Late-stage veins have been studied in the Rhum layered intrusion as part of a wider investigation into postcumulus processes – in particular, to study compositions of residual liquids and their effects on host cumulates. Studies have concentrated on Unit 10 of the Eastern Layered Series (ELS), though brief investigations have also been made in the Western Layered Series (WLS).

The Unit 10 veins are generally concordant with the layering, although locally they are transgressive with dilational relationships. Two principal types can be recognized: ultrabasic and basic. Detailed microprobe studies have shown that the liquids responsible for the veins have substantially modified the compositions of the cumulus minerals in the wall-rock peridotite up to 1 cm from the vein margin.

The style of veining in the WLS is more complex than in the ELS, and several generations of vein can be identified. Both normal and harrisitic cumulates have undergone localized chemical modification near to the veins. Modification occurs regardless of the vein orientation.

The ELS and WLS veins are geochemically distinct from the numerous gabbro intrusions associated with the layered series, and it is concluded that they represent fractionated intercumulus liquids injected into early formed fractures in the cumulates. The observed chemical modification is explained by a model involving mixing of the intrusive vein melt with trapped intercumulus melt in the adjacent peridotite, followed by diffusion-controlled re-equilibration of the cumulus minerals. This type of highly localized compositional modification along fractures is termed channelled metasomatism.

In the allivalite of cyclic unit 9 in the Eastern Layered Series of the Rhum intrusion there is an undulatory contact between a relatively dense pyroxene-rich rock and a less dense pyroxene-poor rock. The three-dimensional form of this contact resembles that developed between two fluids deforming under the influence of gravity. Minor layers, laminae and lamination in these rocks in places cut across the contact between the pyroxene-rich and pyroxene-poor rock types. They are, therefore, inferred to post-date the formation of the contact and hence to have formed within a porous crystal pile on the chamber floor. Possible mechanisms of formation are discussed but their origin remains unclear. These observations cast doubt on the common assumption that the combination of granular (cumulus) textures and layering structures is sufficient evidence on which to infer equilibrium of granular crystals with a contemporary magma.

Feldspathic peridotites on Rhum contain teschenite segregations and veins. Bulk rock and mineral compositional trends define a path of alkali and iron enrichment related to discontinuous isolation of evolving intercumulus melts. Compositionally zoned and skeletal crystals of actinolite, zircon, apatite, sphene, epidote and chlorite are enclosed by clear analcime, and appear to have grown directly from a water-rich fluid at some undercooling, and below dry equilibrium solidus temperatures. The occurrence of late stage residua with undersaturated alkalirich characteristics in the Rhum ultrabasic complex suggests that the patental magma was alkaline rather than tholeiitic, and had similarities to the Plateau Magmas on Skye.