Download Zircon Behaviour in Deeply Subducted Rocks

Zircon Behaviour in
Deeply Subducted Rocks
Photomicrograph of a
zircon crystal containing
inclusions of diamond,
Kokchetav Massif,
Kazakhstan
Daniela Rubatto and Jörg Hermann*
Z
ircon is of fundamental importance in the investigation of deeply
subducted crustal rocks in which it is a trace constituent. Tiny mineral
inclusions within zircon may be the only indicators that rocks were
subducted to a depth of up to 150 km. Because zircon is resistant to physical
and chemical changes, it preserves stages of the subduction and exhumation
history within submillimetre-size grains. Advanced in situ techniques allow
us to date zircon domains and to determine their trace element composition.
We can thus acquire a detailed knowledge of the temperature–pressure–time
paths that these extraordinary rocks have experienced. Zircon studies
provide evidence that subduction and exhumation act at plate tectonic
speeds of 1–3 cm/year.
KEYWORDS: zircon, high-pressure metamorphism, trace elements,
eclogite, U–Pb geochronology
In the last twenty years, the discovery of coesite and diamond in metamorphosed continental crust has completely
changed the paradigm concerning the depth in the mantle
to which the least dense parts of the lithosphere can be subducted. The formation of coesite – a mineral with the same
composition as quartz but with denser structure – requires
pressures of 2.5 GPa, corresponding to a depth of at least
90 km. Microdiamonds in gneisses provide evidence for
even deeper subduction of crustal rocks, to depths of up to
~150 km. These extreme conditions (i.e. great depth) result
in ‘ultrahigh-pressure’ (UHP) metamorphism. Exposures of
high-pressure (HP) and UHP rocks provide a unique natural
laboratory to study an important aspect of plate tectonics:
subduction and exhumation of crustal rocks. While the
presence of coesite and diamond unequivocally demonstrates that crustal rocks have been subducted to great
depth, the processes acting at depth were, until recently,
essentially unknown. How can minerals formed at such
great depth be preserved? How fast can crustal rocks be
buried and exhumed?
FIGURE 1 illustrates increasing degrees of structural modification of zircon in response to HP metamorphism. Zircon
in equilibrated eclogite-facies rocks may be unaffected by
metamorphism and represents the only magmatic relict in
the mineral assemblage. The preservation of older zircon
grains (inheritance) is the rule, particularly in HP rocks that
experienced relatively low temperatures (<650°C). Metamorphic zircon first forms along fractures, probably in the
presence of fluids (FIG. 1A). Commonly, inherited magmatic
crystals have irregular domains where the original zoning is
replaced by chaotic, patchy zircon (FIGS. 1B, E). The altered
zircon is often porous and rich in micro-inclusions, and
shows signs of corrosion (FIG. 1C). As a result, such altered
zircon may be isotopically disturbed, yielding ‘ages’ that are
geologically meaningless (Rubatto and Hermann 2003;
Tomaschek et al. 2003; Spandler et al. 2004). An insight
into zircon recrystallization is provided by FIGURE 1D, in
which a magmatic zircon has been replaced by an aggregate
Zircon, a common accessory mineral in HP rocks, has been
fundamental in constraining processes acting at such
extreme metamorphic conditions because of three extraordinary characteristics it possesses: (1) zircon contains measurable amounts of the radioactive element uranium and
hence can be used as a chronometer for metamorphic
processes; (2) zircon often preserves different growth zones
within a single grain, and thus may document different
stages of the subduction–exhumation cycle; and (3) zircon
protects mineral inclusions formed at high pressures from
* Research School of Earth Sciences,
The Australian National University, Canberra 0200, Australia
E-mail: [email protected]
PP.
31–35
WHAT HAPPENS
TO ZIRCON DURING
SUBDUCTION?
Metamorphic zircon that forms
during the subduction and exhumation of the crust is texturally distinctive. Two main features are observed: partial
or complete replacement of a zircon crystal by a zircon of
different composition (also called recrystallization; see
Geisler at al. 2007 this issue) and new growth of zircon,
often forming on relict (inherited) grains (FIG. 1). The
processes involved in the formation of HP zircon are still
poorly understood. Dissolution–precipitation, i.e. dissolution of existing crystals, or parts of them, in a fluid or melt
and the coupled reprecipitation of zircon with distinct texture and composition, appears to play a key role in both
replacement and modification of zircon. Zirconium liberated from the breakdown of other phases (e.g. garnet, magmatic pyroxene, volcanic glass) may contribute to new zircon
growth. As a general rule, replacement is common under
subsolidus conditions (when no melt is present), whereas
new growth is very common when melt is present.
