Deep-mantle-derived noble gases in metamorphic diamonds from the Kokchetav massif, Kazakhstan

Metamorphic diamonds from the Kokchetav massif in northern Kazakhstan are considered to have crystallized from a C–O–H fluid during ultra-high-pressure metamorphism of metasedimentary rocks subducted to 190–280 km depth. Noble gases contained in the diamonds offer great potential to constrain the noble gas state of deep mantle reservoirs. Previous studies have revealed that secondary processes during the diamond residence in the host rock drastically modified the original noble gas signature of the diamonds. However, nanometer-sized solid/fluid inclusions in the microdiamonds, which represent the former diamond-forming fluid, might preserve the noble-gas signature trapped at the time of diamond formation. We performed noble-gas analyses of the Kokchetav microdiamonds applying two gas extraction techniques: in vacuo crushing and stepwise heating. The former selectively extracts noble gases from inclusions with less noble gas extraction from the diamond lattice. Diamond crushing extracted most of the 3He, indicating that 3He occurs within inclusions trapped during diamond formation. The inclusion-hosted 3He/ 4He of (3.3–6.5) × 10 −5 is significantly higher than that of the MORB-source mantle (1.1 × 10 −5), but close to the maximum value observed in OIBs (ca. 7 × 10 −5) containing noble gases enriched in a primordial component and delivered from the deep mantle by plumes. Neon isotope ratios obtained using stepwise heating also support the presence of a plume-like component. Results show that plume-like, primordial-enriched noble gases were involved in the Kokchetav microdiamond formation, implying metasomatism of the continental lithosphere by a plume prior to its subduction, or interaction of the continental slab and a fragment of the very deep mantle. The deep-mantle-derived fragment might have been delivered to the mantle wedge of the subduction channel by large-scale mantle convection originating from a deeper lower mantle source. ► We performed noble-gas analyses of the Kokchetav microdiamonds. ► Plume-like He and Ne were involved in the Kokchetav microdiamond formation. ► Subducted SCLM may have been metasomatized by a plume prior to its subduction. ► Large-scale convection might deliver a fragment of deep mantle to the mantle wedge.