North American tektite debris and impact ejecta from DSDP Site 612

Abstract— A layer of tektite glass and shock-metamorphosed grains found in an upper Eocene section of core 21 from DSDP Site 612 taken on the continental slope off New Jersey may belong to the North American tektite strewn field. However, the Site 612 glasses generally have higher K₂O and lower Na₂O contents for a given SiO₂ content and different Sr and Nd isotopic compositions. In order to better define the layer, a series of samples was taken continuously through the layer at 1 cm intervals. Tektite fragments are in an 8 cm thick layer; microtektites are concentrated in the upper 4 cm, while spherules with “crystalline” textures (microkrystites) are concentrated in the lower half of the layer. Millimeter-size splash forms are mostly in the lower part of the tektite-bearing layer. Rock and mineral grains showing evidence of shock metamorphism are abundant in the upper half of the tektite-bearing layer. Coesite is abundant, and stishovite was found in one rock fragment. The size and abundance of the tektite glass and the abundance of shocked debris indicate that Site 612 is relatively close to the source crater, which may be to the north of Site 612 on the coastal plain or adjacent continental shelf.     

Upper Eocene tektite and impact ejecta layer on the continental slope off New Jersey

Abstract— During Leg 150 of the Ocean Drilling Project (ODP), two sites (903C and 904A) were cored that have sediments of the same biostratigraphic age as the upper Eocene tektite-bearing ejecta layer at Deep Sea Drilling Project (DSDP) Site 612. Core 45X from ODP Site 904A (～4 km north of Site 612) contains a 5 cm thick tektite-bearing ejecta layer, and Core 56 from Site 903C (～8 km north-northwest of Site 904) contains a 2 cm thick layer of impact ejecta without any tektite or impact glass. Shocked quartz and feldspar grains, with multiple sets of planar deformation features (PDFs), and abundant coesite-bearing grains are present at both sites. The major oxide contents, trace element compositions, and rare earth element (REE) patterns of the Site 904 tektites are similar to those of the Site 612 tektites and to North American tektites (especially bediasites). The ?_Sr and ?_Nd values for one composite tektite sample from Site 904 fall within the range previously obtained for the Site 612 tektites, which defines a linear trend that, if extrapolated, would intersect the values obtained for North American tektites. The water contents of eight tektite fragments from Site 904 range from 0.017 to 0.098 wt%, and, thus, are somewhat higher than is typical for tektites. The heavy mineral assemblages of the 63–125 μm size fractions from the ejecta layers at Sites 612, 903, and 904 are all similar. Therefore, we conclude that the ejecta layer at all three sites is from the same impact event and that the tektites at Sites 904 and 612 belong to the North American tektite strewn field. Clinopyroxene-bearing (cpx) spherules occur below, or in the lower part of, the main ejecta layer at all three sites. At all three sites, the cpx spherules have been partly or completely replaced with pyrite that preserved the original crystalline textures. Site 612, 903, and 904 cpx spherules are similar to those found in the Caribbean Sea, Gulf of Mexico, central equatorial Pacific, western equatorial Pacific, and eastern Indian Ocean. The cpx event appears to have preceded the North American tektite event by 10–15 ka or less. The fining-upward sequence at all three sites and concentration of the denser, unmelted impact ejecta at the top of the tektite layer at Sites 612 and 904 suggest that the tektite-bearing ejecta layers are not the result of downslope redeposition and that the unmelted ejecta landed after the glass. Geographic variations in thickness of the tektite-bearing ejecta layer, the lack of carbonate clasts in the ejecta layer, and the low CaO content of the tektite glass suggest that the ejecta (including the tektite glass) were derived from the Chesapeake Bay structure rather than from the Toms Canyon structure. A sharp decline in microfossil abundances suggests that local environmental changes caused by the impact may have had adverse effects on benthic foraminifera, radiolaria, sponges, and fish as well as the planktic foraminifera.     

