Kaersutite and Kaersutite Eclogite from Kakanui, New Zealand — Water-Excess and Water-Deficient Melting to 30 Kilobars

A natural kaersutite megacryst (compositionally equivalent to olivine nephelinite) and a kaersutite eclogite nodule (equivalent to olivine basanite) from the Mineral Breccia, Kakanui, New Zealand, were reacted in sealed platinum and Pd₇₀Ag₃₀ alloy capsules, both with excess water and no additional water present, using half-inch piston-cylinder apparatus. Near-liquidus assemblages include orthopyroxene at pressures greater than about 15 kb in water-rich portions of the olivine-basanite system but not in the olivine-nephelinite system. Reversed high-pressure limits of the amphibole stability fields (excess water) have negative values of dP/dT, which crosses 25 kb at 1075°C and 30 kb at 925°C in the kaersutite system, but which crosses 25 kb at 1025°C and 30 kb at about 775°C in the kaersutite eclogite system. Comparison with experimental results reported elsewhere indicates that amphiboles persist to highest temperatures in basaltic liquids with greatest TiO₂ contents but with lowest Na₂O/(Na₂O + K₂O) ratios and lowest SiO₂ contents.

Experimental results suggest that many natural nephelinite and basanite magmas evolve from hydrous picritic parent magmas through deep-seated fractionation of olivine, possibly with clinopyroxene and garnet but excluding orthopyroxene. Although some olivine-rich basanitic liquids may be generated by partial fusion of hydrous mantle peridotite, it is unlikely that orthopyroxene fractionation is important in their subsequent evolution.

Experimental observations, together with chemical and petrographic relations, support the following model petrogenetic history for the Kakanui Mineral Breccia: pyrope-rich garnet and omphacitic pyroxene precipitated from ascending hydrous alkali basaltic magma (75 to 85 km, 1200° to 1300°C), then became trapped in deep-seated pockets within Iherzolitic mantle, together with inter-cumulus liquid that precipitated kaersutite on cooling. Resulting kaersutite eclogite assemblages re-equilibrated subsolidus (75 to 85 km, 700° to 800°C) prior to being incorporated into a rapidly ascending hydrous nephelinite magma, which was coprecipitating garnet, clinopyroxene, and probably kaersutite at depths >75 km (1100° to 1200°C). These accidental eclogitic inclusions underwent partial melting during the subsequent rapid ascent, which was terminated by an explosive eruption.