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Measurements of strong‐field magnetization over the temperature range −196° to 700°C have been made on forty‐eight drill core samples of tholeiitic basalt from Alae and Makaopuhi lava lakes, Kilauea volcano, Hawaii. These samples were originally obtained at temperatures ranging from 50° to 1020°C. Nearly all samples contain abundant hemoilmenite with Curie temperatures in the range −100° to −160°C. Samples quenched from high temperatures (800° to 1000°C) have second Curie temperatures ranging from 150° to 290°C, due to unoxidized titanomagnetite, and samples obtained at lower temperatures (50° to between 400° and 700°C) have second Curie temperatures ranging from 500° to 580°C. This transition from medium to high Curie temperatures occurs between 850° and 300°C, varying from one drill hole to another, and is accompanied by a marked increase in the strong‐field magnetization at room temperature. Oxidation of original titanomagnetite to Ti‐poor titanomagnetite containing ilmenite lamellas is the cause of the increase in Curie temperature. Comparison of the compositions of the oxide minerals with the oxygen fugacity data of Sato and Wright and the equilibrium reaction data of Buddington and Lindsley shows that oxygen fugacity was controlled largely by the buffering action of the oxide minerals; hence titanomagnetite was oxidized, whereas the more abundant hemoilmenite was little changed as the lava cooled. This oxidation occurred at temperatures well below equilibrium, the difference being generally of the order of 100°C but as much as 400°C. We conclude that in some basaltic lavas the magnetic minerals may form through subsolidus reactions at temperatures well below their final Curie temperatures. In such lavas the natural remanent magnetization is a mixture of thermoremanent magnetization and high‐temperature chemical remanent magnetization.

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Kilauea and Mauna Loa, Hawaii's two active shield volcanoes, are composed of tholeiitic basalt having MgO contents ranging from more than 20 percent to less than 4 percent. Most eruptive vents are located either within the central caldera or on two rift zones extending to the east and southwest from each volcano's summit. Mauna Loa also has a few isolated vents on its northwest slope that are apparently unrelated to any rift zone. The chemical variability of Mauna Loa and Kilauea lava having more than about 6.8 percent MgO is principally explained by addition or removal of olivine (olivine control), but other minerals are involved to a lesser degree. The chemical variability of these lavas is described and interpreted in this paper. Lavas having less than 6.8 percent MgO are fractionated by separation of pyroxene, plagioclase, and Fe-Ti oxides in addition to olivine. Fractionated basalt from Kilauea is confined to the rift zones. The rare fractionated lavas from Mauna Loa are also confined to the rift zones and are less fractionated (higher MgO) than those from Kilauea. The chemistry of nonfractionated lava from single eruptions of Kilauea may be explained by olivine control alone, and this process is consistent with the observation that olivine is the only true phenocryst in unfractionated Kilauea lavas. The chemistry of nonfractionated lava from single eruptions of Mauna Loa can be explained by a more complex mineral control involving olivine, hypersthene, augite, and plagioclase, all being commonly present as phenocrysts. The addition or removal of these minerals is inferred to take place at pressures not exceeding 2 kilobars in shallow magma reservoirs located beneath the two volcanoes. The chemistry of Mauna Loa lavas shows no correlation with either the time of eruption or location of the eruptive vents. By contrast the lavas erupted at Kilauea summit may be subdivided into three groups pre-1750, 18th-19th century, and 20th century on the basis of their chemical composition compared at the same MgO content. The younger lavas are richer in lower melting constituents; for example, K2O, PaOs, and TiOa. The average composition of Mauna Loa lavas is similar to what one would obtain by extrapolating the Kilauea chemical variation backwards in time. It is possible that the oldest unexposed Kilauea lavas would be similar in composition to Mauna Loa lava. The similarity in the ratio of K20:P2Os both within the lavas of each volcano and between the two volcanoes suggests that Kilauea and Mauna Loa originated by partial melting in a mantle of uniform composition and that no processes other than either melting or crystal-liquid fractionation are needed to explain the chemical variations. cent into shallow crustal reservoirs at 2-4 km depth. 4. Settling of olivine during storage at 2-4 km and periodic eruption of the upper parts of these reservoirs, or rarely, the entire reservoir including settled olivine.

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Introduction ______________________________ 1 Acknowledgments ____________________________ 2 Previous work ___________________________ 2 The eruption of March 5-15, 1965 __________________ 2 Chronology ________________________ 2 Initial conditions following formation of a permanent crust __________________________________ 3 Definition of "crust" and "melt" __ ___ _ _ __ _____ __ 4 Methods of study __ ___ _ ___ __ __ ____ _ __ _ _ _ 5 Measurement of surface altitude changes _________ 5 Core drilling-_____________________________ 5 Sampling of melt and casing drill holes below the crust-melt interface ________________________ 8 Measurement of temperature _______________ 8 Additional field studies ________________________ 9 Observations _______________________________ 10 Thermal history ______________________^__10 Temperature of the crust-melt interface ___ _ ___16 Cooling of the crust (T<1,070°C) _____________16 Cooling of the melt (T>1,070°C)______________18 Page Observations-Continued Oxygen fugacity measurements ______________ 18 Changes in surface altitude_________-____ 24 Chemical and petrographic studies ____________-25 Major element chemistry ______________-25 Petrography ______________________-31 Distribution of olivine ________________ 36 Variation in grain size ____-_________ 36 Core density and vesicle distribution ______ 36 Discussion__________________________-37 Chemical differentiation in the lava lake _________-40 Gravitative settling of olivine ________________-40 Flow differentiation of olivine-augite-plagioclase-47 Segregation veins ____-_______-_________ 42 Convective cooling in the lava lake _____________-44 High-temperature oxidation of basalt _____ ___ _ ___-45 Interpretation of surface altitude changes __________-46 Summary: Cooling and solidification history of Makaopuhi lava lake__________________________________46 References cited

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