Nature of the Mantle Beneath the Argyle AK1 Lamproite Pipe: Constraints from Mantle Xenoliths, Diamonds, and Lamproite Geochemistry

Abstract

Abstract The richly diamondiferous ~1180 Ma Argyle AK1 lamproite pipe at the margin of the Kimberley craton of Western Australia is underlain by a depleted Archean lithospheric root composed mostly of garnet-poor lherzolite. Peridotite xenoliths (some diamondiferous) define a cratonic paleogeotherm with a ~200-km-thick lithosphere, comparable with estimates from present-day seismic S-wave tomography. The Argyle lamproite is highly enriched in incompatible elements and formed by very small degrees of partial melting under reduced H2O- and HF-rich conditions of depleted lithospheric mantle that had undergone long-term (>2 Ga) geochemical enrichment. Multiple metasomatic and thermal events including episodic formation of diamond have impacted on the lithosphere of the Kimberley craton, both predating and postdating Paleoproterozoic reworking of its margins and amalgamation within the larger North Australian craton. At Argyle, the inventory of older (Archean?) peridotitic diamonds in the craton root was augmented by Proterozoic (1.58 Ga) eclogitic diamonds with distinctive light carbon isotope compositions to generate Argyle’s rich diamond grades. The Argyle lamproite and other brief episodes of kimberlite, lamprophyre, and lamproite magmatism (some diamondiferous) on the Kimberley craton all bear their own geochemical and isotopic signature, reflecting variable relative contributions from asthenospheric and enriched lithospheric mantle sources. Mantle melting and eruption of small volumes of these diverse magmas may have been triggered by small short-lived thermal perturbations from the asthenosphere and/or tectonic events elsewhere on the Australian continent, triggered by global plate reconfiguration.