SHRIMP Ion Microprobe Research Articles

Late Paleoproterozoic deposition and Mesoproterozoic metamorphism of detrital material in the southernmost Baltic Sea region (Gdańsk IG1 borehole): monazite versus zircon and chemical versus isotopic age record

ABSTRACT The SHRIMP ion microprobe zircon and monazite U–Th–Pb isotope geochronology of a biotite-sillimanite paragneiss derived from the Gdańsk IG1 borehole that penetrated the Precambrian basement in the southernmost Baltic Sea area, northern Poland, has constrained the ages of detrital zircon cores and metamorphic overgrowths. In addition, monazites have been dated by electron beam chemical technique either as a grain separate or in thin sections. The metasediments show a specific provenance pattern with a dominant input from the late Paleoproterozoic sources. The ages range from 2749 to 1643 Ma with peaks at 1779, 1815, 1840 and 1870 Ma. The maximum depositional age, defined by the youngest detrital zircons, was determined at 1643 Ma. Zircon rims and monazite isotope analyses record a metamorphic event at 1485, and 1498 Ma, respectively, whereas Th–U–Pb chemical dating on monazite revealed an age of 1494 Ma. These ages correspond with the time of the AMCG magmatism in the basement of northern Poland. The metamorphic monazite predates the age of the zircon rims by over 10 m.y. This discrepancy is likely caused by the different behavior of the two accessories since the zircon metamorphic crystallization requires fluid/melt-related support. Thus, zircon is not expected to record the time of peak metamorphism. In consequence, monazite tends to be a more sensitive metamorphic geochronometer. Moreover, the EPMA method provides greater spatial resolution. Additional older fragments, with ages of 1539, 1596, 1600 and 1620 Ma, were identified within monazites reflecting earlier stages of metamorphic evolution.

Comparative Lower-Middle Ordovician conodont oxygen isotope palaeothermometry of the Argentine Precordillera and Laurentian margins

Stratigraphic, palaeontologic, and palaeomagnetic data support a hypothesis that argues for the Argentine Precordillera rifting from the southwestern margin of Laurentia in low latitudes during the Cambrian, migrating across the Iapetus Ocean, colliding with the Gondwanan margin in the late Middle Ordovician, and receiving glaciogenic sediments in the Late Ordovician. An alternative model proposes that the Precordillera originated as a low-latitude segment of Gondwana, migrated southward through major transform faulting toward high latitudes in the late Middle Ordovician, to reach its present position in the Devonian. New conodont oxygen isotope compositions (δ18Ophos) have been determined by ion microprobe SHRIMP II using samples from both the Precordillera and Laurentia (Marathon area of Texas, Wilcox Pass in Alberta, and western Newfoundland). Significantly, the δ18O values of conodonts from all four widely separated areas show a consistent pattern of a cyclic but overall increasing trend in δ18O (ca. 16 to 18‰) hence ocean cooling through the Early and Middle Ordovician. An apparent change occurs at the basal Late Ordovician, where δ18O values obtained from conodonts in the uppermost sample from the Precordillera are significantly higher (+1.5‰) than those from Laurentia. Albeit from a single sample, this higher value implies significantly cooler conditions, as would be anticipated with a southerly (poleward) migration of the Precordillera (irrespective of either hypothesis). The virtual absence of conodont-bearing carbonates in most of the Precordilleran Upper Ordovician precluded analysis of younger samples. When combined with existing macrofaunal and palaeomagnetic data, the oxygen isotope data would tend to favour the model of a drift of the Precordillera from tropical to higher latitudes during the Ordovician; however, further studies are needed to determine unequivocally whether the Precordillera originated from southern Laurentia (Ouachita embayment). These new oxygen isotope values provide the best and regionally most consistent data through the Early-Middle Ordovician.

