Oscillatory zoning and trace element incorporation in hydrothermal minerals: insights from calcite growth experiments

Oscillatory zoning and fine‐scale variations in trace element chemistry are commonly observed in hydrothermal minerals. It has been suggested that fine‐scale chemical variations are caused by extrinsic changes in the parent hydrothermal system, such as varying fluid composition, pressure or temperature, as well as changes in mineral growth rate. In this study, LA–ICP–MS (laser ablation, inductively coupled plasma mass spectrometer) analyses were carried out on calcite crystals grown in Ca–NH₃–Cl solutions doped with rare earth elements (REE). The variety of crystal morphologies observed (euhedral to acicular), likely relate to variations in trace element abundance and calcite supersaturation state. Crystals display oscillatory and sector zoning, with significant variations in REE concentrations among zones. Cyclic variations in REE concentrations (exceeding 10‐fold) occur over distances of <1 mm along the growth direction of acicular calcite crystals. In general, trace element concentrations decrease during progressive crystal growth, implying that the concentration of trace and REEs within crystals reflects the overall composition of the growth solution. However, bulk changes in crystal composition are modulated by fine‐scale (<1 mm) variations, which are inferred to be caused by growth‐rate‐controlled incorporation of trace elements. These results have important implications for using hydrothermal minerals to infer fluctuations in fluid compositions in ancient, exhumed hydrothermal systems.     

Contesting Commerce: Gibbons v. Ogden, Steam Power, and Social Change

The U.S. Supreme Court case Gibbons v. Ogden (1824) ¹ represents one of the most significant yet least understood cases in the history of American jurisprudence. Most accounts depict the case as a constitutional showdown between former New Jersey Governor Aaron Ogden and his estranged business partner, a Georgian businessman and planter named Thomas Gibbons. Ogden charged Gibbons with operating a steamboat on the Hudson River in violation of the Fulton–Livingston Steamboat monopoly that controlled steam travel in the state of New York. In March 1824, Chief Justice John Marshall ruled for the Supreme Court that Gibbons' federal coasting license trumped a state grant issued to Ogden by the Fulton–Livingston syndicate. ²     

The application of failure mode diagrams for exploring the roles of fluid pressure and stress states in controlling styles of fracture-controlled permeability enhancement in faults and shear zones

Permeability enhancement associated with deformation processes in faults and shear zones plays a key role in facilitating fluid redistribution between fluid reservoirs in the crust. Especially in high fluid flux hydrothermal systems, fracture-controlled permeability can be relatively short-lived, unless it is repeatedly regenerated by ongoing deformation. Failure mode diagrams in pore fluid factor and differential stress space, here termed λ–σ failure mode diagrams, provide a powerful tool for analysing how fluid pressure and stress states drive failure, associated permeability enhancement and vein styles during deformation in faults and shear zones. During fault-valve behaviour in the seismogenic regime, relative rates of recovery of pore fluid factor, differential stress and fault cohesive strength between rupture events impact on styles of veining and associated, fracture-controlled permeability enhancement in faults and shear zones. Examples of vein-rich fault zones are used to illustrate how constraints can be placed, not just on fluid pressure and stress states at failure, but also on the fluid pressurization and loading paths associated with failure and transitory permeability enhancement in faults and shear zones. This provides insights about when, during the fault-valve cycle, various types of veins can form. The use of failure mode diagrams also provides insights about the relative roles of optimally oriented faults and misoriented faults as hydraulically conductive structures. The analysis highlights the dynamics of competition between fluid pressures and loading rates in driving failure and repeated permeability regeneration in fracture-controlled, hydrothermal systems.     

Fred Halliday,Marxism and the Cold War

Though possibly best known today as a specialist on the Middle East and Islam, it is often forgotten how central the Cold War was in defining Fred Halliday's understanding of world politics before 1989 and indeed even after. Building on the earlier work of Isaac Deutscher and E. H. Carr, Halliday developed a distinct theory of the Cold War which afforded him great insights but ultimately failed in explaining the complexities of the East–West relationship, and why it came to an abrupt conclusion in the late 1980s.