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1.
We report the results of a detailed study of the basaltic eucrite Northwest Africa (NWA) 7188, including its mineralogical and bulk geochemical characteristics, oxygen isotopic composition, and 147,146Sm‐143,142Nd mineral isochron ages. The texture and chemical composition of pyroxene and plagioclase demonstrate that NWA 7188 is a monomict eucrite with a metamorphic grade of type 4. The oxygen isotopic composition and the Fe/Mn ratios of pyroxene confirmed that NWA 7188 belongs to the howardite–eucrite–diogenite meteorite suite, generally considered to originate from asteroid 4 Vesta. Whole‐rock TiO2, La, and Hf concentrations and a CI chondrite‐normalized rare earth element pattern are in good agreement with those of representative Stannern‐group eucrites. The 147,146Sm‐143,142Nd isochrons for NWA 7188 yielded ages of 4582 ± 190 and 4554 +17/?19 Ma, respectively. The closure temperature of the Sm‐Nd system for different fractions of NWA 7188 was estimated to be >865 °C, suggesting that the Sm‐Nd decay system has either been resistant to reheating at ~800 °C during the global metamorphism or only partially reset. Therefore, the 146Sm‐142Nd age of NWA 7188 corresponds to the period of initial crystallization of basaltic magmas and/or global metamorphism on the parent body, and is unlikely to reflect Sm‐Nd disturbance by late reheating and impact events. In either case, NWA 7188 is a rare Stannern‐group eucrite that preserves the chronological information regarding the initial crustal evolution of Vesta.  相似文献   

2.
Abstract— 40Ar‐39Ar data are presented for the unbrecciated lunar basaltic meteorites Asuka (A‐) 881757, Yamato (Y‐) 793169, Miller Range (MIL) 05035, LaPaz Icefield (LAP) 02205, Northwest Africa (NWA) 479 (paired with NWA 032), and basaltic fragmental breccia Elephant Moraine (EET) 96008. Stepped heating 40Ar‐39Ar analyses of several bulk fragments of related meteorites A‐881757, Y‐793169 and MIL 05035 give crystallization ages of 3.763 ± 0.046 Ga, 3.811 ± 0.098 Ga and 3.845 ± 0.014 Ga, which are comparable with previous age determinations by Sm‐Nd, U‐Pb Th‐Pb, Pb‐Pb, and Rb‐Sr methods. These three meteorites differ in the degree of secondary 40Ar loss with Y‐793169 showing relatively high Ar loss probably during an impact event ?200 Ma ago, lower Ar loss in MIL 05035 and no loss in A‐881757. Bulk and impact melt glass‐bearing samples of LAP 02205 gave similar ages (2.985 ± 0.016 Ga and 2.874 ± 0.056 Ga) and are consistent with ages previously determined using other isotope pairs. The basaltic portion of EET 96008 gives an age of 2.650 ± 0.086 Ga which is considered to be the crystallization age of the basalt in this meteorite. The Ar release for fragmental basaltic breccia EET 96008 shows evidence of an impact event at 631 ± 20 Ma. The crystallization age of 2.721 ± 0.040 Ga determined for NWA 479 is indistinguishable from the weighted mean age obtained from three samples of NWA 032 supporting the proposal that these meteorites are paired. The similarity of 40Ar‐39Ar ages with ages determined by other isotopic systems for multiple meteorites suggests that the K‐Ar isotopic system is robust for meteorites that have experienced a significant shock event and not a prolonged heating regime.  相似文献   

3.
Northwest Africa 757 is unique in the LL chondrite group because of its abundant shock‐induced melt and high‐pressure minerals. Olivine fragments entrained in the melt transform partially and completely into ringwoodite. Plagioclase and Ca‐phosphate transform to maskelynite, lingunite, and tuite. Two distinct shock‐melt crystallization assemblages were studied by FIB‐TEM analysis. The first melt assemblage, which includes majoritic garnet, ringwoodite plus magnetite‐magnesiowüstite, crystallized at pressures of 20–25 GPa. The other melt assemblage, which consists of clinopyroxene and wadsleyite, solidified at ~15 GPa, suggesting a second veining event under lower pressure conditions. These shock features are similar to those in S6 L chondrites and indicate that NWA 757 experienced an intense impact event, comparable to the impact event that disrupted the L chondrite parent body at 470 Ma.  相似文献   

