Volatiles in basaltic glasses from the Easter‐Salas y Gomez Seamount Chain and Easter Microplate: Implications for geochemical cycling of volatile elements
Volatiles in basaltic glasses from the Easter‐Salas y Gomez Seamount Chain and Easter Microplate: Implications for geochemical cycling of volatile elements
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DOI：
10.1029/2001gc000173
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发表时间：
2001-12
期刊：
影响因子：
3.7
通讯作者：
K. Simons;J. Dixon;J. Schilling;R. Kingsley;R. Poreda
中科院分区：
文献类型：
--
作者：
K. Simons;J. Dixon;J. Schilling;R. Kingsley;R. Poreda
本文研究了来自东南太平洋Easter Microplate （EMP）和Easter - Salas y Gomez海山链（ESC）系统的66个玄武岩玻璃中的H2O、CO2和Cl浓度。EMP - ESC系统的特征是枯竭的海中脊玄武岩（MORB）地幔源（DMM）和不相容的元素和放射性同位素富集源Salas y Gomez地幔柱（SyG）之间的二元混合。羽流物质被引导到微板东部裂谷上以~ 27°S为中心的脊峰。EMP上的水浓度在东部裂谷的~ 27°S处最高，并逐渐向南北降低，遵循其他地球化学示踪剂的空间格局。EMP玄武岩在脱气过程中没有损失H2O，但吸收了一定量的富Cl -热液成分。相比之下，一些海山玄武岩由于浅脱气而失去水分，但很少获得Cl，这表明富Cl物质的同化很少。在岩浆通过岩石圈上升的过程中，一些ESC海山玻璃可能已经吸收了含水成分，例如蛇纹石化的辉锌矿。在未受浅层过程影响的样品基础上，主要羽流成分的H2O/Ce为~ 210±20，相对于其他类似不相容元素，它在H2O中既不优先富集也不优先耗尽。耗尽的MORB源的H2O/Ce为~ 150±10。SyG源的地幔挥发性浓度估计为750±210 ppm H2O和40±11 ppm Cl，平均EPR源的地幔挥发性浓度为120±27 ppm H2O和4.5±1.4 ppm Cl， DMM源的地幔挥发性浓度为54±12 ppm H2O和1.7±0.4 ppm Cl。H2O和Cl与相似的不相容元素的耦合行为，加上3He/4He比值的升高，表明挥发物主要是幼稚的，代表了地幔柱的共同成分，少量来自循环岩石圈成分的贡献。
We present H2O, CO2, and Cl concentrations in 66 basaltic glasses from the Easter Microplate (EMP) and Easter‐Salas y Gomez Seamount Chain (ESC) system in the southeastern Pacific. The EMP‐ESC system is characterized by binary mixing between a depleted mid‐ocean ridge basalt (MORB) mantle source (DMM) and an incompatible element and radiogenic isotope enriched source, the Salas y Gomez mantle plume (SyG). Plume material is channeled toward the ridge crest centered at ∼27°S on the east rift of the microplate. Water concentrations on the EMP are highest on the east rift at ∼27°S and become progressively lower to the north and south, following the spatial pattern of other geochemical tracers. EMP basalts have not lost H2O to degassing but have assimilated variable quantities of a Cl‐rich hydrothermal component. In contrast, some seamount basalts have lost water by shallow degassing, but very few have gained Cl, indicating little assimilation of Cl‐rich materials. Several ESC seamount glasses may have assimilated a hydrous component, for example, serpentinized harzburgite, during magma ascent through the lithosphere. On the basis of samples unaffected by shallow processes, the main plume component has H2O/Ce of ∼210 ± 20 and is neither preferentially enriched nor depleted in H2O relative to other similarly incompatible elements. The depleted MORB source has H2O/Ce of ∼150 ± 10. Estimated mantle volatile concentrations are 750 ± 210 ppm H2O and 40 ± 11 ppm Cl for the SyG source, 120 ± 27 ppm H2O and 4.5 ± 1.4 ppm Cl for an average EPR source, and 54 ± 12 ppm H2O and 1.7 ± 0.4 ppm Cl for the DMM source. The coupled behavior of H2O and Cl with similarly incompatible elements, coupled with elevated 3He/4He ratios, suggests that the volatiles are dominantly juvenile, representative of a component common to mantle plumes, with minor contribution from recycled lithospheric components.