Collaborative Research: Using Osmium-Lead isotope variations in mid-ocean ridge and abyssal peridotite sulfides to understand fundamental properties of Earth's mantle

合作研究：利用大洋中脊和深海橄榄岩硫化物中的锇铅同位素变化来了解地幔的基本特性

• 批准号: 1737031
• 负责人: Jonathan Snow
• 金额: $ 9.31万
• 依托单位: University of Houston
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-08-01 至 2020-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1737031&HistoricalAwards=false
• 关键词: Collaborative; Research; Using; Osmium; Lead

Fundamental knowledge of how the Earth works and how and why continents move across its surface over time is critical for our understanding of seafloor volcanism. This movement is driven by forces deep in the Earth and are transmitted to the crust by movement and convection in the mantle. Chemical and isotopic variations in volcanic rocks, generated along Earth's mid-ocean ridge system provide insights into the composition and evolution of Earth's mantle. Abyssal peridotites, direct samples of the mantle, are occasionally exposed though tectonic processes along the mid-ocean ridge. These rocks are a complement to the composition of Earth's mantle provided by mid-ocean ridge lavas. Recent studies have revealed discrepancies in the picture of mantle composition and evolution painted by these two different types of rocks. The discrepancy likely results from the failure of current models to account for complexities in the processes by which melts are generated and extracted from the mantle. This research tests the hypothesis that the differences between these two independent sources of information can be explained by the preferential melting of chemically distinct veins in the mantle and the chemical interaction of these enriched melts with the surrounding mantle material during the ascent of the magma to the surface. Isotopic analysis of small sulfide inclusions in both abyssal peridotites and in the lavas erupted at mid-ocean ridges can help test this "marble cake mantle" hypothesis. Results of the research will enhance our understanding of how oceanic crust, which covers nearly ¾ of the Earth's surface, forms. The project will also support the education and training of two graduate students from two Texas universities, one of which is a minority-serving institution and the other is the first Tier-1 university to be designated as Hispanic-serving. Collaboration between students and investigators at the two institutions will complement ongoing efforts to expand outreach in science to underserved communities.This research examines Osmium (Os) and Lead (Pb) isotope variations in sulfides from abyssal peridotites and mid-ocean-ridge basalts. Os-isotopes in magmatic sulfides, while slightly more radiogenic than average abyssal peridotites, overlap with values in peridotite-derived sulfides. Grain-scale Os- and Pb-isotope heterogeneity documented in many peridotites are postulated to reflect either the long-term isolation and evolution of phases with variable parent/daughter ratios or the recent metasomatism by isotopically-enriched melts. This research tests the latter hypothesis and implicates eclogite/pyroxenite melt generation as the ultimate source of radiogenic Os- and Pb-isotope signatures in both interstitial and magmatic sulfides. This work examines Os- and Pb-isotopes in interstitial and included sulfides from exceptionally fresh abyssal peridotites from the Gakkel Ridge, an ultra-slow mid-ocean ridge spreading center that exposes significant amounts of virtually unaltered mantle rock. X-ray CT imaging will be used to examine the size, spacing, and textural relationships of sulfides and other phases prior to sulfide extraction and analysis. These data will be integrated with sulfide Os-Pb analyses from Gakkel and other North Atlantic mid-ocean ridge basalts spanning a wide range in composition. Specific questions being addressed include: (1) do grain-scale Os- and Pb-isotope variations in peridotite interstitial and included sulfides reflect "internal isochrones" and long-term preservation of heterogeneities in isolated, Os- and Pb-rich phases or do they represent recent metasomatic overprinting from eclogite- or pyroxenite-derived melts percolating through the mantle; (2) do Os- and Pb-isotopes in sulfides from mid-ocean ridge basalts correlate with other petrologic or geochemical signals potentially related to pyroxenite melting (e.g., Nickel-in-olivine, Na/Ti, or other long-lived radiogenic tracers); (3) do areas of low melt productivity preferentially sample mafic components during melt generation; and (4) are there systematic differences in the Os-isotope signatures of sulfides from Gakkel Ridge basalts versus basalts from faster spreading ridge segments with higher melt productivity. A primary goal of this work is to use Os- and Pb-isotope variations in magmatic and mantle sulfides to constrain the role of lithologic heterogeneity and reactive melt transport in the generation of both mid-ocean ridge basalts and abyssal peridotites. The integration of Os- and Pb-isotope data from both sulfides from the basalts and those from abyssal peridotites with other geochemical and petrologic data from the same samples will allow the role that lithologic heterogeneity plays in mid-ocean ridge basalt generation to be determined in addition to better gauging the role of recent and ancient mantle melting and melt/rock reaction in generating chemical and isotopic variability in abyssal peridotites. Results of the work will dramatically improve our ability to use mid-ocean ridge basalt chemistry to infer the complex depletion and refertilization history of Earth's convecting upper mantle.

