The Study of Olivine Inclusions in Diamonds from Liaoning, China and the Evaluation of Related Thermometers

Olivine is the most abundant mineral in mantle peridotite and a typical inclusion in diamonds, providing essential evidence for the characterization of the diamondiferous lithospheric mantle. Three olivine inclusions in diamonds (OlDia) from Liaoning in the North China Craton (NCC) were exposed for in situ measurements, and the compositional data of 62 other OlDia from Liaoning were collected based on previous reports. The enrichment of TiO2 (>0.1 wt.%) with high Cr# (>50; Cr# = 100Cr/(Cr + Al) by atom) was revealed, despite the predominance of depleted TiO2 contents and high Mg# (92.8–93.2; Mg# =100Mg/(Mg + Fe) by atom) for OlDia. Silica fluid accompanying olivine still trapped in the host diamond was recognized using Raman spectroscopy. Three thermometers were applied to the OlDia, based on the data from Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Electron Probe Micro-Analysis (EPMA), and a temperature range (TLA and TEPMA) of 1080–1380 °C was yielded. With respect to the TLA, Al-in-olivine thermometers were preferred, although there was a smaller deviation between TEPMA and TLA when using the Cr-in-olivine thermometer. The results of these thermometers show a high correlation with TEPMA, enabling their application based on EPMA data. Projections onto 39–42 mW/m2 model geotherms underline a diamondiferous base of the lithospheric mantle beneath the NCC. The lithospheric mantle is characterized by refractory and depleted sections, where enrichment metasomatism may have occurred at the lower roots (161–178 km).