Lu-Hf garnet geochronology: Closure temperature relative to the Sm-Nd system and the effects of trace mineral inclusions

The highly elevated Lu/Hf of garnets with respect to other minerals, couple d with the new capability of routinely analyzing small samples (25 ng of Hf ) by multiple-collector ICP-MS (MC-ICP-MS), makes the Lu-Hf garnet system a viable geochronometer. The robustness of Lu-Hf garnet-whole rock (gt-wr) a ges, however, needs to be evaluated, and their closure temperature (T-C) an d potential effects of trace mineral inclusions need to be established. To constrain the T-C of Lu-Hf relative to that of Sm-Nd in gt-wr systems, we u sed thermal ionization mass spectrometry (TIMS) and MC-ICP-MS techniques to determine the Lu-Hf and Sm-Nd ages of garnet-bearing rocks for which the g eneral thermochronology had been previously established. Samples include th e Huiznopala Gneiss (Hidalgo, Mexico), the Gore Mountain amphibolite (New Y ork, USA), a xenolith from the Bearpaw Mountains (Montana, USA), and the Sm ith Grade Granite (California, USA). In addition to whole rocks and garnet, the Lu-Hf isotope compositions of hornblende, zircon. and monazite were al so measured. Our data suggest that the T-C of Lu-Hf is greater than or equa l to the T-C of Sm-Nd in gt-wr systems that cooled slowly (<10 degrees C/m. y.) from granulite facies conditions. There is no single T-C for Lu-Hf or e ven a restricted range of T-C that applies to all garnets, as is the case f or Sm-Nd. Leaching experiments and trace element modeling show that monazit e and apatite inclusions may severely affect the Sm-Nd systematics of garne t, but they have little or no effect on the Lu-Hf system. In contrast, zirc on, with its high Hf content, can strongly influence the Lu-Hf systematics of garnets and whole rocks. Zircon from two samples did not achieve Hf isot ope equilibrium with the rest of the rock at the time indicated by gt-wr is ochrons. Zircon is thus capable of preserving an inherited Hf component thr ough periods of high-grade metamorphism. If present in the matrix only, suc h zircon will cause erroneously old Lu-Hf ages, while such zircon present o nly in the garnet will yield ages that are too young. If inherited zircon i s distributed evenly throughout the garnet and matrix, the competing age ef fects will partially cancel, but the age will be too young if the rock cont ains a phase (e.g., hornblende) that buffers the matrix against the influen ce of inherited zircon Hf. For rocks that contain inherited zircon, the max imum effects on the gt-wr age must be determined before the age can be inte rpreted with confidence. Garnets that have significant zircon inclusion con tents (i.e., Lu-176/Hf-177 less than or similar to 0.3 in this study) shoul d be avoided for Lu-Hf gt-wr geochronology. Copyright (C) 2000 Elsevier Sci ence Ltd.