The origin of the Brunflo fossil meteorite and extraterrestrial chromite in mid‐Ordovician limestone from the Gärde quarry (Jämtland,central Sweden)

Abstract— The Brunflo fossil meteorite was found in the 1950s in mid‐Ordovician marine limestone in the Gärde quarry in Jämtland. It originates from strata that are about 5 million years younger than similar limestone that more recently has yielded >50 fossil meteorites in the Thorsberg quarry at Kinnekulle, 600 km to the south. Based primarily on the low TiO₂ content (about 1.8 wt%) of its relict chromite the Brunflo meteorite had been tentatively classified as an H chondrite. The meteorite hence appears to be an anomaly in relation to the Kinnekulle meteorites, in which chromite composition, chondrule mean diameter and oxygen isotopic composition all indicate an L‐chondritic origin, reflecting an enhanced flux of meteorites to Earth following the disruption of the L chondrite parent body 470 Ma. New chondrule‐size measurements for the Brunflo meteorite indicate that it too is an L chondrite, related to the same parent‐body breakup. Chromite maximum diameters and well‐defined chondrule structures further show that Brunflo belongs to the L4 or L5 type. Chromites in recently fallen L4 chondrites commonly have low TiO₂ contents similar to the Brunflo chromites, adding support for Brunflo being an L4 chondrite. The limestone in the Gärde quarry is relatively rich (about 0.45 grain kg⁻¹) in sediment‐dispersed extraterrestrial chromite grains (>63 μm) with chemical composition similar to those in L chondrites and the limestone (1–3 grains kg⁻¹) at Kinnekulle, suggesting that the enhanced flux of L chondrites prevailed, although somewhat diminished, at the time when the Brunflo meteorite fell.     

Extraterrestrial chromite in Middle Ordovician marine limestone at Kinnekulle,southern Sweden—Traces of a major asteroid breakup event

Abstract— The distribution of sediment‐dispersed extraterrestrial chromite grains and other Cr‐rich spinels (>63 μm) has been studied in Middle Ordovician Orthoceratite Limestone from two quarries at Kinnekulle, southern Sweden. In the Thorsberg quarry, an ?3.2 m thick sequence of beds previously shown to be rich in fossil meteorites is also rich in sediment‐dispersed extraterrestrial chromite grains. Typically, 1–3 grains are found per kilogram of limestone. In the nearby Hällekis quarry, the same beds show similarly high concentrations of extraterrestrial chromite grains, but in samples representing the 9 m downward continuation of the section exposed at this site, only 5 such grains were found in a total of 379 kg of limestone. The extraterrestrial (equilibrated ordinary chondritic) chromite grains can be readily distinguished by a homogeneous and characteristic major element chemistry, including 2.0–3.5 wt% TiO₂ and stable V₂O₃ concentrations close to 0.7 wt%. Terrestrial Cr‐rich spinels have a wide compositional range and co‐exist with extraterrestrial chromite in some beds. These grains may be derived, for example, from mafic dykes exposed and weathered at the sea floor. Considering lithologic and stratigraphic aspects variations in sedimentation rate cannot explain the dramatic increase in extraterrestrial chromite seen in the upper part of the composite section studied. Instead, the difference may be primarily related to an increase in the ancient flux of extraterrestrial matter to Earth in connection with the disruption of the L chondrite parent body in the asteroid belt at about this time. The coexistence in some beds of high concentrations of chondritic chromite and terrestrial Cr‐rich spinels, however, indicates that redistribution of heavy minerals on the sea floor, related to changes in sea level and sea‐floor erosion and currents, must also be considered.     

Unusual sources of fossil micrometeorites deduced from relict chromite in the small size fraction in ~467 Ma old limestone

