Large Iron Meteorites Research Articles

The noble gas inventory in metal samples and troilite inclusions from IIIAB iron meteorites: Reinvestigating the live 129I‐129Xe dating method

AbstractThe live 129I‐129Xe dating technique, which internally corrects for shielding, is particularly well suited for large iron meteorites, for which shielding corrections might be difficult to obtain. In addition, the half‐life of 129I of 16 Ma would allow the study of the important question of the constancy—or variability—of the galactic cosmic rays over a time scale not covered by other cosmogenic nuclides. Here, we present the results of a noble gas study of metal samples and adjacent troilite inclusions from the four IIIAB iron meteorites Cape York (including Agpalilik), Casas Grandes, Trenton, and Grant. The major result is that we can directly determine 129Xenc concentrations caused by (live) 129I decay for Cape York, Casas Grandes, Trenton, and Grant. The 129Xenc concentrations can be used, if combined to 129I activity concentrations (not measured by us), to calculate cosmic ray exposure (CRE) ages using the (live) 129I‐129Xe chronometer. For the light noble gases measured in metal and troilite samples, the new data confirm the earlier estimates of a production rate ratio 21Necos(troilite)/21Necos(metal) in the range 3.08–3.53. Surprisingly, 38Arcos in troilite from Agpalilik and Casas Grandes is only, respectively, ~36% and ~44% relative to that in the respective metal phases. Considering that 38Arcos is only produced from iron but not from sulfur, the 38Arcos concentration measured in troilite is expected to be ~64% relative to that in adjacent metal, that is, some 38Arcos is missing. Considering krypton in troilite, only 80Kr, 82Kr, and 83Kr are higher than the blank, likely indicating a spallogenic contribution. On average, ~4.2% of measured 80Kr, ~2.5% of 82Kr, and ~11.6% of 83Kr are spallogenic.

Methods for determination of the source of iron in precontact Inuit and Dorset culture artifacts from the Canadian Arctic

The period during which Dorset Culture peoples and precontact Inuit used iron was widespread and overlapped the appearance of the Norse in Greenland. In Greenland and Arctic Canada, three sources of iron in artifacts have been reported: terrestrial native iron, Norse wrought iron and meteoritic iron from the Cape York iron meteorite. We investigated the source of iron from fourteen samples of iron artifacts recovered from one precontact Inuit and three Dorset culture sites. Polished mm-scale samples mounted in polyester resin blocks yielding a roughly rectangular 1 × 2 mm cross section to preserve as much of each sample as possible were examined. Petrographic examinations by reflected light microscopy and SEM observation coupled with energy-dispersive analysis revealed the presence of Widmanstätten structure (an intergrowth of kamacite and taenite), a suite of minerals comprising kamacite, taenite, daubreelite and schreibersite and nickel-content at iron meteorite levels indicating all fourteen samples were unambiguously of meteoritic origin. With the meteoritic origin confirmed, Goldstein’s (1965) model relating half-width of the kamacite lamellae in the Widmanstätten patterns to the Ni-content of the lamellae was used to determine whether the meteoritic iron originated from the Cape York meteorite. A test of the method used the mean kamacite bandwidth and the bulk Ni-content of the Cape York mass Savik I, which demonstrated the validity of the basis of the method. Using the experimentally-derived relation between kamacite half-width of the bands and composition of the kamacite, the nickel-content of the kamacite lamellae ranged from 6.13 to 6.61 wt% Ni. These compositions are consistent with bandwidths that are narrower than the mean for Cape York, the anticipated source. The discrepancy may be attributed to flattening of the kamacite lamellae by cold working in the manufacture of the artifacts. As well, it has been shown that a range of bandwidths is typical in large iron meteorites such as Cape York. Thus, this method cannot definitively identify Cape York as the source of the meteoritic iron in the fourteen samples; however, the results can serve to identify meteoritic artifacts if the material that has not been intensively cold worked and if sufficient bandwidths can be measured in order to establish a mean thickness.

