GRADUATE THESIS PROPOSAL EARTH SCIENCES 6300

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1 GRADUATE THESIS PROPOSAL EARTH SCIENCES 6300 LAST NAME: KELTIE FIRST NAME: ERIN STUDENT NUMBER: B DEGREE PROGRAMME: M.SC SUPERVISOR(S): DR. JAMES BRENAN TITLE OF PROPOSAL: AN EXPERIMENTAL INVESTIGATION OF THE EFFECT OF CONTAMINATION ON CHROMITE CRYSTALLIZATION IN THE RING OF FIRE COMPLEX, ONTARIO KEY WORDS (up to 10): CHROMITE, CONTAMINATION, RING OF FIRE COMPLEX, EXPERIMENTAL PETROLOGY, CHROMITITES FIELD(S) OF SPECIALIZATION: EXPERIMENTAL PETROLOGY ESTIMATED COST OF PROPOSED RESEARCH: Year 1 Year 2 Year 3 Year 4 $ $ N/A N/A STUDENT S SIGNATURE: SUPERVISOR S SIGNATURE: DATE: 1

2 SUMMARY Chromite (FeCr 2 O 4 ) is the only ore mineral of chromium, which is an important commodity used in the production of stainless steel. Economic chromitite deposits are rare despite chromite being a common accessory phase in primitive mafic rocks, however the processes that cause chromite concentration in chromitite deposits remain poorly understood. A popular hypothesis for chromitite formation is that contamination of a komatiitic magma by more siliceous country rocks induces chromite crystallization, but there have been no experimental studies that confirm this hypothesis. This thesis aims to test the contamination hypothesis by conducting experiments in which komatiite melt is contaminated by various potential country rock compositions at fixed temperature, pressure, and oxygen fugacity. The primary magma and contaminant compositions used will reflect those from the Ring of Fire chromitite deposits in Ontario to test the validity of the contamination hypothesis in explaining their formation. In the long term, a better understanding of chromitite formation will aid future exploration campaigns targeting chromitite deposits. TIMELINE 2

3 STATEMENT OF PROBLEM Economically significant chromitite deposits are rare despite chromite being a common accessory mineral in primitive mafic rocks. Until the discovery of the Ring of Fire (ROF) chromite deposits in 2007, there were no known economic deposits in Canada. The processes responsible for ore-grade chromite accumulation can be simplified to two hypotheses: chromite deposits form (1) due to fluid dynamic processes involving crystal sorting from komatiite melts in large conduits, or (2) due to komatiitic magma interacting with specific contaminants during transit through country rocks. This thesis will test the second hypothesis by conducting experiments in which komatiitic magma similar in composition to the ROF intrusion will be combined with contaminants resembling ROF country rocks while controlling temperature, pressure, and oxygen fugacity, then observing the resultant changes in chromite solubility and composition. The composition and relative crystallization timing of experimentally-produced chromite will be compared to ROF samples to assess the validity of the contamination hypothesis. Development of an accurate ore-formation model will facilitate effective strategies for discovering new deposits, as focusing exploration efforts to locations the most potential for forming ore-grade chromite deposits may decrease the cost of exploration programs and result in more discoveries. 3

4 BACKGROUND There are many theorized ways to induce significant accumulation of chromite, including the settling of pre-existing chromite to the bottom of the magma chamber (Mondal and Mathez 2007), changes in physical conditions such as pressure (Lipin 1993), and changes in chemical conditions such as addition of water to the magma (Nicholson and Mathez 1991), change in oxygen fugacity (fo 2 ) (Barnes 1986, Murck and Campbell 1986, Roeder and Reynolds 1991), magma mixing between a primitive and evolved magma (Irvine 1977; Hoover and Irvine 1977), or contamination of a primitive magma by country rocks (Irvine 1975, Spandler et al. 2005). Mondal and Mathez (2007) investigated the origin of chromitite layers in the Bushveld Complex, a layered mafic intrusion in South Africa. They analyzed the chemistry of orthopyroxenites directly above and below a chromitite layer and found that the orthopyroxenites showed very little chemical variance below and above the chromitite (Mondal and Mathez 2007). They suggested that this indicates that the chromitite did not originate due to changes in the physical or chemical conditions during emplacement, but rather that the chromite crystallized in a deeper magma chamber and settled to the bottom of the intrusion due to its relatively high density (Mondal and Mathez 2007). One major criticism of this hypothesis is the enormous volume of primitive magma required to crystallize the amount of chromite within the Bushveld Complex. Lipin (1993) investigated the hypothesis that the chromitite layers at the Stillwater Complex, a layered mafic intrusion in Montana, formed due to 4

