The stability of sapphirine + quartz: calculated phase equilibria in FeO–MgO–Al2O3–SiO2–TiO2–O

The presence in rocks of coexisting sapphirine + quartz has been widely used to diagnose conditions of ultra‐high‐temperature (UHT) metamorphism (>900 °C), an inference based on the restriction of this assemblage to temperatures >980 °C in the conventionally considered FeO–MgO–Al₂O₃–SiO₂ (FMAS) chemical system. With a new thermodynamic model for sapphirine that includes Fe₂O₃, phase equilibra modelling using thermocalc software has been undertaken in the FeO–MgO–Al₂O₃–SiO₂–O (FMASO) and FeO–MgO–Al₂O₃–SiO₂– TiO₂–O (FMASTO) chemical systems. Using a variety of calculated phase diagrams for quartz‐saturated systems, the effects of Fe₂O₃ and TiO₂ on FMAS phase relations are shown to be considerable. Importantly, the stability field of sapphirine + quartz assemblages extends down temperature to 850 °C in oxidized systems and thus out of the UHT range.     

Early Holocene environment on Bjørnøya (Svalbard) inferred from multidisciplinary lake sediment studies

Bjørnøya, a small (178 km²) island situated between the mainland of Norway and southern Spitsbergen, provides the opportunity for the reconstruction of early Holocene terrestrial and limnic palaeoenvironments in the southwestern Barents Sea. The AMS ¹⁴C dating technique, geochemical, mineral magnetic, micro and macrofossil analyses were applied to sediments recovered from lake Stevatnet and the results are interpreted in terms of palaeoenvironmental conditions between 9800 and 8300 ¹⁴C bp. After the disappearance of local glaciers before ca 9800¹⁴C BP, the lake productivity increased rapidly at the same time as pioneer plant communities developed on soils which gradually became more stable. Insect data indicates that strong seasonal contrasts with mean July temperatures around 9°C and mean January temperatures around −12°C prevailed between 9500 and 8300 ¹⁴C BP. These high summer temperatures, possibly as much as 4-5°C higher than the present, favoured the development of a flora including Dryas and Angelica cf. archangelica . The enhanced freeze/thaw processes led to an increased erosion of minerogenic and organic material. After 8000 ¹⁴C BP the temperatures may have gradually declined. The environmental reconstruction derived from our data set supports the conceptual insolation model which proposes maximum Holocene seasonality for the Northern Hemisphere at ca 9000 ¹⁴C BP.     

Rapid in situ silicification of microbes at Loburu hot springs, Lake Bogoria, Kenya Rift Valley

Microbial mats, located along the margins of hot-spring pools and outflow channels at Lake Bogoria, Kenya, are commonly silicified forming friable laminated crusts. Columnar microstromatolites composed of silica and calcite are also forming at several springs in sites of oscillating water level or spray. Silicification of the microbes involves impregnation of organic tissue by very fine amorphous silica particles and encrustation by small (< 2 μm) silica spheroids. Rapid silicification of the microbes, which may begin while some are still alive, can preserve sheaths and in some examples, the filaments, capsules and cells. Although this provides evidence of their general morphology, the biological features that are required for taxonomic identifications are commonly poorly preserved.
The silica precipitation results mainly from evaporative concentration and rapid cooling of spring waters that have been drawn upward through the mats and microstromatolites by capillary processes. Almost all the silica at the Loburu springs nucleates on microbial substrates. This affinity of silica for functional groups on microbial surfaces contributes to the rapid silicification of the microbes and their preservation in modern and ancient cherts.     

Diagenesis of late Cenozoic diatomaceous deposits and formation of the bottom simulating reflector in the southern Bering Sea*

Diatom ooze and diatomaceous mudstone overlie terrigenous mudstone beds at Leg 19 Deep Sea Drilling Project sites. The diatomaceous units are 300-725 m thick but most commonly are about 600 m. Diagenesis of diatom frustules follows a predictable series of physical and chemical changes that are related primarily to temperature (depth of burial and local geothermal gradient). During the first 300-400 m of burial frustules are fragmented and undergo mild dissolution. By 600 m dissolution of opal-A (biogenic silica) is widespread. Silica reprecipitates abundantly as inorganic opal-A between 600 and 700 m sub-bottom depth. Inorganic opal-A is rapidly transformed by crystal growth to opal-CT. The result is formation of silica cemented mudstone and porcelanite beds. A regional acoustic reflector (called the bottom-simulating reflector, or BSR) occurs near 600 m depth in the sections. This acoustic event marks the upper surface where silicification (cementation) is active. In Bering Sea deposits, opal-A is transformed to opal-CT at temperatures between 35° and 50°C. This temperature range corresponds to a sub-bottom depth of about 600 m and is the area where silicification is most active. Thus, the BSR represents an isothermal surface; the temperature it records is that required to transform opal-A to opal-CT. Deposition of at least 500 m of diatomaceous sediment was required before the temperature at the base of the diatomaceous section was appropriate (35°-50°C) for silica diagenesis to occur. Accordingly, silica diagenesis did not begin until Pleistocene time. Once silicification began, in response to sediment accumulation during the Quaternary, the diagenetic front (the BSR) moved upsection in pace with the upward migrating thermal boundary. X-ray diffractograms and SEM photographs show three silica phases, biogenic opal-A, inorganic opal-A’, and opal-CT. These have crystallite sizes of 11-16 A, 20-27 A, and 40-81 A, respectively, normal to 101. The d(101) reflection of opal-CT decreases with depth of burial at DSDP Site 192. This occurs by solid-state ordering and requires at least 700 m of burial. Most clinoptilolite in Leg 19 cores forms from the diagenesis of siliceous debris rather than from the alteration of volcanic debris as is commonly reported.     

