Evidence for K‐rich terranes on Vesta from impact spherules

Abstract— The howardite‐eucrite‐diogenite (HED) clan is a group of meteorites that probably originate from the asteroid Vesta. Some of them are complex breccias that contain impact glasses whose compositions mirror that of their source regions. Some K‐rich impact glasses (up to 2 wt% K₂O) suggest that in addition to basalts and ultramafic cumulates, K‐rich rocks are exposed on Vesta's surface. One K‐rich glass (up to 6 wt% K₂O), with a felsic composition, provides the first evidence of highly differentiated K‐rich rocks on a large asteroid. They can be compared to the rare lunar granites and suggest that magmas generated in a large asteroid are more diverse than previously thought.     

The basaltic shergottite Northwest Africa 856: Petrology and chemistry

Abstract— We report on the discovery of a new shergottite from South Morocco. This single stone weighing 320 g is referenced as Northwest Africa (NWA) 856 with Djel Ibone as a synonymous name. It is a fresh, fine‐grained basaltic rock consisting mainly of two pyroxenes (total ?68 vol%: 45% pigeonite, En_61‐16Wo_9–22Fs_26–68; 23% augite, En_46‐26Wo_34‐29Fs_21–43) and plagioclase converted to maskelynite (?23 vol%, Ab_43–57Or_1–5An_54‐36). Accessory minerals include merrillite, Cl‐apatite, pyrrhotite, ilmenite, ulvöspinel, silica (stishovite and glass), amorphous K‐feldspar and baddeleyite. Amorphous mixtures of maskelynite and silica occur most commonly as median layers inside maskelynite laths. In addition, melt pockets (?2 vol%) were recognized with relics of maskelynite, pyroxene and both dense silica glass and stishovite occurring as both grains and submicrometer needles. The compositions of the melt pockets are consistent with mixtures of maskelynite and pyroxenes with an average of ?50 vol% maskelynite. The meteorite is highly fractured at all scales. The bulk composition of NWA 856 has been measured for 44 elements. It is an Al‐poor ferroan basaltic rock which strongly resembles Shergotty and Zagami in its major and trace element composition. The nearly flat rare earth element (REE) pattern (La/Lu)_n = 0.9, is similar to that of Shergotty or Zagami and differs significantly from NWA 480, another Moroccan shergottite recently described. According to the U, Ba and Sr abundances, NWA 856 is not significantly weathered. The oxygen isotopes (δ18O = +5.03%, δ17O = +3.09%, and Δ17O = +0.47%) are in agreement with the martian origin of this meteorite. On the basis of grain size, pyroxene zoning and composition, abundance of silica inclusions associated with maskelynite, trace element abundances, REE pattern and oxygen isotopes, pairing with NWA 480 is excluded. The similarity with Shergotty and Zagami is striking. The only significant differences are a larger grain size, a greater abundance of silica and melt pockets, a slightly more restricted range of pyroxene compositions and the absence of significant mesostasis.     

Determination of parental magmas of HED cumulates: The effects of interstitial melts

Abstract— An evaluation of trapped melts effects during crystallization and subsolidus equilibration of cumulates is necessary to constrain the composition of their parental magmas. In this paper, a simple mass balance approach is described. It allows estimation of trace element abundances in these parental melts from phase compositions. It is used to discuss the genesis of cumulate eucrites and diogenites. The REE behavior is in full agreement with the view that cumulate eucrites formed from melts similar to noncumulate eucrites. Trapped melt fractions ranging from <10 wt% for Moama to ?30 wt% for Moore County were involved. The origin of diogenites is more complex. The assumption that eucrites and diogenites shared the same parental melts cannot satisfactorily explain the diversity of incompatible trace element ratios (e.g., Dy/Yb) observed in diogenitic orthopyroxenes, even if interstitial melt effects are taken into account. Moreover, some diogenites unambiguously crystallized from magmas displaying significant HREE (heavy rare earth elements) enrichments. More likely, diogenites formed from distinct batches of parental magmas, as previously proposed by Mittlefehldt (1994), Fowler et al. (1995), and Shearer (1997).     

