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1.
Abstract— We have used radiochemical neutron activation analysis (RNAA) to determine 15 trace elements, including 10 moderately to highly volatile ones—Rb, Ag, Se, Cs, Te, Zn, Cd, Bi, Tl, In (in increasing volatility order), in 6 H chondrite falls with low-3He contents. These (plus prior RNAA data) provide a compositional database of 92 H4-6 chondrite falls. Three suites of samples can be identified from their noble gas contents: 44 with “normal” contents and, therefore, “normal” orbits and cosmic-ray exposure histories; 8 that lost radiogenic gases, presumably by shock late in their histories; and 17 that lost cosmogenic gases by heating during close solar approach. We used the standard multivariate statistical techniques of linear discriminant analysis and logistic regression to compare contents of the 10 moderately and highly volatile trace elements, listed above, in these three suites. We found no significant differences. This contrasts sharply with similar comparisons involving random falls and H4-6 chondrites that landed on Earth at specific time intervals. Apparently, contents of volatile trace elements in H4-6 chondrites were established early in their histories, and they are so retentively sited that loss during later heating episodes did not occur.  相似文献   

2.
Abstract– Although iron isotopes are increasingly used for meteorites studies, no attempt has been made to evaluate the effect of terrestrial weathering on this isotopic tracer. We have thus conducted a petrographic, chemical, and iron isotopic study of equilibrated ordinary chondrites (OC) recovered from hot Moroccan and Algerian Saharan deserts environment. As previously noticed, we observe that terrestrial desertic weathering is characterized by the oxidation of Fe‐Ni metal (Fe0), sulfide and Fe2+ occurring in olivine and pyroxene. It produces Fe‐oxides and oxyhydroxides that partially replace metal, sulfide grains and also fill fractures. The bulk chemical compositions of the ordinary chondrites studied show a strong Sr and Ba enrichment and a S depletion during weathering. Bulk meteoritic iron isotope compositions are well correlated with the degree of weathering and S, Sr, and Ba contents. Most weathered chondrites display the heaviest isotopic composition, by up to 0.1‰, which is of similar magnitude to the isotopic variations resulting from meteorite parent bodies’ formation and evolution. This is probably due to the release of isotopically light Fe2+ to waters on the Earth’s surface. Hence, when subtle Fe isotopic effects have to be studied in chondrites, meteorites with weathering grade above W2 should be avoided.  相似文献   

3.
Abstract— The nature and isotopic composition of carbonaceous components in a variety of ordinary chondrites have been studied using stepped combustion. The samples were chosen to include falls, finds and Antarctic meteorites; specimens from all three chemical groups (H, L and LL) have been analysed. Effort was concentrated mostly on the low petrologic type meteorites (i.e., type 3); however, types 4–6 were also included in the study. Apart from terrestrial contaminants and weathering products, some of the unequilibrated ordinary chondrites appear to contain an indigenous organic component. In addition, most of the samples studied show evidence for an amorphous/graphitic component. This exists as C-rich aggregates or as carbon associated with “Huss” matrix. There does not appear to be any difference in δ13C for this carbon between Antarctic and non-Antarctic meteorites. In contrast, low temperature carbon in Antarctic samples is characterized by a 13C-enrichment. This is thought to be due to the influence of terrestrial weathering products introduced in the Antarctic. Curiously, the low temperature carbon in non-Antarctic finds appears to be intermediate in δ13C between Antarctic finds and non-Antarctic falls. This suggests that the weathering processes which are so obviously apparent from Antarctic samples may also extend, albeit in a more limited way, to non-Antarctic meteorites.  相似文献   