Measurement of in-situ oxygen isotope ratios in monazite by SHRIMP ion microprobe: Standards, protocols and implications

Monazite forms at sub-solidus conditions in a variety of metamorphic rocks and has been proven to be reactive to fluids, and thus is a potential monitor of fluid–rock interaction. As monazite can preserve multiple growth zones, in order to explore the potential of monazite as a fluid tracer in metamorphic conditions, microbeam analysis is required. We performed oxygen isotope analysis of monazite using the SHRIMP ion microprobe and newly characterized standards, for which we obtained laser fluorination δ18O values (USGS-44069 monazite 7.67±0.26‰ and Itambé monazite 0.46±0.20‰). Reproducibility of δ18O ion microprobe analyses for USGS-44069 and Itambé monazites is in the order of 0.4–0.6‰, standard deviation at 95%c.l., similar to what is routinely obtained for silicates. This reproducibility is comparable to that of the analyses of experimental P-rich glasses, which are assumed to be homogeneous and free of geological imperfections.The variable composition of natural monazite has the potential to produce matrix effects during ion microprobe measurements. Monazite grains from the Malagasy syenite (Madagascar) and the Dora Maira whiteschists (Italy) display a scatter in δ18O values that show a negative correlation with Th content and is related to the huttonite [ThSiO4] and cheralite [CaTh(PO4)2] substitutions in monazite. The matrix effect on oxygen isotope measurements can be significant and is estimated to produce a shift in δ18O of circa −0.85 or −1.9‰ for every 10wt.% Th introduced by the huttonite and cheralite components, respectively. Corrections for this matrix effect are proposed on the basis of the natural samples investigated.Oxygen isotope fractionation factors for monazites of different compositions, cheralite and huttonite were calculated with the increment method. The results suggest that the substitution of trivalent LREE by tetravalent Ce and Th results in consistent enrichment of 18O in the monazites, whereas the substitution of tetravalent Th by divalent Ca results in the depletion of 18O in cheralite-rich monazite.Monazites from high-grade metasediments (Mount-Stafford, Central Australia) preserve inherited cores, but are homogeneous in oxygen composition. This suggests that diffusion may efficiently erase the oxygen isotope signature in monazite that experienced ~800°C metamorphism.

U–Th–Pb dating of collision in the external Alpine domains (Urseren zone, Switzerland) using low temperature allanite and monazite

Dating metamorphism in external domains of orogenic belts is a challenging task due to the few chronometers available and common inheritance or disturbance of the isotopic systems at low-temperature metamorphic conditions. In the external domains of the Central Alps (Urseren zone), the occurrence of distinct populations of monazite and allanite in one single outcrop offers a unique chance to evaluate the potential of these two REE-minerals to date successive metamorphic stages within a collisional cycle. The studied outcrop (locality of Garvera) exposes Mesozoic metasediments that were metamorphosed under greenschist facies conditions (T<450°C). Under these conditions, REE-mineral grains (or domains) are typically small (~5–50μm), and in this case also occur as porphyroblast in pelites at the contact with carbonate layers or veins (allanite grains >200μm). Based on the texture and mineral assemblages, REE-mineral growth conditions were attributed to be detrital (Mnz1), syn-kinematic (Mnz2 and Aln1), post-kinematic at the thermal peak (Aln2) and retrograde during late-stage deformation (Mnz3). To constrain temporally the successive crystallisation conditions, REE-minerals have been dated using SHRIMP ion microprobe. U–Th–Pb analyses show that the Alpine REE-minerals have a high Th/U and while most of the 206Pb is non-radiogenic (f206 that can reach up to 99% for allanite), Th–Pb ages can be obtained using 206Pbc-corrected Th–Pb isochron. The age of different generations of allanite and monazite constrains the timing of successive collisional stages: for the prograde accretion at 22.5±1.5Ma (Mnz1 and Aln1), thermal peak at 19.3±2.0Ma (Aln2) and late stage hydrothermal veining at 13.6±1.4Ma (Mnz3). These novel geochronological data confirm the regional diachroneity through the Central Alps with younger ages (burial, thermal peak and exhumation) toward the external domains. The similarity between metamorphic ages of samples taken in the Urseren zone and the nearby crystalline massifs suggests that the metasedimentary cover of the Urseren zone remained closely juxtaposed to the external crystalline basement during the Alpine collision cycle. The monazite age of the late stage vein testifies for active tectonic deformation at 13.6±1.5Ma, which likely played a crucial control on the exhumation of the external massifs in the Middle Miocene.

