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In the Palghat–Cauvery Shear/Suture Zone of the Southern Granulite Terrane, the Sittampundi Layered Complex occurs as a mappable unit. The Sittampundi Layered Complex consists of mafic, ultramafic and anorthositic rocks with chromitite layers and has been interpreted as an arc/ophiolite complex that formed in an Archaean suprasubduction zone arc setting. In the Sittampundi Layered Complex, reddish-black metabasites occur as layers or boudins with a rim of amphibolite within anorthosite. The peak metamorphic assemblage of the metabasites is garnet + clinopyroxene + quartz + rutile ± plagioclase ± orthopyroxene, and symplectite consisting of amphibole and plagioclase formed around the garnet during retrograde metamorphism. The protolith of metabasite may have intruded in a suprasubduction zone arc setting during the late Neoarchaean (c. 2540–2520 Ma), and then underwent high-pressure granulite-facies peak metamorphism (900–800 °C and 11–14 kbar) in the early Palaeoproterozoic (c. 2460–2440 Ma), followed by amphibolite-facies metamorphism (550–480 °C and 5.5–4.5 kbar) in the middle Palaeoproterozoic (c. 1900–1850 Ma). The results obtained in this study, together with previous studies, indicate that: (1) the high-pressure granulite-facies metamorphism in the study area indicates that subduction occurred during the Archaean–Proterozoic boundary with a higher apparent average geothermal gradient (∼20–16 °C km−1) than the modern-day Earth, and (2) the apparent average geothermal gradient of the subduction zone was ∼29–14 °C km−1 during the Archaean–Proterozoic boundary, which was still too high to enter the realm of eclogite-facies metamorphism.

In far North Greenland at the head of Victoria Fjord (∼81°30′N), a ∼1000 km2 exposure of Precambrian crystalline basement rocks is a window through the region's extensive latest Neoproterozoic and Paleozoic Arctic Platform sedimentary cover sequence. These basement rocks, named here the Victoria Fjord terrane, are dominated by weakly-foliated granodioritic orthogneisses, with lesser amounts of migmatite. These are intercalated with strips of supracrustal rocks dominated by paragneisses. This poorly-exposed Archean terrane at Greenland's northern tip is succeeded to the south by the extensive east-west trending Paleoproterozoic Inglefield Mobile Belt of juvenile arc rocks. The Victoria Fjord terrane granodiorites have zircon U-Pb ages from 3292 ± 14 Ma to ∼3260 Ma. Reconnaissance U-Pb dating on schlieric migmatite zircons yielded ages of ∼3280, 3400 and 3500 Ma. Paragneiss detrital zircons have ages of mostly ∼2710 Ma, with a minority of 3000–2900 Ma grains. This demonstrates that these sedimentary rocks were not derived from the local basement. Post- Archean magmatic or metamorphic zircon was not detected in any of the samples. Initial eHf of 3290–3260 Ma granodiorite and granite zircons ranges from −4 to +2, and the magmatic protoliths are interpreted as sourced from the melting of somewhat older (3500–3400 Ma) Paleoarchean crust, represented by paleosome in the migmatites. The paragneiss 3000–2900 Ma detrital zircons were derived from juvenile felsic crust of that age. The ∼2710 Ma group show negative correlation of Th/U and initial eHf, such that those with high Th/U up to >2.0 have eHf values of −6 to −10. The ∼2710 zircons were sourced from mafic rocks formed by melting of much older (Paleoarchean?) enriched mantle which was variably contaminated by 3000–2900 Ma crust during its ascent. The Victoria terrane is unique amongst the Archean basement terranes in Greenland because it is dominated by 3500–3260 Ma crust. However, the Arctic basin contains detrital zircons of that age, and rocks of that age also occur in eastern Siberia and northwestern Canada. Implications for high Arctic early Precambrian geodynamics are discussed.

Isotopic analyses of elements in the natural reactor materials have often been performed to understand the distribution behaviors of the fission products and to evaluate the function of nuclear reactions since the first discovery of a natural reactor in 1972. Several types of unique microminerals, including significant amounts of fission products, have been found in and around the Oklo and the Bangombé natural reactors. In the past two decades, microbeam techniques using ion and laser probe facilities have been effectively applied for the in situ isotopic analyses of individual microminerals to investigate the migration behaviors of fissiogenic radioisotopes produced in the reactors. This paper presents a review of interpretations of the isotopic results of microminerals found in and around the natural reactors.

The Spongtang Massif is a remnant of Neotethyan ocean crust emplaced onto the Indian passive margin along the Indus-Yarlung-Tsangpo Suture in the NW Himalayan region of Ladakh. The age, tectonic evolution and timing of ophiolite obduction are critical to our understanding of the mechanisms via which entire oceans are formed, consumed and partly preserved before the onset of terminal continent-continent collisions. Geochemistry of the gabbro and basaltic units suggest the presence of both MORB-type and primitive arc-related mafic rocks. Zircons extracted from the Spongtang Massif gabbros yield U-Pb (SHRIMP) ages of 136–133 Ma with initial eHf values of +14 to +16, indicating Early Cretaceous juvenile, depleted mantle sources devoid of contamination by older continental crust. Previously, Middle Jurassic (~177 Ma) zircon ages were obtained from gabbro and we suggest these represent MORB-type Neotethyan oceanic crust through which a younger intra-oceanic island-arc (Spong arc) developed in response to subduction initiation during the Early Cretaceous (~136 Ma). Our zircon ages are consistent with Early Cretaceous ages obtained for radiolarian cherts within the Spong Arc complex. Subduction beneath the Spong Arc continued until its collision with the northern Indian continental margin during the early Eocene. We suggest that the Spongtang Massif is equivalent to the nearby Dras island arc terrane. Intra-oceanic subduction beneath this system was possibly initiated along NNE-SSW trending transform faults in the Neotethyan Ocean, along which different ages of ocean crust was juxtaposed, thereby development of the Early Cretaceous Spong Arc is superimposed on the older Jurassic Spongtang N-MORB crust. The juvenile ɛHf signature indicates the subduction system that spawned the Spong island arc was not related to the coeval Trans-Himalayan (Ladakh-Gangdese) arc that developed along the southern margin of Eurasia. The age, composition and nature of geological relationships with the underlying Indian rocks indicate the Spong Arc was a juvenile, intra-oceanic terrane that first collided with India before the onset of final continent-continent collision. Therefore, final late Eocene Neotethys closure was between the Kohistan-Ladakh (Eurasian) continental arc and the already inactive Indian + Spongtang margin.

We present zircon U-Pb (SHRIMP) and monazite U-Th-total Pb (EPMA) ages from a suite of high-grade rocks of the Phulbani domain of the Eastern Ghats Belt, India. These ages are integrated with the metamorphic and magmatic histories of the Phulbani domain to put precise time constraints on the tectonothermal evolution of the northern part of the Eastern Ghats Belt. Zircon from the coarse-grained charnockite rock shows crystallization age of ca. 970 Ma. Aluminous granulite contains spinel and quartz-bearing mineral assemblage indicating ultrahigh temperature metamorphism (>900 °C at 8–10 kbar) which possibly occurred during ca. 987 Ma as revealed from monazite included in porphyroblastic garnet. Monazite in the aluminous granulite and the migmatitic felsic gneiss grew dominantly at 966 ± 4 Ma and 968 ± 4 Ma ages respectively, which are interpreted as the cooling ages subsequent to peak metamorphism. Oscillatory-zoned zircon grains of the felsic augen gneiss yield ca. 1173 Ma age which is interpreted as the crystallization age of the granitic protolith. This ca. 1173 Ma age granite possibly composed a part of the Proterozoic basement of the Eastern Ghats Province. A monazite age of 781 ± 15 Ma from the aluminous granulite may indicate the timing of a localized shear-induced thermal process in the Phulbani domain. The presently studied rock suite thus recorded three distinct events (ca. 1173 Ma, ca. 1000–900 Ma and ca. 781 Ma) of magmatism, metamorphism and deformation of Eastern Ghats Belt. Metamorphic and magmatic histories of the Phulbani domain are similar to those of the adjacent Visakhapatnam domain and the Rayner Province of East Antarctica during the time frame ca. 1000–900 Ma.

U–Pb zircon geochronology of two granite intrusions containing bluish‑tint K-feldspars and two biotite-hornblende gneisses in the Kagasawa area, situated at the center part of the Hida metamorphic complex, southwest Japan, reveals the early Triassic low P/T metamorphism in the Hida metamorphic complex (251–247 Ma). The granite intrusions into the Hida gneiss occurred at two different stages (235.4 ± 1.6 Ma and 183.6 ± 1.8 Ma) and thermally affected the Hida gneiss. In the Unazuki area, situated at the northeast margin of the Hida metamorphic complex, a quartzo-feldspathic schist and a biotite granite containing a quartzo-feldspathic schist as xenolith yielded U–Pb zircon ages of 258.0 ± 2.3 Ma and 253.0 ± 1.9 Ma, respectively, which indicates that the medium P/T metamorphism in the Unazuki area occurred in late Permian. The similarity between the geochemical compositions in whole rocks and zircons of the quartzo-feldspathic schist and the granite suggests that felsic volcanics were deposited on basement rocks at 258 Ma and, after affecting the medium P/T metamorphism, residual felsic magma was exhumed while taking the Unazuki schists as a xenolith at 253 Ma. Although the time interval between protolith formation and post-peak regional metamorphism in the Unazuki area (0.8–9.2 Ma) is similar to that in the Kagasawa area (1–8.6 Ma), the different timing of the regional metamorphism indicates that the protolith of the Unazuki metasediments was deposited not on the proximal Hida metamorphic complex but on ancient rocks containing Eoarchean-Paleoproterozoic components of the North China Craton.

