ABSTRACT
The modified increment method has been applied to the calculation of oxygen isotope fractionation factors for the polymorphic phases of MgSiO3 and Mg2SiO4. The results suggest the following sequence of 13O-enrichment in the different structures of the mantle silicates: pyroxene Mg2Si2O6 > olivine Mg2SiO4 > spinel- structured Mg2SiO4 > ilmenite-structured MgSiO3 > perovskite-structured MgSiO3. Assuming isotopic equilibrium between the various phases of the mantle, the chemical structure of the mantle can be described by the following sequence of 13O-enrichment: upper mantle > transition zone > lower mantle. Such an oxygen isotope layering results from differences in the chemical composition and crystal structure of mineral phases at different mantle depths.
Oxygen isotope fractionations between pyroxene and olivine in mantle assemblages can be negative under closed-system conditions, provided that pyroxene has a derivation of the lower mantle from the polymorphic transition of the perovskite-structured MgSiO3 without isotopic reequilibration. It can thus have inherited the oxygen isotope composition of the lower mantle minerals without isotopic reequilibration despite the change in crystal structure. Oxygen isotope inheritance in mineral formation by polymorphic transformations is very common in nature and in laboratory experiments. Therefore, the occurrence of the lower mantle materials can be discovered by studies on the oxygen isotope fractionation relationship in the mineral assemblages of the mantle-derived rocks. Moreover, spinel or corundum formed by the decomposition of garnet in the mantle could be out of isotopic equilibrium as normally defined with coexisting pyroxene and olivine due to the oxygen isotope inheritance in the Mg-O-Al of Al-O structural units of garnet.
