Article

A global rain of micrometeorites following breakup of the L-chondrite parent body - Evidence from solar wind-implanted Ne in fossil extraterrestrial chromite grains from China

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Abstract

Previous studies of limestone beds of mid-Ordovician age from both Sweden and China show that the Earth saw an at least two orders of magnitude increase in the influx of extraterrestrial material approximately 470 Ma, following the disruption of an L-chondrite parent body in the asteroid belt. Recovered extraterrestrial material consists of fossil meteorites and sediment-dispersed extraterrestrial chromite (SEC) grains, both with L-chondritic origin. Ne isotope analysis of SEC grains from one of the Swedish limestone sections revealed that the vast majority of the grains were delivered to Earth as micrometeorites. In this study, we extend the previous work, both in time and geographically, by measuring concentrations and isotopic ratios of Ne in individual SEC grains (60-120 μm in diameter) from three different beds from a contemporary Middle Ordovician limestone section in China. All of the Chinese SEC grains, 44 in total, contain surface-implanted Ne of fractionated solar wind composition, implying that these grains were, as in the case of the Swedish SEC grains, delivered to Earth as micrometeorites. This gives further compelling evidence that the two to three orders of magnitude increase in the influx of micrometeoritic material following the breakup of the L-chondrite parent body was indeed a global event. The rain of micrometeorites prevailed for at least 2 Myr (the estimated time of the deposition of the topmost Chinese bed) after the breakup event.

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... The loss of radiogenic noble gases ( 4 He and 40 Ar) in these meteorites can be used to date the age of the gas-loss event to about 0.5 Ga ago (Anders, 1964;Bogard, 1995;Haack et al., 1996), with the most recent Ar-Ar ages converging on 470 ± 6 Ma (Korochantseva et al., 2007;Weirich et al., 2012). Condensed (i.e., slowly deposited) marine sediments formed on Earth around this time, in the Middle Ordovician time-period, record a sudden increase in the flux of extraterrestrial material, ranging from sub-millimeter-sized extraterrestrial dust grains found in coeval sediments from Sweden (Schmitz et al., 2003), China (Cronholm and Schmitz, 2010;Alwmark et al., 2012) and Russia (Korochantsev et al., 2009;Lindskog et al., 2012), to centimetersized fossil meteorites found so far only at different localities in Sweden (Thorslund and Wickman, 1981;Nyström et al., 1988;Tassinari et al., 2004). Also, an increase in the number of craters formed within a few tens of Ma after the LCPB asteroid disruption event has been suggested (e.g., Schmitz et al., 2001;Korochantseva et al., 2007). ...
... Instead, Meier et al. (2010) suggested that these pre-exposed grains must have resided for a total of several tens of millions of years in the topmost few meters of the regolith layer of the pre-break-up LCPB asteroid. The analysis of more SEC grains from Sweden and China showed that the deposition of large numbers of solar-wind-rich grains at 470 Ma is a global, stratigraphic event (Alwmark et al., 2012). In the present work, we study the He and Ne concentration of SEC grains from yet another mid-Ordovician site, the Lynna River section in Russia. ...
... Grains were placed individually into the holes of an aluminum sample holder, which was then placed into an extraction line underneath a viewport, and pumped to ultra-high vacuum (~10 -10 mbar) for 48 h. The noble gas analysis was done with the same instruments and protocols as used earlier by Meier et al. (2010) and Alwmark et al. (2012), therefore we will only give an abridged description here. We use a low blank extraction line with ZrO 2 /TiO 2 getters and three cold traps cooled with liquid N 2 to remove background gases such as H 2 , CH 4 , H 2 O, Ar and CO 2 . ...
Article
We measured the He and Ne concentrations of 50 individual extraterrestrial chromite grains recovered from mid-Ordovician (lower Darriwilian) sediments from the Lynna River section near St. Petersburg, Russia. High concentrations of solar wind-like He and Ne found in most grains indicate that they were delivered to Earth as micrometeoritic dust, while their abundance, stratigraphic position and major element composition indicate an origin related to the L chondrite parent body (LCPB) break-up event, 470 Ma ago. Compared to sediment-dispersed extraterrestrial chromite (SEC) grains extracted from coeval sediments at other localities, the grains from Lynna River are both highly concentrated and well preserved. As in previous work, in most grains from Lynna River, high concentrations of solar wind-derived He and Ne impede a clear quantification of cosmic-ray produced He and Ne. However, we have found several SEC grains poor in solar wind Ne, showing a resolvable contribution of cosmogenic 21Ne. This makes it possible, for the first time, to determine robust cosmic-ray exposure (CRE) ages in these fossil micrometeorites, on the order of a few hundred-thousand years. These ages are similar to the CRE ages measured in chromite grains from cm-sized fossil meteorites recovered from coeval sediments in Sweden. As the CRE ages are shorter than the orbital decay time of grains of this size by Poynting–Robertson drag, this suggests that the grains were delivered to Earth through direct injection into an orbital resonance. We demonstrate how CRE ages of fossil micrometeorites can be used, in principle, to determine sedimentation rates, and to correlate the sediments at Lynna River with the fossil meteorite-bearing sediment layers in Sweden. In some grains with high concentrations of solar wind Ne, we nevertheless find a well-resolved cosmogenic 21Ne signal. These grains must have been exposed for up to several 10 Ma in the regolith layer of the pre-break-up L chondrite parent body. This confirms an earlier suggestion that such regolith grains should be abundant in sediments deposited shortly after the break-up of the LCPB asteroid.
... The chondrite chromite grains from mid-Ordovician fossil meteorites and limestone also hold a record of noble gases induced by the solar wind and galactic cosmic rays (Heck et al., 2004(Heck et al., , 2008Meier et al., 2010;Alwmark et al., 2012). The Ne from cosmic rays is a spallation product formed when high-energy protons hit the target material, whereas solar wind Ne represents implanted atoms traveling with the solar wind. ...
... Myr, and the cosmic ray exposure ages generally increase upward in the sediment column ( Fig. 21) (Heck et al., 2004). The difference in cosmic ray exposure age between the stratigraphically highest and lowest meteorite, ∼1 Myr, is similar to the time difference in deposition on the sea floor based on biostratigraphic estimates of sedimentation rates (see Schmitz et al., 1996;Alwmark et al., 2012). The meteorites in the oldest bed quarried, Arkeologen, occur stratigraphically only ∼80 cm above the first level (in the Hällekis section) with abundant sediment-dispersed EC grains, i.e., the level most likely representing the time for the breakup of the L chondrite parent body (Fig. 11). ...
... The distribution of EC grains in China, Russia and Sweden in mid-Ordovician sediments formed after the L chondrite breakup event, provides new information about relative variations in sedimentation rates, including on a bed-by-bed basis. The EC grains at all three sites are dominantly of a micrometeoritic origin, based on noble gas measurements of the grains (Meier et al., 2010(Meier et al., , 2013Alwmark et al., 2012). After the breakup event, the micrometeorites were transported to Earth in a large cloud of dust by Poynting-Robertson drag. ...
Article
Relict spinel grains (~25–250 μm in diameter) from decomposed extraterrestrial material in Archean to Recent sediments can be used to reconstruct variations in the flux of different types of meteorites to Earth through the ages. Meteorite falls are rare and meteorites weather and decay rapidly on the Earth surface, making it a challenge to reconstruct ancient fluxes. Almost all meteorite types, however, contain a small fraction of spinel minerals that survive weathering and can be recovered by acid-dissolution of large samples (100–1000 kg) of slowly deposited sediments of any age. The spinel grains originate from either micrometeorites, meteorites or asteroids, and can give detailed information on the types of extraterrestrial matter that fell on Earth at specific times in the geological past. Inside the spinels, synchrotron-light X-ray tomography can identify 1–30 μm inclusions of most of the other minerals that made up the original meteorite. With advanced microanalyses of the spinels, such as Ne isotopes (from solar wind, and produced by cosmic rays), oxygen isotopes (meteorite class and group) and cosmic ray tracks, it may be possible to unravel from the geological record fundamental new information about the solar system at specific times through the past ~3.5 Gyr. Variations in flux and types of meteorites may reflect large-scale perturbations of the orbits of planets and other bodies in the solar system, as well as the sequence of disruptions of the parent bodies for the meteorite types known and not yet known. Orbital perturbations may be triggered by near-by passing stars, giant molecular clouds, the galactic gravitational field, supernova shock waves or unusual planetary alignments.
