Dynamics of evolving magma chambers: textural and chemical evolution of cumulates at the arrival of new liquidus phases

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 fr