GEOLOGY AND TERRESTRIAL AGE OF THE DERRICK PEAK METEORITE OCCURRENCE,ANTARCTICA

Sixteen iron meteorites together weighing 320 kg were recovered from the north-eastern flank of Derrick Peak, northern Britannia Range, Antarctica (156°30′E, 80°05′S), in December 1978. The well preserved meteorites rested cleanly upon an elevated, lag covered, glacially carved post-Middle Miocene to Pliocene bench cut into Devonian orthoquartzites intruded by Jurassic dolerite, and at a lower elevation upon Middle Pleistocene glacial drifts. In considering that the irons are in situ, and based on drift correlations along the Transantarctic Mountains, a maximum terrestrial age of 200,000–300,000 years B.P. is favoured.     

Further finds of the Derrick Peak meteorite,Transantarctic Mountains,and implications for terrestrial age

Abstract— Nine additional iron meteorite fragments weighing a total of 72 kg were recovered from the Derrick Peak area by a Canterbury Museum geological party in late 1988. One iron was located in the Onnum Valley, 6 km south of the previous finds. Geochemical analysis indicates that all irons belong to a single meteorite shower, greatly increasing the known extent of the fall zone. Kamp and Lowe (1982) have previously estimated the terrestrial age of the meteorite from glacial geological evidence. The location of the 1988 finds supports Kamp and Lowe's interpretation that the meteorites lie in situ, but recent revisions of the chronology of Cenozoic glacial history of the region reduce the interpreted terrestrial age. An age of between Oxygen Isotope stages 6 and 2 is probable (190–125 to 35–12 ka BP). This conflicts with a terrestrial age estimate of 1.0 ± 0.1 Ma BP from cosmogenic radionuclides.     

THE LOOP CHONDRITE: PETROLOGY,MINERAL CHEMISTRY,AND OPAQUE MINERALOGY

The Loop meteorite was found in 1962 in Gaines County, Texas, at a location very close to that where the Ashmore chondrite was found in 1969. The two specimens were assumed to be fragments of the same meteorite. The Loop meteorite is a type L6 chondrite composed of olivine (Fo_75.4Fa_24.6), orthopyroxene (En_77.6Wo_1.5Fs_20.9), clinopyroxene (En_47.5Wo_45.1Fs_7.4), plagioclase (Ab_84.3Or_5.5An_10.2), Fe-Ni metal, troilite, and chromite. Fe-Ni metal is represented by kamacite (5.8-6.4 wt % Ni, 0.88-1.00 wt % Co), taenite (30.0–52.9 wt % Ni, 0.16-0.34 wt % Co), and plessite (16.8–28.5 wt % Ni, 0.38-0.54 wt % Co). Native copper occurs as rare inclusions in Fe-Ni metal. Both chondrules and matrix have similar mineral compositions. The mineral chemistry of the Loop meteorite is quite different from that of the Ashmore, which was classified as an H5 chondrite by Bryan and Kullerud (1975). Therefore, the Ashmore and Loop meteorites are two different chondrites, even though they were recovered from the same geographic location.     

