High liquidus temperature rhyoliteMELTS

[vladimirgeo]

Hello everyone,

I have a problem with the liquidus temperature calculation. I am using the 1.2.0 rhyolite+MELTS+CO₂+new-H₂O. When I calculate T for anhydrous compositions, it gives me quite promising estimates (for instance 1120°C). However, when I add a little bit of water, let's say 3.5wt%, it suddenly gives me a temperature of 5000°C or so. I know I need to click something ... I tried everything, can you please help me?

Vlad

[Paula]

Hi Vlad,

The problem is that in MELTS, generally, any phase that is not silicate liquid is classed as 'solid'. The 'Find Liquidus' function in the graphical user interface (GUI) searches for the point where one or more 'solid' phases are just saturated. In a fluid-rich system where vapor is exsolved, the 'Find Liquidus' function will keep pushing the temperature up to try to melt the 'solid' phase.

MELTS_Excel has a 'Find  wet  liquidus' function which  searches  for  the  point  where  any  solid phase  except  water (or H₂O-CO₂ fluid) is  just  saturated. So you could try that. The next update of the alphaMELTS 2 beta (see here) will have a 'Find wet liquidus'-like function for the rhyolite-MELTS+H₂O+CO₂ fluid model too.

Otherwise, for the GUI, your best option is to first locate the anhydrous liquidus. Then for the hydrous system start at the anhydrous liquidus and step down-temperature in 1^oC increments, say, and note where any other phase (other than fluid) joins the assemblage.

Paula