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 46 毫秒
1.
Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of two Martian basalts (Los Angeles and Zagami) and two Martian olivine-phyric basalts (Dar al Gani (DaG) 476 and North West Africa (NWA) 1068). With a ∼50 μm diameter focused infrared laser beam, it has been possible to distinguish between argon isotopic signatures from host rock (matrix) minerals and localized shock melt products (pockets and veins). The concentrations of argon in analyzed phases from all four meteorites have been quantified using the measured J values, 40Ar/39Ar ratios and K2O wt% in each phase. Melt pockets contain, on average, 10 times more gas (7-24 ppb 40Ar) than shock veins and matrix minerals (0.3-3 ppb 40Ar). The 40Ar/36Ar ratio of the Martian atmosphere, estimated from melt pocket argon extractions corrected for cosmogenic 36Ar, is: Los Angeles (∼1852), Zagami (∼1744) and NWA 1068 (∼1403). In addition, Los Angeles shows evidence for variable mixing of two distinct trapped noble gas reservoirs: (1) Martian atmosphere in melt pockets, and (2) a trapped component, possibly Martian interior (40Ar/36Ar: 480-490) in matrix minerals. Average apparent 40Ar/39Ar ages determined for matrix minerals in the four analyzed meteorites are 1290 Ma (Los Angeles), 692 Ma (Zagami), 515 Ma (NWA 1068) and 1427 Ma (DaG 476). These 40Ar/39Ar apparent ages are substantially older than the ∼170-474 Ma radiometric ages given by other isotope dating techniques and reveal the presence of trapped 40Ar. Cosmic ray exposure (CRE) ages were measured using spallogenic 36Ar and 38Ar production. Los Angeles (3.1 ± 0.2 Ma), Zagami (2.9 ± 0.4 Ma) and NWA 1068 (2.0 ± 0.5 Ma) yielded ages within the range of previous determinations. DaG 476, however, yielded a young CRE age (0.7 ± 0.25 Ma), attributed to terrestrial alteration. The high spatial variation of argon indicates that the incorporation of Martian atmospheric argon into near-surface rocks is controlled by localized glass-bearing melts produced by shock processes. In particular, the larger (mm-size) melt pockets contain near end-member Martian atmospheric argon. Based on petrography, composition and argon isotopic data we conclude that the investigated melt pockets formed by localized in situ shock melting associated with ejection. Three processes may have led to atmosphere incorporation: (1) argon implantation due to atmospheric shock front collision with the Martian surface, (2) transformation of an atmosphere-filled cavity into a localized melt zone, and (3) shock implantation of atmosphere trapped in cracks, pores and fissures.  相似文献   

2.
The thermal histories of Martian meteorite are important for the interpretation of petrologic, geochemical, geochronological, and paleomagnetic constraints that they provide on the evolution of Mars. In this paper, we quantify 40Ar/39Ar ages and Ar diffusion kinetics of Martian meteorites Allan Hills (ALH) 84001, Nakhla, and Miller Range (MIL) 03346. We constrain the thermal history of each meteorite and discuss the resulting implications for their petrology, paleomagnetism, and geochronology. Maskelynite in ALH 84001 yields a 40Ar/39Ar isochron age of 4163 ± 35 Ma, which is indistinguishable from recent Pb-Pb (Bouvier et al., 2009a) and Lu-Hf ages (Lapen et al., 2010). The high precision of this result arises from clear resolution of a reproducible trapped 40Ar/36Ar component in maskelynite in ALH 84001 (40Ar/36Ar = 632 ± 90). The maskelynite 40Ar/39Ar age predates the Late Heavy Bombardment and likely represents the time at which the original natural remanent magnetization (NRM) component observed in ALH 84001 was acquired. Nakhla and MIL 03346 yield 40Ar/39Ar isochron ages of 1332 ± 24 and 1339 ± 8 Ma, respectively, which we interpret to date crystallization. Multi-phase, multi-domain diffusion models constrained by the observed Ar diffusion kinetics and 40Ar/39Ar age spectra suggest that localized regions within both ALH 84001 and Nakhla were intensely heated for brief durations during shock events at 1158 ± 110 and 913 ± 9 Ma, respectively. These ages may date the marginal melting of pyroxene in each rock, mobilization of carbonates and maskelynite in ALH 84001, and NRM overprints observed in ALH 84001. The inferred peak temperatures of the shock heating events (>1400 °C) are sufficient to mobilize Ar, Sr, and Pb in constituent minerals, which may explain some of the dispersion observed in 40Ar/39Ar, Rb-Sr, and U-Th-Pb data toward ages younger than ∼4.1 Ga. The data also place conservative upper bounds on the long-duration residence temperatures of the ALH 84001 and Nakhla protolith to be  °C and  °C over the last ∼4.16 Ga and ∼1.35 Ga, respectively. MIL 03346 has apparently not experienced significant shock-heating since it crystallized, consistent with the fact that various chronometers yield concordant ages.  相似文献   

