Unshocked equilibrated H-chondrites: A common low-temperature record from orthopyroxene iron-magnesium ordering

Abstract A suite of eight unshocked equilibrated H-chondrites ranging from petrographic type 4 to 6 (La Villa, Raguli, FRO 90076, Miami, FRO 90049, Estacado, FRO 90156 and Acfer 314) was selected for a systematic study of orthopyroxene cation ordering temperatures. The closure temperatures of the Fe-Mg exchange lie within the 384 ± 48 °C to 480 ± 28 °C range, regardless of the petrographic type. Data suggests that metamorphosed H-chondrites share a similar thermal evolution close to 380–480 °C. Indeed, this indicates that H4, H5 and H6 chondrites were located in petrogenetic environments characterized by similar temperature-time conditions when cooling through the above temperature interval.     

Metal phases in ordinary chondrites: Magnetic hysteresis properties and implications for thermal history

Magnetic properties are sensitive proxies to characterize FeNi metal phases in meteorites. We present a data set of magnetic hysteresis properties of 91 ordinary chondrite falls. We show that hysteresis properties are distinctive of individual meteorites while homogeneous among meteorite subsamples. Except for the most primitive chondrites, these properties can be explained by a mixture of multidomain kamacite that dominates the induced magnetism and tetrataenite (both in the cloudy zone as single‐domain grains, and as larger multidomain grains in plessite and in the rim of zoned taenite) dominates the remanent magnetism, in agreement with previous microscopic magnetic observations. The bulk metal contents derived from magnetic measurements are in agreement with those estimated previously from chemical analyses. We evidence a decreasing metal content with increasing petrologic type in ordinary chondrites, compatible with oxidation of metal during thermal metamorphism. Types 5 and 6 ordinary chondrites have higher tetrataenite content than type 4 chondrites. This is compatible with lower cooling rates in the 650–450 °C interval for higher petrographic types (consistent with an onion‐shell model), but is more likely the result of the oxidation of ordinary chondrites with increasing metamorphism. In equilibrated chondrites, shock‐related transient heating events above approximately 500 °C result in the disordering of tetrataenite and associated drastic change in magnetic properties. As a good indicator of the amount of tetrataenite, hysteresis properties are a very sensitive proxy of the thermal history of ordinary chondrites, revealing low cooling rates during thermal metamorphism and high cooling rates (e.g., following shock reheating or excavation after thermal metamorphism). Our data strengthen the view that the poor magnetic recording properties of multidomain kamacite and the secondary origin of tetrataenite make equilibrated ordinary chondrites challenging targets for paleomagnetic study.     

Effect of NaCrSi2O6 component on Lindsley's pyroxene thermometer: An evaluation based on strongly metamorphosed LL chondrites

In Lindsley's thermometry, a revised sequence of calculation of components is proposed for clinopyroxene, in which kosmochlor component is added. Temperatures obtained for the components calculated by the revised method are about 50 °C lower than those obtained for the components calculated by the Lindsley's original method and agree well with temperatures obtained from orthopyroxenes. Ca‐partitioning between clino‐ and orthopyroxenes is then thought to be equilibrated in types 5 to 7 ordinary chondrites. The temperatures for Tuxtuac (LL5), Dhurmsala (LL6), NWA 2092 (LL6/7), and Dho 011 (LL7) are 767–793°, 818–835°, 872–892°, and 917–936°C, respectively, suggesting that chondrites of higher petrographic types show higher equilibrium temperatures of pyroxenes. The regression equations which relate temperature and Wo and Fs contents in the temperature‐contoured pyroxene quadrilateral of 1 atm of Lindsley (1983) are also determined by the least squares method. It is possible to reproduce temperatures with an error less than 20 °C (2SE) using the regression equations.     

