Problems in obtaining precise and accurate Sr isotope analysis from geological materials using laser ablation MC-ICPMS

This paper reviews the problems encountered in eleven studies of Sr isotope analysis using laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) in the period 1995–2006. This technique has been shown to have great potential, but the accuracy and precision are limited by: (1) large instrumental mass discrimination, (2) laser-induced isotopic and elemental fractionations and (3) molecular interferences. The most important isobaric interferences are Kr and Rb, whereas Ca dimer/argides and doubly charged rare earth elements (REE) are limited to sample materials which contain substantial amounts of these elements. With modern laser (193 nm) and MC-ICPMS equipment, minerals with >500 ppm Sr content can be analysed with a precision of better than 100 ppm and a spatial resolution (spot size) of approximately 100 μm. The LA MC-ICPMS analysis of 87Sr/86Sr of both carbonate material and plagioclase is successful in all reported studies, although the higher 84Sr/86Sr ratios do suggest in some cases an influence of Ca dimer and/or argides. High Rb/Sr (>0.01) materials have been successfully analysed by carefully measuring the 85Rb/87Rb in standard material and by applying the standard-sample bracketing method for accurate Rb corrections. However, published LA-MC-ICPMS data on clinopyroxene, apatite and sphene records differences when compared with 87Sr/86Sr measured by thermal ionisation mass spectrometry (TIMS) and solution MC-ICPMS. This suggests that further studies are required to ensure that the most optimal correction methods are applied for all isobaric interferences

Combined U-Pb and Lu-Hf isotope analyses by laser ablation MC-ICP-MS : methodology and applications

O sistema isotópico Lutécio-Hafnio representa uma das ferramentas mais recentes e poderosas para estudos isotópicos e geocronológicos. Análises combinadas in situ de U-Pb e Lu-Hf sobre zircão pelo LA-MC-ICP-MS permitem caracterizar isotopicamente o magma onde ele cristalizou, fornecendo valiosas informações para estudos de proveniência de sedimento e de evolução crustal. Nesse trabalho descrevemos a sistemática de Lu-Hf pelo LA-MC-ICP-MS implantada no laboratório de Geocronologia da Universidade de Brasília e reportamos os resultados obtidos de repetidas análises de três padrões de zircão: GJ-1 = 0.282022 ± 11 (2SD, n=56), Temora 2 = 0.282693 ± 14 (2SD, n=25) and UQ-Z = 0.282127 ± 33 (2SD, n=11). Foi também caracterizada arazão isotópica 176Hf/177Hf(0.282352 ± 22, 2SD, n=14) de um zircão usado como padrão interno do laboratório. Como aplicação geológica, analisamos dois zircões complexos selecionados a partir de uma amostra de migmatito da Província de Borborema, NE do Brasil. Sobre a base dos dados U-Pb e Lu-Hf foram identificados em ambos os zircões dois eventos de cristalização. Um evento mais antigo de 2.05 Ga nos núcleos herdados, representa um evento magmático Paleoproterozoico bem conhecido na Província Borborema. Um segundo evento de ~ 575 Ma, reconhecido nas bordas, representa um evento magmático-metamórfico Neoproterozóico (Brasiliano). ________________________________________________________________________________________ ABSTRACTThe Lutetium-Hafnium isotopic system represents one of the most innovative and powerful tools for geochronology and isotopic studies. Combined U-Pb and Lu-Hf in situ analyses on zircon by LA-MC-ICP-MS permit to characterize isotopically the host magma from which it crystallized furnishing significant information for sediment provenance and crustal evolution studies. In this paper e describe the Lu-Hf systematic by LA-MC-ICP-MS developed in the laboratory of Geochronology of the University of Brasilia and report the results obtained by repeated analyses of 176Hf/177Hf isotopic ratio of three zircon standards: GJ-1 = 0.282022 ± 11 (n=56), Temora 2 = 0.282693 ± 14 (n=25) and UQZ = 0.282127 ± 33 (n=11). The 176Hf/177Hf ratio (0.282352 ± 22, n=14) of gem quality zircon used as in-house standard have been also characterized. As a geological application, we analyzed two complex zircons selected from a migmatitic rocks from the Borborema Province, NE Brazil. On the basis of U-Pb and Lu-Hf data, two main crystallization events have been identified in both studied zircons. An older event at ca. 2.05 Ga recognized in the inherited cores represents a well-characterized paleoproterozoic magmatic event that affected the whole Borborema Province. A second crystallization event at ~ 575 Ma, recognized at the rims, represents a Neoproterozoic (Brazilian) high grade metamorphic-magmatic event

