Denis Fougerouse

Nanogeochronology of discordant zircon measured by atom probe microscopy of Pb-enriched dislocation loops

Atom probe yields geologically meaningful ages from nanoscale Pb-enriched dislocation loops in discordant zircon. Isotopic discordance is a common feature in zircon that can lead to an erroneous age determination, and it is attributed to the mobilization and escape of radiogenic Pb during its post-crystallization geological evolution. The degree of isotopic discordance measured at analytical scales of ~10 μm often differs among adjacent analysis locations, indicating heterogeneous distributions of Pb at shorter length scales. We use atom probe microscopy to establish the nature of these sites and the mechanisms by which they form. We show that the nanoscale distribution of Pb in a ~2.1 billion year old discordant zircon that was metamorphosed c. 150 million years ago is defined by two distinct Pb reservoirs. Despite overall Pb loss during peak metamorphic conditions, the atom probe data indicate that a component of radiogenic Pb was trapped in 10-nm dislocation loops that formed during the annealing of radiation damage associated with the metamorphic event. A second Pb component, found outside the dislocation loops, represents homogeneous accumulation of radiogenic Pb in the zircon matrix after metamorphism. The 207Pb/206Pb ratios measured from eight dislocation loops are equivalent within uncertainty and yield an age consistent with the original crystallization age of the zircon, as determined by laser ablation spot analysis. Our results provide a specific mechanism for the trapping and retention of radiogenic Pb during metamorphism and confirm that isotopic discordance in this zircon is characterized by discrete nanoscale reservoirs of Pb that record different isotopic compositions and yield age data consistent with distinct geological events. These data may provide a framework for interpreting discordance in zircon as the heterogeneous distribution of discrete radiogenic Pb populations, each yielding geologically meaningful ages.

Nanoscale gold clusters in arsenopyrite controlled by growth rate not concentration: Evidence from atom probe microscopy

Abstract Auriferous sulfides, most notably pyrite (FeS2) and arsenopyrite (FeAsS), are among the most important economic minerals on Earth because they can host large quantities of gold in many of the world’s major gold deposits. Here we present the first atom probe study of gold distribution in arsenopyrite to characterize the three-dimensional (3D) distribution of gold at the nanoscale and provide data to discriminate among competing models for gold incorporation in refractory ores. In contrast to models that link gold distribution to gold concentration, gold incorporation in arsenopyrite is shown to be controlled by the rate of crystal growth, with slow growth rate promoting the formation of gold clusters and rapid growth rate leading to homogeneous gold distribution. This study yields new information on the controls of gold distribution and incorporation in sulfides that has important implications for ore deposit formation. More broadly this study reveals new information about crystal-fluid interface dynamics that determine trace element incorporation into growing mineral phases.

Nebula sulfidation and evidence for migration of “free-floating” refractory metal nuggets revealed by atom probe microscopy

Disk models have been proposed that imply particles migrate rapidly in a protoplanetary disk. However, the only physical constraints on these processes from meteorites are observations of refractory inclusions in cometary material from the NASA Stardust mission. Atom probe microscopy (APM) of sub-micrometer refractory metal nuggets (RMNs) contained within a Sc-Zr–rich ultrarefractory inclusion (URI) from the ALH 77307 carbonaceous Ornans (CO) 3.0 meteorite revealed the presence of sulfur at 0.06–1.00 atomic percent (at%) abundances within RMNs. The mineralogical assemblage, petrographic texture, and flat chondrite-normalized highly siderophile element ratios indicate S exposure was unlikely to have occurred after the RMNs were incorporated into the URI. APM analyses suggest these RMNs were likely “free floating” when they were exposed to a S-condensing gas. This requires early, rapid migration of RMNs to cooler regions of the disk to incorporate S and then cycling back to the Ca-Al–rich inclusion (CAI)–forming region for incorporation in the URI, or conditions in the CAI-forming region that promote the incorporation of S into RMNs.