ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00592.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00593.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00594.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—Systematic examination of dating results from various craters indicates that about 90% of the rocks affected by an impact preserve their pre‐shock ages because shock and post‐shock conditions are not sufficient to disturb isotopic dating systems. In the other 10% of target lithologies, various geochronometers show significant shock‐induced effects. Major problems in dating impactites are caused by their non‐equlibrated character. They often display complex textures, where differently shocked and unshocked phases interfinger on the sub‐mm scale. Due to this, dating on whole rock samples or insufficiently pure mineral fractions often yielded ambiguous results that set broad age limits but are not sufficient to answer reliably questions such as a possible periodicity in cratering on Earth, or correlation of impact events with mass extinctions. Dating results from shock recovery experiments indicate that post‐shock annealing plays the most important role in resetting isotopic clocks. Therefore, the major criterion for sample selection in and around craters is the post‐shock thermal regime. Based on their different thermal evolution, the following geological impact formations can be distinguished: (1) the coherent impact melt layer, (2) allochthonous breccia deposits, (3) the crater basement, and (4) distant ejecta deposits.Samples of the coherent impact melt layer are the most suitable candidates for dating. Excellent ages of high precision can be obtained by internal Rb‐Sr, and Sm‐Nd isochrons, U‐Pb analyses on newly crystallized accessory minerals, and K‐Ar (39Ar‐40Ar) dating of clast‐free melt rocks. Fission track counting on glassy material has yielded correct ages, and paleomagnetic measurements have been successfully applied to post‐Triassic craters. In the ideal case of a fast‐cooling impact melt layer, all these different techniques should give identical ages.Allochthonous breccias contain shocked, unshocked, and/or glassy components in various proportions; and, hence, each of these ejecta deposits has its own individual thermal history, making sample evaluation difficult Glassy melt particles in suevitic breccias are well suited for fission track and Ar‐Ar dating. Weakly shocked material may yield reliable Ar‐Ar and fission track ages, if formation temperatures were high, and cooling rates moderate. In contrast, highly shocked but rapidly cooled lithologies show only disturbed and not reset isotopic systems. For ejecta deposits and the crater wall of young craters, dating with cosmogenic nuclides is a new and powerful technique.Crater basement lithologies have a high potential in impact dating, although it has not been exploited so far. A prerequisite for resetting of isotopic clocks in these lithologies is the presence of an overlaying impact melt layer, which causes thermal metamorphism. Fission track and K‐Ar techniques are most promising, because both systems are easily reset at low temperatures. Good candidates for impact dating are long‐term annealed rocks, even if shock metamorphic overprint is very weak. In addition, Ar‐Ar dating dating of pseudotachylites appears promising. In large impact structures, where high temperatures persist for long times, polymict “footwall” breccias beneath the melt sheet are also appropriate for dating, using the isochron approach and U‐Pb on accessory minerals.Distant ejecta material have undergone very fast cooling, and the ejecta deposits have ambient formation temperatures. Among this material, tektites and impact melt glass are ideal objects for Ar‐Ar and fission track impact dating. Dating on other material from distant ejecta deposits, such as U‐Pb analyses on zircons, offers new possibilities. Efforts to correlate distant ejecta with distinct craters critically depend on proper error assignment to a specific age. Thi

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00595.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—We analyzed ropy glasses from Apollo 12 soils 12032 and 12033 by a variety of techniques including SEM/EDX, electron microprobe analysis, INAA, and39Ar‐40Ar age dating. The ropy glasses have KREEP‐like compositions different from those of local Apollo 12 mare soils; it is likely that the ropy glasses are of exotic origin. Mixing calculations indicate that the ropy glasses formed from a liquid enriched in KREEP and that the ropy glass liquid also contained a significant amount of mare material. The presence of solar Ar and a trace of regolith‐derived glass within the ropy glasses are evidence that the ropy glasses contain a small regolith component Anorthosite and crystalline breccia (KREEP) clasts occur in some ropy glasses. We also found within these glasses clasts of felsite (fine‐grained granitic fragments) very similar in texture and composition to the larger Apollo 12 felsites, which have a39Ar‐40Ar degassing age of 800 ± 15 Ma (Bogardet al, 1992). Measurements of39Ar‐40Ar in 12032 ropy glass indicate that it was degassed at the same time as the large felsite although the ropy glass was not completely degassed. The ropy glasses and felsites, therefore, probably came from the same source. Most early investigators suggested that the Apollo 12 ropy glasses were part of the ejecta deposited at the Apollo 12 site from the Copernicus impact Our new data reinforce this model. If these ropy glasses are from Copernicus, they provide new clues to the nature of the target material at the Copernicus she, a part of the Moon that has not been sa