4.
The flux of small meteoroids, originating primarily from comets, consists of sporadic, random objects and others whose orbits are related. Here, we summarize data relevant to the question of whether the flux of large meteoroids of asteroidal origin (recoverable as meteorites) also consists of objects with random orbits, as well as coorbital objects. After reviewing some relevant properties of planetary materials, applications of two nuclear techniques - radiochemical neutron activation analysis (RNAA) and accelerator mass spectrometry (AMS) - to this question are discussed. Contents of ten thermally labile trace and ultratrace elements determined by RNAA (Ag, Bi, Cd, Cs, In, Rb, Se, Te, Tl, Zn) act as thermometers for thermal metamorphism in parent sources. These data, together with spectral reflectivity information, establish the nature of surfaces on abundant C-, G-, B- and F-class asteroids. Data for these ten cosmothermometers in H4-6 type ordinary chondrites, when treated by multivariate statistical techniques, demonstrate that a suite chosen by one set of criteria (the circumstances of their fall in May, between 1855 and 1895) is distinguishable by another set, i.e. compositionally, from all other such falls analyzed. Hence, this suite, H Cluster 1, has an average thermal history distinguishable from those of all other falls, demonstrating that near-Earth source regions for H chondrite falls changes rapidly. AMS measurements of cosmogenic36Cl (301 kyr half-life), quantify nominal terrestrial ages for Antarctic H chondrites whose contents of thermometric trace elements were also established by RNAA. While multivariate statistical analysis of RNAA data from Antarctic H chondrites with nominal terrestrial ages 50 kyr are not distinguishable from those of falls, older Antarctic H chondrites are compositionally distinguishable from falls. Assertions that these highly significant compositional differences reflect terrestrial or methodologic causes are refutable. This result argues that near-Earth source regions of H chondrites have changed over a long time, as well. Thus, the Earth receives a highly biased sampling of planetary objects in the Solar System in any one time-period.  相似文献   

5.
The valence of iron has been used in terrestrial studies to trace the hydrolysis of primary silicate rocks. Here, we use a similar approach to characterize the secondary processes, namely thermal metamorphism and aqueous alteration, that have affected carbonaceous chondrites. X‐ray absorption near‐edge structure spectroscopy at the Fe‐K‐edge was performed on a series of 36 CM, 9 CR, 10 CV, and 2 CI chondrites. While previous studies have focused on the relative distribution of Fe0 with respect to oxidized iron (Feox = Fe2+ + Fe3+) or the iron distribution in some specific phases (e.g., Urey–Craig diagram; Urey and Craig 1953), our measurements enable us to assess the fractions of iron in each of its three oxidation states: Fe0, Fe2+, and Fe3+. Among the four carbonaceous chondrites groups studied, a correlation between the iron oxidation index (IOI = [2(Fe2+) + 3(Fe3+)]/[FeTOT]) and the hydrogen content is observed. However, within the CM group, for which a progressive alteration sequence has been defined, a conversion of Fe3+ to Fe2+ is observed with increasing degree of aqueous alteration. This reduction of iron can be explained by an evolution in the mineralogy of the secondary phases. In the case of the few CM chondrites that experienced some thermal metamorphism, in addition to aqueous alteration, a redox memory of the aqueous alteration is present: a significant fraction of Fe3+ is present, together with Fe2+ and sometimes Fe0. From our data set, the CR chondrites show a wider range of IOI from 1.5 to 2.5. In all considered CR chondrites, the three oxidation states of iron coexist. Even in the least‐altered CR chondrites, the fraction of Fe3+ can be high (30% for MET 00426). This observation confirms that oxidized iron has been integrated during formation of fine‐grained amorphous material in the matrix (Le Guillou and Brearley 2014; Le Guillou et al. 2015; Hopp and Vollmer 2018). Last, the IOI of CV chondrites does not reflect the reduced/oxidized classification based on metal and magnetite proportions, but is strongly correlated with petrographic types. The valence of iron in CV chondrites therefore appears to be most closely related to thermal history, rather than aqueous alteration, even if these processes can occur together (Krot et al. 2004; Brearley and Krot 2013).  相似文献   

6.
Abstract— Samples of Holbrook (an L6 chondrite that fell in Arizona in 1912) were analysed by 57Fe Mössbauer spectroscopy to determine the proportion of the total Fe that had been converted to Fe3+ by weathering processes. Although samples recovered in 1912 showed negligible (<1%) alteration, a spectrum recorded from a specimen recovered in 1968 showed that 9.7% of the total Fe was present as Fe3+. Given the existence of samples in similar semiarid environments with terrestrial ages >40 ka, a linear rate of weathering is unrealistic, rapidly producing very high levels of oxidation in samples with low terrestrial ages. However if weathering follows an approximate power law, then we can model a rapid initial phase and the levels of oxidation that are more typical in samples with much longer terrestrial residence times recovered from Roosevelt County. Our analysis, together with work on terrestrial age dated samples, indicates that hot desert weathering of ordinary chondrites is at least a two-stage process, with an initial rapid phase lasting <500 years before oxidation is passivated.  相似文献   

