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

基本信息

批准号：
1736995
负责人：
John Lassiter
金额：
$ 32.83万
依托单位：
University of Texas at Austin
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 2022-07-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1736995&HistoricalAwards=false
关键词：
Collaborative Research Using Osmium Lead

项目摘要

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大陆如何以及为何随着时间的推移在其表面移动的基础知识对于我们理解海底火山活动至关重要。这种运动是由地球深处的力量驱动的，并通过地幔的运动和对流传递到地壳。沿着地球洋中脊系统产生的火山岩的化学和同位素变化为了解深渊橄榄岩（深渊橄榄岩的直接样本）的成分和演化提供了见解。地幔，偶尔会通过洋中脊的构造过程暴露出来，这些岩石是对洋中脊熔岩所提供的地幔成分的补充。最近的研究揭示了这些岩石所描绘的地幔成分和演化的差异。两种不同类型的岩石之间的差异可能是由于当前模型未能解释熔体产生和从地幔中提取的过程的复杂性所致。信息来源可以通过地幔中化学性质不同的矿脉的优先熔化以及在岩浆上升到地表期间这些富集熔体与周围地幔物质的化学相互作用来解释。而在洋中脊喷发的熔岩可以帮助检验这种“大理石饼地幔”假说，研究结果将增强我们对洋壳如何形成的理解。该项目还将支持德克萨斯州两所大学的两名研究生的教育和培训，其中一所是少数族裔服务机构，另一所是第一所被指定的一级大学。两个机构的学生和研究人员之间的合作将补充不断扩大科学范围到服务不足的社区的努力。这项研究检查了深海硫化物中的锇 (Os) 和铅 (Pb) 同位素变化。岩浆硫化物中的橄榄岩和洋中脊玄武岩虽然比普通深海橄榄岩的放射性稍强，但与许多橄榄岩中记录的粒级橄榄岩中的锇和铅同位素异质性重叠。假设反映了具有可变亲子比例的阶段的长期隔离和演化，或者反映了最近的交代作用这项研究检验了后一种假设，并表明榴辉岩/辉石岩熔体的生成是间质和岩浆硫化物中放射性 Os 和 Pb 同位素特征的最终来源。包括来自加克尔海脊（Gakkel Ridge）的异常新鲜的深海橄榄岩的硫化物，这是一个超慢速的中洋脊扩展中心暴露出大量几乎未改变的地幔岩石，在硫化物提取和分析之前，将使用 X 射线 CT 成像来检查硫化物和其他相的尺寸、间距和结构关系。 Gakkel 和其他北大西洋洋中脊玄武岩的 Os-Pb 分析涵盖了广泛的成分，正在解决的具体问题包括：(1) 颗粒尺度的 Os-和 Pb-同位素。橄榄岩间质和内含硫化物的变化反映了“内部等时线”以及分离的、富含锇和铅的相中异质性的长期保存，或者它们是否代表了榴辉岩或辉石岩衍生的熔体渗透到地幔中的近期交代叠印（ 2) 大洋中脊玄武岩硫化物中的锇和铅同位素与其他岩石学或地球化学信号是否相关可能与辉石岩熔化有关（例如，橄榄石中的镍、Na/Ti 或其他长寿命放射性示踪剂）；（3）在熔体生成过程中优先对镁铁质成分进行采样；（4）是否存在Gakkel 山脊玄武岩的硫化物与来自熔体生产率更高的扩张速度更快的山脊部分的玄武岩的 Os 同位素特征存在系统差异。这项工作的主要目标是利用 Os 和岩浆和地幔硫化物中的铅同位素变化限制了岩性异质性和反应性熔体传输在洋中脊玄武岩和深海橄榄岩生成中的作用来自玄武岩硫化物的锇和铅同位素数据的整合。来自深海橄榄岩的数据以及来自同一样本的其他地球化学和岩石学数据将允许岩性非均质性发挥作用除了更好地衡量近代和古代地幔熔融和熔融/岩石反应在深海橄榄岩中产生化学和同位素变异性中的作用之外，还需要确定大洋中脊玄武岩的生成过程，这项工作的结果将极大地提高我们利用中脊玄武岩的能力。 -海洋基底脊化学，用于推断地球对流上地幔的复杂消耗和再受精历史。