- Title
- Natural carbonation of ultramafic rocks in the Great Serpentinite Belt, New South Wales, Australia
- Creator
- Oskierski, Hans Christoph
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2013
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Mineral carbonation, the reaction of Mg-, Ca- or Fe-rich silicates or oxides with CO₂ to form stable, environmentally benign carbonates, is a promising strategy to mitigate rising CO₂ concentrations in the atmosphere and the associated adverse climate effects. Serpentinite, considered as a highly suitable feedstock for mineral carbonation, due to its high Mg-content and worldwide abundance, naturally transforms into Mg-carbonate, providing insight into the carbonation process. Understanding of these natural analogues to mineral carbonation is an important step in the implementation and optimisation of processes for industrial carbon storage in minerals. This study characterises the natural carbonation of serpentinite and associated ultramafic rocks in the Great serpentinite Belt (GSB), New South Wales, Australia. Based on the formation of the Attunga magnesite deposit and the carbonation of ultramafic mine tailings at the Woodsreef Asbestos Mine, and in comparison to the well document hydrothermal formation of silica-carbonate rock, it provides a record of the textural, geochemical and mineralogical evolution during natural carbonation and identifies the conditions and mechanisms promoting natural storage of CO₂. Nodular, cryptocrystalline magnesite at Attunga occurs in irregular veins and stock-works that are spatially associated with the weathering horizon. Textures and stable oxygen isotope signatures of the magnesite are in accordance with a supergene formation from meteoric waters at surfacial temperatures. Low δ¹³C signatures suggest C3 photosynthetic plants as the main carbon source and a typical shift of stable isotopic composition towards low δ¹³C and high δ¹⁸O can be observed in other weathering-affected carbonate occurrences in the study area. The supergene character of the precipitating fluids is confirmed by low radiocarbon concentrations of the magnesite and the previously suggested incorporation of carbon from decarboxylation of sub-ducted sediments into this type of deposit can be excluded. Some textural evidence indicates the presence of a pre-existing, possibly hydrothermal, magnesite deposit, which has been overprinted by dissolution and re-precipitation during weathering, but textural observations on the outcrop- and micro-scale indicate that weathering of the serpentinite host rock is the major magnesite mineralisation-process. Consequently, the formation of the Attunga magnesite has occurred during Quaternary weathering of the serpentinite host rock and a pre-existing magnesite deposit and is best described by a low temperature hypogene-supergene genetic model involving meteoric waters, which acquire CO₂ from the soil zone. Progress of carbonation at Attunga is strongly dependent on access of CO₂-bearing fluids, which initially is structurally controlled, but dominated by dissolution-derived porosity during progressive stages of alteration. Incipient weathering of tectonised serpentinite by moderately acidic, meteoric waters induces initially incongruent dissolution of least stable minerals (olivine, andradite, diopside and lizardite) and creates a range of porous and reactive alteration products, such as the deweylite assemblage. Progressive dissolution of the serpentinite host rock leads to a concomitant rise in Mg²⁺ concentrations and pH, which, together with the presence of pre-existing magnesite nucleation sites and minor evaporation of fluids, promote carbonate precipitation. Desiccated surfaces and secondary porosity suggest that, magnesite formation has progressed via dehydration of a basic, hydrated precursor carbonate such as hydromagnesite. Late stage precipitation of opal-A, derived from complete serpentine disintegration and advanced carbonation, clogs pore spaces and leads to subsequent cessation of carbonation at Attunga. Serpentinite-carbonation at Attunga occurs by both direct replacement in pervasive alteration zones and in a two step process, involving dissolution of serpentine and precipitation of carbonate in separate rock volumes. Weathering creates a permeability front in advance of the reaction front, which enhances fluid infiltration and reactivity for subsequent carbonation. The amount of net CO₂ sequestration and the rates of carbonation can be estimated based on radiocarbon age constraints and textural observations. Since closure of the Woodsreef Asbestos Mine in 1983, recessive weathering of fine grained material has led to the formation of extensive carbonate-rich crusts on the surface of the mine tailings pile. A relationship between the mode of carbonate occurrence, the mineralogy and the isotopic fingerprint of carbonates of the collected samples can be observed. Hydrated Mg-carbonate hydromagnesite is the main component of vertical carbonate crusts and has precipitated from evaporating meteoric waters incorporating atmospheric CO₂, as reflected in high δ¹⁸O, δ¹³C and F¹⁴C signatures, respectively. Bedrock carbonate, which has formed during alteration of the serpentinite bedrock before mining, is present in the form of low and variable concentrations of magnesite, dolomite and calcite. It is characterised by moderately high δ¹⁸O, low δ¹³C and F¹⁴C, a signature typical for ‘weathering-derived’ magnesite deposits in the GSB. The carbonate fraction of deep cement samples, collected from 70 and 120 cm below the surface, representing the bulk tailings material at depth, predominantly consists of pyroaurite. Despite stable isotope signatures similar to bedrock, the deep cement samples contain significant radiocarbon. Pyroaurite, forming under different conditions as hydromagnesite, may thus represent an additional trap for atmospheric CO₂ in the Woodsreef mine tailings. The radiocarbon content of crust samples strongly suggests atmospheric CO₂ as the dominant carbon source, but δ¹³C is lower than expected for carbonate that forms in exchange equilibrium with atmospheric CO₂. This could either be a result of admixing of carbon from a modern, organic carbon source, such as microbial respiration, or of kinetic isotope effects during uptake of atmospheric CO₂ into moderately high pH waters. The distribution of carbonates and SiO₂-phases, together with the absence of isotopic mixing trends between bedrock carbonate and atmospheric-derived carbonate, strongly suggest that, dissolution and re-precipitation (‘recyling’) of bedrock carbonate is not a dominant process in the Woodsreef tailings. Cations for carbonate precipitation are instead derived from the dissolution of serpentine minerals (lizardite and chrysotile) and brucite. Quantitative methods are used with X-ray diffraction data to estimate the abundance of the two major carbonate minerals hydromagnesite and pyroaurite and to constrain minimal and maximal carbonation rates, considering only the former or both minerals together, respectively. The formation of weathering-derived magnesite deposits, carbonate crusts on mine tailings and silica-carbonate rock reflect several sets of conditions promoting natural carbonation and accordingly produce texturally, geochemically and mineralogically distinct carbonation products. Both the formation of the Attunga magnesite deposit and the carbonate crusts on mine tailings at Woodsreef are closely associated with weathering of serpentinite, i.e. dissolution of serpentine minerals in CO₂-bearing meteoric water and precipitation of hydrated magnesite precursor minerals at ambient temperatures and pressures in a system that is open with respect to CO₂. Distinct isotope signatures reflect a higher degree of fluid evaporation and the recent sequestration of atmospheric CO₂ into carbonates at Woodsreef, as opposed to organically-derived carbon stored during Quaternary weathering at Attunga. The rates of carbonation at Attunga are higher than the background uptake rate of CO₂ by chemical weathering, but, due to the higher surface area of the tailings and possibly to the presence of reactive brucite and chrysotile, they are significantly slower than carbonation at Woodsreef. The reactive surface area of coherent, non-disintegrated serpentinite at Attunga is initially created by faulting and fracturing during tectonic activity and physical weathering, but dominated by dissolution reactions during later, more advanced stages of alteration. Silica-carbonate rocks in the GSB form due to hypogene serpentinite alteration at temperatures of 165 to 225 °C by distinct metamorphic/magmatic fluids in a closed system with respect to CO₂. It is likely that the abundance and massive character of the silica-carbonate rock in the GSB reflects the more effective natural carbonation of serpentinite at elevated temperatures and reaction rates. Each of the studied natural analogues has the potential for application to a process of enhanced industrial storage of CO₂ in serpentinite rocks. The development of porosity and fluid access during low temperature weathering-carbonation of coherent serpentinite at Attunga bears important implications for in-situ mineral carbonation. The acceleration of natural carbonation rates and the direct sequestration of atmospheric CO₂ in carbonate crust on the mine tailings at Woodsreef represent a promising strategy for a low-cost, low energy CO₂ storage process. Silica-carbonate rock formation occurs under conditions that are optimal for injection of hot CO₂-bearing fluids during in-situ carbonation, and, due to elevated temperatures and pressures, resembles ex-situ mineral carbonation approaches.
- Subject
- sequestration of CO₂; mineral carbonation; serpentinite; magnesite; weathering; stable isotopes; radiocarbon; quantitative XRD; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1038903
- Identifier
- uon:13592
- Rights
- Copyright 2013 Hans Christoph Oskierski
- Language
- eng
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