7.
Abstract— The recovery of large numbers of meteorites from Antarctica has dramatically increased the amount of extraterrestrial material available for laboratory studies of solar system origin and evolution. Yet, the great age of Antarctic meteorites raises the concern that significant amounts of terrestrial weathering has corrupted their pre‐terrestrial record. Organic matter found in carbonaceous chondrites is one of the components most susceptible to alteration by terrestrial processes. To assess the effects of Antarctic weathering on both non‐Antarctic and Antarctic chondritic organic matter, a number of CM chondrites have been analyzed. Mössbauer spectroscopy has been used to ascertain pre‐terrestrial and terrestrial oxidation levels, while pyrolysis‐gas chromatography‐mass spectrometry was used to determine the constitution of any organic matter present. Increased oxidation levels for iron bearing minerals within the non‐Antarctic chondrites are likely to be a response to increased amounts of parent body aqueous alteration. Parent body processing also appears to remove ether bonds from organic material and alkyl side chains from its constituent units. The iron in Antarctic chondrites is generally more oxidized than that in their non‐Antarctic counterparts, reflecting terrestrial weathering. Antarctic weathering of chondritic organic matter appears to proceed in a similar way to parent body aqueous alteration and simply enhances the organic responses observed in the non‐Antarctic data set. Degradation of the record of preterrestrial processes in Antarctic chondrites should be taken into account when interpreting data from these meteorites.  相似文献   

8.
Abstract— Mössbauer spectroscopy is a very useful tool for identifying ferric iron weathering products in meteorites because of the capability to quantify the relative amounts of ferric iron in them. Mössbauer measurements were made of 33 Antarctic H chondrites (predominately H5) and two paired Antarctic CR chondrites. The primary goals of this study are to determine if Mössbauer spectroscopy can be used to determine which phases are weathering in Antarctic meteorites and if the relative amounts of ferric iron correlate with terrestrial age. Determining which minerals are weathering in ordinary chondrites appears very difficult due to variations in composition for different ordinary chondrites of the same meteorite class and possible problems in preparing homogeneous samples. The analysis of the two paired CR chondrites appears to indicate that metallic iron is predominately weathering to produce ferric iron for this class of meteorite. No correlation is seen between the relative amounts of ferric iron and terrestrial age for ordinary chondrites. One Antarctic H5 chondrite (ALHA77294) with a short 14C age of 135 ± 200 years from the dating of interior carbonate weathering products does have a relatively low amount of ferric iron, which is consistent with this meteorite being exposed on the surface for a relatively short time.  相似文献   

9.
Abstract— We report radiochemical neutron activation analysis (RNAA) data for U, Co, Au, Sb, Ga, Rb, Cs, Se, Ag, Te, Zn, In, Bi, Tl, and Cd (ordered by increasing ease of vaporization and loss from the Murchison CM2 chondrite during open-system heating) in nine Antarctic C2 and C3 chondrites. These meteorites exhibit properties (obtained by reflectance spectroscopy, O isotopic mass spectrometry and/or mineralogy-petrology) suggesting thermal metamorphism in their parent bodies. Five of these meteorites (Asuka (A) 881655, Yamato (Y) 793495, Y-790992, Pecora Escarpment (PCA) 91008, and Y-86789—paired with Y-86720) exhibit significant depletion of the most thermally mobile 1–5 trace elements, which is consistent with open-system loss during extended parent-body heating (under conditions duplicated by week-long heating of the Murchison C2 chondrite, heated at 500–700 °C in a low-pressure (initially 10?5atm) H2 atmosphere). From earlier data, three other C3 chondrites—Allan Hills (ALH) 81003, ALH 85003, and Lewis Cliffs (LEW) 85332—show significant Cd depletion. Nine additional C2 and C3 chondrites show no evidence of mobile trace element depletion—including Y-793321, which by all other criteria was mildly metamorphosed thermally. Either metamorphism of these nine meteorites occurred under closed conditions and/or alteration took place under such mild conditions that even Cd could not be lost. The RNAA data suggest that 10 of the 46 Antarctic carbonaceous chondrites (including 4 of 37 from Victoria Land and 6 of 9 from Queen Maud Land) exhibit open-system loss of at least some thermally mobile trace elements by heating in their parent bodies, whereas none of the 25 non-Antarctic falls experienced this. These results are consistent with the idea that the Antarctic sampling of near-Earth material differs from that being sampled today.  相似文献   

