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首页> 外文期刊>Contributions to Mineralogy and Petrology >Lu-176-Hf-176 geochronology of garnet II: numerical simulations of the development of garnet-whole-rock Lu-176-Hf-176 isochrons and a new method for constraining the thermal history of metamorphic rocks
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Lu-176-Hf-176 geochronology of garnet II: numerical simulations of the development of garnet-whole-rock Lu-176-Hf-176 isochrons and a new method for constraining the thermal history of metamorphic rocks

机译：石榴石的Lu-176-Hf-176年代学II：石榴石-整岩Lu-176-Hf-176等时线发展的数值模拟和一种限制变质岩热史的新方法

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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.
机译：我们给出了Lu-176-Hf-176和Sm-147-Nd-143石榴石-全岩（WR）等时线发展的数值模拟结果，并显示Lu-176-Hf-176系统有时会产生假石榴石衰老是由于Lu3 +相对于Hf4 +在石榴石中扩散更快，并且导致相对于Lu-176优先保留了Hf-176。发现当Lu-176-Hf-176石榴石-WR年龄因扩散而受损时，相应的年龄通常倾向于分别在变质岩和变质岩中的石榴石生长之前和之后。由于母核素和子核素的几乎相同的扩散动力学特性，Sm-147-Nd-143石榴石的年龄不受子产物优先保留的影响。石榴石Hf同位素很少会通过在峰值温度下扩散而与周围基体相重新平衡。结果，与从Sm-147-Nd-143系统确定的年龄不同，石榴石Lu-176-Hf-176的年龄很少代表凉爽的年龄。当Lu-176-Hf-176石榴石年龄由于子产物的优先保留而变得虚假时，如果峰值PT条件和冷却速率独立地存在，仍然可以使用这些年龄来限制主体岩石的渐进热历史。受约束的。该技术已通过对加拿大Pikwitonei颗粒域的变质岩的依赖粒度的Lu-176-Hf-176和Sm-147-Nd-143石榴石年龄进行了举例说明。尽管这项研究的重点是石榴石中的Lu-176-Hf-176衰变系统，但子产物的优先保留可能会影响任何母体核素比子同位素扩散快得多的地质年代系统。

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