ISOTOPE EVIDENCE FOR THE INVOLVEMENT OF RECYCLED SEDIMENTS IN DIAMOND FORMATION

DIAMONDS, as chemically robust containers of material from the mantle, have provided important information regarding the evolution of the Earth's interior 1; yet despite much effort 2-9, the origins of diamonds themselves have remained controversial 10-13. Sulphur isotope ratios measured in most mantle or meteoritic samples exhibit minimal deviation from the meteoritic standard, whereas the ratios in crustal materials deviate by as much as +/- 70 parts per thousand (ref. 14): for this reason, isotopic studies of sulphide mineral inclusions in diamonds and mantle material are an attractive means of investigating the possible role of crust-mantle interaction in the formation of diamonds 15. Recent ion microprobe studies have found departures from mantle values in the sulphur isotope compositions of individual diamond inclusions 16 and some mantle rocks 17, which would be consistent with subduction of altered ocean crust into the region of diamond growth. Here we present new sulphur and lead isotope data from diamond sulphide inclusions, which suggest that sedimentary material might also be subducted into the mantle and that the crustal recycling process has been operative for at least 1,000 million years.