10.
Abstract— To examine the thermal history of the parent body/bodies of equilibrated H chondrites, we treated data for 11 volatile trace elements (Co, Rb, Ag, Se, Cs, Te, Zn, Cd, Bi, Tl, and In in order of putative volatility) in 90 falls: 15 H4; 46 H5, and 29 H6. Using univariate statistical tests, contents of few of these elements differ significantly between any two of these suites. One element, Cs, differs systematically between all three pairs of suites; Co and Tl differ between two pairs of suites. For Co and Cs, contents varied as H4 > H5 > H6; while for Tl, contents varied as H4 < H5 < H6. Using multivariate statistical tests, all three suites can be distinguished compositionally, with trace element contents in the H5 suite being intermediate to those of H4 and H6. Surprisingly, the multivariate distinguishability reflects contents of less volatile Co, Rb, Ag, Se and Cs, and not of highly volatile Te, Zn, Cd, Bi, Tl and In. The compositional trends apparently reflect heterogeneous accretion >600 K, with the suites deriving from a stratified parent body/bodies.  相似文献   

11.
The weathering products present in igneous terrestrial Antarctic samples were analyzed, and compared with those found in the four Miller Range nakhlite Martian meteorites. The aim of these comparisons was to determine which of the alteration phases in the Miller Range nakhlites are produced by terrestrial weathering, and what effect rock composition has on these phases. Antarctic terrestrial samples MIL 05031 and EET 96400, along with the Miller Range nakhlites MIL 03346 and 090032, were found to contain secondary alteration assemblages at their externally exposed surfaces. Despite the difference in primary mineralogy, the assemblages of these rocks consist mostly of sulfates (jarosite in MIL 05031, jarosite and gypsum in EET 96400) and iddingsite‐like Fe‐clay. As neither of the terrestrial samples contains sulfur‐bearing primary minerals, and these minerals are rare in the Miller Range nakhlites, it appears that SO42?, possibly along with some of the Na+, K+, and Ca+ in these phases, was sourced from wind‐blown sea spray and biogenic emissions from the southern ocean. Cl enrichment in the terrestrially derived “iddingsite” of MIL 05031 and MIL 03346, and the presence of halite at the exterior edge of MIL 090032, can also be explained by this process. However, jarosite within and around the olivine‐bound melt inclusions of MIL 090136 is present in the interior of the meteorite and, therefore, is probably the product of preterrestrial weathering on Mars.  相似文献   

12.
E.A. Cloutis  P. Hudon  T. Hiroi  M.J. Gaffey 《Icarus》2012,217(1):389-407
Powdered samples of a suite of 14 CR and CR-like chondrites, ranging from petrologic grade 1 to 3, were spectrally characterized over the 0.3–2.5 μm interval as part of a larger study of carbonaceous chondrite reflectance spectra. Spectral analysis was complicated by absorption bands due to Fe oxyhydroxides near 0.9 μm, resulting from terrestrial weathering. This absorption feature masks expected absorption bands due to constituent silicates in this region. In spite of this interference, most of the CR spectra exhibit absorption bands attributable to silicates, in particular an absorption feature due to Fe2+-bearing phyllosilicates near 1.1 μm. Mafic silicate absorption bands are weak to nonexistent due to a number of factors, including low Fe content, low degree of silicate crystallinity in some cases, and presence of fine-grained, finely dispersed opaques. With increasing aqueous alteration, phyllosilicate: mafic silicate ratios increase, resulting in more resolvable phyllosilicate absorption bands in the 1.1 μm region. In the most phyllosilicate-rich CR chondrite, GRO 95577 (CR1), an additional possible phyllosilicate absorption band is seen at 2.38 μm. In contrast to CM spectra, CR spectra generally do not exhibit an absorption band in the 0.65–0.7 μm region, which is attributable to Fe3+–Fe2+ charge transfers, suggesting that CR phyllosilicates are not as Fe3+-rich as CM phyllosilicates. CR2 and CR3 spectra are uniformly red-sloped, likely due to the presence of abundant Fe–Ni metal. Absolute reflectance seems to decrease with increasing degree of aqueous alteration, perhaps due to the formation of fine-grained opaques from pre-existing metal. Overall, CR spectra are characterized by widely varying reflectance (4–21% maximum reflectance), weak silicate absorption bands in the 0.9–1.3 μm region, overall red slopes, and the lack of an Fe3+–Fe2+ charge transfer absorption band in the 0.65–0.7 μm region.  相似文献   