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00596.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—New data are reported from five previously unanalyzed Apollo 12 mare basalts that are incorporated into an evaluation of previous petrogenetic models and classification schemes for these basalts. This paper proposes a classification for Apollo 12 mare basalts on the basis of whole‐rock Mg# [molar 100*(Mg/(Mg+Fe))] and Rb/Sr ratio (analyzed by isotope dilution), whereby the ilmenite, olivine, and pigeonite basalt groups are readily distinguished from each other. Scrutiny of the Apollo 12 feldspathic “suite” demonstrates that two of the three basalts previously assigned to this group (12031, 12038, 12072) can be reclassified: 12031 is a plagioclase‐rich pigeonite basalt (Nyquistet al, 1979); and 12072 is an olivine basalt Only basalt 12038 stands out as a unique sample (Nyquistet al., 1981) to the Apollo 12 she, but whether this represents a single sample from another flow at the Apollo 12 site or is exotic to this site is equivocal.The question of whether the olivine and pigeonite basalt suites are co‐magmatic is addressed by incompatible trace‐element chemistry: the trends defined by these two suites when Co/Sm and Sm/Eu ratios are plotted against Rb/Sr ratio demonstrate that these two basaltic types cannot be co‐magmatic. Crystal fractionation/accumulation paths have been calculated and show that neither the pigeonite, olivine, or ilmenite basalts are related by this process. Each suite requires a distinct and separate source region. This study also examines sample heterogeneity and the degree to which whole‐rock analyses are representative, which is critical when petrogenetic interpretation is undertaken. Sample heterogeneity has been investigated petrographically (inhomogeneous mineral distribution) with consideration of duplicate analyses, and whether a specific sample (using average data) plots consistently upon a fractionation trend when a number of different compositional parameters are considered. Using these criteria, four basalts have been identified where reported analyses arenotrepresentative of the whole‐rock composition: 12005, an ilmenite basalt; 12006 and 12036, olivine basalts; and 12031 previously classified as a feldspathic basalt, but reclassified as part of the pigeonite suite (N

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00597.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—The petrogenesis of Apollo 12 mare basalts has been examined with emphasis on trace‐element ratios and abundances. Vitrophyric basalts were used as parental compositions for the modelling, and proportions of fractionating phases were determined using the MAGFOX program of Longhi (1991). Crystal fractionation processes within crustal and sub‐crustal magma chambers are evaluated as a function of pressure. Knowledge of the fractionating phases allows trace‐element variations to be considered as either source related or as a product of post‐magma‐generation processes. For the ilmenite and olivine basalts, trace‐element variations are inherited from the source, but the pigeonite basalt data have been interpreted with open‐system evolution processes through crustal assimilation. Three groups of basalts have been examined: (1) Pigeonite basalts — produced by the assimilation of lunar crustal material by a parental melt (up to 3% assimilation and 10% crystal fractionation, with an “r” value of 0.3). (2) Ilmenite basalts — produced by variable degrees of partial melting (4–8%) of a source of olivine, pigeonite, augite, and plagioclase, brought together by overturn of the Lunar Magma Ocean (LMO) cumulate pile. After generation, which did not exhaust any of the minerals in the source, these melts experienced closed‐system crystal fractionation/accumulation. (3) Olivine basalts — produced by variable degrees of partial melting (5–10%) of a source of olivine, pigeonite, and augite. After generation, again without exhausting any of the minerals in the source, these melts evolved through crystal accumulation. The evolved liquid counterparts of these cumulates have not been sampled. The source compositions for the ilmenite and olivine basalts were calculated by assuming that the vitrophyric compositions were primary and the magmas were produced by non‐modal batch melting. Although the magnitude is unclear, evaluation of these source regions indicates that both be composed of early‐ and late‐stage Lunar Magma Ocean (LMO) cumulates, requi

