Density of Carbonated Magmas and Stability of Carbonatite and Kimberlite at the Earth's Upper Mantle and Transition Zone

Seismological and electrical conductivity studies show that the presence of LVZ atop of the 410-km discontinuity which reveal the possible existence of a melt at this boundary [e.g., Reveanugh and Sipkin, 1994; Toffelmier and Tybruczy, 2007]. The anomalies from these studies support conceptual model [Bercovici and Karato, 2003]. Density measurements of anhydrous basaltic melts indicate that it is denser than the surrounding mantle near 410-km depth [Ohtani and Maeda, 2001]. Hydrous peridotitic and basaltic melts are denser than peridotite at the top of the 410 km discontinuity and therefore can be accumulated at the base of the upper mantle [Sakamaki et al., 2006]. CO2 is one of the important volatile in the mantle and it could be also important to constraints the conceptual models experimentally for the explanation of LVZ near a 410 km depth. In the present study, we have measured the density of carbonated basaltic melt at pressures from 16-20 GPa and 2573 K by using sink-float experiment using a diamond marker. We determined the partial molar volume of CO2 in magmas at around 20 GPa. Using the partial molar volume estimated by several authors in the lower pressure range, the compression behavior of the partial volume in magmas can be expressed by the Vinet equation of state with K= 16GPa and dK/dP= 5.2. Using the pressure dependency of the partial molar volume of CO2 in magmas, we can estimate the density of various carbonated magmas at high pressure. Our results show that the basaltic melt can contain up to ~3.5 wt% CO2 and the peridotite melt can contain up to ~4.0 wt% CO2 to be denser than the surrounding mantle at the top of the 410 km discontinuity. These amounts of CO2 are comparable with the amount of H2O in the hydrous basaltic (~3.0 wt%) and peridotitic (~6.7 wt%) melts, which is stable atop of the 410 km discontinuity [Sakamaki et al., 2006]. However carbonated melt can be formed only at significant degree of melting of mantle materials (e.g., peridotite or eclogite), whereas at low degree of partial melting (1-5 %) carbonatite melt, which is thought to be significantly less denser than peridotite, is formed [Dasgupta and Hirschmann, 2006, 2007]. The melt formed by higher degrees of melting of carbonated peridotite is kimberitic containing ~17-32 wt% of CO2 in the pressure range of the bottom of the upper mantle from 10 GPa to 20 GPa [Ghosh et al., 2005]. The present results indicate that the kimberlitic melt formed by partial melting of the carbonated mantle is less dense than the surrounding mantle, and it can ascend even from the depths of the base of the upper mantle and transition zone. This is consistent with the existence of mantle xenoliths containing diamond with majorite or perovskite inclusions in some kimberites. If we combine our data with hydrous basaltic melt [Sakamaki et al., 2006] and consider the linear mixing between H2O and CO2 then the basaltic melt with 1.5 wt% H2O and ~1.3 wt% CO2 and peridotitic melt with 3.3 wt% H2O and 2.0 wt% CO2could be stable at the top of 410 km discontinuity.