On the origin of rim textures surrounding anhydrous silicate grains in CM carbonaceous chondrites

Abstract— Fine‐grained, optically opaque rims coat individual olivine and pyroxene grains in CM matrices and chondrules. Bulk chemical analyses and observations of these rims indicate the presence of phyllosilicates and disseminated opaques. Because phyllosilicates could not have survived the chondrule formation process, chondrule silicate rims must have formed entirely by late‐state aqueous reactions. As such, these textures provide a useful benchmark for isolating alteration features from more complex CM matrix materials. Both chondrule silicate and matrix silicate rims exhibit morphological features commonly associated with advancing stages of replacement reactions in terrestrial serpentinites. Contacts between many matrix silicate rims and the adjacent matrix materials suggest that these rims formed entirely by aqueous reactions in a parent‐body setting. This contrasts with previous assertions that rim textures can only form by the accretion of nebular dust but does not imply an origin for the rims surrounding other types of CM core components, such as chondrules.     

Wet-oxidation and automated colorimetry for simultaneous determination of organic carbon, nitrogen and phosphorus dissolved in seawater

We developed a simple and reliable method which allows simultaneous determination of organic forms of carbon (DOC), nitrogen (DON) and phosphorus (DOP) dissolved in seawater. Conversion of dissolved organic matter (DOM) to inorganic products (carbon dioxide, nitrate+nitrite and soluble reactive phosphate) is performed by a persulfate wet-oxidation in low alkaline condition. After oxidation, the concentration of the inorganic products dissolved in the sample was measured automatically by colorimetry using a 3-channel Technicon AutoAnalyzer system. A number of pure organic compounds were tested in the concentration range encountered in coastal and open ocean, indicating a high efficiency of the digestion procedure. The recovery range is similar to that obtained by other wet-oxidation procedures and by high-temperature catalytic oxidation techniques. Direct comparisons with usual methods used for separate determination of DOC, DON and DOP indicated a high efficiency of the procedure. Reproducibility tests demonstrated a very good precision (around 5%) for lagoonal and coastal waters, while precision was sometimes around 10–25% in oligotrophic oceanic waters, especially for DOP where values approached limits of detection for measuring phosphate. This method is highly suitable for routine analysis and especially appropriate for shipboard work.     

Isotopic constraints on time scales and mechanisms of slab material transport in the mantle wedge: evidence from the Simcoe mantle xenoliths, Washington, USA

Spinel harzburgite and websterite mantle xenoliths from Simcoe volcano in southern Washington represent fragments of mantle lithosphere from the back-arc side of the Cascade arc front. Previous studies have shown that metasomatism by either silica-rich fluids or hydrous melts crystallized phlogopite, imparted high oxygen fugacities (0.3 to 1.4 log units above QFM), and more radiogenic Os isotopic compositions on these peridotites. These features are consistent with part or all of the metasomatic agent being derived from the Juan de Fuca slab. New Re–Os, Sm–Nd, Sr, and U–Th–Pb isotopic data shed further light on the origin and composition of the metasomatic agent. The clinopyroxenes from the xenoliths have correlated Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb=18.63–19.55, ²⁰⁷Pb/²⁰⁴Pb=15.56–15.63, ²⁰⁸Pb/²⁰⁴Pb=38.22–38.87). The most radiogenic Pb isotopic compositions extend beyond the most radiogenic Pb isotopic compositions for the Cascade arc lavas and display a shallower trend. Mixtures between Juan de Fuca basalts and pelagic or terrigenous sediments would result in Pb isotopic compositions that are not radiogenic enough in ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb at the high ²⁰⁶Pb/²⁰⁴Pb end of this array. Therefore, models for rapid transfer of components from the slab to the mantle lithosphere are not viable in this case. Instead, a multi-stage model is preferred. In the first stage, the slab component is transferred via fluid or melt into, and reacts with the hanging wall mantle. This results in a residual slab depleted in Pb relative to U and Th, and consequent high U/Pb and Th/Pb. Additional dehydration or melting of the slab imparts this chemical signature to the peridotite in the hanging wall. In the second stage, the hybridized hanging wall peridotite evolves for tens of million years until corner flow drags it down to deeper levels in the mantle wedge where melting occurs in response to higher temperatures. In the third stage, this melt migrates upward where it metasomatizes the mantle lithosphere represented by the Simcoe xenoliths. Trace element compositions of the clinopyroxenes, and the presence of high alkali glasses in the xenoliths, are consistent with the metasomatic agent derived from the hybridized hanging wall being alkali-rich, and possibly similar to potassic-rich lavas found in arc and back-arc settings. These data therefore demonstrate the importance of the hybridized hanging wall mantle above slabs as a source for melts which can be metasomatic agents in the upper mantle, and as a site for storage of material derived from the slab for periods of at least tens of million years.     

