CHONDRITES. S. B. Simon, S. R. Sutton, A. J. Brearley, A. N. Krot, and K. Nagashima.Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131(sbs8@unm.edu). Dept. Geophysical Sci., The University of Chicago, Chicago, IL 60637. Center for Advanced Radiation Sources (CARS), The Univ. of Chicago. Dept. Earth and Planetary Sci., Univ. of New Mexico. HIGP/SOEST, Univ. of Hawai’i, Honolulu, HI. Geosci. Inst./Mineralogy, Goethe University Frankfurt, Germany.
Northwest Africa (NWA) 12379 is a new metal-rich chondrite with unique characteristics distinguishing it from all previously described meteorites. It contains high Fe,Ni-metal content (similar to 70 vol.%) and completely lacks interchondrule matrix; these characteristics are typical only for metal-rich carbonaceous (CH and CB) and G chondrites. However, chondrule sizes (60 to 1200 mu m; mean = 370 mu m), their predominantly porphyritic textures, nearly equilibrated chemical compositions of chondrule olivines (Fa(18.1-28.3), average Fa(24.9) (+/- 3.2), PMD = 12.8; Cr2O3 = 0.03 +/- 0.02 wt.%; FeO/MnO = 53.2 +/- 6.5 (wt.-ratio); n = 28), less equilibrated compositions of low-Ca pyroxenes (Fs(3.2-18.7)Wo(0.2-4.5); average Fs(14.7 +/- 3.7)Wo(1.4) (+/- 1.3); n = 20), oxygen-isotope compositions of chondrule olivine phenocrysts (Delta O-17 similar to 0.2-1.4 parts per thousand, average similar to 0.8 parts per thousand), and the presence of coarse-grained Ti-bearing chromite, Cl-apatite, and merrillite, all indicate affinity of NWA 12379 to unequilibrated (type 3.8) ordinary chondrites (OCs). Like most OCs, NWA 12379 experienced fluid-assisted thermal metamorphism that resulted in formation of secondary ferroan olivine (Fa(27)) that replaces low-Ca pyroxene grains in chondrules and in inclusions in Fe,Ni-metal grains Delta O-17 of the ferroan olivine (similar to 4 parts per thousand) is similar to those of aqueously-formed fayalite in type 3 OCs, but its delta O-18 is significantly higher (15-19 parts per thousand, average = 17 parts per thousand vs. 3-12 parts per thousand, average = 8 parts per thousand, respectively). We suggest classifying NWA 12379 as the ungrouped metal-rich chondrite with affinities of its non-metal fraction to unequilibrated OCs and speculate that it may have formed by a collision between an OC-like body and a metal-rich body and subsequently experienced fluid-assisted thermal metamorphism. Trace siderophile element abundances and isotopic compositions (e.g., Mo, Ni, Fe) of the NWA 12379 metal could help to constrain its origin.
CONDENSATION FROM O–POOR GAS IN THE OUTER SOLAR SYSTEM R. C. Ogliore, J. B. Lewis, K. L. Utt, K. Nagashima, A. N. Krot, D. J. Joswiak, D. E. Brownlee. Department of Physics, Washington University in St. Louis, St. Louis, MO 63130, USA, Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA, Department of Astronomy, University of Washington, Seattle, WA 98195, USA.
Introduction: NASA’s Genesis mission collected and returned with samples of solar wind to Earth. Isotopic measurements revealed that solar wind is O-rich compared to sampled planets (i.e., Earth, Mars) and asteroids. The Sun’s DO (=dO–0.52 ́dO) value was inferred to be –28.4±3.6‰ (2s) [1]. While many nebular materials preserved in chondritic meteorites are more O-rich than Earth (e.g., [2-6]), compositions as O-rich as the Sun are exceedingly rare in the meteorite record. For example, most Ca-, Al-rich inclusions (CAIs), the oldest dated materials that formed in the Solar System [7], appear to have formed with a DO of ~ –24‰ to –23‰ [2–5], suggesting that a resolved difference exists between the Sun and many of these early formed objects. Here, we report preliminary O, Ca, Ti, and Al-Mg systematics for a rare CAI (1-2-4) from Murchison with solar O isotopic composition. We also explore the reasons for the scarcity of such objects in the meteorite record. Methods: O, Al-Mg, and Ca-Ti isotopes were analyzed with the UH Mānoa ims-1280 with conditions similar to those reported in [8], [9], and [5,6,10], respectively. Results: Petrologic characteristics: Mineralogically, CAI 1-2-4 resembles spinel-hibonite inclusions known as SHIBs in CM chondrites [11]. This angular, friable aggregate is ~100 μm across and consists of spinel and hibonite crystals (Fig. 1). Picked from an acid residue of Murchison, voids could be primary or the result of dissolution of silicate minerals. The hibonite grains in this CAI are zoned in MgO and TiO2, as seen in many SHIBs [5], but usually not in PLAC-type hibonites [6].
