The Hf isotope composition of cratonic mantle: LAM-MC-ICPMS analysis of zircon megacrysts in kimberlites

Zircon megacrysts represent a late stage in the crystallisation of the magm as that produced the low-Cr megacryst suite (Ol+Opx+Cpx+Gnt+Ilm+Phl+Zir) fo und in many kimberlites, and may carry information on the sources of the pa rent magmas and the interaction of these magmas with the cratonic lithosphe re. The isotopic composition of Hf has been measured in 124 mantle-derived zircon megacrysts from African, Siberian and Australian kimberlites, using a laser-ablation microprobe (LAM) and a multi-collector (MC) ICPMS. The zir cons range in age from 90 Ma to ca 2500 Ma, allowing indirect analysis of m antle-derived Hf over a long time span. Most values of epsilon(Hf) fall bet ween 0 and +10, but zircon suites from several kimberlites range down to ep silon(Hf) = -16. Combined with published Nd data on the silicate members of the low-Cr megacryst suite, these data indicate crystallisation of zircon from magmas lying well below the terrestrial epsilon(Hf)-epsilon(Nd) array. LAM-ICPMS analyses of garnets and clinopyroxenes from mantle-derived perid otite xenoliths suggest that cratonic lithospheric mantle has Hf/Nd (0.3-0. 5) greater than estimated Bulk Silicate Earth. The depleted and metasomatis ed Iherzolites and harzburgites that make up much of the Archean lithospher ic mantle have Lu/Hf ratios (less than or equal to 0.15) low enough to acco unt for the lowest epsilon(Hf) observed in the zircons, over time spans of 1-3.5 Ga. We therefore suggest that the magmas from which the kimberlitic z ircons crystallised were derived from Depleted Mantle or OzdIB-type sources , and developed negative epsilon(Hf) through reaction with the subcontinent al lithospheric mantle. Copyright (C) 2000 Elsevier Science Ltd.