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1.
Approximately 100 glasses and 52 lithic fragments from Apollo 11 lunar fines and microbreccias were analyzed with the electron microprobe. Ranges in bulk composition of lithic fragments are considerably outside the precision (<±1%) and accuracy (±2–5%) of the broad electron beam technique. Results of this study may be summarized as follows: i) A large variety of rock types different from the hand specimens (basalt) were found among the lithic fragments, namely anorthosites, troctolitic and noritic anorthosites, troctolites, and norites (different from Apollo 12 norites). ii) In analogy to the hand specimens, the basaltic lithic fragments may be subdivided into low-K and high-K groups, both of which extend considerably in composition beyond the hand specimens. iii) Glasses were divided into 6 groups: Group 1 are the compositional analogs of the anorthositic-troctolitic lithic fragments and were apparently formed in single-stage impact events directly from parent anorthosites and troctolites. iv) Group 2 glasses are identical in composition to Apollo 12 KREEP glass and noritic lithic fragments, but have no counterparts in our Apollo 11 lithic fragment suite. Occurrence of KREEP in Apollo 11,12, and 14 samples is indicative of its relatively high abundance and suggests that the lunar crust is less depleted in elements that are common in KREEP (e.g. K, rare earths, P) than was originally thought on the basis of Apollo 11 basalt studies. v) Group 3 glasses are the compositional analogs of the basaltic lithic fragments, but low-K and high-K glasses cannot be distinguished because of loss of K (and Na, P) by volatilization in the vitrification process. vi) Group 4 glasses have no compositional analogs among the lithic fragments and were probably derived from as yet unknown Fe-rich, moderately Ti-rich, Mg-poor basalts. vii) Group 5 (low Ti-high Mg peridotite equivalent) and 6 (ilmenite peridotite equivalent) glasses have no counterparts among the Apollo 11 lithic fragments, but rock equivalents to group 5 glasses were found in Apollo 12 samples. Group 6 glasses are abundant, have narrow compositional ranges, and are thought to be the products of impact melting of an as yet unrecognized ultramafic rock type. iix) The great variety of igneous rocks (e.g. anorthosites, troctolites, norites, basalts, peridotites) suggests that large scale melting or partial melting to considerable depth must have occurred on the moon.  相似文献   

2.
The LL-group chondrites Krähenberg (Krbg) and Bhola are heterogeneous agglomerates containing a variety of lithic fragments and chondrules as well as crystal fragments. The FeFe + Mg content of most olivine grains is uniform (Fa28), although a few with distinctly lower Fe contents were found (Fa19). Both meteorites contain large, cm-sized, fragments with high enrichments of K (~12×), Rb (~45×) and Cs (~70×) relative to LL-chondrites, while the REE concentrations are normal (except for a negative Eu anomaly); Na and Sr are depleted (~0.5×) and the NaK weight ratio is 0.33 compared to 11 in the host. However, there is no difference in the sum of Na + K atoms. Also, the major elements, Si, Al, Mg, Ca and Fe, are nearly the same in fragments as in the host material. The K-rich igneous lithic fragments have a microporphyritic texture of euhedral to skeletal olivines in a partly devitrified glass with ~4% K2O. The main pans of both Krbg and Bhola contain mesostasis glasses in porphyritic chondrules and lithic fragments with varying K content (0.1–8.6% K2O) and NaK ratios (0.2–100). Crystalline plagioclase is depleted in K with an average NaK ratio of 22, i.e. higher than that for ordinary chondritic plagioclase, 8.4. Olivines in the large, K-rich fragments and in the host meteorites have the same iron content (Fa28), indicating that both formed under the same oxygen fugacity and probably on the same parent body.Conceivable mechanisms for the formation of the K-rich rocks from normal LL-chondrite parent material are: 1, magmatic differentiation: 2. Na-K exchange via a vapor phase; 3. silicate liquid immiscibility; 4. volatilization and condensation in impact events. Process 2 appears most feasible for forming a rock enriched only in K and heavier alkalies and depleted in Na without noticeably changing other elements including the REE.  相似文献   

3.
The Luna 20 regolith sample contains crystalline lithic fragments of mare basalt, the anorthosite-norite-troctolite group, and feldspathio basalt. Discrete mineral fragments and mineral fragments in regolith breccias can generally be assigned, based on chemical criteria, to one or the other of the first two rock types. A complex history is indicated for the regolith fragments involving repeated metamorphism and melting of the highlands due to impact events. The glass fragments and the feldspathic basalts probably are the result of this melting and their composition may be representative of a large portion of the regolith at this site.  相似文献   

