Isotopic Discordance Research Articles

Perda de Pb por Difusão Contínua em Zircão Ígneo: Estudo da Espectroscopia BSE, ID-TIMS e Raman em Granito Mesoproterozoico do SW do Cráton Amazônico

This work presents a detailed investigation on zircon crystals of a Mesoproterozoic granite from the SW Amazonian Craton using a combination of back scattered electron (BSE) imaging, thermal ionisation mass spectrometry (TIMS) and Raman scattering spectroscopy. Six zircon grains were analysed. The results plotted in the Concordia diagram yielded an upper intercept age of 1423.0 ± 3.8 Ma, which is interpreted as the crystallization age of the rock. The scattering of the analyses in such diagram indicates that U-Pb isotopes are volume-dependent in the studied zircon grains. BSE reveals zonation in the zircon crystals, and Hf is the main element causing the variability in the BSE intensity and U has a secondary effect. Both elements have much higher atomic number than the principal constituents of zircon (Zr, Si and O), so the substitution of Zr by U and Hf results in increasing image brightness. BSE images from the zircon grains exhibit a euhedral external shape and fine-scale euhedral oscillatory zoning. The predominance of magmatic zoned crystals points out that the granite has not suffered metamorphic heating or any other processes that induce the formation of new phases. The results of the Raman scattering technique indicate that a SiO 4 tetrahedral internal vibrational structure is sufficiently sensitive to determine the increasing degree of metamictization. Furthermore, evidence for metamictization is given by the heterogeneity of the half-width values of the Raman scattering peaks, which exhibit a decreasing trend from the core to the rims of the analysed grains. We concluded that the combination of the three techniques provides information on the Pb loss and that the degree of U-Pb isotopic discordance correlates closely with the volume of the zircon grain. The data, in addition to the lateral degree of metamictization detected by the Raman scattering technique, indicate that the loss of radiogenic Pb may be linked to the continuous diffusion of this element. The correlation of the U-Pb discordance and metamictization emphasizes the importance of the Raman scattering spectroscopy analysis to performing zircon dating. Indeed, it is a helpful tool for geochronologists.

Combined Nd and Hf isotope evidence for deep-seated source of Isua lavas

We present the first combined 146,147Sm– 142,143Nd and 176Lu– 176Hf study of mafic rocks (amphibolites) from the western part of the Isua Supracrustal Belt (ISB, SW Greenland). Sm–Nd and Lu–Hf whole-rock isochrons yield identical ages within error: 3.72 ± 0.08 and 3.67 ± 0.07 Ga, respectively. 142Nd measurements confirm and extend the database of 142Nd excesses in Isua samples (μ 142Nd = 7–16 ppm relative to the terrestrial Nd standard), indicating that early-differentiated reservoirs escaped complete homogenization by mantle convection until the Archean. Combined 146,147Sm– 142,143Nd systematics suggest differentiation of the Isua amphibolite reservoir at maximum ages of ~ 4.53, ~ 4.47, and ~ 4.32 Ga. These ages are relative to, respectively, ordinary-chondritic, enstatite-chondritic, and higher-than-chondritic mantle compositions. The intercept of the Sm–Nd whole-rock isochron is in accordance with 142Nd results and consistent with a superchondritic initial 143Nd/ 144Nd ratio (ε 143Nd 3.7Ga = + 1.41 ± 0.98). In contrast, the corresponding initial ε 176Hf 3.7Ga = − 1.41 ± 0.57 is subchondritic. Since Lu/Hf and Sm/Nd fractionate similarly during mantle processes, the Sm–Nd and Lu–Hf isotope systems display incongruent relative parent–daughter behavior in the source of Isua amphibolites. Based on high-pressure and -temperature phase partition coefficients, we propose a model that satisfies 147Sm– 143Nd, 176Lu– 176Hf, and 142Nd results, as well as trace element characteristics. A deep-seated source composed largely of magnesium perovskite (98% MgPv) and containing 2% calcium perovskite satisfactorily explains the Nd and Hf isotopic discordance observed for Isua amphibolites. The negative HFSE anomalies characterizing Isua basalts likewise could have been inherited from such an early (4.53–4.32 Ga) deep mantle cumulate. Since the ~ 4 Ga old Acasta Gneisses (Northwest Territories, Canada) have radiogenic 176Hf/ 177Hf (Scherer et al., 2010), dissociation of the Lu–Hf and Sm–Nd isotope systems may not be a ubiquitous feature of the Hadean mantle, but rather reflects that a deep-seated source was involved in the formation of ISB lavas.

