Abstract The Auckland Volcanic Field (AVF) is one of the most intensively studied monogenetic basalt fields in the world yet its origin remains enigmatic. Magmatism in the AVF occurred from ~193 ka to 500 years bp. The trace element and isotopic diversity of AVF basalts require “small-scale” compositional heterogeneity in the underlying mantle that is comparable in volume to, or slightly larger than, the scale of individual eruptions in the AVF. Olivine from tephra and lava, representing the range of AVF compositions, was crushed and analysed for 3He/4He, and He and CO2 concentrations in an attempt to further characterize and explain the significance of the AVF “end-members”. AVF basalts show a negative covariation between the amount of CO2 released by crushing of olivine and the whole rock concentrations of highly incompatible trace elements, such as Ba, Rb, Nb, Zr, Ti and K. In contrast, the amount of He released by crushing shows no simple relations with the same incompatible elements or their ratios. This leads to a significant variation in CO2/3He ratios (9.4 × 107–3.5 × 109) that may relate to differences in magma ascent dynamics, as well as to different magma sources. The measured CO2/He ratios may have been influenced by varying amounts of CO2 diffusion into vapour bubbles within melt inclusions that depend on melt composition and magma ascent rate. However, petrographically there is no evidence for systematic differences in the size or quantity of vapour bubbles in olivine-hosted melt inclusions. 3He/4He ratios in 14 AVF samples studied here show a narrow range from 6.57 to 7.26 RA (mean of 7.10 ± 0.26). This may imply a dominance of the mantle helium budget by small-scale heterogeneities. Alternatively, the 3He/4He results suggest that the tectonic and magmatic history of the mantle beneath the AVF has effectively hybridized the 3He/4He ratio to a larger extent than for highly incompatible trace element ratios and Pb-Nd-Sr isotopes. The value of ~7 RA for the mantle source of AVF basalts, in light of other evidence, suggests that there is a relatively homogeneous He isotope composition for the Zealandia-Antarctic mantle domain.

中文翻译：

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奥克兰火山场异常均匀的氦同位素组成及其对下伏地幔的影响

摘要 奥克兰火山区 (AVF) 是世界上研究最深入的单生玄武岩区之一，但其起源仍然是个谜。AVF 中的岩浆活动发生在大约 193 ka 到 500 年 bp 之间。AVF 玄武岩的微量元素和同位素多样性需要下伏地幔中的“小规模”成分异质性，其体积与 AVF 中单个喷发的规模相当或略大。来自火山岩和熔岩的橄榄石代表 AVF 的成分范围，被压碎并分析 3He/4He、He 和 CO2 浓度，以试图进一步表征和解释 AVF“末端成员”的重要性。AVF 玄武岩显示橄榄石压碎释放的 CO2 量与高度不相容的微量元素的整个岩石浓度之间存在负协变，Ba、Rb、Nb、Zr、Ti 和 K。相比之下，破碎释放的 He 量与相同的不相容元素或它们的比例没有简单的关系。这导致 CO2/3He 比率（9.4 × 107–3.5 × 109）的显着变化，这可能与岩浆上升动力学的差异以及不同的岩浆来源有关。测得的 CO2/He 比值可能受到不同量的 CO2 扩散到熔体包裹体内的蒸汽泡的影响，这取决于熔体成分和岩浆上升速率。然而，在岩相学上，没有证据表明橄榄石熔体包裹体中蒸汽泡的大小或数量存在系统差异。此处研究的 14 个 AVF 样品中的 3He/4He 比值范围很窄，从 6.57 到 7.26 RA（平均值为 7.10 ± 0.26）。这可能意味着小尺度异质性在地幔氦预算中占主导地位。或者，3He/4He 结果表明，与高度不相容的微量元素比和 Pb-Nd-Sr 同位素相比，AVF 下方地幔的构造和岩浆历史已在更大程度上有效地混合了 3He/4He 比。根据其他证据，AVF 玄武岩地幔源的~7 RA 值表明西兰蒂亚-南极地幔域存在相对均匀的氦同位素组成。