53Mn and 60Fe in iron meteorites—New data, model calculations

AbstractWe measured specific activities of the long‐lived cosmogenic radionuclides 60Fe in 28 iron meteorites and 53Mn in 41 iron meteorites. Accelerator mass spectrometry was applied at the 14 MV Heavy Ion Accelerator Facility at ANU Canberra for all samples except for two which were measured at the Maier‐Leibnitz Laboratory, Munich. For the large iron meteorite Twannberg (IIG), we measured six samples for 53Mn. This work doubles the number of existing individual 60Fe data and quadruples the number of iron meteorites studied for 60Fe. We also significantly extended the entire 53Mn database for iron meteorites. The 53Mn data for the iron meteorite Twannberg vary by more than a factor of 30, indicating a significant shielding dependency. In addition, we performed new model calculations for the production of 60Fe and 53Mn in iron meteorites. While the new model is based on the same particle spectra as the earlier model, we no longer use experimental cross sections but instead use cross sections that were calculated using the latest version of the nuclear model code INCL. The new model predictions differ substantially from results obtained with the previous model. Predictions for the 60Fe activity concentrations are about a factor of 2 higher, for 53Mn, they are ~30% lower, compared to the earlier model, which gives now a better agreement with the experimental data.

Geophysical expression of the Meyers Crater, a new meteorite impact crater discovered in the Coolgardie Goldfield of Western Australia

SummaryDetailed airborne electromagnetic (VTEM), gravity and passive seismic HVSR surveying, followed up by RC and diamond drilling, has identified an isolated circular and geologically young meteorite impact crater filled with sediments and located next to the town of Coolgardie in Western Australia. It was discovered in 2017 (Meyers, 2017), is 800 m in diameter and estimated to be >140 m deep based on passive seismic results, because only angled drilling was carried out for gold exploration along the western side of the crater. The crater has been filled with impact debris and sedimentary deposits to form a flat modern topographic feature. The pre-impact bedrock is Archean greenstone, which hosts numerous gold occurrences and mines surrounding the crater. The crater is filled with a polymict impact breccia of mixed greenstone rocks from below and surrounds near its base. A thin (<2 m) tuffaceous looking ejecta layer occurs at the boundary between the impact breccia and underlying in situ brecciated greenstone bedrock (high Mg basalt and ultramafic lava). This transition zone is also imaged as two “bedrock” layers in passive seismic HVSR cross-sections. The impact breccia is overlain by a very thick deposit of anoxic peat and carbonised wood fragments containing framboidal pyrite, as well as slump blocks of greenstone rocks, starting from over 120 m deep to a depth of 14 m from surface, where this organic layer is capped by a fine clay deposit, which was likely transported into the crater after its raised rim was breached by erosion. The crater and its fill deposits cannot explain the entire circular gravity anomaly low of -5mGal, and the brecciated in situ greenstone bedrock around and below the crater is modelled to contribute to the gravity anomaly low. It is estimated that the meteorite projectile was about 40 m in diameter, came from an easterly trajectory, and impacted Coolgardie between the Miocene to Pleistocene, with age dating and Ir analysis of the impact layer and overlying plant material soon to be carried out. A high-resolution ground magnetic survey using 10 m line spacing was carried out to try and detect large iron meteorite fragments, but only a weak and diffuse anomaly pattern occurs at the crater centre, indicating that the meteorite was highly fragmented on impact and/or later demagnetised by weathering. A vertical diamond drillhole down the centre of the crater is required to properly study this relatively young impact structure and analyse the preserved plant material filling the crater over a vertical column of 100 m or more, continuously recording local environmental changes over a considerable time period.