4.
Many carbonaceous chondrites contain discrete olivine fragments that have been considered to be primitive material, i.e. direct condensates from the solar nebula or pre-solar system material. Olivine occurring in chondrules and as isolated grains in C3(0) chondrites has been characterized chemically and petrographically. Type I chondrules contain homogeneous forsterite grains that exhibit a negative correlation between FeO and CaO. Type II chondrules contain zoned fayalite olivines in which FeO is positively correlated with CaO and MnO. The isolated olivines in C3(0) chondrites form two compositional populations identical to olivines in the two types of porphyritic olivine chondrules in the same meteorites. Isolated olivines contain trapped melt inclusions similar in composition to glassy mesostasis between olivines in chondrules. Such glasses can be produced by fractional crystallization of olivine and minor spinel in the parent chondrule melts if plagioclase does not nucleate. The isolated olivine grains are apparently clastic fragments of chondrules. Some similarities between olivines in C3(0), C2, and Cl chondrites may suggest that olivine grains in all these meteorites crystallized from chondrule melts.  相似文献   

5.
The Plainview. Texas, meteorite is a polymict-brecciated H-group chondrite composed of recrystallized light-colored portions embedded in a well-compacted, dense, somewhat recrystallized, dark-colored matrix. Both portions consist of equilibrated silicates (H5 classification), but a small number of silicate grains and unequilibrated lithic fragments not compatible with equilibrated ordinary H-group material are present in the dark-colored matrix. Lithic fragments include: (i) dark-colored, more or less altered, type II carbonaceous chondrites. (ii) unequilibrated ordinary chondrites and (iii) light-colored, unequilibrated and equilibrated fragments, some of which are compositionally similar to the host. Also present are fragment-like dark areas that are highly-shocked host material and not true lithic fragments (pseudo-fragments). Conclusions: Plainview represents a complex regolith breccia formed by repeated impact episodes. Recrystallized, light-colored portions represent surface or near-surface material of a small (asteroidal-sized) parent body. Impacts broke up this material to form fine-grained, dark material which enclosed light-colored protolith. Lithic fragments (i-iii) and some unequilibrated silicate grains and chondrules (apparently derived from unequilibrated chondrites) were embedded in the dark matrix during these repeated impacts. Xenolitlils of carbonaceous and unequilibrated ordinary chondrites are either residues of projectiles that impacted the Plainview parent body, or material from coexisting regoliths impact-splashed into Plainview regolith. Chondrules and silicate grains in the dark matrix which differ from H-group material are likely related to these xenoliths and their regoliths. Light-colored lithic fragments may represent shock-melted chondritic material, sometimes compositionally-modified, or new, achondritic meteoritic types. Unequilibrated and carbonaceous lithic fragments in the dark-colored host matrix indicate that equilibration of the host occurred before incorporation of the fragments and that compaction and lithification of the Plainview regolith to form a coherent meteorite must have occurred at temperatures below 300°C and/or on a short time scale.  相似文献   

6.
Bulk analyses of 157 lithic fragments of igneous origin and analyses of their constituent minerals (plagioclase, pyroxene, olivine, Mg-Al spinel, chromite, ilmenite, armalcolite, baddeleyite, zirkelite, K-feldspar, interstitial glass high in SiO2 and K2O) have been used to characterize the lunar highland rock suites at the Luna 20 site. The predominant suite is composed of ANT (anorthositic-noritic-troctolitic) rocks, as found at previous Apollo and Luna sites. This suite consists of an early cumulate member, spinel troctolite, and later cumulate rocks which are gradational from anorthosite to noritic and troctolitic anorthosite to anorthositic norite and troctolite; anorthositic norite is the most abundant rock type and its composition is close to the average composition for the highland rocks at this site. Spinel troctolite is a distinctive member of this suite and is characterized by the presence of Mg-Al spinel, magnesian olivine (average, Fo83), and plagioclase. High-alumina basalt with low alkali content is another important rock type and melt of this composition may be parental to the cumulate ANT suite. Alkalic high-alumina basalt (KREEP) was not found in our sample, but may be genetically related to the ANT suite in that it may have formed by partial melting of rocks similar to those of the ANT suite. Fractional crystallization of low alkali, high-alumina basalt probably cannot produce alkalic high-alumina basalt because the enrichment in KREEP component is many times greater than the simultaneous change in major element components. Formation of alkalic high-alumina basalt by mechanical mixing of ANT rocks with very KREEP-rich components is not likely because the high-alumina basalt suite falls on a cotectic in the anorthiteolivine-silica system. Mare basalts may also be genetically related in that they may have been derived by remelting of rocks formed from residual liquids of fractional crystallization of parental low-alkali, high-alumina basalt, plus mafic cumulate crystals; the resultant melt would have a negative Eu anomaly and high FeMg and pyroxeneplagioclase ratios.  相似文献   

