首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
A combined in situ SIMS and LA-(MC)-ICPMS study of U-Pb ages, trace elements, O and Lu-Hf isotopes was conducted for zircon from eclogite-facies metamorphic rocks in the Sulu orogen. The two microbeam techniques sampled various depths of zircon domains, revealing different element and isotope relationships between residual magmatic cores and new metamorphic rims and thus the geochemical architecture of metamorphic zircons which otherwise cannot be recognized by the single microbeam technique. This enables discrimination of metamorphic growth from different subtypes of metamorphic recrystallization. Magmatic cores with U-Pb ages of 769 ± 9 Ma have positive δ18O values of 0.1-10.1‰, high Th/U and 176Lu/177Hf ratios, high REE contents, and steep MREE-HREE patterns with negative Eu anomalies. They are interpreted as crystallizing from positive δ18O magmas during protolith emplacement. In contrast, newly grown domains have concordant U-Pb ages of 204 ± 4 to 252 ± 7 Ma and show negative δ18O values of −10.0‰ to −2.2‰, low Th/U and 176Lu/177Hf ratios, low REE contents, and flat HREE patterns with weak to no Eu anomalies. They are interpreted as growing from negative δ18O fluids that were produced by metamorphic dehydration of high-T glacial-hydrothermally altered rocks during continental subduction-zone metamorphism. Differences in δ18O between different domains within single grains vary from 0.8‰ to 12.5‰, suggesting different degrees of O isotope exchange between the positive δ18O magmatic core and the negative δ18O metamorphic fluid during the metamorphism. The magmatic zircons underwent three subtypes of metamorphic recrystallization, depending on their accessibility to negative δ18O fluids. The zircons recrystallized in solid-state maintained positive δ18O values, and REE and Lu-Hf isotopes of protolith zircon, but their U-Pb ages are lowered. The zircons recrystallized through dissolution exhibit negative δ18O values similar to the metamorphic growths, almost completely reset U-Pb ages, and partially reset REE systems. The zircons recrystallized through replacement show variably negative δ18O values, and partially reset REE, and U-Pb and Lu-Hf isotopic systems. Therefore, this study places robust constraints on the origin of metamorphic zircons in eclogite-facies rocks and provides a methodological framework for linking the different types of metamorphic zircons to petrological processes during continental collision.  相似文献   

2.
In‐situ SIMS analyses of O and U‐Pb isotopes were carried out for zircons from a quartz vein hosted by ultrahigh‐pressure metagranite (UHP) in the Dabie orogen. The results are integrated to decipher the property of unusual U‐rich aqueous fluids and their effects on both metamorphic and magmatic zircons during exhumation of the UHP metagranite. In CL images, most zircon grains show distinct core‐rim structures. Relict cores are bright and exhibit oscillatory or patchy zonation, giving Neoproterozoic upper‐intercept ages of 795 ± 26 Ma. Newly grown rims are dark and exhibit no zoning, yielding Triassic concordant ages of 215 ± 5 Ma. The cores give Th contents of 59 to 463 ppm and U contents of 98 to 558 ppm, with Th/U ratios of 0.263 to 1.423. The rims yield reduced Th contents of 11 to 124 ppm but significantly elevated U contents of 1051 to 3531 ppm, with Th/U ratios of 0.010 to 0.035. Comparison with the cores of magmatic origin, the unusual enrichment in U but depletion in Th in the rims of metamorphic origin are interpreted as zircon growth from Cl‐rich oxidized vein‐forming aqueous fluids that were produced by dehydration reactions of the wallrock during continental exhumation. The cores have variably positive δ18O values with concordant or discordant Neoproterozoic U‐Pb ages, suggesting their solid‐state modification of both O and U‐Pb isotopes through interaction with the fluids. The rims yield negative δ18O values, indicating their growth from the negative δ18O fluids. Taken together, the proposed Cl‐rich oxidized negative‐δ18O vein‐forming aqueous fluids have such an ability to not only cause variable metamorphic recrystallization in the relict magmatic zircons but also produce dramatic fractionation of U over Th in the metamorphic zircons during quartz veining, and potentially impact on the overlain metasomatite in the mantle wedge.  相似文献   

