Sensitive High-resolution Ion Microprobe Analyses Research Articles

Anatexis of late Neoarchean-Early Paleoproterozoic and late Paleoproterozoic garnet-bearing leucogranite from the Khondalite Belt in the North China Craton

ABSTRACT Two episodes of late Neoarchean-early Paleoproterozoic and late Paleoproterozoic garnet-bearing leucogranite, named the Baotou and Jining–Liangcheng garnet-bearing leucogranite are recognized from the Khondalite Belt in the North China Craton. Geochemical studies show that garnet-bearing leucogranite has high Al2O3 content and FeOT/MgO ratio, with a large variation in the CaO content and K2O/Na2O ratio. Additionally, the garnet-bearing leucogranite is enriched in light rare earth elements and large ion lithophile elements and depleted in Nb, Ta, P, and Ti. However, there are some variations in Eu anomalies, as indicated by the Eu-enriched and Eu-depleted patterns. The sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb dating of the Baotou garnet-bearing leucogranite and Jining–Liangcheng garnet-bearing leucogranite show that their anatectic ages were 2.35–2.43 Ga and 1.90–1.92 Ga, respectively, with zircon εHf (t) values of −0.01–3.93 and −4.36–1.99, and two-stage model ages (TDM2) of 2.76–3.02 Ga and 2.57–2.83 Ga, respectively. U-Th-Pb SHRIMP analyses on zircon reveal ages of ca. 2.35–2.47 Ga for the Baotou metapelite gneiss and ca. 1.90–1.91 Ga for the Jining–Liangcheng metapelite gneiss, which represent the ages of tectonic metamorphic events related to the anatexis. Our results, combined with geological and petrographical observations, geochemistry, and geochronology, suggest that (1) Garnet-bearing leucogranites formed as a result of anatexis of metapelite gneisses; (2) They are mixtures of leucosomes, residual minerals and mantle-derived materials; (3) Two episodes of garnet-bearing leucogranites have been identified; (4) Both two episodes of garnet-bearing leucogranites corresponded to late Neoarchean-early Paleoproterozoic and late Palaeoproterozoic tectonothermal events, respectively.

Zircon as a Recorder of Trace Element Changes during High-Grade Metamorphism of Neoarchean Lower Crust, Shevaroy Block, Eastern Dharwar Craton, India

AbstractSystematic changes in whole-rock chemistry, mineralogy, mineral textures, and mineral chemistry are seen along a ca. 95-km traverse of late Archean granitoid orthogneisses in the Shevaroy Block, Eastern Dharwar Craton, southern India. The traverse passes from amphibolite-grade gneisses in the north to granulite-grade rocks (charnockite) in the south. Changes include whole-rock depletion of Rb, Cs, Th, and U in the granulite grade rocks as relative to the amphibolite grade gneisses, and oxidation trends regionally from highly oxidised granulite-facies rocks near the magnetite–haematite buffer to relatively reduced amphibolite-facies rocks below the fayalite-magnetite-quartz. Rare earth elements show limited mobility and are hosted a variety of minerals whose presence is dependent on the metamorphic grade ranging from titanite and allanite in the amphibolite-facies rocks to monazite in the vicinity of the orthopyroxene-in isograd to apatite in the granulite-grade charnockite. Cathodoluminescence and back-scattered electron sub-grain imaging and sensitive high-resolution ion microprobe analysis of zircon from 29 samples of dioritic, tonalitic, and granitic orthogneiss from the traverse reveals magmatic zircon cores that record the emplacement of the granitoid protoliths mostly about 2580 to 2550 Ma, along with a few older mid to late Archean tonalites. Protolith zircon was modified during metamorphism by overgrowth and/or replacement. Relative to igneous cores, U-enriched metamorphic zircon, dominant in the amphibolite-grade gneisses, formed at ca. 2530 Ma, predating retrograde titanite growth at ca. 2500 Ma. Uranium-depleted mantles grew on zircon between 2530 and 2500 Ma in granulite-grade samples south of the orthopyroxene-in isograd. In some of these samples, the U-depleted metamorphic zircon is preceded by mantles of U-undepleted zircon, indicating a progression of metamorphic zircon growth with increasingly depleted compositions between 2530 and 2500 Ma. With increasing metamorphic grade (from amphibolite to granulite) and oxidation state, allanite and monazite disappear from the assemblage and zircon became depleted in U and Th. Whole-rock U-Th compositions became decoupled from relict magmatic zircon compositions, reflecting the development of U-depleted metamorphic zircon and indicating that whole-rock chemical differences along the traverse were produced during metamorphism, rather than just reflecting differences in dioritic vs granitic protoliths. Although in situ anatexis and melt extraction may have played a role, whole-rock and zircon depletion of trace elements can be explained by the action of externally derived, oxidising, low-H2O activity hypersaline fluids migrating up through the mid to lower crust. Fluids and element migration during metamorphism may be the end result of subduction related processes that cumulated in the collision and concatenation of island arcs and continental blocks. These tectonic processes assembled the Dharwar Craton at the end of the Archean.