U-Pb isotopic dating of zircons (SHRIMP-II) from granulites of the Ol’khon region of Western Baikal area

Based on the first isotopic geochronological data obtained by the classic U–Pb method by zircons [1–3], it was established that magmatic and metamorphic rocks of the Ol’khon region are of Early Paleozoic age. The age of granulite metamorphism of the Chernor� udskaya zone rocks spreading as a narrow band (with a width of <1 km) along the boundary of the Siberian Craton was 485 ± 5 Ma. Data of the SHRIMP ion microprobe for U–Pb isotopic dating of zircons [4, 5] confirmed previously obtained results: age estimations were 507 ± 8 Ma, for granulites in the northeastern part of the Chernorudskaya zone (Khadarta Penin� sula) and 498 ± 7 Ma, for the Khoboi Cape (Ol’khon Island). However, problems concerning age estima� tions for the protolith of these rocks remained unsolved. Our study is aimed at solution of these prob� lems.

Brief history of isotope geology at the Australian National University

Isotope geology began in the Department of Geophysics, the Australian National University, Canberra, in the late 1950s. A K–Ar dating laboratory was set up in 1960, with development of Rb–Sr dating facilities taking place soon after. These laid the foundations for extensive programs of isotopic dating with emphasis on providing quantitative age information related to the geological evolution of the Australian continent. Demonstration of progressive younging of granite emplacement to the northeast in the Tasman Fold Belt of eastern Australia was a notable initial achievement. Early successes on broader questions included the demonstration of the ability to measure K–Ar ages on young volcanic rocks, leading on to a significant role in the establishment of the geomagnetic polarity time-scale. In the early 1970s, dating of returned lunar samples by the Rb–Sr method was another most notable success. The development of the SHRIMP ion microprobe, commencing in the mid-1970s, resulted in U–Pb dating of zircon (and other minerals) becoming possible on very small areas of single crystals, revolutionising the approach to dating, with wide application not only to rocks in Australia, but indeed worldwide. Isotope geochemical study of the Earth's mantle and crust was also an important activity, especially in the 1980s. Subsequently, emphasis has been redirected toward studies of the Quaternary, including U-series isotopic dating as well as high-resolution isotopic and element ratio measurements, especially on corals, to track climate change. Many other developments in isotopic dating and isotope geochemistry have taken place over the last 45 years with installation and, in some cases, development of appropriate instrumentation, facilitating the updating of capabilities and expanding the range of techniques, continuing to the present time.

SHRIMP U-Pb zircon geochronology of the High Himalayan rocks in the Nyalam region, Tibet