3.
唐俊  郑永飞  吴元保  龚冰 《地球科学》2005,30(6):692-706
在苏鲁超高压变质带东北端山东威海地区皂埠镇发现锆石δ18O值低至-7.8‰左右的花岗片麻岩, 与前人在苏鲁超高压变质带西南端江苏东海青龙山地区发现的锆石δ18O值为-7‰~-9‰左右的花岗片麻岩一致.对这些低δ18O值花岗片麻岩进行了锆石SHRIMP法UPb定年和系统的激光氟化法矿物氧同位素分析, 结果对低δ18O值锆石成因和花岗片麻岩的原岩性质提供了制约.研究得到: (1) 这些低δ18O值锆石以新元古代岩浆锆石为主, 但部分岩浆锆石在印支期超高压变质作用过程中发生了不同程度的重结晶作用.δ18O值为-7.08‰的岩浆核锆石UPb定年得到的花岗片麻岩原岩谐和年龄和不一致线上交点年龄分别为(760±49) Ma和(751±27) Ma, 变质谐和年龄和不一致线下交点年龄分别为(232±4) Ma和(241±33) Ma, 指示其原岩为新元古代花岗岩并经历了印支期变质作用; (2) 锆石δ18O值在局部范围内变化于-7.76‰~5.40‰之间, 低δ18O值岩浆锆石表明它们是从新元古代低δ18O值岩浆中直接结晶形成, 锆石δ18O值的局部变化表明其原岩岩浆的氧同位素组成具有不均一性, 指示低δ18O值岩浆源区物质曾经在地表与极度亏损18O的大气降水发生过不同程度的高温水岩反应; (3) 低δ18O值花岗片麻岩在印支期板块俯冲和折返过程中基本没有与外部发生显著的氧同位素交换, 在退变质作用过程中花岗片麻岩内部缓冲流体对原岩岩浆锆石的δ18O值影响不大.威海皂埠镇地区和东海青龙山地区的花岗片麻岩在原岩时代、变质时代和氧同位素组成等方面基本相同, 指示它们应具有相同的原岩性质, 并经历了相同的变质作用和水岩相互作用过程.因此, 极度亏损18O的新元古代双峰式基性-酸性岩浆岩可能分布于整个大别-苏鲁造山带.   相似文献   

4.
Oxygen isotope analysis and U-Pb dating were carried out on zircons from granite, granitic gneiss and eclogite in the Dabie-Sulu orogenic belt of east-central China. The results show a wide variation in zircon δ18O values from −10.9 to 8.5‰. Most of the values are lower than normal mantle zircon and almost half have prominently negative values. The low δ18O zircons have protolith ages of 700 to 800 Ma and metamorphic ages of 205 to 250 Ma, respectively. Fluid availability within the metaigneous rocks dictates the extent of metamorphic recrystallization and overgrowth. The igneous zircons have preserved their magmatic zoning and middle Neoproterozoic U-Pb age during the Triassic metamorphism, indicating low fluid availability. Widespread low δ18O values are identified in the magmatic zircons of middle Neoproterozoic age, within an outcrop area of over 20,000 km2 along the northern edge of the South China Block. The low δ18O zircons record the presence of large volumes of low δ18O igneous rocks that were derived from remelting of meteoric-hydrothermally altered rocks at some time between 700 and 800 Ma.The U-Pb ages for metaigneous protoliths and granites are correlated not only with the timing of rifting accompanying the breakup of Rodinia, but also with contemporary glacial deposits in the South China Block at paleolatitudes of 30 to 40°N. Melting of glacial ice and snow is suggested, instead of the direct involvement of meteoric water, to produce the low δ18O fluid with oxygen isotopic signatures like the cold-climate meteoric water. The rifting created conditions favorable to anatexis of meteoric-hydrothermally altered rocks. Glaciated regions supplied copious water for the water-rock interaction during magma emplacement along rifting zones. Both rifting and glaciation favored the generation of the low δ18O magmas in the region. The low δ18O zircons are thus interpreted to have crystallized from the low δ18O magmas of middle Neoproterozoic age, and onset of the generation of low δ18O magmas in the northern margin of the South China Block is estimated to occur at about 758 ± 15 Ma. The large-scale remelting of hydrothermally altered crust not only results from repetitive emplacement of mafic magmas along the rifting zones with protracted episodes of water-rock interaction, but also involves rift systems that rapidly introduce large volumes of fluid through confined pathways and traps in a short space of time in response to tectonic triggers. Occurrence of the large-scale 18O depletion during the middle Neoproterozoic may be a manifestation of the cold paleoclimate related to the snowball Earth event.  相似文献   

5.
A loop of soft iron wire or a paper clip or a ferromagnetic grid mounted between the poles of an electromagnet picks up and allows further magnetic separation of zircons previously found to be non-magnetic on a Frantz Isodynamic Separator. Tests on previously analysed samples indicate that most such zircons that are fairly discordant (say ~10%) can be picked up and isolated from associated grains that are more concordant. Tests on new samples indicate that even when most grains can be picked up the last few percent of the sample contain less uranium, and are more concordant than the bulk sample.The degree of discordance is the dominant factor affecting the uncertainty of U-Pb zircon ages both because of the error amplification in projections, and because the assumption of a simple two-stage system may not be valid. Only by eliminating or reducing discordance can errors approaching the uncertainty in a single analysis, say ±2 m.y. for 2700 m.y. rocks, be achieved.Rutile normally concentrated with zircon as non-magnetic has been successfully removed from a small amount of low uranium zircon, using the high intensity separation technique.  相似文献   

6.
Unusual 18O depletion, with δ18O values as negative as −10‰ to −4‰ relative to VSMOW, was reported in zircons from ultrahigh-pressure eclogite-facies metamorphic rocks in the Dabie-Sulu orogenic belt, China. But it is critical for the negative δ18O zircons to be distinguished between magmatic and metamorphic origins, because the 18O depletion can be acquired by high-T eclogite-facies metamorphism of meteoric-hydrothermally altered low δ18O rocks. While zircon O diffusion kinetics has placed a reasonable constraint on this, zircon trace element compositions can provide a straightforward distinction between the magmatic and metamorphic origins. This paper reports our finding of unusual 18O depletion in zircon from granitic gneiss in the northeastern end of the Sulu orogen. Zircon δ18O values vary from −7.8‰ to −3.1‰ along a profile of 50 m length at Zaobuzhen. They are close to extremely low δ18O values of −9.0‰ to −5.9‰ for metagranite at Qinglongshan and adjacent areas in the southwestern end of the Sulu orogen. CL imaging suggests that the low δ18O zircons at Zaobuzhen are primarily of magmatic origin, but underwent different degrees of metamorphic modification. Zircon U-Pb dating yields middle Neoproterozoic ages of 751 ± 27 to 779 ± 25 Ma for protolith crystallization and Triassic ages of 214 ± 10 to 241 ± 33 Ma for metamorphic resetting. However, no metamorphic modification occurs in zircon REE patterns that only indicate magmatic recrystallization and hydrothermal alteration, respectively. Thus, the negative δ18O zircons are interpreted as crystallizing from negative δ18O magmas due to melting of meteoric-hydrothermally altered negative δ18O rocks in an active rift setting at about 780 Ma. The variation in zircon δ18O values indicates considerable O isotope heterogeneity in its granitic protolith. Zircon Lu-Hf isotope analyses give positive εHf(t) values of 1.6-4.1 and Hf model ages of 1.18-1.30 Ga. This suggests that the granitic protolith was derived from the mid-Neoproterozoic reworking of late Mesoproterozoic juvenile crust. The metagranites at Zaobuzhen and Qinglongshan, about 450 km apart, are two known occurrences of the unusually low δ18O zircons below −6‰ so far reported in the Sulu orogen. They are similar to each other in both protolith and metamorphic ages, so that they share the same nature of both Neoproterozoic protolith and Triassic metamorphism. Therefore, the locally negative δ18O zircons may register centers of low δ18O magmatism during the supercontinental rifting.  相似文献   

7.
Zircon and titanite were investigated in impactites of the Gardnos structure, a crater formed in Sveconorwegian (ca. 1 Ga) crust, which was then overridden in the Devonian by Caledonian nappes. Observed deformation features in zircons are granular texture, planar microstructures, and likely the incorporation of organic carbon during impact causing black staining of the zircon grains. The grains were studied by scanning electron microscopy (SEM) and cathode luminescence (CL) and dated by U-Pb isotope dilution - thermo-ionization mass spectrometry (ID-TIMS). Zircon grains without impact related features have U-Pb data showing moderate discordance (5-13%) and indicating formation ages mostly in the range of 1600-1000 Ma, except detrital zircon ages as old as >2481 Ma, reflecting the diversity of target rocks in the area. Titanite with concordant ages of 995-999 Ma dates metamorphism during final juxtaposition of the Telemarkia on the Idefjorden terrane to the east. Zircon grains with demonstrated or presumed shock features yield highly discordant (14-40%) U-Pb data, with a majority of them plotting along an array with a lower intercept of about 340 Ma reflecting the influence of the Caledonian orogeny and recent Pb-loss. One zircon grain was totally reset at 379 Ma during late Caledonian metamorphism, which also caused local growth of new titanite. A specific group of zircon grains yields data with relatively high discordance for moderate U contents, and five of these analyses, including that of a grain with proven granular or aggregate texture, fit a discordia line with an upper intercept of 546 ± 5 Ma. These features are interpreted as indicating zircon break-down to an amorphous state during impact, with subsequent recrystallization into microcrystalline aggregates causing extensive to complete Pb loss. We further suggest that their crystallinity prevented Pb loss during the Caledonian orogeny, while the small subgrain size and increasing metamictisation allowed more recent disturbances. We thus interpret the 546 Ma age as the approximate time of impact.  相似文献   

