A mantle origin for Paleoarchean peridotitic diamonds from the Panda kimberlite, Slave Craton: Evidence from ~(13)C-, ~(15)N- and ~(33,34)S-stable isotope systematics

摘要

In order to address diamond formation and origin in the lithospheric mantle underlying the Central Slave Craton, we report N- and C-stable isotopic compositions and N-contents and aggregation states for 85 diamonds of known paragenesis (73 peridotitic, 8 eclogitic and 4 from lower mantle) from the Panda kimberlite (Ekati Mine, Lac de Gras Area, Canada). For 12 peridotitic and two eclogitic sulfide inclusion-bearing diamonds from this sample set, we also report multiple-sulfur isotope ratios. The 73 peridotitic diamonds have a mean δ~(13)C-value of - 5.2‰ and range from - 6.9 to - 3.0‰, with one extreme value at -14.1‰. The associated δ~(15)N-values range from -17.0 to + 8.5‰ with a mean value of -4.0‰. N-contents range from 0 to 1280 ppm. The 8 eclogitic diamonds have δ~(13)C-values ranging from -11.2 to -4.4‰ with one extreme value at -19.4‰. Their δ~(15)N ranges from -2.1 to + 7.9‰ and N-contents fall between 0 and 3452 ppm. Four diamonds with an inferred lower mantle origin are all Type II (i.e. nitrogen-free) and have a narrow range of δ~(13)C values, between - 4.5 and - 3.5‰. The δ~(34)S of the 14 analyzed peridotitic and eclogitic sulfide inclusions ranges from -3.5 to +5.7‰. None of them provide evidence for anomalous δ~(33)S-values; observed variations in δ~(33)S are from +0.19 to - 0.33‰, i.e. within the 2 sigma uncertainties of mantle sulfur (δ~(33)S = 0‰). At Panda, the N contents and the δ~(13)C of sulfide-bearing peridotitic diamonds show narrower ranges than silicate-bearing peridotitic diamonds. This evidence supports the earlier suggestion established from eclogitic diamonds from the Kaapvaal that sulfide-(±silicate) bearing diamonds sample a more restricted portion of sublithospheric mantle than silicate-(no sulfide) bearing diamonds. Our findings at Panda suggest that sulfide-bearing diamonds should be considered as a specific diamond population on a global-scale. Based on our study of δ~(34)S, Δ~(33)S, δ~(15)N and δ~(13)C, we find no evidence for subduction-related isotopic signatures in the mantle sampled by Panda diamonds.