Abstract Deep-rooted mantle plumes provide a link for carbon between Earth’s interior and its surface. Low-degree partial melting of CO2-bearing mantle is predicted to produce carbonated melts in the deep mantle, but carbonated melts are rarely found in modern oceanic settings, and the role of primary carbonated silicate melts in mantle-plume volcanism remains unclear. Because low-degree mantle melting tends to generate CO2-rich melts, we studied a suite of volcanic rocks from Pohnpei Island formed by the waning Caroline mantle plume based on whole-rock major- and trace-element, Sr–Nd–Pb–Hf isotopic, and high-precision in situ mineral compositions. We demonstrate that the mineralogy and geochemistry of these volcanic rocks are consistent with those of high-MgO primary carbonated silicate melts, e.g., they have the most SiO2-depleted and CaO-enriched compositions reported for ocean-island basalts, they are enriched in rare-earth elements and have negative anomalies of high-field-strength-elements. The Sr–Nd–Pb–Hf isotopic compositions of these volcanic rocks exhibit mixing between the Pacific-type depleted mantle and an enriched-mantle II (EM2)-like source. We found that primary carbonated silicate melts were modified to become typical ocean-island alkali basalts through reactive evolution and CO2 degassing during magma ascent in the lithosphere, and also identified a closed-system stage of evolution after these melts reached equilibrium with the lithospheric mantle. We suggest that low-degree melting of a waning mantle plume facilitates carbonated-silicate-melt volcanism, with the carbonated silicate melts being converted to typical alkali ocean-island basalt in the overlying thick lithospheric mantle. The anomalously low olivine Ca and Mn contents and the Ca partition coefficient between olivine and melt suggest that the primary magmas had a CO2 content of 11–15 wt.%. These volcanic rocks also contain Hawaii-like high-Ni olivines, which we interpret as reflecting a pyroxenite-rich and olivine-poor mantle source. The proposed reactive evolution of carbonated melts in the lithospheric mantle tends to cause loss of CO2 in the deep mantle and regulates the fate of carbonated melts, which may explain the widespread occurrence of alkali basalts and the rarity of carbonated melts in intra-oceanic-plate settings.

中文翻译：

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CO2 在逐渐减弱的卡罗琳地幔柱的火山活动中起重要作用的证据

摘要 根深蒂固的地幔柱为地球内部和地表之间的碳提供了联系。预计含二氧化碳地幔的低度部分熔融会在深部地幔中产生碳酸化熔体，但在现代海洋环境中很少发现碳酸化熔体，并且原生碳酸化硅酸盐熔体在地幔柱火山作用中的作用仍不清楚。由于低度地幔熔融往往会产生富含 CO2 的熔体，我们研究了一系列来自波纳佩岛的火山岩，这些火山岩是基于全岩主要和微量元素 Sr-Nd-Pb-Hf 由逐渐减弱的卡罗琳地幔柱形成的同位素和高精度原位矿物成分。我们证明这些火山岩的矿物学和地球化学与高 MgO 原生碳酸盐硅酸盐熔体的矿物学和地球化学一致，例如，它们具有最多的 SiO2 耗尽和 CaO 富集成分，它们富含稀土元素，并具有高场强元素的负异常。这些火山岩的 Sr-Nd-Pb-Hf 同位素组成表现出太平洋型耗尽地幔和富集地幔 II (EM2) 样源之间的混合。我们发现原生碳酸盐化硅酸盐熔体在岩石圈岩浆上升过程中通过反应演化和 CO2 脱气被改造为典型的海岛碱性玄武岩，并在这些熔体与岩石圈地幔达到平衡后确定了一个封闭系统演化阶段。我们认为，逐渐减少的地幔柱的低度熔化促进了碳酸盐硅酸盐熔体火山活动，碳酸盐化的硅酸盐熔体在上覆厚厚的岩石圈地幔中转化为典型的碱性海岛玄武岩。异常低的橄榄石 Ca 和 Mn 含量以及橄榄石和熔体之间的 Ca 分配系数表明原生岩浆的 CO2 含量为 11-15 wt.%。这些火山岩还含有类似夏威夷的高镍橄榄石，我们将其解释为反映了富含辉石岩而贫橄榄石的地幔源。岩石圈地幔中碳酸盐熔体的反应演化往往会导致深部地幔中CO2的损失并调节碳酸盐熔体的命运，这可能解释了碱性玄武岩的广泛存在和大洋板块内碳酸盐熔体的稀有性设置。异常低的橄榄石 Ca 和 Mn 含量以及橄榄石和熔体之间的 Ca 分配系数表明原生岩浆的 CO2 含量为 11-15 wt.%。这些火山岩还含有类似夏威夷的高镍橄榄石，我们将其解释为反映了富含辉石岩而贫橄榄石的地幔源。岩石圈地幔中碳酸盐熔体的反应演化往往会导致深部地幔中CO2的损失并调节碳酸盐熔体的命运，这可能解释了碱性玄武岩的广泛存在和大洋板块内碳酸盐熔体的稀有性设置。异常低的橄榄石 Ca 和 Mn 含量以及橄榄石和熔体之间的 Ca 分配系数表明原生岩浆的 CO2 含量为 11-15 wt.%。这些火山岩还含有类似夏威夷的高镍橄榄石，我们将其解释为反映了富含辉石岩而贫橄榄石的地幔源。岩石圈地幔中碳酸盐熔体的反应演化往往会导致深部地幔中CO2的损失并调节碳酸盐熔体的命运，这可能解释了碱性玄武岩的广泛存在和大洋板块内碳酸盐熔体的稀有性设置。我们将其解释为反映了富含辉石岩和贫橄榄石的地幔源。岩石圈地幔中碳酸盐熔体的反应演化往往会导致深部地幔中CO2的损失并调节碳酸盐熔体的命运，这可能解释了碱性玄武岩的广泛存在和大洋板块内碳酸盐熔体的稀有性设置。我们将其解释为反映了富含辉石岩和贫橄榄石的地幔源。岩石圈地幔中碳酸盐熔体的反应演化往往会导致深部地幔中CO2的损失并调节碳酸盐熔体的命运，这可能解释了碱性玄武岩的广泛存在和大洋板块内碳酸盐熔体的稀有性设置。