skill-melting

name	skill-melting
description	MDOODZ partial melting module — melt model indices, solidus/liquidus parameterizations, melt fraction computation, melt weakening, per-phase melt configuration, kinetic relaxation, and melting scenario examples.

Partial Melting

Overview

MDOODZ supports pressure-dependent partial melting via parameterized solidus/liquidus curves. Melting is computed per marker using the marker's temperature and pressure. The melt fraction field (X) is stored at cell centres and written to HDF5 as Centers/X.

Enabling Melting

Global Switches (in .txt file)

Parameter	Default	Description
melting	0	Enable melting globally (0=off, 1=on)
force_melt_weak	0	Enable melt-induced viscosity weakening (0=off, 1=on)
melt_weak	0.0	Activation volume override for weakening [J/mol] — typical: 30.0

Per-Phase Parameter (in .txt PHASE PROPERTIES blocks)

Parameter	Default	Description
melt	0	Melt model index (0=off, see table below)
tau_kin	0.0	Kinetic relaxation time constant [s] — 0.0 = instantaneous equilibrium

Melt Model Indices

melt	Name	Solidus	Liquidus	Latent heat	Reference
1	Simple T-dependent	600°C + 273 K (fixed)	1200°C + 273 K (fixed)	320 kJ/kg	Stüwe (1995)
10	Crustal/sediment (P-dependent)	P-dependent formula	1423 + 0.105·P (MPa)	380 kJ/kg	—
11	Felsic/crustal (P-dependent)	P-dependent formula	1262 + 0.09·P (MPa)	300 kJ/kg	Schenker et al. (2012)
40	Mantle peridotite (P-dependent)	P-dependent formula	2073 + 0.114·P (MPa)	400 kJ/kg	—
41	Mantle wet olivine (P-dependent)	P-dependent formula	2073 + 0.114·P (MPa)	300 kJ/kg	Schenker et al. (2012)

Choosing the Right Model

Phase type	Recommended melt	Rationale
Continental crust (e.g., quartzite, anorthite, granite)	11	Felsic solidus, Schenker 2012
Sediments	10 or 11	Lower solidus than mantle
Lithospheric mantle (dry olivine)	41	P-dependent peridotite
Asthenosphere (wet olivine)	41	Same model, wet conditions
Diabase / mafic lower crust	11	Treated as crustal

Melt Fraction Computation

Source: MDLIB/MeltingRoutines.c, function PartialMelting()

Equilibrium melt fraction:

φ_eq = (exp(α·T) − exp(α·T_s)) / (exp(α·T_l) − exp(α·T_s))

where T_s = solidus temperature, T_l = liquidus temperature, both P-dependent.

With kinetic relaxation (tau_kin > 0):

φ = (dt / (τ_k + dt)) · φ_eq + (τ_k / (τ_k + dt)) · φ_0

where φ_0 is the previous melt fraction and τ_k = tau_kin.

Clamped to [0, 1].

Melt Weakening

When force_melt_weak = 1 and melt_weak > 0, the code overrides the dislocation creep activation volume for melt-bearing phases:

if (model->force_melt_weak == 1) mat->apwl[k] = model->melt_weak;

This effectively reduces viscosity in regions with high melt fraction. The melt_weak value replaces the power-law activation volume (Vpwl), making melt-bearing regions weaker.

Typical value: melt_weak = 30.0 (used in RiftingMelting, RiftingCombinedYield, and other scenarios).

Interaction with Other Features

With eta_vp (viscoplastic regularisation)

When combining melting with plasticity (plast = 1 or plast = 2), eta_vp must be large enough to stabilise the weakened rheology.

Model scale	Without melting	With melting
Lithospheric rifting	eta_vp ≥ 1e19	eta_vp ≥ 2.5e20
Localised chamber	eta_vp ≥ 1e18	eta_vp ≥ 1e19

Stability warning: eta_vp = 1e19 with melting = 1 at lithospheric scale can produce instabilities (negative viscosity, values ~1e60) after ~100 steps. Use eta_vp = 2.5e20 or higher.

With combined yield (plast = 2)

No special interaction — plast = 2 works with melting. Ensure T_st and eta_vp are set per phase.

Example Configuration

### Global (in header of .txt)
melting           = 1
force_melt_weak   = 1
melt_weak         = 30.0

### Per phase
ID   = 0           // Continental crust
melt = 11          // Schenker 2012 felsic

ID   = 1           // Lithospheric mantle
melt = 41          // Schenker 2012 mantle

ID   = 2           // Asthenosphere
melt = 41          // Schenker 2012 mantle

Scenarios Using Melting

Scenario	Melt models used	Application
RiftingMelting	11 (crust), 41 (mantle)	Extension with decompression melting
RiftingCombinedYield	11 (crust), 41 (mantle)	Rifting + combined mode-I/II yield
SubductionMelting	Fluid-released melting	Slab-edge weakening
TCMagmaticSystem	10, 11	Thermo-chemical magmatic evolution
PressurizedMagmaChamber	— (thermal only)	Pressurised intrusion
MeltingOverpressure	—	Melting-induced overpressure
ChristmasTree	Multi-branch	Dike geometry

Key Source Files

• Melting computation: MDLIB/MeltingRoutines.c
• Melt weakening: MDLIB/FlowLaws.c (activation volume override)
• Parameter parsing: MDLIB/InputOutput.c
• HDF5 output: Centers/X (melt fraction field)