Petrologic tools to understand ore-formation in felsic igneous systems

一些对社会至关重要的关键金属的矿床与地球沉积地壳熔融产生的长英质岩浆结晶的末期有关。这些长英质岩浆系统的一个独特属性是其极端的化学分馏水平，它已被假定，在这种行为的岩浆气相起着重要的作用。确定这些矿床如何形成以及在哪里找到新矿床的关键在于了解晶体-液体分离，岩浆氧化还原状态和岩浆气相（MVP）的发展如何影响元素行为。黑云母和磷灰石是这类岩浆中常见的矿物，它们含有多种元素，可以产生这样的信息。然而，有意义的解释这些矿物中的元素丰度需要在已知的压力，温度和氧逸度条件下的晶体/熔体分区的校准。关键分馏元素，包括地球化学双胞胎铌和钽的相对熔融-蒸汽分配的信息是稀缺的，但必不可少的评估熔融和蒸汽在成矿中的作用。拟议的发现赠款项目将产生关于黑云母熔体和黑云母磷灰石对大量元素的分配的新数据，包括关键金属和卤素，这些数据可用于开发结晶的定量模型，评估MVP形成的时间，并为岩浆氧气压测量的新方法提供基础。进一步的研究将评估MVP分馏铌钽的作用，因此更好地了解这些独特的岩浆系统所表现出的极端水平的化学分馏的起源。由此，可以确定相对“正常”的地壳关键元素丰度可以集中到经济水平的方式，并解开导致地壳内化学分馏的过程。

Mineral deposits of some of the critical metals essential to society are associated with the end stages of crystallization of the felsic magmas produced by melting of Earth's sedimentary crust. A unique attribute of these felsic magmatic systems is their extreme level of chemical fractionation, and it has been postulated that a magmatic vapour phase plays a significant role in this behaviour. Key to determining how these deposits form, and where to find new ones, lies in understanding how crystal-liquid separation, magma redox state and the development of a magmatic vapour phase (MVP) affect element behaviour. Biotite and apatite are common minerals in such magmas, and incorporate a variety of elements that can yield such information. However, meaningful interpretation of element abundances in these minerals requires calibration of crystal/melt partitioning at known conditions of pressure, temperature and oxygen fugacity. Information on the relative melt-vapour partitioning of key fractionated elements, including the geochemical twins Nb and Ta, is scarce, but essential to evaluating the roles of melt and vapour in ore formation. The proposed Discovery Grant projects will generate new data on biotite-melt and biotite-apatite partitioning of a large number of elements, including the critical metals and halogens, which can be used to develop quantitative models of crystallization, assess the timing of MVP formation and provide the basis for new methods of magma oxygen barometry. Additional research will evaluate the role of the MVP on fractionating Nb from Ta, and therefore better understand the origin of the extreme level of chemical fractionation exhibited by these unique magmatic systems. From this, it can be established the ways by which relatively "normal" crustal abundances of critical elements can be concentrated to economic levels and unravel the processes that result in chemical fractionation within Earth's crust.