Zinc-rich Pyrite from the TAG Active Mound, the TAG Hydrotherma Field, Mid-Atlantic Ridge

Abstract: Pyrite rich in Zn, up to 3.1 wt%, was found in the TAG active mound of the TAG hydrothermal field, the slow-spreading Mid-Atlantic Ridge at 26°08'N and 44°49'W. The Zn-rich pyrite is characterized by an optical homogeneity, a homogeneous distribution of Zn in the back-scattered electron images, both at a magnification of about 500, a negative correlation between Fe and Zn contents of the pyrite and a rather small unit cell edge (a₀ = 5.4117 ± 0.0008Å), strongly indicating that the detected Zn is present in the pyrite in solid solution. Such Zn concentrations are observed exclusively in dendritic pyrite, suggesting that the Znrich pyrite grew from hydrothermal fluids of a high degree of supersaturation due to quenching on the seafloor.     

AstroGrid: A place for your science

Nicholas A Walton, Silvia Dalla, Eduardo Gonzalez-Solarez, Anita Richards and Jonathan Tedds show off the fast-developing astronomical applications of the UK's virtual observatory service.     

Fracture in Westerly Granite under AE Feedback and Constant Strain Rate Loading: Nucleation, Quasi-static Propagation, and the Transition to Unstable Fracture Propagation

New observations of fracture nucleation are presented from three triaxial compression experiments on intact samples of Westerly granite, using Acoustic Emission (AE) monitoring. By conducting the tests under different loading conditions, the fracture process is demonstrated for quasi-static fracture (under AE Feedback load), a slowly developing unstable fracture (loaded at a `slow' constant strain rate of 2.5 × 10<sup>−6</sup> /s) and an unstable fracture that develops near instantaneously (loaded at a `fast' constant strain rate of 5 × 10⁻⁵ /s). By recording a continuous ultrasonic waveform during the critical period of fracture, the entire AE catalogue can be captured and the exact time of fracture defined. Under constant strain loading, three stages are observed: (1) An initial nucleation or stable growth phase at a rate of ~ 1.3 mm/s, (2) a sudden increase to a constant or slowly accelerating propagation speed of ~ 18 mm/s, and (3) unstable, accelerating propagation. In the ~ 100 ms before rupture, the high level of AE activity (as seen on the continuous record) prevented the location of discrete AE events. A lower bound estimate of the average propagation velocity (using the time-to-rupture and the existing fracture length) suggests values of a few m/s. However from a low gain acoustic record, we infer that in the final few ms, the fracture propagation speed increased to 175 m/s. These results demonstrate similarities between fracture nucleation in intact rock and the nucleation of dynamic instabilities in stick slip experiments. It is suggested that the ability to constrain the size of an evolving fracture provides a crucial tool in further understanding the controls on fracture nucleation.     

Resource-based development in the Russian Far East: Problems and prospects

The paper examines the prospects for resource-based development in the Russian Far East. It adopts a critical perspective on the potential for resource-based development by examining problems with Far East resource industries, specifically by looking at the experience of the other resource economies more generally. In particular, it highlights the new geo-economic context for resource development in the Far East of Russia as the world economy approaches the much touted, and now questioned, ‘Pacific Century’.     

Rare earth elements in Huronian (Lower Proterozoic) sedimentary rocks: Composition and evolution of the post-Kenoran upper crust

The Huronian sequence (Lower Proterozoicl. north of Lake Huron, contains tillites and abundant fine-grained sedimentary rocks. Analyses of rare earth elements (REE) in the matrix of tillite samples from the Gowganda Formation (~ 2.3 Gal is considered to be a reasonable estimate of upper crustal REE abundances for the region north of Lake Huron at the time of Gowganda deposition. The average is characterized by a moderately steep pattern (σLREEσHREE = 9.1) and a slight negative europium anomaly ($\text{Eu}\text{Eu1}\text{= 0.89}$). This pattern is similar to estimates of the composition of the surface of the Canadian Shield and is intermediate between estimates of typical Archean and post-Archean sedimentary rocks. REE patterns for framework granitoid clasts from the tillite suggest that K-rich granites, which were apparently unimportant in the formation of Archean sedimentary rocks, were abundant in the source regions of the Gowganda Formation. This may explain the intermediate nature of the Gowganda pattern.Comparison of the tillites and associated Gowganda mudstones suggests that previous estimates of upper crustal REE abundances, which were based on the analyses of fine-grained sedimentary rocks, may be systematically high. Relative distributions, however, are the same.Analyses of mudstones from the McKim. Pecors. Serpent Gowganda Lorrain and Gordon Lake Formations suggest rapid evolution in the composition of the exposed upper crust at the close of the Kenoran orogeny. REE patterns at the base of the Huronian are similar to typical Archean sedimentary rocks. REE characteristics change up section: patterns at the top resemble typical post-Archean sedimentary rocks.It is inferred that an essentially episodic change from an early exposed upper crust dominated by a tonalite-greenstone suite to one approximating granodioritic composition is recorded in Huronian sedimentary rocks. A deviation from the evolutionary trend of the Huronian, documented in the Gowganda Formation, may be related to the glacial origin of the Gowganda.