LuHf geochronology applied to dating Cenozoic events affecting lower crustal xenoliths from Kilbourne Hole, New Mexico

Abstract

Melt-extraction or crystal accumulation events that affected garnet-bearing, granulite xenoliths from Kilbourne Hole, New Mexico, have been dated using the LuHf isotope system. Two garnet-bearing granulites from Kilbourne Hole have extreme 176 Lu 177 Hf ratios of 0.95 and 1.3 ( Lu Hf ≈ 28 and 39 × chondritic), but relatively ‘normal’ ϵ Hf 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; 176 Lu 177 Hf = 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Pb 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.