The Atlantis Bank Oceanic Batholith is a 660 km2 gabbro massif representing the plutonic foundation of a major ridge magmatic center. It was continuously accreted, emplaced, and exposed in the rift mountains of the paleo-SW Indian Ridge from 13 to 10·3 Ma. Ocean Drilling Program (ODP) Hole 735B, drilled to 1508 m at Atlantis Bank, recovered evolved intercalated olivine and oxide gabbros representing the upper levels of the lower ocean crust. Within this section, ∼5·6 m of primitive chromian-spinel-bearing troctolites (0·1–2 m thick), with sharp modal contacts with the host gabbros, were cored between 410 and 500 m depth. Here we present new mineral (chromian spinel, clinopyroxene, olivine, and plagioclase) major and trace element and petrographic data for the troctolite suites (i.e. troctolites, clinopyroxene-rich troctolites, troctolitic gabbros) from 410 to 528 mbsf (meters below seafloor) and olivine gabbros in the 0–274 mbsf and 410–500 mbsf intervals, and examine the origin of these troctolite layers. Equilibrium melts for the troctolites are primitive to moderately evolved, with Mg# [= 100Mg/(Mg + Fe), 48–68 mol%] comparable with those of Atlantis Bank basalts. By contrast, equilibrium melts for both the 0–274 mbsf and 410–500 mbsf host olivine gabbros are highly to moderately evolved (Mg# 27–63 mol%). The 410–500 mbsf troctolite suites maintain high clinopyroxene Mg# (81–89 mol%) and mineral concentrations of compatible elements (i.e. clinopyroxene Cr2O3 0·1–1·2 wt%, Ni 151–330 μg g–1 and olivine Ni 1055–1559 μg g–1) with the increase in incompatible trace element abundances (i.e. clinopyroxene TiO2 0·3–2·1 wt%, Zr 5·8–112 μg g–1 and olivine Ti up to 293 μg g–1). Combined with abundant dissolution–reprecipitation textures, our results indicate that the troctolites were formed by the reaction between a spinel-bearing, troctolitic mush, from which they inherited the high Mg#, Ni and Cr, and percolating melts adding incompatible trace elements. Moreover, the spinel (NiO versus TiO2) and olivine (Ti versus Y) trace element compositions indicate that the troctolites were affected by low-degree Fe–Ti-rich melt metasomatism. In contrast, both the 0–274 mbsf and 410–500 mbsf olivine gabbros display a prominent decrease in clinopyroxene Mg# (from 88 to 66) and mineral compatible element concentrations (i.e. clinopyroxene Cr2O3 from 1·1 to ∼0 wt%, Ni from 208 to 34 μg g–1 and olivine Ni from 1200 to 136 μg g–1) with increasing incompatible trace element abundances (e.g. clinopyroxene Zr from 4·8 to 157 μg g–1). These features are compatible with the reactive porous flow of slightly to highly evolved melts through the cooling crystal mush zone. Our results, combined with the literature data, indicate that most olivine gabbros between 410 and 500 mbsf were formed prior to the troctolite layers. We document that the troctolites represent conduits for mid-ocean ridge basalt melt transport through the lower oceanic crust, whereas the olivine gabbros represent crystallization of a large crystal mush, recording initial gabbro emplacement, hyper- and sub-solidus deformation, and melt–rock reaction owing to upward penetrative flow of intercumulus melt.

中文翻译：

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MORB 熔体运输通过亚特兰蒂斯银行海洋岩基（西南印度洋脊）

亚特兰蒂斯银行海洋岩基是一个 660 平方公里的辉长岩地块，代表了一个主要山脊岩浆中心的深成基础。13～10·3 Ma在古西南印度洋海脊裂谷山脉中不断增生、侵位和暴露。海洋钻探计划 (ODP) 钻孔 735B 在亚特兰蒂斯河岸钻探至 1508 m，回收了代表下洋壳上层的演化的夹层橄榄石和氧化物辉长岩。在该剖面内，在 410 到 500 米深度之间取芯了约 5·6 米的原始含铬尖晶石（0·1-2 米厚），与宿主辉长岩有尖锐的模态接触。在这里，我们展示了新矿物（铬质尖晶石、单斜辉石、橄榄石和斜长石）的主要元素和微量元素以及岩石学数据，用于托克托石组（即托克托石、富含单斜辉石的托克托石、410 到 528 mbsf（海底以下米）的 troctolitic gabbros）和 0-274 mbsf 和 410-500 mbsf 间隔的橄榄石辉长岩，并检查这些 troctolite 层的起源。troctolites 的平衡熔体是原始到适度演化的，Mg# [= 100Mg/(Mg + Fe), 48-68 mol%] 与亚特兰蒂斯银行玄武岩相当。相比之下，0-274 mbsf 和 410-500 mbsf 主橄榄石辉长岩的平衡熔体是高度到中度演化的（Mg# 27-63 mol%）。410–500 mbsf troctolite 套件保持高单斜辉石 Mg# (81–89 mol%) 和相容元素的矿物浓度（即单斜辉石 Cr2O3 0·1–1·2 wt%、Ni 151–330 μg g–1 和橄榄石 Ni 1055-1559 μg g-1）随着不相容的微量元素丰度的增加（即单斜辉石 TiO2 0·3-2·1 wt%、Zr 5·8-112 μg g-1 和橄榄石 Ti 高达 293 μg g-1 ）。结合丰富的溶解-再沉淀织构，我们的研究结果表明，托托石是由含有尖晶石的托托石糊状物（它们继承了高 Mg#、Ni 和 Cr）与添加不相容微量元素的渗流熔体之间的反应形成的。此外，尖晶石（NiO 与 TiO2）和橄榄石（Ti 与 Y）微量元素组成表明，troctolites 受到低度富 Fe-Ti 熔体交代作用的影响。相比之下，0-274 mbsf 和 410-500 mbsf 橄榄石辉长岩都显示出单斜辉石 Mg#（从 88 到 66）和矿物相容元素浓度（即单斜辉石 Cr2O3 从 1·1 到 ∼0 wt%、Ni从 208 到 34 μg g-1 和橄榄石 Ni 从 1200 到 136 μg g-1），不相容的微量元素丰度增加（例如单斜辉石 Zr 从 4·8 到 157 μg g-1）。这些特征与轻微到高度演化的熔体通过冷却结晶糊区的反应性多孔流动相兼容。我们的结果与文献数据相结合，表明大多数介于 410 和 500 mbsf 之间的橄榄石辉长岩是在 troctolite 层之前形成的。我们记录了 troctolites 代表洋中脊玄武岩熔体通过下洋壳输送的管道，而橄榄石辉长岩代表大晶体糊状物的结晶，记录初始辉长岩侵位、超固相线和亚固相线变形以及熔岩由于积云熔体向上渗透流​​动而发生反应。表明大多数介于 410 和 500 mbsf 之间的橄榄石辉长岩是在 troctolite 层之前形成的。我们记录了 troctolites 代表洋中脊玄武岩熔体通过下洋壳输送的管道，而橄榄石辉长岩代表大晶体糊状物的结晶，记录初始辉长岩侵位、超固相线和亚固相线变形以及熔岩由于积云熔体向上渗透流​​动而发生反应。表明大多数介于 410 和 500 mbsf 之间的橄榄石辉长岩是在 troctolite 层之前形成的。我们记录了 troctolites 代表洋中脊玄武岩熔体通过下洋壳输送的管道，而橄榄石辉长岩代表大晶体糊状物的结晶，记录初始辉长岩侵位、超固相线和亚固相线变形以及熔岩由于积云熔体向上渗透流​​动而发生反应。