Controls on peat accumulation and depositional environments of a coal-bearing coastal plain succession of a pull-apart basin; a petrographic, geochemical and sedimentological study, Lower Jurassic, Denmark

The paralic, Lower-Middle Jurassic Bagå Formation of the Island of Bornholm, Denmark, was deposited in a fault-bounded, subsiding, pull-apart basin. The formation is up to 400 m thick and contains more than 50 coal seams. Twelve of these have been investigated petrographically and geochemically to provide basic information on the composition of the relatively unknown Jurassic coals. The peat-forming environments represented by the seams and the associated siliciclastic sediments are interpreted.The seams represent three types of environments with organic matter deposition. Peat accumulation occurred in low-lying areas situated between river channels in a coastal plain environment undergoing overall transgression. The coals have a relatively uniform, huminite-rich petrographic composition, indicating that the precursor mires were dominated by persistent, water-saturated and anoxic conditions. The swamps were probably occupied by a small-statured flora with cellulose-rich tissues. Significant bacterial activity in the peat swamps is suggested by an abundance of hopanes. Influence from marine water was not common but occurred occasionally. During peat accumulation, the depositional conditions were stable and quiet. The small thicknesses of the seams (8–57 cm thick) indicate relatively short periods of peat formation (average c. 2300 yr), due to continued base-level rise, controlled by subsidence, and an overall eustatic rise, causing repeated changes in the sedimentary regimes. The coal seams are of low rank and were buried to a depth of 1100–1200 m before uplift, due to Late Cretaceous-early Tertiary basin inversion and Neogene uplift.     

The enrichment of 34S in the solfataras of the Nea Kameni volcano,Santorini archipelago,Greece

Sulfur isotope investigations carried out on elemental sulfur and sulfates of the Nea Kameni solfataras, Santorini, Aegean Sea, Greece, show a clear enrichment in the heavy sulfur isotope ³⁴S against the assumed primordial ${}^{32}\text{S}{}^{34}\text{S}$ ratio of 22,220. Within the same crater, different vents, only a few meters apart from each other, produced δ differences up to 10‰, which remained constant for several years. This enrichment is most probably due to contamination by heavy sulfur from a nonvolcanic source. An enrichment in the same order of magnitude was observed in sulfur of recent and older lavas ($\text{δ}{}^{34}\text{S}\text{= ?1 ? +11‰}$).Potential contaminants like sulfide sulfur in hydrothermal ore veins of Athinios has a δ ³⁴S mean value close to 0‰, sulfide and sulfate in the sedimentary basement has a δ ³⁴S mean value of $\text{+2.6‰.}$ Seawater sulfate from the area gives a value of $\text{δ}{}^{34}\text{S}\text{= 20‰}$, while sulfide from bacterial reduction of pore-water sulfate in recent iron ore sediments has δ ³⁴S values between ?8 and $\text{?5‰.}$ Sulfate remaining in the pore solutions gave $\text{δ}{}^{34}\text{S}\text{= +27‰}$.The most probable explanation for the observed high δ ³⁴S values in the solfataric sulfur and in some of the lavas of the Santorini area is contamination of the volcanic vents by Mediterranean Sea water.     

Crystallization processes in the Bjerkreim-Sokndal layered intrusion,south Norway: evidence from the boundary between two macrocyclic units

