GEMOC ARC National Key Centre

W.L. Griffin^1,2, N.J. Pearson¹, E. Belousova¹, S.E. Jackson¹, E. van Achterbergh¹, Suzanne Y. O'Reilly¹ and S.R. Shee³¹

1. GEMOC Macquarie
2. CSIRO Exploration and Mining³Stockdale Prospecting Ltd.

Introduction: Zircon is a widespread member of the low-Cr megacryst suite that occurs in kimberlites worldwide, and the isotopic composition of Hf in this mineral carries information on lithospheric processes. Here we combine analyses of Hf isotopes in zircon megacrysts with data on the Lu and Hf contents of other mantle minerals to speculate on the Hf-isotope budget of the lithospheric mantle.

Methods: The isotopic composition of Hf was measured in-situ in 122 mantle-derived zircon megacrysts from African, Siberian and Australian kimberlites using a Merchantek EO LUV (266 nm Nd:YAG) laser-ablation microprobe and a Nu Plasma multi-collector ICPMS. Analyseswere done with a beam diameter of ca 80 µm, a 10Hz repetition rate, and energies of 0.6-1.3 mJ/pulse. This gave total Hf signals of 1-6x10^-11 A for Hf contents ranging from 0.5-1.5%. Typical ablation times were 80-120 seconds, resulting in flat-bottomed pits 40-60 µm deep. The total amount of Hf consumed in a typical single analysis was ca 50 ng. Interference of ¹⁷⁶Lu and ¹⁷⁶Yb on ¹⁷⁶Hf was corrected using measured intensities of ¹⁷⁵Lu and ¹⁷²Yb. Internal precision on ¹⁷⁶Hf/¹⁷⁷Hf is typically ±0.000020 (2sd). Accuracy was established by 40 analyses of the 91500 zircon standard: ¹⁷⁶Hf/¹⁷⁷Hf = 0.282286±44 (2sd); TIMS value [1] = 0.282290±37. These values are relative to our ¹⁷⁶Hf/¹⁷⁷Hf = 0.282158±40 for the JMC475 Hf and 0.282167±20 for the same standard spiked to ¹⁷⁶Yb/¹⁷⁷Hf = .055 [3].

Results: The analysed zircons range in age from 90 Ma to 1460 Ma, as determined either directly by U-Pb dating or from kimberlite eruption age, and allow indirect analysis of mantle-derived Hf over a long time span and wide geographic area. Most values of ε_Hf fall between 0 and +10, but zircon suites from the Orapa (Botswana) Leicester (South Africa) and Ruslovaya (NE Siberia) kimberlites range down to ε_Hf= -16 (Fig. 1).

Fig. 1. ε_Hf vs age for analysed zircons

Discussion: Combined with published Nd data on the silicate members of the low-Cr megacryst suite [2], these data indicate derivation from magmas with ε_Hf as low as -20. LAM-ICPMS analyses of garnets and clinopyroxenes from mantle-derived peridotite xenoliths show that cratonic mantle has high Hf contents (0.4-1 ppm) and Hf/Nd (0.3-0.5) greater than estimated Bulk Silicate Earth. Depleted harzburgites, which are common in Archean lithospheric mantle, have Lu/Hf ratios low enough (<0.1) to account for the lowest ε_Hfobserved in the zircons, over time spans of 1-3.5 Ga. We therefore suggest that the magmas from which the zircon megacrysts crystallised were derived from Depleted Mantle or OIB sources, and developed negative ε_Hf through reaction with the subcontinental lithospheric mantle. This type of mantle has ε_Hf , ε_Ndand Hf/Nd values appropriate to the "missing reservoir" required by global Hf/Nd systematics.

References:
[1] M. Weidenbeck et al. (1995) Geostandards Newsletter 19, 1-24.
[2] R.A. Jones (1987) In (P.H. Nixon, ed.) Mantle Xenoliths. Wiley. pp.711-724.
[3] W.L. Griffin et al. (1999) Geochim. Cosmochim. Acta (subm.)