Postshock annealing and postannealing shock in equilibrated ordinary chondrites: implications for the thermal and shock histories of chondritic asteroids

In addition to shock effects in olivine, plagioclase, orthopyroxene and Ca-pyroxene, petrographic shock indicators in equilibrated ordinary chondrites (OC) include chromite veinlets, chromite-plagioclase assemblages, polycrystalline troilite, metallic Cu, irregularly shaped troilite grains within metallic Fe-Ni, rapidly solidified metal-sulfide intergrowths, martensite and various types of plessite, metal-sulfide veins, large metal and/or sulfide nodules, silicate melt veins, silicate darkening, low-Ca clinopyroxene, silicate melt pockets, and large regions of silicate melt. The presence of some of these indicators in every petrologic type-4 to -6 ordinary chondrite demonstrates that collisional events caused all equilibrated OC to reach shock stages S3-S6. Those type-4 to -6 OC that are classified as shock-stage S1 (on the basis of sharp optical extinction in olivine) underwent postshock annealing due to burial beneath materials heated by the impact event. Those type-4 to -6 OC that are classified S2 (on the basis of undulose extinction and lack of planar fractures in olivine) were shocked to stage S3-S6, annealed to stage S1 and then shocked again to stage S2. Some OC were probably shocked to stage ≥ S3 after annealing. It seems likely that many OC experienced multiple episodes of shock and annealing.Because ⁴⁰Ar-³⁹Ar chronological data indicate that MIL 99301 (LL6, S1) was annealed ∼ 4.26 Ga ago, presumably as a consequence of a major impact, it seems reasonable to suggest that other equilibrated S1 and S2 OC (which contain relict shock features) were also annealed by impacts. Because some type-6 S1 OC (e.g., Guareña, Kernouvé, Portales Valley, all of which contain relict shock features) were annealed 4.44- 4.45 Ga ago (during a period when impacts were prevalent and most OC were thermally metamorphosed), it follows that impact-induced annealing could have contributed significantly to OC thermal metamorphism.     

The metallic microstructures and thermal histories of severely reheated chondrites

The metallographic structures of eight severely reheated chondrites (Farmington, Ramsdorf, Orvinio, Wickenburg, Lubbock, Rose City, Arapahoe and Tadjera) have been studied by optical, scanning electron microscope and electron microprobe techniques. Unreheated chondrites and experimentally heat treated chondritic material have also been examined.The following metallographic characteristics can be used to estimate the post-shock residual temperature of reheated chondrites: melted appearance of metal-troilite, presence of martensite, P enrichment of the metal, and the averaging of central metal grain compositions. Metallographic characteristics used to estimate the cooling rates of the severely reheated chondrites are the Ni content of troilite, the Ni gradients in metal grain rims, and the presence of secondary kamacite and phosphides. Farmington, Ramsdorf, Orvinio, Lubbock, Rose City and several of the heat treatment specimens have substantial P in solution in the metal grains ( > 0.1 wt%). P enrichment is apparently caused by reduction of phosphates upon severe reheating and partial melting of metal-troilite areas in chondritic meteorites.The eight severely reheated chondrites studied showed evidence of reheating to temperatures ranging from ～ 950°C to ～ 1250°C. Ramsdorf has the highest reheating temperature (1200–1250°C) and the fastest cooling rate ～ 100°C/day. Wickenberg has the lowest reheating temperature (950–1000°C) and the slowest cooling rate, ～ 1°C/100yrs. Cooling rate estimates correspond to post-reheating burial depths of less than 1 to ～ 1000m.     

Equilibration temperatures of the ordinary chondrites: A new evaluation

Kretz (1982) and Lindsley (1983) have each calibrated the Ca-pyroxene thermometer. These new calibrations are applied to 24 analyzed pyroxenes from 24 H6, L6 and LL6 chondrites. Both thermometers agree that the H group equilibrated to a significantly lower temperature, 820°–830°C, than the L and LL groups. The two thermometers disagree on the value of the higher temperature of equilibration of the L and LL groups, giving values of 860°C (Lindsley) to 930°C (Kretz).     