4.
Here we report in situ secondary ionization mass spectrometry Ca-phosphate U-Pb ages for an L-impact melt breccia (NWA 7251), which are integrated with petrological and mineral chemical studies of this meteorite. NWA 7251 is a heavily shocked rock that is composed mainly of the chondrite host, impact melt portion, and melt veins (crosscutting and pervasive type). The host is an L4 chondrite that has been shocked to S4. The impact melt portion has a fine-grained igneous texture, and is composed mainly of olivine, low-Ca pyroxene, high-Ca pyroxene, and albitic glass. The impact melt was generated at pressure of >30–35 GPa and temperature of >1300–1500 °C during an impact event. The Ca-phosphate grains in the host were affected by a shock heating event. Most of the Ca-phosphate grains in the melt were neocrystallized, but relatively large grains enclosed by or adjacent to metal veins or melt globules are likely inherited. The U-Pb isotopic systematics of Ca-phosphates in NWA 7251 yield an upper intercept age of 4457 ± 56 Ma and a lower intercept age of 574 ± 82 Ma on the normal U-Pb concordia diagram. The age of 4457 ± 56 Ma is interpreted to be related to an early shocking event rather than the thermal metamorphism of the parent body. The impact melt and veins in NWA 7251 were generated at 574 ± 82 Ma, resulting from disruption of the L chondrite parent body.  相似文献   

5.
The large collection of howardite‐eucrite‐diogenite (HED) meteorites allows us to study the initial magmatic differentiation of a planetesimal. We report Pb‐Pb ages of the unequilibrated North West Africa (NWA) 4215 and Dhofar 700 diogenite meteorites and their mass‐independent 26Mg isotope compositions (26Mg*) to better understand the timing of differentiation and crystallization of their source reservoir(s). NWA 4215 defines a Pb‐Pb age of 4484.5 ± 7.9 Myr and has a 26Mg* excess of +2.3 ± 1.6 ppm whereas Dhofar 700 has a Pb‐Pb age of 4546.4 ± 4.7 Myr and a 26Mg* excess of +25.5 ± 1.9 ppm. We interpret the young age of NWA 4215 as a thermal overprint, but the age of Dhofar 700 is interpreted to represent a primary crystallization age. Combining our new data with published Mg isotope and trace element data suggests that approximately half of the diogenites for which such data are available crystallized within the first 1–2 Myr of our solar system, consistent with a short‐lived, early‐formed magma ocean undergoing convective cooling. The other half of the diogenites, including both NWA 4215 and Dhofar 700, are best explained by their crystallization in slowly cooled isolated magma chambers lasting over at least ~20 Myr.  相似文献   

6.
A silicious impact melt rock from polymict impact breccia of the northern part of the alkali granite core of the Araguainha impact structure, central Brazil, has been investigated. The melt rock is thought to represent a large mass of impact‐generated melt in suevite. In particular, a diverse population of zircon grains, with different impact‐induced microstructures, has been analyzed for U‐Pb isotopic systematics. Backscattered electron and cathodoluminescence images reveal heterogeneous intragrain domains with vesicular, granular, vesicular plus granular, and vesicular plus (presumably) baddeleyite textures, among others. The small likely baddeleyite inclusions are not only preferentially located along grain margins but also occur locally within grain interiors. LA‐ICP‐MS U‐Pb data from different domains yield lower intercept ages of 220, 240, and 260 Ma, a result difficult to reconcile with the previous “best age” estimate for the impact event at 254.7 ± 2.7 Ma. SIMS U‐Pb data, too, show a relatively large range of ages from 245 to 262 Ma. A subset of granular grains that yielded concordant SIMS ages were analyzed for crystallographic orientation by EBSD. Orientation mapping shows that this population consists of approximately micrometer‐sized neoblasts that preserve systematic orientation evidence for the former presence of the high‐pressure polymorph reidite. In one partially granular grain (#36), the neoblasts occur in linear arrays that likely represent former reidite lamellae. Such grains are referred to as FRIGN zircon. The best estimate for the age of the Araguainha impact event from our data set from a previously not analyzed type of impact melt rock is based on concordant SIMS data from FRIGN zircon grains. This age is 251.5 ± 2.9 Ma (2σ, MSWD = 0.45, p = 0.50, n = 4 analyses on three grains), indistinguishable from previous estimates based on zircon and monazite from other impact melt lithologies at Araguainha. Our work provides a new example of how FRIGN zircon can be combined with in situ U‐Pb geochronology to extract an accurate age for an impact event.  相似文献   

