(Table 1) Geochemistry of sector-zoned crystal at DSDP Hole 45-395A

Variations in crystal morphologies in pillow basalts and probable sheet flows sampled from the region of the East Pacific Rise drilled during Leg 54 are related both to differences in composition and to an extreme range of cooling rate experienced upon extrusion. The basalts range in composition from olivine-rich tholeiites to tholeiitic ferrobasalts, and include some more alkaline basalts. The kinetics of crystal growth in some samples appears to have been influenced by the amount of initial superheating (or supercooling) of the magma, or possibly by differential retention of volatiles. Olivine in quartznormative ferrobasalts apparently formed metastably at high undercooling. Despite these effects, reliable petrographic criteria are established to distinguish the principal rock types described regardless of the crystallinity and grain size. Microphenocrysts formed prior to pillow formation correspond closely to mineral assemblages inferred from normative plots and variation diagrams to control crystal fractionation at various stages. The details of spherulitic and dendritic growth also provide some clues about composition. Petrographic evidence for magma mixing is scant. Only some Siqueiros fracture zone basalts contain zoned plagioclase phenocrysts with glass inclusions similar to those used to infer mixing among Mid-Atlantic Ridge basalts. All basalts from the summit and flanks of the East Pacific Rise are aphyric. One possible petrographic consequence of mixing between olivine tholeiites and ferrobasalts - formation of clinopyroxene phenocrysts - is not evident in any fracture zone or Rise crest basalt. Highly evolved ferrobasalts with liquidus low-Ca clinopyroxene have not been sampled, nor does textural evidence indicate that any basalts sampled are hybrid compositions between such magmas and less fractionated compositions. Evidently the sampled ferrobasalts are close to the most evolved compositions that occur in any abundance on this portion of the East Pacific Rise.本研究针对东太平洋海隆（East Pacific Rise）第54航次钻探所采集的枕状玄武岩（pillow basalts）及疑似席状岩流（sheet flows）中的晶体形态变异特征开展系统分析，其变异程度既与岩石成分差异相关，也受喷出后冷却速率的极端变化影响。所采集的玄武岩成分跨度宽泛，从富橄榄石拉斑玄武岩（olivine-rich tholeiites）延伸至拉斑质铁玄武岩（tholeiitic ferrobasalts），且包含部分碱性玄武岩（alkaline basalts）。部分样品的晶体生长动力学特征，似乎受岩浆初始过热度（或过冷度）（initial superheating (or supercooling)）影响，也可能与挥发分（volatiles）的差异留存有关。石英标准型铁玄武岩（quartznormative ferrobasalts）中的橄榄石，显然是在高过冷条件下亚稳形成的。 尽管存在上述影响因素，但已建立可靠的岩石学判别标准，可无需考量结晶度与晶粒尺寸，对本文所述的主要岩石类型进行有效区分。枕状构造形成前生成的微斑晶（microphenocrysts），与通过标准矿物图（normative plots）及变异图解（variation diagrams）推断的、用于表征各阶段晶体分异作用（crystal fractionation）的矿物组合高度吻合。球粒状与树枝状生长的细节特征，也可为岩石成分提供一定参考线索。 岩浆混合（magma mixing）的岩石学证据相对匮乏。仅部分西基罗斯断裂带（Siqueiros fracture zone）的玄武岩含有环带斜长石斑晶（zoned plagioclase phenocrysts），其内部的玻璃包裹体（glass inclusions）与大西洋中脊（Mid-Atlantic Ridge）玄武岩岩浆混合成因研究所用的包裹体特征相似。东太平洋海隆峰顶与侧翼的所有玄武岩均为无斑晶（aphyric）结构。橄榄石拉斑玄武岩与铁玄武岩混合可能形成的单斜辉石斑晶（clinopyroxene phenocrysts），在所有断裂带及海隆峰顶玄武岩中均未观测到。迄今未采集到含有液相线低钙单斜辉石（liquidus low-Ca clinopyroxene）的高度分异铁玄武岩，且结构学证据也未表明所采集的玄武岩存在此类岩浆与分异程度较低岩浆之间的混合成分。可见本次采集的铁玄武岩，已接近该段东太平洋海隆区域内广泛产出的最高度分异岩浆组分。