https://hdl.handle.net/11436/921 2026-07-09T10:12:22Z https://hdl.handle.net/11436/10932 Chromite as a key player on highly siderophile elements and osmium isotope compositions of the refractory mantle Su, Ben-Xun; Uysal, İbrahim; Akmaz, Recep Melih; Pan, Qi-Qi; Demir, Yılmaz; Ackerman, Lukáš; Robinson, Paul T. Chromitites in ophiolites are recognized for their high concentrations of highly siderophile elements (HSE), notably IPGE (Os, Ir and Ru). However, the effects of this enrichment on ophiolitic rocks and mantle heterogeneity remain poorly understood and to address this issue, we investigated chromitites from the Pozantı-Karsantı and Kızıldağ ophiolites in Türkiye. The results reveal HSE enrichment, particularly IPGE, in chromite separates compared to matrix, with indistinguishable Os isotope compositions between the two phases. This observation aligns with the presence of platinum-group minerals (PGM) occurring as inclusions within chromite grains. The IPGE enrichment and similar Os isotope compositions are also present in chromitites relative to dunites, suggesting close affinity of HSE with chromite fractions relative to silicate melts during chromite crystallization and aggregation. A global compilation of data on chromitites in ophiolites demonstrates covariations of HSE and their ratios with chromite Cr# values, indicating that chromite composition modulates its capacity to retain HSE. Peridotites containing chromite with low-Cr# (<45) exhibit consistent HSE concentrations and a decrease in 187Os/188Ost ratios as chromite Cr# values increase, reflecting different partial melting degrees. In contrast, the refractory peridotites (with chromite Cr# > 45) show more scattered HSE variations and distinct trends with chromite Cr#, generally trending towards the chromitite field. This variability likely reflects the compositional effects of chromite on HSE variations, suggesting significant metasomatism and interaction with parental melts of chromitites or evolved melts. Therefore, chromite plays a critical role in the variations of HSE contents and 187Os/188Os within the refractory mantle. 2025-01-01T00:00:00Z https://hdl.handle.net/11436/10706 Subduction initiation of the Neotethyan Ocean recorded in chromite deposits: A case study of the Kızıldağ ophiolite, southern Türkiye Uysal, İbrahim; Akmaz, Recep Melih; Su, Ben-Xun; Demir, Yılmaz; Şen, Ahmet Dündar The chromitites of the Kızıldağ ophiolite (KO) are primarily characterized by massive and disseminated textures within refractory mantle peridotites, predominantly harzburgite and dunite, with occasional occurrences of banded and nodular chromitites. Based on chromite composition, these chromitites can be classified into two groups exhibiting a broad compositional range: intermediate (0.55 ≤ Cr# < 0.70, TiO2 = 0.08–0.51 wt%) and high-Cr (0.70 ≤ Cr# ≤ 0.77, TiO2 = 0.05–0.28 wt%). The compositions of minor and trace elements align consistently with variations in Cr#. Total PGE (Os, Ir, Ru, Rh, Pt, Pd) contents in these chromitites are generally less than 500 ppb, with many samples showing enrichment of IPGE (Os, Ir, Ru) relative to PPGE (Rh, Pt, Pd). However, certain samples from both groups display elevated PGE concentrations (ƩPGEintermediate = 718 ppb, ƩPGEHigh-Cr = 866 ppb) compared to other samples. The dominant platinum group mineral inclusions in chromite are Ru-rich laurite [(Ru, Os)S2], with a single osmiridium (Os-Ir alloy) grain also observed suggesting formation under high-temperature and low ƒS2 conditions. Primary base metal mineral inclusions include millerite and pentlandite. Additionally, some chromite crystals contain single or multi-phase silicate inclusions, such as clinopyroxene, orthopyroxene, olivine, and amphibole. Parental melt compositions, calculated from Al2O3 and TiO2 contents of chromites, indicate that most high-Cr chromitites originated from boninitic melts, whereas intermediate chromitites and some high-Cr samples derived from melts with intermediate compositions. Trace element compositions of amphibole and clinopyroxene inclusions in the chromite crystals from the intermediate group further support crystallization from hydrous intermediate melts. It is proposed that the intermediate chromitite group formed from partial melts influenced by fluids from the subducted oceanic crust during the early stages of subduction initiation, while high-Cr chromitites originated from hydrous melts derived from a more depleted mantle wedge in later stages. 