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

Abstract

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.