Argyle Diamonds: How Subduction Along the Kimberley Craton Edge Generated the World’s Biggest Diamond Deposit
Authors: Stachel T. , Harris J. W. , Hunt L. , Muehlenbachs K. , Kobussen A.F. ,   Edinburgh Ion Micro-Probe Facility (EIMF)
Published: February 2020

Based on the mineral inclusion content, diamonds from the Argyle mine, Western Australia, derive primarily (~90%) from eclogitic sources with a minor peridotitic contribution from both harzburgitic and lherzolitic lithologies. The eclogitic inclusions cover a large compositional range and, in part, show unusually high concentrations of mantle-incompatible elements (P, Ti, Na, and K). Coherent trends in major elements (e.g., of Ti or Na versus Mg-number) suggest that the eclogitic diamond source was created by a single process, namely igneous fractionation. Calculated bulk-rock chondrite-normalized rare earth element (REEN) patterns match a section of oceanic crust reaching from lavas and sheeted dikes to upper gabbros. Positive Eu anomalies for garnet and clinopyroxene, with calculated bulk-rock REEN patterns similar to upper (nonlayered) gabbros, are strong evidence for plagioclase accumulation, which is characteristic for the gabbroic portions of oceanic crust. Linking previously published oxygen isotope analyses of eclogitic garnet inclusions with their major element composition reveals a correlation between δ18O (mean of 7.2‰) and Na content, consistent with coupled 18O and Na enrichment during low-temperature alteration of oceanic crust. The carbon isotope composition of Argyle eclogitic diamonds forms a normal distribution around a δ13C value of −11‰, indicative of mixing and homogenization of mantle-and crustal (organic matter)-derived carbon prior to diamond precipitation. Previously published noble gas data on Argyle diamonds support this two-component model. Inclusion-and nitrogen-in-diamond–based thermometry suggests an unusually hot origin of the eclogitic diamond suite, indicative of derivation from the lowermost 25 km (about 180–205 km depth) of the local lithospheric mantle. This is consistent with emplacement of an oceanic protolith during subduction along the Kimberley craton margin, likely during the Halls Creek orogeny (about 1.85 Ga). For Argyle eclogitic diamonds, the relationship between the rate of platelet degradation and mantle residence temperature indicates that both temperature and strain play an important role in this process. Therefore, ubiquitous platelet degradation and plastic deformation of Argyle diamonds are consistent with derivation from a high-temperature environment (softening the diamond lattice) close to the lithosphere-asthenosphere boundary (inducing strain). In combination, the Argyle data set represents a uniquely strong case for a subduction origin of an eclogitic diamond source, followed by mixing of mantle and crustal components during diamond formation.

Some lherzolitic inclusions show a similarity to the eclogitic suite in incompatible element enrichments (elevated P, Na, and K). The presence of a mildly majoritic lherzolitic garnet further supports a link to eclogitic diamond formation, as very similar majoritic components were found in two eclogitic garnet inclusions. The carbon isotope composition of peridotitic diamonds shows a mode between −5 and −4‰ and a tail extending toward the eclogitic mode (−11‰). This suggests the presence of multiple generations of peridotitic diamonds, with indications for an origin linked to the eclogitic suite being restricted to diamonds of lherzolitic paragenesis.