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Zheng - insight into mountain building during continental collision. Dr. Yong-Fei Zheng
From the Special Topic of Zircon Dating. Dr. Zheng has also sent along images of his work.

According to our February 2008 Special Topic on Zircon Dating, the top paper in the Research Front Map on Zircon Geochronology and Isotope Geochemistry is, “Stable isotope geochemistry of ultrahigh pressure metamorphic rocks from the Dabie-Sulu orogen in China: implications for geodynamics and fluid regime” (Zheng YF, et al., Earth Sci. Rev. 62[1-2]: 105-61, July 2003), with 152 citations.

The lead author of this paper is Dr. Yong-Fei Zheng. According to Essential Science IndicatorsSM from Thomson Scientific, Dr. Zheng’s record in the field of Geosciences includes 127 papers cited 1,863 times in the past 10 years. He is ranked at #112 of the 2,142 scientists comprising the top 1% of this field.

Dr. Zheng is Professor of Geochemistry in the School of Earth and Space Sciences at the University of Science and Technology of China (USTC) in Hefei, where he is the director of the Key Laboratory of Crust-Mantle Materials and Environments in the Chinese Academy of Sciences (CAS), as well as the Deputy Dean of the School of Earth and Space Sciences. He is an Executive Editor of the Chinese Science Bulletin, an Associate Editor of Geochemical Journal, and a member of the editorial board for Lithos. In 2005, he was named a Fellow of the Mineralogical Society of America.

In the interview below, talks with Dr. Zheng about his highly cited paper.

Would you please describe the significance of your paper and why it is highly cited?

Our paper represents a successful extension of isotope geodynamics to the field of continental collision and ultrahigh-pressure metamorphism (UHPM), with highlighting of a conceptually new approach for geochemical studies of metamorphic and igneous rocks in subduction zones. This was realized by integrating stable isotope geochemistry with zircon U-Pb geochronology and petrotectonics in the Dabie-Sulu orogenic belt, the largest UHPM belt in the world. For the first time, it has provided geochemical documentation of the following three important issues:

  1. the short timescale for a bulk cycle of continental subduction from prograde, peak, and retrograde metamorphism under high- to ultrahigh-pressure conditions, which can be described as a rapid process like ice cream frying in boiling oil (Fig. 1). The timescale has been quantitatively confirmed by zircon U-Pb dates on different types of metamorphic zircon;

  2. the temporal link between the oxygen isotope record of premetamorphic protolith and the snowball Earth event, with assumption of ice-fire interaction during supercontinental rifting (Fig. 2). This link has been substantiated by precise U-Pb dating of 18O-depleted zircons from granitic minerals that have the world-record lowest oxygen isotope compositions;

  3. activity of metamorphic fluid during exhumation of deeply subducted rocks, with a provocative hypothesis that the decompressional exsolution of structural hydroxyl in nominally anhydrous minerals provides sufficient fluid for amphibolite-facies retrogression (Fig. 3). This hypothesis has gained more and more support from the measurement of hydroxyl content in nominally anhydrous minerals from ultrahigh-pressure eclogites, the experimental determination of hydroxyl solubility in the minerals, and the U-Pb dating of vein zircons within eclogites.

In summary, our paper has provided a timely outline of important progresses in studies of continental collision and ultrahigh-pressure metamorphism, explored and predicted a number of challengeable scientific questions, and highlighted possible forefronts of research in the near future. It has provided an important complement to chemical geodynamics of oceanic subduction-zone metamorphism. It has also served as a background paper for people who work on metamorphic chemical geodynamics of continental subduction-zones. Thus it is an overview of many studies, and is simply a convenient reference. These may be the reason why our paper has been highly cited in the past five years.

How did you become involved in this research, and were there any particular successes or obstacles that stand out?

Figure 1: +enlarge
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Figure 2:
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Figure 3:
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Findings of coesite and diamond in continental rocks in the 1980s and early 1990s were revolutionizing the plate tectonic theory. The early 1990s was a time when I planned to have my career in China after obtaining my Ph.D. from the University of Göttingen and doing my post-doc at the University of Tübingen, Germany. I was faced with an opportunity to decipher crust-mantle interaction and geochemical reactions in the processes of continental subduction to mantle depths over 100 km. I immediately became involved in the field of continental collision and UHPM since my work at USTC, with the exciting finding of extreme 18O-depleted eclogite in the Dabie-Sulu orogenic belt. While I was puzzled with preservation of meteoric oxygen isotope signature in the eclogite within the upper mantle, the chemical kinetics of oxygen isotope exchange between minerals gave me hints on the timescale of UHPM at mantle depths. It stimulated me to envision "fast-in" and "fast-out" tectonic processes for both subduction and exhumation of continental curst. This expectation of short duration has been verified by later geochronological studies, marking the first success of my studies in the subduction factory.

