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1.
Thermal history recorded by the Apollo 17 impact melt breccia 73217   总被引：1，自引：0，他引：1  
Lunar breccia 73217 is composed of plagioclase and pyroxene clasts originating from a single gabbronorite intrusion, mixed with a silica-rich glass interpreted to represent an impact melt. A study of accessory minerals in a thin section from this breccia (73217,52) identified three different types of zircon and anhedral grains of apatite which represent distinct generations of accessory phases and provide a unique opportunity to investigate the thermal history of the sample. Equant, anhedral zircon grains that probably formed in the gabbronorite, referred to as type-1, have consistent U-Pb ages of 4332 ± 7 Ma. A similar age of 4335 ± 5 Ma was obtained from acicular zircon (type-2) grains interpreted to have formed from impact melt. A polycrystalline zircon aggregate (type-3) occurs as a rim around a baddeleyite grain and has a much younger age of 3929 ± 10 Ma, similar to the 3936 ± 17 Ma age of apatite grains found in the thin section. A combined apatite-type-3 zircon age of 3934 ± 12 Ma is proposed as the age of the Serenitatis impact event and associated thermal pulse. X-ray mapping and electron probe analyses showed that Ti is inhomogeneous in the zircon grains on the sub-micrometer scale. However, model temperatures estimated from SHRIMP analyses of Ti-concentration in the 10 μm diameter spots on the polished surfaces of type-1 and type-2 zircons range between about 1300 and 900 °C respectively, whereas Ti-concentrations determined for the type-3 zircon are higher at about 1400-1500 °C. A combination of U-Pb ages, Ti-concentration data and detailed imaging and petrographic studies of the zircon grains shows that the gabbronorite parent of the zircon clasts formed shortly before the 4335 ± 5 Ma impact, which mixed the clasts and the felsic melt and projected the sample closer to the surface where fast cooling resulted in the crystallization of acicular zircon (type-2). The 3934 ± 12 Ma Serenitatis event resulted in partial remelting of the glass and formation of polycrystalline zircon (type-3). This event also reset the U-Pb system of apatite, formed merrillite coronas around some apatite grains, and probably re-equilibrated some pyroxenes in the clasts. Although there have been arguments for pre-3.9 Ga impacts based on other types of samples, the age of the acicular zircon at 4335 ± 5 Ma provides the first evidence of impact melt significantly predating the lunar cataclysm. Our data, combined with other chronological results, demonstrate the occurrence of pre-3.9 Ga impacts on the Moon and suggest that the lunar impact history consisted of a series of intense bombardment episodes interspersed with relatively calm periods of low impact flux.  相似文献   

2.
We report the results of a SIMS U-Pb study of 112 zircons from breccia samples from the Apollo 14 and 17 landing sites. Zircon occurs in the breccia matrices as rounded, irregular shaped, broken and rarely euhedral grains and as constituent minerals in a variety of lithic clasts ranging in composition from ultra-mafic and mafic rocks to highly evolved granophyres. Crystallisation of zircon in magmatic rocks is governed by the zirconium saturation in the melt. As a consequence, the presence of zircon in mafic rocks on the Moon implies enrichment of their parent melts in the KREEP component. Our SIMS results show that the ages of zircons from mafic to ultramafic clasts range from ca. 4.35 Ga to ca. 4.00 Ga demonstrating multiple generations of KREEPy mafic and ultramafic magmas over this time period. Individual zircon clasts in breccia matrices have a similar age range to zircons in igneous clasts and all represent zircons that have been incorporated into the breccia from older parents. The age distributions of zircons from breccias from both the Apollo 14 and Apollo 17 landing sites are essentially identical in the range 4.35-4.20 Ga. However, whereas Apollo 14 zircons additionally show ages from 4.20 to 3.90 Ga, no zircons from Apollo 17 samples have primary ages less than ca. 4.20 Ga. Also, in contrast to previous suggestions that the magmatism in the lunar crust is continuous our results show that the zircon age distribution is uneven, with distinct peaks of magmatic activity at ca. 4.35 Ga, ca. 4.20 Ga in Apollo 14 and 17 and a possible third peak in zircons from Apollo 14 at ca. 4.00 Ga. To explain the differences in the zircon age distributions between the Apollo 14 and 17 landing sites we propose that episodes of KREEP magmatism were generated from a primary reservoir, and that this reservoir contracted over time towards the centre of Procellarum KREEP terrane. We attribute the peaks in KREEP magmatism to impact induced emplacement of KREEP magma from a primary mantle source or to a progressive thermal build-up in the mantle source until the temperature exceeds the threshold for generation of KREEP magma, which is transported into the crust by an unspecified possibly plume-like process.  相似文献   

