Daniela Rubatto

Zircon trace element geochemistry: partitioning with garnet and the link between U–Pb ages and metamorphism

Abstract With the aim to link zircon composition with paragenesis and thus metamorphic conditions, zircons from eclogite- and granulite-facies rocks were analysed for trace elements using LA-ICP-MS and SHRIMP ion microprobe. Metamorphic zircons from these different settings display a large variation in trace element composition. In the granulites, zircon overgrowths formed in equilibrium with partial melt and are similar to magmatic zircon in terms of high Y, Hf and P content, steep heavy-enriched REE pattern, positive Ce anomaly and negative Eu anomaly. They are distinguishable from magmatic zircon because of their low Th/U ratio. Independently of whole rock composition, metamorphic zircon domains in eclogite-facies rocks have low Th/U ratio and reduced HREE enrichment and Eu anomaly. In a low grade metamorphic vein, zircon has low Th/U ratio but is extremely enriched in Y, Nb and HREE. Petrological and geochronological data demonstrate that metamorphic zircon overgrowths crystallised at granulite-facies conditions in equilibrium with unzoned garnet. It is thus possible for the first time to calculate trace element distribution coefficients between zircon and garnet. Hf is the elements that most strongly partition into zircon. Y, Nb and REE have distribution coefficients between 90 and 0.9 with minimum values for the MREE. In eclogite-facies rocks, the HREE depletion in metamorphic zircon domains is attributed to concurrent formation of garnet under sub-solidus conditions. In one sample, the zircon/garnet trace elements partitioning indicates that metamorphic zircon formed in equilibrium with the garnet rim, i.e. at the eclogitic peak. The reduced Eu anomaly in the metamorphic zircon is interpreted as indicating absence of feldspars and thus supports zircon formation in eclogite facies. In a metamorphic vein within the eclogite-facies rocks, zircons have larger Eu anomaly with respect to high-pressure zircon. Together with geochronological evidence, the Eu anomaly suggests that these zircons formed during prograde metamorphism, before the break down of feldspars at high pressure. The REE composition of zircon can therefore relate zircon formation to specific metamorphic stages such as eclogite, granulite or greenschist facies. This allows linking zircon U–Pb ages with pressure–temperature conditions, a fundamental step in constraining rates of metamorphic processes.

Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia

Abstract We report an extensive field-based study of zircon and monazite in the metamorphic sequence of the Reynolds Range (central Australia), where greenschist- to granulite-facies metamorphism is recorded over a continuous crustal section. Detailed cathodoluminescence and back-scattered electron imaging, supported by SHRIMP U–Pb dating, has revealed the different behaviours of zircon and monazite during metamorphism. Monazite first recorded regional metamorphic ages (1576 ± 5 Ma), at amphibolite-facies grade, at ∼600 °C. Abundant monazite yielding similar ages (1557 ± 2 to 1585 ± 3 Ma) is found at granulite-facies conditions in both partial melt segregations and restites. New zircon growth occurred between 1562 ± 4 and 1587 ± 4 Ma, but, in contrast to monazite, is only recorded in granulite-facies rocks where melt was present (≥700 °C). New zircon appears to form at the expense of pre-existing detrital and inherited cores, which are partly resorbed. The amount of metamorphic growth in both accessory minerals increases with temperature and metamorphic grade. However, new zircon growth is influenced by rock composition and driven by partial melting, factors that appear to have little effect on the formation of metamorphic monazite. The growth of these accessory phases in response to metamorphism extends over the 30 Ma period of melt crystallisation (1557–1587 Ma) in a stable high geothermal regime. Rare earth element patterns of zircon overgrowths in leucosome and restite indicate that, during the protracted metamorphism, melt-restite equilibrium was reached. Even in the extreme conditions of long-lasting high temperature (750–800 °C) metamorphism, Pb inheritance is widely preserved in the detrital zircon cores. A trace of inheritance is found in monazite, indicating that the closure temperature of the U–Pb system in relatively large monazite crystals can exceed 750–800 °C.

Zircon formation during fluid circulation in eclogites (Monviso, Western Alps): implications for Zr and Hf budget in subduction zones

Abstract The zircons from an eclogite and an enclosed eclogite-facies vein from the Monviso ophiolite (Western Alps) display contrasting chemical and morphologic features and document different stages of the evolution of the ophiolite. The zircons from the eclogite show a typical magmatic zoning and are enriched in heavy rare earth elements (HREEs) over middle rare earth elements (MREEs) and have an accentuated negative Eu anomaly, which indicates that the grains co-crystallised with plagioclase. These magmatic zircons document the formation of oceanic crust at 163 ± 2 Ma. In contrast, zircons from the vein contain inclusions of garnet, omphacite, and rutile, which indicate that they crystallised under eclogite-facies conditions. The vein zircons have Th/U ratios In the vein, the apparent coexistence of zircon, omphacite, and garnet permits the determination of a set of trace element distribution coefficients among these minerals at high pressure. This set of partitioning can demonstrate chemical equilibrium among these phases in rocks that show less clear evidence of textural equilibrium. In addition, zircon age can now be linked to sensors of metamorphic pressure-temperature conditions. The presence of zircon and rutile in the vein is another example of high field strength element (HFSE) mobility over short distances in aqueous fluids at eclogite-facies conditions. However, the concentrations of Zr and Hf in the aqueous fluid are estimated to be at least a factor of 10 less than primitive mantle values. Mass balance calculations demonstrate that zircon hosts > 95% of the bulk Zr, 90% of Hf, and ∼25% of U in the vein. Zircon is a residual phase in subducted basalts and sediments up to temperatures of at least 800 to 900 °C. Therefore, residual zircon in subducted crust, together with rutile, control the HFSE in liberated subduction zone fluids/melts and might be partly responsible for negative Zr and Hf anomalies in subduction zone magmas.