7.
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8.
In the suevite breccia of the Ries impact crater, Germany, glasses occur as bombs, and small particles in the groundmass. These glasses were formed from melt produced by shock fusion of crystalline basement rocks. Ejection from the crater resulted in the formation of aerodynamically shaped bombs, a few homogeneous spherules and a large mass of small glass particles which were deposited in the suevite breccia. Bombs and small particles included within chilled bottom and top layers of suevite deposits have been preserved in vitreous state, whereas glasses within the interior of the suevite devitrified, due to slower cooling rates.This paper summarizes the results of petrographical and chemical investigations of suevite glasses and their devitrification products. Conclusions are derived on origin and history of bombs and glass particles.Vitreous bombs and glass particles consist of schlieren-rich glass, mineral fragments (mainly quartz), rock fragments and vesicles. Wet chemical, trace element and microprobe analyses reveal that a primary melt was formed by shock fusion of a basement complex, consisting of about 80% biotite granite and 20% amphibolite. The, originally, more than 1800° C hot melt then incorporated shocked and desintegrated rocks of outer zones of the impact. Partial fusion of the rock debris resulted in a polyphase mixture consisting of melts, different in composition, accumulations of refractory mineral fragments and vesicles.Devitrified bombs and glass particles which are found in the interior of suevite deposits show alterations of texture and composition, due to microcrystallite growth and action of hydrothermal and weathering solutions. Incipient devitrification is indicated by brown staining of the glasses, originating, probably, by exsolution of minute magnetite particles. By optical microscopy and X-ray analysis, plagioclase and pyroxenes have been identified as main devitrification products. Shapes and textures of microcrystallites indicate fast crystal growth in a viscous and supercooled medium. Hot fluids permeating the suevite deposited microcrystalline quartz in vesicles and cracks. Later, montmorillonite was precipitated by solutions corroding the glass. Action of solutions on glasses which were weakened in coherence by devitrification resulted in oxidation of iron, leaching of iron and magnesium, and enrichment in alkalis.  相似文献   

9.
R.N. Thompson 《Lithos》1975,8(1):9-14
An intensely-welded ignimbrite has been identified in the upper part of the thick Palaeocene lava pile of Ubekendt Ejland, west Greenland. It consists of 2.3 volume present of phenocrysts and 2 percent of lithic fragments (basalt and trachyte) in a compact nitreous matrix of flattened and welded rhyolitic pumice fragments and glass shards. The phenocrysts are mostly of plagioclase (An17), together with minor amounts of sanidine (Or87), hydrothermally-altered olivine and augite, ilmenite and zircon. The major elements chemistry of the magmatic fraction of the rock is deduced from microprobe analyses of its constituent phases. It is suggested that this erupted pyroclastic rock may be a surface expression of high-level granite emplacement in southern Ubekendt Ejland. The source could have been an early acid member of the currently-exposed Sarqâta qáqâ plutonic complex. Recent Rb/Sr isotopic studies have shown that the acid and basic rocks of this intrusive centre, together with the surrounding basaltic lavas, define a single isochron (65 m.y.) with an initial 87Sr/86Sr ratio of 0.7045, appropriate to the upper mantle. The ultimate source of the Ubekendt Ejland ignimbr.es may therefore be sub-crustal.  相似文献   