8.
本文主要报道内蒙古中部白乃庙地区侵入古生界徐尼乌苏组沉积地层中的石英二长闪长岩脉的锆石LA-ICP-MS 法U-Pb定年结果。测试结果显示大量锆石为捕获锆石并获得了从古元古代1.9Ga到中元古代1.26Ga左右的谐和年龄。其中中元古代年龄可划分为1.7Ga, 1.6~1.47Ga和1.26Ga等三个不同阶段;前两阶段年龄分别与华北克拉通化之后初始裂谷和非造山岩浆作用以及白云鄂博裂谷中火成碳酸盐的侵位时代相对应;而1.26Ga的年龄则与全球格林威尔构造-岩浆热事件时代相吻合。结果表明华北北缘存在中元古1.26Ga左右岩浆热事件。该结果为进一步认识华北北缘地壳演化及华北板块是否参与过罗迪尼亚超大陆演化提供了重要线索。  相似文献   

9.
We have undertaken petrologic and SHRIMP U-Th-Pb isotopic studies on zircons from basaltic eucrites (Yamato [Y]-75011, Y-792510, Asuka [A]-881388, A-881467 and Padvarninkai) with different thermal and shock histories. Eucritic zircons are associated with ilmenite in most cases and have subhedral shapes in unmetamorphosed and metamorphosed eucrites. Some zircons in highly metamorphosed eucrites with granulitic texture occur alone in pyroxene, and typically have rounded to subrounded shapes due to recrystallization. Superchondritic Zr/Hf ratios of eucritic zircons indicate that they crystallized from incompatible element-rich melts after crystallization of ilmenite. Concentrations of uranium and thorium in zircons in the unmetamorphosed eucrite Y-75011 are higher than those in metamorphosed eucrites.The U-Pb systems of eucritic zircons are almost concordant but some zircon grains show reverse discordance. Radiogenic lead-loss up to 48% from zircons is observed in the shock-melted eucrite Padvarninkai. The 207Pb-206Pb ages of zircon in Y-75011 (4550 ± 9 Ma, n = 5) are nearly identical, within analytical uncertainty, to the ages of zircons from the metamorphosed eucrite Y-792510 (4545 ± 15 Ma, n = 13), the highly metamorphosed eucrites A-881388 (4555 ± 54 Ma, n = 5) and A-881467 (4558 ± 13 Ma, n = 8), and the shock-melted eucrite Padvarninkai (4555 ± 13 Ma, n = 18). The averaged 207Pb-206Pb age of zircon from five eucrites analyzed in this study is 4554 ± 7 Ma (95% confidence limits, n = 49), indistinguishable from the averaged U-Pb age (4552 ± 9 Ma) of the same samples. Because of the high closure temperature of lead in zircon (Tclosure = ∼1050°C with a cooling rate of 0.2°C/yr), the 207Pb-206Pb ages of eucritic zircon do not represent metamorphic ages but crystallization ages of extrusive lavas.This fact strongly suggests that volcanism of the eucrite parent body occurred at a very early stage of the Solar System history, 7-20 Ma after CAI formation (4567.2 ± 0.6 Ma), thus basaltic eucrites crystallized from parental magmas within a short interval following the differentiation of their parent body. The U-Pb ages of eucritic zircons are older than the U-Pb, Sm-Nd and Rb-Sr ages of some basaltic eucrites, which is consistent with differences in closure temperatures of each isotopic system, and suggests that thermal and shock metamorphism affected the isotopic systems of pyroxene, plagioclase and phosphates.  相似文献   

