Evidence of subduction and crust–mantle mixing from a single diamond Daniel J. Schulze a, * , Ben Harte b , John W. Valley c , Dominic M. DeR. Channer d a Department of Geology, Erindale College, University of Toronto, Mississauga, Ontario, Canada L5L 1C6 b Department of Geology and Geophysics, University of Edinburgh, Edinburgh EH9 3JW, UK c Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USA d Guaniamo Mining Company, Centro Gerencial Mohedano, 9D Urb. La Castellana, Caracas, Venezuela Received 27 June 2003; accepted 17 February 2004 Available online 18 May 2004 Abstract Cathodoluminescence (CL) imaging of polished sections of a diamond from the Guaniamo region of Venezuela suggests a history of the diamond involving two periods of growth separated by a period of resorption and possibly brittle deformation. In situ electron probe analysis of multiple eclogitic garnet inclusions reveals a correlation between garnet composition and location in the stone. An early-formed garnet in the diamond core has higher Ca/(Ca + Mg) and lower Mg/(Mg + Fe) values than later garnets associated with the second period of diamond growth. This variation conforms to an extensive trend of variation in the suite of eclogitic garnets extracted from Venezuelan diamonds. The diamond is zoned in carbon isotope composition (in situ secondary ion mass spectrometry, SIMS, data). The core compositions (d 13 C PDB), corresponding to the first stage of growth, average À 17.7x. The second period of growth is apparently in two sub-sets of CL zones with mean values of À 13.0x and À 7.9x. Nitrogen contents of diamond are low (30 – 300 atomic ppm) and do not correlate with carbon isotope composition. Oxygen isotope ratios of the garnet inclusions are elevated substantially above those expected for ‘‘common mantle’’; d 18 O VSMOW of early garnet is approximately + 10.5x and two late garnets average + 8.8x. The evolutionary trend of magnesium enrichment in garnet is unlikely to represent igneous fractionation. The stable isotope data are consistent with diamond formation in subducted meta-basic rocks that had interacted with sea water at low temperatures at or near the sea floor and contained a substantial biogenic carbon component. During or following subduction, diamonds continued to form in an evolving system that was progressively modified by interaction with mantle material. D 2004 Elsevier B.V. All rights reserved. Keywords: Diamond; Garnet; Carbon isotopes; Oxygen isotopes; Subduction 1. Introduction The study of the minute minerals found as prima- ry (syngenetic) inclusions in diamonds has provided a wealth of information bearing on the origin of diamonds within Earth’s interior (e.g., Sobolev, 1976; Meyer, 1987; Gurney, 1989). Due to technical limitations, and expediency, however, most studies to date have involved breaking or burning diamonds to liberate their inclusions. In many cases, multiple inclusions of the same mineral in a single diamond have been found to have virtually the same compo- 0024-4937/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2004.04.022 * Corresponding author. Tel.: +1-905-828-3970; fax: +1-905- 828-3717. E-mail address: dschulze@utm.utoronto.ca (D.J. Schulze). www.elsevier.com/locate/lithos Lithos 77 (2004) 349 – 358