IMPACT PLUME CHEMISTRY DETERMINED FROM FERROPSEUDOBROOKITE INCLUSIONS IN ACCRETIONARY SPHERULES AT THE CRETACEOUS-PALEOGENE BOUNDARY, SOUTH CAROLINA

The record of the Cretaceous-Paleogene boundary impact in the coastal plain strata of the Southeastern Atlantic Seaboard was nearly erased by deep estuarine valleys during the middle Tertiary. We previously described the occurrence of a boundary sequence near Columbia, South Carolina perched atop the divide between two large middle Eocene basins (GSA, 2014). The bottom of the deposit consists of a 10 cm section of dark kaolinitic clays, exhibiting a conspicuous zone of dioctohedral smectite, lepidocrocite, and charred leaf debris in addition to a strong iridium anomaly. The clays are capped by a 7 cm laminated layer of densely packed accretionary spherules. Although some of the spherules have been modified by biogenic processes and their matrixes have been replaced by phosphates of the woodhouseite-svanbergite-goyazite series, they contain inclusions including shocked quartz and glass fragments indicative of their impact origin.

An unusual population of Ti-Fe oxides most clearly shows the high-T history of the spherules. Quantitative composition (WDS) data were collected using a JEOL 8600 electron microprobe. The data show that the dominant phase is ferropseudobrookite, Fe_0.75Ti_2.1O₅. The grains contain elevated Cr (1360-4260 ppm) and occasionally Ni (~1200 ppm). Stoichiometry suggests that all iron is divalent and some of the titanium is reduced. Assuming plume conditions approximating equilibrium at high T and applying a number of FeO-TiO thermometers, a range of temperatures was calculated between 1259 ̊C and 1505 ̊C. These conditions, and correspondingly low fO₂, are consistent with the observation of native Fe, Ni, and Al-Mg alloy in the spherules. Abundant graphite veins in the ferropseudobrookite can then be explained by reduction of a carbonate-rich fluid, the likely matrix material ultimately replaced by phosphate. As many Cretaceous-Paleogene “melt” ejecta seem to have altered to goyazite, we are investigating the timing of diagenesis to determine if something was unique about the post-impact environment endemic to the Chicxulub event.

Although the ferropseudobrookite may be explained by high T and low fO₂ in the impact plume, it also is possible that they were inherited from the bolide. The coexisting Ti-Fe phase is a Cr, Nb-rich rutile, most commonly derived from deep mantle or meteoritic sources.