Revealing Repton: bringing landscape to life at Sheringham Park

The year 2012 marked 200 years since Humphry Repton (1752–1818) produced his design for Sheringham Park in north Norfolk, bound as one of his Red Books. On paper, Repton is England’s best-known and most influential landscape gardener. On the ground, his work is much harder to identify, focused as it was on light touches that equated more to landscape makeover than the landscape making of his predecessor Lancelot “Capability” Brown. This paper documents and evaluates a project that celebrated this bicentenary through a temporary exhibition within the visitor centre of Sheringham Park, whilst also making reference to the commemoration of his work in other places and on paper. In attempting to reveal Repton at Sheringham, we explore the context of the 1812 commission and the longer landscape history of the site, as well as the different methods of representing Repton on site that are open to site owners and managers

Investigation of Fluid-driven Carbonation of a Hydrated, Forearc Mantle Wedge using Serpentinite Cores in High-pressure Experiments

High-pressure experiments were performed to investigate the effectiveness, rate and mechanism of carbonation of serpentinites by a carbon-saturated COH fluid at 1 5-2 5 GPa and 375-700 °C. This allows a better understanding of the fate and redistribution of slab-derived carbonic fluids when they react with the partially hydrated mantle within and above the subducting slab under pressure and temperature conditions corresponding to the forearc mantle. Interactions between carbon-saturated CO2-H2O-CH4 fluids and serpentinite were investigated using natural serpentinite cylinders with natural grain sizes and shapes in piston-cylinder experiments. The volatile composition of post-run fluids was quantified by gas chromatography. Solid phases were examined by Raman spectroscopy, electron microscopy and laser ablation inductively coupled plasma mass spectrometry. Textures, porosity and phase abundances of recovered rock cores were visualized and quantified by three-dimensional, high-resolution computed tomography. We find that carbonation of serpentinites is efficient at sequestering CO2 from the interacting fluid into newly formed magnesite. Time-series experiments demonstrate that carbonation is completed within ∼96 h at 2 GPa and 600 °C. With decreasing CO2,aq antigorite is replaced first by magnesite quartz followed by magnesite talc chlorite in distinct, metasomatic fronts. Above antigorite stability magnesite enstatite talc chlorite occur additionally. The formation of fluid-permeable reaction zones enhances the reaction rate and efficiency of carbonation. Carbonation probably occurs via an interface-coupled replacement process, whereby interconnected porosity is present within reaction zones after the experiment. Consequently, carbonation of serpentinites is self-promoting and efficient even if fluid flow is channelized into veins. We conclude that significant amounts of carbonates may accumulate, over time, in the hydrated forearc mantle.This study was funded by an ARC Discovery Grant (DP14010089) to G.M.Y. and J.H., and by an ANU PhD stipend and the Mervyn and Katalin Paterson fellowship to M.J.S

Varying behaviour of Li in metasomatised spinel peridotite xenoliths from western Victoria, Australia

Lithium concentrations have been measured in minerals in a suite of spinel peridotite xenoliths from western Victoria, Australia. Carbonatite metasomatism has previously been documented in most of these samples, while others have evidence of interaction with a mafic silicate melt [Earth Planet. Sci. Lett. 107 (1991) 305; J. Petrol. 39 (1998) 1917]. In addition, all samples contain small, dispersed pockets of siliceous aluminous alkali-rich melt that frequently contain fine-grained phenocrysts. This melt is mostly of local derivation and developed very late, just prior to or during transport to the surface. Equilibrium partitioning of Li is observed between phenocryst minerals and between phenocrysts and melt. In contrast, an apparent disequilibrium Li distribution exists between the olivine and clinopyroxene affected by metasomatism. Olivine exhibits preferential Li enrichment compared to clinopyroxene in the carbonatite metasomatised xenoliths. Two samples with evidence of mafic silicate melt metasomatism contain preferentially enriched clinopyroxene. These results are in accordance with [Chem. Geol. 166 (2000) 47], who suggested that the interphase Li distribution can discriminate different metasomatic agents. An explanation for this varied behaviour could be a change in DLi (ol/cpx) due to a compositional effect in one or other phase, or both. The olivine that exhibits preferential Li enrichment is also P-rich, suggesting a coupled substitution involving P and Li. In contrast, there is an anti- correlation between P and Li in clinopyroxene, suggesting different substitution mechanisms for Li in olivine and clinopyroxene. Coupled Li-P substitution cannot be the only mechanism important for Li incorporation in olivine, however, and a combination of compositional parameters is expected to play a role in determining DLi (ol/cpx)

Reduced methane-bearing fluids as a source for diamond

Diamond formation in the Earth has been extensively discussed in recent years on the basis of geochemical analysis of natural materials, high-pressure experimental studies, or theoretical aspects. Here, we demonstrate experimentally for the first time, the spontaneous crystallization of diamond from CH4-rich fluids at pressure, temperature and redox conditions approximating those of the deeper parts of the cratonic lithospheric mantle (5-7 GPa) without using diamond seed crystals or carbides. In these experiments the fluid phase is nearly pure methane, even though the oxygen fugacity was significantly above metal saturation. We propose several previously unidentified mechanisms that may promote diamond formation under such conditions and which may also have implications for the origin of sublithospheric diamonds. These include the hydroxylation of silicate minerals like olivine and pyroxene, H2 incorporation into these phases and the "etching" of graphite by H2 and CH4 and reprecipitation as diamond. This study also serves as a demonstration of our new high-pressure experimental technique for obtaining reduced fluids, which is not only relevant for diamond synthesis, but also for investigating the metasomatic origins of diamond in the upper mantle, which has further implications for the deep carbon cycle

Origins of compositional heterogeneity in olivine-hosted melt inclusions from the Baffin Island picrites

The Baffin Island picrites are highly magnesian (< 22 wt% MgO) olivine tholeiites, erupted through felsic continental crust. Plots of most major and minor element oxides against MgO for the lavas define very tight trends consistent with modification of melts parental to the erupted suite by olivine fractionation or accumulation. However, melt inclusions trapped in primitive olivine phenocrysts in these lavas have much more diverse compositions. After correction for post-entrapment modification, the inclusions are systematically slightly lower in Al2 O3, and significantly higher in SiO2, K2O and P2O5 than the lavas' fractionation trends. CaO, Na2O and TiO2 contents lie within the lavas' fractionation trends. Similarly, most inclusions are higher in Sr/Nd, K/Nb, Rb/Ba, Rb/Sr, U/Nb and Ba/Th than the lavas. These characteristics resulted from up to ≈15% contamination of evolving picritic-basaltic liquids by locally-derived, broadly granitic partial melts of the quartz + feldspar-rich crust through which the picrites erupted. Contamination was minor in the bulk lavas (<1%), suggesting that the inclusions' compositions partly reflect a link between wall rock reaction and precipitation of liquidus olivine. Rapid crystallisation of liquidus olivine from the picrites, along with melting of felsic crustal wall rocks of magma chambers or conduits, were likely during emplacement of hot picritic magmas into cooler felsic crust. Inclusion compositions may thus reflect mixing trends or may be constrained to phase boundaries between olivine and a phase being resorbed, for example, an olivine-plagioclase cotectic. The extent of contamination was probably a complex function of diffusion rates of components in the magmas, and phenocryst growth rates and proximity to wall rock. These results bear on the common observation that melt inclusions' compositions are frequently more heterogeneous than those of the lavas that host them