IM-LU” ,NG ISOTOPE GEOSCIENCE ELSEVIER Chemical Geology 142 (1997) 63-78 Lu-Hf geochronology applied to dating Cenozoic events affecting lower crustal xenoliths from Kilbourne Hole, New Mexico Erik E. Scherer a3 *, Kenneth L. Cameron a, Clark M. Johnson b, Brian L. Beard b, Karin M. Barovich b3c, Kenneth D. Collerson d a Earth Sciences Department, Unioersity qf California, Santa Cruz, CA 95064, USA b Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USA ’ Department of Geology and Geophysics, University of Adelaide, Adelaide, S.A. 5005, Australia d Department of Earth Sciences, The University of Queensland, Brisbane, QLD. 4072, Australia Received 4 October 1996; accepted 15 May 1997 Abstract Melt-extraction or crystal accumulation events that affected garnet-bearing, granulite xenoliths from Kilboume Hole, New Mexico, have been dated using the Lu-Hf isotope system. Two garnet-bearing granulites from Kilboume Hole have extreme ‘76Lu/ ‘77Hf ratios of 0.95 and 1.3 (Lu/Hf = 28 and 39 X chondritic), but relatively ‘normal’ ~~~ values (+ 5, and + 12) necessitating either garnet accumulation or melt-extraction from a garnet-bearing protolith in the Cenozoic. Hf isotope evolution curves for these two samples intersect those of depleted mantle and Proterozoic crust at high angles and at similar times, demonstrating the potential of Hf isotope model ages to yield true age significance even if the initial Hf isotope composition is not well constrained. A third garnet-granulite xenolith (CKH63; ‘76Lu/ ‘77Hf = 0.025) contains zircon, which buffered this sample against changes in Lu/Hf during the Cenozoic differentiation event. The three garnet granulites lie closely about a 25 Ma Lu-Hf reference line, demonstrating the potential of the Lu-Hf system for detecting garnet-controlled differentiation events in the Cenozoic; given the range in Lu/Hf ratios measured, events as young as 5 Ma may be detected using the Lu/Hf system. Conventional U-PI, zircon data from sample CKH63 reveal both a ca. 1.4 Ga inherited component and a component of recent Pb-loss (or new zircon growth), supporting the Cenozoic event documented by Hf isotope model ages. 0 1997 Elsevier Science B.V. 1. Introduction crust (Unruh et al., 1983; Salters and Hart, 1989; The Lu-Hf system is demonstrably sensitive to the mineralogy of magma sources in the mantle and Beard and Johnson, 1993; Johnson and Beard, 1993; Johnson et al., 1996). As a result, the Lu-Hf system provides a view of crust-mantle evolution that is * Corresponding author. Tel.: + 1 (408) 459-5228; fax: + 1 (408) 459-3074: e-mail: escherer@cats.ucsc.edu unique as compared to more commoniy used isotopic systems such as Rb-Sr, Sm-Nd, and U-Pb. How- ever, geochronological application of the Lu-Hf sys- 0009-2541/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOOOS-2541(97)00076-4