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(玄武岩、霞石)是洋岛和海山下岩浆作用的常见产物，但其成因尚不清楚。由于这些岩石包含有关地幔中的地球化学和地球动力学过程的关键信息，因此对它们成因的限制对于我们更广泛地了解地球内部是至关重要的。目前，关于大洋岛玄武岩(OIB)地幔来源的热活力，以及再循环物质--通过俯冲返回地幔的古代地壳，以及H2O和CO2等挥发性成分--参与其成因的可能性，仍存在相当大的争议和不确定性。解决这些争论需要限制岩浆成分与其来源的温度、压力和成分之间的关系。这一提议将支持该团队最近的实验工作的扩展，该实验表明，被认为构成上地幔大部分的石榴石橄榄岩的小度部分熔融不能解释大洋岛玄武岩的组成。这意味着要么没有在合适的条件下进行实验，要么在大洋界面的来源中需要额外的(辉石岩或交代橄榄岩)岩性。他们将继续使用在上一个资助期开发和改进的迭代实验方法(MISE)来确定石榴石二辉橄榄岩的小度部分熔体的组成，并探索压力、熔体分数以及CO2、K2O、H2O和FeO的来源富集度对近固相线部分熔体组成的影响。这些新的实验将允许更好地确定石榴石橄榄岩在初始部分熔融过程中产生的液体的组成和比例。如果碱性玄武岩起源于石榴石橄榄岩的小度熔融，并且熔融的温度和压力可以由可能的母液的主元素化学所制约，则实验将把大洋橄榄岩的岩石学特征与其熔融深度和源区的地幔温度联系起来。第二组实验的目的是根据大洋岛玄武岩的微量元素组成，了解其熔融的来源和条件。特别是，第一行跃迁元素(FRTE)已成为OIB起源的重要探索者。与MORB相比，OIB中Ni和Ti的浓度以及Mn/Fe和Zn/Fe比值是不同的，可能需要辉石岩或成分改性橄榄岩的贡献。因此，建议用小度部分熔融实验和3-5 Gpa的结晶实验测量FRTE的石榴石二辉橄榄岩矿物(橄榄石、辉石和石榴石)之间的分配系数。