Timescales of Crystallization, Ore Formation, and

岩浆房的热演化是控制岩浆的地球化学演化、喷发潜力以及有价值的铂族元素和热液矿体发育的主要因素。传统上，大型岩浆室被认为冷却和结晶非常缓慢，允许发生重大的物理和化学重组。来自世界上最大的裸露岩浆室-南非的布什维尔德杂岩--来自世界上最大的裸露岩浆室--南非布什维尔德杂岩中的矿物锆石中的U-铅和矿物中的Ar-Ar的年代学数据最近挑战了这一长期存在的假设，并提出布什维尔德从熔融状态(约1200-1300°C)迅速冷却到环境地壳温度(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之间)，以及从固态向下到周围地壳地热的冷却路径。这一结果将对两个与社会相关的问题产生重大影响。首先，对大型岩浆室中凝固时间尺度的量化将为现代岩浆补充事件和火山喷发期间的火山灾害监测提供信息。其次，量化岩浆凝固和演化的速度将有助于我们理解贵重和战略性金属矿床的形成时间和方式(南非的布什维尔德杂岩包含世界70%以上的面积？S已探明的铂储量和许多其他重要和战略性金属)。此外，中低温冷却速率结果还将提供20亿年前古地磁逆转(其中在布什维尔德杂岩中有7例)的估计。磁反转的速度与地球发电机的内核凝固和形成的动力学有关，这一点在过去5亿年后还没有得到很好的了解。拟议的工作将使用六个独立的地温计和地质计时器的组合，并具有一定的关闭温度范围(TC)。PI和学生将使用斜长石-辉石和/或辉石-辉石稀土温度计来量化每一层地层的液相线温度(研究1)。高温冷却速度将利用斜方辉石中的钙和两个辉石温度计中的钙-镁交换来确定(研究2)。固相线温度和固相线的绝对年龄将通过已经在进行中的U-Pb锆石热年代学和钛In-锆石温度计(研究3)进行量化。中低温冷却历史将通过钙在橄榄石(研究4)和铁钛氧化物(研究5)中的扩散来量化。斜长石、黑云母和角闪石矿物对的低温冷却年龄将由Ar-Ar热年代学确定(研究6)。这种方法的优点是，由扩散测温法和Solvus测温法确定的冷却速度将由U-Pb和Ar-Ar热年代学的绝对年龄在高温和低温下进行区分。上述未提及的其他影响包括：量化分离结晶过程中氧逸度的演化；以及研究中低温热液循环和接触变质过程中热液成矿(特别是锌、氟、锡)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.