DATING LAYERED INTRUSIONS

Mafic layered intrusions crystallize from basaltic magma to form large bodies of igneous rocks that preserve stunning rock records of the processes by which magma evolves in crustal magma chambers. Accurate and precise dates are required to establish how different parts of layered intrusions are related to each other, to define genetic relations with associated ore bodies, to determine links to mafic dike swarms and volcanic rocks within large igneous provinces, and to correlate with potential environmental impacts. Yet, surprisingly, even classic layered intrusions in petrology (e.g., Stillwater, Bushveld, Muskox, Skaergaard) do not have robust geochronological frameworks. Prospective samples from mafic-ultramafic cumulates are characterized by heterogeneous textures and macroscopic evidence for interstitial minerals that crystallized from evolved interstitial melt at near-solidus temperatures. Dateable accessory minerals, including zircon, baddeleyite, rutile, and apatite, are typically present in minute quantities (10s to 100s of grains per 10-20 kg of sample), but are in sufficient quantity to provide high-precision geochronologic results. Major advances in sample pretreatment, instrument sensitivities, and data reduction protocols for U-Th-Pb and ⁴⁰Ar/³⁹Ar geochronology, combined with establishment of the “EARTHTIME Initiative”, have led to significantly improved precision and accuracy. For U-Pb dating of single zircon grains, internal uncertainties of significantly less than 0.1% can now be achieved. Uncertainties of ±500 ka or less are attainable for Paleoproterozoic to Archean intrusions (e.g., Bushveld, Stillwater) and ±10-20 ka for some Phanerozoic intrusions, approaching the timescales of residence, magma storage, and differentiation of magmas in the crust. The geochronological tools are available to address the age and duration of layered intrusions throughout Earth history, whether major layered intrusions are simple stratigraphic sequences of cumulates younging upwards towards the roof, and whether discontinuities in cumulates can be identified and time gaps measured. Answers to these and other important questions will directly impact our understanding of the emplacement, crystallization, and cooling of these exceptional bodies of igneous rocks.