POSSIBLE ORIGIN OF ENIGMATIC HEMATITE INCLUSIONS IN SHOCKED QUARTZ FROM THE LATE DEVONIAN ALAMO IMPACT BRECCIA

Shocked quartz grains within the Alamo Breccia (AB) contain ubiquitous hematite inclusions. The largest inclusions protrude halfway into and out of each quartz grain, studding their margins. Smaller inclusions are observed deeper within each grain. They have not been observed in target strata or post-impact units and appear uniquely associated with ejecta from the Alamo event. It is easy to speculate that they are tied to the impact although the exact connection has been elusive.

Quartz grains within Argentine impact melt breccias may provide a novel solution. Hematite in the AB inclusions formed after Fe-sulfides. FeS spherules are found attached to and embedded in shocked quartz grains from Argentina. They appear to have impacted the quartz grains in high-velocity collisions. In a unique case from Mar del Plata (MdP), we can demonstrate the extremes at which these collisions occur because a micro-impacted quartz grain also split and sheared an olivine crystal, smearing it across its surface. The olivine shows the signature of being derived from a mesosiderite (P₂O₅=2 wt%, Fo=72, Fe/Mn=37, Fe/Mg=0.4) and may be a fragment from the MdP impactor. Pyrrhotite spherules, possibly molten or condensed material from the impactor, are embedded in the quartz grain and, similar to many AB inclusions, are associated with percussion fractures.

We propose that the larger AB inclusions have a similar origin as micro-impacting particles in the plume or ejecta curtain. Our hypothesis is supported by the observation that planar deformation features (PDFs) in some studded grains are deflected around the inclusions. SEM shows that the smaller inclusions have grown from individual isolated fluid inclusions along the PDF decoration trails. This suggests that Fe-sulfide, from the larger inclusions or from the environment, was mobilized along the originally amorphous PDFs prior to annealing.

Eltanin and some Pampean events demonstrate that significant portions of an impactor can survive deep-water and soft sediment impacts. Both are appropriate to consider for the Alamo event. If the Fe-sulfide inclusions were not micro-impactors, they still may have grown as the result of PDF-assisted diffusion from hot waters concentrated with FeS from the impactor. We are investigating their geochemistry to determine if a connection can be confirmed.