Metasomatic origin of clinopyroxene in Archean mantle xenoliths from Hebi, North China Craton: Trace-element and Sr-isotope constraints

Abstract

Clinopyroxene is a major host for lithophile elements in the mantle lithosphere, and therefore its origin is critical for constraints on mantle evolution and melt generation. This study present detailed in-situ trace-element and Sr-isotope analyses of clinopyroxene in the mantle xenoliths from Hebi, North China Craton (NCC). The Archean lithospheric mantle beneath the NCC has been heterogeneously thinned and replaced by juvenile asthenospheric mantle. The Hebi mantle xenoliths mainly consist of spinel harzburgites with highly refractory compositions; most olivines are Fo91.1–92.5. Eight samples have 187Os/188Os ratios of 0.11067–0.11688, giving Paleoproterozoic-Archean model ages (TMA; 2.3–3.4Ma) calculated relative to the primitive upper mantle (PUM). This confirms the existence of Arhcean mantle relics beneath the Hebi area during the Cenozoic. Clinopyroxene in the Hebi mantle xenoliths shows strong enrichment in LILEs (e.g., Th, U and LREE), but depletion in both HREE and HFSEs (e.g., Nb, Ta, Zr and Hf), which suggests that the Hebi mantle xenoliths have been metasomatized by carbonatitic melts. Strontium isotopes in clinopyroxene have been determined in-situ in 13 samples, giving 87Sr/88Sr ratios of 0.70309–0.70556, which are lower than the bulk-rock values of the Paleozoic mantle xenoliths. Therefore, we suggest that clinopyroxenes in the Hebi mantle xenoliths crystallized from metasomatic melts. However, the metasomatic process has not significantly disturbed the Os isotope ages of the Hebi mantle xenoliths.