FIGURE 1. Detrital zircon age spectra from Nyalam High Himalaya (Sample NY93-1) U-Pb age patterns of individual detrital zircons are a potentially powerful tool for constraining metasedimentary provenance. Each zircon grain has a characteristic age reflecting its genesis, and the overall population of detrital zircons represents the spectrum of primary zircon-bearing source rocks, including detritus derived from a number of sedimentary cycles (Cawood et al. 1999). The complex of sillimanite-, kayaniteand garnet-bearing gneiss, calc-silicates and augen gneiss in Nyalam High Himalaya are intruded by granitoids. The garnet-sillimanite paragneisses have experienced an early Paleozoic metamorphism (Gehrel et al. 2003) and Tertiary metamorphism. The source of metasedimentary rocks of the Nyalam High Himalaya zone has been studied using SHRIMP ion microprobe at Beijing SHRIMP Lab, Chinese Academy of Geological Sciences. SHRIMP U/Pb age data from the metasedimentary rocks of the High Himalayan terranes in South Tibet range from ca.23 Ma to 3221 Ma. The U-Pb data allow grouping of the zircons into five major age components with the exception of Cenozoic ages (Figure 1). (1) Archean grains with a maximium age frequency between 3221 and 2509 Ma, (2) Paleoproterozoic grains ranging from 2453 to 1631 Ma, (3) Mesoproterozoic to early Neoproterozoic grains ranging in age between 1530 Ma and 944 Ma, (4) Neoproterozoic grains ranging between 852 and 540 Ma, and (5) Pan-African ages (543-443 Ma). Potential source regions for the detrital zircons occur within the Gondwana terranes: zircons of Archean age correspond to the age of rock unit formed during major a mgmatic and tectonothermal pulses in the Bhandara Craton or Singhbum craton, zircons with paleo-Mesoproterozoic ages reflect tectonic or magmatic events related to or older than the assembly of India (Catlos et al. 2002). Neoproterozoic zircons may have been derived from Lesser Himalayan rocks or the Indian craton (Myrow et al. 2003). Pan-African zircons are the Pan-African orogen event along the Himalayan orogenic belts. The U-Pb age data suggest the sedimentary sources of the studied gneisses in the northern part of the Indian plate. During Late Precambrian and the Palaeozoic, the Gondwanian India bounded to the north by the Cimmerian Super terranes, was part of Gondwana and was separated from Eurasia by the Paleotethys Ocean. During the periods, the northern part of India was affected by a Pan-African event, Numerous granitic intrusions dated at around 500Ma are attributed to this event. The Pan-African event is marked by an unconformity between Ordovician continental conglometrates and the underlying Cambrian marine sediments (Zhu Tongxin et al. 2003). The PanAfrican event form a wide belt stretching from the Alps over the Arabic peninsula, Africa, India, Australia and down to Antarctica. These evidences suggests the presence of the Pan-African orogenic event in the Himalaya. It is tempting to correlate the early Palaeozoic thermal event with a late extensional stage of the long-lasting Pan-African orogenic events, which ended with the formation of the Gondwana supercontinent. The protolith age of the High Himalayan metamorphic rocks is generally regarded to be Precambrian to early Paleozoic. It seems plausible that the High Himalayan metamorphic rocks represent a minimum depositional rage at ∼500 Ma. Our data show strong similarities to previously published spectra for the Greater Himalayan zone, Lesser Himalayan zone and Tethyan Himalayan zone (Parrish et al. 1996; DeCelles et al. 2000; Myrow et al. 2003). Detrital zircon spectra from the High Himalayan range is as young as 500 Ma, so the High Himalaya and Tethyan Himalaya were deposited contemporaneously.

Timing of magmatism and metamorphism in the Gruinard Bay area of the Lewisian Gneiss Complex: comparison with the Assynt Terrane and implications for terrane accretion—reply

U-Pb zircon data from selected gneiss samples provide estimates of the timing of igneous protolith emplacement and granulite facies metamorphism in the Gruinard Bay, Badcall Bay and Lochinver areas of the mainland Lewisian Gneiss Complex of north-west Scotland. Zircons from the gneisses contain complex zoning patterns making them amenable to SHRIMP ion microprobe analysis, with inherited zircon distinguishable from magmatic growth and metamorphic recrystallisation. From Gruinard Bay, a tonalitic granulite yielded a protolith age of 2,825±8 Ma for oscillatory-zoned zircon cores, with recrystallised low-U rims preserving an age of 2,733±12 Ma. An inherited 2,905±15 Ma age component was recognised in a sample of trondhjemitic gneiss, the protolith of which is estimated to have been emplaced at 2,858±11 Ma. Low-U rims yielding an age of 2,729±20 Ma are also present, which together with the rim age from the granulite are considered to date granulite facies metamorphism in the Gruinard region. For comparison with the Gruinard Bay gneisses, a trondhjemitic partial melt and trondhjemite sheet were analysed from the granulite facies rocks at Badcall Bay in the Assynt terrane. Age data do not show definitive relationships with zoning patterns in the zircons, and show a spread along concordia typical of previous ion microprobe age data from the Scourie gneisses. Both samples do, however, show a significant isotopic disturbance at 2,480 Ma, the most recent estimate for the Badcallian metamorphism. Protolith ages for components of the Assynt terrane range between ca. 2,900 and 3,030 Ma, and appear to be older than the 2,860–2,825 Ma gneiss protoliths from the Gruinard Bay area. The timing of granulite facies metamorphism also appears to be different between the two areas, suggesting that the two granulite regions of the mainland may not be related as previously thought. Instead, they are interpreted in terms of a terrane accretion model whereby the two areas evolved as separate blocks prior to their juxtaposition during the Palaeoproterozoic. The boundary between the two terranes is thought to lie to the south of Lochinver, corresponding with the Strathan Line, because of the observed 2,480 Ma age signature to the north and 2,730 Ma signature to the south. By defining a new mainland terrane, the Gruinard Terrane, the number of mainland blocks, which includes the Assynt (central), Rhiconich (northern) and Southern terranes, has been expanded.

