Thermodynamic calculations of the polybaric melting phase relations of spinel lherzolite

Abstract

This study presents a new thermodynamic model for the calculation of phase relations during the melting of anhydrous spinel lherzolite at pressures of 1-2.5 GPa. The model is based on the total energy minimization algorithm for calculating phase equilibria within multicomponent systems and the thermodynamic configuration of Ueki and Iwamori [2013]. The model is based on a SiO 2 2Al 2 O 3 2FeO2Fe 3 O 4 2MgO-CaO system that includes silicate melt, olivine, clinopyroxene, orthopyroxene, and spinel as possible phases. The molar Gibbs free energy of the melt phase is modeled quasi-empirically, and the thermodynamic parameters for silicate melt end-member components are calibrated with a polybaric calibration database. The temperatures and pressures used in this newly compiled calibration data set are 1230-1600 C and 0.9-3 GPa, corresponding to the stability range of spinel lherzolite. The modeling undertaken during this study reproduces the general features of experimentally determined melting phase relations of spinel lherzolite at 1-2.5 GPa, including the solidus temperature, the melt composition, the chemical reaction during melting, and the degree of melting. This new thermodynamic modeling also reproduces phase relations of various bulk compositions from fertile to deplete spinel lherzolite and can be used in the modeling of polybaric mantle melting within various natural settings. Comparing the results derived from this new modeling with those produced using previous models indicates that the new approach outlined here, involving a combination of total energy minimization and the direct calibration of melt thermodynamic parameters at pressure and temperature conditions corresponding to mantle melting with a relatively simple melt thermodynamic equation, can accurately model polybaric melting phase relations.

Geochemical Discrimination and Characteristics of Magmatic Tectonic Settings: A Machine‐Learning‐Based Approach

“…A deep melting source for OI magmas is also consistent with the negative loading for SiO 2 , because melt SiO 2 concentrations decrease with increasing pressure (Ueki & Iwamori, ).…”

A Simple Thermodynamic Model for Melting of Peridotite in the System NCFMASOCr

“…After the review stages of this paper the publication of Ueki & Iwamori (2014) became available. This offers an alternative model for exploring phase relationships limited to spinel lherzolite melting in the smaller FCMASO subsystem, but has not been evaluated in our work.…”

Density and seismic velocity of hydrous melts under crustal and upper mantle conditions

“…Our model and parameter set provides strong constraints on the construction of thermodynamic models for hydrous melt‐bearing systems, such as predicting melting phase relations [ Ghiorso and Sack , ; Ghiorso et al ., ; Gualda et al ., ; Ueki and Iwamori , ; Jennings and Holland , ] and modeling the solubility of H 2 O in melts at high pressures [ Newman and Lowenstern , ]. Our results show that the Birch–Murnaghan equation can be used for the equation of state of the H 2 O end‐member component in hydrous melts.…”

“…In addition, H 2 O dramatically changes the phase relation of the melt‐bearing system [ Kushiro et al ., ; Green , ; Médard and Grove , ; Green et al ., ]. Detailed information on the pressure ( P )–volume ( V ) relation of a hydrous melt is important for constructing a polybaric thermodynamic model for a melt‐bearing system [ Ghiorso and Sack , ; Ghiorso et al ., ; Gualda et al ., ; Ueki and Iwamori , ; Jennings and Holland , ], and is a necessary constraint on water solubility models [e.g., Newman and Lowenstern , ]. Density and compressibility of hydrous melts also provide tight constraints on geophysical inversion to predict physical and chemical conditions of the melting region based on seismic velocity data [e.g., Nakajima et al ., ].…”