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1.
Abstract— Iodine-xenon analyses of chondrules from the Bjurböle L4 and Parnallee LL3.6 meteorites have been made using a continuous wave laser microprobe and the resonance ionisation mass spectrometer RELAX. The excess 129Xe content released from the Bjurböle chondrule is lower than previous stepped-heating studies have found, suggesting that the technique does not completely degas the samples. Nonetheless, clear isochrons were produced, and data for initial 129Xe/130Xe are consistent with earlier work. A correlation is evident in each chondrule between 131Xe* and 128Xe* perhaps indicating a common host-phase for their parent nuclides, a condition possibly fulfilled by a Te- and I-bearing sulfide. The I-Xe system of a Parnallee macrochondrule exhibits no excess 129Xe, possibly as a result of thermal alteration or deformation before accumulation of the meteorite. A cristobalite-bearing chondrule depleted in 16O yields an I-Xe age of 4.5 ± 0.5 Ma after the mean Bjurböle age.  相似文献   

2.
Abstract— We review the development of the I‐Xe technique and how its data are interpreted, and specify the best current practices. Individual mineral phases or components can yield interpretable trends in initial 129I/127I ratio, whereas whole‐rock I‐Xe ages are often hard to interpret because of the diversity of host phases, many of which are secondary. Varying standardizations in early work require caution; only samples calibrated against Shallowater enstatite or Bjurböle can contribute reliably to the emerging I‐Xe chronology of the early solar system. Although sparse, data for which I‐Xe and Mn‐Cr can be compared suggest that the two systems are concordant among ordinary chondrite samples. We derive a new age for the closure of the Shallowater enstatite standard of 4563.3 ± 0.4 Myr from the relationship between the I‐Xe and Pb‐Pb systems. This yields absolute I‐Xe ages and allows data from this and other systems to be tested by attempting to construct a common chronology of events in the early solar system. Absolute I‐Xe dates for aqueous and igneous processes are consistent with other systems. Consideration of the I‐Xe host phases in CAIs and dark inclusions demonstrates that here the chronometer records aqueous alteration of pre‐existing material. The ranges of chondrule ages deduced from the Al‐Mg and I‐Xe systems in Semarkona (LL3.0) and Chainpur (LL3.4) are consistent. Chainpur I‐Xe data exhibit a greater range of ages than Semarkona, possibly reflecting a greater degree of parent body processing. However individual chondrules show little or no evidence of such processing. Determining the host phase(s) responsible for high temperature correlations may resolve the issue.  相似文献   

3.
Abstract— Forty-six chondrules from Chainpur (LL3.4) and 39 chondrules and clasts from Parnallee (LL3.6) have been sectioned and searched for Na-, Cl-rich phases by electron probe microanalysis (EPMA). Oxygen isotopic compositions, I-Xe ages and ion probe data were also obtained on some of these chondrules. Sodium-, Cl-rich glass and microcrystalline sodalite (Na4Al3Si3O12Cl), nepheline (NaAlSiO4), scapolite (Na4Al3Si9O24Cl) have been identified in 7% of the Chainpur and 8% of the Parnallee samples. These phases are present in chondrule mesostases or, in one case, the plagioclase of a barred-olivine chondrule. None of the chondrules contain >5 vol% Na-, Cl-rich phases. In the Chainpur chondrules, they originated through partial devitrification of silica-undersaturated, rare-earth-element-(REE), Na- and Cl-rich mesostases. Two processes have been identified that led to the formation of these mesostases. In two of the chondrules, which consist mainly of low-Ca pyroxene, the extended, metastable crystallization of low-Ca pyroxene created silica-undersaturated, REE-rich residua. Barium- and Cl-enrichments in nepheline and scapolite of one chondrule suggest that there was also an influx of alkalis and Cl during crystallization of the low-Ca pyroxene. Similarly, another one of the Chainpur chondrules, mainly composed of olivine phenocrysts, is markedly enriched in Cl (10 × OC). As there is no evidence of corrosive metasomatism in any of the chondrules, Cl- (and alkali) enrichment is believed to have occurred when they were still partially molten. The chondrules were derived from normal O-isotopic reservoirs, so the postulated influx of Ba, Na and Cl did not occur on an exotic parent body. Trace amounts of nepheline and sodalite, present in two Parnallee chondrules, crystallized from small Na-, Cl-, REE-rich residua following extended crystallization of anorthite. An I-Xe age of 5.0 Ma post-Bjurböle obtained on one of these Parnallee chondrules dates the crystallization of feldspathoid and, thus, formation of the chondrule.  相似文献   

