Abstract
The age-old process of crystal fractionation leading to the diversity of the igneous rocks and Earth itself is an exceedingly well-understood chemical process in magmatism and physical chemistry. But the broader physical aspects of this and related processes have proven elusive on many fronts, especially in its relation to the spatial variations in rock composition, texture, and macroscopic features like layering. Magmatic systems, be they volcanic, dikes, sills, or plutons, are generally analyzed with a problem at hand and an end result in mind. The processes invoked to solve these problems, which are most often purely chemical, are often unique to each problem with few if any general principles emerging that are central to understanding the wider perspective of magmatic processes and problems. An attempt is made at the outset to provide a list of inviolate Magmatic First Principles that are relevant to analyzing most magmatic problems. These involve: initial conditions; critical crystallinity; solidification fronts; transport and emplacement fluxes; phenocrysts, xenocrysts, primocrysts; crystal size; layering and crystal sorting; thermal convection; magmatic processes are physical. Along with these principles, two reference magmatic systems are suggested where the initial conditions and outcome are unequivocal: the Sudbury impact melt sheet and the Hawaiian lava lakes. Sudbury formed in ~5 min by superheated magma crystallized to a near uniform sequence, while the tiny lava lakes, formed of crystal-laden slurries, form a highly differentiated layered sequence. The major difference is in the initial conditions of formation, especially the nature of the input materials. The challenge is to construct and analyze magmatic systems (i.e., magma chambers, sills, dikes, and lavas) using these reference end members and the suggested principles. The Hawaiian 500,000 year volcanic record exhibits what can be expected as input materials, namely a highly varied output of magma of an overall composition reflecting the abundance of entrained olivine primocrysts. The provenance of these crystals is varied, and within any single sample, the population may be highly heterogeneous in composition from crystal to crystal, yet the overall pattern of chemical fractionation is exceedingly regular and well defined. If similar inputs go to form large intrusions, these systems will undoubtedly be dominated by crystal-rich slurries, which provide a vast set of physical processes promoting exotic layering and, at the same time, given the effects of annealing and continued crystal growth, a final chemical record adhering to all the time-honored effects of crystal fractionation. The long assumed initial condition of instantaneously emplaced crystal-free magmas cannot reasonably produce the observed rock records.
Highlights
The sharp break in grain size between phenocrysts and groundmass is correlated with some corresponding change in conditions prevailing during freezing of the magma
A key ingredient to gaining new and unusual insight into the broader fundamentals of planetary magmatism and differentiation is the appreciation of the intimate spatial connection of chemical and physical processes
It is the specific physical processes that largely remain elusive, namely those governing the general functioning of magmatic systems leading to protracted differentiation via crystal fractionation, the distinctive layering in mafic plutons, and the connection between deep magmatic processes and volcanic processes
Summary
The sharp break in grain size between phenocrysts and groundmass is correlated with some corresponding change in conditions prevailing during freezing of the magma. The general association of silicic or rhyolitic magma with massive basaltic systems is sometimes abundantly clear (e.g., Snake River, Thingmuli, Torfajokull), and convincing chemical evidence, commonly invoking permissive AFC processes, can be presented linking the entire compositional sequence. Scenarios are presented that match the evidence, but the scenario itself is most often unique to that body or that researcher These principles are unique in the sense that different ones are routinely separately applied to understand volcanic sequences, sills, dikes, small intrusions, large intrusions, and granitic versus basaltic magmas. In this context, it is not the separate specific processes that are in question; it is in the choice of the inclusion and exclusion in their stringing together to make a satisfactory scenario that is in question.
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