3.
Northwest Africa (NWA) 4472 is a polymict lunar regolith meteorite. The sample is KREEP-rich (high concentrations of potassium, rare earth elements and phosphorus) and comprises a heterogeneous array of lithic and mineral fragments. These clasts and mineral fragments were sourced from a range of lunar rock types including the lunar High Magnesian Suite, the High Alkali Suite, KREEP basalts, mare basalts and a variety of impact crater environments. The KREEP-rich nature of NWA 4472 indicates that the sample was ejected from regolith on the nearside of the Moon in the Procellarum KREEP Terrane and we have used Lunar Prospector gamma-ray remote sensing data to show that the meteorite is most similar to (and most likely sourced from) regoliths adjacent to the Imbrium impact basin.U-Pb and Pb-Pb age dates of NWA 4472 phosphate phases reveal that the breccia has sampled Pre-Nectarian (4.35 Ga) rocks related to early episodes of KREEP driven magmatism. Some younger phosphate U-Pb and Pb-Pb age dates are likely indicative of impact resetting events at 3.9-4 Ga, consistent with the suggested timing of basin formation on the Moon. Our study also shows that NWA 4472 has sampled impact melts and glass with an alkali-depleted, incompatible trace element-rich (high Sc, low Rb/Th ratios, low K) compositional signature not related to typical Apollo high-K KREEP, or that sampled by KREEPy lunar meteorite Sayh al Uhaymir (SaU) 169. This provides evidence that there are numerous sources of KREEP-rich protoliths on the Moon.  相似文献   

4.
Sample 73235 is one of several aphanitic impact melt breccias collected by the Apollo 17 mission at stations 2 and 3 on the slopes of the South Massif. This study presents a detailed investigation of internal structures and U-Pb ages of large zircon grains from this breccia sample. New data combined with the results of previous studies of zircon grains from the same location indicate that most zircon clasts in breccias from stations 2 and 3 formed during multiple magmatic events between 4.37 and 4.31 Ga, although the oldest zircon crystallized at about 4.42 Ga and the youngest at 4.21 Ga. In addition, zircons from the aphanitic breccias record several impact events prior to the ∼3.9 Ga Late Heavy Bombardment. The results indicate that the zircons probably crystallized at different locations within the Procellarum KREEEP Terrane and were later excavated and modified by several impacts and delivered to the same locality within separate ejecta blankets. This locality became a source of material that formed the aphanitic impact melt breccias of the South Massif during a ∼3.9 Ga impact. However, the zircons, showing old impact features, are not modified by this ∼3.9 Ga impact event suggesting that (i) this common source area was located at the periphery of excavation cavity, and (ii) the > 3.9 Ga ages recorded by the zircon grains could date large (basin-forming) events as significant as major later (∼3.9 Ga) collisions such as Imbrium and Serenitatis.  相似文献   

