DETAILED TEXTURAL, GEOCHEMICAL, AND U-PB GEOCHRONOLOGICAL INVESTIGATION OF DETRITAL ZIRCONS WITHIN TRINITITE POST-DETONATION MATERIAL

The world’s first atomic test conducted at the Trinity site on July 16^th, 1945 near Alamogordo (New Mexico) produced a blast melt known as Trinitite. The nuclear explosion generated temperatures >1470 °C, which melted the surrounding arkosic sand and produced a highly heterogeneous (mineralogical, textural, chemical) glass. The latter contains melted residues from both bomb components and ground zero infrastructure, as well as unmelted precursor mineral phases (e.g. quartz, feldspar, calcite, zircon) present within the arkosic sand. Thus, detailed investigation of Trinitite post-detonation material (PDM) can provide valuable forensic information on the atomic device, and its effects on the natural geological background. In this study, we investigate the textural, chemical, and isotopic changes incurred by detrital zircons within Trinitite subsequent the nuclear explosion at ground zero.

The presence of zirconium-enriched areas within a series of Trinitite petrographic thin sections was identified through major element chemical maps obtained by μ-XRF. These areas were subsequently investigated using scanning electron microscopy (SEM) imaging, and revealed that zircons were present as either un-weathered grains, variably damaged, or fully melted (resorbed) into the Trinitite melt glass. Most of the zircon grains investigated are surrounded by Zr-enriched blast melt and in some extreme cases the grain appears mostly melted and Zr enrichment (3-4wt%) is observed up to a distance of 500 μm. In-situ U-Pb dating of the larger (>30 microns) zircon grains by laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) yields a highly variable range of ages, between 0 and ~2500 Ma old; however, a vast majority of the zircon analyses are highly discordant (with 0 Ma lower intercept age) and/or contain significant amounts of common Pb. The latter derives from the surrounding blast melt and was most likely included subsequent the nuclear explosion. Several detrital zircon grains that are texturally intact yield near concordant Proterozoic ages at ~1300 and ~1700 Ma.