Probing the Mantle Cargo of Kimberlites – A Geochemical Investigation of Different Mineral Components and Evaluation of their Potential as Petrogenetic and Exploration Tools

• Author / Creator

  Bussweiler, Yannick

• This thesis applies microanalytical tools to the geochemical investigation of different mineral components in the volcanic rock kimberlite. This work significantly advances the application of one of these tools, deep UV (193 nm) laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), to the investigation of the most common mineral in kimberlite and the upper mantle – olivine. The investigation is focused on, but not limited to, samples from the Lac de Gras kimberlite field in the Central Slave Craton, Canada. The overall aim is to further illuminate the applicability of clinopyroxene, garnet, and olivine as petrogenetic and exploration indicator minerals using advanced analytical techniques of electron microprobe microanalysis (EPMA), quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and secondary ion mass spectrometry (SIMS). One petrogenetic topic of interest is the formation of megacrysts, large (> 1 cm) crystals commonly found in kimberlites. Cr-diopside clinopyroxene and Cr-pyrope garnet megacrysts from Lac de Gras kimberlites, Slave Craton, Canada, are investigated. Based on their large crystal size and compositional overlap with lherzolitic phases, the samples can be classified as Cr-rich megacrysts, constituting the first report of such megacrysts in Lac de Gras kimberlites. While trace element systematics and Sr isotope isotopes suggest a genetic link to kimberlite-like melts, a strictly cognate relationship is ruled out, based on evidence for disequilibrium with the transporting kimberlite. Instead, an older metasomatic origin is adopted for their formation, presumably linked to precursor kimberlite melts that stalled at mantle depths and reacted with the wall rocks. The Cr-rich megacrysts also contain fully crystallized former melt inclusions. A detailed petrographical and geochemical study suggests that these polymineralic inclusions constitute an early snapshot of the transporting kimberlite melt which infiltrated the megacrysts during ascent. Within Cr-diopsides, polymineralic inclusions are dominated by calcite and olivine and are commonly surrounded by reaction rims with abundant fluid inclusions (CO2-rich). This mineralogy appears to record long-proposed decarbonation reactions that may play an important role in the evolution of calcite-bearing kimberlites, such as: diopside + dolomite (melt) = olivine + calcite + CO2. In examining new applications of olivine geochemistry using LA-ICP-MS to kimberlite evolution and mantle studies, it is necessary to develop a strict protocol for the measurement of trace elements in olivines. Comparative experiments reveal a more pronounced and complex beam-size dependent inter-element fractionation behavior for olivine that requires careful optimization of ablation parameters and calibration strategies. Methods developed in this thesis have been externally verified in laboratories at the University of Melbourne and at the Geological Survey of Canada. A main conclusion of this study is that matrix-matched calibration is necessary in order to avoid inaccuracies caused by calibration and fractionation effects, especially when using small (< 100 μm) laser spot sizes. Thus, an important output of this study is the characterization of appropriate olivine standards for matrix-matched LA-ICP-MS calibration. The development of a fast and accurate method for analyzing trace elements in olivine opens up new possibilities to pinpoint the different origins of olivine in kimberlite and to explore its potential as a petrogenetic indicator mineral and as a possible diamond exploration tool. Using natural samples, this investigation presents new trace element discriminators to distinguish between olivines derived from spinel- and garnet-peridotites. Using the high-pressure, high-temperature experiments performed by Brey et al. (1990), it is possible to derive the first experimental calibration of the Al-in-olivine thermometer at upper mantle conditions and hence evaluate the empirical thermometer of De Hoog et al. (2010). Al measurements in the experiments are performed at high spatial resolution (~8 μm) by SIMS, including SIMS mapping in order to investigate element distribution in the experiments. Based on Al concentrations in the experimental olivines, an improved and simplified calibration of the Al-in-olivine geothermometer applicable to garnet peridotites can be derived. Evaluation of this thermometer against olivine from natural garnet peridotites indicates that it may be the most reliable geothermometer currently available for garnet harzburgites.

• Subjects / Keywords

  • Al-in-olivine thermometry
  • Lithospheric Mantle
  • Kimberlite
  • LA-ICP-MS
  • Megacrysts
  • Diamond Exploration
  • Kimberlite Formation

• Graduation date

  Spring 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3KP7V39Q

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.