PICKING IN THE DARK: FLUORESCENCE IMAGING OF ZIRCON ON A PETROGRAPHIC SCOPE, A PRACTICAL GRAIN-PICKING TOOL

Zircon grain selection for U/Pb geochronology is typically done on a binocular scope under incident or transmitted white light and/or on a petrographic scope in polarized light. In many techniques (e.g. SIMS, LA-ICPMS) the grains are then cross-sectioned and imaged with backscattered electrons (BSE) and cathodoluminescence (CL), commonly revealing intricate and complex zoning patterns that guide analytical spot placement and age interpretation. Unfortunately this complex zoning is often imperceptible using existing grain-picking methods and so the selection of the “rosetta stone” grains that record the most geochronologic information is left to chance.

Here we present fluorescence images of whole zircon grains excited by 365 nm light (long UV) from a tin-halide bulb fitted to a standard petrographic microscope. Comparison of the whole-grain fluorescence response with panchromatic CL and BSE response of the same grains after cross-sectioning shows that in most grains there is a direct correlation between the fluorescence and the BSE and CL responses. In other words, the fluorescence response can be used to select zircon grains for polishing that have the desired internal zoning characteristics. The applications are myriad. A few of the demonstrated uses include the selection of zircon grains with: 1) metamorphic rims of sufficient thickness to date, 2) multiple compositions of healed fractures and, 3) unusual fluorescence characteristics in a detrital zircon population. Our observations indicate that picking zircon under long-UV excitation is a simple, inexpensive, and useful addition to the standard grain-picking methods.

To explore the origins of the observed correlations we also present preliminary spectroscopic (Raman and fluorescence) data from zircon excited with 473 nm and 532 nm laser-light. The data demonstrate a reduction in the 1000 cm^-1 SiO4 Raman response in zones with high U-content (and presumably high radiation damage) as documented elsewhere. These zones, which are dark in CL and bright in BSE, also exhibit broad fluorescence bands in the 475-490 nm and 565-585 nm regions while those zones that are low in U and bright in CL have narrower and more discrete fluorescence peaks within these same wavelength regions. Work is underway to further document and understand these relationships.