Formation of syngenetic and early diagenetic iron minerals in the late Archean Mt. McRae Shale, Hamersley Basin, Australia: New insights on the patterns, controls and paleoenvironmental implications of authigenic mineral formation

A section through the late Archean Mt. McRae Shale comprising, in ascending order, a lower shale interval (LSI), a banded iron formation (BIF), an upper shale (USI) and a carbonate (C1) has been analyzed for total Fe and Al contents and authigenic Fe present as carbonate, oxide, sulfide and silicate phases. The authigenic mineralogy is controlled by the episodic addition of Fe from hydrothermal activity and removal of Fe by sulfide, relative to rates of clastic sedimentation. The LSI and BIF have mean Fe_T/Al values of 2 and 5, respectively, that record iron enrichment from hydrothermal sources. Iron was precipitated primarily as siderite accompanied by Fe-rich chlorite from anoxic bottom waters rich in dissolved Fe. Pyrite formation was probably limited by the availability of sulfate, which was present at low concentrations and became rapidly depleted. The USI has generally lower Fe_T/Al values (0.6-1.3), similar to those found in Paleozoic shales, with the exception of one interval where enrichment may reflect either a weak hydrothermal source or the operation of an iron shuttle. This interval contains authigenic Fe predominantly as pyrite, where high values for DOP (>0.8) indicate the existence of a water column that became rich in dissolved sulfide (euxinic) when sulfate concentrations increased due to a transient or secular increase in ocean/atmosphere oxygenation. High concentrations of dissolved sulfide maintained low concentrations of dissolved Fe, which allowed only minor amounts of Fe to be precipitated as carbonates and silicates. The USI also has elevated concentrations of organic matter that most probably reflect increased productivity and likely limited euxinia to midportions of the water column on the basin margin. The carbonate C1 represents a basinal chemistry where sulfide has been removed and Fe_T/Al values are ∼1 indicating that hydrothermal activity again produced dissolved Fe-rich bottom waters. Detailed iron speciation of the Mt. McRae Shale can be used to recognize spatial and temporal variations in iron and sulfur inputs to the late Archean Hamersley Basin, just prior to the Paleoproterozoic rise in atmospheric oxygenation, and our refined methods have relevance to all Fe-rich deposits.     

An Adaptive Code for Radial Stellar Model Pulsations

We describe an implicit 1–D adaptive mesh hydrodynamics code that is specially tailored for radial stellar pulsations. In the Lagrangian limit the code reduces to the well tested Fraley scheme. The code has the useful feature that unwanted, long lasting transients can be avoided by smoothly switching on the adaptive mesh features starting from the Lagrangean code. Thus, a limit cycle pulsation that can readily be computed with the relaxation method of Stellingwerf will converge in a few tens of pulsation cycles when put into the adaptive mesh code. The code has been checked with two shock problems, viz. Noh and Sedov, for which analytical solutions are known, and it has been found to be both accurate and stable. Superior results were obtained through the solution of the total energy (gravitational + kinetic + internal) equation rather than that of the internal energy only. This revised version was published online in July 2006 with corrections to the Cover Date.     

Eclogite-facies fluid infiltration: constraints from δ18O zoning in garnet

In situ analysis reveals that eclogite-facies garnets are zoned in δ¹⁸O with lower values in the core and rims that are ~1.5 to 2.5 ‰ higher. This pattern is present in 9 out of 12 garnets analyzed by SIMS from four orogenic eclogite terranes, and correlates with an increase in the mole fraction of pyrope and Mg/Fe ratio from core to rim, indicating prograde garnet growth. At the maximum temperatures and the time-scales experienced by these garnets, calculated intragranular diffusion distances for oxygen are small (<5 μm), indicating that δ¹⁸O records primary growth zoning and not diffusive exchange. The oxygen isotope gradients are larger than could form due to temperature changes during closed-system mineral growth. Thus, gradients reflect the compositions of fluids infiltrating during prograde metamorphism. Values of δ¹⁸O in garnet cores range from ?1 to 15 ‰, likely preserving the composition of the eclogite protoliths. Two garnet cores from the Almenningen eclogite in the Western Gneiss Region, Norway, have δ¹⁸O ~?1 ‰ and are the first negative δ¹⁸O eclogites identified in the region. In contrast with orogenic eclogites, seven high δ¹⁸O garnets (>5 ‰) from two kimberlites are homogeneous in δ¹⁸O, possibly due to diffusive exchange, which is possible for prolonged periods at higher mantle temperatures. Homogeneity of δ¹⁸O in garnets outside the normal mantle range (5–6 ‰) may be common in kimberlitic samples.     

