Category: Planetary Physics & Geodynamics
Archean cratons—the ancient nuclei of continental plates formed between 2.5 and 3.8 billion years ago—have endured billions of years of supercontinental rifting, mantle plume bombardment, and Wilson cycles without undergoing destructive convective recycling. Seismic tomographic imaging reveals that cratons are underlain by massive lithospheric roots or “keels” extending to depths of 200 to 250 kilometers. Under standard thermodynamic conditions, these deep roots should be cold, dense, and gravitationally unstable, causing them to delaminate and plunge into the convecting asthenosphere. Resolving why cratonic keels remain stable over billion-year timescales is explained by Jordan’s classic isopycnic hypothesis and melt-depletion rheology.
1. The Isopycnic Hypothesis: Compositional vs. Thermal Density Compensation
The stability of cratonic keels rests on the balance between thermal contraction and chemical depletion. Because cratonic keels reside along a conductive continental geotherm, their temperature at 150–200 km depth is significantly lower (T ≈ 900–1,100°C) than the surrounding convective asthenosphere (T ≈ 1,350°C). This thermal difference (ΔT ≈ 250–450 K) generates thermal contraction, which should increase keel density by Δρ_th ≈ +50 to +70 kg/m³.
High-Degree Melt Extraction
However, mantle xenoliths preserved in diamond-bearing kimberlite pipes prove that cratonic keels consist of refractory harzburgites and dunitic peridotites that underwent extreme partial melting (30–45% melt extraction) in the Archean mantle. This intense extraction stripped iron, aluminum, and calcium (which partition preferentially into basaltic melt), leaving a residue dominated by magnesium-rich forsteritic olivine (Mg# = Mg / [Mg + Fe] ≈ 92.5–94.0):
Δρ_total = Δρ_th(T) + Δρ_chem(X) ≈ 0
Because magnesium is substantially lighter than iron, this chemical depletion produces an intrinsic compositional density deficit (Δρ_chem ≈ -50 to -70 kg/m³) that precisely cancels the thermal contraction. The net buoyancy of the cratonic root is approximately zero relative to ambient asthenosphere—a condition termed the isopycnic state.
2. Rheological Stiffening via Hydrogen Extraction
Even if neutrally buoyant, cratonic keels would eventually be sheared away by turbulent mantle convective currents unless protected by an immense effective viscosity contrast. The primary mechanism responsible for this mechanical rigidity is dehydration stiffening.
The Role of Trace Hydrogen in Olivine
In the ambient asthenosphere, trace quantities of hydrogen (water dissolved structurally as hydroxyl defect pairs within olivine, ~50–150 ppm wt H₂O) drastically lower mantle viscosity by facilitating hydrolytic weakening of Si-O bonds during dislocation creep. During high-degree Archean partial melting, hydrogen behaves as an incompatible element, partitioning cleanly into the extracting komatiitic melt.
The remaining cratonic harzburgite keel is completely desorbed of structural water (< 5–10 ppm wt H₂O). This dry state elevates the effective dynamic viscosity of cratonic peridotite by two to three orders of magnitude (η_keel ≈ 10²³–10²⁴ Pa·s versus η_asth ≈ 10²⁰–10²¹ Pa·s). The cratonic keel behaves as a rigid mechanical anchor that deflects convective flow beneath it.
3. Destruction and Re-fertilization: The Decratonization Frontier
While cratonic roots are remarkably resilient, they are not immortal. Under specific geodynamic conditions, cratons undergo destructive “decratonization”—most famously illustrated by the North China Craton during the Mesozoic.
Metasomatic Re-enrichment and Delamination
Long-term stagnant slab subduction in the mantle transition zone beneath a craton releases hydrous carbonatitic fluids and silicate melts. These ascending metasomatic fluids infiltrate the dry cratonic peridotite, re-introducing iron, calcium, and water while converting olivine to pyroxene. This re-fertilization destroys the isopycnic buoyancy balance and reduces keel viscosity, triggering gravitational detachment (convective delamination) of the lower keel into the deep mantle.
The Planetary Scale Metric
Keel Thickness L ≈ 200–250 km | Olivine Forsterite Content Fo = 92.8–93.5% | Effective Viscosity Contrast η_keel / η_ambient > 100–1,000
| Global Craton | Crustal Age | Keel Depth (Seismic) | Mean Fo Content | Structural Integrity Status |
|---|---|---|---|---|
| Kaapvaal Craton (S. Africa) | 3.6–2.7 Ga | ~ 200–220 km | 92.8% | Intact; archetypal stable cratonic keel |
| Slave Craton (Canada) | 4.0–2.6 Ga | ~ 220–250 km | 93.2% | Intact; stratified ultra-depleted shallow root |
| Siberian Craton | 3.4–2.5 Ga | ~ 240–280 km | 93.5% | Thick diamondiferous keel; partially modified |
| North China Craton (Eastern Block) | 3.8–2.5 Ga | ~ 60–80 km | 90.5% | Destroyed (Mesozoic decratonization) |
4. Key Takeaways and Geophysical Summary
- Isopycnic Balance: Thermal contraction of cold cratonic keels is neutrally offset by compositional depletion of iron and aluminum (Δρ_chem ≈ -60 kg/m³).
- Dehydration Stiffening: Extreme melt extraction purges trace hydrogen, elevating keel viscosity by 100–1,000× relative to convective asthenosphere.
- Convective Isolation: Buoyant, high-viscosity cratonic keels deflect ascending mantle plumes and lateral mantle shear around their margins.
- Decratonization Vulnerability: Slab-derived metasomatic fluid infiltration can re-fertilize keels with iron and water, destroying their buoyancy and causing catastrophic lithospheric delamination.
