Secondary dolomitisation processes in the Estonian Secondary dolomitisation processes in the Estonian Paleozoic basement Paleozoic basement Kristjan Urtson & Alvar Soesoo Kristjan Urtson & Alvar Soesoo Institute of Geology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia Institute of Geology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia Kristjan.Urtson@gi.ee Kristjan.Urtson@gi.ee Introduction Introduction The replacement of calcite with dolomite in calcareous sedimentary rocks by interaction The replacement of calcite with dolomite in calcareous sedimentary rocks by interaction with magnesium-bearing fluids is the main mechanism that accounts for the extensive with magnesium-bearing fluids is the main mechanism that accounts for the extensive genesis of dolomites through the Earth's history. Besides the secondary replacement, lesser genesis of dolomites through the Earth's history. Besides the secondary replacement, lesser amount of primary dolomites is precipitated directly from supersaturated solutions in tidal amount of primary dolomites is precipitated directly from supersaturated solutions in tidal and lagoonal evaporite environments. The relative scarcity of recent dolomites compared to and lagoonal evaporite environments. The relative scarcity of recent dolomites compared to their wide distribution in the Paleozoic rock record has caused a lot of puzzling over the their wide distribution in the Paleozoic rock record has caused a lot of puzzling over the dolomitisation kinetics and is therefore referred to as the “Dolomite Problem”. dolomitisation kinetics and is therefore referred to as the “Dolomite Problem”. During the dolomitisation rocks often attain an enhanced porosity both due to the smaller During the dolomitisation rocks often attain an enhanced porosity both due to the smaller volume of dolomite mineral and the greater durability of dolomite rock to the compaction volume of dolomite mineral and the greater durability of dolomite rock to the compaction which makes dolomites excellent reservoir rocks for hydrocarbon accumulation. In fact, a which makes dolomites excellent reservoir rocks for hydrocarbon accumulation. In fact, a great number of important hydrocarbon occurrences in the world are associated with great number of important hydrocarbon occurrences in the world are associated with dolomites. Also, dolomitisation of limestones is closely related to ore forming processes, dolomites. Also, dolomitisation of limestones is closely related to ore forming processes, being a precursor process that provides the depositional space for sulphides of base metals being a precursor process that provides the depositional space for sulphides of base metals such as lead and zinc. such as lead and zinc. Abstract Abstract Paleozoic limestone successions in the Estonian basement are extensively dolomitised during Paleozoic limestone successions in the Estonian basement are extensively dolomitised during several distinct episodes. The early diagenetic dolomitisation occurs in constrained levels several distinct episodes. The early diagenetic dolomitisation occurs in constrained levels between unaltered limestones and retains the primary sedimentary layering and fabrics of between unaltered limestones and retains the primary sedimentary layering and fabrics of precursor rocks. Later events are often associated with subvertical fracture systems where precursor rocks. Later events are often associated with subvertical fracture systems where both formerly dolomitised and unaltered limestones were subjected to new dolomitisation and both formerly dolomitised and unaltered limestones were subjected to new dolomitisation and recrystallisation processes that overprinted earlier rock textures. In fracture zones the recrystallisation processes that overprinted earlier rock textures. In fracture zones the transition from unaltered limestones to dolomites can be observed along a short distance that transition from unaltered limestones to dolomites can be observed along a short distance that provides the opportunity to study the evolution of rock fabrics and dynamics of calcite provides the opportunity to study the evolution of rock fabrics and dynamics of calcite replacement during progressive dolomitisation. replacement during progressive dolomitisation. Estonian dolomites Estonian dolomites Early diagenetic Paleozoic dolomites in the Estonian basement were formed shortly after the deposition of their precursor Early diagenetic Paleozoic dolomites in the Estonian basement were formed shortly after the deposition of their precursor limestones at shallow-water inner shelf environment. Normal to dilute marine water has been proposed to be the source of Mg, limestones at shallow-water inner shelf environment. Normal to dilute marine water has been proposed