When did the continents grow? At present, the formation and the evolution of the continents remains poorly understood. The question is whether the present mass of continents formed early in the earth's history or grew with time.
The particularity of the Sm-Nd isotopic system is that parent and daughter belong to the same chemical family, which is the group of the Rare Earth Elements. Previous studies by Nance and Taylor (1976) and by McCulloch and Wasserburg (1978) have shown that Sm and Nd are not fractionated by erosion, transport and diagenesis processes. Consequently, the Sm/Nd ratio in shales integrate the isotopic diversity of the source materials from which they derive. The neodymium isotopic composition in shales is controlled by two processes: Direct inputs from the mantle by volcanism and recycling of older continental crust controlled by tectonic and erosion processes. The problem is to distinguish between the juvenile part and the recycled part of each crustal segment at each time. Additionally, it is poorly known if and how continental crust continues to be created.
We report on Nd isotopic composition and Sm/Nd ratios measured by TIMS from shales of the stratigraphically well
documented Montagne Noire's sequence (South of France). It is the most complete and the least metamorphic sedimentary record of the French Palaeozoic. Our sampling was restricted to only fine grained sediments because they represent a well mixing of rocks from different sources. In order to constrain the continental origin of our samples more precisely, chemical compositions, especially the concentrations of Ni and Cr, were measured by ICP-MS. They allow us to eliminate the volcanogenic contribution.
Initial 143Nd/144Nd isotopic ratios range from -4.3 to -10 eNd(T) units for the whole stratigraphic column. Using the growth curve of the depleted mantle as a reference, Nd model ages were calculated that range from 1.15 to 1.90 Ga. From first sight, the isotopic composition seem to be not correlated with the stratigraphic age. Going more into detail, we see that the highest ratios are found for the sediment sequences that are ~300 Ma, ~460 Ma and ~560 Ma old, and therefore correspond to well known and important orogenic or volcanic episodes. A quantitative model of crustal growth will be done and comparison with a neighbouring segment (eastern Pyrénées) is in progress.
McCulloch, M.T. & Wasserburg, G.J., Science 200, 1003-1011 (1978).
Nance, W.B. & Taylor, S.R., Geochim. Cosmochim. Acta 40, 1539-1551 (1976).