2025-01-01T00:00:00Z https://hdl.handle.net/11436/10035 Diatom Communities in the Littoral Sediments of the Robert Island’s Lakes, South Shetland Islands, Antarctica Akar, Bülent; Güney, Ayşegül; Özkan, Korhan; Avcı, Utku; Kandemir, Raif The study area is situated at Robert Island, which is located in the South Shetland Islands in the northwest of the Antarctic Peninsula. The diatom samples were taken as part of the Turkish Antarctic Expedition (TAE-II) which took place between March and April 2018. The samples were collected from the surface of sediments (c. 5 cm) at a depth of approximately 20 cm, 50–100 cm from the littoral zones of an inland glacial lake (L1) and three coastal lakes (L2, L3 and L4) situated on glacier-free area on Robert Island. The main objective of this study was to reveal the diversity and the abundance of diatom communities in the littoral sediments of the lakes on Robert Island. The lakes were shallow and oligotrophic freshwater lakes, although the coastal lagoons had a high conductivity and nutrient concentration. Twenty-six diatom taxa belonging to Achnanthes (1), Fragilaria (1), Frustulia (1), Gomphonema (1), Halamphora (1), Hantzschia (1), Hippodonta (1), Humidophila (2), Luticola (3), Melosira (1), Nitzschia (4), Pinnularia (3), Planothidium (2), Psammothidium (2), Stauroneis (1) and Stephanopterobia (1) were identified in the samples from these lakes. The coastal lake L2 showed the highest richness of diatom communities among the in studied lakes, with 21 identified taxa. 2024-01-01T00:00:00Z https://hdl.handle.net/11436/10014 K-Pg transition in neritic limestones: Deciphering the first bio/chemo-stratigraphic records and tectono-sedimentary evolution, eastern Neotethys (NE Türkiye) Köroğlu, Fatih; Sarı, Bilal; Kandemir, Raif; Yılmaz, İsmail Ömer; Özer, Sacit Interestingly, the K-Pg transition from the Cretaceous to the Paleogene is the closest to our time and, from today's perspective, the most dramatic extinction that has ever occurred on Earth. Using different methods and lithologies, K-Pg transition records have been identified in different areas around the Earth. In this study, the neritic carbonate successions in the arc sedimentary cover of eastern Neotethys (Eastern Sakarya Zone, NE Türkiye) were studied using high-resolution bio-chemo/stratigraphy and sedimentary geology methods. The Ziyarettepe Formation, which is the subject of this study, is characterized by its neritic and hemipelagic sedimentary features. Notwithstanding, the clastic and irregular distribution of macro- and microfossils in the Ziyarettepe Formation was realized in parallel with sea level changes, regional tectonism, and global event effects. As a result of these events, the shell structure of the fossils shows the processes of transport from shallow habitats, transport to the platform-platform edge-slope edge, and deposition under these conditions. On the other hand, the neritic and hemipelagic limestones of the Ziyarettepe Formation allow the study of benthic and planktonic foraminifera biostratigraphy, as well as other fossil groups in the stratigraphic interval around the K-Pg transition. According to the results, the stratigraphic age of the Ziyarettepe Formation is given as late Campanian-Thanetian, both chemostratigraphically and biostratigraphically (mainly foraminifera). The Ziyarettepe Formation (Tosunlu Tunnel) stratigraphic section was examined for stable δ13C and δ18O isotopes, rare earth elements, and whole rock carbonate geochemistry records within the Ziyarettepe Formation. Geochemical trends across the K-Pg transition are distinguished by anomalies resulting from many impact signals, including mainly the Deccan Traps and the Chicxulub Impact. The geochemical profile of the K-Pg transition was influenced by paleoenvironmental impacts, as in this example of a neritic succession in the eastern Neotethys. Together with multi-chemostratigraphic analyses, it could be shown for the first time that the K-Pg transition was also recorded in this basin. According to the tectono-sedimentary evolution in the region, these neritic successions were deposited in a back-arc environment during the northward subduction of the northern branch of Neotethys. 2025-01-01T00:00:00Z