Abstract A number of polyphase or single-phase ceramic waste forms have been considered as options for the disposal of nuclear waste in geological repositories. Of critical concern in the scientific evaluation of these materials is their performance in natural systems over long periods of time (e.g., 10 3 to 10 6 years). This paper gives an overview of the aqueous durability of the major titanate host phases for actinides (e.g., Th, U, Np, Pu, Cm) and important fission products (e.g., Sr and Cs) in alternative crystalline ceramic waste forms. These host phases are compared with reference to some basic acceptance criteria, including the long-term behaviour determined from studies of natural samples. The available data indicate that zirconolite and pyrochlore are excellent candidate host phases for actinides. These structures exhibit excellent aqueous durability, crystal chemical flexibility, high waste loadings, and well-known processing conditions. Although both pyrochlore and zirconolite become amorphous due to alpha-decay processes, the total volume swelling is only 5–6% and there is no significant effect of radiation damage on aqueous durability. Hollandite also appears to be an excellent candidate host phase for radioactive Cs isotopes. Brannerite and perovskite, on the other hand, are more prone to alteration in aqueous fluids and have a lower degree of chemical flexibility. With the exception of hollandite, many of the properties of these potential host phases have been confirmed through studies of natural samples.

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 H 2 O- 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.