The Bjerkreim-Sokndal (BKSK) layered intrusion belongs to the Rogaland anorthosite province in southern Norway. The northwestern part of BKSK consists of a ca. 6 km-thick Layered Series, made up of macrocyclic units (MCU) arranged in a syncline. Each MCU, which resulted from the crystallization of a major-magma influx, can be subdivided into a series of cumulate zones. The MCU III/IV boundary has been studied in seven profiles across its strike length of 24 km. Massive piC¹ at the base of MCU IV overlies laminated and modally layered phimC in the central part of the chamber and phimacC towards the flanks; there is a discordance of between 2 and 6° between the base of MCU IV and phase layering in MCU III. The MCU IV piC is overlain by 75–100 m of massive poiC (the Svaalestad unit of Michot 1960; a similar olivine-bearing unit occurs near the base of MCU III) which has more primitive compositions than the underlying piC. This is followed by laminated and modally layered phiC, phimC and phimacC. The reversal to more primitive mineral assemblages across the MCU III/IV boundary is accompanied by a cryptic reversal; plagioclase and Ca-poor pyroxene have compositions of about An 44/Mg no. 71 at the top MCU III and about An 52/Mg no. 77 near the base of MCU IV. Olivines in the MCU IV poiC vary unsystematically from Fo 66 to 76. Macrocyclic units III and IV crystallized from monzonoritic parental magma. The BKSK magma chamber had a broad saucer-like shape with a small thickness to breadth ratio. The magma in the chamber during crystallization of MCU III was compositionally zoned and crystallized on an inward-sloping floor by down-dip accretion. Just before the major-magma influx at the base of MCU IV, phimC was crystallizing from the basal-magma layer at the centre of the chamber, while phimacC was crystallizing towards the flanks. The new, dense magma fountained into and mixed with the basal-magma layers already in the chamber. This hybrid magma crystallized during continued influx to produce massive piC at the base of MCU IV. This hybrid unit is thickest near the centre of the chamber and smoothed out the floor to an essentially horizontal surface. Continued influx resulted in the dense, primitive magma ponding on the floor; this crystallized fairly rapidly to produce the massive poiC unit. The return of normal fractional crystallization conditions is marked by the overlying sequence of modally and cryptically layered cumulates which duplicate the succession in MCU III. The variation in thickness of the upper part of MCU IV indicates that crystallization of the BKSK Layered Series was accompanied by sinking of the floor at a greater rate near the centre of the chamber than towards the flanks. This was accompanied by compaction of the underlying cumulates, promoting the development of lamination and the expulsion of intercumulus melt to encourage the development of adcumulates. ¹ p plagioclase - i ilmenite - h Ca-poor pyroxene - o olivine - m magnetite - a apatite - c Ca-rich pyroxene - C cumulate     

The occurrence and formation of Ti-Aegirines in peralkaline syenites

The formation and stabilities of Fe²⁺-NAT (NaTi_1/2Fe²⁺ _1/2Si₂O₆) and Mg-NAT (NaT_1/2Mg_1/2 Si₂O₆) clinopyroxene end-members in peralkaline nepheline syenites of a shonkinite-nepheline syenite suite in the Tertiary ultramafic alkaline Gardiner complex, East Greenland is described. Based on the composition of coexisting aegirines and Ti-minerals it is suggested that the formation of Mg-NAT is favoured by high f _O2 and high Nds- and TiO₂-activity. The instability of ilmenite at a late magmatic stage leads to the breakdown of a peralkaline acmite-ilmenite oxygen-buffer. Lack of buffering results in strong enrichment of aegirines in Fe²⁺-NAT. Up to 8% TiO₂ and 20% Mg-NAT and 20% Fe²⁺-NAT is recorded from these aegirines. The most evolved compositions show decreasing NAT and increasing Ac. The breakdown of sphene at a late magmatic to subsolidus stage results in increasing Wo in coexisting aegirines. Based on comparison with other occurrences it is concluded that NAT endmembers form at low pressure, but not necessarilly at low T., and that high f _O2 favors Mg-NAT formation and low f _O2 Fe²⁺-NAT formation.     

A preliminary prediction analysis of radiation fog

Summary The system of physical equations describing temperature changes near the ground in fog-free air as well as in radiation fog is solved numerically. The variation of the exchange coefficient with height is taken into account using different models while time variations are still disregarded. Temperature changes due to latent heat effects are incorporated in this study. Moreover, the presence of radiative flux divergence is included in an approximate manner.The solution of the problem is presented in terms of graphs showing the development of temperature and water droplet profiles as function of time and height. Computed liquid water content as well as temperature profiles are in general agreement with observations while the vertical growth of fog usually proceeds too rapidly. Concrete suggestions are given of how to improve the model.