5.
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   

6.
The results of our combined U-Pb, Rb-Sr, and Sm-Nd isotope study of mare basalt 10017 contribute to the understanding of the petrogenetic processes involved in the origin of geochemical diversity in lunar mare basalt sources, as well as the U-Pb isotope systematics of the Moon. The Rb-Sr, Sm-Nd, and 238U-206Pb isotope systems yield concordant crystallization ages of 3.633 ± 0.057 Ga, 3.678 ± 0.069 Ga, and 3.616 ± 0.098 Ga, respectively. The 235U-207Pb isochron yields an older, though still concordant, age of 3.80 ± 0.12 Ga. Neither the 206Pb-207Pb system nor U-Pb concordia system yields an age for 10017 that is concordant with the age determined from the Sm-Nd, Rb-Sr, and 238U-206Pb systems. The initial 87Sr/86Sr of 10017 is 0.69941 ± 7 and the initial εNd is +3.2 ± 0.4. Initial Pb isotopic compositions, determined from the U-Pb isochrons, are 206Pb/204Pbi = 31 ± 11 and 207Pb/204Pbi = 34 ± 15. Together, these initial Pb compositions constrain the μ value of the 10017 source to be 70 ± 30, assuming a single-stage Pb growth model. This is considerably lower than μ values typically estimated for mare basalt sources (∼100-600). Regardless, the μ values calculated for the sources of mare basalts, as well as other lunar samples, show a range that is larger than can be explained by fractionation of U from Pb solely by crystallization of silicate phases and ilmenite during magma ocean solidification and formation of lunar mantle sources. The U-Pb isotope systematics may reflect late-stage formation of a sulfide phase, which strongly fractionates Pb from U but has minimal effect on Rb/Sr or Sm/Nd compositions, during crystallization of the lunar magma ocean.  相似文献   

7.
Hafnium isotope analyses of a large number of metamorphic zircon grains of two garnet-kyanite-staurolite schist samples from the Shackleton Range yielded 176Hf/177Hf of 0.28160 ± 0.00003 and 0.28142 ± 0.00003, respectively. The variations of these analyses are less than ±1.2 epsilon units and indicate that all metamorphic zircon grains in the two rocks formed in environments with nearly homogenous Hf isotopic composition. The metamorphic origin of the zircon grains is constrained by textures as well as by their low Th/U (<0.2), 176Lu/177Hf (<0.0003), and 176Yb/177Hf ratios (<0.009), indicating that they grew in the presence of garnet. Furthermore, the grains yield Pb-Pb ages of c. 1.7 Ga, which is the time of amphibolite-facies metamorphism. In combination with petrological results, it is suggested that the observed 176Hf/177Hf homogeneity was caused by a fluid- and deformation-assisted dissolution of detrital zircon grains, followed by new zircon re-precipitation that was accompanied by Hf transport on at least a hand-specimen scale. This interpretation is supported by results obtained from an additional paragneiss sample that contains zoned zircon grains with xenocrystic cores formed at 2.6-1.8 Ga and metamorphic rims with a U-Pb age of 1.7 Ga. The 176Hf/177Hf variation of the zircon rims is mostly at ±0.0003, which is much less than that of the magmatic cores (±0.0019). The metamorphic fluid for the dissolution-homogenization-re-precipitation process most likely resulted from prograde reactions among the minerals chlorite-muscovite-biotite-garnet-staurolite-apatite, in agreement with thin section observations and P-T pseudosection calculations.  相似文献   

