Bridgmanite may have held as much water as is present in today's oceans?

Question

BlackTom AI · Accepted Answer

Bridgmanite, the dominant mineral of the lower mantle (Mg,Fe)SiO3 in a perovskite structure, can incorporate hydrogen as hydroxyl defects within its crystal lattice. High-pressure, high-temperature experiments and theoretical work show bridgmanite can dissolve hydrogen at the level of hundreds to over a thousand parts per million (ppm) by weight under lower-mantle conditions, so it is plausible that it could store an amount of water comparable to Earth’s surface oceans.

Relevant concepts:
- Nominally anhydrous minerals (NAMs) can host water as OH defects even though they lack free H2O.
- The lower mantle is compositionally dominated by bridgmanite, so even small H concentrations per unit mass can sum to a large total reservoir.
- Ocean mass and mantle mass allow a simple mass-balance check to test plausibility.

Step-by-step reasoning and calculation:
1. Mass of Earth’s oceans ≈ 1.4 × 10^21 kg.
2. Mass of the whole mantle ≈ 4.0 × 10^24 kg; bridgmanite constitutes roughly ~70% of the mantle by mass → bridgmanite mass ≈ 0.70 × 4.0×10^24 kg = 2.8 × 10^24 kg.
3. Required bridgmanite water concentration to match ocean mass = (ocean mass) / (bridgmanite mass) = (1.4×10^21) / (2.8×10^24) = 5.0×10^-4 by mass = 0.05% = 500 ppm.
4. Laboratory and theoretical estimates show bridgmanite solubilities of order hundreds to >1,000 ppm H2O under lower-mantle pressures and temperatures, so a ~500 ppm average is within experimentally demonstrated ranges.

Why the correct answer is True:
Because bridgmanite comprises a very large mass fraction of the lower mantle, even modest H2O solubility (hundreds of ppm) integrates to a global reservoir comparable to the oceans. Experimental and seismological evidence (and analogous findings for transition-zone minerals like ringwoodite) support this possibility, so the statement that bridgmanite may have held as much water as today’s oceans is well founded.

Why “False” would be incorrect:
False would deny the experimental/theoretical and mass-balance plausibility. Although uncertainties remain about exact amounts and distribution, current data make the statement plausible rather than implausible.

Conclusion: The correct answer is True, matching the provided answer.

Bridgmanite may have held as much water as is present in today's oceans?True/False

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