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Keil, K.

Publications and source records attributed to Keil, K..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2009

Impact Histories of Vesta and Vestoids inferred from Howardites, Eucrites, and Diogenites

The parent body of the howardites, eucrites and diogenites (HEDs) is thought to be asteroid (4) Vesta [1]. However, several eucrites have now been recognized, like NWA 011 and Ibitira, with major element compositions and mineralogy like normal eucrites but with different oxygen isotope compositions and minor element concentrations suggesting they are not from the same body [2, 3]. The discoveries of abnormal eucrites and V-type asteroids that are probably not from Vesta [see 4] raise the question whether the HEDs with normal oxygen isotopes are coming from Vesta [3]. To address this issue and understand more about the evolution of Vesta in preparation for the arrival of the Dawn spacecraft, we integrate fresh insights from Ar-Ar dating and oxygen isotope analyses of HEDs, radiometric dating of differentiated meteorites, as well as dynamical and astronomical studies of Vesta, the Vesta asteroid family (i.e., the Vestoids), and other V-type asteroids.

Scott, E. R. D.↗
NASA NTRS2004

Experimental Study of Serpentinization Reactions

Current carbonaceous chondrite parent-body thermal models [1-3] produce scenarios that are inconsistent with constraints on aqueous alteration conditions based on meteorite mineralogical evidence, such as phase stability relationships within the meteorite matrix minerals [4] and isotope equilibration arguments [5, 6]. This discrepancy arises principally because of the thermal runaway effect produced by silicate hydration reactions (here loosely called serpentinization, as the principal products are serpentine minerals), which are so exothermic as to produce more than enough heat to melt more ice and provide a self-sustaining chain reaction. One possible way to dissipate the heat of reaction is to use a very small parent body [e.g., 2] or possibly a rubble pile model. Another possibility is to release this heat more slowly, which depends on the alteration reaction path and kinetics.

Cohen, B. A.↗
NASA NTRS2004

Shock Effects in the Metal-rich Chondrites QUE 94411, Hammadah al Hamra 237 and Bencubbin

We have studied the metal-rich chondrites QUE94411 (QUE), Hammadah al Hamra 237 (HH237) and Bencubbin with an emphasis on the petrographical and mineralogical effects of the shock processing that these meteorite assemblages have undergone. Iron-nickel metal and chondrule silicates are the main components in these meteorites. These high-temperature components are held together by shock melts consisting of droplets of dendritically intergrown FeNi metal/sulfide embedded in ferrous silicate glass. The silicate glass component of the shock melt is substantially more FeO-rich (FeO: 30 to 40 wt%) than the prevailing chondrule silicates (FeO less than 5 wt%). Fine-grained matrix material, which is a major component in most other chondritic assemblages, is extremely scarce in QUE4411 and HH237; it has not been observed in Bencubbin. This material occurs as rare, hydrated, and fine-grained matrix lumps with major and minor element abundances roughly similar to the ferrous silicate shock melts (and CI). We infer that hydrated, fine-grained material, compositionally similar to these matrix lumps, was originally present between the FeNi metal grains and chondrules, but was preferentially heated by the shock wave and melted. Other shock-related features in QUE94411, HH237, and Bencubbin include a distinct alignment and occasionally strong plastic deformation of metal and chondrule fragments. However, the existence of chemically zoned and metastable FeNi metal condensates condensates in direct contact with shock melts indicates that the shock did not substantially increase the average temperature of the aggregate. TEM analyses of the FeNi metal droplets embedded in the shock melts show martensitic crystal structures, indicating rapid cooling, essentially quenching, of the shock melts. These findings indicate that the preshock temperature of the assemblage was low. Because porphyritic olivine-pyroxene chondrules are absent in QUE94411, HH237, and Bencubbin, it is difficult to determine the precise shock stage of these meteorites, but the shock was probably relatively light (S2- S3; 5-20 GPa) consistent with a bulk temperature increase of the assemblages of less than 200 C. The higher pressures within this range (i.e. 15-20 GPa) are consistent with the findings of shock produced diamonds in Bencubbin. The many mineralogical, petrological and isotopic (e.g. O and N) similarities between Bencubbin, Weatherford, Gujba and QUE/HH237 have been used to argue for a common origin of these meteorites on a single asteroidal parent body. The findings of this study, which indicate similar shock processing, lend support to this view.

