Stromatolites of the lower Missoula Group (Middle Proterozoic), Belt Supergroup,Glacier National Park,Montana

The mode of formation and environmental setting of stromatolites from the lower Missoula Group (ca. 1.1·10⁹ years old) in Glacier National Park, Montana, have been determined. The stromatolite-bearing interval in the lower Missoula Group was deposited in a shallow, intermittently exposed setting of very low relief, the stromatolites forming during periods of submergence. In situ carbonate precipitation was the dominant process involved in the formation of encrusting stromatolitic laminae. This precipitate was deposited within, and probably beneath, algal mats, most likely as a result of the photosynthetic removal of carbon dioxide by the mat-building microscopic algae. Calcite also was precipitated in several types of open-space structures occurring within these stromatolites. Other laminae were produced by the organic stabilization of detrital particles; by the solely physical accumulation of terrigenous material; and probably, by bacterially induced precipitation of iron sulfide which was later oxidized to form hematite layers.Three forms of filamentous microfossils, two of which appear to be oscillatoriacean cyanophytes and the third of which is probably either a cyanophyte or filamentous bacterium, have been detected in these structures. In addition, hematitic pillar-shaped microstructures, interpreted to have been produced by filamentous bacteria, are abundant locally.In gross morphology, the lower Missoula Group stromatolites are simple, unbranched, domal structures ranging from several millimeters to several decimeters in both height and diameter. Physical conditions played a major role in determining the macrostructure of these stromatolites. Of particular importance were the shape of the positive sediment-surface irregularities upon which the stromatolites initially formed, the rate of sedimentation between stromatolite colonies, and the deposition of layers of terrigenous material on stromatolite growth surfaces. The effect of biological factors on stromatolite structure is clearly seen in those portions of stromatolites relatively free of terrigenous material; biological activity was apparently greatest on positive irregularities of the growth surface, resulting in preferential enhancement of such irregularities and development of second- and higher-order hemispheroidal structures.     

Estimation of seismic hazard and risks for the Himalayas and surrounding regions based on Unified Scaling Law for Earthquakes

To estimate seismic hazard, the basic law of seismicity, the Gutenberg–Richter recurrence relation, is applied in a modified form involving a spatial term: $\log N\left( {M,\;L} \right) = A - B\left( {M - 5} \right) + C\log L$ , where N(M,L) is the expected annual number of earthquakes of a certain magnitude M within an area of linear size L. The parameters A, B, and C of this Unified Scaling Law for Earthquakes (USLE) in the Himalayas and surrounding regions have been studied on the basis of a variable space and time-scale approach. The observed temporal variability of the A, B, and C coefficients indicates significant changes of seismic activity at the time scales of a few decades. At global scale, the value of A ranges mainly between ?1.0 and 0.5, which determines the average rate of earthquakes that accordingly differs by a factor of 30 or more. The value of B concentrates about 0.9 ranging from under 0.6 to above 1.1, while the fractal dimension of the local seismic prone setting, C, changes from 0.5 to 1.4 and larger. For Himalayan region, the values of A, B, and C have been estimated mainly ranging from ?1.6 to ?1.0, from 0.8 to 1.3, and from 1.0 to 1.4, respectively. We have used the deterministic approach to map the local value of the expected peak ground acceleration (PGA) from the USLE estimated maximum magnitude or, if reliable estimation was not possible, from the observed maximum magnitude during 1900–2012. In result, the seismic hazard map of the Himalayas with spatially distributed PGA was prepared. Further, an attempt is made to generate a series of the earthquake risk maps of the region based on the population density exposed to the seismic hazard.     

Incorporation in soil of barytes derived from mineral veins in Southwest England

Highest Ba concentrations in the Bridford area of Devon, U.K., are found in the A horizon of soil downslope from barytes mineralization. Most Ba is present as BaSO₄ mechanically weathered from barytes veins. Ba values are greatest in the coarse silt fraction of the A, B, and C horizons. Owing to the large sand-size content of the soil, however, 80–90% of the total Ba occurs in the sand fraction. Two distinct size populations occur resulting from independent abrasion mechanisms. Two main stages are considered responsible for dispersion of BaSO₄. Cryoturbation and solifluction processes operating in the Pleistocene initially shattered, mixed and transported barytes during head formation. Subsequent soil development and creep in Recent times has led to anomalous A horizon values. Implications for prospecting are that in similar areas both A and B strata could prove most useful and that exact estimates of lode positions could be made if the modifying influence of soil creep is taken into consideration.     

