Metamorphic and post-metamorphic fluid flow in the low-grade rocks of the Harlech Dome, north Wales

A combination of fluid inclusion, stable isotope and geochemical techniques has been used to study the nature of fluids present and their behaviour during Caledonian low-grade metamorphism of the Harlech Dome, north Wales. Fluid inclusion studies show that in most of the metasedimentary sequence the peak metamorphic fluid was an aqueous Na–K–Cl brine but in the graphitic Clogau Formation and in parts of the overlying Maentwrog Formation immiscible H₂O-rich and CH₄-rich fluids coexisted. Late-stage inclusions are of calcium-rich brine and a dilute aqueous fluid. The chemical composition of chlorite in metamorphic veins and rocks varies between different formations and quartz-oxygen isotopic compositions show considerable variation between different units. Both of these features are taken to indicate that there was little or no pervasive movement of fluid between different units at the peak of metamorphism. After the metamorphic peak there was focused flow of fluid upward through the sequence along fractures, in response to end-Caledonian uplift and unloading. Where the migrating fluid crossed the graphitic shales, interaction between the fluid and the shales gave rise to the formation of the auriferous veins of the Dolgellau Gold Belt. Subsequent to this mineralizing event there was widespread development of ¹⁸O-enriched calcites and micas. In the case of vein minerals it is possible that these crystallized directly from late-stage fluids at lower temperature than the quartz in the same veins. Alternatively, the original vein minerals may have re-equilibrated with later ¹⁸O-enriched or cooler fluid. In the case of muscovites in the rock matrix it is proposed that the isotopically heavy compositions are the result of re-equilibration of initially light grains with an introduced fluid, requiring considerable influx of fluid. This event may relate to either of two late-stage fluids observed as inclusions.     

The parallel roads of Glen Roy and adjacent glens, Scotland

The width and angle of outward slope of the parallel roads of glens Roy, Gloy and Spean have been measured at 787 points and road volume has been approximately determined at 236 points. Analyses of the measurements show that road widths are highly correlated inversely with the slopes of the hill sides on which they occur. Road volume is unrelated to hillside slope or to geology, but is highly correlated with length of fetch from south-west. Locally the roads resulted from fluvial (including fluvioglacial) depostion, but normally they are the result of erosion at the back and deposition at the front. It is maintained that the roads were not formed only by wave action but also by powerful frost action in the critical environment of the slightly varying lake level. It is inferred that each road was at first formed extremely quickly and, thereafter, developed very slowly     

Petrology and geochemistry of the Northwest Africa 3368 eucrite

Abstract– Analysis of the mineralogy, isotopic, and bulk compositions of the eucrite meteorites is imperative for understanding their origin on the asteroid 4 Vesta, the proposed parent body of the HED meteorites. We present here the petrology, mineral compositions, and bulk chemistry of several lithic components of the new brecciated basaltic eucrite Northwest Africa (NWA) 3368 to determine if all the lithologies reflect formation from one rock type or many rock types. The meteorite has three main lithologies: coarse‐ and fine‐grained clasts surrounded by a fine‐grained recrystallized silicate matrix. Silicate compositions are homogeneous, and the average rare earth element pattern for NWA 3368 is approximately 10× CI chondrites with a slight negative Eu anomaly. Major and trace element data place NWA 3368 with the Main Group‐Nuevo Laredo trend. High‐Ti chromites with ilmenite exsolution lamellae provide evidence of NWA 3368’s history of intense metamorphism. We suggest that this meteorite underwent several episodes of brecciation and metamorphism, similar to that proposed by Metzler et al. (1995) . We conclude that NWA 3368 is a monomict basaltic eucrite breccia related to known eucrites in texture and in mineral, bulk, and oxygen isotopic composition.     

