The relative role of petrogenetic variables at accreting plate margins varies according to the tectonic setting of the spreading centre in question. Spreading rate, heat flow, proximity to fracture zones and proximity to areas of 'anomalous' mantle are particularly important factors. This can be illustrated with reference to three well-studied areas: the FAMOUS segment of the Mid Atlantic Ridge; the Troodos ophiolite complexes and the active rift centres on Iceland. Geochemical modelling based on some critical major and trace element ratios reveals that crystal accumulation and fractional crystallization dominate variation both of the Troodos lavas (perhaps because a high degree of partial melting yielded a homogeneous primary magma) and of lavas from fast-spreading ridges (perhaps because of buffering in a large axial magma chamber). Source heterogeneities dominate variation of the Iceland lavas (perhaps because of 'anomalous' mantle beneath the centre of Iceland). The FAMOUS lavas exhibit a combination of primary magma and fractional crystallization variation.