Volatilization of B4C control rods in Fukushima Daiichi nuclear reactors during meltdown: B–Li isotopic signatures in cesium-rich microparticles

Boron carbide control rods remain in the fuel debris of the damaged reactors in the Fukushima Daiichi Nuclear Power Plant, potentially preventing re-criticality; however, the state and stability of the control rods remain unknown. Sensitive high-resolution ion microprobe analyses have revealed B–Li isotopic signatures in radioactive Cs-rich microparticles (CsMPs) that formed by volatilization and condensation of Si-oxides during the meltdowns. The CsMPs contain 1518–6733 mg kg–1 of 10+11B and 11.99–1213 mg kg–1 of 7Li. The 11B/10B (4.15–4.21) and 7Li/6Li (213−406) isotopic ratios are greater than natural abundances (~4.05 and ~12.5, respectively), indicating that 10B(n,α)7Li reactions occurred in B4C prior to the meltdowns. The total amount of B released with CsMPs was estimated to be 0.024–62 g, suggesting that essentially all B remains in reactor Units 2 and/or 3 and is enough to prevent re-criticality; however, the heterogeneous distribution of B needs to be considered during decommissioning.

A new model for the formation of nephrite deposits: A case study of the Chuncheon nephrite deposit, South Korea

The Chuncheon nephrite deposit is the largest in South Korea with an estimated reserve of 300,000 tons. Located in mid-eastern South Korea, the deposit is in the form of lenses in the contact zone between dolomitic marble and amphibole schist in the upper part of a biotite schist. To date, the origin, process of formation, and age of the Chuncheon nephrite deposit remain unclear, which hinders our further understanding of the formation of nephrite. This study involved petrography, chemical analyses, and nephrite age determinations using optical mineralogy, electron microprobe-analyses (EMPA), X-ray fluorescence spectroscopy (XRF), and SHRIMP (Sensitive High-Resolution Ion MicroProbe) analyses of zircon. Petrographic studies indicated that the Chuncheon nephrite consists mainly of tremolite. The low contents of Cr (2.15–4.74 ppm) and Ni (2.95–4.90 ppm) are typical of dolomite-related nephrites and lower than in serpentine-related nephrites. The total REE contents are <18.5 ppm. The REE chondrite-normalized patterns display right-leaning LREE and almost flat HREE patterns, which are consistent with the REE model of dolomitic marble. The δ18O isotope compositions of the zircons (6.34‰–12.5‰) from the nephrite overlap those of the Chuncheon dolomite (2.4‰–18.2‰), indicates clearly that dolomite played an important role in the formation of the Chuncheon nephrite. The petrography and geochemistry suggest that it had a metasomatic origin involving the reactions dolomitic marble → diopside, diopside → tremolite, and tremolite → chlorite. The δD and δ18O isotope compositions of our specimens of nephrite (δD = − 95.3‰ to − 78.3‰ and − 118‰ to − 105‰; 18O = − 9.9‰ to − 7.9‰) and the hydrothermal fluids in equilibrium with the nephrite (δD = − 73.6‰ to − 56.6‰ and − 96.3‰ to − 83.3‰, 330–450 °C; δ18O = − 9.41‰ to − 6.38‰, 330–430 °C) suggest that the hydrothermal fluids were predominantly meteoric water. The zircons contain almost no mineral inclusions, which is quite different from zircons in other nephrite deposits. The variable SHRIMP U–Pb ages of the zircons (908.9 ± 4.8 to 459.1 ± 1.9 Ma (n = 7), 436.3 ± 16.9 to 359.5 ± 6.4 Ma (n = 5), and 249.9 ± 5.2 to 79.4 ± 1.9 Ma (n = 5)) in the Chuncheon nephrite may record the ages of a regional multi-stage metamorphism. Due to the generally low temperature of formation of the nephrite, we speculate that the ages of zircons only record the two relatively high-temperature initial stages of nephrite formation (dolomitic marble → diopside, and diopside → coarse-grained tremolite) and failed to record the age of the lowest-temperature third stage of high-quality nephrite formation (coarse-grained tremolite → fine-grained tremolite). It has generally been considered that dolomite-related nephrite forms at the contacts between granite/granodiorite and dolomitic marble, whereas serpentinite-related nephrite forms at the contacts between serpentinite and silica-rich rocks. However, the genesis of the Chuncheon nephrite deposit does not fit with either of these well-known types. Thus, we developed a new model for the Chuncheon nephrite in which heat was provided by regional multi-stage metamorphism, meteoric water dominated the nephrite-forming fluids, and dolomitic marble and biotite schist provided the material for the nephrite.