THE ROLE OF ZIRCON IN
HIGH-PRESSURE ROCKS
ELEMENTS, VOL. 3,
retrogression during exhumation.
In this contribution we summarise
processes and conditions that can
lead to formation of zircon during
deep subduction and highlight the
role of zircon as a mineral container, chemical tracer and time
capsule. We show that the wealth
of information contained in zircon
can only be exploited if the conditions of zircon formation can be
linked to the metamorphic evolution of its host rock.
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A
B
C
D
E
F
G
H
I
and polygonally zoned, and may contain inclusions of HP
minerals (FIG. 1G). In subducted rocks that have reached
temperatures high enough to cause partial melting
(T > 650°C), inherited zircon can be completely dissolved or
lost, and new metamorphic zircon, which tends to be euhedral and exhibits regular zoning, may be precipitated
(FIGS. 1H, I; Hermann et al. 2001; Katayama et al. 2001).
These features resemble those of melt-related zircon formed
at high temperatures but lower pressures in granulites and
migmatites (see Harley et al. 2007 this issue).
Internal structure of zircon crystals from subducted rocks.
A, B and E–I are cathodoluminescence images (see Glossary),
C is a secondary electron image and D is a backscattered electron
image. (A) Zircon with preserved magmatic shape and zoning, transected by fractures filled with metamorphic zircon (Zermatt, Switzerland;
Rubatto et al. 1998). (B) Zircon in eclogite with metamorphic alteration
replacing the original magmatic zoning (Monviso, Italy; Rubatto and
Hermann 2003). (C) Morphology of a zircon from an eclogite-facies
rock showing a surface cut by corrosion channels (Syros, Greece;
Tomaschek et al. 2003). (D) Zircon recrystallized as small crystals intergrown with HP allanite and omphacite (Lanzo, Italy). (E) Zircon from
eclogite with a preserved magmatic core exhibiting low-grade alteration,
subsequently overgrown by a HP metamorphic rim (New Caledonia;
Spandler et al. 2004). (F) Zircon in eclogitic metasediment with a HP
metamorphic rim on a detrital core (Aosta Valley, Italy; Rubatto et al.
1999). The ovals indicate the location of the SHRIMP analyses, which are
small enough to resolve rim from core. (G) Hydrothermal zircon with
rutile inclusions from a HP vein (Monviso, Italy; Rubatto and Hermann
2003). (H and I) Metamorphic zircon crystals from UHP rocks
(Kokchetav Massif, Kazakhstan; Hermann et al. 2001). The crystal in H
contains inclusions of coesite in the core and quartz in the rim. The crystal
in I has two distinct metamorphic growth zones, which formed at different pressures and temperatures.
FIGURE 1
Metamorphic zircon found in HP rocks has usually been
interpreted as forming at the pressure peak, i.e. the maximum subduction depth, which in most cases also corresponds to the temperature peak. However, zircon in HP
rocks can in fact form over a wide range of conditions, from
prograde subduction through to post-peak exhumation
(FIG. 2). This explains why zircon often preserves multiple
growth zones formed at different stages of HP metamorphism (FIGS. 1E, H, I). In rare cases, metamorphic zircon can
form in low-pressure (<1.0 GPa) veins that are produced on
the prograde burial path during subduction and accompanying dehydration. This occurs before the subducting crust
reaches the depth at which eclogite-facies mineral assemblages are formed (Liati and Gebauer 1999; Rubatto et al.
1999; Spandler et al. 2004). There is some evidence that,
once the rock is at great depth, metamorphic zircon forms
even before the pressure peak. The formation of prograde
HP zircon can explain the range of ages (~240–215 Ma) documented in HP zircon of the Dabie-Sulu orogen of eastern
China (e.g. Wan et al. 2005; Wu et al. 2006). Importantly,
zircon formation has also been documented during postpeak exhumation of subducted crust, i.e. at pressures less
of small zircon crystals, intergrown with HP minerals. Discrete zircon rims or domains are common features of zircon
in subducted rocks. These rims form on inherited magmatic
(FIG. 1E) or detrital cores (FIG. 1F) and often provide reliable
ages for the metamorphism (see below). Occasionally, completely new zircon grains are found in HP metamorphic
veins (e.g. Liati and Gebauer 1999; Rubatto et al. 1999;
Rubatto and Hermann 2003). This growth requires dissolution of Zr from other sources (most likely magmatic zircon
in the country rock) and very high fluid/rock ratios. These
hydrothermal zircon crystals lack inheritance, are euhedral
ELEMENTS
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F EBRUARY 2007
than the maximum attained. This is particularly common
in rocks that maintained relatively high temperatures during exhumation, such as those of the Kokchetav Massif in
Kazakhstan or the Dabie-Sulu belt in China. In these cases,
the retrograde formation of zircon is generally attributed to
recrystallization in the presence of melts produced as a result
of decompression-melting reactions.