to be the source of Mg, dolomitisation occurred in the upper part of unlithified carbonate sediments close to the sediment-water interface. dolomitisation occurred in the upper part of unlithified carbonate sediments close to the sediment-water interface. Contemporaneous sedimentation and dolomitisation has generally retained original sedimentary and biogenic structures. Although Contemporaneous sedimentation and dolomitisation has generally retained original sedimentary and biogenic structures. Although laterally widely distributed, early diagenetic dolomites have a limited and well-defined time span in the sedimentary record [1]. laterally widely distributed, early diagenetic dolomites have a limited and well-defined time span in the sedimentary record [1]. After lithification, early dolomites and unaltered limestones have undergone late diagenetic changes at deep burial conditions After lithification, early dolomites and unaltered limestones have undergone late diagenetic changes at deep burial conditions including repeated dolomitisation and recrystallisation by the action of hydrothermal fluids. Later changes overprint the earlier including repeated dolomitisation and recrystallisation by the action of hydrothermal fluids. Later changes overprint the earlier structures and do not follow certain stratigraphic units. Remnant magnetism studies reveal distinct diagenetic episodes of structures and do not follow certain stratigraphic units. Remnant magnetism studies reveal distinct diagenetic episodes of Devonian, Carboniferous and Triassic age which can be associated with global tectonic events at the margins of Baltica Devonian, Carboniferous and Triassic age which can be associated with global tectonic events at the margins of Baltica paleocontinent. In association with later dolomitisation stages, occurrences of Zn and Pb sulphide mineralisation have been paleocontinent. In association with later dolomitisation stages, occurrences of Zn and Pb sulphide mineralisation have been reported. Late burial dolomites mainly follow tectonic disturbance zones, where alteration processes occur in relatively limited reported. Late burial dolomites mainly follow tectonic disturbance zones, where alteration processes occur in relatively limited space adjacent to fractures (Fig. 1). space adjacent to fractures (Fig. 1). Knowledge Based Materials Dolomite mineral & dolomitisation mechanism Dolomite mineral & dolomitisation mechanism The ideal dolomite (CaMg(CO The ideal dolomite (CaMg(CO 3 3 ) ) 2 2 ) lattice contains equal amounts of Ca and Mg ions which are ) lattice contains equal amounts of Ca and Mg ions which are ordered into separate monolayers alternating with CO ordered into separate monolayers alternating with CO 3 3 layers (Fig. 2A). The smaller ionic layers (Fig. 2A). The smaller ionic radius of Mg causes about 13% volume decrease of dolomite unit cell with respect of that of radius of Mg causes about 13% volume decrease of dolomite unit cell with respect of that of precursor calcite. However, ideal stoichiometric ordered dolomite is quite rare due to the precursor calcite. However, ideal stoichiometric ordered dolomite is quite rare due to the strong hydration of Mg ion which promotes the substitution of larger Ca ion into Mg strong hydration of Mg ion which promotes the substitution of larger Ca ion into Mg monolayer and disordering of the lattice structure (Fig.2B). To overcome this kinetic barrier, monolayer and disordering of the lattice structure (Fig.2B). To overcome this kinetic barrier, high Mg/Ca ratios in the dolomitising fluids are high Mg/Ca ratios in the dolomitising fluids are required required (e.g. 5.2 in modern seawater). (e.g. 5.2 in modern seawater). The conversion of minerals is carried out via small-scale dissolution of calcite and The conversion of minerals is carried out via small-scale dissolution of calcite and simultaneous precipitation of dolomite from the pore solution. The process starts with simultaneous precipitation of dolomite from the pore solution. The process starts with heterogeneous nucleation of dolomite on calcite surface where the critical radius of the nuclei heterogeneous nucleation of dolomite on calcite surface where the critical radius of the nuclei is controlled by the degree of fluid supersaturation and the surface free energy of the is controlled by the degree of fluid supersaturation and the surface free energy of the precipitate. Ca-rich disordered dolomite with lower activation energy is precipitated first, precipitate. Ca-rich disordered dolomite with lower activation energy is precipitated first, followed by the recrystallisation through several metastable phases with compositions followed by the recrystallisation through several metastable phases with compositions progressively closer to the stable ordered stoichiometric end product (the process also known progressively closer to the stable ordered stoichiometric end product (the process also known as Ostwald Step Rule). Recrystallisation operates in combination with surface free energy as Ostwald Step Rule). Recrystallisation operates in combination with surface free energy controlled Ostwald ripening or ageing that drives the system towards equilibrium with controlled Ostwald ripening or ageing that drives the system towards equilibrium with crystals approximately the same size [2]. crystals approximately the same size [2]. References: References: 1. Teedumäe, A., Shogenova, A. & Kallaste, T., 2006. 