Metamorphic evolution of Daqingshan supracrustal rocks and garnet granite from the North China Craton: Constraints from phase equilibria modelling, geochemistry, and SHRIMP U–Pb geochronology

Supracrustal rocks and garnet granite within the high-grade metamorphic complex in Daqingshan, at the northern margin of the Palaeoproterozoic Khondalite Belt, North China Craton (NCC), preserve various mineral assemblages that record three distinct stages of their metamorphic evolution. The mineral assemblages representative of prograde metamorphism (M1) comprise plagioclase (Pl) + quartz (Q) ± biotite (Bio) inclusions preserved in garnet porphyroblasts in gneiss and granite members. The metamorphic peak mineral assemblages (M2) are represented by garnet (Gt) + Pl + Bio + Q + sillimanite (Sil) + K-feldspar (Kfs) in each rock’s matrix. Retrograde mineral assemblages (M3) that formed during post-peak, near-isothermal decompression (ITD) occur between relict garnet porphyroblasts and matrix minerals. These M3 assemblages consist of Bio + cordierite (Crd) + Pl + Gt + Q + Sil + Kfs. Phase equilibria modelling shows that the supracrustal rocks record clockwise pressure–temperature (P–T) trajectories from 650 to 750 °C/5.3–8.9 kbar (M1) up to 800–830 °C/9.8–11.2 kbar (M2), to 760–830 °C/4.8–6.2 kbar (M3). Geochemical similarities between the Daqingshan supracrustal rocks (i.e. garnet-biotite gneisses) and garnet granites, such as relative enrichment in K, Rb, and Ba, and depletion in Th, U, Ta, Nb, P, and Ti, imply a petrological affinity. Sensitive high-resolution ion microprobe analysis (SHRIMP) U–Pb dating of zircons reveals that the protolith of the Daqingshan supracrustal rocks formed at ~2.5 Ga and subsequently metamorphosed at ~2.45–2.37 Ga. Anatexis likely occurred at ~2.43–2.40 Ga during the retrograde near-ITD stage. Combined with previous studies, the timing of metamorphism and anatexis in the Daqingshan area may be connected with the regional mafic magmatism.

New insight into disturbance of U-Pb and trace-element systems in hydrothermally altered zircon via SHRIMP analyses of zircon from the Duluth Gabbro

Redistribution and retention behavior of major (Zr, Si, O, and Hf) and trace (Li, Al, P, K, Ca, Ti, Mn, Fe, rare earth elements (REE), Pb, U, and Th) elements in zircon during hydrothermal alteration were investigated by a sensitive high-resolution ion microprobe (SHRIMP). AS3 zircons collected from the Duluth Complex, U.S.A. were classified into three domains: Type-A comprise domains with a darker backscattered electron (BSE) and cathodoluminescence (CL) response; Type-B comprise domains with fine fractures, and; Type-C comprise domains where the dark response areas in CL are bright in BSE and vice versa, and are free of fractures. The Type-A domains are characterized by discordant U-Pb ages, light REE enrichment, depletion of Zr and Si, and higher contents of Ca, Mn, Fe, Al, Li, and K, which suggests that elemental redistribution occurred during hydrothermal alteration. Although the Type-B domains also show depletion of Zr and enrichments of Ca, Mn, Fe, Al, Li and K, the Zr contents and the contents of Ca, Mn, Fe, Al, Li, and K are higher and lower than those of the Type-A domains, respectively. This feature of the Type-B domains suggests existence of thin altered domains surrounding the fractures like “clads”. Therefore, there is a possibility that the Type-B domains are mixtures of the altered clads and unaltered domains. The Ca contents show a correlation with the Zr contents, which indicates that the Ca content is a suitable criterion for identifying hydrothermally altered zircon. There is no obvious correlation between Ca content and disturbance of the U-Pb system, but enrichment of some trace elements and REE's are correlated to the degree of Ca enrichment. The monovalent elements (Li and K) and the trivalent elements (especially light REE) are enriched at Ca contents of >1 ppm and >10 ppm, respectively. Mobility of the tetravalent elements (Hf and Ti) is increased at Ca contents >100 ppm. In spite of the elemental redistribution during the hydrothermal alteration, oxygen isotopic compositions are homogeneous through the three types of domains. Additionally, the contents of monovalent elements in zircon are potential indicators of the hydrothermal alteration of zircon.

