A study of ordinary chondrites by Mössbauer spectroscopy with high‐velocity resolution

Abstract— An improvement in the velocity resolution and quality of Mössbauer spectra has been applied to a group of ordinary chondrites. This improvement permitted us to carry out a more detailed study of the iron bearing phases in these samples than has previously been possible. Mössbauer spectra of 11 ordinary chondrites of L and H chemical groups were measured using 4096 channels and presented for further analysis in 1024 channels. Subspectra of the metal grains of several chondrites demonstrated the presence of at least two magnetic sextets related to the main Fe(Ni, Co) phases. Moreover, Mössbauer study of extracted metal grains from Tsarev L5 revealed three sextets and one singlet spectral components related to various α‐Fe(Ni, Co), α‘‐Fe(Ni, Co), α₂‐Fe(Ni, Co), and γ‐Fe(Ni, Co) phases. Each subspectrum of olivine and pyroxene in Mössbauer spectra of ordinary chondrites was fitted by superposition of two quadrupole doublets related to M1 and M2 sites in minerals for the first time. An analysis of relative areas and Mössbauer hyperfine parameters was performed and some differences for L and H chondrites as well as for M1 and M2 sites were observed. Mössbauer parameters of troilite and oxidized iron were analyzed. In contrast to a previous study with 512‐channel spectra, the presence of oxidized iron was found in all chondrites.     

Weathering in Antarctic H and CR chondrites: Quantitative analysis through Mössbauer spectroscopy

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.     

Systematics of Mössbauer absorption areas in ordinary chondrites and applications to a newly fallen meteorite in Jodhpur,India

Abstract— Mössbauer absorption areas corresponding to ⁵⁷Fe in olivine, pyroxene, troilite, and the metallic phase in ordinary chondrites are shown to exhibit certain systematic behaviors. H chondrites occupy 2 distinct regions on the plot of metallic phase absorption area versus silicate absorption area, while L/LL chondrites fall in a separate region. Similar separation is also observed when pyroxene absorption area is plotted against olivine absorption area. The one‐dimensional plot for the ratio of olivine area to pyroxene area separates L and LL chondrites. Based on these systematics, a newly fallen meteorite at Jodhpur, India is suggested to be an LL chondrite.     

Classification of the Didwana‐Rajod meteorite: A Mössbauer spectroscopic study

Abstract— Mössbauer spectroscopic studies of the Didwana‐Rajod chondrite, which fell on 1991 August 12 in western Rajasthan, India, are presented. The results are compared with the Mössbauer data of several enstatite and ordinary chondrites including the Dhajala chondrite for which Mössbauer data were acquired during the present study. The Didwana‐Rajod chondrite's iron phases and its oxidation states strongly suggest that it should be classified as an H‐type ordinary chondrite instead of the earlier suggestion (based on petrographic studies) that it could be an enstatite chondrite. The present study demonstrates that Mössbauer spectroscopy is a very powerful technique for aiding in the classification of meteorites.     

A coordinated spectral, mineralogical, and compositional study of ordinary chondrites

Mineral compositions and abundances derived from visible/near-infrared (VIS/NIR or VNIR) spectra are used to classify asteroids, identify meteorite parent bodies, and understand the structure of the asteroid belt. Using a suite of 48 equilibrated (types 4-6) ordinary (H, L, and LL) chondrites containing orthopyroxene, clinopyroxene, and olivine, new relationships between spectra and mineralogy have been established. Contrary to previous suggestions, no meaningful correlation is observed between band parameters and cpx/(opx + cpx) ratios. We derive new calibrations for determining mineral abundances (ol/(ol + px)) and mafic silicate compositions (Fa in olivine, Fs in pyroxene) from VIS/NIR spectra. These calibrations confirm that band area ratio (BAR) is controlled by mineral abundances, while Band I center is controlled by mafic silicate compositions. Spectrally-derived mineralogical parameters correctly classify H, L and LL chondrites in ∼80% of cases, suggesting that these are robust relationships that can be applied to S(IV) asteroids with ordinary chondrites mineralogies. Comparison of asteroids and meteorites using these new mineralogical parameters has the advantage that H, L and LL chemical groups were originally defined on the basis of mafic silicate compositions.     

