Search for extinct aluminum‐26 and titanium‐44 in nanodiamonds from the Allende CV3 and Murchison CM2 meteorites

Abstract– We report Mg‐Al and Ca‐Ti isotopic data for meteoritic nanodiamonds separated from the Allende CV3 and Murchison CM2 meteorites. The goal of this study was to search for excesses in ²⁶Mg and ⁴⁴Ca, which can be attributed to the in situ decay of radioactive and now extinct ²⁶Al and ⁴⁴Ti, respectively. Previous work on presolar SiC and graphite had shown that ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti ratios in presolar grains can be used to discriminate between different types of stellar sources. Aluminum and Ti concentrations are low in the meteoritic nanodiamonds of this study. Murchison nanodiamonds have higher Al and Ti concentrations than the Allende nanodiamonds. This can be attributed to contamination and the presence of presolar SiC in the Murchison nanodiamond samples. ²⁶Mg/²⁴Mg and ⁴⁴Ca/⁴⁰Ca ratios are close to normal in Allende nanodiamonds with upper limits on the initial ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti ratios of approximately 1 × 10^?3. These ratios are factors of 10–1000 and, respectively, 1–1000 lower than those of presolar SiC and graphite grains from supernovae. The ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti data for nanodiamonds are compatible with an asymptotic giant branch star or solar system origin, but not with a supernova origin of a major fraction of meteoritic nanodiamonds. The latter possibility cannot be excluded, though, as the diamond separates may contain significant amounts of contaminating Al and Ti, which would lower the inferred ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti ratios considerably.     

Noble gas adsorption with and without mechanical stress: Not Martian signatures but fractionated air

Abstract– Sample preparation, involving physical and chemical methods, is an unavoidable step in geochemical analysis. From a noble gas perspective, the two important effects are loss of sample gas and/or incorporation of air, which are significant sources of analytical artifacts. This article reports on the effects of sample exposure to laboratory air without mechanical influence and during sample grinding. The experiments include pure adsorption on terrestrial analog materials (gibbsite and olivine) and grinding of Martian meteorites. A consistent observation is the presence of an elementally fractionated air component in the samples studied. This is a critical form of terrestrial contamination in meteorites as it often mimics the heavy noble gas signatures of known extra‐terrestrial end‐members that are the basis of important conclusions about the origin and evolution of a meteorite. Although the effects of such contamination can be minimized by avoiding elaborate sample preparation protocols, caution should be exercised in interpreting the elemental ratios (Ar/Xe, Kr/Xe), especially in the low‐temperature step extractions. The experiments can also be transferred to the investigation of Martian meteorites with long terrestrial residence times, and to Mars, where the Mars Science Laboratory mission will be able to measure noble gas signatures in the current atmosphere and in rocks and soils collected on the surface in Gale crater.     

The Puerto Lápice eucrite

Abstract— Puerto Lápice is a new eucrite fall (Castilla‐La Mancha, Spain, 10 May 2007). In this paper, we report its detailed petrography, magnetic characterization, mineral and bulk chemistry, oxygen and noble gas isotope systematics, and radionuclide data. Study of four thin sections from two different specimens reveal that the meteorite is brecciated in nature, and it contains basaltic and granulitic clasts, as well as recrystallized impact melt and breccia fragments. Shock veins are ubiquitous and show evidence of at least three different shock events. Bulk chemical analyses suggest that Puerto Lápice belongs to the main group of basaltic eucrites, although it has a significantly higher Cr content. Oxygen isotopes also confirm that the meteorite is a normal member of the HED suite. Noble gas abundances show typical patterns, with dominant cosmogenic and radiogenic contributions, and indicate an average exposure age of 19 ± 2 Ma, and a Pu‐fission Xe age well within typical eucrite values. Cosmogenic radionuclides suggest a preatmospheric size of about 20–30 cm in diameter.     

An Example of Island-Arc Petrogenesis: Geochemistry and Petrology of the Southern Luzon Arc, Philippines

