Cores recovered by International Ocean Discovery Program Expedition 345 to the Hess Deep Rift (HDR) include lower crustal cumulates from the East Pacific Rise (EPR) and primitive basalts from the Cocos–Nazca Rift (CNR). This study presents major and trace element compositions of channel samples—the continuous strips of rock removed during core splitting—from this expedition. Consistently high Eu/Eu* anomalies (1·37–5·22) and strong correlations among major element oxides in samples of cumulates indicate that rock composition at the meter scale is controlled by the accumulation and segregation of plagioclase and olivine. However, constant Mg# (82·22 ± 0·66) among 13 samples through a ∼50 m interval suggests that this cumulus was host to percolating, replenishing melt(s). Modeling finds this rock composition to be in equilibrium with melts having Mg# = 58–61. This is identical to the mean value of EPR lavas (57·7 ± 6·2) and suggests that melt buffering by permeable crystal mush is a common and important process in controlling mid-ocean ridge basalt compositions at fast-spreading ocean centers. Analyses of the cumulates provide the most comprehensive composition of in situ, fast-spreading lower oceanic crust currently available. These are compiled with analyses of gabbros, dikes, and lavas from across the HDR to calculate the bulk composition of fast-spreading oceanic crust produced at the equatorial EPR. This bulk composition is strikingly similar to the composition of the primitive basalts from the CNR, and these compositions have nearly identical modeled fractional crystallization histories. Lower abundances of incompatible elements in the primitive basalt suggest that CNR magmatism is the result of the resumption of decompression melting in mantle that previously produced EPR crust. However, higher abundances of chalcophile elements in the CNR basalt point to a diversity of mantle melts that is not evident in calculations of the composition of bulk oceanic crust.

中文翻译：

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快速扩张的海洋地壳的整体组成：来自东太平洋隆起下部堆积层和赫斯深科科斯-纳斯卡裂谷玄武岩的见解

国际海洋探索计划 345 远征海斯深裂谷 (HDR) 回收的岩心包括来自东太平洋隆起 (EPR) 的下地壳堆积岩和来自科科斯-纳斯卡裂谷 (CNR) 的原始玄武岩。这项研究展示了这次考察中通道样品的主要和微量元素组成——在岩心分裂过程中移除的连续岩石条带。堆积物样品中持续较高的 Eu/Eu* 异常（1·37-5·22）和主要元素氧化物之间的强相关性表明，米级的岩石成分受斜长石和橄榄石的堆积和分离控制。然而，13 个样品中约 50 m 间隔的恒定 Mg# (82·22 ± 0·66) 表明该积云是渗透、补充熔体的宿主。建模发现这种岩石成分与 Mg# = 58–61 的熔体处于平衡状态。这与 EPR 熔岩的平均值 (57·7 ± 6·2) 相同，表明通过可渗透的结晶糊进行熔体缓冲是控制快速扩张的海洋中心洋中脊玄武岩成分的常见且重要的过程。对堆积物的分析提供了目前可用的最全面的原地、快速扩散的下洋壳组成。这些是通过对整个 HDR 的辉长岩、堤坝和熔岩的分析进行汇总的，以计算在赤道 EPR 产生的快速扩散的海洋地壳的整体组成。这种块状成分与来自 CNR 的原始玄武岩的成分惊人地相似，并且这些成分具有几乎相同的模拟分级结晶历史。原始玄武岩中不相容元素的较低丰度表明，CNR岩浆作用是先前产生EPR地壳的地幔减压熔融恢复的结果。然而，CNR 玄武岩中较高丰度的嗜热元素表明地幔熔体的多样性在大洋地壳组成的计算中并不明显。