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00598.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—FeO‐rich (Fs6–34) pyroxene lacking cathodoluminescence (CL), hereafter black pyroxene, is a major constituent of some of the chondrules and fragments in unequilibrated (type 3) enstatite chondrites (UECs). It contains structurally oriented zones of Cr‐, Mn‐, V‐rich, FeO‐poor enstatite with red CL, associated with mm‐sized blebs of low‐Ni, Fe‐metal and, in some cases, silica. These occurrences represent clear evidence of pyroxene reduction. The black pyroxene is nearly always rimmed by minor element (Cr, Mn, V)‐poor enstatite having a blue CL. More commonly, red and blue enstatites, unassociated with black pyroxene, occur as larger grains in chondrules and fragments, and these constitute the major silicate phases in UECs. The REE abundance patterns of the black pyroxene are LREE‐depleted. The blue enstatite rims, however, have a near‐flat to LREE‐enriched pattern, ∼0.5–4x chondritic. The petrologic and trace element data indicate that the black pyroxene is from an earlier generation of chondrules that formed in a nebular region that was more oxidizing than that of the enstatite chondrites. Following solidification, these chondrules experienced a more reducing nebular environment and underwent reduction. Some, perhaps most, of the red enstatite that is common throughout the UECs may be the product of solid‐state reduction of black pyroxene. The blue enstatite rims grew onto the surfaces of the black pyroxene and red enstatite as a result of condensation from a nebular gas.The evolutionary history of some of the enstatite and chondrules in enstatite chondrites can be expressed in a four‐stage model that includes: Stage 1. Formation of chondrules in an oxidizing nebular environment Stage 2. Solid‐state reduction of the more oxidized chondrules and fragments to red enstatite in a more reducing nebular environment Stage 3. Formation of blue enstatite rims on the black pyroxene as well as on the red enstatite. Stage 4. Reprocessing, by various degrees of melting, of

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00599.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—The 1.13‐km‐diameter Pretoria Saltpan impact crater is located about 40 km NNW of Pretoria, South Africa. The crater is situated in 2.05 Ga old Nebo granite of the Bushveld Complex that is locally intruded by about 1.3 Ga old volcanic rocks. In 1988, a borehole was drilled in the center of the crater. At depths>90 m, breccias were found that contained minerals with characteristic shock‐metamorphic features, thus confirming the impact origin of the crater. Fragments of impact glass were recovered from the melt breccias and several hundred sub‐millimeter‐sized glass fragments were subjected to fission track analysis. The measurements were complicated by the inhomogeneous composition of the impact glasses, but analysis of a large number of tracks yielded an age of 220 ± 52 ka for the Sa

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00600.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—The genesis of the 1.13‐km‐diameter Pretoria Saltpan crater has long been the focus of a controversy. Its origin has been explained by either meteorite impact or “cryptoexplosive” volcanic activity, but it was recently confirmed, through detailed petrographic and chemical analysis of a breccia layer forming part of the crater fill, that the crater was formed by impact. As the limited previous geophysical work failed to support an impact origin, a more detailed gravity and magnetic study was conducted. A possible 400‐m‐diameter circular crater located 3 km to the southwest of the main crater was also investigated with geophysical methods, including resistivity, seismics and ground‐probing radar. The gravity signature of the main crater is compatible with that of a simple impact crater and the magnetic signature (no magnetic anomaly could be detected) rules out the possibility of a central magnetic volcanic body below the crater‐fill sediments. The results for the possible twin or satellite crater are inconclusive. As it is the only such feature in the entire region, it should not be overlooked. A drilling program may reveal in

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00601.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY