【作者】 杨静；

【导师】 尹功明； 郑德文；

【作者基本信息】 中国地震局地质研究所 ， 构造地质学， 2013， 博士

【摘要】 对于中国西北部地区干旱化的时间及原因，国内外的科学家进行了大量的研究，但是现在还存在激烈的争论。黄钾铁矾等含钾硫酸盐矿物通常形成于干旱-半干旱气候条件下，并保存于极端干旱气候条件下，因此对这类矿物进行⁴⁰Ar/³⁹Ar定年，不仅可以得到大陆化学风化的时间序列，还可以反演区域的古气候特征。本文通过对吐哈盆地两个矿床氧化带：红山铜金矿床和彩华沟铜—硫多金属矿床中表生含钾硫酸盐矿物⁴⁰Ar/³⁹Ar定年的研究，探索和验证了风化矿物黄钾铁矾、斜钾铁矾等用于⁴⁰Ar/³⁹Ar定年研究的适用性和可行性，建立了样品的前处理流程和实验测试流程。结合这类矿物的形成条件和环境，分析了吐哈盆地的古气候和古环境，为建立全球气候变化与中国西北部吐哈盆地干旱环境演化的联系提供了年代学证据。主要取得以下认识：建立了样品的前处理流程和实验测试流程：首先在双目镜下对样品进行观察，以了解其矿物组成和共生关系、以及是否有原生矿物（如伊利石、斜长石等）的混染。将样品破碎到0.2-2mm，在双目镜下挑选2-3g的矿物颗粒。在进行⁴⁰Ar/³⁹Ar同位素分析之前先进行X衍射分析、扫描电镜分析，以检查样品的纯净度、组成、粒度大小和结晶度。样品经过X衍射分析、扫描电镜分析确定所挑选的样品纯净后，从中挑选大约20-30mg的矿物，将其与中子通量监测标准样品一起进行快中子进行照射。经过一段时间的放置在用于⁴⁰Ar/³⁹Ar分析。在⁴⁰Ar/³⁹Ar分析上增加了-130℃的冷阱来消除水汽和氧化硫的影响。根据扩散理论，应用阶段加热的方法对一种新的含钾矿物斜钾铁矾进行Ar的扩散特性研究，在此基础上，应用简单的内生长—扩散模型模拟了可能的高地表温度和矿物颗粒大小对样品年龄的影响。实验结果表明斜钾铁矾中Ar的扩散频率因子logD₀/a²=13.71/s，活化能E_a=71.30kcal/mol，封闭温度Tc为294℃（假设冷却速率为10℃/Myr），活化能和封闭温度较高。而且，模拟结果表明斜钾铁矾形成后，在外界环境下温度和矿物颗粒大小对年龄也几乎没有影响。在误差范围内重复性测试结果基本一致，约3.3Ma，进一步证明斜钾铁矾适用于⁴⁰Ar/³⁹Ar定年。对一个黄钾铁矾样品的三个子样品进行了⁴⁰Ar/³⁹Ar定年：每个子样品的坪年龄与反等时线年龄、概率分布年龄基本一致及这三个子样品年龄的重现性再次证实了⁴⁰Ar/³⁹Ar定年技术可以对表生含钾硫酸盐矿物进行年龄测定，并且其年龄是其真实的沉淀时的年龄。通过测得的红山铜金矿床的风化矿物的年龄，可以看出硫酸盐矿物从上到下生成年代逐渐变新。根据风化剖面的高程与对应位置风化矿物的年龄的关系就可以计算出风化前锋的拓展速率。这样计算得到氧化带中中新世到早上新世期间风化前锋的平均拓展速率约为2.3m/Ma，这也代表了该地区的最大剥蚀速率，如此低的剥蚀速率有利于氧化带的保存。通过对中国新疆吐哈盆地的两个硫化物矿床氧化带（红山铜金矿床和彩华沟铜—硫多金属矿床氧化带）中含钾硫酸盐矿物进行⁴⁰Ar/³⁹Ar定年，得到了高精度的年龄，范围在30.3Ma到3.3Ma。结合其他学者测得的K-Ar年龄，我们认为在中国西北部地区，在渐新世以来经历了较长时间的风化作用和表生氧化作用，但是各个时期的强度是不同的。对得到的矿物年龄做的一个概率分布图，并通过与全球深海氧同位素的对比表明吐哈盆地干旱化的时间与全球气候的两次降温相一致。吐哈盆地可能在渐新世早期就已经出现干旱半干旱气候；在中新世，10.5-7.3Ma年龄出现的概率达到最大，表明这期间一直盛行干旱-半干旱气候条件，之后气候开始逐渐向极端干旱气候开始转变，是全球气候变冷和区域隆升共同作用的结果。4-3.3Ma斜钾铁矾的出现代表了4-3.3Ma之后的一次干旱化增强事件，可能反映了由北极冰盖的快速增长导致的全球变冷引起的干旱化在这一时段的增强。综合来说，我们认为新生代吐哈盆地的干旱化主要受全球气候的影响，青藏高原的隆升和天山山脉的隆升起了较小的作用。需要进一步开展的工作：继续进行系统的野外采样，争取采集被开挖出来的完整的氧化带剖面，以确保获得比较完整的年龄数据库。由于采集的黄钾铁矾样品中的K部分会被Na替代，造成含钾量较低，如何能寻找到含钾量高的黄钾铁矾是一个难点。对于原生矿物的混染，进一步尝试用HF对黄钾铁矾进行前处理分离得到纯净的矿物。更多还原

【Abstract】 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.更多还原