The Formation of Meteoritic Iron Oxide

Meteoritic iron oxide occurs in three forms: coarsely lamellar, finely lamellar, and massive. The lamellar forms result from oxidation proceeding parallel to the surface of the metal. The thickness of the lamellae may be proportional to the rate of oxidation. Massive oxide results when electrolysis causes the oxide to invade the metal or when redeposition of dissolved oxide obliterates the original lamellar structure. The action of lawrencite produces a friable, porous type of oxide which is confined largely to specimens in collections. The effect of electrolysis on the differential oxidation of meteoritic minerals is briefly discussed. Three new analyses of meteoritic oxides (by F. G. Hawley) are given (Table 1).     

The Anoka,Minnesota, Iron Meteorite*

Abstract The Anoka, Minnesota, meteorite was found on the Joe Fields Farm at location coordinates 45° 12′ N, 93° 26′ W. It is a fine octahedrite distinguished by large fields of dense plessite. The chemical analysis of the meteorite is 84.9 percent iron, 11.75 nickel and 0.51 cobalt.     

The age of Wolfe Creek meteorite crater (Kandimalal), Western Australia

Wolfe Creek crater lies in northwestern Australia at the edge of the Great Sandy Desert. Together with Meteor Crater, it is one of the two largest craters on Earth from which meteorite fragments have been recovered. The age of the impact is poorly constrained and unpublished data places the event at about 300,000 years ago. In comparison, Meteor Crater is well constrained by exposure dating. In this paper, we present new ages for Wolfe Creek Crater from exposure dating using the cosmogenic nuclides ¹⁰Be and ²⁶Al, together with optically stimulated luminescence ages (OSL) on sand from a site created by the impact. We also present a new topographic survey of the crater using photogrammetry. The exposure ages range from ~86 to 128 ka. The OSL ages indicate that the age of the impact is most likely to be ~120 ka with a maximum age of 137 ka. Considering the geomorphic setting, the most likely age of the crater is 120 ± 9 ka. Last, we review the age of Meteor Crater in Arizona. Changes in production rates and scaling factors since the original dating work revise the impact age to 61.1 ± 4.8 ka, or ~20% older than previously reported.     

The Binningup H5 Chondrite: A New Fall from Western Australia

Abstract— Following a brilliant daylight fireball at 10:10 a.m. (local time) on 30 September 1984, a single stone weighing 488.1 grams was recovered from Binningup beach (33°09′23″S, 115°40′35″E), Western Australia. Data from 23 reported sightings of the fireball indicate an angle of trajectory 20–40° from the horizontal, a flight-path bearing N210°E and an end-point (ca. 32°39′S, 115°54.5′E) at a height of ～20–30 km. A recrystallized chondritic texture and the presence of olivine and low-Ca orthopyroxene with compositions of Fa_18.4 (PMD 1.1)and Fs_16.1 (PMD 1.1), respectively, show that Binningup is a typical member of the H-group of ordinary chondrites. Uniform mineral compositions and the presence of generally microcrystalline plagioclase feldspar indicate that the meteorite belongs to petrologic type 5. Pervasive fracturing of silicates suggests mild pre-terrestrial shock loading. Measurements (dpm kg^?1) of cosmogenic radionuclides including ²²Na (61 ± 5), ²⁶Al (49 ± 3) and ⁵⁴Mn (66 ± 10) indicate a normal history of irradiation.     

Surficial properties in Gale Crater, Mars, from Mars Odyssey THEMIS data

We examine the nature of the surface layer in Gale Crater as determined from high-resolution thermal and visible Mars Odyssey Thermal Emission Imaging System (THEMIS) data as well as how our conclusions compare to past analyses. At THEMIS resolution, the thermal surface structure is dominated by local control, thus providing us with detailed images that contain thermophysical information as well. Using these data sets we have created a map of the area, defining units based primarily on their geomorphology as determined from the daytime thermal and visible images and then using the nighttime thermal data to interpret the nature of the surface layer within each unit. Seven units have been defined: (i) partially blanketed knobby plateaus, (ii) crater walls with terrain similar to that on the plateaus on the upper half and exposed, rocky surfaces on the lower half, (iii)-(v) three floor units with varying combinations of bedrock and indurated and/or particulate deposits, (vi) sand sheets, and (vii) a central mound, consisting of indurated and/or rocky material forming layers, terraces, and slides, covered by particulate material that tapers in thickness downslope. Additionally, dozens of channels have been observed on the crater walls and central mound. The results indicate that aeolian processes have played a major role in shaping much of the present surface layer within Gale and may still be active today. Because of the dramatic size and structure of Gale, the winds are most likely controlled by the local topography. Additionally, the presence and frequency of channels within Gale bolster hypotheses involving aqueous episodes in the history of the crater.     

