Diamonds as probes of earth's mantle in space and time

Diamonds probe the otherwise inaccessible interior of Earth since at least ~3.5 Ga. In this thesis, two diamond populations from the Canadian portion of the Laurentian supercraton are utilized to investigate the composition and evolution of Earth’s mantle through a wide range in time and depth. In Chapter II, a batch of more than 3000 diamonds from ~2.73 Ga ultramafic volcaniclastic rocks in the Oxford Lake - Knee Lake greenstone belt provides information on the thermal state of the lithospheric mantle underlying the northwestern Superior Craton during its accretionary period. The Knee Lake diamonds show unusual physical characteristics relative to typical kimberlite-hosted diamonds, which, together with a correlation between crystal shape and nitrogen aggregation, point to a single, rapid growth event in a cool mantle substrate driven by strong C-oversaturation of the growth medium. Pressure-temperature estimates from a garnet-olivine inclusion pair indicate diamond formation at conditions equivalent to a cool, modern-day reference geotherm of 38–39 mW/m2. At ~2.73 Ga the lithospheric column also contained eclogite formed by metamorphism of a recycled crustal protolith, as indicated by an omphacite inclusion and low δ13C values of the host diamond and other diamonds of the suite. The Knee Lake diamond suite likely records cool transient thermal conditions inherited from subduction stacking of lithospheric slabs to form the lithospheric mantle of the western Superior Craton. In Chapter III, secondary ion mass spectrometry analyses of 186 Knee Lake diamonds yielded the lowest (-46.8‰) to the highest (+31.8‰) δ15N values ever measured for terrestrial diamonds. The extreme range and discontinuous distribution of the δ15N values rule out an explanation through mixing of heterogeneous N sources and its origin remains enigmatic. The systematics of N concentration and δ15N variations for most of the diamonds are permissive of a Rayleigh distillation process. The very large δ15N range, that is uncorrelated with δ13C or N concentration, excludes equilibrium fractionation processes. Kinetic isotopic effects associated with decomposition of N-phases could produce the most 15N-enriched compositions, but not the most 15N-depleted. The extreme δ15N values of the Knee Lake diamond suite are correlated to crystal shape, with the most negative values occurring in cuboid and the most positive in octahedral crystals. This suggests the operation of fractionation processes related to different growth-surface structures or growth conditions. Distinct δ15N values in different growth sectors of mixed habit synthetic diamonds are confirmed by the analysis of seven synthetics. However, ten Knee Lake natural diamonds of mixed habit growth show only small and non-systematic internal δ15N variability related to growth sectors. Models of N isotope diffusive fractionation along thermal and chemical gradients are explored. Thermal diffusion cannot explain the large δ15N range of the Knee Lake diamonds and would predict opposite fractionation directions. Diffusion along chemical gradients is the only process that can produce the low δ15N values. These observations call for the operation of various kinetic effects during diamond growth, likely superimposed on source variability. Chapter IV focusses on diamonds from the Candle Lake C29/30 kimberlite, erupted on the northwestern edge of the Sask Craton, Canada. The characteristics of the Candle Lake diamonds suggest a complex population, composed of at least two subgroups. The larger diamonds are more often highly resorbed, internally pure and N-rich, and derive from lithospheric eclogites as substrates, as indicated by their inclusion mineralogy and δ13C values. The N characteristics of the lithospheric diamonds at Candle Lake are consistent with a diamond formation age of ~1.27 Ga reported for the Fort à La Corne deposit, suggesting a widespread Proterozoic diamond growth event in the Sask Craton. Diamonds with pristine octahedral shape, often aggregates of multiple crystals, are commonly of smaller dimensions and N-free. They have C-isotope compositions close to the mantle value and carry sublithospheric mineral inclusions of metaperidotitic paragenesis. Inclusion assemblages and residual pressures attest to their derivation from lower mantle to transition zone depths. The occurrence of carbonate inclusions in this diamond suite suggests diamond formation from carbonatitic melts at oxygen fugacity higher than in ambient mantle. The pristine shape of the sublithospheric diamonds may be explained by their upward transport to lithospheric depth only after the intense resorption event affecting the lithospheric diamonds.

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