3.
Multiple lines of evidence show that the Rb-Sr, Sm-Nd, and Ar-Ar isotopic systems individually give robust crystallization ages for basaltic (or diabasic) shergottite Northwest Africa (NWA) 1460. In contrast to other shergottites, NWA 1460 exhibits minimal evidence of excess 40Ar, thus allowing an unambiguous determination of its Ar-Ar age. The concordant Rb-Sr, Sm-Nd, and Ar-Ar results for NWA 1460 define its crystallization age to be 346 ± 17 Ma (2σ). In combination with petrographic and trace element data for this specimen and paired meteorite NWA 480, these results strongly refute the suggestion by others that the shergottites are ∼4.1 Ga old. Current crystallization and cosmic-ray exposure (CRE) age data permit identification of a maximum of nine ejection events for Martian meteorites (numbering more than 50 unpaired specimens as of 2008) and plausibly as few as five such events. Although recent high resolution imaging of the Martian surface has identified limited areas of sparsely cratered terrains, the meteorite data suggest that either these areas are representative of larger areas from which the meteorites might come, or that the cratering chronology needs recalibration. Time-averaged 87Rb/86Sr = 0.16 for the mantle source of the parent magma of NWA 1460/480 over the ∼4.56 Ga age of the planet is consistent with previously estimated values for bulk silicate Mars in the range 0.13-0.16, and similar to values of ∼0.18 for the “lherzolitic” shergottites. Initial εNd for NWA 1460/480 at 350 ± 16 Ma ago was +10.6 ± 0.5, which implies a time-averaged 147Sm/144Nd of 0.217 in the Martian mantle prior to mafic melt extraction, similar to values of 0.211-0.216 for the “lherzolitic” shergottites. These time-averaged values do not imply a simple two-stage mantle/melt evolution, but must result from multiple episodes of melt extractions from the source regions. Much higher “late-stage” εNd values for the depleted shergottites imply similar processes carried to a greater degree. Thus, NWA 1460/480, the “lherzolitic” shergottites and perhaps EET 79001 give the best (albeit imperfect) estimate of the Sr- and Nd-isotopic characteristics of bulk silicate Mars.  相似文献   

4.
We report 39Ar-40Ar ages of whole rock (WR) and plagioclase and pyroxene mineral separates of nakhlites MIL 03346 and Y-000593, and of WR samples of nakhlites NWA 998 and Nakhla. All age spectra are complex and indicate variable degrees of 39Ar recoil and variable amounts of trapped 40Ar in the samples. Thus, we examine possible Ar-Ar ages in several ways. From consideration of both limited plateau ages and isochron ages, we prefer Ar-Ar ages of NWA 998 = 1334 ± 11 Ma, MIL 03346 = 1368 ± 83 Ma (mesostasis) and 1334 ± 54 Ma (pyroxene), Y-000593 = 1367 ± 7 Ma, and Nakhla = 1357 ± 11 Ma, (2σ errors). For NWA 998 and MIL 03346 the Ar-Ar ages are within uncertainties of preliminary Rb-Sr isochron ages reported in the literature. These Ar-Ar ages for Y-000593 and Nakhla are several Ma older than Sm-Nd ages reported in the literature. We conclude that the major factor in producing Ar-Ar ages slightly too old is the presence of small amounts of trapped martian or terrestrial 40Ar on weathered grain surfaces that was degassed along with the first several percent of 39Ar. A total K-40Ar isochron for WR and mineral data from five nakhlites analyzed by us, plus Lafayette data in the literature, gives an isochron age of 1325 ± 18 Ma (2σ). We emphasize the precision of this isochron over the value of the isochron age. Our Ar-Ar data are consistent with a common formation age for nakhlites. The cosmic-ray exposure (CRE) age for NWA 998 of ∼12 Ma is also similar to CRE ages for other nakhlites.  相似文献   

