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）是大洋岛屿和海山下方岩浆作用的常见产物，但人们对它们的起源知之甚少。 由于这些岩石蕴藏着有关地幔地球化学和地球动力学过程的关键信息，因此对其成因的限制对于我们更广泛地了解地球内部至关重要。目前，关于大洋岛玄武岩（OIB）地幔来源的热活力以及回收材料（通过俯冲作用返回地幔的古代地壳以及H2O等挥发性成分）的可能性，仍然存在相当大的争论和不确定性。和CO2 - 参与了它们的起源。 解决这些争论需要限制岩浆的成分与其来源的温度、压力和成分之间的关​​系。该提案将支持该团队最近实验工作的扩展，该实验工作表明，“典型”石榴石橄榄岩（被认为构成上地幔大部分的材料）的小程度部分熔融无法解释大洋岛屿玄武岩的成分。这意味着要么没有在正确的条件下进行实验，要么 OIB 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间开发和改进的迭代实验方法（“MISE”）来确定石榴石二辉橄榄岩小程度部分熔体的成分，并探索压力、熔体分数和来源富集的影响CO2、K2O、H2O 和 FeO 对近固相线部分熔体成分的影响。这些新实验将有助于更好地确定石榴石橄榄岩初期部分熔化过程中产生的液体的成分和比例。如果碱性玄武岩起源于石榴石橄榄岩的小程度熔融，并且如果熔融温度和压力可以受到可能母体液体的主要元素化学的限制，那么实验将把OIB的岩石学特征与熔融深度和地幔电位联系起来源头的温度。第二组实验旨在根据玄武岩的微量元素成分了解其来源和融化条件。 特别是，第一行过渡元件（FRTE）已成为 OIB 起源的重要探针。 与 MORB 相比，OIB 中 Ni 和 Ti 的浓度以及 Mn/Fe 和 Zn/Fe 比率不同，可能需要辉石岩或成分改性橄榄岩的贡献。 因此，建议通过小程度部分熔融实验和 3-5 GPa 下的结晶实验来测量 FRTE 石榴石二辉橄榄石矿物（橄榄石、辉石和石榴石）之间的分配系数。