Arsenic release from biotite into a Holocene groundwater aquifer in Bangladesh

Continuous core sediments (to a depth of 90.1 m) taken at a transitional area of Holocene and Pleistocene deposits in Sonargaon, Bangladesh were characterized for their mineralogy and chemistry. Among the sediments of the lower part of the Holocene aquifer (depth: 18–29 m), where most domestic wells are installed, As is mostly fixed in biotite and organic phases. A positive correlation of As concentration with those of Al and Fe but not that of total organic C clearly suggests that biotite is a primary source of As. Although microbial reduction–dissolution of As-containing Fe oxyhydroxides is thought to cause As-enriched groundwater in the Ganges–Brahmaputra–Meghna delta plain, the authors conclude that chemical weathering of biotite is the primary formation mechanism and prevailing reducing conditions contribute to the expansion of As-enriched groundwater in the study area.     

Transition metals in the transition zone: partitioning of Ni,Co, and Zn between olivine,wadsleyite, ringwoodite,and clinoenstatite

Ni, Co, and Zn are widely distributed in the Earth’s mantle as significant minor elements that may offer insights into the chemistry of melting in the mantle. To better understand the distribution of Ni²⁺, Co²⁺, and Zn²⁺ in the most abundant silicate phases in the transition zone and the upper mantle, we have analyzed the crystal chemistry of wadsleyite (Mg₂SiO₄), ringwoodite (Mg₂SiO₄), forsterite (Mg₂SiO₄), and clinoenstatite (Mg₂Si₂O₆) synthesized at 12–20 GPa and 1200–1400 °C with 1.5–3 wt% of either NiO, CoO, or ZnO in starting materials. Single-crystal X-ray diffraction analyses demonstrate that significant amounts of Ni, Co, and Zn are incorporated in octahedral sites in wadsleyite (up to 7.1 at%), ringwoodite (up to 11.3 at%), olivine (up to 2.0 at%), and clinoenstatite (up to 3.2 at%). Crystal structure refinements indicate that crystal field stabilization energy (CFSE) controls both cation ordering and transition metal partitioning in coexisting minerals. According to electron microprobe analyses, Ni and Co partition preferentially into forsterite and wadsleyite relative to coexisting clinoenstatite. Ni strongly prefers ringwoodite over coexisting wadsleyite with \({D}_{\text{Ni}}^{\text{Rw}/\text{Wd}}\)?=?4.13. Due to decreasing metal–oxygen distances with rising pressure, crystal field effect on distribution of divalent metal ions in magnesium silicates is more critical in the transition zone relative to the upper mantle. Analyses of Ni partitioning between the major upper-mantle phases implies that Ni-rich olivine in ultramafic rocks can be indicative of near-primary magmas.     

Mass dispersal and angular momentum transfer during collisions between rubble-pile asteroids

We perform N-body simulations of impacts between initially non-rotating rubble-pile asteroids, and investigate mass dispersal and angular momentum transfer during such collisions. We find that the fraction of the dispersed mass (Mdisp) is approximately proportional to , where Qimp is the impact kinetic energy; the power index α is about unity when the impactor is much smaller than the target, and 0.5?α<1 for impacts with a larger impactor. Mdisp is found to be smaller for more dissipative impacts with small values of the restitution coefficient of the constituent particles. We also find that the efficiency of transfer of orbital angular momentum to the rotation of the largest remnant depends on the degree of disruption. In the case of disruptive oblique impacts where the mass of the largest remnant is about half of the target mass, most of the orbital angular momentum is carried away by the escaping fragments and the efficiency becomes very low (<0.05), while the largest remnant acquires a significant amount of spin angular momentum in moderately disruptive impacts. These results suggest that collisions likely played an important role in rotational evolution of small asteroids, in addition to the recoil force of thermal re-radiation.     

Numerical Investigations of Mean Winds Within Canopies of Regularly Arrayed Cubical Buildings Under Neutral Stability Conditions

Recently, several attempts have been made to model the wind velocity in an urban canopy in order to accurately predict the mixing and transport of momentum, heat, and pollutants within and above the canopy on an urban scale. For this purpose, unverified assumptions made by Macdonald (Boundary-Layer Meteorol 97:25–45, 2000) to develop a model for the profile of the mean wind velocity within an urban canopy have been used. In the present study, in order to provide foundations for improving the urban canopy models, the properties of the spatially-averaged mean quantities used to make these assumptions have been investigated by performing large-eddy simulations (LES) of the airflow around square and staggered arrays of cubical blocks with the following plan area densities: λ _p = 0.05, 0.11, 0.16, 0.20, 0.25, and 0.33. The LES results confirm that the discrepancy between the spatial average of wind velocity and Macdonald’s five-point average of wind velocity can be large in both types of arrays for large λ _p . It is also confirmed that Prandtl’s mixing length varies significantly with height within the canopy, contrary to Macdonald’s assumption for both types of arrays and for both small and large λ _p . On the other hand, in accordance with Macdonald’s assumption, the sectional drag coefficient is found to be almost constant with height except in the case of staggered arrays with high λ _p .     