The metal phase in unequilibrated ordinary chondrites and its implications for calculated accretion temperatures

The abundance, composition and grain size of the metal available to volatile siderophile elements strongly affect the condensation of these elements. These parameters are redefined on the basis of published chemical analyses and new mechanical analyses of the unequilibrated ordinary chondrites. The results suggest that previous workers have seriously overestimated the amount of metal present and available during condensation, and seriously underestimated the heat of solution of Bi in chondritic metal. Correction of these parameters results in nominal accretion temperatures for Bi which are substantially (95–110°K) lower than those calculated earlier, and which are discordant with the temperatures inferred for chalcophile trace elements.     

Composition of the metal phases in ordinary chondrites: implications regarding classification and metamorphism

EMP determinations of Fe, Co and Ni in the metal phases of ordinary chondrites confirm the report of Sears and Axon that kamacite Co contents show restricted, nonoverlapping ranges in the three groups; ranges are 3.3–4.8 mg/g in H, 6.7–8.2 mg/g in L and 15–110 mg/g in LL. Experimental data by Widge and Goldstein show that the Ni concentration of the $\text{α}\text{(α + γ)}$ boundary increases with increasing Co concentration: unexpectedly, we find lower kamacite Ni concentrations in unequilibrated LL chondrites (44–55 mg/g) than in H and L chondrites (57–69 mg/g). We infer that, at temperatures below 550° C increasing Co causes a decrease in the equilibrium kamacite Ni concentration of an α-γ system. Although some evidence indicates that the equilibrated L chondrites Barratta, Knyahinya and Shaw have siderophile concentrations lower than the normal L-group range, they have kamacite and taenite Co concentrations in the L-group range.Metal-phase studies of petrologic type-3 ordinary chondrites having highly unequilibrated silicates showed a wide range in the degree of matrix kamacite equilibration ranging from nearly equilibrated in Mezö-Madaras to highly unequilibrated in Bishunpur, Ngawi and Semarkona. Kamacite in chondrule interiors is highly unequilibrated in all 9 chondrites, and in each setting taenite data are consistent with the expectation that it should be less equilibrated than kamacite. Our kamacite Co data confirm that Sharps is H and Hallingeberg. Khohar and Mezö-Madaras are L chondrites. Chainpur and Parnallee have kamacite Co concentrations between the L and LL ranges: we present evidence indicating that they are truly intermediate, i.e. neither L nor LL. Highly unequilibrated Ngawi is either LL or, less likely, still more oxidized. Bishunpur and Semarkona have mean kamacite Co concentrations in the H range but too unequilibrated to be used for classification. The highly heterogeneous compositions of the metal in Bishunpur, Ngawi and Semarkona indicate that their metal partially preserves properties established during nebular processes. Most of the taenite in these chondrites has high Ni contents (>470 mg/g) and is essentially unzoned; much of the kamacite is polycrystalline with crystals ?5μm across. Metamorphism causes tiny grains to disappear, increases the grain size of both kamacite and taenite, tends to equilibrate metallic minerals and, during cooling, can produce zoned taenite.A petrologic type-5 clast in the Ngawi LL3 chondrite has 3 coexisting metal phases, clear taenite (540 mg/g Ni, 21 mg/g Co), kamacite (30 mg/g Ni, 120 mg/g Co) and a phase tentatively identified as ordered FeCo (8.5 mg/g Ni, 370 mg/g Co).     

Classification of five new ordinary chondrites from North West Africa

We classified five new ordinary chondrites from North West Africa. NWA 3010 is an L6(S5), NWA 3011 is an L5(S5), NWA 3012 is an LL4(S5), NWA 3013 is an L5(S5), and NWA 3014 is an H4(S1). The meteorites experienced a range of terrestrial alteration, with NWA 3010 equal to weathering grade W2, NWA 3011 equal to W3, NWA 3012 equal to W3, NWA 3013 equal to W2, and NWA 3014 equal to W4.     

