Mantle peridotites of ophiolites rarely preserve reliable records of paleo-oceanic lithospheric mantle

Mantle peridotites of ophiolites have traditionally been interpreted as fragments of residual oceanic lithospheric mantle depleted by partial melting. However, recent petrological, geochemical and isotopic data suggest that this hypothesis needs to be reconsidered. Spinel grains in ophiolitic mantle peridotites, previously believed to record melt extraction, exhibit significant compositional variations that are incompatible with partial melting. These grains occur mostly as interstitial, anhedral crystals associated with clinopyroxene or as inclusions in silicates; they commonly also contain a variety of inclusions. Their elemental compositions are highly variable at both intra- and inter-grain, as well as intra- and inter-sample scales. In addition to association with spinel, interstitial clinopyroxene is also widespread in ophiolitic mantle peridotites. Individual clinopyroxene grains show positive correlations between Al2O3 and Cr2O3 contents, akin to those observed in orthopyroxene, which deviate from partial melting trends. Reactive and replacive features are well-developed in orthopyroxene grains. Olivine has anomalously variable Li isotope compositions and generally heavier Fe and Cr isotope compositions than coexisting spinel, contrary to theoretical predictions and results from mantle peridotite xenoliths. Additionally, aggregates of olivine, orthopyroxene, clinopyroxene, spinel and amphibole are also commonly observed in many samples. All of these characteristics suggest significant post-crystallization modifications of the constituent minerals in the mantle peridotites of ophiolites. Amphibole, a widespread hydrous mineral in mantle peridotites of ophiolites, serves as robust evidence of the past presence of hydrous melts/fluids. It typically exhibits variable compositions that can be distinguished from grains in other lithologies or different tectonic settings. These melts/fluids are most likely related to t