Ca isotope fingerprints of early crust-mantle evolution

At face value, these results imply the Zimbabwe crust is juvenile whereas most Greenland samples include an earlier crustal component. Yet the west Greenland craton, as with many Archaean localities, has experienced a complex geological history and the interpretation of age-corrected initial isotope values requires great care. Both felsic and mafic samples from Greenland display εCa(3.7) so radiogenic that they are not readily explained by crustal growth scenarios. The presence of such radiogenic 40Ca/44Ca found in low K/Ca plagioclases requires Ca isotope exchange between plagioclase and whole rock during later metamorphic event(s). In addition the unexpectedly radiogenic Ca isotope ratios in some mafic samples reflect anomalous K/Ca ratios as a result of intense K-metasomatism ∼3.6 Ga. Thus Ca isotope measurements are not a robust tracer of crustal growth in the presence of intense tectono-metamorphic processes. Coupled with other isotope data, however, the degree of overprint can be estimated and the 40Ca/44Ca ratio of a little disturbed sample hints at a small contribution of Hadean protocrust in the coastal part of the GodthÃ¥bsfjord area (Southwest Greenland). In the majority of Zimbabwe TTG samples, unradiogenic initial Ca isotope ratios point to very little prior crustal history and minor subsequent disturbance. We thus infer that the modest initial εNd ∼0.8 of the Zimbabwean samples is representative of the depleted mantle at ∼3.6 Ga. Furthermore, Ca isotope systematics provide little support for a “steady state” model of crustal growth.