THE MAJOR ELEMENT CHEMISTRY OF LIBYAN DESERT GLASS AND THE MINERALOGY OF ITS PRECURSOR

Pieces of high-silica, natural glass (Libyan Desert Glass), found on the desert surface of western Egypt, have been treated as an enigma for 50 years although it is virtually certain they are similar to tektites in being impact-derived products. New major element analyses of four Libyan Desert Glass specimens agree extremely well with the only other recent analysis and demonstrate that the original bulk analyses reported by Spencer (1939) are in error. The five modern analyses define a very tight chemical range for SiO₂ (97.38–98.25 wt %), Al₂O₃ (1.16–2.26 wt %), total Fe (0.15–0.60 wt % as Fe₂O₃) and TiO₂ (0.13–0.19 wt %). Measurable MgO (0.04–0.20 wt %) was found in one specimen. No other elements are present in greater than trace amounts. Microprobe analyses show that Al, Fe and Ti are all positively correlated with one another and are almost ubiquitously distributed throughout the glass. They must also have been so distributed in the LDG precursor material as mechanical mixing and elemental diffusion in the short-lived melt were limited. In contrast, Mg is sharply restricted in occurrence and correlates only with Fe, strongly suggesting a precursor Mg-Fe oxide or silicate mineral present as rare, discrete grains. Aside from rare accessory minerals, the parent material was a sand or sandstone composed of quartz grains coated with a mixture of kaolinite, hematite and anatase. This conclusion is based solely on the elemental distribution in the glass but is buttressed by the occurrence of both sand and sandstone, in southwestern Egypt, with the requisite mineralogy. However, mineralogic identity need not, in general, translate to a chemical match and it is entirely possible that the specific sand or sandstone facies involved in the glass formation no longer exists after 28 million years. Consequently, it may well be that evidence other than chemical comparisons will be needed to identify the presently unknown parent crater.     

JERSLEV,THE FIRST IRON METEORITE FROM DENMARK

Jerslev is a new iron meteorite of 40 kg, found 1976 on the island of Sjaelland, Denmark. The coordinates are 55°36'N, 11°13'E, and the altitude 20 m. It was excavated from moraine deposits from a depth of about 0.5 m. Jerslev is a coarsest octahedrite of group IIB, related to Mount Joy and Sikhote-Alin. It shows intergranular corrosion from a long exposure to terrestrial groundwater rich in chlorides.     

Petrology and Pairing of Mesosiderites from Victoria Land,Antarctica

Abstract— The Allan Hills mesosiderites A77219, A81059 and A81098 are classified as subgroup 1B (Hewins, 1984), on the basis of very fine-grained silicate matrix, low plagioclase content and absence of highly ferroan pyroxenes. Since they are so similar petrologically, it is reasonable to pair them. ALHA 81208, a highly weathered orthopyroxenite, is probably a clast from one of the Allan Hills mesosiderites. Reckling Peak A80258, is a Floran 2B or Hewins 4B mesosiderite. It contains reversely zoned orthopyroxene clasts in a sparse matrix with interstitial/poikilitic plagioclase and highly magnesian (chadacryst) orthopyroxene (close to En₈₀). All pyroxene is much lower in FeO/MnO than Allan Hills mesosiderite pyroxene of similar FeO content. The other Reckling Peak mesosiderites A79015, A80229, A80246 and A80263 contain orthopyroxene and recrystallized orthopyroxenite clasts in a metal-troilite matrix. Orthopyroxenite clasts in ALH A81059 are very similar texturally and modally to RKPA 79015 orthopyroxenite, but differ in pyroxene composition. Orthopyroxene in RKPA 79015 (Prinz et al., 1982) is very similar in Fe/Mg and Fe/Mn to the cores of (reversely zoned) pyroxene clasts in RKPA 80258. On this basis, RKPA 80258 is related to the other Reckling Peak mesosiderites and they could all be paired, assuming that three components (metal, pyroxenite and silicate matrix) were very irregularly mixed in these breccias. Pairing is problematical, in that specimens of a polymict breccia can be so different that they would not be paired if they were not known to have fallen together. The silicate fraction of mesosiderites ranges from diogenitic (RKPA 79015) to analogous to polymict eucrite (Dyarrl Island) although the silicate fractions are not equivalent in detail. The mesosiderite subdivision scheme is amended recognizing this, permitting the classification of the formerly “anomalous” RKPA 79015.     

