Temperatures and Heat Production in the Slave Craton Lower Crust: Evidence from Xenoliths in the Diavik A-154 Kimberlite

• Author / Creator

  Gruber, Benjamin Hess

• Lower crustal heat production is poorly constrained due to the relative inaccessibility of
  lower crustal samples and their inherent complexity. To obtain the requisite information, the
  current project conducts spatially resolved geochemical analyses on minerals in 15 lower crustal
  xenoliths erupted via the Diavik A-154 kimberlite of the Northwest Territories, Canada. The aims
  are to: 1) conduct geothermometric measurements on lower crustal minerals, 2) construct a heatproducing
  element budget of the lower crust of the Slave craton, and 3) test the validity of these
  measurements in a parameter space relevant to geodynamic modeling and diamond exploration.
  The Diavik lower crustal xenolith suite comprises two main lithologies, mafic granulite
  (garnet-plagioclase-clinopyroxene ± orthopyroxene) and metasedimentary granulite (garnetplagioclase-
  orthopyroxene ± quartz ± K-feldspar ± kyanite), which are present in proportions of
  approximately 80:20, respectively. Application of mineral-pair, iron-magnesium exchange
  geothermometers (garnet-biotite, garnet-amphibole, and garnet-clinopyroxene) to these xenoliths
  indicates that the lower crust was at a maximum temperature of roughly 500 °C at the time of
  kimberlite eruption (~ 55 Ma). The actual temperature of the lower crust is likely lower than 500
  °C as the geothermometers probably record the closure temperature of diffusional Fe2+-Mg
  exchange between touching mineral pairs rather than the ambient temperature of the rocks prior to
  their entrainment in the kimberlite magma.
  Heat-producing element (HPE) concentration measurements show that the lower crustal
  heat production of the Slave craton is likely 0.14 ± 0.02 μW/m3, which is lower than most values
  in the literature but broadly comparable to some geophysical estimates. This estimate is the result
  of (20:80) bimodal mixing of idealized lower crustal endmembers: a metasedimentary lower crust
  (0.37 ± 0.06 μW/m3) and a mafic lower crust (0.08 ± 0.01 μW/m3). These endmembers were
  iii
  calculated via a reconstructed bulk rock calculation utilizing trace element concentrations of
  constituent lower crustal minerals and idealized lithologies from the lower crustal xenoliths.
  Using these heat production estimates and other crustal parameters such as continental heat
  flux, mantle heat flux, crustal thickness, and crustal thermal conductivity, I modeled a Moho
  temperature for the Slave craton of 425 °C, which is consistent with maximum lower crustal
  temperature estimate given by geothermometry. Adjusting the lower crustal heat production in the
  geotherm modeling program FITPLOT changes the temperature of the Moho in a similar fashion
  to the calculated models; however, the diamond propensity of the mantle lithosphere (partially a
  function of Moho temperature and heat production) does not appear to be strongly affected by a
  changing Moho temperature and is more strongly controlled by the conditions of the mantle P-T
  array.

• Subjects / Keywords

  • Heat Producing Elements
  • Deep Crustal Temperatures
  • Crustal Xenoliths
  • Slave Craton
  • Diavik
  • Moho

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-mxym-n231

• License

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