Ultra-depleted peridotites of New Caledonia: a reappraisal

In spite of pervasive serpentinisation and prominent weathering, the New Caledonia Peridotitic Nappe is one of the largest and best exposed supra-subduction mantle section in the world and hence provides a unique insight on upper mantle processes. The sequence is dominated by harzburgite-dunite association and minor lherzolites, but it also includes mafic/ultramafic cumulates. Recent geochemical studies revealed that harzburgites experienced several phases of melting, melt-rock interaction and re-melting, that led to overall ultra-depletion, making their geochemical characterization an analytical challenge. This presentation deals with the preliminary results of a reappraisal based upon a new set of unserpentinised, or only slightly serpentinised, peridotite samples collected on the whole island. Harzburgites are highly refractory rocks, as attested by the absence of primary clinopyroxene, very high Fo content in olivine (90.7-92.9 mol%), high Mg# in orthopyroxene ([Mg/(Mg+Fe)]= 91.0-92.7) and Cr# in spinel ([100 • Cr/(Cr+Al)]= 40-71). Secondary, interstitial and undeformed clino- and orthopyroxenes have also been observed in harzburgites. These phases indicate percolation after partial melting and re-equilibration at lithospheric conditions. Their chemical compositions, i.e. low Al2O3 and CaO contents in orthopyroxene and very low Na2O and TiO2 in clinopyroxene, suggest a metasomatic origin by SiO2-rich fluids and/or depleted melts in a subduction-related setting. In contrast, lherzolites exhibit a fairly fertile nature, indicated by lower Fo in olivine (88.5-90.0), low Cr# in spinel ([100 • Cr/(Cr+Al)]= 13-17) and relatively high Na2O (up to 0.80 wt%) and Al2O3 (3.1-6.7 wt%) in clinopyroxene. The peridotites are low strain tectonites with porphyroclastic textures partially overprinted by mosaic equigranular textures. They record an asthenospheric HT origin followed by sub-solidus re-equilibration, as also testified by geothermometric estimates (930–1145°C and 870-1080°C on porphyroclastic assemblages for harzburgites and lherzolites respectively; 830°C-980°C for spinel facies recrystallization in both lithotypes). Mineral compositional variations (e.g. Mg# (Ol) vs Cr# (Spl), Cr# (Spl) vs Mg# (Spl)), show that most investigated harzburgites plot in the field of fore-arc peridotites, while lherzolites are more akin to (variably refertilized) abyssal peridotites. These features have also been confirmed by whole-rock REE contents, that display U-shaped pattern for harzburgites, commonly attributed to a fore-arc setting and abyssal-type patterns for lherzolites. These results are consistent with multi-stage history of melting, deformation, recrystallization and melt-rock interaction. Major and trace elements combined with radiogenic isotope analyses (in progress) will provide a clue for deciphering the depletion vs. refertilization evolution of the different peridotite types and will add new constraints on their geodynamical significance.