Establishing the link between the Chesapeake Bay impact structure and the North American tektite strewn field: The Sr‐Nd isotopic evidence

Abstract— The Chesapeake Bay impact structure, which is about 35 Ma old, has previously been proposed as the possible source crater of the North American tektites (NAT). Here we report major and trace element data as well as the first Sr‐Nd isotope data for drill core and outcrop samples of target lithologies, crater fill breccias, and post‐impact sediments of the Chesapeake Bay impact structure. The unconsolidated sediments, Cretaceous to middle Eocene in age, have ?_Sr^{t = 35.7 Ma} of +54 to +272, and ?_Nd^{t = 35.7 Ma} ranging from ?6.5 to ?10.8; one sample from the granitic basement with a T^Nd_CHUR model age of 1.36 Ga yielded an ?_Sr^{t = 35.7 Ma} of +188 and an ?_Nd^{t = 35.7 Ma} of ?5.7. The Exmore breccia (crater fill) can be explained as a mix of the measured target sediments and the granite, plus an as‐yet undetermined component. The post‐impact sediments of the Chickahominy formation have slightly higher T^Nd_CHUR model ages of about 1.55 Ga, indicating a contribution of some older materials. Newly analyzed bediasites have the following isotope parameters: +104 to +119 (?_Sr^{t = 35.7 Ma}), ?5.7 (?_Nd^{t = 35.7 Ma}), 0.47 Ga (T^Sr_UR), and 1.15 Ga (T^Nd_CHUR), which is in excellent agreement with previously published data for samples of the NAT strewn field. Target rocks with highly radiogenic Sr isotopic composition, as required for explaining the isotopic characteristics of Deep Sea Drilling Project (DSDP) site 612 tektites, were not among the analyzed sample suite. Based on the new isotope data, we exclude any relation between the NA tektites and the Popigai impact crater, although they have identical ages within 2s? errors. The Chesapeake Bay structure, however, is now clearly constrained as the source crater for the North American tektites, although the present data set obviously does not include all target lithologies that have contributed to the composition of the tektites.     

Rb‐Sr and Sm‐Nd isotopic systematics of the lherzolitic shergottite GRV 99027

Abstract– Rb‐Sr and Sm‐Nd isotopic analyses of the lherzolitic shergottite Grove Mountains (GRV) 99027 are reported. GRV 99027 yields a Rb‐Sr mineral isochron age of 177 ± 5 (2σ) Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio (I_Sr) of 0.710364 ± 11 (2σ). Due to larger uncertainties of the Sm‐Nd isotopic data, no Sm‐Nd isochron age was obtained for GRV 99027. The ε¹⁴³Nd value is estimated approximately +12.2, assuming an age of 177 Ma. The I_Sr of GRV 99027 is distinguishable from other lherzolitic shergottites, confirming our previous conclusion that it is not paired with them ( Lin et al. 2005 ). The new data of GRV 99027 support the same age of approximately 180 Ma for most lherzolitic shergottites, and fill the small gap of I_Sr between Allan Hills A77005 and Lewis Cliff 88516 ( Borg et al. 2002 ). All available data are consistent with a single igneous source for the intermediate subgroup of lherzolitic shergottites.     

The origin of dark inclusions in Allende: New evidence from lithium isotopes

Abstract— Aqueous and thermal processing of primordial materials occurred prior to and during planet formation in the early solar system. A record of how solid materials were altered at this time is present in the carbonaceous chondrites, which are naturally delivered fragments of primitive asteroids. It has been proposed that some materials, such as the clasts termed “dark inclusions” found in type III chondrites, suggest a sequence of aqueous and thermal events. Lithium isotopes (⁶Li and ⁷Li) can reveal the role of liquid water in dark inclusion history. During aqueous alteration, ⁷Li passes preferentially into solution leaving ⁶Li behind in the solid phase and, consequently, any relatively extended periods of interaction with ⁷Li‐rich fluids would have left the dark inclusions enriched in the heavier isotope when compared to the meteorite as a whole. Our analyses of lithium isotopes in Allende and its dark inclusions reveal marked isotopic homogeneity and no evidence of greater levels of aqueous alteration in dark inclusion history.     

