Nitrogen-bearing,aqueous fluid inclusions in some eclogites from the Western Gneiss Region of the Norwegian Caledonides

Minerals in eclogites from different localities in the Western Gneiss Region of the Norwegian Caledonides (age 425 Ma) contain a variety of fluid inclusions. The earliest inclusions recognized are contained in undeformed quartz grains, protected by garnet, and consist of H₂O+N₂ (with ). The reconstructed P-V-T-X properties of these fluid inclusions are compatible with peak or early-retrograde metamorphic conditions. Matrix minerals (quartz, garnet, apatite, plagioclase) contain a complex pattern of mostly truly secondary inclusions, dominated by CO₂ and N₂. The textural patterns and P-V-T-X properties of these inclusions are incompatible with the high pressures of the eclogite-forming metamorphic event, but suggest that they were formed during uplift, by a combination of remobilization of preexisting inclusions and influx of external fluids. The fluid introduced at a late stage was dominated by CO₂, and did not contain N₂. The present data agree with theoretical predictions of eclogite fluids from mineral equilibria, and highlight the differences between granulite (CO₂) and eclogite (H₂O+N₂) fluid regimes. The provenance of the nitrogen in the eclogite fluid inclusions represents an important, but unsolved question in the petrology of high-pressure metamorphic rocks.Contribution no. 68 to the Norwegian programme of the International Lithosphere Project     

Collisional plateaus

In the fifteen years since the importance of collisional plateaus with thickened continental crust began to be recognized as one of the inevitable consequences of the processes of plate tectonics, rapid progress in their understanding has come from studies of the world's only active terminal collision zones in the Himalayan-Tibetan and Turkish-Iranian plateaus.Ancient collisional plateaus are being recognized throughout the geological record (back to 3.8 Ga) from the occurrence of extensive areas (typically > 500,000 km²) of 8 kbar metamorphism in granulite facies or from the occurrence of extensive areas of higher level minimum-melt composition granite rocks whose isotopic signatures indicate reactivation of existing continental crust rather than addition of new crust from the mantle at the time of collision. Recognition of strike-slip faulting in the ancient collisional plateau areas indicates that “tectonic escape” may have been as important in the past as it is today.Earth may not be the only planet on which collisional plateaus are important. The highlands of Venus (approximately 7% of the surface with elevations over 1.5 km above mean planetary radius) can only exist as a result of crustal thickening, and not as a product of lithospheric thinning. Most of these highlands can be explained by models involving volcanic construction. However, the highest peaks, including Maxwell Montes, the highest mapped area of Venus rising over 10 km above mean planetary radius, require much greater crustal thickening to support them than can reasonably be explained by a volcanic mechanism. Geological features of Maxwell Montes inferred from radar images suggest some analogy between Maxwell Montes and the Tibetan plateau.It is somewhat paradoxical that extensional tectonics are commonly associated with continental collision, and that collision-related rifts may be the only sites where the uppermost layers of a collision-thickened crust are preserved from erosion. Extensional stress fields are generated during continental collision, primarily in areas associated with strike-slip faulting and “tectonic escape”. Additional extensional stresses are gravitationally generated associated with the topography and thickened crust in a collision zone. Tectonically thickened crust is particularly susceptible to rifting as its lithosphere is weak as a result of heating associated with magmatism. This lithosphere is also compositionally weak because of the relatively thick crust, dominated by a weak quartz rheology, and thin mantle lithosphere, dominated by a strong olivine rheology, in comparison with a lithosphere with a more normal crustal thickness. Thus, the common association of rifts and collision zones may be a consequence of both stresses generated during collision and modification of the lithosphere by collision.     

The deglaciation of the western sector of the Irish Ice Sheet from the inner continental shelf to its terrestrial margin

This paper provides a new deglacial chronology for retreat of the Irish Ice Sheet from the continental shelf of western Ireland to the adjoining coastline, a region where the timing and drivers of ice recession have never been fully constrained. Previous work suggests maximum ice-sheet extent on the outer western continental shelf occurred at ~26–24 cal. ka BP with the initial retreat of the ice marked by the production of grounding-zone wedges between 23–21.1 cal. ka BP. However, the timing and rate of ice-sheet retreat from the inner continental shelf to the present coast are largely unknown. This paper reports 31 new terrestrial cosmogenic nuclide (TCN) ages from erratics and ice-moulded bedrock and three new optically stimulated luminescence (OSL) ages on deglacial outwash. The TCN data constrain deglaciation of the near coast (Aran Islands) to ~19.5–18.5 ka. This infers ice retreated rapidly from the mid-shelf after 21 ka, but the combined effects of bathymetric shallowing and pinning acted to stabilize the ice at the Aran Islands. However, marginal stability was short-lived, with multiple coastal sites along the Connemara/Galway coasts demonstrating ice recession under terrestrial conditions by 18.2–17. ka. This pattern of retreat continued as ice retreated eastward through inner Galway Bay by 16.5 ka. South of Galway, the Kilkee–Kilrush Moraine Complex and Scattery Island moraines point to late stage re-advances of the ice sheet into southern County Clare ~14.1–13.3 ka, but the large errors associated with the OSL ages make correlation with other regional re-advances difficult. It seems more likely that these moraines are the product of regional ice lobes adjusting to internal ice-sheet dynamics during deglaciation in the time window 17–16 ka.     

Geological interpretation of current subsidence and uplift in the London area,UK, as shown by high precision satellite-based surveying

Long term planning for flood risk management in coastal areas requires timely and reliable information on changes in land and sea levels. A high resolution map of current changes in land levels in the London and Thames estuary area has been generated by satellite-based persistent scatterer interferometry (PSI), aligned to absolute gravity (AG) and global positioning system (GPS) measurements. This map has been qualitatively validated by geological interpretation, which demonstrates a variety of controlling influences on the rates of land level change, ranging from near-surface to deep-seated mechanisms and from less than a decade to more than 100,000 years’ duration.     

