Distributions of major (Si, Al, Ca, Mg, Fe) and minor elements (Cs, Rb, K, Na, Sr, Ba, Pb, Cr, Sc, Y, Yb, Ho, Sm, Nd, La, Ti, Zr, Hf, U, Th and Nb) between majorite garnet, MgSiO3-perovskite, CaSiO3-perovskite and coexisting liquids have been determined experimentally in ultrabasic and basic compositions at pressures of 15-25 GPa and temperatures of 1400-2200°C using an MA-8 apparatus. The results demonstrate the capacity of silicate perovskites to accept a wide range of normally "incompatible" elements possessing diverse ionic radii and charges into their crystal structures. All of the minor elements investigated are preferentially partitioned in Mg- or Ca-perovskites as compared to majorite garnet under subsolidus conditions. In subsolidus assemblages containing garnet (gnt) and Mg-perovskite (mpv), Sc, Ti, Zr, Hf and Nb are preferentially partitioned into perovskite with D mpv gnt values of 3-14 whereas there is little fractionation of rare earth elements (REE), all of which have D mpv gnt values of 1-2. In subsolidus assemblages containing Mg-perovskite and Ca-perovskite, Sc, Nb, Zr, and Hf are preferentially partitioned into Mg-perovskite whereas all other minor elements are strongly partitioned into Ca-perovskite. Majorite garnet which appears on the liquidus in ultrabasic compositions at ∼ 16 GPa and ∼ 2100°C is enriched in Al2O3 compared to the coexisting ultrabasic liquid by factors of 2-3 and is depleted in CaO and TiO2 by similar factors. The partition coefficients D gnt liq which are applicable to liquidus majorite garnet in ultrabasic compositions are Sc (1.7), Yb (1.4), Y (1.3), Hf (0.8), Zr (0.6), Sm (0.2) and less than 0.1 for K, Sr, La, Th, U, Ba, Rb and Cs. Partition coefficients (D mpv liq) between Mg-perovskite and ultrabasic liquids were obtained by combining garnet/liquid and subsolidus garnet/Mg-perovskite partition data obtained at similar temperatures. It was found that Sc, Hf, Zr, Ti and HREE are enriched in liquidus Mg-perovskite with D mpv liq values ranging between 2 and 14. Partition coefficients between Ca-perovskite (cpv), garnet and liquid were determined in a basaltic composition at ∼ 20 GPa and ∼ 2000°C. D cpv liq values for U, Th and Pb are remarkably high (10-20) and are also high for REE and Sr (2-6) whereas K, Rb, Cs and Ba behave as incompatible elements with D cpv liq values less than 0.5. The results provide strong constraints upon hypotheses which maintain that the mantle experienced extensive melting during formation of the earth followed by fractional crystallization-differentiation processes involving majorite garnet and/or MgSiO3-perovskite. The measured partition coefficients show that fractionation of garnet would be accompanied by sharp decreases of Al Ca and Sc Sm ratios in residual liquids. Likewise, fractionation of Mg-perovskite would cause marked variations of Lu Hf and especially of Sc Sm and Hf Sm ratios in residual liquids. The observation that the present upper mantle possesses near-chondritic relative abundances of Ca, Al, Sc, Yb, Sm, Zr and Hf categorically excludes models which propose that the bulk mantle once possessed chondritic relative abundances of Mg, Si, Al, Ca and other lithophile elements, but has differentiated to form a perovskitic lower mantle ( Mg Si < 1) and a peridotitic upper mantle ( Mg Si > 1). The chemical and isotopic compositions of ancient (∼ 4.2 Ga) Western Australian zircons imply that even if the mantle were extensively melted and differentiated around 4.5 Ga, it must somehow have become effectively rehomogenised by solid state convection by 4.2 Ga. Since this scenario appears implausible on dynamic grounds, it is concluded that the mantle probably did not experience extensive melting during the formation of the Earth.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science