Mineralogical evidence for the origin of diamond in ureilites

Abstract— The x‐ray powder diffraction patterns of 50–100 μm C‐rich grains from five ureilitic meteorites—Kenna, Allan Hills (ALH) 78019, Yamato (Y)‐82100, Y‐791538, and ALH 77257—were obtained by using a Gandolfi camera. The results reveal that the basal spacing of part of the graphite coexisting with diamond is slightly smaller compared to the normal spacing. Compressed graphite is experimentally known to occur at the initial stage of the direct transformation from graphite to diamond structures at high pressures and temperatures. The presence of the compressed graphite in ureilites, therefore, gives clear evidence that the diamond formed by high‐pressure conversion of graphite. The modes of occurrence of C minerals observed with reflected light through an optical microscope reveal that graphite coexisted with olivine and pyroxene during igneous or metamorphic processes and, furthermore, that part of the graphite was converted to diamond by impact. The relative x‐ray intensity of diamond to graphite increases in the following order: ALH 78019 and Y‐82100 < Y‐791538 < Kenna < ALH 77257. This correlates with the shock level that is estimated mainly on the basis of the shock features of silicates. Therefore, the relative amounts of diamond to graphite suggested by x‐ray intensities may be useful as a measure of the degree of shock.     

Sr and Nd analyses of upper Eocene spherules and their implications for target rocks

Abstract— Upper Eocene impact ejecta has been discovered all over the world. The number of upper Eocene impact layers and the geographic distribution of each layer, based on major chemical composition and biostratigraphic data, are not agreed upon. We have performed four Sr‐Nd isotopic analyses of clinopyroxene‐bearing spherules (cpx spherules) and three Sr‐Nd analyses of microtektites from five Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) sites in the South Atlantic and Indian Oceans. Our data support the hypothesis that there is only one cpx spherule layer in upper Eocene sediments. We also find that the microtektites associated with the cpx spherule layer in the South Atlantic and Indian Oceans are not part of the North American tektite strewn field, but belong to the same event that produced the cpx spherules. The microtektites, together with cpx spherules, are more heterogeneous than microtektites/tektites from other strewn fields. No direct link has been established between the microtektites from this study and possible target rock at the Popigai crater.     

Rb‐Sr and Sm‐Nd isotopic and REE studies of igneous components in the bulk matrix domain of Martian breccia Northwest Africa 7034

The bulk matrix domain of the Martian breccia NWA 7034 was examined petrographically and isotopically to better understand the provenance and age of the source material that make up the breccia. Both ¹⁴⁷Sm‐¹⁴³Nd and ¹⁴⁶Sm‐¹⁴²Nd age results for mineral separates from the bulk matrix portion of breccia NWA 7034 suggest that various lithological components in the breccia probably formed contemporaneously ~4.44 Ga ago. This old age is in excellent agreement with the upper intersection ages (4.35–4.45 Ga) for U‐Pb discordia and also concordia defined by zircon and baddeleyite grains in matrix and igneous‐textured clasts. Consequently, we confirm an ancient age for the igneous components that make up the NWA 7034 breccia. Substantial disturbance in the Rb‐Sr system was detected, and no age significance could be gleaned from our Rb‐Sr data. The disturbance to the Rb‐Sr system may be due to a thermal event recorded by bulk‐rock K‐Ar ages of 1.56 Ga and U‐Pb ages of phosphates at about 1.35–1.5 Ga, which suggest partial resetting from an unknown thermal event(s), possibly accompanying breccia formation. The NWA 7034 bulk rock is LREE enriched and similar to KREEP‐rich lunar rocks, which indicates that the earliest Martian crust was geochemically enriched. This enrichment supports the idea that the crust is one of the enriched geochemical reservoirs on Mars that have been detected in studies of other Martian meteorites.     

Zinc isotopic composition of iron meteorites: Absence of isotopic anomalies and origin of the volatile element depletion

High‐precision Zn isotopic compositions measured by MC‐ICP‐MS are documented for 32 iron meteorites from various fractionally crystallized and silicate‐bearing groups. The δ⁶⁶Zn values range from ?0.59‰ up to +5.61‰ with most samples being slightly enriched in the heavier isotopes compared with carbonaceous chondrites (0 < δ⁶⁶Zn < 0.5). The δ⁶⁶Zn versus δ⁶⁸Zn plot of all samples defines a common linear fractionation line, which supports the hypothesis that Zn was derived from a single reservoir or from multiple reservoirs linked by mass‐dependent fractionation processes. Our data for Redfields fall on a mass fractionation line and therefore refute a previous claim of it having an anomalous isotopic composition due to nonmixing of nucleosynthetic products. The negative correlation between δ⁶⁶Zn and the Zn concentration of IAB and IIE is consistent with mass‐dependent isotopic fractionation due to evaporation with preferential loss of lighter isotopes in the vapor phase. Data for the Zn concentrations and isotopic compositions of two IVA samples demonstrate that volatile depletion in the IVA parent body is not likely the result of evaporation. This is important evidence that favors the incomplete condensation origin for the volatile depletion of the IVA parent body.     

