Oxygen Isotope Exchange Kinetics Between Coexisting Minerals and Water in the Aral Granite Pliiton of the Altay Mountains,Northern Xinjiang

Abstract Coexisting quartz, feldspar and biotite vary widely in their δ¹⁸O values and display a remarkable ¹⁸O / ¹⁶O disequilibrium relation; especially, a quartz–feldspar reversal (Δ ¹⁸O_{Quartz?feldspar}<0) exsists in the Aral granite pluton of the Altay Mountains, northern Xinjiang. The ¹⁸O / ¹⁶O exchange reaction definitely occurred between granite and water. Initial δ¹⁸O values of the granite and exotic fluid are evaluated by the mass balance consideration. The conventional method of discrimination between various magma derivations simply with δ¹⁸O values of either whole rock or separate minerals is misleading and unreliable. Experiments carried out by the authors show that the ¹⁸O / ¹⁶O exchange reaction is not accompanied by what geologists describe as petrological and mineralogical alteration effect. This decoupling relation implies that exchange reaction occurs at a relatively high temperature during subsolidus–postmagmatic cooling of magmas. The exchange mechanism is mainly diffusion–controlled. It is demonstrated through quantitative modelling that the hydrothermal system associated with the Aral pluton is long–lived (0.8—6 Ma), with a relatively high fluid flow rate (3 x 10^?14 mol. s^?1) and high W/ R ratio (0.79-6.14). This means that an intense flow and convection may exist at the midcrustal level of orogenic magmatic arcs.