1. Teedumäe, A., Shogenova, A. & Kallaste, T., 2006. Dolomitization and sedimentary cyclicity of the Ordovician, Silurian, and Devonian rocks in South Estonia. Dolomitization and sedimentary cyclicity of the Ordovician, Silurian, and Devonian rocks in South Estonia. Proceedings of Estonian Academy of Proceedings of Estonian Academy of Sciences, Geology Sciences, Geology 55, 67-87 55, 67-87 2. Warren, J., 2000. Dolomite: occurrence, evolution and economically important associations. 2. Warren, J., 2000. Dolomite: occurrence, evolution and economically important associations. Earth-Science Reviews Earth-Science Reviews 52, 1-81. 52, 1-81. 3. Sibley, D.F. & Gregg, J.M., 1987. Classification of dolomite rock textures. 3. Sibley, D.F. & Gregg, J.M., 1987. Classification of dolomite rock textures. Journal of Sedimentary Petrology Journal of Sedimentary Petrology 57, 967–975. 57, 967–975. Analytical techniques: Analytical techniques: Petrographic study of thin sections Petrographic study of thin sections Cathodoluminescence- growth zonality and crystal relationships (Fig.5) Cathodoluminescence- growth zonality and crystal relationships (Fig.5) Electron microprobe- micro-scale chemical composition within a single Electron microprobe- micro-scale chemical composition within a single crystal crystal X-ray diffractometry- dolomite macro-scale composition and ordering X-ray diffractometry- dolomite macro-scale composition and ordering Numerical modelling of nucleation and crystal growth Numerical modelling of nucleation and crystal growth Glossary: Glossary: Dolomitisation Dolomitisation - the process by which limestone is wholly or partly converted to dolomite rock or dolomitic limestone by the replacement of the original calcite (CaCO - the process by which limestone is wholly or partly converted to dolomite rock or dolomitic limestone by the replacement of the original calcite (CaCO 3 3 ) by dolomite (CaMg(CO ) by dolomite (CaMg(CO 3)2 3)2 ), ), usually through the action of magnesium-bearing water (seawater or percolating meteoric water). It can occur usually through the action of magnesium-bearing water (seawater or percolating meteoric water). It can occur contemporaneously contemporaneously or shortly after deposition of the limestone, or during and after or shortly after deposition of the limestone, or during and after lithification at a later period. lithification at a later period. Diagenesis Diagenesis - any change occurring within a sediment after its deposition and during and after its lithification, exclusive of weathering. It includes such processes as compaction, cementation, - any change occurring within a sediment after its deposition and during and after its lithification, exclusive of weathering. It includes such processes as compaction, cementation, replacement, and crystallisation, under normal surficial conditions of pressure and temperature. replacement, and crystallisation, under normal surficial conditions of pressure and temperature. Cathodoluminescence Cathodoluminescence is an optical and electrical phenomenon whereby a beam of electrons generated by an electron gun (e.g. cathode ray tube) impacts on a luminescent material causing the is an optical and electrical phenomenon whereby a beam of electrons generated by an electron gun (e.g. cathode ray tube) impacts on a luminescent material causing the emission of visible light. It is used to examine internal structures of semiconductors, rocks, ceramic, glass etc. in order to get information on the composition, growth and quality of the material. emission of visible light. It is used to examine internal structures of semiconductors, rocks, ceramic, glass etc. in order to get information on the composition, growth and quality of the material. Electron microprobe Electron microprobe is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials. It uses a high-energy focused beam of electrons to is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials. It uses a high-energy focused beam of electrons to generate X-rays characteristic of the elements present within a sample generate X-rays characteristic of the elements present within a sample