Geochronology and Thermochronology

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❤ : Lu-hf dating method

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These differences can confer some advantages to the Lu-Hf isotope system for both geochronology and tracer isotopic work as will be discussed below. Yet the discrepancies in the determinations by the direct physical counting experiments remain unexplained and unresolved. Determination of the radioisotope decay constants and half-lives: Rubidium-87 87Rb.

Model ages are also applied to sediments to identify wr Hf model ages 03. The Bulk Silicate Earth and CHUR Because of the analytical challenges in measuring Hf isotope compositions in Hf-poor ~0. Chemical Geology , 167 , 257 —270.

The vertical shaded areas indicate the two standard deviations 2σ from the mean values vertical lines for each method. The uncertainties associated with direct half-life determinations are, in most cases, still at the percent level at best, which is still significantly better than any radioisotope method for determining the ages of rock formations. True or False: If a U-Pb dating result on a zircon grain is reproducible, it is thereby self-checked, and must necessarily give a reliable crystallization age for the host rock. Earth and Planetary Science Letters204, 167 —181. The revision in the value for the 176 Lu decay constant from the Patchett 1983b and Sguigna et al. The two isotopes, 176Lu and 176Hf, in the system are measured as ratio to the reference stable isotope of 177Hf. The current practice employed in most labs is to remove the majority of the Yb recall that Yb is an even element and so is much more abundant than the odd numbered Lu and use the Yb isotope measurements to determine the mass bias and then apply this to the Lu measurement. On the other hand, the r-process, which differs from the s-process by its faster rate of neutron capture of more than one neutron, entails a succession of rapid neutron captures hence the name r-process by heavy seed nuclei, typically 56Fe or other more neutron-rich heavy isotopes, before β decay takes place. This approach entails multi-chronometric lu-hf dating method of terrestrial rocks or a mineral or minerals from them for example, Scherer, Münker, and Mezger 2001; Söderlund et al 2004or meteorites and a mineral from them for example, Amelin 2005; Patchett et al. Lu-Hf lu-hf dating method H2SO4 leaching 23 Diffusion limited REE uptake Fig. This introduces a time-dependent concentration into lu-hf dating method time-independent linear equation.

Lutetium - Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event. Conventional isochron ages are obtained from bulk garnet separates and are only an estimate of the average age of the overall growth of garnet.

IntroductionThe Lu-Hf isotope system, with applications to geo- and cosmochemistry, was first investigated in the early 1980s Patchett 1983b; Patchett and Tatsumoto 1980a, b, c, 1981 following the successful implementation of the Rb-Sr and Sm-Nd isotope systems several years earlier. There are some obvious similarities between the Lu-Hf and Sm-Nd isotope systems and, as a result, they have long been used in concert in a wide range of studies e. There are some obvious similarities between the Lu-Hf and Sm-Nd isotope systems and, as a result, they have long been used in concert in a wide range of studies e. In these two systems all elements are lithophile and refractor y with high condensation temperatures. Because of these characteristics it has long been assumed that their abundances in the Earth can be approximated by chondritic meteorites see discussion below. In addition, all element s in these systems behave incompatibly during melting and are concentrated in the melt over the residual solid. In both systems, the daughter element e. In the case of the Lu-Hf isotope system, the parent is the heaviest of the rare-earth elements REEs and has broad geochemical similarities with all other +3 valence REEs. These differences can confer some advantages to the Lu-Hf isotope system for both geochronology and tracer isotopic work as will be discussed below. Although the Lu-Hf system was introduced to the geochemical and cosmochemical communities in the 1980s, analytical challenges limited the widespread use of this technique until the late 1990s. As a result there were only a few practitioners using this system in the 1980s and much of the 1990s. This revolutionized the use of the Lu-Hf isotope system for geochronology and tracer isotope applications by allowing the routine analysis not