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.

大陆是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，了解大陆是如何形成和演化的，以及是什么使它们稳定，对人类来说至关重要。 大陆最古老的部分，即那些在25亿年前形成的区域，被称为“大陆”，通常是地球上最稳定的表面特征。 它们没有活火山活动或地震，大陆地壳下厚而坚硬的岩石层延伸到200-250公里深处，通常被认为是这些古老地区稳定的关键。 大约38亿年前的华北古陆是这一总体图景的一个例外。 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存款形成都发生在这个克拉通。 有强有力的证据表明，最初存在于华北陆壳下的厚岩层在数亿年前被移走，取而代之的是更薄、更弱的岩层，但这一转变的时间和过程尚不确定。 本提案中概述的项目旨在通过研究在过去4亿年中爆发的火山所取样的这一深层的碎片来确定华北盆地下的厚岩层何时以及如何被替换，这一时间段跨越了替换事件。 通过岩石学、Re-Os和Lu-Hf同位素地球化学以及地幔捕虏体中铂族元素（PGE）丰度的测定，揭示了华北地区中新生代地幔岩石圈的演化历史。 将在目标地点收集大量具有统计学意义的铼-锇数据集，以便将后太古代岩石圈年龄与对流上地幔中观察到的铼-锇光谱区分开来。 从选择捕虏体的单斜辉石将测量Lu和Hf同位素，以比较两个系统之间的年龄系统，并评估Lu-Hf系统确定岩石圈形成年龄的实用性。 铂族元素数据将被用来评估个别样品的Os模型年龄的准确性，也将提供信息的过程，如再施肥和熔体岩石的相互作用，可能已经对地幔捕虏体。 这些结果将有助于阐明华北克拉通不同地区岩石圈地幔的现今年龄分布，中生代岩石圈与更古老和现今岩石圈地幔的对比，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，确定岩石圈地幔置换的横向和纵向范围。 反过来，这将为一个稳定的克拉通转变为当今世界上最活跃的构造地区之一的过程提供信息。