Porphyry Copper Deposits (PCDs) dominantly form in convergent plate margins genetically related to volcanic arcs. However, their temporal relationships with active volcanism are less clear. Active volcanic activity is often considered to impede the formation of PCDs and some workers have concluded that PCD formation must take place at times of volcanic quiescence or at a waning stage of volcanism. However, several studies have documented evidence suggesting syn-eruption PCD formation. A combination of radiometric dating, geochemistry studies and observations of rock cores from the Cerro Colorado PCD, a BHP Billiton property located in N. Chile, provides evidence compatible with syn-volcanic PCD formation within a volcanic edifice. Textural observations at both hand specimen and microscopic scales combined with new LA-ICPMS and SHRIMP dating of zircons suggest the intrusive history of the Cerro Colorado area can be separated into two main units: a porphyritic tonalite ca. 53.5 Ma and a porphyritic quartz monzonite ca. 50 Ma. There is also evidence for crystallisation at ca. 60 Ma, but a corresponding distinct lithology was not identified. A quartz porphyry unit previously identified as the youngest intrusive body is reinterpreted as an altered equivalent of the monzonite. The youngest intrusive units are represented by porphyritic granodiorite with an age 51–50 Ma. These units only show minor metal mineralisation suggesting that the main phase of mineralisation in the Cerro Colorado mine occurred over a few million years, bracketed by the two main pulses of igneous intrusion, which are consistent with documented Ar–Ar ages. Re–Os ages reported by previous studies suggest an earlier phase of mineralisation at ca. 56–54 Ma. U–Pb dating of zircon from a volcanic-related breccia that cuts the host rocks of the Cerro Colorado Mine reveals a phase of volcanic activity at ca. 57 Ma, after formation of the regional Cretaceous Cerro Empexa Formation. This age range overlaps the ages derived for the intrusive porphyry units suggesting a close temporal relationship between volcanism and the ore-forming subvolcanic intrusions in Cerro Colorado. A link between active volcanism and mineralisation is reinforced by the ultrafine-grained matrix and the presence of strong brecciation associated with the monzonite-quartz porphyry stock suggesting decompression and volatile release have taken place during emplacement.

The Eastern Ghats Belt (EGB) is an orogenic belt which is known to evolve through Proterozoic time before cratonisation with Proto-India and holds an important key to the reconstruction of erstwhile supercontinents. To track the comprehensive evolution of the EGB with respect to Proto-India prior to and during its cratonisation, we carried out a detailed petrological and high-resolution geochronological investigation (zircon U-Pb SHRIMP and monazite U-Th-total Pb EPMA) on systematically sampled rocks from a transect across the western boundary of the EGB. Our results reveal that along a narrow zone near the thrust-bound margin between the EGB and Proto-India, the entire crustal segment, i.e. from deep-crustal granulites to mid-crustal cratonic amphibolites to shallow-crustal quartz breccia, was involved actively in the thrust-related tectonics during cratonisation of the EGB at ~ 550–500 Ma. This youngest imprint of the ~ 550–500 Ma event erased all the earlier age histories from the zircon and monazite grains of the marginal granulites. On the other hand, the older age-imprints are better preserved in the interior of the orogeny (~ 30 km east of thrust-boundary between the EGB and the BC and further eastward) with little or no age-imprints of the final tectonic pulse. Combining all the new and earlier geochronological data with P-T calculations and textural characterisation, we propose a comprehensive and geochronologically well-constrained tectonic evolution of the EGB, which encompasses its earliest deep-crustal metamorphic to latest shallow-crustal sedimentation histories. The outcome of this study also reveals, for the first time, that the entire crustal section participated actively during the cratonisation. The new geochronological data coupled with field evidence and textural evidence from the shallow-crustal rock finally suggest that the thrusting vis-a-vis cratonisation was continued, at least, up to Late Cambrian, thus post-dating the Pan-African orogeny.

Mesosiderites are a group of stony-iron meteorites, which are thought to be the result of mixing of silicates with Fe-Ni metal. In this study, we combined textural observations with geochemical and chronological studies of two zircon grains found in the Estherville mesosiderite. One of the zircons (Zrc1) occurs with pyroxene, plagioclase, troilite, and silica, and the other (Zrc2) is located at a boundary between Fe-Ni metal and a silicate part mainly composed of pyroxene and plagioclase. The textural observations demonstrate that Zrc1 is relatively homogenous, whereas Zrc2 is composed of at least two chemically distinct domains. Trace element analyses of Zrc2 resolve large concentration gradients within this single grain with variations that are an order of magnitude for rare earth elements (REE) and two orders of magnitude for U and Th. The lowest trace element concentration in Zrc2 is more than an order of magnitude lower than those of lunar and eucritic zircons. However, it is similar to those of Zrc1 and a zircon from the Vaca Muerta mesosiderite. The calculated REE composition of the melt in equilibrium with Zrc2 shows that Zrc2 and perhaps also Zrc1 did not crystallize from a melt that was produced by fractional crystallization of the primary magmatic mineral assemblages. The zircons with low REE, U, and Th concentrations can be interpreted to have formed in a residual melt after incorporation of large amounts of REE, U, and Th into secondary phosphate minerals, which formed during the metal-silicate mixing event. The large concentration gradients observed in Zrc2 suggest significant heterogeneities in the melt from which the zircon crystallized. Alternatively, either mixing or diffusion between a relict zircon and a newly formed zircon could explain the observed concentration gradients. However, the REE patterns of Zrc2 cannot be explained by mixing or diffusion between the two distinct generations of zircons. These considerations suggest that Zrc1 and Zrc2 formed during a high-temperature reheating event, which is probably related to the metal-silicate mixing event. The weighted average 207 Pb- 206 Pb age obtained by SIMS from both zircons is 4521 ± 26 Ma (2σ). This age is younger than that of a primary magmatic zircon from Vaca Muerta (4563 ± 15 Ma) and probably corresponds to the timing of the metal-silicate mixing event or a later impact event.

The present study aims at tracking sediment provenance for a Mesoproterozoic craton-margin basin in eastern India, i.e. the Ampani Basin. Occurring on the eastern flank of the Archean Bastar Craton, the basin occupies a very strategic geographic position coinciding with the boundary between Archean craton and a Proterozoic orogenic belt with spatially varied tectonothermal history. The studied basin succession includes unmetamorphosed siliciclastic sediments deposited under three different environmental set-up viz. fluvial, shoreface and shelf. The basin sediment package defines a fining-upward succession with fluvial products at its base, shoreface deposits in the middle and shelf sandstone–siltstone–shale sediments at the top. In this study sandstone–siltstone and shale units encompassing the entire basin succession are analyzed geochemically and with U–Pb detrital zircon geochronology in order to characterize the sediment source/s. The basin was fed mainly by the Paleoarchean to late Paleoproterozoic granitoid source with minor contribution from mafic source particularly at the closing part of its depositional history. Except for relative enrichment, REE and trace element patterns of the three arenaceous units, viz. fluvial, shoreface, shelf sandstone are quite similar. U–Pb analysis of detrital zircon grains from the three units indicate that in the initial phase of basin deposition, sediments were sourced from Paleo- to Neoarchean and Palaeoproterozoic (with an age spread from ca. 3700 to 2400 Ma) provenance. From the middle shoreface part upwards, the Paleo- to Neoarchean sources fade away and a new Mesoproterozoic source (ca. 2000–1700 Ma) was added with the continued Paleoproterozoic source/s. The youngest zircon populations of the three analyzed units yield an upper age limit of sedimentation at 2079 ± 44 Ma (lower being close to 1446 ± 21 Ma, as evident from the age of tuffaceous rocks on the middle shoreface part of the basin). The change in detrital zircon age spectra from the middle part of the stratigraphy might be due to some basin-adjacent tectonic activities.