Quantitative characterization of plastic deformation of zircon and geological implications

The deformation-related microstructure of an Indian Ocean zircon hosted in a gabbro deformed at amphibolite grade has been quantified by electron backscatter diffraction. Orientation mapping reveals progressive variations in intragrain crystallographic orientations that accommodate 20° of misorientation in the zircon crystal. These variations are manifested by discrete low-angle (<4°) boundaries that separate domains recording no resolvable orientation variation. The progressive nature of orientation change is documented by crystallographic pole figures which show systematic small circle distributions, and disorientation axes associated with 0.5–4° disorientation angles, which lie parallel to rational low index crystallographic axes. In the most distorted part of the grain (area A), this is the [100] crystal direction. A quaternion analysis of orientation correlations confirms the [100] rotation axis inferred by stereographic inspection, and reveals subtle orientation variations related to the local boundary structure. Microstructural characteristics and orientation data are consistent with the low-angle boundaries having a tilt boundary geometry with dislocation line [100]. This tilt boundary is most likely to have formed by accumulation of edge dislocations associated with a 〈001〉{100} slip system. Analysis of the energy associated with these dislocations suggest they are energetically more favorable than TEM verified 〈010〉{100} slip. Analysis of minor boundaries in area A indicates deformation by either $${{\left[ {0\bar{1}0} \right]}}$$ (001) edge, or [100](100) and [001](100) screw dislocations. In other parts of the grain, $${{\left[ {1\bar{1}0} \right]}}$$ cross slip on (111), $${{\left({11\bar{1}}\right)}}$$ and (112) planes seems likely. These data provide the first detailed microstructural analysis of naturally deformed zircon and indicate ductile crystal-plastic deformation of zircon by the formation and migration of dislocations into low-angle boundaries. Minimum estimates of dislocation density in the low-angle boundaries are of the order of ∼3.1010 cm−2. This value is sufficiently high to have a marked effect on the geochemical behavior of zircon, via enhanced bulk diffusion and increased dissolution rates. Therefore, crystal plasticity in zircon may have significant implications for the interpretation of radiometric ages, isotopic discordance and trace element mobility during high-grade metamorphism and melting of the crust.

Zircon reaction and stability of the U-Pb isotope system during interaction with carbonate fluid: experimental hydrothermal study

The interpretation of metamorphically induced U-Pb isotopic discordance requires a thorough understanding of zircon-fluid interactions. With this aim we have studied the behaviour of metamict and crystalline zircon phases and their U-Pb systems by cathodoluminescence after treatment by 2M Na2CO3 solution at T = 200–800 °C and P = 1–5 kbar for 3–14 days, X-ray diffraction, microprobe and isotope dilution analysis. The data indicate that zircon transformation under hydrothermal conditions depends on the experimental conditions and the degree of structural damage. Reconstitution of defective and impurity-enriched zones of metamict zircon (homogenization of impure element concentrations and increase of crystallinity) was observed at 400 °C and P = 1 kbar. Considerable lead and uranium loss occurred under these conditions. As a result of zircon dissolution, newly formed baddeleyite accommodating U from 2M Na2CO3 solution and Zr-Na-silicate were recognized. This process intensified with increasing pressure. Study of crystalline zircon indicates that migration of U and Pb took place only during dissolution of zircon at T above 650 °C. In the presence of carbonate-ions essential U and Pb amounts are lost from metamict zircon at a lower P-T than is typical for greenschist facies metamorphism.

An electron microprobe study of the U–Th–Pb systematics of metamorphosed monazite: the role of Pb diffusion versus overgrowth and recrystallization

An electron microprobe (EMP) was used to obtain U–Th–Pb chemical ages from monazite that underwent an amphibolite facies metamorphic overprint in the Monashee complex, southern Canadian Cordillera. U–Pb isotope dilution dating of single grains yielded discordant ages that plot in linear arrays on a concordia diagram; upper intercepts of the arrays indicate primary growth in the Proterozoic and Cretaceous, and lower intercepts are coeval with a Tertiary metamorphic event. The microprobe's high spatial resolution (∼2 μm) and ability to quickly differentiate between domains with greatly contrasting U, Th, and Pb concentrations made it the ideal tool for investigating the cause of the U–Pb isotopic discordance. The microprobe dating shows that Proterozoic monazite lies directly adjacent to generally small (<20 μm thick) Tertiary domains that exist along embayments into the Proterozoic domains, in inclusion-rich zones, and in cracks. Tertiary monazite is typically U- and Th-richer than the Proterozoic monazite, although it locally has normal to slightly lower U and Th concentrations. U-rich domains also rim Cretaceous grains, but low Pb concentrations preclude differentiation between Cretaceous and Tertiary domains. The uniformity of the Proterozoic primary growth ages and the sharpness of the boundary between Proterozoic and Tertiary domains require that Pb diffusion was negligible, despite the attainment of substantial temperatures (600°C–650°C) in the Tertiary (based on the metamorphic mineral assemblage in overlying schists). The presence of Tertiary domains in locations that are susceptible to fluid interaction suggests that fluids locally dissolved Proterozoic monazite and precipitated Tertiary monazite during the overprint; this mechanism caused most of the U–Pb isotopic discordance. We use Pb diffusion data gathered by previous studies to contend that the absence of Pb diffusion was due to a short duration of elevated temperatures, perhaps only a few million years.

The interpretation of complex zircon U–Pb systems in Archaean granitoids and gneisses from the Jack Hills, Narryer Gneiss Terrane, Western Australia

SHRIMP U–Pb ages of zircons from Archaean TTG granitoids and gneisses from the Jack Hills region of Western Australia show a range of complexity which reflects the origin and geological history of the rocks and the stability of the zircon U–Pb systems under metamorphic and magmatic conditions. Strong light rare earth element (LREE) enrichment and similarities in the major and trace element chemistry suggest that TTG gneisses formed by fractional melting of a garnet-bearing amphibolitic source. Maximum ages, of 3.75 to 3.6 Ga, obtained from low U zircon cores are interpreted as the period of formation of the TTG parent rocks. However, most zircons in these populations are complex, consisting of zoned and unzoned parts, and have disturbed U–Pb systems which form a concordant to discordant array between 3.7 and 3.3 Ga on a concordia plot, suggesting these grains experienced one or more disturbance events within this time span. The range of complexity of U–Pb systems observed in a number of the zircon populations is also present within individual zircon crystals indicating that secondary movement and loss of Pb and possibly U has occurred within and from zircons, causing variable isotopic discordance and yielding a range of apparent ages within a single grain. Disturbance of the zircon isotopic systems appears to be due to a combination of early Archaean events together with recent Pb loss. The presence of 3.3 Ga zircon rims around older cores in samples of tonalite and porphyritic granodiorite, together with the exclusive presence of 3.3 Ga zircons in one sample of porphyritic granite, is interpreted as evidence of a major magmatic and possibly metamorphic event at this time. The presence of ca 3.5 Ga ages in a number of unzoned zircons in the tonalite and two samples of granodiorite suggests that this could also have been a time of magmatism and zircon crystallization. Evidence for 3.3 Ga magmatism appears to be restricted to zircons from TTG rocks south of the Jack Hills Metasedimentary Belt, raising the possibility that the present position of the belt defines a major suture within the Narryer Gneiss Terrain. The post-tectonic monzogranites have LREE enriched patterns with strong negative Eu anomalies, similar to the older gneisses, but in contrast to these rocks, zircons from the monzogranites have relatively simple U–Pb systems, which define an age of 2654±7 Ma, and only rarely do zircons have inherited cores with nearly concordant ages of ca 3.6 Ga. The very low content of inherited zircon raises doubts about the formation of the granites by reworking of older TTG gneisses and an alternative explanation that the monzogranites formed from new 2.65 Ga crust, with minor 3.6 Ga crustal contamination, is seen as more likely.