only of Hf isotopes in general but also of much smaller amounts of Hf. This made several important things possib le. First , it allowed for the more precise and accurate determination of the 176 Lu decay constant Scherer et al. Second , the new MC-ICPMS instrumentation also facilitated the analysis of high Lu-Hf, but Hf-poor, phases like garnet or apatite, which made the use of these phases for geochronology possible. Third , it also made it possible to analyze Hf-poor rocks such as chondritic meteorites, which allowed more accurate constraints to be placed on the chondritic Lu-Hf value e. Finally , MC-ICPMS technology has made it possible to determine the Lu-Hf isotope composition in very small, Hf-rich phases such as zircon. This latter application, particularly when done in conjunction with U-Pb geochronology, has enjoyed an explosion of use in recent years e. Background Lutetium is the heaviest of the rare-earth elements REEs and shares a +3 valence with the lanthanides as a whole. It also has an odd atomic number odd number of protons; Z ¼71 , which, in part, makes it the least abundant of the REEs 67 ppb; McDonough and Sun 1995. Lu has two isotopes Table 1 : 175 Lu is stable, is an odd-even nuclide, and is the most abundant isotope 97. It is relatively under-abundant in the Earth 0. One isotope, 176 Hf 5. By conven- tion the reference stable isotope for the Lu-Hf system is 177 Hf 18. This technique involves the addition of measured amounts of tracers of known isotopic composition highly enriched in 176 Lu and 180 Hf, typically, with some labs using 178 Hf. These spikes are added to the sample solution following or during dissolution, equilibrated with the solution, and become part of the isotopic mixture of the sample. In order to correct for mass dependent fractionation during mass spectrometry i. Mass bias for Lu isotopic analyses is complicated by virtue of the fact that Lu has only two isotopes, making internal normalization impossible. In the early TIMS era of Lu-Hf isotope measurements, the small degree of mass bias ~ 0. With the advent of the MC-ICPMS analyses, however, mass fractionation that is over an order of magnitude larger than TIMS necessitated an accurate mass bias correction. The current approach is to use a different element with at least one invariant isotope ratio and which has a similar mass bias response, such as Yb, to correct for mass bias in the Lu isotopic measurement. The current practice employed in most labs is to remove the majority of the Yb recall that Yb is an even element and so is much more abundant than the odd numbered Lu and use the Yb isotope measurements to determine the mass bias and then apply this to the Lu measurement. This approach is somewhat complicated by the uncertain isotopic composition of Yb e. The interference of 176 Yb on the spike isotope, 176 Lu, requires that most of the Yb be removed from the sample. In detail there are probably small differences in the mass bias of Yb and Lu, and there are approaches to quantify these differences T able 1 Isotopic composition of Lu and Hf 175 Lu 176 Lu Ratio to 176 Lu 37. The JMC-475 Hf standard solution is available from the author on request. Decay System Fundamentals Lutetium-176 decays to 176 Hf by beta decay with a half-life of 37. In practice, we reference these isotopes to 177 Hf, a stable isotope of Hf. An important part of using these radiogenic equations for geochronology and for determining precise initial isotopic compositions is not only measuring the present-day ratios accurate ly but also knowing the decay constant precisely and accurately. The 176 Lu decay constant has been a source of uncertainty, however , and this has limited the full application of the Lu-Hf isotope system. Using a presumed age of 4. This value was later revised by Patchett 1983b t o l 176 Lu ¼ 1. This revised value was similar to a decay constant value of 1. The agreement between these results led, in part, to the use of the l 176 Lu value of 1. This early history of 176 Lu decay constant determinations is reviewed more thor- oughly by Begemann et al. The value determined for l 176 Lu changed dramatically following age comparison studies by Scherer et al. This included two pegmatites from Norway with ages ~1. In a similar study, Söderlund et al. Additional determinations by Scherer et al. As a result of this agreement, the isotopic community is now using the l 176 Lu value of 1. The half-life corresponding to this value is 37. The revision in the value for the 176 Lu decay constant from the Patchett 1983b and Sguigna et al. As was true