Characterization and significance of shocked quartz from the Woodleigh mpact structure,Western Australia

Abstract— We re‐examined the buried Woodleigh structure in Western Australia, which has been inferred to be a multi‐ringed, 120 km diameter impact crater, because the proposed size and possible synchronicity with one of the pre‐Mesozoic mass extinction events has attracted controversy. We undertook a detailed study of the petrology and mineralogy of a number of samples of core from the Woodleigh‐1 borehole that was drilled into the central uplift of the structure. Crystalline Proterozoic basement rocks comprising granites and gneisses in the Woodleigh‐1 core contain minor brecciation in discrete veins and reveal clear evidence of shock metamorphism over the full extent of the core. Imaging of laboratory‐etched quartz showed that a large number of grains contain shock deformation lamellae. Microstructural and crystallographic analysis of these lamellae by TEM showed that they are planar deformation features (PDFs) that have subsequently undergone annealing and water assisted recrystallization. The available geological, petrographic, and mineralogical evidence suggest that Woodleigh is an eroded impact crater that is nearer to 60 km than 120 km in diameter. Future drilling projects should better constrain the level of erosion, and may reveal any preserved impact lithologies.     

Petrography and sedimentology of the ~2490 Ma DS4 impact spherule layer revisited,Brockman Iron Formation (Hamersley Group,Western Australia)

The ~2490 Ma DS4 impact layer in the Dales Gorge Member is the only bed in the Brockman Iron Formation (Hamersley Group, Western Australia) known to contain “splash form” impact spherules. At a newly discovered site in Munjina Gorge (MG), the internal stratigraphy of the DS4 impact layer differs from previously known occurrences; it ranges from 36 to 57 cm in total thickness and consists of two distinct subunits. The lower subunit contains abundant cobble‐ to boulder‐scale intraclasts and spherules supported by a finer matrix. We interpret this subunit as the product of poorly cohesive debris flows. The upper subunit is 11–15 cm of low‐density turbidites. The DS4 layer also consists of two newly recognized subunits at Yampire Gorge (YG). The lower subunit is rich in well‐sorted spherules, 0–22 cm thick, and comprises an unstratified bedform with an irregular or swaley upper surface. This is overlain by 2 dm‐scale, fine‐grained, irregularly laminated beds that we interpret as low density turbidites laterally equivalent to the upper subunit at MG. The bedform at YG could be the lateral equivalent of the debrite at MG, genetically related to the overlying turbidites, or a product of impact tsunami‐induced bottom return flow. Other DS4 layer sites that have debrites similar to the one at MG are geographically separated from one another by sites that both lack debrite facies and feature well‐sorted spherules like YG. These characteristics suggest the DS4 layer had a complex depositional history that generated multiple debrites.     

Shocked quartz in polymict impact breccia from the Upper Cretaceous Yallalie impact structure in Western Australia

Yallalie is a ~12 km diameter circular structure located ~200 km north of Perth, Australia. Previous studies have proposed that the buried structure is a complex impact crater based on geophysical data. Allochthonous breccia exposed near the structure has previously been interpreted as proximal impact ejecta; however, no diagnostic indicators of shock metamorphism have been found. Here we report multiple (27) shocked quartz grains containing planar fractures (PFs) and planar deformation features (PDFs) in the breccia. The PFs occur in up to five sets per grain, while the PDFs occur in up to four sets per grain. Universal stage measurements of all 27 shocked quartz grains confirms that the planar microstructures occur in known crystallographic orientations in quartz corresponding to shock compression from 5 to 20 GPa. Proximity to the buried structure (~4 km) and occurrence of shocked quartz indicates that the breccia represents either primary or reworked ejecta. Ejecta distribution simulated using iSALE hydrocode predicts the same distribution of shock levels at the site as those found in the breccia, which supports a primary ejecta interpretation, although local reworking cannot be excluded. The Yallalie impact event is stratigraphically constrained to have occurred in the interval from 89.8 to 83.6 Ma based on the occurrence of Coniacian clasts in the breccia and undisturbed overlying Santonian to Campanian sedimentary rocks. Yallalie is thus the first confirmed Upper Cretaceous impact structure in Australia.     

