From the mid-Ordovician into the Late Silurian: Changes in the micrometeorite flux after the L chondrite parent breakup

We present the first reconstruction of the micrometeorite flux to Earth in the Silurian Period. We searched 321 kg of condensed, marine limestone from the Late Silurian Cellon section, southern Austria, for refractory chrome-spinel grains from micrometeorites that fell on the ancient sea floor. A total of 155 extraterrestrial spinel grains (10 grains >63 μm, and 145 in the 32-63 μm fraction) were recovered. For comparison, we searched 102 kg of similar limestone from the mid-Ordovician Komstad Formation in southern Sweden. This limestone formed within ~1 Ma after the breakup of the L chondrite parent body (LCPB) in the asteroid belt. In the sample we found 444 extraterrestrial spinel grains in the >63 μm fraction, and estimate a content of at least 7000 such grains in the 32-63 μm fraction. Our results show that in the late Silurian, ~40 Ma after the LCPB, the flux of ordinary equilibrated chondrites has decreased by two orders of magnitude, almost down to background levels. Among the ordinary chondrites, the dominance of L-chondritic micrometeorites has waned off significantly, from >99% in the post-LCPB mid-Ordovician to ~60% in the Late Silurian, with ~30% H-, and ~10% LL-chondritic grains. In the Late Silurian, primitive achondrite abundances are similar to today's value, contrasting to the much higher abundances observed in pre-LCPB mid-Ordovician sediments.