Lu–hf garnet geochronology: closure temperature relative to the Sm–Nd system and the effects of trace mineral inclusions

Abstract

The highly elevated Lu/Hf of garnets with respect to other minerals, coupled 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) ages, however, needs to be evaluated, and their closure temperature ( T C ) and 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 used 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 general thermochronology had been previously established. Samples include the Huiznopala Gneiss (Hidalgo, Mexico), the Gore Mountain amphibolite (New York, USA), a xenolith from the Bearpaw Mountains (Montana, USA), and the Smith Grade Granite (California, USA). In addition to whole rocks and garnet, the Lu–Hf isotope compositions of hornblende, zircon, and monazite were also measured. Our data suggest that the T C of Lu–Hf is greater than or equal to the T C of Sm–Nd in gt-wr systems that cooled slowly (<10°C/m.y.) from granulite facies conditions. There is no single T C for Lu–Hf or even a restricted range of T C that applies to all garnets, as is the case for Sm–Nd. Leaching experiments and trace element modeling show that monazite and apatite inclusions may severely affect the Sm–Nd systematics of garnet, but they have little or no effect on the Lu–Hf system. In contrast, zircon, 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 isotope equilibrium with the rest of the rock at the time indicated by gt-wr isochrons. Zircon is thus capable of preserving an inherited Hf component through periods of high-grade metamorphism. If present in the matrix only, such zircon will cause erroneously old Lu–Hf ages, while such zircon present only in the garnet will yield ages that are too young. If inherited zircon is distributed evenly throughout the garnet and matrix, the competing age effects will partially cancel, but the age will be too young if the rock contains a phase (e.g., hornblende) that buffers the matrix against the influence of inherited zircon Hf. For rocks that contain inherited zircon, the maximum effects on the gt-wr age must be determined before the age can be interpreted with confidence. Garnets that have significant zircon inclusion contents (i.e., 176Lu/ 177Hf ≲ 0.3 in this study) should be avoided for Lu–Hf gt-wr geochronology.