EVALUATING INTRA-CRYSTALLINE ZR STABLE ISOTOPE ZONATION IN A ZIRCON MEGACRYST

Zirconium (Zr) plays a key role in the development of phases like zircon and baddeleyite in magmatic systems, which are crucial minerals in the study of geologic time. Constraining the Zr cycle in the silicate Earth is essential to better understand how the occurrence of these phases is linked to crustal evolution across a variety of geologic settings and time scales. According to first principles of stable isotope fractionation theory, heavy stable isotopes preferentially occupy shorter bond lengths and lowest coordination state. In zircon and baddeleyite, Zr occurs in 8-fold and 7-fold coordination, respectively. In contrast, in silicate melts, Zr is found in a lower (6-fold) coordination state. Therefore, as Zr-rich phases crystallize, they are predicted to be isotopically lighter than the melt. This coordination change is hypothesized to drive Zr stable isotope fractionation. Although preliminary studies in bulk igneous rocks have suggested that δ94/90Zr [i.e., ((94Zr/90Zr)sample/ (94Zr/90Zr)Std-1)*1000] values correlate positively with increasing SiO2 wt.%, a detailed study of single zircon and baddeleyite crystals from a closed igneous system has shown that these phases are isotopically heavy in Zr relative to the coexisting melt, opposite to the prediction from stable isotope theory. One way to elucidate which of these hypotheses is correct is to measure the internal isotopic zoning of a zircon along a core-to-rim transect; zircons are expected to record the changing isotopic chemistry of the melt with which they are in equilibrium as differentiation and concurrent Zr isotopic fractionation take place. Here, we conducted such an experiment by micro-drilling a zircon megacryst across a core-to-rim transect and measuring each drilled core using the double spike technique. In addition, all fragments are in the process of being dated using CA-ID-TIMS U-Pb geochronology to ascertain the differing ages (if any) recorded by crystal growth.