The thermal history of equilibrated ordinary chondrites and the relationship between textural maturity and temperature

The compositions and textures of phases in eleven equilibrated ordinary chondrites from the H, L, and LL groups spanning petrographic types 4-6 were studied and used to constrain the thermal histories of their parent bodies. Based on Fe-Mg exchange between olivine and spinel, average equilibration temperatures for type 4-6 chondrites encompass a small range, 586-777 °C, relative to what is commonly assumed for peak temperatures (600-950 °C). The maximum temperatures recorded by individual chondrites, which are minima relative to peak metamorphic temperatures, increase subtly but systematically with metamorphic type and are tightly clustered for H4-6 (733-754 °C) and LL4-6 (670-777 °C). For the Ls, Ausson (L5) records a higher maximum olivine-spinel temperature (761 °C) than does the L4 chondrite Saratov (673 °C) or the L6 chondrite Glatton (712 °C). Our data combined with olivine-spinel equilibration temperatures calculated for other equilibrated ordinary chondrites using mineral compositions from the literature demonstrate that, in general, type 4 chondrites within each chemical group record temperatures lower than or equal to those of types 5-6 chondrites.For H chondrites, the olivine-spinel closure temperature is a function of spinel grain size, such that larger grains, abundant in types 5-6 chondrites, record temperatures of ∼740 °C or more while smaller grains, rare in types 5-6 but abundant in type 4 chondrites, record lower temperatures. Olivine-spinel temperatures in the type 6 chondrites Guareña and Glatton are consistent with rapid (50-100 °C/Myr) cooling from high temperatures in the ordinary chondrite parent bodies. With one exception (∼500 °C/Myr), olivine-spinel data for St.-Séverin (LL6) are consistent with similar cooling rates. Cooling rates of order 100 °C/Myr at ∼750 °C for type 6 chondrites are considerably higher than previously determined cooling rates for lower temperatures (?550 °C) based on metallography, fission tracks, and geochronology. For H chondrites, current thermal models of an “onion shell” parent body are inconsistent with a small range of peak temperatures based on olivine-spinel and two pyroxene thermometry combined with a wide dispersion of cooling rates at low temperatures. Equilibrated chondrites may have sampled regions near a major transition in physical properties such as near the base of a regolith pile.     

The matrices of unequilibrated ordinary chondrites: Implications for the origin and history of chondrites

The matrices of sixteen unequilibrated ordinary chondrites (all witnessed falls) were studied microscopically in transmitted and reflected light and analyzed by electron microprobe. Selected specimens were also studied by scanning electron microscopy. These studies indicate that the fine-grained, opaque, silicate matrix of type 3 unequilibrated chondrites is compositionally, mineralogically and texturally distinct from the chondrules and chondrule fragments and may be the low temperature condensate proposed by Larimer and Anders (1967, 1970). Examination of the matrices of unequilibrated chondrites also shows that each meteorite has been metamorphosed, with the alteration ranging in intensity from quite mild, where the matrix has been only slightly altered, to a more severe metamorphism that has completely recrystallized the opaque matrix. Most of the metamorphic changes in the matrix occurred without significant effects on the compositions or textures of the chondrules. The metamorphic alteration probably resulted from a combination of processes including thermal metamorphism and the passage of shock waves. The present appearance of each unequilibrated chondrite is a result of the particular temperature and pressure conditions under which it and its components formed, plus the subsequent metamorphic alteration it experienced.     

Compositions of three low-FeO ordinary chondrites: Indications of a common origin with the H chondrites

Burnwell, EET 96031, and LAP 04575 are ordinary chondrites (OC) that possess lower than typical olivine Fa content than has been established for the H chondrites (<∼17 mol%). Mean low-Ca pyroxene Fs contents are typically lower than mean Fa content, with generally ?16 mol% Fs. We have investigated these three low-FeO chondrites by measuring their trace element abundances, oxygen isotopic compositions, and examining their three-dimensional (3D) petrography with synchrotron X-ray microtomography. We compare our results with those established for more common OC. The low FeO chondrites studied here have bulk trace element abundances that are identical to the H chondrites. From bulk oxygen isotopic analysis, we show that Burnwell, EET 96010, and LAP 04757 sampled oxygen reservoirs identical to the H chondrites. Burnwell, EET 96031, and LAP 04575 possess common 3D opaque mineral structures that could be distinct from the H chondrites, as evidenced by X-ray microtomographic analysis, but our comparison suite of H chondrites is small and unrepresentative. Overall, our data suggest a common origin for the low-FeO chondrites Burnwell, EET 96010, and LAP 04757 and the H chondrites. These three samples are simply extreme members of a redox process where a limiting nebular oxidizing agent, probably ice, reacted with material containing slightly higher amounts of metal than typically seen in the H chondrites.     

Can the ordinary chondrites have condensed from a gas phase?