with the earlier work of Patchett and Tatsumoto 1980a , however, the basaltic and cumulate eucrites plot at opposite ends of the isochron with the basaltic eucrites at the low Lu-Hf end and the cumulate eucrites at the high Lu-Hf end, which may indicate these are not truly cogenetic. Further, these samples appear to have different formation ages with the cumulate eucrites arguably younger Mittlefehldt et al. In another study using meteorite age comparison, Bizzarro et al. The Bizzarro et al. As was true with the eucrite- based determination, this study included a diverse suite of meteorites of potentially different ages and metamorphic histories, and therefore, the decay constant determination may be affected in some way by the use of samples that do not strictly meet the requirements for an isochron. These caveats notwithstanding, there are indications that meteorite isochrons yield a faster decay constant. One possible explanation for this apparent difference between the terrestrial and meteorite determina- tions is the presence of excess 176 Hf in meteorites. The exact mechanism of how this might have occurred is still a matter of debate Albarède et al. Unlike some chronometers such as U-Pb in zircon where there is little, if any, daughter isotope at the time of mineral growth, in the Lu-Hf system, there is invariably an abundance of daughter isotopes present i. For this reason, it is not possible to determine the age of a single mineral sample by measuring the accumulation of daughter isotopes in the sample. Thus all Lu-Hf geochronometry relies on the isochron approach, which utilizes coexisting rocks and minerals to determine an age. Important assumptions in this approach include the following: 1 All the mineral s formed at the same time i. The systematics of the Lu-Hf isochron method, using garnet geochronology as an example, is shown in Fig. In most cases the rock is certainly not in perfect isotopic equilibrium i. In addition, the time different phases in the rock form may not be precisely the same, especially in the case of prolonged garnet growth e. Examples of the former may be represented by the eucrite isochrons combining both basaltic and cumulate eucrites Blichert-Toft et al. In practice, therefore, Lu-Hf whole-rock isochrons are rarely employed for geochronology, especially for terrestrial samples. The magmatic rocks that have the most promise for useful geochronometry are those that contain apatite as a crystallizing phase. As demonstrated by the decay constant work of Scherer et al. In addition, these phases are likely in isotopic equilibrium with the magma at the time of their formation and likely formed at the same time as the other phases in the rock. Garnet Lu-Hf Geochronology The most successful geochronologic application of the Lu-Hf system has been garnet geochronol- ogy. Following this early proof-of-concept work, there have been a large number of studies in a wide variety of metamorphic rocks: garnet-bearing pelitic schists, para- and orthogneisses, amphibolites, eclogites, and granulites Anczkiewicz et al. Reliable garnet Lu-Hf ages have been reported for rocks as old as Archean e. Although there are many parallels between Lu-Hf and Sm-Nd garnet geochronology, there are important differences between these two systems. First , the decay rate of the parent isotope in the Lu-Hf system, 176 Lu, is ~3 times faster than 147 Sm in the Sm-Nd system. This results in a higher rate of ingrowth in the radiogenic daughter, 176 Hf, compared to 143 Nd. Third , Lu is partitioned into garnet most strongly of all the REEs, while Hf is incorporated into garnet only weakly Table 2. In this way, we would expect slightly older Lu-Hf garnet ages compared with Sm-Nd ages in a slowly growing garnet Lapen et al. Debate continues on closure temperature estimates of the Sm-Nd chronometer e. This can lead to differences between the garnet Lu-Hf and Sm-Nd ages with the Lu-Hf ages T able 2 Melt-solid partition coefficients for Lu and Hf in various minerals Mineral D Lu D Hf Melt References Olivine 0. In some granulite terranes that may have remained buried at mid- to lower-crustal levels for a prolonged time during and following garnet growth, these age differences can be large — as much as 50 — 70 m. Finally , the presence of accessory phases e. For the Lu-Hf system, zircons provide the biggest challenge. The incorporation of zircon Hf not in isotopic equilibrium with the protolith at the time of metamorphism in the analyses, either in garnet or as a dispersed accessory phase in the rock matrix, can result in erroneous ages