... 465±5 Ma ago), encompassing four of the impacts listed in Table 1, and many more candidates of similar (but non-robust) ages listed in the EIDB. A plausible explanation for this cluster is the break-up of a large asteroid near a powerful orbital resonance in the asteroid belt, resulting in a "shower" of fragments delivered to Earthcrossing orbits (Heck et al., 2004;Nesvorný et al., 2009;Ormö et al., 2014;Schmitz et al., 2001), confirmed independently by Ar-Ar ages of meteorites (Bogard, 2011), the recovery of fossil meteorites from Sweden (Heck et al., 2004;Schmitz et al., 2001) and micrometeoritic dust from different Ordovician sediment layers in Sweden, China and Russia (Alwmark et al., 2012;Meier et al., 2014Meier et al., , 2010. Nesvorný et al., (2009) suggested that the Gefion asteroid family in the main asteroid belt was formed by the same event. ...
Preprint
Rampino & Caldeira (2015) carry out a circular spectral analysis (CSA) of the terrestrial impact cratering record over the past 260 million years (Ma), and suggest a ~26 Ma periodicity of impact events. For some of the impacts in that analysis, new accurate and high-precision ("robust"; 2SE<2%) 40Ar-39Ar ages have recently been published, resulting in significant age shifts. In a CSA of the updated impact age list, the periodicity is strongly reduced. In a CSA of a list containing only impacts with robust ages, we find no significant periodicity for the last 500 Ma. We show that if we relax the assumption of a fully periodic impact record, assuming it to be a mix of a periodic and a random component instead, we should have found a periodic component if it contributes more than ~80% of the impacts in the last 260 Ma. The difference between our CSA and the one by Rampino & Caldeira (2015) originates in a subset of "clustered" impacts (i.e., with overlapping ages). The ~26 Ma periodicity seemingly carried by these clusters alone is strongly significant if tested against a random distribution of ages, but this significance disappears if it is tested against a distribution containing (randomly-spaced) clusters. The presence of a few impact age clusters (e.g., from asteroid break-up events) in an otherwise random impact record can thus give rise to false periodicity peaks in a CSA. There is currently no evidence for periodicity in the impact record.
... Only a small fraction of the ordinary chondrites falling to Earth today are derived from regoliths (3% of the L chondrites, 8% of the LL chondrites, and 20% of the H chondrites; Bischoff et al., 2018). However, most Antarctic micrometeorites contain fractionated solar He and Ne, and a significant fraction of them derive from ordinary chondritic parent bodies (Osawa et al., 2010;Baecker et al., 2018). While in interplanetary space, micrometeoroids are exposed to bombardment by solar photons, leading to orbital decay on a time-scale of 10 4 -10 6 Ma, depending on size, density, and original heliocentric distance (Burns et al., 1979). ...
Article
We have reconstructed the distribution of extraterrestrial chrome spinels in a marine limestone section across the Frasnian–Famennian stratotype section at Coumiac in southern France, providing the first insights on the types of micrometeorites and meteorites that fell on Earth at this time. The data can test whether the small cluster of roughly coeval, large impact structures is related to an asteroid breakup and shower with possible bearings also on the late Devonian biodiversity crisis. A total of ∼180 extraterrestrial spinel grains (>32μm) were recovered from 957 kg of rock. Noble-gas measurements of individual grains show high solar-wind content, implying an origin from decomposed micrometeorites. Element analyses indicate a marked dominance of ordinary chondritic over achondritic grains, similar to the recent flux. The relation between H, L and LL meteorites is ∼29–58–13%, similar to the late Silurian flux, ∼31–63–6%, but different from the distribution, ∼45–45–10%, in the recent and the Cretaceous flux. Our data show no indication of a generally enhanced late Devonian micrometeorite flux that would accompany an asteroid shower. However, in a single limestone bed that formed immediately before the Upper Kellwasser horizon, that represents the main end-Frasnian species-turnover event, we found an enrichment of ∼10 ordinary chondritic grains (>63μm) per 100 kg of rock, compared to the ∼1–3 grains per 100 kg that characterisebackground. The anomalously abundant grains are of mixed H, L and LL types and may be related to an enhanced flux of extraterrestrial dust during postulated minima in both the 405 ka and 2.4 Ma Earth-orbit eccentricity cycles at the onset of the Upper Kellwasser event. In the present solar system the dust accretion at Earth is the highest at eccentricity minima because of the spatial distribution of dust bands of the zodiacal cloud. Besides this small grain anomaly the data here and in previous studies support a stable meteorite flux through the late Silurian and Devonian, in contrast to the mid-Ordovician, when achondritic meteorites that are rare on Earth today were common, followed by the influx of a flood of debris related to the breakup of the L-chondrite parent body. Our accumulated data for six time windows through the Phanerozoic indicate that the ordinary chondrites make up a major fraction in the meteorite flux since at least the mid-Ordovician. We note that the sources in the asteroid belt of the H and L meteorites, the two most common types of meteorites today and through much of the Phanerozoic, remain elusive.
... Only a small fraction of the ordinary chondrites falling to Earth today are derived from regoliths (3% of the L chondrites, 8% of the LL chondrites, and 20% of the H chondrites; Bischoff et al., 2018). However, most Antarctic micrometeorites contain fractionated solar He and Ne, and a significant fraction of them derive from ordinary chondritic parent bodies (Osawa et al., 2010;Baecker et al., 2018). While in interplanetary space, micrometeoroids are exposed to bombardment by solar photons, leading to orbital decay on a time-scale of 10 4 -10 6 Ma, depending on size, density, and original heliocentric distance (Burns et al., 1979). ...
Article
We have reconstructed the distribution of extraterrestrial chrome spinels in a marine limestone section across the Frasnian–Famennian stratotype section at Coumiac in southern France, providing the first insights on the types of micrometeorites and meteorites that fell on Earth at this time. The data can test whether the small cluster of roughly coeval, large impact structures is related to an asteroid breakup and shower with possible bearings also on the late Devonian biodiversity crisis. A total of ∼180 extraterrestrial spinel grains (>32μm) were recovered from 957 kg of rock. Noble-gas measurements of individual grains show high solar-wind content, implying an origin from decomposed micrometeorites. Element analyses indicate a marked dominance of ordinary chondritic over achondritic grains, similar to the recent flux. The relation between H, L and LL meteorites is ∼29–58–13%, similar to the late Silurian flux, ∼31–63–6%, but different from the distribution, ∼45–45–10%, in the recent and the Cretaceous flux. Our data show no indication of a generally enhanced late Devonian micrometeorite flux that would accompany an asteroid shower. However, in a single limestone bed that formed immediately before the Upper Kellwasser horizon, that represents the main end-Frasnian species-turnover event, we found an enrichment of ∼10 ordinary chondritic grains (>63μm) per 100 kg of rock, compared to the ∼1–3 grains per 100 kg that characterisebackground. The anomalously abundant grains are of mixed H, L and LL types and may be related to an enhanced flux of extraterrestrial dust during postulated minima in both the 405 ka and 2.4 Ma Earth-orbit eccentricity cycles at the onset of the Upper Kellwasser event. In the present solar system the dust accretion at Earth is the highest at eccentricity minima because of the spatial distribution of dust bands of the zodiacal cloud. Besides this small grain anomaly the data here and in previous studies support a stable meteorite flux through the late Silurian and Devonian, in contrast to the mid-Ordovician, when achondritic meteorites that are rare on Earth today were common, followed by the influx of a flood of debris related to the breakup of the L-chondrite parent body. Our accumulated data for six time windows through the Phanerozoic indicate that the ordinary chondrites make up a major fraction in the meteorite flux since at least the mid-Ordovician. We note that the sources in the asteroid belt of the H and L meteorites, the two most common types of meteorites today and through much of the Phanerozoic, remain elusive.
