Melting study of an alkali basalt JB-1 up to 12.5 GPa: behavior of potassium in the deep mantle

In order to study the role of alkali elements in global mantle circulation, we studied melting phase relations of an alkali basalt JB-1 up to 12.5 GPa under dry conditions. Compatibility of Na and K in clinopyroxene solid solution changes dramatically in the studied pressure range. The liquidus clinopyroxene/meltpartition coefficient for Na2O increases from 0.08 at 1 atm to 2.0 at 10 GPa and that for K2O increases discontinuously at around 7 GPa. Subsolidus clinopyroxene of JB-1 contains over 1 wt.% K2O at pressures above 7 GPa, whereas it contains less than 0.1% at pressures below 5 GPa. Accordingly, subsolidus assemblage of the alkali basalt contains small amount of potassium feldspar up to 6 GPa but contains only clinopyroxene, garnet and coesite at pressures above 7 GPa. There is a huge temperature increase in the JB-1 solidus at pressures between 6 and 7.5 GPa where the subsolidus mineral assemblage changes. The geometry of the solidus curve indicates that partial melting of the alkali basaltic materials must occur at 6-7 GPa in adiabatically ascending mantle flow with broad range of potential mantle temperatures (PMT greater than or equal to 1200 degrees C). Origin of the occurrence of ultrapotassic magmas in continental alkali basalt provinces and that for mildly potassium rich magmas in ocean island provinces are discussed in the light of present experiments. (C) 1998 Elsevier Science B.V. All rights reserved.