7.
U‐Pb ages of zircon in four different Apollo 14 breccias (14305, 14306, 14314, and 14321) were obtained by secondary ion mass spectrometry. Some of the analyzed grains occur as cogenetic, poikilitic zircon grains in lithic clasts, revealing magmatic events at ~4286 Ma, ~4200–4220 Ma, and ~4150 Ma. The age distribution of the crystal clasts in the breccias exhibits a minor peak at ~4210 Ma, which can be attributed to a magmatic event, as recorded in zircon grains located in noritic clasts. An age peak at ~4335 Ma is present in all four breccias, as well as zircon grains from different Apollo landing sites, enhancing the confidence that these grains recorded a global zircon‐forming event. The overall age distribution among the four breccias exhibits minor differences between the breccias collected farther away from the Cone Crater and the ones collected within the continuous ejecta blanket of the Cone Crater. A granular zircon grain yielded a 207Pb/206Pb age of 3936 ± 8 Ma, which is interpreted as an impact event. A similar age of 3941 ± 5 Ma (n = 17, MSWD = 0.89, P = 0.58) was obtained for a large zircon grain (~430 × 340 μm in size). This grain might have crystallized in the same impact melt sheet which formed the granular zircon or the age is representative of the final extrusion of KREEP magma. The majority of zircon grains, however, occur as isolated crystal clasts within the matrix and their ages cannot be correlated with any real events (impact or magmatic) nor can the possibility be excluded that these ages represent partial resetting of the U‐Pb system.  相似文献   

8.
Novato, a newly observed fall in the San Francisco Bay area, is a shocked and brecciated L6 ordinary chondrite containing dark and light lithologies. We have investigated the U‐Pb isotope systematics of coarse Cl‐apatite grains of metamorphic origin in Novato with a large geometry ion microprobe. The U‐Pb systematics of Novato apatite reveals an upper intercept age of 4472 ± 31 Ma and lower intercept age of 473 ± 38 Ma. The upper intercept age is within error identical to the U‐Pb apatite age of 4452 ± 21 Ma measured in the Chelyabinsk LL5 chondrite. This age is interpreted to reflect a massive collisional resetting event due to a large impact associated with the peak arrival time at the primordial asteroid belt of ejecta debris from the Moon‐forming giant impact on Earth. The lower intercept age is consistent with the most precisely dated Ar‐Ar ages of 470 ± 6 Ma of shocked L chondrites, and the fossil meteorites and extraterrestrial chromite relicts found in Ordovician limestones with an age of 467.3 ± 1.6 Ma in Sweden and China. The lower intercept age reflects a major disturbance related to the catastrophic disruption of the L chondrite parent body most likely associated with the Gefion asteroid family, which produced an initially intense meteorite bombardment of the Earth in Ordovician period and reset and degassed at least approximately 35% of the L chondrite falls today. We predict that the 470 Ma impact event is likely to be found on the Moon and Mars, if not Mercury.  相似文献   

9.
Previous age estimates of the Imbrium impact range from 3770 to 3920 Ma, with the latter being the most commonly accepted age of this basin‐forming event. The occurrence of Ca‐phosphates in Apollo 14 breccias, interpreted to represent ejecta formed by this impact, provides a new opportunity to date the Imbrium event as well as refining the impact history of the Moon. We present new precise U‐Pb analyses of Ca‐phosphates from impact breccia sample 14311 that are concordant and give a reliable weighted average age of 3938 ± 4 Ma (2σ). Comparison with previously published U‐Pb data on phosphate from Apollo 14 samples indicate that all ages are statistically similar and suggest phosphates could have been formed by the same impact at 3934 Ma ± 3 Ma (2σ). However, this age is older than the 3770 to 3920 Ma range determined for other samples and also interpreted as formed during the Imbrium impact. This suggests that several impacts occurred during a 20–30 Ma period around 3900 Ma and formed breccias sampled by the Apollo missions.  相似文献   