The conditions under which ordinary chondrites containing iron in three different chemical states can form in thermodynamic equilibrium with a gas phase are calculated. Hydrogen depletion factors of 10²–10³ are obtained and the formation of liquid condensates from residual gases occurs at pressures (prior to hydrogen depletion) of ?1 atm.     

The origin and history of ordinary chondrites: A study by iron isotope measurements of metal grains from ordinary chondrites

Chondrules and chondrites provide unique insights into early solar system origin and history, and iron plays a critical role in defining the properties of these objects. In order to understand the processes that formed chondrules and chondrites, and introduced isotopic fractionation of iron isotopes, we measured stable iron isotope ratios ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe in metal grains separated from 18 ordinary chondrites, of classes H, L and LL, ranging from petrographic types 3-6 using multi-collector inductively coupled plasma mass spectrometry. The δ⁵⁶Fe values range from −0.06 ± 0.01 to +0.30 ± 0.04‰ and δ⁵⁷Fe values are −0.09 ± 0.02 to +0.55 ± 0.05‰ (relative to IRMM-014 iron isotope standard). Where comparisons are possible, these data are in good agreement with published data. We found no systematic difference between falls and finds, suggesting that terrestrial weathering effects are not important in controlling the isotopic fractionations in our samples. We did find a trend in the ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe isotopic ratios along the series H, L and LL, with LL being isotopically heavier than H chondrites by ∼0.3‰ suggesting that redox processes are fractionating the isotopes. The ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe ratios also increase with increasing petrologic type, which again could reflect redox changes during metamorphism and also a temperature dependant fractionation as meteorites cooled. Metal separated from chondrites is isotopically heavier by ∼0.31‰ in δ⁵⁶Fe than chondrules from the same class, while bulk and matrix samples plot between chondrules and metal. Thus, as with so many chondrite properties, the bulk values appear to reflect the proportion of chondrules (more precisely the proportion of certain types of chondrule) to metal, whereas chondrule properties are largely determined by the redox conditions during chondrule formation. The chondrite assemblages we now observe were, therefore, formed as a closed system.     

Metallic minerals,thermal histories and parent bodies of some xenolithic,ordinary chondrite meteorites

We have studied metal grains in the hosts and lithic fragments of widely differing petrologic types in four xenolithic chondrftes, using reflected-light microscopy and electron-probe analysis. In Weston and Fayetteville, which both contain solar-flare tracks and solar-wind gases, kamacite, taenite and tetrataenite (ordered FeNi) and troilite show a variety of textures. On a Wood plot of central Ni content vs dimension, taenite analyses scatter as if metal grains cooled at rates of 10–1000 and 1–100 K/Myr respectively through 700 K, although metal in an H6 clast in Fayetteville plots coherently with a cooling rate of 50 K/Myr. We propose that metal grains cooled at these rates in chondritic clasts at different locations before host and clasts were compacted, and were not subsequently heated above 650 K. We predict a similar history for all gas-rich ordinary chondrites.By contrast, metallic minerals throughout Bhola and Mezö-Madaras show more uniform textures and plot coherently giving cooling rates in the range 750 to ~600 K of 0.1 and 1 K/Myr, respectively. We conclude that host and xenoliths in both chondrites were slowly cooled after compaction. Thus clasts in these chondrites experienced peak metamorphic temperatures and slow cooling through 700 K in different environments.According to the conventional onion-shell model for H, L or LL chondrite parent bodies, material of petrologic types 3–5 was arranged in successive shells around a type 6 core prior to catastrophic collisions which mixed all types intimately. But if peak metamorphic temperatures were reached during, not after accretion, as seems plausible, maximum metamorphism may have occurred in planetesimals <10 km in radius. Cooling through 700 K may then have occurred in larger bodies that accreted from these planetesimals. Iron meteorites, mesosiderites and some achondrites may also have experienced melting in planetesimals and slow cooling in larger bodies.     

