The Ardón L6 ordinary chondrite: A long‐hidden Spanish meteorite fall     

Josep M. Trigo‐Rodríguez  Jordi Llorca  Mona Weyrauch  Addi Bischoff  Carles E. Moyano‐Cambero  Klaus Keil  Matthias Laubenstein  Andreas Pack  José María Madiedo  Jacinto Alonso‐Azcárate  My Riebe  Rainer Wieler  Uli Ott  Mar Tapia  Narcís Mestres 《Meteoritics & planetary science》2014,49(8):1475-1484

We report and describe an L6 ordinary chondrite fall that occurred in Ardón, León province, Spain (longitude 5.5605°W, latitude 42.4364°N) on July 9th, 1931. The 5.5 g single stone was kept hidden for 83 yr by Rosa González Pérez, at the time an 11 yr old who had observed the fall and had recovered the meteorite. According to various newspaper reports, the event was widely observed in Northern Spain. Ardón is a very well‐preserved, fresh, strongly metamorphosed (petrologic type 6), and weakly shocked (S3) ordinary chondrite with well‐equilibrated and recrystallized minerals. The mineral compositions (olivine Fa_23.7±0.3, low‐Ca pyroxene Fs_20.4±0.2Wo_1.5±0.2, plagioclase An_10.3±0.5Ab_84.3±1.2), magnetic susceptibility (log χ = 4.95 ± 0.05 × 10^?9 m³ kg^?1), bulk density (3.49 ± 0.05 g   cm^?3), grain density (3.58 ± 0.05 g   cm^?3), and porosity (2.5 vol%) are typical for L6 chondrites. Short‐lived radionuclides confirm that the meteorite constitutes a recent fall. The ²¹Ne and ³⁸Ar cosmic ray exposure ages are both about 20–30 Ma, similar to values for many other L chondrites. The cosmogenic ²²Ne/²¹Ne ratio indicates that preatmospheric Ardón was a relatively large body. The fact that the meteorite was hidden in private hands for 83 yr makes one wonder if other meteorite falls may have experienced the same fate, thus possibly explaining the anomalously low number of falls reported in continental Spain in the 20th century.  相似文献   

Northwest Africa 032: Product of lunar volcanism     

T. J. Fagan  G. J. Taylor  K. Keil  T. E. Bunch  J. H. Wittke  R. L. Korotev  B. L. Jolliff  J. J. Gillis  L. A. Haskin  E. Jarosewich  R. N. Clayton  T. K. Mayeda  V. A. Fernandes  R. Burgess  G. Turner  O. Eugster  S. Lorenzetti 《Meteoritics & planetary science》2002,37(3):371-394

Abstract— Mineralogy, major element compositions of minerals, and elemental and oxygen isotopic compositions of the whole rock attest to a lunar origin of the meteorite Northwest Africa (NWA) 032, an unbrecciated basalt found in October 1999. The rock consists predominantly of olivine, pyroxene and chromite phenocrysts, set in a crystalline groundmass of feldspar, pyroxene, ilmenite, troilite and trace metal. Whole‐rock shock veins comprise a minor, but ubiquitous portion of the rock. Undulatory to mosaic extinction in olivine and pyroxene phenocrysts and micro‐faults in groundmass and phenocrysts also are attributed to shock. Several geochemical signatures taken together indicate unambiguously that NWA 032 originated from the Moon. The most diagnostic criteria include whole‐rock oxygen isotopic composition and ratios of Fe/Mn in the whole rock, olivine, and pyroxene. A lunar origin is documented further by the presence of Fe‐metal, troilite, and ilmenite; zoning to extremely Fe‐rich compositions in pyroxene; the ferrous oxidation state of all Fe in pyroxene; and the rare earth element (REE) pattern with a well‐defined negative europium anomaly. This rock is similar in major element chemistry to basalts from Apollo 12 and 15, but is enriched in light REE and has an unusually high Th/Sm ratio. Some Apollo 14 basalts yield a closer match to NWA 032 in REE patterns, but have higher concentrations of Al₂O₃. Ar‐Ar step release results are complex, but yield a whole‐rock age of ?2.8 Ga, suggesting that NWA 032 was extruded at 2.8 Ga or earlier. This rock may be the youngest sample of mare basalt collected to date. Noble gas concentrations combined with previously collected radionuclide data indicate that the meteorite exposure history is distinct from currently recognized lunar meteorites. In short, the geochemical and petrographic features of NWA 032 are not matched by Apollo or Luna samples, nor by previously identified lunar meteorites, indicating that it originates from a previously unsampled mare deposit. Detailed assessment of petrographic features, olivine zoning, and thermodynamic modelling indicate a relatively simple cooling and crystallization history for NWA 032. Chromite‐spinel, olivine, and pyroxene crystallized as phenocrysts while the magma cooled no faster than 2 °C/h based on the polyhedral morphology of olivine. Comparison of olivine size with crystal growth rates and preserved Fe‐Mg diffusion profiles in olivine phenocrysts suggest that olivine was immersed in the melt for no more than 40 days. Plumose textures in groundmass pyroxene, feldspar, and ilmenite, and Fe‐rich rims on the phenocrysts formed during rapid crystallization (cooling rates ?20 to 60 °C/h) after eruption.  相似文献   

Northwest Africa 773: Lunar origin and iron‐enrichment trend     

T. J. Fagan  G. J. Taylor  K. Keil  T. L. Hicks  M. Killgore  T. E. Bunch  J. H. Wittke  D. W. Mittlefehldt  R. N. Clayton  T. K. Mayeda  O. Eugster  S. Lorenzetti  M. D. Norman 《Meteoritics & planetary science》2003,38(4):529-554

Abstract— The meteorite Northwest Africa 773 (NWA 773) is a lunar sample with implications for the evolution of mafic magmas on the moon. A combination of key parameters including whole‐rock oxygen isotopic composition, Fe/Mn ratios in mafic silicates, noble gas concentrations, a KREEP‐like rare earth element pattern, and the presence of regolith agglutinate fragments indicate a lunar origin for NWA 773. Partial maskelynitization of feldspar and occasional twinning of pyroxene are attributed to shock deformation. Terrestrial weathering has caused fracturing and precipitation of Carich carbonates and sulfates in the fractures, but lunar minerals appear fresh and unoxidized. The meteorite is composed of two distinct lithologies: a two‐pyroxene olivine gabbro with cumulate texture, and a polymict, fragmental regolith breccia. The olivine gabbro is dominated by cumulate olivine with pigeonite, augite, and interstitial plagioclase feldspar. The breccia consists of several types of clasts but is dominated by clasts from the gabbro and more FeO‐rich derivatives. Variations in clast mineral assemblage and pyroxene Mg/(Mg + Fe) and Ti/(Ti + Cr) record an igneous Fe‐enrichment trend that culminated in crystallization of fayalite + silica + hedenbergite‐bearing symplectites. The Fe‐enrichment trend and cumulate textures observed in NWA 773 are similar to features of terrestrial ponded lava flows and shallow‐level mafic intrusives, indicating that NWA 773 may be from a layered mafic intrusion or a thick, differentiated lava flow. NWA 773 and several other mafic lunar meteorites have LREE‐enriched patters distinct from Apollo and Luna mare basalts, which tend to be LREE‐depleted. This is somewhat surprising in light of remote sensing data that indicates that the Apollo and Luna missions sampled a portion of the moon that was enriched in incompatible heatproducing elements.  相似文献   

Advanced Technology Solar Telescope     

Stephen L. Keil  Thomas R. Rimmele  Jeremy Wagner 《Earth, Moon, and Planets》2009,104(1-4):77-82

High-resolution studies of the Sun’s magnetic fields are needed for a better understanding of the fundamental processes responsible for solar variability. The generation of magnetic fields through dynamo processes, the amplification of fields through the interaction with plasma flows, and the destruction of fields are poorly understood. There is incomplete insight into physical mechanisms responsible for chromospheric and coronal structure and heating, causes of variations in the radiative output of the Sun, and mechanisms that trigger flares and coronal mass ejections. Progress in answering these critical questions requires study of the interaction of the magnetic field and convection with a resolution sufficient to observe scale fundamental to these processes. The planned 4 m aperture ATST will be a unique scientific tool, with excellent angular resolution, a large wavelength range, and low scattered light. With its integrated adaptive optics, the ATST will achieve a spatial resolution nearly 10 times better than any existing solar telescope. The ATST design and development phase began in 2001 and it is now ready to begin construction in 2009.  相似文献   

The Lueders,Texas, IAB iron meteorite with silicate inclusions     

T. J. McCoy  A. J. Ehlmann  G. K. Benedix  K. Keil  J. T. Wasson 《Meteoritics & planetary science》1996,31(3):419-422

Abstract The Lueders iron meteorite with silicate inclusions was recovered as a single specimen of ～35.4 kg in Shackelford County, Texas, in 1973 and recognized as a meteorite in 1993. Siderophile element concentrations indicate chemical classification as a low-Ni IAB iron meteorite closely related to Landes; like Landes, it has a Cu content ～4σ above the main IAB-IIICD trend and therefore we also designate Lueders as an anomalous member of IAB. The metallic host is composed of equigranular kamacite but with a suggestion of octahedral structure and with a bandwidth of 1.4 mm, suggesting structural classification as a coarse octahedrite (Og). The meteorite contains ～23 wt% of roughly millimeter to centimeter-sized angular silicate inclusions. Classification as a IAB is confirmed by O isotopic analysis of silicate inclusions. These inclusions contain an assemblage rich in silicates, troilite and graphite; lack certain minor phases (e.g., daubreelite); and have angular shapes. A variety of processes (e.g., fragmentation, partial melting, reduction) appear to have played a significant role in the formation of Lueders and all IAB iron meteorites. Petrologic and chemical differences confirm that Lueders is not paired with the widely distributed Odessa meteorite.  相似文献   

CLASSIFICATION OF A SECOND GROUP OF ORDINARY CHONDRITES FROM TEXAS     

Arthur J. Ehlmann  Klaus Keil 《Meteoritics & planetary science》1987,22(1):17-23

Based on optical microscopy and electron microprobe analyses of mafic minerals, six previously undescribed or poorly known ordinary chondrites from Texas were classified into compositional groups, petrologic types, and shock facies. These meteorites are Junction, L5d; Anton, H4b; Venus, H4d; Dalhart, H5a; Rosebud, H5c; and Cranfills Gap, H6c.  相似文献   

Organic–mineral interactions in marine sediments studied using density fractionation and X-ray photoelectron spectroscopy     

Thorarinn S. Arnarson  Richard G. Keil 《Organic Geochemistry》2001,32(12):434

Interactions between organic matter (OM) and minerals are important for the preservation of organic material in marine sediments. Recent evidence suggests that these interactions may not be due to protection of the organic matter via simple sorption to minerals, but rather that the organic matter is acting as a glue between mineral particles resulting in aggregate formation. Density fractionation into multiple density intervals is a powerful approach for looking at the patchy nature of organic matter distribution (Bock, M.J., Mayer, L.M., 2000. Mesodensity organo-clay associations in a near-shore sediment. Marine Geology 163, 65–75), because isolates with varying organic matter loadings can be obtained. The aggregate surfaces can be examined using X-ray photoelectron spectroscopy (XPS), because this technique is restricted to the top 10 nm of the sample surface. XPS gives quantitative information on all the elements on the surface (except H and He) and, in addition, gives information on the chemical bonding of carbon and nitrogen. We used these techniques, along with more standard geochemical tools (CHN, surface area and amino acid analyses), to study a sediment sample of high organic matter content from the oxygen deficient zone off the western coast of Mexico. We found that amino acid composition indicates that the fractions get progressively more degraded as density increases, while the fraction of carbon bonded to oxygen suggests that the oxidation state of the organic matter is similar for all fractions. In addition, the comparison of surface carbon concentrations from XPS and OC:SA suggests that the OM is located in discrete spots on the surface and that as density increases (OC:SA decreases) the organic matter decreases in thickness while retaining the same proportional areal extent.  相似文献   

Microchondrule-bearing clast in the Piancaldoli LL3 meteorite: a new kind of type 3 chondrite and its relevance to the history of chondrules     

Alan E. Rubin  Edward R.D. Scott  Klaus Keil 《Geochimica et cosmochimica acta》1982,46(10):1763-1776

In the Piancaldoli LL3 chondrite, we found a mm-sized clast containing ~100 chondrules 0.2–64 μm in apparent diameter (much smaller than any previously reported) that are all of the same textural type (radial pyroxene; FS_1–17). This clast, like other type 3 chondrites, has a fine-grained Ferich opaque silicate matrix, sharply defined chondrules, abundant low-Ca clinopyroxene and minor troilite and Si- and Cr-bearing metallic Fe,Ni. However, the very high modal matrix abundance (63 ± 8 vol. %), unique characteristics of the chondrules, and absence of microscopically-observable olivine indicate that the clast is a new kind of type 3 chondrite. Most chondrules have FeO-rich edges, and chondrule size is inversely correlated with chondrule-core FeO concentration (the first reported correlation of chondrule size and composition). Chondrules acquired Fe by diffusion from Fe-rich matrix material during mild metamorphism, possibly before final consolidation of the rock. Microchondrules (those chondrules ? 100 μm in diameter) are also abundant in another new kind of type 3 chondrite clast in the Rio Negro L chondrite regolith breccia. In other type 3 chondrite groups, microchondrule abundance appears to be anticorrelated with mean chondrule size, viz. 0.02–0.04 vol. % in H and CO chondrites and ?0.006 vol. % in L, LL, and CV chondrites.Microchondrules probably formed by the same process that formed normal-sized droplet chondrules: melting of pre-existing dustballs. Because most compound chondrules in the clast and other type 3 chondrites formed by collisions between chondrules of the same textural type, we suggest that dust grains were mineralogically sorted in the nebula before aggregating into dustballs. The sizes of compound chondrules and chondrule craters, which resulted from collisions of similarly-sized chondrules while they were plastic, indicate that size-sorting (of dustballs) occurred before chondrule formation, probably by aerodynamic processes in the nebula. We predict that other kinds of type 3 chondrites exist which contain chondrule abundances, size-ranges and proportions of textural types different from known chondrite groups.  相似文献   

Geochemical modelling of basalts from DSDP Leg 65, East Pacific Rise,Gulf of California     

A.M. Kudo  S.E. Barker  K. Keil  Marvin H. Beeson 《Geochimica et cosmochimica acta》1982,46(12):2427-2434

Major and rare earth element (REE) data for basalts from Holes 483, 483B, and 485A of DSDP Leg 65, East Pacific Rise, mouth of the Gulf of California, support a simple fractional crystallization model for the genesis of rocks from this suite. The petrography and mineral chemistry (presented in detail elsewhere) provide no evidence for magma mixing, but rather a simple multistage cooling process. Based on its lowest TiO₂ content (0.88%), $\text{FeO}{}^1\text{MgO}$ ratio (0.95 with total Fe as FeO), and Mg# (100 $\text{Mg}\text{Mg + Fe}\text{″ = 70}$), sample 483-17-2-(78–83) has been selected as the most primitive primary magma of the samples analyzed. This is supported by the REE data which show this sample has the lowest total REE content, a $\text{La}\text{Sm}{}_{\mathrm{cn}}$ (chondrite-normalized) = 0.36, and $\text{Eu}\text{Sm}{}_{\mathrm{cn}}$ = 1.05. Because other samples analyzed have higher SiO₂, lower Mg#, and a negative Eu anomaly ($\text{Eu}\text{Sm}{}_{\mathrm{cn}}$ as low as 0.89), they are most likely derivative magmas. Wright-Doherty and trace element modelling support fractional crystallization of 14.1% plagioclase (An₈₈), 6.7% olivine (Fo₈₆), and 4.7% clinopyroxene (Wo₄₁En₄₉Fs₁₀) from 483-17-2-(78–83) to form the least differentiated sample with Mg# = 63. The $\text{La}\text{Sm}{}_{\mathrm{cn}}$ of this derivative magma is almost identical to the parent magma (0.35 to 0.36), but the other samples have higher $\text{La}\text{Sm}{}_{\mathrm{cn}}$ (0.45 to 0.51), more total REE, and lower Mg# (60 to 56). Both Wright-Doherty and trace element modelling indicate that the primary magma chosen cannot produce these more evolved samples. For the major elements, the TiO₂ and P₂O₅ are too low in the calculated versus the observed (1.38 to 1.90; 0.11 to 0.17, respectively, for example). Rayleigh fractionation calculates a lower $\text{La}\text{Sm}{}_{\mathrm{cn}}$ and requires about 60% crystal removal versus 40% for the Wright-Doherty. These more evolved samples must be derived from a parent magma different from the one selected here and, unfortunately, not sampled in this study. A magma formed by a smaller degree of partial melting with slightly more residual clinopyroxene left in the mantle than for sample 483-17-2-(78–83) is required.  相似文献   

Metamorphism of the H-group chondrites: implications from compositional and textural trends in chondrules     

Gayle Lux  Klaus Keil  G.Jeffrey Taylor 《Geochimica et cosmochimica acta》1980,44(6):841-855

Major and minor element bulk compositions of 373 individual chondrules from 18 H3 to H6 chondrites were determined in polished thin sections by broad-beam electron probe analysis. Bulk chondrule FeO and Al₂O₃ increase and TiO₂ and Cr₂O₃ decrease with increasing petrologic type; normative fayalite, albite and plagioclase increase through the petrologic sequence. Chondrule diameters correlate with phenocryst sizes in porphyritic chondrules of type 3 chondrites, but this correlation is diminished in the higher petrologic types. Furthermore, for a given chondrule diameter, phenocryst sizes are larger in the higher petrologic types. We attribute most compositional trends in chondrules through the petrologic sequence to diffusion and equilibration among chondrules and between chondrules and matrix in response to increasing degrees of thermal metamorphism. Increased phenocryst sizes in the higher petrologic types are probably the result of grain growth during metamorphism.We suggest that H-group chondrites formed by accretion of high-temperature (chondrules) and low-temperature (matrix) materials. Parent materials of each of the petrologic types resembled type 3 chondrites, but had slight compositional differences (e.g. volatiles, rare gases, total iron) inherited during accretion. These differences were predominantly functions of decreasing temperature in the nebula as accretion progressed. Internal reheating of the parent materials to different temperatures and (probably) for different times, as a function of depth in the parent body, caused compositional equilibration, grain coarsening, and reduction of FeO to Fe° by carbon.  相似文献