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Pyroxenites, comprising only approximately 2 %–10 % by volume of the upper mantle, are among the most important lithological heterogeneity in the mantle. The formation of pyroxenite veins within peridotites is often attributed to migrating melts which provides an important mechanism for crust-mantle interaction. The Re–Os isotope system provides valuable insight into crust-mantle interaction, as there is a significant contrast in 187Os/188Os between crustal and mantle rocks. Previous Os isotope studies have shown that the forearc mantle wedge mainly consists of highly heterogeneous ancient peridotites. However, limited Re–Os isotope data of forearc mantle pyroxenites exist. Here, two types of mantle pyroxenites from the New Caledonia forearc ophiolite were selected for a comparative study. Contrasting microtextures and geochemical compositions support that these pyroxenites were formed by different processes. The Massif du Sud pyroxenites, hosted in harzburgites, have a cumulate origin and crystallized from boninitic melts generated by remelting of depleted mantle during subduction initiation, whereas the Tiebaghi pyroxenites within the host lherzolites are the products of melt-peridotite reactions. Both types of pyroxenites display highly fractionated highly siderophile element (HSE) patterns. The Massif du Sud pyroxenites are strongly enriched in palladium group platinum-group elements (PPGE) relative to iridium group platinum-group elements (IPGE) with remarkable positive Pt anomalies, which are similar to those of primitive arc lavas and mantle wedge pyroxenite xenoliths. In contrast, the Tiebaghi pyroxenites have flat IPGE but variable PPGE and Re patterns. Compared to the forearc mantle peridotites, the Tiebaghi pyroxenites have higher Os isotope ratios. Ancient basaltic melt percolation followed by long-term (>1 Ga) radiogenic ingrowth can successfully produce the observed correlations of 187Os/188Os with Os or Al 2 O 3 contents. Besides, the oldest Re-depletion model age (T RD) of ∼1.35 Ga has been obtained for the host lherzolites. Therefore, the Tiebaghi pyroxenites and their host lherzolites probably represent a stranded old mantle relict that has undergone ancient partial melting and melt percolation events. Our study supports the presence of highly radiogenic mantle domains in the forearc and provides significant constraints on the role of ancient basaltic melt percolation for generating mantle heterogeneity. [ABSTRACT FROM AUTHOR]

Both felsic and mafic intrusions occur in mantle peridotites of the New Caledonia ophiolite (Peridotite Nappe) and record important information regarding the magmatic evolution during subduction initiation. A systematic petrological, geochronological and geochemical study on gabbro/diorite samples from the Peridotite Nappe is performed to explore their petrogenesis. The gabbro is mainly composed of plagioclase, clinopyroxene and amphibole. Most clinopyroxene have been altered to hornblende or actinolite, whereas some amphibole with high Al2O3 (>8 wt%) and low SiO2 (<46 wt%) contents are of a magmatic origin. This suggests that the gabbro crystallized from hydrous melts. The gabbro shows depleted Sr-Nd-Hf isotope compositions, suggesting that the parental magma of the gabbro could have been derived from the depleted MORB-type mantle (DMM) asthenosphere. The gabbro zircon have δ18O values of +4.74-+6.63‰, which are slightly higher than the mantle-like δ18O values of +5.3 ± 0.6‰, indicating the involvement of subduction-related fluids in their genesis. The diorite mainly consists of albite (An1-4) and tremolite. The felsic dikes within the New Caledonia ophiolite, including granitoids, high-Mg felsic dikes and diorites, are oceanic plagiogranites. Extremely low TiO2 contents, roughly negative correlation of Yb and SiO2 contents, REE patterns of plagiogranites and mafic rocks, and compositions of zircon suggest that the plagiogranites were not products of fractional crystallization of MORB but were generated by hydrous melting of mafic protoliths. The diorite zircon show core-rim textures. Both rims and cores yield identical U-Pb ages of 54.6 ± 0.9 Ma vs 54.6 ± 0.6 Ma, and no old inherited cores were identified based on significantly older dates. Our studies show zircon U-Pb ages of ~55 Ma for the gabbro/diorite and titanite U-Pb age of ~53 Ma for the gabbro, which are older than the New Caledonia CE-boninites (51–47 Ma). Therefore, the magmatic evolution of the New Caledonia ophiolite during subduction initiation has been reconstructed. [ABSTRACT FROM AUTHOR]

