The geochemistry of the PGE Subzone in the Selukwe Subchamber, Great Dyke: an intraformational layer model for Platinum Group Element enrichment in layered intrusions

Summary ?The zone of enrichment of platinum group elements (PGE) in the pyroxenite hosted Main Sulphide Zone (MSZ) of the Great Dyke coincides with the stage at which the magma had become saturated in sulphide. No fundamental characteristics of the MSZ have previously been recognized other than the concentration of PGE at the base of the sulphide zone. This zone of enrichment, referred to here as the PGE Subzone, as well as the footwall and hangingwall sequences to the PGE Subzone, are recognized in this work as comprising a series of petrologically and chemically distinct rock layers. Characterization is on the basis of (a) distinct Pd:Pt ratios, (b) cyclicity in pyroxene compositions and, (c) cyclicity in the concentrations of incompatible elements in the whole rock. The incompatible element contents in whole rocks (eg. Zr) are closely linked with the TiO₂ contents of the orthopyroxene. The strong interdependence of these parameters indicates an integrated primary process for the PGE mineralization and does not support an origin by which late stage metasomatising fluids transported PGE into the mineralized zone. It also does not support previous models for the MSZ based on progressive fractional segregation of sulphide as a continuous process extracting PGE. An intraformational layer model is suggested by which liquid layers in a double diffusive convective system break down and become sequentially incorporated into the crystallizing zone resulting in offset chemical trends in silicate compositions together with extraction of PGE into sulphide of effectively constant values for Pd:Pt ratio. Reequilibration of pyroxene is recognized for each of the sublayers as well as minor redistribution of PGE across the boundaries of the sublayers.

---

Zusammenfassung ?Die Geochemie der PGE – Subzone in der Selukwe Subchamber, Great Dyke: ein intraformationales Modell für Anreicherung von Platingruppenelementen in geschichteten Intrusionen Die Anreicherungszone von Platinelementen in der Main Sulphide Zone (MSZ) in den Pyroxeniten des Great Dyke entspricht dem Entwicklungsstadium, in dem das Magma an Sulfiden ges?ttigt wurde. Bisher wurden keine grunds?tzlichen Eigenschaften der MSZ erkannt, abgesehen von der Anreicherung der PGE an der Basis der Sulfidzone. Diese Anreicherungszone, die wir hier als PGE Subzone bezeichnen, wie auch die Abfolgen in ihrem Hangenden und im Liegenden enthalten eine Serie von petrologisch und chemisch eindeutig charakterisierten Gesteinsschichten. Diese Erkenntnis beruht auf (a) klaren Pd-Pt-Verh?ltnissen, (b) der Zyklizit?t der Pyroxenzusammensetzungen und (c) den Konzentrationen inkompatibler Elemente im Gesamtgestein. Letztere (z.B. Zr) sind eng mit den TiO₂-Gehalten der Orthopyroxene verbunden. Die deutliche gegenseitige Abh?ngigkeit dieser Parameter weist auf einen integrierten prim?ren Prozess für die PGE-Vererzung hin und liefert keine Unterstützung für eine Entstehung, bei der sp?te metasomatische Fluide PGE in die mineralisierte Zone transportiert haben. Es unterstützt auch nicht frühere Modelle für die MSZ, die auf einer progressiven fraktionalen Trennung von Sulfiden als einem kontinuierlichen Anreicherungsprozess für PGE beruhten. Ein intraformationales Schichtmodell wird hier vorgeschlagen, wobei flüssige Lagen in einem doppelt diffusiven Konvektionssystem aufbrechen und sequentiell in die kristallisierte Zone inkorporiert werden. Dies resultiert in abweichenden chemischen Trends für Silikat-Zusammensetzungen und ebenso auch in einer Extraktion der PGE in Sulfide mit konstanten Pd/Pt-Verh?ltniszahlen. Reequilibrierung von Pyroxenen wird für jede der Sublayers nachgewiesen, wie auch geringfügige Umverteilung der PGE über die Grenzen der einzelnen Sublayers hinaus.

---

---

Received November 30, 1998;/revised version accepted July 12, 1999     

Neogene sutures in eastern Indonesia

Five suture zones are described from the zone of collision between the Eurasian, Indian–Australian and Pacific–Philippine Sea plates within the eastern Indonesia region. These are the Molucca, Sorong, Sulawesi, Banda and Borneo sutures. Each of these sutures has a relatively short history compared to most pre-Neogene orogenic belts, but each preserves a record of major changes in tectonics including subduction polarity reversals, elimination of volcanic arcs, changing plate boundaries, and important extension within an overall contractional setting. Rapid tectonic changes have occurred within periods of less than 5 Ma. Many of these events, although important, would be overlooked in older orogenic belts because the age resolution required to identify them, even when the evidence is preserved, is simply not possible.     

Cooling rates of hyaloclastites: applications of relaxation geospeedometry to undersea volcanic deposits

 Glass fragments from three different hyaloclastites have been used to evaluate the range of cooling rates experienced by undersea volcanic deposits. We found that the glass fragments retain structures with a range of apparent quench rates from 25 to 0.15 K min^–1. The most rapid cooling rates are interpreted to be those resulting from cooling of the lava near the water interface. Simple conductive cooling models produce a range of quench rates comparable to those of the more rapidly cooled samples. The very slow apparent quench rates are unlikely to result from simple linear cooling through the glass transition, because of the onset of crystallization; instead, they are indicators of a more complex thermal history that involves the annealing of glasses at temperatures within the glass transition interval for a dwell time sufficient to allow the relaxation of the glass to lower temperature structures. The thermal history recorded in these samples illustrates the complexity of eruptive processes and demonstrates that quench rates for natural glasses retain information relevant to more complex cooling models. Received: 10 February 1999 / Accepted: 9 September 1999     

S<Subscript>2</Subscript> tide aliasing in GRACE time-variable gravity solutions

Errors in high-frequency ocean tide models alias to low frequencies in time-variable gravity solutions from the Gravity Recovery and Climate Experiment (GRACE). We conduct an observational study of apparent gravity changes at a period of 161 days, the alias period of errors in the S₂ semidiurnal solar tide. We examine this S₂ alias in the release 4 (RL04) reprocessed GRACE monthly gravity solutions for the period April 2002 to February 2008, and compare with that in release 1 (RL01) GRACE solutions. One of the major differences between RL04 and RL01 is the ocean tide model. In RL01, the alias is evident at high latitudes, near the Filchner-Ronne and Ross ice shelves in Antarctica, and regions surrounding Greenland and Hudson Bay. RL04 shows significantly lower alias amplitudes in many of these locations, reflecting improvements in the ocean tide model. However, RL04 shows continued alias contamination between the Ronne and Larson ice shelves, somewhat larger than in RL01, indicating a need for further tide model improvement in that region. For unknown reasons, the degree-2 zonal spherical harmonics (C₂₀) of the RL04 solutions show significantly larger S₂ aliasing errors than those from RL01.     

Isotopic variation (δN, δC,and δS) with body size in post-larval estuarine consumers

Stable isotope analyses were conducted on fish and crabs in Apalachicola Bay, Florida (USA) to determine whether δ¹⁵N values are correlated with length within these species. Our objective was to define the smallest trophic unit of the system as a prelude to further isotopic evaluations of consumer trophic level in Apalachicola Bay. In some cases, the smallest trophic unit is the individual species; however, previous gut content analysis on organisms in this system indicated that subdivision by size class within species might be required. However, over the size ranges observed, there was no systematic change in the δ¹⁵N values of individual consumers. Analysis of the δ¹³C and δ³⁴S values of these species reveal no obvious shifts in the utilization of organic matter sources with length that could be obscuring a trend in the δ¹⁵N signal. We find no isotopic evidence for a systematic trophic level increase within the species examined over the size ranges observed in this estuary. These results indicate that species rather than size class is the best predictor of trophic habit among these Apalachicola Bay consumers.     

An equilibrium profile model for retreating marsh shorelines in southeast Louisiana

Louisiana's coastal marshes are experiencing the highest wetland loss rates in the U.S., in part due to subsidence-driven relative sea-level rise. These marshes are also vulnerable to the erosive power of wave attack: 1) on the marsh edge adjacent to open-water bodies, and 2) after the marsh platform is submerged. Marsh shorelines in Barataria Bay, Breton Sound, and the active Balize delta of southeastern Louisiana were examined in areas where the subaerial marsh platform had disappeared since 1932. Vibracore transects of marsh and adjacent bay surface sediments (to 2 m depth) were analyzed using geotechnical, stratigraphic, and radiochemical (137-Cs and 210-Pb) methods, and the subaerial-to-subaqueous transition of the marsh was mapped for elevation using standard stadia rod transit and fathometer measurements. Results indicate that marsh edge erosion of the platform takes place subaqueously until water depths of 1.5 m are reached. This is observed even in interior pond regions, but the shoreface elevation profiles are a function of fetch: exposed open bay sites display greater incision (depth and rate) of the marsh platform than protected interior bay or pond sites. Core stratigraphy reveals that the outer part of the subaqueous platform switches from erosional to depositional as retreat proceeds, covering the incised marsh deposits unconformably with estuarine shelly muds. 137-Cs and excess 210-Pb activity indicates that these muds are deposited within a few decades of subaerial marsh loss. The consistency of the cross-shore profile results suggests that a single profile of equilibrium can approximate the morphology of eroding marsh edges in southeast Louisiana: platform stratigraphy and resistance to erosion have a limited effect on profile shape. This equilibrium profile and remote sensing images of shoreline change are used to estimate the sediment yield to adjacent estuarine areas by this process. On average, 1.5 m³ of sediment are yielded per m shoreline length annually from both Barataria Bay and Breton Sound. Due to the highly organic nature of the eroded sediment (30%), this supply of organic-rich material could significantly impact estuarine productivity and hypoxia on the Louisiana continental shelf.