Experimental determination of REE partition coefficients between zircon,garnet and melt: a key to understanding high‐T crustal processes

The partitioning of rare earth elements (REE) between zircon, garnet and silicate melt was determined using synthetic compositions designed to represent partial melts formed in the lower crust during anatexis. The experiments, performed using internally heated gas pressure vessels at 7 kbar and 900–1000 °C, represent equilibrium partitioning of the middle to heavy REE between zircon and garnet during high‐grade metamorphism in the mid to lower crust. The D_REE (zircon/garnet) values show a clear partitioning signature close to unity from Gd to Lu. Because the light REE have low concentrations in both minerals, values are calculated from strain modelling of the middle to heavy REE experimental data; these results show that zircon is favoured over garnet by up to two orders of magnitude. The resulting general concave‐up shape to the partitioning pattern across the REE reflects the preferential incorporation of middle REE into garnet, with D_Gd (zircon/garnet) ranging from 0.7 to 1.1, D_Ho (zircon/garnet) from 0.4 to 0.7 and D_Lu (zircon/garnet) from 0.6 to 1.3. There is no significant temperature dependence in the zircon–garnet REE partitioning at 7 kbar and 900–1000 °C, suggesting that these values can be applied to the interpretation of zircon–garnet equilibrium and timing relationships in the ultrahigh‐T metamorphism of low‐Ca pelitic and aluminous granulites.     

Formation of ferriolivine and magnesioferrite from Mg — Fe-olivine: Reactions and kinetics of oxidation

Olivine samples (Fa 11) have been oxidized in air (f _O2 = 0.2 atm) at temperatures ranging from 350–700 °C and examined by Mössbauer spectroscopy, transmission electron microscopy, X-ray powder diffraction and thermomagnetic analysis. Oxidation of olivine was found to result in ferriolivine, magnesioferrite (major oxide phase) and magnetite (minor oxide phase) formation. Ferriolivine forms planar (001) precipitates, 0.6 nm in thickness, in the olivine host; the composition is likely to be Mg_0.5 v _0.5(Fe³⁺)_1.0SiO₄. Magnesioferrite MgFe₂O₄ exsolves as fine-grained precipitates (5–6 nm in size) filling interstices between the ferriolivine planar precipitates. Oxidation kinetic data at 700 °C show two stages of oxidation corresponding to formation of ferriolivine in the first stage and magnesioferrite in the second stage. The linear rate law with a rate constant k _Fol = 1.23 · 10^-3 s^-1 was found for the first stage whereas a parabolic rate-law with a constant of k _oxi = 3.28 · 10^-3 s^-1 was determined for the second stage of oxidation. It was found that ferriolivine is not an intermediate metastable phase in the oxidation process, terminated by magnesioferrite formation. The ferriolivine and magnesioferrite are considered to have formed by independent reactions which do not necessarily proceed simultaneously.     

Post-peak,fluid-mediated modification of granulite facies zircon and monazite in the Trivandrum Block,southern India

The quarry at Kottavattom in the Trivandrum Block of southern India contains spectacular examples of fluid-assisted alteration of high-grade metamorphic rocks. Garnet-biotite gneiss has undergone a change in mineral assemblage to form submetre scale orthopyroxene-bearing patches, later retrogressed to form an amphibole-bearing lithology. These patches, often referred to as arrested or incipient charnockite, crosscut the original metamorphic foliation and are typically attributed to passage of a low aH₂O fluid through the rock. Whilst this conversion is recognised as a late stage process, little detailed chronological work exists to link it temporally to metamorphism in the region. Zircon and monazite analysed from Kottavattom not only record metamorphism in the Trivandrum Block but also show internal, lobate textures crosscutting the original zoning, consistent with fluid-aided coupled dissolution-reprecipitation during formation of the orthopyroxene-bearing patches. High-grade metamorphism at the quarry occurred between the formation of metamorphic monazite at ~585 Ma and the growth of metamorphic zircon at ~523 Ma. The fluid-assisted alteration of the garnet-biotite gneiss is poorly recorded by altered zircon with only minimal resetting of the U–Pb system, whereas monazite has in some cases undergone complete U–Pb resetting and records an age for fluid infiltration at ~495 Ma. The fluid event therefore places the formation of the altered patches at least 25 Myr after the zircon crystallisation in the garnet-biotite gneiss. The most likely fluid composition causing the modification and U–Pb resetting of zircon and monazite is locally derived hypersaline brine.     