Metamictization and U-PB isotopic discordance in single zircons: a combined Raman microprobe and SHRIMP ion probe study

We report results of a combined SHRIMP ion microprobe and Raman microprobe study of the correlation between metamictization and U-Pb isotopic discordance of zircon. The spatial resolution of the SHRIMP and Raman probe used are about 200 μm3 and 80 μm3, respectively. This allows a direct comparison of U-Pb isotopic discordance and metamictization of small areas within zircon crystals. We show that the impact of the oxygen ion beam on the zircon during the SHRIMP analysis does not cause significant amorphization or recrystallization in the remaining zircon on a scale of 1 μm. Consequently, it is possible to determine the initial degree of metamictization of zircon within and adjacent to a SHRIMP analytical spot by Raman microprobe measurements after performing SHRIMP analyses. A combination of the two microprobe techniques gives information on the concentration and distribution of radionuclides as well as the degree of metamictization and its heterogeneity and the lateral “age distribution” within the grain. We found that the degree of U-Pb isotopic discordance correlates closely with the degree of metamictization within single zircon grains, on a scale of 15 μm, which is consistent with previous results on the scale of single and multiple zircon grains showing that metamictization enhances the relative potential for secondary loss of radiogenic lead in zircon.

Stable isotope composition of soil organic matter and phytoliths as paleoenvironmental indicators

The stable C isotope composition of organic matter and opal phytoliths in diverse ecosystems demonstrate that soils carry a C isotopic signature that reflects long-term inputs of above- and below-ground C 3 or C 4 biomass. The utility of these isotopic characterization data for paleoenvironmental study is based on a knowledge of dominant soil forming processes as well as geomorphic and climatic conditions. This paper reports on both the theory and applicability of isotopic characterization of soil organic C and opal phytoliths. The theoretical perspectives are discussed in light of broad environmental applications. Details of the sample collection and preparation are provided with insights into quantities and pretreatment required for stable C isotopic characterization. Two case studies from the central Great Plains region are presented. Paleoclimatic interpretations are discussed for a portion of the central Great Plains. These interpretations are based on the stable C isotope data recovered from Holocene paleosols. These data indicate higher proportions of C 3 vegetation persisted during the early Holocene. The concordance in the C isotopic signatures of soil organic matter and phytoliths provide strong biological evidence of regionally cooler conditions. This C isotopic concordance also appears during the mid-Holocene; C isotope values indicate an increase in the proportion of C 4 vegetation, which reflects regionally warmer climatic conditions than present. Isotopic discordance in soil organic C and phytoliths can indicate the degree of diagenesis resulting from pedological alteration. Geomorphic and pedologic evidence indicate that the C isotope discordance between soil organic matter and opal phytoliths is the result of local topographic variations and spatial heterogeneity associated with plant distribution.

Heterogeneous metamictization of zircon on a microscale

The ability of modern Raman microprobes to determine the crystallinity of zircon with a lateral resolution of about 1 μm and a volume resolution of less than 5 μm 3 opens up new opportunities for correlating structural, chemical, and isotopic data on the same micro-area within a zircon grain. Combining this information we confirm by direct measurement the inhomogeneity in the degree of crystallinity of complex zircon grains and the correlation between U-Th concentration, isotopic discordance, and degree of metamictization in micro-areas of these grains. Micro-areas with a higher degree of metamictization are more chemically heterogeneous than neighbouring more crystalline areas and contain higher concentrations of Ca, Fe, U, Th, and Y but not always Hf, which in some cases shows higher concentration in the less metamict areas. Raman observations that there are no “radiation-damage halos” extending over distances greater than 1–1.5 μm preclude alpha and beta radiation as well as fission recoil as viable causes for amorphization, and confirm that alpha recoil is the most important mechanism causing zircons to disorder into the metamict state.

