MC-HR-SIMS oxygen isotope analysis of ferropericlase inclusions in diamond

Ferropericlase (Mg, Fe)O is an important terrestrial mineral. While relatively rare in the crust, it constitutes approximately 17% by volume of the lower mantle, from ~660km to ~2900km. Importantly, the assemblage of ferropericlase and perovskite (which together are diagnostic of a lower mantle origin) are rarely found together as inclusions in diamond and provide our only solid samples of a region that occupies over 50% of Earths volume [1].

Studies of these ultra-deep diamond inclusions have shown that the periclase inclusions can have a variable Mg#, and have demonstrated the presence of Eu anomalies in coexisting perovskite [2-3]. This has led some workers to suggest that the ultra-deep mineral assemblages trapped in the
diamond formed from subducted crust that has passed through the transition zone and into the lower mantle [3]. If verified, this would provide an important geochemical verification of whole mantle convection.

Oxygen isotope ratios of crustal rocks can be distinctly different those of the mantle (δ¹⁸O_VSMOW values of ca. 5.5‰) and can therefore provide a crucial test of the crustal or mantle heritage of ferropericlase inclusions. Diamond inclusions of ferropericlase are <100μm in size and therefore best analysed by microbeam methods. We have performed preliminary experiments on the ¹⁸O/¹⁶O measurement of synthetic and natural crustal periclase on the newly installed Cameca IMS1280 multicollector secondary ion mass spectrometer (MC-SIMS) at CCIM,. The SIMS analytical methods largely follow established procedures, using Cs^- primary ions, normal incidence electron gun, and multi-collection on faraday cups. Synthetic and natural RM’s are being characterized by laser fluorination. Preliminary analysis shows that internal spot errors of <0.1‰ (1$) for ¹⁸O/¹⁶O are readily achieved. Spot-to-spot reproducibility and potential matrix effects due to varying Mg#s will be examined.