10.
This study addresses the issue of what fraction of the impact glass in the regolith of a lunar landing site derives from local impacts (those within a few kilometers of the site) as opposed to distant impacts (10 or more kilometers away). Among 10,323 fragments from the 64-210-μm grain-size fraction of three Apollo 16 regolith samples, 14% are impact glasses, that is, fragments consisting wholly or largely of glass produced in a crater-forming impact. Another 16% are agglutinates formed by impacts of micrometeorites into regolith. We analyzed the glass in 1559 fragments for major- and minor-element concentrations by electron probe microanalysis and a subset of 112 of the fragments that are homogeneous impact glasses for trace elements by secondary ion mass spectrometry. Of the impact glasses, 75% are substantially different in composition from either the Apollo 16 regolith or any mixture of rocks of which the regolith is mainly composed. About 40% of the impact glasses are richer in Fe, Mg, and Ti, as well as K, P, and Sm, than are common rocks of the feldspathic highlands. These glasses must originate from craters in maria or the Procellarum KREEP Terrane. Of the feldspathic impact glasses, some are substantially more magnesian (greater MgO/FeO) or have substantially lower concentrations of incompatible elements than the regolith of the Apollo 16 site. Many of these, however, are in the range of feldspathic lunar meteorites, most of which derive from points in the feldspathic highlands distant from the Procellarum KREEP Terrane. These observations indicate that a significant proportion of the impact glass in the Apollo 16 regolith is from craters occurring 100 km or more from the landing site. In contrast, the composition of glass in agglutinates, on average, is similar to the composition of the Apollo 16 regolith, consistent with local origin.  相似文献   

11.
In three brecciated meteorites, Bencubbin, Cumberland Falls and Plainview, the oxygen isotopic compositions of different rock types within each meteorite were determined to seek genetic relationships between them. In all cases the isotopic compositions are not consistent with derivation from a single parent body. There is no evidence that chondrites and achondrites could be derived from a common parent body. The chondritic inclusions in Bencubbin and Cumberland Falls cannot be identified with any of the ordinary chondritic meteorites. The carbonaceous chondritic fragments in Bencubbin are smilar to, but not identical with, C2 meteorites. The achondritic portion of Bencubbin has a very unusual isotopic composition, which, along with its close relative Weatherford, sets it in a class distinctly apart from other achondrites. Lithic fragments in brecciated meteorites provide a wider range of rock types than is represented by known macroscopic meteorites. Collisions between some meteorite parent bodies were of sufficiently low velocity that fragments of both are preserved in breccias.  相似文献   

12.
The lithic units of polymict breccia 14321 (Big Bertha) have been grouped according to composition, texture, degree of metamorphism, and additional criteria based on a systematic study of the interrelationships of all clast-matrix pairs. From this information it has been possible to reconstruct the assembly and metamorphic history of this breccia. The earliest formed fragmental component of 14321 (microbreccia-1) is dominated by KREEP-rich norite, extruded and subsequently brecciated and lithified in an ejecta blanket at approximately 1000°C in the general region of Mare Imbrium after the Serenitatis impact but prior to the Imbrium impact. This early microbreccia component and lesser amounts of mare-type basalt, microgranite, rhyolite glass, anorthosite and olivine microbreccia were assembled at the Apollo 14 site as part of the Fra Mauro ejecta blanket from the Imbrium impact. The resulting microbreccia-3 incorporates all the lithic types above and accretionary lapilli structures (microbreccia-2) in a dark matrix annealed at approximately 700°C. A later impact on the Fra Mauro excavated and mutually abraded microbreccia-3 and a local, 14321-type, basalt which were assembled into polymict breccia 14321. Final placement of 14321 at its sampling location was accomplished during the minor Cone Crater impact event.  相似文献   

13.
2013年2月15日,俄罗斯车里雅宾斯克(Chelyabinsk)发生了伴随罕见的空中爆炸的大规模陨石雨事件。本文对3块代表不同冲击变质程度的车里雅宾斯克陨石碎块进行了研究。它们都具有部分熔壳,其中1块仅出现碎裂,1块含有冲击熔融细脉,1块基本由冲击熔融囊和冲击熔脉组成。冲击变质程度最低的样品,代表了该陨石母体小行星的原始岩石矿物学特征:即具有粗粒的岩石结构和均一的矿物化学组成,但仍保留一些残余球粒,表明受到了明显的热变质作用,其岩石类型可划分为5型。铁镁质硅酸盐高的Fe O含量(橄榄石Fa:27.9mol%~28.2mol%,辉石Fs值:23.3mol%~23.7mol%)、以及较低的Fe-Ni金属含量,表明其化学群属于低铁低金属的LL群。我们所分析的样品与前人报导的结果相似,未发现不同岩性的岩屑,表明车里雅宾斯克陨石的原始岩矿特征较为均一。3块陨石碎块中,随着冲击程度的增强,其冲击变质特征依次表现为硅酸盐矿物的破碎、熔长石化更为普遍、陨硫铁与铁镍合金共熔、硅酸盐熔脉的形成、铬铁矿与长石共熔、以及大量熔融囊的发育等。但是,在冲击熔融囊和熔脉中,以及相邻围岩中均未发现高压矿物相。熔脉中的橄榄石晶屑和相邻围岩的橄榄石颗粒表现为化学成分的不均一,在背散射电子图像中呈不同灰度的结构。这与其他强烈冲击变质陨石中橄榄石的林伍德石或瓦茨利石相变相似。该陨石中林伍德石或瓦茨利石的缺失很可能是由于强烈撞击后高温产生的退变质。这也表明车里雅宾斯克陨石的母体小行星可能遭受了非常强烈的撞击事件。  相似文献   

