Leaching and differential recrystallization of metamict zircon under experimental hydrothermal conditions

We report results of hydrothermal experiments on four alluvial zircons from Sri Lanka, which cover a wide range of radiation damage, at 450 degreesC a nd 1.3 kbar for 744 h with 2 M CaCl2 solution as reactive fluid. After the hydrothermal treatment, the most metamict samples show micrometer-thick rea ction rims, which surround apparently unreacted zircon, as revealed by cath odoluminescence (CL) and Nomarski differential interference contrast (NDIC) images. These rims have sharp, curved, and transgressive boundaries with u nreacted zircon and are, in some cases, spread out along cracks. The thickn ess of reaction rims increases with increasing cumulated cn-dosage of the s tarting materials. The reaction rims are strongly enriched in Ca (up to 700 0 ppm) and a water species and depleted in radiogenic Pb, Zr, and Si, as re vealed by electron microprobe analyses. A significant Th loss from the reac tion rims was detected in the case of the most metamict sample, whereas U r emained in the structure. FT-infrared spectrometry and X-ray diffraction me asurements revealed that the bulk run products were recrystallized. Using m icro-Raman spectrometry, we were able to demonstrate that differential recr ystallization took place. The reaction rims are strongly recrystallized, wh ereas the unreacted grain interiors underwent only minor recrystallization. Recrystallization of the rims is accompanied by an enhancement of the inte gral CL intensity. It is suggested that recrystallization in the reaction r ims was catalyzed by water infiltration and ion exchange and prevented sign ificant congruent zircon dissolution under the given experimental condition s. Previous zircon studies have shown that (1) a transgressive morphology, (2) a reduced Th-U ratio, and (3) an enhanced CL emission are also characte ristics of rims in zircons from high-grade metamorphic rocks. Based on thes e similarities between natural and experimentally produced rims, it is sugg ested that leaching-catalyzed recrystallization is an important alteration process in zircon under wet geological conditions and can account for many complex core-rim structures found in natural zircons. Furthermore, the stro ng enrichment of Ca in the reaction rims supports previous assumptions that high Ca concentrations in natural zircons are of secondary origin. It is s uggested that lower U-Pb concordia intercept ages obtained from single-phas e zircons with high Ca contents date a leaching event.