Evidence for igneous differentiation in Sudbury Igneous Complex and impact-driven evolution of terrestrial planet proto-crusts

High-precision U-Pb zircon dating identifies a major magmatic event on the Moon at 4.338 Ga

The Moon has had a complex history, with evidence of its primary crust formation obscured by later impacts. Existing U-Pb dates of >500 zircons from several locations on the lunar nearside reveal a pronounced age peak at 4.33 billion years (Ga), suggesting a major, potentially global magmatic event. However, the precision of existing geochronology is insufficient to determine whether this peak represents a brief event or a more protracted period of magmatism occurring over tens of millions of years. To improve the temporal resolution, we have analyzed Apollo 14, 15, and 17 zircons that were previously dated by ion microprobe at ~4.33 Ga using isotope dilution thermal ionization mass spectrometry. Concordant dates with sub-million-year uncertainty span ~4 million years from 4.338 to 4.334 Ga. Combined with Hf isotopic ratios and trace element concentrations, the data suggest zircon formation in a large impact melt sheet, possibly linked to the South Pole-Aitken basin.

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

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.

Fossilized solidifications fronts in the Bushveld Complex argues for liquid-dominated magmatic systems

Chemical differentiation of magma on Earth occurs through physical separation of liquids and crystals. The mechanisms of this separation still remain elusive due to the lack of information on solidification fronts in plutonic magmatic systems. Here, we present records of fossilized solidification fronts from massive magnetitites of the Bushveld Complex in South Africa, obtained by two-dimensional geochemical mapping on field outcrops. The chemical zoning patterns of solidification fronts indicate that nucleation and crystallization occur directly at the chamber floor and result in near-perfect fractionation due to convective removal of a compositional boundary layer from in situ growing crystals. Our data precludes the existence of thick crystal mushes during the formation of massive magnetitites, thus providing no support for the recent paradigm that envisages only crystal-rich and liquid-poor mushy reservoirs in the Earth’s crust.

Magma storage zones are debated to either be crystal-dominated mush zones or large liquid-dominated magma chambers. Here, the authors discover fossilized solidification fronts of magnetitite in the Bushveld pluton, which indicate nucleation and crystal growth occurred at the magma chamber floor, precluding the existence of a thick crystal mush zone in this region.