14.
Measured were the abundance and distribution of nitrogen in glasses of glass inclusions in olivines of CV3, CO3, CR, C4, CH3, and LL chondritic meteorites by means of the 14N(d, p)15N nuclear reaction. Similar to what was observed with carbon, nitrogen is present in low concentrations (<20 ppm) in the structure of olivines but can by stored in variable amounts in glasses of glass inclusions. These primitive glasses, characterized by a Si-Al-Ca-rich composition, have highly variable nitrogen contents (30 to 1500 ppm) and highly inhomogeneous nitrogen distribution. Nitrogen contents are independent of the chemical composition of the glasses. The heterogeneous distribution is a common feature of all studied inclusions, as is evidenced by the variable contents of nitrogen in glass inclusions occurring in the same olivine grain. Nitrogen heterogeneity is suggestive of trapping of solid nitrogen carrier phases during formation of the constituents of chondrules. However, part of the originally trapped nitrogen appears to have been lost, possibly, by ulterior oxidation and subsequent transformation into volatile species.  相似文献   

15.
Summary The mineralogy and petrology of three lithic fragments of alkalic highalumina basalt (Kreep) composition from the Apollo 12 coarse fines was studied in detail, using an electron microprobe, in order to gain insight into their crystallization histories. Most rocks of this composition are brecciated and our study indicates that a variety of environments of crystallization can be distinguished for mineral fragments and matrices. Mineral fragments are derived from members of the ANT suite (probably troctolites) in fragments 2 and 5, and the alkalic high-alumina basalt suite in fragment 3. The rocks from which they were derived were coarse-grained, recrystallized and equilibrated, as indicated by major, and especially, minor elements. Minor elements in plagioclase, olivine, pyroxene, and zircon are consistently lower in mineral fragments as compared with matrix minerals. The origin of large zircon fragments is problematic but they are probably from the alkalic high-alumina basalt suite. Mineral fragments may have been derived from plutonic rocks (none have yet been recognized from the alkalic high-alumina basalt suite), but possibly also from breccia fragments which were recrystallized in hot, thick ejecta blankets. The matrix of the lithic fragments is of alkalic high-alumina basalt composition and is either igneous or metamorphic, or both. Hence, lithic fragments 2 and 5 are polymict breccias whereas fragment 3 is a monomict breccia. Matrix glasses in fragments 2 and 3 represent melts fractionated along the orthopyroxeneplagioclase cotectic in the olivine-anorthite-silica pseudoternary system. If these liquids could be separated from the residuum and crystallized they would be, as yet, unrecognized members of the alkalic high-alumina basalt suite. The alkalic high-alumina basalt mixing component of fragment 5 (a polymict breccia) has such a composition and may be derived from such a fractionated rock. A mineral fragment of pyroxene intergrown with ilmenite, approximately parallel to (001), is interpreted as decorated shock lamellae rather than as a deep-seated intergrowth, as found in kimberlites. A glass coating on one side of fragment 3 has SiO2-rich and feldspathic schlieren and appears to be derived, by impact melting, from a rock of granite composition.
Petrologie einiger Gesteinsfragmente mit alkalic high-alumina basalt Chemismus aus dem Grobanteil von Apollo 12 Bodenproben
Zusammenfassung Drei Gesteinsfragmente mit alkalic high-alumina basalt (Kreep) Chemismus aus der Grobraktion von Apollo 12 Bodenproben wurden mittels einer Elektronenstrahl-Mikrosonde einer detaillierten Studie unterzogen, um Einblick in ihre Genese zu gewinnen. Der überwiegende Teil von Gesteinen dieser Zusammensetzung ist brekziös und unsere Studie zeigt, daß unterschiedliche Kristallisationsbedingungen für die Mineralfragmente und Matrizes herrschten. Die Mineralfragmente in den Fragmenten 2 und 5 stammen von Gesteinen der ANT- (Anorthositisch-Noritisch-Troctolitischen) Reihe (wahrscheinlich von Troctoliten) und in Fragment 3 von Gesteinen der alkalic high-alumina basalt-Reihe.Die Verteilung der Haupt- und Nebenelemente in den Mineralfragmenten zeigt, daß diese von rekristallisierten und equilibrierten, grobkörnigen Gesteinen stammen. Die Konzentrationen der Nebenelemente sind in allen Mineralfragmenten (Plagioklas, Olivin und Zirkon) deutlich geringer als in den Mineralen der Matrix. Die Herkunft der großen Zirkon-Fragmente ist nicht genau zu klären. Sie stammen jedoch wahrscheinlich von Gesteinen der alkalic high-alumina