MINERALOGY,PETROLOGY AND CHEMISTRY OF THE MESSINA (ITALY) CHONDRITE

The meteorite which fell near Messina, Italy, on 16 July 1955 is a typical olivine-hypersthene (L-group) chondrite. Its mineralogical composition is: olivine (Fa24), orthopyroxene (Fs20) with some polysynthetically twinned clynopyroxene, plagioclase (An10) and merrillite. Opaque phases present are: copper, kamacite, taenite, plessite, chalcopyrrhotite, mackinawite, troilite and chromite. The stone contains abundant chondrules. The matrix consists chiefly of broken chondrules with tiny fragments of crystals and rare amorphous material. Chondrules form more than 42% of the meteorite by volume. Some unusual features of the fabric of this meteorite include silicate grains showing deformation; silicates with fusion spots of dark glass containing blebs of metallic iron; iron and troilite with marginal fusion yielding globules and droplets sometimes showing flow structures. The classification of this chondrite is confirmed by bulk chemical analysis.     

Barred olivine chondrules in ordinary chondrites: Constraints on chondrule formation

In general, barred olivine (BO) chondrules formed from completely melted precursors. Among BO chondrules in unequilibrated ordinary chondrites, there are significant positive correlations among chondrule diameter, bar thickness, and rim thickness. In the nebula, smaller BO precursor droplets cooled faster than larger droplets (due to their higher surface area/volume ratios) and grew thinner bars and rims. There is a bimodal distribution in the olivine FeO content in BO chondrules, with a hiatus between 11 and 19 wt% FeO. The ratio of (FeO rich)/(FeO poor) BO chondrules decreases from 12.0 in H to 1.6 in L to 1.3 in LL. This is the opposite of the case for porphyritic chondrules: the mean (FeO rich)/(FeO poor) modal ratio increases from 0.8 in H to 1.8 in L to 2.8 in LL. During H chondrite agglomeration, most precursor dustballs were small with low bulk FeO/(FeO + MgO) ratios and moderately high melting temperatures. The energy available for chondrule melting from flash heating was relatively low, capable of completely melting many ferroan dusty precursors (to form FeO-rich BO chondrules), but incapable of completely melting many magnesian dusty precursors (to form FeO-poor BO chondrules). When L and LL chondrites agglomerated somewhat later, significant proportions of precursor dustballs were relatively large and had moderately high bulk FeO/(FeO + MgO) ratios. The energy available from flash heating was higher, capable of completely melting higher proportions of magnesian dusty precursors to form FeO-poor BO chondrules. These differences may have resulted from an increase in the amplitude of lightning discharges in the nebula caused by enhanced charge separation.     

Impact melting of the largest known enstatite meteorite: Al Haggounia 001, a fossil EL chondrite

Al Haggounia 001 and paired specimens (including Northwest Africa [NWA] 2828 and 7401) are part of a vesicular, incompletely melted, EL chondrite impact melt rock with a mass of ~3 metric tons. The meteorite exhibits numerous shock effects including (1) development of undulose to weak mosaic extinction in low‐Ca pyroxene; (2) dispersion of metal‐sulfide blebs within silicates causing “darkening”; (3) incomplete impact melting wherein some relict chondrules survived; (4) vaporization of troilite, resulting in S₂ bubbles that infused the melt; (5) formation of immiscible silicate and metal‐sulfide melts; (6) shock‐induced transportation of the metal‐sulfide melt to distances >10 cm; (7) partial resorption of relict chondrules and coarse silicate grains by the surrounding silicate melt; (8) crystallization of enstatite in the matrix and as overgrowths on relict silicate grains and relict chondrules; (9) crystallization of plagioclase from the melt; and (10) quenching of the vesicular silicate melt. The vesicular samples lost almost all of their metal during the shock event and were less susceptible to terrestrial weathering; in contrast, the samples in which the metal melt accumulated became severely weathered. Literature data indicate the meteorite fell ~23,000 yr ago; numerous secondary phases formed during weathering. Both impact melting and weathering altered the meteorite's bulk chemical composition: e.g., impact melting and loss of a metal‐sulfide melt from NWA 2828 is responsible for bulk depletions in common siderophile elements and in Mn (from alabandite); weathering of oldhamite caused depletions in many rare earth elements; the growth of secondary phases caused enrichments in alkalis, Ga, As, Se, and Au.     

