Two terrestrial lead isotope paradoxes, forward transport modelling, core formation and the history of the continental crust

New insights into the ancient silver production of the Wallis area, Switzerland

“…During the LT D, the silver supply seems to be supra-regional, because we found Kramers and Tolstikhin 1997) silver artefacts with the same isotopic fingerprint in the adjacent area, namely the Ticino. During Augustan times, we observe also a regional silver supply, since metal from the Goppenstein mines have only been handcrafted for silver jewellery from Randogne/Bluche.…”

“…3). This linear trend can be explained by lead derived from the partial fusion of the upper crust; the ore fluid formed in this way is then enriched in lead from the old upper crust by lixiviation during metamorphism, before its deposition (Kramers and Tolstikhin 1997). The ore bodies of the mines of Goppenstein are characterised by a complex lead isotopic fingerprint.…”

“…They Pb permits to explain the formation of the mineralisations of these Wallis mines: lead derived by partial fusion of the upper crust (= brown line: young upper crust); the ore fluid formed in this way is then enriched in lead from the old upper crust (blue line) by lixiviation during metamorphism, before its deposition (Kramers and Tolstikhin 1997). When the ore body is formed by only one mineralisation event, the lead isotope signature shows then one straight tendency (grew arrow).…”

“…The identification of this linear trend is a useful tool for characterising an ore body: It allows determining (a) the origin of the mineralisation fluid (in our case the upper crust. Other possibilities are the lower crust or the mantle as described by the model of Kramers and Tolstikhin 1997), (b) the influence of the host rock on the lead isotope composition, (c) the age, the mineralisation was formed (younger mineralisation lies on the right, older in the left) and finally (d) the number of mineralisation events are almost contemporary: The Wallis samples (red circles) date from the LT D (150-30 BC), whereas the Ticino samples from the LT C2/D1 (200-80 BC). The silver seems to be a mix of silver metal from two sources with variable proportions.…”

Crustal redistribution, crust–mantle recycling and Phanerozoic evolution of the continental crust

“…Although it is clear that the continental crust was formed as a result of differentiation from the mantle there continues to be debate as to how this was generated and exactly when the bulk of the crust was formed. In some models the volume of continental crust has remained essentially unchanged since the earliest Archean (Fyfe, 1978;Armstrong and Harmon, 1981;Phipps Morgan and Morgan, 1999), while others have argued that the volume of crust has grown through time (Hurley and Rand, 1969;Moorbath, 1978;O'Nions et al, 1979;Albarède and Brouxel, 1987;Kramers and Tolstikhin, 1997). Dating studies of zircons from continental cratons now argue for much of the crust being generated in a series of pulses timed at approximately 1.2, 1.9, 2.7 and 3.3 Ga (Gastil, 1960;Hurley and Rand, 1969;Kemp et al, 2006), a fact consistent with 187 Re/ 187 Os isotope data that imply mantle depletion events at 1.2, 1.9, 2.7 Ga (Parman, 2007;Pearson et al, 2007).…”