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摘要
We present numerical modeling results for the development of Lu-176-Hf-176 and Sm-147-Nd-143 garnet-whole-rock (WR) isochrons and show that the Lu-176-Hf-176 system can sometimes produce spurious garnet ages due to the faster diffusion of Lu3+ relative to Hf4+ in garnet, and resulting preferential retention of Hf-176 relative to Lu-176. It is found that when Lu-176-Hf-176 garnet-WR ages are compromised by diffusion, the corresponding ages generally tend to pre- and post-date garnet growth in metapelites and metabasites, respectively. Sm-147-Nd-143 garnet ages are not compromised by preferential retention of the daughter product, due to the nearly identical diffusion kinetic properties of the parent and daughter nuclides. Garnet Hf isotopes are rarely re-equilibrated with those of the surrounding matrix phase(s) via diffusion at peak temperature; as a consequence, unlike ages determined from the Sm-147-Nd-143 system, garnet Lu-176-Hf-176 ages seldom represent cooling ages. When Lu-176-Hf-176 garnet ages are spurious due to preferential retention of the daughter product, it is still possible to use these ages to constrain the prograde thermal history of the host rocks, provided the peak P-T conditions and cooling rate are independently constrained. This technique has been exemplified by modeling the grain-size-dependent Lu-176-Hf-176 and Sm-147-Nd-143 garnet ages of metapelites from the Pikwitonei granulite domain, Canada. Although this study focuses on the Lu-176-Hf-176 decay system in garnet, preferential retention of the daughter product could potentially impact any geochronological system in which the parent nuclide diffuses significantly faster than the daughter isotope.
- 出版日期2015-2