Reply to the comment by M. Genge and M. Van Ginneken on paper entitled “Unmelted cosmic metal particles from the Indian Ocean”

Reply to the comment by M. Genge and M. Van Ginneken on paper entitled “Unmelted cosmic metal particles from the Indian Ocean”

Meteorites at Meridiani Planum provide evidence for significant amounts of surface and near‐surface water on early Mars

Abstract– Six large iron meteorites have been discovered in the Meridiani Planum region of Mars by the Mars Exploration Rover Opportunity in a nearly 25 km‐long traverse. Herein, we review and synthesize the available data to propose that the discovery and characteristics of the six meteorites could be explained as the result of their impact into a soft and wet surface, sometime during the Noachian or the Hesperian, subsequently to be exposed at the Martian surface through differential erosion. As recorded by its sediments and chemical deposits, Meridiani has been interpreted to have undergone a watery past, including a shallow sea, a playa, an environment of fluctuating ground water, and/or an icy landscape. Meteorites could have been encased upon impact and/or subsequently buried, and kept underground for a long time, shielded from the atmosphere. The meteorites apparently underwent significant chemical weathering due to aqueous alteration, as indicated by cavernous features that suggest differential acidic corrosion removing less resistant material and softer inclusions. During the Amazonian, the almost complete disappearance of surface water and desiccation of the landscape, followed by induration of the sediments and subsequent differential erosion and degradation of Meridiani sediments, including at least 10–80 m of deflation in the last 3–3.5 Gy, would have exposed the buried meteorites. We conclude that the iron meteorites support the hypothesis that Mars once had a denser atmosphere and considerable amounts of water and/or water ice at and/or near the surface.

Noble gases in the Xinjiang (Armanty) iron meteorite––A big object with a short cosmic‐ray exposure age

Abstract– We measured the concentrations and isotopic ratios of the cosmogenic noble gases He, Ne, and Ar in the very large iron meteorite Xinjiang (IIIE). The 3He and 4He data indicate that a significant portion of the cosmogenic produced helium has been lost via diffusion or in a recent impact event. High 22Ne/21Ne ratios indicate that contributions to the cosmogenic 21Ne from sulfur and/or phosphorous are significant. By combining the measured nuclide concentrations with model calculations for iron meteorites we were able to determine the preatmospheric diameter of Xinjiang to 260–320 cm, which corresponds to a total mass of about 70–135 tons. The cosmic‐ray exposure age of Xinjiang is 62 ± 16 Ma, i.e., relatively short compared to most of the other iron meteorites. With the current database we cannot firmly determine whether Xinjiang experienced a complex irradiation history. The finding of 3He and 4He losses might argue for a recent impact event and therefore for a complex exposure.

Evidence for mechanical and chemical alteration of iron-nickel meteorites on Mars: Process insights for Meridiani Planum

[1] The weathering of meteorites found on Mars involves chemical and physical processes that can provide clues to climate conditions at the location of their discovery. Beginning on sol 1961, the Opportunity rover encountered three large iron meteorites within a few hundred meters of each other. In order of discovery, these rocks have been assigned the unofficial names Block Island, Shelter Island, and Mackinac Island. Each rock presents a unique but complimentary set of features that increase our understanding of weathering processes at Meridiani Planum. Significant morphologic characteristics interpretable as weathering features include (1) a large pit in Block Island, lined with delicate iron protrusions suggestive of inclusion removal by corrosive interaction; (2) differentially eroded kamacite and taenite lamellae in Block Island and Shelter Island, providing relative timing through crosscutting relationships with deposition of (3) an iron oxide–rich dark coating; (4) regmaglypted surfaces testifying to regions of minimal surface modification, with other regions in the same meteorites exhibiting (5) large-scale, cavernous weathering (in Shelter Island and Mackinac Island). We conclude that the current size of the rocks is approximate to their original postfall contours. Their morphology thus likely results from a combination of atmospheric interaction and postfall weathering effects. Among our specific findings is evidence supporting (1) at least one possible episode of aqueous acidic exposure for Block Island; (2) ripple migration over portions of the meteorites; (3) a minimum of two separate episodes of wind abrasion; alternating with (4) at least one episode of coating-forming chemical alteration, most likely at subzero temperatures.

