Timescales of Crystallization, Ore Formation, and

岩浆房的热演化是控制岩浆地球化学演化、岩浆喷发潜力、铂族元素和热液成矿的主要因素。传统上，大型岩浆房被认为冷却和结晶非常缓慢，允许发生重大的物理和化学重组。 来自世界各地的锆石U-Pb和黑云母、斜长石Ar-Ar年代学新数据？南非最大的暴露岩浆房布什维尔德杂岩最近对这一长期存在的假设提出了挑战，并提出布什维尔德从熔融状态（约1200-1300°C）非常迅速地冷却到环境地壳温度（150- 300° C）。 这里提出的工作将具体测试从液态到岩浆完全凝固点（800-900°C之间）的几种可能的冷却路径，以及从固态到环境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现代岩浆补给事件和火山爆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了解有价值和战略性金属矿床的形成时间和方式（南非布什维尔德综合体占世界的70%以上）。已探明的铂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还将提供20亿年前古地磁反转速率的估计（其中布什维尔德复合体有7次）。 磁场逆转的速率与内核凝固和地球发电机形成的动力学有关，这在过去的5亿年里还没有得到很好的理解。 拟议的工作将使用六个独立的地质温度计和地质年代计，并具有一系列闭合温度（Tc）。 PI和学生将使用斜长石-辉石和/或辉石-辉石REE温度计（研究1）量化每个地层水平的液相线温度。 高温冷却速率将使用两个辉石温度计中斜方辉石中的Ca和Ca-Mg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正在进行的U-Pb锆石热年代学和锆石中钛温度计（研究3）进行量化。 中低温冷却历史将通过橄榄石（研究4）和Fe-Ti氧化物（研究5）中的Ca扩散进行量化。 低温冷却年龄将通过斜长石、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，由扩散和固溶线测温法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来确定。 上面没有提到的其他影响包括：量化的演化过程中的氧逸度分离结晶;和，调查的利率和热液成矿过程（特别是锌，氟，锡）在中低温热液循环和接触变质。

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.