Enstatite chemical composition and microstructures in the La Villa H4 chondrite

Abstract— La Villa is an unshocked H4 chondrite. Chemical compositions require crystallization at temperatures >1250 °C for enstatite and >1211 °C for augite. Widespread (100) polysynthetic twins and (001) contraction cracks in enstatite indicate crystallization as protoenstatite, inverted to either ortho‐ or clinoenstatite or both on cooling. High‐resolution transmission electron microscopy shows a range of ortho‐clinoenstatite intergrowths: heavily faulted clinoenstatite in radial and poikilitic chondrules, almost regular orthoenstatite in a microgranular chondrule and in the matrix. In the former, the clinoenstatite lamellae are both even or odd multiples of the 9Å periodicity, a few unit cells thick, twinned and interleaved with minor orthoenstatite. In the latter, orthoenstatite lamellae are regularly stacked for more than 2000 Å. Localized annealing effects, reversing clinoenstatite to orthoenstatite, are revealed by “U‐shaped” and “Z‐shaped” terminations. The variable microstructures suggest different cooling rates for the different chondrule types, soon after the liquidus‐to‐solidus transition (1200 to 1300 °C) but prior to accretion. In particular, clinoenstatite‐rich crystals from radial and poikilitic chondrules give cooling rates on the order of 100 and 10 °C/h. Comparisons with previous works on dynamic crystallization experiments and orthopyroxene Fe‐Mg cation ordering indicate a nonlinear cooling path from the high chondrule formation temperatures to a postaccretionary low‐temperature (340–480 °C) evolution.     

Reclassification and thermal history of Trenzano chondrite

Abstract— We present a new single‐crystal X‐ray diffraction (XRD) study performed on a suite of six orthopyroxene grains from the low‐shocked H6 Trenzano meteorite. The quenched intracrystalline Fe²⁺‐Mg ordering state in orthopyroxene preserves the memory of the cooling rate near closure temperature T_c, thus yielding useful constraints on the last thermal event undergone by the host rock. The orthopyroxene T_c of 522 ± 13 °C, calculated using a new calibration equation obtained by Stimpfl (2005b), is higher than in previously published H chondrite data. The orthopyroxene cooling rate at this T_c is about 100 °C/kyr. This fast rate is inconsistent with the much slower cooling rate expected for H6 in the onion shell structural and thermal model of chondrite parent bodies. A petrographic study carried out at the same time indicated that the Trenzano meteorite is an H5 chondrite and not an H6 chondrite, as it is officially classified. Furthermore, the two‐pyroxene equilibrium temperature of Trenzano (824 ± 24 °C), calculated with QUILF95, is similar to the two‐pyroxene temperature of 750–840 °C obtained for the Carcote (H5) chondrite (Kleinschrot and Okrusch 1999).     

Compositional properties of coexisting orthopyroxene and spinel in some Antarctic diogenites: Implications for thermal history

Abstract— We analyzed the compositional profiles of coexisting orthopyroxenes and spinels in six diogenite samples from the Antarctic meteorite collection and used the data to constrain their thermal histories. The closure temperatures of Fe²⁺‐Mg exchange between spinel and orthopyroxene in these samples vary between ～630 and 830 °C. However, those in other diogenite samples, for which the compositional data are available in the literature, extend up to ～1125 °C. This wide range of closure temperatures suggests repeated excavation of the diogenites from their original sites over a long time interval during cooling. The orthopyroxene grains were found to be homogeneous in composition while two of the relatively large spinel grains in the samples Elephant Moraine (EET) 87530 and Thiel Mountains (TIL) 82410 showed compositional zoning near the rim. Modeling of the spinel zoning in TIL 82410 suggests that it developed during cooling under a regolith or ejecta blanket, possibly at a depth of ～80–120 m, and that the spinel composition was homogeneous at ～900 °C. A nonlinear cooling model in which the cooling rate is given by ηT(K)², with η = 5.8 times 10^?3 K^?1Ma^?1, leads to simulated retrograde zoning profile in spinel which match the observed profile in TIL 82410 very well.     

Cooling rates of diogenites: A study of Fe2+-Mg ordering in orthopyroxene by single-crystal x-ray diffraction

Abstract— The iron-magnesium exchange between M1 and M2 sites in orthopyroxene is a reversible reaction that records the latest event in the thermal evolution of the host rock. A kinetic analysis of this process has been applied to 16 orthopyroxene single crystals from 7 different diogenites to constrain the cooling history of their parent body. The Fe²⁺-Mg ordering degrees have been determined by single-crystal x-ray diffraction. The Fe²⁺-Mg ordering closure temperatures were very homogeneous within each sample and ranged, for all diogenites studied, between 311 ± 29 °C and 408 ± 10 °C. Cooling rates at these closure temperatures were calculated using a numerical method developed by Ganguly (1982). These ranged between ～5 °C/10⁴ year in Johnstown and ～0.8 °C/year in Roda. A comparison with other achondrites studied with the same method showed that increasing closure temperatures correspond to increasing cooling rates and that meteorites from a same parent body exhibit similar closure temperature and cooling rate values. The cooling rates obtained for these diogenites, at their low closure temperatures, should probably be ascribed to a complex thermal history of their parent body, thus confirming Miyamoto and Takeda's theory (1994a) of excavation of deep crustal material due to impact events. The differences on cooling rate values for different diogenites could be due to different burial depths in the fragment ejected from the parent body.     

Stones from Mohave County,Arizona: Multiple falls in the “Franconia strewn field”

One of the most productive and well‐sampled dense collection areas for meteorites on Earth is the “Franconia strewn field” in Mohave County, Arizona, which since 2002 has yielded hundreds of meteorites in an ellipsoidal area approximately 5 × 16 km across. Based on petrographic, mineral‐chemical, and terrestrial age data, we conclude that among 14 meteorites examined, there are at least 6 and possibly 8 distinct meteorites represented, which fell over a period of approximately 0–20 kyr ago. These include equilibrated H‐chondrites such as Franconia (H5) and Buck Mountains (BM) 001 (H6); H3–6 breccias such as Buck Mountains Wash and BM 004; and L6 chondrites such as BM 002 and BM 003 (which may be paired), Palo Verde Mine, and BM 005. To confidently pair such meteorites often requires thorough petrographic examination, mineral‐chemical analyses, and terrestrial ages. We estimate that 50 ± 10% of the larger specimens in this area are paired, yielding a relatively high value of approximately 2.3–2.9 distinct meteorites km^?2. The meteorite flux estimated for Franconia area is higher than the flux inferred from contemporary fireball data for larger masses. We suggest that one large H3–6 meteoroid fell in the area, most likely that of Buck Mountains Wash approximately 4 kyr ago, which produced an elliptical strewn field with masses generally increasing toward one end, and which raised the meteorite productivity in the recovery area.     

TWO NEW CHONDRITE-FALLS IN JAPAN

In the summer of 1984, two meteorites fell in the northern part of Honshu, Japan; Aomori, at 1:50 p.m. on June 30, and Tomiya, at 1:35 p.m. on August 22. Coordinates of the falls of the Aomori and the Tomiya are at 140°47.1'E., 40°48.6'N., and 140°51.9'E., 38°22.0'N., respectively. Results of chemical analyses of major elements, ratios of Fe_total/SiO₂ (0.546 and 0.803) and Fe_metal/Fe_total (0.332 and 0.581), and molar compositions of olivines (Fa₂₅ and Fa₁₉) indicate that the Aomori and the Tomiya are typical L- and H-group ordinary chondrites, respectively. In the Aomori, chondrules are present as relicts in the well-recrystallized matrix. Olivine and pyroxene are homogeneous in composition, and coarse clear feldspar, up to 100 micrometers in size, is well developed in the chondrules and matrix. Though the Aomori is a petrologic type 6 based on its texture and mineralogy, it includes a few grains of multiple twinned clinobronzite which is rarely observed in highly equilibrated ordinary chondrites. In the Tomiya, chondrules possess a fine-grained mesostasis, and both orthopyroxene and clinobronzite are noticeable in thin sections. Plagioclase is mostly microcrystalline, but is also sparsely present as tiny, visible grains. Thus, the Tomiya was classified to be petrologic type between 4 and 5. The deformation texture of olivine, pyroxene and plagioclase indicates that both meteorites were shocked by 0.2-0.25 Mb. In conjunction with the discussion of the frequency of meteorite-falls, all observed falls of meteorites in Japan are tabulated in this paper.     

GOMEZ,TERRY COUNTY,TEXAS: A NEW METEORITE FIND

The Gomez meteorite, weighing slightly over 47 kg, was found near the town of Gomez, Terry County, Texas (33° 10′53 “N, 102° 24′5” W) prior to 1974. It is a highly weathered, equilibrated L-6 chondrite of composition Fa 26, Fs 23. A large number of chromite grains and possibly partially weathered lawrencite grains were noted.     

Metallographic cooling rates of group IIF iron meteorites

Abstract— Metallographic cooling rates have been calculated for all five members of the iron meteorites group IIF using two different techniques. We have determined cooling rates of ～5 °C/Ma based on Ni profiles through the taenite rim enclosing kamacite spindles. Ni profiles through the kamacite phase are less precise cooling rate indicators, but suggest a cooling rate of ～1 °C/Ma within an order of magnitude at lower temperatures (360–400 °C). Based on the kamacite bandwidth and the Ni profiles through the taenite, we estimate that the kamacite nucleated 130–200 °C below the temperature predicted from the phase diagram. The size of and the distance between the large kamacite spindles is found to be consistent with the thermal history that we have determined on the basis of Ni profiles in kamacite and taenite. We find that previously published kamacite bandwidth cooling rates for the five group IIF members are most likely in error because of the presence of large schreibersite spindles in some kamacite spindles and because undercooling of kamacite was ignored. Contrary to previous workers we find that the metallographic cooling rates are consistent with cooling in a common core.     

Petrographic classification of Middle Ordovician fossil meteorites from Sweden

Abstract— The maximum diameter of chromite (FeCr₂O₄) grains within L chondrites reflects the petrographic type of the sample. On the basis of our measurements of nine recent L chondrites, L3 chromite D_max = 34–50 μm, L4 = 87–150 μm, L5 = 76–158 μm, and L6 = 253–638 μm. This variation reflects the crystallization of the chromite grains during parent body thermal metamorphism. We use this calibration to classify six fossil meteorites from the Middle Ordovician in Sweden as type 3 (or 4) to 6. The high flux of L chondrites at 470 Ma contained a range of petrographic types and may have had a higher proportion of lower petrographic type meteorites than are found in recent L chondrite falls. The fossil meteorites have in places preserved recognizable chondrule textures, including porphyritic olivine, barred olivine, and radiating pyroxene. A large relict clast and fusion crust have also been tentatively identified in one fossil meteorite. Apart from chromite, all of the original meteorite minerals have been replaced by carbonate (and sheet silicate and sulfate) during diagenesis within the limestone host. The preservation of chondrule definition has allowed us to measure the mean diameters of relict chondrules. The range (0.4–0.6 mm) is consistent with measurements made in the same way on recent L chondrites.     

Intermineral oxygen three‐isotope systematics of silicate minerals in equilibrated ordinary chondrites