5.
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   

6.
Detailed Rb-Sr and Sm-Nd isotopic analyses have been completed on the lherzolitic shergottites ALH77005 and LEW88516. ALH77005 yields a Rb-Sr age of 185 ± 11 Ma and a Sm-Nd age of 173 ± 6 Ma, whereas the Rb-Sr and Sm-Nd ages of LEW88516 are 183 ± 10 and 166 ± 16 Ma, respectively. The initial Sr isotopic composition of ALH77005 is 0.71026 ± 4, and the initial εNd value is +11.1 ± 0.2. These values are distinct from those of LEW88516, which has an initial Sr isotopic composition of 0.71052 ± 4 and an initial εNd value of +8.2 ± 0.6. Several of the mineral and whole rock leachates lie off the Rb-Sr and Sm-Nd isochrons, indicating that the isotopic systematics of the meteorites have been disturbed. The Sm-Nd isotopic compositions of the leachates appear to be mixtures of primary igneous phosphates and an alteration component with a low 143Nd/144Nd ratio that was probably added to the meteorites on Mars. Tie lines between leachate-residue pairs from LEW88516 mineral fractions and whole rocks have nearly identical slopes that correspond to Rb-Sr ages of 90 ± 1 Ma. This age may record a major shock event that fractionated Rb/Sr from lattice sites located on mineral grain boundaries. On the other hand, the leachates could contain secondary alteration products, and the parallel slopes of the tie lines could be coincidental.Nearly identical mineral modes, compositions, and ages suggest that these meteorites are very closely related. Nevertheless, their initial Sr and Nd isotopic compositions differ outside analytical uncertainty, requiring derivation from unique sources. Assimilation-fractional-crystallization models indicate that these two lherzolitic meteorites can only be related to a common parental magma, if the assimilant has a Sr/Nd ratio near 1 and a radiogenic Sr isotopic composition. Further constraints placed on the evolved component by the geochemical and isotopic systematics of the shergottite meteorite suite suggest that it (a) formed at ∼4.5 Ga, (b) has a high La/Yb ratio, (c) is an oxidant, and (d) is basaltic in composition or is strongly enriched in incompatible elements. The composition and isotopic systematics of the evolved component are unlike any evolved lunar or terrestrial igneous rocks. Its unusual geochemical and isotopic characteristics could reflect hydrous alteration of an evolved Martian crustal component or hydrous metasomatism within the Martian mantle.  相似文献   

7.
Combined 147Sm-143Nd and 176Lu-176Hf chronology of the martian meteorite Larkman Nunatak (LAR) 06319 indicates an igneous crystallization age of 193 ± 20 Ma (2σ weighted mean). The individual 147Sm-143Nd and 176Lu-176Hf internal isochron ages are 183 ± 12 Ma and 197 ± 29 Ma, respectively, and are concordant with two previously determined 147Sm-143Nd and 87Rb-87Sr internal isochron ages of 190 ± 26 Ma and 207 ± 14 Ma, respectively (Shih et al., 2009). With respect to the 147Sm-143Nd isotope systematics, maskelynite lies above the isochron defined by primary igneous phases and is therefore not in isotopic equilibrium with the other phases in the rock. Non-isochronous maskelynite is interpreted to result from shock-induced reaction between plagioclase and partial melts of pyroxene and phosphate during transformation to maskelynite, which resulted in it having unsupported 143Nd relative to its measured 147Sm/144Nd ratio. The rare earth element (REE) and high field strength element (HFSE) compositions of major constituent minerals can be modeled as the result of progressive crystallization of a single magma with no addition of secondary components. The concordant ages, combined with igneous textures, mineralogy, and trace element systematics indicate that the weighted average of the radiometric ages records the true crystallization age of this rock. The young igneous age for LAR 06319 and other shergottites are in conflict with models that advocate for circa 4.1 Ga crystallization ages of shergottites from Pb isotope compositions, however, they are consistent with updated crater counting statistics indicating that young volcanic activity on Mars is more widespread than previously realized (Neukum et al., 2010).  相似文献   

8.
The 40Ar/39Ar stepwise crushing technique is applied for the first time to date garnet from ultra-high-pressure metamorphic (UHPM) eclogites. Three garnet samples from the Bixiling eclogites analyzed by 40Ar/39Ar stepwise crushing yield regular, predictable age spectra, and a clear separation between excess 40Ar and concordant plateau and isochron ages. All three age spectra begin with high apparent ages followed by step by step decreasing ages, and finally age plateaux with apparent ages in the range from 427 ± 20 to 444 ± 10 Ma. The data points constituting the age plateaux yield excellent isochrons with radiogenic intercept ages ranging from 448 ± 34 to 459 ± 58 Ma, corresponding to initial 40Ar/36Ar ratios from 292.1 ± 4.5 to 294.5 ± 6.7, statistically indistinguishable from the modern air. The high initial ages are interpreted to derive from secondary fluid inclusions containing excess 40Ar, whereas the plateau ages are attributed to gas from small primary fluid inclusions without significant excess 40Ar. The plateau ages are interpreted to approximate the time of garnet growth during initial UHPM metamorphism. Phengite analyzed by laser stepwise heating yielded a complicated two-saddle age spectrum with a scattered isochron corresponding to age of 463 ± 116 Ma and initial 40Ar/36Ar ratio of 1843 ± 1740 indicative of the presence of extraneous 40Ar within phengite. These concordant isochron ages measured on minerals diagnostic of eclogite grade metamorphism strongly suggest that Dabie UHPM eclogites were first formed in the early Paleozoic, during the same event that caused the Qinling-Northern Qaidam Basin-Altyn Tagh eclogites.  相似文献   