4.
Abstract– Neon was measured in 39 individual olivine (or olivine‐rich) grains separated from individual chondrules from Dhajala, Bjurböle, Chainpur, Murchison, and Parsa chondrites with spallation‐produced 21Ne the result of interaction of energetic particle irradiation. The apparent 21Ne cosmic ray exposure (CRE) ages of most grains are similar to those of the matrix with the exception of three grains from Dhajala and single grains from Bjurböle and Chainpur, which show excesses, reflecting exposure to energetic particles prior to final compaction of the object. Among these five grains, one from chondrule BJ2A5 of Bjurböle shows an apparent excess exposure age of approximately 20 Ma and the other four from Dhajala and Chainpur have apparent excesses, described as an “age,” from 2 to 17 Ma. The precompaction irradiation effects of grains from chondrules do not appear to be different from the effects seen in olivine grains extracted from the matrix of CM chondrites. As was the case for the matrix grains, there appears to be insufficient time for this precompaction irradiation by the contemporary particle sources. The apparent variations within single chondrules appear to constrain precompaction irradiation effects to irradiation by lower energy solar particles, rather than galactic cosmic rays, supporting the conclusion derived from the precompaction irradiation effects in CM matrix grains, but for totally different reasons. This observation is consistent with Chandra X‐Ray Observatory data for young low‐mass stars, which suggest that our own Sun may have been 105 times more active in an early naked T‐Tauri phase ( Feigelson et al. 2002 ).  相似文献   

5.
Abstract— I‐Xe analyses were carried out for chondrules and refractory inclusions from the two CV3 carbonaceous chondrites Mokoia and Vigarano (representing the oxidized and reduced subgroups, respectively). Although some degree of disturbance to the I‐Xe system is evident in all of the samples, evidence is preserved of aqueous alteration of CAIs in Mokoia 1 Myr later than the I‐Xe age of the Shallowater standard and of the alteration of a chondrule (V3) from Vigarano ～0.7 Myr later than Shallowater. Other chondrules in Mokoia and Vigarano experienced disturbance of the I‐Xe system millions of years later and, in the case of one Vigarano chondrule (VS1), complete resetting of the I‐Xe system after decay of essentially all 129I, corresponding to an age more than 80 Myr after Shallowater. Our interpretation is that accretion and processing to form the Mokoia and Vigarano parent bodies must have continued for at least 4 Myr and 80 Myr, respectively. The late age of a chondrule that shows no evidence for any aqueous alteration or significant thermal processing after its formation leads us to postulate the existence of an energetic chondrule‐forming mechanism at a time when nebular processes are not expected to be important.  相似文献   

6.
If chondrules were exposed to cosmic rays prior to meteorite compaction, they should retain an excess of cosmogenic noble gases. Beyersdorf‐Kuis et al. (2015) showed that such excesses can be detected provided that the chemical composition of each individual chondrule is precisely known. However, their study was limited to a few samples as they had to be irradiated in a nuclear reactor for instrumental neutron activation analysis. We developed a novel analytical protocol that combines the measurements of He and Ne isotopic concentrations with a fast method to correct for differences in chemical composition using micro X‐ray computed tomography. Our main idea is to combine noble gas, nuclear track, and petrography data for numerous chondrules to understand the precompaction exposure history of the chondrite parent bodies. Here, we report our results for a total of 77 chondrules and four matrix samples from NWA 8276 (L3.00), NWA 8007 (L3.2), and Bjurböle (L/LL4). All chondrules from the same meteorite have within uncertainty identical 21Ne exposure ages, and all chondrules from Bjurböle have within uncertainty identical 3He exposure ages. However, most chondrules from NWA 8276 and a few from NWA 8007 show small but resolvable differences in 3He exposure age that we attribute to matrix contamination and/or gas loss. The finding that none of the chondrules has noble gas excesses is consistent with the uniform track density found for each meteorite. We conclude that the studied chondrules did not experience a precompaction exposure longer than a few Ma assuming present‐day flux of galactic cosmic rays. A majority of chondrules from L and LL chondrites thus rapidly accreted and/or was efficiently shielded from cosmic rays in the solar nebula.  相似文献   