Spinel-bearing,Al-rich chondrules in two chondrite finds from Roosevelt County,New Mexico: Indicators of nebular and parent body processes

Abstract— Two rare, spinel-bearing, Al-rich chondrules have been identified in new chondrite finds from Roosevelt County, New Mexico—RC 071 (L4) and RC 072 (L5). These chondrules have unusual mineralogies, dominated by highly and asymmetrically zoned, Al-Cr-rich spinels. Two alternatives exist to explain the origin of this zoning—fractional crystallization or metamorphism. It appears that fractional crystallization formed the zoning of the trivalent cations (Al, Cr) and caused a localized depletion in chromites around the large Al-Cr-rich spinels. The origin of the zoning of the divalent cations (Fe, Mg, Zn) is less certain. Diffusive exchange and partitioning of Fe and Mg between olivine and spinel during parent body metamorphism can explain the asymmetric zoning of these elements. Unfortunately, appropriate studies of natural and experimental systems to evaluate the formation of zoning of the divalent cations by fractional crystallization have not yet been conducted. The bulk compositions of the chondrules suggest affinities with the Na-Al-Cr-rich chondrules, as would be expected from the abundance of Al-Cr-rich spinels. Melting of rare and unusual precursors produced the compositions of Na-Al-Cr-rich chondrules, possibly including a spinel-rich precursor enriched in Cr₂O₃ and ZnO. The two chondrules we studied have larger modal abundances of Al-Cr-rich spinels than reported in other Na-Al-Cr-rich chondrules of similar composition, and Al-rich chondrules even more enriched in spinel are reported in the literature. These differences indicate that factors other than bulk composition control the mineralogy of the chondrules. The most important of these factors are the temperature to which the molten chondrule was heated and the cooling rate during crystallization. These two chondrules cooled rapidly from near the liquidus, as indicated by the zoning, occurrence and sizes of spinels, radiating chondrule textures and localized chromite depletions. The range of mineralogies in other Al-rich chondrules of similar composition reflect a range of peak temperatures and cooling rates. We see no reason to believe that this range is fundamentally different from the range of thermal histories experienced by “normal” Fe-Mg-rich chondrules.     

Microchondrules in two unequilibrated ordinary chondrites: Evidence for formation by splattering from chondrules during stochastic collisions in the solar nebula

The diversity of silicate, glassy spherules analogous to chondrules, called microchondrules, and the implications for their presence in unequilibrated ordinary chondrites (UOCs) were investigated using different electron microscope techniques. Our observations show that the abundance of microchondrules in UOCs is much larger than the values proposed by previous studies. We identified two different types of microchondrules, porous and nonporous, embedded within fine‐grained matrices and type I chondrule rims. The porous microchondrules are characterized by distinctive textures and chemical compositions that have not been recognized previously. Additionally, we show detailed textures and chemical compositions of protuberances of silicate materials, connected to the chondrules and ending with microchondrules. We suggest that microchondrules and protuberances formed from materials splattered from the chondrules during stochastic collisions when they were still either completely or partially molten. The occurrence and distinct morphologies of microchondrules and protuberances suggest that rather than just a passive flash melting of chondrules, an additional event perturbed the molten chondrules before they underwent cooling. The bulk chemical compositions suggest that (1) nonporous microchondrules and protuberances were formed by splattering of materials that are compositionally similar to the bulk silicate composition of type I chondrules, and (2) the porous microchondrules could represent the splattered melt products of a less evolved, fine‐grained dust composition. The preservation of protuberances and microchondrules in the rims suggests that the cooling and accretion rates were exceptionally fast and that they represent the last objects that were formed before the accretion of the parent bodies of OCs.     