Extraterrestrial chrome spinel and chromite extracted from the sedimentary rock record are relicts from coarse micrometeorites and rarely meteorites. They are studied to reconstruct the paleoflux of meteorites to the Earth and the collisional history of the asteroid belt. Minor element concentrations of Ti and V, and oxygen isotopic compositions of these relict minerals were used to classify the meteorite type they stem from, and thus to determine the relative meteorite group abundances through time. While coarse sediment-dispersed extraterrestrial chrome-spinel (SEC) grains from ordinary chondrites dominate through the studied time windows in the Phanerozoic, there are exceptions: We have shown that ~467 Ma ago, 1 Ma before the breakup of the L chondrite parent body (LCPB), more than half of the largest (>63 μm diameter) grains were achondritic and originated from differentiated asteroids in contrast to ordinary chondrites which dominated the meteorite flux throughout most of the past 500 Ma. Here, we present a new data set of oxygen isotopic compositions and elemental compositions of 136 grains of a smaller size fraction (32–63 μm) in ~467 Ma old pre-LCPB limestone from the Lynna River section in western Russia, that was previously studied by elemental analysis. Our study constitutes the most comprehensive oxygen isotopic data set of sediment-dispersed extraterrestrial chrome spinel to date. We also introduce a Raman spectroscopy-based method to identify SEC grains and distinguish them from terrestrial chrome spinel with ~97% reliability. We calibrated the Raman method with the established approach using titanium and vanadium concentrations and oxygen isotopic compositions. We find that ordinary chondrites are approximately three times more abundant in the 32–63 μm fraction than achondrites. While abundances of achondrites compared to ordinary chondrites are lower in the 32–63 μm size fraction than in the >63 μm one, achondrites are approximately three times more abundant in the 32–62 μm fraction than they are in the present flux. We find that the sources of SEC grains vary for different grain sizes, mainly as a result of parent body thermal metamorphism. We conclude that the meteorite flux composition ~467 Ma ago ~1 Ma before the breakup of the LCPB was fundamentally different from today and from other time windows studied in the Phanerozoic, but that in contrast to the large size fraction ordinary chondrites dominated the flux in the small size fraction. The high abundance of ordinary chondrites in the studied samples is consistent with the findings based on coarse extraterrestrial chrome-spinel from other time windows.     

Reflectance spectroscopy of insoluble organic matter (IOM) and carbonaceous meteorites

Insoluble organic matter (IOM) is the major organic component of chondritic meteorites and may be akin to organic materials from comets and interplanetary dust particles (IDPs). Reflectance spectra of IOM in the range 0.35–25 μm are presented as a tool for interpreting organic chemistry from remote measurements of asteroids, comets, IDPs, and other planetary bodies. Absorptions in the IOM spectra were strongly related to elemental H/C (atom) ratio. The aliphatic 3.4 μm absorption in IOM spectra increased linearly in strength with increasing H/C for H/C > 0.4, but was absent at lower H/C values. When meteorite spectra from the Reflectance Experiment Laboratory (RELAB) spectral catalog (n = 85) were reanalyzed at 3.4 μm, this detection limit (H/C > 0.4) persisted. Aromatic absorption features seen in IOM spectra were not observed in the meteorite spectra due to overlapping absorptions. However, the 3.4 μm aliphatic absorption strength for the bulk meteorites was correlated with both H/C of the meteorite's IOM and bulk C (wt%). Gaussian modeling of the 3 μm region provided an additional estimate of bulk C for the meteorites, along with bulk H (wt%), which is related to phyllosilicate abundance. These relationships lay the foundation for determining organic and phyllosilicate abundances from reflectance spectra. Both the full IOM spectra and the spectral parameters discussed here will aid in the interpretation of data from asteroid missions (e.g., OSIRIS‐REx, Hayabusa2), and may be able to place unknown spectral samples within the context of the meteorite collection.     

Mechanisms of ringwoodite formation in shocked meteorites: Evidence from L5 chondrite Dhofar 1970

The formation of the high‐pressure compositional equivalents of olivine and pyroxene has been well‐documented within and surrounding shock‐induced veins in chondritic meteorites, formed by crystallization from a liquid‐ or solid‐state phase transformation. Typically polycrystalline ringwoodite grains have a narrow range of compositions that overlap with those of their olivine precursors, whereas the formation of iron‐enriched ringwoodite has been documented from only a handful of meteorites. Here, we report backscattered electron images, quantitative wavelength‐dispersive spectrometry (WDS) analyses, qualitative WDS elemental X‐ray maps, and micro‐Raman spectra that reveal the presence of Fe‐rich ringwoodite (Fa_44‐63) as fine‐grained (500 nm), polycrystalline rims on olivine (Fa_24‐25) wall rock and as clasts engulfed by shock melt in a previously unstudied L5 chondrite, Dhofar 1970. Crystallization of majorite + magnesiowüstite in the vein interior and metastable mineral assemblages within 35 μm of the vein margin attest to rapid crystallization of a superheated shock melt (>2300 K) from 20─25 GPa to ambient pressure and temperature. The texture and composition of bright polycrystalline ringwoodite rims (Fa_44‐63; MnO 0.01─0.08 wt%) surrounding dark polycrystalline olivine (Fa_8‐14; MnO 0.56─0.65 wt%) implies a solid‐state transformation mechanism in which Fe was preferentially partitioned to ringwoodite. The spatial association between ringwoodite and shock melt suggests that the rapidly fluctuating thermal regimes experienced by chondritic minerals in contact with shock melt are necessary to both drive phase transformation but also to prevent back‐transformation.     

