PGE, Re-Os and Lu-Hf Isotopic Signature of Lithosphere Removal Beneath the North China Craton

华北克拉通岩石圈去除的PGE、Re-Os和Lu-Hf同位素特征

• 批准号: 0911096
• 负责人: Roberta Rudnick
• 金额: $ 33.03万
• 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: 2009
• 资助国家: 美国
• 起止时间: 2009-09-15 至 2014-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0911096&HistoricalAwards=false
• 关键词: PGE; Re; Os; Lu; Hf

Continents form the platform on which our species has evolved, and nearly all of our natural resources derive from the continents. Understanding how continents form and evolve and what makes them stable is, therefore, of importance for humans. The oldest portions of continents, those regions that formed before 2.5 billion years ago and termed 'cratons', are typically the most stable surface features on Earth. They experience no active volcanism or earthquakes, and the presence of a thick and stiff rock layer beneath the continental crust, extending down to depths of 200-250 km, is generally considered key for the stability of these ancient regions. The ~3.8 billion year old North China Craton is an exception to this general picture. Some of the most devastating earthquakes in human history, as well as relatively recent volcanism and ore deposit formation have occurred in this craton. There is strong evidence that the thick rock layer, which was originally present beneath the crust of the North China Craton, was removed and replaced with a thinner and weaker rock layer several hundred million years ago, but the timing and process of this transformation is uncertain. The project outlined in this proposal seeks to determine when and how the thick rock layer was replaced beneath the North China Craton by studying pieces of this deep layer sampled by volcanoes that erupted over the last 400 million years, a time period that spans the replacement event. The research proposed involves the use of petrography, Re-Os and Lu-Hf isotope geochemistry, as well as measuring the abundances of the platinum group elements (PGE) in mantle xenoliths to unravel the history of the mantle lithosphere beneath the North China Craton during the Mesozoic and Cenozoic. Large, statistically significant data sets for Re-Os will be collected at targeted locations in order to distinguish post-Archean lithospheric ages from the Re-Os spectrum observed in the convecting upper mantle. Clinopyroxene from select xenoliths will be measured for Lu and Hf isotopes in order to compare age systematics between the two systems and evaluate the utility of the Lu-Hf system for determining lithosphere formation ages. PGE data will be used to evaluate the veracity of Os model ages for individual samples, and will also provide information on processes, such as refertilization and melt-rock interaction, that may have acted on the mantle xenoliths. The results are expected to clarify the present-day age distribution of lithospheric mantle beneath different portions of the North China Craton, how the Mesozoic aged lithosphere compares with older and present-day lithospheric mantle, and how this information can be linked with available seismological images in order to deduce the deep architecture of the craton and determine the lateral and vertical extent of lithospheric mantle replacement. This, in turn, will inform models for the processes involved in the transformation of a stable craton into one of the most tectonically active regions of the world today.

大陆构成了我们物种进化的平台，我们几乎所有的自然资源都来自大陆。因此，了解大陆如何形成和演化以及是什么使它们保持稳定对于人类来说非常重要。 大陆最古老的部分，也就是 25 亿年前形成的区域，被称为“克拉通”，通常是地球上最稳定的表面特征。 它们没有经历过活跃的火山活动或地震，大陆地壳下方存在厚而坚硬的岩层，延伸至 200-250 公里的深度，通常被认为是这些古老地区稳定的关键。 大约 38 亿年前的华北克拉通是这幅图景中的一个例外。 人类历史上一些最具破坏性的地震以及相对较新的火山活动和矿床形成都发生在这个克拉通。 有强有力的证据表明，华北克拉通地壳下原本存在的厚岩层在几亿年前就被去除并被更薄、更弱的岩层所取代，但这种转变的时间和过程尚不确定。 该提案中概述的项目旨在通过研究过去 4 亿年（跨越更换事件的时间段）火山喷发采样的深层岩层碎片，确定华北克拉通下方的厚岩层何时以及如何被更换。 拟议的研究涉及使用岩相学、Re-Os和Lu-Hf同位素地球化学，以及测量地幔捕虏体中铂族元素（PGE）的丰度，以揭示华北克拉通地下地幔岩石圈的历史。中生代和新生代。 将在目标地点收集大量具有统计意义的 Re-Os 数据集，以便从对流上地幔中观察到的 Re-Os 光谱中区分太古代后岩石圈年龄。 将测量来自选定捕虏体的单斜辉石的 Lu 和 Hf 同位素，以便比较两个系统之间的年龄系统学并评估 Lu-Hf 系统在确定岩石圈形成年龄方面的效用。 PGE 数据将用于评估单个样本的 Os 模型年龄的准确性，还将提供有关可能对地幔捕虏体产生作用的过程的信息，例如再施肥和熔岩相互作用。 研究结果有望阐明华北克拉通不同部分下方岩石圈地幔的当今年龄分布、中生代岩石圈与更古老和当今岩石圈地幔的比较，以及如何将这些信息与现有的地震图像联系起来从而推断克拉通的深部结构并确定岩石圈地幔置换的横向和纵向范围。 反过来，这将为稳定克拉通转变为当今世界构造最活跃的区域之一的过程模型提供信息。