The survival of tracers of primordial mantle heterogeneity investigated through 142Nd/144Nd and 3He/4He isotope decoupling in the Gorgona Island lavas
The survival of tracers of primordial mantle heterogeneity investigated through 142Nd/144Nd and 3He/4He isotope decoupling in the Gorgona Island lavas
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通过 142Nd/144Nd 和 3He/4He 同位素解耦研究戈尔戈纳岛熔岩中原始地幔异质性示踪剂的存留
DOI：
10.1016/j.epsl.2023.118409
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发表时间：
2023
影响因子：
5.3
通讯作者：
Rojas-Agramonte, Yamirka
中科院分区：
文献类型：
--
作者：
Hyung, Eugenia;Ibañez-Mejia, Mauricio;Rojas-Agramonte, Yamirka
摘要 虽然 142 Nd/144 Nd、3 He/4 He 和 182 W/184 W 同位素比被认为并用作原始地幔异质性的示踪剂，但它们的分布和现代地幔变化程度背后的原因仍不清楚。与加拉帕戈斯羽流相关的熔岩表现出最陡峭的 3 He/4 He 与 μ 182 W 同位素相关性，因此有可能阐明同位素变异性保存所涉及的过程。戈尔戈纳岛是与加拉帕戈斯火山喷发有关的各种喷发之一，该岛因其显生宙科马提岩而具有重要的地质意义。我们报告了 Gorgona 科马提岩的首次高精度 142 Nd/144 Nd 同位素测量，以及一系列熔岩的 3 He/4 He 和主量元素和微量元素数据，包括科马提岩、辉长岩、d-辉长岩（“贫化”）和 e-玄武岩（“富集”）。六个科马提岩样品的 142 Nd/144 Nd 比率与大多数洋中脊和洋岛玄武岩采样的现代陆地幔无法区分，误差在±2.4 ppm (2σ) 之内。为科马提岩和苦味岩确定的 3 He/4 He 值从 4.6 到 45.1 R A。 3 He/4 He 测量值中有八个高于 MORB 平均值 (8±1 R A)，因此强烈表明对未脱气储层进行采样的羽流源。尤其是 R A 值为 45.1，是迄今为止在戈尔戈纳岛和其他地方测得的最高 3 He/4 He 比率之一，接近于巴芬岛显生宙溢流玄武岩的观测结果。使用主要元素和稀土元素以及 ε 143 Nd 特征对熔岩来源进行的评估表明，科马提岩和苦铁矿源自枯竭的橄榄岩来源。同时，e-玄武岩被推断源自由大约 8% 的辉石岩和 92% 的橄榄岩组成的混合来源。不同熔岩类型来源中辉石岩的推断和计算程度与观测到的 3 He/4 He 比率的大小一致。据报道，e-玄武岩中辉石矿源表现出 182 W/184 W 同位素异常，模型计算证明了它们的存在，而 142 Nd/144 Nd 同位素比率的类似计算表明均一化发生在更快的时间尺度上。据推断，与 142 Nd/144 Nd 和 3 He/4 He 比率相比，182 W/184 W 异常可能比与回收辉石岩的混合更持久。因此，现代 142 Nd/144 Nd、3 He/4 He 和 182 W/184 W 同位素变异的解耦可能反映了每个系统对不同地幔过程的敏感性，这些过程有助于它们在整个时间的保存。
Abstract While 142 Nd/144 Nd, 3 He/4 He, and 182 W/184 W isotope ratios are recognized and applied as tracers of primordial mantle heterogeneity, their distribution and reasons behind the extent of variability in the modern-day mantle remain unclear. Lavas associated with the Galapagos plume exhibit the steepest 3 He/4 He vs. μ 182 W isotope correlations and thus hold the potential to elucidate the processes involved in the preservation of isotope variability. Among the various eruptions suggested to be related to the Galapagos plume is Gorgona Island, which is geologically significant for their Phanerozoic komatiites. We report the first high precision 142 Nd/144 Nd isotope measurements for the Gorgona komatiites, as well as 3 He/4 He and major and trace element data for an array of lavas including komatiites, a picrite, a d-gabbro (“depleted”), and an e-basalt (“enriched”). The 142 Nd/144 Nd ratios of six komatiite samples are indistinguishable to within±2.4 ppm (2σ) from the modern-day terrestrial mantle as sampled by most mid-ocean ridge and ocean island basalts. The 3 He/4 He values determined for the komatiites and a picrite are from 4.6 to 45.1 R A. Eight of the 3 He/4 He measurements are higher than the MORB average (8±1 R A), and thus strongly point to a plume source sampling an undegassed reservoir. The R A value of 45.1 in particular ranks among the highest 3 He/4 He ratios measured to date at Gorgona Island and elsewhere, close to what is observed for Phanerozoic flood basalts from Baffin Island. Evaluation of the sources of the lavas using major and rare earth elements along with ε 143 Nd signatures suggest that the komatiites and picrites are derived from a depleted peridotite source. Meanwhile, the e-basalts are deduced to have been derived from a hybrid source consisting of roughly 8% pyroxenite to 92% peridotite. The inferred and calculated degree of pyroxenite in the sources of the different lava types align with the magnitude of 3 He/4 He ratios observed. Model calculations involving pyroxenite sources in the e-basalts, which are reported to exhibit 182 W/184 W isotope anomalies, attest to their survival, while analogous calculations with 142 Nd/144 Nd isotope ratios suggest homogenization occurring on a faster timescale. It is deduced that 182 W/184 W anomalies are likely to outlast mixing with recycled pyroxenite compared to 142 Nd/144 Nd and 3 He/4 He ratios. The decoupling of modern-day 142 Nd/144 Nd, 3 He/4 He, and 182 W/184 W isotope variabilities thus likely reflect the sensitivity each system has to different mantle processes that contribute to their preservation throughout time.
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DOI：
10.1073/pnas.2215903120
发表时间：
2023-01-24
影响因子：
11.1
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The Journal of geology
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2001
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--
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2010
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