3D Diffusion of Water in Melt Inclusion‐Bearing Olivine Phenocrysts.

Olivine‐hosted melt inclusions are an important archive of pre‐eruptive processes such as magma storage, mixing and subsequent ascent through the crust. However, this record can be modified by post‐entrapment diffusion of H+ through the olivine lattice. Existing studies often use spherical or 1D models to track melt inclusion dehydration that fail to account for complexities in geometry, diffusive anisotropy and sectioning effects. Here we develop a finite element 3D multiphase diffusion model for the dehydration of olivine‐hosted melt inclusions that includes natural crystal geometries and multiple melt inclusions. We use our 3D model to test the reliability of simplified analytical and numerical models (1D and 2D) using magma ascent conditions from the 1977 eruption of Seguam volcano, Alaska. We find that 1D models underestimate melt inclusion water loss, typically by ∼30%, and thus underestimate magma decompression rates, by up to a factor of 5, when compared to the 3D models. An anisotropic analytical solution that we present performs well and recovers decompression rates within a factor of 2, in the situations in which it is valid. 3D models that include multiple melt inclusions show that inclusions can shield each other and reduce the amount of water loss upon ascent. This shielding effect depends on decompression rate, melt inclusion size, and crystallographic direction. Our modeling approach shows that factors such as 3D crystal geometry and melt inclusion configuration can play an important role in constraining accurate decompression rates and recovering water contents in natural magmatic systems. Plain Language Summary: The water content of olivine‐hosted melt inclusions can reveal important information about the generation and storage of magma beneath basaltic volcanoes. Diffusion of hydrogen (as H+) through the olivine host crystal, however, can modify the water content of melt inclusions over minutes to hours. Here we develop a new 3D diffusion model for water loss from olivine‐hosted melt inclusions which includes natural crystal shapes and multiple melt inclusions. We use our model to test the reliability of different types of analytical and numerical models using conditions of magma ascent from the 1977 eruption of Seguam volcano, Alaska. We find that 1D and 2D numerical models underestimate water loss and magma decompression rates because they do not account for additional water loss from all directions. An anisotropic analytical solution that we present compares well with the 3D model giving decompression rates within a factor of 2. Multiple melt inclusions can also shield each other and help to reduce water loss. Our modeling approach shows that factors such 3D crystal geometry and melt inclusion configuration can play an important role in constraining accurate decompression rates, and recovering water contents in natural magmatic systems. Key Points: New 3D multiphase finite element diffusion model and anisotropic analytical solution for water loss from melt inclusions1D and 2D numerical models underestimate magma decompression rates compared to 3D models. The analytical solution performs wellShielding effect from multiple melt inclusions may limit water loss [ABSTRACT FROM AUTHOR]