e. Fortunately, what makes zircon resistant to isotopic equilibration during metamorphism also makes it resistant to dissolution, and thus zircon inclusions can be largely avoided with a nonaggressive dissolution protocol e. The effect of apatite inclusions on garnet Lu-Hf geochronology, however , has not yet been examined in the literature. The effect of these inclusions is generally to drastically reduce the spread in the isochron. Lawsonite Lu-Hf Geochronology A recent addition to the Lu-Hf geochronology toolbox is the use of the mineral lawsonite to date subduction zone processes Mulcahy et al. Lawsonite is an important index mineral formed during low temperature-high pressur e subduction zone metamoprhism and potentially provides one of the few tools we have available to date subduction zone processes. The full potential of this geochronologic tool is just now being explored. Hf Radiogenic Isotope Geochemistry As mentioned above, both Lu and Hf are lithophile and refractory elements with high condensation temperatures. Because of these characteristics, it has long been assumed that the abundance of these elements in the bulk Earth can be approximated by chondritic meteorites. It has also been long assumed that the isotopic composition of a refractory element such as Hf was homogeneous in the solar nebula and can also be approximated by chondritic meteorites. This is known as the Chondritic Uniform Reservoir model, or CHUR. Recent work, primarily based on the short-lived 146 Sm- 142 Nd isotopic system, has questioned whether the Earth is strictly chondritic in composition see discus- sion in Sm-Nd section. Regardless of whether the Earth is strictly chondritic, the CHUR model provides an important reference value for a plausible bulk Earth composition. The Lu-Hf and Sm-Nd systems are used in tandem for planetary evolutionary studies for the important reason that these are the only two such isotopic systems in which all elements are lithophile and refractory. The Bulk Silicate Earth and CHUR Because of the analytical challenges in measuring Hf isotope compositions in Hf-poor ~0. There have been two approaches used to determine a CHUR value for the Lu-Hf and Sm-Nd isotope systems. This works because the total Sm-Nd variation in chondrites is modest. With the advent of the MC-ICPMS and its application to measuring Hf isotopes came the ability to analyze Hf-poor chondrites more precisely. In order to constrain the Lu-Hf CHUR values, Blichert- T oft and Albarede 1997 analyzed 25 chondritic meteorit es including carbonaceous, ordinary, and enstatite chondrites. These are the values currently in use by the geochemical and cosmo- chemical communities. Although these are very close to the values of Jacobsen and Wasserburg 1980 , 1984 , the Bouvier et al. As is true for the Sm-Nd system, Hf isotope data are often expressed in epsilon values, which are variations in parts per 10,000 from the CHUR reference. It is important to note that the CHUR value also needs to be calculated back to its composition at time, t, from the present-day values. Lu-Hf T racers of Geologic Processes The current paradigm for the evolution of the Earth holds that the crust was formed from silicate melts extracted in some way from the mantle. The crustal rocks resulting from this process are enriched in incompatible elements hence the term enriched crust , and the mantle left from this extraction of crust is depleted in incompatible elements hence the term depleted mantle. In terms of epsilon values relative to bulk Earth i. The concept of a model age in the Lu-Hf isotope system follows that of Sm-Nd model ages DePaolo 1981 and is illustrated in Fig. The time, corresponding to this intersection, is a Hf model age — in this case, the Hf depleted mantle model age Hf T DM. This model age has also been termed a crust-formation age, Hf T CR , signifying the average time since separation from the depleted mantle or time of residence in a crustal reservoir DePaolo 1981. Hf isotope evolution 0. In this way it is simply another means of conveying the Hf isotopic compositions in the context of time — in much the same way as an epsilon Hf isotope diagram places the Hf isotope compositions in a time context. Model ages are also applied to sediments to identify wr Hf model ages 03. Also shown in this diagram are the depleted mantle model ages T DM and the CHUR model ages T CHUR. Note the different trajectories for the two samples and the differences in T CHUR. As can be seen from this diagram, the uncertainty in the T DM increases dramatically the further the initial