Rapid ascent conditions of diamond-bearing kimberlitic magmas: Findings from high pressure-temperature experiments and finite element modeling

- Simulation of rapid ascent conditions of kimberlitic magmas in high P-T experiment
- Diamond to graphite transitions as a function of magma ascent rates
- Electrical resistivity as a parameter for the estimation of graphitization
- A threshold ascent value of 3 m/s for retention of diamond in metastable state
- Evaluation of the critical shape and density of magma pools for tensile fracturing

This paper deals with the problem of rapid ascent mechanism of kimberlite magmas with a multi-directional approach: 1) the kinetics of diamond-graphite transition; 2) settling velocity of diamond phenocrysts in magmas and 3) formation of ruptures required for magma ascent with a high speed. Based on the diamond-graphite transition, we present an estimate of the ascent rates from high pressure and temperature experiments using Walker-type multi-anvil apparatus. The experiments were conducted with diamond, placed within a synthetic kimberlitic assemblage, keeping an initial pressure and temperature of 6 GPa and 1350 °C, respectively. It was observed that the volume fraction of diamond to graphite conversion strongly depended on the ascent rates. Using electrical resistivity and X-ray diffraction studies, we measured the degree of graphitization as a function of the ascent rate (u). For u < 3 m/s, diamond underwent almost complete graphitization (conversion > 90%), whereas it remained nearly intact (conversion < 10%) when u > 10 m/s. Our theoretical calculations of the settling velocity of mantle xenoliths again confirm that diamond can exist when u > 3 m/s. We performed numerical experiments with finite element (visco-elastic) models to analyze the dynamics of tensile failure at the tip of magma pools, leading to dilatational vertical fractures for magma transport. Considering the tensile strength of mantle in the order of 0.5 kb, our models show this failure process as a function of the critical shape (Ar: ratio of vertical and horizontal dimensions) and density contrast (Δρ) of magma pools. The critical Δρ is estimated nearly 200 kg/m3 when Ar is very large (> 4).