We introduce and propose zircon M257 as a future reference material for the determination of zircon U-Pb ages by means of secondary ion mass spectrometry. This light brownish, flawless, cut gemstone specimen from Sri Lanka weighed 5.14 g (25.7 carats). Zircon M257 has TIMS-determined, mean isotopic ratios (2s uncertainties) of 0.09100 ± 0.00003 for 206pb/238U and 0.7392 ± 0.0003 for 207pb/235U. Its 206pb/238U age is 561.3 ± 0.3 Ma (unweighted mean, uncertainty quoted at the 95% confidence level); the U-Pb system is concordant within uncertainty of decay constants. Zircon M257 contains ∼ 840 μg g−1 U (Th/U ∼ 0.27). The material exhibits remarkably low heterogeneity, with a virtual absence of any internal textures even in cathodoluminescence images. The uniform, moderate degree of radiation damage (estimated from the expansion of unit-cell parameters, broadening of Raman spectral parameters and density) corresponds well, within the “Sri Lankan trends”, with actinide concentrations, U-Pb age, and the calculated alpha fluence of 1.66 × 1018 g−1. This, and a (U+Th)/He age of 419 ± 9 Ma (2s), enables us to exclude any unusual thermal history or heat treatment, which could potentially have affected the retention of radiogenic Pb. The oxygen isotope ratio of this zircon is 13.9%o VSMOW suggesting a metamorphic genesis in a marble or calc-silicate skarn.



Nous presentons et proposons a la communaute le zircon M257 qui pourrait devenir un materiau de reference pour la determination des âges U-Pb sur zircon par spectrometrie de masse a ions secondaires (SIMS). Ce specimen scie d'une gemme brun clair, sans defauts, provenant du Sri Lanka, pese 5.14 g (25.7 carats). Les moyennes des rapports isotopiques (et les incertitudes associees 2s) mesurees par TIMS sur ce zircon M257 sont 0.09100 ± 0.00003 en 206pb/238U et 0.7392 ± 0.0003 en 207pb/235U. Son âge 206pb/238U est de 561.3 ± 0.3 Ma (moyenne non ponderee, incertitude: 95% intervalle de confiance); le systeme U-Pb y est concordant, aux erreurs sur les constantes de desintegration pres. Le zircon M257 contient ∼ 840 μg g−1 U (Th/U ∼ 0.27). Ce materiel montre de tres faibles heterogeneites, avec une quasi absence de textures internes, meme a I'examen par cathodoluminescence. Le degre de dommage lie aux radiations (estime par I'expansion des parametres de la cellule unitaire, I'elargissement des parametres des spectres Raman et la densite) est modere et uniforme. II se correle bien, (sur I'alignement “Sri Lanka”) avec les concentrations en actinides, I'âge U-Pb et la fluence calculee de 1.66 × 1018 g−1 Ceci, couplea son âge (U+Th)/He de 419 ± 9 Ma (2s), nous permet d'exclure I'existence d'evenement thermique passe, ou de traitement par la chaleur, qui aurait potentiellement pu affecter le processus de retention de Pb radiogenique. Le rapport isotopique de I'oxygene de ce zircon est 13.9%o VSMOW, suggerant une genese par metamorphisme, au sein d'un marbre ou d'un skarn calco-silicate.

Zircon M257 ‐ a Homogeneous Natural Reference Material for the Ion Microprobe U‐Pb Analysis of Zircon

The Re–Os (rhenium–osmium) chronometer applied to molybdenite (MoS2) is now demonstrated to be remarkably robust, surviving intense deformation and high-grade thermal metamorphism. Successful dating of molybdenite is dependent on proper preparation of the mineral separate and analysis of a critical quantity of molybdenite, unique to each sample, such that recognized spatial decoupling of 187Re parent and 187Os daughter within individual molybdenite crystals is overcome. Highly precise, accurate and reproducible age results are derived through isotope dilution and negative thermal ion mass spectrometry (ID-NTIMS). Spatial decoupling of parent–daughter precludes use of the laser ablation ICP-MS microanalytical technique for Re–Os dating of molybdenite. The use of a reference or control sample is necessary to establish laboratory credibility and for interlaboratory comparisons. The Rb–Sr, K–Ar and 40Ar/39Ar chronometers are susceptible to chemical and thermal disturbance, particularly in terranes that have experienced subsequent episodes of hydrothermal/magmatic activity, and therefore should not be used as a basis for establishing accuracy in Re–Os dating of molybdenite, as has been done in the past. Re–Os ages for molybdenite are almost always in agreement with observed geological relationships and, when available, with zircon and titanite U–Pb ages. For terranes experiencing multiple episodes of metamorphism and deformation, molybdenite is not complicated by overgrowths as is common for some minerals used in U–Pb dating (e.g. zircon, monazite, xenotime), nor are Re and Os mobilized beyond the margins of individual crystals during solid-state recrystallization. Moreover, inheritance of older molybdenite cores, incorporation of common Os, and radiogenic Os loss are exceedingly rare, whereas inheritance, common Pb and Pb loss are common complications in U–Pb dating techniques. Therefore, molybdenite ages may serve as point-in-time markers for age comparisons.

