Geochemistry of diamonds and their mineral inclusions constraining the composition of the lithospheric mantle and recycling of crustal materials
Creators: Lai, Mei Yan
Publisher: name: University of Alberta Library
Year: 2022

<p>Bimineralic eclogite xenoliths from Koidu have been extensively studied previously, but less attention has been directed towards diamondiferous eclogite xenoliths as well as eclogitic and peridotitic diamonds from this locality. To gain insight into the geologic history of the lithospheric mantle of the West African Craton beneath Koidu, a suite of 105 diamonds and their mineral inclusions and six diamond bearing eclogites from Koidu are analyzed for major element, trace element and stable isotopic compositions in this study. Of 105 Koidu diamonds, 78% contain eclogitic mineral inclusions, 17% contain peridotitic mineral inclusions and 5% contain co-occurring eclogitic and peridotitic mineral inclusions indicating a mixed paragenesis. All diamonds were analyzed in situ for carbon (δ13C) and nitrogen (δ15N) isotope compositions, nitrogen concentrations and nitrogen aggregation states. Based on the δ13C-δ15N systematics, Koidu diamonds can be classified into three distinct compositional clusters: (1) diamonds derived from recycled crustal carbon and nitrogen (δ13C = -33.2 to -14.4‰; δ15N = -5.3 to +10.1‰); (2) diamonds derived from mixing of carbon and nitrogen from subducted and mantle sources (δ13C = -6.0 to -1.1‰; δ15N = -4.2 to +9.7‰); and (3) diamonds derived from mantle-derived carbon and nitrogen (δ13C = -7.8 to -3.6‰; δ15N = -7.9 to -2.1‰). The distinct isotopic signatures of the three diamond clusters suggest episodic diamond growth during multiple fluid/melt pulses. Peridotitic diamonds contain olivine inclusions with high Mg# (92.2–94.7), indicating highly-depleted dunites or harzburgites as the preferred substrates for peridotitic diamond formation at Koidu. High-Ti spinel inclusions (TiO2 = 2.8 wt%) occurring in one peridotitic diamond indicate melt-metasomatic Ti re-enrichment which may have converted some dunites or harzburgites into lherzolites, as lherzolitic garnet dominates in heavy mineral concentrate recovered from Koidu. Eclogitic diamonds with coexisting garnet and omphacite inclusions were used to estimate the depth distribution of eclogitic substrates through the Koidu lithospheric mantle. Diamond-bearing eclogites with gabbroic oceanic crust protoliths derived from shallower lithospheric depths (150 to 190 km) compared to those with basaltic protoliths (≥ 200 km). Eclogite whole-rock trace-element compositions reconstructed from co-existing garnet and omphacite inclusions indicate that a subset of eclogites experienced a high-degree of melt extraction (&gt; 20% batch melting), followed by metasomatic LREE re-enrichment prior to or coeval with diamond formation. A high abundance of coesite inclusions (in 44% of studied diamonds) is in apparent contradiction to the high modeled melt degrees but can be reconciled if the coesite is of secondary nature, introduced via metasomatism subsequent to melt extraction. A set of 16 eclogitic garnet inclusions were analyzed for oxygen isotope composition (δ18O). One eclogitic garnet inclusion has a positive Eu-anomaly and δ18O of +5.4 ‰, suggesting derivation from gabbroic layers of oceanic crust. Fourteen eclogitic garnet inclusions have much higher δ18O (+9.9 to +12.1 ‰) consistent with extensive low-temperature seawater alteration of upper basaltic layers of oceanic crust protoliths. The one remaining garnet inclusion has a mild majorite component (Si = 3.03), with δ18O of +6.3 ‰ and relatively high Mg# (79.0) and Cr# (0.90), suggesting its formation involved interaction with ambient peridotitic mantle. Covariations between major element compositions (Mg#, Ca# and Cr#) and δ18O in garnet inclusions suggest possible chemical exchange between eclogite and ambient mantle, which may have been facilitated by eclogite-derived melt. However, δ13C of the host diamonds do not show correlations with δ18O of these garnets, suggesting that there was no significant mixing between crust- and mantle-derived carbon during diamond formation.</p>