Oxygen isotope zoning in garnets from Franciscan eclogite blocks: evidence for rock–buffered fluid interaction in the mantle wedge

The oxygen isotope compositions of eclogite and amphibolite garnets from Franciscan Complex high-grade blocks and actinolite rinds encasing the blocks were determined to place constraints on their fluid histories. SIMS oxygen isotope analysis of single garnets from five eclogite blocks from three localities (Ring Mountain, Mount Hamilton, and Jenner Beach) shows an abrupt decrease in the δ¹⁸O value by ~1–3 ‰ from core to rim at a distance of ~120 ± 50 μm from the rim in nine out of the 12 garnets analyzed. In contrast, amphibolite garnets from one block (Ring Mountain) analyzed show a gradual increase in δ¹⁸O value from core to rim, implying a different history from that of the eclogite blocks. Values of δ¹⁸O in eclogite garnet cores range from 5.7 to 11.6 ‰, preserving the composition of the eclogite protolith. The abrupt decrease in the δ¹⁸O values of the garnet rims to values ranging from 3.2 to 11.2 ‰ suggests interaction with a lower δ¹⁸O fluid during the final stages of growth during eclogite facies metamorphism (450–600 °C). We hypothesize that this fluid is sourced from the serpentinized mantle wedge. High Mg, Ni, and Cr contents of actinolite rinds encasing the blocks also support interaction with ultramafic rock. Oxygen isotope thermometry using chlorite and phengite versus actinolite of rinds suggests temperatures of 185–240 °C at Ring Mountain and Mount Hamilton. Rind formation temperatures together with the lower δ¹⁸O garnet rims suggest that the blocks were in contact with ultramafic rock from the end of garnet growth through low-temperature retrogression. We suggest a tectonic model in which oceanic crust is subducted at the initiation of subduction and becomes embedded in the overlying mantle wedge. As subduction continues, metasomatic exchange between high-grade blocks and surrounding ultramafic rock is recorded in low δ¹⁸O garnet rims, and later as temperatures decrease, with rind formation.     

A strategy for detecting derelict fishing gear at sea

Derelict fishing gear (DFG) is a highly persistent form of marine pollution known to cause environmental and economic damage. At-sea detection of DFG would support pelagic removal of this gear to prevent and minimize impacts on marine environments and species. In 2008, experts in marine debris, oceanography, remote sensing, and marine policy outlined a strategy to develop the capability to detect and ultimately remove DFG from the open ocean. The strategy includes three interrelated components: understanding the characteristics of the targeted DFG, indirectly detecting DFG by modeling likely locations, and directly detecting pelagic DFG using remote sensing. Together, these components aim to refine the search area, increase the likelihood of detection, and decrease mitigation response time, thereby providing guidance for removal operations. Here, we present this at-sea detection strategy, relate it to relevant extant research and technology, and identify gaps that currently prevent successful at-sea detection and removal of DFG.     

Effect of finite Larmor radius on the gravitational instability of a conducting plasma layer of finite thickness surrounded by a non-conducting matter

The gravitational instability of an incompressible, infinitely conducting plasma layer of finite thickness surrounded a non-conducting matter has been investigated taking into account the effect of the finite Larmor radius. The magnetic field is assumed to be directed parallel to the interfaces. Only the perturbations transverse to the magnetic field are considered, though both the symmetric and asymmetric nature of the perturbations are taken into account. Using the normal mode technique, dispersion relations are obtained.It is found that the finite larmor radius has, in general, a stabilizing influence on the configuration. Even when the system is thoroughly unstable, it has been shown that there exists a critical value of the wave-number, such that the motion is stabilized for wave-numbers of perturbations exceeding this critical value.