Magmatic Water Content Controls Magma Storage Depth

Magma storage depths inform our understanding of volcanic hazards, crustal structure, and continent formation. However, controls on the depth of magma stagnation are poorly understood. Some have argued for the importance of intrinsic (e.g., buoyancy, viscosity) controls [a], while others have emphasized the importance of extrinsic (e.g., crustal structure) controls [b]. We investigate the influence of magmatic water content, a key intrinsic variable, on magma storage depth. Water is thought to be important because decompression induced degassing of water during magma ascent results in an increase in melt viscosity and magma crystallinity, both inhibiting ascent.

We investigate 21 arc volcanoes worldwide, focusing on mafic-intermediate systems. Reservoir depths, mostly based on geodetic or seismic observations, are compiled from the literature. Magmatic water contents are based on the maximum water content measured in large melt inclusion suites, which include our new melt inclusion suites for several Aleutian volcanoes and previously published data. Storage depths ( 2-15 km below the surface) correlate positively with maximum melt inclusion water contents ( 2-7 wt.%). There are two possible explanations for the correlation: (1) magmatic water content controls magma stalling or (2) melt inclusion water re-equilibrates at the storage depth. If (2) were the dominant control, then melt inclusion water contents would not correlate with non-volatile trace elements (e.g., Nb/Ce and Ba/La), and yet they often do for the volcanoes studied here (our work in the Aleutians, [c], [d], [e]). Furthermore, storage depths tend to correspond to the depth at which degassing models predict that water degassing begins. Such systematics support (1), where intrinsically wetter magmas degas and crystallize deeper than dry magmas, resulting in deeper storage prior to eruption. This trend exists globally despite a wide range of extrinsic variables, demonstrating the global importance of intrinsic controls on the depth of reservoir formation.

[a] Zellmer et al. (2016). AmMin. [b] Chaussard & Amelung (2014). G³. [c] Sadofsky et al. (2008). CMP. [d] Plank et al. (2013). EPSL. [e] Walowski et al. (2016). EPSL.