Citation by Michael O. Garcia
The eminent research career of Professor Rhodes spans >6 decades and is continuing with new work on the 2022 Mauna Loa eruption. After completing a B.Sc. at Bristol University in 1959, "Mike" worked for the Australian Bureau of Mineral Resources (BMR) mapping extensive areas of the outback of Australia. His tales of 'life in the bush' are hair-raising. Subsequently, he coordinated geochronology work with Australian National University (ANU), involving pure mineral separations including K-feldspars from granites. He discovered that K and Na abundances in K-feldspar were related to ordering (triclinicity), whereas Rb, Sr, Ba were related to the bulk composition. Mike developed a novel method to evaluate feldspar order/disorder by combining the separation X-ray diffraction lines 131 and 130 with 2V measured with a universal stage (Rhodes, 1969a). This work so impressed the famous mineralogist Thomas Barth that it was included in his seminal book on Feldspar.
Rhodes' love affair with XRF. The young Rhodes recognized that XRF was the tool of the future. He pursued a Ph.D. with then XRF guru Bruce Chappell (ANU) but discovered that the data reduction methods were painfully slow involving desktop calculators. He decided to learn Fortran and multivariate statistics to speed up the process. This software became an appendix to his 1970 Ph.D. dissertation. His XRF laboratory at the University of Massachusetts, where he was the 5 College Professor, has been and still is open to researchers worldwide producing high quality data. Furthermore, unlike many lab supervisors, his name is not added to publications in exchange for data. Mike Rhodes exemplifies the spirit/credo of GSA to advance and communicate science collaboratively via the outstanding quality of his geochemical analyses and his keen interest in diverse research questions. A few examples are given below.
Basalt and more basalt. When I first met Mike in Hawai‘i (1979), he was already an esteemed scientist based on seminal work on Australian granites (Rhodes, 1965, 1968), the development of new analytical XRF methods (Rhodes, 1971), his many contributions as a PI in the lunar program (Apollo 14-17), study of mantle xenoliths (Rhodes and Dawson, 1975), and his highly influential work on some of the first basalts recovered from mid-ocean ridges. Mike discerned that magma mixing is an important petrologic process (Rhodes and Duncan, 1979), unlike others at the time. His fascination with basalt led to a comparison of basalt from mid-ocean ridges, Hawai‘i, and the Moon in two insightful chapters of the Basaltic Volcanism on Terrestrial Planets (Rhodes and others, 1981a, b). Over the last 30+ years, Mike and I have worked closely on Hawaiian volcanism and co-authored 22 papers in major scientific journals. Our work on Kilauea's recent eruptions has resulted in many 100's of high-quality major and trace element analysis of lavas creating a unique timeseries database that provide important insights into crustal and mantle processes for the 35-year-long Pu‘u Ō‘ō eruption (summarized in Garcia et al., 2021).
Mauna Loa. Rhodes is the expert on the petrology of the world's largest active volcano. Over the last four decades, he has sampled and written about its subaerial lava surface flows (including during the 1984 eruption) and flows in its near vertical caldera walls, drill core (HSDP) that penetrated the last 100 ka of Mauna Loa volcanism, and on 3 marine expeditions using robotic and human-occupied vehicles to sample down to 5 km below sea level from the oldest exposed parts of this volcano (~550 ka). Mike provided criteria leadership in championing and coordinating research on Mauna Loa that resulted in an AGU monograph Mauna Loa Revealed (Rhodes and Lockwood, 1995). His paper in that volume presented the first systematic isotopic investigation of historical Mauna Loa lavas (Rhodes and Hart, 1995). This work showed that Mauna Loa lava geochemistry progressively changed reflecting open-system processes and variable extents of melting of small-scale heterogeneities in the Hawaiian mantle plume. This was an astounding discovery that has been confirmed at many other volcanoes. Mike's classic work on Hawaiian basalts includes two Nature papers. The "Will the real primary magma please stand up" paper (Rhodes, 1982) recognized primary magmas are "Rosetta stones" providing essential clues to mantle composition and the nature of melting processes. He emphasized not only the importance of these stones but also their rarity. This paper was written at a time when many others were mistakenly interpreting olivine-rich rocks as primary magma. Mike provided criteria for the recognition of 'true' primary magmas. The Rhodes et. al (1989) Nature paper “Geochemical evidence for the invasion of Kilauea's magmatic plumbing system by Mauna Loa magma” was the first to suggest that these adjacent Hawaiian volcanoes were periodically sharing magma from the Hawaiian mantle plume. This radical, new idea has been supported by subsequent geophysical studies and his continuing geochemical work.
Granites. Mike rekindled his love affair with granites as he and his students explored the superbly exposed coastal Paleozoic granites of Maine (via kayaks) to characterize magma chamber processes (e.g., Chapman and Rhodes, 1992). They discovered that dense mafic intrusions ponded on the floor of the granite magma bodies providing field evidence for the Sparks and Sigurdsson model (Nature, 1977) on how explosive eruptions are triggered. This granite work led to an epiphany when the DOE initiated a program to explore for geothermal energy. Mike spearheaded an effort to evaluate the geothermal potential of New England granites (which constitute 35% of the surface area of Massachusetts). Instead of drilling numerous expensive wells, Mike developed an ingenious method to measure the heat production using K, Th, U concentrations in granites by XRF, and determined their density and thermal conductivity to predict heat flow and temperature/depth profiles. He discovered that several granite bodies should be sufficiently hot at accessible drilling depths (70-90°C at depths of 2-4 km) to warrant further exploration.
