U-total Pb Research Articles

Возраст монацита из метапелитовых сланцев серий Атомфьелла и Моссель (полуостров Ню-Фрисланд, о. Западный Шпицберген) по данным Th-U-Pb (CHIME)-датирования

The article presents the results of CHIME (chemical Th–U-total Pb isochron method) dating of monazite from metamorphic rocks of Precambrian complexes located in the north part of the West Spitsbergen Island. It is shown that for rocks of Atomfjella Series and Mossel Series, monazite ages are coeval within error (Atomfjella Series: 381 ± 18 Ma, Mossel Series: 377 ± 23 Ma). These age estimates show that metamorphism of the crystalline basement possibly took place during the Late Caledonian orogeny.

Geochronological constraints on uranium mineralization within the Hüttenberg siderite deposit (Eastern Alps, Austria)

A small-scale hydrothermal uranium mineralization hosted within the siderite deposit at Hüttenberg (Eastern Alps, Austria) was re-investigated using modern scanning electron microscope (SEM) and microprobe (EMPA) methods. The uranium mineralization comprises brannerite, coffinite and uraninite, and is spatially associated with Fe-, Ni- and Co-arsenides (loellingite, rammelsbergite, safflorite), bismuth and bismuthinite, as well as rare accessory silver-bearing minerals and gold locally. The U-bearing fluids also carried P, Y and REEs, which precipitated as xenotime and P–Y coffinite. The uranium mineralization paragenetically postdates the metasomatic formation of the host siderite ore.Uraninite allows for precise single spot EMPA ages to be calculated (±2 Ma) due to its high radiogenic lead content. There is an excellent internal consistency in the uraninite data with calculated dates ranging between 77 and 84 Ma. We interpret that these record the crystallization age of the uraninite, and that any influences of lead loss or common Pb are minimal. Brannerite was also analyzed but U-total Pb microprobe data scatter (c.30–80 Ma) with evidence of lead loss, particularly in altered domains. It does, however, provide a minimum constraint on the timing of mineralization that is consistent with the crystallization age of the uraninite. Coffinite proved completely unsuitable for U-total Pb geochronology due to extreme lead loss. However, hydrothermal xenotime yields a U-total Pb age of 78 ± 5 Ma, consistent with the uraninite age. Therefore, the U-total Pb geochronological data support a late Cretaceous age for the U-mineralization of c. 80 Ma. Considering this timing constraint together with other geological and indirect thermochronological aspects, we conclude that the siderite deposit of Hüttenberg is older and formed at c. 90−80 Ma.

Constraining paleoproterozoic (∼1.7 Ga) collisional orogenesis between the Eastern Dharwar and Bastar cratons: New Sm–Nd garnet isochron and Th–U-total Pb monazite chemical ages from the Bhopalpatnam orogen, central India

The timing of assembly into India by amalgamation of cratonic fragments, is controversial with estimates ranging from Tonian, earliest Mesoproterozoic to Neoarchean. All models have as the nucleus the South Indian Block of Dharwar, Bastar and Singhbhum cratons. Although the time of assembly of the Bastar and Singhbhum cratons is uncertain, the implication of a proposed 1891–1883 Ma-aged Large Igneous Province, straddling the Bastar and Eastern Dharwar cratons is that their amalgamation occurred by Paleoproterozic time (≥1.9 Ga ago) and is at variance with the ages reported from the Bhopalpatnam granulite belt between 1.7-1.6 Ga ago. To test whether this belt formed by convergent collisional tectonics along the margins of these two cratons, petrochronologic analysis was conducted on high-grade rocks of this belt using garnet Sm–Nd and texturally-controlled Th–U-total Pb monazite chronometry.The whole rock-garnet Sm–Nd isochron from garnet-clinopyroxene-amphibolite in migmatite (M1a-stage), yielded an isochron age of 1692±13Ma with Ndi of 0.51042±0.00006 (2σ, MSWD = 2.5 n = 4) whereas a whole rock-garnet Sm–Nd isochron from interlayered garnet-sillimanite-rutile metapelite (M1b-stage), yielded an isochron age of 1675±7 Ma with Ndi of 0.50997±0.00003 (2σ, MSWD = 1.9 n = 3) Monazite inclusions within garnet porphyroblasts in this metapelite have a weighted mean age of 1703±28 Ma (2σ, MSWD = 0.95, n = 8), within error of the age of monazite inclusions within matrix biotite at 1665±16 Ma (2σ, MSWD = 0.56, n = 17). Post M1b stage lower temperature hydration-retrogression events likely occurred by late Mesoproterozoic time. The metamorphism followed a clockwise P-T path (high-pressure amphibolite-facies stage M1a P ≥ 12 kbar at ∼750 °C, followed by M1b granulite-facies stage with T ≥ 850 °C at lower pressures), interpreted as a response to crustal thickening during Paleoproterozoic continental collision. We propose that the Bhopalpatnam orogen is the exhumed locus of the leading edge of the Archean Bastar craton that was thrust under the EDC craton.

