An alteration zone at the base of a Carboniferous lava flow on the island of Bute resembles modern and ancient weathering profiles, but it is ‘upside down’ in the sense that alteration is most intense at the base and decreases upwards. Values for a Chemical Index of Alteration (CIA) increase from fresh lava and corestones downwards to fine-grained structureless material. The altered material is Mg- and Cu-rich, possibly as a result of migration of these elements from underlying sediments. Rare earth elements (REE) display considerable and systematic mobility and fractionation. In general heavy rare earth elements (HREE) are concentrated during alteration, whereas the light rare earth elements (LREE) are lost. Mobility of the REE appears to be related to atomic weight, with La (the lightest REE) being the most depleted through to Lu, which is the most concentrated REE in the highly altered material. Similar systematic fractionation is shown by some weathering profiles developed on mafic igneous rocks. Movement of water into the volcanic rocks was probably driven by a steep thermal gradient between the hot lava and its sedimentary substrate.