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171.
Alexandra Navrotsky Hans D Zimmermann Richard L Hervig 《Geochimica et cosmochimica acta》1983,47(8):1535-1538
High temperature solution calorimetry of glasses in the system CaMgSi2O6 (Di)-CaAl2SiO6 (CaTs) show them to have negative enthalpies of mixing with a regular enthalpy parameter, WH, of -11.4 ± 0.7 kcal. Negative heats of mixing between alumina-rich and alumina-poor glasses seem to be a general phenomenon in aluminosilicates and are not confined only to glassy systems containing anorthite as a component. The thermodynamic behavior of glasses in the system SiO2-Ca0.5;AlO2-CaMgO2 appears to vary in a smooth fashion, with small positive heats of mixing near SiO2 and substantial negative heats of mixing for other compositions. The exothermic behavior with increasing may be related to local charge balance of M2+ and Al3+. The negative heats of mixing in MgCaSi2O6-CaAl2SiO6, MgCaSi2O6-CaAl2Si2O8 and NaAlSi3O8-CaAl2Si2O8 glasses are in contrast to the positive heats of mixing found in MgCaSi2O6-CaAl2SiO6 (pyroxene) and NaAlSi3O8-CaAl2Si2O8 (high plagioclase) crystalline solid solutions. 相似文献
172.
173.
Charles R. Stern Rolf Kilian Bettina Olker Eric H. Hauri T. Kurtis Kyser 《Lithos》1999,48(1-4):217-235
Garnet peridotite xenoliths in the Quaternary Pali-Aike alkali olivine basalts of southernmost South America are samples of the deeper portion of continental lithosphere formed by accretion along the western margin of Gondwanaland during the Phanerozoic. Core compositions of minerals in garnet peridotites indicate temperatures of 970 to 1160°C between 1.9 and 2.4 GPa, constraining a geothermal gradient which suggests a lithospheric thickness of approximately 100 km below this region. Previously, this lithosphere may have been heated and thinned to ≤80 km during the Jurassic break-up of Gondwanaland, when widespread mafic and silicic volcanism occurred in association with extension in southern South America. Subsequent cooling, by up to >175°C, and thickening, by about 20 km, of the lithosphere is reflected in low-temperature (<970°C) spinel peridotites by chemical zonation of pyroxenes involving a rimward decrease in Ca, and in moderate- and high-temperature (>970°C) peridotites by textural evidence for the transformation of spinel to garnet. A recent heating event, which probably occurred in conjunction with modal metasomatism related to the genesis of the Pali-Aike alkali olivine basalts, has again thinned the lithosphere to <100 km. Evidence for this heating is preserved in moderate- and high-temperature (>970°C) peridotites as chemical zonation of pyroxenes involving a rimward increase in Ca, and by kelyphitic rims around garnet. The majority of moderate- and high-temperature (>970°C) xenoliths are petrochemically similar to the asthenospheric source of mid-oceanic ridge basalts: fertile (>20% modal clinopyroxene and garnet), