Abstract

The Middle Ordovician (466 Ma) Osmussaar breccia, situated along the northwestern coast of Estonia, is rich in angular chromite grains of extraterrestrial origin (>13 grains kg−1) and shocked quartz. The angularity of the chromite grains implies that they have not been transported or reworked to any large extent, connoting that the brecciation is the result of a contemporary impactor, either as a direct consequence of the impact or as a result of an earthquake triggered by the impact, and thus is not, as previously suggested, redeposited material from the nearby ∼70 m.y. older Neugrund impact structure. The chemical composition of the chromite indicates that the impactor was an ordinary chondrite of L-type, which concurs well with the hypothesis that the influx of large bodies to Earth increased during this period due to the breakup of the L-chondrite parent body. This in turn gives support to the recent suggestion that abundant coeval megabreccias worldwide are impact triggered. The presence of extraterrestrial chromite also strengthens the theory that physical pieces of a large celestial body can survive upon impact with Earth.

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