Abstract Basaltic magma chambers are best exemplified by layered intrusions – fossilized natural laboratories that historically constrain most fundamental principles of igneous petrology. Progressive fractional crystallization of basaltic melts in layered intrusions results in successive appearance of 5-10 liquidus minerals. Most minerals appear in the stratigraphy of layered intrusions via two distinct stages. They first emerge as ‘oikocrysts’ – large, irregularly-shaped and interstitial crystals that form in a crystal-liquid framework and then as ‘primocrysts’ – relatively small, idiomorphic and cumulus crystals that crystallize from a main magma body. The transition from oikocrysts to primocrysts is commonly marked by the following textural and chemical features: (a) a similarity in chemical composition of the last-forming oikocrysts and the first-forming primocrysts, (b) slow arrival, modal overabundance and finer-than-normal size of the first primocrysts, (c) a low abundance of trapped liquid in rocks with the first-forming primocrysts, (d) a step-like increase in mineral dihedral angle in rocks slightly below the level of the primocryst arrival, and (e) the rims on plagioclase primocrysts that are identical in composition to cores of plagioclase primocrysts at the first appearance of new liquidus phases higher up in the section. The phase equilibria control on the cumulus stratigraphy of layered intrusions require large, long-lived and largely molten magma chambers and, therefore, their origin is not compatible with several recent concepts that deny the existence of such reservoirs in the Earth’s crust. We therefore suggest that the systematic changes associated with the arrival of cumulus phases in layered intrusions are best explained by a traditional concept that considers layered intrusions as large initially crystal-free melt bodies that gradually lose heat and crystallize from margins inwards through thin solidification fronts. The entire magma body during its internal evolution is thought to be kinetically supercooled (i.e. kept slightly below its liquidus temperature) with respect to cumulus phases that crystallize at the margins. Each new cumulus phase arrives on the liquidus with some kinetic delay and nucleates heterogeneously against pre-existing crystals at the floor, roof and sidewalls of basaltic magma chambers. This implies that internal differentiation of basaltic magma chambers mostly occurs through convective separation of evolved liquid from in situ growing crystals and its mixing with the main magma body.

中文翻译：

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岩浆房演化动力学：新液相线到达时堆积物的结构和化学演化

摘要 玄武岩岩浆房最好的例证是分层侵入体——化石化的自然实验室，历史上限制了火成岩学的最基本原理。层状侵入体中玄武岩熔体的逐步分级结晶导致 5-10 种液相线矿物的连续出现。大多数矿物通过两个不同的阶段出现在层状侵入体的地层中。它们首先以“oikocrysts”的形式出现——在晶体液体框架中形成的大的、不规则形状的间隙晶体，然后以“primocrysts”的形式出现——从主岩浆体结晶的相对较小的自形和积云晶体。从 oikocrysts 到 primocrysts 的转变通常具有以下结构和化学特征：(a) 最后形成的 oikocrysts 和第一次形成的原晶的化学成分相似，(b) 到达缓慢、模态过多和第一个原晶的尺寸比正常尺寸更细，(c) 被困液体的丰度低在具有最初形成的初晶石的岩石中，(d) 略低于初晶石到达水平的岩石中矿物二面角的阶梯状增加，以及 (e) 斜长石原晶上的边缘与斜长石核的成分相同新液相线第一次出现时的原晶在剖面较高处。层状侵入体的积云地层的相平衡控制需要大的、长寿命的和大部分熔融的岩浆房，因此，它们的起源与最近几个否认地壳中存在此类储层的概念不符。因此，我们建议与积云相到达层状侵入体相关的系统变化最好用一个传统概念来解释，该概念认为层状侵入体是最初无晶体的大熔体，逐渐失去热量并通过薄的凝固前沿从边缘向内结晶。相对于在边缘结晶的积云相而言，在其内部演化过程中，整个岩浆体被认为是动力学过冷的（即保持略低于其液相线温度）。每个新的积云相都会在一定的动力学延迟下到达液相线，并与玄武岩岩浆房底部、顶部和侧壁上的预先存在的晶体形成不均匀的成核。