basalt-Reihe. Alle Mineralfragmente könnten von plutonischen Gesteinen stammen (solche sind von der alkalic highalumina basalt-Reihe zur Zeit noch nicht bekannt), sie könnten ihren Ursprung jedoch auch in prä-existenten Brekzien haben, welche in dichten, heißen Auswurfdecken rekristallisierten. Die Matrix der Gesteinsfragmente hat durchwegs eine alkalic high-alumina basalt Zusammensetzung und ist entweder magmatisch oder metamorph, oder beides. Die Fragmente 2 und 5 sind daher als polymikte und das Fragment 3 als monomikte Brekzie zu bezeichnen.Die Matrixgläser in den fragmenten 2 und 3 repräsentieren Rest-schmelzen, welche entlang der Orthopyroxen-Plagioklas-Kotektik im Olivin-Anorthit-SiO2-System fraktionierten. Diese Schmelzen würden-könnten sie vom System getrennt werden-bisher noch nicht bekannte Glieder der alkalic high-alumina basalt-Reihe darstellen. Eine derartige Zusammensetzung hat jedoch die Mischkomponente im Fragment 5 (eine polymikte Brekzie), welche von einem auf diesem Wege fraktioniertem Gestein stammen könnte.Ilmenit-Lamellen [subparallel zu (001)] in einem Pyroxenfragment stellen eher dekorierte Schocklamellen als Verwachsungen, wie sie aus Kimberliten bekannt sind, dar. Fragment 3 ist einseitig mit einem schlierigen Glas bedeckt, dessen Schlieren angenähert die Zusammensetzung von Alkalifeldspat und reinem SiO2 haben. Dieses Glas ist offensichtlich eine Impakt-Schmelze eines Gesteines von granitischer Zusammensetzung.

With 5 Figures  相似文献   

16.
The angrites are a small and heterogeneous group of achondritic meteorites with highly unusual chemical and mineralogical features. The abundant presence of glasses in D'Orbigny makes this rock a unique member of the angrite group. Glasses fill open spaces, form pockets, and occur as inclusions in olivines. Their physical settings exclude an incorporation from an external source. Major and trace element (rare earth elements [REE], Li, B, Be, transition elements, N and C) contents of these glasses and host olivines were measured combining laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), secondary-ion mass spectrometry (SIMS), Nuclear Reaction Analysis (NRA), and EMP techniques. Based on the major element composition, glasses filling voids could represent either a melt formed by melting an angritic rock or a melt from which angrites could have crystallized. Trace element contents of these glasses strongly indicate a direct link to the D'Orbigny bulk meteorite. They are incompatible with the formation of the glasses by partial melting of a chondritic source rock or by shock melting. The refractory elements (e.g., Al, Ti, Ca) have about 10 × CI abundances with CaO/TiO2 and FeO/MnO ratios being approximately chondritic. Trace element abundances in the glasses appear to be governed by volatility and suggest that the refractory elements in the source had chondritic relative abundances. Although the glasses (and the whole rock) lack volatile elements such as Na and K, they are rich in some moderately volatile elements such as B, V, Mn, Fe (all with close to CI abundances), and Li (about 3-5 × CI). These elements likely were added to the glass in a sub-solidus metasomatic elemental exchange event. We have identified a novel mechanism for alteration of glass and rock compositions based on an exchange of Al and Sc for Fe and other moderately volatile elements in addition to the well-known metasomatic exchange reactions (e.g., Ca-Na and Mg-Fe).Because glass inclusions in olivine were partly shielded from the metasomatic events by the host crystal, their chemical composition is believed to be closer to the original composition than that of any other glasses. The relative trace element abundances in glasses of glass inclusions in olivine and glass pockets are also unfractionated and at the 10 to 20 × CI level. These glasses are chemically similar to the common void-filling glasses but show a much wider compositional variation. Inclusion glasses demonstrate that at least olivine grew with the help of a liquid. In analogy to olivines in carbonaceous chondrites, initial formation could also have been a vapor-liquid-solid condensation process. At that time, the glass had a purely refractory composition. This composition, however, was severely altered by the metasomatic addition of large amounts of FeO and other moderately volatile elements. The presence of volatile elements such as carbon and nitrogen in glasses of glass inclusions is another feature that appears to give these glasses a link with those hosted by olivines of carbonaceous chondrites. All these features point to an origin from a vapor with relative abundances of condensable elements similar to those in the solar nebula.  相似文献   