The making of Martian meteorite Block Island

An analysis of the circumstances leading to the placement of the large iron meteorite, unofficially named Block Island, on Mars is presented. We investigate the possibility that Block Island fell during the late Noachian period on Mars when its atmosphere was much denser (and hence much more massive) than at the present time. Indeed, we find that in order to produce a non-crater-forming, non-fragmenting meteorite with the characteristics of Block Island, the surface pressure of the Martian atmosphere must have been at least one to two orders of magnitude larger than it is at the present epoch.

Irradiation Histories of Iron Meteorites

Typical examples of cosmic ray irradiation effects in extraterrestrial materials have been introduced by two large iron meteorites, Campo del Cielo and Gibeon. The Gibeon meteorite is known for having the longest strewn field of all meteorites extending 400 km. Stable light noble gases were determined simultaneously by sensitive mass-spectrometers. Based on the contents of products determined, we have discussed an unique complex history of the meteorite. We could classify Gibeon meteorite as a mixture of two main sub groups. The members of the first group have a simple long irradiation history of 330 million years, which is similar to Campo del Cielo. On the other hand, those of the 2nd group are composed of the secondary products of various contents of noble gases showing irradiation records equivalent to an average of 20 million years. The reductions of noble gases were most probably due to a serious break up of the meteorite in space. The deep interior of the main body of Group 1 has been exposed by th...

Galactic cosmic ray‐produced 129Xe and 131Xe excesses in troilites of the Cape York iron meteorite

Abstract— The flux of galactic cosmic rays (GCR) in the solar system appears to change with time. Based on the abundances in iron meteorites of cosmogenic nuclides of different half lives, Lavielle et al. (1999) found that the GCR flux increased in recent times (&lt;100 Ma) by about 38% compared to average flux in the past 150 Ma to 700 Ma ago. A promising technique for calibrating the GCR flux during the past ˜50 Ma, based on the 129I and 129Xe pair of nuclides, was discussed earlier (Marti 1986; Murty and Marti 1987). The 129I‐129Xen chronometer provides a shielding‐independent system as long as the exposure geometry remained fixed. It is especially suitable for large iron meteorites (Te‐rich troilite) because of the effects by the GCR secondary neutron component. Although GCR‐produced Xe components were identified in troilites, several issues require clarifications and improvements; some are reported here. We developed a procedure for achieving small Xe extraction blanks which are required to measure indigenous Xe in troilites. The 129Xe and 131Xe excesses (129Xen, 131Xen) due to neutron reactions in Te are correlated in a stepwise release run during the troilite decomposition. Our data show that indigenous Xe in troilite of Cape York has isotopic abundances consistent with ordinary chondritic Xe (OC‐Xe), in contrast to a terrestrial signature which was reported earlier. Two methods are discussed which assess and correct for an interfering radiogenic 129Xer component from extinct 129I. The corrected 129Xen concentration in troilite D4 of Cape York yields a cosmic ray exposure (CRE) age of 82 ± 7 Ma consistent, within uncertainties, with reported data (Murty and Marti 1987; Marti et al. 2004).