Biogeochemical Model and Simulation of the Effect of Precambrian Algae on the Formation Process of Certain Laminated Cherts

The source of silica in the formation of the Precambrian laminated cherts has long remained a problem to be solved. Through experiments on cherts and living blue-green algae, the authors found that the collected chert samples probably come from primary deposits, and there is a great biomass of fossil algae in chert, among which the filamentous algae can be compared with the living blue-green algae Oscillatoria, that a higher Pco_2 of the gas would be favourable for the increase of the biomass of living blue-green algae and consequently raises the pH value of the water body; and that lack of free oxygen and a higher concentration of SiO_2 in the water have no apparent influence on the biomass of blue-green algae. Based on the evidence above, a biogeochemical model concerning the origin of Precambrian laminated chert has been set up, in which the. photosynthesis of algae under the presumed atmospheric conditions of the Precambrian might raise the pH value of the water body and promote the dissolution of silicate minerals, thus providing a source of colloid SiO_2 for the formation of Precambrian laminated chert.Furthermore, a simulation experiment device has been designed successfully, which can control the temperature(30 ± 0.5°), Pco_2(50662.5 Pa) and Po_2(about zero Pa) of the gas, the rate of photosynthesis of algae and the movement of the water carrying opal. In the simulation experiments, separate measurements have been made on the rate of photosynthesis of algae, pH value and concentration of SiO_2 of the water body, with the results indicating that under the conditions similar to the presumed Precambrian atmosphere, the photosynthesis of algae can make the pH value of the water body go up steadily to over 9.7, leading to the dissolution of the silicate minerals, with the concentration of SiO_2 measured reaching as high as 84 mg/l. Finally, through the vaporation of water, a phenomenon of colloid floccilation has been observed.The simulation experiment has verified the proposed biogeochemical model illustrating the origin of Precambrian laminated chert. Moreover, the device and method of its kind may also be applied to the research on the relationship of the Precambrian algae with the formation of some other mineral deposits such as of Fe, Mn, U and carbonates.     

Monazite, iron oxide and barite exsolutions in apatite aggregates from CCSD drillhole eclogites and their geological implications

We have identified abundant exsolutions in apatite aggregates from eclogitic drillhole samples of the Chinese Continental Scientific Drilling (CCSD) project. Electron microscope and laser Raman spectroscopy analyses show that the apatite is fluorapatite, whereas exsolutions that can be classified into four types: (A) platy to rhombic monazite exsolutions; (B) needle-like hematite exsolutions; (C) irregular magnetite and hematite intergrowths; and (D) needle-like strontian barite exsolutions. The widths and lengths of type A monazite exsolutions range from about 6-10 μm (mostly 6 μm) and about 50-75 μm, respectively. Type B exsolutions are parallel with the C axis of apatite, with widths ranging from 0.5 to 2 μm, with most around 1.5 μm, and lengths that vary dramatically from 6 to 50 μm. Type C exsolutions are also parallel with the C axis of apatite, with lengths of ∼30-150 μm and widths of ∼10 to 50 μm. Type D strontian barite exsolutions coexist mostly with type B hematite exsolutions, with widths of about 9 μm and lengths of about 60-70 μm. Exsolutions of types B, C and D have never been reported in apatites before. Most of the exsolutions are parallel with the C axis of apatite, implying that they were probably exsolved at roughly the same time. Dating by the chemical Th-U-total Pb isochron method (CHIME) yields an U-Pb isochron age of 202 ± 28 Ma for monazite exsolutions, suggesting that these exsolutions were formed during recrystallization and retrograde metamorphism of the exhumed ultrahigh pressure (UHP) rocks. Quartz veins hosting apatite aggregates were probably formed slightly earlier than 202 Ma. Abundant hematite exsolutions, as well as coexistence of magnetite/hematite and barite/hematite in the apatite, suggest that the oxygen fugacity of apatite aggregates is well above the sulfide-sulfur oxide buffer (SSO). Given that quartz veins host these apatite aggregates, they were probably deposited from SiO₂-rich hydrous fluids formed during retrogression of the subducted slab. Such SiO₂-rich hydrous fluids may act as an oxidizing agent, a feasible explanation for the high oxygen fugacity in convergent margin systems.     