... 465±5 Ma ago), encompassing four of the impacts listed in Table 1, and many more candidates of similar (but non-robust) ages listed in the EIDB. A plausible explanation for this cluster is the break-up of a large asteroid near a powerful orbital resonance in the asteroid belt, resulting in a "shower" of fragments delivered to Earthcrossing orbits (Heck et al., 2004;Nesvorný et al., 2009;Ormö et al., 2014;Schmitz et al., 2001), confirmed independently by Ar-Ar ages of meteorites (Bogard, 2011), the recovery of fossil meteorites from Sweden (Heck et al., 2004;Schmitz et al., 2001) and micrometeoritic dust from different Ordovician sediment layers in Sweden, China and Russia (Alwmark et al., 2012;Meier et al., 2014Meier et al., , 2010. Nesvorný et al., (2009) suggested that the Gefion asteroid family in the main asteroid belt was formed by the same event. ...
Article
Rampino & Caldeira (2015) carry out a circular spectral analysis (CSA) of the terrestrial impact cratering record over the past 260 million years (Ma), and suggest a ~26 Ma periodicity of impact events. For some of the impacts in that analysis, new accurate and high-precision ("robust"; 2SE<2%) 40Ar-39Ar ages have recently been published, resulting in significant age shifts. In a CSA of the updated impact age list, the periodicity is strongly reduced. In a CSA of a list containing only impacts with robust ages, we find no significant periodicity for the last 500 Ma. We show that if we relax the assumption of a fully periodic impact record, assuming it to be a mix of a periodic and a random component instead, we should have found a periodic component if it contributes more than ~80% of the impacts in the last 260 Ma. The difference between our CSA and the one by Rampino & Caldeira (2015) originates in a subset of "clustered" impacts (i.e., with overlapping ages). The ~26 Ma periodicity seemingly carried by these clusters alone is strongly significant if tested against a random distribution of ages, but this significance disappears if it is tested against a distribution containing (randomly-spaced) clusters. The presence of a few impact age clusters (e.g., from asteroid break-up events) in an otherwise random impact record can thus give rise to false periodicity peaks in a CSA. There is currently no evidence for periodicity in the impact record.
... The presence of surface-implanted solar-wind-derived helium and neon in sediment-dispersed extraterrestrial chrome spinels (SECs) that were recovered from similar sediments from several younger Ordovician beds from sites in Sweden, China and Russia is evidence that the SECs were parts of micrometeorites [11][12][13] . Because the abundance ratio of the two ordinary chondrite groups H and L chondrites in recently fallen coarse micrometeorites 14,15 is similar to this ratio in macroscopic meteorites, micrometeorites bearing coarse chromite grains can be used as a proxy for meteorites 7 . ...
Article
Full-text available
Most meteorites that fall today are H and L type ordinary chondrites, yet the main belt asteroids best positioned to deliver meteorites are LL chondrites 1,2 . This suggests that the current meteorite flux is dominated by fragments from recent asteroid breakup events 3,4 and therefore is not representative over longer (100-Myr) timescales. Here we present the first reconstruction of the composition of the background meteorite flux to Earth on such timescales. From limestone that formed about one million years before the breakup of the L-chondrite parent body 466 Myr ago, we have recovered relict minerals from coarse micrometeorites. By elemental and oxygen-isotopic analyses, we show that before 466 Myr ago, achondrites from different asteroidal sources had similar or higher abundances than ordinary chondrites. The primitive achondrites, such as lodranites and acapulcoites, together with related ungrouped achondrites, made up ~15–34% of the flux compared with only ~0.45% today. Another group of abundant achondrites may be linked to a 500-km cratering event on (4) Vesta that filled the inner main belt with basaltic fragments a billion years ago 5 . Our data show that the meteorite flux has varied over geological time as asteroid disruptions create new fragment populations that then slowly fade away from collisional and dynamical evolution. The current flux favours disruption events that are larger, younger and/or highly efficient at delivering material to Earth.
... Micrometeorites (MMs) are extraterrestrial dust particles <2 mm in size that are recovered from the Earth's surface and have been collected from Antarctic blue ice and snow, Antarctic traps, and aeolian deposits [Maurette et al., 1991;Taylor et al., 2000;Duprat et al., 2007;Rochette et al., 2008]. The flux of MMs captured by Earth is currently 40,000 t a À1 [Love and Brownlee, 1993]; however, changes over geological time occur due to secular and planetary perturbation of dust orbits [Kortenkamp and Dermott, 2001] and major dust production events in the solar system [Alwmark et al., 2012]. Changes in the flux of extraterrestrial dust have been detected in the 3 He content of ocean sediments and have been suggested to affect terrestrial climate [Farley and Patterson, 1995]. ...
Article
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A new type of cosmic spherule is reported with textures suggesting settling of olivine during atmospheric deceleration. Numerical simulations of entry heating reveal that relict forsterite, which survives melting, can settle over the 1-2 s of flight at high entry angles and entry velocities up to 16 km s-1. Enhanced crystallisation of phenocrysts by heterogeneous nucleation on accumulated relict forsterites is the most likely origin of the observed cumulate textures in cosmic spherules. Such textures in cosmic spherules reveal interplanetary dust with higher encounter velocity with the Earth that correspond to orbital eccentricities >0.3. The relative abundance of cumulate spherules suggests 14% of ordinary chondrite-related, S(IV)-type asteroid dust over the last 800 kyr had relatively high orbital eccentricity owing to secular and planetary perturbations. The textures of cosmic spherules collected from sediments can therefore be used to trace dust orbital variations with time which may influence terrestrial climate.
... 90 fossil L-chondritic meteorites in a Middle Ordovician limestone section 16 , which were deposited over a period of ,2 m.y. 3,17 , support the theory that the meteorite flux to Earth, and likely also the cratering rate, was enhanced by one to two orders of magnitude for at least a few million years as a consequence of the large breakup event in the MAB ,470 Ma 3,18 . The increased influx was further corroborated by the analysis of the approximately 5 m.y. ...
Article
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Approximately 470 million years ago one of the largest cosmic catastrophes occurred in our solar system since the accretion of the planets. A 200-km large asteroid was disrupted by a collision in the Main Asteroid Belt, which spawned fragments into Earth crossing orbits. This had tremendous consequences for the meteorite production and cratering rate during several millions of years following the event. The 7.5-km wide Lockne crater, central Sweden, is known to be a member of this family. We here provide evidence that Lockne and its nearby companion, the 0.7-km diameter, contemporaneous, Ma˚lingen crater, formed by the impact of a binary, presumably ‘rubble pile’ asteroid. This newly discovered crater doublet provides a unique reference for impacts by combined, and poorly consolidated projectiles, as well as for the development of binary asteroids.
... These SEC grains are again L chondritic in their major and minor element (Schmitz et al. 2003), and oxygen isotopic composition (Heck et al. 2010). Aside from Sweden, similarly high concentrations of SEC grains have recently also been identified in mid-Ordovician rocks from China (Cronholm and Schmitz 2010;Alwmark et al. 2012) and Russia (Korochantsev et al. 2009;Lindskog et al. 2012;Meier et al. 2014), demonstrating that the sudden deposition of many L chondritic SEC grains was a global event. The SEC grains contain He and Ne with an isotopic composition corresponding to that of the (fractionated) solar wind. ...
Article
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. ). 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.
