Constraining the Source of Ancient, Surface-derived Sulfur in the Bushveld Complex

南非的布什维尔德复合体（Bushveld Complex）是世界上最大的岩石体，它是20多亿年前在地球表面下从熔融岩石中结晶出来的。它是地球上最大的铂族元素（PGE）资源的所在地，也是铬（Cr）和钒（V）的重要资源。布什维尔德杂岩中这些元素的富集可能与若干因素有关，包括熔体的原始来源以及熔体上升和侵位进入地壳期间发生的过程。研究布什维尔德不仅对了解铂族元素矿床的形成很重要，而且对理解导致如此巨大和不寻常的岩石体的地球过程也很重要。布什维尔德熔体富含镁，这表明它最初来自地幔，但令人困惑的地壳同位素特征，包括Sr，Nd，Pb，O和S的异质同位素组成，表明它经历了复杂的上升历史，与周围环境的相互作用以及可能的地壳合并。最近发现的一个厚的继承性的高镁岩石（基底超镁铁岩序列）下面被普遍接受的最低层的布什维尔德提供了一个独特的机会，调查岩石来自最原始的熔体与布什维尔德和测试的假设，这些地壳的签名来源。一种使用元素硫（S）的同位素组成的新方法将用于研究深层岩浆如何与较浅的地壳相互作用以产生这些岩石，并限制导致地球上最大的PGE存款形成的地球过程。基底超镁铁质序列提供了研究岩石的机会，这些岩石在侵位和结晶过程中经历了最小的蚀变（Wilson 2012; 2015）。现有的化学和矿物学数据以及计划的Sr和Nd同位素分析将与相同样品的Del 33 S测量结果进行比较。需要解决的问题包括：（1）在这些最低的布什维尔德火成岩中是否存在地表来源的S信号，（2）这些最低的单元的Del 33 S值是否一致，以及（3）S同位素组成如何与地壳的其他指标（如Sr和Nd同位素）相关。对地壳同位素信号来源的几种假设进行了检验，包括：（1）来自亚大陆岩石圈地幔的布什维尔德的地壳特征，（2）来自可能在深部地壳分级室中的大陆地壳的地壳特征，（3）来自侵位时与围岩相互作用的地壳特征，（4）硅酸盐熔体与不混溶硫化物熔体之间的交换作用，使硫化物富集于地壳中。布什维尔德复杂的Sr同位素组成的变化，整个火成岩地层，这种变化已被解释为在不同的岩浆中的地壳物质的不同数量的合并的结果？对于不同岩浆脉冲的数量具有不同的意见（例如，Sharpe，1985; Kruger，1994）。到目前为止，看来，Sr和S同位素系统是没有联系在布什维尔德杂岩，因为有不同的Sr同位素的变化，但S同位素组成似乎是统一的。然而，没有研究表明这两种系统都是在相同的样品上测量的。该项目提供了对这两种同位素系统进行研究的机会，这两种同位素系统都是地壳的指标，都来自同一样本。

The Bushveld Complex, South Africa, is the world's largest body of rock that crystallized beneath the Earth's surface from molten rock over two billion years ago. It is host to Earth's largest resource of platinum group elements (PGE) and also a significant resource for chromium (Cr) and vanadium (V). The enrichment of the Bushveld Complex in these elements is likely related to a number of factors, including the original source of the melt and processes occurring during its ascent and emplacement into the crust. Studying the Bushveld is important not only for the understanding the formation of PGE ore deposits but also to comprehend the Earth processes that led to such a large and unusual body of rock. The Bushveld melt was rich in magnesium, which suggests it was originally derived from the Earth's mantle, but puzzling crustal isotopic signatures, including heterogeneous isotopic compositions of Sr, Nd, Pb, O and S, suggest that it endured a complicated history of ascent, interactions with its surroundings and possible incorporation of crust. The recent discovery of a thick succession of highly magnesian rocks (the Basal Ultramafic Sequence) below what have generally been accepted as the lowermost layers of the Bushveld provides a unique opportunity to investigate rocks derived from the most pristine melts associated with the Bushveld and to test hypotheses about the sources of these crustal signatures. A novel approach that uses the isotopic composition of the element sulfur (S) will be used to investigate how deep magma interacted with the shallower crust to yield these rocks and constrain which Earth processes that led to the formation of the largest PGE deposit on Earth. The Basal Ultramafic Sequence provides the opportunity to investigate rocks that have experienced minimal alteration during emplacement and crystallization (Wilson 2012; 2015). Existing chemical and mineralogical data and planned Sr and Nd isotope analyses will be compared to measurements of Del33S on the same samples. Questions to be addressed include (1) whether there is a signal of surface-derived S in these lowermost Bushveld igneous rocks, (2) whether these lowermost units are uniform in Del33S values, and (3) how S isotopic compositions correlate with other indicators of crust such as Sr and Nd isotopes. Several hypotheses for the source of the crustal isotopic signals will be tested including (1) crustal signatures of the Bushveld derived from sub-continental lithospheric mantle, (2) crustal signatures derived from continental crust perhaps in a deep crustal staging chamber, (3) crustal signatures derived from interaction with wall rock upon emplacement, and (4) exchange between silicate melt and immiscible sulfide melt which enriched the sulfide in crustal S. The Sr isotopic composition of the Bushveld Complex varies throughout the igneous stratigraphy, and this variation has been interpreted as the result of incorporation of different amounts of crustal material in different magmas ? with varying opinions on the numbers of different pulses of magma (e.g., Sharpe, 1985; Kruger, 1994). So far, it appears that the Sr and S isotopic systems are not linked in the Bushveld Complex, since there are distinct variations in Sr isotopes, but the S isotopic composition appears to be uniform. However, there are no studies in which both these systems have been measured on the same samples. This project provides the opportunity to investigate these two isotopic systems, both indicators of crust, on the same samples.