Composition of the Crystalline Basement and Shock Metamorphism of Crystalline and Sedimentary Target Rocks at the Haughton Impact Crater,Devon Island,Canada

Abstract— About 100 cobble-sized samples collected from the surface of the central polymict breccia formation of Haughton impact crater, Canada, have been studied microscopically and chemically. Breccia clasts derived from the 1700 m deep Precambian basement consist of 13 rock types which can be grouped into sillimanite- and garnet-bearing gneiss; alkali feldspar-rich aplitic or biotite-hornblende-bearing gneiss; biotite and hornblende gneiss; apatite-rich biotite and biotite-hornblende gneiss; calcitediopside gneiss; amphibolite; tonalitic orthogneiss; and basalts. The range of chemical compositions of these rocks is wide: e.g., SiO₂ ranges from 40–85 wt.%; Al₂O₃ from 7–20 wt.%; CaO from 0.01–25 wt.%; or P₂O_s from <0.01–5 wt.%. Nearly all samples of crystalline rocks are shock metamorphosed up to about 60 GPa. Most conspicuous is the absence of whole-rock melts and the very rare occurrence of unshocked rocks. The 45 samples examined can be classified into the following shock stages: stage 0 (<5 GPa): 4.5%, stage Ia (10–20 GPa): 9.0%, stage Ib (20–35 GPa): 33%, stage II (35–45 GPa): 29%, stage III (45–55 GPa): 18%, stage III–IV (55–60 GPa): 6.5%. Among Paleozoic sedimentary rock clasts higher degrees of shock than within crystalline rocks were observed such as highly vesiculated, whole-rock melts of sandstones and shales. Within the northern and eastern sectors of the allochthonous breccia no distinct radial variation of the cobble-sized lithic clasts regarding abundance, rock type, and degree of shock was observed, with the exception that clasts of shock-melted sedimentary rocks and of highly shocked basement rocks (stage III–IV) are strongly concentrated near the center of the crater. Based on our field and laboratory investigations we conclude that vaporization and melting due to the Haughton impact affected the lower section of the sedimentary strata from about 900 to 1700 m depth (Eleanor River limestones and dolomites, Lower Ordovician and Cambrian limestones, dolomites, shales, and sandstones). The 60-GPa shock pressure isobar reached only the uppermost basement rocks so that whole rock melting of the crystalline rocks was not possible.     

The properties of large particles in the zodiacal cloud and in the interstellar medium,and their relation to recent IRAS observations

To explain the scattering of sunlight observed from theF-corona and from the zodiac, the scattering particles must have radii of order 15 m, and must have an imaginary component of the refractive index that requires the presence of from 5 to 10% of free carbon. The particles, therefore, have a composition very like the material of C 1 carbonaceous chondrites and like extraterrestrial particles which have been recovered from the high atmosphere.Such particles absorb sunlight, the absorbed solar energy being reradiated in the infrared with a close approximation to black-body emission, even as far into the infrared as 100 m, a deduction in good agreement with recently published observations from the IRAS satellite.The IRAS observations at high ecliptic latitudes require similar particles to be present in large quantity in the interstellar medium, 10⁶ solar masses or more of them. The presence of such a quantity of material with properties very like the material of the C 1 carbonaceous chondrites is a remarkable outcome of the IRAS observations and is likely to have profound implications in many directions.     

Electron microscopy study of the iron meteorite Santa Catharina

Abstract— Characterization of the microstructural features of the metal of the Santa Catharina meteorite was performed using a variety of electron optical techniques. Sample USNM#6293 is chemically homogeneous on the micron scale and has a Ni content of 28.2 wt.%. Its microstructure is similar to that of the Twin City ataxite and contains clear taenite II, i.e., fcc taenite with domains of tetrataenite, < 10 nm in size. Sample USNM#3043 is a more typical Santa Catharina specimen with dark and light regions as observed with the light optical microscope. The dark regions are inhomogeneous and contain 45–50 wt.% Ni and 7–12 wt.% O. The light regions are homogeneous and contain 35 wt.% Ni and no detectable oxygen. The microstructure is that of cloudy zone, i.e., islands of tetrataenite, ～20 nm in size, in a honeycomb matrix. The honeycomb phase contains Ni rich oxide in the dark regions and contains metal, fcc taenite, in the light regions. The original metal structure of USNM#3043 is cloudy zone which formed during cooling into the low temperature miscibility gap of the Fe-Ni phase diagram. The dark regions were developed from the metal by selective corrosion of the honeycomb structure, transforming it into Ni containing oxides, possibly non-stoichiometric Fe₂NiO₄ while retaining the tetrataenite islands. Using the results of this study, many of the existing discrepancies concerning the microstructure of Santa Catharina can be explained.     