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On the afternoon of Christmas Eve 1965, the village of Barwell, in Leicestershire, England, was the scene of an extraterrestrial invasion. Rocks rained down from the sky causing damage to buildings and vehicles, and consternation among the locals. It was soon established that these stones were actually meteorites and, with more than 40 kg falling in total, it proved to be the largest meteorite fall in UK history. Analysis showed the Barwell meteorites to be stone meteorites of a particular type known as ordinary chondrites. More specifically, they were a variety called L chondrites, which are among the most common type of stony meteorite so it might be assumed that its story prior to arrival in Barwell might not be especially interesting. In fact, nothing could be further from the truth. Common as they might be (by meteorite standards), the Barwell meteorites and other L chondrites are ‘ordinary’ only in name and they actually record some truly astonishing events in the history of the Solar System.
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Delivery of bioavailable iron and other bioessential metals to the sea surface influences the ocean-climate system through stimulation of marine primary productivity and organic carbon export in areas where eolian iron input is low, such as the Southern Ocean. In these regions, extraterrestrial dust may provide a significant fraction of bioavailable iron, due to the high reactivity of re-condensed material following ablation. If so, past episodes of increased extraterrestrial dust flux should be evident in lithologic, and potentially climatic, records. Here we show that the well-documented increase in extraterrestrial flux associated with the Ordovician L-chondrite parent-body breakup is close in time to proliferation of ooidal ironstones on continental shelves. We hypothesize that benthic iron flux from shelf sediments was increased by expanded oxygen minimum zones driven by broader regions of increased primary productivity. A later Mesozoic interval of global proliferation of ooidal ironstones and black shales also occurs at a peak in asteroid family ages formed by major main-belt collisions. Although internal forcings and feedbacks, such as global tectonic and magmatic processes, play the dominant control on long-term changes in biogeochemical cycles and related marine lithologic records, we suggest that extraterrestrial dust flux may play a role as an external forcing on Earth's climate system.
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The Decorah structure, recently discovered in northeastern Iowa, now appears as an almost entirely subsurface, deeply eroded circular basin 5.6 km in diameter and ~200 m deep, that truncates a near-horizontal series of Upper Cambrian to Lower Ordovician platform sediments. Initial analysis of geological and well-drilling data indicated characteristics suggestive of meteorite impact: a circular outline, a shallow basin shape, discordance with the surrounding geology, and a filling of anomalous sediments: (1) the organic-rich Winneshiek Shale, which hosts a distinctive fossil Lagerstätte, (2) an underlying breccia composed of fragments from the surrounding lithologies, and (3) a poorly known series of sediments that includes shale and possible breccia. Quartz grains in drill samples of the breccia unit contain abundant distinctive shock-deformation features in ~1% of the individual quartz grains, chiefly planar fractures (cleavage) and planar deformation features (PDFs). These features provide convincing evidence that the Decorah structure originated by meteorite impact, and current models of meteorite crater formation indicate that it formed as a complex impact crater originally ~6 km in diameter. The subsurface characteristics of the lower portion of the structure are not well known; in particular, there is no evidence for the existence of a central uplift, a feature generally observed in impact structures of comparable size. The current estimated age of the Decorah structure (460-483 Ma) suggests that it may be associated with a group of Middle Ordovician impact craters (a terrestrial "impact spike") triggered by collisions in the asteroid belt at ca. 470 Ma.
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The disruption of the L chondrite parent body (LCPB) at ~470 Ma is currently the best‐documented catastrophic celestial impact event, based on the large number of L chondritic materials associated with this event. Uranium‐lead (U‐Pb) dating of apatite and its high‐pressure decomposition product, tuite, in the Sixiangkou L6 chondrite provides a temporal link to this event. The U‐Pb system of phosphates adjacent to shock melt veins was altered to varying degrees and the discordance of the U‐Pb system correlates closely with the extent of apatite decomposition. This suggests that the U‐Pb system of apatite could be substantially disturbed by high‐temperature pulse during shock compression from natural impacts, at least on the scale of mineral grains. Although many L chondrites can be temporally related to the catastrophic LCPB impact event, the shock conditions experienced by each individual meteorite vary. This could be due to the different geologic settings of these meteorites on their parent body. The shock pressure and duration derived from most meteorites may only reflect local shock features rather than the impact conditions, although they could provide lower limits to the impact conditions. The Sixiangkou shock duration (~4 s), estimated from high‐pressure transformation kinetics, provides a lower limit to the high‐pressure pulse of the LCPB disruption impact. Combined with available literature data of L chondrites associated with this impact event, our results suggest that the LCPB suffered a catastrophic collision with a large projectile (with a diameter of at least 18–22 km) at a low impact velocity (5–6 km s−1). This is consistent with astronomical estimates based on the dynamical evolution of L chondritic asteroids.
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Micrometeorites (MMs) are extraterrestrial dust particles that survive atmospheric entry and can be recovered from sedimentary rocks. Fossil MMs allow events beyond the Earth, such as the collisional break-up of asteroids to be identified. Here the effects of vesicle formation during melting of dust are investigated through numerical modelling and observations of Antarctic MMs. Vesicle formation is shown to cause a parachute effect that causes rapid deceleration, decreasing peak temperature. Vesicular parachuting enhances the abundance of melted MMs formed from phyllosilicate-bearing C-type asteroid dust on the Earth-surface by a factor of two. Micrometeorites recovered from the geological record, therefore, are biased towards break-up events involving hydrated C-type asteroids, whilst those involving phyllosilicate-poor particles are diluted by the enhanced background flux of hydrous dust. The parachute effect is also likely to increase the delivery of 3He to ocean sediments by C-type asteroid dust.
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We report the discovery of significant numbers (500) of large micrometeorites (>100 mm) from rooftops in urban areas. The identification of particles as micrometeorites is achieved on the basis of their compositions, mineralogies, and textures. All particles are silicate-dominated (S type) cosmic spherules with subspherical shapes that form by melting during atmospheric entry and consist of quench crystals of magnesian olivine, relict crystals of forsterite, and iron-bearing olivine within glass. Four particles also contain Ni-rich metal-sulfide beads. Bulk compositions are chondritic apart from depletions in the volatile, moderately volatile, and siderophile elements, as observed in micrometeorites from other sources. The reported particles are likely to have fallen on Earth in the past 6 yr and thus represent the youngest large micrometeorites collected to date. The relative abundance ratio of barred olivine to cryptocrystalline spherule types in the urban particles of 1.45 is shown to be higher than a Quaternary average of ~0.9, suggesting variations in the extraterrestrial dust flux over the past 800 k.y. Changes in the entry velocities of dust caused by quasi-periodic gravitational perturbation during transport to Earth are suggested to be responsible. Variations in cosmic spherule abundance within the geologic column are thus unavoidable and can be a consequence of dust transport as well as major dust production events.
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The high content of extraterrestrial chromite grains was discovered in Russian Ordovician sediments, those are coeval to Swedish limestones, bearing the fossil meteorites and extraterrestrial chromites.
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Sediment-dispersed extraterrestrial chromite grains (63–355 µm) have been recovered in a section across the Arenig-Llanvirn transition in the Killeröd quarry in southeastern Scania. Previous studies of the same stratigraphic interval in the Orthoceratite Limestone at Kinnekulle, ca. 350 km to the north in Västergötland, have shown a two orders of magnitude increase in extraterrestrial chromite beginning close to the Arenig-Llanvirn boundary. The stratigraphic distribution and abundance trends of extraterrestrial chromite are essentially identical at Killeröd and Kinnekulle. In the Killeröd sec-tion extraterrestrial chromite grains are extremely rare (2 grains/125 kg of rock) in the Arenig lower 8 m of the section (Megistaspis simon to Asaphus expansus trilobite zones) and abundant (326 grains/ 162 kg of rock) in the succeeding ca. 3 m of uppermost Arenig and lower Llanvirn (upper A. expan-sus and A. raniceps zones) section. The extraterrestrial chromite grains at Killeröd and Kinnekulle are very similar in chemical composition, including characteristic elevated values and narrow ranges of V 2 O 3 (0.6–0.9 wt%) and TiO 2 (2.0–3.5 wt%). At Killeröd there is a small group (<4%) of chromite grains with relatively low TiO 2 (1.5–2.0 wt%), but otherwise typical extraterrestrial compositions. These grains may reflect a somewhat more aggressive diagenetic environment at Killeröd than at Kinnekulle. As in previous studies, rare chrome spinel grains with terrestrial or uncertain origin have also been found in the limestone. The results of this study support previous suggestions that after the disruption of the L chondrite parent body in the asteroid belt at ca. 470 Ma, the flux of extraterrestrial matter to Earth was enhan-ced by up to two orders of magnitude compared to the present. This is supported by finds in Swe-den of abundant fossil meteorites in uppermost Arenig and lower Llanvirn sediments. Whether these conclusions can be put into a global context awaits further study of Middle Ordovician lime-stones from other continents.