U-total Pb timing constraints on the emplacement of the granitoid pluton of Stolpen, Germany

Abstract Monazite from the Stolpen monzogranite (SE Germany) was studied to constrain the Th-U-total Pb age of pluton formation. Monazite grains demonstrate subtle to distinct patchy zoning related to slight compositional variations. Textural and compositional characteristics indicate that the monazite formed in a single magmatic event in a slightly heterogeneous system, and was only weakly affected by secondary alteration, which did not disturb the Th-U-Pb system. Chemical dating of the monazite gave a consistent age of 299 ± 1.7 Ma. The current study presents the first geochronological data for the Stolpen granite. It provides evidence that Stolpen is the youngest Variscan granitic intrusion in the Lusatian Granodiorite Complex and indicates that magmatic activity related to post-collisional extension in this region lasted at least 5my longer than previously assumed

U-Pb zircon and CHIME monazite dating of granitoids and high-grade metamorphic rocks from the Eastern and Peninsular Thailand — A new report of Early Paleozoic granite

Abstract In order to understand the age and tectonic framework of Eastern to Peninsular Thailand from the viewpoint of basement (metamorphic and plutonic) geology, the LA-ICP-MS U-Pb zircon dating and the chemical Th–U-total Pb isochron method (CHIME) monazite dating were performed in the Khao Chao, Hub-Kapong to Pran Buri, and Khanom areas in Eastern to Peninsular Thailand. The LA-ICP-MS U-Pb zircon dating of the garnet-hornblende gneiss from the Khao Chao area gave 229 ± 3 Ma representing the crystallization age of the gabbro, and that of the garnet-biotite gneisses gave 193 ± 4 Ma representing the timing of an upper amphibolite facies metamorphism. The CHIME monazite dating of pelitic gneiss from the Khao Chao gneiss gave scattered result of 68 ± 22 Ma, due to low PbO content and rejuvenation of older monazite grains during another metamorphism in the Late Cretaceous to Tertiary time. The U-Pb ages of zircon from the Hua Hin gneissic granite in the Hub-Kapong to Pran Buri area scatter from 250 Ma to 170 Ma on the concordia. Granite crystallization was at 219 ± 2 Ma, followed by the sillimanite-grade regional metamorphism at 185 ± 2 Ma. Monazite in the pelitic gneiss from this area also preserves Early to Middle Jurassic metamorphism and rejuvenation by later contact metamorphism by non-foliated granite or by another fluid infiltration event in the Late Cretaceous to Tertiary time. The Khao Dat Fa granite from the Khanom area of Peninsular Thailand gave a U-Pb zircon age of 477 ± 7 Ma. This is the second oldest granite pluton ever reported from Thailand, and is a clear evidence for the Sibumasu block having a crystalline basement that was formed during the Pan-African Orogeny. The Khao Pret granite gives U-Pb zircon concordia age of 67.5 ± 1.3 Ma, which represents the timing of zircon crystallization from the granitic melt and accompanied sillimanite-grade contact metamorphism against surrounding metapelites and gneisses. Metamorphic rocks in the Doi Inthanon area also share the similar plutono-metamorphic history with the Khanom and the Hub-Kapong to Pran Buri areas. This suggests that these three areas belong to the Sibumasu block, and the Sibumasu block records similar plutono-metamorphic history from Northern to Peninsular Thailand. Relative abundance of oceanic components in the Khao Chao gneiss, their Late Triassic magmatic ages, and the Early Jurassic metamorphic ages prefer the interpretation that the Khao Chao gneiss belongs to the Sukhothai Arc.