13.
Abstract— We report noble gas data for 37 H chondrites collected from the Allan Hills by EUROMET in the 1988–1989 field season. Among these are 16 specimens with high levels (>100 krad) of natural thermoluminescence (NTL), originally interpreted as signaling their derivation from a single meteoroid with an orbit that became Earth‐crossin‐100 ka ago. One of these 16 is an H3 chondrite with a cosmic‐ray exposure age of ～33 Ma and clearly represents a separate fall. The other 15 H4–6 chondrites derive from three separate meteoroids, each of which is represented by a five or six member group. These groups have mean exposure ages of 3.7, 4.1, and 6.6 Ma: the middle‐group members all contain solar Ne. The two younger groups also seem to each include a few H chondrites with normal NTL levels. Measurements of cosmogenic 10Be (1.5 Ma), 26AI (710 ka), and 36CI (301 ka) in 14 of the high‐NTL chondrites indicate that all reflect a simple irradiation history. In contrast, many of a different (38 member) randomly selected suite of Antarctic H chondrites seem to have different cosmic‐ray irradiation histories. The 3.7 and 6.6 Ma groups from the 37 member Allan Hills suite come from about 5–30 and about 5–10 cm depths in 80–125 and 60–125 cm radius meteoroids, respectively.  相似文献   

14.
Abstract— We present a new weathering index (wi) for the metallic‐Fe‐Ni‐poor chondrite groups (CK and R) based mainly on transmitted light observations of the modal abundance of crystalline material that is stained brown in thin sections: wi‐0, <5 vol%; wi‐1, 5–25 vol%; wi‐2, 25–50 vol%; wi‐3, 50–75 vol%; wi‐4, 75–95 vol%; wi‐5, >95 vol%; wi‐6, significant replacement of mafic silicates by phyllosilicates. Brown staining reflects mobilization of oxidized iron derived mainly from terrestrial weathering of Ni‐bearing sulfide. With increasing degrees of terrestrial weathering of CK and R chondrites, the sulfide modal abundance decreases, and S, Se, and Ni become increasingly depleted. In addition, bulk Cl increases in Antarctic CK chondrites, probably due to contamination from airborne sea mist.  相似文献   

15.
Phosphorus zoning is observed in olivines in high‐FeO (type IIA) chondrules in H chondrites over the entire range of petrologic grades: H3.1–H6. Features in P concentrations such as oscillatory and sector zoning, and high P cores are present in olivines that are otherwise unzoned in the divalent cations. Aluminum concentrations are low and not significantly associated with P zoning in chondrule olivines. In highly unequilibrated H chondrites, phosphorus zoning is generally positively correlated with Cr. Atomic Cr:P in olivine is roughly 1:1 (3:1 for one zone in one olivine in RC 075), consistent with Cr3+ charge‐balancing P5+ substituting for Si4+. Normal igneous zonation involving the dominant chrome species Cr2+ was observed only in the LL3.0 chondrite Semarkona. In more equilibrated chondrites (H3.5–H3.8), Cr spatially correlated with P is occasionally observed but it is diffuse relative to the P zones. In H4–H6 chondrites, P‐correlated Cr is absent. One signature of higher metamorphic grades (≥H3.8) is the presence of near matrix olivines that are devoid of P oscillatory zoning. The restriction to relatively high metamorphic grade and to grains near the chondrule–matrix interface suggests that this is a response to metasomatic processes. We also observed P‐enriched halos near the chondrule–matrix interface in H3.3–H3.8 chondrites, likely reflecting the loss of P and Ca from mesostasis and precipitation of Ca phosphate near the chondrule surface. These halos are absent in equilibrated chondrites due to coarsening of the phosphate and in unequilibrated chondrites due to low degrees of metasomatism. Olivines in type IA chondrules show none of the P‐zoning ubiquitous in type IIA chondrules or terrestrial igneous olivines, likely reflecting sequestration of P in reduced form within metallic alloys and sulfides during melting of type IA chondrules.  相似文献   