Infrared transmission spectra from 2.5 to 25 μm of various meteorite classes

Infrared transmission spectra from 53 meteorites in the spectral range from 2.5 to 25 μm were measured to permit comparisons with data of astronomical objects that are potential meteorite sources. Data were taken for 14 carbonaceous chondrites, 5 LL ordinary chondrites, 6 L ordinary chondrites, 10 H ordinary chondrites, 1 enstatite chondrite, 4 aubrites, 3 eucrites, 4 howardites, 1 diogenite, 1 mesosiderite, 2 nakhlites, 1 shergottite, and the anomalous achondrite Angra dos Reis. The CO and CV carbonaceous chondrites have spectra similar to each other, with 10-μm features characteristic of olivine. The CM carbonaceous chondrites have distinctive 10-μm features that are attributed to layer lattice silicates. Members of both the CI and CR classes have spectra distinct from those of other carbonaceous chondrites. The LL, L, and H ordinary chondrites have spectra that match those of olivine and pyroxene mixtures. The enstatite chondrites and enstatite achondrites (aubrites) all exhibit spectra diagnostic of the pyroxene enstatite. The angrite, howardites, aucrites, nakhlites, shergottite, and diogenite all have similar spectra also dominated by pyroxene. The single mesosiderite examined had a spectrum distinct from all the other meteorites.     

Weathering of ordinary chondrites from the Atacama Desert,Chile, by Mössbauer spectroscopy and synchrotron radiation X‐ray diffraction

Some terrestrial areas have climatic and geomorphologic features that favor the preservation, and therefore, accumulation of meteorites. The Atacama Desert in Chile is among the most important of such areas, known as DCA. This desert is the driest on Earth, one of the most arid, uninhabitable localities with semiarid, arid, and hyper‐arid conditions. The meteorites studied here were collected from within the DCA of San Juan and Pampa de Mejillones, located, respectively, in the Central Depression and the Coastal Range of the Atacama Desert. ⁵⁷Fe Mössbauer spectroscopy was used for quantitative analysis of the degree of weathering of the meteorites, through the determination of the proportions of the various Fe‐bearing phases and in particular the amount of oxidized iron in terrestrial alteration products. The abundance of ferric ions in weathered chondrites can be related to specific precursor compositions and to the level of terrestrial weathering. The aim of the study was the identification, quantification, and differentiation of the weathering products in the ordinary chondrites found in the San Juan and the Pampa de Mejillones areas of the Atacama Desert. The ⁵⁷Fe Mössbauer spectroscopy study was complemented by synchrotron radiation X‐ray diffraction and magnetic susceptibility measurements. The results allow a clear differentiation of the rate of weathering in meteorite samples collected from the San Juan versus the Pampa de Mejillones areas of the Atacama Desert.     

The Meteoritical Bulletin,No. 80, 1996 July

Abstract— The Meteoritical Bulletin No. 80 lists data for 178 meteorites. Noteworthy are 3 HED meteorites (ALH 88102, Hammadah al Hamra (HaH) 059, and Monticello); 3 ureilites (HaH 064, HaH 126, and Dar al Gani (DaG) 084); 4 irons (Baygorria (IAB), Ste. Croix (IIIAB), Sargiin Gobi (IAB), and Tarahumara (IIE)); an unusual metal-rich meteorite (Vermillion); 8 carbonaceous chondrites (HaH 043 (C03), HaH 073 (C4), DaG 055 (C3) and 5 C03 chondrites (probably paired) from DaG); an R chondrite (DaG 013); and 6 unequilibrated ordinary chondrites (ALH 88105 (L3), Camel Donga 016 (L3), HaH 093 (LL3.9), HaH 096 (LL(L)3), Richfield (LL3.7), and Sarir Quattusah (LL(L)3)). Three recent falls of ordinary chondrites (Coleman (LL5), St. Robert (H5), and Tsukuba (H5-6)) are described.     

