Crystallization and impact history of a meteoritic sample of early lunar crust(NWA 3163)refined by atom probe geochronology

Granulitic lunar meteorites offer rare insights into the timing and nature of igneous,metamorphic and impact processes in the lunar crust.Accurately dating the different events recorded by these materials is very challenging,however,due to low trace element abundances(e.g.Sm,Nd,Lu,Hf),rare micrometerscale U-Th-bearing accessory minerals,and disturbed Ar-Ar systematics following a multi-stage history of shock and thermal metamorphism.Here we report on micro-baddeleyite grains in granulitic mafic breccia NWA 3163 for the first time and show that targeted microstructural analysis(electron backscatter diffraction)and nanoscale geochronology(atom probe tomography)can overcome these barriers to lunar chronology.A twinned(～90°/401)baddeleyite domain yields a ~(232)Th/~(208)Pb age of 4328 ± 309 Ma,which overlaps with a robust secondary ion mass spectrometry(SIMS)~(207)Pb/~(206)Pb age of 4308 ± 18.6 Ma and is interpreted here as the crystallization age for the igneous protolith of NWA 3163.A second microstructural domain,2μm in width,contains patchy overprinting baddeleyite and yields a Th-Pb age of 2175 ±143 Ma,interpreted as dating the last substantial impact event to affect the sample.This finding demonstrates the potential of combining microstructural characterization with nanoscale geochronology when resolving complex P-T-t histories in planetary materials,here yielding the oldest measured crystallization age for components of lunar granulite NWA 3163 and placing further constraints on the formation and evolution of lunar crust.     

Tidal effects on phytoplankton assemblages in a near‐pristine estuary: a trait‐based approach for the case of a shallow tropical ecosystem in Brazil

The study of near‐pristine estuaries can be used as a tool for mitigation projects of harmful effects in anthropogenic eutrophic systems, since one can analyze the effect of temporal and spatial variations generated mainly by natural forces. Phytoplankton taxonomy has been used as a classical indicator to assess changes in transitional water communities, however alternative methods based on morphological, behavioral and physiological traits offer the opportunity to compare sites or moments with different taxonomic compositions. Our goal, in this context, is to evaluate phytoplankton community short‐term and seasonal variability in a near‐pristine estuary, Barra Grande estuary (Ilha Grande, RJ, Brazil), through species functional traits and thus community functional diversity. Samplings were carried out in a mooring in complete tidal cycles, seasonally during 2012. Our results showed a diverse phytoplankton community with 38 frequent and abundant taxa, marked by density variation (1.2 × 10.4–2 × 10.7 cell L^?1) in depth, with abundance inversely related to tidal currents. The functional structure of the phytoplankton community measured by functional diversity (FD), varied seasonally in and across a gradient of tidal energy. A core community, mainly represented by flagellates and dinoflagellates, occurred in all observations and was highly functional (high FD), exploiting a variety of habitats. The chain‐forming diatoms were associated with high tidal energy, and occurred in higher densities during summer. Phytoplankton cell size, cell shape, and the ability to form colonies are extremely plastic traits that can be regulated by the environment, however, isolated tychopelagic diatoms were present in the study area across all seasons, with higher contributions in autumn and winter, reflecting the shallow characteristic of this system. During the winter, an exposed sandbar was formed, and the lack of connection with the ocean resulted in an abundance of riverine and brackish water taxa. In this near‐pristine estuary the densities and occurrences of HAB phytoplankton are low. Trait‐based analyses add information about community structure, which can be impacted by anthropogenic actions in urbanized coastal systems. Thus the information provided by this study regarding phytoplankton functional diversity and its relation to nutrients and hydrography in Barra Grande Estuary can be applied as a baseline model for the development of public policies.     