The micrometeorite flux to Earth during the earliest Paleogene reconstructed in the Bottaccione section (Umbrian Apennines), Italy

Based on sediment‐dispersed extraterrestrial spinel grains in the Bottaccione limestone section in Italy, we reconstructed the micrometeorite flux to Earth during the early Paleocene. From a total of 843 kg of limestone, 86 extraterrestrial spinel grains (12 grains > 63 μm, and 74 in the 32–63 μm fraction) have been recovered. Our results indicate that the micrometeorite flux was not elevated during the early Paleocene. Ordinary chondrites dominated over achondritic meteorites similar to the recent flux, but H chondrites dominated over L and LL chondrites (69%, 22%, and 9%, respectively). This H‐chondrite dominance is similar to that recorded within an enigmatic ³He anomaly (70, 27, and 3%) in the Turonian, but different from just before this ³He anomaly and in the early Cretaceous, where ratios are similar to the recent flux (~45%, 45%, and 10%). The K‐Ar isotopic ages of recently fallen H chondrites indicate a small impact event on the H‐chondrite parent body ~50 to 100 Ma ago. We tentatively suggest that this event is recorded by the Turonian ³He anomaly, resulting in an H‐chondrite dominance up to the Paleocene. Our sample spanning the 20 cm above the Cretaceous–Paleogene (K–Pg) boundary did not yield any spinel grains related to the K–Pg boundary impactor.     

Characterization of weathering and heterogeneous mineral phase distribution in brachinite Northwest Africa 4872

Terrestrial weathering of hot desert achondrite meteorite finds and heterogeneous phase distributions in meteorites can complicate interpretation of petrological and geochemical information regarding parent‐body processes. For example, understanding the effects of weathering is important for establishing chalcophile and siderophile element distributions within sulfide and metal phases in meteorites. Heterogeneous mineral phase distribution in relatively coarsely grained meteorites can also lead to uncertainties relating to compositional representativeness. Here, we investigate the weathering and high‐density (e.g., sulfide, spinel, Fe‐oxide) phase distribution in sections of ultramafic achondrite meteorite Northwest Africa (NWA) 4872. NWA 4872 is an olivine‐rich brachinite (Fo_{63.6 ± 0.5}) with subsidiary pyroxene (Fs_9.7 ± 0.1Wo_{46.3 ± 0.2}), Cr‐spinel (Cr# = 70.3 ± 1.1), and weathered sulfide and metal. Raman mapping confirms that weathering has redistributed sulfur from primary troilite, resulting in the formation of Fe‐oxide (‐hydroxide) and marcasite (FeS₂). From Raman mapping, NWA 4872 is composed of olivine (89%), Ca‐rich pyroxene (0.4%), and Cr‐spinel (1.1%), with approximately 7% oxidized metal and sulfide and 2.3% marcasite‐dominated sulfide. Microcomputed tomography (micro‐CT) observations reveal high‐density regions, demonstrating heterogeneities in mineral distribution. Precision cutting of the largest high‐density region revealed a single 2 mm Cr‐spinel grain. Despite the weathering in NWA 4872, rare earth element (REE) abundances of pyroxene determined by laser‐ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) indicate negligible modification of these elements in this mineral phase. The REE abundances of mineral grains in NWA 4872 are consistent with formation of the meteorite as the residuum of the partial melting process that occurred on its parent body. LA‐ICP‐MS analyses of sulfide and alteration products demonstrate the mobility of Re and/or Os; however, highly siderophile element (HSE) abundance patterns remain faithful recorders of processes acting on the brachinite parent body(ies). Detailed study of weathering and phase distribution offers a powerful tool for assessing the effects of low‐temperature alteration and for identifying robust evidence for parent‐body processes.     

