Thermal-mechanical modeling of the role of thermal stresses and stoping in magma contamination

Mantle-derived magmas emplaced in the upper crust (depth < 10 km) are in thermal disequilibrium with the surrounding country rock. As a consequence of the large temperature contrasts not only will the magma cool quite rapidly, but the surrounding country rock will be heated and affected by large thermal stresses. Using coupled time dependent thermal-mechanical models, we find that: (a) a single-stage 3 km radius magma body has an eruptive life of on the order of 10⁴ years; (b) near the magma-wall-rock boundary large thermal stresses develop over short time spans (10 years); and (c) material stoped into the chamber as a result of these thermal stresses will be assimilated either in the upper reaches of the chamber or at its base leaving a relatively uncontaminated central core. These results indicate that contamination of a primary magma by stoped crustal blocks is an efficient mechanism for producing hybrid lava sequences.