Obituaries Australia

Bruce Chappell, by Tony Feder, 1971

Libraries ACT, 002284

Bruce Chappell was one of the most distinguished geologists of his generation whose contributions to understanding the origins of granites are both insightful and profound. A pioneer in the application of X-ray fluorescence spectrography to the analysis of geological materials, his fresh ideas about magma genesis have dominated and largely determined the global directions of much subsequent research on granites. His restite model—the recognition that most granite magmas are a mixture of melt and solid residual source material—underlies his tenet that granites are images of their source. His consequent recognition, with Allan White, that there are two fundamentally different types of granite magma, I-type (derived from igneous sources) and S-type (derived from weathered sedimentary sources), each with its distinctive evolutionary path and associated mineralisation, continues to underpin research into granites worldwide, and the search for granite-related mineral deposits.

Chappell spent his childhood in Arding, a village near Armidale, where his father, his sole teacher for seven years, was the headmaster of the one-teacher school. As a student at Armidale High School he excelled. His Leaving Certificate result in 1953 remains the best achieved by a student from that school. He topped the state in several subjects, and was awarded first-class honours in mathematics, physics and chemistry.

In 1954, Chappell enrolled at Armidale’s New England University College (later the University of New England [UNE]), where he became increasingly interested in geology, and particularly the role of chemistry in geology. On field trips in New England, he became aware of the vast area of granite in the region and how little was known about it.

During his honours year (1958), Chappell studied the relationships between granites and their host rocks in the Tamworth–Manilla region. The year 1958 was a seminal one in the history of granite studies. The controversy over whether granites were igneous rocks was finally laid to rest by the partial melting experiments of Tuttle and Bowen. Chappell became engrossed in the problem of how granites formed. In 1959 he graduated with first-class honours in geology and was awarded a University Medal, the first to a geology graduate at the UNE.

Two important ideas took root in Chappell’s mind during his honours year. First, he realised that the volcanogenic sediments near Tamworth were almost unchanged in chemical composition from typical island arc andesite. Second, he recognised that the enclaves in the New England granites were distinctly different from any of the country rocks that the granites intruded, so were not xenoliths (foreign rocks) as commonly believed, but intimately related to the origin of the granites.

In 1959, Chappell took a job as a Demonstrator in the Geology Department at the UNE and began work on a PhD. After one year he resigned, withdrew his PhD candidature, and took a job as a Lecturer in the newly established ANU Geology Department, the second appointment to the department after founding Professor David Brown. During 1960–61, while working with Brown to set up the department and lecturing, he studied part-time for his MSc degree, developing his ideas about the close chemical relationship between volcanogenic sediments and the erupted magmas that had given rise to them.

Chappell met Dr Allan White at the ANU, the start of a lifelong scientific partnership and enduring friendship. White had studied granites for his PhD and been a Lecturer at the University of Otago with Professor Brown. In 1962, Chappell enrolled to study for a PhD, which he worked on part-time under White’s supervision, completing the project in 1966. He studied the granites of the Moonbi region, part of the New England Batholith. When he started his work, fundamental questions about granite genesis remained unresolved. Although it was widely agreed that granites were igneous rocks formed from magmas, the source of those magmas was assumed to be metasedimentary rocks. It was firmly believed by most igneous petrologists that there was no connection between granites and volcanic rocks.

With his background in physics, Chappell became interested in the physical methods of chemical analysis. A critical component of his PhD research was the chemical analysis of granites using X-ray fluorescence spectrography (XRF). In 1963 he was given responsibility for the operation of the ANU Geology Department’s XRF laboratory, a position that he held for 34 years. The XRF equipment that he used was very basic, but under the expert guidance of Keith Norrish from the CSIRO in Adelaide, he produced rock analyses with a level of accuracy not previously achieved. His paper with Norrish on X-ray fluorescence spectrography published in Zussman’s Physical Methods in Determinative Mineralogy (1967) remains a benchmark in the development and application of the technique.

The accuracy of Chappell’s analyses made it possible for him to see patterns and relationships in the chemical compositions of granites that had never been seen before. He recognised that bulk chemical composition is a basic parameter by which a granite must be defined, and the most useful parameter for studying granite genesis. He showed that it was possible to recognise genetically related groups of granite plutons (which he later termed suites) that had mineralogical and chemical features in common.