10.
北淮阳新开岭地区花岗岩锆石U-Pb年龄和氧同位素组成   总被引：9，自引：2，他引：7  
对大别造山带北麓的北淮阳新开岭地区岩浆岩进行了锆石阴极发光显微结构观察和SHRIMP法锆石微区UPb定年.在锆石阴极发光图像中, 一个花岗岩样品中的大部分锆石颗粒具有明显的初始岩浆振荡环带, 为典型的岩浆锆石, 少有蚀变的颗粒和/或区域; 而另一个花岗岩样品中的锆石虽然同样具有振荡环带, 但是大部分颗粒中心的初始岩浆环带被扰动, 指示这些锆石为岩浆锆石, 受到了较强的后期热液蚀变的改造.对锆石具有初始岩浆环带和溶蚀结构的区域分别进行SHRIMP法UPb微区定年结果表明, 这些岩浆岩的形成年龄为(820±4) Ma, 热液蚀变作用发生的时间为(780±4) Ma.新开岭地区新元古代花岗质岩石的形成和后期超固相热液蚀变作用分别对应于超大陆裂解之前的约830~795Ma岩浆活动和裂解过程中约780~745Ma的岩浆作用.单矿物激光氟化氧同位素分析结果表明, 这些岩浆岩具有非常低的δ18O值, 其中锆石为1.90‰~5.78‰, 石英为-2.88‰~-7.67‰, 斜长石为-4.01‰~-11.40‰.锆石和其他矿物之间表现出强烈的氧同位素不平衡, 而其他矿物之间则达到了氧同位素的再平衡.结合不同δ18O值锆石的内部结构特征, 认为该地区的热液蚀变作用为超固相条件下的高温热液蚀变.这一过程不但改变了石英等矿物的氧同位素组成, 同时也不同程度地改变了锆石的氧同位素组成, 所以这些样品中低δ18O值锆石可能是超固相条件下热液蚀变的结果.石英中具有异常低的δ18O值表明蚀变流体来源应为寒冷气候大气降水.所以, 新开岭地区亏损18O蚀变岩石的形成与裂谷岩浆作用和雪球地球事件相耦合的高温大气降水热液蚀变有关.   相似文献   

11.
We report oxygen isotopic compositions for 14 zircon grains from a sample of sawdust from lunar breccia 14321. The zircons range in age from ∼4.4 to 3.9 Ga and in U and Th content from a few to several hundred ppm. As such these grains represent a range of possible source rocks, from granophyric to mafic composition, and cover the total age range of the major initial lunar bombardment. Nevertheless, results show that the oxygen isotopic compositions of the zircons fall within a narrow range of δ18O of about 1 per mil and have δ18O values indistinguishable from those observed for terrestrial mid-ocean ridge basalts confirming the coincidence of lunar and Earth oxygen isotopic compositions. In the δ17O vs. δ18O, coordinates data form a tight group with a limited trend on the terrestrial fractionation line. The zircon oxygen isotopes show minimal evidence of the extreme and variable mineral differentiation and element fractionation that have contributed to the formation of their parent rocks.  相似文献   

12.
Electron backscatter imaging, Raman spectroscopy and U-Pb geochronology have been applied to Precambrian zircon grains that were annealed at 1000 and 1450 °C for various times, then leached with HF to constrain the conditions for healing radiation damage and attaining primary U-Pb zircon ages using the chemical abrasion (CA-TIMS) method. SEM images reveal a variety of textures for ZrO2 overgrowths on 1450 °C annealed and leached zircon surfaces that depend on the degree of radiation damage and annealing history. Highly damaged zircon produces finer textures than zircon with less damage.Raman spectroscopy indicates that crystals with different levels of radiation damage are only partially restored by annealing at 1000 °C for 2-3 days. Longer annealing periods of 20 days are not noticeably more effective. Annealing at 1450 °C for 1 h results in partial breakdown of zircon but restores Raman peak widths and wave numbers to values characteristic of undamaged zircon after ZrO2 overgrowths are removed by HF. Raman spectra are much less sensitive to polarization angle for annealed highly damaged grains than for weakly damaged zircon.U-Pb isotopic analyses of low to moderately damaged zircon (alpha fluence ranging up to 1019/g corresponding to an amorphization volume fraction of 80% or more) yield almost concordant data (0.3-0.5% discordance) after high-temperature annealing at 1450 °C followed by HF leaching at 195 °C. Analyses of cracked zircon annealed at 1450 °C and leached may remain discordant but those of uncracked grains are concordant. Most analyses show primary 207Pb/206Pb ages although cracked grains annealed at 1450 °C may produce discordant data with 207Pb/206Pb ages that are too young after leaching. The solubility of highly damaged, very disordered zircon (amorphization level of 99%) is only slightly reduced by annealing, and analyses of leach residues are strongly discordant although primary 207Pb/206Pb ages are obtained.Annealing of highly damaged zircon under any conditions apparently results in a mass of randomly oriented micro-crystals that pseudomorph the original grain. This could explain the fine-scale pattern observed on etched crystal surfaces, reduced anisotropy at the 5 μm scale of the Raman laser beam and high solubility in HF. It may be impossible to restore primary U-Pb isotopic ages in such cases but precise ages can still potentially be determined from 207Pb/206Pb ratios or by application of the air abrasion method.  相似文献   

