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Improving the Use of Eclogitic Garnet As a Diamond Indicator Mineral, and Constraining the Origin

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Improving the Use of Eclogitic Garnet As a Diamond Indicator Mineral, and Constraining the Origin Improving the use of eclogitic garnet as a diamond indicator mineral, and constraining the origin of eclogites in the subcontinental lithospheric mantle by Matthew Fraser Hardman A thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Earth and Atmospheric Sciences University of Alberta © Matthew Fraser Hardman, 2020 Abstract Diamond occurs in the subcontinental lithospheric mantle (SCLM) and is transported to the surface by kimberlite-lamproite volcanism and other deeply-derived volcanic rocks. In the SCLM diamond is often hosted by peridotitic or eclogitic substrates and is a highly sought-after mineral. Even when it occurs in economic abundances in a kimberlite deposit, diamond is extremely scarce, typically below the parts per million level. Therefore, diamond exploration practices often seek out “diamond indicator minerals,” silicate or oxide minerals that may have co-existed with diamond or equilibrated under conditions where diamond may have been stable. These minerals are typically much more abundant than diamond, and the more resistant minerals may be transported by mechanical processes on the surface. By utilising the compositions of diamond indicator minerals recovered in concentrate during kimberlite exploration, it is possible to locate a diamondiferous kimberlite deposit. Diamond exploration practices often employ garnet geochemistry, as garnet is a silicate mineral present in almost all the major diamond-host lithologies in the SCLM. The exploration practices that use high-Cr garnets to locate diamondiferous deposits hosted by peridotitic substrates are robust. Some practices that employ low-Cr garnets to identify diamondiferous deposits hosted by eclogitic substrates, however, are susceptible to error: one of the causes of this is that there is significant compositional overlap between low-Cr garnets from mantle eclogites – which may be diamondiferous – and garnets from lower crustal granulites, which are barren. One existing methods for the discrimination of crustal- and mantle-derived garnets is the Mg# (Mg/[Mg+Fe]) versus Ca# (Ca/[Ca+Mg]) method of Schulze (2003). To determine if the Schulze (2003) method can successfully discriminate garnets derived from granulites from those derived ii from mantle eclogites, I determined the major- and trace-element compositions of garnets from 190 new lower crustal granulite and 529 new mantle eclogite xenoliths, from a variety of kimberlites globally. These data are combined with the major-element compositions of 2977 garnets from published literature. When this combined dataset is applied to the Schulze (2003) method, the full error rate is 17.1 ± 2.1 %, with a misclassification rate of garnets from lower crustal granulites of 39.2 %. One consequence of this for diamond exploration is the possibility of “false positive” signals – crustal garnets classified incorrectly as mantle-derived – which may lead to an erroneous impression of the expected amount of mantle material present in a deposit, if the kimberlite sampled garnet granulite during eruption, or if garnet granulite is also present in till-derived indicator mineral samples. To remedy this situation, in Chapter 2 I derive new probabilistic single-grain discriminants for crustal and mantle garnets using major-element compositions. These discriminants are based on two multivariate statistical methods, namely linear discriminant analysis and logistic regression. The cross-validated error rate of the logistic regression method is 7.5 ± 1.9 %. This error rate is the lowest overall in published literature for the classification of low-Cr garnets derived from crustal and mantle low-Cr garnets. This approach reduces the error rate of garnets from granulites to ~ 7.6 % using the available dataset. These new statistical methods can be applied to single garnets with known major-element compositions and can assign a probability of certainty to every classification. However, even using these new methods the error rates for classification of garnets from granulites and mantle eclogites are non-negligible; major-element geochemical overlap is still present between some low-Cr garnets from crustal and mantle rocks and cannot be resolved even using eight major-elements. Therefore, in Chapter 3, I assess the value of garnet trace-element data in improving classification error rates during diamond exploration. Using a combined iii dataset of garnet trace-element compositions from new xenoliths in this study and data in the published literature, I find that classification error rates for garnets from crustal granulites and mantle eclogites are improved by adding tracing