EarthRef.org Reference Database (ERR) -- Helz 1973

The phase relations of three basalts, the Picture Gorge tholeiite, the 1921 Kilauea olivine tholeiite, and the 1801 Hualalai alkali basalt, were studied at 5 kb water pressure, 680-1000 degrees C, at the oxygen fugacities of the quartz-fayalite-magnetite (QFM) and hematite-magnetite (HM) buffers. In the range 680-850 degrees C, the crystalline assemblage on the QFM buffer is dominantly hornblende + plagioclase + ilmenite, magnetite, sphene, fayalitic olivine, and phlogopitic mica. From 875 to 1000 degrees C the crystalline assemblage is hornblende + olivine + augite + ilmenite + magnetite. A melt phase is present from 700 to 1000 degrees C; a vapor phase was present in all charges. The hornblendes formed on the QFM buffer range in composition from common green hornblendes at low temperatures to kaersutitic hornblendes at 1000 degrees C. Al(IV) and Ti increase steadily with increasing temperature. Al(VI) passes through a maximum near 825 degrees C, decreasing both above and below this temperature. Al(IV) is proportional to the sum Al(VI)+ 2Ti. There is a positive linear correlation of approximately 3:1 between Al(IV) and the number of cations in the A-site. The most likely explanation for this correlation at present is that the substitution of Al(VI) or Ti+4 for a divalent cation creates local charge imbalances in the amphibole structure which can be compensated only by further A-site substitution. There also appears to be a correlation between the a-cell dimension of hornblende and the A-site occupancy. Above a threshold value of approximately 0.5 cations in A, a increases as A-site occupancy increases. Phase relations on the hematite-magnetite buffer are considerably simpler. The hornblendes show relatively little change in composition as temperature increases, and in the tholeiitic compositions break down at or below 970 degrees C, 35-60 degrees C above the first appearance of augite + olivine. The melting of hornblende is incongruent in all cases. The Fe-Ti oxides are pseudobrookite and titanohematite; at 1000 degrees C these oxides make up 10 per cent by weight of the assemblage and contain most of the TiO2 and FeO in the charge. The patterns of hornblende variation observed in this study compare closely with those reported in a wide range of experimental and field data. The appearance of high-TiO2 kaersutitic hornblendes in the tholeiites at 1000 degrees C, PH2O = 5 kb on the QFM buffer implies that the restricted occurrence of kaersutite in nature (where it is associated only with mafic to intermediate alkalic rocks) is controlled by volatile content (H2O, F2) rather than by differences in condensed bulk composition.