Deciphering variable mantle sources and hydrous inputs to arc magmas in Kamchatka
Deciphering variable mantle sources and hydrous inputs to arc magmas in Kamchatka
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DOI：
10.1016/j.epsl.2021.116848
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发表时间：
2021-05
影响因子：
5.3
通讯作者：
Alexander A. Iveson;M. Humphreys;I. Savov;J. D. De Hoog;S. Turner;T. Churikova;C. Macpherson;
中科院分区：
文献类型：
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作者：
Alexander A. Iveson;M. Humphreys;I. Savov;J. D. De Hoog;S. Turner;T. Churikova;C. Macpherson;
原始弧岩石的化学成分为了解地幔楔的成分变化以及俯冲相关岩浆作用的板块衍生输入提供了一个窗口。然而，在地球内部和外部储层之间元素的长期循环中，一个关键的未知因素是通过与俯冲相关的地幔交代作用在俯冲系统内保留可移动元素的重要性。为了解决这些问题，我们分析了来自堪察加弧的橄榄石熔体包裹体和相应的块状岩石。熔体包裹体组记录了在不同母体岩浆成分聚集期间沿熔体混合阵列截留的证据。在所研究的不同喷发中心中，母岩浆 B/Zr、Nb/Zr、Ce/B 和 δ 11 B 的系统变化也很明显。这些元素比例限制了与俯冲相关的交代作用的性质，并为周围地幔的异质性和不同程度的地幔融化提供了证据。弧后岩石中的高 Nb/Zr 和低 B/Zr 表明熔融程度较小，来自板片的输入较低，但地幔成分相对丰富。同样，远离主层锥的小型单成因喷发中心也倾向于喷发板片贡献相对较低且总体熔化程度较小的岩浆。相反，弧前成分反映了更大的板片贡献和更贫乏的周围地幔的更大程度的熔化。 δ 11 B 的跨弧变化（范围从弧后和斯雷丁尼海脊的大约− 6 ‰到堪察加半岛中部凹陷的+ 7 ‰）通常与贫化 MORB 地幔成分中同位素重板片衍生成分的可变添加相一致。然而，个别火山中心（例如巴肯宁火山）显示熔体包裹体δ 11 B 与其他地球化学指标（例如Cl/K 2 O、Ce/B）之间的相关性，需要在经历了不同程度的地壳演化和脱气过程的同位素不同熔体批次之间进行混合。我们的结果表明，虽然熔体包裹体挥发性库存在较浅的熔体储存和聚集过程中很大程度上被套印，但不相容的微量元素比例和B同位素组成更忠实地追踪了初始地幔组成和俯冲输入。此外，我们认为，俯冲早期阶段的长期交代作用在弧下地幔中产生的成分异质性信号可以在后来的岩浆聚集和地壳储存过程中得以保留。
The chemistry of primitive arc rocks provides a window into compositional variability in the mantle wedge, as well as slab-derived inputs to subduction-related magmatism. However, in the long-term cycling of elements between Earth's internal and external reservoirs, a key unknown is the importance of retaining mobile elements within the subduction system, through subduction-related metasomatism of the mantle. To address these questions, we have analysed olivine-hosted melt inclusions and corresponding bulk rocks from the Kamchatka arc. Suites of melt inclusions record evidence for entrapment along melt mixing arrays during assembly of diverse parental magma compositions. Systematic variations in parental magma B/Zr, Nb/Zr, Ce/B, and δ 11 B are also apparent among the different eruptive centres studied. These element ratios constrain the nature of subduction-related metasomatism and provide evidence for ambient mantle heterogeneity and variable degrees of mantle melting. High Nb/Zr and low B/Zr in back-arc rocks indicate smaller degree melts, lower slab-derived inputs, but relatively enriched mantle compositions. Similarly, small monogenetic eruptive centres located away from the main stratocones also tend to erupt magmas with relatively lower slab contribution and overall smaller melting degrees. Conversely, arc-front compositions reflect greater slab contributions and larger degree melts of a more depleted ambient mantle. Across-arc variations in δ 11 B (ranging from ca.− 6‰ in the rear-arc and Sredinny Ridge to+ 7‰ in the Central Kamchatka Depression) are generally consistent with variable addition of an isotopically heavy slab-derived component to a depleted MORB mantle composition. However, individual volcanic centres (eg Bakening volcano) show correlations between melt inclusion δ 11 B and other geochemical indicators (eg Cl/K 2 O, Ce/B) that require mixing between isotopically distinct melt batches that have undergone different extents of crustal evolution and degassing processes. Our results show that while melt inclusion volatile inventories are largely overprinted during shallower melt storage and aggregation, incompatible trace element ratios and B isotope compositions more faithfully trace initial mantle compositions and subduction inputs. Furthermore, we suggest that the signals of compositional heterogeneity generated in the sub-arc mantle by protracted metasomatism during earlier phases of subduction can be preserved during later magma assembly and storage in the crust.