5 ephemeral increases in the overall pressure of the magma chamber. In a system made up of olivine-orthopyroxene-chromite-plagioclase, an increase of pressure from 0.1 MPa to 1000 MPa expands the chromite crystallization field at the expense of the olivine field (Lipin 1993). It follows that if a magma was crystallizing olivine and chromite, an increase in pressure would cause chromite to begin to crystallize alone until the excess pressure is dissipated, at which point olivine will start to crystallize again (Lipin 1993). The increase in pressure could be caused by exsolution of CO 2 during the ascent of mafic magma to a shallow chamber, since the increase in volume of CO 2 is dramatic during the transition from > 6 km depth to the surface (Lipin, 1993). Although it is generally thought that magma chambers have no strength and cannot withstand such an increase in pressure, evidence from Kilauea and Krafla volcanoes show that magma chambers can endure short periods of increased pressure due to changes in CO 2 volume (Lipin, 1993). Nicholson and Mathez (1991) developed a petrogenetic model for the formation of the Merensky Reef chromitites in the Bushveld Complex in which the chromitites formed at the front of hydrous interstitial melt moving through the crystal pile and dissolving orthopyroxene and plagioclase from the layer below. This model was based on experiments (Ford et al. 1972, Sisson and Grove 1993, Gaetani et al. 1994) that showed that adding water to a silicate melt lowers the crystallization temperatures of silicate minerals more than chromite. However, Mondal and Mathez (2007) asserted that this model would require that the 5

6 chromitite dissolve 100 times its mass in orthopyroxene to achieve the observed Cr content of the chromitite, making it geologically improbable. Numerous experimental studies (Barnes 1986, Murck and Campbell 1986, Roeder and Reynolds 1991) have shown that Cr solubility in mafic and ultramafic melts is sensitive to fo 2, and that Cr solubility increases with decreasing fo 2. It follows that inducing an increase in fo 2 of the melt, possibly by mixing two melts with different fo 2 values, could cause the precipitation of chromite. However, Barnes (1986) conducted experiments that showed that the Cr content of orthopyroxene is highly dependent upon fo 2. This, combined with the data on the Bushveld Complex orthopyroxenites from Mondal and Mathez (2007) showing little change in Cr content above chromitite seams, suggests that there was no significant change in fo 2 during emplacement. Irvine (1977) postulated that the chromitites within the Muskox intrusion, a layered intrusion located in Nunavut, Canada, formed due to mixing of a primitive magma and more evolved magma. Using ternary phase diagrams (Fig. 1) modeled from experimental data on the MgO-Cr 2 O 3 -SiO 2 system (Keith 1954), Irvine (1977) showed that due to the curved nature of the cotectic between olivine and chromite stability fields, mixing a primitive magma with a more evolved magma of the same source could cause the crystallization of chromitites (Fig. 1c). However, a more realistic system would be one such as CaO-MgO- Al 2 O 3 -Cr 2 O 3 -SiO 2, wherein the cotectic between olivine and chromite is much more dependent upon melt composition and is poorly constrained, making it difficult to predict the effect of mixing between a primitive and evolved magma 6

7 (Hoover and Irvine, 1977). Furthermore, Mondal and Mathez (2007) rejected this model by explaining that if such mixing did occur in the Bushveld Complex, the Mg/Fe ratio of the orthopyroxenites above the chromitites would be expected to increase, yet no such increase is found. Using the same ternary phase diagram (Fig. 1d), Irvine (1975) proposed that the Muskox intrusion chromitites could have formed due to assimilation of Figure 1. (a) The olivine-chromite-silica system. (b) Arrow shows the uncontaminated crystallization pathway of komatiitic magma. In the A-B segment, olivine crystallizes alone; from B-C, olivine and chromite crystallize; from C-D, orthopyroxene crystallizes. No significant chromite accumulation occurs. (c) Mixing an evolved komatiite with composition at point F with a primitive komatiite with composition at point E shifts the magma composition to point H on the mixing line between E and D. From F-G, chromite crystallizes alone. (d) Contamination causes the magma composition to shift into the chromite-only crystallization field. From H-G, chromite crystallizes alone. From G-C, olivine and chromite crystallize together. After Irvine (1977). 7