A detailed zircon U–Pb (SHRIMP) geochronological study of the amphibolite to granulite-grade rocks of the Rengali Province of eastern India records the growth history of the southern margin of the Singhbhum Craton. Pelitic and mafic granulites from the gneissic belt exhibit contrasting styles of metamorphism. Zircon of the pelitic granulites from the eastern segment yields c. 3528–3064 Ma detrital ages. Charnockitic gneiss from the eastern segment has protolith age of 3058 ± 15 Ma while that from the central segment has protolith age of 2861 ± 30 Ma. The latter rock records high-grade metamorphism at 2818 ± 15 Ma. Hornblende gneiss from the central sector has a protolith age of 2828 ± 9 Ma. Deformed leucogranite in the central and undeformed granitoid in the eastern segment were emplaced at 2807 ± 13 and 2809 ± 13 Ma respectively. The protolith of felsic gneiss from the central sector was emplaced at 2776 ± 24 Ma. Most of the zircon samples contain overgrowths of c. 2500 Ma, inferred to be the age of reworking of the gneissic belt. Our data suggest that the Rengali Province evolved as an orogenic belt in the Neoarchean time (c. 2800–2500 Ma) during southward growth of the Singhbhum Craton. These tectonothermal imprints at the margin of the Singhbhum Craton are possibly related to its inclusion within the supercontinent Ur.

We present high precision geochronological data from the Jahazpur granite and the associated Mangalwar gneiss from Deoli-Jahazpur-Hindoli region of the Aravalli Craton, northwestern India. These data are useful to understand the stratigraphic correlation between the basement gneiss and the low-grade supracrustal rocks of the Hindoli-Jahazpur Group. U–Pb (SHRIMP-IIe) data from zircon suggest emplacement age of 2538 ± 5 Ma for the Jahazpur granite, whereas a similar age of 2520 ± 37 Ma is the timing of the high-grade metamorphism and anatexis of the Mangalwar gneiss. Combining these with whole rock geochemical data, we argue that the Neoarchean Jahazpur granite underwent crustal contamination and emplaced at the same time frame of the Berach granite. Concomitant metamorphism and granitic magmatism imply that overlying Hindoli-Jahazpur Group is younger than c. 2520 Ma. Thus, the Neoarchean Jahazpur granite and Mangalwar gneiss are constituents of the Banded Gneissic Complex which forms the basement of the Hindoli-Jahazpur supracrustal sequence.

The major and trace elements in zircons obtained from basaltic eucrites Yamato [Y]-75011, Y-792510, Y-82082, Asuka [A]-881467, Juvinas, and Stannern were determined using an electron microprobe analyzer and a sensitive high-resolution ion microprobe, and the formation of zircons on the parent body was discussed. The maximum sizes of zircon grains in each sample are likely to be related to its metamorphic grade. The least-metamorphosed basaltic eucrite Y-75011 contains zircons with sizes of a few μm. Conversely, the highly metamorphosed basaltic eucrites Y-792510 and A-881467 include zircons larger than 20 μm. The relationship between the maximum grain size and metamorphic grade suggests that the formation process of zircons is related to metamorphic events. The Zr/Hf ratios of the zircons found in this study show a wide variation (31.6–71.7), as compared to the chondritic values (Zr/Hf = 32.8–34.3) and bulk basaltic eucrites (34.0–34.7). The Zr/Hf ratios of the zircons from Y-792510 and A-881467 are relatively constant, whereas those of the zircons from Y-82082, Juvinas, and Stannern show large variations. The rare earth element (REE) content in the zircons from Y-792510 and A-881467 is given by La = 0.1 × CI and Lu = 1000 × CI. On the other hand, the zircons from Stannern show higher REE content ( La = 0.1 – 1 × CI and Lu = 1000 – 10 000 × CI ) than those from Y-792510 and A-881467. The most reliable REE data of a large zircon from A-881467 show no Ce anomaly and a distinct negative Eu anomaly. Therefore, it is presumed that the zircons formed under reducing condition in which Ce 3+ was stable. The melt compositions coexisting with the studied zircons suggest that the zircons from highly metamorphosed eucrites might react with the REE-enriched melt derived from partial melting of the mesostasis region during metamorphism.

This contribution addresses the time framework of the regional metamorphism in the three crustal provinces making the Archean Dharwar craton. We present results of texturally controlled in situ EPMA chemical dating of monazites, chemical dating of monazite separates, as well as Sm–Nd garnet–whole rock isochrons and SHRIMP U–Pb zircon ages for pelites, amphibolites and granitoids over target areas typical of the various crustal levels of the provinces. The Western Dharwar craton has undergone a major thermal pulse at 2.52 Ga followed by slow cooling to ca. 2.4 Ga and recorded earlier thermal events around 3.0 Ga and 3.1 Ga. The Central Dharwar craton records a major high-grade thermal imprint at ca. 2.55–2.51 Ga followed by cooling up to 2.45 Ga and earlier thermal events at ca. 2.62 and 3.20 Ga. In the Eastern Dharwar craton the widespread thermal pulse between 2.55 and 2.52 Ga is best recorded. From 2.52 Ga on, the entire craton ultimately and contemporaneously undergoes the main event of regional HT–LP metamorphism. The contrasted thermal records of the three provinces reflect their accretion age(s) and their degree of involvement in a wide Latest Archean hot orogen, which sets the capacity of these lithospheric segments to be impact by deformation and mantle fluxes. The tectonic setting of Latest Archean hot orogeny is compatible with active plate margin processes having interacted with mantle instabilities (i.e., plumes?). The tectonic setting of pre-2.5 Ga thermal pulses is difficult to assess, but considering their systematic links with documented magmatic pulses, they may have been generated in contexts comparable that of Latest Archean hot orogeny where lateral constrictional flow of hot orogenic crust achieves gravity driven flow, 3D mass redistribution of viscous lower crust submitted to convergence.

The New England Orogen of easternmost Australia is dominated by suites of Palaeozoic to earliest Mesozoic rocks that formed in supra-subduction zone settings at Gondwana's eastern margin. On the northern New South Wales coast at Rocky Beach, Port Macquarie, a serpentinite melange carries rare tectonic blocks of low-grade, high-pressure, metamorphic rocks derived from sedimentary and igneous protoliths. Dominant assemblages are glaucophane. +. phengite. ±. garnet. ±. lawsonite. ±. calcite. ±. albite blueschists and lawsonite-bearing retrogressed garnet. +. omphacite eclogites. In some blocks with sedimentary protoliths, eclogite forms folded layers within the blueschists, which is interpreted as Mn/(Mn. +. Fe) compositional control on the development of blueschist versus eclogite assemblages. Review of previous studies indicates pressure-temperature conditions of 0.7-0.5. GPa and ≤. 450. °C. Three samples of high-pressure metasedimentary rocks contain Archaean to 251. ±. 6. Ma (Permo-Triassic) zircons, with the majority of the grains being Middle Devonian to Middle Carboniferous in age (380-340. Ma). Regardless of age, all grains show pitting and variable rounding of their exteriors. This morphology is attributed to abrasion in sedimentary systems, suggesting that they are all detrital grains. New in situ metamorphic zircon growth did not develop because of the low temperature (≤. 450. °C) of metamorphism. The Permo-Triassic, Devonian and Carboniferous zircons show strong heavy rare earth element enrichment and negative europium anomalies, indicating that they grew in low pressure igneous systems, not in a garnet-rich plagioclase-absent high pressure metamorphic environment. Therefore the youngest of these detrital zircons provides the maximum age of the metamorphism. A titanite. +. rutile porphyroblast within an eclogite has a U-Pb age of 332. ±. 140. Ma (poor precision due to very low U abundances of mostly

The Early Palaeozoic Ross–Delamerian orogenic belt is considered to have formed as an active margin facing the palaeo-Pacific Ocean with some island arc collisions, as in Tasmania (Australia) and Northern Victoria Land (Antarctica), followed by terminal deformation and cessation of active convergence. On the Cambrian eastern margin of Australia adjacent to the Delamerian Fold Belt, island arc and backarc basin crust was formed and is now preserved in the Lachlan Fold Belt and is consistent with a spatial link between the Delamerian and Lachlan orogens. The Delamerian–Lachlan connection is tested with new zircon data. Metamorphic zircons from a basic eclogite sample from the Franklin Metamorphic Complex in the Tyennan region of central Tasmania have rare earth element signatures showing that eclogite metamorphism occurred at ~ 510 Ma, consistent with island arc–passive margin collision during the Delamerian(− Tyennan) Orogeny. U–Pb ages of detrital zircons have been determined from two samples of Ordovician sandstones in the Lachlan Fold Belt at Melville Point (south coast of New South Wales) and the Howqua River (western Tabberabbera Zone of eastern Victoria). These rocks were chosen because they are the first major clastic influx at the base of the Ordovician ‘Bengal-fan’ scale turbidite pile. The samples show the same prominent peaks as previously found elsewhere (600–500 Ma Pacific-Gondwana and the 1300–1000 Ma Grenville–Gondwana signatures) reflecting supercontinent formation. We highlight the presence of ~ 500 Ma non-rounded, simple zircons indicating clastic input most likely from igneous rocks formed during the Delamerian and Ross Orogenies. We consider that the most probable source of the Ordovician turbidites was in East Antarctica adjacent to the Ross Orogen rather than reflecting long distance transport from the Transgondwanan Supermountain (i.e. East African Orogen). Together with other provenance indicators such as detrital mica ages, this is a confirmation of the Delamerian–Lachlan connection.

