Melt Penetration in Oceanic Lithosphere: Li Isotope Records from the Pozanti-Karsanti Ophiolite in Southern Turkey

Abstract

Understanding melt penetration in ophiolites is important in revealing the formation and evolution of the oceanic lithosphere, as well as the formation of chromite deposits. We conducted in situ Li isotope analysis of olivine grains from the harzburgite, podiform chromitite and dunite envelope of the mantle sequence, and dunite and wehrlite cumulates from the crustal sequence of the PozantiKarsanti ophiolite. Olivine in the different rocks has variable and distinctive Li concentrations (0.95 to 2.01 ppm) and d 7 Li isotopic compositions (-5.43 to 16.65 &). The olivine in the refractory harzburgite of the Pozanti-Karsanti ophiolite extends from MOR-peridotite-like compositions to lower d 7 Li values and higher Li concentrations, suggesting ingressive Li diffusion via melt penetration. The Li isotopic compositions of the olivine in the dunite envelope and podiform chromitite represent the compositions of their parental melts and thus their sources. The dunite envelope has d 7 Li values beyond MORB and OIB variations and overlaps the arc lava range, suggesting an affinity with arc magmatism. The podiform chromitite, on the other hand, shows wider d 7 Li variation and overall lower values, falling in the ranges of granulite and eclogite. The parental melts of the podiform chromitite might: (1) originate from partial melting of a dehydrated and metamorphosed subducted slab or (2) may initially have had the same isotopic composition as the associated dunite, but later experienced compositional modification by fluids. The Li isotopic compositions of the dunite and wehrlite cumulates fall between the OIB and arc fields, supporting a subduction origin for these ultramafic cumulates and their arc-like parental magmas. The Pozanti-Karsanti ophiolite records various melt penetration agents during its formation and evolution. The melts display progressive compositional variations, mainly between OIB and immature and mature oceanic island arc magmatism, pointing to their correlation with subduction initiation. The distinctive Li isotopic compositions of the Pozanti-Karsanti, Luobusa and Trinity ophiolites suggest that the generation and compositions of penetrating melts in oceanic lithosphere and subsequent formation of chromite deposits are strongly controlled by tectonic setting. Consequently, Li isotope systematics can be used as an indicator of the tectonic setting and mineralization of ophiolites.