9.
Portales Valley, Sombrerete, and Northwest Africa (NWA) 176 are three unrelated meteorites, which consist of silicate mixed with substantial amounts of metal and which likely formed at elevated temperatures as a consequence of early impacts on their parent bodies. Measured 39Ar-40Ar ages of these meteorites are 4477 ± 11 Ma and 4458 ± 16 Ma (two samples of Portales Valley), 4541 ± 12 Ma, and 4524 ± 13 Ma, respectively (Ma = million years; all one-sigma errors). The Ar-Ar data for Portales Valley show no evidence of later open system behavior suggested by some other chronometers. Measured 129I-129Xe ages of these three meteorites are 4559.9 ± 0.5 Ma, 4561.9 ± 1.0 Ma, and ∼4544 Ma, respectively (relative to Shallowater = 4562.3 ± 0.4 Ma). From stepwise temperature release data, we determined the diffusion characteristics for Ar and Xe in our samples and calculated approximate closure temperatures for the K-Ar and I-Xe chronometers. Adopting results and interpretations about these meteorites from some previous workers, we evaluated all these data against various thermal cooling models. We conclude that Portales Valley formed 4560 Ma ago, cooled quickly to below the I-Xe closure temperature, then cooled deep within the parent body at a rate of ∼4 °C/Ma through K-Ar closure. We conclude that Sombrerete formed 4562 Ma ago and cooled relatively quickly. NWA 176 likely formed and cooled quickly ∼4544 Ma ago, or later than formation times of most meteorite parent bodies. For all three meteorites, the Ar-Ar ages are in better agreement with I-Xe ages and preferred thermal models if we increase these Ar-Ar ages by ∼20 Ma. Such age corrections would be consistent with probable errors in 40K decay parameters in current use, as suggested by others. The role of impact heating and possible disruption and partial reassembly of meteorite parent bodies to form some meteorites likely was an important process in the early solar system.  相似文献   

10.
Whereas most radiometric chronometers give formation ages of individual meteorites >4.5 Ga ago, the K–Ar chronometer rarely gives times of meteorite formation. Instead, K–Ar ages obtained by the 39Ar–40Ar technique span the entire age of the solar system and typically measure the diverse thermal histories of meteorites or their parent objects, as produced by internal parent body metamorphism or impact heating. This paper briefly explains the Ar–Ar dating technique. It then reviews Ar–Ar ages of several different types of meteorites, representing at least 16 different parent bodies, and discusses the likely thermal histories these ages represent. Ar–Ar ages of ordinary (H, L, and LL) chondrites, R chondrites, and enstatite meteorites yield cooling times following internal parent body metamorphism extending over ∼200 Ma after parent body formation, consistent with parent bodies of ∼100 km diameter. For a suite of H-chondrites, Ar–Ar and U–Pb ages anti-correlate with the degree of metamorphism, consistent with increasing metamorphic temperatures and longer cooling times at greater depths within the parent body. In contrast, acapulcoites–lodranites, although metamorphosed to higher temperatures than chondrites, give Ar–Ar ages which cluster tightly at ∼4.51 Ga. Ar–Ar ages of silicate from IAB iron meteorites give a continual distribution across ∼4.53–4.32 Ga, whereas silicate from IIE iron meteorites give Ar–Ar ages of either ∼4.5 Ga or ∼3.7 Ga. Both of these parent bodies suffered early, intense collisional heating and mixing. Comparison of Ar–Ar and I–Xe ages for silicate from three other iron meteorites also suggests very early collisional heating and mixing. Most mesosiderites show Ar–Ar ages of ∼3.9 Ga, and their significantly sloped age spectra and Ar diffusion properties, as well as Ni diffusion profiles in metal, indicate very deep burial after collisional mixing and cooling at a very slow rate of ∼0.2 °C/Ma. Ar–Ar ages of a large number of brecciated eucrites range over ∼3.4–4.1 Ga, similar to ages of many lunar highland rocks. These ages on both bodies were reset by large impact heating events, possibly initiated by movements of the giant planets. Many impact-heated chondrites show impact-reset Ar–Ar ages of either >3.5 Ga or <1.0 Ga, and generally only chondrites show these younger ages. The younger ages may represent orbital evolution times in the asteroid belt prior to ejection into Earth-crossing orbits. Among martian meteorites, Ar–Ar ages of nakhlites are similar to ages obtained from other radiometric chronometers, but apparent Ar–Ar ages of younger shergottites are almost always older than igneous crystallization ages, because of the presence of excess (parentless) 40Ar. This excess 40Ar derives from shock-implanted martian atmosphere or from radiogenic 40Ar inherited from the melt. Differences between meteorite ages obtained from other chronometers (e.g., I–Xe and U–Pb) and the oldest measured Ar–Ar ages are consistent with previous suggestions that the 40K decay parameters in common use are incorrect and that the K–Ar age of a 4500 Ma meteorite should be possibly increased, but by no more than ∼20 Ma.  相似文献   

