Hf isotopes of MARID (mica-amphibole-rutile-ilmenite-diopside) rutile trace metasomatic processes in the lithospheric mantle

The Hf isotope composition of rutile in mica-amphibole-rutile-ilmenite-diopside (MARID) xenoliths from the Kimberley area of South Africa has been analyzed in situ by laser ablation microprobe-multicollector-inductively coupled plasma mass spectrometry to investigate metasomatic processes in the lithospheric mantle. The ¹⁷⁶Hf/¹⁷⁷Hf ratio shows a wide range, from 0.2811 to 0.2858 (ε_Hf = -55 to +110); much of this range is found within single samples and even within single grains. We suggest that the MARID rutiles initially resulted from the interaction of an asthenospheric melt with ancient depleted harzburgitic mantle with low ¹⁷⁶Hf/¹⁷⁷Hf ratios (<0.2812) and Lu/Hf ratios of ∼0.04, that dominated their Hf budget. The MARID rocks were later metasomatized by a fluid and/or melt that had caused the breakdown of eclogitic or lherzolitic garnet with Lu/Hf ratios of ∼ 0.6, providing a source of highly radiogenic Hf. The low ¹⁷⁶Hf /¹⁷⁷Hf ratios preserved in some MARID rutiles show that the Nd-Hf isotope systematics of kimberlites and lamproites can be explained by mixing between an asthenospheric melt and the ancient subcontinental lithospheric mantle.