OT4 zircon collected from Oto-dani quartz diorite in Unazuki, in the northeastern part of the Hida Belt, Southwest Japan, was evaluated using microbeam analysis for its qualification as a Mesozoic reference zircon based on U–Pb dating. The characterization of OT4 zircon was carried out using a sensitive high-resolution ion microprobe (SHRIMP II). Chemical zoning was revealed in some of the grains with a relatively high concentration of U and Th (up to 988 and 1054 ppm, respectively). The weighted mean 206Pb/238U age corrected by 207Pb was 191.1 ± 0.3 Ma (95% confidence limit) and was found to be almost constant for all the grains. The high crystallinity, the constant REE abundance patterns, and the homogeneous Hf concentrations (8651 ± 466 ppm) indicate that the grains were not affected by any geological events after crystallization. Furthermore, the homogeneous Hf concentrations and consistency of the Ti-in-zircon temperature (733 ± 34°C) and the Zr saturation temperature (728°C) suggest that the OT4 zircon crystallized over a short time. The OT4 zircon contains enough radiogenic 206Pb (>2 ppm) and 207Pb (>0.1 ppm) to be detected by microbeam analysis, which indicates its potential of being an effective Mesozoic reference used in microbeam U–Pb dating.

The Sm and Gd isotopic compositions of silicates from six mesosiderites (Dalgaranga, Estherville, Morristown, Northwest Africa (NWA) 1242, NWA 2932, and Vaca Muerta) and one iron meteorite (Udei Station) were determined to elucidate the cosmic-ray exposure records. All seven samples showed significant 150 Sm/ 149 Sm and 158 Gd/ 157 Gd isotopic shifts from neutron capture reactions corresponding to neutron fluences of (1.3–21.8) × 10 15 n cm −2 . In particular, Vaca Muerta showed a significantly higher neutron fluences than the other six samples. The parameter for the degree of neutron thermalization ( e Sm / e Gd ) also showed a significant difference between Vaca Muerta (0.76) and the other samples (0.93–1.20). These results suggest a two-stage irradiation of the Vaca Muerta silicates in the parent body (>50 Ma) before formation of the mesosiderite and during its transit to Earth (138 Ma). This is consistent with the 81 Kr–Kr cosmic-ray exposure age data of a Vaca Muerta pebble from a previous noble gas isotopic study.

– The isotopic compositions of Sm and/or Gd of nine documented drill core samples of the Jilin H5 chondrite were determined to study the neutron capture records of individual meteorite samples. All the samples showed significant isotopic shifts of 150Sm/149Sm and/or 158Gd/157Gd corresponding to neutron fluences of (1.3–1.7) × 1015 n cm−2. Considering the short 4π irradiation age of 0.32 Ma during the second stage, the 2π irradiation of 7 Ma during the first stage is the main influence on the Sm and Gd isotopic shifts of the Jilin chondrite. Although a depth dependence of the neutron capture effects was expected from the isotopic variations of 150Sm/149Sm and 158Gd/157Gd in the Jilin chondrite core samples that were possibly drilled perpendicular to the surface of the large object with a radius of >10 m in the 2π-geometry, no clear evidence was observed in this study. The data from the combination of the isotopic shifts between Sm and Gd defined as eSm/eGd suggest that the neutrons produced in the 2π-geometry of the Jilin chondrite follow the similar energy spectra as the neutrons in lunar samples, although the present analytical quality is not enough to discuss a critical discussion for the thermalization of the neutron energy levels.

Field studies integrated with cathodoluminescence petrography and SHRIMP U–Pb dating of zircons from >150 orthogneisses and metatonalites from the Eoarchaean Itsaq Gneiss Complex (southern West Greenland) shows that only a minority contain ≥3840 Ma zircons, whereas the majority carry only younger ones. Rocks containing ≥3840 Ma zircons vary from very rare single-phase metatonalites to more common complexly banded tonalitic migmatites. The former metatonalites have simple oscillatory-zoned ≥3840 Ma zircon with limited recrystallisation and overgrowth, whereas the more common migmatites have much more complicated zircon populations with both ≥3840 Ma and 3650–3600 Ma oscillatory-zoned zircon, more extensive recrystallisation and widespread complex core-rim multiple growth relationships. With only 100–160 ppm Zr in the tonalites and likely melt generation temperatures of >1000 °C, the experimentally determined zircon solubility–melt composition relationships established by other workers shows that the precursor melts to the Itsaq Complex tonalites were strongly undersaturated in zircon, thus any entrained xenocrystic zircon would have been rapidly dissolved. Therefore, the ≥3840 Ma oscillatory-zoned zircons crystallised out of tonalitic melt and gives magmatic age of the rock in which they occur. With an established igneous age of ≥3840 Ma established from such relationships, we interpret the correlated variation between the field nature of these rocks and their zircon petrography/age structure as due to superimposition onto ≥3840 Ma tonalite protoliths of variable amounts of heterogeneous strain, heterogeneous distribution of melt patches formed during in situ anatexis at up to ∼800 °C, plus granitic veining. This explains why geologically simple metatonalites have simple zircon populations, whereas complex orthogneisses have complex zircons. The large amount of integrated field, geochemical and zircon data rule out an alternative interpretation, that the ≥3840 Ma zircons represent an igneous xenocrystic component, present in younger rocks to varying degrees. If this were true, then the structurally simple (less reworked) rocks should still display complex zircon populations. Gneisses with ≥3840 Ma zircon are commonest on Akilia and neighbouring islands, in Itilleq fjord (∼65 km east Akilia) and on the north of Ivisaartoq (∼150 km northeast of Akilia). These include from Itilleq a 3891 ± 6 Ma gneissic tonalite (with minor neosome)—which is currently the oldest rock recognised in the Itsaq Gneiss Complex. Overall, the ≥3840 Ma tonalites are a widespread and unevenly distributed in the Itsaq Gneiss Complex, and they are a volumetrically minor component compared with ∼3800, 3750 and 3700 Ma tonalite generations. Using the subset of our data covering Itilleq and the neighbouring fjords, migmatite samples with ≥3800 Ma igneous zircon are mutually exclusive from migmatite samples with ∼3700 Ma igneous zircon. This suggests that prior to an amalgamation event followed by 3660–3600 Ma high-grade metamorphism, ≥3840 Ma tonalites might have resided in a terrane discrete from ∼3700 Ma tonalites. This is in accord with interpretation of the non-migmatised part of the Complex in the Isua area, where a terrane of ∼3800 Ma tonalites with a minor associated ≥3840 Ma component and a terrane with ∼3700 Ma tonalites were tectonically juxtaposed at ∼3660 Ma.

U–Pb zircon geochronology of two Permo-Triassic granites (samples OT-52 and OT-272 with ages of 229 ± 8 Ma and 256 ± 2 Ma, respectively) in the Unazuki area, Hida Metamorphic Belt, southwest Japan, revealed the presence of Eoarchean to Paleoproterozoic inheritance. Inheritance is consistent with both samples showing low zircon saturation temperatures for their bulk compositions. In OT-52, dark in CL, low Th/U zircon domains have a mean 207Pb/206Pb age of 1940 ± 17 Ma, which is consistent with an age of 1937 ± 6 Ma for anatexis in the Precambrian Busan gneiss complex in Korea. Eoarchaean inherited zircons with 207Pb/206Pb ages from ca. 3750 to 3550 Ma are common in OT-272 but are few in OT-52, suggesting a source from rocks with affinities to those in the Anshan area in the northeast China part of the North China Craton. On the other hand, a Hida Metamorphic Belt metasedimentary gneiss into which the granites were intruded contains ca. 1840, 1130, 580, 360, 285 and 250 Ma zircons ( Sano et al., 2000 ). These ages suggest that the Unazuki Mesozoic granites did not originate from proximal Hida Metamorphic Complex rocks, but instead from unrelated rocks obscured at depth. The predominance of Eoarchean to Paleoproterozoic age components, and the marked lack of 900–700 Ma components suggest that the source was the (extended?) fringe of the North China Craton, rather than from Yangtze Craton crust. The Mesozoic evolution of Japan and its linkages to northeast Asia are discussed in the context of these results.

In the Precambrian, parts of the Oklo, Okelobondo and Bangombe uranium deposits of the Republic of Gabon, central Africa, functioned as natural fission reactors. Many elements in the Oklo and Bangombe uranium deposits show variations in isotopic composition caused by a combination of nuclear fission, neutron capture and radioactive decay. Isotopic studies provide useful information to understand the behavior of radionuclides in geological media. In our recent work, in situ REE, Pb and U isotopic analyses of individual tiny minerals in and around reactor zones have been performed using a SHRIMP (Sensitive High Resolution Ion Microprobe). The isotopic results of the SHRIMP analyses on micro-minerals found in and around the Oklo and Bangombe natural reactors are reviewed in this paper. The data suggest the selective uptake behavior of (1) Ra into illite, and (2) Pu into apatite, (3) the formation process of secondary minerals bearing fissiogenic REE and depleted U, (4) evidence of nuggets (ɛ-particles) bearing fissiogenic platinum group elements (PGE), and (5) from the U–Pb systematics of highly altered zircons, the redistribution of U and Pb.

