Abstract Basalts erupted within intra-transform spreading centers can be valuable probes of geochemical components in Earth's upper mantle, and provide constraints on the proportions of mantle carbon that are juvenile (primordial) vs. tectonically recycled. Here we present new results for submarine basalt glasses, erupted within the Garrett Transform Fault (GTF) and to its north and south along the East Pacific Rise (EPR). Analyses of 3He/4He, and He and CO2 concentrations in vesicles and glass were performed, through a series of crushing and melting experiments plus FTIR spectroscopy. Trace elements were analyzed by laser ablation ICP-MS. The GTF basalts provide further tests for the origin of volatile-undersaturated basalts using CO2/Ba and CO2/Nb systematics. CO2 is highly correlated with Ba and Nb in basalts erupted in the transform domain (n = 13, including 5 undersaturated basalts) and does not show the variability expected from mixing between undegassed and variably degassed melts. Rather, the melts appear to originate from a heterogeneous mantle source that was variably depleted through partial melting, and limited mixing of melts is involved in their generation. The CO2/Ba and CO2/Nb weight ratios of 106 ± 8 and 308 ± 27, respectively, are similar to values determined previously for a global suite of undersaturated mid-ocean ridge basalts (MORBs). The ridge and transform domains have distinct 3He/4He ratios. Along the nearby EPR, 3He/4He = 8.5 - 9.1 RA, while within the Garrett Transform Fault 3He/4He = 9.2 - 10.1 RA. These two basalt populations are also distinct in their Pb-Sr-Nd isotope compositions based on earlier regional studies. The distinct populations result from partial melting of two different mantle source compositions. Melting of depleted mantle containing a small amount (∼1 to 5%) of enriched, ancient heterogeneities occurs beneath the EPR. Melting of ultradepleted mantle (in which the heterogeneities have been removed by earlier melting beneath the EPR) occurs beneath the GTF. This explains the distinction between intra-transform and spreading ridge domains for 3He/4He, if the heterogeneities were enriched in U, Th and He and had low 3He/4He as would be found in tectonically recycled material. The enriched mantle component sampled by the EPR basalts has molar CO2/3He = 2 × 10 9 , and it dominates the CO2/3He ratio generally ascribed to the upper mantle source for mid-ocean ridge basalts. In contrast, the ultradepleted MORB mantle component sampled by the GTF basalts has CO2/3He = 3 × 10 8 or less. This indicates that most of the carbon in Earth's upper mantle originates from tectonic recycling.

中文翻译：

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He-CO2-Ba 系统学在超耗竭洋脊玄武岩中揭示了地球上地幔中主要回收的碳

摘要 变形内扩张中心喷发的玄武岩可以作为地球上地幔地球化学成分的有价值的探测，并限制地幔碳的幼年（原始）与构造再循环的比例。在这里，我们展示了海底玄武岩玻璃的新结果，这些玻璃在加勒特转换断层 (GTF) 内以及沿东太平洋海隆 (EPR) 向北和向南喷发。通过一系列破碎和熔化实验以及 FTIR 光谱，对囊泡和玻璃中的 3He/4He 以及 He 和 CO2 浓度进行了分析。通过激光烧蚀 ICP-MS 分析痕量元素。GTF 玄武岩使用 CO2/Ba 和 CO2/Nb 系统学提供了对挥发性不饱和玄武岩来源的进一步测试。CO2 与转化域中喷发的玄武岩中的 Ba 和 Nb 高度相关（n = 13，包括 5 个不饱和玄武岩），并且未显示出未脱气和可变脱气熔体之间混合所预期的可变性。相反，熔体似乎起源于异质地幔源，该地幔源因部分熔融而不同程度地耗尽，并且熔体的有限混合参与了它们的产生。CO2/Ba 和 CO2/Nb 重量比分别为 106 ± 8 和 308 ± 27，与之前为全球欠饱和洋中脊玄武岩 (MORB) 确定的值相似。脊域和变换域具有不同的 3He/4He 比率。沿着附近的 EPR，3He/4He = 8.5 - 9.1 RA，而在 Garrett 变换断层内 3​​He/4He = 9.2 - 10.1 RA。根据早期的区域研究，这两个玄武岩群的 Pb-Sr-Nd 同位素组成也不同。不同的种群是由两种不同地幔源成分的部分熔融造成的。在 EPR 下方，包含少量（~1% 至 5%）富集的远古非均质性的贫化地幔发生熔融。超贫化地幔的熔化（其中 EPR 下方的早期熔化已消除了异质性）发生在 GTF 下方。这解释了 3He/4He 的转化内和扩展脊域之间的区别，如果异质性富含 U、Th 和 He 并且具有低的 3He/4He，就像在构造回收材料中发现的那样。EPR玄武岩采样的富集地幔组分的摩尔CO2/3He = 2 × 10 9 ，并且它主导着大洋中脊玄武岩通常归因于上地幔源的 CO2/3He 比值。相比之下，GTF 玄武岩采样的超耗尽 MORB 地幔组分的 CO2/3He = 3 × 10 8 或更少。这表明地球上地幔中的大部分碳来自构造循环。