ELEVATION OF POTASSIUM CONTENT OF BASALTIC MAGMA BY FRACTIONAL CRYSTALLIZATION - THE EFFECT OF PRESSURE

A wide range of natural quartz-normative liquids crystallizes olivine at low pressure. Addition of K2O to the system results in expansion of the olivine primary phase field and replacement of pigeonite (stable in the K-free system) by hypersthene. Some variation in phase relations results from depression of crystallization temperature towards the temperature at which pigeonite reacts to form augite and hypersthene because of addition of K2O. Another important influence on phase relations results from cation interactions in the liquid related to addition of K2O. Studies of crystallization behavior of materials similar in most elements except K2O show that K2O content markedly alters crystallization behavior for more siliceous liquids but appears to have less effect on liquids with lower SiO2 contents. Low-Ca pyroxenes melt congruently at P>≈5 kbar, so anhydrous liquids coprecipitate olivine, plagioclase, and two pyroxenes. Addition of K2O to the liquid has the same effect as at 1 atm. Hypersthene replaces pigeonite as the Low-Ca pyroxene crystallization from liquids with >≈1.5% K2O and the olivine primary phase field grows at the expense of those of pyroxenes and plagioclase. At 10 kbar, olivine may develop a reaction relationship with liquids containing >≈6% K2O. At 15 kbar, however, liquids evolve to a pseudoeutectic involving alkali feldspar. The systematic variation in phase relations has important consequences for magmatic evolution in different environments. Dry mafic liquids at shallow levels in oceanic areas can crystallize olivine until the liquid is very evolved, resulting in extreme SiO2-enrichment besides enrichment in K2O, and producing potassic dacites. Olivine coexists with liquids with up to 54% SiO2 if K2O=0.6% (Grove and Baker 1984) but as much as 63% SiO2 if K2O≈3.5% (Ussler and Glazner 1989). Magmas rising beneath light continental crust may pond at the Moho and evolve to low-density liquids that can rise to the surface. Coprecipitation of olivine, plagioclase, augite, and a low-Ca pyroxene, produces enrichment in K2O with only slight enrichment in SiO2. This is terminated, at pressures of 6 to, possibly, 12 kbar, by development of a reaction relationship of olivine and liquid that progresses to higher K2O contents with pressure. At pressures as high as 15 kbar, the reaction relation may not develop and only crystallization of alkali feldspar suppresses K2O-enrichment. Any magmatic H2O or crustal contamination may modify phase relations. The phase relations do, however, suggest that variation in K2O:SiO2 of evolved volcanic rocks is related to crustal thickness rather than to variation in the chemical compositions of primary magmas. © 1990 Springer-Verlag.