THE MICROMETEORITE FLUX IN THE ALBIAN/APTIAN AGE (~103-117 MA): A SEARCH FOR TYCHO EJECTA IN PELAGIC SEDIMENTS USING CHROME SPINELS

Numerical models of meteorite delivery from impacts on the Moon have demonstrated that the impact event forming the lunar crater Tycho (~85 km diameter; ~109 Ma age) would have delivered considerable amounts of ejected material to Earth. The ejecta, containing lunar Ti- and V-rich chrome spinels, would have been distributed globally and admixed with sea-floor sediments over a few meters of a typical marine stratigraphic interval. In order to locate such ejecta, samples weighing ~12-25 kg each, with one-meter spacing were extracted over a ~30-meter interval of the deep-sea formed Calera Limestone, Albian and Aptian age (ca. 103-117 Ma), from the Pacifica Quarry, south of San Francisco. The limestone samples were leached in acids and residues searched for possible lunar Ti-rich chrome-spinel grains.

In a total of 689 kg of limestone, 1154 chrome-spinel grains were found. Of these, 319 contain >0.45 wt% V₂O₃, of which 227 originate from equilibrated ordinary chondrites. The majority of the other 92 grains with >0.45 wt% V₂O₃ are most likely from different types of achondritic meteorites. Among these, we found eleven particularly Ti-rich chrome-spinel grains. The elemental abundances of these grains were compared with chrome spinel from lunar, HED and R-chondritic meteorites. This showed that only one of these grains could potentially be of lunar origin. The bulk of the other grains likely originate from HED meteorites based on oxygen isotopic analysis of similar grains in previous studies. Grains with TiO₂ >10 wt%, common among lunar spinels are not found, further supporting an HED source for the Ti-rich grains. In summary, Albian and Aptian strata in the Pacifica quarry do not likely record any major lunar impact event. Either the timing of the impact is located within a ~110-114 Ma unconformity in the middle part of the section or the impact is likely older than the interval searched.