Textural and mineralogical biosignatures in an unusual microbialite from Death Valley, California

For in situ astrobiological studies of Mars or other planets, we must employ strategies that will enable us to verify whether our approach and prototype instruments are actually capable of distinguishing life from non-life. This must be done against a background of rigorously conducted scientific characterization of the environment or sample types being considered for measurement by the instruments under development. In this study we show how a combination of mineralogical and textural features can be considered a biosignature in an early Mars analogue environment, Death Valley, California. We propose that it is a combination of features in context of the geologic matrix which allows determination of biogenicity to be made. Polymineralic microbialites (organosedimentary formations constructed by microorganisms) from a spring pool at Badwater, within Death Valley National Park, are composed of alternating biogenic and abiogenic minerals in a distinct triplet sequence related to wet and dry seasons. A microbial community, occurring as a black biofilm, produced paired layers of two different mineral types: manganese oxyhydroxides and calcite. These biogenic layers are separated from the next pair by a gypsum layer and appear to be laid down in the wet season, with the gypsum (a mineral positively identified on Mars) precipitating in the dry part of the year, abiogenically (i.e., not dependent on microbial metabolic activity for its deposition). In addition, textural features (smaller grain size and less geometric morphology) unique to the biogenic vs the abiogenic layers, were consistently observed so that texture served as a biosignature in this environment.     

Mojave Mars simulant—Characterization of a new geologic Mars analog

We have identified and characterized a basaltic Mars simulant that is available as whole rocks, sand and dust. The source rock for the simulant is a basalt mined from the Tertiary Tropico Group in the western Mojave Desert. The Mojave Mars Simulant (MMS) was chosen for its inert hygroscopic characteristics, its availability in a variety of forms, and its physical and chemical characteristics. The MMS dust and MMS sand are produced by mechanically crushing basaltic boulders. This is a process that more closely resembles the weathering/comminution processes on Mars where impact events and aerodynamic interactions provide comminution in the (relative) absence of water and organics. MMS is among the suite of test rocks and soils that was used in the development of the 2007/8 Phoenix Scout and is being used in the 2009 Mars Science Laboratory (MSL) missions. The MMS development team is using the simulant for research that centers on sampling tool interactions in icy soils. Herein we describe the physical properties and chemical composition of this new Mars simulant.     

Diffraction imaging in depth

High resolution imaging is of great value to an interpreter, for instance to enable identification of small scale faults, and to locate formation pinch-out positions. Standard approaches to obtain high-resolution information, such as coherency analysis and structure-oriented filters, derive attributes from stacked, migrated images. Since they are image-driven, these techniques are sensitive to artifacts due to an inadequate migration velocity; in fact the attribute derivation is not based on the physics of wave propagation. Diffracted waves on the other hand have been recognized as physically reliable carriers of high- or even super-resolution structural information. However, high-resolution information, encoded in diffractions, is generally lost during the conventional processing sequence, indeed migration kernels in current migration algorithms are biased against diffractions. We propose here methods for a diffraction-based, data-oriented approach to image resolution. We also demonstrate the different behaviour of diffractions compared to specular reflections and how this can be leveraged to assess characteristics of subsurface features. In this way a rough surface such as a fault plane or unconformity may be distinguishable on a diffraction image and not on a traditional reflection image.
We outline some characteristic properties of diffractions and diffraction imaging, and present two novel approaches to diffraction imaging in the depth domain. The first technique is based on reflection focusing in the depth domain and subsequent filtering of reflections from prestack data. The second technique modifies the migration kernel and consists of a reverse application of stationary-phase migration to suppress contributions from specular reflections to the diffraction image. Both techniques are proposed as a complement to conventional full-wave pre-stack depth migration, and both assume the existence of an accurate migration velocity.     