7.
Cluster chondrites are characterized by close‐fit textures of deformed and indented chondrules, taken as evidence for hot chondrule accretion (Metzler 2012 ). We investigated seven cluster chondrite clasts from six brecciated LL3 chondrites and measured their bulk oxygen isotopic and chemical composition, including REE, Zr, and Hf. The same parameters were measured in situ on 93 chondrules and 4 interchondrule matrix areas. The CI‐normalized REE patterns of the clasts are flat, showing LL‐chondritic concentrations. The mean chemical compositions of chondrules in clasts and other LL chondrites are indistinguishable and we conclude that cluster chondrite chondrules are representative of the normal LL chondrule population. Type II chondrules are depleted in MgO, Al2O3 and refractory lithophiles (REE, Zr, Hf) by factors between 0.65 and 0.79 compared to type I chondrules. The chondrule REE patterns are basically flat with slight LREE < HREE fractionations. Many chondrules exhibit negative Eu anomalies while matrix shows a complementary pattern. Chondrules scatter along a correlation line with a slope of 0.63 in the oxygen 3‐isotope diagram, interpreted as the result of O‐isotope exchange between chondrule melts and 18O‐rich nebular components. In one clast, a distinct anticorrelation between chondrule size and δ18O is found, which may indicate a more intense oxygen isotope exchange by smaller chondrules. In some clasts the δ18O values of type I chondrules are correlated with concentrations of SiO2 and MnO and anticorrelated with MgO, possibly due to the admixture of a SiO2‐ and MnO‐rich component to chondrule melts during oxygen isotope exchange. Two chondrules with negative anomalies in Sm, Eu, and Yb were found and may relate their precursors to refractory material known from group III CAIs. Furthermore, three chondrules with strong LREE > HREE and Zr/Hf fractionations were detected, whose formation history remains to be explained.  相似文献   

8.
Dhofar 1671 is a relatively new meteorite that previous studies suggest belongs to the Rumuruti chondrite class. Major and REE compositions are generally in agreement with average values of the R chondrites (RCs). Moderately volatile elements such as Se and Zn abundances are lower than the R chondrite values that are similar to those in ordinary chondrites (OCs). Porphyritic olivine pyroxene (POP), radial pyroxene (RP), and barred olivine (BO) chondrules are embedded in a proportionately equal volume of matrix, one of the characteristic features of RCs. Microprobe analyses demonstrate compositional zoning in chondrule and matrix olivines showing Fa‐poor interior and Fa‐rich outer zones. Precise oxygen isotope data for chondrules and matrix obtained by laser‐assisted fluorination show a genetic isotopic relationship between OCs and RCs. On the basis of our data, we propose a strong affinity between these groups and suggest that OC chondrule precursors could have interacted with a 17O‐rich matrix to form RC chondrules (i.e., ?17O shifts from ~1‰ to ~3‰). These interactions could have occurred at the same time as “exotic” clasts in brecciated samples formed such as NWA 10214 (LL3–6), Parnallee (LL3), PCA91241 (R3.8–6), and Dhofar 1671 (R3.6). We also infer that the source of the oxidation and 17O enrichment is the matrix, which may have been enriched in 17O‐rich water. The abundance of matrix in RCs relative to OCs, ensured that these rocks would be apparently more oxidized and appreciably 17O‐enriched. In situ analysis of Dhofar 1671 is recommended to further strengthen the link between OCs and RCs.  相似文献   