He and Ne in individual chromite grains from the regolith breccia Ghubara (L5): Exploring the history of the L chondrite parent body regolith

We analyzed He and Ne in chromite grains from the regolith breccia Ghubara (L5), to compare it with He and Ne in sediment‐dispersed extraterrestrial chromite (SEC) grains from mid‐Ordovician sediments. These SEC grains arrived on Earth as micrometeorites in the aftermath of the L chondrite parent body (LCPB) breakup event, 470 Ma ago. A significant fraction of them show prolonged exposure to galactic cosmic rays for up to several 10 Ma. The majority of the cosmogenic noble gases in these grains were probably acquired in the regolith of the LCPB (Meier et al. 2010 ). Ghubara, an L chondritic regolith breccia with an Ar‐Ar shock age of 470 Ma, is a sample of that regolith. We find cosmic‐ray exposure ages of up to several 10 Ma in some Ghubara chromite grains, confirming for the first time that individual chromite grains with such high exposure ages indeed existed in the LCPB regolith, and that the >10 Ma cosmic‐ray exposure ages found in recent micrometeorites are thus not necessarily indicative of an origin in the Kuiper Belt. Some Ghubara chromite grains show much lower concentrations of cosmogenic He and Ne, indicating that the 4π (last‐stage) exposure age of the Ghubara meteoroid lasted only 4–6 Ma. This exposure age is considerably shorter than the 15–20 Ma suggested before from bulk analyses, indicating that bulk samples have seen regolith pre‐exposure as well. The shorter last‐stage exposure age probably links Ghubara to a small peak of ⁴⁰Ar‐poor L5 chondrites of the same exposure age. Furthermore, and quite unexpectedly, we find a Ne component similar to presolar Ne‐HL in the chromite grains, perhaps indicating that some presolar Ne can be preserved even in meteorites of petrologic type 5.     

Mineral chemistry of the Tissint meteorite: Indications of two‐stage crystallization in a closed system

The Tissint meteorite is a geochemically depleted, olivine‐phyric shergottite. Olivine megacrysts contain 300–600 μm cores with uniform Mg# (~80 ± 1) followed by concentric zones of Fe‐enrichment toward the rims. We applied a number of tests to distinguish the relationship of these megacrysts to the host rock. Major and trace element compositions of the Mg‐rich core in olivine are in equilibrium with the bulk rock, within uncertainty, and rare earth element abundances of melt inclusions in Mg‐rich olivines reported in the literature are similar to those of the bulk rock. Moreover, the P Kα intensity maps of two large olivine grains show no resorption between the uniform core and the rim. Taken together, these lines of evidence suggest the olivine megacrysts are phenocrysts. Among depleted olivine‐phyric shergottites, Tissint is the first one that acts mostly as a closed system with olivine megacrysts being the phenocrysts. The texture and mineral chemistry of Tissint indicate a crystallization sequence of: olivine (Mg# 80 ± 1) → olivine (Mg# 76) + chromite → olivine (Mg# 74) + Ti‐chromite → olivine (Mg# 74–63) + pyroxene (Mg# 76–65) + Cr‐ulvöspinel → olivine (Mg# 63–35) + pyroxene (Mg# 65–60) + plagioclase, followed by late‐stage ilmenite and phosphate. The crystallization of the Tissint meteorite likely occurred in two stages: uniform olivine cores likely crystallized under equilibrium conditions; and a fractional crystallization sequence that formed the rest of the rock. The two‐stage crystallization without crystal settling is simulated using MELTS and the Tissint bulk composition, and can broadly reproduce the crystallization sequence and mineral chemistry measured in the Tissint samples. The transition between equilibrium and fractional crystallization is associated with a dramatic increase in cooling rate and might have been driven by an acceleration in the ascent rate or by encounter with a steep thermal gradient in the Martian crust.     

The variability of ruthenium in chromite from chassignite and olivine‐phyric shergottite meteorites: New insights into the behavior of PGE and sulfur in Martian magmatic systems

The Martian meteorites comprise mantle‐derived mafic to ultramafic rocks that formed in shallow intrusions and/or lava flows. This study reports the first in situ platinum‐group element data on chromite and ulvöspinel from a series of dunitic chassignites and olivine‐phyric shergottites, determined using laser‐ablation ICP‐MS. As recent studies have shown that Ru has strongly contrasting affinities for coexisting sulfide and spinel phases, the precise in situ analysis of this element in spinel can provide important insights into the sulfide saturation history of Martian mantle‐derived melts. The new data reveal distinctive differences between the two meteorite groups. Chromite from the chassignites Northwest Africa 2737 (NWA 2737) and Chassigny contained detectable concentrations of Ru (up to ~160 ppb Ru) in solid solution, whereas chromite and ulvöspinel from the olivine‐phyric shergottites Yamato‐980459 (Y‐980459), Tissint, and Dhofar 019 displayed Ru concentrations consistently below detection limit (<42 ppb). The relatively elevated Ru signatures of chromite from the chassignites suggest a Ru‐rich (~1–4 ppb) parental melt for this meteorite group, which presumably did not experience segregation of immiscible sulfide liquids over the interval of mantle melting, melt ascent, and chromite crystallization. The relatively Ru‐depleted signature of chromite and ulvöspinel from the olivine‐phyric shergottites may be the consequence of relatively lower Ru contents (<1 ppb) in the parental melts, and/or the presence of sulfides during the crystallization of the spinel phases. The results of this study illustrate the significance of platinum‐group element in situ analysis on spinel phases to decipher the sulfide saturation history of magmatic systems.     