Temperature dependence of specific heat capacity and its effect on asteroid thermal models

Abstract— Asteroid thermal models have typically employed a constant value of specific heat capacity, although this parameter is a function of temperature. We examine whether incorporation of the temperature dependence of specific heat capacity is a necessary refinement and find that including it produces considerable differences in thermal history. Though the temperature effect is demonstrated in a representative calculation for a chondritic asteroid with ²⁶Al decay as a heat source, this result also has implications for asteroidal thermal models based on electromagnetic induction heating or heating by impact, as well as for thermal models of the terrestrial planets.     

Simulating compound flooding events in a hurricane

Ocean Dynamics - Compound flooding is usually induced by the concurrence of coastal storm surge and heavy precipitation induced river flooding, with the former involving oceanic processes and the...     

Modification of the deep salinity-maximum in the Southern Ocean by circulation in the Antarctic Circumpolar Current and the Weddell Gyre

The evolution of the deep salinity-maximum associated with the Lower Circumpolar Deep Water (LCDW) is assessed using a set of 37 hydrographic sections collected over a 20-year period in the Southern Ocean as part of the WOCE/CLIVAR programme. A circumpolar decrease in the value of the salinity-maximum is observed eastwards from the North Atlantic Deep Water (NADW) in the Atlantic sector of the Southern Ocean through the Indian and Pacific sectors to Drake Passage. Isopycnal mixing processes are limited by circumpolar fronts, and in the Atlantic sector, this acts to limit the direct poleward propagation of the salinity signal. Limited entrainment occurs into the Weddell Gyre, with LCDW entering primarily through the eddy-dominated eastern limb. A vertical mixing coefficient, κ_V of (2.86 ± 1.06) × 10^?4 m² s^?1 and an isopycnal mixing coefficient, κ_I of (8.97 ± 1.67) × 10² m² s^?1 are calculated for the eastern Indian and Pacific sectors of the Antarctic Circumpolar Current (ACC). A κ_V of (2.39 ± 2.83) × 10^?5 m² s^?1, an order of magnitude smaller, and a κ_I of (2.47 ± 0.63) × 10² m² s^?1, three times smaller, are calculated for the southern and eastern Weddell Gyre reflecting a more turbulent regime in the ACC and a less turbulent regime in the Weddell Gyre. In agreement with other studies, we conclude that the ACC acts as a barrier to direct meridional transport and mixing in the Atlantic sector evidenced by the eastward propagation of the deep salinity-maximum signal, insulating the Weddell Gyre from short-term changes in NADW characteristics.     

Evaluating post-hurricane impacts in co-management areas: a framework for expert consensus

Natural Hazards - Natural ecosystems are characterized as dynamic systems that evolve through natural patterns of disturbance. Land managers can work within this system of natural disturbance by...     

Approximate computer analysis of pile groups subjected to loads and ground movements

This paper describes the development of an approximate approach for the analysis and design of piles subjected to axial and lateral loading and also to vertical and horizontal ground movements. The analysis involves a number of simplifications in order to make it feasible to implement. For example, it considers the behaviour of a ‘representative’ pile in a group to characterize the behaviour of all piles in the group, and adopts approximations to derive free-field interaction factors from the conventional interaction factors for direct loading. The analysis has been implemented via a computer program called EMbankment PIle Group (EMPIG) and has the ability to incorporate the following features:

• 1. single piles or pile groups,
• 2. applied vertical, lateral and moment loading on the pile cap,
• 3. the effects of axial and lateral soil movements caused by embankment construction,
• 4. a layered soil profile,
• 5. non-linear axial and lateral response of the piles.

Comparisons between solutions from EMPIG and other independent programs suggest that it is capable of providing results of adequate accuracy for practical design purposes. The analysis has been used to investigate the effects of pile rake on a typical bridge abutment group. The presence of raked piles can have a detrimental effect on group behaviour, especially in the presence of ground movements. Large lateral deflections can be generated and axial forces and moments in the piles are increased. Comparisons are also made with the results of centrifuge model tests on abutment pile groups. Copyright © 1999 John Wiley & Sons, Ltd.