Cosmic-ray exposure ages of the ordinary chondrites and their significance for parent body stratigraphy

²¹Ne cosmic-ray exposure ages have been calculated from literature data for 201 H, 203 L and 38 LL chondrites, corrected for shielding differences when possible. The distributions of exposure ages again show the familiar peaks at 4.5 and 20 Myr for the H's, but no outstanding events for the L's and LL's. If the L-chondrite distribution is interpreted as a series of discrete events, then at least 6 peaks between 1 and 35 Myr are needed to model it. The observations, that every petrologic type occurs in every large peak and that even the higher petrologic types contain solar wind gases, suggest that the parent bodies have been fragmented and reassembled into a megabreccia. For the H chondrites, both large and small peaks contain about 15% solar-gas bearing meteorites, which could mean that surface material has been mixed to depths represented by the largest event, on the order of a kilometer. In contrast, only 3% of the L's contain solar wind, which may be related to breakup of their parent planet. Those L's with especially low radiogenic He (U,Th-He ages < 1 AE) tend to have low exposure ages: their distribution may be biased by a subgroup that had orbits coupling short capture lifetimes with significant solar heating.     

Response of silicate chemical composition variation on thermal metamorphism of ordinary chondrites and classification of petrologic types: the case of L chondrites from Grove Mountains,Antarctica

Acta Geochimica - Analysis of the thermal metamorphism of the ordinary chondrites is a key premise for gaining insights into the accretion and heating of rocky bodies in the early solar system....     

Composition and formation of metal nodules and veins in ordinary chondrites

Five large metal nodules, a composite sample of five small metal nodules, one troilite nodule and two metal veins from five ordinary chondrites were analyzed by electron microprobe and neutron activation analysis. Metal nodules and veins in H chondrites generally consist of large single crystals of kamacite, whereas L nodules contain significant taenite. Most nodules and veins are depleted by large factors ranging up to 240 in refractory siderophiles (Re, Os, Ir, Pt). Tungsten (normally a refractory siderophile) and Au, As, and Ga (volatile siderophiles) have abundance ratios similar to those of the common siderophiles Fe, Co and Ni. We propose that the metal with extremely low refractory-element contents was produced by shock-induced vaporization of chondritic material. The refractory elements condensed near the point of vaporization and were not transported with the vapor. Because the shock-generated gas was mildly oxidizing, W formed volatile oxides. The low Ni/Fe ratio and the resulting high kamacite contents may reflect dilution by Fe from dissociated FeS. Subsequent condensation of the refractory-depleted vapor into fractures and voids formed the metal veins and nodules.     

普通球粒陨石的物理和岩石学性质及其分类参数

不同球粒陨石群的物理和岩石学性质,包括球粒的平均大小、球粒结构类型、复合球粒、带火成边球粒及含硫化物的比例、化学组成及矿物学特征等可用以划分球粒陨石的化学-岩石类型和小行星类型,这些性质提供了不同球粒陨石群有用的分类参数及其形成环境的信息.由于不同球粒陨石群的△17O与日心距离存在有相关关系,因此,依据不同球粒陨石群形...     

Metamorphism of pelites in NKFMASH – a new petrogenetic grid with implications for the preservation of high-pressure mineral assemblages during exhumation

A petrogenetic grid for metapelites in the system NKFMASH is presented. The P–T range is investigated in three sections: (1) The high‐ and ultrahigh‐pressure range is discussed in the system NFMASH because phengite is the only stable potassic phase. (2) The transition region is characterised by four NKFMASH‐invariant points that separate high‐pressure glaucophane‐bearing from medium‐pressure biotite‐bearing metapelites. (3) The medium‐pressure range contains the fifth NKFMASH‐invariant point. The univariant reactions of this point terminate the stability range of paragonite, which breaks down to form staurolite or kyanite and plagioclase during decompression and/or heating. As the growth of albitic plagioclase by decomposition of paragonite via continuous reactions may be conspicuous already before these staurolite‐ or kyanite‐producing reactions are reached, such albite porphyroblast schists are typical indicators of a former high‐pressure metamorphic history. Considering the preservation of high‐pressure metapelitic assemblages, those crossing the NKFMASH‐transition region during exhumation commonly dehydrate and preservation is unlikely. Three types of metapelites have a fairly good survival potential: (1) low‐temperature metapelites (up to c. 540 °C) with an exhumation path back into the chlorite + albite stability field, (2) assemblages with chloritoid + glaucophane, and (3) the relatively high‐temperature glaucophane + kyanite and jadeite + kyanite bearing parageneses, that are relatively dry at the onset of exhumation. A comparison with data from the literature shows that these rock types are the most abundant in nature.