First U–Pb SHRIMP zircon and 40Ar/39Ar ages of metarhyolites from the Afyon–Bolkardag Zone, SW Turkey: Implications for the rifting and closure of the Neo-Tethys

The Afyon–Bolkardag Zone (ABZ) is one of the most important metamorphic zones in Turkey. This zone consists of metasedimentary and metavolcanic rocks, which have undergone regional greenschist facies metamorphism. The metavolcanic rocks are dominated by rhyolite, dacite, and trachyandesite. In the Ilgin (Konya) area, the low grade metavolcanics were originally rhyolitic lavas and tuffs. We present new precise Sensitive High-Resolution Ion MicroProbe (SHRIMP) 206Pb–238U age analyses of zircons and 40Ar/39Ar age data of phengite from metarhyolites that enable a more precise reconstruction of the ABZ. SHRIMP analyses of euhedral or sub-euhedral magmatic zircons from two metarhyolite samples yield weighted means 230±2Ma and 229±2Ma. These ages are interpreted as formation age of rhyolitic volcanics, implying they were formed in the Late Triassic time (Carnian). We suggest that these Upper Triassic magmatic rocks were formed in an extensional tectonic setting on the northern active margin of Gondwana, which led to the development of the northern branch of the Neo-Tethys. 40Ar/39Ar dating of two phengite samples from metarhyolites gave plateau ages of between 63.7±0.1Ma and 62.6±0.1Ma. These dates are interpreted as the metamorphic age of these volcanic rocks, and as the age of metamorphism that affected the ABZ. Consequently, these earliest Tertiary ages also constrain the timing of the closure of the Neo-Tethys.

Constraints on the Timing of Co-Cu Au Mineralization in the Blackbird District, Idaho, Using SHRIMP U-Pb Ages of Monazite and Xenotime Plus Zircon Ages of Related Mesoproterozoic Orthogneisses and Metasedimentary Rocks

The Blackbird district, east-central Idaho, contains the largest known Co reserves in the United States. The origin of strata-hosted Co-Cu ± Au mineralization at Blackbird has been a matter of controversy for decades. In order to differentiate among possible genetic models for the deposits, including various combinations of volcanic, sedimentary, magmatic, and metamorphic processes, we used U-Pb geochronology of xenotime, monazite, and zircon to establish time constraints for ore formation. New age data reported here were obtained using sensitive high resolution ion microprobe (SHRIMP) microanalysis of (1) detrital zircons from a sample of Mesoproterozoic siliciclastic metasedimentary country rock in the Blackbird district, (2) igneous zircons from Mesoproterozoic intrusions, and (3) xenotime and monazite from the Merle and Sunshine prospects at Blackbird. Detrital zircon from metasandstone of the biotite phyllite-schist unit has ages mostly in the range of 1900 to 1600 Ma, plus a few Neoarchean and Paleoproterozoic grains. Age data for the six youngest grains form a coherent group at 1409 ± 10 Ma, regarded as the maximum age of deposition of metasedimentary country rocks of the central structural domain. Igneous zircons from nine samples of megacrystic granite, granite augen gneiss, and granodiorite augen gneiss that crop out north and east of the Blackbird district yield ages between 1383 ± 4 and 1359 ± 7 Ma. Emplacement of the Big Deer Creek megacrystic granite (1377 ± 4 Ma), structurally juxtaposed with host rocks in the Late Cretaceous ca. 5 km north of Blackbird, may have been involved in initial deposition of rare earth elements (REE) minerals and, possibly, sulfides. In situ SHRIMP ages of xenotime and monazite in Co-rich samples from the Merle and Sunshine prospects, plus backscattered electron imagery and SHRIMP analyses of trace elements, indicate a complex sequence of Mesoproterozoic and Cretaceous events. On the basis of textural relationships observed in thin section, xeno-time and cobaltite formed during multiple episodes. The oldest age for xenotime (1370 ± 4 Ma), determined on oscillatory-zoned cores, may date the time of initial cobaltite formation, and provides a minimum age for the host metasedimentary rocks. Additional Proterozoic xenotime growth events occurred at 1315 to 1270 Ma and ca. 1050 Ma. Other xenotime grains and rims grew in conjunction with cobaltite during Cretaceous metamorphism. However, ages of these growth episodes cannot be precisely determined due to matrix effects on 206 Pb/ 238 U data for xenotime. Monazite, some of which encloses cobaltite, uniformly has Cretaceous ages that mainly are 110 ± 3 and 92 ± 5 Ma. These data indicate that xenotime, monazite, and cobaltite were extensively mobilized and precipitated during Middle to Late Cretaceous metamorphic events.