关于地球如何运作以及如何以及为什么继续在其表面随着时间的流逝而移动的基本知识对于我们对海底火山主义的理解至关重要。这种运动是由地球深处的力驱动的，并通过地幔中的运动和结构传播到地壳。沿地球中部山脊系统产生的火山岩中的化学和同位素变化提供了对地球地幔组成和进化的见解。深渊橄榄岩是地幔的直接样品，偶尔通过沿着海脊沿着构造过程暴露。这些岩石是由中山山脊熔岩提供的地球披风组成的完整。最近的研究揭示了这两种不同类型的岩石绘制的地幔成分和进化的差异。差异可能是由于当前模型未能解释熔体从地幔产生和提取的过程中的复杂性。这项研究检验了以下假设：这两个独立的信息来源之间的差异可以通过在地幔中的化学上不同静脉的首选熔化以及这些富集融化的化学相互作用与周围地幔材料的化学相互作用在岩浆上升到表面上。对深渊橄榄岩和中山脊中爆发的熔岩中小硫化物夹杂物的同位素分析可以帮助检验这一“大理石蛋糕地幔”假设。这项研究的结果将增强我们对覆盖地球表面几乎¾形成的海洋壳的理解。该项目还将支持来自两所得克萨斯大学的两名研究生的教育和培训，其中一所是少数派服务机构，而另一个是第一层被指定为西班牙裔服务的大学。这两个机构的学生与调查人员之间的合作将完成持续的努力，将科学的外展扩展到服务不足的社区。这项研究考试Osmium（OS）和铅（PB）（PB）同位素的硫化物中硫化物的同位素变化来自深渊橄榄岩和中元中米及地盆地的硫化物。岩浆硫化物中的os-同位素，而放射原质量比平均深渊橄榄岩的放射原质略多，但与橄榄岩衍生的硫化物中的值重叠。假定在许多橄榄岩中记录的晶尺度OS和PB - 同位素异质性被认为反映了具有可变父/女儿比例的相的长期隔离和演变，或者是富含同位素增强的融化的近期跨质量。这项研究检验了后来的假设，并实现了叶酸岩/辉石熔体的产生，作为间隙和岩浆硫化物中放射性OS和PB - 同位素特征的最终来源。这项工作检查在间隙中的OS和PB - 同位素包括来自Gakkel Ridge的异常新鲜的深渊橄榄岩的硫化物，Gakkel Ridge是一个非常慢的中山山脊扩散中心，暴露了大量几乎没有更换的地幔岩。 X射线CT成像将用于检查硫化物和其他阶段在硫化物提取和分析之前的大小，间距和质地关系。这些数据将与来自Gakkel和其他北大西洋中部山脊的硫化物OS-PB分析集成在一起，这些玄武岩的组成范围很广。 Specific questions being addressed include: (1) do grain-scale Os- and Pb-isotope variations in peridotite interstitial and included sulfides reflect "internal isochrones" and long-term preservation of heterogeneities in isolated, Os- and Pb-rich phases or do they represent recent metasomatic overprinting from eclogite- or pyroxenite-derived melts percolating through the mantle; （2）从中山脊玄武岩的硫化物中的OS和PB - 同位素与其他可能与辉石融化相关的岩石学或地球化学信号相关（例如，镍中的镍，Na/ti或其他长期寿命的放射性示踪剂）； （3）在熔体产生过程中，低熔体生产力的区域优先采样宏观成分； （4）来自Gakkel Ridge玄武岩的硫化物的OS-同位素特征与来自更快扩散的山脊段的硫化物的OS-同位素特征与具有较高熔体生产率的更快的山脊段相对于玄武岩。这项工作的主要目的是在岩浆和地幔硫化物中使用OS和PB - 同位素变化来限制岩性异质性和反应性熔体转运在中近代山脊玄武岩和深渊甲壳虫的产生中的作用。来自基层硫化物的OS和PB - 同位素数据的整合以及来自同一样品的其他地球化学和岩石学数据的OS和PB - 同位素数据将允许岩性异质性在中质山脊中扮演岩性异质性的作用，除了岩石中部的变化以及在近期和古老的梅尔梅尔特（Melting Melting Melting Melting）的反应外，还可以确定岩性异质性的作用。在深渊橄榄岩中。这项工作的结果将极大地提高我们使用中海脊玄武岩化学来推断地球上层地幔的复杂部署和转诊历史的能力。