Meteorites from Mongolia

Abstract— The mineralogy and composition of six Mongolian meteorites were studied in some detail. Previously, only limited information existed about these rocks, and some were still unclassified. The six meteorites include three ordinary chondrites and three irons. The ordinary chondrite Adzhi-Bogdo (stone) is a regolith breccia (LL3–6) containing various types of clasts (some of foreign origin) embedded within a fine-grained clastic matrix. Tugalin Bulen (H6) and Noyan Bogdo (L6) meteorites are typical, well-metamorphosed ordinary chondrites. Adzhi-Bogdo (iron) has to be regarded as an IA iron meteorite like Campo del Cielo or Canyon Diablo; although the sample studied had been heated to about 900 °C–950 °C some time in the past, thus eradicating all original structural elements. Manlai is structurally closely related to the IIC iron meteorites; but based on its chemistry, which does not fit into this group, it is suggested that Manlai is an anomalous iron meteorite. The third iron, Sargiin Gobi, is certainly a normal member of the IA iron meteorites. The concentrations and isotopic compositions of He, Ne, and Ar were measured for all meteorites and their gas retention ages and exposure ages are discussed.     

Studies and characterizations of the Al Zarnkh meteorite

Abstract— A newly fallen Sudanese meteorite named Al Zarnkh was investigated using room and liquid nitrogen temperature Mössbauer measurements, X‐ray diffraction (XRD), and electron probe microanalysis (EPMA) in conjunction with energy dispersive X‐ray microscopy. The Mössbauer spectra exhibited strong paramagnetic doublets with magnetic sextets. The doublets are assigned to olivine and pyroxene, while the magnetic sextets are assigned to troilite and kamacite. Based on microprobe analyses and textural studies, olivine is the most abundant phase and occurs as fine to medium grained laths both in the groundmass and in barred olivine chondrules. Both orthopyroxenes and clinopyroxenes are present and these tend to be granular. Plagioclase is an abundant interstitial groundmass phase. Chromites were detected in some groundmass olivine and are highly chromiumand iron‐rich with no Fe³⁺ detected. The kamacite contains small amounts of Co. The mole fraction of the Fe end‐member of olivine (fayalite) and orthopyroxene (ferrosilite) are found to be about 28% and 23%, respectively. These values are compared with that obtained from two chondritic meteorites. Based on these results, the studied meteorite is classified as an ordinary LL5 chondrite.     

Glassy magnetic cronstedtite signatures in Mukundpura CM2 chondrite based on magnetic and Mössbauer studies

We have studied the Mukundpura CM2 meteorite for magnetic properties as a function of temperature and magnetic field, as well as its Mössbauer spectrum, at room and low temperatures (up to 5 K). We find that the high temperature paramagnetic phase is followed by two magnetic transitions: a weak transition near 125 K and a strong transition at 8 K. The weak (125 K) magnetic phase can be attributed to complex Fe²⁺–Fe³⁺ constituents present in the meteorite. The absence of the characteristic sextet corresponding to magnetite in Mossbauer spectrum indicates that this magnetic phase is not magnetite, which, if present, must be in insignificant amount. The 8 K magnetic ordering is superimposed with weak ferromagnetic ordering, showing spin‐glass transition. The Mössbauer spectrum taken at 5 K substantiates the observed spin‐glassy nature, as very large hyperfine field ~32 T is recorded, causing localized subordering leading to spin‐glass behavior. The Mössbauer spectra also confirm that iron is mainly present in serpentine‐group minerals, both in ferrous and ferric states. The complete serpentinization of basic silicates indicates aggressive hydrous alteration. These results show that the observed spin‐glass signature is a characteristic feature of the cronstedtite phase in CM meteorites. This feature is unique to carbonaceous CM chondrites and could be used for nondestructive, quick, and independent classification of this rare class of meteorites. Furthermore, the absence of olivine and the presence of cronstedtite in Mossbauer spectra show that the degree of aqueous alteration observed is the most severe in Mukundpura CM2 meteorite, as compared to many other CM2 meteorites. The degree of aqueous alteration in CM2 carbonaceous chondrites increases in the sequence: Paris, Murchison, Murray, Mighei, Nogoya, Cold Bokkeveld, and Mukundpura.     