Pecora Escarpment 91002: A member of the new Rumuruti (R) chondrite group

Abstract— Pecora Escarpment (PCA)91002 is a light/dark-structured chondrite breccia related to Carlisle Lakes and Rumuruti; the meteorite contains ～10–20 vol% equilibrated (type ?5 and ?6) clasts within a clastic groundmass, much of which was metamorphosed to type-3.8 levels. The olivine compositional distribution forms a tight cluster that peaks at Fa_38–40; by contrast, low-Ca pyroxene compositions are highly variable. Opaque phases identified in PCA91002 and its paired specimen, PCA91241, include pyrrhotite, pentlandite, pyrite, chromite, ilmenite, metallic Cu and magnetite. The majority of the rock is of shock stage S3-S4; there are numerous sulfide-rich shock veins and 50-μm plagioclase melt pockets. Instrumental neutron activation analysis shows that, unlike Carlisle Lakes and ALH85151, PCA91002 exhibits no Ca enrichment or Au depletion; because PCA91002 is relatively unweathered, it seems probable that the Ca and Au fractionations in Carlisle Lakes and ALH85151 were caused by terrestrial alteration. The Rumuruti-like (formerly Carlisle-Lakes-like) chondrites now include eight separate meteorites. Their geochemical and petrographic similarities suggest that they constitute a distinct chondrite group characterized by unfractionated refractory lithophile abundances (0.95 ± 0.05x CI), high bulk Δ¹⁷O, a low chondrule/groundmass modal abundance ratio, mean chondrule diameters in the 400 ± 100 μm range, abundant NiO-bearing ferroan olivine, sodic plagioclase, titanian chromite, abundant pyrrhotite and pentlandite and negligible metallic Fe-Ni. We propose that this group be called R chondrites after Rumuruti, the only fall. The abundant NiO-bearing ferroan olivine grains, the occurrence of Cu-bearing sulfide, and the paucity of metallic Fe-Ni indicate that R chondrites are highly oxidized. It is unlikely that appreciable oxidation took place on the parent body because of the essential lack of plausible oxidizing agents (e.g., magnetite or hydrated silicates). Therefore, oxidation of R chondrite material must have occurred in the nebula. A few type-I porphyritic olivine chondrules containing olivine grains with cores of Fa_3–4 composition occur in PCA91002; these chondrules probably formed initially as metallic-Fe-Ni-bearing objects at high nebular temperatures. As temperatures decreased and more metallic Fe was oxidized, these chondrules accreted small amounts of oxidized material and were remelted. The ferroan compositions of the >5-μm olivine grains in the R chondrites reflect equilibration with fine-grained FeO-rich matrix material during parent body metatnorphism.     

Spade: An H chondrite impact‐melt breccia that experienced post‐shock annealing

Abstract— The low modal abundances of relict chondrules (1.8 vol%) and of coarse (i.e., ≥200 μm‐size) isolated mafic silicate grains (1.8 vol%) in Spade relative to mean H6 chondrites (11.4 and 9.8 vol%, respectively) show Spade to be a rock that has experienced a significant degree of melting. Various petrographic features (e.g., chromite‐plagioclase assemblages, chromite veinlets, silicate darkening) indicate that melting was caused by shock. Plagioclase was melted during the shock event and flowed so that it partially to completely surrounded nearby mafic silicate grains. During crystallization, plagioclase developed igneous zoning. Low‐Ca pyroxene that crystallized from the melt (or equilibrated with the melt at high temperatures) acquired relatively high amounts of CaO. Metallic Fe‐Ni cooled rapidly below the Fe‐Ni solvus and transformed into martensite. Subsequent reheating of the rock caused transformation of martensite into abundant duplex plessite. Ambiguities exist in the shock stage assignment of Spade. The extensive silicate darkening, the occurrence of chromite‐plagioclase assemblages, and the impact‐melted characteristics of Spade are consistent with shock stage S6. Low shock (stage S2) is indicated by the undulose extinction and lack of planar fractures in olivine. This suggests that Spade reached a maximum prior shock level equivalent to stage S6 and then experienced post‐shock annealing (probably to stage S1). These events were followed by a less intense impact that produced the undulose extinction in the olivine, characteristic of shock stage S2. Annealing could have occurred if Spade were emplaced near impact melts beneath the crater floor or deposited in close proximity to hot debris within an ejecta blanket. Spade firmly establishes the case for post‐shock annealing. This may have been a common process on OC asteroids.     

