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 K₂O3.5% (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.