Magmatic-subsolidus and oxidation trends in composition of amphiboles from silica-saturated peralkaline igneous rocks

Summary Sub-calcic alkali amphiboles from silica-saturated peralkaline rocks exhibit two main patterns of compositional variation, which we term the magmatic-subsolidus and oxidation trends. The first is a continuous change from magmatic to subsolidus amphibole from barroisite through richterite to arfvedsonite, involving substitutions under reducing conditions, mainly of the type Al^ivCaSi(Na,K). The second trend is towards riebeckite, under the influence of oxidizing hydrothermal fluids, according to a reaction such as Fe²⁺ SiFe³⁺ Al^vi or (K, Na)^A Fe²⁺Fe³⁺. Textural evidence suggests that fluid characteristics are as important as host-rock compositions in controlling the chemical characteristics of amphiboles in peralkaline intrusive and volcanic rocks.

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Magmatische Subsolidus-und Oxydations-Trends der Zusammensetzung von Amphibolen aus Si-gesättigten peralkalinen Magmatiten

Zusammenfassung Alkali-Amphibole aus Si-gesättigten peralkalinen Gesteinen zeigen zwei definierte Trends chemischer Variation, die hier als magmatisch-subsolidus und Oxydations-Trend bezeichnet werden. Der erstere stellt einen kontinuerlichen Übergang von magmatischen zu subsolidus Amphibolen dar, und zwar von Barroisit über Richterit bis zu Arfvedsonit, wobei Substitutionen unter reduzierenden Bedingungen, hauptsächlich des Typs Al^iv CaSi(Na,K), zu beobachten sind. Der zweite Trend zeigt, unter dem Einfluß oxydierender hydrothermaler Lösungen, eine Tendenz auf Riebeckit hin, entsprechend einer Reaktion wie z.B. Fe²⁺ Si Fe³⁺ Al^vi oder (K, Na)^AFe²⁺ Fe³⁺. Mikroskopische Befunde weisen darauf hin, daß die Eigenschaften dieser Lösungen genauso wichtig für die chemische Zusammensetzung der Amphibole in peralkalinen Intrusiv-Gesteinen und Vulkaniten sind, wie die Zusammensetzung der Wirtsgesteine.

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With 6 Figures     

The Quaternary history of the Wash fluvial network, UK

This paper appraises and compares the Middle-Upper Pleistocene sedimentary sequences preserved in the fluvial systems draining into the Fenland Basin and the Wash estuary. Of the main Fenland rivers the longest records, which extend back to the initial Anglian (glacial) formation of the basin, are found in the Great Ouse and its tributaries, particularly the Cam and the Nar. These sequences preserve sediments representing all four post-Anglian interglacials. The Nene also has an extensive post-Anglian history, with evidence for a Hoxnian estuary that is presumed to have been the precursor of the Wash. North of the Nene, however, the Welland and Witham (proto-Trent) have relatively short sequences, which are thought to commence with a later (post-Anglian-pre-Devensian) glaciation that affected Lincolnshire and fed the previously-recognized Tottenhill outwash delta south of Kings Lynn. Prior to Devensian deglaciation the Witham valley was occupied by the Trent, which was the trunk river of the late Middle Pleistocene Wash system. During periods of low sea level the river would have extended north-eastwards across what is now the floor of the North Sea, possibly via the Inner Silver Pit. Several of the central Fenland sequences show evidence of infrequent terrace formation during the late Quaternary, although this might in part be due to poor vertical separation between terraces, so that differentiating them has been difficult; this has been exacerbated by mixed biostratigraphical signals due to the preservation of sediments representing more than one interglacial beneath a single terrace surface. In several of the systems there is evidence for valley rejuvenation to the lowest terrace or valley-floor level during the MIS 4-3 transition. The observed differences within what, during the predominant periods of lower sea level, would have been a single Wash river system are difficult to explain.     

