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The role of carbonate complexes and crystal habit on rare earth element uptake in low-temperature calcite in fractured crystalline rock
Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science. (Geochemistry research group)
Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.ORCID iD: 0000-0001-7230-6509
Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.ORCID iD: 0000-0002-3585-2209
2015 (English)In: Chemical Geology, ISSN 0009-2541, E-ISSN 1872-6836, Vol. 391, 100-110 p.Article in journal (Refereed) Published
Abstract [en]

This study focuses on rare earth element (REE) geochemistry of low-temperature calcite coatings occurring on the walls of fractures throughout the upper kilometer of crystalline rocks of the Baltic Shield. Fifty one calcite coatings were sampled from cores drilled with the triple-tube technique which successfully preserved the fragile calcite coatings on the fracture walls. The calcites, which based on geological and isotopic evidence were precipitated over the last 10 million years, had highly variable Sigma REE concentrations (0.61-2276 ppm) that decreased weakly with the depth the calcite was sampled from. When normalized to shale (and host rock), the REE concentrations of habits with c-axis approximate to a-axes and the closely associated c-axis > a-axes, the most abundant crystal morphologies in the system, decreased strongly and smoothly across the series. In contrast, the REEs of habits with c-axis >> a-axes, identified only in fractures in the uppermost 260m of the bedrock, were flatter and occasionally expressed a weak middle REE enrichment. By using calcite-water partition coefficients derived for REEs in previous laboratory experiments, the La/Yb of the paleogroundwater from which the calcites precipitated was back-calculated and found to be overall similar (range 0.15-452) overlap to the corresponding ratio of the present groundwater (range: 2.1-36.4). In terms of REE/Ca, the values for the back-calculated paleogroundwater (La / Ca 9.9 (*) 10(-11)-3.9 (*) 10(-7); Yb / Ca 1.5 (*) 10(-10)-2.2(*)10(-7)) were similar to those of LaCO3+ / Ca (4.5(*)10(-10)-8.5 (*) 10(-7)) and (YbCO3+ + Yb(CO3)(2)(-)) / Ca (5.4 (*) 10(-11)-1.8 (*) 10(-8)), respectively, in the present groundwater. These patterns indicate that the LREE to HREE and REE to Ca ratios in the groundwater at the site are broadly similar to those existing when the calcites precipitated, and that carbonate complexes present in the paleogroundwater played a crucial role in sequestration and fractionation of REEs in calcite. The findings have implications for bedrock storage of high-level radioactivewaste, which contains actinides for which the REEs can be used as natural analogues. (C) 2014 Elsevier B.V. All rights reserved.

Place, publisher, year, edition, pages
2015. Vol. 391, 100-110 p.
Keyword [en]
Low-temperature calcite, Groundwater, Rare earth elements, Stable isotopes, Partition coefficient
National Category
Geochemistry
Research subject
Natural Science
Identifiers
URN: urn:nbn:se:lnu:diva-39118DOI: 10.1016/j.chemgeo.2014.10.030ISI: 000346565400010OAI: oai:DiVA.org:lnu-39118DiVA: diva2:780950
Available from: 2015-01-15 Created: 2015-01-15 Last updated: 2017-03-08Bibliographically approved
In thesis
1. Origin and mobility of major and key trace elements (Cs, YREEs) in fracture groundwater in the upper 1.2 kilometres of coastal granitoids - Implications for future repositories of spent nuclear fuel
Open this publication in new window or tab >>Origin and mobility of major and key trace elements (Cs, YREEs) in fracture groundwater in the upper 1.2 kilometres of coastal granitoids - Implications for future repositories of spent nuclear fuel
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on the chemical and isotopic features of groundwater residing in the upper 1.2 km of sparsely fractured crystalline bedrock of the Baltic Shield. The work is based on previous understanding of the groundwater origin and its evolution in the fractured bedrock of the Äspö Hard Rock Laboratory (underground tunnel and facilities) and in two candidate coastal areas (Laxemar and Forsmark) investigated by the Swedish Nuclear Fuel and Waste Management Company (SKB) for future construction of a nuclear waste repository. In order to assess the origin and the apparent mobility of major elements and key trace elements (Cs and YREEs) in this difficult-to-reach deep environment, new (and previously published) data of SKBs hydrogeochemical monitoring programme was iteratively characterised and integrated in phenomenological models. The overall aim was to improve the integration between groundwater mixing and in situ water-rock interaction processes in deep coastal crystalline bedrock under natural and/or disturbed (i.e., dynamic) flow conditions induced by the presence of a tunnel system.

