The physico-chemical nature of the exchange between mafic enclaves and their felsic hosts

Abstract:

What happens to recharge magmas as they mix with felsic magma stored in the crust? Under conditions common in nature, a mafic magma quenches against the cooler, more silicic magma and disperses as mafic enclaves, effectively halting the hybridization process at the cm-scale. Here, we present results from an experimental study to investigate the fate of such mafic enclaves as they equilibrate over the course of days (1 h, 10 h, 79 h) with the felsic magma. In detail, we ran diffusion-couple experiments using natural compositions from Quizapu volcano, Chile, at sub-liquidus conditions (T = 1000 ˚C, P = 1.5 kbar, nearly H₂O-saturated, fO₂= FMQ+4) to document the complex interplay of mineral dissolution and precipitation and diffusive exchange near the “enclave” interface. We record the progressive dissolution of opx, cpx, plg, and oxides in addition to major and trace element concentration profiles (electron probe and LA-ICPMS data) following a diffusion-like exchange between mafic and felsic end-members.

As the mafic end-member experiences slow crystal dissolution, the experiments constrain, in the absence of convection, a minimum time scale of survival of mafic enclaves in natural systems. We find that the advancement of the dissolution boundary layer in the mafic end-member correlates with the square-root of the run duration and therefore is diffusion-controlled. Extrapolating the boundary layer progress suggests that under sub-liquidus conditions and without large internal stresses caused by volatile exsolution or advection, cm-size mafic enclaves remain intact for weeks to months. On the other hand, depending on the size of the enclaves, they should be completely consumed/assimilated by the host magma within a few decades even by diffusion-controlled processes alone. Therefore, the presence of mafic enclaves is a testimony to recent mafic recharge, potentially related to or even responsible for eruption triggering.