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（碧玄岩、霞石）是大洋岛屿和海山下岩浆作用的常见产物，但对它们的起源知之甚少。 由于这些岩石包含有关地球地幔中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，仍有相当大的争议和不确定性的地幔源的大洋岛屿玄武岩（OIB）的热活力和回收材料的可能性，已返回到地幔通过俯冲的古地壳，以及挥发性组分，如H2O和CO2 -参与其成因。 解决这些争论需要限制岩浆成分与温度、压力及其来源成分之间的关系。这一提议将支持该团队最近实验工作的扩展，该工作表明，“典型”石榴石橄榄岩的小程度部分熔融-被认为构成上地幔的大部分物质-不能解释大洋岛屿玄武岩的成分。这意味着实验没有在正确的条件下进行，或者在OIB的来源中需要额外的（辉石岩或交代橄榄岩）岩性。他们将继续使用迭代实验方法（“MISE”），开发和改进的最后一个资助期，以确定石榴石二辉橄榄岩的小程度部分熔体的组成，并探讨压力，熔体分数，和源富集CO2，K2 O，H2O和FeO的近固相线部分熔体的组成的影响。这些新的实验将允许一个更好地确定的组成和比例的液体产生的石榴石橄榄岩在初期部分熔融。如果碱性玄武岩起源于石榴橄榄岩的小程度熔融，并且熔融的温度和压力可以由可能的母体液体的主元素化学约束，则实验将OIB的岩石学特征与熔融深度和其源地的地幔位温联系起来。第二组实验的目的是根据微量元素的组成，了解大洋岛屿玄武岩熔融的来源和条件。 特别是第一行过渡元素（FRTE）已成为OIB起源的重要探针。 Ni和Ti的浓度以及Mn/Fe和Zn/Fe比是不同的OIB相比，MORB，可能需要从辉石岩或从成分改性橄榄岩的贡献。 因此，建议测量分配系数之间的石榴石二辉橄榄岩矿物（橄榄石，辉石，石榴石）的FRTE从小程度的部分熔融实验和结晶实验在3-5 GPa。