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3.3 f. Multianvil experiments on chondritic meteorites between 20 and 25 GPa: Implications for K and Na fractionation in the early primitive Earth's lower mantle (D.J. Frost, in collaboration with M. Chen and A. El Goresy/Mainz)

The fractionation of potassium and sodium between silicate minerals and melt at conditions relevant to the Earth's lower mantle is poorly constrained. To gain insight into the behavior of K and Na during the early evolution of the Earth's primitive chondritic mantle high-pressure experiments have been conducted on chondritic meteorite samples.

Two powdered samples of chondrite meteorites (H- and L-chondrites), with supplemented Na2O and K2O contents (3 times normal chondrite concentration) were used as starting materials (Na/K ratio ˜9). Multianvil experiments (7/3 and 10/4 assemblies) were conducted in the pressure range 22-25 GPa and at temperatures between 1800-2100°C. The charges are currently being studied by SEM, electron microprobe and micro-raman spectroscopy.

Preliminary results indicate that garnet, magnesiowüstite, and (K,Na)AlSi3O8 hollandite are stable above the chondrite solidus between 22 and 24 GPa, and that perovskite, magnesiowüstite, and garnet are stable above the solidus at 25 GPa. Electron microprobe analysis of the samples revealed that Na is strongly fractionated from K in the phases garnet and magnesiowüstite. The Na2O/K2O ratios in majorite-rich garnet and magnesiowüstite are ˜60 and ˜170, respectively. K is preferentially incorporated into K-hollandite. In addition, uncharacterized phases with high K-contents were also encountered at 24 and 25 GPa and are being studied further. The residual melt K2O concentrations were lower in the L-chondrite (0.32 wt%) than in the H-chondrite (0.87 wt%). This is due to a higher K-hollandite concentration in the L-chondrite samples, which may be a result of the lower melting temperature and MgO content of L-chondrite compared to H-chondrite.

These results seem to indicate that for chondritic compositions at lower mantle conditions K is a compatible element during melting, preferring to reside in the residual solid phases. This is in contrast to K behavior at upper-mantle conditions. There may be important implications for the genesis of K-rich kimberlitic magmas, which, from these results, would seem unlikely to be formed at lower-mantle conditions. These results may also have implications for the budget of radioactive 40K in the Earth's primitive mantle.

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