Continental flood basalts...arachnids rule OK?

Quite apart from isotope systematics, the elemental compositions of these basaltic lavas and associated dykes can contribute substantially to our understanding of their petrogenesis. Major- and trace-element analyses of worldwide flood-basalt occurrences (including 15 new analyses) are discussed in detail. When their incompatible elements are plotted on chondrite-normalized diagrams ('spidergrams' - hence arachnids), a few show the comparatively smooth downward-facing curves which characterize ocean-island tholeiites and both oceanic and continental alkalic basic lavas, but most show distinctive features which may be modelled by the addition of a few per cent of various types of fusible continental crust to oceanic magmas.-R.A.H.