Abstract

Melting experiments at high-pressure and temperature were conducted at pressures from 16 to 25 GPa using chondritic starting material but with slightly enhanced Na- and K-contents while keeping the chondritic Na/K ratio constant. The experiments revealed that majorite garnet contains enhanced concentrations of Na (3.75–4.71 wt.% Na2O) and moderately enhanced concentrations of K (0.3–0.46 wt.% K2O) from 16 to 20 GPa and it is therefore the Na- and K-bearing phase at this pressure range. The Na2O/K2O ratio (8–15) in garnet is close to the initial chondritic ratio (7) of the starting material, which might indicate that at these pressures only little Na/K fractionation takes place. At pressures above 21 GPa, the high-pressure phases enriched in Na include majorite garnet (3.08–6.22 wt.% Na2O), magnesiowiistite (1.83–3.3 wt.% Na2O), and (Mg, Fe)SiO3-perovskite (0.43–1.25 wt.% Na2O). These phases contain only small amounts of K (<0.1 wt.% K2O). The Na2O content of magnesiowüstite progressively increases with increase of pressure, whereas Na2O contents of garnet and perovskite are negatively correlated with pressures between 23 and 25 GPa. Na/K fractionation takes place among the high-pressure phases (Na2O/K2O ratio: 56–102 in garnet, 5–20 in perovskite, and 43–94 in magnesiowüstite), and between the high-pressure phases and the residual silicate melt above 21 GPa, and most of the K is partitioned into the residual melt. The hollandite-stractured, high-pressure KAlSi3O8polymorph crystallizes from the liquid at 23 GPa. These results demonstrate that Na/K fractionation at P > 21 GPa should have taken place in the early accretional period in the primitive chondritic mantle of the Earth.

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