The Ordovician strata of the South Mayo Trough in western Ireland were deposited as a conformable sequence, first in the forearc of a north-facing oceanic arc, then in a synorogenic basin above the arc–continent collisional Grampian orogen, then in an extensional hanging-wall basin above the exhuming Dalradian metamorphic complex and, finally, in an Andean-type margin successor basin. This depositional history, and its relationship to the Grampian orogeny, to the pre- and post-collision arcs, and to sources in the Laurentian foreland, is recorded in detrital heavy mineral suites, Ar/Ar ages of detrital white micas, and U–Pb ages of detrital zircons. Heavy minerals record the progressive extensional unroofing of an obducted ophiolite complex, then the rapid extensional unroofing of the Grampian Dalradian metamorphic complex. White mica ages give Neoproterozoic, Mesoproterozoic, and Neoarchaean ages in the lower part of the sequence then, at about 466 Ma, synchronously with the appearance of amphibolite-facies metamorphic detritus, a tightly unimodal Grampian metamorphic age. Zircons yield mainly Mesoproterozoic and Neoarchaean ages, indicating a Laurentian source, and a Grampian age of 460–468 Ma. Paralleling the appearance of Grampian-age micas, the colour of zircon changes from dominantly purple to dominantly colourless, reflecting the thermal annealing effect of metamorphism. However, the temperature of Grampian metamorphism was not high enough to reset the U–Pb isotopic systematics of detrital zircons.

In this study, redistribution behaviors of U, Th, and rare earth elements (REE) in association with igneous activity in the Unazuki region of southwest Japan were investigated. At the contact boundary between granite and gabbro, REE minerals such as monazite, xenotime, and REE oxides formed with Fe–Ti oxides in K-feldspar assimilated by gabbro. U was enriched in xenotime (UO2 > 0.72 wt.%) and REE oxides (UO2 > 0.10 wt.%), whereas Th was concentrated in REE oxides (ThO2 > 1.32 wt.%). Considering the similarity between the ionic radii of U4+ and Y3+, U was incorporated into xenotime and REE oxides together with heavy REE. Th incorporation into xenotime (eightfold coordinated site) and monazite (ninefold coordinated site) was constrained by the lower contents of Gd and Dy, respectively. Saussuritization of plagioclase in gabbro and chloritization of biotite in granite at the contact boundary suggest low-temperature hydrothermal alteration at late stages of gabbro intrusion that occurred at 253 ± 1 Ma. In contrast, zircon in the chloritized granite contained altered domains characterized by a dark response between back-scattered electron and cathodoluminescence images; this was caused by a low average atomic number due to the release of Zr and Hf; the incorporation of non-formula elements such as Ca, Fe, and Al; and low crystallinity because of radiation damage. The altered domains showed enrichments of Th (ThO2 > 1.50 wt.%) and light REE (LREE) to middle REE (MREE) as well as positive Eu anomalies; this suggests the preferential incorporation of Th and LREE–MREE (particularly Eu2+) with Ca occurring during the hydrothermal alteration. The redistribution behaviors of U, Th, and REE during low-temperature hydrothermal alteration strongly depend on their ionic radii. Thorite in zircon was probably formed by the exsolution of high-Th altered domains.

The isotopic compositions of Sm and Gd in twelve Martian meteorites, ALH 77005, ALH 84001, DaG 735, Dhofar 019, EET 79001, Lafayette, Los Angeles, Nakhla, SaU 005, Y 000593, Y 000749 and Zagami, were determined to quantify the neutron capture records of individual meteorite specimens. Seven of these twelve samples, ALH 84001, Y 000749, DaG 735, Dhofar 019, EET 79001, SaU 005 and Zagami, showed significant isotopic shifts of 150 Sm/ 149 Sm and/or 158 Gd/ 157 Gd corresponding to neutron fluences of (0.7–3.4) × 10 15 n cm − 2 . Among these seven meteorites, the neutron fluences of ALH 84001, Y 000749, and Dhofar 019 apparently correlated with their cosmic-ray exposure ages, indicating that most of the irradiation took place while the meteoroids were small bodies in space after the ejection from Mars. However, our results suggest an accumulation of their inherited irradiation occurred on Mars. On the other hand, the exposure histories of the other four meteorites (basaltic shergottites), DaG 735, EET 79001, SaU 005, and Zagami, cannot be explained as single- or multistage irradiations in space, or as a single irradiation on the Martian surface. The mixing between basaltic lava with a significantly irradiated Martian regolith is a reasonable interpretation of the excess neutron capture records observed in these four basaltic shergottites.

Zircon U-Pb geochronology was applied to a migmatitic gneiss in the Busan gneiss complex, Gyeonggi massif, Korea. 207Pb/206Pb ages of concordant zircon grains reveal that overgrowth rims with Th/U ratios lower than 0.1 formed at 1937 ± 6 Ma during the migmatization and mantled the inherited cores of ca. 3500–2200 Ma. The 207Pb/206Pb age of the rims is consistent with the monazite Th-total Pb age (ca. 1938 Ma), suggesting that the rims are the product of Paleoproterozoic regional metamorphism. Our zircon age data are not sufficient for discerning the tectonic affinity of the Busan gneiss complex belonging to either Gyeonggi or Yeongnam massif. However, consistency of these ages with the ca. 1930 Ma Pb-Pb age reported from the Bagdalryeong gneiss complex, Gyeonggi massif, supports the tectonic model that the Busan gneiss complex is a part of the Gyeonggi massif. Discordant U-Pb data obtained from the zircon rims (293 ± 31 Ma) and the monazite Th-total Pb age (ca. 289 Ma) suggest that the Busan gneiss complex was affected by the Early Permian regional metamorphism during the Ogcheon (Okcheon) Orogeny. Therefore, we conclude that the Busan gneiss complex is a part of the Gyeonggi massif occurring as either a basement rock of the Ogcheon (Okcheon) metamorphic rocks, or a tectonically emplaced, exotic piece affected by the Early Permian Ogcheon (Okcheon) metamorphism. In addition, our U-Pb data suggest that the Busan complex is possibly linked with the Hida-Oki terrane in Japanese Islands.

In-situ isotopic analyses of Pb and U in the highly altered zircons found in the clay and black shale layers above the Bangombe natural reactor, Republic of Gabon, were analyzed using a sensitive high resolution ion microprobe (SHRIMP) to understand the geological history in this area. The analyzed zircons widely vary in the U contents ranging from

The isotopic compositions of rare earth elements (REE), Pb and U of highly altered zircons from the clay and black shale layers above the Bangombe natural reactor, Gabon, were determined by a sensitive high resolution ion microprobe (SHRIMP) to discuss the redistribution processes of elements into zircons under the supergene weathering. The clay layer trapped most of the fissiogenic Nd, Sm and Eu derived from the reactor and prevented them migrating into the black shale layer. On the other hand, only the Ce isotopic ratios of the clay and black shale layers have about 2 times larger variations than the other REE. This result suggests that a large chemical fractionation between Ce and other REE above the reactor occurred under the oxidizing condition. The U–Pb data of zircons suggest that the U–Pb system was largely disturbed by migration of chemically fractionated Pb and U from the 2.0 Ga-old uraninite in association with recent weathering.

U‐Pb zircon ages of tuffs and sandstones of the Daedong Supergroup (Bansong and Nampo groups) in the Korean Peninsula were determined using a sensitive high‐resolution ion microprobe (SHRIMP) in order to constrain their age of sedimentation and to unravel discrete geologic events as recorded in detrital zircons. The ages of four tuffaceous samples from the Bansong Group imply that the Daedong Supergroup formed at ca. 187–172 Ma in association with the Early‐Middle Jurassic orogeny. These data are in marked contrast with paleomagnetic arguments suggesting that the Bansong and Nampo groups are precollisional Early‐Middle Triassic deposits that are correlative with the North and South China blocks, respectively. Detrital zircons of the Daedong Supergroup define seven age components: (1) Early‐Middle Archean (3.64–2.97 Ga), (2) Late Archean–middle Early Proterozoic (2.63–2.33 Ga), (3) late Early Proterozoic (1.98–1.75 Ga), (4) Middle‐Late Proterozoic (1.2–0.6 Ga), (5) Devonian (400–355 Ma), (6) Early...