FIGURE 2 illustrates the fact that the conditions at which zircon forms in subducted rocks vary over a wide P–T range
(1.8–4.5 GPa and 450–950°C), suggesting that zircon does
not form by a single reaction. In these rocks, zircon forms
by processes that can act over a variety of P–T conditions,
whenever fluids or external factors (deformation, reaction
kinetics, time) permit. Despite recent advances, our understanding of the influence of such factors on zircon formation is still quite limited.
ZIRCON AS A MINERAL CONTAINER
Zircon is also important in the study of HP metamorphism
because of its extraordinary capacity to preserve mineral
inclusions. This is due to its physical and chemical stability
and its reluctance to re-equilibrate or dissolve at lower pressure conditions. In deeply subducted rocks that subsequently underwent intense lower-pressure recrystallization,
zircon may contain the best or only evidence of the HP
stage. For example, in the crustal rocks of the Kokchetav
Massif, phases such as diamond and coesite have been
almost exclusively found as small inclusions in zircon
(FIG. 1H; Parkinson and Katayama 1999; Hermann et al.
2001; Katayama et al. 2001; Ogasawara 2005). Similarly, in
the UHP rocks of the Dabie-Sulu orogen in China, coesite is
extensively preserved as inclusions in metamorphic zircon
(Wan et al. 2005), even in gneisses that have been completely retrogressed under lower-pressure conditions and
preserve no other UHP relicts (Ye et al. 2000). Because of the
remarkable confining strength of zircon, the coesite inclusions it contains can remain under high internal pressures
even when the rocks are exposed at Earth’s surface. For
example, Parkinson and Katayama (1999) demonstrated
using laser Raman spectroscopy that some coesite inclusions in zircon retain pressures up to 2 GPa. Similarly,
zircon formed during low-temperature hydrothermal alteration is able to protect low-grade minerals from later HP
metamorphism, providing a window into pre-subduction
processes (Spandler et al. 2004).
Schematic representation of zircon preservation and formation in subducted rocks. Zircon in subducted rocks
records a number of different processes and forms over a wide range of
P–T conditions. Symbols represent different zircon types. The various
P–T paths refer to specific localities: pink – Kokchetav Massif, Kazakhstan
(Hermann et al. 2001; Katayama et al. 2001); brown – Rhodope, Greece
(Liati and Gebauer 1999); red – Dabieshan, China (e.g. Wu et al. 2006);
light blue – Western Alps, Italy (e.g. Rubatto et al. 1998; Rubatto et al.
1999; Rubatto and Hermann 2003); dark blue – New Caledonia (Spandler et al. 2004); green – Syros, Greece (Tomaschek et al. 2003).
FIGURE 2
ZIRCON AS A CHEMICAL TRACER
In the last decade there has been an ever-increasing capability of microbeam techniques to measure in situ the
chemical and isotopic compositions of small volumes of
materials. As a consequence, the analysis of small grains of
accessory minerals such as zircon has become increasingly
important to geochemical studies. Zircon contains significant amounts of a number of key elements [such as Hf, Y,
the heavy rare earth elements (HREE) and of course Zr] that
are present in trace quantities in rocks but are important
geochemical tracers. The composition of zircon is relevant
to the study of subducted rocks for two main reasons: (1)
the dissolution and crystallization of zircon strongly influence the transport and release of these trace elements at
depth in the ‘subduction factory’, and (2) the trace element
composition of zircon may assist in age interpretation.