Dissemination and fractionation of projectile materials in the impact melts from Wabar Crater,Saudi Arabia

Abstract— We have analyzed small, ballistically dispersed melt samples in the form of aerodynamically shaped spheres, dumbbells, teardrops, etc., from Wabar Crater, Saudi Arabia, and have compared these to our previous study of the more massive, black and white melt specimens. The smaller melt samples differ from the more massive melts in that they are petrographically and chemically more homogeneous, possess fewer, more diffuse schlieren and contain much less clastic detritus. These observations suggest higher peak temperatures for the smaller melt samples than for the massive black and white melts which represent Wabar's major melt-zone. Analyses of the Wabar and Nejed (paired with Wabar) meteorites permit detailed comparison of the unaltered projectile with impactor residues in the melts. Siderophile element concentrations indicate that the small glass beads commonly contain > 10% meteoritic component, compared to < 5% for the massive black and white melts. One glass bead was found to contain ～ 17% meteoritic component. Based on models for melt production during cratering, we deduce that more meteoritic material was mixed with the upper stratigraphic horizons of Wabar's melt zone than with the lower parts. Siderophile elements in all Wabar melt specimens are fractionated relative to the Wabar-Nejed meteorite and have Fe/Ni ratios up to ～ 1.8 times that of Wabar-Nejed for the most siderophile element-rich glasses. The abundance sequence of siderophiles in the melts relative to the projectile is Fe ? Co > Ni ? Ir ? As » Au. Although this sequence seems incompatible with simple vapor fractionation of either elements or oxides, we believe that a complex vapor fractionation process most likely produced the observed siderophile element abundances. Our sample suite should be representative of all materials found in and around the Wabar structure, and we conclude that substantial quantities of the projectile were lost to the atmosphere, most likely as vapor. No fractionation of lithophile elements is observed in the glasses relative to the target rocks. Although fractionation of the impactor must have occurred prior to intimate mixing of projectile and target, details of the actual fractionation mechanism(s) remain poorly understood. The results of this study indicate that caution is necessary when attempting to define impactor types and masses from compositional data for impact melts from other craters.     

The Gove relict iron meteorite from Arnhem Land,Northern Territory,Australia

On February 24, 1979, a deeply oxidized mass of iron meteorite was excavated from bauxite at an open cut mine on the Gove Peninsula, Northern Territory, Australia. The meteorite, measuring 0.75–1 m in diameter and of unknown total weight, was found at coordinates 12°15.8′S, 136°50.3′E. On removal from the ground, the meteorite is reported to have disintegrated rapidly. A preliminary analysis at the mine laboratory reportedly gave 8.5 wt% Ni. A modern analysis of oxidized material gave Ni = 32.9, Co = 3.67 (both mg g^?1), Cr = 168, Cu = 195, Ga = 22.5, Ge = <70, As = 4.16, W = 1.35, Ir = 10.5, Pt = 21.2, Au = 0.672 (all μg g^?1), Sb = <150, and Re = 844 (both ng g^?1). Competent fragments of oxidized material retain a fine to medium Widmanstätten pattern with an apparent average bandwidth of 0.5 mm (range 0.2–0.9 mm in plane section). Primary mineralogy includes rare γ–taenite and daubréelite, and secondary minerals produced by weathering include awaruite (with up to 78.5 wt% Ni) and an, as yet, unnamed Cu‐Cr‐bearing sulfide with the ideal formula CuCrS₂ that is hitherto unknown in nature. Deep weathering has masked many of the features of the meteorite; however, the analysis normalized to the analyses of fresh iron meteorites favors chemical group IIIAB. The terrestrial age of the meteorite is unknown, although it is likely to be in the Neogene (2.5–23 Ma), which is widely accepted as the major period of bauxite formation in the Northern Territory of Australia. Gove is the second authenticated relict meteorite found in Australia.     

Stable nickel isotopes and cosmogenic beryllium-10 and aluminum-26 in metallic spheroids from Meteor Crater,Arizona

Abstract— The isotopic abundances of Ni in 17 metallic spheroids from Meteor Crater, Arizona, were determined by inductively coupled plasma mass spectrometry (ICP-MS). Sixteen spheroids have normal isotopic abundances. A 17th shows a marginally detectable mass fractionation of 0.40 ± 0.14 %/AMU in favor of the heavier isotopes. The general absence of mass fractionation indicates that open system evaporation caused little loss of Ni. Variable activities of the cosmogenic radionuclides ¹⁰Be and ²⁶Al were measured by accelerator mass spectrometry in separate suites of spheroids. Activities of ²⁶Al in most samples and of ¹⁰Be in metal cores separated from spheroids indicate that they either (1) come from greater depths in the parent meteoroid than do hand specimens, or (2) lost Al and Be during the process of spheroid formation. One individual spheroid has ¹⁰Be and ²⁶Al activities comparable to those of bulk specimens. This result suggests that spheroid formation may occasionally include material from the outermost meter or so of the impactor. Relatively high activities of ¹⁰Be, ～3 dpm/kg, in the siliceous shells of Canyon Diablo spheroids very likely have a meteoric origin.     

Meteorites from the Nullarbor Region,Western Australia: II. Recovery and classification of 34 new meteorite finds from the Mundrabilla,Forrest, Reid and Deakin areas

Abstract— The discovery of 34 new stony meteorites is reported from those areas of the Nullarbor Region, Western Australia named after Mundrabilla, Forrest, Reid and Deakin sidings on the Trans Australian Railway line. The recoveries include 15 H-, and 15 L-group equilibrated (types 4–6) ordinary chondrites, two distinct H3 chondrites (Mundrabilla 003 and Forrest 003), a genomict H-group chondrite breccia (Reid 011) comprising types 3–6, and one structurally anomalous chondrite (Deakin 001). Seventy-eight distinct meteorites are now known from the region.     

Carbon,Oxygen, Silicon,Sulphur, Calcium,and Iron Determinations in Twenty-four Dark Clasts and One Dark Inclusion of the Allende Meteorite by 12C(d,p)13C Nuclear Reaction and PIXE

Abstract— A study of carbon contents of millimeter-sized dark clasts and of one dark inclusion of the Allende meteorite with the ¹³C(d,P₀)¹³C method has been undertaken. The results show that C in dark Allende areas varies, perhaps in a “clustered” fashion, from near 0.3%, which is essentially the bulk C-content of the meteorite, to at least 2.1% in the dark rim of a dark inclusion. At least one cluster has now been identified with C-contents in the range 0.4%-0.5%. Raman spectra of C in the dark inclusion ADIR 4E1-fu and in matrix have also been determined. These are identical. Our results are consistent with a comparatively low temperature origin of essentially all elemental carbon in Allende.     

Meteorites from the Nullarbor Region,Western Australia: I. A review of past recoveries and a procedure for naming new finds

Abstract— Currently, 44 distinct meteorites are recorded from the Nullarbor Region in Western Australia. Recovery data for the Billygoat Donga, Cardanumbi, Cocklebiddy, Forrest Lakes, Laundry East, Lookout Hill, North East Reid, North Reid, Reid, Webb, West Forrest, West Reid and Yayjinna meteorites are amended, and North Forrest is recognized as distinct from North West Forrest (H). Since 1971, the recovery of more than 500 specimens (predominantly ordinary chondrites) from the desert has made the Nullarbor Region one of the most productive areas of the world for meteorite recoveries and has caused major problems for meteorite nomenclature. To overcome a lack of geographical names, we have delineated a grid of 47 named ‘areas’ in the Nullarbor Region. Henceforth, distinct meteorites will take the name of the ‘area’ in which they are found and a number (e.g., 001) in order of discovery. In general, the names of past recoveries remain unchanged. The absence of transportation processes in the region, and accurate documentation of the distribution of finds allows ‘pairing’ of specimens to at least 90% level of confidence.     

Textural constraints on the formation of impact spherules: A case study from the Dales Gorge BIF,Paleoproterozoic Hamersley Group of Western Australia

Abstract— Impact ejecta (about 2.5 Gyr old) in the DS4 layer of the Dales Gorge BIF (Hamersley Group, Western Australia) are so well preserved that many original textures such as vesicles and microlites are faithfully preserved. About 65% of the particles in the layer originated as impact ejecta, of which 81% are splash forms. The remaining 19% are angular, but the splash forms and angular particles have the same composition (mainly diagenetic stilpnomelane and K‐feldspar) and share a common suite of internal textures. Some particles contain randomly oriented microlites texturally identical to plagioclase in basalts. Most splash forms have rims of inward‐growing crystals that may have formed from the melt (perhaps nucleated by impinging dust) or via thermal devitrification. The rims clearly formed in flight because in broken particles (which make up about 13% of the splash forms) they are generally not present on broken surfaces. The origin of the angular particles is uncertain, but they may represent solid ejecta. Given the large sizes and variable shapes of the splash forms, they are probably droplets of impact melt emplaced ballistically. This is largely by analogy to the K‐T boundary layer, but DS4 splash forms differ from K‐T spherules in important ways suggesting the K‐T model is not universal. The occurrence of basaltic ejecta from a large impact highlights its scarcity in the stratigraphic record despite the areal abundance of oceanic crust. The diverse textures formed via in‐flight crystallization suggest particle paths in the plume are more complex than is generally appreciated.     

