Diamonds in Earth's oldest zircons from Jack Hills conglomerate, Australia, are contamination

The earliest period of Earth history is the Hadean era (4600-4000 Myr) but to date no rocks are known to be preserved from this time. However, abundant detrital zircons from the Archean Jack Hills conglomerate complex of Western Australia have been found with dates in excess of 4000 Myr and there is a significant literature concerning these zircons from which abundant geochemical and mineralogical information has been obtained. The essence of what has been derived from these zircons and their inclusions concerning conditions of formation is that the zircons crystallized from granitic melts at shallow depths. However, in the last decade two papers were published reporting microdiamond inclusions in a suite of zircons ranging in age from similar to 4200 Myr to similar to 3000 Myr, implying much greater depths and repeated recycling and re-incorporation of diamonds into zircons over at least 1200 Myr. To complicate the issue, subsequent to this discovery, another study surveyed the inclusion suite in a very large sample (>400) of Jack Hills zircons (JHZs) and found no diamonds but abundant inclusions that are consistent with the previous consensus. It is important, therefore, to verify whether or not the reported JHZ diamonds could be laboratory contamination because diamond abrasive paste was used for sample preparation. Here, we report very-high-resolution re-examination of the original diamond-bearing specimens that were kindly provided to us by the authors (see Acknowledgements). We confirm the presence of diamonds in their zircons but the diamonds we find are fragments of polishing compound. These results combined with the lack of diamonds in any other samples of JHZs strongly suggest that there are no indigenous diamonds in the JHZs. (C) 2013 Elsevier B.V. All rights reserved.