Formation conditions of Copeton (NSW) diamonds based on laser Raman spectroscopy of inclusions

Abstract

Laser Raman spectroscopy is used to identify mineral inclusions in situ within six Copeton diamonds and to determine the remnant internal pressure on them, namely 31-34 kbar for coesite, 13.6 and 23 kbar for diopsidic omphacite, and 8 kbar for grossular garnet. Using a published linear model of the host-inclusion volume system, each remnant pressure value generates a PT locus of diamond formation. The intersections of these define a range of diamond formation conditions from 250ºC, 43 kbar to 950ºC, 58 kbar with estimated intersection errors of ±70ºC and ±4 kbar. This range encompasses at least two sets of ultrahigh pressure (UHP) formation conditions derived from different protoliths involved with the termination of subduction, namely subducted oceanic slab (lower temperatures: eclogitic diamonds) and subducted continental rocks (higher temperatures: calcsilicate diamonds). Coesite has been found as a compound inclusion with omphacite (lower temperature set) and with grossular (higher temperature set). The measurements indicate that the Copeton stones are UHP diamonds (not cratonic), and their Carboniferous argon age dates on pyroxene inclusions should be interpreted as ages of crystallisation. The results and implications are consistent with a local source for Copeton diamonds, and imply there is a buried Carboniferous UHP terrane within eastern Australia.