ON THE ORIGIN OF GREY AND BLACK ‘COLOURS’ OF ANCIENT POTTERY: ROLE OF Fe AND Ti4+ IONS

It is shown, from comparisons with the optical absorption of minerals, that the black and grey colours of pottery can arise from Fe²⁺ Fe³⁺ and Fe²⁺ Ti⁴⁺ charge-transfer (or electron-hopping) processes, the cations being located at the centres of adjacent edge-sharing octahedra.     

Demography and ethnic continuity in the Tlailotlacan enclave of Teotihuacan: the evidence from stable oxygen isotopes

Mechanisms for the maintenance of Zapotec ethnicity in Tlailotlacan, an enclave of Teotihuacan, Mexico, are reconstructed using oxygen-isotope analyses of bone and enamel phosphate. The data indicate continual interaction among the enclave inhabitants, their Oaxacan homeland, and other diaspora colonies, with no dominant post-marital residence pattern. There is no apparent association between social status and place of origin, and individual agency is indicated by variable acceptance of Teotihuacan traditions. Striking patterns of movement include the sojourning of children in other regions and the relocation of breastfeeding women. These segments of the population would have played major roles in the social and political articulation of diaspora colonies. Foreign signatures in primary burials indicate that some immigrants died shortly after their arrival in Tlailotlacan. Data from secondary burials indicate the transportation of dead relatives or ancestors to Teotihuacan for burial. We conclude that the behaviour of immigrants in ancient and modern cities is analogous in many ways.     

Solubility of corundum + kyanite in H2O at 700°C and 10 kbar: evidence for Al‐Si complexing at high pressure and temperature

The solubility of the assemblage corundum + kyanite in H₂O was determined at 700°C and 10 kbar, using a piston‐cylinder apparatus and rapid‐quench/fluid‐extraction techniques. Weighted mean concentrations of total Al and Si were 5.80 ± 0.03 mmol kg^?1 H₂O and 0.308 ± 0.003 mol kg^?1 H₂O, respectively (1σ errors). The Al concentration is nearly five times higher than that of corundum solubility in pure H₂O. This difference is interpreted to arise from complexing between Si and Al to form HAlSiO_4,aq species. Charged or more polymerized species are also possible, but their abundance cannot be constrained based on these experiments. Assumption of a single aqueous aluminosilicate complex permits calculation of the thermodynamic consequences of Al‐Si interaction in high‐pressure fluids, as well as phase diagrams for the system Al‐Si‐O‐H. Formation of Al‐Si complexes leads to a large increase in dissolved Al with increasing Si in solution, such that Al concentration in equilibrium with kyanite + quartz is predicted to be 7.1 mmolal, higher than with kyanite + corundum. The elevated concentration of Si in deep‐crustal and mantle aqueous fluids suggests that Al must readily be dissolved and transported by Al‐Si complexing in high‐pressure metamorphic and metasomatic environments. The results provide a simple explanation for the common observation of kyanite + quartz segregations in eclogites and Barrovian metamorphic rocks.     

Between crime and colony: interrogating (im)mobilities aboard the convict ship

Recent literature in carceral geography has attended to the importance of mobilities in interrogating the experience and control of spaces of imprisonment, detention and confinement. Scholars have explored the paradoxical nature of incarcerated experience as individuals oscillate between moments of fixity and motion as they are transported to/from carceral environments. This paper draws upon the convict ship – an example yet to gain attention within these emerging discussions – which is both an exemplar of this paradox and a lens through which to complicate understandings of carceral (im)mobilities. The ship is a space of macro-movement from point A to B, whilst simultaneously a site of apparent confinement for those aboard who are unable to move beyond its physical parameters. Yet, we contend that all manner of mobilities permeate the internal space of the ship. Accordingly, we challenge the binary thinking that separates moments of fixity from motion and explore the constituent parts that shape movement. In paying attention to movements in motion on the ship, we argue that studies of carceral mobility must attend to both methods of moving in the space between points A and B; as micro, embodied and intimate (im)mobilities are also played out within large-scale regimes of movement.     

Permeability of the continental crust: dynamic variations inferred from seismicity and metamorphism

The variation of permeability with depth can be probed indirectly by various means, including hydrologic models that use geothermal data as constraints and the progress of metamorphic reactions driven by fluid flow. Geothermal and metamorphic data combine to indicate that mean permeability ( k ) of tectonically active continental crust decreases with depth ( z ) according to log  k  ≈ −14–3.2 log  z , where k is in m² and z in km. Other independently derived, crustal-scale k – z relations are generally similar to this power-law curve. Yet there is also substantial evidence for local-to-regional-scale, transient, permeability-generation events that entail permeabilities much higher than these mean k – z relations would suggest. Compilation of such data yields a fit to these elevated, transient values of log  k  ≈ −11.5–3.2 log  z , suggesting a functional form similar to that of tectonically active crust, but shifted to higher permeability at a given depth. In addition, it seems possible that, in the absence of active prograde metamorphism, permeability in the deeper crust will decay toward values below the mean k – z curves. Several lines of evidence suggest geologically rapid (years to 10³ years) decay of high-permeability transients toward background values. Crustal-scale k – z curves may reflect a dynamic competition between permeability creation by processes such as fluid sourcing and rock failure, and permeability destruction by processes such as compaction, hydrothermal alteration, and retrograde metamorphism.