SEARCH FOR TRACES OF THE LATE HEAVY BOMBARDMENT ON EARTH – RESULTS FROM HIGH PRECISION CHROMIUM ISOTOPES

High precision mass spectrometric analyses of the chromium isotopic composition of metamorphosed turbiditic and pelagic sedimentary rocks and banded iron stones from the ~3.7 Gyr Isua Supracrustal Belt (ISB) in West Greenland cannot be distinguished from the standard terrestrial ⁵³Mn-⁵³Cr ratio at our present level of resolution. As a consequence, our search for chemical traces of possible impact-derived meteoritic components (asteroidal and/or cometary material, or accreted cosmic dust) in the Earth's oldest chemical and detrital sediments was negative. Our results, based on the ⁵³Mn-⁵³Cr short-lived radionuclide system (half-life of 3.7 Myr), cannot confirm the recent findings of tungsten isotope anomalies (based on the ¹⁸²Hf-¹⁸²W short-lived radionuclide system; half-life of 9 Myr) in these sediments, which were interpreted as indicating a component derived from meteorites. Possible reasons for the failure to trace cosmic material in the ISB metasediments are various: 1. The samples studied are not representative; 2: The sedimentation period did not overlap with the period of late heavy bombardment of the Moon; and 3. The potential chromium anomalies, if present, are too small to be traceable by our present levels of detection. Unequivocal evidence of a late heavy bombardment on the early Earth therefore remains elusive and uncertain.