Ascent Dynamics of Low Degree Mantle Partial Melts, Constrained from CO2 Solubility Experiments.

Abstract:

Low degree partial melting of carbonated mantle peridotite generates strongly silica-undersaturated melts containing substantial amount of carbon dioxide (several tens of wt%). Kimberlite melts are one of these volatile-rich mantle product and are believed to ascent through the upper mantle and crust at great speed (~5 to 50 ms^-1). The role of volatiles in propelling this ascent has remained poorly quantified due to experimental difficulties in quenching such compositions to a glass. In this study, we used a range of melt compositions in the Si-C-Al-Ca-Mg-Fe-Na-K-O system addressing the chemical complexity needed to closely mimic kimberlitic to carbonatitic characteristics. These melts can, furthermore, be quenched fast enough to produce a glass and be used to determine the CO₂ solubility as a function of composition and pressure. Our results suggest that the solubility of CO₂ decreases steadily with increasing amount of network forming cations from ~30 wt% CO₂ at 12 wt% SiO₂ down to ~3 wt% CO₂ at 40 wt% SiO₂ and that pressure has limited effect on the solubility of CO₂ up until very shallow depth (~ last 3 km). This peculiar pressure-solubility relation in kimberlite melt can explain the highly explosive nature of kimberlite magma and characteristic geo-morphological features of their root zone. We present a general CO₂ solubility model based on thermodynamic formalism covering a large range of melt composition from 11 to 53 wt% SiO₂ spanning the transition from carbonatitic to basaltic melts at pressures up to 1500 MPa.