University of Tokyo, Japan
Ikuo Kushiro was awarded the Harry H. Hess Medal at the AGU Spring Meeting Honors Ceremony, which was held on June 2, 1999, in Boston, Massachusetts. The medal recognizes outstanding achievements in the research of the constitution and evolution of Earth and sister planets.
“Characterization of rock-forming processes requires laboratory experiments to determine the physicochemical properties of materials or their analog model compounds at high temperatures and high pressures. The experimental data, however, are not particularly useful unless integrated with observational and theoretical information. Ikuo Kushiro, this year’s winner of the Harry H. Hess Medal, approaches experiments with this integrated view as his guiding light. He does experimental petrology, geochemistry, and geophysics, but always with the understanding that the experimental data provide only a piece of a more complex puzzle. It is perhaps in part because of this attitude that Ikuo and his colleagues have provided the fundamental experimental data on phase equilibria, element partitioning, and equation-of-state of molten silicate applicable to most major rock-forming processes in the Earth. All too often we ask what we can do with our instrumentation and proceed from there. Ikuo first asks the real question and then designs the experiment.
“Ikuo Kushiro’s career as an experimentalist began in earnest shortly after receiving his Ph.D. from the University of Tokyo in 1962. He was then appointed as a research associate at the university, but within months we find him at the Geophysical Laboratory of the Carnegie Institution of Washington, where he spent the next 3 years as a postdoctoral fellow working with Frank Schairer and Hat Yoder. This was an extremely active period where his experimental studies of olivine + 2 pyroxenes + aluminosilicate phases led to the establishment of the principal phase relations governing basalt petrogenesis at high pressure and temperature.
“Upon his return to the University of Tokyo in 1965, Ikuo began a collaboration with S. Akimoto. A primary objective at that time was the search for mineralogical storage of H2O in the Earth’s interior. In 1967, they identified phlogopite as such a mineral with pressure-temperature stabilities well into the upper mantle. In 1970, the H2O-bearing mineral, K-richterite, was found to have a comparable stability field. Much subsequent experimental work has reconfirmed that these minerals are two of the most important minerals with which H2O can be recycled into the Earth’s interior.
“These experimental results on hydrous phases in the upper mantle led naturally to the question of how H2O affects melting phase relations of peridotite. In 1968, Kushiro and his colleagues reported that the solidus temperatures of hydrous peridotite are 400°-500°C lower than those without H2O. This observation implied that temperatures near 1000°C were sufficient to initiate melting of peridotite. That was a key to our understanding of magma genesis near convergent plate boundaries. The effect of H2O on melt composition was established when Kushiro and his colleagues reported in 1968 that enstatite + H2O melts incongruently to olivine + liquid, and that the olivine + clinopyroxene + orthopyroxene + liquid invariant point in the diopside-enstatite-olivine-H2O system lies on the quartz-normative side of the diopside-enstatite join. Andesite in island arcs could then be related to partial melting under hydrous conditions at comparatively low temperatures in the upper mantle.
“Ikuo Kushiro returned to the Geophysical Laboratory as a postdoctoral fellow in 1967 and was appointed to the permanent staff in 1971. During this period, Ikuo continued his experimental work on the role of H2O during partial melting in the upper mantle. Ikuo also began to examine the systematics of liquidus phase relations as a function of the type of oxide components added to simple silicate systems. This led to a simple and elegant model that describes the relationships between silicate activity and the electronic properties of the oxide components in silicate melts.
“Ikuo never thought, however, that experimental studies of phase relations provided all the answers needed to characterize rock-forming processes in the Earth. In the years between 1975 and 1980, he devoted much of his energy to experimental examination of the properties of magmatic liquids, including rheology and equation-of-state of molten silicates. His adaptation of the falling-sphere technique to the solid-media, high-pressure apparatus provided the means for simultaneous determination of melt density, compressibility, and viscosity at magmatic temperatures to pressures of about 3 GPa. One of the most surprising observations reported at the time was that anhydrous magmatic liquids become significantly less viscous with increasing pressure.