Exploring the potential of allanite as a geochronometer of high-grade crustal processes

The REE-rich accessory mineral allanite plays a key role in the storage and mobility of geochemically important trace elements (LREE, Th) in magmatic and high-grade rocks. Allanite occurs in a wide range of rock types, but of particular interest is its common presence in mafic, migmatitic and high-pressure rocks. We report on the response of allanite trace element chemistry and its U-Th-Pb isotopic system to magmatism, partial melting and eclogite-facies metamorphism. In situ U, Th–Pb geochronology of allanite has been carried out using SHRIMP ion microprobe in conjunction with LA ICP-MS analysis. We analyse allanite with a variety of FeO and trace element compositions from anatectic rocks, granodioritic to tonalitic plutons and high-pressure mafic rocks. Allanite Th– Pb ages are capable of reliably addressing geochronological problems with a precision of 1–2% and 2–5% (2r) for magmatic and metamorphic rocks, respectively. This study allows preliminary conclusions to be made for the closure temperature of allanite. In the case studies presented, no indication of inheritance has been observed for allanite. The trace element composition of allanite records changes in paragenesis. Allanite HREE content relative to LREE provides an indicator for the co-crystallization of garnet. Where allanite formed in migmatites display a small negative Eu anomaly, eclogitic allanite lacks a Eu anomaly, which is related to crystallization in the absence of plagioclase. In addition, the Sr content in allanite can be used as an indicator of crystallization above the stability field of plagioclase. This study demonstrates that allanite can be correlated to major metamorphic and rock-forming minerals and therefore the P-T conditions of crystallization.

Geochronology (SHRIMP II) of zircons from Archean stratotectonic associations of Karelian greenstone belts: Significance for stratigraphic and geodynamic reconstructions

Individual grains of zircon from the Archean Kostomuksha, North Karelian and Matkalakhta greenstone belts, which are situated respectively in western, northern and eastern Karelia, are studied using the ion microprobe SHRIMP II. As a result, the oldest 207Pb/206Pb ages of 3151 ± 4.6 and 3329 ± 16 Ma are first determined for detrital zircons from northern and eastern Karelia. The 207Pb/206Pb ages estimated for two subsequent metamorphic events of Archean Eon in eastern Karelia correspond to 3.25 and 3.17–3.10 Ga. The age value of 2711 ± 9.6 Ma is determined for silicic volcano-plutonic complex and quartz stockwork in northern Karelia and the date 2821 ± 15 Ma for magmatic rocks of eastern Karelia. Silicic volcanics from an oceanic plateau section in the Kostomuksha belt are dated at 2791.7 ± 6.1 Ma for the first time in the Archean of Fennoscandia. The oldest detrital zircons from siliciclastic metasediments determine the stabilization time of Archean continental nuclei in East Fennoscandia. The younger generation of greenstone belts is exemplified in the Karelian craton by the Matkalakhta and Kostomuksha structures comprising rock associations less than 2.82 Ga old, mafic rocks of the Kontokki Group included. Geological history of these belts corresponds to geodynamic mesocycle 90–110 Ma long and to the Archean global epoch of metallogeny, which was responsible for origin of most valuable deposits of base and precious metals.

The Age of the Giant Golden Mile Deposit, Kalgoorlie, Western Australia: Ion-Microprobe Zircon and Monazite U-Pb Geochronology of a Synmineralization Lamprophyre Dike