Meibom, A.↗
NASA NTRS2001

Trace-Element Concentrations in Northwest Africa 032

Trace-element concentrations (INAA) are presented for four samples of the NWA 032 lunar meteorite. The mare basalt has a moderately high Th concentration (1.9 ppm) and a higher Th/REE ratio than any other known mare basalt. Additional information is contained in the original extended abstract.

Korotev, R. L.↗
NASA NTRS1994

Catastrophic fragmentation of asteroids: Evidence from meteorites

Meteorites are impact-derived fragments from approximately 85 parent bodies. For seven of these bodies, the meteorites record evidence suggesting that they may have been catastrophically fragmented. We identify three types of catastrophic events: (1) impact and reassemble events greater than 4.4 Gy ago, involving molten or very hot parent bodies (greater than 1200 C); this affected the parent bodies of the ureilites, Shallowater, and the mesosiderites. In each case, the fragments cooled rapidly (approximately 1-1000 C/day) and then reassembled. (2) Later impacts involving cold bodies which, in some cases, reassembled; this occurred on the H and L ordinary chondrite parent bodies. The L parent body probably suffered another catastrophic event about 500 My ago. (3) Recent impacts of cold, multi-kilometer-sized bodies that generated meter-sized meteoroids; this occurred on the parent bodies of the IIIAB irons (650 My ago), the IVA irons (400 My ago), and the H ordinary chondrite (7 My ago).

Keil, K.↗
NASA NTRS1994

Formation of chondrules by electrical discharge heating

A possible mechanism for making chondrules in the solar nebula is electrical discharge ('lightning') heating, which can create high-temperature (greater than 1600 K), short-duration (approximately 10 s) thermal events as indicated by the chondritic record. Lightning occurs in many diverse terrestrial and planetary settings, and may have occurred in the solar nebula; it is thus worthwhile to investigate its implications for chondrule formation. Nebular discharges do not appear a viable source of heat for melting chondrule precursors, regardless of the uncertainty in the details of the mode. Nonetheless, we believe nebular lightning worthy of further investigation than what is presented here. Experiments analogous to those currently underway to investigate terrestrial thunderstorm lightning could be fruitful in refining nebular lightning models, and would be double interesting in application to nonthunderstorm and planetary lightnings.

Love, S. G.↗
NASA NTRS1994

Origin of dark clasts in the Acfer 059/El Djouf 001 CR2 chondrite

The ten specimens of the paired Acfer 059/El Djouf 001 CR2 chondrite contain abundant lithic fragments which we refer to as dark clasts. Petrological and mineralogical studies reveal that they are not related to the CR2 host meteorite but are similar to dark clasts in other CR2 chondrites. Dark clasts consist of chondrule and mineral fragments, phyllosilicate fragments and clusters, magnetite, sulfides and accessory phases, embedded into a very fine-grained, phyllosilicate-rich matrix. Magnetite has morphologies known from CI chondrites: spherules, framboids and platelets. Average abundances of major elements in the dark clasts are mostly in the range of both CR and CV chondrites, but strong depletions in Na and S are apparent. Oxygen isotopic compositions of two dark clasts suggest relationships to type 3 carbonaceous chondrites and dark inclusions in Allende. The dark clasts are clearly different in texture and mineralogical compositions from the host matrix of Acfer 059/El Djouf 001. Therefore, these dark clasts are xenoliths and are quite unlike the Acfer 059/El Djouf 001 CR2 host meteorite. We suggest that dark clasts accreted at the same time with all other components during the formation of Acfer 059/El Djouf 001 whole rock.

Endreb, M.↗
NASA NTRS1993

Roosevelt County 075: A petrologic chemical and isotopic study of the most unequilibrated known H chondrite

Roosevelt County (RC) 075 was recovered in 1990 as a single 258-gram stone. Classification of this meteorite is complicated by its highly unequilibrated nature and its severe terrestrial weathering, but we favor H classification. This is supported by O isotopes and estimates of the original Fe, Ni metal content. The O isotopic composition is similar to that of a number of reduced ordinary chondrites (e.g., Cerro los Calvos, Willaroy), although RC 075 exhibits no evidence of reduced mineral compositions. Chondrule diameters are consistent with classification as an L chondrite, but large uncertainties in chondrule diameters of RC 075 and poorly constrained means of H, L and LL chondrites prevent use of this parameter for reliable classification. Other parameters are compromised by severe weathering (e.g., siderophile element abundances) or unsuitable for discrimination between unequilibrated H, L and LL chondrites (e.g., Co in kamacite delta C-13). Petrologic subtype 3.2 +/- 0.1 is suggested by the degree of olivine heterogeneity, the compositions of chondrule olivines, the thermoluminescence sensitivity, the abundances and types of chondrules mapped on cathodoluminescence mosaics, and the amount of presolar SiC. The meteorite is very weakly shocked (S2), with some chondrules essentially unshocked and, thus, is classified as an H3.2(S2) chondrite. Weathering is evident by a LREE enrichment due to clay contamination, reduced levels of many siderophile elements, the almost total loss of Fe, Ni metal and troilite, and the reduced concentrations of noble gases. Some components of the meteorite (e.g., type IA chondrules, SiC) appear to preserve their nebular states, with little modification from thermal metamorphism. We conclude that RC 075 is the most equilibrated H chondrite yet recovered and may provide additional insights into the origin of primitive materials in the solar nebula.

Mccoy, T. J.↗
NASA NTRS1993

Ar-39 - Ar-40 ages of acapulcoites and lodranites: Evidence for early parent body heating

New age dating of acapulcoites (Acapulco, Monument Draw) and lodranites (Gibson) allow us to reconstruct the history of their parent body in a chronological framework. These meteorites originated on a common parent body -4.55 Ga ago. Non-collisional heating caused partial melting, with lodranites having been heated slightly higher than acapulcoites. This parent body cooled to the Ar closure temperature at -4.51 Ga for acapulcoites and -4.48 Ga for lodranites. The difference in ages is consistent with the higher temperature and longer cooling time for lodranites. Cooling probably occurred at a few to tens of deg C/Ma, with slower cooling rates at lower temperatures.

Bogard, D. D.↗
NASA NTRS1993

Metal-rich meteorites from the aubrite parent body

Three metal-rich meteorites - Mt. Egerton, Horse Creek, and LEW 88055 - were studied and it is suggested that they formed in the aubrite parent body. LEW 85369 and 88631 may also have a common origin, but these rocks have not yet been studied in detail. This body was probably heated to about 1600 C by a very strong heat source. While molten, metal agglomerated into sizeable nodules which never segregated efficiently to form a core, but were trapped in the silicate mantle. Different clasts and lithologies in aubrites solidified and cooled under local equilibrium conditions of oxygen fugacity, and with different thermal histories. Impacts mixed clasts from throughout the parent body, creating the typical aubrite breccias.

Casanova, I.↗
NASA NTRS1993

The granulite suite: Impact melts and metamorphic breccias of the early lunar crust

The granulite suite consists of two major types of rocks. One is coarse-grained and poikilitic with many euhedral crystals of olivine and plagioclase. These characteristics indicate crystallization from a melt; the poikilitic granulites are impact melt breccias. The other group is finer-grained and granoblastic, with numerous triple junctions; the granoblastic granulites are metamorphic rocks. Compositional groups identified by Lindstrom and Lindstrom contain both textural types. Two pyroxene thermometry indicates that both groups equilibrated at 1000 to 1150 C. Calculations suggest that the granoblastic group, which has an average grain size of about 80 microns, was annealed for less than 6 x 10 exp 4 y at 1000 C, and for less than 2500 y at 1150 C. Similar equilibration temperatures suggest that both groups were physically associated after impact events produced the poikilitic melts. Granulitic impactites hold important information about the pre-Nectarian bombardment history of the Moon, and the composition and thermal evolution of the early lunar crust. Granulitic impactites are widely considered to be an important rock type in the lunar crust, but how they formed is poorly understood. Metal compositions and elevated concentrations of meteoritic siderophile elements suggest that most lunar granulites are impact breccias. Their occurrence as clasts in approximately 3.9 Ga breccias, and Ar-(40-39) ages greater than or = 4.2 Ga for some granulites show that they represent a component of the lunar crust which formed prior to the Nectarian cataclysm. Petrographic characteristics of lunar granulites indicate at least two endmember textural variants which apparently formed in fundamentally different ways. One type has granoblastic textures consisting of equant, polygonal to rounded grains, and abundant triple junctions with small dispersions around 120 degrees indicating a close approach to textural equilibrium. As suggested by many authors, granoblastic granulites probably formed by subsolidus annealing and recrystallization of fragmental or glassy protoliths. Examples of this type include 15418, 78155, and 79215. The other textural type consists of poikilitic to poikiloblastic rocks with euhedral to subhedral plagioclase and olivine enclosed by interstitial pyroxene. In some cases, the texture resembles that of an orthocumulate. Examples of this type include 60035, 67955, and 77017. Rounding of grain edges is common in poikilitic granulites, but the regular crystal shapes and widely dispersed dihedral angles show they are far from textural equilibrium. The textures of poikilitic granulites are more consistent with the formation of these rocks by crystallization from a melt than by subsolidus metamorphism. A few samples have been recognized with textural characteristics transitional between those of the granoblastic and poikiloblastic endmembers (e.g., 72559, 78527). Pyroxene compositions taken from the literature and determined for this study by electron microprobe were used to calculate equilibration temperatures. The Kretz Ca transfer (solvus) thermometer and the Lindsley and Anderson graphical method both give similar temperatures, which range from approximately 1000 to 1150 C. There is no apparent temperature difference between granoblastic and poikilitic varieties, but there is a hint in these data that the more ferroan varieties equilibrated to lower temperatures. Additional studies are in progress to test this possibility.

