MULTI-STAGE EVOLUTION OF THE NEO-TETHYAN OCEANIC UPPER MANTLE: EVIDENCE FROM OS ISOTOPE AND HSE SYSTEMATICS OF SPINEL-PERIDOTITES FROM SW TURKEY

Ophiolites exposed across the western Tauride belt in SW Turkey represent fragments of oceanic lithosphere emplaced following the closure of the Neotethys Ocean during the Late Cretaceous. The mantle sections of the ophiolites contain peridotites with diverse suites of geochemical signatures indicative of residual origin by melt depletion in both mid-ocean ridge (MOR) and supra-subduction zone (SSZ) settings. We examined Re and Platinum Group Element (PGE) abundances and ¹⁸⁷Os/¹⁸⁸Os systematics of peridotites in order to identify the nature of the mantle source and the processes effective during variable stage of melt extraction in these discrete tectonic settings. The MOR peridotites are marked by chondritic Os/Ir and Pt/Ir ratios and slightly supra-chondritic Pd/Ir and Rh/Ir ratios, indicating a mantle region compositionally similar to estimates for the Primitive Mantle (PM). Moderate enrichment in PPGE/IPGE ratios with respect to the PM composition in some of the samples, however, reflects compositional modification by sulfide addition in some parts of the mantle during post-melting processes. The ¹⁸⁷Os/¹⁸⁸Os isotopic compositions of the MOR peridotites range from 0.12227 to 0.12544. The sub-chondritic ¹⁸⁷Os/¹⁸⁸Os isotope ratios (γOs = -3.4]) in the most depleted and chemically undisturbed samples are accompanied by depletion in Re/Os ratios, suggesting long-term differentiation of oceanic upper mantle by continuous melt extraction. The observed positive covariance between ¹⁸⁷Re/¹⁸⁸Os and γOs and higher PPGE and Re abundances than expected for low-degree melting (< 9%) residues can most likely be explained by interaction of solid residues with MORB melts most probably produced by melting of relatively more radiogenic components in the mantle source. The SSZ peridotites, on the other hand, display larger variations in relative PGE and Re abundances, reflecting a more complex evolutionary history. Significantly wide range of Os isotopic ratios (0.12086 to 0.13182; γOs = -4.8 to +3.8) in these high-degree melting (> 13%) residues seems to be consistent with multi-stage evolution of oceanic upper mantle and suggests that parts of the lithospheric mantle contain material that have experienced ancient melt extraction processes (~1Ga) which created time-integrated depletion in Re/Os ratios, while some other parts display evidence indicative of interaction with melts from a mantle reservoir carrying a radiogenic component, most probably created by addition of radiogenic ¹⁸⁷Os during fluid assisted re-melting of previously depleted mantle in a SSZ setting.