Geochemistry最新文献

Calc-silicate granulites constitute a relatively small part of the whole granulitic material outcrops characterizing the In Ouzzal terrane (NW Hoggar, South Algeria). However, these rocks preserve a number of spectacular reaction textures that could be effectively used to infer their pressure-temperature-fluid history. These textures are interpreted using P-T and T-X_CO2 grids in the simplified CaO-Al₂O₃-SiO₂-Vapor system. In this process, sequences of reactions have been subdivided into two distinct stages: (i) the early prograde stage that was accompanied by significant rise of temperature from about 800 °C up to 1050 °C at around10 kbar followed by (ii) the decompression stage from about 9 to 6 kbar. During the prograde stage, coarse grained wollastonites were produced according to the reaction calcite + quartz → wollastonite + CO₂. Furthermore, in the peak pressure temperature stage, the reaction producing wollastonite + scapolite from coarse primary garnet consumes CO₂ with temperature increasing from 850 °C to 1000 °C according to the reaction 3grossular + 3CO₂ → 3wollastonite + 2calcite + scapolite. The latest reactions have been occurred during the decompression stage from about 10 kbar to 5 kbar and cooling from 1000 °C to 800 °C. The growth of calcite + quartz around wollastonite besides to garnet coronas between wollastonite, calcite and scapolite are explained by the reaction: calcite + quartz → wollastonite + CO₂ and 3wollastonite + scapolite +2calcite → 3grossular + 3CO₂. The appearance of anorthite around scapolite occurs following a decrease of temperature independently to the fluids according to the reaction scapolite → 3anorthite + calcite. All reactions took place at CO₂ low pressure which was estimated between 0.04 and 0.55.

Limited attention has been given to antimony present in detrital form in the different environmental compartments except for highly polluted systems related in some way to ore deposits. In highly polluted systems, the ultimate sinks of Sb may be the minerals tripuhyite (FeSbO₄) or perhaps schafarzikite (FeSb₂O₄) but how about Sb dynamics in the much more abundant, weakly polluted or ‘non-polluted’ systems? This deficiency in our knowledge is probably related to the perception that the element is mostly present ‘dissolved’ in waters and to a focus on the role of its binding to iron oxyhydroxides in solid phases. Here we evaluate the state of our knowledge in the Sb journey from geological matrices to detrital forms in soils and waters and identify key aspects that require further investigation. In high-temperature environments, Sb demonstrated its striking incompatibility by fractionation into aqueous fluids or sulfide/metallic melts, or by uptake in a few common minerals that accept this element (e.g., rutile or pyrite). In low-temperature environments, Sb enters the structures of minerals with different formation rates and solubilities, creating a confusing impression of being mobile and immobile at the same time. The estimates of Sb concentration in the upper continental crust are scattered and the Sb-bearing mineral(s) there have not yet been identified. Given that sedimentary rocks are consistently enriched in Sb, the carriers could be the clay minerals. In surface water bodies, Sb could be carried predominantly in the particulate fraction, despite the popular belief of the opposite. An important point to consider is the transport of Sb within the suspended particulate matter, not on its surface. In soils, many studies employed sequential extractions to show that Sb accumulates in the ‘residual’ fraction, without ever asking what the nature of this fraction is. Based on these facts (i.e., knowns), we have identified the unknowns regarding detrital Sb on our planet that should preferentially be addressed by future projects if our understanding is to improve.