10.
Northwest Africa (NWA) 11042 is a heavily shocked achondrite with medium‐grained cumulate textures. Its olivine and pyroxene compositions, oxygen isotopic composition, and chromium isotopic composition are consistent with L chondrites. Sm‐Nd dating of its primary phases shows a crystallization age of 4100 ± 160 Ma. Ar‐Ar dating of its shocked mineral maskelynite reveals an age of 484.0 ± 1.5 Ma. This age coincides roughly with the breakup event of the L chondrite parent body evident in the shock ages of many L chondrites and the terrestrial record of fossil L chondritic chromite. NWA 11042 shows large depletions in siderophile elements (<0.01×CI) suggestive of a complex igneous history involving extraction of a Fe‐Ni‐S liquid on the L chondrite parent body. Due to its relatively young crystallization age, the heat source for such an igneous process is most likely impact. Because its mineralogy, petrology, and O isotopes are similar to the ungrouped achondrite NWA 4284 (this work), the two meteorites are likely paired and derived from the same parent body.  相似文献   

11.
In situ U‐Pb measurements on zircons of the Ries impact crater are presented for three samples from the quarry at Polsingen. The U‐Pb data of most zircons plot along a discordia line, leading to an upper intercept of Carboniferous age (331 ± 32 Ma [2σ]). Four zircons define a concordia age of 313.2 ± 4.4 Ma (2σ). This age most probably represents the age of a granite from the basement target rocks. From granular textured zircon grains (including baddeleyite and anatase/Fe‐rich phases, first identified in the Ries crater), most probably recrystallized after impact (13 analyses, 4 grains), a concordia age of 14.89 ± 0.34 Ma (2σ) and an error weighted mean 206Pb*/238U age of Ma 14.63 ± 0.43 (2σ) is derived. Including the youngest concordant ages of five porous textured zircon grains (24 spot analyses), a concordia age of 14.75 ± 0.22 Ma (2σ) and a mean 206Pb*/238U age of 14.71 ± 0.26 Ma (2σ) can be calculated. These results are consistent with previously published 40Ar/39Ar ages of impact glasses and feldspar. Our results demonstrate that even for relatively young impact craters, reliable U‐Pb ages can be obtained using in situ zircon dating by SIMS. Frequently the texture of impact shocked zircon grains is explained by decomposition at high temperatures and recrystallization to a granular texture. This is most probably the case for the observed granular zircon grains having baddeleyite/anatase/Fe‐rich phases. We also observe non‐baddeleyite/anatase/Fe‐rich phase bearing zircons. For these domains, reset to crater age is more frequently for high U,Th contents. We tentatively explain the higher susceptibility to impact resetting of high U,Th domains by enhanced Pb loss and mobilization due to higher diffusivity within former metamict domains that were impact metamorphosed more easily into porous as well as granular textures during decomposition and recrystallization, possibly supported by Pb loss during postimpact cooling and/or hydrothermal activity.  相似文献   

12.
Abstract— Phosphates in martian meteorites are important carriers of trace elements, although, they are volumetrically minor minerals. PO4 also has potential as a biomarker for life on Mars. Here, we report measurements of the U‐Th‐Pb systematics of phosphates in the martian meteorite ALH 84001 using the Sensitive High Resolution Ion MicroProbe (SHRIMP) installed at Hiroshima University, Japan. Eleven analyses of whitlockites and 1 analysis of apatite resulted in a total Pb/U isochron age of 4018 ± 81 Ma in the 238U/206Pb‐207Pb/206Pb‐204Pb/206 Pb 3‐D space, and a 232Th‐208Pb age of 3971 ± 860 Ma. These ages are consistent within a 95% confidence limit. This result is in agreement with the previously published Ar‐Ar shock age of 4.0 ± 0.1 Ga from maskelynite and other results of 3.8–4.3 Ga but are significantly different from the Sm‐Nd age of 4.50 ± 0.13 Ga based on the whole rock and pyroxene. Taking into account recent studies on textural and chemical evidence of phosphate, our result suggests that the shock metamorphic event defines the phosphate formation age of 4018 ± 81 Ma, and that since then, ALH 84001 has not experienced a long duration thermal metamorphism, which would reset the U‐Pb system in phosphates.  相似文献   

