The determination of the Pressure-Temperature-time (P-T-t) path of metamorphic rocks has an essential role in understanding the tectonic evolution of metamorphic rocks. Garnet has played a crucially important part in our understanding of metamorphic and tectonic processes and conditions. The potential of garnet geochronology in metamorphic rock studies lies in the use of the compositional zoning in garnet to elucidate quantitative P-T paths and the coupled application of multiple geochronometers to constrain the timescales of garnet growth. Garnet has long been the mineral of choice for metamorphic chronology because it hosts a remarkable number of long-lived radioactive isotopes and their stable decay products. These include: 238U, 235U and 232Th, which decay via intermediate steps to 206Pb, 207Pb and 208Pb, respectively; 87Rb, which decays to 87Sr; 176Lu, which decays to 176Hf; and 147Sm, which decays to 143Nd. This makes garnet one of the most versatile mineral phases available to geochronologists. As a result of advances in the techniques for sample preparation and measuring Nd-Hf isotopes by Thermal Ionization Mass Spectrometry (TIMS) and Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS), garnet Lu-Hf and Sm-Nd geochronology has been increasingly used to constrain the rates and timing of tectonometamorphic processes in orogenic studies over the last two decades. Metamorphic geochronologists have developed new techniques, such as microsampling, to link garnet ages with textural and petrological fingerprints of particular metamorphic processes, leading to major advances in petrogenesis and tectonics. When combined with petrographic and chemical observations, Lu-Hf and Sm-Nd ages in garnets are able to give unprecedented resolution of the timing and processes of metamorphism, although there are many potential pitfalls in the acquisition and interpretation of these data. This paper provides a brief review of the basic science and development of the garnet Lu-Hf and Sm-Nd systems, highlights the potential of garnet Lu-Hf and Sm-Nd geochronology, and reviews several crucial issues related to the complexities of interpretation of the radiometric ages. Examples, limitations, advantages and potential research directions are presented.
Cheng Hao, Xu Naixiao. Garnet Geochronology of Metamorphic Rocks. Advances in Earth Science[J], 2020, 35(10): 991-1005 DOI:10.11867/j.issn.1001-8166.2020.089
1 引 言
地球科学研究致力于了解地球的现在和过去,展望未来。对各种自然现象(事件)进行观察和描述,进而理解自然现象之间的联系是地球科学研究的基本思路。准确厘定自然现象(事件)的绝对时间则是寻找这种联系的关键钥匙,也是地学最核心的内容之一。基于放射性同位素衰变(或宇宙成核)的现代地质年代学(Geochronology)不仅能对自然现象(事件)进行定年,还能通过使用严格、定量和创新的方法来测定其发生的速率、通量和时间尺度,并从时间的角度来了解自然现象的驱动机制。固体岩石是地质年代学中最早也是最广泛的研究对象。近半个世纪以来,国际上以固体岩石为定年对象的长周期定年体系的进展乏善可陈,基于石榴石的各个定年体系,是近10多年来唯一在理论和实践中得到长足发展的体系。其中的变质岩往往经历了复杂的地质过程,准确厘定变质事件发生的时间、速率和跨越尺度一直是地质年代学中的一个重点和难点。石榴石作为变质岩中广泛存在的造岩矿物相,其不仅记录了变质的压力—温度(P-T)演化轨迹,也封存了变质事件的时间记录。长期以来,石榴石一直是变质年代学的首选矿物,因为它拥有众多长半衰期的放射性同位素及其衰变的稳定产物。这些长半衰期同位素包括:238U、235U和232Th分别经过冗长的衰变链最终变为稳定的206Pb、207Pb和208Pb,87Rb衰变为87Sr,176Lu衰变为176Hf,147Sm衰变为143Nd。这使得石榴石成为地质定年的最通用矿物相。因此,基于石榴石的变质岩年代学是研究变质事件的绝对时间,并将其与构造变质过程的特定热历史联系起来的众多方法中最重要的方法之一[1,2]。随着同位素样品化学分离技术的进步,以及热电离质谱(Thermal Ionization Mass Spectrometry,TIMS)和多收集器电感耦合等离子体质谱(MC-ICP-MS)仪器的发展,上述基于石榴石的U-Pb、Rb-Sr、Lu-Hf和Sm-Nd地质年代学已越来越多地用于限定各种构造背景下的变质过程的速率和时间[3~9]。伴随着各种微区取样技术的日新月异,将石榴石年龄与特定变质过程的岩相学指纹联系起来,正在极大地促进年代学、岩石学和构造地质学等学科的发展[6,7,10~12]。本文对基于石榴石的放射性同位素定年系统的基本原理,及其在变质岩研究中的应用,各体系的优势和局限性,以及今后可能的突破方向等进行了回顾和展望。
Lu-Hf和Sm-Nd体系使用等时线法——利用共存岩石和/或矿物中的放射性母体同位素(176Lu或147Sm)衰变为稳定的子体同位素(176Hf或143Nd)来确定绝对年龄。如果有一系列同源的岩石和/或矿物严格地在封闭系统中演化,那么它们的所有测量值将落在D/R vs. P/R图的一条等时直线上,其中D代表子体同位素,R代表一个稳定的非放射性成因子体同位素,P是母体同位素。等时线法[22]对所有样品的假设包括:同时形成,初始子体同位素平衡,自形成以来一直保持封闭状态。封闭系统中Sm-Nd和Lu-Hf的衰变方程为:
衰变常数可以通过对α、β-或γ活度进行直接计数法来确定。首次对147Sm衰变至143Nd的α放射性的测量可以追溯到19世纪30年代初,从那时开始使用电离室和液体闪烁计数仪进行直接计数。但许多此类实验(考虑到147Sm的半衰期长达约1011年)都无法排除系统误差的干扰,要确定一个最接近真实值的衰变常数并非易事[23]。确定衰变常数的第二种方法是通过年龄对比法:比如为了获得147Sm的衰变常数,可以对已知年龄的地质样品进行Sm-Nd定年,通过年龄对比来获得147Sm的半衰期。Lugmair等[24]通过对Juvinas和Angra dos Reis陨石同时进行Sm-Nd和U-Pb对比定年,获得了目前被地质学家所普遍采用的147Sm的衰变常数6.54×10-12 a-1,尽管这个数值仍在其他学科,比如化学和物理学中存在争论[25,26]。当然,由年龄对比法获得的衰变常数的精度和准确度也受到诸多不确定因素的影响。例如,两个同位素系统是否同时封闭并保持严格封闭,参照体系本身的半衰期的精度和准确度。
Fig.1
Errors in age determinations are controlled by uncertainties in decay constants of different isotopic systems
