The Weaklaw vent in SE Scotland (East Lothian coast), inferred to be Namurian, produced lava spatter and volcanic bombs. The latter commonly contained ultramafic xenoliths. All were metasomatised by carbonic fluids rich in incompatible elements. The lavas and xenoliths are inferred to have been basanites and lherzolites prior to metasomatism. The abundance and size of (carbonated) peridotite xenoliths at Weaklaw denotes unusual rapidity of magma ascent and high-energy eruption making Weaklaw exceptional in the British Isles. The lavas and xenoliths were altered subsequently by low-temperature (<200°C) carbo-hydrous fluids to carbonate, clay and quartz assemblages. A small irregular tuffisite ‘dyke’ that transects the ejecta is also composed dominantly of carbonates and clays. The peridotitic xenoliths are typically foliated, interpreted as originating as pre-entrainment mantle shear-planes.

Analyses of the relic spinels shows them to be compositionally similar to spinels in local unaltered lherzolites from near-by basanitic occurrences. Chromium showed neither significant loss nor gain but was concentrated in a di-octahedral smectite allied to volkonskoite. It is in the complex association of smectite with other clays, chlorite and possibly fuchsite that the diverse incompatible elements are concentrated.

We conclude that late Palaeozoic trans-tensional fault movement caused mantle shearing. Rapid ascent of basanite magma entrained large quantities of sheared lithospheric mantle. This was followed by ascent of an aggressive carbonate-/ hydroxyl-rich fluid causing pervasive metasomatism. The vent is unique in several ways: in its remarkable clay mineralogy and in displaying such high Cr-clays in a continental intra-plate setting; in being more productive in terms of its ‘cargo’ of peridotite xenoliths; in presenting an essentially un-eroded sequence of Namurian extrusives; and, not least, for giving evidence for post-eruptive, surface release of small-melt, deep-source fluids.

Archival and recent boreholes over an area of c. 3 km2 have revealed complex magma-host interaction at the termination of an olivine–dolerite sill in Fife. The sill interior has zones rich in plastically deformed, vesiculated heterogeneous sediment surrounded by amygdaloidal basalt. Sediments at the contacts have been reconstituted and enclose blebs of chilled vesicular basalt. Intrusion into low rank coal seams has produced multicomponent tuffisite. A vertically nested and laterally fingered sill front is envisaged as having propagated down dip under a thin cover (<500 m) of wet Namurian sediments. Non-explosive bulk interaction of fluidised sediment and devolatilising magma occurred at intrusive contacts. Steam explosivity was more vigorous where lobes of magma repeatedly intruded moist lignite, to produce compositionally banded tuffisite rich in basalt clasts and coal fragments. The hydrovolcanic explosions did not give rise to surface eruptions because the low volumes of porewater and the high permeability and low tensile strength of the lignite prevented a build-up of high pressure steam.