13.
Zircons and apatites in clasts and matrix from the Martian breccia NWA 7034 are well documented, timing ancient geologic events on Mars. Furthermore, in this study, zircon trace elemental content, apatite volatile content, and apatite volatile isotopic compositions measured in situ could constrain the evolution of those geologic events. The U‐Pb dates of zircons in basalt, basaltic andesite, trachyandesite igneous clasts, and the matrix are similar (4.4 Ga) suggesting intense volcanism on ancient Mars. However, two metamict zircon grains found in the matrix have an upper intercept date of ~4465 Ma in crystalline, whereas amorphous areas have a lower intercept date of 1634 ± 93 Ma. The younger date is consistent with the date of apatites (1530 ± 65 Ma), suggesting a metamorphic event that completely reset the U‐Pb system in both the amorphous areas of zircon and all apatites. δD values in all apatites negatively correlate with water content in a two‐endmember mixing trend. The D (δD up to 2459‰) and 37Cl heavy core (3.8‰) of a large apatite grain suggest a D‐, 37Cl‐rich fluid during the metamorphic event ~1.6 Ga ago, consistent with the trace elements Y, Hf and Ti and P in zircons. The fluid was also therefore P‐rich. The D‐, 37Cl‐poor H2O‐rich rim (<313‰) suggests the degassing of water from the Martian Cl‐poor interior at a later time. This D‐, 37Cl‐poor Martian mantle reservoir could have derived from volcanic intrusions postdating the younger metamorphic event recorded in NWA 7034.  相似文献   

14.
A recrystallized band of pale feldspathic impact melt in a gneissic impact breccia from the approximately 10 km Paasselkä impact structure in southeast Finland was dated via 40Ar/39Ar step‐heating. The newly obtained plateau age of 228.7 ± 1.8 (2.2) Ma (2σ) (MSWD = 0.32; p = 0.93) is equal to the previously published pseudoplateau age of 228.7 ± 3.0 (3.4) (2σ) for the impact event. According to the current international chronostratigraphic chart and using the most recent published suggestions for the K decay constants, a Carnian (Late Triassic) age for the Paasselkä impact structure of 231.0 ± 1.8 (2.2) Ma (2σ) is calculated and considered the most precise and accurate age for this impact structure. The new plateau age for Paasselkä confirms the previous dating result but is, based on its internal statistics, much more compelling.  相似文献   

15.
Abstract— We report ion microprobe U‐Th‐Pb dating of Shergotty phosphates by means of the sensitive high‐resolution ion microprobe (SHRIMP) recently installed at Hiroshima University, Japan. ten analyses of whitlockite (merrillite) and three analyses of apatite indicate a 238u/206pb isochron age of 225 ± 200 ma and a tera‐wasserburg concordia‐constrained linear three‐dimensional isochron age of 217 ± 110 ma in the 238u/206pb‐207pb/206pb204pb/206pb diagram. These ages agree well with the 232Th‐208pb age of 189 ± 83 Ma, which suggests that primary crystallization or a shock metamorphic event defined the formation age of the phosphate minerals. The average of the later two ages, 204 ± 68 Ma, is consistent with the previously published Rb‐Sr age of 165 ± 11 Ma and U‐Th‐Pb age of ～200 Ma. These show marginal agreement with the 40Ar‐39Ar age of 254 ± 10 Ma but are significantly different from the Sm‐Nd age of 360 ± 16 Ma. Taking into account the closure temperature of the U‐Pb system in apatite, we suggest the time that Shergotty last experienced a temperature of ～900 °C was 204 ± 68 Ma.  相似文献   

