Crustal signature of δ ¹³C and nitrogen content in microdiamonds from Erzgebirge, Germany: Ion microprobe studies

Abstract

The study of δ ¹³CPDB (Pee Dee Belemnite) and nitrogen contents in 1 to 5-μm-diameter microdiamonds included in garnets from the quartz-feldspathic gneisses (Erzgebirge, Germany) was performed in situ with the Nano-scale Secondary Ion Mass Spectrometer. The results revealed that there were two stages of diamond crystallization from a C-O-H supecritical fluid rich in biogenic carbon and diverse minor elements of crustal origin. The δ ¹³C_PDB of the Erzgebirge diamond of the first stage falls in the range −17‰ to 19‰, with an average value of −17.8‰; the average content of nitrogen is 820 ppm. Diamonds of the second stage are characterized by δ ¹³C_PDB=−21.5‰ to −25.5‰, with an average value of −23.24‰; the average nitrogen content is non-homogeneously scattered from 740 ppm to 3 370 ppm among 6 diamonds situated in garnets within the same polished rock slide. Both diamond of the first stage and diamond of the second stage carbon reservoirs belong to biogenic matter, therefore confirming deep subduction of the continental crust sediments and their subsequent exhumation during the Variscan orogeny.

Keywords

metamorphic diamond / carbon isotope / nitrogen content / geochemistry

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