Large spinel grains in a CM chondrite (Acfer 331): Implications for reconstructions of ancient meteorite fluxes

Abstract

Abstract-By dissolving 30-400 kg of marine limestone in HCl and HF acid, our group has previously recovered common relict chromite grains (approximately 63-250 lm) from ordinary chondritic micrometeorites that fell on ancient sea floors, up to 500 Myr old. Here, we evaluate if CM group carbonaceous chondritic material, which makes up an important fraction of the micrometeorite flux today, contains analogous grains that can be searched for in acid residues. We dissolved 8 g of CM2 meteorite Acfer 331 in HF, which yielded a characteristic assemblage of both transparent Mg-Al-and opaque Cr-spinels >28 lm. We find on average 4.6 and 130 Mg-Al-spinel grains per gram in the 63-250 and 28-63 lm size fractions, respectively. These grains are mostly pink or colorless, and often characterized by heterogeneous Cr-content. Black, opaque Cr-spinel grains are absent from the >63 lm fraction, but in the 28-63 lm fraction we find approximately 65 such grains per gram meteorite. The individual grains have a characteristic composition, with heterogeneous major element compositions (e.g., 44.4-61.7 wt% Cr 2 O 3 ), but narrow ranges for maximum TiO 2 (0.6-1.6 wt%) and V 2 O 3 (0.5-1.0 wt%) concentrations. The content of spinel grains in the 28-63 lm fraction of CM meteorites appears comparable at the order of magnitude level with the content of >63 lm sized chromite grains in fossil L-chondrites from Ordovician limestone. Our approach of recovering meteoritic spinel from sediment may thus be extended to include CM meteorites, but the smaller size fraction of the acid residues should be searched.

Fragments of Late Eocene Earth-impacting asteroids linked to disturbance of asteroid belt

“…For the estimate we used chromite data for ordinary chondrites (1000 grains >63 μm per gram) from Bjärnborg and Schmitz (2013). Considering that the average thickness of the regolith of Eros is estimated at 20-40 m (Robinson et al, 2002) the ∼30 cm of regolith shed off from the Popigai impactor may only represent a small fraction of the entire regolith layer on such a body.…”

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

“…The characterization of the K‐Pg impactor deduced from chromium isotopes is that of a CM‐type chondrite, a meteorite type known to be rich in Mg‐Al‐spinel grains in the 32–63 µm fraction (Trinquier et al 2006; Bjärnborg and Schmitz 2013). In a 103 kg limestone sample spanning the interval 0.0–0.2 m above the top of the K‐Pg boundary clay, 113 chromite grains 32–63 µm were found; however, no Mg‐Al‐spinel grains were recovered.…”

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

“…At present, no evidence of detrital extraterrestrial MgAl 2 O 4 spinels has been reported in the literature, but, as they are present in meteorites, they are likely to exist. A recent attempt has been made by Bjärnborg and Schmitz () dissolving a fragment of CM2 chondrite Acfer 331, to verify if these kinds of meteorites could be considered as a possible source for detrital Cr‐spinels. There, they found Cr‐spinels similar to those previously found in the Ordovician limestones but also several MgAl spinels that lack in the detrital fraction.…”