13.
This study reports new zircon U–Pb ages, Lu–Hf isotope data, and oxygen isotope data for Mesozoic Mo-bearing granitoids in the eastern Xing–Meng Orogenic Belt (XMOB) of Northeast China, within the eastern Central Asian Orogenic Belt. Combining these new laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb ages with the results of previous research indicates that two stages of Mo-bearing granitoid magmatism occurred in the eastern XMOB, during the Early–Middle Jurassic (200–165 Ma) and the Early Cretaceous (ca. 111 Ma). The eastern XMOB also contains Mo-bearing granitoids with variable δ18O compositions that record variations in source oxygen isotopic compositions. Combining δ18O data with zircon U–Pb and Hf isotopic data provides evidence of the origin of these granitoids. Three types of zircon have been identified within these granitoids. Type 1 zircons formed during the Mesozoic and having high δ18O values (5.71–7.05‰) that are consistent with the compositions of magmatic zircons from the Luming, Jiapigou, and Kanchuangou areas. These zircons suggest that the Mo-bearing granitoids were derived from a source containing supracrustal materials. The type 2 zircons have extremely low and heterogeneous δ18O values (4.64–4.89‰) that are consistent with the compositions of magmatic zircons from the Jidetun and Fuanpu areas. These magmas were generated by the remelting of juvenile crustal material that was previously significantly modified by interaction with fluids. Type 3 zircons generally have mantle-like δ18O values (5.42–5.57‰), with several zircons yielding higher δ18O values, suggesting that these intrusions formed from mantle-derived magmas that assimilated and were metasomatized by crustal material. Combining these geochemical data with the geology of this region indicates that the Mo-bearing granitoids were generated as a result of subduction of the Palaeo-Pacific Plate beneath the Eurasian continent.  相似文献   

14.
Several lines of isotopic evidence - the most direct of which is from Hadean Jack Hills zircons - suggest a very early history of crust formation on Earth that began by about 4.5 Ga. To constrain both the fate of the reservoir for this crust and the nature of crustal evolution in the sediment source region of the Jack Hills, Western Australia, during the early Archean, we report here initial 176Hf/177Hf ratios and δ18O systematics for <4 Ga Jack Hills zircons. In contrast to the significant number of Hadean zircons which contain highly unradiogenic 176Hf/177Hf requiring a near-zero Lu/Hf reservoir to have separated from the Earth’s mantle by 4.5 Ga, Jack Hills zircons younger than ca. 3.6 Ga are more radiogenic than -13ε (CHUR) at 3.4 Ga in contrast to projected values at 3.4 Ga of -20ε for the unradiogenic Hadean reservoir indicating that some later juvenile addition to the crust is required to explain the more radiogenic younger zircons. The shift in the Lu-Hf systematics together with a narrow range of mostly mantle-like δ18O values among the <3.6 Ga zircons (in contrast to the spread towards sedimentary δ18O among Hadean samples) suggests a period of transition between 3.6 and 4 Ga in which the magmatic setting of zircon formation changed and the highly unradiogenic low Lu/Hf Hadean crust ceased to be available for intracrustal reworking. Constraining the nature of this transition provides important insights into the processes of crustal reworking and recycling of the Earth’s Hadean crust as well as early Archean crustal evolution.  相似文献   

15.
Zircons from Archean granites from the Darling Range Batholith in the Yilgarn Craton of Western Australia have been shown to have complexly discordant U–Pb systems with a strong component of zero age disturbance. The only geological event that has affected the granites in recent times is the pervasive regional weathering. Our aim in this study was to investigate the effects of weathering on the U–Pb and oxygen systems of the zircons, and to this end, we report secondary ion mass spectrometry (SIMS) results of OH, oxygen and U–Pb isotope systems of six typical zircons from a sample of the granite. These results confirmed the presence of OH in highly radiation-damaged parts of the zircons, demonstrating fluid interaction within grains. The presence of OH was accompanied by significant changes in the 18O/16O ratios. The data suggest trends where δ18O values in individual grains both increase and decrease with increasing OH. SIMS measurements showed the U–Pb systems are variably and unsystematically discordant in radiation-damaged parts of the zircons, particularly those with elevated OH contents. The complex U–Pb systems are interpreted in terms of multiple disturbance events between 450 and 0 Ma involving low-temperature fluid-induced movement of radiogenic Pb, decoupled from parent U and Th, within the radiation-damaged zircons, together with some Pb loss.  相似文献   

