Timescales of Crystallization, Ore Formation, and

结晶、矿石形成的时间尺度和

• 批准号: 2038105
• 负责人: Jill VanTongeren
• 金额: $ 15.36万
• 依托单位: Tufts University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-01-01 至 2023-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2038105&HistoricalAwards=false
• 关键词: Timescales; Crystallization; Ore; Formation

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.

岩浆室的热演化是对岩浆地球化学演化、其喷发潜力以及有价值的铂族元素和热液矿体发育的主要控制。传统上，大型岩浆室被认为冷却和结晶非常缓慢，从而允许来自世界上最大的暴露岩浆的矿物锆石中的 U-Pb 和矿物黑云母和斜长石中的 Ar-Ar 的新地质年龄数据将发生。南非布什维尔德综合体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 之间） C) 这里提出的工作将专门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。以及从固态到周围地壳地温的冷却路径，这些结果将对两个与社会相关的问题产生重大影响：首先，大型岩浆室凝固时间尺度的量化将为现代岩浆补给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的战略性金属矿床形成的时间和方式（南非布什维尔德杂岩体包含超过 70% 的岩浆矿床）。此外，中低温冷却速率结果还将提供古地磁反转速率的估计（其中布什维尔德杂岩中有 7 个）。超过 20 亿年前，磁反转速率与内核凝固和地球发电机形成的动力学有关，而在过去 5 亿年之后，人们对这一点的了解还不够深入。拟议的工作将采用六种方法的组合。具有一系列闭合温度 (Tc) 的独立地温计和地质年代计 PI 和学生将使用斜长石-辉石和/或辉石-辉石 REE 温度计量化每个地层水平的液相线温度（研究 1）。冷却速率将使用斜方辉石中的 Ca 和两个辉石温度计中的 Ca-Mg 交换来确定（研究2). 固相线温度和固相线绝对年龄将通过已经进行的U-Pb锆石热年代学来量化，并且中低温冷却历史将通过Ca来量化。橄榄石（研究 4）和 Fe-Ti 氧化物（研究 5）中的扩散 低温冷却年龄将通过 Ar-Ar 热年代学来确定。斜长石、黑云母和角闪石矿物对（研究 6）是通过扩散和固溶线测温法确定的冷却速率将根据 U-Pb 和 Ar-Ar 热年代学的绝对年龄划分在高温和低温下。上面未提及的其他影响包括：分步结晶过程中氧逸度演变的量化；以及热液矿石形成的速率和过程的研究（特别是 Zn、F、 Sn)在中低温热液循环和接触变质过程中。