Estimating periphyton standing crop in streams: a comparison of chlorophyll a sampling and visual assessments

Periphyton standing crop is often measured as chlorophyll a but there is increasing interest in using visual assessments of periphyton cover. Visual methods are rapid and require no sample analysis, but can be regarded as subjective with high inter-operator variability. We investigated variability in periphyton standing crop across operators, rivers and time, as determined visually and from chlorophyll a measurements. We found that visual assessments (comprising percentage cover estimates of up to eight periphyton categories) distinguished sites and occasions as effectively as chlorophyll a. Furthermore, an estimate of chlorophyll a could be derived from the visual assessments. Because our surveys were conducted in only three rivers, general applicability of the derivation of estimated chlorophyll a from the visual assessments warrants further investigation. The current recommendation of 20 views was sufficient for realistic visual assessments of average cover. Overall, our results indicated that inter-operator variability in visual assessments need not be a major concern, given adequate training.     

Metal‐saturated sulfide assemblages in NWA 2737: Evidence for impact‐related sulfur devolatilization in Martian meteorites

NWA 2737, a Martian meteorite from the Chassignite subclass, contains minute amounts (0.010 ± 0.005 vol%) of metal‐saturated Fe‐Ni sulfides. These latter bear evidence of the strong shock effects documented by abundant Fe nanoparticles and planar defects in Northwest Africa (NWA) 2737 olivine. A Ni‐poor troilite (Fe/S = 1.0 ± 0.01), sometimes Cr‐bearing (up to 1 wt%), coexists with micrometer‐sized taenite/tetrataenite‐type native Ni‐Fe alloys (Ni/Fe = 1) and Fe‐Os‐Ir‐(Ru) alloys a few hundreds of nanometers across. The troilite has exsolved flame‐like pentlandite (Fe/Fe + Ni = 0.5–0.6). Chalcopyrite is almost lacking, and no pyrite has been found. As a hot desert find, NWA 2737 shows astonishingly fresh sulfides. The composition of troilite coexisting with Ni‐Fe alloys is completely at odds with Chassigny and Nahkla sulfides (pyrite + metal‐deficient monoclinic‐type pyrrhotite). It indicates strongly reducing crystallization conditions (close to IW), several log units below the fO₂ conditions inferred from chromites compositions and accepted for Chassignites (FMQ‐1 log unit). It is proposed that reduction in sulfides into base and precious metal alloys is operated via sulfur degassing, which is supported by the highly resorbed and denticulated shape of sulfide blebs and their spongy textures. Shock‐related S degassing may be responsible for considerable damages in magmatic sulfide structures and sulfide assemblages, with concomitant loss of magnetic properties as documented in some other Martian meteorites.     

Theoretical studies on metal thioarsenites and thioantimonides: synergistic interactions between transition metals and heavy metalloids

Recently we established that the ternary complex, CuAsS(SH)(OH) has an unusually high stability and makes a large contribution to the total concentrations of both Cu and As in sulfidic solutions equilibrated with Cu and As sulfide minerals. This ternary complex has an unusual structure, containing a bond which is formally Cu(I)–As(III), along with a broken As–S bond. We have now found that complexes with similar structures exist for Au⁺ and Tl⁺ coordinated to AsS(SH)(OH)^-. However, such a direct metal–metalloid bond is not a requirement for stability. In fact, TlAsS(SH)(OH) is unstable while AuAsS(SH)(OH) is highly stable (compared to the aquo ion). Zn²⁺, Cd²⁺, Hg²⁺ and Pb²⁺ also form bonds to the As of AsS(SH)(OH), but without breaking any As–S bonds, and HgAsS(SH)(OH)⁺ and PbAsS(SH)(OH)⁺ are particularly stable complexes. Calculated structures are shown for these complexes, gas-phase energies are calculated, and formation constants in aqueous solution are estimated. The SbS(SH)(OH)^- ion forms analogous complexes, with similar stabilities. However, the Au⁺ complex of SbS(SH)(OH)^- is slightly less stable than the Cu⁺ complex, opposite to the order found for the AsS(SH)(OH)^- ligand. The Au⁺ and AuSH complexes of AsSSHOH^- or AsS(SH)₂ ^- may be implicated in "invisible gold" in arsenian pyrites. Vibrational frequencies are given for the AuAsS(SH)₃ ^- complex and the XANES energies of this complex and Au(SH)₂ ^- are compared. The existence of such strong complexes may explain the many correlations observed between the concentrations of coinage metals and metalloids.     

DETERMINATION OF RARE EARTH ELEMENTS IN SIXTEEN SILICATE REFERENCE SAMPLES BY ICP-MS AFTER TM ADDITION AND ION EXCHANGE SEPARATION

This study reports the results of thirteen rare earth elements (REE) in sixteen geochemical reference samples. The analytical procedure involved dissolution of a whole rock or mineral separate, spiking with Tm, and separation of the REE using a simple ion-exchange chromatography procedure. The resulting REE solutions were analysed by ICP-MS. The results are compared with literature values. The agreement between our data and recommended or ID-TIMS values is very favorable. The precision of the technique is better than 5% (2) for all the REE.     