8.
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   

9.
In order to improve our understanding of impact history and surface geology on the Moon, we obtained 40Ar-39Ar incremental heating age data and major + trace element compositions of anorthositic and melt breccia clasts from Apollo 16 feldspathic fragmental breccias 67016 and 67455. These breccias represent the Descartes terrain, a regional unit often proposed to be ejecta from the nearby Nectaris basin. The goal of this work is to better constrain the emplacement age and provenance of the Descartes breccias.Four anorthositic clasts from 67016 yielded well-defined 40Ar-39Ar plateau ages ranging from 3842 ± 19 to 3875 ± 20 Ma. Replicate analyses of these clasts all agree within measurement error, with only slight evidence for either inheritance or younger disturbance. In contrast, fragment-laden melt breccia clasts from 67016 yielded apparent plateau ages of 4.0-4.2 Ga with indications of even older material (to 4.5 Ga) in the high-T fractions. Argon release spectra of the 67455 clasts are more variable with evidence for reheating at 2.0-2.5 Ga. We obtained plateau ages of 3801 ± 29 to 4012 ± 21 Ma for three anorthositic clasts, and 3987 ± 21 Ma for one melt breccia clast. The anorthositic clasts from these breccias and fragments extracted from North Ray crater regolith (Maurer et al., 1978) define a combined age of 3866 ± 9 Ma, which we interpret as the assembly age of the feldspathic fragmental breccia unit sampled at North Ray crater. Systematic variations in diagnostic trace element ratios (Sr/Ba, Ti/Sm, Sc/Sm) with incompatible element abundances show that ferroan anorthositic rocks and KREEP-bearing lithologies contributed to the clast population.The Descartes breccias likely were deposited as a coherent lithologic unit in a single event. Their regional distribution suggests emplacement as basin ejecta. An assembly age of 3866 ± 9 Ma would be identical with the accepted age of the Imbrium basin, and trace element compositions are consistent with a provenance in the Procellarum-KREEP Terrane. The combination of age and provenance constraints points toward deposition of the Descartes breccias as ejecta from the Imbrium basin rather than Nectaris. Diffusion modeling shows that the older apparent plateau ages of the melt brecia clasts plausibly result from incomplete degassing of ancient crust during emplacement of the Descartes breccias. Heating steps in the melt breccia clasts that approach the primary crystallization ages of lunar anorthosites show that earlier impact events did not completely outgas the upper crust.  相似文献   

10.
Dhofar 1442 is one of the few lunar KREEP-rich meteorites, which contains KREEP norites and KREEP gabbronorite as well as low-Ti basalts and highly evolved granophyres. Zircon is a typical accessory mineral of KREEP rocks. U-Th-Pb dates of 12 zircon grains (four of them were in two lithic clasts, and the others were fragments in the meteorite matrix) indicate that the zircons belong to at least two groups of different age: “ancient” (～4.31 Ga) and “young” (～3.95 Ga), which correspond to two major pulses of KREEP magmatism in the source region of the Dhofar 1442 meteorite. The zircon of the “young” group was most probably related to the crater ejecta of the Mare Imbrium Basin. The rock fragments dated at approximately 3.95 Ga have the composition of KREEP gabbronorite. The parental rocks of the zircon of the “ancient” group in the Dhofar 1442 meteorite are uncertain and could be highly evolved granophyres. This hypothesis is supported by the high Th (100–300 ppm) and U (150–400 ppm) contents. These zircon fragments of the “ancient” group, higher than in the “young” group (<50 ppm Th and <70 ppm U) and are typical of zircon from lunar granitic rocks. The composition of the products of KREEP magmatism in the source region of the Dhofar 1442 meteorite could vary from predominantly granitic to KREEP gabbronoritic at 4.3–3.9 Ga.  相似文献   

