THOLEIITES, ANORTHOSITES, POTASSIC GRANITES, SODIC TRACHYTES, AND TEPHRIPHONOLITES: IS THERE A LINK?

Experimental investigations have determined the conditions under which fractionation can produce the three primary silica-saturated trends of intra-plate magmas (i.e., the sodic alkalic series exemplified by Ascension Island magmas, the potassic alkalic series exemplified by the anorthosite/potassic granite suites and Craters of the Moon volcanics, and the ocean island tholeiitic series exemplified by magmas from Thingmuli) using natural bulk compositions from each of the three trends. The uniqueness of the fractionation conditions, coupled with the compositional similarity of the least-evolved members of each suite-type to olivine tholeiite, suggests the possibility that a common tholeiitic parental liquid fractionating under different conditions could produce these three trends. This possibility was investigated experimentally using a Snake River Plain olivine tholeiite (10.7 wt % MgO). Piston-cylinder experiments were conducted at 9.3 and 4.3 kbar in graphite capsules with varied bulk water contents. These experiments indicate that the typical silica-depletion and Fe-Ti-and P- enrichment of the mafic units of the anorthosite complexes and Snake River Plain volcanics can be produced from olivine tholeiite at 9.3 kbar and water contents below 0.4 wt%. The high-aluminum basalt commonly considered as parental to anorthosite is produced along this crystallization path. At 9.3 kbar, but with bulk water contents above 0.4 wt%, olivine tholeiite produces the typical alkali enrichment trend that leads to sodic alkalic compositions because of the dominance of clinopyroxene and the suppression of plagioclase. Early domination of plagioclase and olivine at lower pressure (4.3 kbar) precludes strong Na enrichment and results in potassic compositions. At even lower pressures (~ 1 atm), the olivine tholeiite produces the typical ocean island tholeiitic trend. Additional experiments conducted at 14.3 kbar with varied water contents produced liquids that evolve to strongly alkalic silica-undersaturated compositions similar to those from Gough Island at the higher water contents and to more extreme silica-undersaturated alkalic compositions under drier conditions.