Browsing by Author "Gulec, Nilgun"

Now showing 1 - 5 of 5

Geological and Geochemical Evolution of the Quaternary Suphan Stratovolcano, Eastern Anatolia, Turkey: Evidence for the Lithosphere-Asthenosphere Interaction in Post-Collisional Volcanism
(Oxford Univ Press, 2014) Ozdemir, Yavuz; Gulec, Nilgun
The Suphan stratovolcano is one of the major Quaternary eruption centers of post-collisional volcanism in eastern Anatolia, located in the zone of convergence between the Arabian and the Anatolian plates. We document the geological and geochemical evolution of Suphan in terms of volcanostratigraphy, geochronology (40Ar-39Ar), isotope composition (Sr, Nd, Pb) and bulk-rock geochemistry. Our new 40Ar-39Ar data, along with previously published K/Ar ages, indicate an age of 0 center dot 76-0 center dot 06 Ma for the eruptive products, which extend over an area of similar to 2000 km(2). The products of Suphan include transitional mildly alkaline to calc-alkaline lavas, domes and pyroclastic rocks ranging in composition from basalt to rhyolite. MELTS modeling suggests that the majority of the Suphan magmas with SiO2 contents between similar to 57 and similar to 65 wt % are the products of isobaric-isenthalpic mixing of basaltic trachyandesitic and rhyolitic magmas at a crustal pressure of 0 center dot 5 kbar. Energy-constrained assimilation and fractional crystallization modeling of trace element and isotope compositions points to 2-10% (upper) crustal contamination that accompanied differentiation. Large ion lithophile element enrichment and the presence of negative high field strength element anomalies in primitive mantle-normalized trace element patterns suggest a mantle source that was modified by subducted sediments. Light to heavy and medium to heavy rare earth element ratios and partial melting models suggest the derivation of the eastern Anatolian parental magmas from variable mixing of lithospheric and asthenospheric melts. Unlike the commonly accepted models for post-collisional volcanism that envisage an increasing contribution from asthenospheric melts with time, melting degree and contribution of the lithospheric mantle seem to have increased from Miocene-Pliocene to Quaternary times in eastern Anatolia.
Origin and Petrogenesis of Magmatism in Collision-Related Environments: Evidence From the Melikler Volcanics on the Kars Plateau-Turkey in the Turkish-Iranian High Plateau
(Oxford Univ Press, 2023) Oyan, Elif; Oyan, Vural; Ozdemir, Yavuz; Gulec, Nilgun
The temporal distribution of enriched source components and magmatism in continental collision zones provides critical information about mantle dynamic processes in collision-related environments. This paper presents petrology, mineralogy, K-Ar ages and whole-rock major and trace elements, as well as Sr-Nd-Pb-Hf isotopic compositions of Melikler volcanism in Kars Plateau (KP) in the East Anatolia Collision Zone, NE Turkey, with the aim to understand the role of the subducting slab, the origin of magmatism and the geodynamic evolution in the collision-related environments. Our K-Ar dating results show the Melikler volcanism erupted between 5.29 and 1.7 Ma and allows us to divide it into an early (5.29-2.53 Ma) and a late (2.24-1.7 Ma) stage. Major-trace element abundances, isotopic compositions, EC(R) AFC (energy-constrained recharge, assimilation, and fractional crystallisation) and MELTS model calculations of both stages indicate that the least evolved samples were not affected by significant crustal contamination and fractional crystallisation. More evolved samples of the late stage underwent AFC processes with up to 6-9% crustal assimilation; however, those of the early stage were differentiated from a parental magma composition via AFC (up to 2-7.5% crustal assimilation) and experienced magma replenishment at pressure of 0.5 kbar; thus, both early and late stages have experienced open system conditions. The least evolved samples of both stages across the KP have arc-enriched geochemical and isotopic signatures, characterised by prevalent negative Nb-Ta anomalies and moderately radiogenic Sr, unradiogenic Nd-Hf and highly radiogenic Pb isotopic compositions. These primary melts could be derived from a depleted MORB mantle source metasomatised by sediment melt from the subducting Neotethys oceanic slab. Combined trace elemental and isotopic modelling results suggest that the least evolved samples of the early stage were formed by 2-4% melting of an amphibole-bearing garnet lherzolitic mantle source, which was metasomatised by 0.3-0.5% contribution of subducted slab component with a ratio of sediment melt/AOC (altered oceanic crust) melt about 90:10. A depleted lherzolitic mantle source containing apatite and garnet through inputs of 0.6-0.8% melts derived from the subducted oceanic slab, with 5-10% partial melting degree, could produce the least evolved samples of the late stage. Thermobarometric calculations reveal that the least evolved samples of the late stage are derived from the lithosphere-asthenosphere boundary at a depth of 77-82 km; in contrast, those of the early stage are produced from the lithosphere at a depth of 66-69 km. Literature data and the findings obtained from this study indicate that the onset of the Arabian-Eurasian collision may have occurred in the Oligocene and lithospheric dripping caused by the hard collision that occurred around the Late Miocene-Early Pliocene may produce the Melikler volcanic rocks.