Large-scale metal zoning in a late-Precambrian skarn-type mineralization, Wadi Kid, SE Sinai, Egypt

A Precambrian skarn-type mineralization is recently discovered in the Wadi Kid area in southeast Sinai, Egypt. Two sulfide ore types define large scale metal zoning; Cu–Zn–Co in calc-silicate rocks and Zn–Pb–As–Ag in metapelites. The sulfides and host rocks underwent amphibolite facies metamorphism (2.1–4.2kbar and 500–620°C). Dating by the chemical Th–U-total Pb isochrone method yields an Th–Pb isochrone age of 660±25Ma for metamorphic monazite from metapelites. Overall structural and textural relationships of silicate and sulfide minerals favor syn-tectonic formation during granitoids emplacement in a continental margin setting. Large-scale metal zoning reflects variable distances from the causative pluton(s). The Wadi Kid area is highly prospective for Cu, Zn, Pb and Ag mineralization.

Alteration of crystalline zircon solid solutions: a case study on zircon from an alkaline pegmatite from Zomba–Malosa, Malawi

A natural, altered zircon crystal from an alkaline pegmatite from the Zomba–Malosa Complex of the Chilwa Alkaline Province in Malawi has been studied by a wide range of analytical techniques to understand the alteration process. The investigated zircon shows two texturally and chemically different domains. Whereas the central parts of the grain (zircon I) appear homogeneous in backscattered electron images and are characterised by high concentrations of trace elements, particularly Th, U, and Y, the outer regions (zircon II) contain significantly less trace elements, numerous pores, and inclusions of thorite, ytttrialite, and fergusonite. Zircon II contains very low or undetectable concentrations of non-formula elements such as Ca, Al, and Fe, which are commonly observed in high concentrations in altered radiation-damaged zircon. U–Pb dating of both zircon domains by LA-ICPMS and SHRIMP yielded statistically indistinguishable U–Pb weighted average ages of 119.3 ± 2.1 (2σ) and 118 ± 1.2 (2σ) Ma, respectively, demonstrating that the zircon had not accumulated a significant amount of self-irradiation damage at the time of the alteration event. Electron microprobe dating of thorite inclusions in zircon II yielded a Th–U-total Pb model age of 122 ± 5 (2σ) Ma, supporting the age relationship between both zircon domains. The hydrothermal solution responsible for the alteration of the investigated zircon was alkaline and rich in CO3 2−, as suggested by the occurrence of REE carbonates and CO2-bearing fluid inclusions. The alteration of the crystalline, trace element-rich zircon is explained by an interface-coupled dissolution-reprecipitation mechanism. During such a process, the congruent dissolution of the trace element-rich parent zircon I was spatially and temporally coupled to the precipitation of the trace element-poor zircon II at an inward moving dissolution-precipitation front. The driving force for such a process was merely the difference between the solubility of the trace element-rich and -poor zircon in the hydrothermal solution. The replacement process and the occurrence of mineral inclusions and porosity in the product zircon II is explained by the thermodynamics of solid solution-aqueous solution systems.

Petrology and CHIME geochronology of Pan-African high K and Sr/Y granitoids in the Nkambe area, Cameroon

The Central African Belt in the Nkambe area, northwestern Cameroon represents a collisional zone between the Saharan metacraton and the Congo craton during the Pan-African orogeny, and exposes a variety of granitoids including foliated and massive biotite monzogranites in syn- and post-kinematic settings. Foliated and massive biotite monzogranites have almost identical high-K calc-alkaline compositions, with 73–67 wt.% SiO 2, 17–13 wt.% Al 2O 3, 2.1–0.9 wt.% CaO, 4.4–2.7 wt.% Na 2O and 6.3–4.4 wt.% K 2O. High concentrations of Rb (264–96 ppm), Sr (976–117 ppm), Ba (3680–490 ppm) and Zr (494–99 ppm), with low concentrations of Y (mostly< 20 ppm with a range 54–6) and Nb (up to 24 ppm) suggest that the monzogranites intruded in collisional and post-collisional settings. The Sr/Y ratio ranges from 25 to 89. K, Rb and Ba resided in a single major phase such as K-feldspar in the source. Garnet was present in the source and remained as restite at the site of magma generation. This high K 2O and Sr/Y granitic magma was generated by partial melting of a granitic protolith under high-pressure and H 2O undersaturated conditions where garnet coexists with K-feldspar, albitic plagioclase. CHIME (chemical Th–U-total Pb isochron method) dating of zircon yields ages of 569 ± 12–558 ± 24 Ma for the foliated biotite monzogranite and 533 ± 12–524 ± 28 Ma for the massive biotite monzogranite indicating that the collision forming the Central African Belt continued in to Ediacaran (ca 560 Ma).

