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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.

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.

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.

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.

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.

Isotopic compositions of Ru, Te and Ag in several types of uraninite sample were determined by thermal ionization mass spectrometry in order to elucidate the spontaneous fission phenomenon of uranium. Ru and Te in a few samples show clear isotopic anomalies for 99 Ru, 101 Ru, 102 Ru, 104 Ru, 128 Te and 130 Te because of probable addition of spontaneous fission fragments to natural isotopes, while no significant evidence of fissiogenic Ag could be obtained from the isotopic measurements of Ag because of its low fission yield. The fission pattern of Ru obtained from one uraninite, Morogoro (Tanzania), indicates the possibility that this uraninite had reached a critical condition for neutron-induced fission of 235 U.