16.
Laser ablation inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) U–Pb geochronology of shocked zircon grains in a vesicular‐fluidal impact melt rock from the ≥54 km Charlevoix impact structure, Québec, Canada, suggests an Ordovician to Silurian age of 450 ± 20 Ma for the impact. This age is anchored by concordant U–Pb results of ~450 Ma for a U‐rich, cryptocrystalline zircon grain in the melt rock, interpreted as a recrystallized metamict zircon crystal; the U–Th–Pb system of the metamict grain was seemingly chronometrically reset by the Charlevoix impact, but withstood later tectonometamorphic events. The new zircon age for Charlevoix is in agreement with a stratigraphically constrained Late Ordovician maximum age of ~453 Ma and corroborates earlier suggestions that the impact occurred most likely in the Ordovician, and not ~100 Myr later, as indicated by previous K/Ar and 40Ar/39Ar geochronologic results. The latter may reflect postimpact thermal overprint of impactites during the Salinian (Late Silurian to Early Devonian) and/or Acadian (Late Devonian) orogenies. U–Pb geochronology of zircon crystals in anorthosite exposed in the central uplift of the impact structure yielded a Grenvillian crystallization age of 1062 ± 11 Ma. The preferred Ordovician age for the Charlevoix impact structure, which is partially overthrusted by the Appalachian front, suggests the impact occurred during a phase of Taconian tectonism and an episode of enhanced asteroid bombardment of the Earth. Our results, moreover, demonstrate that (recrystallized) metamict zircon grains may be of particular interest in impact geochronology.  相似文献   

17.
The petrologic and oxygen isotopic characteristics of calcium‐aluminum‐rich inclusions (CAIs) in CO chondrites were further constrained by studying CAIs from six primitive CO3.0‐3.1 chondrites, including two Antarctic meteorites (DOM 08006 and MIL 090010), three hot desert meteorites (NWA 10493, NWA 10498, and NWA 7892), and the Colony meteorite. The CAIs can be divided into hibonite‐bearing inclusions (spinel‐hibonite spherules, monomineralic grains, hibonite‐pyroxene microspherules, and irregular/nodular objects), grossite‐bearing inclusions (monomineralic grains, grossite‐melilite microspherules, and irregular/nodular objects), melilite‐rich inclusions (fluffy Type A, compact type A, monomineralic grains, and igneous fragments), spinel‐pyroxene inclusions (fluffy objects resembling fine‐grained spinel‐rich inclusions in CV chondrites and nodular/banded objects resembling those in CM chondrites), and pyroxene‐anorthite inclusions. They are typically small (98.4 ± 54.4 µm, 1SD) and comprise 1.54 ± 0.43 (1SD) area% of the host chondrites. Melilite in the hot desert and Colony meteorites was extensively replaced by a hydrated Ca‐Al‐silicate during terrestrial weathering and converted melilite‐rich inclusions into spinel‐pyroxene inclusions. The CAI populations of the weathered COs are very similar to those in CM chondrites, suggesting that complete replacement of melilite by terrestrial weathering, and possibly parent body aqueous alteration, would make the CO CAIs CM‐like, supporting the hypothesis that CO and CM chondrites derive from similar nebular materials. Within the CO3.0‐3.1 chondrites, asteroidal alteration significantly resets oxygen isotopic compositions of CAIs in CO3.1 chondrites (?17O: ?25 to ?2‰) but left those in CO3.0‐3.05 chondrites mostly unchanged (?17O: ?25 to ?20‰), further supporting the model whereby thermal metamorphism became evident in CO chondrites of petrologic type ≥3.1. The resistance of CAI minerals to oxygen isotope exchange during thermal metamorphism follows in the order: melilite + grossite < hibonite + anorthite < spinel + diopside + forsterite. Meanwhile, terrestrial weathering destroys melilite without changing the chemical and isotopic compositions of melilite and other CAI minerals.  相似文献   