16.
U-Pb zircon geochronology is increasingly called upon to achieve the resolution of absolute time at the 0.1% to 1% level for rocks of Phanerozoic to Hadean age. Doing so requires accurate calibration of the several methods (conventional isotope dilution thermal ionization mass spectrometry [ID-TIMS], Pb evaporation, high-resolution ion microprobe [e.g. SHRIMP], and laser ablation inductively coupled plasma mass spectrometry [LA-ICPMS]) currently in use, in numerous laboratories, for the analysis of U and Pb isotopes in accessory minerals. Toward this end, the geochronological community would benefit from the establishment, distribution and widespread analysis of one or more standard reference materials. Among the candidates is natural zircon from the Duluth Complex anorthositic series of the Midcontinent Rift system of North America. These zircons, first dated by conventional ID-TIMS at 1099.1 ± 0.5 Ma, have been subsequently adopted as a geochronological standard by a number of high resolution ion microprobe facilities. A new and independent analysis of the systematics of a large set of single zircons (n = 27) from the same mineral separate yields indistinguishable 207Pb/206Pb, upper intercept, and U-Pb concordia dates for the AS3 zircons. The concordia date, based on a subset of 12 concordant and equivalent zircons, of 1099.1 ± 0.2 Ma (±1.2 Ma considering systematic uncertainties in Pb/U tracer calibration and U decay constants) is indistinguishable from previously published results. We further document the absence of inherited Pb in the AS3 zircons, and discuss strategies for avoiding certain domains within the AS3 zircons exhibiting small amounts of radiation-induced, surface and fracture-correlated, recent Pb loss. Although the AS3 zircons do not represent the ideal (and elusive) homogeneous closed U-Pb system, we conclude that these and similar zircons from the Duluth Complex anorthositic series can provide a suitable geochronological reference standard for numerous U-Pb zircon analytical methods, given appropriate preparation guided by the results of this study. Our high-precision data set also serves as a useful confirmatory test of the currently accepted U decay constants.  相似文献   

17.
Zircon grains were separated from lunar regolith and rocks returned from four Apollo landing sites, and analyzed in situ by secondary ion mass spectrometry. Many regolith zircons preserve magmatic δ18O and trace element compositions and, although out of petrologic context, represent a relatively unexplored resource for study of the Moon and possibly other bodies in the solar system. The combination of oxygen isotope ratios and [Ti] provides a unique geochemical signature that identifies zircons from the Moon. The oxygen isotope ratios of lunar zircons are remarkably constant and unexpectedly higher in δ18O (5.61 ± 0.07 ‰ VSMOW) than zircons from Earth’s oceanic crust (5.20 ± 0.03 ‰) even though mare basalt whole-rock samples are nearly the same in δ18O as oceanic basalts on Earth (~5.6 ‰). Thus, the average fractionation of oxygen isotopes between primitive basalt and zircon is smaller on the Moon [Δ18O(WR-Zrc) = 0.08 ± 0.09 ‰] than on Earth (0.37 ± 0.04 ‰). The smaller fractionations on the Moon suggest higher temperatures of zircon crystallization in lunar magmas and are consistent with higher [Ti] in lunar zircons. Phase equilibria estimates also indicate high temperatures for lunar magmas, but not specifically for evolved zircon-forming melts. If the solidus temperature of a given magma is a function of its water content, then so is the crystallization temperature of any zircon forming in that melt. The systematic nature of O and Ti data for lunar zircons suggests a model based on the following observations. Many of the analyzed lunar zircons are likely from K, rare earth elements, P (KREEP)-Zr-rich magmas. Zircon does not saturate in normal mafic magmas; igneous zircons in mafic rocks are typically late and formed in the last most evolved portion of melts. Even if initial bulk water content is moderately low, the late zircon-forming melt can concentrate water locally. In general, water lowers crystallization temperatures, in which case late igneous zircon can form at significantly lower temperatures than the solidus inferred for a bulk-rock composition. Although lunar basalts could readily lose H2 to space during eruption, lowering water fugacity; the morphology, large size, and presence in plutonic rocks suggest that many zircons crystallized at depths that retarded degassing. In this case, the crystallization temperatures of zircons are a sensitive monitor of the water content of the parental magma as well as the evolved zircon-forming melt. If the smaller Δ18O(zircon–mare basalt) values reported here are characteristic of the Moon, then that would suggest that even highly evolved zircon-forming magmas on the Moon crystallized at higher temperature than similar magmas on Earth and that magmas, though not necessarily water-free, were generally drier on the Moon.  相似文献   