Identification of pressure-indicator minerals as inclusions
in zircon has become increasingly important, not only for
the diagnosis of UHP metamorphism, but particularly for
the interpretation of zircon ages in the context of the metamorphic P–T path. Mineral inclusions provide an excellent
link between zircon formation and P–T conditions, and
thus between age and metamorphism. The use of mineral
inclusions as a tool to link the age of zircon to P–T conditions in subducted rocks has grown considerably from the
pioneering work of Gebauer et al. (1997). Identifying pressure-indicator minerals such as coesite, zircon, rutile,
omphacite, HP garnet and Ti-rich phengite in zircon
(FIGS. 1D, G–H) requires detailed microscopy, spot chemical
analysis and in some cases Raman microspectroscopy.
These techniques are widely available, low-cost and nondestructive, and assist greatly in relating ages to metamorphic evolution (e.g. Hermann et al. 2001; Katayama et al.
2001; Rubatto and Hermann 2003; Gilotti et al. 2004; Spandler et al. 2004; Wan et al. 2005; Zhang et al. 2005), particularly where multiple zircon growth stages with different
inclusion generations are preserved (FIG. 1H).
ELEMENTS
Despite comprising only 0.01% by volume of subducted
oceanic crust, zircon, where present, carries most of the Zr
(>95%) and Hf (~90%) and a significant amount of the U
(~25%) contained in the bulk rock (Rubatto and Hermann
2003). Dissolution of accessory zircon in liberated fluids and
melts will therefore play a fundamental role in the transfer of
Zr and Hf from the subducted slab to the overlying mantle.
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Rubatto (2002) first documented that zircon formed at HP
has a peculiar trace element pattern, with no enrichment in
HREE and no Eu anomaly, two features common in magmatic zircon (FIG. 3). The negative Eu anomaly in the REE
pattern of magmatic zircon can be explained by the presence
of coexisting feldspars, which have a strong positive Eu
anomaly, whereas HREE enrichment is related to the preferred substitution of the smaller REEs on the Zr site. Rocks
equilibrated under HP conditions are characterised by the
presence of abundant garnet (a mineral rich in HREE) and
omphacite, and by the absence of feldspars. Zircon formed
in equilibrium with these eclogitic minerals will thus be
depleted in HREE relative to the middle REE (giving a flat
chondrite-normalised HREE pattern with Lu/Gd < 3) and
enriched in Eu (producing no negative Eu anomaly and
with Eu/Euexpected > 0.75; FIG. 3). The peculiar trace element
composition of HP zircon distinguishes it not only from
most magmatic zircon but also from metamorphic zircon
crystallized under lower-pressure conditions, where feldspar
is present. The chemical signature of HP zircon is remarkably reproducible and has been observed in a wide range of
rock types that have experienced HP metamorphism. This
diagnostic tool is expected to be very robust because diffusion
of REE in zircon is slow enough to be insignificant under
most geological conditions experienced by crustal rocks.
in mixed ages. This problem was surmounted with the
development of the sensitive high resolution ion microprobe (SHRIMP), the first instrument capable of dating mineral domains as small as 20 µm. Since it was first applied to
the study of HP rocks (Gebauer 1996; Gebauer et al. 1997),
there has been an explosion of ion microprobe U–Pb analysis
to date zircon growth zones in subduction-related rocks.
In dating HP rocks that have experienced temperatures
above 650°C, the use of zircon is particularly important
because the U–Pb system is one of the few chronometers
that is resistant to such high temperatures. Dating of UHP
rocks is particularly difficult because a large part of the
decompression history occurs at nearly constant temperature (FIG. 2), making it essential that formation and not
cooling ages are determined to constrain the time span of
exhumation. U–Pb ages date the formation of a particular
zircon domain and not its cooling below the closure temperature of diffusion of the daughter product of the radioactive decay. This temperature resistance, together with the
chemical robustness, allows preservation of multiple zircon
ages, from inherited to prograde, peak and retrograde conditions. Because zircon in HP rocks can preserve inheritance
and can form at different stages of the subduction–exhumation cycle (FIG. 2), the age of any zircon domain cannot
simply be assumed to date the pressure or temperature peak
of the rock, but must be linked to P–T conditions.
In order to link ages to metamorphic conditions, a number
of methods can be used. Initially and most commonly, the
shape and internal zoning of zircon have been used to distinguish between inherited and metamorphic zircon. A
Th/U ratio < 0.1 has long been recognised as a common feature of metamorphic zircon, in contrast to Th/U ratios of
0.2–0.8 in magmatic zircon. The most direct way of determining the conditions of zircon formation is by documenting metamorphic mineral inclusions contained in distinct
zircon domains. These mineral inclusions (for example
coesite, diamond and garnet) are trapped during the growth
of a zircon and permit the zircon domain to be related to
the stable paragenesis in the host rock (e.g. Hermann et al.