(a) Errors in age due solely to uncertainties in the decay constant as a function of time [λ147Sm = (6.54 ± 0.05)×10-12 a-1 (1σ)[24]; λ176Lu = (1.867 ± 0.008)× 10-11 a-1(1σ)[30,31]]; (b) The smaller black bar represents the analytical uncertainty for each age, whereas the larger white bar encompasses the combined analytical and the decay constants uncertainties (conceptually modified after reference[19]). In this case, although the analyses agree poorly within analytical uncertainties (black), they agree well when the day constants uncertainties are considered (gray), modified after reference[21]
一般是采用同位素稀释法,通过对ICP-MS或TIMS测试获得的母子体同位素的比值进行回归,得出Lu-Hf和Sm-Nd等时线年龄。尽管母体同位素147Sm与子体核素143Nd的质量数不同,但参比同位素144Nd与144Sm是同量异位素,为了避免后期校正带来的不确定性,在使用TIMS进行Sm-Nd定年时,需要将Sm与Nd进行分离。由于会抑制三价REE离子的电离[48],所以Ba也需要和REE完全分离。对于几百纳克的Nd,现代质谱仪的同位素比值精度可达10×10-6(2 RSD, Relative Standard Deviation,相对标准偏差)或更高(例如2×10-6[37,49]。通过改进的进样锥和真空系统,结合使用Ta[50],对4 ng Nd的测试可以达到很高的外部精度(10×10-6~35×10-6,2 RSD),使高分辨率的地质年代分析成为可能[8]。
Fig.2
A pioneering application of the garnet Sm-Nd method
(a) Sm-Nd isochron diagram for Whole-Rock (WR)-garnet core and matrix-garnet rim pairs from a graphite-free metapelite, modified after reference [58]; (b) Isochron diagram for 12 concentric zones in a single garnet showing the corresponding drilled annuli, the differences in isotope ratios relative to the composition of the rim are shown in the lower diagram; (c) Ages of different zones within a garnet plotted as a function of radius and volume with indications of rapid growth episodes, modified after reference [8]
Fig.3
Garnet cores and rims from eclogites are distinguishable from Lu-Hf isochrons
(a) Isochron plots illustrating the different ages obtained for two garnet generations present in an eclogite from Trescolmen, the element map for Fe is also shown,modified after reference[62]; (b) Lu-Hf isochron plot for garnet fraction, bulk rock powder separates of the Qianjin eclogite from the Dabie orogen. Inset photomicrograph shows the separated garnet porphyroblasts with distinct optical contrasts,modified after reference[63]
(a) Lu-Hf garnet-only two-point isochrons of consecutive garnet segments showing the compositions of the rims of the garnets as the lower points of the isochrons. An X-ray map of Fe is also shown. Numbered boxes indicate sampled sections of the garnet crystals for dating, modified after reference[10]. (b) Lu-Hf plots for 12 garnet zones in the garnet grain, two mixtures of garnet rims and adhered matrix, and the whole rock. Numbered boxes in the inset photograph of the garnet porphyroblast indicate sampled sections for Lu-Hf dating, modified after reference[11]
Fig.5
Coupled Lu-Hf and Sm-Nd geochronology on a single microsampling garnet
(a) Micro-sawing of a quarter of the mega garnet. Ten rectangle sections were micro-sawed to produce five garnet aliquots for isotope analysis indicated by the dotted white lines; (b) Sm-Nd age resetting profiles as a function of initial cooling rate and peak temperature for a 2.1 cm diameter garnet crystal. Square symbols represent the mean age differences between the calculated Lu-Hf and Sm-Nd ages of the five mega garnet zones, and implies that Lu-Hf ages solely represent prograde to peak metamorphic garnet growth. Note that to reset the Sm-Nd age of the garnet rim, would require peak metamorphic temperature of 720 °C and initial cooling rates of 2.0~1.5 °C/Ma, modified after reference[6]
Fig.7
Inclusion issues in different isochron systems(modified after reference[21])
(a) Sm-Nd plot showing the effects of contamination due to high Sm/Nd inclusions on measured garnet compositions; (b) Lu-Hf plot showing the effects of old, inherited zircon included in garnet and whole rock samples
Fig.8
Diffusional resetting of garnet ages with varying radii(modified after reference [7])
(a) The violin plot of measured fraction sizes (radii) of the large garnet and a cluster of small euhedral grains, presenting the distribution of the length and probability density; (b) Summary of geochronological data reported in this study and interpreted T-t path. The red line indicates our preferred T-t path. When poorly constrained, the line is thick and dashed
Lu-Hf geochronology on cm-sized garnets using microsampling: New constraints on garnet growth rates and duration of metamorphism during continental collision (Menderes Massif, Turkey)
