Geocheminar spring-2016

When do time capsules leak? Zircon alteration and its implications for the geologic record

Despite the robust nature of zircon in most deep crustal and surface environments, chemical alteration, especially associated with radiation damaged regions, can affect its geochemistry as well as that of its mineral inclusion cargo. This consideration is especially important when drawing inferences from the detrital record where the original rock context is missing. Typically, alteration of zircon is qualitatively diagnosed through inspection of zircon REE patterns and the style of zoning shown by cathodoluminescence imaging, since fluid-mediated alteration often causes a flat, high LREE pattern. Due to its much lower abundance in zircon relative both to other crustal materials and to the other REE, disturbance to the LREE pattern is the most likely first sign of disruption to zircon trace element contents. Using a database of 378 (148 new) trace element and 801 (201 new) oxygen isotope measurements on zircons from Jack Hills, Western Australia, we propose a quantitative framework for assessing chemical contamination and exchange with fluids in this population. The Light Rare Earth Element Index is scaled on the relative abundance of light to middle REE from which to estimate the degree of zircon chemical alteration, or LREE-I = (Dy/Nd) + (Dy/Sm). LREE-I values vary systematically with other known contaminants (e.g., Fe, P) more faithfully than other suggested proxies for zircon alteration (Sm/La, various absolute concentrations of LREEs) and can be used to distinguish primary compositions when textural evidence for alteration is ambiguous. Chemical trends in alteration of Jack Hills zircons are consistent with known phases contaminating cracks in the zircons. An examination of apparent alteration to mineral inclusion records in zircon from a variety of settings reveals common spatial patterns of alteration (largely in cracked and isotopically disturbed regions) and a variety of alteration phases