Superdeep Diamonds and Mantle Convection
Andrew Steele, Gaillou (Carnegie Institution of Washington)
Walter, Bulanova, Smith and Kohn (University of Bristol) Passos de Araujo (University of Brasilia)
Why examine superdeep diamonds?
A collaboration has begun (Fall, 2010) between the University of Bristol, the University of Brasilia , and the Carnegie Institution of Washington to examine the prolific suite of superdeep diamonds from Juina State, Brazil. These diamonds and their inclusions will allow us to address fundamental questions regarding deep mantle processes and carbon cycling beneath Brazil.
With our ability to use sulfides to provide ages and isotopic compositions on single diamonds, the oceanic lithosphere recycling model above can be carefully tested. We will attempt to see if the Juina 5 and Collier 4 diamonds turn out to be young so that they can be related to the same geodynamic setting. It will be interesting to see if their initial Os isotopic compositions will support the idea of recycling basaltic components of the oceanic lithosphere or if we can see the evidence for recycling ancient oceanic harzburgite in the inclusions.
These sulfides will be analyzed by our team for Re-Os to determine both the age of the diamond, the isotopic composition of the Os and by inference the source of the diamond-forming fluids.
Another goal will be to understand the role of carbonate melt in diamond and inclusion petrogenesis. SIMS analysis of volatiles will provide evidence for whether transition zone inclusion minerals are higher in water to understand what constraints there are on the composition and amount of water delivered to the transition zone. The ultimate origin of the carbon in the diamond is of great importance to the igneous carbon cycle. Whether biogenic carbon, subducted into the mantle, really contributes to diamond formation at these depths can perhaps be answered with C and where possible, N isotopic analyses.