Subduction and the genesis of mantle roots and diamonds

• Author / Creator

  Regier, Margo E

• Subduction is a dominant force that has driven the evolution of Earth’s atmosphere, the diversity of its crustal lithologies, and the variety of its deep mantle regimes. In this thesis, I present three separate studies that examine the importance of subduction in the creation of ancient cratonic mantle roots and in the formation of diamonds in the lithosphere, asthenosphere, and lower mantle.

  The opening study, in Chapter 2, examines various models of cratonic mantle formation. The chapter begins by documenting the oxygen isotope signatures of mantle xenoliths from five different Archean cratons to determine if Ca-depleted and Si-enriched cratonic peridotites are derived from the subduction of serpentinized oceanic lithosphere. I find that orthopyroxene and olivine mineral separates, including those from Si-enriched and Ca-depleted samples, have δ18O that is equivalent to that of mid ocean ridge (MORB) source mantle. The contrast of these data to the wide range of δ18O documented in serpentinites does not support the model of recrystallized serpentinites as a protolith for cratonic peridotites. Other possible mechanisms for producing these Si-enriched peridotites are tested using isotopic and elemental modeling. This modeling suggests that the infiltration of slab melts is unlikely to produce the observed degrees of silica enrichment without also producing observable δ18O variations in the crystallized Si-rich orthopyroxene. Instead, I suggest that the infiltration of ascending mantle melts or water-fluxed depleted mantle melts may produce Si-enriched peridotite without significant modification of cratonic mantle δ18O. Subsequent collisional compression of these variably Si-enriched, cratonic mantle protoliths created the thick cratonic roots that exist today.

  The third chapter is devoted to the global carbon cycle and the mechanisms and sources of diamond formation in the lithospheric and sublithospheric mantle. Since the largest uncertainty in the carbon cycle is the depth to which carbon persists in sediments and altered oceanic crust (AOC), I examine a suite of lithospheric and sublithospheric inclusions in Kankan diamonds for the stable isotopic tracers of subduction. Oxygen isotope data on inclusions in diamond, combined with carbon and nitrogen isotopic signatures of diamond hosts, demonstrate the importance of AOC as a carbon-source at diamond-forming depths. I propose that the exceptionally elevated oxygen isotopes of asthenospheric to transition zone majoritic garnet inclusions are derived from their crystallization from carbonated slab melts. This contrasts with the lower mantle environment that is sampled by the Kankan diamonds, which appears to be isotopically identical to the convecting mantle. Since diamond is not stable in the reduced lower mantle due to the solubility of carbon in metal alloys, I propose that crystallization of diamond is driven by the mobilization of carbon after the destabilization of metals near a dehydrating subducting slab. This transition from diamond formation in carbonated slab melts in the transition zone to diamond formation in hydrated lower mantle confirms an experimentally hypothesized lower mantle barrier for carbon subduction.

  Finally, the fourth chapter shifts the focus to the formation of the ultra-rare and valuable boron-bearing blue diamonds in the lower mantle. An analytical method for the first boron isotope analyses of blue diamonds is developed, which identifies a subducted-related origin for the boron impurities. An elemental and mineralogical study of the mineral inclusions and matrix impurities in blue diamonds specifies their formation in a Ca-rich melt. This production of a Ca-rich melt may be induced by the decomposition of hydrous silicate minerals (i.e. ‘post-serpentinite’ phases) that are present in peridotitic sections of the slab at high pressures and temperatures. The interaction of this oxidized Ca-rich melt with the reduced convecting mantle would induce redox-driven diamond formation. The chapter concludes with a discussion of the boron isotopic similarity of blue diamonds, ocean island basalts (OIB), and carbonatites. Their isotopic similarity suggests that the deep mantle sources of carbonatites and OIBs may have been contaminated by the same volatile-bearing oceanic lithosphere that produced the blue diamonds in this study.

  All together, these three independent studies identify the key role played by subduction in the creation of cratonic lithosphere, lithospheric diamonds, sublithospheric diamonds, and deep mantle heterogeneities. These findings help clarify our views on the evolution of volatile-bearing subducting slabs over geologic time and with depth in the Earth.

• Subjects / Keywords

  • diamond
  • isotope
  • subduction
  • craton
  • geology

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-5w3n-hg39

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.