STATIC COMPRESSIBILITY OF MAGNESITE TO 20 GPA - IMPLICATIONS FOR MGCO3 IN THE LOWER MANTLE

The bulk modulus of MgCO3 magnesite has been measured to 20 GPa at roo m temperature using X-ray powder diffraction. Since the response to pr essure is primarily compression of the MO6 octahedron, magnesite is su bstantially less compressible than Ca-carbonates of similar structure type. Using the Birch equation of state, with K(T') of 4.0, we obtain K(T) of 142 (+/-9) GPa at room temperature. The full Birch-Murnaghan e quation of state yields K(T) of 151 (+/-7) GPa and K(T)' of 2.5. These results, together with other thermodynamic data, have been applied to calculate the stability of magnesite under lower mantle conditions. A ssuming lower-mantle saturation in (Mg,Fe)O, magnesite should be stabl e to temperatures well in excess of 6000 K at the core-mantle boundary , i.e. at least up to the solidus of peridotitic or pyroxenitic compos itions. This confirms that magnesite is a stable host for carbon in th e mantle.