THE METEORITE FLUX TO EARTH IN THE EARLY CRETACEOUS AS RECONSTRUCTED FROM SEDIMENT-DISPERSED EXTRATERRESTRIAL SPINELS (Invited Presentation)

From Earth's sedimentary strata the evolution of life, climate, and magnetic polarity has been reconstructed for much of our planet's history. Here we show that these same strata can also provide a detailed record of the collisional evolution of the asteroid belt. From 1100 kg of the pelagic Majolica limestone of Berriasian-Valanginian age in Italy we recovered ~60 extraterrestrial spinel grains (32-63 μm) representing relict minerals from coarse micrometeorites. This is a sufficiently large number of micrometeorites to make a first-order estimate of the major types of asteroids delivering extraterrestrial material to Earth at the time. Elemental and oxygen-three isotope analyses were used to characterize the origin of the grains. About three quarters of the extraterrestrial spinels recovered originate from ordinary chondrites. The ratios between the three types of ordinary chondrites, H, L, LL, appear similar to the present, 1:1:0.2, and point to an enigmatic stability for the past 150 Myr in the delivery of these meteorites from the asteroid belt. As much as a quarter of the grains found in the Majolica beds originate from meteorites that are uncommon on Earth today. Several have very different oxygen isotopic (e.g., Δ¹⁷O of -1.8‰ to -3.8‰) and elemental compositions compared to ordinary chondrites. We found no Mg-Al spinels of the type common in carbonaceous chondritic meteorites. Because most meteorite groups have lower chrome spinel content than the ordinary chondrites, our data indicate that the ordinary chondrites did not dominate the flux to the same extent as today. Different types of primitive and ungrouped achondrites that are rare on Earth today played a larger role in the early Cretaceous meteorite flux, a finding consistent also with our reconstructions of the micrometeorite flux during older periods of the Phanerozoic. The Majolica limestone is exceptionally "clean", i.e. having very low contents of terrestrial mineral grains, making it next to ideal for reconstructions of the micrometeorite flux in small size ranges, such as the 32-63 μm range used here.