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Author Posting. © Nature Publishing Group, 2007. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 1 (2008): 49-53, doi:10.1038/ngeo.2007.37. The rise and diversification of shelled invertebrate life in the early Phanerozoic took place in two major steps. During the Cambrian Explosion at ca. 540 Ma a large number of new phyla appeared over a short time interval. Biodiversity at the family, genus and species level, however, remained low until the Great Ordovician Biodiversification Event (GOBE) in the mid-Ordovician. This event represents the most intense phase of species radiation during the Paleozoic and the biological component of planet's seafloors was irreversibly changed. The causes of the GOBE remain elusive mainly because of a lack of detailed data relating faunal to environmental change. Here we show that the onset of the major phase of the GOBE coincides at ca. 470 Ma with the disruption in the asteroid belt of the L chondrite parent body, the largest documented asteroid breakup event during the last few billion years. The precise coincidence between an event in space and on Earth is established by bed-by-bed records of extraterrestrial chromite, osmium isotopes and invertebrate fossils in mid- Ordovician strata in Baltoscandia and China. We argue that frequent impacts on Earth of kilometer-sized asteroids accelerated the biodiversification. This is supported also by abundant mid-Ordovician fossil meteorites and impact craters. This study was supported by funds to B.S. from the National Geographic Society, Swedish Research Council (VR) and Crafoord Foundation and to D.A.T.H. from the Carlsberg Foundation.
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Very large collisions in the asteroid belt could lead temporarily to a substantial increase in the rate of impacts of meteorites on Earth. Orbital simulations predict that fragments from such events may arrive considerably faster than the typical transit times of meteorites falling today, because in some large impacts part of the debris is transferred directly into a resonant orbit with Jupiter. Such an efficient meteorite delivery track, however, has not been verified. Here we report high-sensitivity measurements of noble gases produced by cosmic rays in chromite grains from a unique suite of fossil meteorites preserved in approximately 480 million year old sediments. The transfer times deduced from the noble gases are as short as approximately 10(5) years, and they increase with stratigraphic height in agreement with the estimated duration of sedimentation. These data provide powerful evidence that this unusual meteorite occurrence was the result of a long-lasting rain of meteorites following the destruction of an asteroid, and show that at least one strong resonance in the main asteroid belt can deliver material into the inner Solar System within the short timescales suggested by dynamical models.
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This chapter provides an overview of available noble gas data for solar system bodies apart from the Earth, Mars, and asteroids. Besides the Sun, the Moon, and the giant planets, we will also discuss data for the tenuous atmospheres of Mercury and the Moon, comets, interplanetary dust particles and elementary particles in the interplanetary medium and beyond. In addition, we summarize the scarce data base for the Venusian atmosphere. The extensive meteorite data from Mars and asteroidal sources are discussed in chapters in this volume by Ott (2002), Swindle (2002a,b) and Wieler (2002). Data from the Venusian and Martian atmospheres are discussed in more detail in chapters by Pepin and Porcelli (2002) and Swindle (2002b). Where appropriate, we will also present some data for other highly volatile elements such as H or N. The solar system formed from a molecular cloud fragment—traditionally called the solar nebula—that was rather well mixed. Therefore, isotopic abundances in almost all available solar system materials are very similar to each other, and elemental abundances in primitive meteorites are also similar to the values in the Sun. The major exceptions to this rule are the noble gases. Because they are chemically inert and volatile, they are very strongly depleted in solid matter. As a consequence, numerous noble gas “components” can be recognized throughout the solar system which are not necessarily related to the composition of the bulk nebula. Still, one major question in cosmochemistry is to what extent planetary bodies contain reservoirs that reflect the noble gas composition in the nebula or the presolar cloud. To discuss this, we first need a proxy for the nebula composition. The obvious choice is the bulk Sun, except for He, which has been produced in the Sun throughout its history. We will discuss the …
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Numerous fossil meteorites and high concentrations of sediment-dispersed extraterrestrial chromite (EC) grains with ordinary chondritic composition have previously been documented from 467 ± 1.6 Ma Middle Ordovician (Darriwilian) strata. These finds probably reflect a temporarily enhanced influx of L-chondritic matter, following the disruption of the L-chondrite parent body in the asteroid belt 470 ± 6 Ma. In this study, a Volkhovian-Kundan limestone/marl succession at Lynna River, northwestern Russia, has been searched for EC grains (>63 μm). Eight samples, forming two separate sample sets, were collected. Five samples from strata around the Asaphus expansus-A. raniceps trilobite Zone boundary, in the lower-middle Kundan, yielded a total of 496 EC grains in 65.5 kg of rock (average 7.6 EC grains kg-1, but up to 10.2 grains kg-1). These are extremely high concentrations, three orders of magnitude higher than "background" levels in similar condensed sediment from other periods. EC grains are typically about 50 times more abundant than terrestrial chrome spinel in the samples and about as common as terrestrial ilmenite. Three stratigraphically lower lying samples, close to the A. lepidurus-A. expansus trilobite Zone boundary, at the Volkhov-Kunda boundary, yielded only two EC grains in 38.2 kg of rock (0.05 grains kg-1). The lack of commonly occurring EC grains in the lower interval probably reflects that these strata formed before the disruption of the L-chondrite parent body. The great similarity in EC chemical composition between this and other comparable studies indicates that all or most EC grains in these Russian mid-Ordovician strata share a common source--the L-chondrite parent body.
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We have measured helium and neon concentrations, elemental and isotopic ratios of 91 individual presolar graphite grains from the KFC1 density separate of the Murchison meteorite. Eleven grains contain measurable amounts of either 4He, 20Ne, 21Ne or 22Ne, or a combination thereof. We report the first detection of 21Ne from an individual presolar graphite grain and the first detection of 4He and 20Ne in individual KFC1 graphite grains. Six of the gas-rich grains originate from asymptotic giant branch (AGB) stars, while another five are likely derived from core-collapse supernovae. The mono-isotopic 22Ne detected in one supernova grain is either radiogenic and compatible with condensation in the O/Ne zone, or nucleosynthetic and derived from the He/C zone. Two grains with 20Ne and 12C/13C
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This Ph.D. thesis is dedicated to the study of helium and neon, from dust-sized extraterrestrial samples, to gain insight on their provenance; in particular presolar silicon carbide (SiC) stardust from primitive meteorites (Murchison and Murray) and relict chromite grains from fossil meteorites and micrometeorites. Noble gas analyses were conducted with an ultra-high sensitivity noble gas mass spectrometer coupled with infrared laser gas extraction. The primary objective of the first study was to determine the fraction of single sub-micron to micron-sized presolar SiC grains containing detectable nucleosynthetic noble gases. This was combined with NanoSIMS analyses of silicon, carbon and nitrogen isotopes to constrain the origin of the grains. About 40 percent of the samples studied contain nucleosynthetic noble gases. The majority formed as circumstellar condensates around low-mass carbon stars on the Asymptotic Giant Branch (AGB). A small fraction of the gas-rich grains might have originated in other types of stars and in the ejecta of supernovae and arguably novae. The second part is dedicated to fossil meteorites and sediment-dispersed extraterrestrial chromite grains from 480 Myr old marine sediments from Sweden to constrain the delivery of extraterrestrial material from the asteroid belt to Earth. An important result are the unusually low cosmic-ray exposure ages (about 100 kyr to 1 Myr) of a suite of fossil meteorites, determined from noble gas concentrations in relict chromite grains. Exposure ages increase in younger sediments and the difference between the meteorites found in the youngest sediments and the ones from the oldest is ~1 Myr, corresponding to the age difference of the host sediments. The short exposure ages are consistent with model predictions of space residence times of meteorites generated by a major asteroid collision close to regions where orbits are unstable due to resonances. Surprisingly, extraterrestrial chromite grains dispersed in the same sediments as the fossil meteorites are very rich in solar noble gases. The solar gas-richness suggests they were produced as dust in the same asteroid break-up and came to Earth as sub-mm sized micrometeorites. In an additional pilot study, helium and neon isotopes in stratospheric interplanetary dust particles (IDPs) were analyzed.