High‐precision oxygen three‐isotope ratios were measured for four mineral phases (olivine, low‐Ca and high‐Ca pyroxene, and plagioclase) in equilibrated ordinary chondrites (EOCs) using a secondary ion mass spectrometer. Eleven EOCs were studied that cover all groups (H, L, LL) and petrologic types (4, 5, 6), including S1–S4 shock stages, as well as unbrecciated and brecciated meteorites. SIMS analyses of multiple minerals were made in close proximity (mostly <100 μm) from several areas in each meteorite thin section, to evaluate isotope exchange among minerals. Oxygen isotope ratios in each mineral become more homogenized as petrologic type increases with the notable exception of brecciated samples. In type 4 chondrites, oxygen isotope ratios of olivine and low‐Ca pyroxene are heterogeneous in both δ¹⁸O and Δ¹⁷O, showing similar systematics to those in type 3 chondrites. In type 5 and 6 chondrites, oxygen isotope ratios of the four mineral phases plot along mass‐dependent fractionation lines that are consistent with the bulk average Δ¹⁷O of each chondrite group. The δ¹⁸O of three minerals, low‐Ca and high‐Ca pyroxene and plagioclase, are consistent with equilibrium fractionation at temperatures of 700–1000 °C. In most cases the δ¹⁸O values of olivine are higher than those expected from pyroxene and plagioclase, suggesting partial retention of premetamorphic values due to slower oxygen isotope diffusion in olivine than pyroxene during thermal metamorphism in ordinary chondrite parent bodies.     

Igneous cooling history of olivine‐phyric shergottite Yamato 980459 constrained by dynamic crystallization experiments

Dynamic crystallization experiments were performed on a liquid having the bulk composition of olivine‐phyric shergottite Yamato 980459, to constrain the igneous thermal history of this meteorite. Key characteristics of the meteorite's mineralogy and texture, including several morphologically distinct olivine and pyroxene crystal populations and a glassy mesostasis devoid of plagioclase, were replicated upon cooling from 1435 to 909 °C at 1 atmosphere under reducing conditions. Three sequential cooling ramps are required to produce synthetic samples with textures and compositions matching Yamato 980459. Olivine phenocrysts formed at <1 °C h^?1, presumably at depth in the Martian crust. Pyroxene phenocrysts formed mainly at ~10 °C h^?1, consistent with crystallization within a lava flow at depths of 25–45 cm. Increased cooling rate (~100 °C h^?1) in a third stage suppressed the formation of plagioclase and produced groundmass crystals, consistent with crystallization at lava flow depths of 5–7 cm. Although Y 980459 is unique among Martian meteorites (i.e., preserving a primary glassy mesostasis), its emplacement did not require unique physical conditions. Rather, the second and third cooling stages may reflect cooling within the core of a pāhoehoe‐like flow and subsequent breakout on the surface of Mars.     

Terrestrial age measurements using natural thermoluminescence of a drained zone under the fusion crust of Antarctic ordinary chondrites

Abstract— Miono et al. (1990) and Miono and Nakanishi (1994) have proposed that the build‐up of natural thermoluminescence (TL) in a drained layer directly below the meteorite fusion crust can be used to determine terrestrial ages of meteorites in the 40 to 200 ka range. We have measured the natural TL of the drained layer of 15 meteorites. The data indicate that this technique could be used to determine terrestrial ages of meteorites with ages <200 ka, after which TL equilibrium is reached. Comparison of TL build‐up with terrestrial ages for a suite of Antarctic meteorites suggests that the meteorites have been exposed to temperatures of 0 to 5 °C. The close correspondence between natural TL levels and surface exposure TL growth curves suggest that Allan Hills meteorites with ages <200 ka have spent a significant portion of their terrestrial history exposed on the ice surface, rather than being buried in the ice sheet. The technique is, however, sensitive to thermal history; and, for Antarctic meteorites with terrestrial ages <200 ka, natural TL of the drained zone largely reflects exposure on the ice surface.     

Peak metamorphic temperatures in type 6 ordinary chondrites: An evaluation of pyroxene and plagioclase geothermometry

