Description: Chromite is a typical refractory igneous mineral, precipitated from mafic magmas at relatively high temperatures. Chromites commonly occur in sedimentary, metamorphic, and metasomatic rocks, where they are interpreted as relics of an igneous phase and serve as the source of Cr for low-temperature Cr-bearing minerals. We present evidence for the nucleation of chromite within hydrothermal solutions. We have found minute euhedal chromite grains enclosed by uvarovite (Ca-Cr garnet) in a diopsidite, metasomatically replacing the layered gabbro of the Oman ophiolite. The uvarovite shows oscillatory concentric zoning in terms of Cr no. [Cr/(Cr+Al)], and the chromite is embedded only in the high-Cr-no. zones of the uvarovite. Another diopsidite, replacing peridotite in the underlying upper mantle section, contains xenocrystic chromite, which is partly dissolved. This suggests that a hydrothermal solution collected Cr by partial to total dissolution of chromite within the upper mantle and precipitated chromite, along with high-Cr-no. uvarovite, within the lower crust upsection. The metasomatic agent involved was a CO 2 -, SO 2 -, and Cl-bearing hydrothermal solution containing appreciable silicate components that could carry Cr, possibly as a complex. The hydrothermal chromite is similar in chemistry to that commonly found in igneous rocks [e.g., Cr no. = 0.8, Mg/(Mg+Fe 2+ ) = 0.1–0.2, TiO 2 〈 0.3 wt% and Fe 3+ /(Cr+Al+Fe 3+ ), up to 0.3], but its Cr no. is clearly different from that of mantle chromite (0.6–0.7) in peridotites and chromitites from the Oman ophiolite. The results from this study suggest that a hydrothermal origin is possible for chromites in ultramafic rocks that have experienced fluid activity assuming that there is sufficient chromite at the fluid source.

Description: Secondary orthopyroxenes occur as veinlets (〈0.1 mm thick) cutting an olivine grain in a two-pyroxene peridotite xenolith from the Shiribeshi Seamount in the Sea of Japan. These orthopyroxenes are characterized by low Al 2 O 3 (0.4–1.7 wt%), Cr 2 O 3 (〈0.1 wt%), and CaO (0.3–0.4 wt%) contents, which are the same signatures of the secondary orthopyroxenes in peridotite xenoliths from island arcs. The trace-element patterns of the melts in equilibrium with the secondary orthopyroxenes show enrichment in light rare earth elements and Sr and depletion in heavy rare earth elements, Nb and Ti. These trace-element characteristics are highly consistent with those of slab-derived adakites. The involvement of slab-derived melts in the mantle beneath the Sea of Japan has been inferred from the geochemical characteristics of the volcanic rocks formed during opening of the Sea of Japan. The source mantle of the enriched basalts in the Sea of Japan is likely to have been metasomatized by adakitic melts in the same manner as the peridotite-hosted veinlet. The secondary orthopyroxenes in the peridotite xenolith from the Shiribeshi Seamount provide direct evidence for the metasomatic influx of adakitic melts into the back-arc mantle beneath the Sea of Japan. Adakitic metasomatism, as documented in the Sea of Japan, potentially plays an important role in mantle evolution and magma generation beneath global back-arc basins.