EXPERIMENTAL DIFFUSION KINETICS OF GEOCHRONOLOGICAL SYSTEMS AND INTERPRETATIONS OF MINERAL AGES IN TERRESTRIAL ROCKS AND METEORITES

Calculation of closure temperatures (T_c) of geochronological systems in minerals and interpretations of mineral-whole rock (WR) isochron require knowledge of the diffusion kinetic parameters of the species of interest. The ¹⁷⁶Lu-¹⁷⁶Hf (t_1/2 = 37 Ga) decay system has been used extensively to date metamorphic garnets, whereas ²⁶Al-²⁶Mg (t_1/2 = 0.72 Ma) decay system has found wide applications in the field of early solar system chronology. In this paper we present experimental diffusion kinetic data for these two systems and discuss their chronological and other implications. Our results show that D(Hf⁴⁺ ) is around a factor of 30 slower than D(Lu³⁺) in garnet, leading to significant difference in their closure temperatures during cooling of metamorphic rocks. As a consequence, the growth of ¹⁷⁶Hf/¹⁷⁷Hf ratio in a Lu-Hf isochron diagram would be nonlinear because of the diffusive exchange of the parent nuclide (¹⁷⁶Lu) between garnet and matrix phases after the closure of the radiogenic daughter product, ¹⁷⁶Hf (or *Hf) in garnet. This would cause rotation of the Grt-WR “isochron” with the consequence that the (apparent) age derived from the slope of an isochron would be different from the time lapse since the closure of Hf in garnet. In metapelites the ages would be older, with the extent of difference from the closure age of ¹⁷⁶Hf depending on the peak temperature (T_p) experienced by the rock, especially at T_p > 750 ^oC and CR < 5 ^oC/Myr. In addition, significant proportion of the *Hf produced during the prograde path could be retained within typical metapelitic garnets if T_p < 900 ^oC, causing additional complication in the interpretation of Grt-WR “isochrons”. With regard to early solar system chronology using²⁶Al-²⁶Mg decay system, comparison of D(Mg) data and the consequent T_c(Mg) in melilite with those in other important minerals in CAIs shows that the melilite-age is least susceptible and spinel-age is most susceptible to diffusive resetting during cooling in planetary parent bodies. The evidence of Mg isotopic exchange between melilite and anorthite during nebular processes for a period of ~ 300,000 years, as put forth by Young et al., (2005: Science), implies thermal pulses during nebular processes exceeding 1100-1200 ^oC.