LINKING THOLEIITES AND ALKALIC ROCKS: THE ROLE OF DISSOLVED WATER

Stepwise fractionation experiments on hy-normative rocks from hotspot and early continental rift environments suites indicate that at low water contents and pressures of 9.3 to 4.3 kbar, alkalic basalt, ocean island tholeiite, continental tholeiite, and tholeiitic gabbro associated with anorthosite all follow a similar trend of liquid evolution. This trend is characterized by early silica depletion accompanied by strong enrichment in Fe, Ti, and P. At higher water content this trend changes to one characterized by silica-enrichment. The critical range of water contents over which this transition occurs is being investigated experimentally.

Basalts of the alkalic Nandewar suite crystallize the high temperature assemblage olivine + clinopyroxene + plagioclase for bulk water contents of 0.01, 0.13, 0.36, and 0.55 wt% at 9.3 kbar. This assemblage gives rise to Fe-Ti-P-enriched, Si-poor liquids that are absent in alkalic suites but characteristic of volcanic associations such as Craters of the Moon and the plutonic anorthosite suites. This trend is also produced in ocean island tholeiite from Pinzon, Galapagos for low water contents and 9.3 kbar. Water contents of 0.8 wt% and higher in the alkalic basalt instead yield the typical hy-normative alkalic trend hawaiiite-mugearite-benmoreite-trachyte-rhyolite as early plagioclase crystallization is suppressed. The increase in alkalic characteristics diminishes once kaersutite becomes volumetrically dominant.

Experiments at 4.3 kbar on a fine-grained high aluminum gabbro from the Laramie Anorthosite Complex produced silica-depletion and Fe-Ti-P-enrichment for 0.13 wt% bulk H₂O, but a fractionation trend towards potassic granite for 0.8 wt% H₂O. Similarly, experiments on a Craters of the Moon ferrobasalt with 0.9 wt % water produced a silica-enrichment trend. Ongoing experiments are focusing on defining this range more closely.

The critical range of water contents separating the silica-depletion trend from the silica-enrichment trend lies only slightly above the water content of typical tholeiite and appears to be the same for basalt of the three hy-normative suite-types. Thus, slight variations in the water content of a tholeiitic parental liquid may produce widely different evolutionary trends in hotspot associated suites.