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The Solar Initial Abundance of Hafnium-176 Revealed by Eucrite Zircon
Abstract
We determine the initial abundance of hafnium-176 of the solar system through the first high-precision Lu-Hf isotopic analysis of meteorite zircon.
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From measurements of Hf–Yb mixtures, we have found that the correction of isobaric interferences involving accepted Yb isotope ratios and reasonable estimates of mass bias result in a significantly under-corrected 176Hf, which is proportional to the amount of Yb added. This can be explained by (1) a significant difference in the instrumental mass bias between Hf and Yb, and (2) that the accepted values for isotopic ratios within the Yb and/or Hf systems are incorrect. We have evaluated these possibilities by measuring mixed solutions of Yb and Hf on two MC-ICP-MS instruments and undertaking a series of REE fractionation experiments using a thermal ionisation mass spectrometer (TIMS). Our results indicate that the presently accepted abundances of the Yb isotopes are not appropriate. We present new values for Yb isotopic abundances based on the TIMS and MC-ICP-MS results. Using the newly defined Yb values, we demonstrate that Yb and Hf have similar levels of mass bias in plasma ionisation instruments, and that Hf isotope ratios can be used to correct Yb mass bias before subsequent correction of isobaric interference. A laser ablation comparison of Yb and Hf indicates that similar relationships exist, and can be applied to micro-analytical techniques where chemical separation is not possible.
Events occurring within the first 10 million years of the Solar System's approximately 4.5 billion-year history, such as formation of the first solids, accretion, and differentiation of protoplanetary bodies, have determined the evolutionary course of our Solar System and the planetary bodies within it. The application of high-resolution chronometers based on short-lived radionuclides is critical to our understanding of the temporal sequence of these critical events. However, to map the relative ages from such chronometers onto the absolute time scale, they must be "anchored" to absolute ages of appropriate meteoritic materials using the high-precision lead-lead (Pb-Pb) chronometer. Previously reported Pb-Pb dates of the basaltic angrite meteorites, some of which have been used extensively as time anchors, assumed a constant (238)U/(235)U ratio (= 137.88). In this work, we report measurements of (238)U/(235)U ratios in several angrites that are distinct from the previously assumed value, resulting in corrections to the Pb-Pb ages of ≥ 1 million years. There is no resolvable variation in the (238)U/(235)U ratio among the angrite bulk samples or mineral separates, suggesting homogeneity in the U isotopic composition of the angrite parent body. Based on these measurements, we recalculated the Pb-Pb age for the commonly used anchor, the D'Orbigny angrite, to be 4563.37 ± 0.25 Ma. An adjustment to the Pb-Pb age of a time anchor (such as D'Orbigny) requires a corresponding correction to the "model ages" of all materials dated using that anchor and a short-lived chronometer. This, in turn, has consequences for accurately defining the absolute timeline of early Solar System events.
The (142)Nd/(144)Nd ratio of the Earth is greater than the solar ratio as inferred from chondritic meteorites, which challenges a fundamental assumption of modern geochemistry--that the composition of the silicate Earth is 'chondritic', meaning that it has refractory element ratios identical to those found in chondrites. The popular explanation for this and other paradoxes of mantle geochemistry, a hidden layer deep in the mantle enriched in incompatible elements, is inconsistent with the heat flux carried by mantle plumes. Either the matter from which the Earth formed was not chondritic, or the Earth has lost matter by collisional erosion in the later stages of planet formation.
Large deviations in ɛHf(T) from bulk silicate Earth seen in > 4 Ga detrital zircons from Jack Hills, Western Australia, have been interpreted as reflecting a major differentiation of the silicate Earth at 4.4 to 4.5 Ga. We have expanded the characterization of 176Hf/177Hf initial ratios (Hf) in Hadean zircons by acquiring a further 116 laser ablation Lu–Hf measurements on 87 grains with ion microprobe 207Pb/206Pb ages up to 4.36 Ga. Most measurements employed concurrent Lu–Hf and 207Pb/206Pb analyses, permitting assessment of the age of the volumetrically larger domain sampled by laser drilling against the spatially more restricted ion microprobe ages. Our new results confirm and extend the earlier observation of significant negative deviations in ɛHf(T) throughout the Hadean, although no positive ɛHf(T) values were documented in this study. Monte Carlo modelling of these data yields an essentially uniform spectrum of model ages between 4.56 and 4.20 Ga for extraction of the zircons' protoliths from a chondritic reservoir. To assess whether the five data plotting close to solar system initial Hf (Hfo) are statistically robust, we derived the error propagation equation for a parameter, ɛo, which measures the difference of a sample from Hfo. Our analysis suggests that this limited data is indicative of source sequestration in a crustal-type Lu/Hf environment prior to 4.5 Ga. Oxygen isotope data and Ti thermometry from Hadean zircons show little obvious correlation with Hf, consistent with their derivation through fusion of a broad suite of crustal rock types under water-saturated conditions. Together with other isotopic and trace element data obtained from these ancient zircons, our results indicate essentially continuous derivation of crust from the mantle from 4.5 to 4.2 Ga with concurrent recycling into the mantle and internal crustal re-working. These results represent further evidence that by 4.35 Ga, portions of the crust had taken on continental characteristics.