Metamorphism and aqueous alteration in low petrographic type ordinary chondrites

Abstract— In order to investigate the relative importance of dry metamorphism and aqueous alteration in the history of chondrules, chondrules were hand-picked from the Semarkona (petrographic type 3.0), Bishunpur (3.1), Chainpur (3.4), Dhajala (3.8) and Allegan (5) chondrites, and matrix samples were extracted from the first three ordinary chondrites. The thermoluminescence (TL) properties of all the samples were measured, and appropriate subsets of the samples were analyzed by electron-microprobe and radiochemical neutron activation and the water and H-isotopic composition determined. The TL data for chondrules from Semarkona and Bishunpur scatter widely showing no unambiguous trends, although group B1 chondrules tend to have lower sensitivities and lower peak temperatures compared with group A5 chondrules. It is argued that these data reflect the variety of processes accompanying chondrule formation. The chondrules show remarkably uniform contents of the highly labile elements, indicating mineralogical control on abundance and volatile loss from silicates and loss and recondensation of mobile chalcophiles and siderophiles in some cases. Very high D/H values (up to ～8000‰ SMOW) are observed in certain Semarkona chondrules, a confirmation of earlier work. With increasing petrographic type, mean TL sensitivities of the chondrules increase, the spread of values within an individual meteorite decreases, and peak temperatures and peak widths show trends indicating that the TL is mainly produced by feldspar and that dry, thermal metamorphism is the dominant secondary process experienced by the chondrules. The TL sensitivities of matrix samples also increase with petrographic type. Chainpur matrix samples show the same spread of peak temperatures and peak widths as Chainpur chondrules, indicating metamorphism-related changes in the feldspar are responsible for the TL of the matrix. The TL data for the Semarkona and Bishunpur matrix samples provide, at best, only weak evidence for aqueous alteration, but the matrix contains H with approximately terrestrial D/H values, even though it contains much water. Secondary processes (probably aqueous alteration) presumably lowered the D/H of the matrix and certain chondrules. While chondrule properties appear to be governed primarily by formation processes and subsequent metamorphism, the matrix of Semarkona has a more complex history involving aqueous alteration as a meteorite-wide process.     

Mineralogy,petrography, and thermometry of the H5 chondrite Carcote,Chile

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 (Fa_16.5–20), orthopyroxene (Fs_14–17.5), diopsidic clinopyroxene (FS_6–7), plagioclase (An_15–20). 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.     

Shape,metal abundance,chemistry, and origin of chondrules in the Renazzo (CR) chondrite

Abstract— We used synchrotron X‐ray microtomography to image in 3‐dimensions (3D) eight whole chondrules in a ?1 cm³piece of the Renazzo (CR) chondrite at ?17 μm per volume element (voxel) edge. We report the first volumetric (3D) measurement of metal/silicate ratios in chondrules and quantify indices of chondrule sphericity. Volumetric metal abundances in whole chondrules range from 1 to 37 volume % in 8 measured chondrules and by inspection in tomography data. We show that metal abundances and metal grain locations in individual chondrules cannot be reliably obtained from single random 2D sections. Samples were physically cut to intersect representative chondrules multiple times and to verify 3D data. Detailed 2D chemical analysis combined with 3D data yield highly variable whole‐chondrule Mg/Si ratios with a supra‐chondritic mean value, yet the chemically diverse, independently formed chondrules are mutually complementary in preserving chondritic (solar) Fe/Si ratios in the aggregate CR chondrite. These results are consistent with localized chondrule formation and rapid accretion resulting in chondrule + matrix aggregates (meteorite parent bodies) that preserve the bulk chondritic composition of source regions.     