Open-pit and underground mines in the Golden Mile at Kalgoorlie, Western Australia, have produced more than 1,475 metric tons (t) of gold since 1893. Despite the economic importance of the deposit, the age of the mineralized shear zone array and its temporal relationship to other structures and to porphyry intrusions in the host greenstone terrane are poorly constrained. In this study, the SHRIMP ion microprobe has been used to date zircons and monazites recovered from a chlorite-carbonate–altered, synmineralization lamprophyre dike intruded into sericite-ankerite–altered Paringa basalt below the high-grade Oroya hanging-wall shear zone. Analyses of magmatic-hydrothermal zircons define a weighted mean 207 Pb/ 206 Pb age of 2642 ± 6 Ma ( n = 37, MSWD = 0.88). Analyses of cogenetic hydrothermal monazites yield a concordant 207 Pb/ 206 Pb age of 2637 ± 20 Ma ( n = 9, MSWD = 1.8). The two alteration and/or mineralization ages, determined independently in two different minerals, are inconsistent with recent structural models relating mineralization to the early compressional events of the D 1 to D 4 deformation sequence for the terrane. Instead, they confirm earlier structural models relating mineralization to the postfolding wrench tectonic regime which generated regional sinistral faults (D 3 ) coeval with mantle-derived lamprophyric and initial felsic magmatism in the middle to lower Archean crust.

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

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.

U–Pb evidence of ∼1.7 Ga crustal tectonism during the Nimrod Orogeny in the Transantarctic Mountains, Antarctica: implications for Proterozoic plate reconstructions

The Pacific margin of East Antarctica records a long tectonic history of crustal growth and breakup, culminating in the early Paleozoic Ross Orogeny associated with Gondwanaland amalgamation. Periods of older tectonism have been proposed (e.g. Precambrian Nimrod and Beardmore Orogenies), but the veracity of these events is difficult to document because of poor petrologic preservation, geochronologic uncertainty due to isotopic resetting, and debated geological field relationships. Of these, the Nimrod Orogeny was originally proposed as a period of Neoproterozoic metamorphism and deformation within crystalline basement rocks of the Nimrod Group, based on ∼1000 Ma K–Ar mineral ages. Later structural and thermochronologic study attributed major deformation features in the Nimrod Group to Ross-age basement reactivation. Yet, new SHRIMP ion microprobe U–Pb zircon age data for gneissic and metaigneous rocks of the Nimrod Group indicate a period of deep-crustal metamorphism and magmatism between ∼1730–1720 Ma. Igneous zircons from gneissic Archean protoliths show metamorphic overgrowths of ∼1730–1720 Ma, and an eclogitic block preserved within the gneisses contains zircons yielding an average metamorphic crystallization age of ∼1720 Ma. Deformed granodiorite that intrudes the gneisses and associated metasedimentary rocks yields a concordant zircon crystallization age of ∼1730 Ma. Despite scant petrologic evidence for these metamorphic and igneous events, the zircon ages from these diverse rock types indicate major crustal thickening, possibly due to collision, in the late Paleoproterozoic. We therefore recommend revival of the term Nimrod Orogeny to describe Paleoproterozoic tectonic events in rocks of the East Antarctic shield. Similarities in the ages of igneous and metamorphic events in the Nimrod Group and geologic units elsewhere in present-day East Antarctica, southern Australia and southwestern North America suggest they may have played a role in early supercontinent assembly. In particular, similarity with the Laurentian Mojave province is consistent with Proterozoic plate reconstructions joining ancestral East Antarctica with western Laurentia.

Multiple zircon growth during fast exhumation of diamondiferous, deeply subducted continental crust (Kokchetav Massif, Kazakhstan)

