HR: 16:00h
AN: V54B-01    [Abstracts]
TI: No Indications from Cr Isotopes for the Late Heavy Bombardment on Earth in Early Archean Metasediments from Isua, Greenland.
AU: * Frei, R
EM: robertf@geol.ku.dk
AF: Geological Institute, University of Copenhagen, Oster Voldgade 10, Copenhagen, DK-1350 Denmark
AU: Schoenberg, R
EM: r.schoenberg@mineralogie.uni-hannover.de
AF: Institute of Mineralogy, University of Hannover, Callinstrasse 3, Hannover, D-30167 Germany
AU: Rosing, M T
EM: rosing@savik.geomus.ku.dk
AF: Geological Museum, Oster Voldgade 5-7, Copenhagen, DK-1350 Denmark
AB: The short-lived $^{53}$Mn-$^{53}$Cr (half-life of 3.7 Ma) isotope system has been widely used in cosmochemical studies and excess $^{53}$Cr relative to the terrestrial $^{53}$Cr/$^{52}$Cr 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 $^{53}$Cr 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.
DE: 4808 Chemical tracers
DE: 3662 Meteorites
DE: 3670 Minor and trace element composition
DE: 4294 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting