Lu-Hf Data Research Articles

Meteorite Phosphates Show Constant 176 Lu Decay Rate Since 4557 Million Years Ago

The use of radioactive decay of 176Lu to 176Hf to study the evolution of the Earth requires a precise and accurate value for the 176Lu decay constant. Recent determinations of this decay constant by age comparison to the more precisely calibrated U-Pb isotopic system produced internally consistent but discrepant values between terrestrial minerals and meteorites. New highly radiogenic Lu-Hf data for phosphate minerals from Richardton (ordinary chondrite) and Acapulco (primitive achondrite) yield decay constant values of 1.864 x 10(-11) +/- 0.016 x 10(-11) and 1.832 x 10(-11) +/- 0.029 x 10(-11) year(-1), respectively, identical to the value determined from terrestrial minerals.

Timing of Late Neoproterozoic glaciation on Baltica constrained by detrital zircon geochronology in the Hedmark Group, south‐east Norway

AbstractThe Moelv Tillite is the Late Neoproterozoic Varanger glacial deposit recorded in the Hedmark Group, SE Norway. Paired U–Pb and Lu–Hf data collected on detrital zircons in the Rendalen Formation underlying the Moelv Tillite have identified an uncommon 677 ± 15 to 620 ± 14 Ma population, that constrain the deposition of the Moelv Tillite to be younger than 620 ± 14 Ma. The youngest detrital zircons may be derived from granite magmatism related to the 616 ± 3 Ma Egersund dolerite magmatism, situated in the western part of the Sveconorwegian orogen. The Moelv Tillite, which is not overlain by a cap carbonate, possibly correlates with the c. 580 Ma Squantum‐Gaskiers glacial deposits of Avalonia. Available palaeomagnetic data for the Late Neoproterozoic suggest that Baltica was located at intermediate to high latitude between 620 and 555 Ma.

MANTLE MELTING PROCESSES DISCERNED FROM A GEOCHEMICAL STUDY OF THE TINAQUILLO PERIDOTITE MASSIF, VENEZUELA

The Tinaquillo orogenic spinel peridotite massif in north central Venezuela is a sub-horizontal, 3-km thick sheet overlying and in thrust contact with phyllites of the Cordillera de la Costa belt in the north and underlying and in extensional fault contact with the gabbroic to felsic Tinaco complex to the south (Ostos, 1990). While early workers interpreted the complex as a magmatic or crystal-mush intrusion (e.g., MacKenzie, 1960), Seyler and Mattson (1989, 1993) and Seyler et al. (1998) interpreted it as a fragment of the upper mantle possibly having ascended in the form of a diapir, but having been mylonitized during transit through the lower crust. The ultramafic rocks comprise ca. 75% harzburgite locally grading into lherzolite, 20% dunite, and 5% serpentinite. Pyroxenite and hornblendite veins intruding the complex are volumetrically small, but quite conspicuous where present. Minerals in the peridotite have a bimodal grain-size distribution owing to survival of remnant porphyroclasts of olivine, orthopyroxene, and minor clinopyroxene, and the late development of a mylonitic fabric with considerable grain-size reduction probably related to emplacement. The massif is thought to be a fragment of lithospheric mantle emplaced into the Caribbean belt during the late Cretaceous, but not exhumed until the late Eocene to middle Miocene by northwestward-directed thrusting along the Manrique Fault (Pindell et al., 1988; Pindell and Barrett, 1990). Seven peridoitites and three amphibole-rich veins have been investigated for major and trace element concentrations, and Sr-Nd-Pb-Hf isotopic compositions. The peridotites have geochemical characteristics of residues after moderate degrees of partial melting (estimated to be 8 to 16%) assuming a homogeneous primitive mantle source. In addition, they have strong light rare earth element (REE)- depleted to spoon-shaped REE patterns, but flat heavy REE (La/Yb at 0.04-0.55 times primitive mantle). The (La/Yb)N ratios increase with the refractoriness of the peridotites, reflecting secondary metasomatism, which is a common feature of the sub-continental lithospheric mantle xenoliths world-wide. A primitive mantle-normalized element distribution diagram for the peridotites reveals an overall depletion in the highly incompatible elements relative to less incompatible ones, but moderate enrichments in the strongly incompatible elements (Ba, Th, U), without depletion in Nb and Ta relative to La. The peridotites have isotopic compositions of 87Sr/86Sr = 0.70277-0.71044, 143Nd/144Nd = 0.513095-0.514000, 206Pb/204Pb = 19.00-19.15, 208Pb/204Pb = 38.18-38.75, and 176Hf/177Hf = 0.282692-0.284703. The Nd, Pb and Hf isotopic signatures show ranges that overlap with present-day mid-ocean ridge basalts (MORB) but also extending to more depleted compositions. The unusually radiogenic Sr for some samples appears to be the result of pervasive infiltration by seawater at some stage in the massif’s history. The Sr, Nd, Pb and Hf isotopic signatures of the veins (87Sr/86Sr = 0.70231-0.70256, 143Nd/144Nd = 0.513052-0.513289, 206Pb/204Pb = 18.30-18.52, 208Pb/204Pb = 37.66-37.87, 176Hf/177Hf = 0.282988-0.283303) are similar to the Pacific and Atlantic MORB; lack of the highly radiogenic Sr component in the veins indicates its addition to the peridotites prior to vein emplacement. The Lu-Hf data yield an errorchron suggesting a Phanerozoic age for stabilization of this piece of the lithosphere. We attribute the origin of the amphibole-rich veins in the peridotites to melting during subduction of the proto-Caribbean Plate in a back-arc tectonic setting, fairly recently in the massif’s history, possibly during the late Jurassic.

