K-Ar ve ''Ar yaş tayini yöntemlerinin karşılaştırılması: Güney Türkiye ofiyolit tabanı metamorfiklerinden örnekler

Bu çalışmada, Toros Kuşağı ofiyolitlerinin, metamorfik taban kayaçlarına ait amfibolitlerinden ve mikaşistlerinden ayıklanan amfibol ve mika mineralleri için K-Ar ve 40Ar-39Ar yaş tayini yöntemleri uygulanmıştır. K-Ar yaş tayini yöntemi ile amfiboller 82.3 ± 5 My ile 124.2 ± 3 My arasında değişen geniş bir yaş aralığı vermiştir. Aynı yöntemle mika minerallerinden 83.9 ± 2 My ile 93.7 ± 2 My arasında değişen yaşlar elde edilmiştir. Buna karşın, aynı kayaçlardan seçilen amfibollerden 40Ar-39Ar yöntemi kullanılarak, 90.7 ± 0.5 My ile 93.8 ± 1.7 My arasında yaşlar elde edilmiştir. Aynı yöntemle mikalar 91.2 ± 2.3 My ile 93.6 ± 0.8 My arasında yaş sonuçları vermişlerdir. Adı geçen iki yöntemi karşılaştırabilmek, ya da K-Ar yaş tayini yönteminin güvenilirliğini anlayabilmek için aynı kayaçlardan amfibol ya da mika mineralleri analiz edilmiş ve sonuçların genellikle uyumsuz oldukları saptanmıştır. Örneğin, Beyşehir ofiyolitik melanjı içinde yeralan bir amfibolitten elde edilen amfibol minerallerinden K-Ar yöntemi ile 107.8 ± 4 My yaş sonucuna ulaşılmışken, aynı kayaçtaki amfibollerden 40Ar-39Ar yöntemi ile 90.9 ± 1.3 My elde edilmiştir. Her iki yöntemden elde edilen yaş verilerinin birbirleriyle uyuşmamasının başlıca nedenleri; analiz edilen minerallerin heterojenliği ve düşük potasyum içerikleri, ısıl (örn., hidrotermal alterasyon) süreçler ya da dış etkenler (örn., yeraltısuyu) ile sistemden 40Ar kaybı yada ilavesi ve bu etkilerin K-Ar yöntemi ile belirlenememesi olarak sayılabilir. Ayrıca K-Ar yönteminde 40Ar ve 40K içeriklerinin farklı yöntemlerle saptanması, analitik hata olasılığını yükseltmektedir.

Comparison of the K-Ar and ''Ar-''Ar dating methods: examples from the metamorphic sole rocks of the southern Turkish ophiolites

In this study, K-Ar and 40Ar-39Ar dating methods were used for amphibole and mica separated from the amphibolites and micaschists of the metamorphic sole rocks from the Tauride Belt ophiolites. Amphiboles and micas from the metamorphic sole rocks of the Tauride Belt ophiolites yielded K-Ar ages between 82.3 ± 5 Ma and 124.2 ± 3 Ma, and between 83.9 ± 2 Ma to 93.7 ± 2 Ma, respectively. However, 40Ar-39Ar ages from the same rocks are between 90.7 ± 0.5 Ma and 93.8 ± 1.7 Ma for amphiboles, and between 91.2 ± 2.3 Ma to 93.6 ± 0.8 Ma for whitemicas. Both methods were performed on the amphibole and white mica grains from the same sample to compare or to understand the reliability of K-Ar method. The results generally do not show an agreement. For instance, amphibole samples from the amphibolite of the Beyşehir ophiolite yielded 107.8 ± 4 Ma from K-Ar method of dating. The amphiboles from the same amphibolite sample yielded 90.9 ± 1.3 Ma by using 40Ar-39Ar method of dating. The main reasons of the age differences obtained from the both methods can be the heterogenity and the low potassium content of the minerals analysed, thermal processes (e.g. hydrothermal alteration) or the external factors (e.g. groundwater) induced the lost or addition of 40Ar in the system which could not be detected by K-Ar method. In addition, in the K-Ar method, determination of 40Ar and 40K contents with different methods results in an increase in the possibility of the analytical error.

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