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Science 7 May 2004:
Vol. 304. no. 5672, pp. 853 - 855
DOI: 10.1126/science.1094668
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Reports

Early Proterozoic Ultrahigh Pressure Metamorphism: Evidence from Microdiamonds

Pierre Cartigny,1* Ingrid Chinn,2 K. S. (Fanus) Viljoen,2 Derek Robinson2

Microdiamonds from the Akluilâk minette dykes (Nunavut, Canada) are similar to diamonds formed in subducted metamorphic rocks. High concentrations of unaggregated nitrogen and positive {delta}15N suggest that the microdiamonds formed within rocks subducted to ultrahigh pressures before being sampled by the minette magma 1.8 billion years ago. This ultrahigh pressure metamorphism in North America, probably related to the Trans-Hudson orogen (about 2 billion years ago), extends the occurrence of ultrahigh pressure metamorphism from 0.6 billion years to before 1.8 billion years ago and suggests that Phanerozoic-type subductions were active by the Early Proterozoic.

1 Laboratoire de Géochimie des Isotopes Stables, Institut de Physique du Globe, UMR 7047, 2 Place Jussieu, F-75251 Paris, Cedex 05, France.
2 GeoScience Centre, De Beers Consolidated Mines Limited, Post Office Box 82232, Southdale, 2135 Johannesburg, South Africa.

* To whom correspondence should be addressed. E-mail: cartigny{at}ipgp.jussieu.fr

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