The Eoarchaean (>3,600 Ma) Itsaq Gneiss Complex of southern West Greenland is dominated by polyphase orthogneisses with a complex Archaean tectonothermal history. Some of the orthogneisses have c. 3,850 Ma zircons, and they vary from rare single phase metatonalites to more common complexly banded migmatites. This is due to heterogeneous strain, in situ anatexis and granitic veining superimposed during younger tectonothermal events. In the single-phase tonalites with c. 3,850 Ma zircon, oscillatory-zoned prismatic zircon is all 3,850 Ma old, but shows patchy ancient loss of radiogenic Pb. SHRIMP spot analyses and laser ablation ICP-MS depth profiling show that thin (usually 850°C, zircon solubility–melt composition relationships show that they were only one-third saturated in zircon. Any zircon entrained in the precursor magmas would thus have been highly soluble. Combined with the cathodoluminesence imaging, this demonstrates that the c. 3,850 Ma oscillatory zoned zircon crystallised out of the melt and hence gives a magmatic age. Thus the rare well-preserved tonalites and palaeosome in migmatites testify that c. 3,850 Ma quartzo–feldspathic rocks are a widespread (but probably minor) component in the Itsaq Gneiss Complex. C. 3,850 Ma zircon with negative Eu anomalies (showing growth in felsic systems) also occurs as detrital grains in rare c. 3,800 Ma metaquartzites and as inherited grains in some 3,660 Ma granites (sensu stricto). These demonstrate that still more c. 3,850 Ma rocks were present, but were recycled into Eoarchaean sediments and crustally derived granites. The major and trace element characteristics (e.g. LREE enrichment, HREE depletion, low MgO) of the best-preserved c. 3,850 Ma rocks are typical of Archaean TTG suites, and thus argue for crust formation processes involving important contributions from melting of hydrated mafic crust to the earliest Archaean. Five c. 3,850 Ma tonalites were selected as the best preserved on the basis of field criteria and zircon petrology. Four of these samples have overlapping initial ɛNd (3,850 Ma) values from +2.9 to +3.6± 0.5, with the fourth having a slightly lower value of +0.6. These data provide additional evidence for a markedly LREE-depleted early terrestrial mantle reservoir. The role of c. 3,850 Ma crust should be considered in interpreting isotope signatures of the younger (3,800–3,600 Ma) rocks of the Itsaq Gneiss Complex.

A zircon ‘microvein’ composed of several hundred crystals occurs in peralkaline granitic gneiss of western Ethiopian Precambrians. U–Pb ages and trace element (U, Th, Hf, Y, REE, P, Ca, Al, Fe, and Mn) abundances of the ‘microvein’ and host granitic gneiss zircon were determined using a sensitive high mass resolution ion microprobe (SHRIMP) and electron probe microanalyzer (EPMA). Back-scattered electron (BSE) imaging of the ‘microvein’ zircon and host granite zircon, hereafter referred to as Type-I and Type-II zircon, respectively, reveal prevalent low and high mean atomic number contrast domains within individual crystals. Ubiquitous fluorite microinclusions in bright BSE domains of Type-I and less commonly, Type-II zircon suggest an early formation of fluorite that buffers F activity, causing zircon supersaturation and precipitation from a late-magmatic melt/fluid-enriched in high field strength elements (HFSEs) including Zr. The textural make up of the host peralkaline granitic gneiss and internal structural features of Type-I and Type-II zircon indicate that dark-grey BSE domains were formed by dissolution–reprecipitation owing to fluid infiltration and interaction with the primary zircon crystals. The bright and dark-grey BSE domains in Type-I zircon yield U–Pb ages of 779 ± 69 Ma and 780 ± 35 Ma, and similar domains in Type-II zircon dated at 778 ± 49 Ma and 780 ± 31 Ma, respectively. The primary and recrystallized domains in both zircon types have indistinguishable ages, suggesting initial crystallization shortly followed by fluid-driven alteration. The ages are identical, within analytical uncertainties, to the 776 ± 12 Ma zircon U–Pb emplacement age of a protolith of a leucocratic granitic gneiss determined from a different sample. Hence, zircon crystals forming ‘microvein’ and aggregate structures, the relatively high Th/U ratios (reaching up to 1.5) in the primary domains, high LREE/HREE, and the formation of Type-I and Type-II zircon during emplacement support a late-magmatic–hydrothermal origin. Extensive alteration of the host rock, recrystallization of young and non-metamict zircon corroborate the infiltration of orthomagmatic or hydrothermal fluids containing fluorides as a major constituent, which expelled a considerable amount of trace elements, namely, Hf, U, Th, Y, and the REEs, from the recrystallized domains of Type-I and Type-II zircon. The trace element depleted recrystallized domains characteristically contain microfractures apparently caused by differential volume expansion of the U and Th enriched primary domains or volume change during cation exchange reactions, and anomalously high Th/U ratios (∼ 0.5 to 1.0). Furthermore, the ca. 780–776 Ma emplacement age of the protolith of the peralkaline granitic gneiss and late-stage orthomagmatic or hydrothermal activity shed light on the occurrence of older anorogenic granitoid magmatism and associated structures in western Ethiopian Precambrian terranes.

The isotopic compositions of Sm and Gd in lunar regolith samples from the Apollo 16 and 17 deep drill stems showed clear isotopic shifts in 150 Sm / 149 Sm ( e = +124 to +191 for A-16, and +37 to +111 for A-17) and 158 Gd/ 157 Gd ( e = +107 to +169 for A-16, and +31 to +84 for A-17) corresponding to neutron fluences of (5.68–9.03) × 10 16 n cm −2 for A-16 and (1.85–5.04) × 10 16 n cm −2 for A-17. The depth profiles of neutron fluences suggest that the regoliths at both sites were due to incomplete mixing of three different slabs which experienced individual two-stage irradiation before and after deposition of the upper slabs. The variations in REE compositions provide chemical evidence for incompletely vertical mixing of regoliths especially at upper layers of the two sites. The thermal neutron energy index estimated from the combination of Sm and Gd isotopic shifts, defined as e Sm / e Gd , shows a small variation (0.61–0.64) in the A-16 core except for the surface layer. On the other hand, a large variation in e Sm / e Gd = 0.67 to 0.83 in the A-17 core may result from complicated history such as two-stage irradiation and incomplete mixing during the gardening processes. Isotopic enrichments of 152 Gd and 154 Gd correlated with Eu/Gd elemental abundances and neutron fluences were also observed in almost all of 15 samples, showing evidence of neutron-capture from 151 Eu and 153 Eu, respectively.

Enstatite achondrites (aubrites) when compared to other stone meteorites have unusually long cosmic-ray exposure (CRE) ages. We report here the 150 Sm/ 149 Sm and 158 Gd/ 157 Gd ratios in six different structural phases, i.e., light and dark (shocked) grains and in matrix materials of Pesyanoe, in three different fragments from Pena Blanca Spring, and in one from Norton County, Shallowater, and Khor Temiki, to investigate the regolith history on the aubrite parent body. The results from phases components of Pesyanoe confirm earlier reported evidence for regolith irradiation of several aubrites. The inferred neutron fluences for six Pesyanoe separates vary between (2.13 and 2.82) × 10 16 n cm −2 . The fluences also significantly exceed those expected from cosmic-ray irradiation during transit to Earth and approach those observed in the lunar regolith. These observations confirm that the brecciated Pesyanoe meteorite, which contains solar wind (SW) gases only in dark phases, was processed in a regolith and that structural phases were differentially irradiated before compaction. On the other hand, in some aubrites (Mt. Egerton, Shallowater, Pena Blanca Spring, Norton County) neutron capture effects may entirely be due to space irradiation.

The polymict Kamiaso Conglomerate (Mino Terrane, Japan) contains Jurassic to Palaeoproterozoic clasts—probably derived from Korean basement that lay nearby to the northwest at time of deposition. Clast K2 broke cleanly into two halves during sampling (but the halves were recombined for zircon separation). A third of the K2 zircons are colourless euhedral prisms with oscillatory zoning, with no inheritance and yielded a SHRIMP U/Pb date of 743±17 Ma. Two thirds of K2 zircons are brown oscillatory-zoned corroded prisms with a date of 1860±8 Ma, with inherited cores up to ∼2460 Ma. A likely explanation for this could be that clast K2 might have been composite, and contained undistinguished 743 Ma and 1860 Ma granites. Kamiaso granitic clast K3 igneous zircons gave a date of 179.3/−2.1 Ma (Toarcian–Early Jurassic), with 2100–2300 Ma and ∼1860 Ma inherited cores. ∼740 Ma A-type magmatism related to the extension and break up of Rodinia occurs in both Korea (Gyeonggi Block) and the main part of the South China Craton, but is unknown in the Sino-Korean Craton. Thus from recognition of a 743 Ma clast, the Kamiaso detritus was probably derived from the northernmost part of the South China Craton in Korea.

Isotopic analyses of U, Pb and REE in 144 zircon grains from the Bidoudouma stream, the Republic of Gabon, were performed to study correlation of elemental distribution with geological alteration effects. The U–Pb isotopic results reveal that the zircons formed around 2.8–2.1 Ga ago and altered around 500 Ma ago by igneous activity in association with dolerite dyke intrusion. The REE abundance patterns of the U–Pb discordant zircons are characterized by high contents of REE (977–98,154 ppm), small LREE–HREE fractionations and distinctly positive Eu anomalies. The discordant zircons also contain significant amounts of non-formula elements such as Ca, Mn, Al and Fe; whereas, their contents of Zr and Si are 1–15% lower than those of concordant zircon. Non-formula elements and REE, especially LREE, were incorporated into metamict zircon in association with Pb-loss, which indicates that the non-formula elements and REE substituted for Zr and Si. Judging from this correlation among the contents of non-formula elements, when zircon was initially altered, Fe substituted for major elements of zircon. After the saturation of Fe, Ca and Mn were incorporated into zircon together with REE and Al followed by Ca. The metamict zircons contain large amount of U (∼8215 ppm), which indicates that U had not migrated from zircon grain in association with the Pb-loss.