ELEMENTAL COMPOSITIONS OF MAJOR SILICIC PHASES IN CHONDRULES OF UNEQUILIBRATED CHONDRITIC METEORITES

Elemental compositions of olivine, low-Ca pyroxene and mesostasis in chondrules from type-3 ordinary chondrites (OC), CV3, CO3, CM2 and EH3 chondrites were compiled in a search for mineral compositional differences among chondrules of different chondrite groups. Such differences are demonstrated. A few elements occur in silicic phases in amounts proportional to their bulk chondrule concentrations: e.g., Mn in OC chondrules, Ti in CV chondrules, Cr in EH chondrules. However, OC chondrules have higher bulk Cr than CM-CO chondrules, higher Cr in mesostasis, but lower Cr in olivine and low-Ca pyroxene. The higher oxidation state of OC chondrules implies that Cr is more likely to be trivalent, and thus, less likely to enter the olivine crystal structure and more likely to concentrate in pyroxene and mesostasis. CV and OC chondrules have similar high bulk Fe and mesostasis Fe, but OC chondrules have much more FeO in olivine and low-Ca pyroxene. The remaining Fe in CV chondrules is reduced and occurs as metal blebs in the mesostasis. Relative to OC chondrules, EH chondrules have lower bulk Ca, lower Ca in pyroxene and mesostasis, but higher (by a factor of 2) Ca in olivine. EH chondrules may have been incompletely melted, preserving relict refractory lithophile-rich olivine nuclei. OC chondrules are richer than EH chondrules in FeO; they have a lower melting temperature and may have been more completely melted during chondrule formation.     

Evidence for fractional condensation and reprocessing at high temperatures in CH chondrites

Abstract— We performed a detailed study of silica‐rich components (SRC) in the paired CH chondrites Acfer 182 and 207. These SRCs appear either as chondrules or fragments, and they contribute <0.1 vol% to the bulk meteorite. They usually contain a silica and a silicate portion. Both portions are, in most cases, cryptocrystalline and have bulk SiO₂‐concentrations between 65 and 85 wt%. The silicate generally has a pyroxene normative composition. The silica often appears as blebs within the silicate matrix or vice versa. If there are no blebs, silica and silicate still form rounded interfaces. The SRCs are depleted in refractory elements like Ca, Al, and Ti relative to CI. A few SRC‐like objects are extremely rich in Mn and show no depletion in refractory elements. We conducted micro‐Raman studies on the silica portions of the SRCs to determine their structure, and we identified several silica phases: α‐quartz, cristobalite, glass, and a yet unidentified polymorph. The silicate portion is glass when the silica is glass and crystalline when the silica is crystalline. The low contents of Al and Ca make an igneous origin of the SRCs very unlikely, and the absence of metal excludes the formation by reduction of pyroxene. We suggest, instead, a fractional condensation origin of the SRCs from a Si‐enriched gas after removal of gaseous Mg by forsterite condensation. Additional evidence for fractional condensation is provided by a unique layered object with olivine in the core, pyroxene and metal at the rim, and silica at the outermost border; these layers record the condensation sequence. Two chondrules were found with several percent of Mn and high Cr, Na, and K contents, providing further evidence for condensation from a fractionated gas. The texture of the SRCs and the occurrence of cristobalite and silica glass, however, require formation by liquid immiscibility at high temperatures, above 1968 K, and subsequent fast cooling. Therefore, we propose a 2‐stage model for the formation of SRCs in CH chondrites: 1) fractional condensation of forsterite, enstatite, and SiO₂‐rich phases; and 2) reheating of SiO₂‐rich components to temperatures above 1968 K followed by rapid cooling. All other phases identified in CH chondrites can be understood within the framework of this model. Thus, the extremely unequilibrated CH chondrites provide a wealth of evidence for fractional condensation processes in the early solar nebula, in metals (Meibom et al. 1999), and in silicates.     

