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Séminaires externes 2018-2019

Lundi 3 septembre 2018 à 13h : Katie Smart Insights into Archean volatile cycles and magmatism from diamonds and kimberlites

Diamonds from the 2.7 - 3 Ga Witwatersrand supergroup (South Africa) provide a unique window into carbon and nitrogen cycles on Mesoarchean Earth. Isotope compositional information from the diamonds suggest the presence of recycled nitrogen and relatively oxidized carbon in the upper mantle prior to 3 Ga. The carrier magma for the Witwatersrand diamonds remains speculative due to the scarcity of global kimberlite magmatism before 1.2 Ga, a feature that can be linked to the secular cooling of Earth.

Jeudi 27 septembre 2018 à 13h30 : Osvaldo P. Gonzalez Maurel Triggering of Neogene-Quaternary mafic eruptions in the northern Chilean Andes

The western margin of South America is the active magmatic arc of the Andean Central Volcanic Zone (CVZ) formed on a thick continental crust (70-74 km). The primitive magma composition is difficult to determine due to the scarcity of true basalts, with basaltic-andesite being the least-evolved volcanic product. This lack of mafic rocks is directly related to the significant crustal contribution to the parental magmas during their ascent. Most mafic magmas identified in Neogene and Quaternary volcanoes in the CVZ are isolated eruptions of basaltic-andesite to andesite lavas. Evidence of such rare mafic eruptions is concentrated in the Western Cordillera of northern Chile between 21º10’S to 22º50’S as lava fields erupted from stratovolcanoes and/or scoria cones. Based on new automated mineralogical analyses, whole rock geochemical data, and Sr and Nd isotope ratios of the aforementioned mafic volcanism and combined with the geological framework, yields information about the magmatic evolutionary processes and its spatio-temporal evolution, and even constrains a potential source for the parental magmas. According to macroscopic observations and microscopic petrography, combined with automated mineralogical analyses, all studied rocks are quite similar (overall colour, grain size), that is : porphyritic, containing four main mineral phases as phenocrysts (0.2 - 2.5 mm), plagioclase, orthopyroxene, clinopyroxene and/or olivine. Three main types of mafic rocks were identify based on the ferromagnesian assemblages : (i) olivine-bearing, (ii) olivine- and two-pyroxene-bearing, and (iii) two-pyroxene-bearing. In contrast, whole-rock geochemical and Sr and Nd isotopic data reveal a compositional heterogeneity for the parental magmas (52.5 - 64.8 wt% SiO2 ; 1.76 - 6.33 wt% MgO ; 2 - 382 ppm Cr ; 3 - 80 ppm Ni ; 87Sr/86Sr = 0.70554 - 0.70875 ; 143Nd/144Nd = 0.512208 - 0.512513). The petrogenesis of these rare mafic rocks is resembled by the interplay of magma mixing, assimilation and fractional crystallization (AFC), and assimilation during turbulent ascent (ATA) processes. Mineralogy and geochemistry indicate temporal compositional changes, including the processes involved in the magmatic evolution in time, nevertheless the final composition of the mafic input erupted is related to the spatial framework of the Altiplano-Puna Magma Body (APMB). The spatial distribution of the studied volcanoes define that the least differentiated lava fields are concentrated at the borders of the large partial melt body, where the minimum thickness of the vertical mush column is located. To erupt these Neogene-Quaternary mafic rocks, the parental magmas required a combined effect of a restricted degree of interaction with the APMB melts, which is linked to a rapid ascent due to thermal ductility and faulting features in the mid-upper crust.

Jeudi 11 octobre 2018 à 13h00 : Stacy Carolin, University of Insbruck Speleothems reveal Western Asia rainfall response to abrupt millennial events and orbital forcing, 190-70 kaBP

Western Asia, from the Levant across the Tigris-Euphrates river basin and through Iran, is comprised of several areas labeled as high overall water risk. Dust storms are also a particular health hazard in the area. More palaeoclimate records are needed to improve our understanding of rainfall variability beyond the instrumental record in this region, which currently relies on knowledge gained mostly from lake records and loess deposits. Here we present a speleothem record from northeast Iran spanning 190-70 ka. The site today receives a small amount of rainfall in the winter to spring months, 250 mm total annually, making caves in the region under modern conditions particularly dry. Oxygen isotopes in the calcite layers are also difficult to interpret due to the general location of the cave, which is far inland, causing precipitation to form from water vapor that may have traveled along many rainout pathways and originated from various source waters. Nonetheless, we present a stalagmite oxygen isotope record that reveals shifts to more positive values on a millennial scale that are strikingly similar to the Greenland stadial events. We use additional metal elements (local rainfall proxies) that have precipitated within the speleothem calcite, and a limited number of speleothem oxygen isotope records available in other parts of Western Asia to interpret the cause of the oxygen isotope enrichment in northeast Iran and the associated rainfall changes. The northeast Iran record also reveals significant precession-length period variations, with higher precession in phase with lighter oxygen isotope values. We use atmospheric climate model experiments and several speleothem records with precession-length period variations across Asia to interpret these signals as rainfall pattern or amount changes in either the winter or summer season. Finally, we note the sole deviation of the low-resolution oxygen isotope record from its linear relationship with precessional forcing, which occurs between 155-135 ka. We suggest that the relationship has broken down here due to the presence of large Eurasian ice sheets, which likely had a significant impact on the atmospheric circulation system that typically carries moisture from the Atlantic across Europe and the Mediterranean into Iran.

publié mercredi 14 août 2013