Abstract
As accurate measurement of geological time is the key to understanding processes that occurred in the Earth's history, development and testing of time scales is a first order problem in the geological sciences. One of the most versatile dating methods for the younger part of the Earth's history is undoubtedly
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40Ar/39Ar dating. This study was initiated to build an extensive set of high quality 40Ar/39Ar data for volcanic ash layers intercalated in Mediterranean Neogene sedimentary successions for which an excellent time control exists based on the astronomical dating technique. This database allowed a rigorous and direct intercalibration of radio-isotopic and astronomical time with the aim to provide an independent test of the accuracy of conventional K/Ar ages of mineral dating standards and to investigate the potential of providing an astronomically dated 40Ar/39Ar standard.
Chapter 1 describes the 40Ar/39Ar procedures in detail. Error propagation methods are improved, system performance is monitored by a new and more accurate method to determine mass discrimination and a new intercalibration data set of TCR and FCT sanidine mineral dating standards is presented. Chapter 2 describes the theoretical background of the astronomical dating technique and the potential uncertainties with regard to astronomical ages of volcanic ash layers. Ash layers are divided in three categories based on the reliability of their astronomical ages. New 40Ar/39Ar data for eastern Mediterranean Neogene ash layers are presented in chapter 3. The Pliocene Ptolemais ashes could be divided into upper and lower ashes, where the 40Ar/39Ar ages of the upper ashes were ~2% younger and the older ashes ~1% younger than their astronomical counterparts. The Cretan A1 showed a discrepancy of ~1% comparable with the older Ptolemais ashes. Intercalibration with FCT sanidine resulted in astronomically calibrated FCT ages of respectively 28.21 ± 0.04 Ma based on A1, 28.28 ± 0.21 Ma for the lower Ptolemais ashes and 28.61 ± 0.20 Ma based on the upper Ptolemais ashes. Following these partially inconsistent results, this research was extended to the western Mediterranean Sorbas, Nijar and Melilla Basins. Chapter 4 describes the development of an astronomical time frame for sediments and intercalated ash layers in the Melilla Basin. Chapter 5 presents the 40Ar/39Ar data of these ash layers. The proximity of the source volcano in Melilla resulted in large crystal sizes allowing single crystal dating. Intercalibration of in total 183 experiments of 16 Melilla ash layers resulted in an astronomically calibrated age of 28.24 ± 0.01 Ma for FCT (1 s.e.m.) in agreement with the lower Ptolemais and A1 ash layers. Biotite experiments on Sorbas/Nijar ashes showed consistently older ages for biotite compared to sanidine. Chapter 6 describes the results for Middle Miocene ash layers. Instead of being ~1% younger, the isotopic ages appeared to be equal (feldspar fractions) or older (biotite fractions) than the astronomical ages. Due to the analyses of multiple grains in smaller grain-size fractions xenocrystic contamination might have gone unnoticed. In chapter 7 a first effort is made to intercalibrate the U/Pb system with the 40Ar/39Ar and astronomical methods. Zircons of Ptolemais ash layer SR3M were dated using TIMS and SHRIMP techniques, which required either a lot of material (TIMS) or was on the edge of practical limitations (SHRIMP). Unfortunately, the zircon of this ash layer seemed to be affected by an inherited component and no strong recommendations concerning intercalibration of the several systems could be made. Finally, in chapter 8 the implications of this research are summarized. The most probable astronomically calibrated FCT sanidine age is 28.25 ± 0.01Ma. Further, a few of the Melilla ash layers are proposed as -direct astronomically dated- standards for use in the 40Ar/39Ar dating method
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