Abstract The Doldrums transform system offsets the Equatorial Mid Atlantic Ridge by ~630 km at 7–8° N. This transform system consists of four intra-transform spreading centers (ITRs) bounded by five transform faults. The northernmost ITR is linked to the MAR axis by a ~ 180 km-long transform. Here, during two R/V A. N. Strakhov expeditions (S06 and S09), mantle peridotites were dredged along the transverse and median ridge of the transform, across the western flank of the ITR valley. Residual harzburgites were mainly sampled along the northern Doldrums transform valley, whereas plagioclase-bearing peridotites showing evidence for melt-rock interaction characterize the ITR domain. Petrological and geochemical observations reinforced by geochemical modelling are used to define the behaviour of trace elements during melt extraction and melt-rock reaction in our rocks. Results suggest that residual peridotites derive from mantle rocks that have undergone a degree of partial melting up to 12%, with melting likely starting at the transition of garnet-spinel stability fields, whereas peridotites which suffered melt-rock reactions have been divided into two types: (i) pl-impregnated peridotites, formed by migration of melts at high porosity and high melt-rock ratio; and (ii) refertilized peridotites, generated at reduced porosity, when small fractions of the same percolating melt crystallized clinopyroxene and minor plagioclase. We suggest that the refertilizing agent was a melt highly depleted in incompatible trace elements, in turn produced by an ultra-depleted mantle source. This mantle experienced previous degrees of melt extraction at the ridge axis, before being transposed laterally along the transform where it melted a second time during the opening of the intra-transform spreading segment.

中文翻译：

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地幔橄榄岩在一个大的内部转化区域（Doldrums 断裂带；7-8°N，大西洋中脊）中的超贫化熔体再肥

摘要 Doldrums 变换系统在 7–8° N 处将赤道大西洋中脊偏移约 630 公里。该变换系统由以五个变换断层为边界的四个变换内扩展中心 (ITR) 组成。最北端的 ITR 通过约 180 公里长的变换与 MAR 轴相连。在这里，在两次 R/V AN Strakhov 探险（S06 和 S09）期间，地幔橄榄岩沿着转换的横向和中脊，穿过 ITR 山谷的西侧被挖掘。残余的斜长岩主要沿北部的 Doldrums 转换谷取样，而含斜长石的橄榄岩显示出熔岩相互作用的证据，是 ITR 域的特征。通过地球化学模型加强的岩石学和地球化学观测用于定义我们岩石中熔体提取和熔岩反应过程中微量元素的行为。结果表明，残余橄榄岩来源于部分熔融程度高达 12% 的地幔岩石，熔融可能始于石榴石-尖晶石稳定场的转变，而遭受熔融-岩石反应的橄榄岩则分为两种类型: (i) 浸渍橄榄岩，由高孔隙率和高熔岩比的熔体迁移形成；(ii) 再肥橄榄岩，当相同渗透熔体的小部分结晶单斜辉石和少量斜长石时，在孔隙率降低的情况下生成。我们认为补肥剂是一种高度消耗不相容微量元素的熔体，反过来又由超贫化地幔源产生。这个地幔在脊轴上经历了先前程度的熔体提取，然后沿着变换横向转换，在变换内扩展段打开期间它第二次熔化。