Akmaz, Recep Melih; Uysal, Ibrahim; Su, Ben-Xun; Sen, Ahmet D.; Hu, Wen-Jun; Seitz, Hans-Michael; Mueller, Dirk; Eslami, Alireza

This study presents a complete alteration sequence in chromitites from the Kizildag ophiolite (southern T & uuml;rkiye), documenting three distinct chromite types that record processes related to Neotethyan subduction. Chromitites exhibit predominantly lenticular forms and occasionally banded textures, embedded within mantle peridotites and the mantle-crust transition zone, specifically harzburgite and dunite. Our analysis categorizes chromite grains into three groups: fresh, altered, and porous. Fresh chromites are characterized by high contents of Cr2O3 ranging from 42.69 to 57.08 wt% and Al2O3 from 12.46 to 25.34 wt%. In contrast, altered chromites, typically located along fractures or at the edges of fresh grains, show significant variability in chemical composition, with Cr2O3 content between 43.34 and 63.94 wt% and Al2O3 between 2.91 and 14.84 wt%. Porous chromites, distinguished by their porous structure, present Cr2O3 contents from 44.58 to 52.66 wt% and Al2O3 from 3.00 to 8.38 wt%. Notably, altered and porous chromites demonstrate substantial enrichments in Fe and trace elements such as Mn, Ni, Zn, Co, and Ga, indicating a shift due to metamorphic processes. The observed high Cr# (0.68-0.93) and low Mg# (0.25-0.65) values in rims of the altered and porous chromite further illustrate the metamorphic influence on chromite, with greenschist-facies conditions indicated by the Cr-Al-Fe3+ diagram at temperatures below 450 degrees C. The alteration dynamics reflect a progression that initiates with the olivine matrix and advances through reactions involving fresh chromites. This progression results in mass loss, the formation of chlorite, and the porous structures. Our findings contribute to the understanding of metamorphic alteration processes in chromitites and provide a detailed account of the geochemical trends (such as Fe/Mg exchange and trace element enrichment) that reflect greenschist-facies metamorphism. The evidence of chlorite formation and the prevalence of porous textures highlight the influence of metamorphic fluids over serpentinization, offering a model for chromite alteration in supra-subduction zones. This research elucidates the metamorphic history of chromitites in the Kizildag ophiolite.

Uysal, Ibrahim; Akmaz, Recep Melih; Su, Ben-Xun; Demir, Yilmaz; Sen, Ahmet Dundar

The chromitites of the K & imath;z & imath;lda & gbreve; 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 (Sigma PGE(intermediate) = 718 ppb, Sigma PGE(High-Cr) = 866 ppb) compared to other samples. The dominant platinum group mineral inclusions in chromite are Ru-rich laurite [(Ru, Os)S-2], with a single osmiridium (Os-Ir alloy) grain also observed suggesting formation under high-temperature and low & fnof;S-2 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.