The Petrology and Geochemistry of Cone-sheets from the Cuillin Igneous Complex, Isle of Skye: Evidence for Combined Assimilation and Fractional Crystallization during Lithospheric Extension

The origin and evolution of the magma(s) involved in the formationof the olivine tholeiite cone-sheets which invade the 60-MaCuillin Igneous Complex, Isle of Skye, have been investigatedusing mineral chemistry data together with whole-rock major-and trace-element and isotope (Sr and Nd) geochemistry data.The most primitive compositions identified are almost identicalto those of the basalts being erupted at the present day alongthe spreading centre in Iceland. Rare examples of lavas fromthe slightly older lava pile of west-central and north Skye,together with a large number of dykes from the axial zone ofthe Skye Main Swarm, are of similar composition and are referredto as being of the Preshal More Basalt type (spelling is accordingto current Ordnance Survey of Great Britain maps). The intra-suitecompositional variation exhibited by the cone-sheets can beexplained in terms of relatively low-pressure fractionationof the three-phase assemblage olivine+clinopyroxene+plagioclasein their cotectic proportions of 10: 35: 55. Modelling of major-and compatible trace-element data indicates that the most evolvedcomposition may be derived by 60% crystallization of the leastevolved composition. Incompatible trace-element modelling impliesgreater degrees of crystal-liquid fractionation: Y and Zr indicate64 and 68% crystallization, respectively, whereas the rare earthelements (Eu, Yb, Gd, and Ce) indicate between 71 and 75% crystallization.This decoupled behaviour of compatible and incompatible elementsis attributed to the cone-sheet magma(s) evolving in a subjacentmagma chamber, before final emplacement in the overlying crust.Sr- and Nd-isotope data indicate that the cone-sheet magma(s)assimilated upper-crustal lithologies during fractional crystallization,most likely involving amphibolite facies gneiss of the LewisianGneiss Complex. This style of simultaneous assimilation andfractional crystallization (AFC) process in the upper crustfor the Skye cone-sheet magma(s) is in direct contrast to theprocesses identified for the magmas which produced the bulkof the lava field of west-central and north Skye, where assimilationoccurred after fractionation. The cone-sheet magmas were generatedby relatively large degrees of partial melting of a depletedmantle source associated with significant lithospheric stretching.The cause of this depletion, together with the temporal relationshipsbetween the cone-sheet magmas and the dominant transitionalmagmas of the Skye lava field and sill complex, are also discussed. ^* Reprint request to B. R. Bell     

Deep‐water foreland basin deposits of the Cerro Toro Formation,Magallanes basin,Chile: architectural elements of a sinuous basin axial channel belt

Coarse‐grained deep‐water strata of the Cerro Toro Formation in the Cordillera Manuel Señoret, southern Chile, represent the deposits of a major channel belt (4 to 8 km wide by >100 km long) that occupied the foredeep of the Magallanes basin during the Late Cretaceous. Channel belt deposits comprise a ca 400 m thick conglomeratic interval (informally named the ‘Lago Sofia Member’) encased in bathyal fine‐grained units. Facies of the Lago Sofia Member include sandy matrix conglomerate (that show evidence of traction‐dominated deposition and sedimentation from turbulent gravity flows), muddy matrix conglomerate (graded units interpreted as coarse‐grained slurry‐flow deposits) and massive sandstone beds (high‐density turbidity current deposits). Interbedded sandstone and mudstone intervals are present locally, interpreted as inner levée deposits. The channel belt was characterized by a low sinuousity planform architecture, as inferred from outcrop mapping and extensive palaeocurrent measurements. Laterally adjacent to the Lago Sofia Member are interbedded mudstone and sandstone facies derived from gravity flows that spilled over the channel belt margin. A levée interpretation for these fine‐grained units is based on several observations, which include: (i) palaeocurrent measurements that indicate flows diverged (50° to 100°) once they spilled over the confining channel margin; (ii) sandstone beds progressively thin, away from the channel belt margin; (iii) evidence that the eroded channel base was not very well indurated, including a stepped margin and injection of coarse‐grained channel material into surrounding fine‐grained units; and (iv) the presence of sedimentary features common to levées, including slumped units inferring depositional slopes dipping away from the channel margin, lenticular sandstone beds thinning distally from the channel margin, soft sediment deformation and climbing ripples. The tectonic setting and foredeep architecture influenced deposition in the axial channel belt. A significant downstream constriction of the channel belt is reflected by a transition from more tabular units to an internal architecture dominated by lenticular beds associated with a substantially increased degree of scour. Differential propagation of the fold‐thrust belt from the west is speculated to have had a major control on basin, and subsequently channel, width. The confining influence of the basin slopes that paralleled the channel belt, as well as the likelihood that numerous conduits fed into the basin along the length of the active fold‐thrust belt to the west, suggest that proximal–distal relationships observed from large channels in passive margin settings are not necessarily applicable to axial channels in elongate basins.     

Performance evaluation and work-load estimation for geographic information systems

Abstract

Agencies acquiring GIS hardware and software are faced with uncertainty at two levels: over the degree to which the proposed system will perform the functions required, and over the degree to which it is capable of doing so within proposed production schedules. As the field matures the second concern is becoming more significant. A formal model of the process of acquiring a GIS is presented, based on the conceptual level of defining GIS sub-tasks. The appropriateness of the approach is illustrated using performance data from the Canada Land Data System. It is possible to construct reasonably accurate models of system resource utilization using simple predictors and least squares techniques, and a combination of inductive and deductive reasoning. The model has been implemented in an interactive package for MS-DOS systems.