SHRIMP in situ isotopic analyses of REE, Pb and U in micro-minerals bearing fission products in the Oklo and Bangombé natural reactors: A review of a natural analogue study for the migration of fission products

In the Precambrian, parts of the Oklo, Okélobondo and Bangombé uranium deposits of the Republic of Gabon, central Africa, functioned as natural fission reactors. Many elements in the Oklo and Bangombé 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 Bangombé 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 contribution of the sensitive high-resolution ion microprobe (SHRIMP) to lunar geochronology

The sensitive high-resolution ion microprobe (SHRIMP) developed at the Australian National University (ANU) was the first of the high-resolution ion microprobes. The impact of this instrument on geochronological research over the last twenty years has been immense. This is particularly so for lunar geochronology where it has opened up avenues of research that were not possible using conventional TIMS techniques. The great advantage of SHRIMP is that it provides a means for determining precise U–Pb isotopic ratios on selected micron-size areas on polished grains of zircon and other U-bearing minerals. One of the first projects undertaken on the newly invented SHRIMP I was an investigation of U–Pb ages of lunar zircon. Using SHRIMP, multiple analyses could be made on areas of individual zircons to test the stability of U–Pb systems in shocked grains. Also, by analysing grains “ in situ”, textural relationships between the analysed zircon and the components of the sample breccia could be used in the interpretation of the SHRIMP data. As a result of this research it was realised that most lunar zircons have ages up to 500 Ma older than the Imbrium and Serenitatis impacts at ca. 3.9 Ga, demonstrating that the zircons have not been affected by the these impact events although heating and shock effects have profoundly disturbed other dating systems. This has opened the way for research into the early lunar magmatic and bombardment record. For example, recent SHRIMP results have revealed profound differences in the ages of zircons from breccias from the Apollo 14 and Apollo 17 sample sites, raising new questions about the evolution of lunar magmatism. Also, multiple SHRIMP analyses on complex lunar zircons have shown that these grains can record U–Pb disturbance by later impact events. SHRIMP U–Pb age determinations on phosphates in lunar meteorites has identified lunar events not recognised in samples from the Apollo program. SHRIMP-based research on lunar materials is ongoing and, in combination with other chemical and structural evidence, continues to stimulate new ideas on the early evolution of the Moon.

Sensitive high-resolution ion microprobe analysis of zircon reequilibrated by late magmatic fluids in a hybridized pluton

Zircon from a microgranular enclave in the ca. 315 Ma postcollisional Karkonosze pluton (Western Sudetes, northeastern Bohemian Massif) is characterized by unusual morphologies and reequilibration textures. Blocky, clustered, and skeletal Th-U–rich zircon grains are internally corroded along discrete boundary zones, and subsequently replaced by porous microgranular aggregates of zircon and various other minerals, including thorite. The boundary zones have textures and compositions characteristic of diffusion-controlled chemical reaction fronts, including enrichment in Ca, Ba, and light rare earth elements, whereas microgranular domains are typical of zircon replacement and regrowth by coupled dissolution and precipitation. Initial zircon crystallization occurred with the mingling of mafic magma into a cooler granitic melt, whereas zircon modification is attributed to the reaction of late magmatic fluids from the host granite with the enclave. Precise dating of reequilibrated zircon as 304 ± 2 Ma indicates that fluid activity, which is also responsible for scheelite mineralization, postdates the emplacement of the main part of the pluton by several millions of years.

In-situ U-Pb analyses of highly altered zircon from sediments overlying the Bangombé natural fission reactor, Gabon

In-situ isotopic analyses of Pb and U in the highly altered zircons found in the clay and black shale layers above the Bangombé 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 <100 to 59000 ppm. To confirm the problem on insufficient Pb/U calibration of high-U zircon in the SHRIMP technique, 206Pb/238U isotopic ratios from the SHRIMP analysis were compared with those from the analytical combination of U/Pb elemental ratios by electron probe micro analyzer (EPMA) with Pb isotopic ratios by SHRIMP. A significant deviation was found in the 206Pb/238U data of high-U zircons between two analytical techniques. Then, we concluded that the analytical data with U concentrations less than 2500 ppm can be used as reliably calibrated SHRIMP U-Pb data and those with high U concentration greater than 2500 ppm were determined by the analytical combination by EPMA and SHRIMP. The U-Pb data of zircons provide chronological information on the igneous activity associated with the basement rock formation at ca. 2.8 Ga. Moreover, nearly constant 207Pb/206Pb data corresponding to the mean age of 1959 ± 12 Ma and wide variation of 238U/206Pb data suggest that U and Pb were originated from 2.05-Ga old uraninite and migrated into zircon grains independently under the recent severe weathering.