The Maja Valles outflow is traced for 1400 km from its source in Juventae Chasma to its termimation on central Chryse Planitia. The outflow impounded as a large take on the northern Lunae Planum surface. Spillover from this lake across the rugged Xanthe Terra carved an interconnected series of anastomosing channels that drained onto the lower Chryse Planitia surface. The flood surge from Lunae Planum is traced through the channel system, and hydrographs are constructed for the discharge at the mouths of the channels on Chryse Planitia. The initial surge crossed Xanthe Terra in about 4 hours, but flow through multiple channel segments and ponding in breached craters delayed flood crests in some valley segments as much as 15 hours and introduced multiple crests in the discharge at the mouths of trunk valleys. Anastomosing flow was primarily responsible for reducing the initial flood crest and extending the discharge at the valley mouth. Lag times introduced by ponding in craters was responsible for the greatest retardations and separation of flow into multiple flood crests at the mouths of the canyons. The outflow from the paleolake on Lunae Planum required from two to eight months to drain the lake. Thus, the duration of die flow and its resulting erosional and depositional history along an outflow course is much more complex than the history of release at the source.
Magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy is an extremely sensitive and reproducible probe of disorder in synthetic quartz powder samples that have been experimentally shock loaded to peak pressures up to 22 GPa. Careful curve fitting and deconvolution of Si-29 NMR spectra of shocked quartz leads to four primary observations: (1) the width of the resonance increases systematically with shock pressure, indicating a greater distribution of local silicon environments caused by shock-induced disorder; (2) a much broader peak component appears, indicating the presence of highly disordered material, which TEM observation indicates may be due to very high dislocation densities, (3) the disordered material appears to have a significantly shorter T1 relaxation time than the crystalline material; and (4) there is no evidence for the presence of high-pressure silica phases (coesite or stishovite) in the shocked samples. The net effect of shock loading on the Si-29 NMR spectrum of quartz is an increase in peak width that strongly and consistently correlates with known shock pressure. Peak width is therefore a useful ''shock barometer'' for quartz in this pressure range for the experimental loading conditions. By contrast, Na-23, Al-27, and Si-29 MAS NMR spectra of shocked and unshocked An60 plagioclase feldspar powders do not change significantly with increasing shock pressure. There is no evidence for either amorphous or high-pressure phases in the Plagioclase spectra for shock pressures up to 22 GPa.
A group of KREEPy basalts has been discovered in Apollo 14 soils. These samples exhibit similarities to both HA and VHK basalts, albeit with much higher REE abundances, and contain up to 2 vol pct whitlockite and can be explained by assimilation of a K-, REE- and P-rich fluids by an original HA or VHK basalt. This KREEP component could have been produced late in the evolution of the lunar magma ocean and is similar in composition to QMD at Apollo 14. Two rocks have trace element compositions that are representative of actual KREEP. One of the samples appears to be petrographically pristine and could represent an actual KREEP basalt rock. Five subophitic high-Al basalts represent sampling of either a slowly cooled impact melt sheet or, more likely, the same basalt flow. Two 'quasi-pristine' highland rocks confirm the postulate of a connection between KREEP and the alkali suite. A newly discovered alkali anorthosite is a plagioclase cumulate with about 15 percent trapped KREEPy liquid.
Micrometer-sized glass spheres from a mature highland lunar soil (61181) were analyzed for major elements with a transmission electron microscope. The majority of the glasses analyzed (50%) have refractory, high-alumina, silica-poor (HASP) compositions. Most of the HASP glasses are derived from the bulk soil by impact vaporization of silica, Fe, and volatile elements, Two HASP trends (a highland- and a mare-type) occur in the soil. A group of volatile-rich alumina-poor (VRAP) glasses were identified with compositions that are complementary to the HASP glasses. These VRAP glasses result from the condensation of impact-produced vapors.
