Lavas that have erupted at near‐axis seamounts provide windows into mid‐ocean ridge mantle heterogeneity and melting systematics which are not easily observed on‐axis at fast‐spreading centers. Beneath ridges, most heterogeneity is obscured as magmas aggregate toward the ridge, where they efficiently mix and homogenize during transit and within shallow magma chambers prior to eruption. To understand the deeper magmatic processes contributing to oceanic crustal formation, we examine the compositions of lavas erupted along a chain of near‐axis seamounts and volcanic ridges perpendicular to the East Pacific Rise. We assess the chemistry of near‐ridge mantle using a ∼200 km‐long chain at ∼8°20′N. High‐resolution bathymetric maps are used with geochemical analyses of ∼300 basalts to evaluate the petrogenesis of lavas and the heterogeneity of mantle feeding these near‐axis eruptions. Major and trace element concentrations and radiogenic isotope ratios are highly variable on <1 km scales, and reveal a continuum of depleted, normal, and enriched basalts spanning the full range of ridge and seamount compositions in the northeast Pacific. There is no systematic compositional variability along the chain. Modeling suggests that depleted mid‐ocean ridge basalt (DMORB) lavas are produced by ∼5%–15% melting of a depleted mid‐ocean ridge (MOR) mantle. Normal mid‐ocean ridge basalts (NMORB) form from 5% to 15% melting of a slightly enriched MOR mantle. Enriched mid‐ocean ridge basalts (EMORB) range from <1% melting of 10% enriched mantle to >15% melting of 100% enriched mantle. The presence of all three lava types along the seamount chain, and on a single seamount closest to the ridge axis, confirms that the sub‐ridge mantle is much more heterogeneous than is commonly observed on‐axis and heterogeneity exists over small spatial scales.

中文翻译：

---

从8°20′N近轴海山链揭示了拉瓦斯中洋中脊地幔的极端非均质性

在近轴海山喷发的拉瓦斯提供了进入大洋中脊地幔非均质性和融化系统的窗口，而在快速扩展的中心很难在轴上观测到。在山脊下面，由于岩浆聚集到山脊上，大部分异质性都被掩盖了，它们在穿越过程中以及在喷发之前在浅岩浆室内有效混合并均匀化。为了了解导致海洋地壳形成的更深的岩浆过程，我们研究了沿与东太平洋上升垂直的近轴海山和火山山脊链喷发的熔岩的成分。我们在〜8°20′N时使用〜200 km长的链评估近垄地幔的化学性质。高分辨率测深图与约300个玄武岩的地球化学分析一起用于评估熔岩的成岩作用以及这些近轴喷发的地幔非均质性。主要和微量元素的浓度以及放射性同位素比值在<1 km的范围内变化很大，并且揭示了东北太平洋范围内整个山脊和海山组成的枯竭，正常和富集的玄武岩的连续体。沿链没有系统的组成变异性。模拟表明，枯竭的中脊玄武岩（DMORB）熔岩是由枯竭的中脊（MOR）地幔融化约5％-15％产生的。正常的中洋脊玄武岩（NMORB）形成程度略微丰富的MOR地幔的5％至15％融化。富集的中洋脊玄武岩（EMORB）范围从<1％的10％富集地幔融化到> 15％融化的100％富集地幔。沿着海山链以及在最靠近山脊轴线的单个海山上所有三种熔岩类型的存在，证实了亚山脊地幔的异质性比通常在轴向上观察到的要多得多，并且在小空间尺度上存在着异质性。