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,
ScienceWatch.com 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:
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;
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;
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?
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