Maribo—A new CM fall from Denmark

Abstract– Maribo is a new Danish CM chondrite, which fell on January 17, 2009, at 19:08:28 CET. The fall was observed by many eye witnesses and recorded by a surveillance camera, an all sky camera, a few seismic stations, and by meteor radar observatories in Germany. A single fragment of Maribo with a dry weight of 25.8 g was found on March 4, 2009. The coarse‐grained components in Maribo include chondrules, fine‐grained olivine aggregates, large isolated lithic clasts, metals, and mineral fragments (often olivine), and rare Ca,Al‐rich inclusions. The components are typically rimmed by fine‐grained dust mantles. The matrix includes abundant dust rimmed fragments of tochilinite with a layered, fishbone‐like texture, tochilinite–cronstedtite intergrowths, sulfides, metals, and carbonates often intergrown with tochilinite. The oxygen isotopic composition: (δ¹⁷O = ?1.27‰; δ¹⁸O = 4.96‰; Δ¹⁷O = ?3.85‰) plots at the edge of the CM field, close to the CCAM line. The very low Δ¹⁷O and the presence of unaltered components suggest that Maribo is among the least altered CM chondrites. The bulk chemistry of Maribo is typical of CM chondrites. Trapped noble gases are similar in abundance and isotopic composition to other CM chondrites, stepwise heating data indicating the presence of gas components hosted by presolar diamond and silicon carbide. The organics in Maribo include components also seen in Murchison as well as nitrogen‐rich components unique to Maribo.     

Geochemistry of intermediate olivine‐phyric shergottite Northwest Africa 6234, with similarities to basaltic shergottite Northwest Africa 480 and olivine‐phyric shergottite Northwest Africa 2990

Abstract— The newly found meteorite Northwest Africa 6234 (NWA 6234) is an olivine (ol)‐phyric shergottite that is thought, based on texture and mineralogy, to be paired with Martian shergottite meteorites NWA 2990, 5960, and 6710. We report bulk‐rock major‐ and trace‐element abundances (including Li), abundances of highly siderophile elements, Re‐Os isotope systematics, oxygen isotope ratios, and the lithium isotope ratio for NWA 6234. NWA 6234 is classified as a Martian shergottite, based on its oxygen isotope ratios, bulk composition, and bulk element abundance ratios, Fe/Mn, Al/Ti, and Na/Al. The Li concentration and δ⁷Li value of NWA 6234 are similar to that of basaltic shergottites Zagami and Shergotty. The rare earth element (REE) pattern for NWA 6234 shows a depletion in the light REE (La‐Nd) compared with the heavy REE (Sm‐Lu), but not as extreme as the known “depleted” shergottites. Thus, NWA 6234 is suggested to belong to a new category of shergottite that is geochemically “intermediate” in incompatible elements. The only other basaltic or ol‐phyric shergottite with a similar “intermediate” character is the basaltic shergottite NWA 480. Rhenium‐osmium isotope systematics are consistent with this intermediate character, assuming a crystallization age of 180 Ma. We conclude that NWA 6234 represents an intermediate compositional group between enriched and depleted shergottites and offers new insights into the nature of mantle differentiation and mixing among mantle reservoirs in Mars.     

Sedimentological–ichnological model for tide‐dominated shelf sandbodies: Lower Cambrian Gog Group of western Canada