“Ikuo Kushiro was appointed full professor at the University of Tokyo in 1974, but remained a staff member at the Geophysical Laboratory until 1981, when he needed to choose between the two locations and returned to Tokyo on a permanent basis. There, he immediately focused on experimental examination of igneous processes in island arcs, but this period also coincided with rich new meteorite finds in Antarctica. These finds naturally drew Ikuo’s attention to the extraterrestrial environment. He quickly designed an experimental apparatus for high-temperature, high-vacuum experiments. The result was a series of experimental studies that focused on the petrogenesis of calcium, aluminum-rich inclusions, and chondrules in chondritic meteorites.
“The past decade saw Ikuo Kushiro drawing on his vast research experience, teaching students, and producing graduates who quickly established themselves in their chosen subdisciplines. The administration at the University of Tokyo obviously also noticed Ikuo’s success and appointed him Dean of the Faculty of Science in 1991 and Vice-President of the University of Tokyo in 1993 until his retirement in 1994. Most people would probably have found these jobs to be full-time endeavors, but not Ikuo. He continued an active research career during this period with an average of about four published papers per year. Among these were several papers on the partial melting of mantle peridotite, utilizing a novel diamond sponge technique to extract small fractions of partial melt from the crystalline residue. This was an important innovation because until that time experimental studies of low degree of partial melting had remained a major problem because the melt fractions required for quantitative electron microprobe analysis effectively limited experiments to conditions under which analysis of the melt could be carried out. The diamond sponge technique removed this obstacle and in the following years became the method of choice for experiments of this type.
“Following his retirement from the University of Tokyo, Ikuo moved to the Institute for the Study of the Earth’s Interior (ISEI) of Okayama University. During his 5-year tenure there while simultaneously pursuing his own research interests, he quickly became the Institute Director and proceeded to develop ISEI, which is now one of the preeminent such facilities in the world.
“It is difficult to identify anyone more worthy of the Harry H. Hess Medal than this year’s recipient. Therefore, I have the great pleasure of introducing Ikuo Kushiro to you as the medalist for 1999.”
—BJORN MYSEN, Geophysical Laboratory, Carnegie Institution of Washington, D.C.
“I am much honored to receive the Harry H. Hess Medal from the American Geophysical Union, and I am most grateful to the members of the Hess Medal Committee and the people who nominated and supported me. I also thank Bjorn for a nice citation of my work, which is more than I deserve. I first heard the name of Harry Hess in the petrology lecture given by the late Hisashi Kuno in 1956 while I was an undergraduate student of the University of Tokyo. Kuno had been a ‘scientifically close friend’ of Hess since 1951; they had common interests in pyroxenes, mantle materials, and lunar rocks, and they published a joint paper on pyroxene. When Kuno died on August 6, 1969, Hess wrote me a kind letter of mourning for him. Sad to say, Hess passed away only 20 days later. Kuno greatly respected Hess. I admired Hess for his many pioneering works and original ideas in igneous and mantle petrology and in the dynamics of the crust and upper mantle of the Earth.
“I was fortunate to have had outstanding teachers and colleagues during my career. My first teacher was Hisashi Kuno during the undergraduate and graduate courses of the University of Tokyo. I also learned petrology from Akiho Miyashiro. Under the supervision of Kuno, I started a study on the differentiation of basalt magma in the intrusive bodies in northeast Japan; however, during the course of that study, I became more interested in the genesis of basalt magma, and I greatly wanted to study this problem by means of high-pressure experiments.
“Soon after I was awarded a Ph.D. in 1962, I was fortunate to become a postdoc at the Geophysical Laboratory, where Hat Yoder and Frank Schairer were very stimulating teachers. I learned from them the fundamentals of experimental petrology as well as the ‘spirit’ of the Geophysical Laboratory. My first work was to examine the effect of pressure on the composition of melts formed in the upper mantle. Using relatively simple systems such as the forsterite-diopside-silica and forsterite-nepheline-silica systems, I recognized a systematic change with pressure in the composition of partial melts formed in the upper mantle, which supported Kuno’s argument for the genesis of basalt magmas. I also worked on subsolidus phase relations in the system MgO-CaO-Al2O3-SiO2 at high pressures with Hat Yoder. We were excited to find that forsterite and anorthite reacted to form pyroxene and spinel, because this reaction located the stability boundary between plagioclase-lherzolite and spinel-lherzolite, as well as the upper stability limit of olivine gabbro.