Post-Earthquake Anomalies in He-Co2 Isotope and Relative Abundance Systematics of Thermal Waters: the Case of the 2011 Van Earthquake, Eastern Anatolia, Turkey
(Elsevier, 2015) Aydin, Harun; Hilton, David R.; Gulec, Nilgun; Mutlu, Halim
We report the helium and carbon isotope (He-3/He-4, delta C-13) and relative abundance (CO2/He-3) characteristics of hydrothermal gases from eastern Anatolia sampled similar to 1 month after the October 23, 2011 Van earthquake (Mw: 7.2, focal depth: 19 km). Seven sites were sampled which comprise three localities along the Caldiran Fault Zone, at a distance of 58-66 km to the epicenter (Group I), two localities north of the Caldiran Fault, about 90-113 km from the epicenter (Group II), and two localities in the vicinity of the historically-active Nemrut Caldera at a distance of 110-126 km from the epicenter (Group III). All sites were previously sampled for their He-CO2 systematics in 2009 (Mutlu et al., 2012) facilitating direct comparison with the post-earthquake dataset. The post-earthquake values cover a wide range of He-3/He-4, delta C-13 and CO2/He-3 ratios, from 0.84 to 6.37 RA (where RA = air He-3/He-4), -5.30 to + 0.49% (vs. VPDB), and 4.9 x 10(10) to 6.85 x 10(13), respectively. Group I samples show a consistent post-earthquake increase in He-3/He-4 whereas both Group II localities decreased in He-3/He-4. No change was recorded for the two Group III localities. He isotope variations are consistentwith simple changes in the proportions of mantle and crustal volatiles, with all Group I sites showing an increase in the mantle He contribution. We hypothesize that the enhanced mantle He signal is derived from asthenospheric melts intruded into the crust, with seismic perturbations responsible for bubble formation and growth leading to overpressure and gas loss. The strike-slip Caldiran Fault Zone provides the permeable pathway for the liberated volatiles to reach hydrothermal systems at shallow levels of the crust and the surface. Release of crustal He dominates the He mass balance of Group II samples as locations are further from the earthquake epicenter. Group III samples are even further away from the earthquake and show no perturbations in He isotopes. Whereas binary mixing dominates the He isotope systematics, CO2 shows additional effects involving the hydrothermal system. Consequently, changes in the balance between mantle and crustal CO2 are masked and more difficult to discern. The results emphasize the sensitivity of He isotopes to seismic perturbations in the crust and illustrate how location of sampling sites - on permeable segments of faults - and distance from seismic events influence resulting changes involving gas chemistry. (C) 2015 Elsevier B.V. All rights reserved.
Spatial Variations in Gas and Stable Isotope Compositions of Thermal Fluids Around Lake Van: Implications for Crust-Mantle Dynamics in Eastern Turkey
(Elsevier, 2012) Mutlu, Halim; Gulec, Nilgun; Hilton, David R.; Aydin, Harun; Halldorsson, Saemundur A.
We investigate the helium (He-3/He-4) and carbon (delta C-13) isotope compositions and relative abundance ratios (CO2/He-3) of gas samples together with the stable isotope compositions of dissolved carbon and sulfur and the oxygen and hydrogen isotopic compositions of the associated water phase from a number of geothermal fields located around Lake Van in eastern Anatolia, Turkey. The mantle-derived helium component, which is likely transferred to the crust beneath eastern Turkey by recent magmatism, is found to constitute up to 96% (e.g. Nemrut Caldera) of the total He content in fluids. As regards the spatial distribution of He, samples collected from areas of Pliocene-Quaternary volcanics are characterized by a wide and generally higher range of R/R-A ratios (0.93 to 7.76 R-A) compared to those of non-volcanic regions ((1.85 to 1.0 R-A). CO2/He-3 ratios vary over a wide range (2.4 x 10(5)-3.8 x 10(13)) but are mostly higher than that of the nominal upper mantle (similar to 2 x 10(9)). Oxygen-hydrogen isotope values of the waters are conformable with the Global Meteoric Water Line and indicate a local meteoric origin. Sulfate in waters is most probably derived from dissolution of marine carbonates and terrestrial evaporite units. Temperatures calculated by SO4-H2O isotope geothermometry lie between 40 and 199 degrees C, and are in poor agreement with reservoir temperatures estimated from silica geothermometers. Discordant temperatures may be due to either the relatively slow rate of isotopic equilibrium between water and sulfate or mixing of geothermal water with sulfate-bearing shallow waters which may modify the delta O-18 value. The delta C-13 (CO2) values of gas samples are consistently lower than those of their water counterparts, consistent with loss of CO2 from waters by degassing. Mixing between mantle and various crustal C-sources appears to be the main control on the C-isotope composition. The principal origin of CO2 in all samples is crustal lithologies, mainly limestone (similar to 85 to 98% of the total carbon inventory): thus, the crustal carbon flux is at least 10 times that from the mantle. There is a broad correlation between high He-3/He-4 values and thinner crust in the western part of the Lake Van region, where several historically-active volcanoes are located. This observation indicates that localized volcanic and magmatic activity exerts the primary control on the balance between mantle and crustally-derived volatiles in the region. (C) 2012 Elsevier B.V. All rights reserved.
Volcano Stratigraphy and Petrogenesis of the Nemrut Stratovolcano (East Anatolian High Plateau): the Most Recent Post-Collisional Volcanism in Turkey - Reply
(Elsevier, 2007) Ozdemir, Yavuz; Karaoglu, Ozguer; Tolluoglu, Arif Umit; Gulec, Nilgun