Modes of sediment transport in channelized water flows with ramifications to the erosion of the Martian outflow channels

Depending on their grain sizes (settling velocities), sediments are transported in rivers as bed load, in suspension, or as wash load. The coarsest material rolls or bounces along the bottom as bed load whereas finer material is placed into suspension by the water turbulence. The finest sediments are transported as wash load, evenly distributed through the water depth and effectively moving at the same rate as the water. The criteria for quantitatively determining which grain-size ranges are being transported in terrestrial rivers as bed load, suspended load and wash load are applied to an analysis of sediment transport in the large Martian outflow channels, assuming their origin to have been from water flow. Of importance is the balance of the effects of the reduced Martian gravity on the water flow velocity versus the reduction in grain settling velocities. Analyses were performed using grain densities ranging from 2.90 g/cm³ (basalt) to 1.20 g/cm³ (volcanic ash). The results show that the Martian flows could have transported cobbles in suspension and that nearly all sand-size material and finer would have been transported as wash load. Wash-load transport requires little or no net expenditure of the water-flow power, so the sands and finer could have been carried in nearly unlimited quantities. A comparison with terrestrial rivers indicates that concentrations as high as 60–70% by weight of wash-load sediment could have prevailed in the Martian flows, resulting in the very rapid erosion of the channels.     

COMPLEX SHOCK-INDUCED Fe-Ni-S-Cr-C MELTS IN THE HAIG (IIIA) IRON METEORITE

The Haig (IIIA) iron meteorite material (BM 1968,280) in the Collection of the British Museum (Natural History) displays pre-terrestrial shear deformation which transects a small troilite-daubréelite-cohenite nodule. Five globular areas (< 1 mm) with dendritic structures indicating rapid solidification from melts occur within and around the larger part of the transected nodule. All shock deformation structures and shock-heating effects exhibited by Haig. are of pre-terrestrial origin and at least four successive alterations to the original structure are evident. Partial bulk analyses of the melt globules indicate complex mixtures of approximately 55 wt % troilite, 24 wt % metal, 14 wt % daubréelite and 7 wt % cohenite. It is suggested that a compressive environment maintained the melts as discrete pools of liquid. The diameters of the globules (< 0.5 mm) and spacing of the dendrite arms (< 1 μm) indicate solidification at > 10⁶ °C sec^?1 which is the fastest cooling rate yet recorded in meteorites.     

Uranus: Disk structure within the 7300-Å methane band

Orthogonal narrow-band (100 Å) photoelectric slit scan photometry of Uranus has been used to infer the basic two-dimensional structure of the disk within the 7300-Å methane band. Numerical image reconstruction and restoration techniques have been applied to quantitatively estimate the degrees of polar and limb brightening on the planet. Through partial removal of atmospheric smearing, an effective spatial resolution of approximately 0.9 arcsec has been achieved. Peak polar, limb, and central intensities on the disk are in the respective proportions 3:2:1. In addition, the bright polar feature is displaced from the geometric pole towards the equator of the planet.     

The Old Woman,California, IIAB iron meteorite

Abstract– The Old Woman meteorite, discovered in March 1976 by two prospectors searching for a fabled lost Spanish gold mine in mountains ～270 km east of Los Angeles, has achieved the status of a legend among meteorite hunters and collectors. The question of the ownership of the 2753 kg group IIAB meteorite, the second largest ever found in the United States (34°28′N, 115°14′W), gave rise to disputes involving the finders, the Bureau of Land Management, the Secretary of the Department of the Interior, the State of California, the California members of the U.S. Congress, various museums in California, the Smithsonian Institution, and the Department of Justice. Ultimately, ownership of the meteorite was transferred to the Smithsonian under the powers of the 1906 Antiquities Act, a ruling upheld in a U.S. District Court and a U.S. Court of Appeals. After additional debate, the Smithsonian removed a large cut for study and curation, and for disbursement of specimens to qualified researchers. The main mass was then returned to California on long‐term loan to the Bureau of Land Management’s Desert Discovery Center in Barstow. The Old Woman meteorite litigation served as an important test case for the ownership and control of meteorites found on federal lands. The Old Woman meteorite appears to be structurally unique in containing both hexahedral and coarsest octahedral structures in the same mass, unique oriented schreibersites within hexahedral areas, and polycrystalline parent austenite crystals. These structures suggest that different portions of the meteorite may have transformed via different mechanisms upon subsolidus cooling, making the large slices of Old Woman promising targets for future research.     