Chappell showed that within each pluton and suite there were commonly strong linear correlations between the abundances of different elements. He interpreted this to indicate that granite magmas consisted of a mixture of partial melt and solid material (restite) that became separated to different extents (unmixed) during magma transport. In essence, Chappell recognised what he stated directly over a decade later—granites are images of their source. These findings and ideas were radical for the time, but because work on the Lunar program then intervened, they were not published until several years later.

In 1967, the ANU XRF laboratory was re-equipped with a new generation of instruments, which used Chappell-developed techniques for the analysis of trace element abundances with parts per million accuracy. He was the first person to use XRF to measure trace elements accurately on large numbers of samples of a wide range of rock types. Working with technical staff from the Geology Department and Steve Butler of Torrens Industries, he designed and built an automatic sample loader for the XRF spectrometer, for which he was jointly awarded a Certificate of Merit by the Australian Industrial Design Council for the best industrial design in the field of electronics in 1971 and was a finalist for the Prince Phillip Prize for Industrial Design. This equipment made it possible for the first time to make large numbers of highly precise geochemical analyses and is now standard on most XRF spectrometers.

As an essentially non-destructive analytical technique with high sensitivity, accuracy and precision, XRF was ideal for the analysis of samples returned from the Moon. Chappell was an Associate Investigator for the entire Lunar program, analysing samples returned from all six manned Lunar missions. Of the first 65 Lunar sample analyses reported at the Apollo 11 Lunar Science Conference, Chappell contributed 10, more than any other laboratory. Only four laboratories, including Chappell’s, reported full analyses of major elements plus trace elements. Much of each small sample that Chappell used for his determinations was recovered afterwards and used by his ANU collaborator Principal Investigator Dr William Compston for isotopic analyses. Largely as a result of Chappell’s work, XRF soon replaced emission spectroscopy as the preferred technique for the analysis of Lunar rocks.

In 1978 Chappell supplemented his XRF analyses by Instrumental Neutron Activation Analysis (INAA), making significant improvements to both techniques. This experience with solid state detectors led him to appreciate the potential of XRF analysis using polarised X-rays, a method that provided much greater sensitivity than conventional XRF for many of the elements of interest to geochemistry. In 1998 he purchased an XRF spectrometer from Spectro Analytical Instruments in Germany. Linking up with the manufacturers, he subsequently visited Germany regularly to provide practical advice on how the spectrometer could be improved.

While Chappell was mostly focused on his Lunar work, Allan White and students from the ANU were systematically mapping and characterising granites in the Berridale region, southern NSW. They found that the granites formed two mineralogically and chemically distinct groups. Alongside White, Chappell came to the realisation that the difference between the granite types was fundamental, reflecting derivation of the magmas from different sources. They called the granites with infracrustal igneous sources I-type, and those with weathered supracrustal sedimentary sources S-type. This concept was published in a conference abstract in 1974. It was listed by Yoder (1993) in his article ‘Timetable of Petrology’ as one of only seven critical discoveries in the development of ideas in petrology that originated in Australia.

After visiting California in 1972, Chappell started a collaboration with Dr Paul Bateman (United States Geological Survey) and later Professor Leon T. Silver (California Institute of Technology) in the study of the Cordilleran granites of the western United States. He found those granites to be different in many basic features from those that he had studied in Australia. Unlike the relatively homogeneous granite plutons in Australia, those in the western USA were commonly chemically and mineralogically zoned. In addition, rather than there being strong linear relationships between the abundances of different elements, some elements commonly defined curved trends when plotted against SiO2 or total Fe. This, he realised, was due to the process of fractional crystallisation, in which some elements are extracted from the cooling granite magma as different minerals crystallise, while others become concentrated in the melt phase.

Chappell had embarked on a lifelong quest to understand the petrogenesis of granites through their chemical compositions. In the course of his career he analysed, with unprecedented accuracy, many thousands of samples of granites and their associated volcanic rocks, carefully collected and documented by himself and his co-workers. The scientific concepts that he developed were underpinned by extensive geological mapping of large areas of eastern Australia that he, White and their students undertook as a basis for understanding the relationships between, and distribution patterns of, the different types of granite and granite suites. That work culminated in preparation of a 1:1,250,000-scale map, published by the Bureau of Mineral Resources, Geology and Geophysics (BMR) in 1991, in which all the main granites and related rocks of the Lachlan Fold Belt were distinguished, delineated, named and, where possible, classified.

