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Variations in the Lava of the 1959 Eruption in Kilauea Iki!

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Variations in the Lava of the 1959 Eruption in Kilauea Iki! Variations in the Lava of the 1959 Eruption in Kilauea Iki! GORDON A. MACDONALD and TAKASHI KATSURA 2 IN · RECENT YEARS the Hawaiian Islands have trated into the molten lava beneath. The results become of great importance in studies concerned of study of the drill-hole samples and samples with igneous petrogenic theory. This is partly of the earlier lava of the eruption are reported because of the total absence in that region of in this paper. sialic crust that might affect the formation of Acknowledgments. The core hole in the crust other rock types by its assimilation in rising of the Kilauea Iki lava pool was drilled for the magma. Partly, also, it is because of the exten­ Lawrence Radiation Laboratory of the Univer­ sive lateral and vertical exposure of successions sity of California in cooperation with the Uni­ of rocks, the structural and stratigraphic rela­ versity of Hawaii, and the chemical analyses of tionships of which are now well known. These the core samples were done at the University conditions have attracted to the area workers of Hawaii for the Lawrence Radiation Labora­ from many parts of the world, and progress in tory. The entire program is a part of the Law­ knowledge of Hawaiian rocks has been rapid. rence Radiation Laboratory 's Plowshare Program The knowledge is, of course, the accumulation for the development of peaceful uses of atomic of the findings of many workers, starting with energy. The results of other aspects of the in­ the visit of J. D. Dana to the islands in 1840, vestigation will be published by members of as a member of the U. S. Exploring Expedition. the staff of the Lawrence Radiation Laboratory. Outstanding among recent workers are Hisashi The drilling was done by a crew in the em­ Kuno of Tokyo University, P. Niggli of the ploy of Nat Whiton of Honolulu, under the University of Zurich, C. E. Tilley of Cambridge general supervision of. Walter Bennett and University, H. S. Yoder of the Geophysical Lab­ Donald E. Rawson of the Lawrence Laboratory. oratory of the Carnegie Institution of Washing­ General scientific supervision was furnished by ton , and H. A. Powers of the United States Macdonald. Thanks are due to the National Geological Survey. Park Service for permitting the drilling for During 1960 and 1961 two studies have scientific purposes within Hawaii National Park, largely confirmed, but also extended and some­ and to the Lawrence Radiation Laboratory for what modified, the previous petrogenic picture. perinitting us to publish the chemical analyses In April, 1960, the junior author of this paper, and other data on the core samples. on leave from the Tokyo Institute of Technol­ The sample of Pele's hair analyzed was col­ ogy, commenced a series of chemical analyses lected and given to us by Mr . H. Ikawa, of of Hawaiian lavas at the University of Hawaii the Department of Agronomy, University of under a National Science Foundation grant to Hawaii. the senior author. At the end of March, 1961, Two analyses (S-l and S-2) in Table 1 are approximately 150 new analyses, primarily of by ]. H. Scoon of Cambridge University. the hitherto largely neglected "primitive" lavas We wish to express our thanks to A. T. Ab­ of the Hawaiian volcanoes, had been completed. bott, of the University of Hawaii, for critical These have helped fill important gaps in the reading of the manuscript. basic knowledge of Hawaiian rocks. Study of them is continuing. HAWAIIAN ROCK SUITES During July, 1960, a core hole was drilled in the crust of the recently erupted lava pool in In the classical Mull Memoir ( 1924) ,E. B. Kilauea Iki crater of Kilauea volcano, and pene- Bailey and his associates distinguished three principal rock types, which they termed the plat­ eau, central porphyritic, and central nonporphy­ 1 Hawaii Institute of Geophysics, Contribution 23. Manuscript received March 21, 1961. ririe types. In 1933, W. Q. Kennedy applied the 2 University of Hawai i. names olivine basalt and tholeiite to the first and 358 Kilauea Iki, 1959-MACDONALD and KATSURA 359 last types, respectively. In 1935, H. A. Powers tholeiites. They grade into other rocks that are recognized the presence of both these types otherwisesimilar but contain normative olivine. in the Hawaiian Islands, though he did not em­ The extreme of the latter group is picrire-basalr ploy the same terminology. Later Macdonald of oceanite type, which may contain more than (1949a: 88) also pointed out that, although it 50 per cent olivine phenocrysts. These rocks are had been thought by Kennedy to be absent from chemically undersaturated with silica, and even the oceanic areas, tholeiite is presentin Hawaii. · with the attainment of complete equilibrium In 1955, Powers emphasized the essentially on crystallization should contain modal olivine. silica-saturated nature of the lavas of Mauna As in the