11.
A synthetic composition representing the Yamato 980459 martian basalt (shergottite) has been used to carry out phase relation, and rare earth element (REE) olivine and pyroxene partitioning experiments. Yamato 980459 is a sample of primitive basalt derived from a reduced end-member among martian mantle sources. Experiments carried out between 1-2 GPa and 1350-1650 °C simulate the estimated pressure-temperature conditions of basaltic melt generation in the martian mantle. Olivine-melt and orthopyroxene-melt partition coefficients for La, Nd, Sm, Eu, Gd and Yb (DREE values) were determined by LA-ICPMS, and are similar to the published values for terrestrial basaltic systems. We have not detected significant variation in D-values with pressure over the range investigated, and by comparison with previous studies carried out at lower pressure.We apply the experimentally obtained olivine-melt and orthopyroxene-melt DREE values to fractional crystallization and partial melting models to develop a three-stage geochemical model for the evolution of martian meteorites. In our model we propose two ancient (∼4.535 Ga) sources: the Nakhlite Source, located in the shallow mantle, and the Deep Mantle Source, located close to the martian core-mantle boundary. These two sources evolved distinctly on the ε143Nd evolution curve due to their different Sm/Nd ratios. By partially melting the Nakhlite Source at ∼1.3 Ga, we are able to produce a slightly depleted residue (Nakhlite Residue). The Nakhlite Residue is left undisturbed until ∼500 Ma, at which point the depleted Deep Mantle Source is brought up by a plume mechanism carrying with it high heat flow, melts and isotopic signatures of the deep mantle (e.g., ε182W, ε142Nd, etc.). The plume-derived Deep Mantle Source combines with the Nakhlite Residue producing a mixture that becomes a mantle source (herein referred to as “the Y98 source”) for Yamato 980459 and the other depleted shergottites with the characteristic range of Sm/Nd ratios of these meteorites. The same hot plume provides a heat source for the formation of enriched and intermediate shergottites. Our model reproduces the REE patterns of nakhlites and depleted shergottites and can explain high ε143Nd in depleted shergottites. Furthermore, the model results can be used to interpret whole rock Rb-Sr and Sm-Nd ages of shergottites.  相似文献   

12.
New 40Ar/39Ar thermochronology results and thermal modeling support the hypothesis of Hollister et al. (2004), that reheating of the mid-Cretaceous Ecstall pluton by intrusion of the Coast Mountains Batholith (CMB) was responsible for spatially variable remagnetization of the Ecstall pluton. 40Ar/39Ar ages from hornblende and biotite from 12 locations along the Skeena River across the northern part of the Ecstall pluton decrease with proximity to the Quottoon plutonic complex, the nearest member of the CMB to the Ecstall pluton. The oldest 40Ar/39Ar ages are found farthest from the Quottoon plutonic complex, and are 90 ± 3 Ma for hornblende, and 77.9 ± 1.2 Ma for biotite. The youngest 40Ar/39Ar ages are found closest to the Quottoon plutonic complex, and are 51.6 ± 1.2 Ma for hornblende, and 45.3 ± 1.7 Ma for biotite. No obvious relationship between grain size and age is seen in the Ecstall pluton biotites. Spatial trends in 40Ar/39Ar ages are consistent with model results for reheating by a thermal wall at the location of the Quottoon plutonic complex. Although no unique solution is suggested, our results indicate that the most appropriate thermal history for the Ecstall pluton includes both reheating and northeast side up tilting of the Ecstall pluton associated with intrusion of the Quottoon plutonic complex. Estimates of northward translation from shallow paleomagnetic inclinations in the western part of the Ecstall pluton are reduced to ∼3000 km, consistent with the Baja-BC hypothesis, when northeast side up tilting is accounted for.  相似文献   

13.
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   

14.
A study was undertaken to determine the chronology, petrogenesis and relationships among the shergottites, Shergotty and Zagami and the unique achondrite ALHA77005. These meteorites are the product of a variety of complex processes.Petrogenesis: Chondrite-normalized abundance patterns of Shergotty and Zagami are very similar and show pronounced depletions of both the light REE (La-Nd) and heavy REE (Dy-Lu) relative to Sm-Gd. These characteristic depletions are even more pronounced for ALHA77005. The light REE depletion is qualitatively consistent with the presence of cumulus pyroxene and/or olivine in these meteorites, but trace element models show that the parental magmas of all three meteorites were probably also light REE depleted. Both trace element model calculations and combined Rb-Sr and Sm-Nd isotopic systematics show that the meteorites could not have been co-magmatic nor can ALHA77005 be representative of the source material of the shergottites. Light REE depletion of the parental magmas also implies light REE depletion of the source material. The Sm-Nd systematics of the shergottites require a time-averaged sub-chondritic (light REE enriched) Sm-Nd ratio since 4.6 AE ago. The Sm-Nd systematics of ALHA77005 permit a time-averaged super-chondritic (light REE depleted) Sm/Nd ratio if its crystallization age is less than TICE = 0.72 AE.Chronology. Rb-Sr internal isochrons for all three meteorites and a Sm-Nd internal isochron for Zagami are concordant at ~ 180 Myr. 39Ar-40Ar plateau ages of Shergotty and Zagami maskelynite are ~250–260 Myr. These ages apparently reflect resetting of these isotopic systems by shock metamorphism which converted the feldspar to maskelynite. The concordance of these ages suggests a single shock event during which the meteorites were in close physical proximity. The time of this event is most precisely given by the Rb-Sr age of 180 ± 4 Myr for Zagami.The crystallization ages of the meteorites were not precisely determined. Extreme upper limits are determined by Sm-Nd model ages relative to an eucrite initial 143Nd144Nd = 0.505835 at 4.6 AE ago. These model ages for Shergotty, Zagami and ALHA77005 are 3600, 3500 and 2850 Myr, respectively. The Sm-Nd whole rock age of 1340 ± 60 Myr for the three meteorites gives the crystallization age if the Sm/Nd ratios of the precursor materials were always the same. We consider this 1340 Myr age as a “best estimate” upper limit. “Best estimate” lower limits for Shergotty and Zagami are taken from the average 39Ar-40Ar ages of 1200 and 900 Myr of pyroxene separates. The average 39Ar-40Ar age of a whole rock sample of ALHA77005 was 1600 Myr and can be partitioned between a low temperature (feldspar) phase and a high temperature (olivine + pyroxene + inclusions) “phase”. The average apparent 39Ar-40Ar age of the low temperature phase is ~1050 Myr, which is chosen as the “best estimate” lower limit to the age. The crystallization ages of Shergotty, Zagami and ALHA77005 probably lie within the ranges of 1200–1300, 900–1300 and 1000–1300 Myr, respectively. The Rb-Sr whole rock age of 4400 ± 400 Myr and single-stage BABI model ages of ~4800–5100 Myr are interpreted as reflecting differentiation of the parent body at ~4600 Myr ago.The complex geochemical and isotopic evolution recorded by these meteorites suggests a geologically active parent body capable of sustaining melting at two or more epochs in its history.  相似文献   

15.
Infrared laser probe 40Ar/39Ar geochronology, instrumental neutron activation analysis (INAA) and analytical electron microscopy have been performed on four 0.5 × 1.0 × 0.3 cm polished rock tiles of Apollo 16 and 17 granulitic breccias (60035, 77017, 78155, and 79215). Pyroxene thermometry indicates that these samples were re-equilibrated and underwent peak metamorphic sub-solidus recrystallization at 1000-1100 °C, which resulted in homogeneous mineral compositions and granoblastic textures.40Ar/39Ar data from this study reveal that three samples (60035, 77017, and 78155) have peak metamorphic ages of ∼4.1 Ga. Sample 79215 has a peak metamorphic age of 3.9 Ga, which may be related to Serenitatis basin formation. All four samples contain moderately high concentrations of meteoritic siderophiles. Enhanced siderophile contents in three of the samples provide evidence for projectile contamination of their target lithologies occurring prior to peak metamorphism.Post-peak metamorphism, low-temperature (<300 °C) events caused the partial resetting of argon in the two finer-grained granulites (60035 and 77017). These later events did not alter the mineralogy or texture of the rocks, but caused minor brecciation and the partial release of argon from plagioclase. Interpretation of the low-temperature data indicates partial resetting of the argon systematics to as young as 3.2 Ga for 60035 and 2.3 Ga for 77017. Cosmic ray exposure ages range from 6.4 to ∼339 Ma.Our results increase the amount of high-precision data available for the granulitic breccias and lunar highlands crustal samples. The results demonstrate the survival of pre-Nectarian material on the lunar surface and document the effects of contact metamorphic and impact processes during the pre-Nectarian Epoch, as well as the low-temperature partial resetting of ages by smaller impact events after 3.9 Ga.The mineralogy and chemical composition of these rocks, as well as exhumation constraints, indicate that the source of heat for metamorphism was within kilometers of the surface via burial beneath impact-melt sheets or hot ejecta blankets.  相似文献   