Hf value is away from the depleted mantle reference line. Further, unrecognized Pb loss in the zircon, which would yield anomalously young crystallization ages, exacerbates the difference between crystallization and depleted mantle ages. This is identical to the approach used for Sm-Nd model ages in sediments with the caveat that Hf model ages are often more variable than Nd model ages because of the presence or absence of Hf-rich and unradiogenic zircons in the sediments e. A recent adaptation of Hf model ages — and one that has seen an explosion of use in recent years — is the use of Hf model ages of zircons. The way this works is shown in Fig. As was the case with the other model ages, the intersection with the depleted mantle reference line yields the zircon Hf depleted mantle model age. This technique has recently been applied to detrital zircon studies because of the facility of acquiring U-Pb crystallization ages and Lu-Hf isotope compositions on the same zircon grain by laser ablation-inductively coupled plasma mass spectrometry LA-ICPMS. The further the composition of a zircon is from the depleted mantle or other reference, the greater the uncertainty in the model age. Another limitation of this technique is that the model ages rely on the intersection of the zircon + precursor rock trajectory with a depleted mantle reference curve. The depleted mantle, however, is not a discrete entity with a discrete isotopic composition. Finally , it is not possible to determine the true ages of some detrital zircons precisely, due to the potential for ancient Pb loss and other factors. If ancient Pb loss exists in these grains, this would greatly exacerbate the differences between the crystallization and the Hf model ages with the former being too young and the latter being too old. Conclusions The Lu-Hf isotope system is an important chronometer in geochemistry and cosmochemist ry. Its utility as a chronometer in whole-rock samples, however, is limited by the relatively small degrees of Lu-Hf fractionation during magmatic processes. In rocks with high Lu-Hf phases, the Lu-Hf isotope system can be a powerful chronome ter; this is especially true for garnet-bea ring metamorphic rocks and magmatic rocks with apatite as a crystallizing phase. There now appears to be widespread agreement on a value for the 176 Lu decay constant of 1. This value has important implications not only for Lu-Hf geochronology but also for early Earth differentiation and evolution based on calculated initial Hf isotope values. Similarly, there is currently widespread acceptance of the Bouvier et al. Notwithstanding discussions of whether or not the Earth is strictly chon- dritic, these values provide a self-consistent baseline on which to reference the isotopic evolution of the Earth. Tracer isotopic applications provide equal utility of the Lu-Hf isotopic system and, in conjunction with independent age information, can be used to determine the Hf isotopic composition of rocks and minerals back into time. This information is useful in constraining both the evolution and source regions of igneous systems as well as the provenance of sedimentary rocks. By far the most heavily utilized mineral for tracer isotope studies is zircon, which provides a unique opportunity for determining both age and Hf isotope compositions in individual crystals and even separate growth zones within a single crystals , such as in the case of inheritance or overgrowths. For this reason, the integrated U-Pb and Hf isotope information from zircon has seen a rapid increase in use in recent years and will likely continue. Many of these studies often report zircon Hf model ages. While these provide useful qualitative, model-based data, these are not in any way truly ages and should not be interpreted as containing quantitative chronologic information. Gamma-ray irradiation in the early solar system and the conundrum of the 176 Lu decay constant. Geochimica et Cosmochimica Acta , 70 , 1261 — 1270. Geochemical test for branching decay of 176 Lu. Geochimica et Cosmochimica Acta , 69 , 465 — 473. Early-middle Archaean crustal evolution deduced from Lu-Hf and U-Pb isotopic studies of single zircon grains. Geochimica et Cosmochimica Acta, 64 , 4205 —4225. Improving precision of Sm-Nd garnet dating by H 2 SO 4 leaching — a simple solution to the phosphate inclusion problem. In V ance, D. 