Diamondiferous rocks from the Kokchetav Massif, Kazakhstan, represent deeply subducted continental crust. In order to constrain the age of ultra high pressure (UHP) metamorphism and subsequent retrogression during exhumation, zircons from diamondiferous gneisses and metacarbonates have been investigated by a combined petrological and isotopic study. Four different zircon domains were distinguished on the basis of transmitted light microscopy, cathodoluminescence, trace element contents and mineral inclusions. Mineral inclusions and trace element characteristics of the zircon domains permit us to relate zircon growth to metamorphic conditions. Domain 1 consists of rounded cores and lacks evidence of UHP metamorphism. Domain 2 contains diamond, coesite, omphacite and titanian phengite inclusions providing evidence that it formed at UHP metamorphic conditions (P>43 kbar; T~950 °C). Domain 3 is characterised by low-pressure mineral inclusions such as garnet, biotite and plagioclase, which are common minerals in the granulite-facies overprint of the gneisses (P~10 kbar; T~800 °C). This multi-stage zircon growth during cooling and exhumation of the diamondiferous rocks can be best explained by zircon growth from Zr-saturated partial melts present in the gneisses. Domain 4 forms idiomorphic overgrowths and the rare earth element pattern indicates that it formed without coexisting garnet, most probably at amphibolite-facies conditions (P~5 kbar; T~600 °C). The metamorphic zircon domains were dated by SHRIMP ion microprobe and yielded ages of 527±5, 528±8 and 526±5 Ma for domains 2, 3 and 4 respectively. These indistinguishable ages provide evidence for a fast exhumation beyond the resolution of SHRIMP dating. The mean age of all zircons formed between UHP metamorphic conditions and granulite-facies metamorphism is 528±3 Ma, indicating that decompression took place in less than 6 Ma. Hence, the deeply subducted continental crust was exhumed from mantle depth to the base of the crust at rates higher than 1.8 cm/year. We propose a two-stage exhumation model to explain the obtained P–T–t path. Fast exhumation on top of the subducted slab from depth >140 to ~35 km was driven by buoyancy and facilitated by the presence of partial melts. A period of near isobaric cooling was followed by a second decompression event probably related to extension in a late stage of continental collision.

Quantitative comparison of large sets of geochronological data using multivariate analysis: a provenance study example from Australia

A variety of modern isotopic analytical techniques now allow the relatively rapid acquisition of large sets of geochronological data, particularly in provenance studies. Traditional interpretation of this type of data often relies on visual comparison techniques that are vulnerable to subjective bias and this problem becomes acute with increasingly large quantities of data. Multivariate techniques are presented in this article to objectively evaluate age components within the detrital zircon age data from 31 beach sand and sedimentary samples in Australia for provenance studies. Using the SHRIMP Ion Microprobe each sample typically contains 60 or more individual age measurements with a combined total of 2150 analyses. Principal components derived from the data set allow the construction of provenance models that are independent of expected age components. In this instance the principal components are interpreted in terms of existing knowledge about Australian geology, in particular the provenance relationship between “local” Phanerozoic-aged and “exotic” late Proterozoic-aged protosources. Using principal components analysis this relationship is seen clearly and objectively in one diagram illustrating provenance evolution along the eastern coastline.

U–Th–Pb chemical dating of monazites using the proton microprobe

Proton Induced X-ray Emission (PIXE) analyses using the proton microprobe have been performed to obtain precise concentrations of major and trace elements in a range of monazite samples. With the proton microprobe, detection limits for U, Th and Pb are in the ppm range, thus making the instrument viable for chemical dating, even in young monazite grains with low radiogenic Pb concentrations. The method has been tested successfully on monazites previously analysed by the SHRIMP ion microprobe, conventional mass spectrometry and chemical dating with an electron microprobe. The high sensitivity of the proton microprobe has potential to be used for chemical dating of other minerals with U, Th and Pb abundances below the detection limit of the electron microprobe (e.g., zircon, sphene, rutile).

SHRIMP U-Pb zircon ages for Archaean granitoid rocks, Ataa area, north-east Disko Bugt, West Greenland

Zircons from four samples of granitoid rocks from the Ataa area have been studied by SHRIMP ion microprobe. A trondhjemite from the Atâ intrusive complex (Atâ tonalite) yielded an age of 2803 ± 4 Ma, in agreement with earlier age determinations. A sample from the regional migmatitic biotite gneisses gave 2815 ± 4 Ma. A homogeneous granitoid rock, from field observations believed to be younger than the regional gneisses, has two main zircon populations, 2835 ± 4 Ma and c. 2800 Ma, respectively, and a granite that intrudes the Atâ complex yielded an age of 2758 ± 2 Ma.

