Search for Hadean Protocrust in Modern Mantle

在现代地幔中寻找冥古宙原地壳

• 批准号: 0510589
• 负责人: Mukul Sharma
• 金额: --
• 依托单位: Dartmouth College
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-06-01 至 2008-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0510589&HistoricalAwards=false
• 关键词: Search; Hadean; Protocrust; Modern; Mantle

The Hadean time period - spanning the first 500 million years of the Earth's history - has no rock record and is the subject of intense debate. It has been recently demonstrated that during the Hadean magma extraction from Earth's primordial mantle created a depleted layer from which 3.8 billion year old rocks from Isua, Greenland were derived. One of the key issues arising from these observations is that of the fate the primordial crust or protocrust. Did the protocrust get isolated from the depleted mantle or did it not form in the first place? The former scenario would be possible if the protocrust either foundered into the deep mantle. The latter scenario would be possible if the early Earth's magma ocean was covered by a stagnant lid comprising dominantly of mafic-ultramafic lithologies. Using the isotopic effects in Nd-142 produced from the decay of short-lived Sm-146 (half life of 103 million years) this project will look for the evidence of protocrust and its location in modern mantle. If an enriched protocrust formed in the first 300 million years of Earth's history and has been preserved it would display a deficit in Nd-142. Two potential locations of the protocrust in the mantle are: the D" region of the lower mantle and the subcontinental lithospheric mantle below ancient continents. High precision neodymium isotopes in volcanic rocks from Iceland hotspot and Deccan flood basalt province will be examined; initial investigations of the latter have already revealed the presence of negative Nd-142 effects in some lava flows that have been inferred to have significant subcontinental lithospheric mantle component. This project will constitute a major part of a Ph.D. and a senior thesis. It promotes teaching and training by encouraging graduate students and laboratory technicians to teach sophisticated laboratory skills to undergraduate students and provides graduate students an opportunity to present results in national conferences. The origin and evolution of continental crust is a fascinating topic for the general public and it is believed that the outcome of the project will be widely read and disseminated.

冥古宙时期--跨越地球历史的前5亿年--没有岩石记录，是激烈争论的主题。最近的研究表明，在冥古宙岩浆从地球的原始地幔中提取时，产生了一个耗尽层，格陵兰岛Isua的38亿年前的岩石就是从这个耗尽层中产生的。 从这些观测中产生的一个关键问题是原始地壳或原地壳的命运。原地壳是从耗尽的地幔中分离出来的，还是一开始就没有形成？如果原地壳沉入地幔深处，前一种情况是可能的。如果早期地球的岩浆海洋被一个主要由镁铁质-超镁铁质岩石组成的停滞盖所覆盖，后一种情况是可能的。利用短寿命Sm-146（半衰期为1.03亿年）衰变产生的Nd-142的同位素效应，该项目将寻找原地壳及其在现代地幔中位置的证据。如果富集的原地壳在地球历史的前3亿年中形成并被保存下来，那么它将显示出Nd-142的赤字。原地壳在地幔中的两个可能位置是：下地幔的D”区和古大陆之下的陆下岩石圈地幔。将对冰岛热点和德干溢流玄武岩省火山岩中的高精度钕同位素进行检查;对后者的初步调查已经揭示了一些熔岩流中存在负Nd-142效应，这些熔岩流被推断具有重要的次大陆岩石圈地幔成分。该项目将构成博士学位的主要部分。还有毕业论文它通过鼓励研究生和实验室技术人员向本科生教授先进的实验室技能来促进教学和培训，并为研究生提供在全国会议上介绍成果的机会。大陆地壳的起源和演化是公众感兴趣的话题，相信该项目的成果将被广泛阅读和传播。