17.
Enstatite-rich meteorites include EH and EL chondrites, rare ungrouped enstatite chondrites, aubrites, a few metal-rich meteorites (possibly derived from the mantle of the aubrite parent body), various impact-melt breccias and impact-melt rocks, and a few samples that may be partial-melt residues ultimately derived from enstatite chondrites. Members of these sets of rocks exhibit a wide range of impact features including mineral-lattice deformation, whole-rock brecciation, petrofabrics, opaque veins, rare high-pressure phases, silicate darkening, silicate-rich melt veins and melt pockets, shock-produced diamonds, euhedral enstatite grains, nucleation of enstatite on relict grains and chondrules, low MnO in enstatite, high Mn in troilite and oldhamite, grains of keilite, abundant silica, euhedral graphite, euhedral sinoite, F-rich amphibole and mica, and impact-melt globules and spherules. No single meteorite possesses all of these features, although many possess several. Impacts can also cause bulk REE fractionations due to melting and loss of oldhamite (CaS) – the main REE carrier in enstatite meteorites. The Shallowater aubrite can be modeled as an impact-melt rock derived from a large cratering event on a porous enstatite chondritic asteroid; it may have been shock melted at depth, slowly cooled and then excavated and quenched. Mount Egerton may share a broadly similar shock and thermal history; it could be from the same parent body as Shallowater. Many aubrites contain large pyroxene grains that exhibit weak mosaic extinction, consistent with shock-stage S4; in contrast, small olivine grains in some of these same aubrites have sharp or undulose extinction, consistent with shock stage S1 to S2. Because elemental diffusion is much faster in olivine than pyroxene, it seems likely that these aubrites experienced mild post-shock annealing, perhaps due to relatively shallow burial after an energetic impact event. There are correlations among EH and EL chondrites between petrologic type and the degree of shock, consistent with the hypothesis that collisional heating is mainly responsible for enstatite-chondrite thermal metamorphism. Nevertheless, the apparent shock stages of EL6 and EH6 chondrites tend to be lower than EL3-5 and EH3-5 chondrites, suggesting that the type-6 enstatite chondrites (many of which possess impact-produced features) were shocked and annealed. The relatively young Ar–Ar ages of enstatite chondrites record heating events that occurred long after any 26Al that may have been present initially had decayed away. Impacts remain the only plausible heat source at these late dates. Some enstatite meteorites accreted to other celestial bodies: Hadley Rille (EH) was partly melted when it struck the Moon; Galim (b), also an EH chondrite, was shocked and partly oxidized when it accreted to the LL parent asteroid. EH, EL and aubrite-like clasts also occur in the polymict breccias Kaidun (a carbonaceous chondrite) and Almahata Sitta (an anomalous ureilite). The EH and EL clasts in Kaidun appear unshocked; some clasts in Almahata Sitta may have been extensively shocked on their parent bodies prior to being incorporated into the Almahata Sitta host.  相似文献   

