Rare meteorites common in the Ordovician period
Philipp R. Heck, Birger Schmitz, William F. Bottke, Surya S. Rout, Noriko T. Kita, Anders Cronholm, Céline Defouilloy, Andrei Dronov & Fredrik Terfelt
Nature Astronomy 1, Article number: 0035 (2017)
Asteroids, comets and Kuiper belt Meteoritics
Received: 27 July 2016 Accepted: 14 December 2016 Published online: 23 January 2017
Source/Fonte: Lund University
Most meteorites that fall today are H and L type ordinary chondrites, yet the main belt asteroids best positioned to deliver meteorites are LL chondrites 1,2 . This suggests that the current meteorite flux is dominated by fragments from recent asteroid breakup events 3,4 and therefore is not representative over longer (100-Myr) timescales. Here we present the first reconstruction of the composition of the background meteorite flux to Earth on such timescales. From limestone that formed about one million years before the breakup of the L-chondrite parent body 466 Myr ago, we have recovered relict minerals from coarse micrometeorites. By elemental and oxygen-isotopic analyses, we show that before 466 Myr ago, achondrites from different asteroidal sources had similar or higher abundances than ordinary chondrites. The primitive achondrites, such as lodranites and acapulcoites, together with related ungrouped achondrites, made up ~15–34% of the flux compared with only ~0.45% today. Another group of abundant achondrites may be linked to a 500-km cratering event on (4) Vesta that filled the inner main belt with basaltic fragments a billion years ago 5 . Our data show that the meteorite flux has varied over geological time as asteroid disruptions create new fragment populations that then slowly fade away from collisional and dynamical evolution. The current flux favours disruption events that are larger, younger and/or highly efficient at delivering material to Earth.
To investigate the past meteorite flux, we searched for relict chrome-spinel grains of coarse micrometeorites in condensed marine sediments in northwestern Russia, in a time window of ~10–100 kyr in the geological epoch of the Middle Ordovician period, which ranges from 470 to 458 Myr ago (Fig. 1; see Methods). Chrome spinels are the only minerals of meteorites and coarse micrometeorites that survived diagenesis in Ordovician limestone 6 . They retained their elemental and oxygen isotopic composition, enabling reliable classification based on single-grain microanalysis 7,8 . We also dissolved 32 meteorites of different types in HF or HCl acid to quantify their content of chrome-spinel grains. The sediment sample that we studied is about a million years older than the ~466-Myr-old sediments that contain the first collisional fragments from the L-chondrite parent body breakup (LCPB), the largest known asteroid disruption event in the past three billion years. The sampling level was chosen to exclude the extreme flux enhancement (more than two orders of magnitude 6,7 ) of L-chondritic fragments after the LCPB that obscures the background flux for more than 1 Myr (refs 7,8,9 ). The low, 50- to 100-kyr, cosmic-ray exposure ages of the oldest recovered fossil L chondrites 9 imply that any fragments from the LCPB that might have arrived on Earth before should have even shorter exposure ages. This indicates that a sample separation of one million years before the strata containing the first abundant L chondrites is large enough to assess the pre-LCPB flux. The interval sampled represents a time average of about 10 to 100 kyr and was selected with the aim of determining whether the composition of the meteorite flux to Earth was similar to or different from that of today. This is the first reconstruction of the background flux of the different meteorite types in a geological time perspective. Similar reconstructions are ongoing for other periods in the Earth’s geological past 10 .
How to cite this article: Heck, P. R. et al. Rare meteorites common in the Ordovician period. Nat. Astron. 1, 0035 (2017).
The study was supported by an ERC-Advanced Grant (ASTROGEOBIOSPHERE) to B.S. P.R.H. acknowledges funding from the Tawani Foundation. A.D. acknowledges support from the Russian Governmental Program of Competitive Growth of Kazan Federal University and RFBR (grant 16-05-00799). W.F.B’s participation was supported by NASA’s SSERVI program “Institute for the Science of Exploration Targets (ISET)” through institute grant number NNA14AB03A. We thank K. Deppert and P. Eriksson for support at Lund University, F. Iqbal for the laboratory work, and B. Strack for maintenance of the Field Museum’s SEM laboratory. WiscSIMS is partly supported by the National Science Foundation (EAR03-19230, EAR13-55590). We thank J. Kern for SIMS support. The 3D microscopy was performed in the Keck-II facility of the Northwestern University NUANCE Center, supported by NSEC (NSF EEC–0647560), MRSEC (NSF DMR-1121262), the Keck Foundation, the State of Illinois and Northwestern University.
Robert A. Pritzker Center for Meteoritics and Polar Studies, The Field Museum of Natural History, 1400 South Lake Shore Drive, Chicago, Illinois 60605, USA
Philipp R. Heck, Birger Schmitz & Surya S. Rout
Chicago Center for Cosmochemistry and Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA
Philipp R. Heck & Surya S. Rout
Astrogeobiology Laboratory, Department of Physics, Lund University, PO Box 118, SE-22100 Lund, Sweden
Birger Schmitz, Anders Cronholm & Fredrik Terfelt
Department of Space Studies, Southwest Research Institute, 1050 Walnut Street, Suite 300, Boulder, Colorado 80302, USA
William F. Bottke
WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, Wisconsin 53706-1692, USA
Noriko T. Kita & Céline Defouilloy
Geological Institute, Russian Academy of Sciences, Pyzhevsky Pereulok 7, 119017 Moscow, Russia
Kazan (Volga Region) Federal University, Kremlevskaya ulitsa 18, 420008 Kazan, Russia
P.R.H. and B.S. conceived the study and wrote the paper with input from all authors. W.F.B. provided expertise on the collisional and dynamical evolution of the asteroid belt and meteoroid delivery models. B.S., F.T. and A.D. conducted the fieldwork. B.S., F.T. and A.C. extracted and prepared the samples for SEM/EDS and SIMS. A.C. performed the quantitative SEM/EDS analysis. P.R.H. and S.S.R. prepared the samples for SIMS and performed the SIMS and post-SIMS analyses. N.T.K. and C.D. set up SIMS analysis conditions and assisted with the analyses.
The authors declare no competing financial interests.
Correspondence to Philipp R. Heck.
1. Supplementary Information Supplementary Figures 1 and 2, Supplementary Table 1, description of Supplementary Data files.
1. Supplementary Data 1 Data table with Δ17O, TiO2 and V2O3 values and classification of fossil micrometeorites.
2. Supplementary Data 2 Chrome spinel abundances in different types of meteorites.