No Indications from Cr Isotopes for the Late Heavy Bombardment on Earth in Early Archean Metasediments from Isua, Greenland.

Details No Indications from Cr Isotopes for the Late Heavy Bombardment on Earth in Early Archean Metasediments from Isua, Greenland. No Indications from Cr Isotopes for the Late Heavy Bombardment on Earth in Early Archean Metasediments from Isua, Greenland.

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.v54b..01f&link_type=abstract

American Geophysical Union, Fall Meeting 2004, abstract #V54B-01

Physics

4808 Chemical Tracers, 3662 Meteorites, 3670 Minor And Trace Element Composition, 4294 Instruments And Techniques

Scientific paper

The short-lived 53Mn-53Cr (half-life of 3.7 Ma) isotope system has been widely used in cosmochemical studies and excess 53Cr relative to the terrestrial 53Cr/52Cr ratio has been detected in a variety of ancient solar system objects. Inspired by the highly debatted, apparent indications from W isotopes for a meteoritic component in ~3.8 Ga sediments from Isua ([1]) and the potential of the Mn-Cr extinct isotope system to trace extraterrestrial components in K-T sections ([2]), we have initiated a long-term study of Cr isotope systematics in these sediments, from which Rosing ([3]) reported graphitic microparticles with isotopically light C, indicative of a biogenic origin. We document our ability to reproduce the terrestrial Cr standard to +/- 0.15 \epsilon-units, applying a second order mass bias correction to acccount for residual mass fractionation effects after application of a strict exponential law. While we are able to detect an apparent deficit of 53Cr in the order of -0.3 \epsilon-units in the K-T boundary clay from Stevns Klint and the carbonaceous chondrite Allende (and thus confirm results by [2]), several pelagic shales (with Cr concentrations from 30-70 ppm) from Isua give statistically indistinguishable values from the terrestrial standard. We are thus not capable of detecting a Cr-component of extraterrestrial origin within the limits of our mass spectrometric precision in the Isua sediments and therefore are unable to confirm the W-isotope evidence for early meteorite bombardment apparently recorded by them. [1] Schoenberg R., Kamber B.S., Collerson K.D. and Moorbath S. (2002), Nature 418, 403-405; [2] Shukolyukov A. and Lugmair G.W. (1998), Science 282, 927-929, [3] Rosing M.T. (1999), Science 283, 674-676.

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