Mixing of stratified liquids by the motion of gas bubbles: application to magma mixing

• Published in: Earth and planetary science letters Vol. 115; no. 1; pp. 161 - 175
• Main Authors: Thomas, Nathalie, Tait, Stephen, Koyaguchi, Takehiro
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.03.1993; Elsevier
• Subjects: Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Experimental petrology; interfaces; plumes; mixing; gaseous phase; magmas; hybridization; layered materials; magma chambers; xenoliths; viscosity; migration; volatile elements; laboratory studies
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/0012-821X(93)90220-4

Mafic magma underlying more silicic magma in a reservoir can exsolve volatiles as it crystallizes, forming bubbles that segregate upwards to the interface with the overlying silicic magma. We describe laboratory experiments designed to investigate how gas bubbles migrate into the more viscous upper layer. In one regime, bubbles move individually across the interface, entraining a small quantity of lower layer fluid and the two layers become progressively stirred into a homogeneous mixture. In a second regime, bubbles form a thin foam layer at the interface which becomes gravitationally unstable, giving rise to two-phase plumes which rise and produce a coarse mixture of the liquids. For a given viscosity ratio, the transition between these regimes occurs when the gas flux is greater than a critical value, which we found to decrease with increasing viscosity ratio. We estimate the critical gas flux for magmatic conditions, and how vesiculation may vary with pressure, degree of crystallization and the composition of volatile components. Mafic inclusions, which are often found in silicic to intermediate lavas, can form in the plume regime; analysis of the instability of the foam predicts wavelengths of between 1 cm and 1 m, which is consistent with the observed sizes of inclusions. Volatiles from the mafic magma may be efficiently transported by this mechanism into the viscous silicic magma as bubble plumes despite low Stokes' velocities for individual vesicles. At small viscosity ratios or low gas fluxes, the bubbling regime can produce hybrid magmas.