Lu–Hf and Sm–Nd isotopic systematics in chondrites and their constraints on the Lu–Hf properties of the Earth

Sm–Nd and Lu–Hf isotopic data are presented for 19 chondritic meteorites: six carbonaceous chondrites, five L-chondrites, seven H-chondrites, and a single enstatite chondrite. The primary goal of the study is to better define the Bulk Silicate Earth (BSE) reference values for Hf isotopes. Except for one sample with lower Sm/Nd, the Sm–Nd data define a cluster around the accepted reference values for chondrites and terrestrial planets, giving a mean 147Sm/ 144Nd of 0.1960±0.0005, and a mean 143Nd/ 144Nd of 0.512631±0.000010 (uncertainties are two standard errors). It seems appropriate to retain the presently accepted Sm–Nd reference parameters, 147Sm/ 144Nd=0.1966 and 143Nd/ 144Nd=0.512638 (when fractionation-corrected to 146Nd/ 144Nd=0.7219).Lu–Hf isotopic data are not clustered, but spread along an approximate 4.5-Ga isochron trend, with a range of 176Lu/ 177Hf from 0.0301 to 0.0354. The data are similar to many of the samples of chondrites presented by Bizzarro et al. [Nature 421 (2003) 931], but lack the range to lower Lu/Hf shown by those authors. Our chondrite data define a regression line of 4.44±0.34 Ga when 1.867×10 −11 year −1 is used for the decay constant of 176Lu [Science 293 (2001) 683; Earth Planet. Sci. Lett. 219 (2004) 311–324]. Combining our data with the main population of analyses from Bizzarro et al. [Nature 421 (2003) 931] yields 4.51±0.24 Ga. Unless samples of eucrite meteorites and deviating replicates of chondrites with 176Lu/ 177Hf less than 0.030 are employed, no combination of the main population of chondrite Lu–Hf data yields a regression with sufficiently low error to constrain the decay constant of 176Lu. Sample heterogeneity seems to hinder the acquisition of reproducible Lu–Hf analyses from small, manually ground pieces of chondrites, and we suggest that analysis of powders prepared from large volumes of meteorite will be needed to adequately characterize the Lu–Hf isotope systematics of chondritic reservoirs and of BSE. Our results for carbonaceous chondrites show higher average 176Lu/ 177Hf and 176Hf/ 177Hf than ordinary chondrites, and the mean of carbonaceous chondrites also coincides with replicate analyses of a powder representing a large volume of meteorite, the Allende powder from the Smithsonian Institution. Use of the carbonaceous chondrite mean for BSE Lu–Hf characteristics results in a BSE Hf–Nd point that lies well within the array of terrestrial compositions, and leads to plausible initial ε Hf values for Precambrian rocks. An improved objective resolution of meteorite data and of meteoritic models for the Earth needs to occur before BSE can be established for Lu–Hf.

Mid-Proterozoic magmatic arc evolution at the southwest margin of the Baltic Shield

