Rheology of the upper mantle: some geodynamic and magmatogenic implications
European Journal of Mineralogy Volume 5 Number 6 (1993), p. 1057 - 1064
published: Dec 1, 1993
manuscript accepted: Jun 21, 1993
manuscript received: Dec 29, 1992
ArtNo. ESP147050506017, Price: 29.00 €
Abstract Basic magmatism originates by partial melting in the upper mantle. In order to place this process in a geodynamic context, this paper focuses on the long-term rheological properties of upper mantle materials and on their lateral variations. Geotherms vary between cases where the solidus temperature of wet peridotite is not reached at any depth (e.g., Precambrian Shields), and cases where the solidus is reached not far below the Moho (e.g., continental extension zones). Given the exponential temperature dependence of the rheology, lateral temperature variations result in viscosities in the upper mantle which vary in the horizontal direction by orders of magnitude. Two factors that affect the rheological properties are degree of partial melting and volatile content. While a small (less than a few percent) amount of melting may not significantly change the steady-state rheology, the presence of volatiles can have a considerable softening effect. If conditions are favourable, melt is produced in sufficient quantities and at a sufficient rate, and migrates upwards. The most likely form of magma transport across the lithosphere is through the propagation of fluid-filled fractures. The rheological properties of both magma and host rock are well within realistic limits for this process to be an efficient mechanism of mass and heat transfer.