HR: 0800h
AN: U21B-0411    [Abstracts]
TI: Diamonds From Dachine Komatiites (French Guiana) : Obvious Link to Enigmatic Carbonados and a Possible Primordial Heterogeneity Below the Guiana Shield.
AU: * Cartigny, P
EM: cartigny@ipgp.jussieu.fr
AF: Laboratoire de Géochimie des Isotopes Stables de l'IPG-Paris, 4 Place Jussieu, Paris, 75251, France
AB: We studied 150 diamonds from the Dachine area (French Guiana). These diamonds are the only samples recovered so far from a volcaniclastic komatiite [1]. The samples are monocrystalline (i.e. non polycristalline) macrodiamonds (> 500 microns) mostly (>95%) irregular in shape with resobred octahedron and cubes being occasionnally recognised. No syngenetic inclusion has been identified yet. Most (>90%) samples are nitrogen free (Type II) and the few samples containing nitrogen (Type I) are characterised by low aggregation states. Their carbon isotope compositions range from -31.9 to +0.15‰ with an average of - 24.4°‰ The observed general ¹³C-depletion among diamonds from Dachine including both octahedron and cuboids (and their respective resorbed forms), among both Type II and Type I diamonds demonstrates that an entire diamond population with ¹³C-depletion can originate from the mantle. Furthermore, the nitrogen speciation (that reflects the time-temperature history of diamond) is little advanced (Type Ib-IaA diamonds). This suggests that Dachine diamonds spent virtually no time in the mantle and thus most likely precipitated from their host komatiite. Our results have two implications, the long-standing argument that carbonados could not form in the mantle as inferred from their distinct carbon-isotopes compositions is not valid anymore. The similarties between carbonado and Dachine diamonds being striking from a carbon-isotope but also from a nitrogen aggregation point of view. The second implication concerns the origin of such a carbon isotope distribution. So far we favor a primordal heterogeneity below the Guiana Shield rather than the contribution of subducted material. A result that can be checked further using radiogenic isotope geochemistry. [1] Capdevila et al., Nature 399, 456-458 (1999)
DE: 1038 Mantle processes (3621)
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Union [U]
MN: 2007 Fall Meeting