Tracking chemical alteration in magmatic zircon using rare earth element abundances

Trace elements in magmatic zircon provide a wealth of petrogenetic information about host magmas. However, they are susceptible to alteration through post-magmatic interaction with hydrothermal fluids. Trace element analyses can also be biased by the inadvertent inclusion of exotic materials, such as mineral or glass inclusions, in analyzed volumes of zircon. In order to screen out samples with altered chemical signatures, zircons with high, flat light rare earth element (LREE) patterns are typically considered to be altered. However, visual selection of such patterns is qualitative and does not address ambiguous cases. The light rare earth element index (LREE-I = Dy/Nd + Dy/Sm) provides an approach for quantitative screening for aqueous alteration and contamination of zircon by exotic materials and was used to assess secondary processes in the greenschist facies Jack Hills detrital zircon suite. However, in addition to aqueous alteration, the LREE-I is also sensitive to melt compositional evolution, and its applicability to alteration in settings other than the Jack Hills quartzite is thus far undetermined. We investigate igneous zircon populations from a variety of geologic settings that show evidence for alteration by contact metamorphism during magma intrusion and deuteric fluid interactions during pluton crystallization. In suites with a high proportion of texturally altered zircons, low LREE-I values are common and this parameter correlates well with other contamination indicators (based on the observed secondary phases deposited in the zircons during fluid flow). Filtering zircon trace element compositions based on the LREE-I appears to remove the majority of chemically altered zircon, in many cases revealing previously obscured magmatic signals.