Roles for fluid and/or melt advection in forming high-P mafic migmatites, Fiordland, New Zealand

A series of striking migmatitic structures occur in rectilinear networks through western Fiordland, New Zealand, involving, for the most part, narrow anorthositic dykes that cut hornblende‐bearing orthogneiss. Adjacent to the dykes, host rocks show patchy, spatially restricted recrystallization and dehydration on a decimetre‐scale to garnet granulite. Although there is general agreement that the migration of silicate melt has formed at least parts of the structures, there is disagreement on the role of silicate melt in dehydrating the host rock. A variety of causal processes have been inferred, including metasomatism due to the ingress of a carbonic, mantle‐derived fluid; hornblende‐breakdown leading to water release and limited partial melting of host rocks; and dehydration induced by volatile scavenging by a migrating silicate melt. Variability in dyke assemblage, together with the correlation between dehydration structures and host rock silica content, are inconsistent with macroscopic metasomatism, and best match open system behaviour involving volatile scavenging by a migrating trondhjemitic liquid.     

IN SITU MEASUREMENTS OF BASAL WATER QUALITY AND PRESSURE AS AN INDICATOR OF THE CHARACTER OF SUBGLACIAL DRAINAGE SYSTEMS

Continuous subglacial measurements of turbidity and electrical conductivity — two indicators of basal water quality — can be used to help characterize subglacial drainage systems. These indicators of water quality yield information that complements that provided by water pressure measurements. Quantitative attributes of subglacial drainage systems, such as water velocity and subglacial residence time, as well as qualitative behaviour — for example, spatial and temporal variations in system morphology — can be deduced using water quality measurements. Interpretation is complicated by the many potential influences on turbidity and electrical conductivity, but when these complications are appreciated a richer interpretation results. To demonstrate the utility of basal water quality measurements, observations from Trapridge Glacier, Yukon Territory, Canada were examined. The data reveal complex behaviour of the drainage system, but constraints imposed by basal water quality measurements help to clarify the nature of the subglacial flow system. The measurement and interpretation methods described and demonstrated are applicable to other glaciers. As such, they should prove useful for characterizing different subglacial drainage configurations and behaviours, thereby improving our general understanding of the hydrology and dynamics of wet-based glaciers.     

Decompressional coronas and symplectites in granulites of the Musgrave Complex, central Australia

Abstract In granulite facies metapelitic rocks in the Musgrave Complex, central Australia, reaction between S1 garnet and sillimanite involves the development in S2 of both garnet + cordierite + hercynitic spinel + biotite and hercynitic spinel + cordierite + sillimanite + biotite. The S2 assemblages occur either in coronas and symplectites, mainly around garnet, or, in rocks in which S2 is more strongly developed, as recrystallized assemblages. Ignoring the presence of biotite and ilmenite, the mineral textures can be accounted for qualitatively by a consideration of the model system FeO-MgO-Al₂O₃-SiO₂ (FMAS); the textural relationships accord with decompression accompanying the change from S1 to S2. However, since biotite and ilmenite are involved in the assemblages, the parageneses are better accounted for in terms of equilibria in the expanded model system K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂-TiO₂-Fe₂O₃ (KFMASHTO), i.e. AFM + TiO₂+ Fe₂O₃. The coronas reflect the tectonic unroofing of at least part of the Musgrave Complex from peak S1 conditions of about 8 kbar to S2 conditions of about 4 kbar.     

Mbosi: An anomalous iron with unique silicate inclusions

Abstract— The Mbosi iron meteorite contains millimeter size silicate inclusions. Mbosi is an ungrouped iron meteorite with a Ge/Ga ratio >10, which is an anomalous property shared with the five-member IIF iron group, the Eagle Station pallasites and four other ungrouped irons. Neither the IIF group nor the four other ungrouped irons are known to have silicate inclusions. Chips from three Mbosi inclusions were studied, but most of the work concentrated on a whole 3.1 mm circular inclusion. This inclusion consists of a mantle and a central core of different mineralogies. The mantle is partially devitrified quartz-normative glass, consisting of microscopic crystallites of two pyroxenes and plagioclase, which are crystalline enough to give an x-ray powder diffraction pattern but not coarse enough to permit analyses of individual minerals. The core consists of silica. The bulk composition does not match any known meteorite type, although there is a similarity in mode of occurrence to quartz-normative silicate inclusions in some HE irons. Mbosi silicate appears to be unique. The bulk rare earth element (REE) pattern of the mantle is flat at ? 7×C1; the core is depleted in REE but shows a small positive Eu anomaly. The O-isotope composition of bulk silicate lies on a unit slope mixing line (parallel and close to the C3 mixing line) that includes the Eagle Station pallasites and the iron Bocaiuva (related to the IIF irons); all of these share the property of having Ge/Ga ratios >10. It is concluded that Mbosi silicate represents a silica-bearing source rock that was melted and injected into metal. Melting occurred early in the history of the parent body because the metal now shows a normal Widmanstätten structure with only minor distortion that was caused when the parent body broke up and released meteorites into interplanetary space. The cause of Ge/Ga ratios being >10 in these irons is unknown. The fact that silicates in Mbosi, Bocaiuva (related to IIF irons) and the Eagle Station trio of pallasites, all characterized by a Ge/Ga ratio >10, lie on a unit slope mixing line in the O-isotope diagram suggests that their origins are closely related. The C3 chondrites appear to be likely precursors for silicates in Mbosi, Bocaiuva and the Eagle Station pallasites.     

Age, Origin and Cooling History of the Coronel Joao Sa Pluton, Bahia, Brazil

In north-east Brazil, Archean and Paleoproterozoic cratonicblocks are enclosed within a network of Brasiliano-age (0·7–0·55Ga) metasedimentary foldbelts. The unfoliated Coronel JoãoSá granodiorite pluton, which contains magmatic epidoteand strongly resorbed clinopyroxene, intrudes the SergipanoFoldbelt. Zircons yield a concordant U–Pb crystallizationage of 625 ± 2 Ma; titanite ages are approximately 621Ma. Discordant zircons suggest inheritance from at least twomagma sources of ages <1·8 and >2·2 Ga.Model calculations based on diffusion parameters and Rb–Srisotope data from separated minerals indicate that the plutoncooled at a rate of 36°C/Myr. Whole-rock element compositionsand initial Sr–Nd isotopic compositions that are heterogeneouson all length scales suggest magma mixing. Trace-element concentrationsand Nd isotope data argue against a contribution from a contemporaneousmantle-derived magma. Values of magmatic _Nd (at 625 Ma) resemblecontemporary _Nd for local supracrustal rocks and basement, compatiblewith anatexis of a crustal source. In north-east Brazil, cratonicblocks could have amalgamated with foldbelts that originatedas: (1) a mosaic of island arcs and arc basins (traditionalallochthonous model), or as (2) extensional continental sedimentarybasins (but not oceanic crust) later involved in collision (autochthonousmodel). The Coronel João Sá isotopic and chemicaldata support an autochthonous origin. KEY WORDS: Brasiliano Orogeny; granodiorite pluton; Rb–Sr isotopes, Sm–Nd isotopes; U–Pb isotopes, magma cooling rate     

Uncertainties in mean discharges from two large South American rivers due to rating curve variability

Abstract

The aim of this study was to assess the effect of land application of sewage sludge on phosphorus (P) losses during intense rainfall. Three rainfall simulations (40 mm h^?1 of 30 min duration) were conducted on a field amended with sewage sludge. The overland flow water (OFW) was monitored and sampled every minute. The suspended solid, the dissolved and total phosphorus (respectively SS, TP and DP) concentrations were analysed. The forms of particulate bound P (PP) were investigated. Several results stem from this experiment: (a) sludge application induced a large increase in the DP content of the OFW; the concentrations obtained (0.15–0.57 mg l^?1) were shown to result from desorption processes from the SS; and (b) in contrast, sludge application affected neither the SS content nor the TP concentration of OFW (9.5 g mg l^?1 P, consisting of PP for 95%). However, sludge preserved the structure of soil surface and led to a 45% decrease in runoff rate (150 m³ ha^?1 collected on the test surface compared to 290 m³ ha^?1 on a reference). This indirectly reduced TP losses (2.7 kg ha^?1 on the reference surface compared to 1.4 kg ha^?1 on the test surface).     

Large-scale current-induced erosion and deposition in the path of the 1929 Grand Banks turbidity current

New observations concerning the degree of current-induced erosion and deposition in the path of the 1929 Grand Banks turbidity current are presented. Most of the observations are available from Eastern Valley, Laurentian Fan. Seabeam and SeaMARC I data reveal widespread current erosion along the valley over a distance of 200 km from the shelfbreak. Erosional valley-floor channels are preferentially developed adjacent to the valley margins and the flanks of intravalley highs. Asymmetric transverse bedforms (herein termed gravel waves) are moulded in a deflationary pebble and cobble lag that overlies the eroded valley floor. In contrast, at the distal limit of Eastern Valley, thick deposits of massive granule gravel indicate deposition beneath a decelerating turbidity current. Symmetrical transverse bedforms (herein termed macrodunes) are developed within these granule gravel sediments. The spatial distribution of both bedforms and the areas of erosive excavation suggest that the turbidity current in 1929 was accelerating over the first 100 km from the shelfbreak and was eroding and entraining sediment from the valley floor over a distance of at least 200 km. With the loss of lateral constraint at the distal limit of Eastern Valley the turbidity current spread laterally and started depositing sediment as it decelerated. Current-induced erosion of the valley floor represented a potential source of between 50 and 100 km3 of sediment for incorporation into the resulting turbidite.