Nd isotopes in French Phanerozoic shales: external vs. internal aspects of crustal evolution

Abstract

Nd isotopic composition has been determined on shales of Paleozoic (Brittany and Languedoc) and Mesozoic (Lorraine) age. ϵ Nd ( T) values range from −6 to −12 while Nd crustal residence ages are typically in excess over their stratigraphic ages by some 1.4 Ga. Exceptions to this rule are the sediments coeval with Hercynian, Caledonian and Cadomian orogenic events ( ca. 300, 440 and 560 m.y.), the ϵ Nd ( T) values of which suggest additional of mantle material to the sediment in the form of volcanoclastic detritus. In Brittany, this is confirmed by the local zircon chronology which provides upper intercepts of Concordia up to 800 m.y. in excess of Nd crustal residence ages. Comparison of crustal residence ages with stratigraphie ages through geologic time suggests a three stage history: 1. a) for rocks older than 2 Ga, stratigraphie and crustal residence ages coincide, 2. b) from 2 to I Ga, crustal residence ages level off at ca. 2 Ga and then 3. c) decreases down to 1.4 Ga in recent sediments. Two extreme models can account for the observed variations: an internally driven model, in which variable quantities of mantle material are added to the crust, and an externally driven model, in which the rate of crustal recycling is low in the Archean but increases rapidly at the onset of the atmospheric oxygen buildup and development of soil supported life. A quantitative model is calculated which enables the balance of both processes to be taken into account.