2001; Katayama et al. 2001). On the other hand, textural
relationships between zircon and other HP minerals have
often proven to be inconclusive because of common inheritance – a zircon included in a HP mineral might easily be
the relict of a magmatic or detrital crystal. The trace element signature of HP zircon discussed above is effective in
distinguishing it from inherited or low-pressure zircon.
Additionally, a recently developed geothermometer based
on the Ti content in zircon in equilibrium with rutile (TiO2)
is an important tool in relating zircon growth to temperature (Watson et al. 2006). As is often the case in science,
there is no magic wand for interpreting the age obtained
from zircon. The most successful zircon dating studies of
deeply subducted rocks are those that combine a number of
methods to interpret the age and that consider the complexity of the rock system.
Rare earth element (REE) composition of zircon normalized
to chondrite abundances. Thick lines represent HP zircon
from different rocks and conditions. When compared to magmatic zircon
(gabbro from Monviso) and low-pressure metamorphic zircon (amphibolite facies, Kokchetav Massif), HP zircon has relatively low HREE contents, shows flat HREE patterns and lacks a negative Eu anomaly. The value
for La in magmatic and Sesia Zone metasediment was not measured,
but inferred from detection limits. Sources: light blue – HP metasediment, Sesia Zone (Rubatto 2002); pink – UHP gneiss, Kokchetav Massif
(Hermann et al. 2001); dark blue – HP vein, Monviso (Rubatto and Hermann 2003); red – kyanite eclogite, Greenland (Gilotti et al. 2004);
brown – retrogressed eclogite, Rhodope (Liati 2005). LREE: light REE,
MREE: middle REE; HREE: heavy REE.
FIGURE 3
ZIRCON SPEEDOMETRY
Zircon is often the only mineral that retains age information from different stages in the subduction–exhumation
cycle, and thus zircon has been used to determine the speed
at which rock units are subducted and exhumed. The diamond-bearing gneisses of the Kokchetav Massif are an
excellent example where multiple zircon growth stages
have been documented (FIG. 2). Surprisingly, the age
obtained from zircon domains containing diamonds is
indistinguishable within error from the age of zircon
domains formed at lower pressure, indicating that exhumation from depths of >140 km to 30 km took place in less
ZIRCON AS A TIME CAPSULE
Zircon has long been one of the most commonly used
geochronometers for crustal rocks, but its application to HP
rocks has become prominent only in the last 15 years. This
is because zircon in HP rocks generally consists of multiple
domains formed at different stages of metamorphism
(FIG. 1), such that dating of entire crystals will often result
ELEMENTS
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F EBRUARY 2007
then 6 Ma. This implies very fast exhumation rates of more
than 1.8 cm/year (Hermann et al. 2001). Such high exhumation rates have also been found in the Eocene–Oligocene
UHP Dora Maira unit in the Alps (2.2–3.4 cm/year; Gebauer
et al. 1997; Rubatto and Hermann 2001). The discovery of
very young zircon of 4 Ma in (U)HP rocks from Papua New
Guinea (Baldwin et al. 2004) provides unequivocal proof of
fast exhumation rates for deeply subducted crust. These
rates are comparable to rates of seafloor spreading and plate
convergence, indicating that exhumation can be as fast as
subduction. Such fast exhumation rates exceed the known
rates of erosion, providing evidence that exhumation of
deeply subducted crust must be related to tectonic processes
within the slab.
other geochronometer. The initial data available indicate
that the duration of an entire subduction–exhumation
cycle varies significantly and may last from a few million
years (Liati and Gebauer 1999) to tens of millions of years
(Wu et al. 2006).
ACKNOWLEDGMENTS
We thank guest editors Simon Harley and Nigel Kelly for
the invitation to contribute to this issue. The work presented is the result of collaborations over the years with R.
Compagnoni (Torino, Italy), D. Gebauer (Zürich, Switzerland),
A. Korsakov (Novosibirsk, Russia), and C. Spandler and O.
Müntener (Bern, Switzerland). Internal reviews by M. Norman
and R. Rapp, and journal reviews by I. Parsons, J. Mattinson
and D. Root helped to improve the manuscript. This work
was supported by the Australian Research Council. .
What remains unresolved is the time that HP rocks spend at
depth. This problem is due to the relative scarcity of prograde geochronological information from zircon or any
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