Strontium and neodymium isotopic study of Libyan Desert Glass: Inherited Pan‐African age signatures and new evidence for target material

Abstract— Libyan Desert Glass (LDG) is an impact‐related, natural glass of still unknown target material. We have determined Rb‐Sr and Sm‐Nd isotopic ratios from seven LDG samples and five associated sandstones from the LDG strewn field in the Great Sand Sea, western Egypt. Planar deformation features were recently detected in quartz from these sandstones. ⁸⁷Sr/⁸⁶Sr ratios and ?‐Nd values for LDG range between 0.71219 and 0.71344, and between –16.6 and –17.8, respectively, and hence are distinct from the less radiogenic ⁸⁷Sr/⁸⁶Sr ratios of 0.70910–0.71053 and ?‐Nd values from –6.9 to –9.6 for the local sandstones from the LDG strewn field. Previously published isotopic ratios from the Libyan BP and Oasis crater sandstones are generally incompatible with our LDG values. LDG formation undoubtedly occurred at 29 Ma, but neither the Rb‐Sr nor the Sm‐Nd isotopic system were rehomogenised during the impact event, as we can deduce from Pan‐African ages of ?540 Ma determined from the regression lines from a total of 14 LDG samples from this work and the literature. Together with similar Sr and Nd isotopic values for LDG and granitoid rocks from northeast Africa west of the Nile, these findings point to a sandy matrix target material for the LDG derived from a Precambrian crystalline basement, ruling out the Cretaceous sandstones of the former “Nubian Group” as possible precursors for LDG.     

Meteoritic material in a Boulder from the Apollo 17 Site: Implications for its origin

Sixteen samples of Boulder 1 from Station 2 at the Apollo 17 site were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd, Cs, Ge, Ir, Ni, Rb, Re, Sb, Te, Tl, U, and Zn. Two clast samples contam no meteoritic material and appear to consist of relatively pristine igneous rocks: an unusual, KREEP-rich pigeonite basalt of very high Ge content, and an alkali-poor coarse norite. Nine grey or black breccia samples contain a unique, Group 3 meteoritic component of Ir/Au ratio 0.65–0.82, which appears to separate into subgroups 3H and 3L on the basis of Ni, Ge, and Re content. It is quite distinct from the Group 2 component (Ir/Au - 0.46–0.54) that dominates at the Apollo 17 site.The unique black-rimmed clasts from this boulder show striking compositional zoning. The cores of anorthositic breccia are very low in Rb, Cs, and U, and have a distinctive 5L meteoritic component (Ir/Au1.1). The black rinds are 5- to 10-fold richer in Rb, Cs, and U and have a Group 3 meteoritic component. The cores may represent breccias formed in an earlier impact that became coated with alkali-rich ejecta during the event that produced the boulder.Because of the rarity of the Group 3 meteoritic component at the Apollo 17 site, this boulder cannot represent ordinary Serenitatis ejecta, with their characteristic admixture of the Group 2 Serenitatis projectile. It may represent pre-Serenitatis material excavated from the fringes of the crater during late stages of the Serenitatis impact, but only lightly shocked and hence uncontaminated by the Serenitatis projectile.     

Petrography and geochemistry of five new Apollo 16 mare basalts and evidence for post‐basin deposition of basaltic material at the site