18.
Analytical protocols for SHRIMP‐SI oxygen isotope analysis (δ18O) of a suite of zircon reference materials (RMs) are presented. Data reduction involved a robust estimate of uncertainties associated with the individual spot as well as for groups where the spot data are combined. The repeatability of δ18O measurements is dependent on both the analytical conditions and the choice of the primary reference material. Under optimised conditions, repeatability was often better than 0.4‰ (2s) allowing sample uncertainties to be obtained to better than 0.2‰ (at 95% confidence limit). Single spot uncertainty combined the within‐spot precision with the scatter associated with repeated measurements of the primary zircon reference material during a measurement session. The uncertainty for individual spots measured under optimised conditions was between 0.3 and 0.4‰ (at 95% confidence). The analytical protocols described were used to assess a variety of zircon RMs that have been used for geochronology and for which laser fluorination oxygen isotope data are available (Temora 2, FC1, R33, QGNG and Ple?ovice), as well as zircons that have been used as RMs for trace element or other types of determination (Mud Tank, Monastery, 91500, AS57, AS3, KIM‐5, OG1, SL13, CZ3 and several other Sri Lankan zircons). Repeated analyses over nine sessions and seven different mounts show agreement within analytical uncertainty for Temora 2, FC1, R33, QGNG, Ple?ovice and 91500, when normalised to Mud Tank. For existing ion microprobe mounts with these materials, an appropriate δ18O can be determined. However, care should be taken when using zircons from the Duluth Complex (i.e., FC1, AS57 and AS3) as reference materials as our data indicated an excess scatter on δ18O values associated with low‐U zircon grains.  相似文献   

19.
华北克拉通胶东地区粉子山群碎屑锆石SHRIMPU-Pb定年   总被引：18，自引：16，他引：2  
胶东是华北克拉通重要的变质基底出露区之一。对该区古元古代粉子山群和荆山群中碎屑锆石的年龄分布特征的研究,可以为胶东地区早前寒武纪演化提供重要依据。由于Pb丢失作用,前人仅在粉子山群中获得少量锆石谐和年龄。本文对粉子山群长石石英片岩中碎屑锆石进行了SHRIMP定年。52颗锆石的61个数据点分析,获得36个谐和年龄(不谐和度10%)。所有具谐和年龄的锆石都具有高的Th/U比值,大多显示明显的岩浆环带,原为岩浆成因。谐和锆石年龄分布在2033~3429Ma之间,主要峰值分别为~2.19Ga和~2.48Ga。该样品中还含有2颗年龄大于3.3Ga的锆石,位于谐和线上的2颗最年轻碎屑锆石的年龄为~2.08Ga。结合前人研究,可得出如下结论:1)粉子山群和荆山群原岩沉积时代可进一步限制在1.9~2.1Ga之间;2)粉子山群与荆山群的碎屑锆石年龄分布形式存在明显区别,指示其源区可能不同;3)胶东地区可能存在古太古代地壳物质。  相似文献   

20.
王明  李才  解超明  吴彦旺  苏犁  胡培远 《岩石学报》2012,28(12):4101-4108
聂荣微陆块呈透镜体状夹持在班公湖-怒江板块缝合带内,其上出露有黑云母花岗片麻岩以及二长花岗片麻岩等不同类型的古老片麻岩,本文报道产出于聂荣微陆块上花岗片麻岩的锆石LA-ICP-MS U-Pb定年结果,样品锆石的岩浆震荡环带十分发育,多数锆石具有典型的岩浆结晶锆石的特征。所有测点中有16个测点的Th/U值较高,介于0.58~1.24之间,平均值为0.79,获得的206Pb/238U年龄基本一致,加权平均值为819.6±5.2Ma,笔者等认为该年龄代表了花岗片麻岩的原岩结晶时代为新元古代。我们在羌塘南部地区的碎屑岩中获得了同样的年龄峰值(809Ma)。此外,我们认为年龄中具有507Ma左右的信息反映了泛非事件之后另外一次强烈构造热事件的影响在该区的存在。  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号