Petrology and oxygen isotopic compositions of calcium‐aluminum‐rich inclusions in primitive CO3.0‐3.1 chondrites

The petrologic and oxygen isotopic characteristics of calcium‐aluminum‐rich inclusions (CAIs) in CO chondrites were further constrained by studying CAIs from six primitive CO3.0‐3.1 chondrites, including two Antarctic meteorites (DOM 08006 and MIL 090010), three hot desert meteorites (NWA 10493, NWA 10498, and NWA 7892), and the Colony meteorite. The CAIs can be divided into hibonite‐bearing inclusions (spinel‐hibonite spherules, monomineralic grains, hibonite‐pyroxene microspherules, and irregular/nodular objects), grossite‐bearing inclusions (monomineralic grains, grossite‐melilite microspherules, and irregular/nodular objects), melilite‐rich inclusions (fluffy Type A, compact type A, monomineralic grains, and igneous fragments), spinel‐pyroxene inclusions (fluffy objects resembling fine‐grained spinel‐rich inclusions in CV chondrites and nodular/banded objects resembling those in CM chondrites), and pyroxene‐anorthite inclusions. They are typically small (98.4 ± 54.4 µm, 1SD) and comprise 1.54 ± 0.43 (1SD) area% of the host chondrites. Melilite in the hot desert and Colony meteorites was extensively replaced by a hydrated Ca‐Al‐silicate during terrestrial weathering and converted melilite‐rich inclusions into spinel‐pyroxene inclusions. The CAI populations of the weathered COs are very similar to those in CM chondrites, suggesting that complete replacement of melilite by terrestrial weathering, and possibly parent body aqueous alteration, would make the CO CAIs CM‐like, supporting the hypothesis that CO and CM chondrites derive from similar nebular materials. Within the CO3.0‐3.1 chondrites, asteroidal alteration significantly resets oxygen isotopic compositions of CAIs in CO3.1 chondrites (?¹⁷O: ?25 to ?2‰) but left those in CO3.0‐3.05 chondrites mostly unchanged (?¹⁷O: ?25 to ?20‰), further supporting the model whereby thermal metamorphism became evident in CO chondrites of petrologic type ≥3.1. The resistance of CAI minerals to oxygen isotope exchange during thermal metamorphism follows in the order: melilite + grossite < hibonite + anorthite < spinel + diopside + forsterite. Meanwhile, terrestrial weathering destroys melilite without changing the chemical and isotopic compositions of melilite and other CAI minerals.     

Eutectic metal + troilite + Fe‐Mn‐Na phosphate + Al‐free chromite assemblage in shock‐produced chondritic melt of the Yanzhuang chondrite

An assemblage with FeNi metal, troilite, Fe‐Mn‐Na phosphate, and Al‐free chromite was identified in the metal‐troilite eutectic nodules in the shock‐produced chondritic melt of the Yanzhuang H6 meteorite. Electron microprobe and Raman spectroscopic analyses show that a few phosphate globules have the composition of Na‐bearing graftonite (Fe_,Mn,Na)₃(PO₄)₂, whereas most others correspond to Mn‐bearing galileiite Na(Fe,Mn)₄(PO₄)₃ and a possible new phosphate phase of Na₂(Fe,Mn)₁₇(PO₄)₁₂ composition. The Yanzhuang meteorite was shocked to a peak pressure of 50 GPa and a peak temperature of approximately 2000 °C. All minerals were melted after pressure release to form a chondritic melt due to very high postshock heat that brought the chondrite material above its liquidus. The volatile elements P and Na released from whitlockite and plagioclase along with elements Cr and Mn released from chromite are concentrated into the shock‐produced Fe‐Ni‐S‐O melt at high temperatures. During cooling, microcrystalline olivine and pyroxene first crystallized from the chondritic melt, metal‐troilite eutectic intergrowths, and silicate melt glass finally solidified at about 950–1000 °C. On the other hand, P, Mn, and Na in the Fe‐Ni‐S‐O melt combined with Fe and crystallized as Fe‐Mn‐Na phosphates within troilite, while Cr combined with Fe and crystallized as Al‐free chromite also within troilite.     

