Magmatic karst reveals dynamics of crystallization and differentiation in basaltic magma chambers

Abstract

An understanding of magma chamber dynamics relies on answering three important yet highly controversial questions: where, why, and how magma chambers crystallize and differentiate. Here we report on a new natural phenomenon—the undercut-embayed chamber floor in the Bushveld Complex—which allows us to address these questions. The undercut-embayed floor is produced by magmatic karstification (i.e. erosion by dissolution) of the underlying cumulates by replenishing magmas that form basal flows on the chamber floor. This results in a few metres thick three-dimensional framework of spatially interconnected erosional remnants that separate the floor cumulates from the overlying resident melt. The basal flow in this environment is effectively cooled through the floor, inducing heterogeneous nucleation and in situ growth against much of its three-dimensional framework. The solidification front thus propagates in multiple directions from the surfaces of erosional remnants. Fractional crystallization may occur within this environment by convective removal of a compositional boundary layer from in situ growing crystals and is remarkably efficient even in very confined spaces. We propose that the way magma crystallizes and differentiates in the undercut-embayed chamber floor is likely common for the evolution of many basaltic magma chambers.

Magnetite layer formation in the Bushveld Complex of South Africa

“…The resulting perturbation in the flow field led to concentric Cr enrichment around this autolith (Fig. 3d), producing spatial variations very similar to those shown by Kruger and Latypov [18][19] . This also partly accounts for the confusing phenomenon that Cr-rich growth nodes are found in the nooks beneath the autoliths but not directly on the upper surfaces of autoliths [18][19] .…”

“…1d), and it becomes constant at higher levels, none of which it could do during fractional crystallization unless the melt composition or basic physical parameters changed repeatedly during the process [14][15] . A further complicating matter is the common occurrence of exceptionally Cr-rich nodes decorating the basal contact of the MML, showing strong lateral as well as vertical compositional gradients 16,[18][19] . These features have been taken as evidence that magnetite initially nucleated and crystallized around discrete nodes which coalesced as they grew outwards and upwards 16,18 .…”

“…The sizes of the primary grains (~0.5-2 mm) are similar with that of disseminated magnetite in associated cumulate rocks, but far less than the crystals in silicate-free regions (~5-20 mm) 13 , indicating extensive overgrowth, compaction, and annealing to form the adcumulates. If it formed by a process of fractional crystallization as a loose pile of settled crystals [3][4] or as a loose arrangement of chains of magnetite crystals nucleated against others in situ 16,[18][19] and accumulated faster than the rate of compaction, the initial magnetite cumulate may have had a high porosity (~0.52) 31 . Compaction is driven by the large density contrast between solid (~4955 kg•m -3 ) and liquid (~2683 kg•m -3 ) (Methods).…”

“…Others have inferred that concentration contours concentric around node-like structures at the base of the MML were caused by in situ growth that was initiated at discrete points on the floor 16,[18][19] . Instead, we used our two-dimensional model of percolative reactive transport to address the alternative hypothesis that local variations in porosity in the underlying anorthosite mush may allow some sites to become preferred pathways for underlying melt during upward percolation.…”

“…One deposit that has long been used as a laboratory to address the controls on layer formation in layered intrusions is the Main Magnetite Layer (MML) in the Upper Zone of the Rustenburg Layered Suite of the Bushveld Complex of South Africa [9][10][11][12][13][14][15][16][17][18][19] . Here we compare observations of the MML with some forward thermodynamic and fluid dynamic models, appealing to various combinations of fractional crystallization, compositional convection during adcumulus crystal growth, and reactive transport of upward-migrating Cr-rich melt from beneath the MML.…”