Lower mantle diamonds from Rio Soriso

The morphology, colour, fluorescence, cathodoluminescence, nitrogen content and aggregation state, internal structure and mineral inclusions have been studied for 69 alluvial diamonds from the Rio Soriso (Juina area, Mato Grosso State, Brazil). Nitrogen in most diamonds (53%) is fully aggregated as B centres, but there is also a large proportion of N-free stones (38%). A strong positive correlation between nitrogen and IR-active hydrogen concentrations is observed. The diamonds contain (in order of decreasing abundance) ferropericlase, CaSi-perovskite, magnetite, MgSi-perovskite, pyrrhotite, ‘olivine’, SiO2, perovskite, tetragonal almandine-pyrope phase and some other minerals represented by single grains. The Rio Soriso diamond suite is subdivided into several subpopulations that originated in upper and lower mantle of ultramafic and mafic compositions, with the largest subgroup forming in the ultramafic lower mantle. Analysed ferropericlase grains are enriched in Fe (Mg#=0.43–0.89), which is ascribed to their origin in the lowermost mantle. The Juina kimberlites may be unique in sampling the material from depths below 1,700 km that ascended in a plume formed at the core–mantle boundary. Introduction Diamonds and the minerals trapped within them provide a unique opportunity to directly study materials from great depths (>150 km). Less than 1% of diamonds worldwide contain minerals that are predicted by experiments to exist only in the lower mantle (>660 km) (Stachel 2001). The Juina area, Brazil (Fig. 1a) has historically produced the highest proportion of diamonds containing lower mantle inclusions (Wilding 1991; Harte and Harris 1994; Hutchison 1997; Harte et al. 1999; Kaminsky et al. 2001), with diamonds from these studies having mostly been recovered from streams that feed into the Rio Cinta Larga (Fig. 1b). This study examines 69 diamonds from alluvial deposits of the Rio Soriso (Fig. 1b) mined by Diagem International Resource Corporation. A range of techniques was employed to divide the Rio Soriso suite into paragenetic subgroups. We examined crystal morphology, colour, fluorescence (FL), cathodoluminescence (CL), nitrogen concentration and aggregation, internal morphology and mineral inclusion chemistry of the diamonds. We also modified the traditional methodology of studying diamond inclusions (DI) to enable a check on the primary or secondary origin of included grains and to increase the rate of DI recovery. The results from this study significantly expand the database for lower mantle mineral compositions. Following previous studies of Juina samples, we found that most Rio Soriso diamonds formed in the lower mantle. We propose that Fe-enriched ferropericlase found among Rio Soriso inclusions represents ferropericlase from the lowermost mantle, which could be the natural analogue to either the low-spin Fe ferropericlase produced experimentally and predicted theoretically (Badro et al. 2003), or ferropericlase that equilibrated with postperovskite at the base of the mantle (Murakami et al. 2004a). Electronic Supplementary Material Supplementary material is available for this article at http://dx.doi. org/10.1007/s00410-0050657-8 Communicated by T.L. Grove P. C. Hayman Æ M. G. Kopylova (&) Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC, Canada, V6T 1Z4 E-mail: mkopylov@eos.ubc. ca Tel. : +1-604-8220865 Fax: +1-604-8226088 F. V. Kaminsky KM Diamond Exploration Ltd., 2446 Shadbolt Lane, West Vancouver, BC, Canada, V7S 3J1 Contrib Mineral Petrol (2005) 149: 430–445 DOI 10.1007/s00410-005-0657-8