As well as analysing nearly every significant granite body in eastern Australia, from Torres Strait to southern Tasmania, Chappell carried out major studies on the igneous rocks of Papua New Guinea, Japan, the western USA, Scotland and Cornwall. Through this work he refined his ideas and recognised the similarities in, and differences between, the source rocks and magma-forming processes in different tectonic settings. The granites of the western USA, for example, were formed by the high temperature melting of igneous sources and subsequent fractional crystallisation. Many of the granite magmas in Cornwall were also modified by fractional crystallisation, but because their source rocks were predominantly sedimentary, the end products were different, resulting in ore-grade enrichments of Sn.

Chappell considered the restite model to be his most important contribution to the study of granite petrogenesis. After first publishing the concept with White in 1977, he developed the model and explored its implications, publishing his findings in a carefully reasoned article in the Journal of Petrology (1987) and expanding them further in his Mawson Lecture for the Australian Academy of Science, delivered in 1998. The key point of the restite model is that granites image their source rocks—their compositions and the paths by which the magmas evolve are determined by their source ‘DNA’. The model recognises that many granites exposed at the surface of the Earth closely reflect the nature and composition of their deeper source materials, and from the patterns of compositional variation within granites, the nature of the source rocks can be inferred. The fact that many granite magmas are formed at relatively low temperatures has important implications for the amount of heat required to induce magmatism, the mechanism by which the Earth’s crust has become vertically fractionated, and the role of granite magmatism in forming mineral deposits. In Chappell’s stated opinion, the restite model was ‘the only fundamentally new process dealing with the evolution of igneous rock suites since [the publication of The Evolution of the Igneous Rocks by] Bowen in 1928’.

By 2004 Chappell had made significant progress towards a unified model for granite genesis. The model was based on premises that he held to be true: most granites form by processes initiated by partial melting of the crust; source rock compositions provide the genetic code for granites, predetermining to a significant degree the physical and chemical properties of magmas, and their physical and compositional evolution; each granite source responds to heating in a manner that is a physicochemical expression of its composition; crystal fractionation is the dominant mechanism by which the compositions of granites have evolved.

Chappell’s unified model used as its basis the work of Tuttle and Bowen, who showed that partial melting produced a low temperature melt of granitic composition only if the starting material contained a sufficient abundance of four critical components: SiO2, K2O, Na2O and H2O. Chappell took the idea one step further and asked the question: what would the composition of the melt be, and what sort of magma would be produced, if higher temperatures were needed to form a mobile magma because the source rock was deficient in any one of those components? His answer provided an explanation for much of the range in the compositions of granites worldwide.

Chappell and White realised that there were economic implications to the recognition of two main types of granite—Sn mineralisation, for example, appeared to be confined to the most felsic of the S-type granites, whereas W and porphyry-type Cu and Mo deposits were associated with the I-types. These ideas were developed in following years as Chappell’s eastern Australian geochemical data base expanded, vast amounts of information being disseminated to assist the mineral exploration industry through workshops and meetings, particularly through the Australian Mineral Industry Research Association (AMIRA). Chappell’s first AMIRA project (Geochemistry of Granites as an Aid to Mineral Exploration) ran from 1984 to 1986. It collated all the geochemical results on granites of the Lachlan Fold Belt that had been obtained at the ANU by Chappell and his students. The project was subsequently extended twice to encompass analyses of granites from the New England Orogen and North Queensland, providing coverage of much of the exposed Palaeozoic crust in eastern Australia.

The work on granite-related mineralisation accelerated when economic geologist Dr Phillip Blevin joined the team. Starting with a ground-breaking paper in 1992, Blevin and Chappell set about establishing a range of criteria by which the mineral industry could use granites and their related volcanic rocks to assess the prospectivity of a given area for a range of different mineral deposits. These criteria were based on a deep understanding of the chemical processes that govern the transport and concentration of metals in the different chemical environments present in granite magmas and magma sources of different compositions.