Continental crust forms from, and thus chemically depletes, the Earth's mantle. Evidence that the Earth's mantle was already chemically depleted by melting before the formation of today's oldest surviving crust has been presented in the form of Sm-Nd isotope studies of 3.8-4.0 billion years old rocks from Greenland(1-5) and Canada(5-7). But this interpretation has been questioned because of the possibility that subsequent perturbations may have re-equilibrated the neodymium-isotope compositions of these rocks(8). Independent and more robust evidence for the origin of the earliest crust and depletion of the Archaean mantle can potentially be provided by hafnium-isotope compositions of zircon, a mineral whose age can be precisely determined by U-Pb dating, and which can survive metamorphisms(4). But the amounts of hafnium in single zircon grains are too small for the isotopic composition to be precisely analysed by conventional methods. Here we report hafnium-isotope data, obtained using the new technique of multiple-collector plasma-source mass spectrometry(9), for 37 individual grains of the oldest known terrestrial zircons (from the Narryer Gneiss Complex, Australia, with U-Pb ages of up to 4.14 Gyr (refs 10-13)). We find that none of the grains has a depleted mantle signature, but that many were derived from a source with a hafnium-isotope composition similar to that of chondritic meteorites. Furthermore, more than half of the analysed grains seem to have formed by remelting of significantly older crust, indicating that crustal preservation and subsequent reworking might have been important processes from earliest times. Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/62681/1/399252a0.pdf
We propose an analytical protocol for the accurate in situ determination of 176Hf/177Hf isotope ratios in zircon crystals by a 193 nm ultraviolet excimer laser ablation (193ExLA) multiple-collector inductively coupled plasma mass spectrometer (MC-ICPMS). Specifically, we used a simultaneous laser and dry solution aerosol dual sample introduction (DSI) system equipped with the 193ExLA-MC-ICPMS and an ARIDUS II desolvating nebulizer. The DSI-MC-ICPMS analyzed the laser aerosol from the zircon crystal while deionized water was aspirated from the ARIDUS II. Hf standard solutions were analyzed using the ARIDUS II with the laser switched off. This technique provides the same analytical conditions for both the laser and solution aerosols minimizing changes in the plasma condition. Therefore, the instrumental mass bias correction using a standard solution is accurately achieved. The instrumental mass bias for 176Hf/177Hf was corrected by measuring 179Hf/177Hf. 176Yb interference correction on 176Hf was performed using the 176Yb/173Yb ratio with a 173Yb/171Yb internal mass bias correction for Yb. Similarly, 176Lu interference was adjusted for using 176Lu/175Lu with 173Yb/171Yb or 179Hf/177Hf external mass bias correction. These correction factors were carefully determined using standard solutions. The correction factors were stable over three months, as observed by our routine monitoring. Standard zircons of 91500, TEMORA 2, Plešovice, and FC-1 were analyzed using 193ExLA with laser fluence at 20 J/cm2 and a repetition rate of 5-10 Hz. The ablation conditions resulted in craters of 40 μm diameter and 40 μm depth after a few minutes. Standard bracketing using JMC475 solution was applied every one to five spot laser analyses, thereby correcting for any remaining instrumental mass bias. The resultant 176Hf/177Hf isotope ratios were all in excellent agreement with the isotope ratios reported by TIMS and by other LA-MC-ICPMS analyses.
The long-favored paradigm for the development of continental crust is one of progressive growth beginning at ∼4 billion years ago (Ga). To test this hypothesis, we measured initial ¹⁷⁶Hf/¹⁷⁷Hf values of 4.01- to 4.37-Ga detrital zircons from Jack Hills, Western Australia. ϵHf (deviations of ¹⁷⁶Hf/¹⁷⁷Hf from bulk Earth in parts per 10⁴) values show large positive and negative deviations from those of the bulk Earth. Negative values indicate the development of a Lu/Hf reservoir that is consistent with the formation of continental crust (Lu/Hf ≈ 0.01), perhaps as early as 4.5 Ga. Positive ϵHf deviations require early and likely widespread depletion of the upper mantle. These results support the view that continental crust had formed by 4.4 to 4.5 Ga and was rapidly recycled into the mantle.
Given the global dearth of Hadean (>4 Ga) rocks , 4.4-4.0 Ga detrital zircons from Jack Hills, Narryer Gneiss Complex (Yilgarn Craton, Western Australia) constitute our best archive of early terrestrial materials. Previous Lu-Hf investigations of these zircons suggested that felsic (low Lu/Hf) crust formation began by ∼4.4-4.5 Ga and continued for several hundred million years with evidence of the least radiogenic Hf component persisting until at least ∼4 Ga. However, evidence for the involvement of Hadean materials in later crustal evolution is sparse, and even in the detrital Jack Hills zircon population, the most unradiogenic, ancient isotopic signals have not been definitively identified in the younger (<3.9 Ga) rock and zircon record. Here we show Lu-Hf data from <4 Ga Jack Hills detrital zircons that document a significant and previously unknown transition in Yilgarn Craton crustal evolution between 3.9 and 3.7 Ga. The zircon source region evolved largely by internal reworking through the period 4.0 to 3.8 Ga, and the most ancient and unradiogenic components of the crust are mostly missing from the record after ∼4 Ga. New juvenile additions to the crust at ca. 3.9-3.8 Ga are accompanied by the disappearance of unradiogenic crust ca. 3.9-3.7 Ga. Additionally, this period is also characterized by a restricted range of δ18O after 3.8 Ga and a shift in several zircon trace element characteristics ca. 3.9-3.6 Ga. The simultaneous loss of ancient crust accompanied by juvenile crust addition can be explained by a mechanism similar to subduction, which effects both processes on modern Earth. The oxygen isotope and trace element information, although less sensitive to tectonic setting, also supports a transition in zircon formation environment in this period.
Evidence of 176Hf excess in select meteorites older than 4556Ma was suggested to be caused by excitation of long lived natural radionuclide 176Lu to its short lived isomer 176mLu, due to an irradiation event during accretion in the early Solar System. A result of this process would be a deficit in 176Lu in irradiated samples by between 1-7‰. Previous measurements of the Lu isotope ratio in rock samples have not been of sufficient precision to resolve such a phenomenon. We present a new analytical technique designed to measure the 176Lu/175Lu isotope ratio in rock samples to a precision of ~0.1‰ using a multi-collector inductively coupled mass spectrometer (MC-ICP-MS). To account for mass bias we normalized all unknowns to Ames Lu. To correct for any drift and instability associated with mass bias, all standards and samples are doped with W metal and normalized to the nominal W isotopic composition. Any instability in the mass bias is then corrected by characterizing the relationship between the fractionation factor of Lu and W, which is calculated at the start of every analytical session. After correction for isobaric interferences, in particular 176Yb, we were able to measure 176Lu/175Lu ratios in samples to a precision of ~0.1‰. However these terrestrial standards were fractionated from Ames Lu by an average of 1.22±0.09‰. This offset in 176Lu/175Lu is probably caused by isotopic fractionation of Lu during industrial processing of the Ames Lu standard. To allow more straightforward data comparison we propose the use of NIST3130a as a bracketing standard in future studies. Relative to NIST3130a, the terrestrial standards have a final weighted mean δ176Lu value of 0.11±0.09‰. All samples have uncertainties of better than 0.11‰, hence our technique is fully capable of resolving any differences in δ176Lu of greater than 1‰.
Representing a suite of well-preserved basaltic meteorites with reported ages from 4566.18±0.14Ma to 4557.65±0.13Ma, angrites have been recurring targets for cross-calibrating extinct and absolute chronometers. However, inconsistencies exist in the available chronological data set, including a 4566.18±0.14Ma Pb–Pb age reported by Baker et al. [Baker J., Bizzarro M., Wittig N., Connelly J. and Haack H. (2005) Early planetesimal melting from an age of 4.5662Gyr for differentiated meteorites. Nature436, 1127–1131] for Sahara 99555 (herein SAH99555) that is significantly older than a Pb–Pb age for D’Orbigny, despite the two meteorites yielding indistinguishable Hf–W and Mn–Cr ages. We re-evaluate the Pb–Pb age of SAH99555 using a stepwise dissolution procedure on a whole rock fragment and a pyroxene separate. The combined data set yields a linear array that reflects a mixture of radiogenic Pb and terrestrial contamination and corresponds to an age of 4564.58±0.14Ma, which is 1.60±0.20Ma younger than that reported by Baker et al. [Baker J., Bizzarro M., Wittig N., Connelly J. and Haack H. (2005) Early planetesimal melting from an age of 4.5662Gyr for differentiated meteorites. Nature436, 1127–1131]. Our conclusion that SAH99555 crystallized at 4564.58±0.14Ma requires that all initial Pb was removed in the first progressive dissolution steps, an assertion supported by linearity of data generated by stepwise dissolution of a single fragment and the removal of an obvious highly-radiogenic component early in the dissolution process. We infer that the linear array defined by Baker et al. [Baker J., Bizzarro M., Wittig N., Connelly J. and Haack H. (2005) Early planetesimal melting from an age of 4.5662Gyr for differentiated meteorites. Nature436, 1127–1131] and their older age reflects a ternary mixture of Pb with constant relative proportions of highly-radiogenic initial Pb and radiogenic Pb with varying amounts of a terrestrial contamination. This requires that the phase harboring the initial Pb is insoluble in 2M HCl, the only acid applied to the samples by Baker et al. [Baker J., Bizzarro M., Wittig N., Connelly J. and Haack H. (2005) Early planetesimal melting from an age of 4.5662Gyr for differentiated meteorites. Nature436, 1127–1131] prior to dissolution.
The record of early events in the solar system is presently sought, together with information on the isotopic composition of primordial lead, in the lead isotope relations of whole rock and separated phases of Mezo-Madaras (L3) and Sharps (H3) chondrites; the respective ages of 4.48 and 4.47 billion years are not significantly changed when Canyon Diablo troilite lead is included in the data sets, suggesting that the initial Pb isotopic composition in both meteorites was the same as that in the troilite. The 4.48 billion year age, which is younger than the well established 4.54-4.56 billion years of the Allende chondrite and Angra dos Reis achondrite, appears to date an early metamorphic event rather than the formation of the chondrites.
Zircon and baddeleyite U–Pb geochronological dating is widely used in solid Earth sciences and the advent of rapid in-situ methods of analysis, such SIMS and ICP-MS, has re-emphasized the importance of having uniform standards. Recently, it has been shown that Hf isotopic data can provide important information on these minerals since they contain high concentrations of Hf, but have low Lu/Hf ratios, which results in negligible age correction. However, the complex internal structures that result from multiple thermal events, such as inherited cores and metamorphic overgrowths, require that the Hf isotopic data be measured with high spatial resolution. However, the isobaric interferences of 176Yb and 176Lu on 176Hf hamper the precise determination of the 176Hf/177Hf ratio during in-situ laser ablation MC-ICPMS analysis. It is shown here that mass biases of Yb (βYb) and Hf (βHf) change with time during analyses and behave differently for solutions and solid material. Therefore, it is suggested that the mean βYb value of the individual spot be used to obtain the precise isotopic composition for in-situ zircon and baddeleyite Hf isotopic analyses. For low Yb/Hf (176Yb/177Hf0.001) zircons and baddeleyites, since the interference of 176Yb on 176Hf is significant. Using the mean βYb value of the individual spot and newly published Yb isotopic abundance data, six standard zircons and two standard baddeleyites, have been investigated using a Neptune MC-ICPMS, with 193 nm laser. For zircons, the obtained 176Hf/177Hf ratios are 0.282307±31 (2SD) for 91500, 0.282680±31 (2SD) for TEMORA, 0.281729±21 (2SD) for CZ3, 282177±17 (2SD) for CN92-1, 0.282983±17 (2SD) for FM0411, and 281234±11 (2SD) for Phalaborwa. The baddeleyites from Phalaborwa and SK10-2 have 176Hf/177Hf ratios of 0.281238±12 and 0.282738±13 (2SD). These results agree well with the values obtained by the solution method and indicate that these standards have different Hf isotopic compositions, in which the extremely low 176Lu/177Hf and 176Yb/177Hf values of CZ3 zircon and Phalaborwa baddeleyite make them excellent standards for machine calibration during in-situ zircon Hf isotopic measurement, with the other standards being more suitable for the development of the correction method.
Super-chondritic 142Nd signatures are ubiquitous in terrestrial, Martian and lunar samples, and indicate that the terrestrial planets may have accreted from material with Sm/Nd ratio higher than chondritic. This contradicts the long-held view that chondrites represent a reference composition for the 147Sm–143Nd system. Using coupled 146Sm–142Nd and 147Sm–143Nd systematics in planetary samples, we have proposed a new set of values for the 147Sm/144Nd and 143Nd/144Nd ratios of the bulk silicate Earth (Caro et al., 2008). Here, we revise the Bulk Silicate Earth estimates for the 87Rb–87Sr and 176Lu–176Hf systems using coupled Sr–Nd–Hf systematics in terrestrial rocks. These estimates are consistent with Hf–Nd systematics in lunar samples. The implications of a slightly non-chondritic silicate Earth with respect to the geochemical evolution of the mantle–crust system are then examined. We show that the Archean mantle has evolved with a composition indistinguishable from that of the primitive mantle until about 2Gyr. Positive ε143Nd and ε176Hf values ubiquitous in the Archean mantle are thus accounted for by the non-chondritic Sm/Nd and Lu/Hf composition of the primitive mantle rather than by massive early crustal formation, which solves the paradox that early Archean domains only have a limited extension in the present-day continents. The Sm–Nd and Lu–Hf evolution of the depleted mantle for the past 3.5Gyr can be entirely explained by continuous extraction of the continents from a well-mixed mantle. Thus, in contrast to the chondritic Earth model, Sm–Nd mass balance relationships can be satisfied without the need to call upon hidden reservoirs or layered mantle convection. This new Sm–Nd mass balance yields a scenario of mantle evolution consistent with trace element and noble gas systematics. The high 3He/4He mantle component is associated with 143Nd/144Nd compositions indistinguishable from the bulk silicate Earth, suggesting that the less degassed mantle sources did not experience significant fractionation for moderately incompatible elements.
Discoveries of >4Ga old zircon grains in the northwest Yilgarn of Western Australia led to the conclusion that evolved crust formed on the Earth within the first few 100Ma after accretion. Little is known, however, about the fate of the first crust that shaped early Earth's surface. Here we report combined solution and laser-ablation Lu–Hf–U–Pb isotope analyses of early Archean and Hadean detrital zircon grains from different rocks of the Narryer Gneiss Complex (NGC), Yilgarn Craton, Western Australia. The zircons show two distinct groups with separate evolutionary trends in their Hf isotopes. The majority of the zircon grains point to separation from a depleted mantle reservoir at ∼3.8–3.9Ga. The second Hf isotope trend implies reworking of older Hadean zircon grains. The major trend starting at 3.8–3.9Ga defined by the Hf isotopes corresponds to a Lu/Hf that is characteristic for felsic crust and consequently, the primary sources for these zircons presumably had a chemical composition characteristic of continental crust. Reworked Hadean crust appears to have evolved with a similar low Lu/Hf, such that the early crust was probably evolved with respect to Lu–Hf distributions. The co-variation of Hf isotopes vs. age in zircon grains from Mt. Narryer and Jack Hills zircon grains implies a similar crustal source for both sediments in a single, major crustal domain. Age spectra and associated Hf isotopes in the zircon grains strongly argue for ongoing magmatic reworking over hundreds of millions of years of the felsic crustal domain in which the zircon grains formed. Late-stage metamorphic zircon grains from the Meeberrie Gneiss unit yield a mean U–Pb age of 3294.5±3.2Ma with initial Hf isotopes that correspond to the evolutionary trend defined by older NGC zircon grains and overlap with other detrital zircon grains, proving their genetic relationship. This ‘Meeberrie event’ is interpret here as the last reworking event in the precursor domain before final deposition. The continuous magmatic activity in one crustal domain during the Archean is recorded by the U–Pb ages and Hf isotope systematics of zircon grains and implies reworking of existing crust. We suspect that the most likely driving force for such reworking of crustal material is ongoing crustal collision and subduction. A comparison of Hf isotope signatures of zircon grains from other Archean terranes shows that similar trends are recognised within all sampled Archean domains. This implies either a global trend in crustal growth and reworking, or a genetic connection of Archean terranes in close paleo-proximity to each other. Notably, the Archean Acasta gneiss (Canada) shows a similar reworking patterns to the Yilgarn Craton of Hadean samples implying either a common Hadean source or amalgamation at the Hadean–Archean transition.
There is a growing need for new zircon reference materials for in situ Hf-isotope analysis by laser ablation-multicollector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS). In this contribution we document the results of a preliminary investigation of seven natural zircons, conducted in order to test their suitability in this regard. Solution MC-ICP-MS data on separated Lu and Hf fractions provided reference compositional data while the results of ca. 750 in situ LA-MC-ICP-MS analyses allowed assessment of potential micrometre-scale heterogeneity. On the basis of these analyses and additional relevant considerations such as availability, size and (Lu)Yb/Hf ratio, we suggest that, of the currently available zircons, Temora-2 and Mud Tank are most likely to provide robust reference materials for Hf isotope determinations both at the present time and into the future. The former has the advantage of also being well-characterised for U-Th-Pb systematics and suitable for in situ age determination, while the latter is the most readily available and is of very large grain size. Additional materials such as BR266, and 91500, although limited in supply, show more consistent Lu/Hf ratios and are thus of use in monitoring elemental fractionation during ICP-MS analysis.
The Acasta Gneiss Complex of northwestern Canada comprises early Archean orthogneisses and includes the oldest known terrestrial rocks (3.94–4.03 Ga). Previous zircon geochronological studies revealed the presence of zircon xenocrysts with ages up to 4.2 Ga in the oldest rocks, indicating that the source of early Archean granitoids contained Hadean (N 4.03 Ga) crust. In this study, we have determined the Lu–Hf isotopic compositions of zircon grains extracted from the early Archean Acasta gneisses to evaluate the extent of the Hadean crust contribution to the formation of these granitoids. Analyses of the Lu–Hf isotopes were carried out using in-situ laser ablation inductively coupled plasma mass spectrometry combined with cathodolu-minescence images of the internal structure of the grains. Two ca. 3.59 Ga granitoids have ε Hf (T) of −2.4 ± 2.2 and − 3.2 ± 2.5, suggesting that the source of the granitoids was extracted from the mantle as far back as 3.8 Ga. This is consistent with the presence of abundant zircon xenocrysts with ages up to 3.9 Ga. The Hf isotopic composition of a 3.72 Ga granitoid is significantly less radiogenic, with an ε Hf (T) of −6.1 ± 2.5. This indicates that the granitoid formed by remelting of very old crust. The ε Hf (T) values for the two oldest rocks, the ca. 3.97 Ga tonalites, are −1.2 ± 3.3 and −3.3 ± 1.7, respectively, indicating that even the oldest known granitoids contain reworked older crustal materials. These results suggest that Hadean crust had significantly contributed to the genesis of some of the early Archean Acasta granitoids.
We present initial 176Hf/177 Hf ratios for many samples of continental crust 3.7-0.3 Gy old. Results are based chiefly on zircons (1% Hf) and whole rocks: zircons are shown to be reliable carriers of essentially the initial Hf itself when properly chosen on the basis of U-Pb studies. Pre-3.0 Gy gneisses were apparently derived from an unfractionated mantle, but both depleted and undepleted mantle are evident as magma sources from 2.9 Gy to present. This mantle was sampled mainly from major crustal growth episodes 2.8, 1.8 and 0.7 Gy ago, all of which show gross heterogeneity of 176Hf/177Hf in magma sources from Hf=0 to +14, or about 60% of the variability of the present mantle.The approximate Hf=2Nd relationship in ancient and modern igneous rocks shows that 176Lu/177Hf fractionates in general twice as much as 147Sm/144Nd in mantle melting processes. This allows an estimation of the relative value of the unknown bulk solid/liquid distribution coefficient for Hf. DLu/DHf= 2.3 holds for most mantle source regions. For garnet to be an important residual mantle phase, it must hold Hf strongly in order to preserve Hf-Nd isotopic relationships.The ancient Hf initials are consistent with only a small proportion of recycled older cratons in new continental crust, and with quasi-continuous, episodic growth of the continental crust with time. However, recycling of crust less than 150 My old cannot realistically be detected using Hf initials. The mantle shows clearly the general positive Hf resulting from a residual geochemical state at least back to 2.9 Gy ago, and seems to have repeatedly possessed a similar degree of heterogeneity, rather than a continuously-developing depletion. This is consistent with a complex dynamic disequilibrium model for the creation, maintenance and destruction of heterogeneity in the mantle.
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 εHf 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.
Two different groups of values for the 176Lu decay constant have been determined by recent high-precision experiments. The λ176Lu values of 1.86–1.87 × 10−11 a−1 were determined by age comparisons using terrestrial minerals of Proterozoic and late Archean age, whereas values of ∼1.94 × 10−11 a−1 were determined in age comparison studies of meteorites.A possible branched decay of 176Lu could be the cause of this discrepancy. The β+ decay of 176Lu to 176Yb was detected in the early studies of radioactivity of 176Lu, with reported values of λβ+/(λβ+ + λβ−) in the total 176Lu ranging from less than 0.03 to 0.67. If the β+ decay fraction is close to the upper limit of the reported values, it can explain the 4%–6% difference between the apparent λ176Lu values.To get a reliable estimate for the β+ decay of 176Lu, we have measured Yb isotopic composition in 2.7 Ga zircons with Lu/YbN (chondrite-normalized) ratios of 1.40 and 1.45, in 1.0 Ga xenotime with Lu/YbN = 1.23, using Yb from the 28.4 Ma Fish Canyon Tuff (FCT) zircon and titanite as the modern reference value. Multiple analyses yielded the following weighted mean values (± 2σ) for the 176Yb/174Yb ratio: 0.4022134 ± 0.0000017 for the FCT zircon and titanite, 0.4022134 ± 0.0000019 for the 1.0 Ga xenotime, and 0.4022124 ± 0.0000033 for the 2.7 Ga zircons. These data yield λβ+/(λβ+ + λβ−) = −0.005 ± 0.015 (2σ) and establish an upper limit of 0.9% of total decays for the β+ decay branch. Branching decay can therefore be eliminated as the cause of the discrepancy in 176Lu decay constant estimates. We discuss other possible causes of the λ176Lu terrestrial vs. meteorite discrepancy.
