Niveo-eolian Sediment Deposits in Coastal South Victoria Land, Antarctica: Indicators of Regional Variability in Weather and Climate

A 35 year chronology from 1965 to 2000 of the deposition of wind-blown sediment is constructed from snowpits for coastal southern Victoria Land, Antarctica. Analysis of local meteorology, contemporary eolian sedimentation, and mineralogy confirm a Victoria Valley provenance, while the presence of volcanic tephra is ascribed to an Erebus volcanic province source. Winter foelm winds associated with anticyclonic circulation are considered responsible for transporting fine-grained sediment from the snow- and ice-free Victoria Valley east toward the coast, while cyclonic storms transport tephra north along the Scott Coast. No trend could be identified in the occurrence of either tephra or wind-blown sediments sourced from the Victoria Valley and retrieved from the snowpits; excavated on the Victoria Lower and Wilson Piedmont Glaciers. We infer this to indicate that the region has not undergone a significant change in weather patterns for at least the last 35 years. Our results also confirm the McMurdo Dry Valleys as a regionally significant source of wind-blown sediment.

Atomic composition profile change of SiGe islands during Si capping

The 6% Ge isocomposition profile change of individual SiGe islands during Si capping at 640 degrees C is investigated by atomic force microscopy combined with a selective etching procedure. The island shape transforms from a dome to a {103}-faceted pyramid at a Si capping thickness of 0.32 nm, followed by the decreasing of pyramid facet inclination with increasing Si capping layer thickness. The 6% Ge isocomposition profiles show that the island with more highly Si enriched at its one base corner before Si capping becomes to be more highly Si intermixed along pyramid base diagonals during Si capping. This Si enrichment evolution inside an island during Si capping can be attributed to the exchange of capped Si atoms that aggregated to the island by surface diffusion with Ge atoms from inside the island by both atomic surface segregation and interdiffusion rather than to the atomic interdiffusion at the interface between the island and the Si substrate. In addition, the observed Si enrichment along the island base diagonals is attempted to be explained on the basis of the elastic constant anisotropy of the Si and Ge materials in (001) plane. (c) 2006 American Institute of Physics.

40Ar/39Ar dating of Mn-iron ore of the Sapecado Mine, Qadrilatero Ferrifero, Brazil

The SW region of Amazonian craton presents policyclic evolution between 1.80-1.00 Ga and is comprised of the Rio Negro-Juruena, Rondoniana and Sunsas Provinces. The evolution of this region has being characterized by four orogens: Alto Jauru (1.79-1.74 Ga), Cachoeirinha (1.58-1.52 Ga), Suíte Santa Helena (1.45-1.42 Ga) e Sunsas/Aguapeí (1.0-0.9 Ga). The Alto Jauru orogen consists of TTG gneissic associations, greenstone sequences and intrusive granitoids origined in volcanic arc setting. Eight 40Ar/39Ar step-heating analyses were carried out in minerals (biotiteand hornblende) to investigate the thermal history and crustal evolution of this region. From the Alto Jauru orogen was sampled the gneiss banded and two biotite grains provide large dispersion of apparent ages, suggesting heterogenity in reservoir of the argon. Apparent age diagram yielded integrated ages of 1472 ± 6 Ma, interpreted as minimum ages of regional cooling episode. Three analyses of hornblende present ages varing from 1310 to 1400 Ma, possibly because smaller grain size become more susceptible to argon loss. 40Ar/39Ar step-heating methodology applied on biotite of pyroclastic tuff (U-Pb age about 1758 ± 7 Ma) presented integrated age of 1507 ± 7 Ma. The results found for this terrane demonstrated a geochronological correlation with metamorphic process linked Cachoeirinha orogen. Biotite and hornblende grains separates from granite and a tonalite origined during Cachoeirinha orogen were analyzed and the apparent age diagrams indicated well-defined plateau ages of 1520-1540 Ma. Biotite grains from a granitic sample were analized, and integrated ages about 1526 ± 2 Ma were obtained due argon loss in the initial steps. Thermochronologic history of SW region Amazonian craton is coherent with regional policyclic events and 40Ar/39Ar ages here presented probably correspond to regional cooling period of Cachoeirinha orogen.