U-Pb systematics of garnet: Dating the growth of garnet in the late Archean Pikwitonei granulite domain at Cauchon and Natawahunan Lakes, Manitoba, Canada
Initial Pb-Sr(-Nd) isotopic heterogeneity in a single allanite-epidote crystal: Implications of reaction history for the dating of minerals with low parent-to-daughter ratios
176Lu-176Hf geochronology of garnet I: Experimental determination of the diffusion kinetics of Lu3+ and Hf 4+ in garnet, closure temperatures and geochronological implications
Sm-Nd isotope systematics in garnet from different lithologies (Eastern Alps): Age results, and an evaluation of potential problems for garnet Sm-Nd chronometry
Correction: A rapid and efficient ion-exchange chromatography for Lu-Hf, Sm-Nd, and Rb-Sr geochronology and the routine isotope analysis of sub-ng amounts of Hf by MC-ICP-MS
Progress in linking accessory mineral growth and breakdown to major mineral evolution in metamorphic rocks: A thermodynamic approach in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-ZrO2 system
Microsampling Lu-Hf geochronology on mm-sized garnet in eclogites constrains early garnet growth and timing of tectonometamorphism in the North Qilian orogenic belt
Pseudosection modelling and garnet Lu-Hf geochronology of HP amphibole schists constrain the closure of an ocean basin between the northern and southern Lhasa blocks, central Tibet
Ultrahigh temperature granulite metamorphism (1 050 °C, 12 kbar) and decompression in garnet (Mg70)-orthopyroxene-sillimanite gneisses from the Rauer Group, East Antarctica
Diffusional homogenization of light REE in garnet from the Day Nui Con Voi Massif in N-Vietnam: Implications for Sm-Nd geochronology and timing of metamorphism in the Red River shear zone
[J]. , 2012, 318/319: 16-30.
ChengHao, DuFraneS A, VervoortJ D, et al.
Protracted oceanic subduction prior to continental subduction: New Lu-Hf and Sm-Nd geochronology of oceanic-type high-pressure eclogite in the western Dabie orogen
... (a) Micro-sawing of a quarter of the mega garnet. Ten rectangle sections were micro-sawed to produce five garnet aliquots for isotope analysis indicated by the dotted white lines; (b) Sm-Nd age resetting profiles as a function of initial cooling rate and peak temperature for a 2.1 cm diameter garnet crystal. Square symbols represent the mean age differences between the calculated Lu-Hf and Sm-Nd ages of the five mega garnet zones, and implies that Lu-Hf ages solely represent prograde to peak metamorphic garnet growth. Note that to reset the Sm-Nd age of the garnet rim, would require peak metamorphic temperature of 720 °C and initial cooling rates of 2.0~1.5 °C/Ma, modified after reference[6] ...
... (a)大颗粒石榴石和小颗粒石榴石集合中内部裂隙分离开的小区域尺度的小提琴图; (b)定年结果指示石榴石生长的T-t过程的时间和事件记录,红色直线表示优选的T-t路径,不确定的部分用虚线标示Diffusional resetting of garnet ages with varying radii(modified after reference [7])
(a) The violin plot of measured fraction sizes (radii) of the large garnet and a cluster of small euhedral grains, presenting the distribution of the length and probability density; (b) Summary of geochronological data reported in this study and interpreted T-t path. The red line indicates our preferred T-t path. When poorly constrained, the line is thick and dashed ...
High resolution Sm-Nd garnet geochronology reveals the uneven pace of tectonometamorphic processes
5
2010
... 一般是采用同位素稀释法,通过对ICP-MS或TIMS测试获得的母子体同位素的比值进行回归,得出Lu-Hf和Sm-Nd等时线年龄.尽管母体同位素147Sm与子体核素143Nd的质量数不同,但参比同位素144Nd与144Sm是同量异位素,为了避免后期校正带来的不确定性,在使用TIMS进行Sm-Nd定年时,需要将Sm与Nd进行分离.由于会抑制三价REE离子的电离[48],所以Ba也需要和REE完全分离.对于几百纳克的Nd,现代质谱仪的同位素比值精度可达10×10-6(2 RSD, Relative Standard Deviation,相对标准偏差)或更高(例如2×10-6[37,49].通过改进的进样锥和真空系统,结合使用Ta[50],对4 ng Nd的测试可以达到很高的外部精度(10×10-6~35×10-6,2 RSD),使高分辨率的地质年代分析成为可能[8]. ...
... (a) Sm-Nd isochron diagram for Whole-Rock (WR)-garnet core and matrix-garnet rim pairs from a graphite-free metapelite, modified after reference [58]; (b) Isochron diagram for 12 concentric zones in a single garnet showing the corresponding drilled annuli, the differences in isotope ratios relative to the composition of the rim are shown in the lower diagram; (c) Ages of different zones within a garnet plotted as a function of radius and volume with indications of rapid growth episodes, modified after reference [8] ...
Lu-Hf geochronology on cm-sized garnets using microsampling: New constraints on garnet growth rates and duration of metamorphism during continental collision (Menderes Massif, Turkey)
4
2015
... 地球科学研究致力于了解地球的现在和过去,展望未来.对各种自然现象(事件)进行观察和描述,进而理解自然现象之间的联系是地球科学研究的基本思路.准确厘定自然现象(事件)的绝对时间则是寻找这种联系的关键钥匙,也是地学最核心的内容之一.基于放射性同位素衰变(或宇宙成核)的现代地质年代学(Geochronology)不仅能对自然现象(事件)进行定年,还能通过使用严格、定量和创新的方法来测定其发生的速率、通量和时间尺度,并从时间的角度来了解自然现象的驱动机制.固体岩石是地质年代学中最早也是最广泛的研究对象.近半个世纪以来,国际上以固体岩石为定年对象的长周期定年体系的进展乏善可陈,基于石榴石的各个定年体系,是近10多年来唯一在理论和实践中得到长足发展的体系.其中的变质岩往往经历了复杂的地质过程,准确厘定变质事件发生的时间、速率和跨越尺度一直是地质年代学中的一个重点和难点.石榴石作为变质岩中广泛存在的造岩矿物相,其不仅记录了变质的压力—温度(P-T)演化轨迹,也封存了变质事件的时间记录.长期以来,石榴石一直是变质年代学的首选矿物,因为它拥有众多长半衰期的放射性同位素及其衰变的稳定产物.这些长半衰期同位素包括:238U、235U和232Th分别经过冗长的衰变链最终变为稳定的206Pb、207Pb和208Pb,87Rb衰变为87Sr,176Lu衰变为176Hf,147Sm衰变为143Nd.这使得石榴石成为地质定年的最通用矿物相.因此,基于石榴石的变质岩年代学是研究变质事件的绝对时间,并将其与构造变质过程的特定热历史联系起来的众多方法中最重要的方法之一[1,2].随着同位素样品化学分离技术的进步,以及热电离质谱(Thermal Ionization Mass Spectrometry,TIMS)和多收集器电感耦合等离子体质谱(MC-ICP-MS)仪器的发展,上述基于石榴石的U-Pb、Rb-Sr、Lu-Hf和Sm-Nd地质年代学已越来越多地用于限定各种构造背景下的变质过程的速率和时间[3~9].伴随着各种微区取样技术的日新月异,将石榴石年龄与特定变质过程的岩相学指纹联系起来,正在极大地促进年代学、岩石学和构造地质学等学科的发展[6,7,10~12].本文对基于石榴石的放射性同位素定年系统的基本原理,及其在变质岩研究中的应用,各体系的优势和局限性,以及今后可能的突破方向等进行了回顾和展望. ...
... (a) Lu-Hf garnet-only two-point isochrons of consecutive garnet segments showing the compositions of the rims of the garnets as the lower points of the isochrons. An X-ray map of Fe is also shown. Numbered boxes indicate sampled sections of the garnet crystals for dating, modified after reference[10]. (b) Lu-Hf plots for 12 garnet zones in the garnet grain, two mixtures of garnet rims and adhered matrix, and the whole rock. Numbered boxes in the inset photograph of the garnet porphyroblast indicate sampled sections for Lu-Hf dating, modified after reference[11] ...
... (a) Lu-Hf garnet-only two-point isochrons of consecutive garnet segments showing the compositions of the rims of the garnets as the lower points of the isochrons. An X-ray map of Fe is also shown. Numbered boxes indicate sampled sections of the garnet crystals for dating, modified after reference[10]. (b) Lu-Hf plots for 12 garnet zones in the garnet grain, two mixtures of garnet rims and adhered matrix, and the whole rock. Numbered boxes in the inset photograph of the garnet porphyroblast indicate sampled sections for Lu-Hf dating, modified after reference[11] ...
Unraveling histories of hydrothermal systems via U-Pb laser ablation dating of skarn garnet
2
2018
... 地球科学研究致力于了解地球的现在和过去,展望未来.对各种自然现象(事件)进行观察和描述,进而理解自然现象之间的联系是地球科学研究的基本思路.准确厘定自然现象(事件)的绝对时间则是寻找这种联系的关键钥匙,也是地学最核心的内容之一.基于放射性同位素衰变(或宇宙成核)的现代地质年代学(Geochronology)不仅能对自然现象(事件)进行定年,还能通过使用严格、定量和创新的方法来测定其发生的速率、通量和时间尺度,并从时间的角度来了解自然现象的驱动机制.固体岩石是地质年代学中最早也是最广泛的研究对象.近半个世纪以来,国际上以固体岩石为定年对象的长周期定年体系的进展乏善可陈,基于石榴石的各个定年体系,是近10多年来唯一在理论和实践中得到长足发展的体系.其中的变质岩往往经历了复杂的地质过程,准确厘定变质事件发生的时间、速率和跨越尺度一直是地质年代学中的一个重点和难点.石榴石作为变质岩中广泛存在的造岩矿物相,其不仅记录了变质的压力—温度(P-T)演化轨迹,也封存了变质事件的时间记录.长期以来,石榴石一直是变质年代学的首选矿物,因为它拥有众多长半衰期的放射性同位素及其衰变的稳定产物.这些长半衰期同位素包括:238U、235U和232Th分别经过冗长的衰变链最终变为稳定的206Pb、207Pb和208Pb,87Rb衰变为87Sr,176Lu衰变为176Hf,147Sm衰变为143Nd.这使得石榴石成为地质定年的最通用矿物相.因此,基于石榴石的变质岩年代学是研究变质事件的绝对时间,并将其与构造变质过程的特定热历史联系起来的众多方法中最重要的方法之一[1,2].随着同位素样品化学分离技术的进步,以及热电离质谱(Thermal Ionization Mass Spectrometry,TIMS)和多收集器电感耦合等离子体质谱(MC-ICP-MS)仪器的发展,上述基于石榴石的U-Pb、Rb-Sr、Lu-Hf和Sm-Nd地质年代学已越来越多地用于限定各种构造背景下的变质过程的速率和时间[3~9].伴随着各种微区取样技术的日新月异,将石榴石年龄与特定变质过程的岩相学指纹联系起来,正在极大地促进年代学、岩石学和构造地质学等学科的发展[6,7,10~12].本文对基于石榴石的放射性同位素定年系统的基本原理,及其在变质岩研究中的应用,各体系的优势和局限性,以及今后可能的突破方向等进行了回顾和展望. ...