Methane production by large iron meteorite impacts on early Earth

The continuous existence of life on the Earth is thought to have begun around 4 Ga, which is near the end of the heavy bombardment period. Impacts of asteroids and comets may have produced and delivered organic matter on the Hadean Earth. However, the nature of those processes has not been understood well yet. In this paper, we propose a new process of organic synthesis induced by asteroid impacts. We consider the effect of impact vapor condensates reentering the Earth's early atmosphere. Vapor condensates produced by a large‐scale impact are dispersed around the globe and reenter the atmosphere at very high speed. The reentering condensates are heated and decelerated by the friction with the atmosphere and form a “hot condensate layer” around the globe. We calculate the temperature and atmospheric pressure of the hot condensate layer as a function of time. A Fischer‐Tropsch reaction on the surface of the reentering iron and nickel condensates will lead to efficient methane production. The reaction rate of Fischer‐Tropsch catalysis is estimated based on a kinetic model. Calculation results indicate that the amount of methane produced by the impact of an iron meteorite with a diameter of 10 km at 15 km sec−1of velocity reaches ∼1012–1013kg. Some of the methane formed by this process is photodissociated to HCN and organic aerosols. The amount of the resulting HCN is estimated to be ∼1010–1012kg. This is 103–104times the preimpact inventory of HCN on the Earth, which is maintained by UV radiation and electric discharge in a mildly reducing (i.e., CO‐CO2dominant) atmosphere. Such an episodic increase in the organic inventory on the surface of the early Earth may have played an important role in the origin of life.

Cosmogenic nuclides in the Brenham pallasite

Abstract— Cosmic‐ray‐produced (cosmogenic) nuclides were studied in fragments of the Brenham pallasite, a large stony iron meteorite. The contents of light noble gases (He, Ne, and Ar) and long‐lived radionuclides (10Be, 26Al, 36Cl, and 53Mn), produced by nuclear reactions with cosmic rays, were measured in the separated metal and olivine phases from numerous samples representing a wide range of shielding conditions in the meteoroid. The distribution of cosmogenic nuclide concentrations in the metal follows patterns similar to that observed in large iron meteorites. Shielding effects were estimated from the relative proportions of low‐ and high‐energy reaction products. The production rates varied, from surface to interior, by a factor of more than several hundred. The 36Cl‐36Ar cosmic‐ray exposure age of Brenham is 156 ± 8 Myr. This determination is based on a multiple nuclide approach that utilizes cosmogenic nuclide pairs. This approach not only yields a “shielding independent” exposure age but also demonstrates that the production of cosmogenic nuclides occurred in a single stage. The depth profiles of 10Be in the stone phase and 53Mn in the metal phase are shown superimposed on corresponding profiles from the Apollo 15 long drill core.Surprisingly low abundances of lithophile elements, such as K, U, and Th, provided a unique opportunity to examine the production systematics of those nuclides whose inventories typically have significant contributions from non‐cosmogenic sources, particularly radiogenic contributions. The U and Th contents of the olivine samples are extremely low, allowing detection of cosmogenic 4He production from oxygen, magnesium, silicon, and iron.

An attempt to detect changes in the Sun's luminosity during the last glaciation by applying the Poynting-Robertson effect to cosmic spherules from a dated deep-sea core

In previous papers by Parkin, Sullivan & Bull and Parkin, the Poynting-Robertson effect was applied to iron-type cosmic spherules, from two North Pacific cores, in the hope of proving that cosmic spherules existed as round bodies in space. It was assumed that they were produced as melt-droplets in asteroidal collisions. The outcome was unsatisfactory because better methods of measuring spherule density were required. If this was done, it was realized that it might be possible to detect changes in the Sun's luminosity. The present work is a further development. Density measurement is now highly accurate, and only iron spherules undamaged in atmospheric flight are accepted. Both iron and stony spherules have been extracted from a dated (δ18O stage boundaries) Kastenlot core, from the North Atlantic at ∼41°N, 23°W. This 174 cm ≈125 ka core covered the period of the last glaciation but, unfortunately, several sections were missing, one at a critical time. In a plot of the product Dδ (diameter D, density δ) for pristine-looking iron spherules against their time of arrival, the points are mainly confined within a wedge, and seemingly form lines with slopes roughly parallel to the edge of the wedge. By applying the Poynting-Robertson effect, it is suggested that very large iron meteorites impacted Mars, each line denoting an impact. The accompanying stony spherules are thought to be molten ejecta of Martian rock. Because of marine corrosion, the densities of stony spherules are meaningless; however, their sizes are useful if a common density of 3 g cm−3 is assumed, in space. To my eye, the sloping lines have bends which match the variations in δ18O. This would mean that the Sun began to dim more markedly at ∼40 ka than previously, and reached a minimum at ∼20 ka. It then rapidly brightened, perhaps to more than the present-day luminosity. The data are too crude to warrant any estimate of luminosity. Numerous iron and stony spherules have been extracted from a single section 54 to 62 cm down another Kastenlot core from the North Atlantic, at ∼26°N, 31°W. This slowly sedimenting, carbonate-free core is not dated; but perhaps 54 cm ≈210 ka. This section shows that irons and stones are ejected together at a collision; and an iron spherule accompanies a stony one during Poynting-Robertson spiralling, if they have the same Dδ value.