“In 1965, I returned to the University of Tokyo and started experimental studies on the role of H2O in the mantle. Of special interest were studies in collaboration with Syun-Iti Akimoto and Yasuhiko Syono on the melting of mantle peridotite under hydrous conditions and the stability of hydrous minerals such as phlogopite at high pressures. The discovery of a drastic decrease (>600°C) in the solidus temperature of peridotite under hydrous conditions was exciting. During these investigations, I wanted to know the effect of H2O on the composition of magma formed in the mantle, and in late 1967 Hat Yoder again provided me with a chance to work at the Geophysical Laboratory. Soon after I started the experiments with Hat Yoder, we found that incongruent melting of enstatite to produce olivine and silica-saturated melt persisted to at least 3 GPa in the presence of H2O as compared to ‹1 GPa under anhydrous conditions. The experiments were extended to more complex systems to confirm that this effect exists. From 1969 to 1971, my work on this problem was interrupted by another exciting study on the Apollo lunar samples, which I carried out at the University of Tokyo with Yasuo Nakamura, Syun-Iti Akimoto, Hiroshi Haramura, and Yukio Ikeda.
“In 1975, I began my work on the properties of silicate melts at high pressures at the Geophysical Laboratory, and in 1976 I found an unexpected decrease in the viscosity of jadeite melt with increasing pressure at constant temperature. I wondered if natural magmas also showed such a decrease in viscosity, and I worked with Bjorn Mysen and Hat Yoder on the viscosity of basaltic and andesitic melts at high pressures. We found that the viscosity of these melts also decreases with increasing pressure at constant temperature. To understand such an anomalous viscosity change in silicate melts, I tried to measure the diffusivity of ions in melts, especially network-forming ions, which is closely related to viscosity. Shortly thereafter, the work with Nobu Shimizu on the diffusivity of O and Si in jadeite and diopside melts at high pressures demonstrated a beautiful inverse relation between the viscosity and diffusivity of oxygen. I also measured the density of silicate melts at high pressure with Toshitsugu Fujii, and we showed the possibility that basaltic magmas become denser than calcic plagioclase near the base of the continental crust. Intensive studies of the structure of silicate melts were then carried out at the Geophysical Laboratory by Dave Virgo and Bjorn Mysen using Raman spectroscopy. I enjoyed participating in their work.
“At the University of Tokyo, my research after 1978 was concerned mainly with the origin of magmas, island arc crust, and chondrites. In these studies, I enjoyed working with a number of able graduate students and colleagues, particularly Hiroyuki Fukuyama, Masanori Sakuyama, and Yoshiyuki Tatsumi on the genesis of island arc magmas; Eiichi Takahashi on the partial melting of mantle peridotite and origin of mid-ocean ridge basalt; Hiroko Nagahara and Akira Tsuchiyama on the formation of chondrules and evaporation and condensation processes in the primitive solar nebula, and Susumu Umino on the origin of boninites. It was very sad, however, that Hiroyuki Fukuyama and Masanori Sakuyama were killed in an accident in Iceland in 1984, which is unforgettable for me.
“In 1992, I resumed experiments on the partial melting of mantle peridotite at high pressures. Together with Kevin Johnson and Kei Hirose, I developed a new method for segregating partial melts using aggregates of diamond grains, a hint of which was given by Joe Boyd more than 20 years ago. Initially, we tried to simulate fractional melting of mantle peridotite, which Kevin wanted to pursue, and later applied this technique to determining the composition of partial melts and melt fraction. The first successful analysis of quenched partial melts segregated into the diamond aggregates was exciting.
“In 1994, I moved to the Institute for Study of the Earth’s Interior, at Okayama University in Misasa, where I began experimental work with Eizo Nakamura on the partitioning of trace elements between mineral and melt and the diffusivity of trace elements in silicate melts at high pressures using ion probe analysis. On March 31, 1999, I retired from Okayama University.
“In receiving this honor, I would like to thank all the people mentioned above and many others whom I did not mention here for their teaching, collaboration, and encouragement throughout my research career of almost four decades. I would like to share the honor with those people.”
—IKUO KUSHIRO, University of Tokyo, Japan