Origin of the Earth and Moon Conference 4017.pdf

THE LATE HEAVY BOMBARDMENT ON THE EARTH? — A SHOCK PETROGRAPHIC AND GEO- CHEMICAL SURVEY OF SOME OF THE WORLD’S OLDEST ROCKS. C. Koeberl1, W. U. Reimold2, I. McDonald3, and M. Rosing4, 1Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria ([email protected]), 2Department of Geology, University of the Witwatersrand, Johannesburg 2050, South Africa, 3School of Earth and Environmental Sciences, University of Greenwich, Chatham Maritime, Kent ME4 4AW, UK, 4Geologisk Museum, Oster Voldgade 5-7, DK-1350 Kobenhavn K, Denmark.

A variety of data obtained from Apollo rocks indicated that the Moon had been subjected to intense post- accretionary bombardment around 4.6 to 3.9 Ga ago. In addition, there is some evidence (although disputed by some workers) of a short and intense late heavy bombardment period, around 3.9 ± 0.1 Ga ago. During this period, some of the multi-ringed basin seem to have formed, which were later filled by mare basalts. If a late heavy bombardment oc- curred on the Moon, the Earth must have been subjected to an impact flux at least as intense. The consequences for the Earth must have been devastating. Geochemical data (mainly in the form of Nd isotope data) exist that indicate the presence of an already differentiated upper mantle at the time of formation of the oldest known rocks on Earth (Greenland; Australia). This could suggest that small amounts of crust had formed prior to 4.0 Ga, but had later been mixed in with the upper mantle. The late heavy bombardment may have provided the necessary stirring of the Earth’s surface. To study the question if there might be any chances to recover some record of such a late heavy bombardment period on the Earth, we have begun a study of some the oldest rocks on Earth, from Isua in Greenland. The approach of the study is two-fold: first, we are attempting to identify any remnant evidence of shock metamorphism in these rocks by petrographical studies, and second, we are searching for the possible presence of an extraterrestrial compo- nent in these rocks by geochemical analyses. Regarding the petrographical analyses, we focused on the mineral zircon, which is present in trace amounts in most of these rocks, and which has been shown to contain a range of shock- induced features at the optical and electron microscope level. Furthermore, zircon is very resistant to erosion and other forms of alteration; while planar deformation features (PDFs) in quartz may have long been annealed, those in zircon have a good chance to survive for several Gyrs as shown by the presence of shocked zircons in the about 2 Ga old Vredefort and Sudbury impact structures. The identification of suitable rocks for a shock-petrographical study is difficult. Some type of sedimentary rock might be best suited, but there is some controversy as to whether terrigenous clastic sediments occur at Isua. Most units that have been interpreted as metasediments turn out to be strongly metasomatized intrusive gneisses. There are a few graded turbidite units, but they also appear to be dominated by volcanics. We focused on some of the samples that have not been positively proven to be plutonic, and which have mixed zircon populations - either because they represent multi phase intrusives, or because they are eroded from a mixed source. 5 samples of the most well- constrained sediments (Bouma sequence turbidite) yielded, unfortunately, no zircons. Zircons were separated from felsic schists, which are of somewhat more dubious origin. Our petrographic study of grain mounts of hundreds of zircon crystals showed a wide variety of zircon shapes and zonation patterns. Shapes range from well-rounded to very high aspect ratios. Grain sizes are also variable, but mostly between 60 and 150 um. Where extensive zonation is ob- served, the outer zones are generally of euhedral shape with nicely developed pyramidal faces. Inner zones/cores may be well- rounded or euhedral to subhedral in shape. Many crystals contain inclusions, for example, of apatite needles. While many grains are strongly fractured, most fractures are of irregular shape or even of curved appearance. Single planar fractures do occur, but never as part of sets. In fact, sets of irregular fractures were not observed either. In con- clusion, no evidence of optically visible shock deformation was be observed in these samples. However, a large number of samples from different locations should be studied before we can come to any conclusions regarding shock history. A variety of rocks were analyzed for their chemical composition, including the PGE abundances. The samples included turbidite sediments from well preserved Bouma sequence; a greywacke/felsic gneiss, and two conclomer- ate/felsic metasomatite samples; as well as BIF from the Bouma sequence succession, which are representative of quartz-magnetite BIF, and are from two different localities and possible two different stratigraphic units. Analyses for PGEs were done by ICP-MS after Ni-sulfide fire assay with Te co-precipitation. Of nine samples that could be ana- lyzed, two yielded measurable amounts of Ir (detection limit: 0.03 ppb); both samples are from the BIF sequence; these same samples also have Ru and Rh above detection limit. Pt, Pd, and Au content in the samples are highly variable. Chondrite-normalized abundance patterns show variations by a factor of 3 to 4, although the background concentra- tion of Pd and Pt (which are too high) remains unclear. Taken at face value, the elevated Ir content may indicate a remnant meteoritic phase, but no clear evidence for large amounts of extraterrestrial matter in the Isua rocks studied so far was found.