Exhumation as fast as subduction

We produced a pressure-temperature-time path in order to determine the exhumation rate of the deepest subducted Alpine rocks. In situ dating of peak-metamorphic titanite in an eclogite facies calc-silicate rock indicates that subduction to pressures of ∼3.5 GPa was reached at 35.1 ± 0.9 Ma. Titanite formed during two decompression stages, at 1 ± 0.15 GPa and ∼0.4–0.5 GPa, and yielded ages of 32.9 ± 0.9 Ma and 31.8 ± 0.5 Ma, respectively. Combining the age data and making assumptions about the conversion of pressure to depth yield mean exhumation rates of 3.4 cm/yr and 1.6 cm/yr. These rates imply that exhumation acted at plate tectonic speeds similar to subduction, and was significantly faster than erosion. We suggest that fast exhumation is driven by a combination of tectonic processes involving buoyancy and normal faulting.

Dating of eclogite-facies zircons : The age of Alpine metamorphism in the Sesia-Lanzo Zone (Western Alps)

Abstract Zircons from eclogite-facies rocks of the Sesia–Lanzo Zone have been investigated by cathodoluminescence and dated by ion microprobe (SHRIMP). Most of the zircons show a oscillatory or sector zoned core, which is surrounded by a weakly zoned or unzoned rim, irregular in shape. The U–Th–Pb analyses revealed that, opposite to the cores, the zircon rims have low U- and Th-contents, very low Th/U ratios and that their U–Pb systems were reset or they lost lead at the time of metamorphism. This study provides evidence that the zircon rims recrystallized and changed their internal zoning as well as their chemical and isotopic composition during high-pressure (15–18 kbar) moderate temperature (550–600°C) metamorphism. In the Sesia–Lanzo Zone, eclogite-facies zircons from three different localities and four different rock types were analysed. The age of metamorphic zircon rims was obtained from an eclogite from Monte Mucrone (65±5 Ma), an eclogitic micaschist from the Aosta Valley (65±3 Ma) and two mafic rocks from Cima di Bonze (68±7 Ma). The ∼65 Ma age is further supported by the ages of the youngest zircon rims in three other samples. These results constrain the age of eclogite-facies metamorphism in the Sesia–Lanzo Zone. An age of 76±1 Ma determined on zircons from a hydrothermal vein is interpreted as dating the prograde formation of the vein at temperatures well below 550°C. The age results suggest a maximum rate of 0.7 cm/y for the subduction of the Sesia–Lanzo Zone to eclogite-facies conditions. The Cretaceous–Tertiary age for the eclogite-facies metamorphism in the Sesia–Lanzo Zone implies that this continental unit was not subducted together with either portions of the Adriatic margin or the neighbouring Mesozoic Tethys. We suggest that the Sesia–Lanzo Zone behaved as a separate continental block at the time of Alpine convergence.

Use of Cathodoluminescence for U-Pb Zircon Dating by Ion Microprobe: Some Examples from the Western Alps

The first report on cathodoluminescence (CL) of zircon goes back to the second half of the nineteenth century (Crookes 1879), however, only in the last 40 years has this phenomenon been addressed more frequently. The main CL emission bands have been related to different trace elements such as Mn and V (Leverenz 1968), Hf and Y (Ono 1976), Dy (e.g. Mariano 1978), Gd and Tb (e.g. Ohnenstetter et al. 1991). These elements would act as “activators” and enhance the “intrinsic” luminescence of pure zircon by non-stoichiometry, lattice damage or by structural defects (Marshall 1988). It has also been pointed out that elements such as Y can have a “quenching” effect on CL of zircon (Ohnenstetter et al. 1991) as they reduce the CL emission. A second approach to CL relates to the studies of CL zoning patterns to assist in the interpretation of U-Pb dating. This tool has proved to be indispensable when combined with U-Pb zircon dating by ion microprobe (e.g. SHRIMP). CL allows the identification of different types of zircon domains that then may be dated in situ by SHRIMP, with spatial resolutions between ca. 30 µm (e.g. Gebauer et al. 1988) and 15–20 µm (e.g. Gebauer 1996; Vavra et al. 1996).