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Very different palaeogeographical reconstructions have been produced by a combination of palaeomagnetic and faunal data, which are re-evaluated on a global basis for the period from 500 to 400 Ma, and are presented with appropriate confidence (or lack of it) on six maps at 20 Ma intervals. The palaeomagnetic results are the most reliable for establishing the changing palaeolatitudes of Baltica, Laurentia and Siberia. However, global palaeomagnetic reliability dwindles over the 100 Ma, and more evidence for relative continental positioning can be gleaned from study of the distribution of the faunas in the later parts of the interval. The new maps were generated initially from palaeomagnetic data when available, but sometimes modified, and terranes were positioned in longitude to take account of key faunal data derived from the occurrences of selected trilobites, brachiopods and fish. Kinematic continuity over the long period is maintained. The many terranes without reliable palaeomagnetic data are placed according to the affinities of their contained fauna. The changing positions of the vast palaeocontinent of Gondwana (which has hitherto been poorly constrained) as it drifted over the South Pole during the interval have been revised and are now more confidently shown following analysis of both faunal and palaeomagnetic data in combination, as well as by the glacial and periglacial sediments in the latest Ordovician. In contrast, the peri-Gondwanan and other terranes of the Middle and Far East, Central Asia and Central America are poorly constrained.
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Abstract— The noble gases He, Ne, Ar, Kr, and Xe were measured in 27 individual Antarctic micrometeorites (AMMs) in the size range 60 to 250 μm that were collected at the Dome Fuji Station. Eleven of the AMMs were collected in 1996 (F96 series) and 16 were collected in 1997 (F97 series). One of the F97 AMMs is a totally melted spherule, whereas all other particles are irregular in shape. Noble gases were extracted using a Nd-YAG continuous wave laser with an output power of 2.5-3.5 W for ˜5 min. Most particles released measurable amounts of noble gases. 3He/4He ratios are determined for 26 AMMs ((0.85-9.65) × 10−4). Solar energetic particles (SEP) are the dominant source of helium in most AMMs rather than solar wind (SW) and cosmogenic He. Three samples had higher 3He/4He ratios compared to that of SW, showing the presence of spallogenic 3He. The Ne isotopic composition of most AMMs resembled that of SEP as in the case of helium. Spallogenic 21Ne was detected in three samples, two of which had extremely long cosmic-ray exposure ages (> 100 Ma), calculated by assuming solar cosmic-ray (SCR) + galactic cosmic-ray (GCR) production. These two particles may have come to Earth directly from the Kuiper Belt. Most AMMs had negligible amounts of cosmogenic 21 Ne and exposure ages of <1 Ma. 40Ar/36Ar ratios for all particles (3.9–289) were lower than that of the terrestrial atmosphere (296), indicating an extraterrestrial origin of part of the Ar with a very low 40Ar/36Ar ratio plus some atmospheric contamination. Indeed, 40Ar/36Ar ratios for the AMMs are higher than SW, SEP, and Q-Ar values, which is explained by the presence of atmospheric 40Ar. The average 38Ar/36Ar ratio of 24 AMMs (0.194) is slightly higher than the value of atmospheric or Q-Ar, suggesting the presence of SEP-Ar which has a relatively high 38Ar/36Ar ratio. According to the elemental compositions of the heavy noble gases, Dome Fuji AMMs can be classified into three groups: chondritic (eight particles), air-affected (nine particles), and solar-affected (eight particles). The eight AMMs classified as chondritic preserve the heavy noble gas composition of primordial trapped component due to lack of atmospheric adsorption and solar implantation. The average of 129Xe/132Xe ratio for the 16 AMMs not affected by atmospheric contamination (1.05) corresponds to the values in matrices of carbonaceous chondrites (˜1.04). One AMM, F96DK038, has high 129Xe/132Xe in excess of this ratio. Our results imply that most Dome Fuji AMMs originally had chondritic heavy noble gas compositions, and carbonaceous chondrite-like objects are appropriate candidate sources for most AMMs.
Article
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% TiO 2 and stable V 2 O 3 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.
Article
Abstract— Radiochronometry of L chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 Myr ago. Fossil meteorites from Sweden indicate a highly increased influx of extraterrestrial matter in the Middle Ordovician ∼480 Myr ago. An association with the L-chondrite parent body event was suggested, but a definite link is precluded by the lack of more precise radiometric ages. Suggested ages range between 450 ± 30 Myr and 520 ± 60 Myr, and can neither convincingly prove a single breakup event, nor constrain the delivery times of meteorites from the asteroid belt to Earth. Here we report the discovery of multiple 40Ar-39Ar isochrons in shocked L chondrites, particularly the regolith breccia Ghubara, that allow the separation of radiogenic argon from multiple excess argon components. This approach, applied to several L chondrites, yields an improved age value that indicates a single asteroid breakup event at 470 ± 6 Myr, fully consistent with a refined age estimate of the Middle Ordovician meteorite shower at 467.3 ± 1.6 Myr (according to A Geologic Time Scale 2004). Our results link these fossil meteorites directly to the L-chondrite asteroid destruction, rapidly transferred from the asteroid belt. The increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate, which implies severe environmental stress.
Article
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.
Article
Chromite is the only common meteoritic mineral surviving long-term exposure on Earth, however, the present study of relict chromite from numerous Ordovician (470 Ma) fossil meteorites and micrometeorites from Sweden, reveals that when encapsulated in chromite, other minerals can survive for hundreds of millions of years maintaining their primary composition. The most common minerals identified, in the form of small (<1–10 μm) anhedral inclusions, are olivine and pyroxene. In addition, sporadic merrillite and plagioclase were found.Analyses of recent meteorites, holding both inclusions in chromite and corresponding matrix minerals, show that for olivine and pyroxene inclusions, sub-solidus re-equilibration between inclusion and host chromite during entrapment has led to an increase in chromium in the former. In the case of olivine, the re-equilibration has also affected the fayalite (Fa) content, lowering it with an average of 14% in inclusions. For Ca-poor pyroxene the ferrosilite (Fs) content is more or less identical in inclusions and matrix. By these studies an analogue to the commonly applied classification system for ordinary chondritic matrix, based on Fa in olivine and Fs in Ca-poor pyroxene, can be established also for inclusions in chromite. All olivine and Ca-poor pyroxene inclusions (>1.5 μm) in chromite from the Ordovician fossil chondritic material plot within the L-chondrite field, which is in accordance with previous classifications. The concordance in classification together with the fact that inclusions are relatively common makes them an accurate and useful tool in the classification of extraterrestrial material that lacks matrix silicates, such as fossil meteorites and sediment-dispersed chromite grains originating primarily from decomposed micrometeorites but also from larger impacts.