Volcanism throughout the Luzon arc is associated with eastwardsubduction of the South China Sea floor along the Manila Trench.The southern section of the arc, the focus of this study, extendsfrom the Lingayen-Dingalan fault to the small islands just southof Luzon. Two segments appear to exist along this section ofthe arc the northern Bataan and southern Mindoro segments whichare separated by the Macolod Corridor. The volcanic rocks have typical arc phenocryst mineralogies:olivine, clinopyroxene, plagioclase, and titanomagnetite inthe most mafic rocks and clinopyroxene, plagioclase, orthopyroxene,titanomagnetite, ? amphibole in the more felsic samples. Complexzoning, sieve textures, and decoupling of incompatible traceelements suggest that processes such as assimilation have takenplace. The rocks from the study area range from basalts to rhyolitesand show typical calc-alkaline features. The rocks of the MacolodCorridor and Mindoro segment are particularly enriched in largeion lithophile elements (LILE), light rare earth elements (LREE),and radiogenic Sr compared with the Bataan segment. The datafall within the mantle array on Sr-Nd isotopic diagrams andgrade toward higher Sr and lower Nd isotopic values from northto south. A likely source for the volcanics of this study is either amid-ocean ridge basalt (MORB)-type mantle that undergoes higherdegrees of partial melting than regions involved in MORB generationor a previously depleted source. We suggest that the high fieldstrength element (HFSE) anomalies have been derived throughdifferential element partitioning during fluid transport fromthe subducted lithosphere to the mantle wedge. Continental crustal material seems to play a significant roleparticularly in the Macolod Corridor and the Mindoro segment,based on the high LILE, La/Sm ratios, radiogenic Sr isotopes,and ¹⁸O values. The Macolod Corridor and the Mindoro segmenthave undergone source contamination by crustal material fromthe North Palawan-Mindoro crustal block either during the collisionof this block with the Manila Trench or by subduction of sedimentsrich in this crustal material. A similar component has alsobeen detected in the Bataan segment but in minor amounts. Thetrace element and isotopic differences between the northernand southern sections of the arc are interpreted in terms ofvariable composition (i. e., variable amounts of a crustal componentintroduced from the Palawan-Mindoro crustal terrain) of themetasomatic fluids released into the source.     

Exploring till bed kinematics using AMS magnetic fabrics and pebble fabrics: the Weedsport drumlin field,New York State,USA

Gentoso, M. J., Evenson, E. B., Kodama, K. P., Iverson, N. R., Alley, R. B., Berti, C. & Kozlowski, A. 2012 (January): Exploring till bed kinematics using AMS magnetic fabrics and pebble fabrics: the Weedsport drumlin field, New York State, USA. Boreas, Vol. 41, pp. 31–41. 10.1111/j.1502‐3885.2011.00221.x. ISSN 0300‐9483. Thick, relatively homogeneous basal tills exposed in the drumlins and flutes of the Weedsport drumlin and flute field in New York State exhibit anisotropy of magnetic susceptibility (AMS) and pebble fabrics that are consistently oriented parallel to the streamlined bedforms. The pebble fabrics and AMS fabrics are concordant. In this study, six drumlins and five flutes were sampled. Thermally induced, incremental reduction of isothermal remanent magnetization indicates that AMS is caused by primarily elongate maghaemite grains. The orientations of principal axes of maximum susceptibility (k₁) are generally parallel to pebble long‐axis orientations, and tend to plunge mildly up‐glacier. Fabric directions are generally parallel to drumlin long‐axis orientations, but deviate by 12°–23° from flute directions. Fabrics of the flutes are stronger and more unidirectional than those of the drumlins. These results support the use of AMS as a fast and objective method for characterizing fabrics in tills, and suggest hypotheses about basal processes linked to glacially streamlined landforms.     

Late Weichselian glaciation history of the northern North Sea

Based on new data from the Fladen, Sleipner and Troll areas, combined with earlier published results, a glaciation curve for the Late Weichselian in the northern North Sea is constructed. The youngest date on marine sedimentation prior to the late Weichselian maximum ice extent is 29.4 ka BP. At this time the North Sea and probably large parts of southern Norway were deglaciated (corresponding to the Alesund interstadial in western Norway). In a period between 29.4 and c. 22 ka BP, the northern North Sea experienced its maximum Weichselian glaciation with a coalescing British and Scandinavian ice sheet. The first recorded marine inundation is found in the Fladen area where marine sedimentation started close to 22 ka BP. After this the ice fronts receded both to the east and west. The North Sea Plateau, and possibly parts of the Norwegian Trench, were ice-free close to 19.0 ka, and after this a short readvance occurred in this area. This event is correlated with the advance recorded at Dimlington, Yorkshire, and the corresponding climatostratigraphic unit is denoted the Dimlington Stadial (18.5 ka to 15.1 ka). The Norwegian Trench was deglaciated at 15.1 ka in the Troll area. The data from the North Sea, together with the results from Andwa, northern Norway (Vorren et al . 1988; Møller et al . 1992), suggest that the maximum extent of the last glaciation along the NW-European seaboard from the British Isles to northern Norway was prior to c . 22 ka BP.     

Carbonates in fractures of Martian meteorite Allan Hills 84001: Petrologic evidence for impact origin

Abstract— Carbonates in Martian meteorite Allan Hills 84001 occur as grains on pyroxene grain boundaries, in crushed zones, and as disks, veins, and irregularly shaped grains in healed pyroxene fractures. Some carbonate disks have tapered Mg-rich edges and are accompanied by smaller, thinner and relatively homogeneous, magnesite microdisks. Except for the microdisks, all types of carbonate grains show the same unique chemical zoning pattern on MgCO₃-FeCO₃-CaCO₃ plots. This chemical characteristic and the close spatial association of diverse carbonate types show that all carbonates formed by a similar process. The heterogeneous distribution of carbonates in fractures, tapered shapes of some disks, and the localized occurrence of Mg-rich microdisks appear to be incompatible with growth from an externally derived CO₂-rich fluid that changed in composition over time. These features suggest instead that the fractures were closed as carbonates grew from an internally derived fluid and that the microdisks formed from a residual Mg-rich fluid that was squeezed along fractures. Carbonate in pyroxene fractures is most abundant near grains of plagioclase glass that are located on pyroxene grain boundaries and commonly contain major or minor amounts of carbonate. We infer that carbonates in fractures formed from grain boundary carbonates associated with plagioclase that were melted by impact and dispersed into the surrounding fractured pyroxene. Carbonates in fractures, which include those studied by McKay et al. (1996), could not have formed at low temperatures and preserved mineralogical evidence for Martian organisms.     

Beryllium and Other Trace Elements in Paragneisses and Anatectic Veins of the Ultrahigh-Temperature Napier Complex, Enderby Land, East Antarctica: the Role of Sapphirine

Anatectic veins containing the Be minerals khmaralite and berylliansapphirine as primary phases (or surinamite derived therefrom)are associated with Mg–Al-rich paragneisses at three localitiesin the ultrahigh-temperature Napier complex, Antarctica, a uniqueBe mineralization in the granulite facies. Likely precursorsof the paragneisses are volcaniclastic deposits that were hydrothermallyaltered by heated seawater prior to metamorphism. Regular distributionof Be among minerals in the paragneisses suggests an approachto equilibrium with Be greatly concentrated in sapphirine (25–3430ppm Be) or cordierite (560–930 ppm Be) relative to plagioclaseAn53–66 (14–43 ppm Be) > cores of coarse-grainedorthopyroxene (0·7–29 ppm Be) > coronitic orthopyroxene(0·4–14 ppm Be) sillimanite (0·1–26ppm Be) plagioclase An18–33 (0·6–15 ppmBe) > biotite (0·06–8 ppm Be) > K-feldspar,quartz, garnet (0·05–0·7 ppm Be). Sapphirine-bearingparagneisses have average Be concentrations, 4·9 ±2·4 ppm (13 samples), about twice that of typical pelites,whereas paragneisses lacking sapphirine and primary cordieritehave only 2·9 ± 2·1 ppm Be (12 samples),implying some loss of Be during metamorphism. The likely sourcerocks for the Be-rich melts were biotitic rocks lacking theBe sinks sapphirine and cordierite. These gneisses were probablyless competent than the sapphirine-bearing gneisses, so themelts were drawn to the latter and collected in spaces openedduring deformation and boudinage of the more competent paragneisses.Fractionation of the melts concentrated Be to the extent thatBe minerals could crystallize. The final result was Be-mineralizedanatectic veins hosted by relatively Be-rich sapphirine-bearingparagneisses. KEY WORDS: Antarctica; beryllium; granulite facies; microprobe; sapphirine     

Sample site variation in settling velocity distribution subpopulations using curve dissection analysis

ABSTRACT It is generally believed that grain-size distributions can be broken down into subpopulations which may be characteristic of a specific mode of transport, some source signature, or the nature of sediment packing in the depositional process. This paper evaluates the variation at a sample site of subpopulation parameters of settling velocity distributions derived using a non-linear least squares algorithm for curve dissection. Results incorporate artificial samples constructed using spherical glass beads in the 125-840 μm range and natural samples collected as close as possible from a single sedimentation unit in marine, fluvial and aeolian environments. If subpopulation distributions from the same sample site are compared then 90% of artificial bead samples and 65% of natural sands have cumulative curves diverging from each other by less than 20%. Similarity in means and standard deviations of subpopulations collected at the same site was not easily obtained. However, if it is important to be able to identify a specific process or source signature correctly, then 86% of all subpopulations from the same natural sample site grouped together. If subpopulations of grain-size distributions are to be used for process identification, it is advisable to analyse more than one sample from a site.