Possible ecological and environmental significance of the predominance of even-carbon number C20-C30n-alkanes

Even-carbon number C₂₂-C₂₈n-alkanes, are present in higher concentrations than their C₂₁-C₂₉ odd-carbon number homologs in an organic-rich rock of Mississippian (ca. 310 × 10⁶-yr-old) age, which contains algae-like remnants. Environmental indicators associated with this organic-rich rock permitted environmental comparison with ancient and modern environments that contain n-alkanes which exhibit similar distributions. The results of these comparisons suggest that even-carbon number n-alkanes are preferentially produced in highly saline, carbonate environments where aerobic and anaerobic bacteria have subsisted on the remains of blue-green algae.     

Trace element composition of magnetite from the Xinqiao Fe–S(–Cu–Au) deposit,Tongling, Eastern China: constraints on fluid evolution and ore genesis

The Xinqiao deposit is one of several polymetallic deposits in the Tongling ore district. There are two types of mineralization in the Xinqiao: skarn-type and stratiform-type. The skarn-type mineralization is characterized by iron oxides such as magnetite and hematite, whereas stratiform-type mineralization is characterized by massive sulfides with small amounts of magnetite and hematite. We defined three types of ores within the stratiform-type mineralization by the mineral assemblages and ore structures. Type I ore is represented by magnetite crosscut by minor calcite veins. Type II is a network ore composed of magnetite and crosscutting pyrite. Type III is a massive ore containing calcite and hematite. Type I magnetite is characterized by highly variable trace element content, whereas Type II magnetite has consistently higher Si, Ti, V, and Nb. Type III magnetite contains more In, Sn, and As than the other two types. Fluid–rock interaction, oxygen fugacity (fO₂), and temperature (T) are the main factors controlling element variation between the different magnetite types. Type I magnetite was formed by more extensive fluid–rock interaction than the other two types at moderate fO₂ and T conditions. Type II magnetite is thought to have formed in relatively low fO₂ and high-T environments, and Type III in relatively high fO₂ and moderate-T environments. Ca?+?Al?+?Mn and Ti?+?V discrimination diagrams show that magnetite in the Xinqiao deposit is hydrothermal in origin and is possibly linked with skarn.     

Phase transformations in ABO 4 type compounds under high pressure

For ABO ₄ type ternary oxides, high pressure phase transformations known up to the present are reviewed, and an attempt is made to explain and predict crystal structures of their high pressure phases. When ABO ₄ type compounds are plotted based on the two variables, k=r _A /r _B and t=(r _A +r _B )/2r _O, where r _A , r _B , and r _O are the ionic radii of A and B cations and divalent oxygen, they can be classified into the major structure types. It is found empirically that a compound basically transforms to the structure type isostructural with a compound lying in a classified area with the same k and larger t values in the diagram.     

Occurrence of long chain 1,14-diols in Apedinella radians

Long chain 1,14-diols have been reported in diatoms of the genus Proboscia and applied as specific biomarker lipids for such algae. We report here the presence of saturated C₂₈, C₃₀ and C₃₂ 1,14-diols in a culture of the marine heterokont alga Apedinella radians (Class Dictyochophyceae, order Pedinellales). Apedinella species occur globally, although predominantly in estuarine waters, so the finding has potential implications for the use of long chain 1,14-diols as biomarkers of Proboscia diatoms and as an indicator of upwelling.     

Dynamics of the stationary eddies in the lower stratospheric circulation in the wavenumber domain and monsoon activity

Meridional transports of momentum and sensible heat have been computed using the Fourier coefficients of the geopotential heights at 50 mb and 30 mb levels for each month during the years 1972 to 1979. Seasonal variations in these transports and in the energy conversionsC (K _E ,K _Z ) andC (A _z ,A _E ) in the contrasting years of monsoon activity have been discussed. The transport and the conversion terms undergo annual variations only with winter maxima and summer minima. Large northward transport of momentum and of sensible heat in the low latitude summer and in the high latitude preceeding winter suggests a linkage between monsoon activity and high latitude stratosphere. Seasonal variations in the energy conversion termsC (K _E ,K _Z ) andC (A _Z ,A _E ) conform with the changes in the system in the years of widely differing monsoon activity both in low and high latitudes. Large conversion of energy fromA _Z toA _E in low latitude summer indicates weakening of the existing meridional temperature gradient, in bad monsoon years.     