Abstract— Quantifying the peak temperatures achieved during metamorphism is critical for understanding the thermal histories of ordinary chondrite parent bodies. Various geothermometers have been used to estimate equilibration temperatures for chondrites of the highest metamorphic grade (type 6), but results are inconsistent and span hundreds of degrees. Because different geothermometers and calibration models were used with different meteorites, it is unclear whether variations in peak temperatures represent actual ranges of metamorphic conditions within type 6 chondrites or differences in model calibrations. We addressed this problem by performing twopyroxene geothermometry, using QUILF95, on the same type 6 chondrites for which peak temperatures were estimated using the plagioclase geothermometer (Nakamuta and Motomura 1999). We also calculated temperatures for published pyroxene analyses from other type 6 H, L, and LL chondrites to determine the most representative peak metamorphic temperatures for ordinary chondrites. Pyroxenes record a narrow, overlapping range of temperatures in H6 (865–926 °C), L6 (812–934 °C), and LL6 (874–945 °C) chondrites. Plagioclase temperature estimates are 96–179 °C lower than pyroxenes in the same type 6 meteorites. Plagioclase estimates may not reflect peak metamorphic temperatures because chondrule glass probably recrystallized to plagioclase prior to reaching the metamorphic peak. The average temperature for H, L, and LL chondrites (～900 °C), which agrees with previously published oxygen isotope geothermometry, is at least 50 °C lower than the peak temperatures used in current asteroid thermal evolution models. This difference may require minor adjustments to thermal model calculations.     

Mineral and chemical composition of the Jezersko meteorite—A new chondrite from Slovenia

The Jezersko meteorite is a newly confirmed stony meteorite found in 1992 in the Karavanke mountains, Slovenia. The meteorite is moderately weathered (W2), indicating short terrestrial residence time. Chondrules in partially recrystallized matrix are clearly discernible but often fragmented and have mean diameter of 0.73 mm. The meteorite consists of homogeneous olivine (Fa_19.4) and low‐Ca pyroxenes (Fs_16.7Wo_1.2), of which 34% are monoclinic, and minor plagioclase (Ab₈₃An₁₁Or₆) and Ca‐pyroxene (Fs₆Wo_45.8). Troilite, kamacite, zoned taenite, tetrataenite, chromite, and metallic copper comprise about 16.5 vol% of the meteorite. Phosphates are represented by merrillite and minor chlorapatite. Undulatory extinction in some olivine grains and other shock indicators suggests weak shock metamorphism between stages S2 and S3. The bulk chemical composition generally corresponds to the mean H chondrite composition. Low siderophile element contents indicate the oxidized character of the Jezersko parent body. The temperatures recorded by two‐pyroxene, olivine‐chromite, and olivine‐orthopyroxene geothermometers are 854 °C, 737–787 °C, and 750 °C, respectively. Mg concentration profiles across orthopyroxenes and clinopyroxenes indicate relatively fast cooling at temperatures above 700 °C. A low cooling rate of 10 °C Myr^?1 was obtained from metallographic data. Considering physical, chemical, and mineralogical properties, meteorite Jezersko was classified as an H4 S2(3) ordinary chondrite.     

Microstructures of metal grains in ordinary chondrites: Implications for their thermal histories

Abstract— This paper reports one of the first attempts to investigate by analytical transmission electron microscopy (ATEM) the microstructures and compositions of Fe‐Ni metal grains in ordinary chondrites. Three ordinary chondrites, Saint Séverin (LL6), Agen (H5), and Tsarev (L6) were selected because they display contrasting microstructures, which reflects different thermal histories. In Saint Séverin, the microstructure of the Ni‐rich metal grains is due to slow cooling. It consists of a two‐phase assemblage with a honeycomb structure resulting from spinodal decomposition similar to the cloudy zone of iron meteorites. Microanalyses show that the Ni‐rich phase is tetrataenite (Ni = 47 wt%) and the Ni‐poor phase, with a composition of ～25% Ni, is either martensite or taenite, these two occurring adjacent to each other. The observation that the Ni‐poor phase is partly fcc resolves the disagreement between previous transmission electron microscopy (TEM) and Mössbauer studies on iron meteorites and ordinary chondrite metal. The Ni content of the honeycomb phase is much higher than in mesosiderites, confirming that mesosiderites cooled much more slowly. The high‐Ni tetrataenite rim in contact with the cloudy zone displays high‐Ni compositional variability on a very fine scale, which suggests that the corresponding area was destabilized and partially decomposed at low temperature. Both Agen and Tsarev display evidence of reheating and subsequent fast cooling obviously related to shock events. Their metallic particles mostly consist of martensite, the microstructure of which depends on local Ni content. Microstructures are controlled by both the temperature at which martensite forms and that at which it possibly decomposes. In high‐Ni zones (>15 wt%), martensitic transformation started at low temperature (<300 °C). Because no further recovery occurred, these zones contain a high density of lattice defects. In low‐Ni zones (<15 wt%), martensite grains formed at higher temperature and their lattice defects recovered. These martensite grains present a lath texture with numerous tiny precipitates of Ni‐rich taenite (Ni = 50 wt%) at lath boundaries. Nickel composition profiles across precipitate‐matrix interfaces show that the growth of these precipitates was controlled by preferential diffusion of Ni along lattice defects. The cooling rates deduced from Ni concentration profiles and precipitate sizes are within the range 1–10 °C/year for Tsarev and 10–100 °C/year for Agen.     