The multiple origins (glacial, marine, meteoric and allochthonous) of the fracture groundwater resulted in a large range of concentrations for dissolved major and trace elements in the studied bedrock. Dependent on the current flow conditions, the apparent mobility of dissolved elements was generally challenging to identify in the deep fractured bedrock under natural flow conditions. There, the relatively long residence time of most of the various groundwater types prevented to clearly differentiate the (apparent) fast retention processes from slow but active processes on a long-term perspective. Both processes alter the primary hydrochemical composition mainly imposed by the mixing between the dominant sources of groundwater. Nevertheless, in the particular case of YREEs, their generally low natural concentrations and predominant binding to organic colloids in most palaeo- (and modern meteoric) groundwaters (independently of the flow conditions) indicated strong active sorption onto minerals and physical filtration of organic colloids in the fractures. Together, these properties tend to minimise the mobility of dissolved YREEs and to stabilise their concentrations and fractionation patterns during the long residence time of the groundwaters.

At the Äspö HRL, an analogue (in a broad sense) of future repositories for high-level and long-lived radioactive wastes, changes in groundwater origin and salinity took place rapidly in subvertical fracture zones and progressively within the sparsely fractured deep rock domains. The changes resulted either from partial-to-substantial replacement of palaeo-groundwater by modern surface/shallow ground-water or induced dynamic up-flow of deep-lying saline groundwater. The hydrogeochemical instability near the underground facility during excavation to operational phase helped to assess qualitatively – and in some case differentiate quantitatively – the combined role of mixing, short-term and long-term reactions on the chemical composition of groundwater and the mobility of major elements and Cs within fracture zones and the sparsely fractured rock domains.

Collectively, the findings of the individual studies showed that the composition of intruded past or modern marine groundwater was likely to affect the natural retention properties/reactivity of the bedrock towards dissolved species at repository depth. For instance, the intrusion of modern seawater induced a desorption process of some dissolved species originally present on the exchange sites of the clayish fault gouge material in the fractures. This contributed to an apparent increase of the abundance level of dissolved cations naturally occurring in relatively moderate (i.e., K and Mg) and trace (i.e., Cs) concentrations in the fracture groundwaters.

The general understanding of the current hydrogeochemical conditions in deep crystalline bedrock is crucial when predicting future changes in groundwater chemistry (i.e., climatic cycles), which in turn might be of relevance to the long-term integrity of the KBS-3 repository method developed for isolating the nuclear waste from the surficial environment and biosphere.

Place, publisher, year, edition, pages
Växjö: Linnaeus University Press, 2015. 214 p.
Series
Linnaeus University Dissertations, 235/2015
Keyword
fracture groundwater, groundwater mixing, seawater intrusion, water-rock interactions, cation exchange, Cesium, Rare Earth Elements, colloidal speciation, humic substances, dual porosity, inverse modelling, Äspö HRL, Laxemar, Forsmark, eaux de fractures, mélange d'eaux, intrusion d'eau de mer, interactions eau-roche, échanges cationiques, Césium, Elements Terres Rares, spéciation colloïdale, substances humiques, double porosité, modélisation inverse, Äspö HRL, Laxemar, Forsmark, sprickvatten, grundvatten mixing, havsvattenintrusion, vatten-berg interaction, katjonbyte, cesium, sällsynta jordartsmetaller, kollidal speciation, humusämnen, dual porositet, invers modellering, Äspölaboratoriet, Äspötunneln, Laxemar, Forsmark
National Category
Natural Sciences
Research subject
Natural Science, Environmental Science
Identifiers
urn:nbn:se:lnu:diva-47607 (URN)978-91-87925-86-3 (ISBN)
Public defence
2015-12-17, Hörsalen Fullriggaren, Landgången 4, Kalmar, 09:30 (English)
Opponent
Supervisors
Available from: 2015-12-02 Created: 2015-11-26 Last updated: 2017-01-27Bibliographically approved

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