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Updated solar photospheric abundances are compared with meteoritic abundances. The uncertainties of solar abundances of many trace elements are considerably reduced compared to the 2003 compilation. Some of the solar rare earth elements have now assigned errors of ± 5%, approaching the accuracy of meteorite analyses. The agreement between solar abundances and CI chondrites is further improved. Problematic elements with comparatively large differences between solar and meteoritic abundances are manganese, hafnium, rubidium, gallium, and tungsten. The CI chondrites match solar abundances in refractory lithophile, siderophile, and volatile elements. All other chondrite groups differ from CI chondrites. With analytical uncertainties, there are no obvious fractionations between CI meteorites and solar abundances. Further progress will primarily come from improved solar abundance determinations. The limiting factor in the accuracy of meteorite abundances is the inherent heterogeneity of CI chondrites, primarily the Orgueil meteorite. The interstellar medium (ISM) from which the solar system formed has the same composition as the Sun for volatile and moderately volatile elements within a factor of 2. The more refractory elements of the ISM are depleted from the gas and are concentrated in grains.
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Unusual sources of fossil micrometeorites deduced from relict chromite in the small size fraction in ~467 Ma old limestone
ABSTRACT Extraterrestrial chrome spinel and chromite extracted from the sedimentary rock record are relicts from coarse micrometeorites and rarely meteorites. They are studied to reconstruct the paleoflux of meteorites to the Earth and the collisional history of the asteroid belt. Minor element concentrations of Ti and V, and oxygen isotopic compositions of these relict minerals were used to classify the meteorite type they stem from, and thus to determine the relative meteorite group abundances through time. While coarse sediment‐dispersed extraterrestrial chrome‐spinel (SEC) grains from ordinary chondrites dominate through the studied time windows in the Phanerozoic, there are exceptions: We have shown that ~467 Ma ago, 1 Ma before the breakup of the L chondrite parent body (LCPB), more than half of the largest (>63 μm diameter) grains were achondritic and originated from differentiated asteroids in contrast to ordinary chondrites which dominated the meteorite flux throughout most of the past 500 Ma. Here, we present a new data set of oxygen isotopic compositions and elemental compositions of 136 grains of a smaller size fraction (32–63 μm) in ~467 Ma old pre‐LCPB limestone from the Lynna River section in western Russia, that was previously studied by elemental analysis. Our study constitutes the most comprehensive oxygen isotopic data set of sediment‐dispersed extraterrestrial chrome spinel to date. We also introduce a Raman spectroscopy‐based method to identify SEC grains and distinguish them from terrestrial chrome spinel with ~97% reliability. We calibrated the Raman method with the established approach using titanium and vanadium concentrations and oxygen isotopic compositions. We find that ordinary chondrites are approximately three times more abundant in the 32–63 μm fraction than achondrites. While abundances of achondrites compared to ordinary chondrites are lower in the 32–63 μm size fraction than in the >63 μm one, achondrites are approximately three times more abundant in the 32–62 μm fraction than they are in the present flux. We find that the sources of SEC grains vary for different grain sizes, mainly as a result of parent body thermal metamorphism. We conclude that the meteorite flux composition ~467 Ma ago ~1 Ma before the breakup of the LCPB was fundamentally different from today and from other time windows studied in the Phanerozoic, but that in contrast to the large size fraction ordinary chondrites dominated the flux in the small size fraction. The high abundance of ordinary chondrites in the studied samples is consistent with the findings based on coarse extraterrestrial chrome‐spinel from other time windows.
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- Award ID(s):
- 2004618
- PAR ID:
- 10489261
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Meteoritics & Planetary Science
- ISSN:
- 1086-9379
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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