Near-zero 33S and 36S anomalies in Pitcairn basalts suggest Proterozoic sediments in the EM-1 mantle plume
Near-zero 33S and 36S anomalies in Pitcairn basalts suggest Proterozoic sediments in the EM-1 mantle plume
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DOI：
10.1016/j.epsl.2022.117422
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发表时间：
2022-04
影响因子：
5.3
通讯作者：
Jabrane Labidi;J. Dottin;M. Clog;C. Hemond;P. Cartigny
中科院分区：
文献类型：
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作者：
Jabrane Labidi;J. Dottin;M. Clog;C. Hemond;P. Cartigny
在皮特凯恩海山之间喷发的火山岩样本显示出极度富集地幔-1特征的地幔热柱。这种特殊的地幔端元的起源仍然存在争议，可能涉及太古代或元古代海洋沉积物的再循环，来自下大陆地壳的拆沉单元，或来自岩石圈地幔的交代橄榄岩。本文报道了来自皮特凯恩海山的15个新鲜海底玄武岩玻璃的硫多同位素特征(32个S，33个S，34个S，36个S)。我们观察到有证据表明，在海平面(MBSL)以下2,000米处，∼喷发的熔体中有硫磺的岩浆脱气作用。硫浓度与喷发深度相关，范围在1300ppm S(采集∼2,500Mbsl)到600ppm S(∼2000Mbsl)之间。在脱气过程中，δ34 S值可用平衡同位素分馏来解释，α气体熔体在1.0020~1.0001之间，起始δ34 S值在−0.9‰~+0.6‰之间。δ34 S的估计与MORB的特征相似或更高，这表明与太平洋上地幔相比，具有∼1‰34 S富集的循环硫的贡献。Δ33 S和Δ36 S的平均签名分别为+0.024±0.007‰和+0.02±0.07CDT(均为1‰)。仅Δ33 S与MORB差异有统计学意义，相差+0.02‰。Δ33 S的浓缩在脱气和硫化物分离过程中是不变的。我们认为这反映了地幔来源的富集物，而不是玄武岩中S的高温分馏作用。尽管33个S-36个S的变化幅度很小，但我们的数据需要大量的回收硫淹没皮特凯恩地幔来源。我们表明，涉及交代橄榄岩、下地壳单元或太古宙沉积物的模型可能是可行的，但仅限于狭窄的环境。相反，涉及元古界海洋沉积物贡献的情景似乎是对皮特凯恩EM-1特征最简明的解释。
Volcanic rocks erupted among Pitcairn seamounts sample a mantle plume that exhibits an extreme Enriched Mantle-1 signature. The origin of this peculiar mantle endmember remains contentious, and could involve the recycling of marine sediments of Archean or Proterozoic ages, delaminated units from the lower continental crust, or metasomatized peridotites from a lithospheric mantle. Here, we report the sulfur multi-isotopic signature (32 S, 33 S, 34 S, 36 S) of 15 fresh submarine basaltic glasses from three Pitcairn seamounts. We observe evidence for magmatic degassing of sulfur from melts erupted∼ 2,000 meters below seawater level (mbsl). Sulfur concentrations are correlated with eruption depth, and range between 1300 ppm S (collected∼ 2,500 mbsl) and 600 ppm S (∼ 2,000 mbsl). The δ 34 S values can be accounted for under equilibrium isotope fractionation during degassing, with α gas-melt between 1.0020 and 1.0001 and starting δ 34 S values between− 0.9‰ and+ 0.6‰. The δ 34 S estimates are similar or higher than MORB signatures, suggesting the contribution of recycled sulfur with a∼ 1‰ 34 S enrichment compared to the Pacific upper mantle. The Δ 33 S and Δ 36 S signatures average at+ 0.024±0.007‰ and+ 0.02±0.07‰ vs. CDT, respectively (all 1σ). Only Δ 33 S is statistically different from MORB, by+ 0.02‰. The Δ 33 S enrichment is invariant across degassing and sulfide segregation. We suggest it reflects a mantle source enrichment rather than a high-temperature fractionation of S in the basalts. Despite the small magnitude of the 33 S-36 S variations, our data require a substantial amount of recycled sulfur overwhelm the Pitcairn mantle source. We show that models involving metasomatized peridotites, lower crust units, or Archean sediments, may be viable, but are restricted to narrow sets of circumstances. Instead, scenarios involving the contribution of Proterozoic marine sediments appear to be the most parsimonious explanation for the EM-1 signature at Pitcairn.