For AMIRA Project P425 (1994–07: Intrusion Related Gold and Copper Deposits of Eastern Australia), Chappell and Blevin joined forces with Dr Gregg Morrison (Klondyke Exploration Services) to examine the relationships between regional-scale igneous associations and many of the major Cu and Au deposits in eastern Australia. Simon Beams (later Director of Terra Search Pty Ltd) and Doone Wyborn (later a Director of Geodynamics Ltd) were major contributors to this large project. AMIRA project P515 (1999–2002: Igneous Metallogenic Systems of Eastern Australia) brought the long-running collaboration with industry to a conclusion. The confidential final report written by Blevin provided a first-pass synthesis of the granite geology and related metallogeny throughout eastern Australia and an assessment of the prospectivity and likely styles of granite-related mineralisation, region by region, from northern Queensland to southern Victoria.

Chappell’s work for AMIRA was only one of his many contributions to industry. The XRF and INAA techniques that he developed made it possible for him to analyse with great accuracy samples of unusual composition, such as commonly are associated with mineralisation. He assisted various mining companies that were having problems with their assays, and samples that he analysed are still being used by some companies to calibrate their assay results. Work that Chappell did with Western Mining Corporation geologist Dr Doug Haynes has been acknowledged by the company as playing an important role in its discovery of the large Olympic Dam Cu-U-Au deposit in South Australia. Over three decades, Chappell and Norrish ran annual courses on X-ray spectrometry for analysts from industry, training over 200 people from major mining companies, government agencies and universities. In addition, Chappell advised companies such as Philips, Siemens and Spectro Instruments on the development of X-ray and related equipment and its use for specific applications.

Although best known for his contributions to the study of granite petrogenesis, Chappell worked on a broad spectrum of projects. These included work on sea-floor basalts, sediments and sedimentary rocks, granulites, the Murchison meteorite, Lunar rocks, volcanic rocks from eastern Australia, Melanesia and New Zealand, kimberlites and lamproites, ophiolites and mantle rocks, oil shales and carbonatites. The most important of these projects were those on kimberlites and Melanesian volcanic rocks. The study of lamproites from the Kimberley region of Western Australia was carried out with Dr Lynton Jaques from the BMR (now Geoscience Australia). The work demonstrated the national value of the ANU laboratory in its ability to analyse rocks of unusual compositions. Without it much of the geochemistry of this economically important suite of rocks could not have been studied in Australia. All known occurrences of lamproite suite rocks in northwestern Australia were thoroughly documented, including the most diamond-rich igneous rocks known, the lamproites from Argyle.

Chappell received several honours and awards in recognition of his contributions to geoscience. He was elected a Fellow of the Mineralogical Society of America in 1983. His paper with Allan White and Rick Hine on granite provinces and basement terranes in the Lachlan Fold Belt was awarded the Stillwell Medal in 1988. In 1990 he was awarded a DSc by the ANU. In 1993 he gave the biennial Clarke Memorial Lecture to the Royal Society of New South Wales, and in 1994 was elected a Fellow of the Geological Society of America. He was elected an Honorary Fellow of the Geological Society (London) in 1995, one of only six Australians with that honour at the time. In 1998 he was awarded the Mawson Medal by the Australian Academy of Science, delivered the Mawson Lecture and was elected a Fellow of the Academy. He was awarded the Centenary Medal ‘for service to Australian society in Earth and planetary science’ in 2001, and in 2007 was elected a Fellow of the Geological Society of Australia. He was posthumously awarded the Keith Norrish AXAA Award for Excellence in X-Ray Fluorescence Analysis in 2014.

Chappell was appointed Professor of Geology at the ANU in 1992, a position that he held until his retirement in 1997. For the following two years, as Emeritus Professor, he worked at the University as a Visiting Fellow. In 2000 he was appointed an Honorary Professor of the University of St Andrews and an Adjunct Professor at Macquarie University. In 2001 he was the Leverhulme Fellow and Visiting Professor at the University of Bristol, and in 2006 was appointed Professor of Earth Sciences at the University of Wollongong. Bruce William Chappell died in Canberra on 22 April 2012, leaving a substantial financial legacy to support future geology PhD students at the ANU. 

* An extended version of this obituary was published by I. S. Williams and K. S. W. Campbell in Historical Records of Australian Science 28(2) (2017): 146–158.