Rift-induced disruption of cratonic keels drives kimberlite volcanism

Kimberlites are volatile-rich, occasionally diamond-bearing magmas that have erupted explosively at Earth's surface in the geologic past1-3. These enigmatic magmas, originating from depths exceeding 150 km in Earth's mantle1, occur in stable cratons and in pulses broadly synchronous with supercontinent cyclicity4. Whether their mobilization is driven by mantle plumes5 or by mechanical weakening of cratonic lithosphere4,6 remains unclear. Here we show that most kimberlites spanning the past billion years erupted about 30 million years (Myr) after continental breakup, suggesting an association with rifting processes. Our dynamical and analytical models show that physically steep lithosphere-asthenosphere boundaries (LABs) formed during rifting generate convective instabilities in the asthenosphere that slowly migrate many hundreds to thousands of kilometres inboard of rift zones. These instabilities endure many tens of millions of years after continental breakup and destabilize the basal tens of kilometres of the cratonic lithosphere, or keel. Displaced keel is replaced by a hot, upwelling mixture of asthenosphere and recycled volatile-rich keel in the return flow, causing decompressional partial melting. Our calculations show that this process can generate small-volume, low-degree, volatile-rich melts, closely matching the characteristics expected of kimberlites1-3. Together, these results provide a quantitative and mechanistic link between kimberlite episodicity and supercontinent cycles through progressive disruption of cratonic keels.

The crustal geophysical signature of a world-class magmatic mineral system

World-class magmatic mineral systems are characterised by fluid/melt originating in the deep crust and mantle. However, processes that entrain and focus fluids from a deep-source region to a kilometre-scale deposit through the crust are unclear. A magnetotelluric (MT) and reflection seismic program across the margin of the Gawler Craton, Australia yield a distinct signature for a 1590 Ma event associated with emplacement of iron-oxide copper gold uranium (IOCG-U) deposits. Two- and three-dimensional MT modelling images a 50 km wide lower-crustal region of resistivity <10 Ωm along an accreted Proterozoic belt. The least resistive (~1 Ωm) part terminates at the brittle-ductile transition at ~15 km, directly beneath a rifted sedimentary basin. Above the brittle-ductile transition, three narrow low-resistivity zones (~100 Ωm) branch to the surface. The least resistive zone is remarkably aligned with the world-class IOCG-U Olympic Dam deposit and the other two with significant known IOCG-U mineral occurrences. These zones are spatially correlated with narrow regions of low seismic reflectivity in the upper crust, and the deeper lower-crust conductor is almost seismically transparent. We argue this whole-of-crust imaging encapsulates deep mineral system and maps pathways of metalliferous fluids from crust and mantle sources to emplacement at discrete locations.