New bulk Hf and Pb isotope data were obtained for 63 leached single zircons from Jack Hills (JH), Western Australia, using solution chemistry and, respectively, MC-ICP MS and ICP-MS. With the exception of one “young” zircon at 3.32 Ga, the remainder of the selected grains were previously dated at > 3.9 Ga by ion-microprobe. This work extends and complements the solution chemistry data of Harrison et al. [Harrison, T.M., Blichert-Toft, J., Müller, W., Albarède, F., Holden, P., Mojzsis, S.J., 2005. Heterogeneous Hadean hafnium: evidence of continental crust at 4.4 to 4.5 Ga. Science 310, 1947–1950.] but uses bulk rather than in situ Pb–Pb ages to interpret the Hf isotope data. This larger data set is used to explore whether the host rocks of the JH zircons formed as a succession of pulses or rather as a single event, and to calculate the age and assess the nature of their crustal protolith. We find that the parent granites of the JH zircons analyzed here formed during a single pulse 4.1 ± 0.1 Ga ago by the remelting of a 4.30–4.36 Ga old protolith. Monte Carlo modeling indicates that the 176Lu/177Hf ratios of this material (< 0.01) are unlike the ratios of modern-type oceanic crust and island arc rocks but rather fit a tonalite–trondhjemite–granodiorite (TTG) source. TTGs themselves derived their inordinately enriched character from a basaltic progenitor which corresponds to the missing enriched reservoir identified by the 143Nd–144Nd, 142Nd–144Nd, and 176Hf/177Hf systematics of Archean rocks. We speculate that crystallization of the magma ocean in the presence of garnet left the upper mantle and an early basaltic crust enriched in incompatible elements. Reaction of this early crust with the overlying hydrosphere and subsequent foundering into the mantle gave rise at ∼ 4.3 Ga to the TTG protolith of the JH granites. Dating the onset of plate tectonics therefore depends on whether TTGs can be considered as subduction zone magmas or not.
We report detailed high precision combined single grain U-Pb and Lu-Hf studies of early zircons to obtain more reliable indications of the extent of mantle depletion and crustal recycling in the Archean. Despite the possibility that MC-ICPMS affords for precise Hf isotopic measurement of single zircons, the complexity of grain populations adds uncertainty to initial isotopic ratios. Multiple episodes of zircon growth and ancient Pb loss, common in early Archaean rocks, result in ²⁰⁷Pb/²⁰⁶Pb ages, and in some cases ¹⁷⁶Hf/¹⁷⁶Hf ratios, that are variable between and within zircon grains. In order to evaluate the role of heterogeneity of zircon populations and to obtain the most reliable εHf(T), we have analysed several abraded zircon grains (from two to eleven) from each of several samples for both Lu-Hf and U-Pb. Hf isotopic analyses with precision better than 1 to 1.5 ε-units (2σ) were obtained from grains weighing between 3 and 10 micrograms. The observed internal variations in age, U-Pb discordance, and Hf isotopic composition have been tested against models of disturbance of isotopic systems in zircon. Application of the U-Pb and Lu-Hf methods to the same zircon grains and analysis of single grains appears to be crucial for finding closed geochemical systems and thereby obtaining reliable Hf isotope data from early Archaean rocks. The precision and accuracy of Hf isotopic data obtained with the approach presented here are limited mainly by the sensitivity of Hf isotopic analyses, and may be greatly improved with the progress of analytical techniques.
A precise determination of the isotopic abundances of tungsten with natural isotopic composition is presented. WO−3 ions are generated by negative thermal ionization (NTI) in a double-filament ion source. La2O3 is used as a chemical substance to reduce the electron work function of the rhenium filament material. An ionization efficiency of 1% is obtained for sample loadings of 100 ng. The isotopic abundances are measured with relative standard deviations of 0.2% for the least abundant 180W isotope and 0.02–0.004% for the other tungsten isotopes. These improved isotopic data are used to recalculate the atomic weight of tungsten as 183.8417 ± 0.0001. The new NTI technique is an ideal tool for the application of isotope dilution mass spectrometry to analyse tungsten traces and for the measurement of isotopic shifts of this element in meteorites produced by the decay of 182Hf.
Detrital zircon crystals from the Jack Hills metasedimentary belt, Western Australia, are the only surviving vestiges of Hadean crust and represent an extraordinary archive into the nature of the early Earth. We report the results of an in situ isotopic study of 68 Jack Hills zircons in which the Hf and Pb isotope ratios were measured concurrently, allowing a better integration of isotope tracer information (176Hf/177Hf) with crystallization age (207Pb/206Pb). These data are augmented by Hf isotope data from zircons of the surrounding Narryer gneisses (3.65–3.30 Ga) and from Neoarchaean granites that intrude the Jack Hills belt. The detrital zircons define a subchondritic εHf–time array that attests to a far simpler evolution for the Hadean Earth than claimed by recent studies. This evolution is consistent with the protracted intra-crustal reworking of an enriched, dominantly mafic protolith that was extracted from primordial mantle at 4.4–4.5 Ga, perhaps during the solidification of a terrestrial magma ocean. There is no evidence for the existence of strongly depleted Hadean mantle, or for juvenile input into the parental magmas to the Jack Hills zircons. This simple Hf isotope evolution is difficult to reconcile with modern plate tectonic processes. Strongly unradiogenic Hf isotope compositions of zircons from several Archaean gneiss terranes, including the Narryer and Acasta gneisses, suggest that Hadean source reservoirs were tapped by granitic magmas throughout the Archaean. This supports the notion of a long-lived and globally extensive Hadean protocrust that may have comprised the nuclei of some Archaean cratons.
