Isotope topology of individual hotspot basalt arrays: Mixing curves or melt extraction trajectories?

Abstract

Arrays of basalts from the same hotspot usually plot within an elongate tube-like field in 87Sr/86Sr-143Nd/144Nd-206Pb/204Pb space (Hart et al., 1992). Each hotspot array tube (HART) is commonly interpreted as the result of melting multiple basalt sources that are variably proportioned mixtures of the hotspot source components. I propose instead that a HART is the isotopic trace of the melt extraction trajectory produced by progressive melting of a heterogeneous source mixture characteristic to that hotspot. melt extraction trajectories form when the sources of the individual hotspot basalts differ in the amount of previous melt extraction from the upwelling and melting plume mantle mixture. Sequential melts along a melt extraction trajectory trace a one-dimensional path in through isotope space, independent of the number of distinct isotopic components in the initial source mixture. Subsequent partial mixing of the melts produced along a melt extraction trajectory would still tends to preserve this tube-like isotope topology. The melting physics that leads to a melt extraction trajectory also provides straightforward explanations for the 187Os/188Os contrasts between mid-ocean ridge basalts and their presumed abyssal peridotite source and for the enigmatic trace element and isotopic patterns of pyroxenite veins and peridotite exposed within orogenic lherzolites.