Cushing, J. A.↗
NASA NTRS1993

I-Xe structure of ILAFEGH 009 and shallowater: Evidence for early formation and rapid cooling of impact-derived enstatite meteorites

Enstatite meteorites have proven to be ideal samples for past studies of the I-Xe system. This work focuses on two enstatite meteorites that were formed by impact processes. Ilafegh 009 is a clast-free impact melt rock from the EL chondrite parent body. The Shallowater aubrite likely formed when a fully molten planetesimal collided with a solid planetesimal, mixing fragments of the solid planetesimal into the enstatite mantle of the molten planetesimal. A complex three-stage cooling history resulted from this mixing and later break-up and reassembly of the parent body. The present study indicates that the I-Xe structure of these two meteorites resulted from in situ decay of live I-129 and that both experienced xenon closure of the iodine host phase at approximately the same time. I-Xe cooling rates are consistent with the cooling rates derived from mineralogic and petrologic studies of these objects. The similarities in ages suggest that the region of the nebula in which enstatite parent bodies formed must have experienced an intense early bombardment.

Kehm, K.↗
NASA NTRS1993

Classificational parameters for acapulcoites and lodranites: The cases of FRO 90011, EET 84302 and ALH A81187/84190

Acapulcoites and lodranites probably sample a common parent body, which has experienced a range of partial melting. We present classificational parameters which allow acapulcoites-lodranites to be distinguished from other groups of meteorites, as well as from each other. Petrography can complement oxygen isotopic compositions in separating these meteorites from other groups of stony-irons and primitive achondrites, while petrographic properties alone distinguish acapulcoites from lodranites. Acapulcoites differ from lodranites in having smaller grain sizes, abundant Fe, Ni-FeS as micron-sized veins and plagioclase which escaped melting. We have applied these criteria to three new members of the group. FRO 90011 is a typical lodranite; EET 84302 is intermediate in many properties between acapulcoites and lodranites; and ALH A81187/84190 are paired meteorites and are first low-FeO acapulcoites. These meteorites provide a wider spectrum of samples from the acapulcoite-lodranite parent body and suggests that this body may have had a complex structure.

Mccoy, T. J.↗
NASA NTRS1993

The dregs of crystallization in Zagami

The Zagami shergottite is a basaltic meteorite which formed when a phenocryst-bearing lava flow was emplaced at or near the surface of Mars. Recently, a cm-sized olivine-rich lithology has been identified in Zagami by Mossbauer spectroscopy. Olivine is extremely rare in shergottites, particularly in Zagami and Shergotty, where it occurs only as minute grains. We report petrologic and microprobe studies of this olivine-rich lithology. This material represents the last few percent of melt and is highly enriched in phosphates, opaques and mesostases, all of which are late-stage crystallization products. Phosphates replaced augite as a phenocryst phase when the magma became saturated in P. This late stage melt also includes a fayalite-bearing, multi-phase intergrowth which crystallized after the melt became too rich in iron to crystallize pigeonite. We can now reconstruct the entire crystallization history of the Zagami shergottite from a deep-seated magma chamber to crystallization of the final few percent of melt in a near-surface dike or thick flow. Small pockets (tens of microns) of late-stage melt pockets are ubiquitous but volumetrically minor in Zagami. We do not know the physical relationship between these areas and the cm-sized olivine-rich material described here. It is possible that these small pockets were mobile, forming larger areas. Perhaps inspection of the entire hand specimen of Zagami would clarify this relationship.

Mccoy, T. J.↗