Ilmenite and pyroxene grain-size separates from 79035 were analyzed for trapped N; the ilmenite was also analyzed for trapped Xe. Ilmenite N contains two or more isotopically distinct trapped components differing in release temperature and therefore plausibly in implantation energy. The isotopically light, higher-temperature (higher-energy?) component has a deltaN-15 value equal to -180 parts per thousand, significantly above the minimum value observed in bulk 79035, suggesting that parts of 79035 were exposed on the lunar surface earlier than the ilmenite. Using trapped Ar-40/Ar-36 and cosmogenic Ne-21 (Benkert, 1989) we have derived a compaction age of about 1 Ga for the ilmenite. This implies a considerably more recent exposure than previously thought, and suggests that the long-term Change in deltaN-15 of regolith N was more rapid than generally believed. Comparison of these results with those for black/orange glass from 74001/74002 [compaction age 3.7 Ga, deltaN-15 in die range -36 to +18 parts per thousand (Kerridge et al., 1991)] indicates that the long-term trend may have followed a complex evolutionary path. Data for 79035 pyroxenes are consistent with the ilmenite compaction age but suggest a more complex exposure history.
The effects of grain size and shape in modeling reflectance spectra of mineral mixtures have been investigated using a simple model called the "isograin model." This model treats reflectance from a particulate surface as a series of grain interactions that are coupled to the optical constants of mineral constituents. The validity of the model is demonstrated using mixtures of glass powders whose absorption coefficients and refractive indexes could be measured directly. An effective grain size is included to account for differences in grain shape and a wavelength-dependent scattering factor is included in the mixing formula to account for grains smaller than die wavelength of light. By choosing appropriate parameters for each mixture, the isograin model calculates spectra in good agreement with measured spectra, and models reflectances within 1% and mixing ratios within 5 wt%.
Amazonis and Utopia Plantiae are two large basins on Mars that have morphologic features commonly associated with former standing bodies of water. The investigation of these areas is an extension of our previous paleolake studies in the Elysium Basin. Like Elysium, the basins exhibit terraces and lineations resembling shorelines, etched and infilled floors marked by sinuous channels in places, inflow channels along their borders, and other geomorphic indicators believed to be related to the presence of water and ice. Moreover, most of the shoreline features have consistent elevations of about -1000 m, which suggests that the bodies of water thought to have occupied the basins may once have been connected. Although our concept of large paleolakes in the northern lowlands of Mars might be expanded to include inland seas, it is still premature to advance this hypothesis at the present stage of investigation. Even though these postulated paleolakes are very young in the martian stratigraphic sequence, they are probably much older than large Pleistocene lakes on Earth, and their shoreline features are less well preserved.
Two different types of spherules, probably impact derived, occur at the Cretaceous-Tertiary (K/T) boundary. One type of spherule occurs in a strewn field in North America and around the Gulf of Mexico. These spherules are probably tektites or altered tektites. The other type of spherule is found almost worldwide and is a crystal-bearing microkrystite, usually altered. Recently, unaltered glassy cores in the K/T ''tektites'' have been found in the Haiti outcrop. Identical but altered spherules occur at the Arroyo de Mimbral (Mexico), Brazos River (Texas), Dogie Creek, Teapot Dome (Wyoming), and DSDP sites 390a, 603 (Northwestern Atlantic), and (possibly) 540 (Gulf of Mexico). No glass has been found yet in K/T microkrystites. However, magnesioferrite spinels have survived at many K/T sites, and we show that only in DSDP site 577 (Shatsky Rise, Pacific) clinopyroxene (En34-35-Wo49-Fs16) quench crystals survived within spherules On the basis of their morphology (splash forms, flowlines), it is concluded that the K/T ''tektites'' were once molten ejecta, solidified in flight. The microkrystites, on the other hand, are smaller, do not show splash forms, and may represent recondensed material from the ejected vapor cloud from the same large impact. The chemical properties (high calcium, sulfur) of the K/T tektite glass can be explained by a carbonate-evaporite composition of the target rock as it existed 65 Ma ago on the location of the Chicxulub structure on the Northern Yucatan peninsula. The chemistry of the microkrystites may be explained as condensate mixing a part of the bolide with a portion of die target rocks (from Chicxulub). The texture and composition of the microkrystites may depend on the place of condensation in the ejected vapor cloud and oxygen fugacity. Reheating on reentry in the atmosphere may also determine the final texture. Subsequently, the vast majority of the microkrystites and K/T ''tektites'' have been altered pseudomorphically Most of the morphological details have been preserved, but the chemical composition has been changed completely in this process as a function of the local diagenetic environment.
