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: 0609967
• 负责人: Marc Hirschmann
• 金额: --
• 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: 2006
• 资助国家: 美国
• 起止时间: 2006-07-01 至 2010-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0609967&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 the origins of such rocks are critical to a broader understanding Earth's interior. Resolving debate regarding the dynamical origin of intraplate basalts requires constraints on the relationship between the compositions of magmas and the temperatures, pressures, and compositions of their sources. Previous experimental studies have failed to reproduce major element compositions of alkalic oceanic island basalts (OIB) from partial melts of peridotite+/-CO2. 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. Evaluation of these possibilities requires new experiments to determine the compositions and proportions of liquids generated during incipient partial melting of garnet peridotite+/-CO2 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.This project has a goal to determine the compositions of partial melts of fertile garnet peridotite at and near its solidus from 0 to 5% melting at pressures of 3 to 7 GPa. It is planned to employ modified iterative sandwich experiments (MISE), which are designed to determine accurately the composition of very low melt fraction melts. These techniques will allow determination of small-degree melts of garnet peridotite, which may be important sources of intraplate magmatism, but for there are few experimental data, owing to difficulties in analyzing high temperature, high MgO partial melts. The team will also use the MISE method to determine the influence of CO2 on near-solidus melting of garnet peridotite by conducting experiments with peridotite with CO2 concentrations (0.1 wt.%) close to those likely in natural alkali basalt source regions.

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。 由于此类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，因此对此类岩石起源的限制对于更广泛地了解地球内部至关重要。解决有关板内玄武岩动力学起源的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。先前的实验研究未能从橄榄岩+/-CO2的部分熔体中重现碱性海洋岛屿玄武岩（OIB）的主要元素组成。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。 对这些可能性的评估需要新的实验来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+/-CO2 碱性玄武岩是否源自石榴石橄榄岩的小程度熔化，以及熔化的温度和压力是否可以受到限制通过可能的母液的主要元素化学，实验将 OIB 的岩石学特征与其源头的熔化深度和地幔潜在温度联系起来。该项目的目标确定在 3 至 7 GPa 压力下，0 至 5% 熔化的肥沃石榴石橄榄岩在其固相线处及其附近的部分熔体的成分。 计划采用改进的迭代夹层实验（MISE），旨在准确确定熔体分数极低的熔体的成分。 这些技术将允许测定石榴石橄榄岩的小程度熔融物，这可能是板内岩浆作用的重要来源，但由于分析高温、高MgO部分熔融物的困难，实验数据很少。该团队还将使用 MISE 方法，通过对二氧化碳浓度（0.1 wt.%）接近天然碱性玄武岩源区的橄榄岩进行实验，确定二氧化碳对石榴石橄榄岩近固相线熔化的影响。