The Lutetium–Hafnium radiogenic isotopic system is widely used as a chronometer and tracer of planetary evolution. In order for this isotopic system to fulfill its potential in planetary studies, the Lu–Hf system parameters need to be more tightly constrained, in particular the Lu–Hf isotopic composition of the chondritic uniform reservoir (CHUR) and, by extension, the bulk silicate Earth (BSE). We present new Lu–Hf and Sm–Nd isotopic compositions of unequilibrated carbonaceous, ordinary, and enstatite chondrites of petrologic types 1, 2, and 3 which define a narrow range of Lu/Hf ratios (3%) identical with that of Sm/Nd. This contrasts with previously published data from mostly equilibrated ordinary chondrites of petrologic types 4, 5, and 6 which have a much larger range in Lu/Hf (28%). This heterogeneity has hampered an unambiguous choice for the Lu–Hf isotopic composition of CHUR. Our new determinations of Lu–Hf CHUR parameters are 176Lu/177Hf = 0.0336 ± 1 and 176Hf/177Hf = 0.282785 ± 11 (2σm), which are higher than previous estimates and, together with average Sm–Nd chondrite compositions of unequilibrated chondrites of 147Sm/144Nd = 0.1960 ± 4 and 143Nd/144Nd = 0.512630 ± 11 (2σm), now provide firm constraints on the chondritic parameters for both Lu–Hf and Sm–Nd isotopic systems. A comparison of Lu–Hf and Sm–Nd data show that terrestrial planets, as well as early differentiated planetesimals, converge toward a common initial Hf and Nd isotope composition corresponding to the average of chondrites. Finally, a compilation of Lu–Hf isotopic data of unequilibrated and equilibrated chondrites demonstrates that the 176Lu decay decay-constant value cannot be resolved by age comparison on metamorphosed or shocked planetary materials which have a complex history.
It has long been customary to assume that in the bulk composition of the Earth, all refractory-lithophile elements (including major oxides Al2O3 and CaO, all of the REE, and the heat-producing elements Th and U) occur in chondritic, bulk solar system, proportion to one another. Recently, however, Nd-isotopic studies (most notably Boyet M. and Carlson R. W. (2006) A new geochemical model for the Earth’s mantle inferred from 146Sm–142Nd systematics. Earth Planet. Sci. Lett.250, 254–268) have suggested that at least the outer portion of the planet features a Nd/Sm ratio depleted to ∼0.93 times the chondritic ratio. The primary reaction to this type of evidence has been to invoke a “hidden” reservoir of enriched matter, sequestered into the deepest mantle as a consequence of primordial differentiation. I propose a hypothesis that potentially explains the evidence for Nd/Sm depletion in a very different way. Among the handful of major types of differentiated asteroidal meteorites, two (ureilites and aubrites) are ultramafic restites so consistently devoid of plagioclase that meteoriticists were once mystified as to how all the complementary plagioclase-rich matter (basalt) was lost. The explanation appears to be basalt loss by graphite-fueled explosive volcanism on roughly 100-km sized planetesimals; with the dispersiveness of the process dramatically enhanced, relative to terrestrial experience, because the pyroclastic gases expand into vacuous space (Wilson L. and Keil K. (1991) Consequences of explosive eruptions on small Solar System bodies: the case of the missing basalts on the aubrite parent body. Earth Planet. Sci. Lett.104, 505–512). By analogy with lunar pyroclastic products, the typical size of pyroclastic melt/glass droplets under these circumstances will be roughly 0.1 mm. Once separated from an asteroidal or planetesimal gravitational field, droplets of this size will generally spiral toward the Sun, rather than reaccrete, because drag forces such the Poynting–Robertson effect quickly modify their orbits (the semimajor axis, in a typical scenario, is reduced by several hundred km during the first trip around the Sun). Assuming a similar process occurred on many of the Earth’s precursor planetesimals while they were still roughly 100 km in diameter, the net effect would be a depleted composition for the final Earth. I have modeled the process of trace-element depletion in the planetesimal mantles, assuming the partial melting was nonmodal and either batch or dynamic in terms of the melt-removal style. Assuming the process is moderately efficient, typical final-Earth Nd/Sm ratios are 0.93–0.96 times chondritic. Depletion is enhanced by a relatively low assumed residual porosity in batch-melting scenarios, but dampened by a relatively high value for “continuous” residue porosity in dynamic melting scenarios. Pigeonite in the source matter has a dampening effect on depletion. There are important side effects to the Nd/Sm depletion. The heat-producing elements, Th, U and K, might be severely depleted. The Eu/Eu∗ ratio of the planet is unlikely to remain precisely chondritic. One of the most inevitable side effects, depletion of the Al/Ca ratio, is consistent with an otherwise puzzling aspect of the composition of the upper mantle. A perfectly undepleted composition for the bulk Earth is dubious.
Due to the large partition coefficients of garnet for Lu compared to Sm, Nd, and Hf, garnet residual from crustal melting events has a large potential for retaining Lu and, over time, producing high reservoirs in the lower crust. Therefore, melts derived from such garnet-bearing residual assemblages may be high in relative to . In order to determine the extent of potential hafnium isotopic heterogeneities in the lower crust from residual garnet, and to detail coupled HfNd isotopic behavior in older crustal rocks in general, we have determined the hafnium and neodymium isotopic compositions of thirty-two Precambrian granites and rhyolites from diverse suites of crustally derived magmatic suites. The majority of these rocks are known, from major-element and neodymium isotopic evidence, to have been derived from older (>300 m.y.) crust.Modeling of and partitioning during melting events in the lower crust indicates that anomalously high compositions should develop within 300–400 m.y. in residual assemblages provided melting was at least 25–30% and 10% or more garnet was left in the residue. In spite of the diverse older crustal sources for the granites and rhyolites in this study, and the potential for garnet to be present in their source regions, none of the granitoids have anomalously high initial compositions: all samples (with one exception) have initial Hf and Nd compositions that plot within a ±8 ϵHf unit wide band of the reference line (ϵHf = 2ϵNd + 2) for juvenile crust. The absence of high initial compositions in these granites and rhyolites most likely implies that either (1) garnet is not a common residual phase in the lower crust or (2) garnet-bearing restite is not easily incorporated into later melts or (3) not enough time was available to develop anomalous Hf compositions.
