Deep subduction of organic matter: evidence from nanoSIMS studies of carbon isotopes in microdiamonds from Erzgebirge, Germany

Evidence that organic matter may be subducted to mantle depths comes from study of stable isotopes of carbon and nitrogen in diamonds. Diamonds from the Kokchetav massif, Kazakhstan are characterized by very narrow range of δ13C-value (-10.57‰) and high nitrogen content 11,150 ppm (diamonds from pyroxenite), and -10.19‰ and 2650 ppm (diamonds from marbles). No carbon isotope studies were yet performed on the microdiamonds from felsic gneisses. We report here first nanoSIMS (secondary ion mass spectrometry) studies of δ13C and nitrogen content in microdiamonds form the Erzgebirge massif, Germany. Based on the detection of secondary ions emitted by the sample under primary ion bombardment, SIMS is one of the most sensitive analysis techniques available. It allows ppm to ppb trace level detection of all elements of the periodic table, together with isotopic information. The nanoSIMS is capable to extend the analysis to extremely small areas or volumes (50 nm size) keeping very high sensitivity. This derives from a revolutionary coaxial optical design of the ion gun and a new concept of the mass analyzer. We have studied microdiamonds 2-10 micron diameter included in garnets in polished thin sections. Six diamonds from six different garnets situated in the same thin section show the following variations of δ13C (in ‰ ): diamond #1:-20.3 to -23.1; #4:-20.3 to -23.1; #5:-23.5 to -27.5; #6:-17.0 to -19.0; #7:-20.3 to -26.2 and #8:-24.2 to -27.0. Nitrogen (ppm) varies: diamond #1:753-783; #4:1013-1558; #5:1262-2042; #6:100-1196; #7:1089-1167, and #8:2627-4647. Variation of δ13C in diamonds from -17 to -27‰ suggests diamond formed from biological carbon. This interpretation is in agreement with a sedimentary origin of the UHPM felsic gneisses of the Erzgebirge massif. The wide variation of N from 100 to 2042 ppm is probably due to inhomogeneous distribution of N-bearing fluid or solid inclusions inside microdiamonds, as shown by our previous TEM/FIB studies. NanoSIMS study of microdiamonds is a new step in understanding the media and mechanism of their crystallization.