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Source components and magmatic processes in the genesis of Miocene to Quaternary lavas in western Turkey: constraints from HSE distribution and Hf-Pb-Os isotopes

2015-08-01
Aldanmaz, Ercan
Pickard, Megan
Meisel, Thomas
Altunkaynak, Şafak
Sayıt, Kaan
Sen, Pinar
Hanan, Barry B.
Furman, Tanya
Hf-Pb-Os isotope compositions and highly siderophile element (HSE) abundance variations are used to evaluate the mantle source characteristics and possible effects of differentiation processes in lavas from western Turkey, where the eruption of Late Miocene to Quaternary OIB-type intraplate mafic alkaline lavas followed pre-Middle Miocene convergent margin-type volcanism. Concentrations of Os, Ir, and Ru (IPGE) in the OIB-type intraplate lavas decrease with fractionation for primitive melts (MgO > 10 wt%), suggesting that these elements reside predominantly in olivine and associated HSE retaining trace phases and behave compatibly during olivine-dominated fractionation. Fractional crystallization trends indicate distinctly lower bulk partition coefficients for IPGE in more evolved lavas, possibly reflecting a change in the fractionating assemblages. Pd and Re in the primitive melts display negative correlations with MgO, demonstrating moderately incompatible behavior of these elements during fractionation, while the significantly scattered variation in Pt against MgO may indicate the effects of micronuggets of a Pt-rich alloy. Os-rich alkaline primary lavas (> 50 ppt Os) exhibit a limited range of Os-187/Os-188 (0.1361-0.1404), with some xenolith-bearing lavas displaying depletions in Os-187/Os-188 (0.1131-0.1232), suggesting slight compositional modification of primitive melts through contamination with highly depleted, Os-rich mantle lithosphere. More radiogenic Os isotope ratios (Os-187/Os-188 > 0.1954) in the evolved lavas reflect contamination of the magmas by high(187)Os/Os-188 crustal material during shallow differentiation. The OIB-type lavas show limited variations in Hf and Pb isotopes with Hf-176/Hf-177 = 0.282941-0.283051, Pb-206/Pb-204 = 18.683-19.091, Pb-207/Pb-204 = 15.57915.646, Pb-208/Pb-204 = 38.550-38.993; Hf-176/Hf-177 ratios correlate negatively with Pb-208*/Pb-206*, suggesting the effects of similar mantle processes on the evolution of time-integrated Th/U and Lu/Hf. These lavas have distinctly higher Hf-176/Hf-177 and lower Pb-208*/Pb-206* than the Early-Middle Miocene lavas of the region, which are interpreted as melts of enriched mantle with an overprint by sediment-derived subduction component. The source region for the OIB-type alkaline melts is interpreted to be a sub-lithospheric reservoir enriched in Hf and Pb isotopes with respect to depleted MORB mantle. Combined evaluation of Hf, Pb, and Os isotopes suggests that the relative enrichment in this domain is related to mixing of ancient oceanic crust with the ambient mantle through long-term plate recycling processes.