The timing of the retrograde partial melting in the Kumdy-Kol region (Kokchetav Massif, Northern Kazakhstan)

The Kokchetav Massif of northern Kazakhstan is the best-known metamorphic diamond locality among numerous ultrahigh-pressure (UHP) terranes in the world. At the Kumdy-Kol deposit, diamondiferous rocks are interbedded with granitic gneisses, and biotite gneisses; some have been migmatized. Some granite gneisses and migmatites were formed by partial melting of diamondiferous rocks. To verify such suggestion, sensitive high resolution ion microprobe (SHRIMP) U–Pb dating of zoned zircons from migmatites at the Kumdy-Kol region was performed to constrain the age of partial melting of the Kokchetav UHP metamorphic rocks. Most age data from core and rim domains of zircon separates are concordant. The apparent 206Pb/ 238U ages for core and rim domains of zirconsare nearly identical within analytical error. All SHRIMP analyses of zircons from four samples fall in the range 508–538 Ma with the weighted mean age for all zircon domains at 526 ± 2.1 (MSWD = 1.7). Our data show that migmatization of UHP pelites occurred later than the peak metamorphism (537 ± 9 Ma) and the decompression partial melting took place during exhumation of diamondiferous rocks from mantle depths to amphibolite-facies conditions at mid-crustal levels (507 ± 8 Ma).

Allanite micro-geochronology: A LA-ICP-MS and SHRIMP U–Th–Pb study

The accessory mineral allanite occurs in a wide range of igneous and metamorphic rocks and contains appreciable amounts of trace elements including the REEs, Sr, Th and U. The high degree of compositional substitution and the variable incorporation of common Pb into the allanite crystal structure, however, have limited its use for U–Th–Pb dating. Procedures have now been developed for the isotopic dating of allanite using Laser Ablation ICP-MS and the Sensitive High Resolution Ion Microprobe (SHRIMP). The accuracy of those procedures has been demonstrated by dating six Phanerozoic allanite samples of known age and with different FeO, REE and Th contents. Both analytical techniques require normalising factors for the measurement of 208Pb/ 232Th and 206Pb/ 238U, necessitating the use of external matrix-matched standards. The wide range of Th/U in allanite from single samples makes it possible to use multiple LA-ICP-MS analyses to construct Th–Pb isochrons from which ages can be calculated with a precision of 1.4–5.8% (95% confidence level) at a spatial resolution of 32 × 32 × 20μm. A 207Pb-based correction is used to estimate the fraction of common Pb in individual LA-ICP-MS analyses with a precision of 0.3–2%. Accurate (± 1–3%) and precise (1–2%, 95% confidence level) SHRIMP 208Pb/ 232Th ages can be measured directly on allanite samples with REE + Th > 0.5 atoms per formula unit, without additional matrix corrections at a spatial resolution of 17 × 21 × 2μm. LA-ICP-MS is an efficient technique for dating melt-precipitated allanite (e.g., from igneous or migmatitic rocks). SHRIMP analysis is preferable for samples that have a relatively small grain size, are isotopically complex or have relatively large common Pb contents (e.g., metamorphic allanite).

Leucogranites and the Time of Extension in the East Greenland Caledonides

Leucogranites provide timing estimates for deformation, particularly extension, in many orogens. On this basis, the time of extension in the East Greenland Caledonides has been estimated at 420–430 Ma. U‐Pb SHRIMP (sensitive, high‐resolution ion‐microprobe) analysis of zircon and monazite from selected leucogranites was undertaken to establish the age of the Payer Land detachment zone, an extensional structure that places low‐grade metasedimentary rocks on granulite facies gneisses that record high‐pressure (1.4–1.6 GPa) metamorphism at ca. 405 Ma. Deformed equigranular granites associated with upper plate lithologies give ages of $$426\pm 3$$ to $$413\pm 3$$ Ma, consistent with the regional leucogranite suite. A second suite of deformed pegmatitic granites occur as sills and dikes with respect to the extensional mylonitic fabrics that are superimposed on high‐pressure granulites of the lower plate and therefore must be younger than the age of high‐pressure metamorphism at 405 Ma and the more voluminous 415–440 Ma granite suite. Older ages for both zircon and monazite indicate that both minerals are inherited components within the pegmatitic granites. These observations suggest that inheritance, particularly of metamorphic monazite from the source rocks, can make it very difficult to establish meaningful emplacement ages for leucogranites in the absence of corroborative evidence. Although multiple periods of extension are permissible, current timing relationships established in Payer Land suggest that much of the extension responsible for exhumation of deep‐level rocks in the East Greenland Caledonides is younger than 405 Ma.