Diverse Dietary Responses by Saltmarsh Consumers to Chronic Nutrient Enrichment

We examined the effect of whole-ecosystem nutrient enrichment on herbivory in saltmarsh creek-wall habitats in the Plum Island Estuary (Massachusetts, USA). Located between the macrophyte-dominated high marsh and adjoining mudflats, creek walls are steep vertical habitats vegetated with productive filamentous algae and associated epiphytes. Annual nitrate and phosphate loading rates were increased approximately ×10–15 in creeks mimicking short-term (2-month) and chronic (6-year) eutrophication. We assessed the diets of epifaunal invertebrates (three gastropods and one amphipod species) that potentially graze on benthic algae using natural isotope abundance data and per capita grazing rate measurements derived from ¹³C prelabeled algae. Substantial dietary contributions from benthic algae were observed in all consumers even though previous research has indicated most rely on Spartina detritus as the principal food resource. The amphipod Orchestia grillus and the snail Melampus bidentatus grazed benthic algae in excess of 500 μg algal C g C^?1 h^?1, whereas the snail Nassarius obsoletus and hydrobiid snails grazed at lower rates. Few dietary changes were detected with short-term enrichment. Algal grazing rates of N. obsoletus and M. bidentatus increased with chronic enrichment probably as a functional response to increased algal productivity. O. grillus grazed at a high rate and parasitic infection did not affect its consumption of benthic algae. The abundance and frequency of occurrence of O. grillus on creek-wall habitats increased with chronic nutrient enrichment suggesting amphipods contribute to top–down control on benthic algae and slow algal growth as nutrient enrichment occurs.     

Geochemical implications of induced changes in C13 fractionation by blue-green algae

The photosynthetic fractionation of carbon isotopes by blue-green algae in laboratory culture is dependent in a non-linear fashion on the CO₂ concentration in the feed gas. For the three species tested, the minimum fractionation occurred at a CO₂ concentration of 0.2% in air and was approximately zero for the two marine species tested. Enrichment of C¹² in the reduced carbon is not an inevitable result of photosynthetic carbon fixation. Temperature and pH had no detectable effect on fractionation. The maximum fractionation observed in the laboratory cultures or in recent blue-green algal mats was 18‰. Differences in the isotope ratio of coexisting oxidized and reduced carbon in Precambrian stromatolites are as great as 31‰. Present carbon isotopic evidence is not consistent with the idea that blue-green algae were major contributors to the organic matter in Precambrian sediments.     

Carbon and hydrogen isotopic compositions of algae and bacteria from hydrothermal environments,Yellowstone National Park

Stromatolites forming today on a small scale in hydrothermal environments are chemical and biological analogues of much larger Precambrian formations. Carbon isotopic composition varied as a function of CO₂ concentration, pH, and species composition. Stratiform, layered stromatolites grew in silica-depositing springs at 55° to 70°C; they consisted mainly of a unicellular alga, Synechococcus, and a filamentous, photosynthetic bacterium, Chloroflexus. These thermophiles become enriched in ¹²C as the concentration of carbon dioxide in the effluent waters increases. At a concentration of 40 ppm total inorganic C, and δ¹³C of organic carbon was ～ ?12%., whereas at 900 ppm total inorganic C, the δ¹³C of similar species was ～ ?25%.. Conical stromatolites or conophytons (principally a filamentous, blue-green alga Phormidium and Chloroflexus) grew at 40°-55°C. In older, broader conophytons, Chloroflexus was the dominant organism. Their δ¹³C values were ～ ?18%. in a variety of hot springs. In carbonate-depositing springs, i.e., carbon dioxide saturated, conophytons and stromatolites consisting of a variety of blue-green algae and photosynthetic bacteria had the most negative δ¹³C values (to ?30%.). These carbon isotope ratios are directly comparable to carbon isotope ratios of kerogen from Precambrian stromatolites. The presence and activity of methanogenic bacteria or heterotrophic, aerobic and anaerobic bacteria did not alter significantly the δ¹³C of the original organic matter.The hydrogen isotopic fractionation between thermophilic organisms and water is 0 to ?74 for temperatures of 85° to 46°C, respectively. Acidophilic algae fractionated hydrogen isotopes to a lesser extent than did the photosynthetic organisms inhabiting neutral pH springs. Because organic matter retains some of its original isotopic signature, relationships of CO₂ levels, pH, temperature, and species composition between modern stromatolites and their environment and those of the Precambrian can be inferred.     