Abstract— We present the petrography and geochemistry of five 2–4 mm basalt fragments from the Apollo 16 regolith. These fragments are 1) a high‐Ti vitrophyric basalt compositionally similar to Apollo 17 high‐Ti mare basalts, 2) a very high‐Ti vitrophyric basalt compositionally similar to Apollos 12 and 14 red‐black pyroclastic glass, 3) a coarsely crystalline high‐Al basalt compositionally similar to group 5 Apollo 14 high‐Al mare basalts, 4) a very low‐Ti (VLT) crystalline basalt compositionally similar to Luna 24 VLT basalts, and 5) a VLT basaltic glass fragment compositionally similar to Apollo 17 VLT basalts. High‐Ti basalt has been reported previously at the Apollo 16 site; the other basalt types have not been reported previously. As there are no known cryptomaria or pyroclastic deposits in the highlands near the Apollo 16 site (ruling out a local origin), and scant evidence for basaltic material in the Apollo 16 ancient regolith breccias or Apollo 16 soils collected near North Ray Crater (ruling out a basin ejecta origin), we infer that the basaltic material in the Apollo 16 regolith originated in maria near the Apollo 16 site and was transported laterally to the site by small‐ to medium‐sized post‐basin impacts. On the basis of TiO₂ concentrations derived from the Clementine UVVIS data, Mare Tranquillitatis (?300 km north) is the most likely source for the high‐Ti basaltic material at the Apollo 16 site (craters Ross, Arago, Dionysius, Maskelyne, Moltke, Sosigenes, Schmidt), Mare Nectaris/Sinus Asperitatis (?220 km east) is the most likely source for the low‐Ti and VLT basaltic material (craters Theophilus, Madler, Torricelli), and a large regional pyroclastic deposit near Mare Vaporum (?600 km northwest) is the most likely source region for pyroclastic material (although no source craters are apparent in the region).     

New evidence of precision premium for Galilean satellites from CCD imaging

After a CCD image of the four Galilean satellites of Jupiter is obtained by a long focal length telescope, we can compare the theoretical positions of these satellites with their pixel positions so as to obtain the calibration parameters of the CCD field of view. In theory, when two of the four satellites have small enough separation, their relative positional measurement will have a good accuracy since the error existing in the solved calibration parameters has a direct proportional effect on the separation of the two satellites. The 347 CCD images taken by 1-m long focal length telescope at Yunnan Observatory in 2002-2005 are used to perform the experimental test. After we improve the centroid algorithm for the satellites and our former halo-removal technique, the results show that the positional measurement of two small-separation satellites has an external precision as good as 0.01-0.03 arcsec. This precision has comparability as that from rarely occurring mutual events of the Galilean satellites. This experiment confirms the finding of the “precision premium” firstly presented by Pascu [1994. An appraisal of the USNO program of photographic astrometry of bright planetary satellites. In: Morrison, L.V., Gilmore, G.F., (Eds.), Galatic and Solar System Optical Astrometry, pp. 304-311] using photographic observations. We believe that this type of observations, besides mutual event observations, might also be used to improve our knowledge of the orbital motions of the Galilean satellites because of its much more opportunities.     

Al‐Mg isotopic evidence for episodic alteration of Ca‐Al‐rich inclusions from Allende

Abstract— Textures, mineral assemblages, and Al‐Mg isotope systematics indicate a protracted, episodic secondary mineralization history for Allende Ca‐Al‐rich inclusions (CAIs). Detailed observations from one type B1 CAI, one B2, one compact type A (CTA), and one fluffy type A (FTA) indicate that these diverse types of CAIs are characterized by two distinct textural and mineralogic types of secondary mineralization: (1) grossular‐rich domains, concentrated along melilite grain boundaries in CAI interiors, and (2) feldspathoid‐bearing domains, confined mostly to CAI margins just interior to the Wark‐Lovering rim sequence. The Al‐Mg isotopic compositions of most secondary minerals in the type B1 CAI, and some secondary minerals in the other CAIs, show no resolvable excesses of ²⁶Mg, whereas the primary CAI phases mostly yield correlated excesses of ²⁶Mg with increasing Al/Mg corresponding to “canonical” initial ²⁶Al/²⁷Al ～ 4.5–5 × 10^?5. These secondary minerals formed at least 3 Ma after the primary CAI minerals. All but two analyses of secondary minerals from the fluffy type‐A CAI define a correlated increase in ²⁶Mg/²⁴Mg with increasing Al/Mg, yielding (²⁶Al/²⁷Al)0 = (4.9 ± 2.8) × 10^?6. The secondary minerals in this CAI formed 1.8–3.2 Ma after the primary CAI minerals. In both cases, the timing of secondary alteration is consistent with, but does not necessarily require, alteration in an asteroidal setting. One grossular from the type B2 CAI, and several grossular and secondary feldspar analyses from the compact type A CAI, have excesses of ²⁶Mg consistent with initial ²⁶Al/²⁷Al ～ 4.5 × 10^?5. Especially in the compact type A CAI, where ²⁶Mg/²⁴Mg in grossular correlates with increasing Al/Mg, these ²⁶Mg excesses are almost certainly due to in situ decay of ²⁶Al. They indicate a nebular setting for formation of the grossular. The preservation of these diverse isotopic patterns indicates that heating on the Allende parent body was not pervasive enough to reset isotopic systematics of fine‐grained secondary minerals. Secondary mineralization clearly was not restricted to a short time interval, and at least some alteration occurred coincident with CAI formation and melting events (chondrule formation) in the nebula. This observation supports the possibility that alteration followed by melting affected the compositional evolution of CAIs.     

