Geochemical and Petrologic Evolution of Chromitites and Other Platinum Group Element-Rich Rocks From Study of the UG2 Layer, Bushvel Complex

在晶体堆积之后，布什维尔德杂岩和其他大型层状侵入体经历了漫长而复杂的演化，而岩石仍处于部分熔融状态。 在此期间，岩石的结构、成分和某些情况下的岩性都发生了变化。 在布什维尔德复杂的铂族元素（PGEs）的基本地球化学的历史承担，因为富含铂族元素的层，即Merensky礁和UG2铬铁矿，表现出广泛的积累后修改的证据。 本研究的目的是了解部分熔融岩石的动力学和化学演化以及在此演化过程中PGE是如何重新分配的。 将在仔细绘制的地下剖面中绘制铂族元素和其他元素的地球化学剖面图。 将收集有关晶体大小、形状和模式的数据，以了解结构和地球化学变化如何相关，以及它们何时以及如何形成。 将分析间隙硅酸盐矿物中的稀土元素，以推断间隙熔体在结构和地球化学改性中的作用。 铬铁矿中含有含锇矿物包裹体。 将测量它们的Os同位素组成，并与结构分析相关联，以确定包裹体形成的时间和方式。 矿区的测绘显示，UG2与几个复杂的岩层有关。 这些将被研究，以确定它们的起源，并深入了解UG2的就位及其可能的作用，作为一个渗透性屏障的流动间隙熔体和流体。梅伦斯基礁被认为是标志着新的岩浆引入到一个现存的岩浆房。 与这一层相关的Sr同位素比值的复杂地层剖面与简单的岩浆混合不一致，可能是在岩石部分熔融和压实时发展起来的。 铅同位素研究的特点是分析个别含铅矿物将进行深入了解的地球化学发展的梅伦斯基礁。

MathezEAR-0106750The Bushveld Complex and other large layered intrusions have experienced a long and complex evolution following crystal accumulation and while the rocks were still partially molten. During this time the textures, compositions, and in some cases lithologies of the rocks were modified. In the Bushveld Complex this history bears on the fundamental geochemistry of the platinum group elements (PGEs) because the PGE-rich layers, namely the Merensky reef and the UG2 chromitite, exhibit evidence for extensive post-accumulation modification. The objectives of this proposal are to understand the dynamic and chemical evolution of the partially molten rocks and how the PGEs were redistributed during this evolution.The meter-thick UG2 chromitite possesses internal stratigraphic variations in texture and PGE contents. Geochemical profiles for PGEs and other elements will be produced in carefully mapped underground sections. Data on crystal size, shape, and mode will be collected to understand how the textural and geochemical variations are related and when and how they formed. The interstitial silicate minerals will be analyzed for rare earth elements to deduce the role of interstitial melt in textural and geochemical modification. Chromite grains contain inclusions of Os-bearing minerals. Their Os isotopic compositions will be measured and associated with the textural analysis to establish when and how the inclusions formed. Mapping in the mine revealed that the UG2 is associated with several complex rock layers. These will be studied to determine their origins and gain insight into the emplacement of the UG2 and its possible role as a permeability barrier to the flow of interstitial melt and fluid.The Merensky reef is believed to mark the introduction of new magma into an extant magma chamber. The complex stratigraphic profiles of Sr isotopic ratios associated with this layer are inconsistent with simple magma mixing and may have developed later, when the rocks were partially molten and compacting. A Pb isotopic study featuring analysis of individual Pb-bearing minerals will be conducted to gain insight into the geochemical development of the Merensky reef.