Describing the chemical character of a magma

Abstract

We introduce the concepts of hard-soft acid-base (HSAB) interactions and derive parameters to characterise a magma that consists either of a solid rock, a melt or its exsolved gaseous phase. Those parameters are the electronegativity, i.e. the chemical potential changed of sign, hardness, electrophilicity and polarisability. They determine the chemical reactivity of each individual element, or its equivalence in the case of a complex system of elements or compounds. The parameter values result from equalisation methods for complex systems, i.e. the equalisation of the respective chemical potentials, or from direct computation through density functional theory (DFT). They help in characterising magma by a single value. They also provide insights into the reactivity of the melt or its fluid phase when in contact with another magma, or when considering the affinity of elements for each phase. The description leads to a better understanding on the mechanisms that control sequestration and transport of metals within the different phases during igneous activity. The presence of halogens tends to reinforce hardness, making the fluid phase attractive for hard metals such as Sn or W. Conversely, the presence of S decreases hardness of the fluid phase that becomes attractive for soft metals such as Au, Ag and Cu. Theoretical principles of maximum hardness and minimum electrophilicity can be used to identify the poles toward which the reactions should naturally evolve. The trends observed during magma evolution are also observed using these parameters, indicating two different trends toward a pole of greater stability. One pole consists in increasing the hardness at constant electrophilicity as observed for felsic magmas toward a silica-rich pole. Conversely mantle-derived magmas evolve at a constant hardness and decreasing electrophilicity in between the two poles of iron-rich and alkali-rich minerals. They respectively correspond to the Bowen and Fenner trends, though the experimental and field evidences have now some chemical explanation.