Diamonds in cratonic and orogenic settings : a study of Jericho and Wawa diamonds

• Main Author: De Stefano, Andrea
• Format: Thesis
• Language: English
• Published: University of British Columbia 2011
• Subjects: Nunavut
• Online Access: http://hdl.handle.net/2429/33784

Diamonds can form in a number of different ways. Physical and chemical properties of diamonds classify them as formed below cratons (xenocrystal cratonic) or in a subducting slab followed by rapid exhumation (orogenic). I studied diamonds from a cratonic (Jericho kimberlite, Nunavut) and a synorogenic (calc-alkaline lamprophyres of Wawa, Ontario) setting to reconstruct the process of diamond formation. Diamonds from these two locations have been analysed for their morphology, nitrogen content and aggregation, cathodoluminescence, composition of mineral inclusions, and stable carbon isotopes. In addition, fluorescence and stable nitrogen isotopes were studied in Wawa diamonds. Mineral inclusions in Jericho diamonds were compared with diamondiferous and non-diamondiferous eclogitic Jericho xenoliths with respect to major and trace element compositions. The majority of Jericho diamonds belong to “eclogitic” (90% of the studied samples) and “websteritic” (7%) assemblages. The Jericho diamonds differ from “eclogitic” diamonds worldwide in magnesian compositions of associated minerals and extremely light C isotopic compositions (δ¹³C = -24 to -41‰). We propose that metasomatism triggered by H₂O fluids may have been involved in the diamond formation. The model is supported by the general similarity of mineral compositions in diamondiferous eclogites to those in diamond inclusions and to secondary magnesian garnet and clinopyroxene in recrystallized barren eclogites. The ultimate products of the metasomatism could be “websteritic” diamond assemblages sourced from magnesian eclogites. Wawa diamonds show the following features typical for a cratonic origin: 1) weakly resorbed, octahedral morphology; 2) Low N content; 3) high N aggregation state; 4) the mantle signature of carbon isotopes. Other characteristics of the Wawa diamonds suggest a subduction-related origin, i.e. 1) the presence of peridotitic and eclogitic minerals within single diamonds in a mixed paragenesis also combining shallow and deep phases, 2) the crustal signature of nitrogen isotopes. The most viable model to explain the origin of Wawa diamonds involves early subduction of crustal carbon and nitrogen followed by the carbon-bearing mantle metasomatism and advection of the diamondiferous mantle to the shallow depth of the lamprophyre magmagenesis.