Neoproterozic Re-Os age of a sulfide inclusion in a superdeep diamond: Implications for mantle convection beneath Juina, Brazil

Abstract:

Diamonds from the Juina area Brazil have long been known for their sublithospheric or superdeep (e.g. from depths of 300-800 km) origins. These diamonds have yielded new information about high pressure mantle mineralogy, deep crustal recycling, diamond source fluids, and mantle transition zone water content.
A type II (low N) diamond (sample J1) from the 93.1±1.5 myr old (Heaman 7IKC 1998 ) Collier 4 kimberlite was studied previously (Walter et al. Nature 2008; Bulanova et al. CMP 2010) as part of a larger suite of eclogitic-composition, inclusion-bearing type II Collier 4 diamonds with complex internal growth structure. Major and trace element analyses of mineral inclusions in these diamonds include Ca-Ti-Si perovskite, Ca-rich majoritic-garnet, clinopyroxene, olivine, jeffbenite, minerals with stoichiometries of CAS and K-hollandite phases, SiO₂, FeO, native iron, low-Ni sulfides, and Ca–Mg-carbonate. The C isotopic compositions of the diamond hosts range from a δ¹³C of -25 to -5‰ ( J1 being -15‰). Collier 4 diamonds have been interpreted to crystallize from carbonatitic melts derived from the recycling of oceanic lithosphere at TransitionZone depths (Walter et al. Nature 2008; Bulanova et al. CMP 2010).
A rare Fe-sulfide inclusion in the core of diamond J1 has been dated with the Re-Os system in order to provide age and compositional constraints on the proposed oceanic slab recycling. The rim of J1 contained inclusions of Ca-Ti-Si perovskite that yielded U-Pb age of 101±7 Ma and eclogitic (low Cr, high Ca) majorite that yielded formation pressures (Si - Al+Cr geobarometry) >8 GPa (Bulanova et al. CMP 2010). The Fe-sulfide analyzed was low Ni pyrrhotite determined to be Fe₁₀S₁₁ of a rare 11T polytype by single-crystal X-ray diffraction. The 27 ug interior pyrrhotite had a high Re/Os (¹⁸⁷Re/¹⁸⁸Os = 854) typical of MORB (Re/Os ~100) and radiogenic Os isotopic composition (¹⁸⁷Os/¹⁸⁸Os = 8.50±1.38) that yields a 602±16 Ma model age relative to normal mantle. Since the sulfide is in the core of the diamond and the perovskite in the rim, the diamond could have grown at 101 Ma or have two episodes of growth separated by 500 Ma. In any case this sulfide age documents recycling of slab material in to the deep mantle significantly before Mesozoic subduction under Gondwanaland.