AL IN ZIRCON AS A PROXY FOR TERRESTRIAL AND EXTRATERRESTRIAL MELT COMPOSITIONS: A PROGRESS REPORT (Invited Presentation)

We present Al concentrations in zircon grown in hydrous granitic melts in the SiO₂-Al₂O₃-Na₂O-K₂O-CaO-ZrO₂-H₂O system. Four felsic compositions with ASI values (ASI=molar ratio of Al₂O₃/(CaO+Na₂O+K₂O)]) ranging between 0.5 and 1.21 were prepared to represent a variety of crustal melts, including two peraluminous, one metaluminous, and one peralkaline melt. The M-value [(K+Na+2Ca)/(Si∙Al)] of these melts – a compositional parameter used for zircon saturation thermometry – ranges between 1.3 and 3.4. Experiments were conducted from 1100 to 1400 °C (1 GPa) and we infer two main conclusions. First, Al-in-zircon concentration is strongly dependent on melt composition. That is, an increase in ASI results in a corresponding increase in Al concentration in zircon. Broad correlations with M-value are also evident. Second, for similar melt compositions, Al-in-zircon is positively related to temperature. Thus, if temperature is constrained independently, Al-in-zircon holds significant potential as a proxy for melt composition, in agreement with previous data collected on natural zircons from diverse felsic rock types. Our new experimental data are presented along with the first lunar Al-in-zircon measurements. These crystals, which are no longer associated with their parent rock, yield ages that range from ca. 3.95 to 4.35 Ga. We infer at least two distinct melt compositions for these Apollo 14 zircons. We are currently investigating whether this information may help distinguish between zircon that crystallized within impact-generated melts and zircon derived from endogenous lunar magmatism.