Geochemistry of minerals in medium-grained diabase from ODP Holes 137-504B and 140-504B

Samples recovered from Hole 504B during Leg 140 include a number of medium-grained, holocrystalline diabases that appear to represent the cores of thick dikes. The plagioclase and pyroxene in these samples occur in a variety of crystal morphologies. Plagioclase occurs as phenocrysts, microphenocrysts, elongate crystals, skeletal crystals, and branching radial clusters. Pyroxene occurs as phenocrysts, microphenocrysts, ophitic crystals, and poikilitic crystals. Plagioclase compositions became progressively poorer in anorthite and MgO and progressively richer in FeO as crystallization proceeded, while the average grain volume decreased and the aspect ratio of individual grains increased. Pyroxene compositions are largely independent of crystal morphology.
The diabase dikes recovered from Hole 504B during Leg 140 appear to have crystallized in situ. Crystal compositions and morphologies are consistent with a rapid cooling rate and solidification times for individual dikes on the order of hours or days. The crystallization rate and nucleation rate of plagioclase lagged behind the cooling rate so that the degree of undercooling progressively increased as crystallization proceeded. Plagioclase crystal morphologies indicate much greater degrees of supersaturation than do pyroxene or olivine crystal morphologies. The 504B diabase magmas appear to have been emplaced with abundant preexisting pyroxene and olivine nuclei, but with few preexisting plagioclase nuclei. The suppression of plagioclase nucleation and crystallization relative to that of pyroxene and olivine could provide a mechanism by which the actual fractionation assemblage is more pyroxene-rich and plagioclase-poor than that predicted from thermodynamic models, or that observed in isothermal crystallization experiments.

在第140航次（Leg 140）期间从504B钻孔（Hole 504B）获取的样品中，包含多块中粒全晶质辉绿岩，其大概率代表了厚大岩脉的岩心。这些样品中的斜长石（plagioclase）与辉石（pyroxene）呈现出多样的晶体形貌：斜长石以斑晶（phenocrysts）、微晶斑晶（microphenocrysts）、长条形晶体、骸晶（skeletal crystals）以及放射状枝状簇晶（branching radial clusters）的形式产出；辉石则以斑晶、微晶斑晶、填隙晶（ophitic crystals）与嵌晶（poikilitic crystals）的形式产出。随着结晶作用持续推进，斜长石的成分中钙长石（anorthite）与氧化镁（MgO）含量逐渐降低，氧化亚铁（FeO）含量逐步升高；与此同时，平均晶粒体积持续减小，单颗粒晶粒的长径比不断增大。辉石的成分基本不受晶体形貌的影响。
本次从第140航次504B钻孔采获的辉绿岩岩脉，似乎是在原位（in situ）结晶形成的。晶体成分与形貌特征均与单条岩脉数小时至数天量级的快速冷却速率和凝固时间（solidification times）相契合。斜长石的结晶速率与成核速率滞后于冷却速率，使得结晶过程中过冷度（undercooling）持续升高。相较于辉石或橄榄石（olivine）的晶体形貌，斜长石的晶体形貌反映出更高的过饱和度（supersaturation）。504B钻孔的辉绿岩岩浆似乎在就位时携带了大量预先形成的辉石与橄榄石晶核，但预先形成的斜长石晶核极少。相较于辉石与橄榄石，斜长石的成核与结晶作用受到抑制，这一机制或可解释：为何实际的岩浆分异组合（fractionation assemblage）相较于热力学模型（thermodynamic models）的预测结果，或是等温结晶实验（isothermal crystallization experiments）中观测到的结果，更富辉石而贫斜长石。