16.
We present a summary of late Paleoproterozoic to Neoproterozoic mafic magmatism in the Siberian craton, including recently published U–Pb and 40Ar–39Ar dates. These new precise ages suggest that at least some of the previously published K–Ar ages of Siberian mafic bodies should be ignored. The time–space geochronological chart, or the ‘barcode’ of mafic magmatic events shows significant differences between northern and southern Siberia. Both are characterized by ∼1900–1700 Ma magmatic events, but then there was an almost 1 Ga mafic magmatic ‘pause’ in south Siberia until ∼800 Ma. Meanwhile there are indications of multiple mafic magmatic events in North Siberia (Anabar shield and Olenek uplift) between ∼1600 and 1000 Ma. A series of magmatic events probably related to the breakup of Rodinia occurred in southern Siberia after ∼800 Ma. So far, there are no indications of late Neoproterozoic mafic magmatism in North Siberia. Ca. 1000–950 Ma mafic sills were reported from Meso- to Neo-Proterozoic sedimentary successions in the Sette-Daban area on the east side of the Siberian craton, but their tectonic setting is debated. Recent Ar–Ar dates of ∼1750 Ma for NW-trending dykes in the Aldan and Anabar shields, together with similar-age NNE-trending Baikal uplift dykes in south-eastern Siberia suggest the existence of a giant radial dyke swarm possibly related to a mantle plume centred in the Vilyui River area.  相似文献   

17.
The lunar meteorite Northwest Africa (NWA) 032 is a low-Ti basalt that has incompatible-element abundances and Th/Sm ratios characteristic of the involvement of late stage magma ocean crystallization products (urKREEP) in its petrogenesis. This sample is very fine-grained and contains terrestrial weather products. A progressive leaching procedure was therefore developed and applied to magnetic separates and whole rock fractions to obtain Rb-Sr and Sm-Nd ages. Although many of the leachates, as well as the unleached mineral and whole rock fractions contain terrestrial alteration products, selected residue fractions yield concordant Rb-Sr and Sm-Nd ages. Rubidium-Sr isotopic analyses yield an age of 2947 ± 16 Ma with an initial 87Sr/86Sr of 0.700057 ± 17. These characteristics indicate NWA 032 is derived from a source region with an 87Rb/86Sr ratio of 0.044 ± 0.001. This value is higher than all but those determined for KREEP basalts, and suggests that NWA 032 is derived from a source region that has higher incompatible-element abundances than other low-Ti basalts. Samarium-neodymium isotopic analysis yield a concordant age of 2931 ± 92 Ma and an initial εNd of +9.71 ± 0.74 corresponding to a source region with 147Sm/144Nd ratio of 0.246 ± 0.004. The initial Nd isotopic composition stands in contrast to the initial Sr isotopic composition by requiring NWA 032 to be derived from a source with lower incompatible-element abundances than most low-Ti basalts. The source of NWA 032 is therefore unlike those of other lunar basalts.Modeling of magma ocean cumulate formation demonstrates that unlike other low-Ti basalt source regions the NWA 032 source is a mixture of olivine, pigeonite, and clinopyroxene bearing cumulates and only a small amount of urKREEP. Furthermore, unlike other mare basalt sources, the NWA 032 source does not contain appreciable quantities of plagioclase. Partial melting models demonstrate that the incompatible-element characteristics of the NWA 032 result from formation by smaller degrees of partial melting than other mare basalts. Thus, the incompatible-element geochemical signature that is observed in NWA 032 appears to reflect the combined effects of generation from an unusual plagioclase-free incompatible-element-depleted source region by very small degrees of partial melting. This study demonstrates that both the presence of urKREEP in the source region and small degrees of partial melting generate magmas with similar, but not identical, incompatible-element characteristics. In addition, it underscores the fact that there is significantly more geochemical diversity on the Moon than is represented by samples collected by the American and Soviet lunar missions.  相似文献   

18.
We report elemental abundances and the isotopic systematics of the short-lived 26Al-26Mg (half-life of ∼0.73 Ma) and long-lived U-Pb radiochronometers in the ungrouped basaltic meteorite Northwest Africa (NWA) 2976. The bulk geochemical composition of NWA 2976 is clearly distinct from that of the eucrites and angrites, but shows broad similarities to that of the paired NWA 001 and 2400 ungrouped achondrites indicating that it is likely to also be paired with these two samples. The major and trace element abundances in NWA 2976 further indicate that it formed by extensive melting and magmatic fractionation processes on its parent body. The Al-Mg and Pb-Pb isotope systematics indicate that this meteorite represents the earliest stages of crust formation on a differentiated parent body in the early Solar System. The absolute Pb-Pb internal isochron age of NWA 2976, obtained from acid leaching residues of three whole-rock samples and two pyroxene separates, is 4562.89 ± 0.59 Ma (MSWD = 0.02). This Pb-Pb age is calculated using the measured 238U/235U ratio of a NWA 2976 whole-rock of 137.751 ± 0.018 (2σ) which was determined relative to the recently revised value of 137.840 ± 0.008 for the SRM 950a U isotope standard. The Al-Mg systematics reveal the presence of 26Mg isotopic anomalies produced by the decay of 26Al with an (26Al/27Al)0 of (3.94 ± 0.16) × 10−7, and indicate a time of formation of 0.26 ± 0.18 Ma after the D’Orbigny angrite. Using the revised Pb-Pb age of 4563.36 ± 0.34 Ma for the D’Orbigny anchor (corrected for its U isotopic composition), we deduce an Al-Mg model age of 4563.10 ± 0.38 Ma for NWA 2976, which is consistent with its Pb-Pb internal isochron age.The concordance of the Pb-Pb and Al-Mg chronometers, when taking into account the differences in the U isotopic compositions of the D’Orbigny and NWA 2976 achondrites (whose parent bodies likely formed in distinct regions of early Solar System as indicated by their different oxygen isotopic compositions), implies that 26Al was homogeneously distributed in the early Solar System. It also suggests that igneous processes on planetesimals, as represented by the formation of various basaltic meteorite groups that likely originated on distinct parent bodies (e.g., eucrites and angrites, as well as ungrouped achondrites), were widespread throughout the protoplanetary disk within the first ∼5 Ma of the history of the Solar System.  相似文献   