18.
 We have investigated new samples from the Gees mantle xenolith suite (West Eifel), for which metasomatism by carbonatite melt has been suggested. The major metasomatic change is transformation of harzburgites into phlogopite-rich wehrlites. Silicate glasses are associated with all stages of transformation, and can be resolved into two major groups: a strongly undersaturated alkaline basanite similar to the host magma which infiltrated the xenoliths during ascent, and Si-Al-enriched, variably alkaline glass present exclusively within the xenoliths. Si-Al-rich glasses (up to 72 wt% SiO2 when associated with orthopyroxene (Opx) are usually interpreted in mantle xenoliths as products of decompressional breakdown of hydrous phases like amphibole. In the Gees suite, however, amphibole is not present, nor can the glass be related to phlogopite breakdown. The Si-Al-rich glass is compositionally similar to glasses occurring in many other xenolith suites including those related to carbonatite metasomatism. Petrographically the silicate glass is intimately associated with the metasomatic reactions in Gees, mainly conversion of harzburgite orthopyroxene to olivine + clinopyroxene. Both phases crystallize as microlites from the glass. The chemical composition of the Si-Al-enriched glass shows that it cannot be derived from decompressional melting of the Gees xenoliths, but must have been present prior to their entrainment in the host magma. Simple mass-balance calculations, based on modal analyses, yield a possible composition of the melt prior to ascent of the xenoliths, during which glass + microlite patches were modified by dissolution of olivine, orthopyroxene and spinel. This parental melt is a calc-alkaline andesite (55–60 wt% SiO2), characterized by high Al2O3 (ca. 18 wt%). The obtained composition is very similar to high-alumina, calc-alkaline melts that should form by AFC-type reactions between basalt and harzburgite wall rock according to the model of Kelemen (1990). Thus, we suggest that the Si-Al-enriched glasses of Gees, and possibly of other suites as well, are remnants of upper mantle hybrid melts, and that the Gees suite was metasomatized by silicate and not carbonatite melts. High-Mg, high-Ca composition of metasomatic olivine and clinopyroxene in mantle xenoliths have been explained by carbonatite metasomatism. As these features are also present in the Gees suite, we have calculated the equilibrium Ca contents of olivine and clinopyroxene using the QUI1F thermodynamical model, to show that they are a simple function of silica activity. High-Ca compositions are attained at low a SiO2 and can thus be produced during metasomatism by any melt that is Opx-undersaturated, irrespective of whether it is a carbonatite or a silicate melt. Such low a SiO2 is recorded by the microlites in the Gees Si-Al-rich glasses. Our results imply that xenolith suites cannot confidently be related to carbonatite metasomatism if the significance of silicate glasses, when present, is not investigated. Received: 2 March 1995 / Accepted: 12 June 1995  相似文献   

19.
The manned Apollo 11, 12, 14 and 15 and the automated Luna 16 lunar missions have provided us with lunar rock and regolith (soil) samples from a number of geologically distinct sites. The mare regions were sampled by Apollo 11, 12 and Luna 16, whereas Apollo 14 landed on a terrain with more relief, the Fra Mauro Formation which represents an ejecta blanket from the Imbrian Basin, and Apollo 15 touched down near the lunar highlands. The samples collected consist of a mixture, mainly of basalt, breccia and regolith (soil-particulate matter, generally < 1 cm in size). The basalts show considerable variation in texture, mineralogy and chemistry and probably represent fragments from various parts of relatively thin and extensive lava flows in the maria. The breccias represent regolith material which was indurated to varying degrees by impact events. The regolith is a product of the breakdown, again by impact, of coherent rock masses of basalt and breccia.  相似文献   

20.
The first cold plasma ICP-MS (inductively coupled plasma mass spectrometer) Fe isotope study is described. Application of this technique to the analyses of Fe isotopes in a number of meteorites is also reported. The measurement technique relies on reduced temperature operation of the ICP source to eliminate pervasive molecular interferences from Ar complexes associated with conventional ICP-MS. Instrumental mass bias corrections are performed by sample-standard bracketing and using Cu as an external mass bias drift monitor. Repeated measurements of a terrestrial basalt reference sample indicate an external reproducibility of ± 0.06 ‰ for δ56Fe and ± 0.25 ‰ for δ58Fe (1 σ). The measured iron isotopic compositions of various bulk meteorites, including irons, chondrites and pallasites are identical, within error, to the composition of our terrestrial basalt reference sample suggesting that iron mass fractionation during planet formation and differentiation was non-existent. Iron isotope compositions measured for eight chondrules from the unequilibrated ordinary chondrite Tieschitz range from −0.5 ‰ < δ56Fechondrules < 0.0 ‰ relative to the terrestrial/meteorite average. Mechanisms for fractionating iron in these chondrules are discussed.  相似文献   

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