Global mantle convection: Evidence from carbon and nitrogen isotopes in super-deep diamonds (Invited)
Abstract
Constraining the convective regime of the Earth’s mantle has profound implications for our understanding of the Earth’s cooling and the geodynamics of plate tectonics. Although subducting plates seem to be occasionally deflected at 660 km, evidence from seismic tomography and fluid dynamics suggest that substantial amounts of material reach the core-mantle boundary. Most geochemists, on the other hand, based on evidence from noble gases, would argue for the presence of separate upper and lower mantle reservoirs. Diamond provides a unique opportunity to sample those parts of the mantle that remains inaccessible by any other means. Some mineral associations in diamond, such as majoritic garnet, calcic and magnesian perovskite and manganoan ilmenite with ferropericlase have been recognised as originated from the transition zone down to the lower mantle (Stachel et al., 1999; Kaminsky et al., 2001). In addition, nitrogen in these diamonds is potentially a good tracer for mantle geodynamics. Exchanges between an inner reservoir (characterised by negative δ15N) via degassing at oceanic ridges with an outer reservoir (characterised by positive δ15N) via recycling at a subduction zones can lead to isotopic contrast in a stratified mantle. Because of common super-deep mineral inclusion assemblages in diamonds from Juina (Brazil) and Kankan (Guinea), we carried out a detailed study of nitrogen and carbon isotopes. The Juina diamonds show broadly similar ranges of δ15N from +3.8‰ down to -8.8‰ for both upper (UM) and lower (LM) mantle diamonds. This important feature is also found for UM and LM diamonds from Kankan, although the range of δ15N differs with values from +9.6‰ down to -39.4‰. Both sets of results suggest extensive material-isotopic exchange through the 660km discontinuity, contrary to the idea of an isolated reservoir. Transition zone (TZ) diamonds are enriched in 13C with δ13C from -3.1‰ up to +3.8‰ at Kankan but those of Juina are depleted in 13C with δ13C from -4.2‰ down to -12.8‰. Three zoned TZ and LM diamonds demonstrate more than 3.5‰ δ13C variations, two from Kankan with δ13C up to +0.1‰ and +1.4‰ and one from Juina with δ13C down to -9.8‰. These diamonds may have initiated their growth in the LM and following slow uplift in a convective mantle have equilibrated in the TZ, and in doing so show an evolution in carbon isotopic composition. Therefore the C- and N- independent isotopic tracers suggest that significant amounts of material are exchanged across the 660km discontinuity. Subsequent preservation of a large range of δ13C and δ15N compositions may be compatible with the model of global thermochemical convection with a small amplitude of density heterogeneities inferred from seismology and mineral physics. Kaminsky, F. V. et al., 2001. Superdeep diamonds from Juina area, Mato Grosso State, Brazil. Contrib Mineral Petrol 140: 734-753. Stachel, T. et al., 2001. Kankan diamonds (Guinea) III: δ13C and nitrogen characteristics of deep diamonds. Contrib Mineral Petrol 142: 465-475.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V13H..06P
- Keywords:
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- 1025 GEOCHEMISTRY / Composition of the mantle;
- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 8120 TECTONOPHYSICS / Dynamics of lithosphere and mantle: general