The Evolution of the EH4 Chondrite Indarch at High Pressure and Temperature: The First Experimental Results

Chondrite groups are characterized by variations in bulk composition and oxidation state, illustrating in part heterogeneity in the early solar nebula. Planetary accretion could be explained by at least two different scenarios: the homogeneous [1] and heterogeneous accretion models [2, 3]. In particular, for the formation of the Earth, some studies (e.g. [2, 3]) assume that one component is highly reduced material comparable to enstatite chondrites, devoid of volatile elements but containing all other elements in C1 abundance ratios. To derive constraints on the understanding of early differentiation processes, studies of the silicate phase relations and their interactions with metal, at relevant P-T-fO2, are required. Melting relations and equilibrium partitioning behaviour have been studied on peridotitic and chondritic starting compositions at pressures and temperatures corresponding to the transition zone and lower mantle [4, 5, 6]. However, enstatite chondrites, which are highly reduced primitive meteorites, have not yet been studied experimentally under such conditions. Thus, multianvil experiments have been performed at 20-25 GPa and 2000-2400 C on the EH4 chondrite Indarch

Variação temporal da biomassa do café arábica arborizado e a pleno sol, através de índices de vegetação.

Abstract. In recent years, several works have been accomplished in order to analyze the spectral response of coffee crop fields cultivated in full sun. However, the number of papers related to the monitoring of shaded coffee systems is reduced although this kind of cultivation is very important from the perspective of climate change and agriculture. In addition, the biomass of coffee systems can store carbon, which is a mitigation action. In this context, this paper presents the assessment of the meteorological variation joined with the biomass increase in three different coffee plantation systems: full sun, shaded coffee associated with legumes (SAF), and shaded coffee with Macadamia trees. We used Landsat-5 images from 2004 to 2010, and the NDVI and SAVI indexes. LAI index measured in the test area during all 2009 year. The results showed that the shaded coffee systems presented the NDVI and SAVI indexes with higher values than those found with the system of planting in full sun what corroborates the idea of greater amount of biomass in this type of planting. All indexes present the seasonal rain variation. One month after a large period of drought the indexes NDVI, SAVI and LAI decrease. The NDVI and SAVI indexes presented high correlation with the measured LAI. The shaded coffee system and the full sun coffee reached high vegetation indexes what indicate a high concentration of biomass. This factor should be considered in the decision making process since is important to take advantage of cultivation systems acting as carbon sinks.SBSR 2011

Experimental investigation of the stability of Fe-rich carbonates in the lower mantle

International audienceThe fate of carbonates in the Earth's mantle plays a key role in the geodynamical carbon cycle. Although iron is a major component of the Earth's lower mantle, the stability of Fe-bearing carbonates has rarely been studied. Here we present experimental results on the stability of Fe-rich carbonates at pressures ranging from 40 to 105 GPa and temperatures of 1450-3600 K, corresponding to depths within the Earth's lower mantle of about 1000-2400 km. Samples of iron oxides and iron-magnesium oxides were loaded into CO2 gas and laser heated in a diamond-anvil cell. The nature of crystalline run products was determined in situ by X-ray diffraction, and the recovered samples were studied by analytical transmission electron microscopy and scanning transmission X-ray microscopy. We show that Fe-(II) is systematically involved in redox reactions with CO2 yielding to Fe-(III)-bearing phases and diamonds. We also report a new Fe-(III)-bearing high-pressure phase resulting from the transformation of FeCO3 at pressures exceeding 40 GPa. The presence of both diamonds and an oxidized C-bearing phase suggests that oxidized and reduced forms of carbon might coexist in the deep mantle. Finally, the observed reactions potentially provide a new mechanism for diamond formation at great depth

Nickel and helium evidence for melt above the core–mantle boundary

High ^(3)He/^(4)He ratios in some basalts have generally been interpreted as originating in an incompletely degassed lower-mantle source. This helium source may have been isolated at the core–mantle boundary region since Earth’s accretion. Alternatively, it may have taken part in whole-mantle convection and crust production over the age of the Earth; if so, it is now either a primitive refugium at the core–mantle boundary or is distributed throughout the lower mantle. Here we constrain the problem using lavas from Baffin Island, West Greenland, the Ontong Java Plateau, Isla Gorgona and Fernandina (Galapagos). Olivine phenocryst compositions show that these lavas originated from a peridotite source that was about 20 per cent higher in nickel content than in the modern mid-ocean-ridge basalt source. Where data are available, these lavas also have high ^(3)He/^(4)He. We propose that a less-degassed nickel-rich source formed by core–mantle interaction during the crystallization of a melt-rich layer or basal magma ocean, and that this source continues to be sampled by mantle plumes. The spatial distribution of this source may be constrained by nickel partitioning experiments at the pressures of the core–mantle boundary

