PETROLOGY OF LAVAS FROM THE PUU OO ERUPTION OF KILAUEA VOLCANO .3. THE KUPAIANAHA EPISODE (1986-1992)

The Puu Oo eruption has been remarkable in the historical record of Ki lauea Volcano for its duration (over 13 years), volume (>1 km(3)) and compositional variation (5.7-10 wt.% MgO). During the summer of 1986, the main vent for lava production moved 3 km down the east rift zone a nd the eruption style changed from episodic geyser-like fountaining at Puu Oo to virtually continuous, relatively quiescent effusion at the Kupaianaha vent. This paper examines this next chapter in the Puu Oo e ruption, episodes 48 and 49, and presents new ICP-MS trace element and Pb-, Sr-, and Nd-isotope data for the entire eruption (1983-1994). Ne arly aphyric to weakly olivine-phyric lavas were erupted during episod es 48 and 49. The variation in MgO content of Kupaianaha lavas erupted before 1990 correlates with changes in tilt at the summit of Kilauea, both of which probably were controlled by variations in Kilauea's mag ma supply rate. These lavas contain euhedral olivines which generally are in equilibrium with whole-rock compositions, although some of the more mafic lavas which erupted during 1990, a period of frequent pause s in the eruption, accumulated 2-4 vol.% olivine. The highest forsteri te content of olivines (similar to 85%) in Kupaianaha lavas indicates that the parental magmas for these lavas had MgO contents of similar t o 10 wt.%, which equals the highest observed value for lavas during th is eruption. The composition of the Puu Oo lavas has progressively cha nged during the eruption. Since early 1985 (episode 30), when mixing b etween an evolved rift zone magma and a more mafic summit reservoir-de rived magma ended, the normalized (to 10 wt.% MgO) abundances of highl y incompatible elements and CaO have systematically decreased with tim e, whereas ratios of these trace elements and Pb, Sr, and Nd isotopes, and the abundances of Y and Yb, have remained relatively unchanged. T hese results indicate that the Hawaiian plume source for Puu Oo magmas must be relatively homogeneous on a scale of 10-20 km(3) (assuming 5- 10% partial melting), and that localized melting within the plume has apparently progressively depleted its incompatible elements and clinop yroxene component as the eruption continued. The rate of variation of highly incompatible elements in Puu Oo lavas is much greater than that observed for Kilauea historical summit lavas (e.g., Ba/Y 0.09 a(-1) v s similar to 0.03 a(-1)). This rapid change indicates that Puu Oo magm as did not mix thoroughly with magma in the summit reservoir. Thus, ex cept for variable amounts of olivine fractionation, the geochemical va riation in these lavas is predominantly controlled by mantle processes .