HR: 13:55h
AN: V53E-02    [Abstracts]
TI: Nitrogen Speciation in Metamorphic Diamond Provides Constraints on T-t Paths
AU: * Cartigny, P
EM: cartigny@ipgp.jussieu.fr
AF: Lab. Stable Isotopes of IPG-Paris, 4 place Jussieu, Paris, 75251 France
AB: Nitrogen content and speciation in diamond can be determined using micro-infrared spectroscopy provided that the sample is bigger than 60-80 microns. Available data show that metamorphic diamonds are characterized by high amounts of structurally bound nitrogen (70% have N-contents > 3500 ppm) compared with diamonds formed in other contexts (99.5% have N-contents < 3500 ppm). This criterion can be used to recognize metamorphic diamonds (and thus new UHP provinces) anywhere on Earth, even far away from HP areas where UHP indicators would usually be searched. So far, metamorphic diamonds have been detected in Northern Canada (Cartigny et al., 2004) but their search remained unsuccessful in Australia. The four main nitrogen-bearing defects are interrelated by a second-order diffusion process, i.e. longer residence times, higher temperatures and/or N-contents leading the initial single N-atoms to cluster into an increasing number of N-atoms (Evans and Qi, 1982). Due to their rather short residence time (of several Ma) at rather low temperatures (<1100°C), metamorphic diamonds are predicted to contain - and it is observed -significant amount of single N atoms (Type Ib diamonds). According to the temperature/time dependence of the nitrogen diffusion, tight constraints can be placed on available T-t paths. Up to now, N-aggregation state was only roughly used and residence times were calculated assuming constant temperatures (Finnie et al. 1995; De Corte et al., 1998). More precise modeling shows that nitrogen aggregation of Kokchetav diamonds is compatible with diamond crystallization at temperature <1025°C and <1000°C with exhumation rates of 5 and 10cm/year respectively. Another important implication is that nitrogen speciation is not compatible with crystallization temperatures higher than 1050°C.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005