Timescales of Crystallization, Ore Formation, and

岩浆房的热演化是控制岩浆地球化学演化、喷发潜力和有价值的PGE和热液矿体发育的主要因素。传统上，大型岩浆房被认为冷却和结晶非常缓慢，从而允许发生重大的物理和化学重组。锆石中U-Pb和黑云母、斜长石中Ar-Ar的年代学新数据最近，世界上最大的暴露岩浆库——南非的Bushveld复合体——挑战了这一长期存在的假设，并提出Bushveld从熔融状态（大约1200-1300°C）迅速冷却到周围地壳温度（150- 300°C）。这里提出的工作将专门测试几种可能的冷却路径，从液态到岩浆完全凝固的点（800-900°C），以及从固态到周围地壳地热的冷却路径。研究结果将对两个与社会相关的问题产生重大影响。首先，大型岩浆库中凝固时间尺度的量化将为岩浆补给事件和火山爆发期间的现代火山灾害监测提供信息。其次，量化岩浆固化和演化的速度将帮助我们了解何时以及如何形成有价值和战略意义的金属矿床(南非的Bushveld Complex包含超过70%的世界？中国已探明的铂金储量和许多其他重要的战略金属)。此外，中低温冷却速率的结果也将提供对20亿年前古地磁逆转速率的估计（在布什维尔德杂岩中有7次）。地磁倒转的速率与地核的凝固动力学和地球发电机的形成有关，这在过去的5亿年里还没有得到很好的理解。拟议的工作将采用六个独立的地温计和地质时计的组合，具有一系列的闭合温度（Tc）。PI和学生将使用斜长石-辉石和/或辉石岩-辉石ree温度计量化地层各层的液体温度（研究1）。利用正辉石中的Ca和两个辉石温度计中的Ca- mg交换来确定高温冷却速率（研究2）。固体温度和固体的绝对年龄将通过已经进行的U-Pb锆石热年代学和ti -in-锆石温度计来量化（研究3）。中低温冷却历史将通过Ca在橄榄石（研究4）和Fe-Ti氧化物（研究5）中的扩散来量化。低温冷却年龄将由斜长石、黑云母和角闪石矿物对的Ar-Ar热年代学确定（研究6）。这种方法的优点是，通过扩散和溶剂测温确定的冷却速率将在高温和低温下由U-Pb和Ar-Ar热年代学的绝对年龄所涵盖。上述未提及的其他含义包括：在分馏结晶过程中氧逸度演变的量化；研究了中低温热液循环和接触变质作用下热液成矿（特别是锌、F、Sn）的速率和过程。

The thermal evolution of a magma chamber is the primary control on the geochemical evolution of a magma, its eruption potential, and the development of valuable PGE and hydrothermal ore bodies. Traditionally, large magma chambers are thought to cool and crystallize very slowly, allowing for significant physical and chemical reorganization to occur. New geochronologic age data from U-Pb in the mineral zircon and Ar-Ar in the minerals biotite and plagioclase from the world?s largest exposed magma chamber, the Bushveld Complex of South Africa, have recently challenged this long-standing assumption, and suggested that the Bushveld cooled very rapidly from its molten state (approximately 1200-1300°C) down to the ambient crustal temperature (between 150- 300°C). The work proposed here will specifically test several possible cooling paths from the liquid state to the point at which the magma is completely solidified (between 800-900°C) as well as cooling paths from the solid state down to the ambient crustal geotherm. The results will have major implications for two societally relevant issues. First, quantification of the timescale of solidification in large magma chambers will inform modern day volcanic hazard monitoring during magma recharge events and volcanic eruptions. Second, quantifying how fast magmas solidify and evolve will inform our understanding of when and how valuable and strategic metal deposits form (the Bushveld Complex of South Africa contains over 70% of the world?s proven Platinum reserves and numerous other important and strategic metals). In addition, the mid-low temperature cooling rate results will also provide an estimate of the rate of paleo-magnetic reversals (of which there are 7 in the Bushveld Complex) over 2 billion years ago. The rate of magnetic reversals is related to the dynamics of inner core solidification and formation of the geodynamo, which is not well understood beyond the last 500 million years. The proposed work will employ a combination of six separate geothermometers and geochronometers with a range of closure temperatures (Tc). The PI and students will quantify the liquidus temperature at each level of stratigraphy using the plagioclase-pyroxene and/or pyroxene-pyroxene REE thermometer (Study 1). The high-temperature cooling rate will be determined using the Ca in orthopyroxene and Ca-Mg exchange in two pyroxenes thermometers (Study 2). The solidus temperature and absolute age of the solidus will be quantified by U-Pb zircon thermochronology already in progress, and the Ti-in-zircon thermometer (Study 3). The mid-low temperature cooling history will be quantified by Ca diffusion in olivine (Study 4) and Fe-Ti oxides (Study 5). The low-temperature cooling ages will be determined by Ar-Ar thermochronology in plagioclase, biotite, and hornblende mineral pairs (Study 6). The advantage of this approach is that the cooling rates determined by diffusion and solvus thermometry will be bracketed at high and low temperature by absolute ages from U-Pb and Ar-Ar thermochronology. Additional implications not mentioned above include: quantification of the evolution of oxygen fugacity during fractional crystallization; and, investigation of the rates and processes of hydrothermal ore formation (specifically Zn, F, Sn) during mid-low temperature hydrothermal circulation and contact metamorphism.