Meteorite flux to Earth in the Early Cretaceous as reconstructed from sediment-dispersed extraterrestrial spinels

Abstract

We show that Earth's sedimentary strata can provide a record of the collisional evolution of the asteroid belt. From 1652 kg of pelagic Maiolica limestone of Berriasian-Hauterivian age from Italy, we recovered 108 extraterrestrial spinel grains (32-250 μm) representing relict minerals from coarse micrometeorites. Elemental and three oxygen isotope analyses were used to characterize the grains, providing a first-order estimate of the major types of asteroids delivering material at the time. Comparisons were made with meteorite-flux time "windows" in the Ordovician before and after the L-chondrite parent-body breakup. In the Early Cretaceous, ~80% of the extraterrestrial spinels originated from ordinary chondrites. The ratios between the three groups of ordinary chondrites, H, L, LL, appear similar to the present, ~1:1:0.2, but differ significantly from Ordovician ratios. We found no signs of a hypothesized Baptistina LLchondrite breakup event. About 10% of the grains in the Maiolica originate from achondritic meteorite types that are very rare (<1%) on Earth today, but that were even more common in the Ordovician. Because most meteorite groups have lower spinel content than the ordinary chondrites, our data indicate that the latter did not dominate the flux during the Early Cretaceous to the same extent as today. Based on studies of three windows in deep time, we argue that there may have been a gradual long-term (a few hundred million years) turnover in the meteorite flux from dominance of achondrites in the early Phanerozoic to ordinary chondrites in the late Phanerozoic, interrupted by short-term (a few million years) meteorite cascades from single asteroid breakup events.

The micrometeorite flux to Earth during the earliest Paleogene reconstructed in the Bottaccione section (Umbrian Apennines), Italy

“…Analytical accuracy was controlled by analyses of the USNM 117075 (Smithsonian) chromite reference standard (Jarosewich et al 1980). We have confirmed the quality and high‐precision reproducibility of our results by analyses of 20 chrome‐spinel grains with different compositions both at the Vienna Natural History Museum and at the Astrogeobiology Laboratory, Lund University (see supplementary material of Schmitz et al 2017).…”

“…Precisions of concentration analyses for each element were typically better than 1 rel% of measured values. At Lund University, additionally 330 of our recovered grains were analyzed with the same SEM/EDS approach as in our previous studies of chrome spinels (e.g., Schmitz et al 2017; Martin et al 2018). The grains were analyzed with an Oxford Inca X181‐sight energy‐dispersive spectrometer with an Si detector mounted on a Hitachi S‐3400 scanning electron microscope.…”

“…The spinel grains are very resistant and they are virtually unaffected by diagenesis. For this reason, the spinels have been used as a proxy in several studies of the micrometeorite flux in different time periods, such as the mid‐Ordovician (Schmitz and Häggström 2006; Heck et al 2017; Schmitz et al 2019a), the Silurian (Martin et al 2018), the late Devonian (Schmitz et al 2019b), the early Cretaceous (Schmitz et al 2017), the late Cretaceous (Martin et al 2019), and the late Eocene (Schmitz et al 2015; Boschi et al 2017).…”

MgAl2O4 spinels from Allende and NWA 763 carbonaceous chondrites: Structural refinement, cooling history, and trace element contents

“…In the past 20 years, several papers have dealt with the presence of detrital extraterrestrial spinels in carbonate successions (Schmitz and references within; Schmitz et al. , ; Heck et al. ; Martin et al.…”

“…In the past 20 years, several papers have dealt with the presence of detrital extraterrestrial spinels in carbonate successions (Schmitz [2013] and references within; Schmitz et al 2016Schmitz et al , 2017Heck et al 2017;Martin et al 2018). These spinels were classified as Crspinels/chromites, and a common and inexpensive way to distinguish them from terrestrial analogs is by their V 2 O 3 contents, which are higher in extraterrestrial Crspinels.…”