Evaluation of oceanic crust evolution based on melting experiments on gabbro

从大洋钻探计划孔1256D中发现的从喷出的熔岩到席状岩脉和最上面的辉长岩的连续切片，提供了有关超高速扩张山脊下的岩浆管道系统的重要信息。熔融实验表明，成分类似于浅层熔融透镜中较原始辉长岩之一的岩浆的分离结晶模型合理地解释了许多熔岩和岩脉的矿物和全岩成分。大约一半的岩墙样品比喷出的熔岩样品具有更多的富铁成分。富铁岩脉的岩浆密度略高于熔岩的岩浆密度，表明这些岩浆不会到达地表。喷出熔岩堆中的Mg#(100X mg/(Fe+mg)摩尔)具有较大的旋回变化。在每个旋回中，Mg#随着深度的增加而增加，这表明熔融透镜间歇性岩浆注入导致了连续的分馏事件。沙茨基隆起由厚(22公里)的玄武岩地壳组成，具有不同的地球化学成分。地球化学资料表明，青藏高原存在四种岩浆类型：正常型、低钛型、高Nb型和U1349型。在年轻的奥里地块上，正常型玄武岩占最古老的建筑物(塔木地块)熔岩单元的~94%，非正常型(低钛、高Nb和U1349型)玄武岩的熔岩单元占~57%，这意味着随着时间的推移，地球化学成分可能已经变得不均匀。

A continuous section from extrusive lavas, through sheeted dikes, and uppermost gabbros recovered from Ocean Drilling Program Hole 1256D provides important information regarding magma plumbing systems beneath superfast spreading ridges. Melting experiments demonstrate that a model of fractional crystallization from a magma of composition similar to one of the more primitive gabbros in a shallow melt lens reasonably explains mineral and whole-rock compositions of many lavas and dikes. About half of the dike samples have more evolved Fe-rich compositions than the extrusive lava samples. Magma densities of the Fe-rich dikes are a little higher than those of lavas, suggesting that these dike magmas would not reach the surface. Major cyclic variations of Mg#(100 X Mg/(Fe+Mg) in mole) occur within the extrusive lava pile. In each cycle, Mg# increases with depth, suggesting successive episodes of fractionation caused by intermittent magma injection to the melt lens. Shatsky Rise consists of thick(> 22 km) basaltic crust with various geochemical compositions. Geochemistry data indicate that four magma types exist on the plateau ; namely normal, low-Ti, high-Nb, and U1349 types. The normal type basalts constitute～94% lava units of the oldest edifice(Tamu Massif) and non-normal type(the low-Ti, high-Nb and U1349 types) basalts are～57% on the younger Ori Massif, implying that geochemical compositions may have become heterogeneous with time.