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Journal of Petrology Volume 41 Number 4 Pages 583-603 2000
© Oxford University Press 2000
Transport and Storage of Potassium in the Earth’s Upper Mantle and Transition Zone: an Experimental Study to 23 GPa in Simplified and Natural Bulk Compositions
GEOPHYSICAL LABORATORY AND CENTER FOR HIGH-PRESSURE RESEARCH, CARNEGIE INSTITUTION OF WASHINGTON, 5251 BROAD BRANCH ROAD NW, WASHINGTON DC, 20015-1305, USA
We have investigated the stability and composition of potassium amphibole and its high-pressure breakdown product phase X in synthetic peralkaline and subalkaline KNCMASH (K2O–Na2O–CaO–MgO–Al2O3–SiO2–H2O) and natural KLB-1 peridotite bulk compositions between 10 and 23 GPa at 800–1800°C. In the KNCMASH system, potassium amphibole reaches its upper pressure stability limit at 13–15 GPa at 
1400°C. In the natural KLB-1 bulk composition, potassium amphibole breaks down between 12 and 13 GPa at 1200°C. Phase X is a hydrous potassium–magnesium silicate with variable stoichiometry, a general formula K2–xMg2Si2O7Hx with x = 0–1, and a maximum possible H2O content of 3·5 wt %. Electron microprobe analytical totals suggest H2O contents of 
1–2 wt % and a decrease in H2O contents with increasing pressure. In both KNCMASH and KLB-1 systems, phase X coexists with Mg2SiO4 + garnet + high-Ca clinopyroxene + low-Ca clinopyroxene ± fluid. Phase X breaks down between 20 and 23 GPa at 1500–1700°C to form K-hollandite + 
-Mg2SiO4 + majorite + Ca-perovskite + fluid. The upper temperature stability limit of phase X was located in the subalkaline KNCMASH system between 1400 and 1600°C at 14 GPa and at >1700°C at 20 GPa, the latter being at least 200°C above an average current mantle adiabat. Thus, phase X could store and transport both water and potassium not only in subduction zone settings, but also in convecting mantle down to the transition zone–lower-mantle boundary. Phase X would also be an eminently suitable host for Rb, Cs, Ba or Pb.
KEY WORDS: experimental study; high pressure; phase X; potassium
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