Near Solidus Partial Melting of Garnet Peridotite and the Origin of Alkali Olivine Basalt

石榴石橄榄岩近固相线部分熔融与碱橄榄石玄武岩成因

• 批准号: 1019744
• 负责人: Marc Hirschmann
• 金额: $ 41.73万
• 依托单位: University of Minnesota-Twin Cities
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-07-01 至 2014-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1019744&HistoricalAwards=false
• 关键词: Near; Solidus; Partial; Melting; Garnet

Alkali basalts and related rocks (basanites, nephelinites) are common products of magmatic processes beneath oceanic islands and seamounts, but their origin is poorly understood. Because such rocks hold key information about geochemical and geodynamical processes in Earth's mantle, constraints on their genesis are critical to our broader understanding of the interior of the planet. Presently, there remains considerable debate and uncertainty regarding the thermal vigor of the mantle sources of oceanic island basalts (OIB) and the possibility that recycled materials - ancient crust that has been returned to the mantle via subduction, as well as volatile components such as H2O and CO2 - was involved in their genesis. Resolving these debates requires constraints on the relationship between the compositions of magmas and the temperatures, pressures, and compositions of their sources. This proposal will support an extension of recent experimental work by the team that has shown that small-degree partial melts of 'typical' garnet peridotite - the material thought to constitute most of the upper mantle - cannot account for the compositions of oceanic island basalts. This means either that experiments have not been conducted under the right conditions or that an additional (pyroxenitic or metasomatized peridotitic) lithology is required in the source of OIB. They will continue to use an iterative experimental method ('MISE'), developed and improved over the last funding period, to determine the composition of small degree partial melts of garnet lherzolite and to explore the effects of pressure, melt fraction, and source enrichment in CO2, K2O, H2O and FeO on the compositions of near-solidus partial melts. These new experiments will allow for a better determination of the compositions and proportions of liquids generated during incipient partial melting of garnet peridotite. If alkali basalts originate from small-degree melting of garnet peridotite, and if the temperature and pressure of melting can be constrained by major element chemistry of plausible parental liquids, the experiments will relate the petrologic character of OIB to the depth of melting and mantle potential temperature in their source. A second set of experiments will be aimed at understanding the sources and conditions of melting of basalts from oceanic island based on their trace element compositions. In particular, first row transition elements (FRTE) have emerged as important probes of the origins of OIB. Concentrations of Ni and Ti as well as Mn/Fe and Zn/Fe ratios are distinct in OIB as compared to MORB and may require contributions from pyroxenite or from compositionally modified peridotite. Consequently, it is proposed to measure partition coefficients between garnet lherzolite minerals (olivine, pyroxenes, and garnet) of FRTE from small degree partial melting experiments and from crystallization experiments at 3-5 GPa.

碱性玄武岩和相关岩石（底石，侄子）是海洋岛屿和封口下的岩浆过程的常见产物，但它们的起源知之甚少。 由于这样的岩石在地球地幔中拥有有关地球化学和地球动力学过程的关键信息，因此对它们的起源的限制对于我们对地球内部的广泛理解至关重要。目前，关于海洋岛玄武岩（OIB）地幔源的热活力的争论和不确定性仍然存在很大的争论和不确定性，以及回收材料的可能性 - 通过俯冲通过俯冲返回地幔以及挥发性组件（例如H2O和CO2）的可能性与H2O和CO2-相关。 解决这些辩论需要限制岩浆组成与其来源的温度，压力和组成之间的关系。该提案将支持团队最近的实验工作扩展，该研究表明，“典型”石榴石橄榄岩的小型部分融化 - 被认为构成大部分上层地幔的材料无法说明海洋岛玄武岩的组成。这意味着要么在正确的条件下尚未进行实验，要么在OIB​​来源中需要额外的（辉石或交代植物植物）岩性。他们将继续使用迭代实验方法（“ MISE”），并在上一个资金期间开发和改进，以确定小保化的小度局部融化石榴石lherzolite的组成，并探索压力，熔体分数以及CO2，K2O，H2O和FEO对近solids solids solids solials soltials Meltials Meltials Meltials seltials sertials的组成的效果。这些新的实验将可以更好地确定石榴石橄榄岩的初始部分融化过程中产生的液体的组成和比例。如果碱玄武岩起源于石榴石橄榄岩的小度熔化，并且如果熔化的温度和压力受到了合理的父母液体的主要元素化学的限制，则实验将与OIB的岩石学特征与融化的降解和熔点的潜在温度有关。第二组实验将旨在根据其痕量元素组成，了解海洋岛上玄武岩熔化的来源和条件。 特别是，第一排过渡元素（FRTE）已成为OIB起源的重要探针。 与MOR相比，OIB的Ni和Ti以及Mn/Fe和Zn/Fe比的浓度与众不同，可能需要来自辉石的贡献或成分修饰的橄榄岩。 因此，提议从小度局部熔化实验和3-5 GPA的结晶实验中测量FRTE的石榴石氟唑矿矿物（橄榄石，辉石和石榴石）之间的分配系数。