There are three ways I study volatile elements under mantle pressure and temperature conditions.  through the study of diamonds,  experimental petrology, and  using the Deep Earth Water Model (this page).
The Deep Earth Water Model is a platform which enables geochemists to study aqueous geochemistry under extreme conditions, from their laptops. This work is novel and exciting and really began in 2014 (Sverjensky et al. 2014). However, it has already forged a global community with researchers from across the world (see website).
To advance a vision of how fluids have linked the deep Earth and the near surface environment through deep time.
Integration of predictive theoretical models with experimental data, field studies, and remote observation of the deep Earth to investigate the nature, migration, and reactivity of deep fluids in Earth.
More detailed information:
In low-temperature geochemistry eH and pH are both considered important variables for understanding the geochemistry of an environment. However, in high-temperature geochemistry only eH is considered, and this is expressed as fO2.
Note, fO2 is a dimension for the quantification of redox state (the exchange of electrons) whereas pH is a measure of acidity (the exchange of protons).
Because of the focus on redox (electron transfer) traditional modelling for the speciation of volatile elements in mantle fluids have long regarded mixtures of neutral gases (CO2, CH4, H2 and H2O; commonly termed COH-fluids), therefore the role of dissolved aqueous ions or species derived from silicate rock components have been overlooked/ignored. As a consequence, fO2 has long been considered paramount to predicting and understanding reactive volatile element reactions in arc systems and the mantle in general (e.g. not including the noble gases; see Frost and McCammon 2008). In the case of carbon (Sverjensky et al., 2014; Sverjensky and Huang, 2015) and nitrogen (Mikhail and Sverjenksy 2014), and Fl, Cl and REE (Williams-Jones Elements Magazine) the large pH shift as a function of petrology will significantly affect the speciation and behaviour of these biologically important elements (in the mantle and in subduction zones), and should therefore have influenced which species of pH sensitive compounds were and are degassed into planetary surface environments.
The advance made by the Deep Earth Water Model
The speciation of aqueous ions, metal complexes, neutral species and minerals can be predicted by applying the Helgeson-Kirkham-Flowers (HKF) equations of state. There existed a historic limitation of pressures < 0.5 GPa, because the dielectric constant of water was not known > 0.5 GPa. This restricted the application of the HKF equations of state to matters concerning deep Earth systems (lower crust, mantle, subduction zones). However, recent advances have solved this caveat and now extend the P-T range for the application of the HKF equations of state for aqueous species up to ≤6 GPa and ≤1200°C (Sverjensky et al. 2014), and this advance has led to the formulation of the Deep Earth Water Model. Therefore, it is now feasible to speciation of aqueous ions, metal complexes, neutral species and minerals across conditions akin to the pressure and temperature pathway followed by a subducted-slab from Earth’s surface to a depth of around 150 km (Mikhail and Sverjenksy 2014 Sverjensky et al., 2014; Sverjensky and Huang, 2015).
 is there stratification of fluid pH with depth in the bulk silicate Earth (pressure effect)?
 how much (if at all) has the pH of mantle fluids changed through time?
 how much of a difference is there for the pH of mantle fluids between the different inner solar system planets (as is the case for fO2)?
 how accurate is the large pH shift as a function of petrology derived by this theoretical approach?
 which reactions or elemental fractionations can be used to retrospectively constrain the pH of a metasomatised high-temperature environment using data from silicate/oxide minerals or melt inclusions?