Element concentrations of plagioclase and clinopyroxene in poikilitic olivine gabbros from ODP Hole 153-923A (Table 4)

Ocean Drilling Program Hole 923A, located on the western flank of the Mid-Atlantic Ridge south of the Kane Fracture Zone, recovered primitive gabbros that have mineral trace element compositions inconsistent with growth from a single parental melt. Plagioclase crystals commonly show embayed anorthitic cores overgrown by more albitic rims. Ion probe analyses of plagioclase cores and rims show consistent differences in trace element ratios, indicating variation in the trace element characteristics of their respective parental melts. This requires the existence of at least two distinct melt compositions within the crust during the generation of these gabbros. Melt compositions calculated to be parental to plagioclase cores are depleted in light rare earth elements, but enriched in yttrium, compared to basalts from this region of the Mid-Atlantic Ridge, which are normal mid-ocean ridge basalt (N-MORB). Clinopyroxene trace element compositions are similar to those predicted to be in equilibrium with N-MORB. However, primitive clinopyroxene crystals are much more magnesian than those produced in one-atmosphere experiments on N-MORB, suggesting that the major element composition of the melt was unlike N-MORB. These data require that the diverse array of melt compositions generated within the mantle beneath mid-ocean ridges are not always fully homogenised during melt extraction from the mantle and that the final stage of mixing can occur efficiently within crustal magma chambers. This has implications for the process of melt extraction from the mantle and the liquid line of descent of MORB位于凯恩断裂带以南大西洋中脊西翼的大洋钻探计划（Ocean Drilling Program, ODP）923A钻孔，采获了原始辉长岩，其矿物微量元素组成与单一母岩浆结晶生长的成因不符。斜长石晶体普遍表现为：被溶蚀的钙长石核心被更长石质环边增生覆盖。对斜长石核心与环边的离子探针分析显示，二者微量元素比值存在稳定差异，表明其各自母岩浆的微量元素特征存在变化。这意味着在该类辉长岩的形成过程中，地壳内至少存在两种不同的岩浆组分。 相较于该大西洋中脊区域产出的正常洋中脊玄武岩（normal mid-ocean ridge basalt, N-MORB），计算得到的斜长石核心母岩浆组分具有轻稀土元素亏损、钇富集的特征。单斜辉石的微量元素组成与N-MORB平衡结晶的理论预测值相近。然而，原始单斜辉石晶体的镁含量远高于基于N-MORB的常压实验所得产物，暗示该熔体的主量元素组分并非N-MORB型。 上述数据表明，洋中脊下方地幔中形成的多样岩浆组分，在从地幔萃取过程中未必总能完全均一化，且最终混合阶段可在地壳岩浆房内高效完成。这一结论对洋中脊的地幔熔体萃取过程以及洋中脊玄武岩的液相演化线具有重要启示。