Metamorphic rates in collisional orogeny from in situ allanite and monazite dating

The prograde sequence of rare earth minerals recorded in metapelites during regional metamorphism reveals a series of irreversible reactions among silicates and phosphates. In individual samples from the northern Lepontine (Central Alps), allanite is partly replaced by monazite at 560–580 °C. Relic allanite retains its characteristic growth zoning acquired at greenschist facies conditions (430–450 °C). Coexisting monazite and allanite were dated in situ to delimit in time successive stages of the Barrovian metamorphism. In situ sensitive high-resolution ion microprobe (SHRIMP) U-Th-Pb dating of allanite (31.5 ± 1.3 and 29.2 ± 1.0 Ma) and monazite (18.0 ± 0.3 and 19.1 ± 0.3 Ma) constrains the time elapsed between 430–450 °C and 560–580 °C, which implies an average heating rate of 8–15 °C/m.y. Combined with new fission track ages (zircon, 10–9 Ma; apatite, 7.5–6.5 Ma), metamorphic rates of the entire orogenic cycle, from prograde to final cooling, can be reconstructed.

Dissolution-reprecipitation of zircon at low-temperature, high-pressure conditions (Lanzo Massif, Italy)

Abstract An eclogite facies meta-plagiogranite from the Lanzo massif (western Alps, Italy) contains crystals of zircon intimately associated with allanite. Zircon displays different microtextures ranging from pristine, euhedral, and magmatic to fractured, porous varieties with mosaic zoning, and pervasive recrystallization into euhedral microcrystals. Fractures and voids in the recrystallized zircon microcrystals are mainly filled by high-pressure Na-rich pyroxene. Electron backscattered diffraction analysis revealed a similar crystallographic orientation for primary magmatic zircon crystals and microcrystals, with less than 2° misorientation among neighboring microdomains. The textural change is coupled with chemical and isotopic modifications: recrystallized zircon domains contain significantly less Th and light- to mid-REE, but are richer in Sr than magmatic zircon crystals. Magmatic zircon preserves the protolith U-Pb age of 163.5 ± 1.7 Ma, whereas zircon microcrystals have a mean age of 55 ± 1 Ma. The coexisting allanite also contains inclusions of Na-rich pyroxene and has chemical features (elevated Sr and Ni contents and lack of Eu anomaly) indicating formation at high pressure. Despite being associated texturally with zircon, allanite yields a younger Th-Pb age of 46.5 ± 3.0 Ma, suggesting that the Lanzo unit remained at relatively high pressure conditions for ~8 m.y. Zircon recrystallization proceeded with volume reduction and loss of material to an alkaline metamorphic fluid that acted as the agent for a coupled dissolution-reprecipitation process. Recrystallization occurred with minimum transport, in a low-strain environment, and was not significantly enhanced by metamictization. The source of the fluid for zircon recrystallization is most probably related to prograde devolatilization reactions in the surrounding serpentinite.

The rift-to-drift transition in the North Atlantic: A stuttering start of the MORB machine?

We report U-Pb and 39Ar-40Ar measurements on plutonic rocks recovered from the Ocean Drilling Program (ODP) Legs 173 and 210. Drilling revealed continental crust (Sites 1067 and 1069) and exhumed mantle (Sites 1070 and 1068) along the Iberia margin and exhumed mantle (Site 1277) on the conjugate Newfoundland margin. Our data record a complex igneous and thermal history related to the transition from rifting to seafloor spreading. The results show that the rift-to-drift transition is marked by a stuttering start of MORB-type magmatic activity. Subsequent to initial alkaline magmatism, localized mid-oceanic ridge basalts (MORB) magmatism was again replaced by basin-wide alkaline events, caused by a low degree of decompression melting due to tectonic delocalization of deformation. Such “off-axis” magmatism might be a common process in (ultra-) slow oceanic spreading systems, where “magmatic” and “tectonic” spreading varies in both space and time.

From passive margins to orogens: The link between ocean-continent transition zones and (ultra)high-pressure metamorphism

A lithostratigraphic association consisting of serpentinized mantle rocks, continent-derived allochthons, mid-oceanic ridge gabbros of Jurassic age and post-rift sediments, typical of an ocean-continent transition, is found in the eclogitic Piemonte units, in the Western Alps. In situ U-Pb geochronology was performed on zircons from an orthogneiss sampled at the bottom of a sliver of continental basement, in contact with serpentinites. Primary magmatic zircons of Permian age were overgrown by a second generation of zircon at ca. 166–150 Ma, likely related to melt infiltration associated with the intrusion of the underlying gabbroic body. This indicates that continental basement slices and oceanic basement rocks were already juxtaposed in the Jurassic and they were probably part of hyper-extended crust related to the opening of the Tethys. Therefore, the complex lithological association described here, which is also characteristic of several (ultra)high-pressure melange zones worldwide, was acquired prior to the orogenic event, during which it was only partly reworked. Ocean-continent transitions are in positions favorable to reach (ultra)high-pressure conditions, following negatively buoyant oceanic lithosphere into subduction, and then being accreted to the orogen, in response to the arrival of more buoyant continental lithosphere, resisting subduction. The ocean-continent transition is now found in the immediate footwall of a 500-m-thick shear zone, which accommodated multiple episodes of deformation during Eocene–Oligocene time, suggesting an important link between Alpine deformation and rift-related structures.