Monazite, iron oxide and barite exsolutions in apatite aggregates from CCSD drillhole eclogites and their geological implications

We have identified abundant exsolutions in apatite aggregates from eclogitic drillhole samples of the Chinese Continental Scientific Drilling (CCSD) project. Electron microscope and laser Raman spectroscopy analyses show that the apatite is fluorapatite, whereas exsolutions that can be classified into four types: (A) platy to rhombic monazite exsolutions; (B) needle-like hematite exsolutions; (C) irregular magnetite and hematite intergrowths; and (D) needle-like strontian barite exsolutions. The widths and lengths of type A monazite exsolutions range from about 6–10 μm (mostly 6 μm) and about 50–75 μm, respectively. Type B exsolutions are parallel with the C axis of apatite, with widths ranging from 0.5 to 2 μm, with most around 1.5 μm, and lengths that vary dramatically from 6 to 50 μm. Type C exsolutions are also parallel with the C axis of apatite, with lengths of ∼30–150 μm and widths of ∼10 to 50 μm. Type D strontian barite exsolutions coexist mostly with type B hematite exsolutions, with widths of about 9 μm and lengths of about 60–70 μm. Exsolutions of types B, C and D have never been reported in apatites before. Most of the exsolutions are parallel with the C axis of apatite, implying that they were probably exsolved at roughly the same time. Dating by the chemical Th–U-total Pb isochron method (CHIME) yields an U–Pb isochron age of 202 ± 28 Ma for monazite exsolutions, suggesting that these exsolutions were formed during recrystallization and retrograde metamorphism of the exhumed ultrahigh pressure (UHP) rocks. Quartz veins hosting apatite aggregates were probably formed slightly earlier than 202 Ma. Abundant hematite exsolutions, as well as coexistence of magnetite/hematite and barite/hematite in the apatite, suggest that the oxygen fugacity of apatite aggregates is well above the sulfide-sulfur oxide buffer (SSO). Given that quartz veins host these apatite aggregates, they were probably deposited from SiO 2-rich hydrous fluids formed during retrogression of the subducted slab. Such SiO 2-rich hydrous fluids may act as an oxidizing agent, a feasible explanation for the high oxygen fugacity in convergent margin systems.

Electron microprobe dating of monazite substantiates ages of major geological events in the southern Brazilian shield

To obtain the chemical Th ∗–Pb isochron ages and surface maps of monazite crystals in igneous and metamorphic rocks from the southern Brazilian Shield, we employ Th–U-total Pb dating by an electron probe microanalyzer. The ages of two Trans-Amazonian metamorphic events are given by a felsic, garnet-bearing granulite from the Santa Maria Chico granulitic complex. The age of the first event, at approximately 2.35 Ga, was obtained by surface mapping in a grain included in garnet. The dating of the second event, 1899±43 Ma, is in agreement with previous data obtained in zircon crystals with sensitive high-resolution ion microprobe. Other determinations belong to the Brasiliano cycle. In the São Gabriel block, an age of 643±129 Ma was obtained on monazite from a staurolite-garnet schist of the Cambaizinho Formation, whereas a staurolite-bearing schist from the Passo Feio complex yielded a 510±68 Ma age. Several units in the Dom Feliciano belt were dated, including the biotite-sillimanite gneisses of the Várzea do Capivarita complex (552±90 Ma), the sillimanite-garnet gneisses of Camboriú complex (565±77 Ma), the Três Figueiras granite (558±57 Ma), and the Plaza Itapema granite (545±55 Ma). The ages presented in this study, obtained through monazite chemical dating, are confirmed through comparison with previous data regarding zircon crystals from the same geological units.