Magnetically enhanced coagulation of very small iron grains

Laboratory experiments, in which very small (approximately 20 nm) grains are produced in the presence of a magnetic field on the order of 100 Gauss in a low-pressure hydrogen atmosphere, have demonstrated that such smokes can become permanently magnetized. We show that magnetization results in an enormous enhancement in the coagulation efficiency of such materials even in the absence of external magnetic fields. Small iron grains should have been produced in the solar nebula by thermal processing of preexisting interstellar grains. If such processing occurred via high-energy electromagnetic events then the resultant magnetized grains could have triggered the formation of centimeter- to meter-sized protoplanetessimals by acting as "nets" capable of sweeping up nonconductive silicates suspended in the gas. It is possible that the presence of conductive fractal aggregates observed in modern-day protostellar disks could be explained by the enhanced coagulation efficiency of very small magnetized iron particles.     

A felsic dyke swarm formed under the sea: the Archean Hunter Mine Group,south-central Abitibi Belt,Quebec, Canada

An extensive rhyolitic dyke swarm has intruded subaqueous pyroclastic deposits, iron-formations, hyaloclastite breccias and lava flows of the 2730 Ma Hunter Mine Group (HMG) in the south-central part of the Archean Abitibi belt, Quebec. The dyke swarm has a minimum width of 500 m and can be traced perpendicular to the section for 2.4 km. Based on crosscutting relationships, chilled margins, quartz content and colour, five distinct dyke generations have been established. Each dyke generation has several magmatic pulses as indicated by parallel rows of columnar joints. Absence of brecciation between parallel rows suggests extremely brief intervals between magma pulses. The central parts of most dykes display inverted V-shaped patterns of columnar-joint convergence, inferred to indicate differential cooling during the late stages of dyke propagation. The dykes commonly display delicate spherulites suggesting rapid cooling, solidification temperatures between 400 and 600°C and penecontemporaneous devitrification. Quartz-feldspar aggregates in the groundmass have locally developed microgranophyric textures. Large spherulites near the chilled margins probably formed at temperatures below 400°C. Percolation of abundant water throughout the dyke complex is suggested by ubiquitous prominent chilled dyke margins. Development of a chilled margin 500 m along one dyke suggests that water percolated at least 500 m below the water/rock interface. Because the dykes intruded subaqueous pyroclastic deposits of similar composition, dyke emplacement below the sea floor is inferred. Interstratification of pillowed flows and brecciated pillowed flows containing rhyolite fragments at the top of the 4–5-km-thick sequence indicates that the central felsic complex probably never emerged during its evolutionary history, supporting the contention that the felsic dyke complex was emplaced beneath the Archean sea floor.     

The ‘suitcase hypothesis’: Can entrainment of meroplankton by eddies provide a pathway for gene flow between Madagascar and KwaZulu-Natal,South Africa?

Similarities in the marine fauna found off the coasts of southern Madagascar and KwaZulu-Natal Province (KZN), South Africa, led to the development of the ‘suitcase project,’ with the aim of establishing whether eddies that form off southern Madagascar may package and transport biological material across the Mozambique Channel, facilitating connectivity and gene flow. Meroplankton (larval stages of fishes and benthic invertebrates) were collected on the Madagascan shelf and along a transect through a cyclonic eddy in the Mozambique Channel. The samples were analysed using microscopy and DNA barcoding, seeking to identify species known to be common to both the southeast coast of Madagascar and the east coast of South Africa and thereby to reveal potential indicators of connectivity between these regions. The greatest zooplankton biovolume in the upper 200 m occurred on the shelf, followed by in the western part of the eddy and in the region outside the eddy to the west, and was lowest in the region outside the eddy to the east. The meroplankton were dominated by taxa of coastal origin and these were also most abundant on the shelf and in the western part of the eddy, with the lowest abundances in the region outside the eddy to the east. The findings show greater zooplankton biovolumes and larval abundances and the presence of reef-associated larval assemblages on the Madagascan shelf and along the transect through the cyclonic eddy, providing support for the suitcase hypothesis that planktonic organisms are entrained within eddies as they propagate south-westwards of the Madagascan shelf.