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Laboratoire de Géochimie comparée et systématique, U.A. 196 CNRS, Institut de Physique du Globe de Paris et UER des Sciences de la Terre, Université Pierre et Marie Curie, 4 place Jussieu, 75230 Paris Cédex 05
Groupe des Sciences de la Terre, Laboratoire Pierre Süe, CNRS, Centre d’Etudes Nucléaires de Saclay, 91191 Gif sur Yvette Cédex, France
On the basis of the properties of hygromagmaphile (HYG) elements we propose a specific method of identifying and modelling the evolution of mantle geochemical properties using simple mass balance equations and taking into account a large volume of data obtained for basaltic series belonging to characteristic oceanic domains. This method uses both binary and ternary HYG element ratio diagrams to distinguish between the effects of partial melting and mantle heterogeneity. It is assumed that batch partial melting is the simpler and more suitable process of basaltic magma generation, which produces linear correlations on binary HYG element ratio diagrams. Melting of a homogeneous source is projected on ternary HYG element ratio diagrams as a single point approximating the composition of the mantle source.
We show that basalt HYG element ratios are closely dependent on four main factors, namely the nature of melting (batch partial melting), the degree of partial melting, the composition of the mantle and the mineralogy of the mantle.
Partial-melting trends and complex mantle heterogeneities are clearly indicated by binary diagrams, and a simple and regular suboceanic mantle HYG element array is obtained in ternary diagrams. Taking into account the close relation with isotopic trends, the HYG element array is considered to be the fingerprint of ancient mantle differentiation. It can be reproduced by a simple fractional crystallization process, and mostly excludes a large-scale and intensive chemical mixing process. These results are in good agreement with a marble cake mantle model, limiting convection effects to a mechanical stirring of the upper mantle, but they do not necessitate any recycling of crustal material into the suboceanic mantle.