Light noble gases in 12 meteorites from the Omani desert,Australia, Mauritania,Canada, and Sweden

We measured the concentrations and isotopic compositions of He, Ne, and Ar in 14 fragments from 12 different meteorites: three carbonaceous chondrites, six L chondrites (three most likely paired), one H chondrite, one R chondrite, and one ungrouped chondrite. The data obtained for the CV3 chondrites Ramlat as Sahmah (RaS) 221 and RaS 251 support the hypothesis of exposure age peaks for CV chondrites at approximately 9 Ma and 27 Ma. The exposure age for Shi?r 033 (CR chondrite) of 7.3 Ma is also indicative of a possible CR chondrite exposure age peak. The three L chondrites Jiddat al Harasis (JaH) 091, JaH 230, and JaH 296, which are most likely paired, fall together with Hallingeberg into the L chondrite exposure age peak of approximately 15 Ma. The two L chondrites Shelburne and Lake Torrens fall into the peaks at approximately 40 Ma and 5 Ma, respectively. The ages for Bassikounou (H chondrite) and RaS 201 (R chondrite) are approximately 3.5 Ma and 5.8 Ma, respectively. Six of the studied meteorites show clear evidence for ³He diffusive losses, the deficits range from approximately 17% for one Lake Torrens aliquot to approximately 45% for RaS 211. The three carbonaceous chondrites RaS 221, RaS 251, and Shi?r 033 all have excess ⁴He, either of planetary or solar origin. However, very high ⁴He/²⁰Ne ratios occur at relatively low ²⁰Ne/²²Ne ratios, which is unexpected and needs further study. The measured ⁴⁰Ar ages fit well into established systematics. They are between 2.5 and 4.5 Ga for the carbonaceous chondrites, older than 3.6 Ga for the L and H chondrites, and about 2.4 Ga for the R chondrite as well as for the ungrouped chondrite. Interestingly, none of our studied L chondrites has been degassed in the 470 Ma break‐up event. Using the amount of trapped ³⁶Ar as a proxy for noble gas contamination due to terrestrial weathering we are able to demonstrate that the samples studied here are not or only very slightly affected by terrestrial weathering (at least in terms of their noble gas budget).     

The Bunburra Rockhole meteorite fall in SW Australia: fireball trajectory,luminosity, dynamics,orbit, and impact position from photographic and photoelectric records

Abstract– We report an analysis of the first instrumentally observed meteorite fall in Australia, which was recorded photographically and photoelectrically by two eastern stations of the Desert Fireball Network (DFN) on July 20, 2007. The meteoroid with an initial mass of 22 kg entered the atmosphere with a low speed of 13.36 km s^?1 and began a luminous trajectory at an altitude of 62.83 km. In maximum, it reached ?9.6 absolute magnitude and terminated after a 5.7 s and 64.7 km long flight at an altitude of 29.59 km with a speed of 5.8 km s^?1. The angle of the atmospheric trajectory to the Earth’s surface was 30.9°. The first organized search took place in October 2008 and the first meteorite (150 g) was found 97 m southward from the predicted central line at the end of the first day of searching (October 3, 2008). The second stone (174 g) was recovered 39 m northward from the central line, both exactly in the predicted mass limits. During the second expedition in February 2009, a third fragment of 14.9 g was found again very close (～100 m) from the predicted position. Total recovered mass is 339 g. The meteorite was designated Bunburra Rockhole (BR) after a nearby landscape structure. This first DFN sample is an igneous achondrite. Initial petrography indicated that BR was a brecciated eucrite but detailed analyses proved that BR is not a typical eucrite, but an anomalous basaltic meteorite ( Bland et al. 2009 ). BR was delivered from an unusual, Aten type orbit (a < 1 AU) where virtually the entire orbit was contained within Earth’s orbit. BR is the first achondrite fall with a known orbit and it is one of the most precise orbits ever calculated for a meteorite dropping fireball.