9.
Abstract— We have studied the relationship between bulk chemical compositions and relative formation ages inferred from the initial 26Al/27Al ratios for sixteen ferromagnesian chondrules in least equilibrated ordinary chondrites, Semarkona (LL3.0) and Bishunpur (LL3.1). The initial 26Al/27Al ratios of these chondrules were obtained by Kita et al. (2000) and Mostefaoui et al. (2002), corresponding to relative ages from 0.7 ± 0.2 to 2.4 ?0.4/+0.7 Myr after calcium‐aluminum‐rich inclusions (CAIs), by assuming a homogeneous distribution of 26Al in the early solar system. The measured bulk compositions of the chondrules cover the compositional range of ferromagnesian chondrules reported in the literature and, thus, the chondrules in this study are regarded as representatives of ferromagnesian chondrules. The relative ages of the chondrules appear to correlate with bulk abundances of Si and the volatile elements (Na, K, Mn, and Cr), but there seems to exist no correlation of relative ages neither with Fe nor with refractory elements. Younger chondrules tend to be richer in Si and volatile elements. Our result supports the result of Mostefaoui et al. (2002) who suggested that pyroxene‐rich chondrules are younger than olivine‐rich ones. The correlation provides an important constraint on chondrule formation in the early solar system. It is explained by chondrule formation in an open system, where silicon and volatile elements evaporated from chondrule melts during chondrule formation and recondensed as chondrule precursors of the next generation.  相似文献   

10.
Abstract— We performed a systematic high‐precision secondary ion mass spectrometry 26Al‐26Mg isotopic study for 11 ferromagnesian chondrules from the highly unequilibrated ordinary chondrite Bishunpur (LL3.1). The chondrules are porphyritic and contain various amounts of olivine and pyroxene and interstitial plagioclase and/or glass. The chemical compositions of the chondrules vary from FeO‐poor to FeO‐rich. Eight chondrules show resolvable 26Mg excesses with a maximum δ26Mg of ?1% in two chondrules. The initial 26Al/27Al ratios inferred for these chondrules range between (2.28 ± 0.73) × 10?5 to (0.45 ± 0.21) × 10?5. Assuming a homogeneous distribution of Al isotopes in the early solar system, this range corresponds to ages relative to CAIs between 0.7 ± 0.2 Ma and 2.4+0.7?0.4 Ma. The inferred total span of the chondrule formation ages is at least 1 Ma, which is too long to form chondrules by the X‐wind. The initial 26Al/27Al ratios of the chondrules are found to correlate with the proportion of olivine to pyroxene suggesting that olivine‐rich chondrules formed earlier than pyroxene‐rich chondrules. Though we do not have a completely satisfactory explanation of this correlation we tentatively interpret it as a result of evaporative loss of Si from earlier generations of chondrules followed by addition of Si to the precursors of later generation chondrules.  相似文献   