We report analyses of the176Hf/177Hf ratio for 25 chondrites from different classes of meteorites (C, O, and E) and the176Lu/177Hf ratio for 23 of these as measured by plasma source mass spectrometry. We have obtained a new set of present-day mean values in chondrites of176Hf/177Hf= 0.282772 ± 29 and176Lu/177Hf= 0.0332 ± 2. The176Hf/177Hf ratio of the Solar System material 4.56 Ga ago was 0.279742 ± 29. Because the mantle array lies above the Bulk Silicate Earth in a143Nd/144Nd versus176Hf/177Hf plot, no terrestrial basalt seems to have formed from a primitive undifferentiated mantle, thereby casting doubt on the significance of high3He/4He ratios. Comparison of observedHf/Nd ratios with those inferred from isotopic plots indicates that, in addition to the two most prominent components at the surface of the Earth, the depleted mantle and the continental crust, at least one more reservoir, which is not a significant component in the source of oceanic basalts, is needed to account for the Bulk Silicate Earth Hf-Nd geochemistry. This unaccounted for component probably consists of subducted basalts, representing ancient oceanic crust and plateaus. The lower continental crust and subducted pelagic sediments are found to be unsuitable candidates. Although it would explain the Lu-Hf systematics of oceanic basalts, perovskite fractionation from an early magma ocean does not account for the associated Nd isotopic signature. Most basalts forming the mantle array tap a mantle source which corresponds to residues left by ancient melting events with garnet at the liquidus.
We have conducted a detailed study of the Mn-Cr systematics of the angrite D'Orbigny. Here, we report Cr isotopic abundances and Mn/Cr ratios in olivine, pyroxene, glass, chromite, and bulk rock samples from D'Orbigny. 53Cr excesses in these samples correlate well with their respective Mn/Cr ratios and define an isochron with a slope that corresponds to an initial 53Mn/55Mn ratio = (3.24 ± 0.04) - 10-6 and initial 53Cr/52Cr ratio of e(53) = 0.30 ± 0.03 at the time of isotopic closure. The 53Mn/55Mn ratio of the D'Orbigny bulk rock is more than two-fold the 53Mn/55Mn ratio of the angrites Lewis Cliff 86010 (LEW) and Angra dos Reis (ADOR) and implies an older Mn-Cr age of 4562.9 ± 0.6 Ma for D'Orbigny relative to a Pb-Pb age of 4557.8 ± 0.5 Ma for LEW and ADOR. One of the most unusual aspects of D'Orbigny is the presence of glass, a phase that has not been identified in any of the other angrites. The Mn-Cr data for glass and a pyroxene fraction found in druses indicate that they formed contemporaneously with the main phases of the meteorite. Since the Mn-Cr age of D'Orbigny is ~5 Ma years older than the angrites LEW and ADOR, D'Orbigny likely represents an earlier stage in the evolution of the angrite parent body.
INFERENCES about the early evolution of the Earth's crust and mantle have come largely from the study of isotope systematics- in particular, those of neodymium1-5. Neodymium isotope data from the oldest preserved rocks have been interpreted4,6-8 as reflecting early large-scale chemical depletion of the mantle (presumably resulting from the extraction of continental crust), but these data have remained controversial, in view of the potential for disturbances to the samarium-neodymium system during these rocks' long history9-11. Here we provide an independent evaluation of the Nd isotope compositions of ten early Archaean (3.6-3.8 Gyr old) gneisses, by investigating the hafnium isotope systematics of zircons from these rocks. The Hf data are consistent with the Nd record in indicating early depletion of the mantle, but fail to verify the scale and variability of this depletion. We conclude that Nd isotopes of early Archaean gneisses do not faithfully record isotopic variations in the early Earth, and therefore that these data need to be examined more critically before they can be used to constrain the early history of crust-mantle differentiation.
Well-defined constants of radioactive decay are the cornerstone of geochronology and the use of radiogenic isotopes to constrain the time scales and mechanisms of planetary differentiation. Four new determinations of the lutetium-176 decay constant (lambda176Lu) made by calibration against the uranium-lead decay schemes yield a mean value of 1.865 +/- 0.015 x 10(-11) year(-1), in agreement with the two most recent decay-counting experiments. Lutetium-hafnium ages that are based on the previously used lambda176Lu of 1.93 x 10(-11) to 1.94 x 10(-11) year(-1) are thus approximately 4% too young, and the initial hafnium isotope compositions of some of Earth's oldest minerals and rocks become less radiogenic relative to bulk undifferentiated Earth when calculated using the new decay constant. The existence of strongly unradiogenic hafnium in Early Archean and Hadean zircons implies that enriched crustal reservoirs existed on Earth by 4.3 billion years ago and persisted for 200 million years or more. Hence, current models of early terrestrial differentiation need revision.
Separation of high field strength elements Hf) and Lu from rock samples for MC-ICPMS measurements Separation of Hf and Lu for high-pre-cision isotope analysis of rock samples by magnetic sector-multiple collector ICP-MS
- C Münker
- S Weyer
- E Scherer
- Mezger
Münker C, Weyer S, Scherer E, Mezger K (2001) Separation of high field strength elements (Nb, Ta, Zr, Hf) and Lu from rock samples for MC-ICPMS measurements. Geochem Geophys Geosyst 2(12):1064. Iizuka et al. PNAS Early Edition | 5 of 6 40. Blichert-Toft J, Chauvel C, Albarède F (1997) Separation of Hf and Lu for high-pre-cision isotope analysis of rock samples by magnetic sector-multiple collector ICP-MS. Contrib Miner Petrol 127:248–260.