Chemical and physical studies of type 3 chondrites XII: The metamorphic history of CV chondrites and their components

Abstract— The induced thermoluminescence (TL) properties of 16 CV and CV-related chondrites, four CK chondrites and Renazzo (CR2) have been measured in order to investigate their metamorphic history. The petrographic, mineralogical and bulk compositional differences among the CV chondrites indicate that the TL sensitivity of the ～130 °C TL peak is reflecting the abundance of ordered feldspar, especially in chondrule mesostasis, which in turn reflects parent-body metamorphism. The TL properties of 18 samples of homogenized Allende powder heated at a variety of times and temperatures, and cathodoluminescence mosaics of Axtell and Coolidge, showed results consistent with this conclusion. Five refractory inclusions from Allende, and separates from those inclusions, were also examined and yielded trends reflecting variations in mineralogy indicative of high peak temperatures (either metamorphic or igneous) and fairly rapid cooling. The CK chondrites are unique among metamorphosed chondrites in showing no detectable induced TL, which is consistent with literature data that suggests very unusual feldspar in these meteorites. Using TL sensitivity and several mineral systems and allowing for the differences in the oxidized and reduced subgroups, the CV and CV-related meteorites can be divided into petrologic types analogous to those of the ordinary and CO type 3 chondrites. Axtell, Kaba, Leoville, Bali, Arch and ALHA81003 are type 3.0–3.1, while ALH84018, Efremovka, Grosnaja, Allende and Vigarano are type 3.2–3.3 and Coolidge and Loongana 001 are type 3.8. Mokoia is probably a breccia with regions ranging in petrologic type from 3.0 to 3.2. Renazzo often plots at the end of the reduced and oxidized CV chondrite trends, even when those trends diverge, suggesting that in many respects it resembles the unmetamorphosed precursors of the CV chondrites. The low-petrographic types and low-TL peak temperatures of all samples, including the CV3.8 chondrites, indicates metamorphism in the stability field of low feldspar (i.e., <800 °C) and a metamorphic history similar to that of the CO chondrites but unlike that of the ordinary chondrites.     

Metamorphism and aqueous alteration in low petrographic type ordinary chondrites

Abstract— In order to investigate the relative importance of dry metamorphism and aqueous alteration in the history of chondrules, chondrules were hand-picked from the Semarkona (petrographic type 3.0), Bishunpur (3.1), Chainpur (3.4), Dhajala (3.8) and Allegan (5) chondrites, and matrix samples were extracted from the first three ordinary chondrites. The thermoluminescence (TL) properties of all the samples were measured, and appropriate subsets of the samples were analyzed by electron-microprobe and radiochemical neutron activation and the water and H-isotopic composition determined. The TL data for chondrules from Semarkona and Bishunpur scatter widely showing no unambiguous trends, although group B1 chondrules tend to have lower sensitivities and lower peak temperatures compared with group A5 chondrules. It is argued that these data reflect the variety of processes accompanying chondrule formation. The chondrules show remarkably uniform contents of the highly labile elements, indicating mineralogical control on abundance and volatile loss from silicates and loss and recondensation of mobile chalcophiles and siderophiles in some cases. Very high D/H values (up to ～8000‰ SMOW) are observed in certain Semarkona chondrules, a confirmation of earlier work. With increasing petrographic type, mean TL sensitivities of the chondrules increase, the spread of values within an individual meteorite decreases, and peak temperatures and peak widths show trends indicating that the TL is mainly produced by feldspar and that dry, thermal metamorphism is the dominant secondary process experienced by the chondrules. The TL sensitivities of matrix samples also increase with petrographic type. Chainpur matrix samples show the same spread of peak temperatures and peak widths as Chainpur chondrules, indicating metamorphism-related changes in the feldspar are responsible for the TL of the matrix. The TL data for the Semarkona and Bishunpur matrix samples provide, at best, only weak evidence for aqueous alteration, but the matrix contains H with approximately terrestrial D/H values, even though it contains much water. Secondary processes (probably aqueous alteration) presumably lowered the D/H of the matrix and certain chondrules. While chondrule properties appear to be governed primarily by formation processes and subsequent metamorphism, the matrix of Semarkona has a more complex history involving aqueous alteration as a meteorite-wide process.     

A new adiabatic kernel for the MC2 model

Abstract

Traditional semi‐implicit formulations of nonhydrostatic compressible models may not be stable in the presence of steep terrain when pressure gradient terms are split and lagged in time. If all pressure gradient terms and the divergence are treated implicitly, the resulting wave equation for the pressure contains off‐diagonal cross‐derivative terms leading to a highly nonsymmetric linear system of equations. In this paper we present a more implicit formulation of the Mesoscale Compressible Community (MC2) model employing a Generalized Minimal Residual (GMRES) Krylov iterative solver and a more efficient semi‐Lagrangian advection scheme. Open boundaries now permit exact upwind interpolation and the ability to reproduce simulations to machine precision is illustrated for one‐way nesting at equivalent resolution. Numerical simulations of hydrostatic and nonhydrostatic mountain waves demonstrate the stability and accuracy of the new adiabatic kernel. The computational efficiency of the model is reported for 1D Jacobi and 3D Alternating Direction Implicit (ADI) line relaxation preconditioned implemented with a parallel data transposition strategy.     

Can the EU emission trading scheme support CDM forestry?

Abstract

The European Commission is mandated to consider the inclusion of credits from land-use projects under the clean development mechanism (CDM) and joint implementation (JI), beginning with the second period of the European Union's emission trading scheme (ETS) in its report due in July 2006. Temporary credits from afforestation and reforestation under the CDM are seen by many as posing a technical problem for their use under the ETS. This article summarizes three feasible, efficient and environmentally sound alternatives for achieving the integration of such temporary credits in the European emissions trading market starting in 2008. The first proposal integrates tCERs and lCERs (temporary credits) into the EU ETS by allowing for their direct use for compliance purposes. The second proposal builds on the idea of swapping temporary credits for EU allowances (EUAs) by Member States. The third proposal would not require a political decision at the EU level. Instead supportive Member States or private carbon fund operators would agree to swap temporary credits for the CERs or ERUs they hold in their accounts. All three solutions would be linked to a risk-mitigation strategy based on levying a fee or fixing an exchange rate, which would allow governments to hedge the risk of losing temporary credits.     

Determination of Trace Element Mass Fractions in Ultramafic Rocks by HR‐ICP‐MS: A Combined Approach Using a Direct Digestion/Dilution Method and Preconcentration by Coprecipitation

A procedure is described for the determination of thirty‐seven minor and trace elements (LILE, REE, HFSE, U, Th, Pb, transition elements and Ga) in ultramafic rocks. After Tm addition and acid sample digestion, compositions were determined both following a direct digestion/dilution method (without element separation) and after a preconcentration procedure using a double coprecipitation process. Four ultramafic reference materials were investigated to test and validate our procedure (UB‐N, MGL‐GAS [GeoPT12], JP‐1 and DTS‐2B). Results obtained following the preconcentration procedure are in good agreement with previously published work on REE, HFSE, U, Th, Pb and some of the transition elements (Sc, Ti, V). This procedure has two major advantages: (a) it avoids any matrix effect resulting from the high Mg content of peridotite, and (b) it allows the preconcentration of a larger trace element set than with previous methods. Other elements (LILE, other transition elements Cr, Mn, Co, Ni, Cu, Zn, as well as Ga) were not fully coprecipitated with the preconcentration method and could only be accurately determined through the direct digestion/dilution method.