Procedures,Resources and Selected Results of the Deep Ecliptic Survey

The Deep Ecliptic Survey is a project whose goal is to survey a largearea of the near-ecliptic region to a faint limiting magnitude (R ～24) in search of objects in the outer solar system. We are collectinga large homogeneous data sample from the Kitt Peak Mayall 4-m and CerroTololo Blanco 4-m telescopes with the Mosaic prime-focus CCD cameras.Our goal is to collect a sample of 500 objects with good orbits to furtherour understanding of the dynamical structure of the outer solar system.This survey has been in progress since 1998 and is responsible for 272designated discoveries as of March 2003. We summarize our techniques,highlight recent results, and describe publically available resources.     

Impulsive brightenings and velocity transients in prominences

Observations of quiescent prominences with the Zeiss Universal Birefringent Filter at Sacramento Peak show short-lived brightenings and velocity transients in H and D₃. The larger events range in area from 25 to 170 square arc sec, have lifetimes of approximately 30 min, velocities of 30 km s^–1, and total energy excesses up to 7 × 10²⁷ ergs. These events do not disrupt the stable structure of the prominence, and are interpreted as either condensation events or low energy flares.Visiting astronomer, Sacramento Peak Observatory, operated by the Association of Universities for Research in Astronomy Inc. under contrast AST-78-17292 with the National Science Foundation.Visiting student, Sacramento Peak Observatory, operated by the Association of Universities for Research in Astronomy Inc. under contract AST-78-17292 with the National Science Foundation.     

RECOVERY AND CLASSIFICATION OF THIRTY NEW METEORITES FROM ROOSEVELT COUNTY,NEW MEXICO

We report the discovery and classification of 30 new meteorites found in or close to Roosevelt County, New Mexico, including two H3 chondrites and a ureilite; the others are equilibrated ordinary chondrites. Over 160 meteorites representing at least 100 different falls have been recovered from this region, mostly from wind blowout areas. As in Antarctica, small specimens predominate and irons, achondrites and C and E chondrites are rare. Paired specimens are also very difficult to identify.     

The diatomic molecules BH,BN, and BO in sunspots and the solar abundance of boron

Absorption band spectra of BH and BO have been searched for and not found in spectra of sunspots.Electronic oscillator strengths are available only for the A ¹ -X ^{1 +} system of the BH molecule. The absence of the (0,0) band of BH at 4332 Å reflects a solar abundance of boron logA _B<2.5.The band spectra of BN are several orders of magnitude weaker in sunspots than those of BH and BO.Kitt Peak National Observatory Contribution No. 488.Visiting Astronomer, Solar Division, Kitt Peak National Observatory - Operated by The Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Constraints on Mercury’s surface composition from MESSENGER and ground-based spectroscopy

