DIFFERENTIATION AND MAGMA MIXING ON KILAUEAS EAST RIFT-ZONE - A FURTHER LOOK AT THE ERUPTIONS OF 1955 AND 1960 .2. THE 1960 LAVAS

New and detailed petrographic observations, mineral compositional data , and whole-rock vs glass compositional trends document magma mixing i n lavas erupted from Kilauea's lower east rift zone in 1960. Evidence includes the occurrence of heterogeneous phenocryst assemblages, inclu ding resorbed and reversely zoned minerals in the lavas inferred to be hybrids. Calculations suggest that this mixing, which is shown to hav e taken place within magma reservoirs recharged at the end of the 1955 eruption, involved introduction of four different magmas. These magma s originated beneath Kilauea's summit and moved into the rift reservoi rs beginning 10 days after the eruption began. We used microprobe anal yses of glass to calculate temperatures of liquids erupted in 1955 and 1960. We then used the calculated proportions of stored and recharge components to estimate the temperature of the recharge components, and found those temperatures to be consistent with the temperature of the same magmas as they appeared at Kilauea's summit. Our studies reinfor ce conclusions reached in previous studies of Kilauea's magmatic plumb ing. We infer that magma enters shallow storage beneath Kilauea's summ it and also moves laterally into the fluid core of the East rift zone. During this process, if magmas of distinctive chemistry are present, they retain their chemical identity and the amount of cooling is compa rable for magma transported either upward or laterally to eruption sit es. Intrusions within a few kilometers of the surface cool and crystal lize to produce fractionated magma. Magma mixing occurs both within bo dies of previously fractionated magma and when new magma intersects a preexisting reservoir. Magma is otherwise prevented from mixing, eithe r by wall-rock septa or by differing thermal and density characteristi cs of the successive magma batches.