Dear Akhila,

Hope to find you well.

Temperature and pressure control the physicochemical activities in metamorphic environments. In the other side the liquid and volatile species with high oxygen contents control the oxygen fugacity in these environments. Some factors such as the temperature level have great influence on the release of oxygen bearing components like H2O and CO2 from the structure of surrounded rock types. On the other side the presence of reduction agents such as graphite and coal can play an important role in this regard. In general, the chemical composition of the Formations and the temperature grades are worthwhile in this regard.

Cheers,

Rahim

Thank you Rahim Masoumi for your crisp and concise answer.

Dear Mr. Akhila,

To address this issue is not as easy as it might look like and a series of side effects have a strong say when it comes to provide a clear answer.

The oxygen fugacity can be deduced from the mineral association, e.g., in

1. the Fe-S-C-O system with critical minerals such as pyrite, pyrrhotite, hematite, magnetite and siderite

2. the chlorite system which has clay minerals accommodating bivalent and trivalent Fe into their lattice (see also amphiboles at higher metamorphic grades)

3. the Mn-Si-O system

That means you need systems which can respond to changes in the oxygen fugacity under variations of the pressure and temperature regimes.

The term “hydrotherms” is rather poorly defined in this question because you have to consider the subcritical and supercritical systems which range from very low grade to high grade stage metamorphism.

Kind regards

H.G.Dill

thank you Harald G. Dill sir for your valuable insight. in case of complex mineral assemblages defining systems itself seems difficult. in one example, chlorite schist with multiple sulphide oxide mineral assemblage if considered as the original environment for another event of mineralisation, what should be our approach in understanding fO2 variations during the event?

Dear Akhila V R: Rahim Masoumi's answer is as complete as it can be! Definitely the presence of graphite in metamorphic rocks controls the oxygen fugacity.

Regards, Sebastian.

Dear Akhila,

If you consider the before made responses you can get a clear response to your question... Only in plus: the tectonically fragmentation of the metamorphic hydrothermal and you have to quant to the pressure of the process too.

Regards,

Laszlo

Dear All,

as far as the C component is considered as a stand-alone parameter it is too simple an approach to describe the oxygen fugacity, because C occurs in a varied series of minerals or C-bearing compounds from impsonite through semigraphite of different order to graphite sensu stricto mostly associated with a great variety of sulfides, mainly Fe sulfides:

DILL, H.G., KUS J., GOLDMANN S., SUÁREZ-RUIZ I., NEUMANN T., and KAUFHOLD S. (2019) The physical-chemical regime of a sulfide-bearing semi-graphite mineral assemblage in metabasic rocks (SE Germany) – A multidisciplinary study of the

missing link between impsonite and graphite.- International Journal of Coal Geology (in print).

With kind regards

H.G.Dill

thank you Harald G. Dill , László Attila Horváth and Sebastián Grande for your valuable comments.