The Distribution and Significance of Evaporitic Weathering Products on Antarctic Meteorites

Abstract— The distribution of white evaporitic deposits differs among different meteorite compositional groups and weathering categories of Antarctic meteorites. Evaporites occur with unusual frequency on carbonaceous chondrites, and are especially common in carbonaceous chondrites of weathering categories A and B. Among achondrites, weathering categories A and A/B show the most examples of evaporite weathering. Unlike carbonaceous chondrites and achondrites, most evaporite-bearing ordinary (H and L) chondrites are from rustier meteorites of weathering categories B and, to a lesser degree, B/C and C. LL chondrites are conspicuous by their complete lack of any evaporitic weathering product. Almost two-thirds of all evaporite-bearing meteorites belong to weathering categories A, A/B, and B. Where chemical data are available, surficial evaporite deposits are associated with elemental anomalies in meteorite interiors. Meteorites of weathering classes B, A/B, and even A may have experienced significant element redistribution and/or contamination as a result of terrestrial exposure. Evaporite formation during terrestrial weathering cannot be neglected in geochemical, cosmochemical, and mineralogical studies of Antarctic meteorites. A lower-case “e” should be added to the weathering classification of evaporite-bearing Antarctic meteorites, to inform meteorite scientists of the presence of evaporite deposits and their associated compositional effects.     

Density,magnetic susceptibility,and the characterization of ordinary chondrite falls and showers

Abstract— Bulk and grain densities of 132 ordinary chondrites from the Vatican Observatory collection were measured and compared with their magnetic susceptibility (for the most part using previously measured values; ten new susceptibility measures were taken for this study). Grain density and magnetic susceptibility combined provide a reliable method of classifying unweathered ordinary chondrites. Unlike traditional chemical tests, this method is fast, nondestructive, and characterizes the whole rock, making it especially appropriate for surveying large collections. The system is less viable for finds; extensive weathering of metallic iron in an H chondrite can cause it to plot among L chondrites, while heavily weathered L chondrites plot among the LL group. This system has revealed outlier stones that may be misclassified meteorites or mislabeled samples; in every case where the magnetic susceptibility of a meteorite does not fit its putative classification, the grain density is also found to be in disagreement in a manner consistent with either severe weathering or misidentification. An analysis of stones from five showers shows that, excluding outliers, these samples tend to cluster tightly within their appropriate groups in a plot of grain versus magnetic susceptibility.     

A Mössbauer spectroscopy and X‐ray diffraction study of ordinary chondrites: Quantification of modal mineralogy and implications for redox conditions during metamorphism

Abstract— We present a method that combines Mössbauer spectroscopy and X‐ray diffraction to quantify the modal mineralogy of unequilibrated ordinary chondrites (UOCs). Despite being a fundamental tool in the interpretation of geological systems, there are no modal mineralogical data available for these meteorites. This is due to their fine‐grained nature, highly heterogeneous silicate mineralogy, and the presence of poorly characterized phases. Consequently, it has not been possible to obtain accurate modal mineralogy by conventional techniques such as point counting. Here we use Mössbauer spectroscopy as a preliminary identification technique and X‐ray diffraction provides the quantification for a suite of recent UOC falls. We find the most primitive UOCs to contain a significant amount of phyllosilicate material that was converted during metamorphism to form ferromagnesian silicates. A complete suite of Antarctic samples is analyzed by each method to observe mineralogical trends and these are compared with trends shown by recent falls. The fact that mineralogical relationships shown by finds and falls are in agreement allows us to be confident that we are observing the products of pre‐terrestrial alteration. Mössbauer spectroscopy reveals evidence of steadily increasing reduction with metamorphism in the UOCs. Because this technique allows comparisons to be made between UOCs and EOCs, our reduction sequence can be combined with other evidence showing progressive oxidation in the EOCs. This yields an integrated model of changing redox conditions on equilibrating ordinary chondrite parent bodies.     