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Journal of Analytical Atomic Spectrometry , 7 , 571 —575. Geodynamics of synconvergent extension and tectonic mode switching: constraints from the Sevier-Laramide orogen. T ectonics , 31 ,1 — 20. High-precision analysis of Pb isotope ratios by multi-collector ICP-MS. Chemical Geology , 167 , 257 —270. Chemical Geology , 209, 121 —135. Chinese Science Bulletin , 53 , 1565 — 1573. Simultaneous determina- tions of U — Pb age, Hf isotopes and trace element compositions of zircon by excimer laser-ablation quadrupole and multiple-collector ICP-MS. Chemical Geology , 247 , 100 — 118. In Presented at 7th International Kimberlite Conference , Cape T own, South Africa. Insights into the thermal evolution of the Belt-Purcell basin; evidence from Lu-Hf garnet geoch ronology. Canadian Journal of Earth Sciences , 47 , 161 —179. Lu — Hf garnet geochronology applied to plate boundary zones: insights from the U HP terrane exhumed within the Woodlark Rift. Earth and Planetary Science Letters , 309 ,5 6 — 66. Zircon Lu-Hf isotopic data are given in electronic Supplement Table S4. Initial epsilon Hafnium εHf i values of zircon grains can provide in- formation on the crust from which they were derived, with positive Hf initials generally pointing to a juvenile mantle-derived source, where- as negative initials are commonly interpreted to indicate derivation from, or mixing with, pre-existing continental crust e. Hf and Nd model ages provide information on the time elapsed since melt separated from a depleted mantle reservoir e. Hf and Nd model ages provide information on the time elapsed since melt separated from a depleted mantle reservoir e. It should be noted however that model ages do not reflect 'true' protolith ages but may place some constraints on the timing and nature of crust- forming events Vervoort, 2015. We report Hf isotope data for 3467 Ma igneous zircons from the Owens Gully Diorite of the Pilbara Craton. These zircons, designated OG1 or OGC, have simple, well-defined U-Pb age systematics. High-pressure and ultra high-pressure U HP metamorphic rocks occur in many of the world's major orogenic belts, suggesting that subduction of continental lithosphere is a geologically important process. Despite the widespread occurrence of these rocks, relatively little is known about the timescales associated with U HP metamorphism. This is because most U HP terranes are tectonically... We have determined Lu-Hf garnet ages from spatially separated garnet bearing localities in northern Idaho. The Lu-Hf ages are diverse and reflect a progression of Mesoproterozoic metamorphic events. The oldest Lu-Hf garnet age determined in this study is 1463 ± 24 Ma for garnet within a kyanite schist exposed in the core-zone of the Boehls Butte metamorphic complex. A garnet schist from the...

Samarium-neodymium dating
The vertical shaded areas indicate the two standard deviations 2σ from the mean values vertical lines for each method. The uncertainties associated with direct half-life determinations are, in most cases, still at the percent level at best, which is still significantly better than any radioisotope method for determining the ages of rock formations. True or False: If a U-Pb dating result on a zircon grain is reproducible, it is thereby self-checked, and must necessarily give a reliable crystallization age for the host rock. Earth and Planetary Science Letters204, 167 —181. The revision in the value for the 176 Lu decay constant from the Patchett 1983b and Sguigna et al. The two isotopes, 176Lu and 176Hf, in the system are measured as ratio to the reference stable isotope of 177Hf. The current practice employed in most labs is to remove the majority of the Yb recall that Yb is an even element and so is much more abundant than the odd numbered Lu and use the Yb isotope measurements to determine the mass bias and then apply this to the Lu measurement. On the other hand, the r-process, which differs from the s-process by its faster rate of neutron capture of more than one neutron, entails a succession of rapid neutron captures hence the name r-process by heavy seed nuclei, typically 56Fe or other more neutron-rich heavy isotopes, before β decay takes place. This approach entails multi-chronometric lu-hf dating method of terrestrial rocks or a mineral or minerals from them for example, Scherer, Münker, and Mezger 2001; Söderlund et al 2004or meteorites and a mineral from them for example, Amelin 2005; Patchett et al. Lu-Hf lu-hf dating method H2SO4 leaching 23 Diffusion limited REE uptake Fig. This introduces a time-dependent concentration into lu-hf dating method time-independent linear equation. اليمن برنامج تعارف في جديد موقع قبطي للدردشة
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