In situ determination of iodine content and iodine-xenon systematics in silicates and troilite phases in chondrules from the LL3 chondrite Semarkona

Abstract— Iodine concentrations in small domains (～10 μm) of silicates and troilite (FeS) phases in three chondrules from the Semarkona (LL3) meteorite were determined by an ion microprobe. Independent determination of I content in some of these phases was accomplished by in situ laser probe mass spectrometric analysis of I-derived ¹²⁸Xe in one of these neutron-irradiated chondrules. The ion microprobe data suggest low I content for olivines (20–45 ppb) and relatively higher values for pyroxene and glass (mesostasis) (40–160 ppb). The broad similarity in the measured I contents in pyroxenes in a porphyritic pyroxene chondrule by ion microprobe (42–138 ppb) and by laser probe (37–76 ppb) demonstrate the feasibility of in situ determination of I content in silicate phases via ion microprobe. The I contents in troilite measured by ion microprobe, however, are prone to uncertainty because of the lack of a sulfide standard. The ion microprobe data suggest I content of > 1 ppm in troilite, if the calibration from our silicate standard is used. However, the noble gas data suggest that the I content in troilite is comparable to that in silicates. We attribute this apparent discrepancy to an enhanced sputter ion yield of I from sulfides. Iodine-derived ¹²⁹Xe excesses were observed in both pyroxene and troilite within this chondrule. The I-Xe model ages of these selected phases are consistent with the I-Xe studies of the bulk chondrule. The individual data points fall on or near the isochron obtained from the bulk chondrule, although all except the most radiogenic data point contain evidence of low-temperature uncorrelated iodine.     

EH3 matrix mineralogy with major and trace element composition compared to chondrules

We investigated the matrix mineralogy in primitive EH3 chondrites Sahara 97072, ALH 84170, and LAR 06252 with transmission electron microscopy; measured the trace and major element compositions of Sahara 97072 matrix and ferromagnesian chondrules with laser‐ablation, inductively coupled, plasma mass spectrometry (LA‐ICPMS); and analyzed the bulk composition of Sahara 97072 with LA‐ICPMS, solution ICPMS, and inductively coupled plasma atomic emission spectroscopy. The fine‐grained matrix of EH3 chondrites is unlike that in other chondrite groups, consisting primarily of enstatite, cristobalite, troilite, and kamacite with a notable absence of olivine. Matrix and pyroxene‐rich chondrule compositions differ from one another and are distinct from the bulk meteorite. Refractory lithophile elements are enriched by a factor of 1.5–3 in chondrules relative to matrix, whereas the matrix is enriched in moderately volatile elements. The compositional relation between the chondrules and matrix is reminiscent of the difference between EH3 pyroxene‐rich chondrules and EH3 Si‐rich, highly sulfidized chondrules. Similar refractory element ratios between the matrix and the pyroxene‐rich chondrules suggest the fine‐grained material primarily consists of the shattered, sulfidized remains of the formerly pyroxene‐rich chondrules with the minor addition of metal clasts. The matrix, chondrule, and metal‐sulfide nodule compositions are probably complementary, suggesting all the components of the EH3 chondrites came from the same nebular reservoir.     

Re‐examining the role of chondrules in producing the elemental fractionations in chondrites

Abstract— The matrices of all primitive chondrites contain presolar materials (circumstellar grains and interstellar organics) in roughly CI abundances, suggesting that all chondrites accreted matrix that is dominated by a CI‐like component. The matrix‐normalized abundances of the more volatile elements (condensation temperatures <750–800 K) in carbonaceous and ordinary chondrites are also at or slightly above CI levels. The modest excesses may be due to low levels of these elements in chondrules and associated metal. Subtraction of a CI‐like matrix component from a bulk ordinary chondrite composition closely matches the average composition of chondrules determined by instrumental neutron activation analysis (INAA) if some Fe‐metal is added to the chondrule composition. Measured matrix compositions are not CI‐like. Sampling bias and secondary redistribution of elements may have played a role, but the best explanation is that ?10–30% of refractory‐rich, volatile depleted material was added to matrix. If most of the more volatile elements are in a CI‐dominated matrix, the major and volatile element fractionations must be largely carried by chondrules. There is both direct and indirect evidence for evaporation during chondrule formation. Type IIA and type B chondrules could have formed from a mixture of CI material and material evaporated from type IA chondrules. The Mg‐Si‐Fe fractionations in the ordinary chondrites can be reproduced with the loss of type IA chondrule material and associated metal. The loss of evaporated material from the chondrules could explain the volatile element fractionations. Mechanisms for how these fractionations occurred are necessarily speculative, but two possibilities are briefly explored.     

