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

限制布什维尔德杂岩中古代地表硫的来源

• 批准号: 1551196
• 负责人: Sarah Penniston-Dorland
• 金额: $ 13.43万
• 依托单位: University of Maryland, College Park
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-07-01 至 2019-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1551196&HistoricalAwards=false
• 关键词: Constraining; Source; Ancient; Surface; derived

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.

南非的布什维尔德复合体是世界上最大的岩石体，它是20多亿年前由熔岩在地表下结晶而成的。它拥有地球上最大的铂族元素（PGE）资源，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许多因素有关，包括熔融的原始来源以及在其上升和嵌入地壳过程中发生的过程。研究布什维尔德不仅对了解PGE矿床的形成很重要，而且对了解导致如此巨大而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的不均匀同位素组成，表明它经历了一个复杂的上升历史，与周围环境相互作用，并可能与地壳合并。最近在通常被认为是布什维尔德最下层的下面发现了一层厚的高镁质岩石（基底超镁质层序），这为研究来自与布什维尔德有关的最原始熔体的岩石提供了一个独特的机会，并检验了关于这些地壳特征来源的假设。一种利用元素硫(S)的同位素组成的新方法将被用来研究深部岩浆如何与较浅的地壳相互作用以产生这些岩石，并限制地球上哪些过程导致了地球上最大的PGE矿床的形成。基底超基性层序为研究在侵位和结晶过程中经历最小蚀变的岩石提供了机会（Wilson 2012; 2015）。现有的化学和矿物学数据以及计划中的Sr和Nd同位素分析将与Del33S在同一样品上的测量结果进行比较。需要解决的问题包括：(1)在这些最底部的Bushveld火成岩中是否存在来自地表的S信号；(2)这些最底部的单元在Del33S值上是否均匀；(3)S同位素组成如何与地壳的其他指标（如Sr和Nd同位素）相关联。对地壳同位素信号来源的几种假设将进行检验，包括：(1)来自次大陆岩石圈地幔的地壳特征；(2)来自大陆地壳的地壳特征，可能是在一个深部地壳分型室中；(3)来自侵位时与围岩相互作用的地壳特征。(4)硅酸盐熔体与不混相硫化物熔体的交换，使地壳s中的硫化物富集。Bushveld杂岩的Sr同位素组成在整个火成岩地层中都有变化，这种变化被解释为不同岩浆中不同数量的地壳物质掺入的结果。对不同岩浆脉冲的数量有不同的看法（例如，Sharpe, 1985; Kruger, 1994）。到目前为止，似乎在Bushveld杂岩中Sr和S同位素系统没有联系，因为Sr同位素有明显的变化，但S同位素组成似乎是一致的。然而，目前还没有研究在同一样本上测量了这两种系统。该项目提供了在同一样品上研究这两种同位素系统的机会，这两种同位素系统都是地壳的指示器。