Raman microspectrometry of fluid inclusions

For many kinds of fluid inclusions, the coupling of microthermometry and Raman microspectrometry is still the only viable option to obtain compositions of single fluid inclusions. A review is given on the basis of 16 years of experience and helped with about 120 references of the instrumentation, analytical conditions and methodology of the application of Raman microspectrometry to gaseous, aqueous and hydrocarbon inclusions, and their daughter minerals.     

The structure and sequence of geological events in the Basement Complex of the Ibadan area,Western Nigeria

The Basement Complex in Western Nigeria in general, and in the Ibadan area in particular, is composed primarily of a banded gneiss in which hornblende-biotite rich bands alternate with quartz-oligoclase rich bands. The banded gneiss, which originated as part of a sedimentary sequence, contains large lenses of granite gneiss and thin intercolated layers of quartzite and amphibolite.Two distinct major structural events can be clearly identified in the early geological history of the Ibadan area. Detailed field studies suggest that the formation of the Ibadan Granite Gneiss, which had yielded an Eburnean Rb-Sr isochron age, was associated with the later of these events. Five phases of dyke or vein formation, two of which pre-date the formation of the Granite Gneiss, have also been identified giving an overall sequence of geological events the first of which may correspond to the beginning of the Liberian orogeny, around 3000 m.y. ago, and the last of which reflects the waning of the Pan-African thermo-tectonic event about 500 m.y. ago.     

Denitrification and N2O production in near-shore marine sediments

Methods were developed for determining rates of denitrification in coastal marine sediments by measuring the production of N₂ from undisturbed cores incubated in gas-tight chambers. Denitrification rates at summer temperatures (23°C) in sediment cores from Narragansett Bay, Rhode Island, were about 50μmol N₂m^?2 hr^?1. This nitrogen flux is equal to approximately one-half of the NH⁺₄flux from the sediments at this temperature and is of the magnitude necessary to account for the anomalously low N/P and anomalously high O/N ratios often reported for benthic nutrient fluxes. The loss of fixed nitrogen as N₂ during the benthic remineralization of organic matter, coupled with the importance of benthic remineralization processes in shallow coastal waters may help to explain why the availability of fixed nitrogen is a major factor limiting primary production in these areas. Narragansett Bay sediments are also a source of N₂O, but the amount of nitrogen involved was only about 0.2 μmol m^?2 hr^?1 at 23°C.     

Neogene north American-Caribbean plate boundary across Northern Central America: Offset along the polochic fault

The Polochic fault was a segment of the North American-Caribbean plate boundary across Central America in the Neogene. Its 130 km of left slip was previously determined by matching structures and stratigraphie outcrop patterns of northwest and central Guatemala across the fault. Additional support for the model and the youthfulness of the recorded offset comes from an essentially perfect match of major geomorphic features across the fault. A reconstruction process which eliminates 123 km of left slip brings together rivers and drainage divides that existed before the Polochic became active.With the reconstruction carried across the isthmus on an east-west fault the regional structural geology assumes the coherent pattern of a continuous orogenic belt whose geometry is compatible with the model of collisional tectonics centered on the Motagua “suture zone”. Confined within this belt, narrowed to some 60 km by the reconstruction, lie the major Laramide thrusts, folds and tectonically emplaced serpentinites of Guatemala. Crystalline rocks of Guatemala re-join the Chiapas Massif and Paleozoic sedimentary rocks, exposed in the core of an almost-continuous anticlinorium, extend from southern Chiapas to Lake Izabal.The Polochic does not bend in eastern Guatemala but continues eastward to the Motagua fault where it dies. Westward drift of the northern block resulted in rifting which extended from eastern Guatemala into the Caribbean along the Cayman trough. The Honduras depression may represent an element of a triple junction along with the Polochic and Izabal-Cayman rift.The Polochic continues westward into the Pacific Ocean and offsets the Middle America trench. The Polochic has offset the Miocene volcanic belt of northern Central America, confirming the previous estimate of a Neogene time of movement.About 300 km of relative east-west Neogene displacement has been recorded on the Mid-Cayman rise, only 130 km of which can be accounted for across the Polochic. It is suggested that cumulative extension on north-south faults south of the Motagua fault zone between the trench and the Honduras depression might make up that difference.     

Identification of the last glacial maximum in the Upper Paleolithic of Portugal using magnetic susceptibility measurements of Caldeirão Cave sediments

We have sampled Upper and Middle Paleolithic sediments in Caldeirão Cave, Portugal, for paleoclimatic analysis. This work involved measuring magnetic susceptibility (MS) on continuous vertical profiles of a series of ∼8 cc sediment samples, and using the MS data as a paleoclimate proxy. Previous work has shown that caves can be ideal recorders of paleoclimatic variations because they are protected environments. Pedogenesis outside the cave during time of cool climate produces sediments with low MS magnitudes, while warmer climates yield higher MS magnitudes. Eroded soils collect in caves in sediment sequences where futher pedogenesis and biological disturbance is minimal. Continuously sampled profiles of those sediments found in an archaeological context then allow paleoclimatic estimates for all archaeological levels. Results of our work here include identification of the last glacial maximum, at ca. 24,000–22,000 B.P. (calibrated), defined by very low MS values found in basal Solutrean levels in the cave. Distinctive ca. 2500 year Neo-glacial cycles, defined by the MS data within the Early Upper Paleolithic and Solutrean, are consistent with published ¹⁴C duration estimates for the Upper Paleolithic in Caldeirão Cave. © 1998 John Wiley & Sons, Inc.