Major element concentrations in spinel, olivine and pyroxene from peridotites of ODP Hole 210-1277 of the Newfoundland margin

Serpentinized spinel peridotites of the Newfoundland margin drilled during ODP Leg 210 at Site 1277 have preserved, relic mineral compositions similar to the most depleted abyssal peridotites worldwide and different from those of the conjugate Iberian margin. The samples are derived from mass flows containing clasts of peridotite and gabbro and from in-situ basement, and are mostly mylonitic cpx-poor spinel harzburgites with Cr-rich spinels (Cr#0.35-0.66). Melting of the Newfoundland mantle occurred in the spinel peridotite field and probably exceeded the cpx-out phase boundary for some samples. Using proposed spinel peridotite melting models and experimentally derived phase diagrams, the Newfoundland harzburgites can be modeled as a residue after extraction of 14 to 20-25% melting. Basalts that are interleaved with mass flow deposits on top of the peridotite basement resemble normal to transitional mid-ocean ridge basalt. This, together with the unusually high Cr# of some spinel harzburgites suggest that the formation of basalts and partial melting of the underlying peridotite are not cogenetic. Among mantle samples some of the Newfoundland harzburgites approach mineral compositions of the Bay of island ophiolite and ophiolites from Japan that represent peridotites formed in an arc-setting. Thus, the peridotites drilled at Site 1277 may represent inherited (Caledonian or older) subarc mantle that was exhumed close to the ocean floor during the rifting evolution of the Atlantic. Compared to the spinel harzburgites from Newfoundland, the peridotites from the conjugate Iberian margin are, on average, less depleted and provide evidence for local equilibration in the plagioclase stability field. This can either be explained by an inherited, primary, Ca-richer composition of the Iberia peridotite, or, alternatively, by local melt impregnation and stagnation during continental rifting, and thus refertilizing previously depleted (arc-related) peridotite.

在大洋钻探计划（Ocean Drilling Program, ODP）第210航次于1277站位钻取的纽芬兰陆缘蛇纹石化尖晶石橄榄岩，保留了与全球范围内亏损程度最高的深海橄榄岩相似的残余矿物组成，且与共轭伊比利亚陆缘的橄榄岩组分截然不同。该类样品源自含橄榄岩与辉长岩岩块的块体流及原地基底，主体为糜棱岩化、贫单斜辉石（clinopyroxene, cpx）的尖晶石方辉橄榄岩，伴生铬值介于0.35至0.66之间的富铬尖晶石。纽芬兰地幔的熔融作用发生于尖晶石橄榄岩稳定域内，且部分样品的熔融程度可能已突破单斜辉石消失的相边界。借助已提出的尖晶石橄榄岩熔融模型与实验构建的相图，可将纽芬兰方辉橄榄岩建模为经14%至20%~25%部分熔融后的残余体。赋存于橄榄岩基底之上、与块体流沉积互层的玄武岩，其特征类似正常至过渡型洋中脊玄武岩（mid-ocean ridge basalt, MORB）。这一特征结合部分尖晶石方辉橄榄岩异常高的铬值，表明玄武岩的形成与下伏橄榄岩的部分熔融并非同源成因。在地幔样品中，部分纽芬兰方辉橄榄岩的矿物组成接近岛湾蛇绿岩与日本蛇绿岩的组分，而后者代表了弧环境中形成的橄榄岩。因此，1277站位钻取的橄榄岩，可能代表了继承自加里东期或更古老时期的弧下地幔，这些地幔在大西洋张裂演化过程中被剥露至近海底位置。相较于纽芬兰的尖晶石方辉橄榄岩，共轭伊比利亚陆缘的橄榄岩平均亏损程度更低，且记录了斜长石稳定域内的局部平衡作用。该现象可通过两种机制解释：一是伊比利亚橄榄岩原本就具有更富钙的原生继承组分；二是在大陆张裂过程中发生了局部熔体浸渍与熔体停滞，从而使此前亏损的（与弧相关的）橄榄岩发生了交代富集。