Characteristics of the Sahara as a meteorite recovery surface

We describe the geological, geomorphological, and paleoclimatic setting of the Sahara of North Africa in particular, focused on the main meteorite dense collection areas (DCA; Morocco, Algeria, Tunisia, and Libya). We report on the outcome of several meteorite recovery field expeditions in Morocco and Tunisia since 2008, by car and by foot, that applied systematic search methods. The number of meteorites collected is 41 ordinary chondrites and one brachinite. The statistics of unpaired ordinary chondrites indicates that H chondrites are more abundant (21) than L chondrites (12), while LL chondrites are rare (2). Our meteorite density estimates for Tunisia and Morocco are in the order of magnitude of 1 met km^?2. An estimate of the total maximum number of meteorites that could be recovered from the Sahara is 780,000 meteorites. We selected 23 meteorites from Aridal, Bou Kra, Bir Zar, and Tieret DCAs for ¹⁴C dating. The results show a wide range of terrestrial ages from 0.4 to more than 40 kyr with a majority of meteorites showing ages between 0.4 and 20 kyr. The weathering degree of these meteorites is ranges from minor (W1) to strong (W4). The highest weathering grades result from repeated oscillations between high and low humidity in the Sahara. However, there appears to be no correlation between weathering grade and terrestrial age of meteorites.     

Chemical analyses of meteorites at the Smithsonian Institution: An update

Abstract— Thirteen new meteorites and three meteorite inclusions have been analyzed. Their results have been incorporated into earlier published data for a comprehensive reference to all analyzed meteorites at the Smithsonian Institution. The six tables facilitate a convenient overlook of meteorite data. Table 1 presents an alphabetical list of analyzed meteorites, Table 2 chemical analyses of stony meteorites, Table 3 chemical analyses of iron meteorites, Table 4 elemental composition of stony meteorites, Table 5 average composition of carbonaceous chondrites and achondrites (falls and finds), and Table 6 presents average composition of H, L, LL, and Antarctic chondrites (falls and finds). The tables are available online at the journal's Web site http:meteoritics.org .     

Mössbauer study of Meridiani Planum,the first iron‐nickel meteorite found on the surface of Mars by the MER Opportunity

Abstract— –Meridiani Planum is the first iron meteorite found on Mars. It was discovered in 2005 by the Mars Exploration Rover Opportunity (MER‐B). Mössbauer spectra (MS) of the unbrushed and brushed meteorite species were acquired in 10 degrees temperature windows in the range of 210–260 K. Earlier examinations of these MS have led to the conclusion that the meteorite, which contains ～～7 wt% Ni, belongs to the IAB meteorite group. Here, making use of a recently developed calibration/folding procedure for MER MS, we report the results of the MS analyses for the single temperature windows m5 (210–220 K), m6 (220–230 K), m7 (230–240 K), and m89 (240–260 K). All spectra consist of a sextet and a ferric doublet. The hyperfine field of the sextet, extrapolated to room temperature, is ～～34.5 T, which is, based on Mössbauer studies of meteorites found on Earth, indeed consistent with the presence of kamacite. The fractional spectral area of the sextet is ～～0.96 of the total spectrum. The ferric doublet has an average quadrupole splitting of 0.70 mm/s and is not diagnostic of any specific Fe mineral.     

