Geochemistry of highly depleted peridotites of the Mid-Atlantic Ridge@en

During ODP Leg 209, a magma-starved area of the Mid-Atlantic Ridge (MAR) was drilled in the vicinity of the Fifteen-Twenty Fracture Zone (FZ) that offsets one of the slowest portions of the spreading ridge. We present here the results of a bulk rock multi-elemental study of 27 peridotites drilled at Sites 1272 and 1274 (to the south and the north of the FZ, respectively). The peridotites comprise mainly of harzburgites with minor dunites. Clinopyroxene (Cpx), which is interstitial and interpreted as secondary, is observed in Site 1274 peridotites. Sites 1272 and 1274 peridotites have low Al2O3 contents (<1 anhydrous wt.%), high Mg# (>91.5), and bulk rock trace elements compositions mostly below 0.1X primitive mantle (PM). These peridotites, and in particular Site 1272 peridotites, represent the most depleted peridotites yet sampled at a slow spreading ridge. Their compositions indicate high degrees of partial melting and melt extraction. A single open-system melting event (melting plus percolation of melts produced within upwelling mantle) can explain their highly depleted yet linear chondrite-normalized REE patterns, characterized by a steady depletion from HREE to LREE. Late melt-rock reactions and precipitation of Cpx explains the slightly less depleted compositions of Site 1274 peridotites. Hence, the differences in composition between Sites 1272 and 1274 peridotites do not provide evidence for regional variations in the degrees of partial melting from the south to the north of the FZ. The occurrence of highly refractory peridotites in the Fifteen-Twenty area suggests we sampled a more actively convecting mantle than generally supposed below slow spreading centers.在大洋钻探计划（Ocean Drilling Program, ODP）第209航次期间，研究团队于大西洋中脊（Mid-Atlantic Ridge, MAR）的岩浆匮乏区域开展钻探作业，作业点位位于十五-二十断裂带（Fifteen-Twenty Fracture Zone, FZ）附近；该断裂带错断了扩张脊中扩张速率最慢的区段之一。本文报道了分别采自该断裂带南侧1272站位与北侧1274站位的27块橄榄岩的全岩多元素分析结果。 本次研究的橄榄岩以方辉橄榄岩为主，伴生少量纯橄榄岩。1274站位橄榄岩中可见赋存于粒间的单斜辉石（Clinopyroxene, Cpx），该单斜辉石被判定为次生矿物。 1272与1274站位的橄榄岩均具有较低的Al₂O₃含量（<1 无水重量百分比）、较高的镁指数（Mg# >91.5），且全岩微量元素组成大多低于原始地幔（primitive mantle, PM）基准值的0.1倍。 此类橄榄岩（尤以1272站位的样品为代表）是慢速扩张脊环境中迄今采获的最亏损橄榄岩。其地球化学特征指示其经历了高程度的部分熔融与熔体抽取过程。单一开放体系熔融事件——即熔融作用与上升地幔中形成的熔体的渗滤作用相结合——即可解释其高度亏损却呈线性分布的球粒陨石标准化稀土元素配分模式，该模式以从重稀土元素到轻稀土元素的持续亏损为典型特征。 晚期熔体-岩石反应以及单斜辉石的沉淀作用，可合理解释1274站位橄榄岩相对较弱的亏损程度。因此，1272与1274站位橄榄岩的地球化学组分差异，并不支持该断裂带南北两侧的部分熔融程度存在区域变化。 十五-二十区域内高难熔橄榄岩的产出，表明本次采样获取的地幔比慢速扩张脊下方普遍认知的地幔对流活动更为活跃。