K-Ar Chronology Of Jarosite In Eastern Tianshan And Its Environmental Significance

Turpan and Hami region in estern Xinjiang Province, China (eastern Tianshan) is a hyperarid desert which is far from the sea, and it is the main area of Gobi in China. Eastern Tianshan is under the continental arid climate. The average rainfall of this area is 25mm/y while the highest evaporation capacity is 3000mm/y. As short of water, there are almost no rivers here. Jarosite is a widespread weathering mineral in oxidization zones of sulfide metal deposits in arid areas, which is mainly formed by oxidation and K+ metasomatism of pyrite. Jarosite precipitates in arid to semiarid conditions and ceases in hyperarid conditions, whereas jarosite can only be reserved in such hyperarid conditions. As a result, the formation age of jarosite reflects the climatic change from arid-semiarid to hyperarid. At the same time, jarosite is suitable to K-Ar (⁴⁰Ar/³⁹Ar) geochronology because there is considerable amount of potassium contained in the crystal of jarosite. Above all, K-Ar dating of jarosite may provide reliable geochronogical information about it's precipitation in the oxidation zones of deposits in Eastern Tianshan, combined with the conditions of the formation of jarosite, it has important significance to discuss the coupling relationship between evolution of the monsoon-arid environment and phased growth of the Tibetan Plateau.The compositions and content of the weathering minerals in oxidization zones of sulfide metal deposits in eastern Tianshan have been obtained by XRD analysis. Some relatively enriched jarosite samples are selected after identification under polarizing microscope. Then we date the samples by K-Ar method. Combined with the conditions of the formation of jarosite, we investigate the palaeoenvironment in eastern Tianshan and provide a new geochronological evidence for discussing the coupling relationship between evolution of the monsoon-arid environment and phased growth of the Tibetan Plateau. The main conclusions are drawn as follows:1. Having studied the correlation between K-Ar data and compositions of the samples, we conclude that the main influence of reliability of the dating results is the contaminations of priamary minerals (Microcline, Albite and Quartz) and secondary minerals (Illite). So, it is very important to select the samples not containing these contaminations. Besides, we have sdudied and proved the feasibility of K-Ar dating of weathering minerals (jarosite). We also set up a method of testing the K-Ar dating applicability of samples and the dating experiment procedure.2. Analysis by XRD confirms that there are 29 kinds of minerals in oxidization zones of eastern Tianshan, among of which there are 11 kinds of sulfate minerals, such as Natrojarosite, Carphosiderite, Jarosite, Tamarugite, etc. we have find jarosite in Shaquanzi, Hongshi, Hongshan and Meiling Cu-Au deposits. After identification under polarizing microscope and analysis by XRD, we have separated relatively enriched fractions from the sample 08XJ-20 successfully.3. The K-Ar data obtained from 7 samples indicate that there three age stages: 11Ma, 57⁶7Ma and 114¹94Ma. Considering the contaminations of primary minerals, the age of formation of jarosite in eastern Tianshan can be determined at ¹1Ma.4. About 10⁷Ma ago, the Aeolian"Red Clay"sediments developed widely on the Chinese Loess plateau, and the aridity in the Asian interior was enhanced. At that time, the jarosite ceased precipitation because of the hyperarid climate of eastern Tianshan, and was reserved until present under the hyperarid climate. Coupling with other changes (such as, the strength of Indian summer monsoon indicated by coastal upwelling in the Arabian Sea, changes in vegetation from C3(forests) to C4(grasses) in Indian subcontinent, the emerge of a major aeolian dust peak in North pacific, and the increase of sedimentation rate in South China Sea ) around Tibetan plateau about 8 Ma ago, it indicates the Asian monsoon and inland aridification were both enhanced simultaneously at that time.5. The hyperarid climate in eastern Tianshan region since 11Ma ago may be the result of late uplift and expansion of Tibetan plateau along its eastern margin since late Miocene(10⁷Ma) . Further uplift of Qilianshan blocked the warm-wet air brought by the summer monsoon and intensified the aridification of the eastern Tianshan region. It is the aridification of eastern Tianshan area that provided the source of aeolian"Red Clay"sediments which developed widely on the Chinese Loess plateau. While the strength of Asian winter monsoon about 8Ma ago provided the power of Aeolian dust, which carried the dust from northwest arid region to the Loess plateau.6 The age of formation of jarosite in eastern Tianshan (¹1Ma) indicates the climate change from arid-semiarid to superarid in eastern Tianshan. It is consistent with the strengthening of Asian north-westerly winter monsoon and the records of large scale Aeolian sediments. And it also accords with the time of uplift and lateral expansion of Tibetan plateau along its northern and northeastern margins. The age of jarosite's formation in eastern Tianshan support the climate-model that simulates the atmospheric circulation change caused by the uplift and lateral extension of Tibetan plateau.7. There are still some problems to be resolved. We are going to get more samples from deposits of which the surface weathering crusts are not denuded in eastern Tianshan to obtain an integrated age data. As the common occurrence of supergene jarosite partially intergrown with primary or other supergene minerals, it requires selection in the field. We should pick the samples where relatively pure jarostie precipitating in veins or cavities as much as possible. These samples are often relatively coarsely crystalline and easy to separate from possible contaminants. The purity of the selected samples should be further tested through XRD and SEM to insure the reliability of dating object and precision of testing data. Moreover, ⁴⁰Ar/³⁹Ar geochronology of pure jarosite and laser step-heating ⁴⁰Ar/³⁹Ar analysis can remove the contamination of atmospheric argon and access argon. It also can recognize the hypogene contaminations and overgrowth of multi-generation of jarosites to obtain more meaningful weathering ages. Besides, the step-heating ⁴⁰Ar/³⁹Ar analysis of jarosite can help us realize the retention and diffusion characteristic of 40Ar in jarosite, as well as study its close temperature and 40Ar released characteristic. The application of K-Ar (⁴⁰Ar/³⁹Ar) dating of jarosite would be further developed via these studies.