Mineralogical and isotopic studies were carried out on the natural nuclear reaction zone 2 from the Oklo deposit to evaluate the mobility of several nuclear reaction products in response to the alteration of the reaction zone and to identify the mechanisms which could retard the transport of released radionuclides. To address these issues, in situ isotopic analyses by SHRIMP and a selective extraction procedure were performed to constrain the structural location of nuclear reaction products (exchangeable and non exchangeable) and their association with mineral phases. The distribution patterns of U, REE, Zr and Mo isotopes reveal that substantial amounts were released from the core and migrated through the hydrothermal alteration halo over metric distances, owing to uraninite dissolution and advective transport by hydrothermal solutions during and soon after criticality. The results emphasize the mobility of Zr at Oklo, this element being often considered as “immobile” during water–rock interactions. The main output is the demonstration of the net effects of sorption and coprecipitation processes. Chlorite and to a lesser extent illite were found to have adsorbed significant amounts of U, REE, Zr (and probably Th) and less sorbing elements such as Mo. Coprecipitation of secondary UO2 and P-rich coffinite within the alteration halo is also an important means of retardation. The concentration of radionuclides released from the reactor were probably high and they display solubility limited transport behaviour. No retention effect was found for Se in the immediate vicinity of the reactor and this element may have moved farther from its source of production. These results have interesting implications for the evaluation of long-term containment of radionuclides. They provide a simple illustration of the performance of a clay barrier in the uptake of radionuclides by sorption onto clays and reincorporation in secondary U-minerals. This study also demonstrates the robustness of these retention processes over extremely long periods of time.

The Ba isotopic composition of primitive meteorites is one of the more useful tracers to study nucleosynthetic processes in the early solar system, because the seven stable Ba isotopes consist of p-, r-, and s-process nucleosynthetic components and possible decay products from presently extinct 135 Cs. Ba isotopic analyses were performed on acid leachates from five carbonaceous chondrites, Murchison (CM2), Sayama (CM2), Allende (CV3), Isuna (CO3) and Maralinga (CK4), and one basaltic achondrite, Juvinas (Eucrite). The Ba isotopic data of acid treatment fractions in carbonaceous chondrites suggest the presence of independent nucleosynthetic components for s- and r-processes in the solar system. The isotopic patterns from the acid residual fractions of two CM2 meteorites, Murchison and Sayama, show an enrichment of s-process isotopes, which is similar to that of presolar SiC grains. On the other hand, their acid leachates have an excess of r-process components among the Ba isotopes. In many cases of carbonaceous chondrites other than CM meteorites, the excess of r-process isotopes is too small for a detailed discussion about the nucleosynthetic contribution. This may be interpreted as metamorphic destruction of presolar grains in meteorite matrices associated with petrographic-type meteorites. Juvinas shows no isotopic anomalies of Ba, because the isotopic record in the early solar system was reset possibly by magmatic processes in the parent body.

Sensitive high resolution ion microprobe (SHRIMP) U–Pb analyses of zircons from a pelitic gneiss clast from the Jurassic Kamiaso conglomerate in central Japan show that most grains are 1850–1950 Ma, but six are >2400 Ma, including a very old one (>3254±14 Ma). The SHRIMP data give chronological evidence for previously unrecognized igneous activity from 1850 to 1950 Ma. The existence of Archean zircons allows us to consider the evolutionary history of the Archean crust in eastern Asia. Chondrite-normalized REE patterns of the individual zircons determined by SHRIMP show that the 2400–3250 Ma zircons have large positive Ce and small negative Eu anomalies, whereas the 1850–1950 Ma zircons have small positive Ce and significantly negative Eu anomalies. These differences in the REE abundances for the 2400–3250 and 1850–1950 Ma zircons may reflect the composition of the melt that produced the zircons, and reveal the possibility of a geological constraint on the origin of Kamiaso zircons.

Here we report the isotopic excess of 135 Ba studied from Ba isotopic measurements of acid leachates of Allende calcium–aluminum rich inclusions (CAIs) and two primitive chondrites, Beardsley (H5) and Zag (H6 clasts). From the excess 135 Ba abundances of CAIs, after subtraction of the r-process nucleosynthetic component in a model-dependent manner, a value for 135 Cs/ 133 Cs=(4.8±0.8)×10 −4 is proposed as the initial ratio in the early solar system. A correlation between excess 135 Ba abundances and Cs/Ba ratios observed in the Beardsley and the Zag meteorites reveals the possible existence of primordial 135 Cs in the two meteorite parent bodies. On the assumption of two cases for 135 Cs/ 133 Cs initial in the early solar system, the 135 Cs– 135 Ba isochron suggests that primitive (aqueous) alteration in the Beardsley and Zag meteorite bodies occurred at 8.2∼11.9 Ma and 13.9∼17.6 Ma after CAI formation, respectively. Our results are apparently consistent with previously reported data from Pb–Pb model age determinations, and 53 Mn– 53 Cr chronometry for chondrites. This is the first report of isotopic excess of 135 Ba in meteorites, which may relate to live 135 Cs in the early solar system and the 135 Cs– 135 Ba chronological application.

Large-scale light rare earth element (LREE), uranium, lead and phosphorus migration has been evidenced in the FA Lower Proterozoic sandstones of the Franceville basin (Gabon) hosting Oklo natural nuclear reaction zones (RZ) in relation with extensive accessory mineral alteration by highly saline diagenetic brines (28.7 wt.% NaCl eq. to 30 wt.% CaCl2 eq.) at about 140°C and 1 kbar. Monazite is the most severely altered accessory mineral in the coarse-grained sandstones of the basal FA formation. Detrital monazite crystals are altered to Th–OH silicate microcrystalline phase with very low concentrations of U and LREE. The Th/La ratio increase from non-altered (Th/La∼0.27) to altered sandstones (Th/La∼1.14) shows that about 76% of the LREE was leached. This corresponds to the leaching of 2.01×109 metric tons at the scale of the FA formation in the Franceville basin. Similarly, the Th/U increase from monazite (Th/U=18.6) to the Th-silicate phase (Th/U=88.7) is interpreted as a result of an alteration by oxidizing brines with leaching of U together with LREE and P. It corresponds to the leaching of 9.06×106 metric tons of uranium. This amount of uranium largely exceeds the known uranium reserves from the Franceville basin. In zircon crystals, the cores are generally homogeneous, weakly fractured and well preserved as attested by the Archean ages (2867±24 and 2865±51 Ma) obtained by ionic microprobe analysis on zircon of the FA Formation, respectively, from the marginal and central parts of the basin. Their composition corresponds to the pure end-member (Zr,Hf)(SiO4), poor in Th and U (Th/U∼1). At the contrary, their rims, which present several growth zones with cracks fillings, are enriched in REE, P, Th and U with higher Th/U ratios (5–10). Both altered monazite and altered zircon contain galena as numerous inclusions in the outer growth zones and as crack fillings. For example, in zircon, the Pb of galena crystals (3–23 wt.%) largely exceeds the amount of Pb (maximum 0.1 wt.%) that would have been produced in situ by radioactive decay in this mineral. Nearly all the lead were introduced into altered zones of accessories. Dissolution of accessory minerals occurred at 2000 Ma, producing a porous and distorted crystal structure which has allowed a later incorporation of Pb. Galena inclusions in altered zircons located in the vicinity of reactor zones have radiogenic lead compositions. Altered zircon rims and galena inclusions in altered zircon located far from reactor zones have non-radiogenic Pb isotopic compositions, confirming the external origin of lead. Pb isotopic evolution models indicate a crystallization age sometime after 1000 Ma, both for galena located close to and far from U mineralizations and reactor zones, which may be synchronous with a regional extension event contemporaneous with intrusion of dolerite dyke swarms, between 1000 and 750 Ma, at the scale of the Franceville basin. The present study also illustrates the different retention capacities of accessory mineral for elements representing analogs of the radiotoxic nuclides in the relatively extreme natural conditions created by the circulation of moderately hot and chloride-rich fluids during the diagenesis of a sedimentary basin.

Isotopic compositions of Sm and Gd in eight chondrites (Mezo-Madaras, Parnallee, Otis, Suizhou, Zhaodong, Allende, Bruderheim and Y-74191) were measured to study neutron capture effects due to the interaction of cosmic rays with planetary materials in space. The anti-correlation of 149Sm/152Sm with 150Sm/152Sm and of 157Gd/160Gd with 158Gd/160Gd in the meteorites provides evidence for neutron capture effects in these chondrites. Isotopic deviations observed in the eight chondrites are ϵ149Sm=−0.35 to −1.93, ϵ150Sm=+0.87 to +4.35, ϵ157Gd=−2.82 to −1.21 and ϵ158Gd=+0.33 to +1.66, which correspond to thermal neutron fluences of (1.03–5.17)×1015 n cm−2. The combination of isotopic data from Sm–Gd in this study and Ne–Kr from the literature provides information on the secondary produced thermal ( 5 MeV) neutron fluences for chondrites, and it is used here to estimate the neutron energy distribution for six L- and LL-chondrites. From the difference of distribution of thermal, epi-thermal and fast neutron fluences in individual chondrites, the depth of the samples inside the meteoroid is discussed. In Allende, the isotopic anomalies observed may be due not only to neutron capture reactions but also to additions of r-process nuclides of Sm and Gd.