element data as classifiers. I present a new trace-element classifier system using the statistical method Classification and Regression Trees (CART). This CART classifier is additive to the outputs of the major-element method in Chapter 2, and adds garnet Eu-anomalies and Sr concentrations as variables. The combination of the trace-element CART method and the major-element logistic regression method results in an error rate as low as 4.7 % on calibration data. Based on these results, an explorationist can weigh the value of acquiring trace-element data at an additional cost, based on its improvement to classification success rates. Finally, in Chapter 4 I undertake a study of eclogite xenoliths from the former Roberts Victor diamond mine, South Africa. Eclogite xenoliths from the Roberts Victor kimberlite have long yielded fundamental insights into the origins of eclogites and deep cratonic roots. I analysed a new suite of 65 eclogite xenoliths from Roberts Victor for their major- and trace-element compositions. In addition to a new dataset of 34 oxygen isotope analyses by SIMS, I report the first triple oxygen isotope data (δ17O, δ18O) for eight kimberlite-derived eclogites. Eight new samples in the dataset have sub-chondritic whole-rock LREE abundances and are low in Sr, HFSE, sodium-in-garnet, potassium-in-clinopyroxene, Zr/Hf, and δ18O (< 4.0 ‰). These samples classify as Group II eclogites, based on textural equilibrium exemplified by interlocking grains with straight grain boundaries. For the larger sample set of Group I eclogites from Roberts Victor, based on their major- and trace–element characteristics, I concur with previous authors that they are metamorphosed basaltic-picritic lavas or gabbroic cumulates from oceanic crust, crystallised from melts of depleted MORB mantle. For the Group II eclogites, however, I iv propose formation as cumulates in deep oceanic crust from melts that were chemically less- enriched than N-MORB due to derivation from a residual mantle source. Previous melting of this depleted mantle source at garnet- ± spinel-facies preferentially extracted incompatible elements and fractionated Zr-Hf in the residue. Cumulates precipitated from the second-stage melts inherited the residual chemical signature of their mantle sources. Coupling the low δ18O values of the Group II eclogites, which fall outside of the canonical mantle range, with their variable europium anomalies, indicates that they crystallised in plagioclase-facies oceanic crust. The Group II protoliths were altered by seawater at high temperatures (> 350 °C), possibly at greater stratigraphic depths than the Group I eclogite protoliths, consistent with the presumed location at depth of their proposed cumulate protoliths. v Preface This thesis is an original work prepared by Matthew F Hardman and contains the findings of his PhD research supervised by D. Graham Pearson and Thomas Stachel. Samples described in Chapters 2 and 3 were provided by Rio Tinto, De Beers Canada, Peregrine Diamonds Ltd., Jon Carlson (Dominion Diamond Mines), Star Diamond Corp. (formerly Shore Gold Inc.), Slava Spetsius (Alrosa), Juanita Bellinger, D. Graham Pearson, and Thomas Stachel. Sets of major- element data included in Chapter 2 were provided by Daniel Schulze, Thomas Chacko, and Larry Taylor. Samples described in Chapter 4 were provided by Thomas Stachel. A modified version of Chapter 2 has been published as Hardman MF, Pearson DG, Stachel T, Sweeney RJ (2018) Statistical approaches to the discrimination of crust- and mantle-derived low-Cr garnet – Major-element-based methods and their application in diamond exploration. Journal of Geochemical Exploration 186:24-35. I completed all sample preparation and analysis and wrote the manuscript. Andrew Locock assisted in determination of major-element compositions of garnets by electron probe microanalysis. D. Graham Pearson and Thomas Stachel assisted with data interpretation and preparation of the manuscript. Russell Sweeney assisted with the application of multivariate statistical techniques. A modified version of Chapter 3 has been published as Hardman MF, Pearson DG, Stachel T, Sweeney RJ (2018) Statistical approaches to the discrimination of mantle- and crust-derived low-Cr garnets using major and trace element data. Mineralogy and Petrology 112:697-706. I completed all sample preparation and analysis and prepared the manuscript. Yan Luo assisted in determination of trace-element compositions of garnets by laser-ablation ICP-MS. D. Graham Pearson and Thomas Stachel provided substantial assistance in preparation of the manuscript vi through discussion and editorial oversight. Russell Sweeney provided important background on the application of statistical techniques and aided in discussion in the manuscript. Chapter 4 constitutes a study on eclogite xenoliths from the former Roberts Victor
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