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

了解地球是如何运作的，以及大陆是如何以及为什么在地球表面移动的，对我们理解海底火山作用至关重要。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提供了对地幔组成和演化的见解。深海橄榄岩是地幔的直接样本，在沿洋中脊的构造过程中偶尔暴露出来。这些岩石是对大洋中脊熔岩提供的地幔成分的补充。最近的研究表明，这两种不同类型的岩石所描绘的地幔组成和演化的图景存在差异。这种差异很可能是由于目前的模型未能考虑到熔体产生和从地幔中提取过程的复杂性。这项研究验证了这样一个假设，即这两个独立信息来源之间的差异可以用地幔中化学性质不同的脉的优先熔融以及这些富集的熔体在岩浆上升到地表期间与周围地幔物质的化学相互作用来解释。对深海橄榄岩和中洋脊喷发熔岩中的小硫化物包裹体进行同位素分析，可以帮助检验这种“大理石蛋糕地幔”假说。这项研究的结果将增进我们对覆盖地球表面近四分之三的海洋地壳是如何形成的理解。该项目还将支持来自德克萨斯州两所大学的两名研究生的教育和培训，其中一所大学是少数族裔服务机构，另一所大学是第一所被指定为西班牙裔服务的一流大学。这两个机构的学生和研究人员之间的合作将补充正在进行的将科学推广到服务不足的社区的努力。本文研究了深海橄榄岩和洋中脊玄武岩中硫化物中锇（Os）和铅（Pb）同位素的变化。岩浆硫化物中的os同位素虽然比一般深海橄榄岩的放射成因略高，但与橄榄岩衍生硫化物的同位素值重叠。在许多橄榄岩中记录的颗粒级Os和pb同位素非均质性被认为反映了具有可变母/子比的相的长期隔离和演化或同位素富集熔体的近期交代作用。本研究验证了后一种假设，并暗示榴辉岩/辉石岩熔体的产生是间隙和岩浆硫化物中放射性成因Os和pb同位素特征的最终来源。这项工作检查了间隙中的Os和pb同位素，包括来自Gakkel Ridge的异常新鲜的深海橄榄岩中的硫化物，Gakkel Ridge是一个超缓慢的洋中脊扩张中心，暴露了大量几乎未改变的地幔岩石。在硫化物提取和分析之前，x射线CT成像将用于检查硫化物和其他相的大小、间距和结构关系。这些数据将与Gakkel和其他北大西洋中洋脊玄武岩的硫化物Os-Pb分析相结合，这些玄武岩的组成范围很广。具体问题包括：(1)橄榄岩间质和含硫化物的晶粒级Os和pb同位素变化是否反映了“内部等时线”和孤立的、富Os和富pb相的非均质性的长期保存，或者它们是否代表了榴辉岩或辉石岩衍生的熔体在地幔中渗透的近期交代叠加；(2)洋中脊玄武岩硫化物中的Os和pb同位素是否与其他可能与辉石岩熔融有关的岩石学或地球化学信号（例如，镍-橄榄石、Na/Ti或其他长寿命放射性示踪剂）相关；(3)熔体生产效率低的地区在熔体生成过程中优先取样基性组分；(4) Gakkel Ridge玄武岩中硫化物的os同位素特征与熔体生产力较高的快速扩张脊段的玄武岩中硫化物的os同位素特征是否存在系统差异。本研究的主要目标是利用岩浆和地幔硫化物中的Os和pb同位素变化来限制岩石非均质性和反应性熔体运输在洋中脊玄武岩和深海橄榄岩形成中的作用。将玄武岩和深海橄榄岩硫化物的Os和pb同位素数据与来自同一样品的其他地球化学和岩石学数据相结合，可以确定岩性非均质性在洋中脊玄武岩生成过程中的作用，以及更好地测量近代和古代地幔熔融和熔融/岩石反应在深海橄榄岩化学和同位素变异产生中的作用。这项工作的结果将极大地提高我们利用洋中脊玄武岩化学来推断地球上地幔对流的复杂枯竭和再气化历史的能力。

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.