8 siliceous country rock by the primitive ultramafic magma. The curved nature of the cotectic between olivine and chromite suggests that adding silica to a primitive magma would cause chromite to crystallize (Irvine, 1975). Spandler et al. (2005) found that the composition of melt inclusions from chromite in the Stillwater Complex, Montana supports this theory, as the melt inclusions had silica compositions suggesting mixing occurred between silica-rich and silicapoor melts. However, Irvine (1977) pointed out that if contamination also introduced alkalis to the primitive magma, the cotectic would be shifted in such a way that geologically improbable amounts of contamination would have to occur for chromite-only crystallization to begin. In addition, Mondal and Mathez (2007) report no change in the chemistry of orthopyroxenites above chromitite layers to suggest contamination occurred during the emplacement of the Bushveld Complex. OBJECTIVES The long term objective of this study is to evaluate the validity of the contamination hypothesis for the formation of the Ring of Fire chromitites by comparing experimentally produced and natural chromite. The short term objectives of this study that will ultimately allow for achievement of the long term objective are: 1. To perform experiments in which komatiite melt is contaminated with a range of crustal contaminants whose compositions are based on those from the Ring of Fire; 8

9 2. To obtain data on how different crustal contaminants affect chromite crystallization, including chromite solubility and composition; and 3. To compare experimental data to measured values from the Ring of Fire chromitite. METHODS This study utilizes three methods to achieve its objectives: experiments using a vertical tube gas-mixing furnace, electron microprobe analysis (EMPA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Vertical tube gas-mixing furnaces The experiments carried out in this study must allow for the isolation of four variables: temperature, pressure, fo 2, and melt composition. Of these five variables, fo 2 is the most difficult to control during experiments. The vertical tube gas-mixing furnace control fo 2 by altering the proportions of two gases (Huebner, 1975). Dalhousie s Experimental Geochemistry Laboratory houses two such furnaces. Figure 2 illustrates the experimental design for this study. 9

10 The experimental temperatures will be based on the liquidus temperatures for komatiite and the chosen contaminants. Experiments in vertical-tube gas-mixing furnaces at atmospheric pressure (0.1 MPa). Melt composition will vary between 100 wt.% komatiite and 50 wt.% komatiite and 50 wt.% contaminant. All materials will be synthetic. Oxygen fugacity will be kept constant at the fayalitemagnetite-quartz (FMQ) buffer using varying proportions of CO 2 and CO gases, as this buffer is a good approximation for most terrestrial igneous rocks Achievement of equilibrium will be verified by comparing the melt compositions of 48 and 96 hour experiments. This type of furnace will allow for effective isolation of the four previously mentioned variables Figure 2. Schematic of the vertical tube gas-mixing furnace. (1) Fused silica glass rod shaped into a hook at the end. (2) Gas outflow tube. (3) Platinum wire loop onto which the sample is attached with polyvinyl alcohol glue. (4) Alumina tube. (5) Gas inflow tube. Samples are quenched by rapidly pushing the silica hook through the bottom of the furnace into water. and the completion of this study s first short term objective. Electron Microprobe Analysis (EMPA) Once experiments are completed, the products will be analyzed quantitatively. Of particular interest to this study is the amount of Cr present in the melt before and after experiments, and the composition of experimentally produced chromite. These quantities will be measured using EMPA. EMPA 10

11 analyzes the major element composition of samples by bombarding them with an electron beam, generating X-rays whose wavelengths can be used to identify different elements. EMPA is capable of measuring the major element composition of samples (Si, Al, Fe, Mg, Ca, Na, K) with detection limits between parts per million using the wavelength dispersive spectroscopy (WDS) method. The energy dispersive spectroscopy (EDS) method is also useful to quickly identify mineral phases. The Robert M. MacKay Microprobe facility at Dalhousie is capable of both these methods. EMPA will help to achieve the second and third short term objectives of this study by gathering data on the composition of experimentally produced chromite and coexisting melts and chromite from the Ring of Fire chromitites. Laser Ablation Inductively Couple Mass Spectrometry (LA-ICP-MS) Although EMPA is sufficient for major element chemistry, a more precise method is required to gather meaningful data on trace element chemistry. The synthetic nature of the materials used in this study eliminates all trace elements except for Cr. Therefore, Cr content of the melt and mineral phases will be measured using LA-ICP-MS. LA-ICP-MS works by introducing the sample in aerosol form to a plasma source. The plasma ionizes the elements within the sample, which are then separated and identified according to their mass to charge ratio. This method has a much lower detection limit than EMPA: for Cr, the detection limit can be as low as 10 parts per trillion. The Magmatic and Ore-Forming Processes 11