Dunes and bars are common elements in tide‐dominated shelf settings. However, there is no consensus on a unifying terminology or a systematic classification for thick sets of cross‐stratified sandstones. In addition, their ichnological attributes have hardly been explored. To address these issues, the properties, architecture and ichnology of compound cross‐stratified sandstone bodies contained in the Lower Cambrian Gog Group of the southern Canadian Rocky Mountains are described here. In these transgressive sandstones, five types of compound cross‐stratified sandstone are distinguished based on foreset geometry, sedimentary structures and internal heterogeneity. These represent four broad categories of subtidal sandbodies: (i) compound‐dune fields; (ii) sand sheets; (iii) sand ridges; and (iv) isolated dune patches; tidal bars comprise a fifth category but are not present in the Gog Group. Compound‐dune fields are characterized by sigmoidal and planar cross‐stratified sandstone in coarsening‐upward and thickening‐upward packages (Type 1); these are mostly unburrowed, or locally contain representatives of the Skolithos ichnofacies, but are intercalated with intensely bioturbated sandstone containing the archetypal Cruziana ichnofacies. Sand‐sheet complexes, also composed of compound dunes, cover more extensive subtidal areas, and comprise three adjacent subenvironments: core, front and margin. The core is characterized by thick‐bedded sets of cross‐stratified sandstone (Type 2). A decrease of bedform size at the front is recorded by wedges of thinner‐bedded, low‐angle and planar cross‐stratified sandstone (Type 3) exhibiting dense Skolithos pipe‐rock ichnofabric. The margin is characterized by interbedded sandstone and mudstone, and hummocky cross‐stratified sandstone. Sand‐sheet deposits exhibit clear trends in trace‐fossil distribution along the sediment transport path, from non‐bioturbated beds in the core to Skolithos ichnofacies at the front, and a depauperate Cruziana ichnofacies at the margin. Tidal sand ridges are large elongate sandbodies characterized by large sigmoid‐shaped reactivation surfaces (Type 4). Sand ridges display clear ichnological trends perpendicular to the axis of the ridge, with no bioturbation or a poorly developed Skolithos ichnofacies in the core, a depauperate Cruziana ichnofacies in lee‐side deposits, and Cruziana ichnofacies at the margin. While both tidal ridges and tidal bars migrate by means of lateral accretion, the latter occur in association with channels while the former do not. Because tidal bars tend to occur in brackish‐water marginal‐marine settings, their ichnofauna are typically of low diversity, representing a depauperate Cruziana ichnofacies. Isolated dune patches developed on sand‐starved areas of the shelf, and are represented by lenticular sandbodies with sigmoidal reactivation surfaces (Type 5); they typically lack trace fossils, but the interfingering muddy deposits are intensely bioturbated by a high‐diversity fauna recording the Cruziana ichnofacies. The variety of sandbody types in the Gog Group reflects varying sediment supply and location on the inner continental shelf. These, in turn, governed substrate mobility, grain size, turbidity, water‐column productivity and sediment organic matter which controlled trace fossil distribution.     

Microscopic calcite dendrites in cold-water tufa: implications for nucleation of micrite and cement

Dendritic calcite forms in an active cold-water tufa system in association with extracellular polymeric substances (EPS) that discontinuously coat bryophytes and cyanobacteria. Dendrites consist of 100–200 nm thick calcite fibres that form 3D lattice-like domains. In each dendrite domain, fibres have three structurally equal orientations, which correspond in disposition to radii from the centre of a calcite unit cell to the convex triple face junctions on its surface. Fibres do not form in the orientation of the c-axis. The external form of each dendrite has the shape of half of a shortened octahedron, with an upper triangular surface parallel to the substrate. Dendrite nucleation takes place on or in microbial EPS, whether microbial cells are present or not, and is probably effected by attraction of Ca²⁺ cations to negatively charged EPS, together with CO₂-degassing and concomitant pH increase of supersaturated spring water in stream splash zones. Ensuing dendrite growth is abiogenic and controlled by diffusion. Dendrite c-axes are perpendicular to the substrate, probably because the negative charge of EPS forces the orientation of Ca²⁺ and CO planes within the developing dendrite crystal to be parallel to the EPS film surface. Dendrites are eventually filled and overgrown by solid, syntaxial calcite, which gradually and completely obliterates the dendrites as more familiar calcite crystal forms develop. No trace of the dendritic nucleus remains in the rock record. Calcite crystal nucleation may take place by this mechanism in many marine and meteoric settings, given that microbial EPS is now assumed to be virtually ubiquitous in these environments. This phenomenon could contribute to the development of familiar fabrics such as marine micrite cement and fibrous calcite cement, radial ooids, peloids, ‘abiogenic’ stromatolites, sea floor precipitates, microbialites, tufa, travertine, speleothems, and some meteoric cements. It may also contribute to the substrate-normal orientation of c-axes of common cement fabrics.     

Interstadial and last interglacial deposits covered by till in Scotland: a reply

Caseldine & Edwards ( Boreas , Vol. 11, pp. 119–122) have misrepresented assessments of published evidence in the paper by Sissons 1981 ( Boreas , Vol. 10, No. 1). They have not used radiocarbon dates objectively and argue illogically. A different interpretation for the Teindland site is suggested.