FURTHER FINDS FROM THE MUNDRABILLA METEORITE SHOWER

Two new large iron meteorites, weighing 840 kg and 800 kg respectively, have been recovered from the Nullarbor Plain in the vicinity of the original Mundrabilla meteorite site, and are now at the Western Australian Museum in Perth. Numerous small knucklebone‐shaped irons were also found at Tookana Rock Hole in the vicinity of Eucla. X‐ray fluorescence spectrometry of these meteorites confirms that the additional finds are members of the Mundrabilla meteorite shower.

A NEW CUTTING TECHNIQUE FOR METEORITIC IRONS

Small iron meteorites may be cut with high‐speed plastic‐bonded carborundum wheels, with hacksaws or automatic modifications of these, and with carbide‐tipped wheels; to date, no simple method has been available for cutting large iron meteorites. The successful sectioning of a 20‐ton meteorite involved the use of a mason's wire‐saw, fed with a slurry of carborundum‐grit. The slab produced measured 1.8 × 1.3 × 0.05 m and could be ground and polished for deep‐etching, with a minimum of weight loss.

The Mundrabilla Meteorite : a New Discovery in Western Australia

IN March 1966, while engaged on a geological survey of a portion of the Eucla Basin in Western Australia, we discovered two large iron meteorites. The two principal masses, lying some 600 ft. apart, are located on the Nullarbor Plain, to the north of the Transcontinental Railway (latitude 30° 47′ S., longitude 127° 33′ E.). Earlier reports of the sighting of a meteorite by a rabbit-trapper, now deceased, have led to several expeditions which failed, however, to discover these masses.

Chlorine-36 produced by Neutron Capture in Meteorites

THE existence of the long-lived radionuclide chlorine-36 in iron meteorites, where it is produced by the spallation of the metal during its exposure to cosmic rays in space, is now well known. The measurements of cosmogenic 36Ar and 36Cl so produced permit one to calculate the so-called cosmic-ray exposure age of a meteorite. In this communication the results of experiments which show that in large iron meteorites the production of this nuclide also takes place via the reaction 35Cl(n,γ)36Cl on the lawrencite inclusions in the metal are presented. This result is of significance in deducing the history of meteorites from the investigation of cosmic-ray reaction products.

Tektites and silica-glass (With Plates XVI and XVII.)

The natural glasses recognized as tektites are characterized by their acidic composition, the silica percentage ranging from 68 to 80. Their origin is still a debatable point. Within this range glasses are known which have undoubtedly been produced by the fusion of siliceous terrestrial materials in the intense heat developed when large iron meteorites have struck the earth's surface; while others of the same origin show silica ranging up to 98 per cent., that is, an almost pure silica-glass.From a collection of 84 chemical analyses of tektites taken from the literature, only 36 (many of them due to A. Lacroix) give data for the specific gravity and refractive index which presumably were determined on the same sample of material as that analysed. These are reproduced in table I (arranged in order of silica percentages) and plotted in figs. 1 and 2. Corresponding with the range in SiO2 from 68·00 to 80·73% there is a nearly uniform decrease in d (2·498–2·339) and n (1·526–1·4867). The specific refractivity, K = (n−1)/d, varies from 0·2050 to 0·2107 (0·2258 in the case of no. 29 with the abnormally high n 1.5380).