12 Laboratory at the University of Toronto houses the LA-ICP-MS that will be used for this study. This method will complement EMPA in achieving the second and third short term objectives of this study by collecting data on the Cr content of experimental melts. Figure 3 illustrates how these outlined methods will facilitate achievement of the objectives of this study. Figure 3. Workflow diagram. Each short-term objective of this study is represented by an arrow with the method(s) used to complete it beneath. Completion of the short-term objectives will allow for the long term objective of evaluating the long-term objective of this study to be completed. ANTICIPATED RESULTS AND SIGNIFICANCE This study will provide data on the effect of contaminant composition and degree of contamination on chromite composition and solubility in komatiitic magmas. The results will fill an important gap in our understanding of chromitite formation and validate or disprove the contamination hypothesis (Irvine 1975). A better understanding of chromitite formation will guide future exploration efforts to magmatic systems with the highest potential to crystallize chromitites. 12

13 REFERENCES Barnes, S.J The distribution of chromium along orthopyroxene, spinel, and silicate liquid at atmospheric pressure. Geochimica et Cosmochimica Acta, 50: Borisov, A. and Palme, H Solubilities of noble metals in Fe-containing silicate melts as derived from experiments in Fe-free systems. American Mineralogist, 85: Ford et al Role of water on the evolution of the lunar crust; an experimental study of sample 14310; an indication of lunar calc-alkaline volcanism. Proceedings of the Third Lunar Science Conference, Geochimica et Cosmochimica Supplement, 1: Gaetani, G.L., Grove, T.L., and Bryan, W.B Experimental phase relations of basaltic andesite from hole 839B under hydrous and anhydrous conditions. In: Hawkins, J., Parson, L. & Allan, J.(eds) Proceedings of the Ocean Drilling Program: Scientific Results, 135.College Station, TX: Ocean Drilling Program, pp Hoover, J.D. and Irvine, T.N Liquidus relations and Mg Fe partitioning on part of the system Mg 2 SiO 4 Fe 2 SiO 4 CaMgSi 2 O 6 CaFeSi 2 O 6 KAlSi 3 O 8 SiO 2. Carnegie Institute of Washington Yearbook, 77: Huebner, J.S Oxygen Fugacity Values of Furnace Gas Mixtures. American Mineralogist, 60: Irvine, T.N Crystallization sequences in the Muskox intrusion and other layered intrusions II. Origin of chromitite layers and similar deposits of other magmatic ores. Geochimica et Cosmchimica Acta, 39: Irvine, T.N Origin of chromitite layers in the Muskox intrusion and other stratiform intrusions: A new interpretation. Geology, 5: Mondal, S.K. and Mathez, E.A Origin of the UG2 chromite later, Bushveld Complex. Journal of Petrology, 48: Keith, M.L Phase equilibria in the system MgO-Cr 2 O 3 -SiO 2. Journal of the American Ceramic Society, 37: Lipin, B.R Pressure Increases, the Formation of Chromitite Seams, and the Development of the Ultramafic Series in the Stillwater Complex, Montana. Journal of Petrology, 34: Mondal, S.K. and Mathez, E.A Origin of the UG2 chromitite layer, Bushveld Complex. Journal of Petrology, 48:

14 Murck, B.W. and Campbell, I.H The effects of temperature, oxygen fugacity, and melt composition on the behaviour of chromium in basic and ultrabasic melts. Geochimica et Cosmochimica Acta, 50: Nicholson, D.M. and Mathez, E.A Petrogenesis of the Merensky Reef in the Rustenburg section of the Bushveld Complex. Contributions to Mineralogy and Petrology, 107: Roeder, P.L. and Reynolds, I Crystallization of Chromite and Chromium Solubility in Basaltic Melts. Journal of Petrology, 32: Sisson, T.W. and Grove, T.L Experimental investigations of the role of H 2 O in calc-alkaline differentiation and subduction zone magmatism. Contributions to Mineralogy and Petrology, 113: Spandler, C., Mavrogenes, J., and Arculus, R Origin of chromitites in layered mafic intrusions: Evidence from chromite-hosted melt inclusions from the Stillwater Complex. Geology, 33: