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At least 37 records · Page 2Linked to original sources
USGS1967

The occurrence and origin of lamellar troilite in iron meteorites

A number of iron meteorites contain elongated inclusions consisting predominantly of troilite, which have been termed Reichenbach lamellae. Two types of inclusions exist, the first up to 6 cm long and 0·2 mm wide, the second up to 2 cm long and 3 mm wide. The first type contains troilite with subordinate daubreelite and is commonly rimmed by schreibersite and swathing kamacite; the second consists largely of troilite with daubreelite and rare graphite and silicate grains. Both types formed from a residual sulfide melt after the solidification of Ni Fe metal. Swatching kamacite surrounding troilite in iron meteorites formed by nucleation at the troilite-metal interface during the formation of the Widmanstätten texture. Meteorites containing troilite inclusions of the second type appear to have cooled more rapidly than most iron meteorites. In view of the confusion surrounding the use of the term Reichenbach lamella, it is suggested that the term be dropped from the literature.

Geochimica et Cosmochimica Acta↗
USGS2011

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

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.

Journal of Geophysical Research E: Planets↗
USGS1967

Cliftonite in meteorites: A proposed origin

Cliftonite, a polycrystalline aggregate of graphite with cubic morphology, is known in ten meteorites. Some workers have considered it to be a pseudomorph after diamond, and have used the proposed diamond ancestry as evidence of a meteoritic parent body of at least lunar dimensions. We have synthesized cliftonite in Fe-Ni-C alloys in vacuum, as a product of decomposition of cohenite [(Fe,Ni) 3 C]. We therefore suggest that a high pressure origin is unnecessary for meteorites which contain cliftonite, and that these meteorites were formed at low pressures. This conclusion is in agreement with other recent evidence.

Science↗
USGS1992

Rhenium-osmium isotope constraints on the age of iron meteorites

Rhenium and osmium concentrations and the osmium isotopic compositions of iron meteorites were determined by negative thermal ionization mass spectrometry. Data for the IIA iron meteorites define an isochron with an uncertainty of approximately ±31 million years for meteorites ∼4500 million years old. Although an absolute rheniumosmium closure age for this iron group cannot be as precisely constrained because of uncertainty in the decay constant of 187 Re, an age of 4460 million years ago is the minimum permitted by combined uncertainties. These age constraints imply that the parent body of the IIAB magmatic irons melted and subsequently cooled within 100 million years after the formation of the oldest portions of chondrites. Other iron meteorites plot above the IIA isochron, indicating that the planetary bodies represented by these iron groups may have cooled significantly later than the parent body of the IIA irons.

Science↗
USGS1968

The geologic classification of the meteorites

The meteorite classes of Prior and Mason are assigned to three proposed genetic groups on the basis of a combination of compositional, mineralogical, and elemental characteristics: l) the calcium-poor, volatile-rich carbonaceous chondrites and achondrites; 2) the calcium-poor, volatile-poor chondrites (enstatite, bronzite, hypersthene, and pigeonite), achondrites (enstatite, hypersthene, and pigeonite), stonyirons (pallasites, siderophyre), and irons; and, 3) the calcium-rich (basaltic) achondrites. Chondrites are correlated with calcium-poor achondrites and the silicate phase of the pallasitic meteorites on Fe contents of olivine and pyroxene; and with metal of the stony-irons and irons on the basis of trace elements (Ga and Ge). Transitions in structure and texture between the chondrites and achondrites are recognized. The Van Schmus-Wood chemical-petrologic classification of the chondrites has been modified and expanded to a mineralogic-petrologic classification of the chondrites and calcium-poor achondrites. Chondrites apparently are the first rocks of the solar system. Paragenetic and textural relations in the Murray carbonaceous chondrite shed new light on the manner of accretion, and on the character of dispersed solid materials ('dust', and chondrules and metal) that existed in the solar system before accretion. Two pre-accretionary mineral assemblages (components) are recognized in the carbonaceous chondrites and in the unequilibrated volatile-poor chondrites. They are: 1) a 'low temperature' water-, rare gas-, and carbon-bearing component; and, 2) a high temperature anhydrous silicate and metal component. Paragenetic relations indicate that component 2 materials predate chondrite formation. An accretionary assemblage (component 3) also is recognized in the carbonaceous chondrites and in the unequilibrated volatile-poor chondrites. Component 3 consists of very fine grains of olivine and pyroxene, which occur as pervasive disseminations, as small irregular aggregates of grains, and as large subround to round, finely granular accretional chondrules. Evidence in Murray indicates that component 3 silicates precipitated abruptly and at low pressures, possibly from a high temperature gas, in an environment that contained dispersed component 1 and 2 materials. All component 3 aggregates in Murray contain component 1 material, most commonly as flakes, and locally as tiny granules and larger spherules, some of which are hollow and some of which were broken prior to their mechanical incorporation in accretionary chondrules. Accretion may have occurred as ices associated with dispersed water-bearing component 1 materials temporarily melted during the precipitation of component 3 silicates, and then abruptly refroze to form an icy cementing material. Group 1 materials may be cometary, and group 2 materials may be asteroidal. Schematic models are proposed. Evidence is reviewed for the lunar origin of the pyroxeneplagioclase achondrites. On the basis of natural remanent magnetism, it is suggested that the very scarce diopside-olivine achondrites may be samples from Mars. A classification of the meteorite breccias, including the calcium-poor and calcium-rich mesosiderites, and irons that contain silicate fragments, is proposed. A fragmentation history of the meteorites is outlined on the basis of evidence in the polymict breccias, and from gas retention ages in stones and exposure ages in irons. Cometal impacts appear to have caused the initial fragmentation, stud possibly the perturbation of orbits, of two inferred asteroidal bodies (enstatite and bronzite), one and possibly both events occurring before 2000 m.y. ago. Several impacts apparently occurred on the inferred hypersthene body in the interval 1000 to 2000 m.y. ago. Major breakups of the three bodies apparently occurred as the result of interasteroidal collisions at about 900 m.y. ago, and 600 to 700 m.y. ago. The breakups were followed by a number of fr

Open-File Report↗
USGS2010

The cali meteorite fell: A new H/L ordinary chondrite

The fall of the Cali meteorite took place on 6 July 2007 at 16 h 32 ?? 1 min local time (21 h 32 ?? 1 min UTC). A daylight fireball was witnessed by hundreds of people in the Cauca Valley in Colombia from which 10 meteorite samples with a total mass of 478 g were recovered near 3??24.3'N, 76??30.6'W. The fireball trajectory and radiant have been reconstructed with moderate accuracy. From the computed radiant and from considering various plausible velocities, we obtained a range of orbital solutions that suggest that the Cali progenitor meteoroid probably originated in the main asteroid belt. Based on petrography, mineral chemistry, magnetic susceptibility, fhermoluminescence, and bulk chemistry, the Cali meteorite is classified as an H/L4 ordinary chondrite breccia.

Meteoritics and Planetary Science↗
USGS1982

Origin and evolution of the Nakhla meteorite inferred from the Sm-Nd and U-Pb systematics and REE, Ba, Sr, Rb and K abundances

Analyses of Sm-Nd and U-Th-Pb systematics, REE, Ba, Sr, Rb and K concentrations were carried out for whole rock and mineral separates from the Nakhla meteorite. The 1.26 ±.07 b . y . Sm-Nd age obtained in this work is in good agreement with those previously obtained by the Rb-Sr and Ar-Ar methods. The high initial ϵ Nd value of +16 suggests that Nakhla was derived from a light REE-depleted, old planetary mantle source. U-Th-Pb data, after correction for pre-analytical terrestrial Pb contamination assuming an age of 1.26 b.y., suggest that the age of the Nakhla source is ⩽4.33 b.y. The agreement in the age determined by three independent radiometric methods and the high initial ϵ Nd value strongly suggest that the 1.3 b.y. age dates one thorough igneous event in the parent body which not only reset these isotopic systems but also established the chemical and petrologic characteristics observed for the Nakhla meteorite. Using a three-stage Sm-Nd evolution model in combination with LIL element data and estimated partition coefficients, we have tested partial melting and fractional crystallization models to estimate LIL element abundances in a possible Nakhla source. Our model calculations suggest that partial melting of the light REE-depleted source followed by extensive fractional crystallization (⩾50%) of the partial melt could account for the REE abundances in the Nakhla constituent minerals. The estimated source is depleted in the light REE, Ba, Rb and K and therefore may resemble the MORB source in the earth's upper mantle or the upper 60–300 km of the moon. The significantly younger age of Nakhla than the youngest lunar rock; the young differentiation age inferred from the U-Th-Pb data, and the estimated LIL element abundances (including those of K, U and Th) in the source suggest that the Nakhla meteorite may have been derived from a relatively large, well-differentiated planetary body such as Mars.

Geochimica et Cosmochimica Acta↗
USGS1984

Intrinsic oxygen fugacity measurements on seven chondrites, a pallasite, and a tektite and the redox state of meteorite parent bodies

Intrinsic oxygen-fugacity ( f O 2 ) measurements were made on five ordinary chondrites, a carbonaceous chondrite, an enstatite chondrite, a pallasite, and a tektite. Results are of the form of linear log f O 2 − 1 T "> fO 2 −1T plots. Except for the enstatite chondrite, measured results agree well with calculated estimates by others. The tektite produced f O 2 values well below the range measured for terrestrial and lunar rocks. The lowpressure atmospheric regime that is reported to follow large terrestrial explosions, coupled with a very high temperature, could produce glass with f O 2 in the range measured. The meteorite Salta (pallasite) has low f O 2 and lies close to Hvittis (E6). Unlike the other samples, results for Salta do not parallel the iron-wüstite buffer, but are close to the fayalite-quartz-iron buffer in slope. Minor reduction by graphite appears to have taken place during metamorphism of ordinary chondrites. f O 2 values of unequilibrated chondrites show large scatter during early heating suggesting that the constituent phases were exposed to a range of f O 2 conditions. The samples equilibrated with respect to f O 2 in relatively short time on heating. Equilibration with respect to f O 2 in ordinary chondrites takes place between grades 3 and 4 of metamorphism. Application of P − T − f O 2 relations in the system C-CO-CO 2 indicates that the ordinary chondrites were metamorphosed at pressures of 3–20 bars, as it appears that they lay on the graphite surface. A steep positive thermal gradient in a meteorite parent body lying at the graphite surface will produce thin reduced exterior, an oxidized near-surface layer, and an interior that is increasingly reduced with depth; a shallow thermal gradient will produce the reverse. A body heated by accretion on the outside will have a reduced exterior and oxidized interior. Meteorites from the same parent body clearly are not required to have similar redox states.

Geochimica et Cosmochimica Acta↗
USGS2004

Luminescence dating of the Wabar meteorite craters, Saudi Arabia

Luminescence dating has been used to find the age of meteorite impact craters at Wabar (Al Hadida) in Saudi Arabia. The luminescence characteristics of the shocked material were determined. Using a variety of luminescence dating techniques applied to impactite formed by the meteorite, and to the underlying sand, the age is found to be 290 ± 38 years. A comparison is made with two possible historically recorded ages. An impact as young as this has implications for the assessment of hazards from the impact on Earth of small meteorites.

Al Hadida, Ar-Rub' Al-Khali (Empty Quarter)↗
USGS1973

Time differences in the formation of meteorites as determined from the ratio of lead-207 to lead-206

Measurements of the lead isotopic composition and the uranium, thorium, and lead concentrations in meteorites were made in order to obtain more precise radiometric ages of these members of the solar system. The newly determined value of the lead isotopic composition of Canyon Diablo troilite is as follows: 206 Pb/ 204 Pb = 9.307, 207 Pb/ 2O4 Pb = 10.294, and 208 Pb/ 204 Pb = 29.476. The leads of Angra dos Reis, Sioux County, and Nuevo Laredo achondrites are very radiogenic, the 206 Pb/ 204 Pb values are about 200, and the uranium-thorium-lead systems are nearly concordant. The ages of the meteorites as calculated from a single-stage 207 Pb/ 206 Pb isochron based on the newly determined primordial lead value and the newly reported 235 U and 838 U decay constants, are 4.528 x 10 9 years for Sioux County and Nuevo Laredo and 4.555 x 10 9 years for Angra dos Reis. When calculated with the uranium decay constants used by Patterson, these ages are 4.593 x 10 9 years and 4.620 x 10 9 years, respectively, and are therefore 40 to 70 x 10 6 years older than the 4.55 x 10 9 years age Patterson reported. The age difference of 27 x 10 6 years between Angra dos Reis and the other two meteorites is compatible with the difference between the initial 87 Sr/ 86 Sr ratio of Angra dos Reis and that of seven basaltic achondrites observed by Papanastassiou and Wasserburg. The time difference is also comparable to that determined by 129 1- 129 Xe chronology. The ages of ordinary chondrites (H5 and L6) range from 4.52 to 4.57 x 10 9 years, and, here too, time differences in the formation of the parent bodies or later metamorphic events are indicated. Carbonaceous chondrites(C2 and C3) appear to contain younger lead components.

Science↗
USGS2004

The Rock Elm meteorite impact structure, Wisconsin: Geology and shock-metamorphic effects in quartz

The Rock Elm structure in southwest Wisconsin is an anomalous circular area of highly deformed rocks, ∼6.5 km in diameter, located in a region of virtually horizontal undeformed sedimentary rocks. Shock-produced planar microstructures (PMs) have been identified in quartz grains in several lithologies associated with the structure: sandstones, quartzite pebbles, and breccia. Two distinct types of PMs are present: P1 features, which appear identical to planar fractures (PFs or cleavage), and P2 features, which are interpreted as possible incipient planar deformation features (PDFs). The latter are uniquely produced by the shock waves associated with meteorite impact events. Both types of PMs are oriented parallel to specific crystallographic planes in the quartz, most commonly to c(0001), xi{11 (2) over bar2}, and r/k{10 (1) over bar1} . The association of unusual, structurally deformed strata with distinct shock-produced microdeformation features in their quartz-bearing rocks establishes Rock Elm as a meteorite impact structure and supports the view that the presence of multiple parallel cleavages in quartz may be used independently as a criterion for meteorite impact. Preliminary paleontological studies indicate a minimum age of Middle Ordovician for the Rock Elm structure. A similar age estimate (450-400 Ma) is obtained independently by combining the results of studies of the general morphology of complex impact structures with estimated rates of sedimentation for the region. Such methods may be applicable to dating other old and deeply eroded impact structures formed in sedimentary target rocks.

Wisconsin↗
USGS1993

An extremely low UPb source in the Moon: U Th Pb, Sm Nd, Rb Sr, and 40Ar 39Ar isotopic systematics and age of lunar meteorite Asuka 881757

We have undertaken U Th Pb, Sm Nd, Rb Sr, and 40 Ar 39 Ar "> 40 Ar 39 Ar isotopic studies on Asuka 881757, a coarse-grained basaltic lunar meteorite whose chemical composition is close to low-Ti and very low-Ti (VLT) mare basalts. The Pb Pb internal isochron obtained for acid leached residues of separated mineral fractions yields an age of 3940 ± 28 Ma, which is similar to the U-Pb (3850 ± 150 Ma) and Th-Pb (3820 ± 290 Ma) internal isochron ages. The Sm-Nd data for the mineral separates yield an internal isochron age of 3871 ± 57 Ma and an initial 143 Nd 144 Nd "> 143 Nd 144 Nd value of 0.50797 ± 10. The Rb-Sr data yield an internal isochron age of 3840 ± 32 Ma ( λ( 87 Rb) = 1.42 × 10 −11 yr −1 "> λ( 87 Rb) = 1.42 × 10 −11 yr −1 ) and a low initial 87 Sr 86 Sr "> 87 Sr 86 Sr ratio of 0.69910 ± 2. The 40 Ar 39 Ar "> 40 Ar 39 Ar age spectra for a glass fragment and a maskelynitized plagioclase are relatively flat and give a weighted mean plateau age of 3798 ± 12 Ma. We interpret these ages to indicate that the basalt crystallized from a melt 3.87 Ga ago (the Sm-Nd age) and an impact event disturbed the Rb-Sr system and completely reset the K-Ar system at 3.80 Ga. The slightly higher Pb-Pb age compared to the Sm-Nd age could be due to the secondary Pb (from terrestrial and/or lunar surface Pb contamination) that remained in the residues after acid leaching. Alternatively, the following interpretation is also possible; the meteorite crystallized at 3.94 Ga (the Pb-Pb age) and the Sm-Nd, Rb-Sr, and K-Ar systems were disturbed by an impact event at 3.80 Ga. The crystallization age obtained here is older than those reported for low-Ti basalts (3.2–3.5 Ga) and for VLT basalts (3.4 Ga), but similar to ages of some mare basalts, indicating that the basalt may have formed from a magma related to a basin-forming event (Imbrium?). The age span for VLT basalts from different sampling sites suggest that they were erupted over a wide area during an interval of at least ~500 million years. The impact event that thermally reset the K-Ar system of Asuka 881757 must have been post-Imbrium (perhaps Orientale) in age. The lead isotopic composition of Asuka 881757 is nonradiogenic compared with typical Apollo mare basalts and the estimated 238 U 204 Pb "> 238 U 204 Pb (μ) value for the basalt source is 10 ± 3. This source-μ value is the lowest so far measured for lunar rocks. A large positive ϵ Nd value (7.4 ± 0.5) and the time averaged 147 Sm 144 Nd "> 1 47 147 Sm 144 Nd "> Sm 144 Nd ratio for the basalt source are similar to those for some Apollo 12, 15, and 17 basalts, suggesting a LREE-depleted mantle, which is consistent with the global magma ocean hypothesis. The U-Th-Pb, Sm-Nd, and Rb-Sr data on Asuka 881757 suggest that the basalt was derived from a low U Pb "> UPb , low Rb Sr "> RbSr , and high Sm Nd "> SmNd source region, mainly composed of olivine and orthopyroxene with minor amounts of plagioclase (or clinopyroxene) and with sulfides enriched in volatile chalcophile elements. The basalt source may be deep in origin and different in chemistry from those previously estimated from studies of Apollo and Luna mare basalts, indicating heterogeneous sources for mare basalts.

Geochimica et Cosmochimica Acta↗
USGS1995

Rhenium-osmium concentration and isotope systematics in group IIAB iron meteorites

Rhenium and osmium abundances, and osmium isotopic compositions were measured by negative thermal ionization mass spectrometry in thirty samples, including replicates, of five IIA and eight IIB iron meteorites. Concentrations in HA irons range from 4800 ppb Re and 66000 ppb Os (Negrillos) to 160 ppb Re and 800 ppb Os (Lombard). In the IIB subgroup, concentrations vary from 28 ppb Re and 180 ppb Os (Navajo) down to 0.8 ppb Re and 9 ppb Os (São Julião de Moreira and Santa Luzia). Log plots of Os vs. Re abundances for HA and IIB irons describe straight lines that approximately converge on Lombard, which has the lowest Re and Os abundances and highest 187 Re/ 188 Os measured in a IIA iron to date. The linear HA trend may be exactly reproduced by fractional crystallization with constant k Re and k Os , but is not well fitted using variable partition coefficients. The IIB iron trend, however, cannot be entirely explained by simple fractional crystallization. One explanation is that small amounts of Re and Os were added to the asteroid core during the final stages of crystallization. Another possibility is that diffusional enrichment of Os may have occurred in samples most depleted in Re and Os. The combined Re-Os isotopic data for HA irons give the following results: slope = 0.07803 ± 0.00076; intercept = 0.09609 ± 0.00045; age = 4584 ± 43 Ma (neglecting the uncertainty in the decay constant of ±3%). Four IIB iron meteorites (Mount Joy, Central Missouri, DRPA 78009, Santa Luzia) also plot within the analytical uncertainty of the HA isochron. These results are consistent with rapid (probably <50 Ma) core segregation, differentiation, and crystallization in the IIAB parent. Several IIB irons (Navajo, Sandia Mountains, Smithsonian Iron, and perhaps São Julião de Moreira) lie beyond analytical uncertainty above the IIA iron isochron, averaging 8 ± 2% higher in 187 Os/ 188 Os. These irons may have crystallized significantly after the HA irons and Mount Joy, but only if the 187 Re/ 188 Os of the melt was ≥2.2. There is no evidence for a IIA iron crystallizing in equilibrium with a melt having such a high ratio. Alternatively, the osmium isotopic systematics of these irons may have been slightly disturbed long after crystallization at ca. 3.3 Ga ago.

Geochimica et Cosmochimica Acta↗
USGS2016

Spectral nature of CO 2 adsorption onto meteorites

Previous studies have identified carbon dioxide (CO 2 ) on the surfaces of Jovian and Galilean satellites in regions of non-ice material that are too warm for CO 2 ice to exist. CO 2 ice would quickly sublimate if not retained by a less-volatile material. To ascertain what non-ice species may be responsible for stabilizing this CO 2 , we performed CO 2 gas adsorption experiments on thirteen powdered CM, CI, and CV carbonaceous chondrite meteorites. Reflectance spectra of the &nu; 3 feature associated with adsorbed CO 2 near 4.27 &mu;m were recorded. Results show that many meteorites adsorbed some amount of CO 2 , as evidenced by an absorption feature that was stable over several hours at ultra-high vacuum (UHV) and high vacuum, (1.0&times;10 &minus;8 and 1.0&times;10 &minus;7 Torr, respectively). Ivuna, the only CI chondrite studied, adsorbed significantly more CO 2 than the others. We found that CO 2 abundance did not vary with &lsquo;water&rsquo; abundance, organics, or carbonates as inferred from the area of the 3-&mu;m band, the 3.2-3.4 &mu;m C-H feature, and the &sim;3.8-&mu;m band respectively, but did correlate with hydrous/anhydrous phyllosilicate ratios. Furthermore, we did not observe CO 2 ice because the position of the CO 2 feature was generally shifted 3-10 nm from that of the 4.27 &mu;m absorption characteristic of ice. The strongest compositional relationship observed was a possible affinity of CO 2 for total FeO abundance and complex clay minerals, which make up the bulk of the CI chondrite matrix. This finding implies that the most primitive refractory materials in the Solar System may also act as reservoirs of CO 2 , and possibly other volatiles, delivering them to parts of the Solar System where their ices would not be stable.

Icarus↗
USGS1983

On the reported optical activity of amino acids in the Murchison meteorite

In analyses of extracts from the Murchison meteorite (a carbonaceous chondrite), Engel and Nagy1 reported an excess of L-enantiomers for several protein amino acids but found that the non-protein amino acids were racemic. They suggested that the excess of L-isomers might have resulted from an asymmetric synthesis or decomposition. Their results disagree with those obtained previously2-4 and they claim this is due to improved methodology. In fact, their extraction method and analytical procedure (gas chromatography-mass spectrometry, GC-MS) was similar to those used in the original report2 of amino acids in the Murchison meteorite except that they used specific ion monitoring in the GC-MS measurements. We found the results of Engel and Nagy odd in that likely contaminants (the protein amino acids ala, leu, glu, asp and pro) were nonracemic while unlikely contaminants (isovaline and ??-amino-n-butyric acid) were racemic. For example, Engel and Nagy report that the leucine is ???90% L-enantiomer in the water-extracted sample whereas isovaline (??-methyl-??-aminobutyric acid) is racemic. It would be most unusual for an abiotic stereoselective decomposition or synthesis of amino acids to occur with protein amino acids but not with non-protein amino acids. We now show here that the explanation of terrestrial contamination is consistent with their results and is much more probable. ?? 1983 Nature Publishing Group.

Nature↗
USGS1968

Gold in meteorites and in the earth's crust

The reported gold contents of meteorites range from 0.0003 to 8.74 parts per million. Gold is siderophilic, and the greatest amounts in meteorites are in the iron phases. Estimates ,of the gold content of the earth's crust are in the range of 0.001 to 0.006 parts per million.

Circular↗
USGS2010

Lunar highland meteorite Dhofar 026 and Apollo sample 15418: Two strongly shocked, partially melted, granulitic breccias

Studies of lunar meteorite Dhofar 026, and comparison to Apollo sample 15418, indicate that Dhofar 026 is a strongly shocked granulitic breccia (or a fragmental breccia consisting almost entirely of granulitic breccia clasts) that experienced considerable post-shock heating, probably as a result of diffusion of heat into the rock from an external, hotter source. The shock converted plagioclase to maskelynite, indicating that the shock pressure was between 30 and 45 GPa. The post-shock heating raised the rock's temperature to about 1200 °C; as a result, the maskelynite devitrified, and extensive partial melting took place. The melting was concentrated in pyroxene-rich areas; all pyroxene melted. As the rock cooled, the partial melts crystallized with fine-grained, subophitic-poikilitic textures. Sample 15418 is a strongly shocked granulitic breccia that had a similar history, but evidence for this history is better preserved than in Dhofar 026. The fact that Dhofar 026 was previously interpreted as an impact melt breccia underscores the importance of detailed petrographic study in interpretation of lunar rocks that have complex textures. The name “impact melt” has, in past studies, been applied only to rocks in which the melt fraction formed by shock-induced total fusion. Recently, however, this name has also been applied to rocks containing melt formed by heating of the rocks by conductive heat transfer, assuming that impact is the ultimate source of the heat. We urge that the name “impact melt” be restricted to rocks in which the bulk of the melt formed by shock-induced fusion to avoid confusion engendered by applying the same name to rocks melted by different processes.

Meteoritics and Planetary Science↗