Article
In an attempt to establish the cause of the short gas-retention ages among the hypersthene chondrites, two lines of evidence were examined. (1) Rare-gas contents were determined in 20 black hypersthene chondrites by mass spectrometry. (2) Symptoms of shock and reheating were studied by X-ray diffraction analysis in olivine from 40 chondrites, including 15 black ones; and by metallographic techniques in 20 chondrites, including 18 black ones.On the basis of the results, it is concluded that an earlier suggestion by Anders that black chondrites are severely shocked is essentially correct. At least 15 black hypersthene chondrites seem to have suffered shocks ranging from a few hundred kilobars up to perhaps 1 or 1.5 Mbar. The shock apparently transformed olivine to a fine-grained polycrystalline state; the crystallites show a range of preferred orientation.From the cosmogenic ratios, it is concluded that black hypersthene chondrites generally suffered He losses of approximately 20–25% at a late stage, during the cosmic-ray bombardment era. The corrected He3 and He4 contents of 12 black hypersthene chondrites define an isochron of 520 ± 60 Myr with a (He4/He3)cosmogenic production ratio of 5.2 ± 0.3.A review of literature data shows that on the basis of their He contents, one-third of the hypersthene chondrites seem to have been definitely involved in the 520 Myr event, another one-third probably so. The large proportion of concordant K-Ar ages among the former strongly supports the contention that the 520 Myr date represents a genuine out-gassing event. These results bear out earlier conclusions by Anders that most, if not all the hypersthenes were on one or at most two parent bodies until relatively late in their history; and by Kirsten, Krankowsky, and Zähringer that short gas-retention ages of chondrites have arisen from collisional reheating of cold parent objects.The 520 Myrcollision seems to have involved the breakup of a parent asteroid of the hypersthene chondrites. The well-known paradox of the cosmic-ray exposure ages being much shorter (0.03 ∼- 60 Myr) can be solved by the hypothesis that kilometer-sized fragments of the parent asteroid were deflected by Mars into Earth-crossing orbits on a tume scale of ≈ 109 yr. Chondrites spalled off from such fragments by secondary collisions have lifetimes against planetary capture of 106–107 yr. The destruction lifetime of chondrites in the asteroid belt can be increased from 107 to ≈ 108 yr with a 10-fold reduction on the cross-section requirement.
Article
The work reported here confirms the existence of (largely) unmelted micrometeorites in sediments from seasonal lakes on the Greenland ice sheet. Approximately1/3 of the dark and irregularly shaped particles selected from disaggregated sediments had sufficient contents of Ne to permit measurement of isotopic compositions. The sediments consisted of a mixture of biologically derived contaminants and minor amounts of terrestrial minerals and extraterrestrial fragments. Although terrestrial mineral contamination varied by a factor of 20 for the two different sediments, a yield of ∼ 1000 cosmic particles/kg of wet cryoconite was found for both samples. The isotopic compositions of the Ne in both rounded and irregular (25% of total) particles that contained enough gas to measure were non-terrestrial, plotting in a field defined by solar wind (SW), solar energetic particle (SEP) and spallation. Exposure ages ranged from < 0.5 Ma to 20 Ma, consistent with previous estimates from10Be and26Al measurements of larger spherules removed from similar sediments. Comparison of the Ne data with compositional-morphological data demonstrates the general validity of criteria previously used to identify extraterrestrial particles; however, two particles identified as of probable terrestrial origin by three independent, experienced meteorite petrologists were shown by the Ne data to be extraterrestrial and probably extraterrestrial, respectively. Future studies of Greenland micrometeorites are outlined and improved collection techniques are discussed.
Article
We have measured the He and Ne concentrations and isotopic ratios of individual sediment-dispersed extraterrestrial chromite grains (63–180 μm in diameter) from an Ordovician limestone in southern Sweden. In the same sediment, many fossil meteorites were found and have been attributed to the L chondrite parent body breakup event ∼ 470 Ma ago. In this analysis of 37 individual extraterrestrial chromite grains of L chondritic major element composition, at least 35 (∼ 95%) contain surface-implanted helium and neon of fractionated solar wind composition, implying that these grains are (relict parts of) fossil micrometeorites of asteroidal origin. Similar to what has been observed in recent micrometeorites collected in the polar regions, significant amounts of cosmogenic 21Ne were found in several grains, resulting in cosmic ray exposure (CRE) ages of up to ∼ 77 Myr. These ages exceed both dynamical lifetimes for asteroidal micrometeorites of this size as well as CRE ages observed in chromites from fossil meteorites from the same sediment beds. Significant contributions from terrestrial sources, like nucleogenic and cosmic-ray muon induced 21Ne can be excluded in the extraterrestrial chromites, since 11 terrestrial chrome spinel grains from the same sediment beds did not contain any measurable 21Ne excesses. Having found micrometeorites of undisputed asteroidal origin with cosmic ray exposure ages of several 107 years implies that high cosmic ray exposure ages alone are not a good indicator of cometary origin, in opposition to what has been suggested for recent micrometeorites and IDPs. We propose instead that these grains collected their cosmogenic 21Ne while residing in the regolith layer of their parent body.
Article
We examined oxygen three-isotope ratios of 48 extraterrestrial chromite (EC) grains extracted from mid-Ordovician sediments from two different locations in Sweden, and one location in south-central China. The ages of the sediments (∼470 Ma) coincide with the breakup event of the L chondrite parent asteroid. Elemental compositions of the chromite grains are generally consistent with their origin from L or LL chondrite parent bodies. The average Δ17O (‰-deviation from the terrestrial mass-fractionation line, measured in situ from 15 μm spots by secondary ion mass spectrometry; SIMS) of EC grains extracted from fossil meteorites from Thorsberg and Brunflo are 1.17 ± 0.09‰ (2σ) and 1.25 ± 0.16‰, respectively, and those of fossil micrometeorites from Thorsberg and Puxi River are 1.10 ± 0.09‰, and 1.11 ± 0.12‰, respectively. Within uncertainty these values are all the same and consistent with the L chondrite group average Δ17O = 1.07 ± 0.18‰, but also with the LL chondrite group average Δ17O = 1.26 ± 0.24‰ (Clayton et al., 1991). We conclude that the studied EC grains from correlated sediments from Sweden and China are related, and most likely originated in the same event, the L chondrite parent body breakup. We also analyzed chromites of modern H, L and LL chondrites and show that their Δ17O values coincide with averages of Δ17O of bulk analyses of H, L and LL chondrites. This study demonstrates that in situ oxygen isotope data measured by SIMS are accurate and precise if carefully standardized, and can be used to classify individual extraterrestrial chromite grains found in sediments.
Article
Meteorites are impact-derived fragments from ≈ 85 parent bodies. For seven of these bodies, the meteorites record evidence suggesting that they may have been catastrophically fragmented. We identify three types of catastrophic events: (a) impact and reassembly events > 4.4 Gy ago, involving molten or very hot parent bodies(> 1200°C); this affected the parent bodies of the ureilites, Shallowater, and the mesosiderites. In each case, the fragments cooled rapidly (≈ 1–1000°C day−1) and then reassembled, (b) Later impacts involving cold bodies which, in some cases, reassembled; this occurred on the H and L ordinary chondrite parent bodies. The L parent body probably suffered another catastrophic event about 500 My ago. (c) Recent impacts of cold, multi-kilometer-sized bodies that generated meter-sized meteoroids; this occurred on the parent bodies of the IIIAB irons (650 My ago), the IVA irons (400 My ago), and the H ordinary chondrite (7 My ago).
Article
Forty fossil meteorites with a total original mass of ∼7.7 kg have been recovered in the first systematic search for fossil meteorites, pursued in an active quarry in Lower Ordovician (480 Ma) marine limestone in southern Sweden. The meteorites represent at least 12 different falls over a seafloor area of ∼6000 m2 during ≤1.75 Myr, making the quarry one of the most meteorite dense areas known in the world. Geochemical analyses of relict chromite grains indicate that all or most of the meteorites are ordinary chondrites and probably L chondrites. Mechanisms for meteorite delivery from the asteroid belt to Earth were the same 480 Ma as today, however, the flux was one to two orders of magnitude higher, most likely reflecting the disruption of the L chondrite parent body at about that time. This is a major event in late solar-system history, which may also have led to an enhanced flux of asteroids to Earth during ∼30 Myr.