A macro-scale flood risk model for Jamaica with impact of climate variability

Floods account for more than half of the global hydrometeorological risks. Severe floods cause significant economic shocks and loss of lives, particularly for developing countries such as Jamaica. There is need for more information on the present and projected flood risks to justify macro-scale planning for climate change adaptation and facilitate the decision-making processes. In this study, a catalogue of 198 flood events occurring between 1678 and 2010 is compiled for Jamaica and used to examine the climatology, occurrence, trends, causes and duration of the island’s severe events. The annual flood risk is estimated to be a loss of life rate of 4 persons and estimated annual damage of USD96.3 million per annum in 2010 values and approximately 0.84 % of GDP per annum. Macro-scale models for flood risks (deaths and damages) are also developed using data from the flood catalogue and maximum precipitation at the town and parish level. The models examine the relationship between flood risks (death and damages) and extreme rainfall depths and intensities. Future climate risks of loss of lives and damages are predicted to increase 11 and 9 %, respectively, to 4.4 persons and USD105.2 million per annum.     

Trace element fractionation trends of tholeiitic magma at moderate pressure: Evidence from an Al-spinel ultramafic-mafic inclusion suite

Trace element data on an Al-spinel ultramafic-mafic inclusion suite in an analcimite support earlier proposals that the various inclusions are comagmatic and represent fragments of a layered tholeiitic ‘pluton’ which differentiated at pressures close to 8 kb. Ultramafic inclusions are dominantly pyroxenites, often websteritic, whereas the mafic inclusions are largely two pyroxene-plagioclase assemblages. Appropriate experimental data and abundances of Sc, Cr and V indicate that subcalcic clinopyroxene or relatively Ca-rich clinopyroxene was the major ferromagnesian phase fractionated, often accompanied by spinel in the early and middle stages of differentiation and, in the later stages, by titanomagnetite. Comparatively moderate decreases in Ni and Co suggest that olivine was a relatively unimportant fractionating phase. Clinopyroxene fractionation at moderate pressures should be assigned only a comparatively insignificant role in the production of evolved basaltic compositions.     

40Ar/39Ar Thermochronology west of the Main Uralian fault, southern Urals, Russia

West of the Main Uralian fault, the main suture in the southern Urals, ⁴⁰Ar/³⁹Ar apparent ages of amphibole, muscovite and potassium feldspar are interpreted as cooling ages. A fast exhumation of the metamorphic complex of Kurtinsky during Upper Carboniferous time is indicated by the small age difference (15 Ma) between cogenetic amphibole and muscovite. Differentiated movement in the footwall of the Main Uralian fault along strike is indicated by the age difference of 70 Ma between the metamorphic complexes of Kurtinsky (north) and Maksyutov (south). No Upper Paleozoic (Uralian) medium- to high-temperature event is recorded in ⁴⁰Ar/³⁹Ar data from the metamorphic complex of Beloretzk (MCB). An amphibole age of 718±5 Ma and the occurrence of mafic intrusions might signal the break-up of Rodinia and therefore indicate the rifting period followed by the separate movement of the "Beloretzk terrane". Muscovite ages of approximately 550±5 Ma, the unique pre-Ordovician tectonometamorphic evolution of the MCB and the Late Vendian sedimentary history of the western Bashkirian Megaanticlinorium (BMA) imply the existence of a Neoproterozoic orogeny at the eastern margin of Baltica. This orogeny might have been initiated by the accretion of the "Beloretzk terrane". The metamorphic grade of the overlain Silurian shales and the K/Ar microcline ages from the "Beloretzk terrane" give evidence for a new thermal event at approximately 370 Ma. A microcline age of 530–550 Ma obtained for the Vendian conglomerate in the western BMA suggests that a maximum temperature of approximately 200°C was reached in Cambrian or Vendian times. An orthoclase age (590–630 Ma) of the Vendian Zigan flysch deposits might be inherited from the eastern source area, the Cadomian orogen. An orthoclase age (910–950 Ma) from the Riphean Zilmerdak conglomerate coincides with a documented decrease in the subsidence rate of the Upper Riphean basin.     

Using FTIR spectroscopy to characterise the soil mineralogy and geochemistry of cryoconite from Aldegondabreen glacier,Svalbard

Mineralogical and geochemical diversity in cryoconite granules from Aldegondabreen glacier was investigated using FTIR spectroscopy. Results suggest that the technique is an effective tool for investigating mineralogy and identifying spatial differences in geochemistry, based upon characteristic spectral signatures.