Tectonic evolution of the southern margin of the Amazonian craton in the late Mesoproterozoic based on field relationships and zircon U-Pb geochronology

New U-Pb zircon geochronological data integrated with field relationships and an airborne geophysical survey suggest that the Nova Brasilândia and Aguapeí belts are part of the same monocyclic, metaigneous and metasedimentary belt formed in the late Mesoproterozoic (1150 Ma-1110 Ma). This geological history is very similar to the within-plate origin of the Sunsás belt, in eastern Bolivia. Thus, we propose that the Nova Brasilândia, Aguapeí and Sunsás belts represent a unique geotectonic unit (here termed the Western Amazon belt) that became amalgamated at the end of the Mesoproterozoic and originated through the reactivation of a paleo-suture (Guaporé suture zone) in an intracontinental rift environment. Therefore, its geological history involves a short, complete Wilson cycle of ca. 40 Ma. Globally, this tectonic evolution may be related with the final breakup of the supercontinent Columbia. Mafic rocks and trondhjemites in the northernmost portion of the belt yielded U-Pb zircon ages ca. 1110 Ma, which dates the high-grade metamorphism and the closure of the rift. This indicates that the breakup of supercontinent Columbia was followed in short sequence by the assembly of supercontinent Rodinia at ca. 1.1-1.0 Ga and that the Western Amazon belt was formed during the accretion of the Arequipa-Antofalla basement to the Amazonian craton

Ar-40/Ar-39 hornblende dating of a microgranodiorite dyke: implications for early Permian extension in the Moldanubian Zone of the Bohemian Massif

Laser fusion Ar-40/Ar-39 ages of titanian pargasite from a microgranodiorite dyke swarm in the southern Bohemian Massif effectively date the early Permian (late Autunian) emplacement of dykes into a cool Moldanubian crust, This intrusion represents the youngest magmatic phase recorded in this part of the Moldanubian Zone. Strontium and neodymium iso topic ratios of microgranodiorites point to magma derivation from re-melting the lower crustal rocks with a possible component of upper mantle composition. Spatial and temporal association of the dykes with movements on a major N-S (NNE-SSW) tectonic discontinuity (Blanice-Kaplice-Rodl fault zone) suggests that their emplacement corresponds to the maximum age of fault movements associated with the E/W- oriented extension in this part of the Bohemian Massif

Differential Neoproterozoic to Palaeozoic postcollisional cooling histories across the Lúrio Belt, northeast Mozambique

The East African-Antarctic Orogen maks one of the major collision zones in the amalgamation of Gondwana. The tectonic style of the orogen changes abruptly across the Lúrio Belt in its southern part in northeast Mozambique. The orogen north of the Lúrio Belt is characterized by W- to NW-directed nappe stacking, whilst to the south, large volumes of late-tectonic granitoids and charnockites point towards elevated heat flow and possibly a delaminated orogenic root. Further evidence for delamination is given by structures associated with penetrative extension, high-temperature/low-pressure metamorphism, migmatisation and late-tectonic constriction perpendicular to the main collision vector. We have traced the postcollisional tectonic development across the Lúrio Belt and its subsequent cooling from high-temperature metamorphism with an extensive new set of U-Pb titanite, 40Ar/39Ar hornblende, and 40Ar/39Ar mica analyses

From the Palaeozoic collapse of the East African-Antarctic Orogen to Gondwana rifting in NE Mozambique

The East African passive margin resulted from complex reactivation of the ca. 600–500 Ma East African-Antarctic Orogen (EAAO). With the help of a large set of new thermochronological data (U-Pb titanite, Ar-Ar hornblende and biotite, as well as zircon, titanite and apatite fission-track analyses) we have modelled the tectono-thermal history of NE Mozambique from the late (Lower Palaeozoic) stages of the East African-Antarctic Orogeny to its transformation into a passive margin in the Mesozoic