Bridgmanite was initially considered to hold little water. Recent research has changed this view. What is the important factor that allows Bridgmanite to hold more water than previously thought?

Question

Bridgmanite was initially considered to hold little water.  Recent research has changed this view.  What is the important factor that allows Bridgmanite to hold more water than previously thought?

BlackTom AI · Accepted Answer

High temperature increases the equilibrium concentration and mobility of point defects in bridgmanite’s crystal lattice, enabling much greater incorporation of hydrogen as hydroxyl (OH–) defects — thus bridgmanite can hold more water when hotter.

1) Relevant knowledge points and concepts
- Bridgmanite (Mg,Fe)SiO3) is a perovskite-structured mantle mineral that stores water as H+ (protons) bound to oxygen as OH groups or as protonated defects.
- Water incorporation requires charge-balancing defects (oxygen vacancies, cation substitutions) and is governed by defect thermodynamics and kinetics.
- Defect concentrations follow Arrhenius-type temperature dependence: higher T → exponentially more defects and greater ionic mobility.
- High-pressure, high-temperature experiments (diamond-anvil and multi-anvil) revealed much higher H solubility at mantle-like temperatures than earlier low-T experiments suggested.

2) Step-by-step reasoning
- Water enters bridgmanite by reactions such as 2H+ + O2– → 2(OH)– or via coupled substitutions that create vacancies or change cation valence.
- The equilibrium constant for defect formation increases with temperature; thus the number of available defect sites (for H incorporation) rises exponentially with T.
- Increased mobility at high T allows protons to diffuse into and stabilize within the crystal during synthesis/formation rather than being excluded.
- Therefore, experiments conducted at realistic high mantle temperatures show bridgmanite can incorporate significantly more H (water) than earlier low-temperature results indicated.

3) Calculations
- No numerical calculation is required here; the key is the Arrhenius relation for defect concentration: n ∝ exp(–E/kT). As T increases, exp(–E/kT) increases, so n increases.

4) Why the correct answer is correct
- Temperature directly controls defect creation and hydrogen mobility; higher T increases both the number of sites that can host OH and the kinetics to occupy them. This mechanistic link explains why hotter bridgmanite holds more water.

5) Why other factors are less explanatory
- Pressure: important, but pressure alone does not create the defect mobility needed; in many experiments pressure was already high, yet water uptake varied with temperature.
- Composition (Fe/Al content): affects solubility (Al, Fe can enhance H incorporation) but cannot by itself explain the large differences observed without the temperature-driven defect increase.
- Grain size or presence of melts: secondary influences on kinetics, not the primary thermodynamic control demonstrated by experiments.

6) Conclusion
The provided answer — “Temperature: Bridgmanite can hold more water when hotter” — is correct and consistent with defect thermodynamics and high-P, high-T experimental evidence.

Bridgmanite was initially considered to hold little water. Recent research has changed this view. What is the important factor that allows Bridgmanite to hold more water than previously thought?单项选择题

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