The composition and chemistry of Mercury’s regolith has been calculated from MESSENGER MASCS 0.3-1.3 μm spectra from the first flyby, using an implementation of Hapke’s radiative transfer-based photometric model for light scattering in semi-transparent porous media, and a linear spectral mixing algorithm. We combine this investigation with linear spectral fitting results from mid-infrared spectra and compare derived oxide abundances with mercurian formation models and lunar samples. Hapke modeling results indicate a regolith that is optically dominated by finely comminuted particles with average area weighted grain size near 20 μm. Mercury shows lunar-style space weathering, with maturation-produced microphase iron present at ∼0.065 wt.% abundance, with only small variations between mature and immature sites, the amount of which is unable to explain Mercury’s low brightness relative to the Moon. The average modal mineralogies for the flyby 1 spectra derived from Hapke modeling are 35-70% Na-rich plagioclase or orthoclase, up to 30% Mg-rich clinopyroxene, <5% Mg-rich orthopyroxene, minute olivine, ∼20-45% low-Fe, low-Ti agglutinitic glass, and <10% of one or more lunar-like opaque minerals. Mercurian average oxide abundances derived from Hapke models and mid-infrared linear fitting include 40-50 wt.% SiO₂, 10-35 wt.% Al₂O₃, 1-8 wt.% FeO, and <25 wt.% TiO₂; the inferred rock type is basalt. Lunar-like opaques or glasses with high Fe and/or Ti abundances cannot on their own, or in combination, explain Mercury’s low brightness. The linear mixing results indicate the presence of clinopyroxenes that contain up to 21 wt.% MnO and the presence of a Mn-rich hedenbergite. Mn in M1 crystalline lattice sites of hedenbergite suppresses the strong 1 and 2 μm crystal field absorption bands and may thus act as a strong darkening agent on Mercury. Also, one or more of thermally darkened silicates, Fe-poor opaques and matured glasses, or Mercury-unique Ostwald-ripened microphase iron nickel may lower the albedo. A major part of the total microphase iron present in Mercury’s regolith is likely derived from FeO that is not intrinsic to the crust but has been subsequently delivered by exogenic sources.     

Increased iron abundances in slope avalanches of certain lunar craters

On the basis of the first images of certain areas of Lunar surface obtained by the Chang??e-2 spacecraft and materials of large-scale image acquisition from the LRO (Lunar Reconnaissance Orbiter) spacecraft, supplemented with remote spectral measurements performed from the Clementine spacecraft, the slope movements of material have been studied in lunar craters Daniell, Burg and Mauri A. It is established that despite a significantly different age of formation of these craters, the slope formations are of similar structure and differ by increased iron abundance in the soil surface layer. All objects are characterized with by increase in FeO abundance to 20 wt % at depths of several hundred meters from the surface. The material of the slope structures is distinguished by a low maturity rate. According to preliminary assessments using the optical maturity index and spectropolarimetric maturity index, the fresher slope formations can have an exposure age from several tens of years to several years.     

Carbon,nitrogen and hydrogen in Saharan chondrites: The importance of weathering

Abstract— The Sahara Desert is a region of high diurnal temperature variation and sporadic rainfall that has recently yielded over 450 meteorites. Eighteen of these Saharan samples are carbonaceous chondrites, of which we have analysed 17 for C content and isotopic composition. Ten of the 18 are paired CR chondrites, of which four have also had N and H contents and compositions determined. A primitive ordinary chondrite (L/LL3.2) found in the region has also been analysed for C, N and H contents and isotopic composition. Saharan samples contain between 21% and 45% of the light elements of their non-Saharan counterparts. Paired Saharan samples show a greater heterogeneity in both C content and isotopic composition than multiple analyses of non-Saharan samples. The cause of the observed isotopic and abundance effects is due to the hot desert weathering processes experienced by these samples. Peak temperatures of meteorites on the desert floor may be in excess of 100 °C, leading to low-temperature hydrous pyrolysis and oxidation reactions, liberating volatile organics and CO₂. This may also cause the remaining material to become partially solubilised and ultimately lost during rainfall. The low δD of the CR and ordinary chondrites can be attributed to the destruction and loss of organic material through dehydrogenation and exchange reactions on the desert surface. The increased ¹³C abundance suggests that the less tightly bound C from the macromolecular organic material is isotopically lighter than the remaining C. Carbon contents and isotopic compositions are also affected by the addition of terrestrial calcitic evaporite deposits, up to 10,000 ppm carbonate has been measured, with a δ¹³C of between 0 and ?10%₀.