Ancient depletion signals in lherzolites from forearc region:Constraints from Lu-Hf isotope compositions

• Published in: 地学前缘（英文版） 2022, Vol.13 (1), p.62-75
• Main Authors: Yang Xu, Chuan-Zhou Liu, Wei Lin, Xue-Fa Shi
• Format: Article
• Language: English
• ISSN: 1674-9871

The sub-arc mantle that experienced hydrous melting is commonly characterized by refractory geochem-ical compositions.Nevertheless,minor lherzolites with fertile compositions have also been reported for mantle peridotites from subduction zone.The petrogenesis and mantle source of the lherzolites are still controversial.The New Caledonia ophiolite (Peridotite Nappe) has been regarded as an allochthonous body of forearc lithosphere.This is supported by refractory compositions of its dominant mantle rocks.A few isolated lherzolitic massifs have also been observed in the northern part of New Caledonia.Those lherzolites are compositionally similar to abyssal peridotites,with negligible subduction-related modification.Here,we present new comprehensive geochemical compositions,in particular high-precision Sr-Nd-Hf isotope data,for the lherzolites.The initial 176Hf/177Hf ratios display moderate corre-lations with sensitive indicators for the extent of melting (i.e.,olivine Fo,whole-rock Mg# and Yb con-tents in clinopyroxene) and whole-rock initial 187Os/188Os ratios.Some samples have ancient radiogenic Hf isotopes and unradiogenic Os isotope compositions,implying the preservation of ancient depletion signals in the lherzolites.The Nd isotope compositions,together with trace elements and min-eral micro-textures,suggest that the lherzolites have been overprinted by a recent melt-rock interaction event.The high equilibrium temperatures of the studied samples have been estimated by the two-pyroxene REE thermometer,yielding temperatures of 1066-1315 ℃.The lherzolites have more depleted Nd-Hf isotope compositions and higher equilibrium temperatures than the New Caledonia harzburgites.This indicates that the lherzolites may represent the residues ofasthenosphere mantle trapped within the forearc region.Our studies on the New Caledonia lherzolites with ancient depletion signals suggest that ancient mantle domains in the convective mantle can be emplaced in forearc region by the upwelling of asthenosphere during the early stage of subduction initiation.