⁴⁰Ar/³⁹Ar Chronology Of Supergene K-bearing Minerals And Its Paleoclmatic Implication

The debate continues over whether global cooling or uplift of the Tibetanplateau can explain the timing and reason for aridification of Northwestern China，although many studies on this issue have been conducted at home and abroad. TheK-bearing sulfate minerals （jarosite, yavapaiite） are usually precipitated in arid tosemiarid conditions and preserved in hyperarid conditions. Therefore, direct andprecise⁴⁰Ar/³⁹Ar dating on these K-bearing sulfate minerals can provide importantconstraints not only on the timing and continental weathering and supergeneenrichments, but also on regional paleoclimatic evolution. This thesis presents theresults of⁴⁰Ar/³⁹Ar dating of supergene K-bearing sulfate minerals sampled from twoweathering profiles: the Hongshan Cu-Au deposit and Caihuagou Cu-sulfidepolymetallic deposit, Tu-Ha basin, Xinjiang. The dating experiment procedures wereestablished to explore and verify the⁴⁰Ar/³⁹Ar dating applicability and feasibility ofthe supergene K-bearing minerals. The⁴⁰Ar/³⁹Ar dating of supergene K-bearingminerals, combined with the conditions of the formation of these minerals, this workanalyzed the coupling relationship between the global cooling and evolution of thearidification in Northwest China. The main conclusions are drawn as follows:The dating experiment procedures were established：The samples are inspectedunder binocular to determine the mineralogy and paragenesis of the sample. Inaddition, optical microscopy was used to determine whether this sample contains theprimary minerals （such as ledikite, felspar, and quartz） contamination. Afterpetrographic examination, suitable samples were crushed;2-3g grains werehandpicked under a binocular microscope. Then the handpicked yavapaiite grainswere characterized by X-ray powder diffraction analysis （XRD） and scanning electronmicroscopy （SEM） to identify the habits of the phases present and to detect anypossible intergrown minerals before⁴⁰Ar/³⁹Ar isotopic analysis. For the⁴⁰Ar/³⁹Argeochronology, about200mg pure grains （grain size1mm-0.5mm） were wrapped inAl foil, sealed in silica glass tube with ZBH biotite standards, and irradiated. Thesamples were analyzed by the incremental heating⁴⁰Ar/³⁹Ar method. The necessaryuse of a cryocooling device operated at-130℃made it possible to separate the othergases, especially H2O, SO3.According to diffusion theory,⁴⁰Ar/³⁹Ar incremental heating experiments for s new sulfate mineral to determine the apparent Ar diffusivity. Then a simple model ofdiffusive loss and radiogenic in-growth was constructed to evaluate the effects ofextreme high ambient temperature and the grain size on the Ar age. The experimentalresults show that the diffusion parameters as follows: the activation energy Eais76.17kcal/mol and the frequency factor logD0/a2is15.07/s, the corresponding closuretemperature Tcis294℃（assuming a cooling rate of10℃/Ma）, and the activationenergy and closure temperature are very high. The simulation results show that thehigh ambient temperature and the grain size have no effect on the Ar age afteryavapaiite was precipitated. Furthermore, the reproducibility of the age attests to thesuitability of supergene yavapaiite for K-Ar and⁴⁰Ar/³⁹Ar dating.Three aliquots of a single-hand jarosite specimen were analyzed by the⁴⁰Ar/³⁹Armethod. For each sample, the isochron age and ideogram are generallyindistinguishable from the plateau age. The results indicate that the three aliquots of asingle-hand jarosite obtained are reproducible, attesting to the suitability of supergenejarosite to⁴⁰Ar/³⁹Ar geochronology, and the results are the true mineral precipitation.According to the ages obtained from the Hongshan Cu-Au deposit, older supergeneminerals occur at the top, whereas more recently precipitated supergene minerals arepresent at the bottom of the profile. The downward advance of the weathering frontdocuments very low maximum denudation rates of2.3m/Ma from the mid-Miocene toearly Pliocene. The low apparent denudation rates reflect the dominantly depositionalenvironment of the Tu-Ha basin.⁴⁰Ar/³⁹Ar dating of supergene K-bearing sulfate minerals sampled from twoweathering profiles: the Hongshan Cu-Au deposit and Caihuagou Cu-sulfidepolymetallic deposit, Tu-Ha basin, Xinjiang, yields ages from30.3Ma to3.2Ma.Combined with results from other scientist, these data imply a prolonged history ofweathering and supergene oxidation, but the weathering intensities of each period aredifferent. Comparison of the probability distribution of supergene ages obtained frompreviously published results and this study with the global deep-sea oxygen isotoperecords indicates that the aridification of the Tu-Ha basin coincides with the trends inglobal cooling. The arid-semiarid climate might have emerged in early Oligocene inthe Tu-Ha basin; In Miocene, the probability reached a maximum at10.5-7.3Ma，suggesting a prevalence of regionally arid-semiarid conditions during this period.After this period the rapid decrease in probability of ages might reflect the initiation of the transition to hyperarid, which may be the result from the combined effects ofboth global cooling and regional uplift. The age4-3.3Ma from yavapaiite mineralsrepresents an enhanced aridity after4-3.3Ma, which likely resulted from the increaseof global cooling induced by the Northern Hemisphere glaciation induced. So, thearidification of the Tu-Ha basin was forced by global cooling during the Cenozoic，and the uplift of the Tibet plateau and Tian Shan might have played a minor role incontrolling the drought climate in the Tu-Ha basin.Some pieces of work are required to be further advanced: To collect moresamples from deposits of which the whole weathering profile is excavated to obtain anintegrated age database. Since the K in jarosite samples will be substituted by Na,resulting in a low potassium content, so a difficult is how to find the jarosite sampleswhich contain high potassium content. For the mixture of primary minerals withjarosite minerals，further attempts should be made to use hydrofluoric acid （HF） toremove primary contaminants without incongruently dissolving jarosite.