19.
The 40Ar/39Ar dating technique is based on the knowledge of the age of neutron fluence monitors (standards). Recent investigations have improved the accuracy and precision of the ages of most of the Phanerozoic-aged standards (e.g. Fish Canyon Tuff sanidine (FCs), Alder Creek sanidine, GA1550 biotite and LP-6 biotite); however, no specific study has been undertaken on the older standards (i.e. Hb3gr hornblende and NL-25 hornblende) generally used to date Precambrian, high Ca/K, and/or meteoritic rocks.In this study, we show that Hb3gr hornblende is relatively homogenous in age, composition (Ca/K) and atmospheric contamination at the single grain level. The mean standard deviation of the 40Ar?/39ArK (F-value) derived from this study is 0.49%, comparable to the most homogeneous standards. The intercalibration factor (which allows direct comparison between standards) between Hb3gr and FCs is RFCsHb3gr = 51.945 ± 0.167. Using an age of 28.02 Ma for FCs, the age of Hb3gr derived from the R-value is 1073.6 ± 5.3 Ma (1σ; internal error only) and ± 8.8 Ma (including all sources of error). This age is indistinguishable within uncertainty from the K/Ar age previously reported at 1072 ± 11 Ma [Turner G., Huneke, J.C., Podosek, F.A., Wasserburg, G.J., 1971. 40Ar-39Ar ages and cosmic ray exposure ages of Apollo 14 samples. Earth Planet. Sci. Lett. 12, 19-35].The R-value determined in this study can also be used to intercalibrate FCs if we consider the K/Ar date of 1072 Ma as a reference age for Hb3gr. We derive an age of 27.95 ± 0.19 Ma (1σ; internal error only) for FCs which is in agreement with the previous determinations. Altogether, this shows that Hb3gr is a suitable standard for 40Ar/39Ar geochronology.  相似文献   

20.
柴北缘锡铁山榴辉岩退变质成因角闪石40Ar/39Ar年代学研究   总被引：2，自引：2，他引：0  
采用激光阶段加热40Ar/39Ar技术,对柴达木盆地北缘锡铁山榴辉岩退变质作用形成的榴闪岩和斜长角闪岩之角闪石进行了定年分析。09NQ44Amp来自榴闪岩,各阶段表观年龄(以现代空气氩40Ar/36Ar比值295.5扣除非放射性成因40Ar)构成了单调下降的阶梯状年龄谱。在反等时线图解上,2~4阶段数据点和5~18阶段数据点分别构成了两条等时线,等时年龄分别为427.6±10Ma和425.1±2.6Ma,对应的初始40Ar/36Ar比值则分别为435.2±6.1和705.3±13。角闪石09NQ43Amp来自榴辉岩强烈退变质作用形成的斜长角闪岩,40Ar/39Ar阶段加热分析也获得单调下降的年龄谱,在反等时线图解上其数据点3~6阶段和7~16阶段分别构成了两条等时线,等时年龄分别为418.9±2.9Ma和418.1±2.1Ma,对应的初始40Ar/36Ar比值则分别为493.7±2.8和685.8±34.3。等时线截距值高于现代大气40Ar/36Ar比值,表明角闪石中含过剩40Ar。同时,由低温和中-高温阶段加热数据点分别构成两条等时年龄基本一致,截距值却明显不同的等时线,表明在角闪石热力学性质不同的源区,存在两期明显不同且未混合的初始捕获Ar组分。等时年龄425~418Ma代表的是锡铁山榴辉岩角闪岩相退变质作用发生的时间。等时线图解法虽然有效的校正了角闪石中的过剩40Ar,但仅根据表观年龄图谱和等时线图谱还无法清晰判断过剩40Ar在角闪石中的赋存状态,有待进一步探讨。  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