POTASSIUM-RICH CLINOPYROXENE IN THE MANTLE - AN EXPERIMENTAL INVESTIGATION OF A K-RICH LAMPROITE UP TO 60 KBAR

Clinopyroxenes with up to 1.6(7) wt% K2O occur in high pressure (50-60 kbar) and temperature (1200 degrees-1500 degrees C) experiments using an armalcolite-phlogopite lamproite composition. These P-T con ditions are considered to approximate those of the early stages in the evolution of kimberlite and olivine lamproitic magmas. With progressively decreasing temperatures from 1500 degrees-1200 degrees C at these pressures the clinopyroxene coexists with garnet, garnet + coesite(?), garnet + coesite(?) + phlogopite + rutile. Liquid occurs in all assemblages. Potassium contents of clinopyroxene systematically decrease with temperature and increase with pressure as does the K-D(cpx/melt). Comparable trends occur with Ca and Na whereas Mg increases with temperature and decreases with pressure. Similarly, Fe contents of these clinopyroxenes decrease with pressure but show no systematic relationship to temperature. Aluminum in sixfold coordination also has a trend similar to that of K, Na, and Ca at 60 kbar but not at 50 kbar where lower Al-[VI] occurs. These results suggest that K is incorporated within clinopyroxene by complex coupled substitution mechanisms such as: (Mg, Fe) --> Al-[VI] + (K + Na + Ca) with charge balance involving incorporation of Al-[VI], Cr, Fe+3, and Ti in the clinopyroxene structure in octahedral sites. The K contents of these clinopyroxenes are greater than those reported for most clinopyroxenes in deep-seated diamondiferous inclusions in eclogites and kimberlites but are comparable to those of some clinopyroxenes from diamond inclusions and from other high pressure experiments. The presence of the large amounts of K2O in the clinopyroxene may be attributed to the high compressibility of K as well as the high K2O in the bulk composition. The paucity of high K-enriched compositions in nature maybe due to the fact that most olivine lamproite and kimberlite magmas have not been subjected to the lower temperature ranges (less than or equal to 1250 degrees-1300 degrees C) at the high pressures (50-60 kbar) necessary for the incorporation of K into the clinopyroxene structure. Clinopyroxene that can form under P-T conditions comparable to those of this study may be an important mantle reservoir for K.