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首页> 外文期刊>Contributions to Mineralogy and Petrology >Melt inclusion evidence for CO2-rich melts beneath the western branch of the East African Rift: implications for long-term storage of volatiles in the deep lithospheric mantle
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Melt inclusion evidence for CO2-rich melts beneath the western branch of the East African Rift: implications for long-term storage of volatiles in the deep lithospheric mantle

机译：东非大裂谷西部分支下富含CO2的熔融物的熔融包裹体证据：对深层岩石圈地幔中挥发物的长期储存的影响

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摘要

We present new major element, trace element, and volatile (H2O, CO2, S, F, and Cl) concentrations of olivine-hosted melt inclusions from five high-K, low-silica basanites from the western branch of the East African Rift System and use these data to investigate the generation of H2O- and CO2-rich melts at up to similar to 150 km depth. Measured H2O and CO2 concentrations reach similar to 2.5 and similar to 1 wt%, respectively, representing some of the highest CO2 concentrations measured in a melt inclusion to date. These measurements represent direct evidence of the high CO2 and H2O concentrations required to generate high-K alkaline lavas, and the CO2 that has been previously inferred to be necessary for the low mantle potential temperatures in the area. Ratios of CO2/Nb, CO2/Ba, and CO2/Cl are used to estimate an initial melt CO2 concentration of 5-12 wt%. The measured CO2 concentrations are consistent with CO2 solubilities determined by molecular dynamics calculations and high-pressure experiments for melt generation at 3-6 GPa; the depth of melting suggested by previous studies in the area. These melt inclusions measurements represent direct evidence for the presence of H2O- and CO2-rich melts in the deep upper mantle that have been proposed based on experimental and seismic evidence. Primitive-mantle normalized trace element patterns more closely resemble those found in subduction settings rather than ocean island basalt, and ratios of slab fluid tracers such as Li/Dy and B/Be indicate that the measured volatile abundances may be related to Neoproterozoic subduction during the assembly of Gondwana, implying the storage of volatiles in the mantle by subduction-related metasomatism.
机译：我们介绍了来自东非大裂谷系统西部分支的五种高钾，低硅质玄武岩中橄榄石基质熔体夹杂物的新主要元素，微量元素和挥发性（H2O，CO2，S，F和Cl）浓度并使用这些数据来研究在高达150 km的深度下富含H2O和CO2的熔体的产生。测得的H2O和CO2浓度分别达到约2.5和约1 wt％，代表了迄今为止熔体夹杂物中测得的一些最高CO2浓度。这些测量结果直接表明生成高K碱性熔岩所需的高CO2和H2O浓度，并且先前已推断该区域低地幔潜在温度所必需的CO2。使用CO2 / Nb，CO2 / Ba和CO2 / Cl的比率估算初始熔融CO2浓度为5-12 wt％。测得的CO2浓度与通过分子动力学计算和高压实验确定的CO2溶解度一致，以3-6 GPa的速度产生熔体;该地区以前的研究表明的融化深度。这些熔体夹杂物的测量代表了深层上地幔中富含H2O和CO2熔体的直接证据，这是根据实验和地震证据提出的。原始幔归一化的痕量元素模式与俯冲环境中发现的模式更相似，而不是与洋岛玄武岩相似，并且平板流体示踪剂（例如Li / Dy和B / Be）的比率表明，在该过程中测得的挥发性丰度可能与新元古代俯冲有关冈瓦纳的集会，暗示着俯冲相关的交代作用将挥发性物质储存在地幔中。

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