U-Pb systematics of garnet: Dating the growth of garnet in the late Archean Pikwitonei granulite domain at Cauchon and Natawahunan Lakes, Manitoba, Canada
Initial Pb-Sr(-Nd) isotopic heterogeneity in a single allanite-epidote crystal: Implications of reaction history for the dating of minerals with low parent-to-daughter ratios
... (a) Errors in age due solely to uncertainties in the decay constant as a function of time [λ147Sm = (6.54 ± 0.05)×10-12 a-1 (1σ)[24]; λ176Lu = (1.867 ± 0.008)× 10-11 a-1(1σ)[30,31]]; (b) The smaller black bar represents the analytical uncertainty for each age, whereas the larger white bar encompasses the combined analytical and the decay constants uncertainties (conceptually modified after reference[19]). In this case, although the analyses agree poorly within analytical uncertainties (black), they agree well when the day constants uncertainties are considered (gray), modified after reference[21] ...
Quantitative P-T paths from zoned minerals: Theory and tectonic applications
... (a) Errors in age due solely to uncertainties in the decay constant as a function of time [λ147Sm = (6.54 ± 0.05)×10-12 a-1 (1σ)[24]; λ176Lu = (1.867 ± 0.008)× 10-11 a-1(1σ)[30,31]]; (b) The smaller black bar represents the analytical uncertainty for each age, whereas the larger white bar encompasses the combined analytical and the decay constants uncertainties (conceptually modified after reference[19]). In this case, although the analyses agree poorly within analytical uncertainties (black), they agree well when the day constants uncertainties are considered (gray), modified after reference[21] ...
... (a)包裹体矿物对石榴石Sm-Nd等时线的影响;(b)包裹体矿物对石榴石Lu-Hf等时线的影响Inclusion issues in different isochron systems(modified after reference[21])
(a) Sm-Nd plot showing the effects of contamination due to high Sm/Nd inclusions on measured garnet compositions; (b) Lu-Hf plot showing the effects of old, inherited zircon included in garnet and whole rock samples ...
Graphic interpretation of discordant age measurements on metamorphic rocks
1
1961
... Lu-Hf和Sm-Nd体系使用等时线法——利用共存岩石和/或矿物中的放射性母体同位素(176Lu或147Sm)衰变为稳定的子体同位素(176Hf或143Nd)来确定绝对年龄.如果有一系列同源的岩石和/或矿物严格地在封闭系统中演化,那么它们的所有测量值将落在D/R vs. P/R图的一条等时直线上,其中D代表子体同位素,R代表一个稳定的非放射性成因子体同位素,P是母体同位素.等时线法[22]对所有样品的假设包括:同时形成,初始子体同位素平衡,自形成以来一直保持封闭状态.封闭系统中Sm-Nd和Lu-Hf的衰变方程为: ...
Call for an improved set of decay constants for geochronological use
2
2001
... 衰变常数可以通过对α、β-或γ活度进行直接计数法来确定.首次对147Sm衰变至143Nd的α放射性的测量可以追溯到19世纪30年代初,从那时开始使用电离室和液体闪烁计数仪进行直接计数.但许多此类实验(考虑到147Sm的半衰期长达约1011年)都无法排除系统误差的干扰,要确定一个最接近真实值的衰变常数并非易事[23].确定衰变常数的第二种方法是通过年龄对比法:比如为了获得147Sm的衰变常数,可以对已知年龄的地质样品进行Sm-Nd定年,通过年龄对比来获得147Sm的半衰期.Lugmair等[24]通过对Juvinas和Angra dos Reis陨石同时进行Sm-Nd和U-Pb对比定年,获得了目前被地质学家所普遍采用的147Sm的衰变常数6.54×10-12 a-1,尽管这个数值仍在其他学科,比如化学和物理学中存在争论[25,26].当然,由年龄对比法获得的衰变常数的精度和准确度也受到诸多不确定因素的影响.例如,两个同位素系统是否同时封闭并保持严格封闭,参照体系本身的半衰期的精度和准确度. ...
