Geochemistry of clinopyroxene megacrysts, xenocrysts, and phenocrysts in diabase dikes from ODP Hole 140-504B@en

We report the major, rare earth, and other trace element compositions of clinopyroxenes from two Leg 140, Hole 504B diabase dikes. These pyroxenes reflect a complex history of crystal growth and magma evolution. The large ranges of composition found reflect incorporation of exotic phenocrysts into the melt, the early formation of crystal clots before dike intrusion during an undercooling event, and in-situ fractionation of melt during and following dike emplacement. Some of the pyroxenes occur in coarse two- and three-phase glomerocrysts, which may be ôprotogabbrosö representing early stages of melt crystallization in the lower crust. Large variations in trace element composition are found. These likely reflect heterogeneous nucleation and growth of plagioclase and pyroxene in the melt, as well as complex interface kinetics that may affect partition coefficients during rapid crystal growth expected during undercooling. This can explain the formation of irregular chemical sector zoning in some equant anhedral phenocrysts. Undercooling of magmas in the lower crust most likely reflects input of fresh hot melt into a stagnating melt-storage zone. Dikes intruded upward from an inflated melt-storage zone during such a cycle are likely to be larger than those intruded from the storage zone between such cycles, when it would be deflated, consistent with the greater overall thickness of the phyric dikes in the Leg 140 section of Hole 504B.本数据集报道了大洋钻探计划（Ocean Drilling Program, ODP）第140航次504B钻孔两处辉绿岩脉（diabase dikes）中单斜辉石（clinopyroxenes）的主量元素、稀土元素及其他微量元素组成。这些单斜辉石记录了复杂的晶体生长与岩浆演化历程。观测到的宽幅组成分异，反映了熔体中捕获外来斑晶（exotic phenocrysts）、过冷却（undercooling）事件期间岩墙侵位前早期形成的晶簇，以及岩墙侵位过程中及侵位后熔体的原位分异（in-situ fractionation）作用。部分辉石以粗粒二相、三相聚晶（glomerocrysts）形式产出，这类聚晶或为原辉长岩（protogabbros），代表下地壳内熔体结晶的早期阶段。微量元素组成存在显著差异，这可能反映了熔体中斜长石与单斜辉石的非均匀成核与生长过程，以及过冷却条件下快速晶体生长过程中影响分配系数（partition coefficients）的复杂界面动力学机制。该机制可解释部分等轴他形斑晶（equant anhedral phenocrysts）中不规则化学环带构造（sector zoning）的形成。下地壳内岩浆的过冷却现象，大概率反映了新鲜热熔体注入停滞的岩浆储集层（melt-storage zone）中。在此类循环中，从膨胀的岩浆储集层向上侵位的岩墙，其规模大概率大于储集层在两次循环之间处于收缩状态时侵位的岩墙，这与504B钻孔第140航次剖面中斑晶岩墙（phyric dikes）整体厚度更大的观测结果一致。