Deep origin of mantle peridotites from the Aladağ ophiolite, Turkey: Implication from trace element geochemistry of pyroxenes and mineralogy of ophiolitic diamonds

Abstract

The Aladağ peridotites mainly consist of harzburgites with subordinary dunites. High olivine Fo [100 × Mg/(Mg + Fe2+) = 90.8–92.0] and spinel Cr# [100 × Cr/(Cr + Al) = 61.5–71.6] values indicate the Aladağ harzburgites have experienced high degrees of partial melting. Significant depletion of middle rare earth element in the clinopyroxene and orthopyroxene implies that the partial melting of mantle source initiated in the garnet stability field. Occurrences of the interstitial clinopyroxene and spinel-hosted hydrous mineral inclusions, as well as variable enrichments of light rare earth element (LREE) and large-ion lithophile element in the clinopyroxene and orthopyroxene, suggest that the harzburgites suffered pervasive melt-rock interaction. The modeled interacting melts show enrichments of LREE and have a boninite-like affinity. In the dunites, olivines have higher contents of Fo (92.3–92.7), Ca (476–1830 ppm), and Ti (3.4–9.8 ppm) than those in the harzburgites (90.8–92.0, 50–319 ppm, and 0.6–6.4 ppm, respectively). Spinels in dunites have high Cr# (73.4–77.4) values and Ti (569–722 ppm) concentrations. Such mineral chemical characteristics imply extensive melt-rock interaction. We propose that the dunite was produced by early-stage interaction of harzburgite with migrating Si-undersaturated melt in asthenosphere. Subsequently, dunite and harzburgite together experienced late-stage interaction with boninite-like melt as incorporated into thermal boundary layer above a supra-subduction zone. An unusual mineral assemblage of ophiolitic diamond, moissanite, zircon, corundum and rutile, separated from the Aladağ harzburgites, indicates that relatively deep processes involved in the formation of the Aladağ mantle peridotites.