The hydrothermal stability of zircon: Preliminary experimental and isotopic studies

Experimental investigations of the stability of the U-Pb isotopic system in nonmetamict zircons show that appreciable losses of Pb and U can be induced at amphibolite-grade conditions (400°C to 600°C, 4 to 6 kb) in 2 M NaCl and 2% HNO 3 solutions. The severity of U loss and, to a lesser extent Pb loss, varies with solution composition: in this case the 2 M NaCl solution induced more Pb and U loss than the 2% HNO 3 solution at the same P- T conditions. Scanning electron microscopy of the run products also revealed a range of corrosion-related surface features, which suggests that some of the observed trends in Pb and U loss must be attributed to zircon dissolution. Backscattered electron (BSE) imaging of the run products further suggests that partial homogenization of chemical zoning patterns occurred during the experiments. Microprobe analyses of treated and untreated grains show that both populations have a similar range of Hf contents. Thus, the apparent loss of sharp, well-defined zoning features is most likely due to small-scale “smearing out” of formerly sharp chemical gradients and is perhaps related to the annealing of lattice defects caused by alpha-recoil damage. Thus, experimentally induced U-Pb isotopic discordance in zircon is a complex function of zircon stability and annealing effects.

Formula omitted] and [(Mg,Fe)Si-Al 2] compositional control on argon behaviour in high-pressure white micas: A [formula omitted] continuous laser-probe study from the Dora-Maira nappe of the internal western Alps, Italy

High-pressure white micas from a restricted area in the Dora-Maira nappe of the western Alps (Italy) were analyzed by the conventional 40Ar 39Ar bulk-sample step-heating method and by the step-heating and the spot-fusion 40Ar 39Ar continuous laser-probe techniques on single grains. High-Si, Ferich phengites from fine-grained gneisses display homogeneous 35–40 Ma plateau and spot-fusion ages, consistent with the upper Eocene 40Ar 39Ar phengite ages documented over the whole internal western Alps. In contrast, phengite separates and single-grains of high-Si and Mg-rich to intermediate composition selected from metapelitic and Mg-rich mica schist samples yield older discordant apparent ages, even when sampled in the same outcrop as the fine-grained gneisses. These local age variations are interpreted to reflect primarily an increasing Ar retentivity with increasing Mg content in phengites. In metapelite-derived white micas, the isotopic discordance is marked by 1. (1) a characteristic bulk-sample upward-convex spectrum shape, 2. (2) single-grain disturbed age spectra, 3. (3) a systematic radially outward decrease in spot ages across individual mica cleavage planes and 4. (4) strong grain-to-grain spot-age variations in rock sections. 1. (1) In Mg-rich mica schists, the isotopic discordance is observed as a reproducible upward-convex bulk-sample age spectrum with high (140 Ma) intermediate apparent ages, 2. (2) upward-convex single-grain age spectra and 3. (3) irregular intra- and intergrain age gradients. On geochemical grounds, it is argued that the isotopic discordance occurring in the metapelites is mainly controlled by the compositional changes introduced by the [(Mg,Fe)Si-Al 2] celadonitic re-equilibration of the high-pressure phengites during greenschist overprinting. In Mg-rich phengites, this chemical re-equilibration may induce Ar loss below their presumed higher blocking temperature. A model of Ar behaviour in high-pressure white micas is proposed which explains the age variations recorded throughout the different lithologies in terms of a simple interaction between the Mg Fe compositional control on Ar retentivity and the [(Mg,Fe)Si-Al 2] modifications occurring in phengites during progressive overprinting.

