CONSTRAINTS ON DIAMOND DEPTHS OF ORIGIN FROM Fe-Mg PARTITIONING

Sublithospheric diamonds host rare, direct samples from Earth's deep mantle, making them invaluable for understanding the mantle's composition and unveiling the subduction processes. Ferropericlase (Mg, Fe)O is the most prevalent type of inclusions reported in sublithospheric diamonds, comprising >40% of the total. Initial studies attributed them to originate from the lower mantle, but later work demonstrated alternative ways to form them in the upper mantle.

Coexisting pairs of mineral inclusions in these diamonds may provide better constraints on the depths at which these diamonds formed. The most studied mineral pairs are ferropericlase and (Mg,Fe)SiO₃ perovskite, as experiments show that ringwoodite decomposes into these two minerals at the bottom of the mantle transition zone. However, coexisting ferropericlase and olivine polymorphs are rarely reported and thus less studied.

Here, we report preliminary work on thermodynamic modeling of Fe and Mg partitioning between the ferropericlase (Mg,Fe)O and olivine polymorph (Mg,Fe)₂SiO₄ to test whether it can be used to help distinguish the three parental forms of (Mg,Fe)₂SiO₄ (olivine, ringwoodite, wadsleyite) and to constrain the minimum depth of their host diamond formation. We use thermodynamic models to compute the Gibbs free energy change of the ion exchange reaction at various temperatures and pressures. Symmetric interaction parameters were used to describe both solid solutions. When equilibrated at a given P-T condition, the X_Fe (molar Fe/(Fe + Mg)) composition of one mineral is thermodynamically controlled by the other. In addition, we report two newly identified pairs of ferropericlase-olivine in diamond samples from Juina with high iron contents (X_Fe = 0.31 for ferropericlase in DIA0000OR and 0.69 in DIA0000M2).

The results show that within the stability field of each (Mg,Fe)₂SiO₄ mineral phase, the curves for each polymorph are well separated when X_Fe^sil is close to 0.3, indicating that Fe-Mg exchange in fpr/ol pairs is most sensitive to depth when the system is relatively Fe-rich. In addition, we found the (Mg,Fe)₂SiO₄ phase in sample DIA0000M2 to be most likely olivine, last equilibrated under P-T conditions 300 K colder than the adiabatic mantle geotherm.