Understanding microtektite formation: Potassium isotope evidence for condensation in a vapor plumeOPEN ACCESS
Piers Koefoed, Luigi Folco, Gianfranco Di Vincenzo, Nicole X. Nie, Billy P. Glass, Mason Neuman, Kun Wang (王昆)
Geochimica et Cosmochimica Acta
Available online 2 July 2024
“Tektite and microtektite formation have important implications on our understanding of impacts both on Earth, the Moon and on other bodies within our solar system. Here, we investigate the formation mechanisms of microtektites by analysing the K isotope systematics and elemental compositions of forty-four Australasian microtektites from various distances from the proposed impact location. Based on the K isotope and concentration data, the microtektites analyzed here are split into two groups, the “ODP group” and the “MB group”. The ODP group were recovered from the Ocean Drilling Project (ODP) sediment cores and consist of microtektites which landed closer to the proposed impact site (∼1220–1240 km) and show limited δ41K variation (–1.06 ‰ to −0.21 ‰) and higher K concentrations (2.48 wt% to 3.66 wt% K2O). In contrast, the MB group were mostly collected from the surface of Miller Butte (MB) in Antarctica and represent microtektites which landed significantly further from the proposed impact site (∼4100–10800 km) and contain large δ41K variations (−4.04 ‰ to 0.57 ‰) and low K concentrations (0.49 wt% to 1.45 wt% K2O). For the microtektites studied here, the overall correlation observed is consistent with condensation whereby a greater extent of K depletion correlates with lighter K isotope compositions. This simple condensation model is in contrast to previous studies which find evidence for complex evolution involving evaporation, condensation, and mixing. For the ODP group microtektites, the isotopic and elemental data suggest condensation from an upper continental crust (UCC) starting composition. Conversely, for the MB group a UCC starting composition is not compatible, as even the most K-rich MB group microtektites are significantly depleted in K and display δ41K values much higher than the UCC. These observations can be explained by a vapor plume with a progressively evolving K isotope composition, with the earliest K condensates depleting and fractionating K within the plume, thus altering the starting K compositions for the later K condensates. From this data we calculate a cooling rate of up to 2,600 K/hour for the ODP group and up to 20,000 K/hour for the MB group, which are comparable to the cooling rates measured for tektites and considerably faster than those theoretically calculated or experimentally determined for chondrules. Overall, when assessed within the context of previous studies, microtektite formation appears very complex with evidence for different volatilization processes to different degrees observed within different microtektites. As such, while condensation appears dominant for K within the Australasian microtektites studied here, more work is needed to fully untangle the processes involved in microtektite formation.”
