Boron isotopic signatures of melt inclusions from North Iceland reveal recycled material in the Icelandic mantle source
Boron isotopic signatures of melt inclusions from North Iceland reveal recycled material in the Icelandic mantle source
复制标题
冰岛北部熔体包裹体的硼同位素特征揭示了冰岛地幔源中的回收材料
DOI：
10.1016/j.gca.2020.11.013
复制
发表时间：
2021
影响因子：
5
通讯作者：
Hartley M
中科院分区：
文献类型：
--
作者：
Hartley M
地幔中的微量元素和挥发性异质性受到俯冲海洋岩石圈再循环的影响。与洋中脊玄武岩相比，大洋岛玄武岩通常具有高浓度的挥发物，但这种富集与再循环地幔域的联系程度仍不清楚。硼是回收俯冲物质的理想示踪剂，因为只需要一小部分回收成分即可改变源地幔的大量 δ11B。因此，原生熔体的硼同位素组成有可能追踪深部地幔中再循环俯冲物质的命运，并限制不同构造环境中岩性和成分异质性的长度尺度。我们提出了对玄武岩玻璃和熔体包裹体中挥发物、轻元素和硼同位素比率的新测量，这些测量在两个端元空间尺度上对地幔进行了采样。雷克雅内斯山脊的海底玻璃在冰岛羽流的大范围内对长波长地幔异质性进行了采样。来自冰岛北部 Askja 和 Bárðarbunga 火山系统的晶体熔体包裹体采样了靠近地幔柱中心的短波长地幔异质性。雷克雅内斯海岭玻璃仅记录到接近冰岛的沿山脊 B 含量非常微弱的富集，并且沿整个山脊段的 δ11B 没有系统变化。这些观测结果限制雷克雅内斯海岭地幔的δ11B为-6.1‰（2SD=1.5‰，2SE=0.3‰）。冰岛北部熔体包裹体的 δ11B 变化范围很大，介于 -20.7 至 +0.6‰ 之间。我们针对地壳污染的影响筛选熔体包裹体，将高 [B] 和低 δ18O 识别为同化过程的指纹。只有 MgO⩾8wt.% 的最原始熔体包裹体才能可靠地记录来自地幔的 δ11B。在冰岛北部，来自Holuhraun的不相容微量元素（ITE）耗尽的原始熔体包裹体记录的aδ11B为-10.6‰，这一信号在冰岛西南部的熔体包裹体中也出现过（Gurenko和Chaussidon，1997）。相比之下，来自附近阿斯贾火山的原始富含 ITE 的熔体包裹体记录的 aδ11B 为 -5.7‰，与我们对雷克雅内斯海岭地幔 δ11B 的新限制重叠。来自冰岛北部更演化的熔体包裹体的耦合 [B]、δ11B 和 δ18O 特征与初级熔体在上升穿过上地壳时同化 <5-20% 的热液蚀变玄武质透明碎屑岩一致。我们的数据揭示了贫化的低 δ11B 和富集的高 δ11B 地幔成分的存在，这两者都是冰岛地幔源所固有的，与雷克雅内斯山脉不同山脊地幔。非模态熔融计算表明，富集和贫化地幔成分均含有~0.085μg/g B，略低于雷克雅内斯海脊地幔计算的0.10-0.11μg/g。这些数据与含有贫硼脱水循环海洋岩石圈的冰岛地幔一致，与雷克雅内斯海岭玻璃或 MORB 相比冰岛熔体包裹体的低 B/Pr 一致。我们的新数据为循环俯冲岩石圈在海洋岛屿熔体生成中的作用提供了强有力的支持，并强调需要仔细筛选熔体包裹体成分，以研究海洋岛屿玄武岩中的全球挥发性循环。
Trace element and volatile heterogeneity in the Earth’s mantle is influenced by the recycling of oceanic lithosphere through subduction. Oceanic island basalts commonly have high concentrations of volatiles compared to mid-ocean ridge basalts, but the extent to which this enrichment is linked to recycled mantle domains remains unclear. Boron is an ideal tracer of recycled subducted material, since only a small percentage of a recycled component is required to modify the bulkδ11B of the source mantle. Boron isotopic compositions of primary melts thus have potential to trace the fate of recycled subducted material in the deep mantle, and to constrain the lengthscales of lithologic and compositional heterogeneities in diverse tectonic settings.We present new measurements of volatiles, light elements and boron isotopic ratios in basaltic glasses and melt inclusions that sample the mantle at two endmember spatial scales. Submarine glasses from the Reykjanes Ridge sample long-wavelength mantle heterogeneity on the broad scale of the Iceland plume. Crystal-hosted melt inclusions from the Askja and Bárðarbunga volcanic systems in North Iceland sample short-wavelength mantle heterogeneity close to the plume centre. The Reykjanes Ridge glasses record only very weak along-ridge enrichment in B content approaching Iceland, and there is no systematic variability inδ11B along the entire ridge segment. These observations constrain ambient Reykjanes Ridge mantle to have aδ11B of −6.1‰ (2SD = 1.5‰, 2SE = 0.3‰). The North Iceland melt inclusions have widely variableδ11B between −20.7 and +0.6‰. We screen melt inclusions against influence from crustal contamination, identifying high [B] and lowδ18O as fingerprints of assimilation processes. Only the most primitive melt inclusions with MgO ⩾ 8 wt.% reliably record mantle-derivedδ11B. In North Iceland, incompatible trace element (ITE)-depleted primitive melt inclusions from Holuhraun record aδ11B of −10.6‰, a signal that has also been seen in melt inclusions from southwest Iceland (Gurenko and Chaussidon, 1997). In contrast, primitive ITE-enriched melt inclusions from nearby Askja volcano record aδ11B of −5.7‰, overlapping with our new constraint on theδ11B of Reykjanes Ridge mantle. Coupled [B],δ11B andδ18O signatures of more evolved melt inclusions from North Iceland are consistent with primary melts assimilating <5–20% of hydrothermally altered basaltic hyaloclastite as they ascend through the upper crust.Our data reveal the presence of a depleted, low-δ11B and an enriched, higher-δ11B mantle component, both intrinsic to the Icelandic mantle source and distinct from Reykjanes Ridge mantle. Non-modal melting calculations suggest that the enriched and depleted mantle components both contain ∼0.085 μg/g B, slightly lower than the 0.10–0.11 μg/g calculated for Reykjanes Ridge mantle. These data are consistent with the Icelandic mantle containing B-depleted dehydrated recycled oceanic lithosphere, in keeping with the low B/Pr of Icelandic melt inclusions in comparison to Reykjanes Ridge glasses or MORB. Our new data provide strong support for the role of recycled subducted lithosphere in melt generation at ocean islands, and highlight the need for careful screening of melt inclusion compositions in order to study global volatile recycling in ocean island basalts.
登录
查看更多内容
1875 年冰岛阿斯贾火山喷发：流纹质岩浆生成过程中分步结晶和选择性污染的结合
影响因子：
2.7
作者：
R. Macdonald;R. Sparks;H. Sigurdsson;D. Mattey;D. McGarvie;R. L. Smith
通讯作者：
R. L. Smith
包埋后结晶对蒸气饱和硅酸盐熔融包裹体的 H2O-CO2 系统影响的热力学模型
影响因子：
3.9
作者：
M. Steele‐MacInnis;Rosario Esposito;R. Bodnar
通讯作者：
R. Bodnar
影响因子：
5
作者：
R. Tanaka;E. Nakamura
通讯作者：
R. Tanaka;E. Nakamura
来源异质性对冰岛冰后拉斑玄武岩 U-Th-Pa-Ra 不平衡的影响
DOI：
--
发表时间：
2012
期刊：
影响因子：
--
作者：
J. Koornneef;A. Stracke;B. Bourdon;K. Grönvold
通讯作者：
K. Grönvold
使用双尖峰铅同位素比研究冰岛和邻近山脊的地幔成分
DOI：
--
发表时间：
2004
期刊：
影响因子：
--
作者：
M. Thirlwall;M. Gee;R. Taylor;B. Murton
通讯作者：
B. Murton