Lawsonite-bearing eclogitic metabasites in the Cazadero area, northern California

Summary In the Cazadero area, northern California, Lawsonite-bearing eclogitic metabasites occur in association with glaucophane schists. Lawsonite-bearing eclogitic metabasites are coarse-grained, and characteristically lack albite. Representative mineral assemblages are; (1) garnet + omphacite + Lawsonite + epidote + glaucophane + chlorite + quartz, (2) garnet -F- omphacite + Lawsonite + pumpellyite + glaucophane + actinolite + quartz, (3) garnet + omphacite + Lawsonite + pumpellyite + epidote + glaucophane + quartz. They can be represented on an A1₂O₃-Fe₂O₃-MgO-Na₂O diagram in which all minerals are projected from quartz, Lawsonite, almandine garnet, and H₂O-predominant fluid. On the basis of the garnet-clinopyroxene geothermometry and phase relations, the metamorphic conditions for the formation of Lawsonite-bearing eclogitic metabasites are estimated at 360-445 °C and more than 9 ± 1 kbar. Lawsonite-bearing eclogitic metabasites formed near the univariant curve albite = jadeite +quartz. A petrogenetic grid constructed by Schreinemakers' method shows that the Lawsonite-bearing eclogitic metabasites in the Cazadero area formed under transitional P-T conditions between those of the garnet-bearing glaucophane schists in New Caledonia and lawsonitebearing eclogitic metabasites in Corsica.Zusammenfassung Im Gebiet um Cazadero, Nordkalifornien, kommen Lawsonit-führende eklogitische Metabasite in Assoziation mit Glaukophanschiefern vor. Den grobkörnigen Lawsonitführenden Metabasiten fehlt charakteristischerweise Albit. Repräsentative Mineralparagenesen sind: (1) Granat + Omphacit + Lawsonit + Epidot + Glaukophan + Chlorit + Quarz, (2) Granat + Omphacit + Lawsonit + Pumpellyit + Glaukophan + Aktinolith + Quarz, (3) Granat + Omphacit +Lawsonit + Pumpellyit + Epidot + Glaukophan + Quarz. Sie lassen sich in einem A1₂O₃-Fe₂O₃-MgO-Na₂O Diagramm, in dem alle Minerale von Quarz, Lawsonit, Almandin-reichem Granat und einem H₂O-dominierten Fluid projiziert werden, darstellen. Die Metamorphosebedingungen der Lawsonitführenden eklogitischen Metabasite werden auf Grund von Granat-Klinopyroxenthermometrie und der Phasenbeziehungen mit 360-445°C und mehr als 9 ± 1 kbar abgeschätzt. Die Lawsonit-führenden eklogitischen Metabasite bildeten sich nahe der univarianten Reaktion Albit = Jadeit +Quarz. Ein petrogenetisches Netz, konstruiert nach der Schreinemakers Methode, zeigt, daß die P-T Bedingungen der Lawsonitführenden eklogitischen Metabasite im Gebiet von Cazadero im übergangsbereich zwischen jenen von Granat-führenden Glaukophanschiefern in Neukaledonien und Lawsonit-führenden eklogitischen Metabasiten in Korsika liegen.

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Lawsonit-führende eklogitische Metabasite im Gebiet um Cazadero, Nordkalifornien

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With 9 Figures     

Community structures of coral reefs around Peninsular Malaysia

Coral community structures at eleven fringing reef sites were investigated along the coast of Peninsular Malaysia. Estimated coverage of coral communities is examined by applying quantitative digital image analysis to the line transect method. Four coral community types were characterized by dominant genera and lifeforms: Acropora branching community, Montipora-Acropora community, Porites massive community, and Heliopora community. Live coral coverage in all study sites ranged from 17.9% to 68.6%. Most reefs were in “fair” condition while some reefs were shown to be in “poor” condition. Coral community structures among the study sites were divided into the west coast of Peninsular Malaysia with a dominant Porites massive lifeform, and the east coast with a variety of lifeform categories of Montipora and Acropora. Physical effects such as the monsoon wind regime and sedimentation are likely to influence the formation of dominant coral communities around Peninsular Malaysia.     

Elemental abundances in chondrules from unequilibrated chondrites: Evidence for chondrule origin by melting of pre-existing materials

Bulk abundances of Na, Mg, Al, Ca, Sc, V, Cr, Mn, Fe, Co, Ni, Zn, La, Sm, Eu, Yb, Lu, Ir, and Au were determined by neutron activation analysis of chondrules separated from unequilibrated H-, L-, and LL-chondrites (Tieschitz, Hallingeberg, Chainpur, Semarkona) and correlated with chondrule petrographic properties. Despite wellknown compositional differences among the whole-rock chondrites, the geometric mean compositions of their respective chondrule suites are nearly indistinguishable from each other for many elements. Relative to the condensible bulk solar system (approximated by the Cl chondrite Orgueil), chondrules are enriched in lithophile and depleted in siderophile elements in a pattern consistent with chondrule formation by melting of pre-existing materials, preceded or attended by silicate/metal fractionation. Relative to nonporphyritic chondrules, porphyritic chondrules are enriched in refractory and siderophile elements, suggesting that these two chondrule groups may have formed from different precursor materials.