16.
Abstract— Fifty‐four fragments of ordinary chondrites from 50 finds representing all searched areas in central Oman and all weathering stages were selected to compare the physical, chemical, and mineralogical effect of terrestrial weathering with 14C terrestrial ages. 14C ages range from 2.0 to >49 kyr with a median value of 17.9 kyr. The peak of the age range, which is between 10–20 kyr, falls in an arid climate period. A comparison of the chemical composition of Omani chondrites with literature data for unweathered H and L chondrites demonstrates a strong enrichment in Sr and Ba, and depletion in S during weathering. Water contents in H chondrites increase with terrestrial age, whereas L chondrites show a rapid initial increase followed by nearly constant water content. Correlating Sr, Ba, and H2O with age indicates two absorption trends: i) an initial alteration within the first 20 kyr dominated by H2O uptake, mainly reflecting Fe‐Ni metal alteration, and ii) a second Ba‐and Sr‐dominated stage correlated with slower and less systematic weathering of troilite that starts after H2O reaches ?2 wt%. Sulfur released from troilite partly combines with Ba and Sr to form sulfate minerals. Other parameters correlated with 14C age are degree of weathering, color of powdered meteorites, and the Ni/Fe ratio. Chemical analyses of 145 soils show a high degree of homogeneity over the entire interior Oman Desert, indicating large‐scale mixing by wind. Soil samples collected from beneath meteorite finds typically are enriched in Ni and Co, confirming mobilization from the meteorites. High Cr and Ni concentrations in reference soil samples, which decrease from NE to SW, are due to detrital material from ultramafic rocks of the Oman Mountains.  相似文献   

17.
Abstract— A series of 59 impacts in the laboratory reduced a coherent 460 g piece of the L6 ordinary chondrite ALH 85017 to a coarse‐grained “regolith.” We then subjected the 125–250 μm fines from this sample to reverberation shock stresses of 14.5–67 GPa in order to delineate the melting behavior of porous, unconsolidated, chondritic asteroid surfaces during meteorite impact. The initial pore space (40–50%) was completely closed at 14.5 GPa and a dense aggregate of interlocking grains resulted. Grain‐boundary melting commenced at <27 GPa and ?50% of the total charge was molten at 67 GPa; this stress corresponds to typical asteroid impacts at ?5 km/sec. Melting of the entire sample most likely mandates >80 GPa, which is associated with impact velocities >8 km/sec. The Fe‐Ni and troilite clasts of the original meteorite melted with particular ease, forming immiscible melts that are finely disseminated throughout the silicate glass. These metal droplets are highly variable in size, extending to <100 nm and most likely to superparamagnetic domains; such opaques are also observed in the natural melt veins of ordinary chondrites. It follows that melting and dissemination of pre‐existing, Fe‐rich phases may substantially affect the optical properties of asteroidal surfaces. It seems unnecessary to invoke reduction of Fe2+ (or Fe3+) by sputtering or impact‐processes—in analogy to the lunar surface—to produce “space weathering” effects on S‐type asteroids. We note that HED meteorites contain ample FeO (comparable to that in lunar basalts) for reduction processes to take place, yet their probable parent object(s), Vesta and its collisional fragments, display substantially unweathered surfaces. Howardites, eucrites, and diogenites (HEDs), however, contain little native metal (typically <0.5%), in contrast to ordinary chondrites (commonly 10–15%) and their S‐type parent objects. These considerations suggest that the modal content of native metal and sulfides is more important for space weathering on asteroids than total FeO.  相似文献   

18.
To shed light on the mechanism of formation of nanophase iron particles (npFe) in space-weathered materials from airless bodies, we analyzed exsolved and unexsolved space-weathered lunar pyroxenes from Apollo 17 sample 71501. The exsolved pyroxene allowed for the observation of the effects of space weathering on similar mineral phases with variable composition. Using coordinated scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy (EELS), we determined that two coexisting pyroxenes in the exsolved grain showed systematic variations in response to space weathering, despite equivalent exposure conditions. The npFe in the space-weathered rim of augite lamellae were smaller and fewer than the npFe in the rim of pigeonite lamellae. EELS spectrum imaging revealed the presence and heterogeneous distribution of Fe0, Fe2+, and Fe3+ in the exsolved pyroxene. Metallic iron occurred in the npFe, a mixture of Fe2+ and Fe3+ occurred in the pigeonite lamellae, and the augite lamellae contained virtually all Fe3+. Approximately 50% of the total Fe measured in the exsolved pyroxene grain was ferric. Partitioning of Fe2+ and Fe3+ among the lamellae is invoked to explain the difference in npFe development in pigeonite and augite. The results of this study, the first to identify Fe3+ in a crystalline lunar ferromagnesian silicate, have implications for our understanding of how space weathering might proceed in oxidized phases. Furthermore, the discovery of an Fe3+-rich pyroxene also supports attribution of the 0.7 μm absorption feature observed in Galileo Solid State Imager data to oxidized Fe in clinopyroxenes.  相似文献   