Origin of the metamorphosed clasts in the CV3 carbonaceous chondrite breccias of Graves Nunataks 06101, Vigarano,Roberts Massif 04143, and Yamato‐86009

We observed metamorphosed clasts in the CV3 chondrite breccias Graves Nunataks 06101, Vigarano, Roberts Massif 04143, and Yamato‐86009. These clasts are coarse‐grained polymineralic rocks composed of Ca‐bearing ferroan olivine (Fa_24–40, up to 0.6 wt% CaO), diopside (Fs_7–12Wo_44–50), plagioclase (An_52–75), Cr‐spinel (Cr/[Cr + Al] = 0.4, Fe/[Fe + Mg] = 0.7), sulfide and rare grains of Fe‐Ni metal, phosphate, and Ca‐poor pyroxene (Fs₂₄Wo₄). Most clasts have triple junctions between silicate grains. The rare earth element (REE) abundances are high in diopside (REE ~3.80–13.83 × CI) and plagioclase (Eu ~12.31–14.67 × CI) but are low in olivine (REE ~0.01–1.44 × CI) and spinel (REE ~0.25–0.49 × CI). These REE abundances are different from those of metamorphosed chondrites, primitive achondrites, and achondrites, suggesting that the clasts are not fragments of these meteorites. Similar mineralogical characteristics of the clasts with those in the Mokoia and Yamato‐86009 breccias (Jogo et al. 2012 ) suggest that the clasts observed in this study would also form inside the CV3 chondrite parent body. Thermal modeling suggests that in order to reach the metamorphosed temperatures of the clasts of >800 °C, the clast parent body should have accreted by ~2.5–2.6 Ma after CAIs formation. The consistency of the accretion age of the clast parent body and the CV3 chondrule formation age suggests that the clasts and CV3 chondrites could be originated from the same parent body with a peak temperature of 800–1100 °C. If the body has a peak temperature of >1100 °C, the accretion age of the body becomes older than the CV3 chondrule formation age and multiple CV3 parent bodies are likely.     

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.     

Kosmochloric Ca‐rich pyroxenes and FeO‐rich olivines (Kool grains) and associated phases in Stardust tracks and chondritic porous interplanetary dust particles: Possible precursors to FeO‐rich type II chondrules in ordinary chondrites

Abstract— Terminal particles and mineral fragments from comet 81P/Wild 2 were studied in 16 aerogel tracks by transmission and secondary electron microscopy. In eight tracks clinopyroxenes with correlated Na₂O and Cr₂O₃ contents as high as 6.0 wt% and 13.0 wt%, respectively, were found. Kosmochloric (Ko) clinopyroxenes were also observed in 4 chondritic interplanetary dust particles (IDPs). The Ko‐clinopyroxenes were often associated with FeO‐rich olivine ± Cr‐rich spinel ± aluminosilicate glass or albitic feldspar, assemblages referred to as Kool grains (Ko = kosmochloric Ca‐rich pyroxene, ol = olivine). Fine‐grained (submicron) Kool fragments have textures suggestive of crystallization from melts while coarse‐grained (>1 μm) Kool fragments are often glass‐free and may have formed by thermal metamorphism in the nebula. Average major and minor element distributions between clinopyroxenes and coexisting FeO‐rich olivines are consistent with these phases forming at or near equilibrium. In glass‐bearing fine‐grained Kool fragments, high concentrations of Na in the clinopyroxenes are inconsistent with existing experimentally determined partition coefficients at equilibrium. We speculate that the availability of Cr in the melt increased the clinopyroxene Na partition coefficient via a coupled substitution thereby enhancing this phase with the kosmochlor component. The high temperature minerals, fine‐grain sizes, bulk compositions and common occurrence in the SD tracks and IDPs support the idea that Kool grains could have been precursors to type II chondrules in ordinary chondrites. These grains, however, have not been observed in these meteorites suggesting that they were destroyed during chondrule formation and recycling or were not present in the nebula at the time and location where meteoritic chondrules formed.     

