Zircon reaction and stability of the U-Pb isotope system during interaction with carbonate fluid: experimental hydrothermal study

The interpretation of metamorphically induced U-Pb isotopic discordance req uires a thorough understanding of zircon-fluid interactions. With this aim we have studied the behaviour of metamict and crystalline zircon phases and their U-Pb systems by cathodoluminescence after treatment by 2M Na2CO3 sol ution at T = 200-800 degrees C and P = 1-5 kbar for 3-14 days, X-ray diffra ction, microprobe and isotope dilution analysis. The data indicate that zir con transformation under hydrothermal conditions depends on the experimenta l conditions and the degree of structural damage. Reconstitution of defecti ve and impurity-enriched zones of metamict zircon (homogenization of impure element concentrations and increase of crystallinity) was observed at 400 degrees C and P = 1 kbar. Considerable lead and uranium loss occurred under these conditions. As a result of zircon dissolution, newly formed baddeley ite accommodating U from 2M Na2CO3 solution and Zr-Na-silicate were recogni zed. This process intensified with increasing pressure. Study of crystallin e zircon indicates that migration of U and Pb took place only during dissol ution of zircon at T above 650 degrees C. In the presence of carbonate-ions essential U and Pb amounts are lost from metamict zircon at a lower P-T th an is typical for greenschist facies metamorphism.