Monitoring and modelling landscape dynamics

International audienceChanges in land cover and land use are among the most pervasive and important sources of recent alterations of the Earth's land surface.This special issue also presents new directions in modelling landscape dynamics. Agent-based models have primarily been used to simulate local land use and land cover changes processes with a focus on decision making (Le 2008; Matthews et al. 2007; Parker et al. 2003; Bousquet and Le Page 2001)

Experimental and computational study of trace element distribution between orthopyroxene and anhydrous silicate melts: substitution mechanisms and the effect of iron

Although orthopyroxene (Opx) is present during a wide range of magmatic differentiation processes in the terrestrial and lunar mantle, its effect on melt trace element contents is not well quantified. We present results of a combined experimental and computational study of trace element partitioning between Opx and anhydrous silicate melts. Experiments were performed in air at atmospheric pressure and temperatures ranging from 1,326 to 1,420°C in the system CaO-MgO-A

Quantifying garnet-melt trace element partitioning using lattice-strain theory: New crystal-chemical and thermodynamic constraints

Many geochemical models of major igneous differentiation events on the Earth, the Moon, and Mars invoke the presence of garnet or its high-pressure majoritic equivalent as a residual phase, based on its ability to fractionate critical trace element pairs (Lu/Hf, U/Th, heavy REE/light REE). As a result, quantitative descriptions of mid-ocean ridge and hot spot magmatism, and lunar, martian, and terrestrial magma oceans require knowledge of garnet-melt partition coefficients over a wide range of conditions. In this contribution, we present new crystal-chemical and thermodynamic constraints on the partitioning of rare earth elements (REE), Y and Sc between garnet and anhydrous silicate melt as a function of pressure (P), temperature (T), and composition (X). Our approach is based on the interpretation of experimentally determined values of partition coefficients D using lattice-strain theory. In this and a companion paper (Draper and van Westrenen this issue) we derive new predictive equations for the ideal ionic radius of the dodecahedral garnet X-site,

Fe-FeS-silicate partitioning of chalcophile and siderophile elements: Implications for core formation

The concentrations of weakly siderophile and chalcophile elements in the primitive mantle provide important constraints on the conditions of accretion and terrestrial core formation, in particular the possible role of late sulphide addition and of late volatile loss. In order to explore the possible impacts of these processes, we have determined the partitioning of V, Cr, Cu, Zn, Ga, Nb, Ag, Tl and Pb between Fe-FeS liquids and a range of liquid silicate compositions at 1.5-14 GPa and temperatures of 1280- 2300 oC. Many of the observed variations in partition coefficient are consistent with the known properties of Fe-alloys. For pure Fe metal coexisting at 1700 oC with a silicate melt of mantle FeO content, the metal/silicate partition coefficients are as follows: V(0.12), Cr(0.3), Cu(36), Zn(0.9), Ga(6), Nb(0.015), Ag(20), Tl(1.1) Pb(3.5). Adding C (at graphite saturation) to the metal increases Metal/silicate partition coefficients of Nb by an order of magnitude, those of V and Cr by a factor of 3, has small negative effects on Cu and Zn and larger negative effects on Ag and Pb partitioning. On going from Fe to FeS liquid the sulphide/silicate partition coefficients change approximately to: V(0.6), Cr(0.9), Cu(100), Zn(2), Ga(3), Nb(0.4), Ag(100), Tl(40), Pb(35). Using these and literature data, a protracted period of accretion under reducing conditions appears to be required to satisfy the observed V and Cr contents of the mantle. This type of accretion path leads to around 30% of earth’s Nb residing in the core. Application of these constraints to the other elements indicates that late sulphide addition to the core can explain the Pb and Tl contents of the mantle and their isotopic signatures, but the amount required (1.5-2%) seems precluded by the strongly chalcophile nature of Cu

Trace element partitioning between majoritic garnet and silicate melt at 25 GPa

We have determined the partitioning of 39 trace elements between majorite-rich garnet and silicate melt at 25 GPa and ∼2300 °C. Concentrations in crystalline and quenched melt phases were determined by both SIMS and laser ablation-ICPMS techniques with, in general, very good agreement in partition coefficients. As anticipated from size and charge considerations, D Lu (where DLu=[Lu]Majorite/[Lu]Melt=0.77) is much greater than DLa (=0.02) and K, U and Th are strongly incompatible. Partition coefficients for isovalent cations entering either the X- or Y-site exhibit a near-parabolic dependence on the radius of the incorporated cations as predicted from the lattice strain model. This underlines the important contribution made by the crystal structure to the control of trace element partitioning. Our data indicate that anhydrous melting at high pressures in the presence of residual garnet could lead to the generation of Al-depleted komatiites. The partitioning data have also been used to assess the potential effects of majoritic garnet fractionation into the deep mantle during a putative magma ocean event in early earth history. Crystallisation of a maximum of 14% majoritic garnet would have slightly depleted the primitive upper mantle in Si and Al without disturbing the chondritic abundance patterns of other refractory elements. The resultant reservoir would have a high Lu/Hf ratio, but a chondritic Sm/Nd ratio and would not be enriched in heat producing elements. © 2004 Published by Elsevier B.V