Noble gases in fossil micrometeorites and meteorites from 470 Myr old sediments from southern Sweden,and new evidence for the L‐chondrite parent body breakup event

Abstract— We present noble gas analyses of sediment‐dispersed extraterrestrial chromite grains recovered from ?470 Myr old sediments from two quarries (Hällekis and Thorsberg) and of relict chromites in a coeval fossil meteorite from the Gullhögen quarry, all located in southern Sweden. Both the sediment‐dispersed grains and the meteorite Gullhögen 001 were generated in the L‐chondrite parent body breakup about 470 Myr ago, which was also the event responsible for the abundant fossil meteorites previously found in the Thorsberg quarry. Trapped solar noble gases in the sediment‐dispersed chromite grains have partly been retained during ?470 Myr of terrestrial residence and despite harsh chemical treatment in the laboratory. This shows that chromite is highly retentive for solar noble gases. The solar noble gases imply that a sizeable fraction of the sediment‐dispersed chromite grains are micrometeorites or fragments thereof rather than remnants of larger meteorites. The grains in the oldest sediment beds were rapidly delivered to Earth likely by direct injection into an orbital resonance in the inner asteroid belt, whereas grains in younger sediments arrived by orbital decay due to Poynting‐Robertson (P‐R) drag. The fossil meteorite Gullhögen 001 has a low cosmic‐ray exposure age of ?0.9 Myr, based on new He and Ne production rates in chromite determined experimentally. This age is comparable to the ages of the fossil meteorites from Thorsberg, providing additional evidence for very rapid transfer times of material after the L‐chondrite parent body breakup.     

Impact ejecta in upper Eocene deposits at Massignano,Italy

Abstract— Previous workers have shown that an impact ejecta layer at Massignano, Italy contains a positive Ir anomaly, flattened spheroids (pancake spherules), Ni‐rich spinel crystals, and shocked quartz with multiple sets of planar deformation features. Because of sample sizes and work by different investigators, it was not clear if the shocked quartz is associated with the Ir anomaly and pancake spherules or if it belongs to a separate impact event. To address this problem, we carried out a high‐resolution stratigraphic study of this ejecta layer. The ejecta layer was sampled continuously at 1 cm intervals in two adjacent columns. The carbonate was removed with dilute HCl, and the non‐carbonate fraction was gently sieved. Pancake spherules were recovered from the 250–500 μm size fraction and counted. At the peak abundance, the number of pancake spherules in the 250–500 μm size fraction is about 6–7/g of sample. The pancake spherules removed from the 250–500 μm size fraction are mostly translucent to opaque pale green, but some have a grey color or dark opaque patches due to a coating of Ni‐ and Cr‐rich spinel crystals. Energy‐dispersive X‐ray analysis and X‐ray diffraction data indicate that the green spherules are composed of iron‐rich smectite, probably nontronite. Black opaque spinel stringers (dark spinel‐rich pancake spherules), usually <200 μm across, can be seen in a polished section of a block that includes the ejecta layer. None of the dark spinel‐rich pancake spherules were recovered from the sieved non‐carbonate fraction due to their fragile nature, but we believe that they are from the same impact event as the green pancake spherules. The <250 μm size fractions from both columns were disaggregated using ultrasonics and re‐sieved. The 63–125 μm size fractions were then searched for shocked quartz using a petrographic microscope. At the peak‐abundance level, the number of shocked quartz grains in the 63–125 μm size fraction is about 7/g of sample. Some of the shocked quartz grains have a “toasted” appearance. These grains have a brownish color and contain a patchy distribution of faint, densely spaced planar deformation features (PDFs). Polymineralic fragments containing one or two shocked quartz grains with one or two sets of PDFs were observed. They appear to have an organic matrix and are probably fragments of agglutinated foraminiferal tests. We searched for, but did not find, coesite or shocked zircons. We found that the peak abundance of the shocked quartz is within a centimeter of the peak abundance of the green pancake spherules. We conclude that the pancake spherules are diagenetically altered clinopyroxene‐bearing spherules and that the shocked quartz, green (and presumably the dark spinel‐rich) pancake spherules, and Ir anomaly all belong to the same impact event. This conclusion is consistent with previous suggestions that the cpx spherule layer may be from the 100 km‐diameter Popigai impact crater in northern Siberia.     