... 一个一直被忽视的问题就是由衰变常数(λ)的不确定性所引起的系统误差对定年结果的影响:随着地质年代学数据的精度和准确度的提高,对比和解读同一样品不同定年体系年龄差异的意义就需要考虑衰变常数的不确定性带来的误差.例如,只考虑由衰变常数不确定性引起的误差,年龄误差是与绝对年龄值呈线性正相关关系的(σt=σλ/λ×t,t为时间).除了极其古老的样品,理想情况下,与同位素比质谱仪测量的分析不确定度相比,衰变常数的不确定度可忽略不计[23].但Lu-Hf/Sm-Nd体系的年龄每增加约234 Ma/131 Ma,由衰变常数不确定性所引起的年龄误差就将增加约1 Ma.考虑到近年来大量报道的Lu-Hf和Sm-Nd年龄精度都小于1 Ma(2σ未引入衰变常数误差)[37],忽略衰变常数的不确定性就会错误地解读这些年龄.以图1a为例,仅考虑衰变常数带来的误差,200 Ma的Sm-Nd年龄的误差为±1.5 Ma,对于2 000 Ma的Sm-Nd年龄则高达±15 Ma;类似的Lu-Hf年龄中由衰变常数误差带来的不确定性分别为±0.8 Ma和±8 Ma.当比较同一样品或不同样品中相同定年体系年龄时,由于衰变常数误差会使两个年龄偏大或者偏小相同的幅度,在这种情况下,可以不考虑衰变常数不确定性.然而,当比较同一样品或不同样品中不同定年体系年龄的时候,衰变常数不确定性传递到年龄的误差则不能忽略.以图1b为例,一个Lu-Hf年龄为(202±2) Ma的样品,对应的Sm-Nd年龄为(196±2) Ma,在不考虑衰变常数误差的情况下,这两个年龄是显著有差别的;如果考虑衰变常数的不确定性,它们将变得难以区分[(202±2.8) Ma vs. (196±3.5) Ma],在误差范围内一致.对年龄的解释也将从前者可能指示两个不同的地质事件,变成后者的一个单一地质事件. ...
Lunar initial 143Nd/144Nd: Differential evolution of the lunar crust and mantle
3
1978
... 衰变常数可以通过对α、β-或γ活度进行直接计数法来确定.首次对147Sm衰变至143Nd的α放射性的测量可以追溯到19世纪30年代初,从那时开始使用电离室和液体闪烁计数仪进行直接计数.但许多此类实验(考虑到147Sm的半衰期长达约1011年)都无法排除系统误差的干扰,要确定一个最接近真实值的衰变常数并非易事[23].确定衰变常数的第二种方法是通过年龄对比法:比如为了获得147Sm的衰变常数,可以对已知年龄的地质样品进行Sm-Nd定年,通过年龄对比来获得147Sm的半衰期.Lugmair等[24]通过对Juvinas和Angra dos Reis陨石同时进行Sm-Nd和U-Pb对比定年,获得了目前被地质学家所普遍采用的147Sm的衰变常数6.54×10-12 a-1,尽管这个数值仍在其他学科,比如化学和物理学中存在争论[25,26].当然,由年龄对比法获得的衰变常数的精度和准确度也受到诸多不确定因素的影响.例如,两个同位素系统是否同时封闭并保持严格封闭,参照体系本身的半衰期的精度和准确度. ...
... (a) Errors in age due solely to uncertainties in the decay constant as a function of time [λ147Sm = (6.54 ± 0.05)×10-12 a-1 (1σ)[24]; λ176Lu = (1.867 ± 0.008)× 10-11 a-1(1σ)[30,31]]; (b) The smaller black bar represents the analytical uncertainty for each age, whereas the larger white bar encompasses the combined analytical and the decay constants uncertainties (conceptually modified after reference[19]). In this case, although the analyses agree poorly within analytical uncertainties (black), they agree well when the day constants uncertainties are considered (gray), modified after reference[21] ...
Half-Life of Samarium-147
1
2003
... 衰变常数可以通过对α、β-或γ活度进行直接计数法来确定.首次对147Sm衰变至143Nd的α放射性的测量可以追溯到19世纪30年代初,从那时开始使用电离室和液体闪烁计数仪进行直接计数.但许多此类实验(考虑到147Sm的半衰期长达约1011年)都无法排除系统误差的干扰,要确定一个最接近真实值的衰变常数并非易事[23].确定衰变常数的第二种方法是通过年龄对比法:比如为了获得147Sm的衰变常数,可以对已知年龄的地质样品进行Sm-Nd定年,通过年龄对比来获得147Sm的半衰期.Lugmair等[24]通过对Juvinas和Angra dos Reis陨石同时进行Sm-Nd和U-Pb对比定年,获得了目前被地质学家所普遍采用的147Sm的衰变常数6.54×10-12 a-1,尽管这个数值仍在其他学科,比如化学和物理学中存在争论[25,26].当然,由年龄对比法获得的衰变常数的精度和准确度也受到诸多不确定因素的影响.例如,两个同位素系统是否同时封闭并保持严格封闭,参照体系本身的半衰期的精度和准确度. ...
Alpha decay half-life of 147Sm in metal samarium and Sm2O3
1
2010
... 衰变常数可以通过对α、β-或γ活度进行直接计数法来确定.首次对147Sm衰变至143Nd的α放射性的测量可以追溯到19世纪30年代初,从那时开始使用电离室和液体闪烁计数仪进行直接计数.但许多此类实验(考虑到147Sm的半衰期长达约1011年)都无法排除系统误差的干扰,要确定一个最接近真实值的衰变常数并非易事[23].确定衰变常数的第二种方法是通过年龄对比法:比如为了获得147Sm的衰变常数,可以对已知年龄的地质样品进行Sm-Nd定年,通过年龄对比来获得147Sm的半衰期.Lugmair等[24]通过对Juvinas和Angra dos Reis陨石同时进行Sm-Nd和U-Pb对比定年,获得了目前被地质学家所普遍采用的147Sm的衰变常数6.54×10-12 a-1,尽管这个数值仍在其他学科,比如化学和物理学中存在争论[25,26].当然,由年龄对比法获得的衰变常数的精度和准确度也受到诸多不确定因素的影响.例如,两个同位素系统是否同时封闭并保持严格封闭,参照体系本身的半衰期的精度和准确度. ...
