The petrography and mineral chemistry of melt-intruded mantle xenoliths from the Chidliak kimberlite field, Baffin Island, Canada

  • Author / Creator
    Myers, Brody
  • The nature of kimberlitic melts at mantle depths and their interaction with mantle wall-rocks is poorly constrained. This is due to the scarcity of mantle xenoliths and xenocrysts that have preserved this interaction without disaggregating during their transport to Earth’s surface. This thesis presents the petrography and geochemistry of a set of unaltered mantle xenoliths containing unique, crystallized melt pockets and veins that represent the interaction of mantle melts with wall-rocks.
    The xenoliths in this study were collected from diamond drill core from the Upper Jurassic CH-6 and Lower Cretaceous CH-7 kimberlite pipes in the Chidliak kimberlite field, Baffin Island, Canada. The sample suite consists of peridotites, a pyroxenite, and an eclogite. Major-, minor-, and trace-element compositions of xenolith minerals and kimberlite whole-rocks from CH-6 and -7 were determined. In addition, the isotopic composition of the CH-6 and -7 kimberlite whole-rocks were determined.
    Using conventional element exchange geothermobarometry, the xenoliths were found to have been entrained from a range of depths and temperatures in the mantle including both the spinel and garnet stability fields, which is consistent with their mineralogy. The eclogite equilibration pressure was determined by calculating the temperature it last equilibrated at in the mantle, then projecting it onto the Chidliak mantle geotherm of Pell et al. (2013). Thermobarometric calculations indicate the xenoliths last equilibrated in the mantle at temperatures between 646 and 1238 °C at depths from ~ 63 to 213 km.
    Melt in garnet-bearing peridotites form both pocket and vein textures. In contrast, the eclogite only contains melt veins that follow grain boundaries and occasionally transect garnet grains. Spinel peridotites contain melt veins with textures indicating brittle fracture associated with injection of fluid/melt. The melts crystallised as complex intergrown assemblages of spinel group minerals, apatite, perovskite, ilmenite, and Ni-sulphides with secondary serpentine, brucite, carbonates, and mica. Melt-pockets contain both Cr-rich and Cr-poor spinels with a range of magnesian ülvospinel-magnetite compositions that extend to more oxidized compositions than found in kimberlite groundmass spinels from CH-6 and CH-7. The mineral assemblage and mineral compositions in the melt intrusions are compatible with kimberlite melt infiltrating these xenoliths. Petrographic evidence of the host kimberlite infiltrating the edges of xenoliths, combined with compositional overlap of spinel group minerals, ilmenite, and perovskite in the melt intrusions with the kimberlite groundmass, suggests that the infiltrating melt was the host kimberlite. Cr-rich spinels in some of the melt intrusions indicate the intrusions crystallized from a relatively un-evolved phase of kimberlite, but the relative scarcity of fine-grained ilmenite in the melt intrusions indicates that the intrusion event did not occur until almost all ilmenite had crystallized as megacrysts and/or in the fine-grained kimberlite groundmass. The evidence suggests that the xenoliths were intruded by kimberlitic melt shortly prior to, or during mantle transit of CH-6 and CH-7 kimberlite magmas from 68 to 20 kbar.
    The highly depleted character of most of the peridotites in the xenolith suite (> 40% melt extraction) indicated by the lack of orthopyroxene and high olivine content (> 90%), suggests that clinopyroxene was re-introduced into the xenoliths after initial exhaustion during partial melting in the spinel stability field. Garnets in the peridotite xenoliths typically have Cr2O3 > 3 wt% and are interpreted to have formed via metamorphism during subduction and lithospheric thickening, based on the model of Brey and Shu (2018) to produce high-Cr2O3 garnets. The high CaO content of the garnets relative to typical garnets from highly depleted peridotites, suggests they have been metasomatically-enriched in CaO. This metasomatic agent could have been the same agent that re-introduced clinopyroxene into the xenoliths. The trace element compositions of the garnets in CH-7 xenoliths reveal they were dominantly metasomatized by an Fe-Ti-rich melt. In contrast, two xenoliths with CaO-rich garnets have trace element compositions consistent with carbonatitic metasomatism.

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • 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.