Mid-Proterozoic calc-alkaline granitoids from southern Norway, and their extrusive equivalents have been dated by LAM-ICPMS U–Pb on zircons to ages ranging from 1.61 to 1.52 Ga; there are no systematic age differences across potential Precambrian terrane boundaries in the region. U–Pb and Lu–Hf data on detrital zircons from metasedimentary gneisses belonging to the arc association show that these were mainly derived from ca. 1.6 Ga arc-related rocks. They also contain a minor but significant fraction of material derived from (at least) two distinct older (1.7–1.8 Ga) sources; one has a clear continental signature, and the other represents juvenile, depleted mantle-derived material. The former component resided in granitoids of the Transscandinavian Igneous Belt, the other in mafic rocks related to these granites or to the earliest, subduction-related magmatism in the region. Together with published data from south Norway and southwest Sweden, these findings suggest that the western margin of the Baltic Shield was the site of continuous magmatic arc evolution from at least ca 1.66 to 1.50 Ga. Most of the calc-alkaline metaigneous rocks formed in this period show major- and trace-element characteristics of rocks formed in a normal continental margin magmatic arc. The exceptions are the Stora Le-Marstrand belt in Sweden and the Kongsberg complex of Norway, which have an arc-tholeiitic chemical affinity. The new data from south Norway do not justify a suggestion that the crust on the west side of the Oslo Rift had an early to mid-Proterozoic history different from the crust to the east. Instead, they indicate that the different parts of south Norway and southwest Sweden were situated at the margin of the Baltic Shield throughout the mid-Proterozoic. Changes from arc tholeitic to calc-alkaline magmatism reflect changes with time in the subduction zone system, or lateral differences in subduction zone geometry. The NW American Cordillera may be a useful present-day analogue for the tectonomagmatic evolution of the mid-Proterozoic Baltic margin.

The Lu–Hf dating of garnets and the ages of the Alpine high-pressure metamorphism

It remains controversial whether burial and exhumation in mountain belts represent episodic or continuous processes1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20. Regional patterns of crystallization and closure ages of high-pressure rocks may help to discriminate one mode from the other but, unfortunately, metamorphic geochronology suffers from several limitations. Consequently, no consensus exists on the timing of high-pressure metamorphic events, even for the Alps—which have been the subject of two centuries of field work. Here we report lutetium–hafnium (Lu–Hf) mineral ages on eclogites from the Alps as obtained by plasma-source mass spectrometry. We find that the Lu/Hf ratio of garnet is particularly high, which helps to provide precise ages. Eclogites from three adjacent units of the western Alps give (from bottom to top) diachronous Lu–Hf garnet ages of 32.8 ± 1.2, 49.1 ± 1.2 and 69.2 ± 2.7Myr. These results indicate that the Alpine high-pressure metamorphism did not occur as a single episode some 80–120Myr ago6,7,10,18, but rather that burial and exhumation represent continuous and relatively recent processes.

Research from the standpoint of the smaller business enterprise

Radiometric data from accessory zircon in long-lived polymetamorphic granulite facies rocks frequently show a large spread, with no clear relationship to zircon textures. For example, the granulites and associated rocks of the North China Craton which have an apparently continuous spread of metamorphic zircon dates through the Palaeoproterozoic from c. 1.95–1.80 Ga. The data are commonly interpreted to reflect long-lived metamorphism or discrete metamorphic events, with intermediate data reflecting variable Pb loss or a mixture of multiple growth domains. Herein, we present an approach that integrates direct dating of the P–T indexed mineral garnet with accessory zircon petrochronology which allows diagnostic time constraints for polymetamorphic processes in high-temperature rocks. We present high-precision LuHf and SmNd dates for two garnet-bearing mafic granulites and the adjoining granite and quartzite from the Gushan area between the Khondalite Belt and the Trans-North China Orogen, North China Craton. Statistically robust LuHf dates of 1815 ± 4 Ma and 1808 ± 3 Ma and corresponding SmNd isochron dates of 1790 ± 6 Ma and 1800 ± 9 Ma were obtained from the two mafic granulites, overlapping the zircon UPb date range of c. 1959–1785 Ma. Samarium–Nd dates from the accompanying garnet-bearing granite and quartzite are indistinguishable at 1794 ± 6 Ma and 1780 ± 6 Ma. The corresponding bulk garnet LuHf isochrons are poorly constrained—two-point garnet–matrix dates range from 1945 ± 10 Ma to 1849 ± 10 Ma for the granite and from 1936 ± 10 Ma to 1840 ± 9 Ma for the quartzite. Upper intercept UPb dates of 1950 ± 24 Ma (granite) and 1936 ± 17 Ma (quartzite) are obtained from the metamorphic zircons in equilibrium with garnet. The close agreement of SmNd isochron-dates (c. 1.80 Ga), the overlapping clusters of zircon UPb dates (c. 1.96 Ga), the identical LuHf and UPb dates of c. 1.80 Ga within uncertainty in the two granulites, and the older LuHf dates and comparable zircon UPb dates (c. 1.96 Ga) obtained in the garnet-bearing granite and quartzite than in those of the mafic granulites, strongly indicate that these rocks experienced coeval metamorphism from c. 1.95–1.80 Ga. These data require a prolonged episodic tectonometamorphic event in the North China Craton, which is probably related to the amalgamation of the Columbia supercontinent.