Internal structures and dating of complex zircons from Meissen Massif monzonites, Saxony

U–Pb zircon ages were determined for accessory zircons from two monzonites from the Meissen Massif (Northern Bohemian Massif, Germany) by SHRIMP ion microprobe. BSE and CL imaging revealed that the monzonite zircons, showing oscillatory growth zoning, often contain inherited cores. Raman microprobe measurements showed that zoned rims (U 277–1426 ppm, Th 144–910 ppm) are slightly disordered to moderately metamict whereas cores have a wide spectrum of structural states, from well-crystallized to highly metamict. The radionuclide (U+Th) content of cores varies from a few hundred to more than 20,000 ppm. SHRIMP ion probe measurements on zoned rims, interpreted as zircon growth during crystallization of the monzonite magma, gave concordant Carboniferous 206 Pb / 238 U ages of 326±6 Ma (monzonite from Leuben) and 330±5 Ma (monzonite from Heidenschanze; 95% confidence), respectively. These ages are consistent with recently published 40 Ar / 39 Ar ages of 329.1±2.8 to 330.4±2.8 Ma (2 σ) on amphiboles from the same rocks. In contrast, SHRIMP measurements on cores range from (slightly discordant) Proterozoic relic ages in low U and Th cores to highly discordant data in the most metamict cores. Previously obtained 207 Pb / 206 Pb ages (single zircon evaporation technique) in the range 340±16 to 357±16 Ma (1 σ) are probably mixed-ages between magmatic and inherited zircon components. The U–Pb ages of the monzonite zircons postdate the Upper Devonian/Lower Carboniferous subduction and collision events in the area by about 20 Ma. Due to their shoshonitic chemical signature which is typical of rocks from destructive plate boundary settings, the Meissen Massif monzonites might be interpreted as originating during these ≈350 Ma events, and the earlier 207 Pb / 206 Pb ages appeared to confirm this interpretation. However, both the 40 Ar / 39 Ar ages and our current results show that only the enrichment of the lithospheric mantle source, but not the monzonite intrusion itself, was directly related to these events. The monzonite intrusion itself took place ≈330 Ma ago, in the framework of strike-slip tectonics in the Elbe Valley zone. Widely occurring zircon cores are indicative of crustal contamination of the magma, causing considerable modification of chemical signatures (trace element content) of the monzonites.

High detection efficiency isotope ratio analysis using a single focussing ICP magnetic sector mass spectrometer

The IsoProbe is a single focusing magnetic sector ICP source multicollector mass spectrometer. A hexapole collision cell between the ICP source and the magnetic sector analyser reduces the energy spread of ions from the source from 15ev to <lev. The result is accurate mass focusing and fiat topped peaks from the multicollector. The sensitivity of the system is very high. With a Meinhard-type nebuliser ion currents of 1E9 cps/ppm are attained, which increases to 1El0 cps/ppm with a CETAC microconcentric desolvating nebuliser. This leads to a total ions per atom detection efficiency of 1% for Pb. The same efficiency is also apparent with laser ablation of NIST glasses and zircons. This high efficiency is equivalent to that reported for the SHRIMP ion microprobe, where in situ Pb isotope ratio measurements have normally been made. We will present the results of the direct, in situ, measurement of lead and hafnium isotope ratios in zircons, and in NIST glasses by laser ablation using a Nd:YAG laser.

Metamictization and U-PB isotopic discordance in single zircons: a combined Raman microprobe and SHRIMP ion probe study

We report results of a combined SHRIMP ion microprobe and Raman microprobe study of the correlation between metamictization and U-Pb isotopic discordance of zircon. The spatial resolution of the SHRIMP and Raman probe used are about 200 μm3 and 80 μm3, respectively. This allows a direct comparison of U-Pb isotopic discordance and metamictization of small areas within zircon crystals. We show that the impact of the oxygen ion beam on the zircon during the SHRIMP analysis does not cause significant amorphization or recrystallization in the remaining zircon on a scale of 1 μm. Consequently, it is possible to determine the initial degree of metamictization of zircon within and adjacent to a SHRIMP analytical spot by Raman microprobe measurements after performing SHRIMP analyses. A combination of the two microprobe techniques gives information on the concentration and distribution of radionuclides as well as the degree of metamictization and its heterogeneity and the lateral “age distribution” within the grain. We found that the degree of U-Pb isotopic discordance correlates closely with the degree of metamictization within single zircon grains, on a scale of 15 μm, which is consistent with previous results on the scale of single and multiple zircon grains showing that metamictization enhances the relative potential for secondary loss of radiogenic lead in zircon.