more silica-saturated types, however, Loa and Kilauea. Although it had been antici­ phenocrystic olivine commonly has separated in pated to some extent by Powers (1935) , C. E. excess of its stoichiometric proportion, and was Tilley ( 1950) was the first to definitely point undergoing reaction with .the liquid at the time out the presence of two distinct rock series in of consolidation, as is indicated by their partial Hawaii. These he termed the tholeiitic and al­ resorption. The groundmass pyroxene is largely kali olivine basalt series. or entirely lime-poor augite and pigeonite. Thus, . Tholeiite has been defined (Tilley, 1950; although they are undersaturated with silica and Kuno et al., 1957) as a rock essentially satu­ therefore not true tholeiites, these rocks conform rated or slightly oversaturated with silica in with Tilley's definition of tholeiite as a rock in which magnesian olivine bears reaction relation­ which magnesian olivine bears a reaction rela­ ship to orthopyroxene and Ca-poor clinopyrox­ tionship to Ca-poor pyroxene, and the entire ene. In contrast, alkali olivine basalt was defined group from the nonporphyritic true tholeiites as an undersaturated rock in which magnesian to the picrire-basalts of oceanite type may be olivine and Ca-rich clinopyroxene undergo par­ termed the tholeiitic suite. allel crystallization. The characteristic ground­ Both Powers (1955) and Macdonald (I944 ) mass pyroxene of tholeiite is pigeonitic, though have pointed out that the variations within the the wollastonite content ranges from about 40 tholeiitic suite can be largely accounted for by per cent to less than 10 per cent. That of alkali settling of olivine phenocrysts in the magma, · olivine basalt is Ca-rich augite. Hypersthene is though movement of minor amounts of pyrox­ present in some tholeiites. ene and plagioclase also probably are involved Recently Kuno (1960) has pointed out the (Muir and Tilley, 1957; Macdonald, 1949b : wide distribution of basalts resembling the cen­ 1576). Locally differentiation has yielded small tral porphyritic type of Mull in their richness amounts of iron-rich basalt and granophyre in alumina, but differing from it in being essen­ (Kuno et al., 1957). tially nonporphyritic, High-alumina basalt of The major tholeiitic part of the Hawaiian this type has not been found in the Hawaiian shields is succeeded by a relatively small amount Islands, though some rocks containing abundant of lavas of other types, including alkali olivine phenocrysts of feldspar are moderately high in basalt, picrite-basalt of ankaramite type, hawai­ alumina. ite (Macdonald, 1960), mugearite, and trachyte. The predominant lavas of the great bulk of This group may be called the alkalic suite, be­ the visible part of the Hawaiian shield volcanoes cause of its relationship to alkali olivine basalt, contain scattered to moderately abundant phe­ and because the great majority of the members nocrysts of olivine, commonly as much as 5 mm. of the group contain a larger proportion of al­ in diameter. These rocks have been called "oli­ kalies than do members of the tholeiitic suite vine basalt" by Macdonald (1949a, b). Typi­ that contain the same amount of silica. The lavas cally, however , the olivine phenocrysts are partly of the alkalic suite constitute only a few per resorbed, and obviously were reacting with the cent of the total bulk of the Hawaiian volcanic remaining liquid at the time of consolidation of mountains. The nature of the transition from the rock. Chemical analyses show many of these the tholeiitic to the alkalic suite will be dis­ rocks to be essentially saturated in silica, and the cussed in detail in a future report describing the pyroxenes are lime-poor. They are thus typical results of the recent chemical analyses. 360 PACIFIC SCIENCE, Vol. XV, July 1961 A still later group of lavas, which may be producing the variations among the tholeiitic called the nephelinic suite, consists characteris­ basalts, but pointed out, as indeed he had earlier tically of nepheline basalt and melilite-nepheline (Powers, 1935), that crystal differentiation alone basalt, but includes also basanites, and alkali oli­ is inadequate to produce alkali olivine basalt vine basalts (" linosaites") in which nepheline from a saturated tholeiite. is present in the norm though not in the mode. It certainly is true that d esilication of a The nephelinic suite is in general separated magma by crystal differentiation can only result from the rocks of the other groups by a pro­ from the removal of crystals containing more found erosional unconformity (Stearns, 1946: silica than the magma. The removal of pyroxene 22 ) . from tholeiite can perpetuate a state of under­ saturation in silica, but cannot bring it about. THEORIES OF ORIGIN OF HAWAllAN N o mineral containing more silica than a satu­ ROCK SUITES rated tholeiite magma is likely to form and sep­ arate except during the very latest stages of There appears to be little or no question that crystallization.
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