The petrogenesis of komatiites and related rocks as evidence for a layered upper mantle

Although the CaO/Al₂O₃ ratio of komatiites has been regarded as one of the distinguishing features of these rocks, a comparison of various komatiite and oceanic tholeiite analyses suggests that there is a continuum of ratios between the two. The extremely high MgO values of peridotitic komatiites suggest that they are the result of high degrees of partial melting of the mantle, leaving a harzburgitic residuum depleted in CaO and Al₂O₃, and hence preserving in the melt the original CaO/Al₂O₃ ratio of the parental material. Available chemical models of the mantle have CaO/Al₂O₃ ratios too low to explain the origin of komatiite by such a process. Shallow-level melting of a layered mantle in which clinopyroxene content decreases and garnet content increases with depth, may explain the chemistry of komatiites and related ultrabasic lavas.