11.
We performed a petrologic, mineralogical, geochemical, and isotopic study of several lithologies in the Y-86032 feldspathic breccia. This study leads us to conclude that Y-86032 likely originated on the lunar farside. Y-86032 is composed of several types of feldspathic clasts, granulitic breccias, and minor basaltic clasts set in a clastic matrix. We identify an “An97 anorthosite” that has An contents similar to those of nearside FANs. Mg′ (= molar Mg/(Mg + Fe) × 100) values vary significantly from ∼45 to ∼80 covering the ranges of both nearside FANs and the Mg′ gap between FANs and the Mg-suite. A light-gray feldspathic (LG) breccia making up ∼20% of the investigated slab (5.2 × 3.6 cm2) mainly consists of fragments of anorthosites (“An93 anorthosite”) more sodic than nearside FANs. LG also contains an augite-plagioclase clast which either could be genetically related to the An93 anorthosite or to slowly-cooled basaltic magma intruded into the precursor rock. The Na-rich nature of both An93 anorthosite and this clast indicates that the LG breccia was derived from a relatively Na-rich but incompatible-element-poor source. The Mg′ variation indicates that the “An97 anorthosite” is a genomict breccia of several types of primary anorthosites. Granulitic breccias in Y-86032 have relatively high Mg′ in mafic minerals. The highest Mg′ values in mafic minerals for the “An97 anorthosite” and granulitic breccias are similar to those of Mg-rich lithologies recently described in Dhofar 489. Basaltic clasts in the dark-gray matrix are aluminous, and the zoning trends of pyroxene are similar to those of VLT or LT basalts. The crystallization of these basaltic clasts pre-date the lithification age of the clastic matrix at ∼3.8 Ga. The low K contents of plagioclase in both the anorthositic and basaltic clasts and generally low incompatible element abundances in all the lithologies in Y-86032 indicate that KREEP was not involved during the formation of the precursor lithologies. This observation further suggests that urKREEP did not exist in the source regions of these igneous lithologies. All these facts support the idea that Y-86032 was derived from a region far distant from the PKT and that the lithic clasts and fragments are indigenous to that region. An An97 anorthositic clast studied here has distinct Sm-Nd isotopic systematics from those previously found for another An97 anorthositic clast and “An93 anorthosite”, and suggests either that An97 anorthosites come from isotopically diverse sources, or that the Sm-Nd isotopic systematics of this clast were reset ∼4.3 Ga ago. These lines of geochemical, isotopic, and petrologic evidence suggest that the lunar crust is geochemically more heterogeneous than previously thought.  相似文献   

12.
We have determined U-Pb ages, trace element abundances and Hf isotopic compositions of approximately 1000 detrital zircon grains from the Mississippi, Congo, Yangtze and Amazon Rivers. The U-Pb isotopic data reveal the lack of >3.3 Ga zircons in the river sands, and distinct peaks at 2.7-2.5, 2.2-1.9, 1.7-1.6, 1.2-1.0, 0.9-0.4, and <0.3 Ga in the accumulated age distribution. These peaks correspond well with the timing of supercontinent assembly. The Hf isotopic data indicate that many zircons, even those having Archean U-Pb ages, crystallized from magmas involving an older crustal component, suggesting that granitoid magmatism has been the primary agent of differentiation of the continental crust since the Archean era. We calculated Hf isotopic model ages for the zircons to estimate the mean mantle-extraction ages of their source materials. The oldest zircon Hf model ages of about 3.7 Ga for the river sands suggest that some crust generation had taken place by 3.7 Ga, and that it was subsequently reworked into <3.3 Ga granitoid continental crust. The accumulated model age distribution shows peaks at 3.3-3.0, 2.9-2.4, and 2.0-0.9 Ga.The striking attribute of our new data set is the non-uniformitarian secular change in Hf isotopes of granitoid crusts; Hf isotopic compositions of granitoid crusts deviate from the mantle evolution line from about 3.3 to 2.0 Ga, the deviation declines between 2.0 and 1.3 Ga and again increases afterwards. Consideration of mantle-crust mixing models for granitoid genesis suggests that the noted isotopic trends are best explained if the rate of crust generation globally increased in two stages at around (or before) 3.3 and 1.3 Ga, whereas crustal differentiation was important in the evolution of the continental crust at 2.3-2.2 Ga and after 0.6 Ga. Reconciling the isotopic secular change in granitoid crust with that in sedimentary rocks suggests that sedimentary recycling has essentially taken place in continental settings rather than active margin settings and that the sedimentary mass significantly grew through addition of first-cycle sediments from young igneous basements, until after ∼1.3 Ga when sedimentary recycling became the dominant feature of sedimentary evolution. These findings, coupled with the lack of zircons older than 3.3 Ga in river sands, imply the emergence of large-scale continents at about 3.3 Ga with further rapid growth at around 1.3 Ga. This resulted in the major growth of the sedimentary mass between 3.3 and 1.3 Ga and the predominance of its cannibalistic recycling later.  相似文献   

