Titanium- and chromium-rich opaque minerals in condensed sediments : chondritic, lunar and terrestrial origins

Evidence from terrestrial sedimentary successions indicate that the amount of extraterrestrial material reaching Earth is not constant, but has been increased during parts of the Phanerozoic. An increase in the flux of extraterrestrial material to Earth has been proposed for the Middle Ordovician and the Late Eocene. These showers of extraterrestrial material on Earth are thought to have resulted from disruption events in the Main Asteroid Belt. An asteroid shower affecting Earth would also have a great influence on the Moon. A projectile that impacts on the Moon can cause lunar matter to be launched into space, some of which will reach the Earth. Increased amounts of lunar impact ejecta reaching Earth has been proposed to have caused a prominent 3He anomaly in Late Eocene sediments from Massignano, Italy, as the lunar regolith is extremely enriched in this rare isotope of helium derived from the solar wind. The lunar impact ejecta should not only be preserved in the form of a 3He anomaly, but minerals representing the ejected matter should also be preserved in terrestrial sediments. Chrome spinel and ilmenite are resistant minerals that are common in both lunar and terrestrial rocks. These minerals occur in dissolved limestone samples from Kinnekulle, Sweden, eastern Yangtze Gorges area, China, and Massignano, Italy, and their provenance can be discussed based on their chemical composition. Chondritic chromite was confirmed among the grains, but due to the complex overlap in terrestrial and lunar Cr-rich spinel composition, the existence of any lunar chrome spinel could not be definitely confirmed. In order to identify lunar chrome spinel other methods, such as isotope geochemistry, need to be applied. The study presented in this paper shows that ilmenite is less suitable than Cr-rich spinel when trying to identify lunar minerals in terrestrial sediments. Ilmenite grains are more likely to become altered, which makes provenance studies harder. When comparing the chemical composition of ilmenite grains, which have not been subjected to severe alteration, from this study with the compositions of lunar and chondritic ilmenite it was clear that the grains most likely have a terrestrial origin, even if a few grains have the potential to be of lunar or chondritic origin. The vast majority of the ilmenite and chrome spinel grains recovered from the dissolved limestone samples come from terrestrial sources. The origin of these grains is discussed in this paper, and the most likely origin for them is erosion of igneous rocks and transportation to the depositional environment by currents.