I’m trying to study the influence of lithospheric properties (such as lithospheric thickness, fracture zones) on plume material melting and melt migration.
I want to model melt migration with the Melt_global material model. In this subsection only melt and background rock seem to be distinguished. However, I need to further define the properties of the background solid.
I wonder if it is possible to represent different background solid by setting varying porosity in the initial compositional field. For example, setting high porosity in a local area of the lithosphere to add a magma channel. I have tried a simple setup-- set the lithospheric velocity to 0 and the initial composition model is defined as shown below:
Sorry for the slow reply (and I guess this also answers my question from the other thread; you do need magma transport for your problem). For what you’re trying to do, I think you need to pick a material model that is closest to what you want to do and then modify it yourself. For example, if you wanted to have a channel that makes it easy for melt to flow though, you could add a compositional field to one of the melt material models and give that a higher permeability, making it easier for melt to flow though a specific place.
The porosity is simply the volume fraction of melt at a given location, so if, for example, your temperature was below the solidus then all your melt would start to crystallize.
But studying melt migration in the setting of a plume interacting with the overlying lithosphere that has a visco-elasto-plastic rheology is a challenging problem; you would expect interactions between the melt and the fractures in the lithosphere, for example, and this is something that we haven’t implemented in ASPECT yet. So can you be a bit more specific about what features you would actually need for your problem, and then we can see what’s currently in ASPECT and what you would have to add and who to talk to. It could also be good to come to one of the ASPECT user meetings on Wednesday.
I read your paper published in 2016 and found that my previous understanding of porosity was wrong.
I want to calculate the magmatic yield (in other words, accretive crustal thickness). Actually, at present my main concern is how does melt get extracted through the lithosphere, rather than the complex interactions between the melt and the lithosphere.
I would like to know if and how Melt_Global implements the melt extraction process. Is it based on extraction model by Schmeling (2006)? How is the critical melt fraction defined?
I’m interested in what you say about the compositional field with higher permeability.
But I’m not quite sure what you mean. To define different permeabilities for different compositional fields, can it be achieved directly by defining the reference permeability parameter in the Melt_Global material model?
Thank you again for your patience and cooperation.
The melt global model does not implement melt extraction through the lithosphere. What we implemented in ASPECT is the flow of magma through a porous medium (so appropriate for the part of the mantle that deforms viscously, with melt fractions below 30%). If you want to extract your melt through the lithosphere, that would require some sort of additional physics.
The problem with some of the simpler melt extraction routines implemented in other codes, where you take out melt from one cell and then place it at the surface, is ASPECT’s adaptive mesh. Taking our melt and putting it somewhere else requires searching through all cells to find the right one, which is very slow with an unstructured mesh (which you need for adaptive mesh refinement). This is especially problematic in models that run in parallel on many processors. Therefore we have not implemented models like that in ASPECT, but if you have a small model and are fine with the computational cost, you could definitely implement something like that yourself.
But this is why I suggested reducing the permeability instead. This is something that would be possible within ASPECT’s finite element framework (we would still solve the same equation). You would have to change the code in the melt global model and assign a different reference permeability for your different compositional fields.