Anomalous Mössbauer parameters in the second generation regolith Ghubara meteorite

Abstract— We conducted Mössbauer spectroscopic studies on the Ghubara meteorite which had been described as at least two‐generation regolith breccia on the macro scale. The isomer shift and quadrupole splitting of the Fe‐Ni part are quite different from those obtained in ordinary chondrites, reflecting shock effects. We observed a large amount of magnetite that may have come from weathering of, primarily, the silicate fraction. We found very similar iron mineralogy in the Densmore meteorite.     

Spectral properties and mineral compositions of acapulcoite–lodranite clan meteorites: Establishing S‐type asteroid–meteorite connections

Except for asteroid sample return missions, measurements of the spectral properties of both meteorites and asteroids offer the best possibility of linking meteorite groups with their parent asteroid(s). Visible plus near‐infrared spectra reveal distinguishing absorption features controlled mainly by the Fe²⁺ contents and modal abundances of olivine and pyroxene. Meteorite samples provide relationships between spectra and mineralogy. These relationships are useful for estimating the olivine and pyroxene mineralogy of stony (S‐type) asteroid surfaces. Using a suite of 10 samples of the acapulcoite–lodranite clan (ALC), we have developed new correlations between spectral parameters and mafic mineral compositions for partially melted asteroids. A well‐defined relationship exists between Band II center and ferrosilite (Fs) content of orthopyroxene. Furthermore, because Fs in orthopyroxene and fayalite (Fa) content in olivine are well correlated in these meteorites, the derived Fs content can be used to estimate Fa of the coexisting olivine. We derive new equations for determining the mafic silicate compositions of partially melted S‐type asteroid parent bodies. Stony meteorite spectra have previously been used to delineate meteorite analog spectral zones in Band I versus band area ratio (BAR) parameter space for the establishment of asteroid–meteorite connections with S‐type asteroids. However, the spectral parameters of the partially melted ALC overlap with those of ordinary (H) chondrites in this parameter space. We find that Band I versus Band II center parameter space reveals a clear distinction between the ALC and the H chondrites. This work allows the distinction of S‐type asteroids as nebular (ordinary chondrites) or geologically processed (primitive achondrites).     

Identifying meteorite source regions through near-Earth object spectroscopy

By virtue of their landing on Earth, meteorites reside in near-Earth object (NEO) orbits prior to their arrival. Thus the population of observable NEOs, in principle, gives important representation of meteorite source bodies. By linking meteorites to NEOs, and linking NEOs to their most likely main-belt source locations, we seek to gain insight into the original Solar System formation locations for different meteorite classes. To forge possible links between meteorites and NEOs, we have developed a three dimensional method for quantitative comparisons between laboratory measurements of meteorites and telescopic measurements of near-Earth objects. We utilize meteorite spectra from the Reflectance Experiment Laboratory (RELAB) database and NEO data from the SpeX instrument on the NASA Infrared Telescope Facility (IRTF). Using the Modified Gaussian Model (MGM) as a mathematical tool, we treat asteroid and meteorite spectra identically in the calculation of 1-μm and 2-μm Geometric Band Centers and their Band Area Ratios (BARs). Using these identical numerical parameters we quantitatively compare the spectral properties of S-, Sq-, Q- and V-type NEOs with the spectral properties of the meteorites in four classes: H, L, LL and HED. For each NEO spectrum, we assign a set of probabilities for it being related to each of these four meteorite classes. Our NEO-meteorite correlation probabilities are then convolved with NEO-source region probabilities to yield a final set of meteorite-source region correlations. While the ν6 resonance dominates the delivery for all four meteorite classes, an excess (significant at the 2.1-sigma level) source region signature is found for the H chondrites through the 3:1 mean motion resonance. This results suggest an H chondrite source with a higher than average delivery preference through the 3:1 resonance. A 3:1 resonance H chondrite source region is consistent with the short cosmic ray exposure ages known for H chondrites.