Sayh al Uhaymir 094: A new martian meteorite from the Oman desert

Abstract— Sayhal Uhaymir (SaU) 094 is a 223.3 g, partially crusted, strongly to very strongly shocked melanocratic olivine-porphyric rock of the shergottite group showing a microgabbroic texture. The rock consists of pyroxene (52.0–58.2 vol%)—dominantly prismatic pigeonite (En_60–68Fs_20–27Wo^7–9) associated with minor augite (En_46–49Fs_15–16Wo_28–31)—brown (shock-oxidized) olivine (Fo_65–69; 22.1–31%), completely isotropic interstitial plagioclase glass (maskelynite; An_50–64Or_0.3-0.9; 8.6–13.0%), chromite and titanian magnesian chromite (0.9-1.0%), traces of ilmenite (Ilm_80–86), pyrrhotite (Fe_92–100; 0.1-0.2%), merrillite (<<0.1%), and pockets (4.8-6.7%) consisting of green basaltic to basaltic andesitic shock glass that is partially devitrified into a brown to black product along boundaries with the primary minerals. The average maximum dimensions of minerals are: olivine (1.5 mm), pyroxene (0.3 mm) and maskelynite (0.3 mm). Primary melt inclusions in olivine and chromite are common and account for 0.1-0.6% of the rock. X-ray tomography revealed that the specimen contains ˜0.4 vol% of shock-melt associated vesicles, up to 3 mm in size, which show a preferred orientation. Fluidization of the maskelynite, melting and recrystallization of pyroxene, olivine and pyrrhotite indicate shock stage S6. Minor terrestrial weathering resulted in calcite-veining and minor oxidation of sulfides. The meteorite is interpreted as paired with SaU 005/008/051. The modal composition is similar to Dar al Gani 476/489/670/735/876, with the exception that neither mesostasis nor titanomagnetite nor apatite are present and that all phases show little zonation. The restricted mineral composition, predominance of chromite among the oxides, and abundance of olivine indicate affinities to the lherzolitic shergottites.     

Mineralbestand und Genesis des Eisgarner Granits im niederösterreichischen Waldviertel

Zusammenfassung Der jüngste Granit im österreichischen Moldanubikum ist der im nordwestlichen Waldviertel aufgeschlossene Eisgarner Granit.Das Gestein ist ein Zweiglimmergranit. Der Plagioklas (20–28 Vol.-%), selten zonar gebaut, hat einen durchschnittlichen An-Gehalt von 13%; daneben treten basischere Plagioklase (An bis 30%) als Einschlüsse im Mikroklin auf. Der Alkalifeldspat (25–38 Vol.-%) ist ein gegitterter, perthitisch entmischter Mikroklin mit Or 67–72%, Ab 24–28%, An 1,8–2,8%. Durch den Gehalt an Spurenelementen unterscheidet er sich stark von dem der älteren moldanubischen Granite. Die Triklinität liegt meist unter 0,90. Quarz tritt stellenweise als frühes Kristallisat auf, gehört aber im allgemeinen zu den Letztausscheidungen. Biotit (4–7 Vol.-%) bildel 2M-Polymorphe, ebenso Muskowit (6,5–7,5%), der entweder durch Reaktion von Andalusit mit der Schmelze oder aus Alkalifeldspat durch Reaktion mit der fluiden Phase hervorgegangen ist. Die Akzessorien Spinell, Rutil, Granat und Andalusit weisen darauf hin, daß der Granit durch Anatexis hochmetamorpher Gesteine (Silimanitgneise?) entstanden ist. Die Kristallisation verlief im Temperaturbereich zwischen ca 700° und 600°C bei einem Wasserdampfdruck von ca 5000 bar.

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Summary The youngest granite in the Moldanubian zone of Austria is theEisgarn granite which is exposed in the northwestern part of the Lower Austrian Waldviertel.The rock is a two-mica granite. The plagioclase (20–28 vol.-%) has an average An-content of 13% and shows only a weak zonary structure. Also more basic plagioclases (up to An 30) occur as inclusions in microcline. The alkalifeldspar (25–38 vol.-%) is a cross-hatched microcline perthite with the composition Or 67–72, Ab 24–28, An 1,8–2,8. It is distinguished from the microcline perthites of older granites by its content in trace elements. Its obliquity lies below 0,90. Partly the quartz is an early crystallisate, it is commonly a very late product, however. Biotite (4–7 vol.-%) forms the 2M polymorphe, as well as muscovite (6,5–7,5 vol.-%) which may develop by reaction of andalusite with the melt or from alkalifeldspar by reaction with the fluid phase. The accessories rutile, spinel, garnet and andalusite indicate a development of the granite by anatexis of high-metamorphic rocks (sillimanite-bearing gneisses?). Crystallization took place in the temperature range of 700° to 600°C under a water-vapor-pressure of 5000 bars.

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Mit 11 Texiabbildungen

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Herrn Prof. Dr.H. Leitmeier zum 80. Geburtstag gewidmet.