Precessing jets interacting with interstellar material as the origin for the light curves of gamma-ray bursts

We present an internal shock model with external characteristics for explaining the complicated light curves of gamma-ray bursts. Shocks produce gamma-rays in the interaction between a precessing beam of relativistic particles and the interstellar medium. Each time the particle beam passes the same line of sight with the observer the interstellar medium is pushed outward. Subsequent interactions between the medium and the beam are delayed by the extra distance to be travelled for the particles before the shock can form. This results in a natural retardation and leads to an intrinsic asymmetry in the light curves produced for gamma-ray bursts. In addition, we account for the cooling of the electron–proton plasma in the shocked region, which gives rise to an exponential decay in the gamma-ray flux. The combination of these effects and the precessing jet of ultrarelativistic particles produces light curves that can be directly compared with observed gamma-ray burst light curves. We illustrate the model by fitting a number of observed gamma-ray bursts that are difficult to explain with only a precessing jet. We develop a genetic algorithm to fit several observed gamma-ray bursts with remarkable accuracy. We find that for different bursts the observed fluence, assuming isotropic emission, easily varies over four orders of magnitude from the energy generated intrinsically.     

Possible evidence for the disc origin for the powering of jets in Sgr A* and nearby elliptical galaxies

Recent VLBA observation indicates the existence of an elongated (jet) structure in the compact radio source Sgr A*. This is hard to explain in the context of advection-dominated accretion flow (ADAF) model for this source. On the other hand, the mass accretion rate favoured by ADAF is 10–20 times smaller than that favoured by the hydrodynamical simulation based on Bondi capture. If the latter were adopted, the predicted radio flux would significantly exceed the observation. A similar situation exists in the case of nearby giant ellipticals, where the canonical ADAF model – the widely assumed standard model for these sources – also significantly overpredicts the radio flux. Based on these facts, in this paper we propose a truncated ADAF model for Sgr A* and three ellipticals M87, NGC 4649 and NGC 4636. We assume that the accretion disc is truncated at a certain radius R _tr within which the jet forms by extracting the energy of the disc. The radio flux is greatly suppressed owing to the radiative truncation of the disc and the fits to the observational data are excellent. For example, for Sgr A*, the model fits the observational spectrum very well from radio including the 'excess' below the break frequency to hard X-ray under a high accretion rate near the simulation value, and the predicted size-frequency relationship is also in excellent agreement with the observation; for M87, the predicted upper limit of the jet location is 24 R _g, in excellent agreement with the observational result that the jet is formed on scales smaller than 30 R _g, and the ≈20 per cent variability at ∼1 keV – which is hard to explain in another model that succeeded in explaining the low radio flux of M87 – is also marginally interpreted. The success of the model supplies possible evidence for the disc rather than the hole origin for the powering of jets.     