13.
Detrital zircon grains from Beit Bridge Group quartzite from the Central Zone of the Limpopo Belt near Musina yield mostly ages of 3.35-3.15 Ga, minor 3.15-2.51 Ga components, and numerous older grains grouped at approximately 3.4, 3.5 and 3.6 Ga. Two grains yielded concordant Late Hadean U-Pb ages of 3881 ± 11 Ma and 3909 ± 26 Ma, which are the oldest zircon grains so far found in Africa. The combined U-Pb and Lu-Hf datasets and field relationships provide evidence that the sedimentary protolith of the Beit Bridge Group quartzite was deposited after the emplacement of the Sand River Gneisses (3.35-3.15 Ga), but prior to the Neoarchean magmatic-metamorphic events at 2.65-2.60 Ga. The finding of abundant magmatic zircon detritus with concordant U-Pb ages of 3.35-3.15 Ga, and 176Hf/177Hf of 0.28066 ± 0.00004 indicate that the Sand River Gneiss-type rocks were a predominant source. In contrast, detrital zircon grains older than approximately 3.35 Ga were derived from the hinterland of the Limpopo Belt; either from a so far unknown crustal source in southern Africa, possibly from the Zimbabwe Craton and/or a source, which was similar but not necessarily identical to the one that supplied the Hadean zircons to Jack Hills, Western Australia. The Beit Bridge Group zircon population at >3.35 Ga shows a general εHft increase with decreasing age from εHf3.9Ga = −6.3 to εHf3.3-3.1Ga = −0.2, indicating that Hadean crust older than 4.0 Ga (TDM = 4.45-4.36 Ga) was rejuvenated during magmatic events between >3.9 and 3.1 Ga, due to a successive mixing of crustal rocks with mantle derived magmas. The existence of a depleted mantle reservoir in the Limpopo’s hinterland is reflected by the ∼3.6 Ga zircon population, which shows εHf3.6Ga between −4.6 and +3.2. In a global context, our data suggest that a long-lived, mafic Hadean protocrust with some tonalite-trondhjemite-granodiorite constituents was destroyed and partly recycled at the Hadean/Archean transition, perhaps due to the onset of modern-style plate tectonics.  相似文献   

14.
The sensitive high-resolution ion microprobe (SHRIMP) developed at the Australian National University (ANU) was the first of the high-resolution ion microprobes. The impact of this instrument on geochronological research over the last twenty years has been immense. This is particularly so for lunar geochronology where it has opened up avenues of research that were not possible using conventional TIMS techniques. The great advantage of SHRIMP is that it provides a means for determining precise U–Pb isotopic ratios on selected micron-size areas on polished grains of zircon and other U-bearing minerals. One of the first projects undertaken on the newly invented SHRIMP I was an investigation of U–Pb ages of lunar zircon. Using SHRIMP, multiple analyses could be made on areas of individual zircons to test the stability of U–Pb systems in shocked grains. Also, by analysing grains “in situ”, textural relationships between the analysed zircon and the components of the sample breccia could be used in the interpretation of the SHRIMP data. As a result of this research it was realised that most lunar zircons have ages up to 500 Ma older than the Imbrium and Serenitatis impacts at ca. 3.9 Ga, demonstrating that the zircons have not been affected by the these impact events although heating and shock effects have profoundly disturbed other dating systems. This has opened the way for research into the early lunar magmatic and bombardment record. For example, recent SHRIMP results have revealed profound differences in the ages of zircons from breccias from the Apollo 14 and Apollo 17 sample sites, raising new questions about the evolution of lunar magmatism. Also, multiple SHRIMP analyses on complex lunar zircons have shown that these grains can record U–Pb disturbance by later impact events. SHRIMP U–Pb age determinations on phosphates in lunar meteorites has identified lunar events not recognised in samples from the Apollo program. SHRIMP-based research on lunar materials is ongoing and, in combination with other chemical and structural evidence, continues to stimulate new ideas on the early evolution of the Moon.  相似文献   

