rakojen laheisyydessa oleva huokoisuusvyohyke, joka paaosin aiheutuu maasalpien muuttumisesta (serisiittiytymisesta), oli tonaliitissa kapeampi kuin kiillegneississa. Tonaliitissa maasalpien muuttumista havaittiin myos huokoisuusvyohykkeen ulkopuolella. Vulkaniitin huokoisuusvyohykkeet olivat kapeita kuten kiillegneississa ja matriisin taustahuokoisuus oli pieni. Avainsanat: kiven huokoisuus, muuttuminen, mikrorakoilu, u C-PMMA-menetelma, radioaktiivinen jate, loppusijoitus ABSTRACT Assessment of bedrock performance for nuclear waste disposal can benefit from a more detailed understanding of the rock matrix properties along actual and potential groundwater flow pathways. The spatial variability along flow paths and the correlation of groundwater flow with rock matrix properties (type of minerals, porosity, internal surface areas) is an input parameter for the quantification of retarding properties and the chemical buffering capacity of the bedrock. In this investigation tonalite, mica gneiss and volcanite samples from Syyry were studied. The samples were taken from a drill core located near water-conducting, open fractures. The objective of the study was to describe the porosity of different altered and weathered rock matrices next to filled fissures and open fractures. Petrographic studies were done from several thin sections using optical microscopy complemented by SEM/EDS. The porosity profiles were determined with the l4C- polymethylmethacrylate (14C-PMMA) method. Porosity profile measurement based on autoradiographs produced by 14C-PMMA impregnation provides valuable qualitative and quantitative information on the thickness of the diffusion-accessible pore space of rock samples near water-conducting fractures and water-bearing fissures. Rock samples from the Syyry site showed that the zones of increased porosity near water­ bearing fissures in tonalites were narrower than comparable zones in mica gneiss. The porosity profile was consistent with plagioclase sericitization in mica gneiss, but not in tonalite samples. The intermediate volcanite samples showed narrow porosity profiles next to laumontite-filled fissures and no consistent correlation with plagioclase sericitization was found. A gradual increase in porosity was observed in mica gneiss, but an abrupt decrease in porosity was found in tonalite samples. Chloritized biotite was found throughout the rock matrix in moderately altered tonalite, but no influence on the increased porosity zones near fissures was found. Keywords: rock porosity, rock alteration, microfracturing, 14C-PMMA method, nuclear waste disposal CONTENTS TIIVISTELMA ABSTRACT 1. INTRODUCTION 1 2. GENERAL GEOLOGY 2 3. EXPERIMENTAL 3 3.1 Materials 3 3.2 14C-PMMA method 5 3.2.1 Infiltration with 14C-methylmethacrylate 5 3.2.2 Digital image processing and analysis 5 4. RESULTS AND DISCUSSION 9 4.1 Porosity profiles 9 4.2 Comparison of porosity profiles 17 5. CONCLUSIONS 20 REFERENCES 21 APPENDICES 1 1. INTRODUCTION The migration behaviour of radioelements through rock matrices is clearly controlled by a complex combination of matrix diffusion and chemical interactions determined by mineral specific compositional and microstructural factors. Diffusion of radionuclides into the matrix of rocks along water conducting fractures and from fractures into water bearing fissures is one of the major retarding mechanisms ensuring the safety of nuclear waste repositories in deep geological formations. The mineral specific porosity and the breadth of the increased porous zone near water bearing fissures are essential parameters when appraising the ability of different rock types to retard by matrix diffusion. Assessment of the performance of bedrock can benefit from a more detailed knowledge of the rock matrix properties along actual and potential groundwater flow pathways. The spatial variability along flow paths and the correlation of groundwater flow with rock matrix properties (type of minerals, porosity profiles, internal surface areas) are input parameters for the quantification of retarding properties and the chemical buffering capacity of the geosphere. In order to increase the predictability of the diffusion-releated properties of rocks in relation to rock type and alteration, tonalite, mica gneiss and volcanite samples were studied. The samples were taken from a drill core located near water-conducting, open fractures. The objective of the study was to describe the porosity of different altered and weathered rock matrices adjacent to filled fissures and open fractures. Because the study was not intended as a comprehensive site-specific investigation, but rather as a contribution to methodological development, it was limited to sections of a single drill core. Petrographic characterization was done analysing several thin sections with optical microscopy and S EM/EDS. Porosity profiles were determined with 14C- polymethylmethacrylate (14C-PMMA) method. The measured values were fitted to a simple equation to allow comparison of different profiles. An attempt was made to compare the characteristics of different rock types on the basis of porosity profiles near filled fissures. 2 2. GENERAL GEOLOGY The main rock types of the Syyry area are felsic plutonic rocks, particularly tonalite, granodiorite and quartz diorite, but in the vicinity there are also schists and gneisses of both volcanic and sedimentary origin. Southwest of Syyry there is a large occurrence of volcanic rocks, mainly andesitic and dacitic porphyry, agglomerate and amphipolite, while to the southeast is an area of mica schist. The more basic plutonic rocks (peridotite, gabbro and diorite) grade into quartz diorite, tonalite and granodiorite without sharp contacts. Granite occurs only in small amounts (Anttila et al. 1993, Sail! 1967). The stratigraphy of the area from oldest to youngest (supracrustal versus plutonic) is as follows: arkose schists with some volcanic material - ultramafic plutonic rocks and gabbrodiorite; greywacke schists - quartz diorite, granodiorite; porphyroblastic mica schists - granites; volcanic and subvolcanic material - various dyke rocks. The dominant rock type in outcrop as well as in drill core from the Syyry site is tonalite, which contains abundant inclusions (xenolithes) of various schist and plutonic rock types. Aplite, quartz, epidote and pegmatite occur as intersecting dykes and veins. Mica gneiss occurs in the outcrops of the study area only as inclusions, but immediately to the south there is a fairly extensive occurence of greywacke-like mica gneiss which, according to magnetic measurements, continues into the study area (Anttila et al. 1993). The main folding episode in the area was isoclinal and the schistosity therefore mostly shows steep or vertical dips. The orientations of fold axes undulate gently from the northwest to southeast. The most intensive fracturing is
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