11.
Abstract— The CV (Vigarano‐type) chondrites are a petrologically diverse group of meteorites that are divided into the reduced and the Bali‐like and Allende‐like oxidized subgroups largely based on secondary mineralogy (Weisberg et al., 1997; Krot et al., 1998b). Some chondrules and calcium‐aluminum‐rich inclusions (CAIs) in the reduced CV chondrite Vigarano show alteration features similar to those in Allende: metal is oxidized to magnetite; low‐Ca pyroxene, forsterite, and magnetite are rimmed and veined by ferrous olivine (Fs40–50); and plagioclase mesostases and melilite are replaced by nepheline and sodalite (Sylvester et al., 1993; Kimura and Ikeda, 1996, 1997, 1998). Our petrographic observations indicate that Vigarano also contains individual chondrules, chondrule fragments, and lithic clasts of the Bali‐like oxidized CV materials. The largest lithic clast (about 1 times 2 cm in size) is composed of opaque matrix, type‐I chondrules (400–2000 μm in apparent diameter) surrounded by coarse‐grained and fine‐grained rims, and rare CAIs. The matrix‐chondrule ratio is about 1.1. Opaque nodules in chondrules in the clast consist of Cr‐poor and Cr‐rich magnetite, Ni‐ and Co‐rich metal, Ni‐poor and Ni‐rich sulfide; low‐Ni metal nodules occur only inside chondrule phenocrysts. Chromium‐poor magnetite is preferentially replaced by fayalite. Chondrule mesostases are replaced by phyllosilicates; low‐Ca pyroxene and olivine phenocrysts appear to be unaltered. Matrix in the clast consists of very fine‐grained (<1 μm) ferrous olivine, anhedral fayalite grains (Fa80–100), rounded objects of porous Ca‐Fe‐rich pyroxenes (Fs10–50Wo50), Ni‐poor sulfide, Ni‐ and Co‐rich metal, and phyllosilicates; magnetite is rare. On the basis of the presence of the Bali‐like lithified chondritic clast—in addition to individual chondrules and CAIs of both Bali‐like and Allende‐like materials—in the reduced CV chondrite Vigarano, we infer that (1) all three types of materials were mixed during regolith gardening on the CV asteroidal body, and (2) the reduced and oxidized CV materials may have originated from a single, heterogeneously altered asteroid.  相似文献   

12.
NWA 10214 is an LL3‐6 breccia containing ~8 vol% clasts including LL5, LL6, and shocked‐darkened LL fragments as well as matrix‐rich Clast 6 (a new kind of chondrite). This clast is a dark‐colored, subrounded, 6.1 × 7.0 mm inclusion, consisting of 60 vol% fine‐grained matrix, 32 vol% coarse silicate grains, and 8 vol% coarse opaque grains. The large chondrules and chondrule fragments are mainly Type IB; one small chondrule is Type IIA. Also present are one 450 × 600 μm spinel‐pyroxene‐olivine CAI and one 85 × 110 μm AOI. Clast 6 possesses a unique set of properties. (1) It resembles carbonaceous chondrites in having relatively abundant matrix, CAIs, and AOIs; the clast's matrix composition is close to that in CV3 Vigarano. (2) It resembles type‐3 OC in its olivine and low‐Ca pyroxene compositional distributions, and in the Fe/Mn ratio of ferroan olivine grains. Its mean chondrule size is within 1σ of that of H chondrites. The O‐isotopic compositions of the chondrules are in the ordinary‐ and R‐chondrite ranges. (3) It resembles type‐3 enstatite chondrites in the minor element concentrations in low‐Ca pyroxene grains and in having a high low‐Ca pyroxene/olivine ratio in chondrules. Clast 6 is a new variety of type‐3 OC, somewhat more reduced than H chondrites or chondritic clasts in the Netschaevo IIE iron; the clast formed in a nebular region where aerodynamic radial drift processes deposited a high abundance of matrix material and CAIs. A chunk of this chondrite was ejected from its parent asteroid and later impacted the LL body at low relative velocity.  相似文献   

13.
Abstract— We studied the elemental and isotopic abundances of noble gases (He, Ne, Ar in most cases, and Kr, Xe also in some cases) in individual chondrules separated from six ordinary, two enstatite, and two carbonaceous chondrites. Most chondrules show detectable amounts of trapped 20Ne and 36Ar, and the ratio (36Ar/20Ne)t (from ordinary and carbonaceous chondrites) suggests that HL and Q are the two major trapped components. A different trend between (36Ar/20Ne)t and trapped 36Ar is observed for chondrules in enstatite chondrites indicating a different environment and/or mechanism for their formation compared to chondrules in ordinary and carbonaceous chondrites. We found that a chondrule from Dhajala chondrite (DH‐11) shows the presence of solar‐type noble gases, as suggested by the (36Ar/20Ne)t ratio, Ne‐isotopic composition, and excess of 4He. Cosmic‐ray exposure (CRE) ages of most chondrules are similar to their host chondrites. A few chondrules show higher CRE age compared to their host, suggesting that some chondrules and/or precursors of chondrules have received cosmic ray irradiation before accreting to their parent body. Among these chondrules, DH‐11 (with solar trapped gases) and a chondrule from Murray chondrite (MRY‐1) also have lower values of (21Ne/22Ne)c, indicative of SCR contribution. However, such evidences are sporadic and indicate that chondrule formation event may have erased such excess irradiation records by solar wind and SCR in most chondrules. These results support the nebular environment for chondrule formation.  相似文献   