Defining the mechanisms that disturb the Sm‐Nd isotopic systematics of the Martian meteorites: Examples from Dar al Gani 476 and Allan Hills 77005

Abstract— Microbeam studies of Martian meteorites Dar al Gani (DaG) 476 and Allan Hills (ALH) 77005 have been conducted to identify potential causes of disequilibrium exhibited in their Sm‐Nd isotopic systematics. Olivine and maskelynite mineral fractions on the DaG 476 isochron are displaced relative to their positions as dictated by measured mineral compositions. The olivine mineral fractions from ALH 77005 not only have a relatively low Sm/Nd ratio, but appear to contain an unradiogenic component that shifts the olivine mineral fraction off the isochron defined by the pyroxene and maskelynite mineral fractions. Trace components such as melt inclusions, impact melt, high‐Si mesostasis, and altered olivine were analyzed using scanning electron microscopy, quantitative electron microscopy, and secondary ion mass spectrometry to determine their potential for disturbing the isotopic systematics of the mineral fractions, assuming that the mineral fractions were not completely pure. Mixing models indicate that the presence of melt inclusions in the DaG 476 olivine mineral fraction lowered its Sm/Nd ratio. The maskelynite mineral fraction contains a related but more evolved mesostasis component that raised the Sm/Nd ratio of the fraction. The position of two olivine mineral fractions below the ALH 77005 isochron is interpreted to reflect small additions of impact melt with a light rare earth element enriched pattern and a non‐indigenous, unradiogenic Nd component. Furthermore, the presence of rare earth elements in olivine and maskelynite from both igneous and non‐igneous components such as melt inclusions, mesostasis, and impact melt is observed on a fine (<30 μm) scale. Despite the addition of this material, the Sm‐Nd ages are not affected. This study demonstrates that detailed mineral separation procedures as employed by modern geochronology laboratories permit reliable ages to be derived from shocked and altered samples.     

Quantified mineralogical evidence for a common origin of 1929 Kollaa with 4 Vesta and the HED meteorites

In this paper, we present the first correlation of derived mineral abundances of V-class Asteroid 1929 Kollaa, 4 Vesta, and the HED meteorites. We demonstrate that 1929 Kollaa has a basaltic composition consistent with an origin within the crustal layer of 4 Vesta, and show a plausible genetic connection between Kollaa and the cumulate eucrite meteorites. These data support the proposed delivery mechanism of HED meteorites to the Earth from Vesta, and provide the first mineralogical constraint derived from the observation of a small V-class, Vesta family asteroid on the crustal thickness of 4 Vesta.     

Morphology and structure of the taurus-littrow highlands (Apollo 17): evidence for their origin and evolution

The Taurus-Littrow region (Apollo 17 landing area) is located in the northeastern quadrant of the Moon in the mountainous area on the southeastern rim of the Serenitatis basin. The highlands in the Taurus-Littrow region can be divided into three broad terrain types. (1)Littrow massifs - massive, 10-20 km diam, steep-sloped (20°–30°), highland blocks often bordered by linear graben-like valleys. (2)Littrow sculptured hills - a series of closely spaced 1-5 km diam domical hills occupying broad highland plateaus which have been cratered and block faulted. Sculptured hill units stretch along the eastern edge of Serenitatis from the Apollo 17 area north to Posidonius. (3)Vitruvius front and plateau - a long irregular but generally north-trending scarp (occasionally rising over 2 km above the surrounding terrain) and its associated uplifted plateau to the east. This terrain is composed of hills ranging from 2-7 km diam, whose morphology is intermediate between the sculptured hills and the massifs. It is concluded that the highland units in the Taurus-Littrow region are primarily related to the origin of the Serenitatis basin because of their marked similarity to more well-preserved basin-related deposits in the younger Imbrium and Orientale basins: (1) the massifs and sculptured terra are morphologically similar to the Imbrium basin-related Montes Alpes and Alpes Formation, (2) the relative geographic position of the Taurus-Littrow highlands and Montes Alpes/Alpes Formation is the same, forming the second ring and spreading distally, and (3) the structures are similar in orientation and development (e.g., massifs are related to radial and concentric structure; Alpes Formation/sculptured terra are not). Interpretation of the massifs and sculptured hills as Serenitatis impact-related deposits lessens the possible role of highland volcanism in the origin and evolution of the Taurus-Littrow terrain, although extensive pre-Serenitatis volcanism cannot be ruled out. The preserved morphology of the sculptured hills suggests that the thickness of post-Serenitatis large basin ejecta (from Imbrium, for instance) is small, compared to the total highland section. This implies that the primary contributions to the highland stratigraphy are from Serenitatis and pre-Serenitatis events. The highland surface, however, may be dominated by ejecta from the latest nearby large event (formation of the Imbrium basin). Structural elements mapped in the Taurus-Littrow area include lineaments, the Vitruvius structural front, two types of grabens, and scarps. The majority of lineaments, as well as some grabens, appear to be related to a dominant NW trend and subordinate N and NE trends. These trends are interpreted to be related to a more regional lunar grid pattern which formed in the area prior to the origin of the Serenitatis basin, causing distinct structural inhomogeneities in the highland terrain. The Serenitatis event produced radial and concentric structures predominantly influenced by this pre-existing trend. Younger grabens are generally circumferential to the Serenitatis basin and appear to be related to readjustment of Serenitatis-produced structures; those that are oblique to Serenitatis follow the pre-Serenitatis structural grain. No obvious structural elements can be correlated with the post-Serenitatis, Nectaris and Crisium basins. It is believed that the origin and hence the geographic concentration of the Littrow massifs is related to the fact that Serenitatis radials in the massif area coincide with lines of pre-existing structural weakness along a general lunar grid direction (NW). Pre-existing structurally weak lunar grid trends seem to have been structurally reactivated by Serenitatis radials, causing preferential uplift of large blocks in this area. Elsewhere in the region radials would be oblique to this direction. Since Serenitatis and Imbrium radials coincide in the massif area, the post-Serenitatis Imbrium event may have reactivated Serenitatis radial fractures, possibly rejuvenating the massif terrain. The geologic and tectonic history of the Taurus-Littrow highlands began prior to the origin of Serenitatis in Tectonic Interval I. The strong NW trending structural elements are believed to have formed as part of a global stress pattern (possibly shear) sometime during this period of probable crustal formation and fragmentation. Tectonic Interval II was initiated by the origin of the Serenitatis basin. The basic topography and morphology of the region and most large grabens resulted from this event and their orientations show that they were controlled at least in part by the pre-existing grid. No other large basins forming during this interval appear to have had a major effect on the area. Tectonic Interval III is dominated by the formation of narrow grabens following structural patterns circumferential to the Serenitatis basin and tangential to it where they coincide with pre-existing grid directions. Serenitatis isostatic rebound or early mare fill may have produced this stress system. The scarp in the vicinity of the Apollo 17 landing site is the youngest obvious structural element.     

