Development of a ’transient’ interfacial tension between carbonatite and silicate magmas: Results from numerical modelling

Category Petrology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Valentini, Luca; Moore, Kathryn
Holding Date 27 September 2008

Emulsion textures have been frequently observed in both natural and experimental carbonatite-silicate pairs. It is widely accepted that their occurrence testifies to the coexistence of carbonatite and silicate liquids. Moreover, it is generally suggested that such textures provide evidence for either: a) immiscible separation of a carbonate magma from a silicate parent magma or: b) mechanical interaction of a carbonate magma with an immiscible and not genetically related silicate magma. However, such globular shapes are not unequivocally related to droplets of a magma immersed in a pool of an immiscible magma, as they have been also observed, both in the field and in laboratory experiments, in magmatic pairs which have been shown to be mutually miscible.
In this work we aim to investigate the possibility for miscible and rheologically distinct magmas to develop a transient surface tension which may lead to the formation of globular or emulsion-like textures. In order to do that we have devised a numerical model based on an advection-diffusion scheme in which the effect of gradient stresses is included. Stress generated by compositional gradients is known as ’Korteweg stress’. In recent years this concept has been successfully applied to both numerical models and experiments into fluid dynamics.
The results of our computer simulations show that interacting miscible magmas may develop globular shapes and smooth menisci via Korteweg stress driven flow. Breakup instabilities observed in our simulations are on analogy to drop breakup induced by surface tension in immiscible liquids. We advocate the use of this novel technique, coupled with diffusion studies, as an auxiliary tool to textural interpretation. In particular, it is proposed that such a combined approach might potentially clarify whether for specific compositions, the lack of miscibility between carbonate and silicate magmas and the formation of emulsion textures may result from rheological barriers rather than thermodynamic immiscibility.
We still believe that immiscibility may play a fundamental role in carbonatite petrogenesis, however the occurrence of emulsion textures should be regarded as circumstantial rather than absolute evidence. Where liquid immiscibility is not supported by geochemical evidence, the effect of Korteweg stress may be a potential alternative explanation to the formation of such textures. These cases include carbonatite-silicate pairs with compositions lying outside experimentally determined solvi and/or displaying chemical inter-diffusion across their interface.