Diamonds with unusual inclusions from unique cratonic settings
Creators: Milne, Sarah Elizabeth Mcadam
Publisher: name: University of Alberta Library
Year: 2024

<p>Diamonds are chemically robust, shielding minerals included within them from external mantle processes. These inclusions preserve vital information about the original chemical and thermal state of the mantle at the time of their encapsulation, sometimes dating back as far as the Paleoarchean. The diamonds themselves offer insights into the behaviour and origin of carbon and its most common impurity, nitrogen. Ninety-nine percent of diamonds originate from the thick subcontinental lithospheric mantle (SCLM; 2.5 Ga. The remaining 1% of diamonds are sampled from the sublithospheric upper mantle, transition zone, and uppermost lower mantle (660-800 km). In this thesis, I discuss diamond populations from two distinct cratons.<br>
In Chapter 2, I characterise a parcel of 374 randomly sampled diamonds from Juína, Mato Grosso State, Brazil, recovered from kimberlite and from alluvial deposits. This locality is unusual as it hosts a large sublithospheric diamond population, which has been well-documented in literature, disproportionately compared to the population of lithospheric diamonds. In this chapter, I assess the proportion of lithospheric to sublithospheric diamonds, both inclusion-bearing and inclusion-free. This evaluation is based upon current knowledge on the differences in diamond morphology, surface features, diamond colour, and nitrogen characteristics between lithospheric and sublithospheric diamonds. I conclude that the overall Juína diamond population is subdivided as follows: 47% sublithospheric, 35% lithospheric, and 18% have an undetermined origin.<br>
In Chapters 3 and 4 I characterise the diamonds from the Sask craton, Canada. The craton is a small crustal fragment with Archean (3.2-2.5 Ga) crustal ages, enclosed within the Palaeoproterozoic (1.9-1.8 Ga) Trans Hudson Orogen. The majority of the Sask SCLM was melt-depleted and stabilised in the Palaeoproterozoic, far later than its Archean crust. Chapter 3 discusses the isotopic compositions of diamonds from the Fort á la Corne (FalC) kimberlites from the Sask craton, as well as the composition of the mineral inclusions within a subset of diamonds. One hundred diamond fragments were analysed for their δ13C-δ15N systematics via SIMS. Many of the diamond fragments were 13C-depleted, with dominant modes at δ13C= -15.3‰ and δ15N= -2.5‰. The low C isotope values indicate involvement of subducted biogenic carbonate/organic matter, while the N isotope compositions are generally mantle-like. The diamonds are characterised by a dominant, unusual paragenesis of transitional websteritic-lherzolitic (W/L-type) inclusions, with less common eclogitic and rare typical peridotitic inclusions. The W/L-type inclusions have major element compositions which are too fertile (e.g., low Mg#) for typical lherzolitic substrates. <br>
In Chapter 4, I utilise Sm-Nd dating of 13 W/L-type silicate inclusions in FalC diamonds (5 cpx and 8 garnets). Regression of all inclusions yields an over-dispersed isochron age of 1261 ±63 Ma. Dividing the data into two groups yields ages of 1205 ±83 Ma (Group 1: 2 cpx and 5 garnets) and 1237 ±51 Ma (Group 2: 3 cpx and 3 garnets). Both ages are within error of each other and overlap the Mackenzie large igneous event and its related dyke swarm at 1270 ± 4 Ma. There is evidence for Mackenzie-aged activity on the Sask craton in the form of the Tremblay sill and six dykes, suggesting that far-field effects from this large igneous event reached the craton. The model for diamond formation involves metasomatic modification of lherzolitic Sask lithospheric mantle through slab-melt infiltration. The metasomatised W/L-type rock later becomes the substrate within which FalC diamonds form. Diamond-forming CHO-fluids or carbonated melts with a distinct subduction signature (13C depletion) would become re-activated during the Mackenzie plume event (~1270 Ma) and lead to FalC diamond formation.<br>
Isotopic inclusion Group 1 has an ɛNdi of -0.8 and a Sr-isotopic composition of 0.7038. Group 2 has ɛNdi of -11.0 and 87Sr/86Sri of 0.7046. Mixing models are consistent with formation of the FalC diamond substrates from two components, specifically, infiltration of lithospheric lherzolite through variable amounts of an incompatible element enriched melt with isotopic characteristics that resemble cratonic lamproite.<br>
Sublithospheric diamonds make up 1% of the FalC population yet are disproportionately represented in production from the Juina district. W/L-type mineral inclusions have only been reported from FalC and the Voorspoed Mine in South Africa. In this study, I highlight these unique diamonds and inclusions as examples of the diversity of diamond forming processes and mantle evolution globally. </p>