HIGH SPATIAL RESOLUTION GEOCHEMISTRY OF K-PG IMPACT SPHERULES

A large number of studies have recognized that impact melt holds key information on important cratering parameters during the formation of an impact structure. Because impact melt basically represents the molten, quenched state of the precursor target, its geochemical signatures should match the composition of the target. In addition, upon impact, a small portion of the meteorite may be incorporated within the melt lithologies. In the proximal ejecta deposits all around the Gulf of Mexico, impact melt from the Chicxulub crater occurs predominantly in the form of glassy spherules (microtektites-like), which are interpreted as droplets of melt that travelled ballistically from the point of impact and quenched rapidly during flight. In most localities, spherules are severely altered. Pristine glass is very scarce and only found in the center of a smectite/chlorite shell. Although the glass has been shown to be heterogeneous even on the µm-scale, geochemical data have solely been reported from bulk samples. Simple binary mixing between the sedimentary carbonate and the underlying crystalline basement, which currently remains poorly constrained in terms of composition, does not fully account for the variety of impact melt products. Based on thin section studies, it appears that two other types of melt lithologies are present in the spherule deposit. Melt occurs as altered, irregularly shaped glass shards and schlieren with a more K-Al rich composition. This difference in composition compared to the alteration products of the glassy spherules is either due to alteration or a different source for this type of melt. Using Laser Ablation- Inductively Coupled Plasma- Mass Spectrometry, we have conducted spatially resolved analysis of trace elements in both pristine, unaltered microtektite-like glasses and their surrounding chlorite/smectite alteration phase from both Beloc and El Mimbral localities. This approach offers the opportunity not only to study in situ the mixing of different target lithologies, but also the variation of the trace element budget during alteration processes. Refining the geochemical signature of (altered) melt lithologies may lead to a better understanding of their origin and distribution during crater formation.