A 3‐D study of mineral inclusions in chromite from ordinary chondrites using synchrotron radiation X‐ray tomographic microscopy—Method and applications

Abstract– A method is described for imaging in 3‐D the interiors of meteoritic chromite grains and their inclusions using synchrotron radiation X‐ray tomographic microscopy. In ordinary chondrites, chromite is the only common mineral that survives long‐term weathering on Earth. Information about the silicate matrix of the original meteorite, however, can be derived from mineral inclusions preserved in the protecting chromite. The inclusions are crucial in the classification of fossil meteorites as well as sediment‐dispersed chromite grains from decomposed meteorites and larger impacts, as these are used for characterizing the past influx of material to Earth, but have previously been difficult to locate. The method is non‐destructive and time efficient for locating inclusions. The method allowed quantitative and morphological studies of both host chromite grains and inclusions in three dimensions. The study of 385 chromite grains from eight chondrites (H4–6, L4–6, LL4, LL6) reveals that inclusions are abundant and equally common in all samples. Almost two‐thirds of all chromite grains contain inclusions, regardless of group and type. The study also shows that the size of the inclusions and the host chromite grains, as well as the number of inclusions, within the host chromite grains vary with petrographic type. Thus, the petrographic type of the host of a suite of chromite grains can be determined based solely on inclusion content. The study also revealed that the amount of fractures in the host chromite can be correlated to previously assigned shock stages for the various chondrites. The study has thus shown that the features and inclusions of fossil chromite grains can give similar information about a former host meteorite as do studies of an unweathered whole meteorite, meaning that this technique is essential in the studies of ancient meteorite flux to Earth.     

Extraterrestrial chromite in latest Maastrichtian and Paleocene pelagic limestone at Gubbio,Italy: The flux of unmelted ordinary chondrites

Abstract— –The distribution of sediment‐dispersed extraterrestrial (ordinary chondritic) chromite (EC) grains (>63 μm) has been studied across the latest Maastrichtian and Paleocene in the Bottaccione Gorge section at Gubbio, Italy. This section is ideal for determining the accumulation rate of EC because of its condensed nature and well‐constrained sedimentation rates. In a total of 210 kg of limestone representing eight samples of 14–28 kg distributed across 24 m of the Bottaccione section, only 6 EC grains were found (an average of 0.03 EC grains kg^?1). In addition, one probable pallasitic chromite grain was found. No EC grains could be found in two samples at the Cretaceous‐Tertiary (K‐T) boundary, which is consistent with the K‐T boundary impactor being a carbonaceous chondrite or comet low in chromite. The average influx of EC to Earth is calculated to ～～0.26 grain m^?2 kyr^?1. This corresponds to a total flux of ～～200 tons of extraterrestrial matter per year, compared to ～～30,000 tons per year, as estimated from Os isotopes in deep‐sea sediments. The difference is explained by the EC grains representing only unmelted ordinary chondritic matter, predominantly in the size range from ～～0.1 mm to a few centimeters in diameter. Sedimentary EC grains can thus give important information on the extent to which micrometeorites and small meteorites survive the passage through the atmosphere. The average of 0.03 EC grain kg^?1 in the Gubbio limestone contrasts with the up to ～～3 EC grains kg^?1 in mid‐Ordovician limestone that formed after the disruption of the L chondrite parent body in the asteroid belt at ～～470 Ma. The two types of limestone were deposited at about the same rate, and the difference in EC abundance gives support for an increase by two orders of magnitude in the flux of chondritic matter directly after the asteroid breakup.     

Lunar meteorite LaPaz Icefield 04841: Petrology,texture, and impact‐shock effects of a low‐Ti mare basalt

Abstract— Found during the 2004 U.S. Antarctic Search for Meteorites season, LaPaz Icefield (LAP) 04841 represents an addition to the LaPaz lunar basalts suite and brings the total mass collected to 1.93 kg. The presence of FeNi grains, troilite, and the anorthositic composition of plagioclase are evidence for the lunar origin of this meteorite. Pyroxene and olivine Mn/Fe values plot along the trend set for lunar basalts. Analyses of chromite grains provide a V/(Al + Cr) ratio of 1.33 ± 13, translating to an fO₂ one log unit below the IW buffer, in accordance with previous fO₂ estimates for lunar basalts. Application of the Zr‐cooling speedometer, for ilmenite and ulvöspinel pairs, gives a cooling rate of 5.2 °C/day, matching previous estimates of cooling rates for the LaPaz lunar meteorites and Apollo mare basalts. Mineral modes and chemistries, as well as trace‐element patterns, provide compelling evidence for pairing of this meteorite to others in the LaPaz lunar basalt suite.