Direct dating of Adirondack massif anorthosite by U-Pb SHRIMP analysis of igneous zircon: Implications for AMCG complexes

The low abundance of igneous zircon in Proterozoic massif anorthosites has presented a major obstacle to the acquisition of direct absolute ages of crystallization for these important rocks. Indirect dating that relies on zircon ages from associated mangerite-charnockite-granite granitoids assumes that they have a coeval relationship with anorthosite that requires documentation. SHRIMP (sensitive, high-resolution ion-microprobe) U-Pb zircon-dating techniques provide a powerful means for directly dating the small populations of zircons in anorthositic rocks and for resolving problems with inheritance. Within the Adirondack Mountains, 10 samples of massif anorthosite have yielded more-than-sufficient quantities of igneous zircon to establish directly the ages of the region9s classic anorthosite occurrences (e.g., the Marcy and Oregon Dome massifs). In addition, a ferrogabbro, a ferrodiorite, and a coronitic olivine metagabbro, all crosscutting massif anorthosite, were dated. The average age of this suite of 13 anorthositic samples is 1154 ± 6 Ma (MSWD [mean square of weighted deviates] = 0.26, probability = 0.99). In addition, eight associated granitoids have been dated by SHRIMP techniques and complement another five previously dated by multi-grain thermal-ionization mass spectrometry (TIMS) methods. The 13 granitoids yield an average age of 1158 ± 5 (MSWD = 0.89, probability = 0.60) and are broadly coeval with the massif anorthosite. The overlapping ages provide evidence that these rocks constitute a single, composite anorthosite-mangerite-charnockite-granite (AMCG) suite intruded at ca. 1155 Ma, an age corresponding to the ages of major AMCG suites in the Grenville province in Canada (e.g., Morin and Lac St-Jean). Although rocks of the Adirondack AMCG suite are now documented as broadly coeval, it does not follow that the constituent AMCG lithologies were comagmatic. Field relationships and mineral disequilibria in transitional zones are inconsistent with derivation from a single parental magma. Moreover, the presence of older (ca. 1.2–1.3 Ga) inherited cores in some zircons from AMCG granitoids conflicts with derivation of these rocks from magmas that formed anorthosite, gabbro, or ferrodiorite, or jotunite, in which zircons are highly soluble. The slightly older ca. 1158 Ma average age of the mangeritic and charnockitic members of the AMCG suite is consistent with an origin as early lower-crustal anatectites that left behind pyroxene-plagioclase restites. This refractory material then reacted (by assimilation–fractional crystallization [AFC]) with ponded, mantle-derived gabbroic magmas to produce plagioclase-rich crystal mushes with crustal isotopic signatures, as proposed much earlier by R.F. Emslie. These magmas are considered to be parental to the Adirondack anorthosite, and upon ascent they were emplaced in proximity to still hot, earlier mangeritic and charnockitic bodies where they underwent further fractionation. The composite nature of the Marcy massif documents that this process was repeated in several sequential pulses.

Correlations between chemical and age domains in monazite, and metamorphic reactions involving major pelitic phases: an integration of ID-TIMS and SHRIMP geochronology with Y–Th–U X-ray mapping

Chemical mapping and in situ U–Th–Pb analyses reveal a link between age domains and zones of relative yttrium (Y) depletion or enrichment within monazite crystals and are correlated with metamorphic reactions involving garnet. Conventional, small-fraction isotope dilution thermal ionization mass spectrometry (ID-TIMS) and sensitive high-resolution ion microprobe (SHRIMP) techniques were utilized to measure U–Th–Pb isotopic compositions in metamorphic monazite from pelitic rocks of the southern Canadian Cordillera. Monazite ID-TIMS data from individual samples commonly demonstrate a 2 to 25 Ma range in U-Pb ages. This is difficult to reconcile using conventional regression techniques due to complexities such as excess 206Pb or bulk mixing of discrete age domains. Backscattered electron (BSE) imaging and X-ray elemental mapping for Y, Th, and U revealed complex internal zonation within many of the monazite crystals and served as a guide for detailed, in situ (∼30 μm) isotopic analysis using the SHRIMP. The Y maps generally provided the clearest indication of growth or recrystallization domains and were critical for targeting SHRIMP analysis because these relationships were not always clear in BSE, U, and Th images. Moreover, the Y domains consistently correlated with distinct age domains, with up to three or more in some crystals. These data clearly illustrate the cause of age dispersion within the analyzed monazite grains and demonstrate the significance of multiple age domains in metamorphic monazite that may be irreconcilable or misinterpreted when using conventional dating techniques such as ID-TIMS. Recent studies have investigated the interaction between accessory minerals such as monazite and major pelitic phases throughout a metamorphic event. Researchers have begun to focus on the partitioning of Y between monazite and garnet because it is highly compatible in both phases. Due to its greater volume, garnet exerts considerable control over the Y budget available during metamorphism in pelitic rocks. Consequently, this is reflected in the production and consumption of monazite, as it is sensitive to the availability of Y, and is preserved as internal zones of relative Y enrichment or depletion. Thus, precise ages of contrasting Y domains within monazite provided by in situ ion probe analysis may be correlated with metamorphic reactions involving garnet and assigned to points along the P– T path.