15.
在北苏鲁经历超高压变质的花岗片麻岩中零星分布着一些变质表壳岩包体或者残片,通常称之为荆山群。北苏鲁荆山群的年代学特征以及构造亲缘性仍不清楚。本文报道了威海市郊和海阳所附近的荆山群露头,有脉状变质辉长岩侵入到荆山群岩系中。对上述两处变质辉长岩中锆石的阴极发光照片、稀土元素配分模式和Th/U比值的综合分析表明,它们均为岩浆锆石,部分岩浆锆石具有弱发光效应和岩浆韵律环带的核部、以及被改造的强发光效应的边部,但未见新生的变质锆石。未被改造的岩浆锆石微区的LA-ICP-MS U-Pb定年结果表明,威海市郊变辉长岩(S4-2)的~(207)Pb/~(206)Pb年龄值变化范围是1831~1966 Ma,加权平均年龄为1870±34 Ma(MSWD=0.6);海阳所变辉长岩(H3)中绝大部分锆石的~(207)Pb/~(206)Pb年龄值变化范围是1769~1887 Ma,加权平均年龄为1839±37 Ma(MSWD=0.5),两件样品共同记录了一致的ca.1.85 Ga岩浆事件。另外,海阳所变辉长岩中3颗捕获的岩浆锆石分别测得ca.2.9 Ga、ca.2.3 Ga和ca.2.1 Ga等三组~(207)Pb/~(206)Pb年龄。两件样品ca.1.85 Ga岩浆锆石具有相似的Hf同位素组成,其ε_(Hf)(t)=-6.1~-10.7,t_(DM2)=2.74~2.98 Ga,捕获ca.2.1 Ga和ca.2.9 Ga锆石的ε_(Hf)(t)值和t_(DM2)年龄分别是-1.9和2.7 Ga、-4.8和3.5 Ga。锆石U-Pb年龄、Lu-Hf同位素特征表明,北苏鲁两处变辉长岩记录的岩浆事件与胶北前寒武纪陆壳的多期演化历史一致,变辉长岩及其围岩荆山群可能是源自胶北的外来地质体。  相似文献   

16.
The lunar meteorites Northwest Africa (NWA) 3163, 4881, and 4483 are paired stones classified as granulitic breccias. At 2.4 kg, these three stones constitute one of the largest known lunar meteorite masses. Here we describe the petrography, mineralogy, and chemistry of NWA 3163, 4881, and 4483, and present 40Ar-39Ar data for two of the meteorites. Two-pyroxene thermometry indicates that the rocks equilibrated at 1050 ± 50 °C, which represents the high-temperature, low-pressure event that generated their characteristic recrystallization textures and reset their Ar systematics. Stepped-heating, in situ infrared laser microprobe 40Ar-39Ar geochronology yields a mean age of 3327 ± 29 Ma for NWA 3163, and a more disturbed release spectrum for NWA 4881. NWA 4881 shows an upward-trending pattern, suggesting that it may have had a 40Ar-39Ar age of >3.0 Ga, but that it was partially reset at ∼2.6 Ga. NWA 3163 et al. exhibit shock effects, including maskelynitized plagioclase, shock veins, and melt pockets, which are absent in the Apollo granulitic breccias. Although the Apollo and meteorite samples are texturally similar and have comparable bulk compositions and equilibration temperatures, their trace and siderophile element contents point to distinct parental lithologies derived from different regions of the Moon. Based on mineralogical and geochemical differences between the Apollo and meteorite samples, we conclude that the parent rock(s) of the paired NWA meteorites came from an area outside the Imbrium region and that they underwent high-temperature (granulite event) metamorphism long after the Late Heavy Bombardment.  相似文献   