A nondestructive analytical method for stone meteorites—and a controversial discrepancy

Abstract— A method is described for whole rock analyses of major elements in stone meteorites using the electron microprobe and requiring only powdering of the sample, most of which can be retrieved after analysis for additional analytical studies, such as instrument neutron activation analysis (INAA), radiochemical neutron activation analysis (RNAA) and O-isotopic analysis. Whole individual chondrules of > 1 mg can be analyzed. The method is especially attractive for meteorites in short supply or of great rarity. A total of 398 meteorites were analyzed by this method. The results compare favorably with wet chemical analyses. A study was made of seventeen ordinary chondrites to compare their whole rock (metal free) compositions with the averaged compositions of eleven to thirty-eight of their respective individual chondrules (a total of 374 chondrules). The oxide ratio Al₂O₃/CaO is generally lower in chondrules than in their respective chondrites, the disparity being larger for petrographic grade 5 than for grade 3. Ordinary chondrites are not simply the sum of their respective chondrules. Furthermore, correlations between CaO, Al₂O₃ and TiO₂ are strong for chondrules in unequilibrated chondrites and nonexistent in equilibrated chondrites. Also H, L and LL chondrite groups have similar bulk compositions within their respective groups, in spite of the different proportions of chondrules, kinds of chondrules, chondrule debris, and matrix. All this brings into question the metamorphic classification in which high petrographic grades are the metamorphosed equivalents of low petrographic grades.     

Quantifying the error of 2D bulk chondrule analyses using a computer model to simulate chondrules (SIMCHON)

Bulk chondrule compositions are important to many questions in cosmochemistry, however, the number of available bulk chondrule data sets is still small. A main reason for this is the difficulties of determining bulk chondrule compositions. A commonly used technique is to obtain 2D bulk chondrule compositions from meteorite sections. This technique has an error that we quantify here for the first time using a mathematical model of a chondrule called SIMCHON. The theoretically calculated errors are compared to errors that we determined from serial sectioning of eight Efremovka chondrules. The errors obtained from both approaches are in excellent agreement, proving that our mathematical model produces reliable errors that can be assigned to 2D bulk chondrule compositions. These errors allow a much better interpretation of 2D bulk chondrule data. We provide a table that contains typical errors for 2D bulk compositions of porphyritic chondrules. The errors are in the range of ±<1–30 relative‐%. This should be acceptable for many problems in cosmochemistry. The effect of a chemical layering inside chondrules and the occurrence of a rim around them, as well as the occurrence of opaque and other accessory phases have been studied. A spreadsheet is provided that enables the calculation of errors for any desired chondrule mineral composition. BO chondrules have a negligible error, but it is impossible to provide reasonable error estimates for BO chondrules with an igneous rim. Radial pyroxene chondrules have negligible errors.     

The Fukang pallasite: Characterization and implications for the history of the Main‐group parent body

We report the results of a study of the Fukang pallasite that includes measurements of bulk composition, mineral chemistry, mineral structure, and petrology. Fukang is a Main‐group pallasite that consists of semiangular olivine grains (Fo 86.3) embedded in an Fe‐Ni matrix with 9–10 wt% Ni and low‐Ir (45 ppb). Olivine grains sometimes occur in large clusters up to 11 cm across. The Fe‐Ni phase is primarily kamacite with accessory taenite and plessite. Minor phases include schreibersite, chromite, merrillite, troilite, and low‐Ca pyroxene. We describe a variety of silicate inclusions enclosed in olivine that contain phases rarely or not previously reported in Main‐group pallasites, including clinopyroxene (augite), tridymite, K‐rich felsic glass, and an unknown Ca‐Cr silicate. Pressure constraints determined from tridymite (<0.4 GPa), two‐pyroxene barometry (0.39 ± 0.07 GPa), and geophysical calculations that assume pallasite formation at the core–mantle boundary (CMB), provide an upper estimate on the size of the Main‐group parent body from which Fukang originated. We conclude that Fukang originated at the CMB of a large differentiated planetesimal 400–680 km in radius.     

