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

了解地球如何运作以及大陆如何以及为什么随着时间的推移在其表面移动的基本知识对于我们了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，并通过地幔中的运动和对流传递到地壳。火山岩的化学和同位素变化，产生沿着地球的洋中脊系统提供了深入了解地球地幔的组成和演化。深海橄榄岩，地幔的直接样品，偶尔通过构造过程沿沿着洋中脊出露。这些岩石是对洋中脊熔岩提供的地幔成分的补充。最近的研究揭示了这两种不同类型岩石所描绘的地幔组成和演化的差异。这种差异可能是由于目前的模型未能解释熔体从地幔中产生和提取的过程的复杂性。本研究测试的假设，这两个独立的信息来源之间的差异可以解释的优先熔融的化学性质不同的静脉在地幔和化学相互作用，这些丰富的熔体与周围的地幔物质在岩浆上升到表面。对深海橄榄岩和大洋中脊喷发的熔岩中的小硫化物包裹体进行同位素分析，有助于检验“大理石蛋糕地幔”假说。研究结果将增进我们对覆盖地球表面近1/4的海洋地壳如何形成的理解。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，其中一所是少数民族服务机构，另一所是第一所被指定为西班牙裔服务的一级大学。学生和研究人员在两个机构之间的合作将补充正在进行的努力，以扩大在科学服务不足的community.This研究探讨锇（Os）和铅（Pb）同位素变化的硫化物从深海橄榄岩和大洋中脊玄武岩的推广。在岩浆硫化物的锇同位素，而略高于平均深海橄榄岩的放射性成因，重叠橄榄岩衍生的硫化物的值。颗粒尺度的Os-和Pb-同位素的不均匀性记录在许多橄榄岩被假定为反映长期的隔离和演化的阶段与变量的母/子比或最近的交代同位素富集熔体。本研究测试后一种假设，并暗示榴辉岩/辉石岩熔体生成的放射性Os和Pb同位素签名的最终来源，在间隙和岩浆硫化物。这项工作研究了间隙中的Os和Pb同位素，包括来自Gakkel山脊的异常新鲜的深海橄榄岩的硫化物，Gakkel山脊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。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尺寸、间距和结构关系。这些数据将与来自Gakkel和其他北大西洋洋中脊玄武岩的硫化物Os-Pb分析相结合，这些玄武岩的成分范围很广。（1）橄榄岩间隙和硫化物中的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熔体在地幔中的交代叠加作用;（2）大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，橄榄石中的镍、Na/Ti或其他长寿命放射成因示踪剂）;（3）熔体生产率低的地区在熔体生成过程中是否优先取样镁铁质成分;（4）加克尔脊玄武岩与熔体生产率较高的快速扩张脊段玄武岩的硫化物Os同位素特征是否存在系统差异。这项工作的一个主要目标是使用Os和Pb同位素变化的岩浆和地幔硫化物的岩性异质性和反应熔体运输的洋中脊玄武岩和深海橄榄岩的生成中的作用进行约束。从玄武岩和深海橄榄岩的硫化物中提取的锇和铅同位素数据与来自相同样品的其他地球化学和岩石学数据相结合，将使我们能够确定岩性不均匀性在洋中脊玄武岩生成中所起的作用，此外还能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方面所起的作用。这项工作的结果将大大提高我们利用洋中脊玄武岩化学来推断地球对流上地幔复杂的消耗和再施肥历史的能力。

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.