Testing Models for the Origin of 186Os/188Os and 187Os/188Os Isotope Variations in the Mantle: Core Signal, Recycled Components, or Intra-mantle Differentiation

地幔中 186Os/188Os 和 187Os/188Os 同位素变化起源的测试模型：核心信号、回收成分或地幔内分化

• 批准号: 1321937
• 负责人: John Lassiter
• 金额: $ 32万
• 依托单位: University of Texas at Austin
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-07-15 至 2017-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1321937&HistoricalAwards=false
• 关键词: Testing; Models; Origin; 186Os; 188Os

This project seeks to determine the physical and chemical process(es) responsible for generating variations in the osmium isotopic compositions of mantle-derived rocks. Previous studies havesuggested that some osmium isotope variations in lavas from Hawaii and Gorgona may reflect chemical interaction between the Earth?s mantle and core. If confirmed, this would have wide ranging implications for the chemical and thermal evolution of Earth?s interior. Testing this hypothesis through detailed study is therefore of high importance for the geologic community. Osmium isotopes are affected by the radioactive decay of rhenium and platinum, and so osmium isotope variations in geologic materials reflect long-term fractionation of the platinum/osmium and rhenium/osmium ratios in different portions of the Earth's interior. Several different processes in addition to core/mantle interaction can affect the osmium isotope compositions of different mantle reservoirs. These processes include recycling of oceanic crust and sediments into the mantle through plate tectonics, ancient mantle melting and melt extraction, and melting of distinct rock types in the mantle during melt generation today. Therefore, osmium isotope variations can provide important constraints on several geologic processes that have helped shape the chemical and physical evolution of Earth?s interior, but only if the different mechanisms for generating these isotopic variations can be distinguished. This study will examine correlations between Osmium isotopes and other geochemical tracers in lavas from Hawaii and Gorgona Island, thought to be related to deep-seated mantle plumes, and in several suites of mantle-derived peridotites that span a range in composition. These analyses will allow the supported researchers to test and confirm or refute several hypotheses that have been proposed to explain unusual enrichments in 186Os (an isotope generated by the decay of 190Pt) in some plume-derived lavas. By combining high-precision 186Os/188Os data with complementary isotopic, major, and trace element data, the supported research will test several proposed mechanisms for generation of elevated 186Os/188Os ratios in mantle materials. The selected lava suites are chosen because they either have been reported to possess elevated 186Os/188Os in earlier studies (Hawaiian Islands, Gorgona), or because they possess geochemical characteristics that suggest a significant role for pyroxenite or eclogite melting in their generation (Australs HIMU lavas). The peridotite samples will help constrain the long-term coupling (or lack thereof) between Pt/Os and Re/Os ratios in the upper mantle, and illuminate the role that melt depletion plays in the 190Pt-186Os evolution of the upper mantle. Combined, this study will place stronger constraints on the different models proposed for the origin of Pt/Os fractionation and 186Os/188Os variation in the mantle than is currently possible, thereby addressing an important and longstanding controversy in the Geological Sciences and increasing the utility of the 190Pt-186Os decay system in future geochemical studies.

该项目致力于确定导致地幔岩石的Os同位素组成发生变化的物理和化学过程。以前的研究表明，夏威夷和戈尔戈纳熔岩中的某些Os同位素变化可能反映了地球？S地幔和地核之间的化学作用。如果得到证实，这将对地球内部的化学和热演化产生广泛的影响？S。因此，通过详细的研究来验证这一假设对地质界具有重要意义。Os同位素受Re和Ph的放射性衰变的影响，因此，地质材料中的Os同位素变化反映了地球内部不同部分铂/Os和Re/Om比值的长期分馏。除了核-地幔相互作用外，还有几种不同的作用可以影响不同地幔储集层的Os同位素组成。这些过程包括洋壳和沉积物通过板块构造循环进入地幔，古老的地幔熔融和熔体提取，以及在今天熔体生成过程中地幔中不同类型的岩石熔融。因此，只有在能够区分产生这些同位素变化的不同机制的情况下，Os同位素变化才能对帮助塑造地球-S内部的化学和物理演化的几个地质过程提供重要的约束。这项研究将检验来自夏威夷和戈尔戈纳岛的熔岩中的Os同位素和其他地球化学示踪剂之间的相关性，这些熔岩被认为与深部地幔热柱有关，以及几套来自地幔的橄榄岩，这些橄榄岩的成分跨度很大。这些分析将使得到支持的研究人员能够测试和确认或驳斥几个假说，这些假说被提出来解释在一些来自羽流的熔岩中186Os(一种由190Pt衰变产生的同位素)的异常丰度。通过将高精度的186Os/188Os数据与互补的同位素、常量和微量元素数据相结合，这项得到支持的研究将测试地幔材料中186Os/188Os比值升高的几种拟议机制。选定的熔岩套之所以被选中，是因为它们要么是因为在早期的研究中(夏威夷群岛、戈尔戈纳)据报道具有较高的186Os/188Os，要么是因为它们具有表明辉石岩或榴辉岩熔融在其同一代中具有重要作用的地球化学特征(澳大利亚HIMU熔岩)。橄榄岩样品有助于约束上地幔中Pt/Os和Re/Os比值之间的长期耦合(或缺乏耦合)，并阐明熔体亏损在上地幔190Pt186Os演化中的作用。总而言之，这项研究将对提出的关于地幔中铂/铯分馏和186Os/188Os变化的不同模型施加比目前可能的更强的约束，从而解决地质科学中一个重要的和长期存在的争议，并增加190Pt-186Os衰变系统在未来地球化学研究中的应用。