Ca Isotopes Fingerprinting the Earliest Crustal Evolution

The mechanisms of continent formation remain unclear and can be explained in two contrasting ways, using either a steady state crustal growth model involving massive crustal recycling or continuous crustal growth models. Recent developments in mass spectrometry manifest in the new Finnigan-Triton allow Ca isotopic measurements precise enough to use the K-Ca isotope system to address the problem of early Archaean crustal evolution. Due to a strong fractionation of ⁴⁰K and ⁴⁰Ca during continent formation and a non-linear growth of ⁴⁰Ca, Archaean continental crust should show radiogenic initial Ca isotopic composition if large volumes of it have already been existed 3.6 Ga ago. Simple 15-step calculations predict a difference in ⁴⁰Ca /⁴⁴Ca of 9 epsilon units at 3.6 Ga between the two crustal growth models. To test this, as well as to study the earliest crust formation processes, plagioclase separates from Archaean provinces reflecting the initial Ca isotopic composition and a range of different whole rock samples have been analysed. Preliminary data for ~ 3.6 Ga old TTGs from Zimbabwe show ⁴⁰Ca /⁴⁴Ca indistinguishable from the mantle. This is in agreement with rather chondritic initial Sr and Nd data and might reflect a short residence time of the juvenile mafic oceanic crust before partial melting forming the first continental crust. In contrast, the first results for 3.65 Ga old samples from the Itsaq Gneiss Complex of southern West Greenland yield a more evolved radiogenic Ca signature. This can be interpreted in two different ways. Either as partial melting of juvenile mafic crust shortly after its formation but incorporating already existing crust as also suggested by the existence of older inherited zircons in these rocks and negative ɛ Hf values. Partial melting of mafic oceanic crust long after its formation so that ⁴⁰K and ⁴⁰Ca had time to evolve would be an alternative explanation. Importantly, there is no evidence so far for high growth and recycling rates prior to 3.6 Ga as required by the most extreme 'big bang' model.