Half-life measurements of Lutetium-176 using underground HPGe-detectors
... (a) Errors in age due solely to uncertainties in the decay constant as a function of time [λ147Sm = (6.54 ± 0.05)×10-12 a-1 (1σ)[24]; λ176Lu = (1.867 ± 0.008)× 10-11 a-1(1σ)[30,31]]; (b) The smaller black bar represents the analytical uncertainty for each age, whereas the larger white bar encompasses the combined analytical and the decay constants uncertainties (conceptually modified after reference[19]). In this case, although the analyses agree poorly within analytical uncertainties (black), they agree well when the day constants uncertainties are considered (gray), modified after reference[21] ...
The 176Lu decay constant determined by Lu-Hf and U-Pb isotope systematics of Precambrian mafic intrusions
... (a) Errors in age due solely to uncertainties in the decay constant as a function of time [λ147Sm = (6.54 ± 0.05)×10-12 a-1 (1σ)[24]; λ176Lu = (1.867 ± 0.008)× 10-11 a-1(1σ)[30,31]]; (b) The smaller black bar represents the analytical uncertainty for each age, whereas the larger white bar encompasses the combined analytical and the decay constants uncertainties (conceptually modified after reference[19]). In this case, although the analyses agree poorly within analytical uncertainties (black), they agree well when the day constants uncertainties are considered (gray), modified after reference[21] ...
147Sm-143Nd and 176Lu-176Hf in eucrites and the differentiation of the HED parent body
Garnet geochronology: Timekeeper of tectonometamorphic processes
2
2013
... 一个一直被忽视的问题就是由衰变常数(λ)的不确定性所引起的系统误差对定年结果的影响:随着地质年代学数据的精度和准确度的提高,对比和解读同一样品不同定年体系年龄差异的意义就需要考虑衰变常数的不确定性带来的误差.例如,只考虑由衰变常数不确定性引起的误差,年龄误差是与绝对年龄值呈线性正相关关系的(σt=σλ/λ×t,t为时间).除了极其古老的样品,理想情况下,与同位素比质谱仪测量的分析不确定度相比,衰变常数的不确定度可忽略不计[23].但Lu-Hf/Sm-Nd体系的年龄每增加约234 Ma/131 Ma,由衰变常数不确定性所引起的年龄误差就将增加约1 Ma.考虑到近年来大量报道的Lu-Hf和Sm-Nd年龄精度都小于1 Ma(2σ未引入衰变常数误差)[37],忽略衰变常数的不确定性就会错误地解读这些年龄.以图1a为例,仅考虑衰变常数带来的误差,200 Ma的Sm-Nd年龄的误差为±1.5 Ma,对于2 000 Ma的Sm-Nd年龄则高达±15 Ma;类似的Lu-Hf年龄中由衰变常数误差带来的不确定性分别为±0.8 Ma和±8 Ma.当比较同一样品或不同样品中相同定年体系年龄时,由于衰变常数误差会使两个年龄偏大或者偏小相同的幅度,在这种情况下,可以不考虑衰变常数不确定性.然而,当比较同一样品或不同样品中不同定年体系年龄的时候,衰变常数不确定性传递到年龄的误差则不能忽略.以图1b为例,一个Lu-Hf年龄为(202±2) Ma的样品,对应的Sm-Nd年龄为(196±2) Ma,在不考虑衰变常数误差的情况下,这两个年龄是显著有差别的;如果考虑衰变常数的不确定性,它们将变得难以区分[(202±2.8) Ma vs. (196±3.5) Ma],在误差范围内一致.对年龄的解释也将从前者可能指示两个不同的地质事件,变成后者的一个单一地质事件. ...
... 一般是采用同位素稀释法,通过对ICP-MS或TIMS测试获得的母子体同位素的比值进行回归,得出Lu-Hf和Sm-Nd等时线年龄.尽管母体同位素147Sm与子体核素143Nd的质量数不同,但参比同位素144Nd与144Sm是同量异位素,为了避免后期校正带来的不确定性,在使用TIMS进行Sm-Nd定年时,需要将Sm与Nd进行分离.由于会抑制三价REE离子的电离[48],所以Ba也需要和REE完全分离.对于几百纳克的Nd,现代质谱仪的同位素比值精度可达10×10-6(2 RSD, Relative Standard Deviation,相对标准偏差)或更高(例如2×10-6[37,49].通过改进的进样锥和真空系统,结合使用Ta[50],对4 ng Nd的测试可以达到很高的外部精度(10×10-6~35×10-6,2 RSD),使高分辨率的地质年代分析成为可能[8]. ...
176Lu-176Hf geochronology of garnet I: Experimental determination of the diffusion kinetics of Lu3+ and Hf 4+ in garnet, closure temperatures and geochronological implications
Sm-Nd isotope systematics in garnet from different lithologies (Eastern Alps): Age results, and an evaluation of potential problems for garnet Sm-Nd chronometry
The group separation of the rare-earth elements and yttrium from geologic materials by cation-exchange chromatography
1
1984
... 一般是采用同位素稀释法,通过对ICP-MS或TIMS测试获得的母子体同位素的比值进行回归,得出Lu-Hf和Sm-Nd等时线年龄.尽管母体同位素147Sm与子体核素143Nd的质量数不同,但参比同位素144Nd与144Sm是同量异位素,为了避免后期校正带来的不确定性,在使用TIMS进行Sm-Nd定年时,需要将Sm与Nd进行分离.由于会抑制三价REE离子的电离[48],所以Ba也需要和REE完全分离.对于几百纳克的Nd,现代质谱仪的同位素比值精度可达10×10-6(2 RSD, Relative Standard Deviation,相对标准偏差)或更高(例如2×10-6[37,49].通过改进的进样锥和真空系统,结合使用Ta[50],对4 ng Nd的测试可以达到很高的外部精度(10×10-6~35×10-6,2 RSD),使高分辨率的地质年代分析成为可能[8]. ...