Terrestrial ages of meteorites from the Nullarbor region,Australia, based on 14C and 14C–10Be measurements

Abstract– We have investigated the terrestrial ages, or residence times, of 78 meteorites (representing 73 discrete falls) recovered in Western Australia, and one from South Australia, using both ¹⁴C measurements and also ¹⁴C/¹⁰Be. The samples studied included two ureilites, one CK and one EL chondrite. We have included ¹⁰Be measurements from 30 meteorites, including some meteorites for which the ¹⁴C terrestrial age was previously determined. We find that the ¹⁴C/¹⁰Be terrestrial ages are more precise than ¹⁴C alone, as we can correct for shielding effects. In general, the two different age determinations age by ¹⁴C–¹⁰Be are precise to 0.5–1 ka and ¹⁴C alone within 1–2 ka. However, measurement of the ¹⁴C age alone gives good agreement with the ¹⁴C–¹⁰Be for most samples. The study of the terrestrial ages of meteorites gives us useful information concerning the storage and weathering of meteorites and the study of fall times and terrestrial age. We have compared the terrestrial ages to weathering, degree of oxidation (estimated from Mössbauer studies) and Δ¹⁷O. In this study, we found that weathering is not well correlated with terrestrial age for Nullarbor meteorites. However, there is a good correlation between degree of oxidation and Δ¹⁷O. The implications for the study of terrestrial ages and weathering from other desert environments will be discussed.     

An analytical electron microscopy (AEM) investigation of opaque inclusions in some type 6 ordinary chondrites

Abstract— A large number of ordinary chondrites contains micron-sized particles of metal and/or troilite dispersed in their silicate grains. Such metallic phases are responsible for the so-called darkening of the silicate grains and might be either precipitates, which formed during reduction of the silicates, or inclusions injected as a melt during a shock event. We have investigated these tiny foreign phases by analytical transmission electron microscopy in three unweathered, metamorphosed ordinary chondrites (Saint Séverin, LL6, Tsarev, L6 and Kernouvé, H6). We also looked for remnant shock indices. Our TEM observations suggest the following sequence of events in the three meteorites. First, a number of relatively strong shock events occurred on the parent body/bodies producing an Fe-FeS melt that was injected into silicate grains along a dense network of open fractures. Most of these shock defects were subsequently erased by high-temperature (700–900 °C) thermal metamorphism. Some remnants of the shock events are the observed trails of tiny metal and/or sulfide inclusions that formed as a result of fracture healing. Chemical homogenization of the silicates and limited oxidation of the metallic blebs also occurred during this high-temperature annealing event, resulting in Ni-rich inclusions. This effect was especially pronounced in the L and LL-chondrites studied. During subsequent cooling of the body/bodies, inclusions of chromite and phosphate precipitated, nucleating preferentially on lattice defects (dislocations, subgrain boundaries) and on the metal and sulfide inclusions. A later shock event of moderate intensity, probably corresponding to the separation of the meteorite from its parent body, produced new shock features in the silicate grains of the Saint Séverin meteorite, including mechanical twins in diopside and straight free screw dislocations in olivine.