T164 Gem Minerals, Localities and Geological Significance

Tuesday, 24 September 2024
8:00 AM - 12:00 PM
207A

CONSTRAINTS ON DIAMOND DEPTHS OF ORIGIN FROM Fe-Mg PARTITIONING

Paper number: T164-12
Program: Technical Sessions
Time: 10:40 AM – 10:55 AM

Authors

  • Sha Chen Presenting Author
    Earth, Planetary, and Space Sciences, Los Angeles, CA
  • Janina Czas
    Earth & Planets Laboratory, Washington, D.C., WA
  • Peng Ni
    Earth, Planetary, and Space Sciences, Los Angeles, CA
  • Steven B. Shirey
    Carnegie Institution for Science, Washington, DC
  • Lars Stixrude
    University of California Los Angeles, Los Angeles, CA

Abstract

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 XFe (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 (XFe = 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 XFesil 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.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-402754

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