14.
Abstract— We have studied Pb‐isotope systematics of chondrules from the oxidized CV3 carbonaceous chondrite Allende. The chondrules contain variably radiogenic Pb with a 206Pb/204Pb ratio between 19.5–268. Pb‐Pb isochron regression for eight most radiogenic analyses yielded the date of 4566.2 ± 2.5 Ma. Internal residue‐leachate isochrons for eight chondrule fractions yielded consistent dates with a weighted average of 4566.6 ± 1.0 Ma, our best estimate for an average age of Allende chondrule formation. This Pb‐Pb age is consistent with the range of model 26Al‐26Mg ages of bulk Allende chondrules reported by Bizzarro et al. (2004) and is indistinguishable from Pb‐Pb ages of Ca‐Al‐rich inclusions (CAIs) from CV chondrites (4567.2 ± 0.6 Ma) (Amelin et al. 2002) and the oldest basaltic meteorites. We infer that chondrule formation started contemporaneously with or shortly after formation of CV CAIs and overlapped in time with formation of the basaltic crust and iron cores of differentiated asteroids. The entire period of chondrule formation lasted from 4566.6 ± 1.0 Ma (Allende) to 4564.7 ± 0.6 Ma (CR chondrite Acfer 059) to 4562.7 ± 0.5 Ma (CB chondrite Gujba) and was either continuous or consisted of at least three discrete episodes. Since chondrules in CB chondrites appear to have formed from a vapor‐melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated (Krot et al. 2005), there were possibly a variety of processes in the early solar system occurring over at least 4–5 Myr that we now combine under the umbrella name of “chondrule formation.”  相似文献   

15.
Abstract– Unequilibrated ordinary chondrites (UOCs) of all groups (H, L, LL) contain unique chondrite clasts, which are characterized by a close‐fit texture of deformed and indented chondrules. These clasts, termed “cluster chondrites,” occur in 41% of the investigated samples with modal abundances between 5 and 90 vol% and size variations between <1 mm and 10 cm. They show the highest chondrule abundances compared with all chondrite classes (82–92 vol%) and only low amounts of fine‐grained interchondrule matrix and rims (3–9 vol%). The mean degree of chondrule deformation varies between 11% and 17%, compared to 5% in the clastic portions of their host breccias and to values of 3–5% found in UOC literature, respectively. The maximum deformation of individual chondrules is about 50%, a value which seemingly cannot be exceeded due to geometric limitations. Both viscous and brittle chondrule deformation is observed. A model for cluster chondrite formation is proposed where hot and deformable chondrules together with only small amounts of co‐accreting matrix formed a planetesimal or reached the surface of an already existing body within hours to a few days after chondrule formation. They deformed in a hot stage, possibly due to collisional compression by accreting material. Later, the resulting rocks were brecciated by impact processes. Thus, cluster chondrite clasts are interpreted as relicts of primary accretionary rocks of unknown original dimensions. If correct, this places a severe constraint on chondrule‐forming conditions. Cluster chondrites would document local chondrule formation, where chondrule‐forming heating events and the accretion of chondritic bodies were closely linked in time and space.  相似文献   

