Thermoelastic Properties of Fe3+‐Rich Jeffbenite and Application to Superdeep Diamond Barometry

• Published in: Geophysical research letters 2024-03, Vol.51 (6), p.n/a
• Main Authors: Qin, Fei, Wang, Fei, Smyth, Joseph R., Zhang, Dongzhou, Xu, Jingui, Jacobsen, Steven D.
• Format: Article
• Language: English
• Subjects: Carbonates; Compression; Data compression; Diamonds; Earth mantle; Entrapment; Equations of state; External pressure; High pressure; Inclusions; Iron; Lower mantle; Modelling; Peridotite; Phase stability; Plate tectonics; Pressure; Stability; Tectonics; Thermal properties; Thermodynamic properties; Thermoelastic properties; Transition zone
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL106908

Jeffbenite (Mg3Al2Si3O12) is a tetragonal phase found in so far only in superdeep diamonds, and its thermoelastic parameters are a prerequisite for determining entrapment pressures as it is regarded as a potential indicator for superdeep diamonds. In this study, the thermoelastic properties of synthetic Fe3+‐jeffbenite were measured up to 33.7 GPa and 750 K. High‐temperature static compression data were fitted, giving (∂KT0/∂T)P = −0.0107 (4) GPa/K and αT = 3.50 (3) × 10−5 K−1. The thermoelastic properties and phase stability are applied to modeling isomekes, or P‐T paths intersecting possible conditions of entrapment in diamond. We calculate that under ideal exhumation, jeffbenite entrapped at mantle transition zone conditions will exhibit a high remnant pressure at 300 K (Pinc) of ∼5.0 GPa. Elastic geobarometry on future finds of jeffbenite inclusions can use the new equation of state to estimate entrapment pressures for this phase with still highly uncertain stability field in the mantle. Plain Language Summary Ongoing superdeep diamonds research is providing new insights into the Earth's deep mantle. Natural superdeep diamonds and its inclusions can show compelling evidence for retrograde conversion from the lower mantle or transition zone precursors; along with carbonate melt‐peridotite reactions. Jeffbenite with a composition of Mg3Al2Si3O12, found in so far only in superdeep diamonds can be regarded as a potential indicator mineral for superdeep diamonds. Recent synthesis of Fe3+‐rich jeffbenite provides an opportunity for in‐situ measurements to study the thermodynamic properties of jeffbenite at deep‐mantle conditions. Thus, in this study, we explored the high pressure and temperature stability and thermoelastic properties of Fe‐bearing jeffbenite up to 33.7 GPa and 750 K. The thermoelastic data and phase stability were measured and the results are applied to modeling the isomekes, or P‐T paths intersecting possible conditions of entrapment and along which the pressure on the inclusion is equal to the external pressure on the diamond host. Our finding can be applied to determining entrapment pressures in such diamond inclusions in future finds and its primary or retrograde history is essential in understanding mantle dynamics and the hidden consequences of plate tectonics. Key Points We measure the phase stability and thermoelastic properties on synthetic Fe‐bearing jeffbenite The thermoelastic data are applied to modeling isomekes and the remnant