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Reef-type platinum-group element (PGE) deposits account for >70% of the world’s exploited PGE. Characterised by mm-scale seams rich in the mineral Cr-spinel, they are also vital records of the interaction between fresh injections of magma into open-system magma chambers [1]. The eastern layered intrusion (ELI) on the Isle of Rum provides a world class example of this style of magmatism and critical metal enrichment. Previous studies into Cr-spinel seams in the ELI uncovered “cone-structures”, characterised by cm-scale local thickening of the seam in downward pointing cones [2]. Through high energy collimated X-ray computed tomography (XCT), these cone-structures have now been characterised in 3D at a scale of 10’s of cm for the first time (Figure 1). Through the aid of deep learning segmentation, the cone structures can be segmented and characterised quantitatively at a sub-mm scale. We hypothesise that the cone structures – which appear at the interface between relatively buoyant anorthositic layer, and overlying denser olivine cumulate – form by Raleigh-Taylor instabilities. Experimental and computational methods have been used to model Raleigh-Taylor instabilities in layered mafic complexes [3]; however, this has never been applied to reef-type deposits/Cr-spinel seams. If Cr-spinel cone-structures form by Raleigh-Taylor instability, characterising their morphology may allow us to further constrain the conditions under which PGE ore forms, including larger scale features such as “pot-holes” in the Bushveld Complex, South Africa – the world’s largest PGE source.

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