18.
Secondary ion mass spectrometry (SIMS) U‐Pb ages of Ca‐phosphates from four texturally distinct breccia samples (72255, 76055, 76015, 76215) collected at the Apollo 17 landing site were obtained in an attempt to identify whether they represent a single or several impact event(s). The determined ages, combined with inferences from petrologic relationships, may indicate two or possibly three different impact events at 3920 ± 3 Ma, 3922 ± 5 Ma, and 3930 ± 5 Ma (all errors 2σ). Searching for possible sources of the breccias by calculating the continuous ejecta radii of impact basins and large craters as well as their expected ejecta thicknesses, we conclude that Nectaris, Crisium, Serenitatis, and Imbrium are likely candidates. If the previous interpretation that the micropoikilitic breccias collected at the North Massif represent Serenitatis ejecta is correct, then the average 207Pb/206Pb age of 3930 ± 5 Ma (2σ) dates the formation of the Serenitatis basin. The occurrence of zircon in the breccias sampled at the South Massif, which contain Ca‐phosphates yielding an age of 3922 ± 5 Ma (2σ), may indicate that the breccia originated from within the Procellarum KREEP terrane (PKT) and the Imbrium basin appears to be the only basin that could have sourced them. However, this interpretation implies that all basins suggested to fall stratigraphically between Serenitatis and Imbrium formed within a short (<11 Ma) time interval, highlighting serious contradictions between global stratigraphic constraints, sample interpretation, and chronological data. Alternatively, the slightly older age of the two micropoikilitic breccias may be a result of incomplete resetting of the U‐Pb system preserved in some phosphate grains. Based on the currently available data set this possibility cannot be excluded.  相似文献   

19.
Abstract— Northwest Africa 482 (NWA 482) is a crystalline impact‐melt breccia from the Moon with highlands affinities. The recrystallized matrix and the clast population are both highly anorthositic. Clasts are all related to the ferroan anorthosite suite, and include isolated plagioclase crystals and lithic anorthosites, troctolites, and spinel troctolites. Potassium‐, rare‐earth‐element‐, and phosphorus‐bearing (KREEP) and mare lithologies are both absent, constraining the source area of this meteorite to a highland terrain with little to no KREEP component, most likely on the far side of the Moon. Glass is present in shock veins cutting through the sample and in several large melt pockets, indicating a second impact event. There are two separate events recorded in the 40Ar‐39Ar system: one at ～3750 Ma, which completely reset the K‐Ar system, and one at ?2400 Ma, which caused only partial degassing. These events could represent, respectively, crystallization of the impact‐melt breccia and later formation of the glass, or the formation of the glass and a later thermal event. The terrestrial age of the meteorite is 8.6 ± 1.3 ka. This age corresponds well with the modest amount of weathering in the rock, in the form of secondary phyllosilicates and carbonates. Based on terrestrial age and location, lithology, and chemistry, NWA 482 is unique among known lunar meteorites.  相似文献   

20.
Abstract— The lake Lappajärvi impact crater lies in Paleoproterozoic Svecofennian metasedimentary rocks, on the western side of the Central Finland granitoid complex (～1.9 Ga). Two conflicting ages have been reported for the meteorite impact: an age of 77.3 ± 0.4 Ma on the basis of Ar‐Ar whole‐rock data from impact melt samples and a paleomagnetic age of 195 Ma. During studies on impact crater indicator minerals at Lappajärvi, zircons with an atypical appearance were found in suevite boulders. These zircons seemed to have been affected by impact shock metamorphism and it was considered that they would be good candidates for ion microprobe U‐Pb dating, allowing a new and independent age estimate for the impact event at Lappajärvi. Four spot analyses on two black‐coated zircons plotted close to the upper intercept end of the concordia curve giving an approximate age of 1.8 Ga for the source rock. Seventeen analyses were done on three dull zircon grains showing patchy impact‐related partial recrystallization. Most of these data fell fairly well on a single discordia line with intercept ages of 73.3 ± 5.3 Ma and 1854 ± 51 Ma. However, five of the data spots near the lower intercept end fell on the younger side of the line. This was interpreted to indicate post‐impact loss of lead. Importantly, the new ion microprobe U‐Pb age of 73.3 ± 5.3 Ma is in a very good agreement with the previously reported Ar‐Ar age.  相似文献   

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