High-precision 142Nd/144Nd measurements in terrestrial rocks: Constraints on the early differentiation of the Earth's mantle
1
2006
... 一般是采用同位素稀释法,通过对ICP-MS或TIMS测试获得的母子体同位素的比值进行回归,得出Lu-Hf和Sm-Nd等时线年龄.尽管母体同位素147Sm与子体核素143Nd的质量数不同,但参比同位素144Nd与144Sm是同量异位素,为了避免后期校正带来的不确定性,在使用TIMS进行Sm-Nd定年时,需要将Sm与Nd进行分离.由于会抑制三价REE离子的电离[48],所以Ba也需要和REE完全分离.对于几百纳克的Nd,现代质谱仪的同位素比值精度可达10×10-6(2 RSD, Relative Standard Deviation,相对标准偏差)或更高(例如2×10-6[37,49].通过改进的进样锥和真空系统,结合使用Ta[50],对4 ng Nd的测试可以达到很高的外部精度(10×10-6~35×10-6,2 RSD),使高分辨率的地质年代分析成为可能[8]. ...
An improved method for TIMS high precision neodymium isotope analysis of very small aliquots (1-10 ng)
1
2009
... 一般是采用同位素稀释法,通过对ICP-MS或TIMS测试获得的母子体同位素的比值进行回归,得出Lu-Hf和Sm-Nd等时线年龄.尽管母体同位素147Sm与子体核素143Nd的质量数不同,但参比同位素144Nd与144Sm是同量异位素,为了避免后期校正带来的不确定性,在使用TIMS进行Sm-Nd定年时,需要将Sm与Nd进行分离.由于会抑制三价REE离子的电离[48],所以Ba也需要和REE完全分离.对于几百纳克的Nd,现代质谱仪的同位素比值精度可达10×10-6(2 RSD, Relative Standard Deviation,相对标准偏差)或更高(例如2×10-6[37,49].通过改进的进样锥和真空系统,结合使用Ta[50],对4 ng Nd的测试可以达到很高的外部精度(10×10-6~35×10-6,2 RSD),使高分辨率的地质年代分析成为可能[8]. ...
Correction: A rapid and efficient ion-exchange chromatography for Lu-Hf, Sm-Nd, and Rb-Sr geochronology and the routine isotope analysis of sub-ng amounts of Hf by MC-ICP-MS
... (a) Sm-Nd isochron diagram for Whole-Rock (WR)-garnet core and matrix-garnet rim pairs from a graphite-free metapelite, modified after reference [58]; (b) Isochron diagram for 12 concentric zones in a single garnet showing the corresponding drilled annuli, the differences in isotope ratios relative to the composition of the rim are shown in the lower diagram; (c) Ages of different zones within a garnet plotted as a function of radius and volume with indications of rapid growth episodes, modified after reference [8] ...
Sm-Nd dating of spatially controlled domains of garnet single crystals: A new method of high-temperature thermochronology
... (a) Isochron plots illustrating the different ages obtained for two garnet generations present in an eclogite from Trescolmen, the element map for Fe is also shown,modified after reference[62]; (b) Lu-Hf isochron plot for garnet fraction, bulk rock powder separates of the Qianjin eclogite from the Dabie orogen. Inset photomicrograph shows the separated garnet porphyroblasts with distinct optical contrasts,modified after reference[63] ...
Protracted garnet growth in high‐P eclogite: Constraints from multiple geochronology and P-T pseudosection
... (a) Isochron plots illustrating the different ages obtained for two garnet generations present in an eclogite from Trescolmen, the element map for Fe is also shown,modified after reference[62]; (b) Lu-Hf isochron plot for garnet fraction, bulk rock powder separates of the Qianjin eclogite from the Dabie orogen. Inset photomicrograph shows the separated garnet porphyroblasts with distinct optical contrasts,modified after reference[63] ...
Progress in linking accessory mineral growth and breakdown to major mineral evolution in metamorphic rocks: A thermodynamic approach in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-ZrO2 system
Microsampling Lu-Hf geochronology on mm-sized garnet in eclogites constrains early garnet growth and timing of tectonometamorphism in the North Qilian orogenic belt
Pseudosection modelling and garnet Lu-Hf geochronology of HP amphibole schists constrain the closure of an ocean basin between the northern and southern Lhasa blocks, central Tibet
Ultrahigh temperature granulite metamorphism (1 050 °C, 12 kbar) and decompression in garnet (Mg70)-orthopyroxene-sillimanite gneisses from the Rauer Group, East Antarctica
Diffusional homogenization of light REE in garnet from the Day Nui Con Voi Massif in N-Vietnam: Implications for Sm-Nd geochronology and timing of metamorphism in the Red River shear zone
0
2012
Protracted oceanic subduction prior to continental subduction: New Lu-Hf and Sm-Nd geochronology of oceanic-type high-pressure eclogite in the western Dabie orogen