Magnetic fields in methanol maser condensations based on data for related regions. Seven sources: Observational parameters

Results of polarization observations of gas-dust condensations obtained on the Nançay radio telescope in the 1665 and 1667 MHz OH lines in all four Stokes parameters are reported. Seven OH maser sources associated with methanol masers were selected for this study. The goal was to estimate the magnetic fields in methanol condensations from the Zeeman splitting of OH maser lines associated with the methanol masers. The Gaussian parameters of features in the OH spectra are presented, and their polarization parameters are estimated: the degree of circular polarization m _C , flux density in linear polarization p, and degree of linear polarization m _L . The magnetic field intensity B has been estimated from the Zeeman splitting of the OH lines and approximation of the Stokes parameter V from the derivative of Stokes parameter I. B varies from ≤0.5 to 1.4 mG for different sources. The association of OH masers with methanol emission has been analyzed; the magnetic fields of OH masers in interstellar condensations associated with Class I methanol masers can be determined more reliably than the fields in interstellar condensations with OH masers associated with Class II methanol emission, and have higher values. The sizes of the studied regions suggest they may be bound structures such as Bok globules, small IRDC clouds, or protoplanetary disks.     

Xenotime–hematite aggregates on opaline filaments: evidence for biomineralization in weathered siliciclastic rocks, Capanema, Quadrilátero Ferrífero of Minas Gerais, Brazil

Dissolution cavities in weathered pebbly quartzite of the ~2.5-Ga Moeda Formation at Capanema, Quadrilátero Ferrífero of Minas Gerais, Brazil, are decorated with suspended filaments of opaline silica. The filaments sustain xenotime–hematite aggregates in the open space. Xenotime occurs as inclusions in buds and botryoidal aggregates of hematite. The filamentous structures consist of strand-forming buds, hypha-like extensions, and thin strands that compose mat-like arrangements. They resemble microbial filaments that were replaced by opaline silica and fossilized. The occurrence of spherical hematite as protuberances on hematite-free opaline hyphae is interpreted as accretion of dissolved iron onto extracellular polymers. Phosphate sites in polymeric substances expelled from the microbial filaments might have adsorbed yttrium and heavy rare-earth elements from groundwater to the iron-accreting polymers. These would have resulted in botryoidal aggregates of hematite with xenotime inclusions. The presence of authigenic xenotime in the weathering zone opens a new possibility to constrain the evolution of lateritic profiles by xenotime geochronology.     

The assemblage stilpnomelane–chlorite–phengitic mica: a geothermobarometer for blueschist and associated greenschist terranes

Two anhydrous equilibria can be written among the components of stilpnomelane, chlorite, white mica and quartz, namely 89 daphnite+131 Fe-celadonite+190 quartz=96 stilpnomelane+71 muscovite, and amesite+Mg-celadonite=muscovite+clinochlore. We assume that the free energy change of reaction, ΔG=ΔG^o+ΣRT lna_i^j, is approximated by ΔG=A?BT +C(P?1)+ΣRT lna_i^j, where ΔG^o is the free energy change of the end-member components at temperature T and pressure P, a_i is the activity of component i whose coefficient in the equilibrium is j, and A, B and C are constants to be determined. Values of C can be approximated by the change in volume on reaction, namely C=406.517 J/bar for the first reaction and C=0.613 J/bar for the second reaction. Constants A and B were determined by using six occurrences of the assemblage stilpnomelane–chlorite–white mica for which P–T have been otherwise estimated. Using solution models from the literature, linear regression gives for the first equation A=?6118.269 kJ, B=?4584.09 J/K, and for the second equation A=19.397 kJ, B=66.72 J/K. These values predict P–T within 0.5 kbar and 25 K for all occurrences, and appear reasonably robust relative to probable analytical errors. P–T are determined by intersection of the curves generated by given compositions in P–T space. Fine-grained and/or zoned chlorite and white mica make application of the geothermobarometer difficult in some instances, but our work in the Bathurst region of New Brunswick suggests that, with patience and care, useful analyses can be obtained, and the database for the geothermobarometer greatly expanded.     