Furthermore, the relationship of hydrogen and oxygen isotopes between minerals of eclogite and garnet amphibolite associations stimulated me to hypothesize that the decompression exsolution of structural hydroxyl from nominally anhydrous minerals in ultrahigh-pressure rocks can form an important source of retrograde fluid. While this is considered as an important source of retrograde fluid, it also provides resolution to debate how to get surface water into deep crust. Success of this study lies in the fact that more and more experimental and natural studies lend support to this hypothesis, with reasonable interpretations of metamorphic reactions by internally derived fluids.

Our group has been engaged in the chemical geodynamics of continental collision and ultrahigh-pressure metamorphism since 1993. When we were asked by aggressive colleagues if we could advance our approaches of isotope geochronology and geochemistry to geodynamic regime, we faced the dilemma of how to resolve the paradoxical interpretations of element and isotope data on igneous and metamorphic rock in collisional orogens. To this end, we have developed a wealth of geochemical methods suitable for various types of metamorphic minerals. A large number of practices have contributed to our great progress in studying geochemistry of metamorphic and igneous rocks.

Where do you see your research and the broader field leading in the future?

Our paper provides insights into a number of key areas of subduction-zone science which are currently the subject of active research concerning revolution of the plate tectonic theory. These include the continental deep subduction, the evolution of the Precambrian Earth, fluid activity in subduction-zone metamorphism. In addition, it has relevance to the development of the snowball Earth event and to the origin of retrograde fluid in high-grade metamorphic rocks. Furthermore, our conclusion that the short timescale was operated in the bulk processes of continental subduction and exhumation is a key to understanding of the preservation of abnormal chemical and isotopic signatures in ultrahigh-pressure metamorphic rocks that were assumed to form at mantle depths of greater 100 km.

There is no doubt that research efforts on ultrahigh-pressure metamorphic rocks will continue to develop strongly, with contributions from geochemists, geophysicists, geologists, and tectonists, who will keep pushing this field a step further towards higher levels of success. Continental subduction and ultrahigh-pressure metamorphism may become competitive with oceanic subduction and crust-mantle interaction for chemical geodynamics. Our research has highlighted the importance of integrating the stable isotopes, which are capable of characterizing the origin and transport of aqueous fluid, with radiogenic isotopes and trace elements in the same targets for the purpose of interpreting the tectonic evolution in the processes of continental collision and UHPM. It also has implications for geochronological applications to dating of water-rock interaction and fluid activity in igneous and metamorphic rocks. In principle, the approaches used in our paper can be further extended to study chemical geodynamics of other continental and oceanic subduction-zones.

What are the implications of your work for this field?

The past two decades have seen a revolution of the plate tectonic theory. This has brought about by the findings of coesite and diamond in supracrustal rocks of continental collision orogens. In comparison to the rocks of oceanic crust, continental rocks have low porosity and thus small amounts of porous water in sedimentary formations. Fluid regime in the processes of continental subduction and exhumation is one of the most important concerns in the study of metamorphism, magmatism, and mineralization in collisional orogens. Intense interest in the fluid regime during continental subduction and exhumation has continued unabated in the aspects of structural mineralogy, petrology, and geochemistry, resulting in a number of prominent progresses in recent years. Our work has demonstrated the availability of metamorphic fluid during continental collision. This can exert significant influences not only on geodynamic processes but also on element mobility in subduction-zone process, with important implications for crust-mantle interaction and syn-exhumation magmatism.

Water is exchanged between the Earth's surface and deep interior via the formation and breakdown of hydrous minerals. Subduction zones are considered to be key sites in the global recycling of water. Water is found in different states in the deeply subducted lithosphere: as a fluid, especially near subduction zones; as a hydrous phase, particularly in cold regions within subducting slabs; and as a hydroxyl point defect in nominally anhydrous minerals. Metamorphic minerals contain water, primarily in the form of molecular H2O or as hydroxyl OH groups. The inter-diffusion of cations, radiogenic and stable isotopes in metamorphic minerals under hydrous conditions is significantly faster than under anhydrous conditions, indicating that water plays a role in homogenizing chemical heterogeneities in a hydrous rock during subduction-zone metamorphism. The bonding characteristics of hydrogen can influence physico-chemical properties, such as electrical conductivity, compressibility and, in a wider context, mineral stability as a function of pressure, temperature, and composition. Our work has made successful measurements of both concentration and hydrogen isotope composition of water in metamorphic minerals. Thus, the characterization of these water structural environments and recognition of the key factors controlling its occurrence and solubility continues to be an important research area in metamorphic mineralogy, petrology, geochemistry, and tectonics.

Yong-Fei Zheng
Professor of Geochemistry
School of Earth and Space Sciences
University of Science and Technology of China
Hefei, China

Special Topics : Zircon Dating : Yong-Fei Zheng - Special Topic of Zircin Dating