Timing of high-grade metamorphism in central Turkey and the assembly of Anatolia

The timing of metamorphism of the crystalline massifs in western and central Turkey (Menderes, Kirșehir, Nigde, Akdag) is important information for understanding the assembly of Anatolia and the thermal–tectonic evolution of the Turkish segment of the Alpine–Himalayan belt. The high-grade basement rocks of the massifs are polymetamorphic, and the complexity of interpreting ages obtained from zircon and other minerals has led to a longstanding debate about the significance and regional extent of Neoproterozoic v. younger (Mesozoic, Cenozoic) metamorphism. New U–Pb SHRIMP (sensitive high-resolution ion microprobe) analyses of monazite from a garnet–sillimanite gneiss in the Kirșehir Massif, central Anatolia, document Late Cretaceous (84.1 ± 0.8 Ma) high-temperature metamorphism. This Late Cretaceous age for central Anatolian metamorphism is at least 50 Ma older than Alpine metamorphism in the Menderes Massif in western Turkey. Models that propose a continuous Anatolide–Tauride tectonic zone, with central Anatolia as a promontory, cannot explain such a large time difference within a small region. Unless the Menderes ages have been reset by Aegean thermal–tectonic events, Anatolia probably formed from a collage of continental fragments rather than by simple collision of an irregular Tethyan platform with Eurasia.

Age constraints on the Paleoproterozoic tectonometamorphic history of the Committee Bay region, western Churchill Province, Canada: evidence from zircon and in situ monazite SHRIMP geochronology

In situ U–Pb sensitive high-resolution ion microprobe (SHRIMP) analyses of monazite from upper amphibolite-facies paragneiss of the Committee Bay supracrustal belt, central Rae domain, Canada, reveal three age populations: ca. 2350, 1850, and 1780 Ma. The ca. 1850 Ma age also corresponds to growth of low Th/U zircon as indicated by U–Pb SHRIMP analyses of zircon separates from melanosome and leucosome. The contextual advantage of the in situ monazite analysis allows evaluation of the geochronological data in terms of the regional structural and metamorphic evolution. The region is dominated by a northeast-striking S2 (±S1) fabric, axial planar to tight, northwest-vergent F2 folds. Early garnet is enveloped by this biotite–sillimanite ± cordierite S2 fabric. GarnetI hosts ca. 1850 Ma monazite inclusions (with ca. 2350 Ma cores), placing a maximum age on garnetI growth and S2 development. D2 metamorphic conditions progressed through ~3.5 kbar (1 kbar = 100 MPa) and 600 °C to near-peak conditions of ~5 kbar and 675 °C. A minimum age for S2 is provided by unstrained ca. 1820 Ma monzogranite that locally, and regionally, truncates S2. Dominantly ca. 1780 Ma matrix monazite is interpreted to date post-S2 garnetII and cordierite, which record ~5 kbar and 675 °C. These data indicate that the Committee Bay region experienced penetrative D2 tectonometamorphism at ca. 1850–1820 Ma, with a subsequent static overprint. The absence of a ca. 1.85 Ga plutonic suite in the region suggests that low-pressure metamorphism was a response to thick-skinned crustal thickening initiated at ca. 1870 Ma. The new data highlight the importance of Paleoproterozoic reworking of the central Rae domain in the hinterland of the Trans-Hudson orogen.

SHRIMP dating of zircons in eclogite from the Variscan basement in north-eastern Sardinia (Italy)

SHRIMP (Sensitive High-Resolution Ion Microprobe) U-Pb ages of zircons from a single sample of mafic eclogite (Punta de li Tulchi, Sardinia, Italy) are reported. The study under cathodoluminescence (CL) reveals two groupsof metamorphic zircons and the SHRIMP analyses allow recognition of three ages: 1) 453 ′ 14 Ma; 2) 400 ′ 10 Ma, and 3) 327 ′ 7 Ma. The age of 453 ′ 14 Ma could be that of the magmatic protolith and an age of 327 ′ 7 Ma can reasonably be attributed to the main Variscan collisional event in Sardinia, which produced Barrovian-type metamorphism, and retrogression of eclogite under amphibolite-facies metamorphism. The intermediate age 400 ′ 10 Ma is difficult to interpret and it could represent either the age of the eclogite facies metamorphism or it is a result of Pb-loss during the main Variscan event at 327 ′ 7 Ma.