Raman spectrum of natural and synthetic stishovite

Raman spectra of natural and synthetic samples of stishovite have been measured with a micro-optical spectrometer system. These spectra have a pattern that is characteristic of rutile-structured oxides. The spectrum of synthetic stishovite is characterized by well-resolved bands at 231, 589, 753, and 967 cm^?1, which are assigned as theB _1g,E _g,A _1g, andB _2g fundamentals, respectively, of the first-order Raman spectrum of the ideal, ordered structure. Natural stishovite obtained from Meteor Crater, Arizona has a first-order Raman spectrum that is fully consistent with that of the synthetic material. The observed spectrum of the natural sample, however, is weaker and has bands in addition to those identified as fundamentals in the spectrum of the synthetic material. A broad band at ～475 cm^?1 may be indicative of glass or contaminants derived from the extraction procedure. Alternatively, this band may arise from multiphonon scattering that is enhanced by poor crystallinity or structural disorder in the natural shocked sample.     

Triassic palaeosols in the upper Narrabeen group of new South Wales. part I: Features of the palaeosols

The sea‐cliffs north of Sydney expose a complex of Triassic palaeosols, pedoliths, and sedimentary rocks.

The most obvious and diagnostic features of the palaeosols are fossil roots in place and markedly leached or reddened, relatively massive, clay‐rich strata. Associated coaly layers and fossil plant remains in adjacent sediments show varying degrees of decomposition. The A horizons of some of these palaeosols have been silicified by plant opal and contain abundant insect, earthworm, and larger animal burrows, cradle knolls, and basket podzols. Many of the palaeosols have well‐preserved peds and their upper horizons slake more readily in water than their lower horizons. Their B horizons may consist of extensive layers of siderite nodules or red claystone with tubular grey mottles around old root channels.

Less mature palaeosols show some relict sedimentary bedding and ripple marks within their profiles. More mature palaeosols, which appear massive in the field, may show anomalous grainsize variation in thin section, remaining from sedimentary bedding.

Soils can be eroded and deposited as pedoliths. Conglomerates of palaeosol clay pebbles and siderite nodules are easy to recognize as pedoliths. Finer‐grained pedoliths have the distinctive mineralogy and some of the small structures, but not the larger structures and field relations, of palaeosols.

Siderite crystals and nodules developed in the gleyed organic and A horizons of some of the clayey palaeosols shortly after they were covered by sediment and subsided below base level. With further compaction and dewatering, the ferric‐oxide minerals became redder by inverting to hematite, mineralized joints developed in some massive B horizons, some peds were accentuated by slickensides, and some root channels and coal cleat were filled with copper minerals.     

A qualitative molecular orbital study of the stability of polyanions in mineral structures

The existence and structures of various polyanion species, A _k ^n? ,occurring in inorganic compounds and minerals are rationalized using qualitative molecular orbital (MO) theory. Polyanions which may be stabilized by high pressure or lattice defects are discussed and compounds likely to show such species are identified. Previously observed high pressure properties of Fe riebeckite and MgO are interpreted in terms of O - O bond formation at high pressure.     

The generation of ductile and brittle shear bands in a low-angle mylonite zone

In the Saint-Barthélemy Massif, French Pyrenees, a ductile thrust zone developed in gneisses during retrogression from lower amphibolite facies conditions to the upper greenschist facies. The last major structures formed in the zone are isolated shear bands, divided into three types.Anastomosing, inhomogenous ultramylonitic shear bands (Type I) are subparallel to the mylonitic foliation in the gneiss (S_g). Most of these bands developed by ductile deformation processes only.Planar, homogeneous ultramylonite bands (Type II) are usually oblique to S_g. They generated as pseudotachylyte bands by brittle fracturing and underwent strong subsequent ductile deformation.Type III shear bands are planar and oblique to S_g. They consist of pseudotachylyte, weakly affected by ductile deformation.Type I, II and III bands seem to represent progressively younger structures on a local scale, linked to falling P-T conditions. The systematic variation in orientation of the different shear bands with respect to S_g is interpreted as being due to a different response of brittle and ductile structures to the orientation of the kinematic frame and the rock anisotropy.