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; 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.

岩浆腔的热演化是岩浆地球化学演化，其喷发潜力以及有价值的PGE和水热矿体的发展的主要控制。传统上，大型岩浆腔室被认为会非常冷却和晶体慢，从而可以进行大量的物理和化学重组。来自U-PB的新的地质年龄数据来自矿物锆石和矿物Biotite矿物质和AR-AR的新地质年龄数据，以及来自世界上最大的暴露的岩浆室，南非的灌木丛综合体，最近对这一长期的假设提出了挑战，它挑战了这一长期的假设，并表明，布什维尔德在其摩尔群岛之间非常迅速地降低了摩尔河的摩尔群（大约1200-1300-1300-1300-1300-1300-1300-1300-1300°C）。 150-300°C）。此处提出的工作将专门测试从液态到岩浆完全固化（在800-900°C之间）的几个可能的冷却路径，以及从固态到环境地壳地热的冷却路径。结果将对两个与社会相关的问题产生重大影响。首先，大型岩浆腔室中固化时间尺度的定量将告知现代火山危害在岩浆充电事件和火山喷发期间的监测。其次，量化岩浆巩固和进化的速度将有助于我们对何时以及如何有价值和战略性金属沉积物形成的理解（南非的灌木丛中包含全球70％以上已证明的铂金储量以及许多其他重要和战略金属）。此外，中低温冷却速率结果还将估算到超过20亿年前的古磁反转速率（在灌木丛中有7个）。磁反转的速率与Geodynamo的内核固化和形成的动力学有关，这在过去5亿年之后还没有很好地理解。拟议的工作将采用六个单独的地热计和具有一系列闭合温度（TC）的地球工程学器的组合。 PI和学生将使用斜长石 - 氯氧烯和/或辉石 - 羟基耐氧烯REE温度计在每个层次摄影层的每个水平上量化液体温度（研究1）。高温冷却速率将使用两个辉石温度计中的邻苯二甲烯和CA-MG交换中的CA确定（研究2）。固体温度和绝对年龄将通过正在进行的U-PB锆石热量学和Ti-In-Zircon温度计（研究3）来量化。中低温冷却史将通过橄榄石（研究4）和Fe-Ti氧化物（研究5）中的Ca扩散来量化。低温冷却年龄将由斜长石，生物岩和角闪闪发光的矿物质对中的AR-AR热团学确定（研究6）。这种方法的优点是，由U-PB和AR-AR热合体的绝对年龄在高温和低温下通过扩散和SOLVUS温度测定确定的冷却速率。上面未提及的其他含义包括：分数结晶过程中氧气的演化数量；并且，在中低温水热循环和接触变质的过程中，研究水热矿石形成的速率和过程。