EARTH
In the literature: contested
How long do crustal magma reservoirs persist as crystal mush?
In plain words
Under many volcanoes magma is stored for long times as a crystal mush, mostly crystals with a little melt between them. How long it stays there and at what temperature is debated.
Precise statement
Determine the thermal history of upper-crustal magma reservoirs: the fraction of their lifetime ($10^{4}\ \text{to}\ 10^{6}\,\mathrm{yr}$) spent as eruptible magma (crystal fraction below about 0.5) versus near-solidus cold storage, using crystal diffusion chronometry, zircon geochronology and thermal models. An answer is a reconciled storage temperature-time history for well-studied systems.
What would settle it
Diffusion and U-Th zircon chronometry from the same crystals reconciled with thermal-mechanical models of incremental reservoir growth.