The forearc mantle has generally experienced high degrees of partial melting and pervasive melt-rock reactions. Large ultramafic massifs of the New Caledonia ophiolite (Peridotite Nappe) provide opportunities for studying processes within the forearc mantle. Samples of harzburgite, dunite and websterite from three massifs (i.e., Me Maoya, Ouassë Bay, Massif du Sud) have been selected in this study. The peridotites have low contents of Al 2 O 3 (0.10–0.78 wt%) and CaO (0.21–0.58 wt%), but high olivine Fo (90.8–92.5) and spinel Cr# (0.55–0.74) values. This indicates that they have experienced high degrees of partial melting, resulting in the complete consumption of primary clinopyroxene. Trace elements of orthopyroxenes have been measured to constrain the evolution of the New Caledonia harzburgites. Results show that orthopyroxenes have low contents of heavy rare earth element (HREE); their Yb contents are 0.04–0.39 times of chondrites, which can be modeled by ~25–30% of fractional melting of a depleted MORB mantle (DMM) source. The orthopyroxenes also show variable enrichment of light and middle rare earth elements (LREEs and MREEs) and large ion lithophile elements (LILEs), implying late-stage metasomatism. The websterites have high CaO (9.02–9.75 wt%) but low Al 2 O 3 (1.59–1.60 wt%) contents. They have depleted Sr-Nd-Hf isotope compositions similar to that of New Caledonia CE-boninites and plot within the MORB field. Both websterites and dunites have 187Os/188Os ratios (0.12348–0.13067) comparable to that of abyssal peridotites and the New Caledonia harzburgites (0.1203–0.1534). Geochemical compositions and modeling of whole-rock and minerals of harzburgites, dunites and websterites suggest that the New Caledonia peridotites have experienced at least two-stage partial melting and reactions with various migrating melts. Moreover, we also demonstrated that trace element of orthopyroxenes can be applied to discriminate ultra-refractory harzburgites from different tectonic settings. • Ultra-refractory harzburgites and websterites occur in New Caledonia ophiolite. • Opx trace elements were used to melt depletion and enrichment of the harzbrugites. • The ultra-refactory harzburgites have experienced ~25% degrees of partial melting. • The websterites have depleted Sr-Nd-Hf isotope compositions. • The websterites were generated during reaction of harzburgites and boninitic magmas. [ABSTRACT FROM AUTHOR]

Compositions of highly siderophile elements (HSEs) in forearc mantle have been little studied and effects of slab dehydration on their abundances in forearc mantle remains unclear. This study reports two different kinds of HSE patterns for peridotites from a New Caledonia forearc ophiolite. The Group-I samples show relatively flat patterns of Ir-group-platinum-group elements (IPGEs) and enrichment of Pt over Pd. Such patterns imply that interstitial sulfides were significantly removed through melt extraction, whereas sulfides enclosed within silicates were mostly unaffected. Meanwhile, Pt-Fe alloys were generated, resulting in suprachondritic Pt/Pd ratios. In contrast, the Group-II samples display convex HSE patterns and are depleted in all HSEs except for Ru, yielding strongly positive Ru anomalies. This indicates that both enclosed and interstitial sulfides were substantially consumed, whereas chromite was generated to stabilize Ru. Compared to abyssal peridotites, subduction-related peridotites commonly have stronger fractionation in the HSEs. Therefore, the HSE data of mantle peridotites are potentially able to discriminate the tectonic settings of ophiolites. [ABSTRACT FROM AUTHOR]