Mineral inclusions in sublithospheric diamonds from Collier 4 kimberlite pipe, Juina, Brazil: subducted protoliths, carbonated melts and primary kimberlite magmatism

Byline: Galina P. Bulanova (1), Michael J. Walter (1), Chris B. Smith (1), Simon C. Kohn (1), Lora S. Armstrong (1), Jon Blundy (1), Luiz Gobbo (2) Keywords: Brazil; Collier 4 kimberlite; Sublithospheric diamonds; Inclusions; Carbon isotopes; Subduction; Carbonatite melt Abstract: We report on a suite of diamonds from the Cretaceous Collier 4 kimberlite pipe, Juina, Brazil, that are predominantly nitrogen-free type II crystals showing complex internal growth structures. Syngenetic mineral inclusions comprise calcium- and titanium-rich phases with perovskite stoichiometry, Ca-rich majoritic-garnet, clinopyroxene, olivine, TAPP phase, minerals with stoichiometries of CAS and K-hollandite phases, SiO.sub.2, FeO, native iron, low-Ni sulfides, and Ca--Mg-carbonate. We divide the diamonds into three groups on the basis of the carbon isotope compositions ([delt[a].sup.13]C) of diamond core zones. Group 1 diamonds have heavy, mantle-like [delt[a].sup.13]C (-5 to -10[per thousand]) with mineral inclusions indicating a transition zone origin from mafic protoliths. Group 2 diamonds have intermediate [delt[a].sup.13]C (-12 to -15[per thousand]), with inclusion compositions indicating crystallization from near-primary and differentiated carbonated melts derived from oceanic crust in the deep upper mantle or transition zone. A 206.sup.Pb/ [.sup.238]U age of 101 +- 7 Ma on a CaTiSi-perovskite inclusion (Group 2) is close to the kimberlite emplacement time (93.1 +- 1.5 Ma). Group 3 diamonds have extremely light [delt[a].sup.13]C (-25[per thousand]), and host inclusions have compositions akin to high-pressure--temperature phases expected to be stable in pelagic sediments subducted to transition zone depths. Collectively, the Collier 4 diamonds and their inclusions indicate multi-stage, polybaric growth histories in dynamically changing chemical environments. The young inclusion age, the ubiquitous chemical and isotopic characteristics indicative of subducted materials, and the regional tectonic history, suggest a model in which generation of sublithospheric diamonds and their inclusions, and the proto-kimberlite magmas, are related genetically, temporally and geographically to the interaction of subducted lithosphere and a Cretaceous plume. Author Affiliation: (1) Department of Earth Sciences, University of Bristol, Queen's Rd, Bristol, BS8 1RJ, UK (2) Rio Tinto Desenvolvimentos Minerais Ltda, Brasilia, Brazil Article History: Registration Date: 11/01/2010 Received Date: 04/06/2009 Accepted Date: 11/01/2010 Online Date: 07/02/2010 Article note: Communicated by M. W. Schmidt.