Silica-rich orthopyroxenite in the Bovedy chondrite

Abstract— A large (≥4.5 × 7 × 4 mm), igneous-textured clast in the Bovedy (L3) chondrite is notable for its high bulk SiO₂ content (57.5 wt%). The clast consists of normally zoned orthopyroxene (83.8 vol%), tridymite (6.2 %), an intergrowth of feldspar (5.8 %) and sodic glass (3.1 %), pigeonite (1.0 %), and small amounts of chromite (0.2 %), augite, and Fe, Ni-metal; it is best described as a silica-rich orthopyroxenite. The oxygen-isotopic composition of the clast is similar, but not identical, to Bovedy and other ordinary chondrites. The clast has a superchondritic Si/Mg ratio, but has Mg/(Mg + Fe) and Fe/Mn ratios that are similar to ordinary chondrite silicate. The closest chemical analogues to the clast are radial-pyroxene chondrules, diogenites, pyroxene-silica objects in ordinary chondrites, and silicates in the IIE iron meteorite Weekeroo Station. The clast crystallized from a siliceous melt that cooled fast enough to prevent complete attainment of equilibrium but slow enough to allow nearly complete crystallization. The texture, form, size and composition of the clast suggest that it is an igneous differentiate from an asteroid or planetesimal that formed in the vicinity of ordinary chondrites. The melt probably cooled in the near-surface region of the parent object. It appears that in the source region of the clast, metallic and silicate partial melt were largely-to-completely lost during a relatively low degree of melting, and that during a higher degree of melting, olivine and low-Ca pyroxene separated from the remaining liquid, which ultimately solidified to form the clast. While these fractionation steps could not have all occurred at the same temperature, they could have been accomplished in a single melting episode, possibly as a result of heating by radionuclides or by electromagnetic induction. Fractionated magmas can also account for other Si-rich objects in chondrites.     

Lavras do Sul: A New Equilibrated Ordinary L5 Chondrite from Rio Grande do Sul,Brazil

The new Brazilian chondrite, Lavras do Sul, was found in 1985 at Lavras do Sul, Rio Grande do Sul State-Brazil (33°30′48″S; 53°54′65″W). It consists of a single mass weighing about 1 kg, covered by a black fusion crust with grayish interior. Four polished thin sections were prepared from a slice weighing 67 g on deposit at the Museu Nacional/UFRJ. It consists mostly of chondrules and chondrule fragments dispersed in a recrystallized matrix. Most chondrules are poorly defined and range in size from 300 to 2,000 μm, although some of them show distinct outlines, particularly when viewed under cross-polarized transmitted and reflected light. The texture of chondrules varies from non-porphyritic (e.g., barred-olivine, radial-pyroxene) to porphyritic ones (e.g., granular olivine as well as olivine-pyroxene). The meteorite contains mainly olivine (Fa_24.9), low-Ca pyroxene (Fs_22.6) and metal phases, with minor amounts of plagioclase, chromite and magnetite. Mössbauer Spectroscopy studies indicate that the metal phase is kamacite, tetrataenite and antitaenite. Veins of secondary iddingsite crosscut the thin section and some ferromagnesian silicates. The chemical composition indicates that Lavras do Sul is a member of the low iron L chondrite group. The poorly delineated chondritic texture with few well-defined chondrules, the occurrence of rare clinopyroxene and plagioclase (and maskelynite) with apparent diameters ranging from 5 to 123 μm led us to classify Lavras do Sul as an equilibrated petrologic type 5. The shock features of some minerals suggest a shock stage S3, and the presence of a small amount of secondary minerals such as iddingsite and goethite, a degree of weathering W₁. The meteorite name was approved by the Nomenclature Committee (Nom Com) of the Meteoritical Society (Meteoritic Bulletin Nº99).