17.
Zircons from granulite xenoliths entrained in a Late Cretaceous mafic dike in the Jiaodong Peninsula, North China Craton (NCC), show three distinct U-Pb age populations. Part of the old zircon grains yield discordant data that project to ages of about 2.4 to 2.5 Ga, a few grains indicate growth at about 2.0 Ga and a third group yield Cretaceous ages with peaks at 120 and 90 Ma. The oldest zircons give Hf TDM model ages of 2.6-2.8 Ga. These results demonstrate the existence of original Archean lower crust in the Jiaodong region. Zircons of 2.0 Ga have similar Hf TDM model ages as the Neoarchean-Paleoproterozoic grains, suggesting that these zircons were products of metamorphic recrystallization due to thermal event without juvenile input. Early Cretaceous zircons yield εHf(t) values of − 21 to − 12 and Late Cretaceous zircons large variable εHf(t) from + 4 to − 50. These data suggest that magmatic underplating occurred in the Neoarchean to Earliest Proterozoic lower crust of the NCC, both in the Early and Late Cretaceous. It is suggested that the Mesozoic magma underplating, which also provided the heat source for the voluminous Mesozoic magmatism in the NCC, significantly modified the composition of the Archean to Paleoproterozoic lower crust of the NCC.  相似文献   

18.
In the eastern Himalayan syntaxis, the southern Lhasa terrane is dominated by middle- to high-grade metamorphic rocks (Nyingchi Complex), which are intruded by felsic melts. U-Pb zircon dating and zircon Hf isotopic composition of these metamorphic and magmatic rocks provide important constraints on the tectono-thermal evolution of the Lhasa terrane during convergent process between Indian and Asian continents. U-Pb zircon data for an orthogneiss intruding the Nyingchi Complex yield a protolith magma crystallization age of 83.4 ± 1.2 Ma, with metamorphic ages of 65-46 Ma. This orthogneiss is characterized by positive εHf (t) values of + 8.3 and young Hf model ages of ~ 0.6 Ga, indicating a derivation primarily from a depleted-mantle or juvenile crustal source. Zircons from a quartz diorite yield a magma crystallization age of 63.1 ± 0.6 Ma, with εHf (t) values of − 8.2 to − 2.7, suggesting that this magma was sourced from partial melting of older crustal materials. Zircon cores from a foliated biotite granite show ages ranging from 347 to 2690 Ma, with age peaks at 347-403 Ma, 461-648 Ma and 1013-1183 Ma; their zircon εHf (t) values range from − 30.6 to + 6.9. Both the U-Pb ages and Hf isotopic composition of the zircon cores are similar to those of detrital zircons from the Nyingchi Complex paragneiss, implying that the granite was derived from anatexis of the Nyingchi Complex metasediments. The zircon rims from the granite indicate crustal anatexis at 64.4 ± 0.7 Ma and subsequent metamorphism at 55.1 ± 1.3 and 41.4 ± 2.3 Ma. Our results suggest that the late Cretaceous magmatism in the southern Lhasa terrane resulted from Neo-Tethys oceanic slab subduction and we infer that Paleocene crustal anatexis and metamorphism were related to the thermal perturbation caused by rollback of the northward subducted Neo-Tethyan oceanic slab.  相似文献   

19.
The isotopic analyses of rare earth elements (REE), Pb and U in several kinds of minerals from the clay and black shale layers above the Bangombé natural reactor, Gabon, were performed using a sensitive high-resolution microprobe (SHRIMP) to investigate the migration and retardation processes of fission products released from the reactor. REE isotopic data of the secondary minerals found in clays and black shales show that most of fission products were effectively trapped in the clays and not distributed into the black shales over the clays, which reveals that the clays play an important role in preventing fission products from spreading.Zircon crystals in the clays heterogeneously contain high-U regions (up to 28.3 wt%) with normal 235U/238U ratios (=0.00725) and significant amounts of fissiogenic REE, which suggest the occurrence of significant chemical fractionation between REE and U during the dissolution of reactor uraninite and the recrystallization of secondary U minerals. The Pb data suggest that galena grains in the clays were also formed by the mixing of the two components during a recent alteration event, and that a significant amount of Pb was derived from 2.05-Ga-old original uraninite rather than reactor uraninite. The U-Pb systematics of zircon provide chronological information on the old igneous activity associated with the basement rock formation at 2.8 Ga and geochemical evidence of the incomplete mixing of independent Pb and U sources. This result is consistent with previous chronological results in this area.  相似文献   

20.
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   

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