Fig. 1. Fracture-related late dolomitisation in the Karinu Fig. 1. Fracture-related late dolomitisation in the Karinu limestone quarry in central Estonia. Dolomitisation intensity limestone quarry in central Estonia. Dolomitisation intensity decreases outwards from the fracture zone in the centre of the decreases outwards from the fracture zone in the centre of the image. image. Fig. 2. A. Dolomite crystal lattice Fig. 2. A. Dolomite crystal lattice consisting of alternating Ca, Mg and consisting of alternating Ca, Mg and carbonate layers B. Mg ion hydration carbonate layers B. Mg ion hydration and substitution by Ca in Mg and substitution by Ca in Mg monolayer. Greater ionic radius of Ca monolayer. Greater ionic radius of Ca causes disordering of the structure by causes disordering of the structure by slight rotation of carbonate groups slight rotation of carbonate groups and increasing the lattice parameter. and increasing the lattice parameter. Figures by [2]. Figures by [2]. Rock fabric evolution Rock fabric evolution The nucleation on the calcite surface occurs preferentially on energetically favourable sites The nucleation on the calcite surface occurs preferentially on energetically favourable sites such as surface kinks. Fine-grained texture provides larger number of active sites for such as surface kinks. Fine-grained texture provides larger number of active sites for nucleation so the microfabrics of the precursor limestone together with the degree of fluid nucleation so the microfabrics of the precursor limestone together with the degree of fluid supersaturation are the main factors that control the nucleation rate. The final texture of the supersaturation are the main factors that control the nucleation rate. The final texture of the dolomite is determined by nucleation and growth rates and by the residence time of the rock dolomite is determined by nucleation and growth rates and by the residence time of the rock in the dolomitising solution [3](Fig. 3). in the dolomitising solution [3](Fig. 3). In relatively closed system with local source of Mg and CO In relatively closed system with local source of Mg and CO 3 3 ions the mineral volume is reduced ions the mineral volume is reduced which enhances rock porosity and permeability. Influx of additional carbonate will contribute which enhances rock porosity and permeability. Influx of additional carbonate will contribute the dolomite crystal growth and the porosity is occluded (Fig. 4). the dolomite crystal growth and the porosity is occluded (Fig. 4). Fig. 3. Rock texture evolution in result of different Fig. 3. Rock texture evolution in result of different degrees of supersaturation with respect to dolomite degrees of supersaturation with respect to dolomite and different residence times. A-C- three scenarios with and different residence times. A-C- three scenarios with decreasing supersaturation degree; D- low decreasing supersaturation degree; D- low supersaturation degree, long residence time. Figure by supersaturation degree, long residence time. Figure by [3]. [3]. Fig. 4. Evolution of porosity and permeability dependent Fig. 4. Evolution of porosity and permeability dependent on availability of additional Mg and carbonate ions. Figure on availability of additional Mg and carbonate ions. Figure by [2]. by [2]. Objectives of the study Objectives of the study The variations of dolomite composition at micro-scale in order to The variations of dolomite composition at micro-scale in order to determine the short-term changes in pore fluid chemistry determine the short-term changes in pore fluid chemistry The evolution of rock microfabrics and dynamics of calcite The evolution of rock microfabrics and dynamics of calcite replacement during progressive dolomitisation according to variable replacement during progressive dolomitisation according to variable fluid chemistry fluid chemistry Development of secondary porosity and permeability dependent on Development of secondary porosity and permeability dependent on variable nucleation and crystal growth rates controlled by the variable nucleation and crystal growth rates controlled by the supersaturation degree of dolomitising fluids and precursor rock supersaturation degree of dolomitising fluids and precursor rock texture texture Fig. 5. Cathodoluminescence image of Fig. 5. Cathodoluminescence image of zonal growth of dolomite crystals. The zonal growth of dolomite crystals. The zonality is caused by the variable content zonality is caused by the variable content of Mn and Fe and reflects changes in the of Mn and Fe and reflects changes in the fluid composition. Horisontal dimension of fluid composition. Horisontal dimension of image is 2.8 mm. Image image is 2.8 mm. Image from the website web.umr.edu Results Results Coming soon... Coming soon...