The major element chemical compositions of lava from four eruptions on the east rift zone of Kilauea between August 1968 and October 1971 reflect three petrologic processes:

1. Production of chemically distinct batches of magma in the mantle.

2. Separation of olivine, augite, and plagioclase from liquid during flow in the rift-zone conduits.

3. Mixing of different magmas during ascent to the surface.

Chemically none of the four Kilauea east-rift eruptions matches the preceding summit eruption in Halemaumau that ended in July 1968. The Mauna Ulu eruption, May 1969 to October 1971 (the last of flie east-rift eruptions), can be divided into five olivine-controlled and chemically distinct variants. Three of these characterize the first seven months of the eruption and are closest in composition to the 1967–8 Halemaumau eruption. Variants 4 and 5 were erupted later and have compositions that are distinctly different from that of the 1967–8 eruption. Major differences are higher Al₂O₃ (0·15–0·23 per cent), and lower K₂O (0·07–0·10 per cent) and TiO₂ (0·12–0·23 per cent) in variants 4 and 5 at the same MgO content. Some lavas from eruptions in August and October 1968 and February 1969, have olivine-controlled magma compositions that are identical to mixtures of Mauna Ulu variants 1–3 and the 1967–8 composition. This observation fits an hypothesis advanced earlier by T. L. Wright and R. S. Fiske that magmas in the central magma chamber become mixed with magmas in the rift zone and can be identified as mixing components of rift eruption magmas before they appear as distinctive magmas in summit eruptions.

Lavas representing mixing of olivine-controlled magma with differentiated magma were erupted in October 1968, February 1969, and in May and December 1969.

The changes in amount of K₂O and TiO₂ during the latter part of the 1969–71 Mauna Ulu eruption are the reverse of the overall secular change in composition of Kilauea summit lavas from pre-1750 through 1967–8. The K₂O and TiO₂ contents of the latest overflows during the 1969–71 Mauna Ulu eruption (April 1971) are comparable to that of lava erupted at Kilauea summit prior to 1750.

The changing chemistry of Kilauea magma is found to be of use as a ‘tracer’ in the complex Kilauea conduit system. Application of these data to older lava sequences is difficul because of the complexity of the processes controlling lava composition and the absence of detailed information about the time-space chemical variation during individual eruptions.