Electron microprobe observations of Pb diffusion in metamorphosed detrital monazites

Electron microprobe analyses have been made on monazite grains from paragneiss samples in the andalusite-sillimanite transition (620 ± 15°C) and sillimanite-orthoclase (680 ± 15°C) zones of the Cretaceous Ryoke metamorphic belt, southwest Japan. Monazites from pelitic gneisses are of metamorphic origin, euhedral to subhedral and chronologically homogeneous, giving chemical ThU-total Pb isochron (CHIME) ages of 98.8 ± 3.3–98.0 ± 3.2 Ma. Two psammitic gneisses of individual metamorphic grade contain both metamorphic monazite grains and detrital ones as old as ca. 1700 Ma. Most detrital monazite grains are heterogeneous in the ThO 2 and UO 2 concentrations and have multiple or single rims as young as ca. 100 Ma. Several detrital monazite grains are well rounded in form, exhibit homogeneous Th and U distributions and show a Pb diffusion profile in the margin. The width of the diffusion zones is approximately constant throughout grains from each psammitic gneiss: 18–22 μm for 620°C and 48–58 μm for 680°C. Assuming the isothermal diffusion of Pb from homogeneous monazite spheres during a 5 Ma duration of peak metamorphism, we obtain diffusion coefficients of 1.9(±0.3) × 10 −21 and 1.5(±0.3) × 10 −20 cm 2/s at 620°C and 680°C, respectively. These data derive an activation energy of 2.44(+2.85/−1.26) × 10 5 J/mol and a frequency factor of 3.4 × 10 −7 (8.5 × 10 −12–2.2 × 10 7 cm 2/s, taking account of uncertainties of ±15°C in the temperatures and of ±20% in the diffusion coefficients. The diffusion parameters obtained from natural samples in this study provide a reliable insight into the closure temperature for Pb in monazite that has been poorly understood so far.

On the latent heat of carbonic acid gas

We have identified abundant exsolutions in apatite aggregates from eclogitic drillhole samples of the Chinese Continental Scientific Drilling (CCSD) project. Electron microscope and laser Raman spectroscopy analyses show that the apatite is fluorapatite, whereas exsolutions that can be classified into four types: (A) platy to rhombic monazite exsolutions; (B) needle-like hematite exsolutions; (C) irregular magnetite and hematite intergrowths; and (D) needle-like strontian barite exsolutions. The widths and lengths of type A monazite exsolutions range from about 6–10 μm (mostly 6 μm) and about 50–75 μm, respectively. Type B exsolutions are parallel with the C axis of apatite, with widths ranging from 0.5 to 2 μm, with most around 1.5 μm, and lengths that vary dramatically from 6 to 50 μm. Type C exsolutions are also parallel with the C axis of apatite, with lengths of ∼30–150 μm and widths of ∼10 to 50 μm. Type D strontian barite exsolutions coexist mostly with type B hematite exsolutions, with widths of about 9 μm and lengths of about 60–70 μm. Exsolutions of types B, C and D have never been reported in apatites before. Most of the exsolutions are parallel with the C axis of apatite, implying that they were probably exsolved at roughly the same time. Dating by the chemical Th–U-total Pb isochron method (CHIME) yields an U–Pb isochron age of 202 ± 28 Ma for monazite exsolutions, suggesting that these exsolutions were formed during recrystallization and retrograde metamorphism of the exhumed ultrahigh pressure (UHP) rocks. Quartz veins hosting apatite aggregates were probably formed slightly earlier than 202 Ma. Abundant hematite exsolutions, as well as coexistence of magnetite/hematite and barite/hematite in the apatite, suggest that the oxygen fugacity of apatite aggregates is well above the sulfide-sulfur oxide buffer (SSO). Given that quartz veins host these apatite aggregates, they were probably deposited from SiO2-rich hydrous fluids formed during retrogression of the subducted slab. Such SiO2-rich hydrous fluids may act as an oxidizing agent, a feasible explanation for the high oxygen fugacity in convergent margin systems.