16.
Abstract– Chondrule compositions suggest either ferroan precursors and evaporation, or magnesian precursors and condensation. Type I chondrule precursors include granoblastic olivine aggregates (planetary or nebular) and fine‐grained (dustball) precursors. In carbonaceous chondrites, type I chondrule precursors were S‐free, while type II chondrules have higher Fe/Mn than in ordinary chondrites. Many type II chondrules contain diverse forsteritic relicts, consistent with polymict dustball precursors. The relationship between finer and coarser grained type I chondrules in ordinary chondrites suggests more evaporation from more highly melted chondrules. Fe metal in type I, and Na and S in type II chondrules indicate high partial pressures in ambient gas, as they are rapidly evaporated at canonical conditions. The occurrence of metal, sulfide, or low‐Ca pyroxene on chondrule rims suggests (re)condensation. In Semarkona type II chondrules, Na‐rich olivine cores, Na‐poor melt inclusions, and Na‐rich mesostases suggest evaporation followed by recondensation. Type II chondrules have correlated FeO and MnO, consistent with condensation onto forsteritic precursors, but with different ratios in carbonaceous chondrites and ordinary chondrites, indicating different redox history. The high partial pressures of lithophile elements require large dense clouds, either clumps in the protoplanetary disk, impact plumes, or bow shocks around protoplanets. In ordinary chondrites, clusters of type I and type II chondrules indicate high number densities and their similar oxygen isotopic compositions suggest recycling together. In carbonaceous chondrites, the much less abundant type II chondrules were probably added late to batches of type I chondrules from different O isotopic reservoirs.  相似文献   

17.
Abstract— Two groups of chondrules in the Murchison CM chondrite, which have previously been identified on the basis of FeO in the chondrule grains, are readily identified from cathodoluminescence (CL) and belong to those of the ordinary chondrite group A and B chondrules of Sears et al. (1992a). All chondrules are surrounded by fine-grained rims containing forsterite with bright red CL, but on group A chondrules an outer thin rim grades into a much thicker rim, with a lower density of forsterite grains, which in turn grades into the central chondrule. Group B chondrules have only the thin outer rim with a high density of small forsterite grains. This is the first time an unequivocal correlation has been observed between chondrule rim thickness and the composition of the object on which the rim is located. We suggest that while all objects in the meteorite (group B chondrules, refractory inclusions, mineral and chondrule fragments, clasts) acquired a very thin rim during processing in a wet regolith, the thick rims on group A chondrules were formed by aqueous alteration of precursor metal- and sulfide-rich rims which are a characteristic of group A chondrules in ordinary chondrites.  相似文献   

18.
Abstract— We have performed a detailed petrologic and mineralogic study of two chondritic clasts from the polymict eucrite Lewis Cliff (LEW) 85300, and performed chemical analyses by INAA and RNAA on one of these. Petrologically, the clasts are identical and are composed of dispersed aggregates, chondrules and chondrule fragments supported by matrix. The aggregates and chondrules are composed of olivine (Fo100–45), orthopyroxene (Wo1–2En98–60), plus some diopside. The matrix consists of fine-grained olivine (Fo60–53), and lesser orthopyroxene and augite. Fine-grained saponite is common in the matrix. The bulk major element composition of the matrix is identical in both clasts and similar to that of CM, CO and CV chondrites. The bulk composition of the clast studied by INAA and RNAA shows unusual abundance patterns for lithophile, siderophile and chalcophile elements but is basically chondritic. The INAA/RNAA data preclude assignment of the LEW 85300,15 clast to any commonly accepted group of carbonaceous chondrite. The unusual rare earth element abundance pattern may, in part, be due to terrestrial alteration.  相似文献   

19.
Abstract— The iodine‐xenon system has been analyzed in samples of 7 chondrules from the CB chondrites Gujba and Hammadah al Hamra (HaH) 237. One sample from Gujba defined a high temperature iodine‐xenon isochron corresponding to closure 1.87 ± 0.4 Ma before closure of Shallowater enstatite. Motivated by this result, we employ outlier rejection to re‐evaluate the Shallowater age, leading to a modified value of 4562.3 ± 0.4 Ma (1s?). In this process, the datum obtained by combining our I‐Xe age for Gujba with the literature Pb‐Pb age is rejected as an outlier, indicating that in this sample the I‐Xe system closed earlier than the accepted Pb‐Pb age of chondrules from CB chondrites. The need for a formation environment distinct from that of chondrules from other meteorites is thus reduced.  相似文献   

20.
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