Article
The old terrane of Baltica occupies the mass of northern Europe eastwards to the Urals and lies mostly to the north of the Trans-European Suture Zone. The core, the East European Craton, is thick and formed of rocks dating back to well over 3 billion yr, and Protobaltica can be identified as forming part of the supercontinent of Rodinia at about 1 billion yr ago. Following Rodinia's break up at about 800 Ma, Protobaltica remained attached to Laurentia until it became the newly independent Baltica at between 570 and 550 Ma, with the inauguration of plate spreading to form the northern part of the Iapetus Ocean. To the south, during the Early Cambrian, Baltica was separated from Gondwana by the relatively narrow Ran Ocean. Baltica remained a separate terrane until its docking, firstly with Avalonia at the very end of the Ordovician (443 Ma), and then with Laurentia during the Silurian in the Scandian part of the Caledonide Orogeny. The terrane was much enlarged in the Vendian to include the areas such as Timan–Pechora now lying to the north as they became accreted to Baltica during the late Precambrian Timanide Orogeny. During the Cambrian and Ordovician, Baltica firstly rotated through more than 120° and then drifted northwards from high to low palaeolatitudes. New maps present Baltica's outline and progressive positioning, its late Precambrian and Lower Palaeozoic history, and the Cambrian, Ordovician and Silurian land, basins and biofacies belts within and around it. Some of the Lower Palaeozoic faunas are reviewed briefly: the oceans surrounding Baltica were so wide during the Early Ordovician that a substantial part of the benthic fauna of trilobites, brachiopods and other phyla were endemic. As those oceans narrowed, so the faunas of Baltica became progressively more similar to those of adjacent terranes. Some plankton distributions augment the palaeomagnetic data in latitudinal positioning.
Article
A diffusion mechanism is proposed which takes into account phenomena observed in ion-implanted solids, in particular the precipitation of rare gas in the form of bubbles and their migration. The composition of rare gases in the bubbles is inferred from the calculated distribution of solar wind rare gas ions as a function of depth in the grains. These calculations are made for the location and average composition of Apollo 11 samples. It is shown by analogy with experimental observations in ion-implanted solids that the bubbles would migrate towards the surface and that the diffusion constant for this migration would be strongly depth dependent. Relative abundances of rare gas nuclides remaining behind due to the resultant degassing are estimated for one Apollo 11 soil sample and are compared with observed relative abundances for this sample. A qualitative explanation of some of the experimental observations of Ducati et al. on individual lunar grains is also offered.
Article
Previous studies of mid-Ordovician limestone in Sweden have shown that over a stratigraphic interval representing a few million years there is a two orders-of-magnitude enrichment in fossil L-chondritic meteorites (Ø = 1–21 cm) and sediment-dispersed extraterrestrial chromite (EC) grains (>63 μm). This has been interpreted as a dramatic increase in the flux of L-chondritic matter to Earth following the breakup of the L-chondrite parent body, which based on Ar–Ar gas retention ages (470 ± 6 Ma) of recently fallen meteorites occurred at about this time. Here we show that the general trend in the distribution of sediment-dispersed EC grains can be reproduced in the Puxi River section in central China. A total of 288 kg of limestone was searched for chrome spinels. In samples spanning the lower 8 m of the section, representing the Paroistodus originalis and Lenodus antivariabilis conodont zones, a total of 110 kg of limestone yielded only one EC grain. Similarly to the Swedish sections, EC grains begin to be common in the overlying L. variabilis Zone and remain common throughout the upper 9 m of the section, representing the L. variabilis, Yangtzeplacognathus crassus and L. pseudoplanus zones. In this part of the section 178 kg of limestone yielded 290 EC grains, with an average chemical composition very similar to chromite from recent L chondrites. In most of the beds over this interval one finds 1–4 EC grains per kilogram rock, a clear two orders-of-magnitude enrichment relative to the lower part of the section. Small bed-by-bed variations in the EC content over the upper interval most likely reflect small variations in sedimentation rates. The Puxi River section contains only very rare terrestrial chrome spinel grains, which can be distinguished already by their rounded, abraded appearance compared to the angular, pristine extraterrestrial spinels. In the mid-Ordovician, based on paleoplate reconstructions, the Puxi River site was positioned at mid-latitudes on the southern hemisphere a couple of thousand kilometers east of the Swedish sites. The prominent enrichment of EC grains over the same stratigraphic interval in China and Sweden is supporting evidence for a dramatic increase in the flux of L-chondritic matter to Earth shortly after the disruption of the L-chondrite parent body in the asteroid belt.
Article
We present the elemental and isotopic composition of noble gases in the bulk solar wind collected by the NASA Genesis sample return mission. He, Ne, and Ar were analyzed in diamond-like carbon on a silicon substrate (DOS) and 84,86Kr and 129,132Xe in silicon targets by UV laser ablation noble gas mass spectrometry. Solar wind noble gases are quantitatively retained in DOS and with exception of He also in Si as shown by a stepwise heating experiment on a flown DOS target and analyses on other bulk solar wind collector materials. Solar wind data presented here are absolutely calibrated and the error of the standard gas composition is included in stated uncertainties. The isotopic composition of the light noble gases in the bulk solar wind is as follows: 3He/4He: (4.64 ± 0.09) × 10−4, 20Ne/22Ne: 13.78 ± 0.03, 21Ne/22Ne: 0.0329 ± 0.0001, 36Ar/38Ar 5.47 ± 0.01. The elemental composition is: 4He/20Ne: 656 ± 5, and 20Ne/36Ar 42.1 ± 0.3. Genesis provided the first Kr and Xe data on the contemporary bulk solar wind. The preliminary isotope and elemental composition is: 86Kr/84Kr: 0.302 ± 0.003, 129Xe/132Xe: 1.05 ± 0.02, 36Ar/84Kr 2390 ± 150, and 84Kr/132Xe 9.5 ± 1.0. The 3He/4He and the 4He/20Ne ratios in the Genesis DOS target are the highest solar wind values measured in exposed natural and artificial targets. The isotopic composition of the other noble gases and the Kr/Xe ratio obtained in this work agree with data from lunar samples containing “young” (∼100 Ma) solar wind, indicating that solar wind composition has not changed within at least the last 100 Ma. Genesis could provide in many cases more precise data on solar wind composition than any previous experiment. Because of the controlled exposure conditions, Genesis data are also less prone to unrecognized systematic errors than, e.g., lunar sample analyses. The solar wind is the most authentic sample of the solar composition of noble gases, however, the derivation of solar noble gas abundances and isotopic composition using solar wind data requires a better understanding of fractionation processes acting upon solar wind formation.
Article
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 Dmax = 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.
Article
Abundant fossil meteorites in marine, condensed Lower Ordovician limestones from Kinnekulle, Sweden, indicate that accretion rates of meteorites were one to two orders of magnitude higher during an interval of the Early Ordovician than at present. Osmium isotope and iridium analyses of whole-rock limestone indicate a coeval enhancement of one order of magnitude in the influx rate of cosmic dust. Enhanced accretion of cosmic matter may be related to the disruption of the L chondrite parent body around 500 million years ago.
Article
Abundant extraterrestrial chromite grains from decomposed meteorites occur in middle Ordovician (480 million years ago) marine limestone over an area of ∼250,000 square kilometers in southern Sweden. The chromite anomaly gives support for an increase of two orders of magnitude in the influx of meteorites to Earth during the mid-Ordovician, as previously indicated by fossil meteorites. Extraterrestrial chromite grains in mid-Ordovician limestone can be used to constrain in detail the temporal variations in flux of extraterrestrial matter after one of the largest asteroid disruption events in the asteroid belt in late solar-system history.
Article
This paper attempts to bring together and evaluate all significant evidence on the origin of meteorites.
Helium and neon in presolar silicon carbide grains and in relict chromite grains from fossil meteorites and micrometeorites as tracers of their origin Fast delivery of meteorites to Earth after a major asteroid collision
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A noble-gas mass spectrometer compressor source with two orders of magnitude improvement in sensitivity (abstract) Abundance and meaning of regolith breccias among meteorites (abstract) Petrographic classification of Middle Ordovician fossil meteorites from Sweden
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