The Pleistocene evolution of the East Antarctic Ice Sheet in the Prydz bay region: stable isotopic evidence from ODP Site 1167

Ocean Drilling Program Leg 188, Prydz Bay, East Antarctica is part of a larger initiative to explore the Cenozoic history of the Antarctic Ice Sheet through direct drilling and sampling of the continental margins. In this paper, we present stable isotopic results from Ocean Drilling Program (ODP) Site 1167 located on the Prydz Channel Trough Mouth Fan (TMF), the first Antarctic TMF to be drilled. The foraminifer-based δ¹⁸O record is interpreted along with sedimentary and downhole logging evidence to reconstruct the Quaternary glacial history of Prydz Bay and the adjacent Lambert Glacier Amery Ice Shelf System (LGAISS). We report an electron spin resonance age date of 36.9±3.3 ka at 0.45 m below sea floor and correlate suspected glacial–interglacial cycles with the global isotopic stratigraphy to improve the chronology for Site 1167. The δ¹⁸O record based on planktonic (Neogloboquadrina pachyderma (s.)) and limited benthic results (Globocassidulina crassa), indicates a trend of ice sheet expansion that was interrupted by a period of reduced ice volume and possibly warmer conditions during the early–mid-Pleistocene (0.9–1.38 Ma). An increase in δ¹⁸O values after 900 ka appears to coincide with the mid-Pleistocene climate transition and the expansion of the northern hemisphere ice sheet. The δ¹⁸O record in the upper 50 m of the stratigraphic section indicates as few as three glacial–interglacial cycles, tentatively assigned as marine isotopic stages (MIS) 16–21, are preserved since the Brunhes/Matuyama paleomagnetic reversal (780 ka). This suggests that there is a large unconformity near the top of the section and/or that there may have been few extreme advances of the ice sheet since the mid-Pleistocene climate transition resulting in lowered sedimentation rates on the Prydz Channel TMF. The stable isotopic record from Site 1167 is one of the few available from the area south of the Antarctic Polar Front that has been linked with the global isotopic stratigraphy. Our results suggest the potential for the recovery of useful stable isotopic records in other TMFs.