Determining Fe2O3 content by setting fO2 at given P-T conditions

[Sati]

Hi everyone! I'm a new user and I'm trying to model the variation in Fe2O3 content as a function of fO2 at given P-T conditions, but I have few things that I'm unsure of...
First of all I'm using pMELT at 1GPa and 1600ºC, the process I'm using is to set an fO2, then computing redox state and then execute, and read directly the Fe2O3 content in the Bulk system window, is this the right approach?

What is the meaning of the bulk system composition with respect to the Fe2O3 content? does it indicate the overall Fe2O3 content of melt+solid system? by selecting "phase: liquid-00" this should give the fO2 of the melt produced in this system?

When I decrease the temperature at fixed fO2 and 1GPa, the bulk system Fe2O3 diminishes from 1600 to 1400C but then it rises from 1400 to 1200C, from my understanding there should be a direct relationship between T and Fe2O3 (e.g., Kress and Carmichael 91) which seems to not being followed here when the temperature goes below 1400 C (?)

Sorry for the very basic questions, I'm not yet familiar with the software  ::)
Thanks!

[asimow]

Dear Sati,

What you are doing sounds correct. The reason the behavior you are seeing is unexpected is that the bulk system Fe2O3 is a combination of two effects: the varying Fe2O3/FeO of the melt (which is what Kress and Carmichael describe) and the varying melt fraction and mineralogy as you crystallize the system. The superposition of these two effects indeed gives a minimum in bulk system Fe2O3, I think at the temperature where clinopyroxene joins the residue.

-- Paul

[Sati]

Thanks Paul for your reply! :)

When I see the change in Fe2O3 behaviour is indeed at the temperature of cpx precipitation in the residue...Thanks!
Sati