Elemental abundances and isotopic compositions of rare earth elements (REE), Th and U of twenty eight samples from four kinds of natural fission reactors at the Oklo and Bangombe uranium ores were determined by mass spectrometry. Fissiogenic and non-fissiogenic REE components of each sample were calculated from the isotopic compositions. The geochemical distribution of both fissiogenic and non-fissiogenic REE differs among the four reactors. Each reactor shows the following features possibly associated with the local geochemical conditions. (1) Reactor 9 at the shallow depth of the Oklo deposit contains higher amounts of non-fissiogenic REE than the other reactors. The distribution of non-fissiogenic REE does not reflect the primordial REE abundance patterns, and might have been disturbed in and around the reactor due to supergene alteration. Our results suggest that significant amounts of fissiogenic lighter REE (LREE), such as La, Ce and Pr, were mobile in the reactor; (2) Reactors 10 and 13 located deep underground have much lower contents of non-fissiogenic REE than reactor 9. All fissiogenic REE show high retention in the reactors. Non-fissiogenic REE of reactor 10 inform of the primordial ore formation without any geochemical disturbance. Little redistribution of both fissiogenic and non-fissiogenic REE in the reactor 10 was observed; and (3) The Bangombe reactor located at shallow depth has apparently well preserved fissiogenic REE. Non-fissiogenic REE in the Bangombe reactor might have been disturbed, possibly by hydrothermal processes. The distribution behavior of fissiogenic REE is considered to be related with physico-chemical reactor conditions such as nuclear reaction temperature and chemical compositions of each reactor, but clear relationship between nuclear parameters and the REE distribution could not be obtained in this study.

We have measured U concentrations, and elemental ratio of 238 U/ 206 Pb, and isotopic compositions of Pb in thirteen zircons extracted from a volcaniclastic rock of the Kamiaso conglomerate in the Mino terrane and sixty three analyses in fifty zircons from the Amo gneiss, a metamorphic rock originated from an intermediate volcanic rock in the Hida terrane of central Japan by means of an ion microprobe. Zircons from the Kamiaso were first analyzed in Australian National University (ANU) and then measured again by Hiroshima University (HU) Sensitive High Resolution Ion MicroProbe. Both radiometric ages of 238 U/ 206 Pb* and 207 Pb*/ 206 Pb* obtained by the ANU instrument agree well with those by HU except for one sample. There are six geochronological episodes in the Kamiaso sample; about 2550 Ma, 2000 Ma, 1300 Ma, 920 Ma, 250 and 220 Ma. U-Pb zircon geochronology of the Amo sample indicates that there are six concordant zircon episodes and three concordia intercept episodes; about 3420 Ma, 2560 Ma, 1840 Ma, 1130 Ma, 580 Ma, 400 Ma, 360 Ma, 285 Ma, and 250 Ma. The 3420 Ma is the oldest age so far reported in Japan. The 2560 Ma event may have a relation to the period of the evolution of the Sino-Korean craton between 2.7‐2.5 Ga. The third age peak of about 1840 Ma suggests that the zircons were derived from the Ryongnam massif in South Korea with the similar formation age. The younger ages of 1130 Ma, 580 Ma, 400 Ma, 360 Ma and 285 Ma may be minor events recorded in the Hida metamorphic belt. The latest age of 250 Ma may be the most important metamorphic event in Hida terrane. Rare earth element abundance patterns of the Amo zircons show progressive increase with atomic number together with positive Ce anomaly, which is a typical zircon signature. Small negative anomaly of Eu is found in zircons with the age of 2560 Ma and 250 Ma while zircons with intermediate ages show large anomaly. There is no correlation between Ce and Eu anomaly, suggesting that both anomalies may not have been simply controlled by the oxidation state during zircon crystallization.

Large isotopic deviations of 149Sm–150Sm and 157Gd–158Gd derived from neutron capture effects were observed in five enstatite achondrites (aubrites); Norton County, ALH78113, Cumberland Falls, Bishopville and Mayo Belwa. The isotopic variations of these aubrites (e149Sm = −32 to minus7, e150Sm = +11 to +57, 8157Gd = –42 to –14 and e158Gd = +9 to +26) correspond to irradiation with neutron fluence of (1.17 – 3.99) × 1016 n cm−2, which is within the range of estimated neutron fluence values of some lunar samples. These results indicate that aubrites may have been located near the surface of their parent body for several hundreds of Ma. Furthermore, the neutron energy spectrum obtained from the combination of Sm and Gd isotope systematics (eSm/eGd) suggests that the energy of thermalized neutrons in five aubrites was in the range of 200–400 K, and was lower than those in lunar samples A-12 and A-15. In addition, we analyzed one anomalous aubrite (Mt. Egerton), and two enstatite chondrites (Happy Canyon and Abee) to investigate the relationship of their origin with aubrites. However, these three meteorites showed no evidence for intensive neutron irradiation observed in the aubrites.

U-Pb isotopes of apatites and zircons extracted from a sample of the Acasta gneisses that occurs along the western margin of the Slave craton in Northwest Territories of Canada have been measured using a SHRIMP II ion microprobe. Eleven apatite analyses (30 μm spot) give 238U/204Pb-206Pb/204Pb and 204Pb/206Pb-207Pb/206Pb isochron ages of 1905 ± 86 Ma (2 σ) and 1936 ± 28 Ma (2 σ), respectively. Analyses (20 μm spot) on the partly recrystallised central portions of fourteen zircons mostly define a mixing array on a 238U/206Pb∗-207Pb∗/206Pb∗ concordia plot with concordia intercepts at 4014 ± 25 Ma (2 σ) and 1967 ± 93 Ma (2 σ). The former agrees with 3.96–4.02 Ga protolith ages obtained on the oldest components of the Acasta gneisses by other workers. The lower concordia intercept for the zircon data is consistent with the U-Pb age of apatites and may be related to a >500°C thermal event, perhaps early in the Palaeoproterozoic Wopmay orogeny.

Ruthenium isotopic compositions of eleven samples selected from four kinds of natural fission reactors at Oklo and Bangombe uranium deposits were determined by inductively coupled plasma mass spectrometry (ICP-MS). The detectable Ru isotopes in all samples were only fissiogenic origin. The fission contribution of 235 U , 238 U and 239 Pu in each reactor was estimated by using 102 Ru / 101 Ru and 104 Ru / 101 Ru isotopic ratios. Some of the samples show unusual Ru isotopic compositions, possibly derived from fast neutron induced fission. Depletion or enrichment of isotopic abundance of fissiogenic 99 Ru , defined as Δ 99 Ru , reveals the occurrence of a chemical fractionation between Ru and Tc in the reactors. A major part of the fissiogenic Ru has been incorporated into micro metallic aggregates containing Ru, Rh, Te, Pb, As and S in the reactors. In situ analyses of metallic aggregates by secondary ion mass spectrometry (SIMS) and electron probe micro analysis (EPMA) were performed to obtain information on formation process of the aggregates. A correlation between Δ 99 Ru and isotopic abundance of 100 Ru was observed in the metallic aggregates. This correlation suggests that the chemical fractionation between Tc and Ru in the reactors had occurred at the first stage of criticality, and that 100 Ru was produced by successive neutron capture during criticality. The chemical compositions indicate that the aggregates were formed as a mixture of PbS and fissiogenic Ru–Rh–Te components.

To illustrate in situ Sensitive High Resolution Ion MicroProbe (SHRIMP) analysis of apatites, Pb isotopes and Pb/U ratios of five apatite samples with different ages have been measured using SHRIMP II ion microprobe at Hiroshima University. The `PRAP' apatite from a carbonatite complex at Prairie Lake, Ontario with a 207 Pb ∗– 206 Pb ∗ age of 1156±45 Ma (2 σ) was used as the standard for 206 Pb + / 238 U + – 238 U 16 O + / 238 U + calibration. Apatites from the Poohbah Lake alkaline complex give 204 Pb / 206 Pb – 238 U / 206 Pb and 204 Pb / 206 Pb – 207 Pb / 206 Pb isochron ages of 2726±145 Ma (2 σ) and 2701±86 Ma (2 σ), in agreement with a conventional TIMS 207 Pb ∗– 206 Pb ∗ age of 2667±10 Ma (2 σ) and a K–Ar isochron age of 2706±46 Ma (2 σ). For the remaining three apatite samples the 204 Pb / 206 Pb – 238 U / 206 Pb and 204 Pb / 206 Pb – 207 Pb / 206 Pb isochron ages obtained by SHRIMP are consistent with radiometric and geological ages in the literature.