A reconnaissance ion-probe study of hafnium isotopes in zircons

A SIMS technique for the isotopic analysis of hafnium in zircons using the SHRIMP ion microprobe has been developed, and a precision of typically 0.5%. (2σ) achieved in the mean reduced 176Hf 177Hf ratio measured at several spots on a single grain. Unfractionated (chondritic) initial Hf isotopic compositions have been measured on a number of Archaean zircon populations. These include the oldest-known terrestrial minerals, the 4.2 Ga-old Mount Narryer detrital zircons, thereby confirming their antiquity. In contrast, positive initial ϵ Hf (relative to the chondritic model composition) has been found in several post-Archaean zircon populations, reflecting the increasing involvement of isotopically evolved depleted mantle sources in the formation of younger crust. The 570 Ma-old Sri Lankan zircon standard SL7 yielded an exceptionally low initial ϵ Hf of −23, implying a metamorphic origin as a reworked product of ancient crust. SHRIMP U-Pb analyses of zircons from Archaean tonalitic gneiss at Watersmeet, Michigan, yield a precise crystallization age of 3636 ± 6 Ma (2σ), and show that a previously reported correlation between 176Hf 177Hf and U-Pb isotopic discordance in bulk zircon samples (Patchett, 1983) was caused by the addition of radiogenic Hf in discrete overgrowths of new zircon ca. 2.7 Ga ago. The original 3.64 Ga grains show no evidence of disturbance to their original (chondritic) Hf isotopic composition. There is presently no evidence for significant isotopic exchange of Hf between zircon and other minerals in crustal rocks.

The Rayner Complex of East Antarctica: complex isotopic systematics within a Proterozoic mobile belt

Abstract New isotopic (Rb–Sr, U–Pb zircon and Sm–Nd) and petrological data are presented for part of an extensive Proterozoic mobile belt (locally known as the Rayner Complex) in East Antarctica. Much of the belt is the product of Mid‐Proterozoic (∼ 1800–2000 Ma) juvenile crustal formation. Melting of this crust at about 1500 Ma ago produced the felsic magmas from which the dominant orthogneisses of this terrain were subsequently derived. Deformation and transitional granulite‐amphibolite facies conditions (which peaked at 750 ± 50°C and 7–8 kbar (0.7–0.8 GPa) produced open to tight folding about E–W axes and syn‐tectonic granitoids about 960 Ma ago. Subsequent felsic magmatism occurred at about 770 Ma and not, as has been widely advocated, at 500–550 Ma, which appears to have been a time of widespread upper greenschist facies (400–500°C) metamorphism, localized shearing and faulting.Sm‐Nd model ages of 1.65–2.18 Ga disprove a previously favoured hypothesis that the Rayner Complex mostly represents reworked Archaean rocks from the neighbouring craton (Napier Complex). Models that involve rehydration of the Napier Complex are no longer required, since the Rayner Complex was its own source of water. Two episodes of Proterozoic crustal growth are identified, the later of which occurred between about 1200 Ma and 1000 Ma, and was relatively minor. Sedimentation took place only shortly before Late Proterozoic orogenesis.The multiphase history of the Rayner Complex has resulted in complex isotopic behaviour. Three temporally discrete episodes of Pb loss from zircon have been identified, the earliest two of which are responses to the c. 960 Ma and 540 Ma tectonothermal events. Fluid leaching was operative during the later event for there is a good correlation between degree of isotopic discordance and secondary mineral growth. Pb loss during the high‐grade event was probably governed by the same process or by lattice annealing. Some zircon suites also document recent Pb loss. Most lower concordia intercepts have no direct geological meaning and are explicable as mixed ages produced by incomplete Pb loss during two or more secondary events. Whereas all zircon separates from the orthogneisses produce U–Pb isotopic alignments, zircons from the only analysed paragneiss produce scattered data, in part reflecting a range of provenance. The 960 Ma event was also associated with the growth of a characteristically low U zircon (∼ 300 μg/g) in rocks of inferred high Zr content.There is ubiquitous evidence for the resetting of Rb–Sr total‐rock isochrons. Even samples separated by up to 10 km fail to produce igneous crystallization ages. Minor mineralogical changes produced by the 540 Ma upper greenschist‐facies metamorphism were sufficient to almost completely reset some Rb–Sr isochrons and to produce open system conditions on outcrop scale, at least in one location.