Re-Os isotopic evidence for Archean lithospheric mantle beneath the Kimberley block, Western Australia

We report Sm-Nd and the first Re-Os isotopic data as well as platinum-group element concentrations for two of the highest-grade diamond deposits in the Kimberley block of Western Australia. Whole-rock Sm-Nd isotopic data for the 1200 Ma Argyle olivine lamproite and an Argyle peridotite xenolith yield unradiogenic initial isotopic compositions (epsilon(Nd) = -3.2 to -6.0) and depleted mantle model ages of 1750 to 2000 Ma. These data indicate that shallow-mantle light rare earth element enrichment of the Argyle lamproite source probably occurred during the Proterozoic Hooper orogeny. Sm-Nd isotopic data for the 800 Ma Seppelt kimberlite yield a radiogenic initial isotopic composition (epsilon(Nd) = +1.8) and a TDM model age of 1200 Ma, isotopic features consistent with worldwide group I kimberlite occurrences. Re-Os isotopic data yield unradiogenic initial isotopic compositions for the Argyle lamproite and peridotite xenoliths (gamma(Os) = -2 to -6) and Seppelt kimberlite and kimberlitic chromites (gamma(Os) = -7 to -8), whereas data for a picroilmenite megacryst from the 800 Ma Maude Creek kimberlite yield a radiogenic initial Os isotopic composition (gamma(Os) = +27). Our modeling suggests that the Os isotopic composition of the Argyle lamproite and peridotites mag be explained as mixtures of a ca 3000 Ma refractory mantle component (as represented by the kimberlitic chromites) and a ca, 1500 Ma enriched-mantle component (as represented by the picroilmenite megacryst). The xenoliths and chromites yield an imprecise Re-Os isochron age of 3400 Ma, giving a strong indication that the Kimberley block is underlain by Archean continental lithospheric mantle and that this mantle was largely unaffected during Early Proterozoic orogenesis.