19.
Abstract— The mid-infrared (4000–450 cm?1; 2.5–22.2 μm) transmission spectra of seven Antarctic ureilites and 10 Antarctic H-5 ordinary chondrites are presented. The ureilite spectra show a number of absorption bands, the strongest of which is a wide, complex feature centered near 1000 cm?1 (10 μm) due to Si-O stretching vibrations in silicates. The profiles and positions of the substructure in this feature indicate that Mg-rich olivines and pyroxenes are the main silicates responsible. The relative abundances of these two minerals, as inferred from the spectra, show substantial variation from meteorite to meteorite, but generally indicate olivine is the most abundant (olivine:pyroxene = 60:40 to 95:5). Both the predominance of olivine and the variable olivine-to-pyroxene ratio are consistent with the known composition and heterogeneity of ureilites. The H-5 ordinary chondrites spanned a range of weathering classes and were used to provide a means of addressing the extent to which the ureilite spectra may have been altered by weathering processes. It was found that, while weathering of these meteorites produces some weak bands due to the formation of small amounts of carbonates and hydrates, the profile of the main silicate feature has been little affected by Antarctic exposure in the meteorites studied here. The mid-infrared ureilite spectra provide an additional means of testing potential asteroidal parent bodies for the ureilites. At present, the best candidates include the subset of S-type asteroids having low albedos and weak absorption features in the near infrared.  相似文献   

20.
Abstract— The Carcote meteorite, detected in 1888 in the northern Chilean Andes, is a brecciated, weakly shocked H5 chondrite. It contains a few barred olivine chondrules and, even more rarely, fan-shaped or granular orthopyroxene chondrules. The chondrules are situated in a fine-grained matrix that consists predominantly of olivine and orthopyroxene with accessory clinopyroxene, troilite, chromite, merrillite, and plagioclase. The metal phase is mainly kamacite with subordinate taenite and traces of native Cu. In its bulk rock composition, Carcote compares well with other H5 chondrites so far analysed, except for a distinctly higher C content. Microprobe analyses revealed the following mineral compositions: olivine (Fa16.5–20), orthopyroxene (Fs14–17.5), diopsidic clinopyroxene (FS6–7), plagioclase (An15–20). Troilite is stoichimetric FeS with traces of Ni and Cr; chromite has Cr/(Cr + Al) of 0.86, Fe2+/(Fe2+ + Mg) of 0.80-0.88 and contains considerable amounts of Ti, Mn, and Zn. Merrillite is close to the theoretical formula Ca18(Mg, Fe)2Na2(PO4)14, although with a Na deficiency not compensated for by excess Ca; the Mg/(Mg + Fe2+) ratio of the Carcote merrilite is 0.93-0.95. Kamacite and taenite have Ni contents of 5.6–7.2 and 17.1–23.4 wt%, respectively. Native Cu contains about 3.1–3.3 wt% Fe and 1.6 wt% Ni. Application of different geothermometers to the Carcote H5 chondrite yielded apparently inconsistent results. The highest temperature range of 850–950 °C (at 1 bar) is derived from the Ca-in-opx thermometer. From the cpx-opx solvus geothermometers and the two-pyroxene Fe-Mg exchange geothermometer, a lower temperature range of 750–840 °C is estimated, whereas lower and more variable temperatures of 630–770 °C are obtained from the Ca-in-olivine geothermometer. Recent calibrations of the olivine-spinel geothermometer yielded a still lower temperature range of 570–670 °C, which fits well to the temperature information derived from the Ni distribution between kamacite and taenite. Judging from crystal chemical considerations, we assume that these different temperatures reflect the closure of different exchange equilibria during cooling of the meteorite parent body.  相似文献   

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