The remarkable Re-Os chronometer in molybdenite : how and why it works

Constraints on the timing of uplift of the Yanshan Fold and Thrust Belt, North China

https://doc.rero.ch/record/5022/files/1_raumer_gdm.pdf

Gondwana-derived microcontinents — the constituents of the Variscan and Alpine collisional orogens

The East European Craton (Baltica) before and during the assembly of Rodinia

https://homepages.uni-tuebingen.de/belka/papers/zirkon-paper.pdf

U-Pb single-grain dating of detrital zircon in the Cambrian of central Poland: implications for Gondwana versus Baltica provenance studies

The Suwalki anorthosite massif, located in extreme northeast Poland beneath more than a kilometer of Phanerozoic cover, hosts major Fe-Ti-V deposits. These deposits, discovered in 1962, are contained in Fe and Ti oxide minerals that coexist with subordinate quantities of Fe, Cu, Ni, and Co sulfides in massif-style anorthosites, norites, and gabbronorites. Accessibility and other considerations preclude development of this natural resource in the present economic climate. Detailed work by Polish geologists during the last 35 years provides a sound geologic framework for this Re-Os study of the age and origin of oxide and sulfide deposits associated with a major, but lesser known anorthosite massif. Rhenium and osmium abundances and Os isotopic compositions were measured for nine sulfides and four titanomagnetites from the Suwalki anorthosite massif. The titanomagnetites are over an order of magnitude lower in Re (0.4–1.5 ppb) and Os (0.036–0.144 ppb) concentrations than co-precipitated pyrrhotite, pyrite, and chalcopyrite that yield consistent concentrations for Re (30–55 ppb) and Os (1–6 ppb). Parallel lines connecting co-existing titanomagnetite and sulfides have slopes of ∼1 on Re versus common Os concentration plots, indicating that both Re and Os behave similarly during crystallization in their high preference for any sulfide phase over magnetite. Samples from three deposits within the anorthosite massif were analyzed. An age of 1559 ± 37 Ma (n=10) with an initial 187Os/188Os of 1.16 ± 0.06 for the Jezioro Okrągle and Krzemianka deposits is essentially identical to an age of 1556 ± 94 Ma (n=3) for the Udryn deposit. Udryn, however, yielded a marginally lower initial 187Os/188Os of 0.87 ± 0.20. The high initial 187Os/188Os combined with the Proterozoic Re-Os age indicates that the source for Suwalki oxides-sulfides is older crust, and hypothetically, could involve Archean rocks. An average crustal value of 50 for 187Re/188Os yields a 2777 Ma age for Suwalki source rocks. Widespread Phanerozoic cover severely limits knowledge of basement rocks in Poland, however, and no Archean rocks are known in the immediate region. More likely, 187Re/188Os ratios may be higher than average continental crust, reflecting mafic crust in the source, and may move the source age for Suwalki anorthosite and mineral deposits toward younger values that easily include ∼2.0 Ga Proterozoic rocks. This more favorable case also accommodates Paleoproterozoic Nd model ages. Regardless of Archean or Proterozoic source age, the high initial 187Os/188Os ratios derived from the Re-Os isochron indicate that the source for the oxide-sulfide mineral deposits is more likely the crust and not the mantle. Given that these deposits are clearly magmatic, the Re-Os results add a new dimension to the long-standing “origin of anorthosite” problem, implying a crustal source for the anorthosite as well. The 1559 Ma Suwalki age is compatible with a well-exposed east-west band of 1530-1660 Ma rapakivi granite-anorthosite magmatism to the immediate north, transecting western Russia, southern Finland, Estonia and Latvia, and central Sweden. In particular, the age and isotopic character of Suwalki are not unlike those of the well-studied Salmi rapakivi granite-anorthosite batholith in western Russia (Karelia).

Re-Os study of Fe-Ti-V oxide and Fe-Cu-Ni sulfide deposits, Suwałki Anorthosite Massif, northeast Poland

http://homepages.uni-tuebingen.de/belka/papers/Mazury_Complex.pdf

U /Pb and Ar /Ar geochronology of anorogenic granite magmatism of the Mazury complex, NE Poland

Ferro-potassic A-type granites and related rocks in NE Poland and S Lithuania: west of the East European Craton

Deeply buried 1.5 Ga Polish anorthosites, accessible only by bore holes, reveal diagnostic features of some massif-type anorthosites (polybarism, jotunitic parent magma), diapirically emplaced in the mid crust together with the rapakivi granites of the EW-trending Mazury complex, intruded along a major crustal discontinuity. Geochemical modelling and isotope data corroborate recent experimental work on the basaltic system in dry conditions: the source rock of the parental magma is a gabbronorite, necessarily lying in the lower crust. Since no Archaean crust is known in the region, high initial 188 Os ⁄ 187 Os ratios for sulphide-oxide isochrons and negative eNd values are best accounted for by melting a � 2.0 Ga mafic crust.

/pdf/the-north-eastern-polish-anorthosite-massifs-petrological-2s68ccuf3s.pdf

Janina Wiszniewska

Papers

U-Pb single-grain dating of detrital zircon in the Cambrian of central Poland: implications for Gondwana versus Baltica provenance studies

Re-Os study of Fe-Ti-V oxide and Fe-Cu-Ni sulfide deposits, Suwałki Anorthosite Massif, northeast Poland

U /Pb and Ar /Ar geochronology of anorogenic granite magmatism of the Mazury complex, NE Poland

Ferro-potassic A-type granites and related rocks in NE Poland and S Lithuania: west of the East European Craton

The north-eastern Polish anorthosite massifs: petrological, geochemical and isotopic evidence for a crustal derivation