It is a great pleasure to see my old friend receive this much deserved lifetime achievement. Few in the MGPV community have done so much over such a long period for Earth science. Congratulations Mike.
Response by John Michael Rhodes
Considering the list of distinguished geoscientists who have previously received this award, I am deeply honored to be included in their company by the MGPV Division of the Geological Society of America. In particular, I wish to thank my friend and colleague, Mike Garcia for initiating my nomination, together with Dave Clague, Dave Walker and Tony Morse. They have all had a significant influence on my career, in various ways, over the past 50 years. I would also like to thank Alan Whittington and Elizabeth Widom for organizing this session.
At age 14, immersed in hiking, camping and climbing , I knew I wanted to be a geologist: specifically a field geologist. But, tellingly, during my B.Sc. at Bristol (UK), I was impressed by Bernard Leake’s rapid analysis lab for major elements in rocks. Nevertheless, my reaction to his short course on X-Ray Fluorescence (XRF) analysis of rocks was “Thank God that’s over."
My first job was in Australia, where I joined the Bureau of Mineral Resources in Darwin, Northern Territory. During this “Crocodile Dundee” stage of my career, I gained a wealth of experience of reconnaissance mapping, exploratory drilling and of the Outback. I became intrigued by granites, the Rum Jungle granite in particular. This project was initiated by John Richards at the Australian National University (ANU). His early work on Pb dating of zircons produced ages of around 2.4 billion years, in stark contrast to the 1.7 billion years from other dating methods. Careful mapping revealed not an intrusive granite but a mantled gneiss dome, similar to others identified in Finland by Eskola and the Adirondacks by Buddinton, thus supporting Richards Pb ages.
This experience propelled me to the ANU and academia. At this time, Bruce Chappell and Keith Norrish, were developing highly quantitative XRF major and trace analyses for all types of rocks. My focus changed and I took a Ph.D. with Bruce, applying XRF methods to a local granite - gabbro association. A Post-Doc at Northwestern followed. By then, the NASA lunar program had started at full-throttle with Apollo 11. Paul Gast was Chief Lunar Scientist at the Manned Spacecraft Center. He, and (Co-Chief Scientist) Robin Brett had decided that, once there was no need to quarantine the returned samples, preliminary geochemical data should be done by XRF. They approached Bruce, who recommended me, pointing out I was already in the U.S. These were heady, productive but stressful times. I knew that my data would be thoroughly scrutinized by some of the world’s leading geochemists. In addition to providing the preliminary geochemical data for each of the Apollo missions, I was a co-investigator on Paul’s research. Subsequently, I became a Lunar P.I. and my research switched from granites to the lunar mare basalts and soils.
Simultaneously, the Glomar Challenger was drilling the world’s oceans as part of the Deep Sea Drilling Project, recovering samples of the oceanic crust. Studying ocean-floor basalts seemed an appropriate use of our lunar-based analytical skills. The major outcome of this research was the realization that the compositions of these basalts were dominated by magma mixing due to magmatic recharge along the ocean ridges, solving the ongoing dispute between petrologists and geophysicists. These were not primary mantle melts as the geophysicists maintained because there were magma chambers, but they were too small to be seen with the current geophysical instrumentation. A major problem with working on lunar samples is that you do not have temporal control. Cores of ocean crust provide some control, but the magnitude of the time interval between successive eruptive units is unknown. It was time to work on active volcanoes! Collaboration with Pete Lipman and Jack Lockwood (USGS) on Mauna Loa, Hawaii, provided the answer.
In 1979 I moved to the University of Massachusetts (UMass) to establish a Regional XRF Analytical Facility. Studies on lunar rocks, Mauna Loa and DSDP ocean-floor basalts continued now with students, were extended to the Juan de Fuca Ridge, in collaboration with John Delaney and Paul Johnson at the University of Washington, where we discovered the first hydrothermal vents on this ridge. The move to UMass brought me in contact with one of my geochemical heroes, Fred Frey. We established a symbiotic relationship in which we and our students used each others’ labs (XRF, INAA). This led to a long friendship and productive collaboration, especially on the highly successful Hawaii Scientific Drilling Project that geochemically documented about 600 ka of Mauna Kea magmatic history.
Mauna Loa remained the focus of my research including a detailed geochemical study of the 1984 and 2022 eruptions. Simultaneously, work with Mike Garcia on the 39 year-long Puu Oo eruption of Kilauea began. Mike also spearheaded submarine research on Mauna Loa’s South West rift zone. Ar/Ar dating by Brian Jicha identified lavas between 190 and 700 kA along the submarine ridge. Remarkably, over ~500 ka while there are differences in the proportions of the mantle source components, the prevailing magmatic processes have remained the same: mixing of an evolved magma reservoir magma with a more primitive, olivine-laden parental magma.
Meanwhile, granites were not forgotten. Work with students on the islands of Vinalhaven and Isle au Haut, Coastal Maine, showed that these granites had been intruded by basaltic magmas, producing quenched pillow lavas and sills. The melts had mingled but not mixed to produce hybrid magmas: strong evidence that basaltic magmatism provides the driving engine for granitic batholiths and large volume silicic eruptions.
What have I learned over the past 60 years? First, ones career may well be unpredictable. Therefore, one should not rule anything out. Next, throughout all the variations in research focus, careful observations and good data are absolutely essential. Ideas may be flawed, or subject to revision, but sound data continues to be of use. If the data is poor or unreliable ideas based on it must be suspect. It has been my experience that if you have good data for a project, you always find something interesting, or useful. Moreover, if you don’t understand your data, it is telling you something of vital importance. Figure it out!