Sr, Nd, Pb and trace element systematics of the New Caledonia harzburgites: Tracking source depletion and contamination processes in a SSZ setting

Abstract

The New Caledonia ophiolite (Peridotite Nappe) consists primarily of harzburgites, locally overlain by mafic-ultramafic cumulates, and minor spinel and plagioclase lherzolites. In this study, a comprehensive geochemical data set (major and trace element, Sr-Nd-Pb isotopes) has been obtained on a new set of fresh harzburgites in order to track the processes recorded by this mantle section and its evolution.The studied harzburgites are low-strain tectonites showing porphyroclastic textures, locally grading into protomylonitic textures. They exhibit a refractory nature, as attested by the notable absence of primary clinopyroxene, very high Fo content of olivine (91–93 mol.%), high Mg# of orthopyroxene (0.91–0.93) and high Cr# of spinel (0.44–0.71). The harzburgites are characterised by remarkably low REE concentrations (<0.1 chondritic values) and display "U-shaped" profiles, with steeply sloping HREE (DyN/YbN = 0.07–0.16) and fractionated LREE-MREE segments (LaN/SmN = 2.1–8.3), in the range of modern fore-arc peridotites. Geochemical modelling shows that the HREE composition of the harzburgites can be reproduced by multi-stage melting including a first phase of melt depletion in dry conditions (15% fractional melting), followed by hydrous melting in a subduction zone setting (up to 15%–18%). However, melting models fail to explain the enrichments observed for some FME (i.e. Ba, Sr, Pb), LREE-MREE and Zr–Hf. These enrichments, coupled with the frequent occurrence of thin, undeformed films of Al2O3, and CaO-poor orthopyroxene (Al2O3 = 0.88–1.53 wt.%, CaO = 0.31–0.56 wt.%) and clinopyroxene with low Na2O (0.03–0.16 wt.%), Al2O3 (0.66–1.35 wt.%) and TiO2 (0.04–0.10 wt.%) contents, point to FME addition during fluid-assisted melting followed by late stage metasomatism most likely operated by subduction-related melts with a depleted trace element signature.Nd isotopic ratios range from unradiogenic to radiogenic (−0.80≤εNdi≤+13.32) and negatively correlate with Sr isotopes (0.70257≤87Sr/86Sr ≤ 0.70770). Pb isotopes cover a wide range, trending from DMM toward enriched, sediment-like, compositions. We interpret the geochemical signature displayed by the New Caledonia harzburgites as reflecting the evolution of a highly depleted fore-arc mantle wedge variably modified by different fluid and melt inputs during Eocene subduction.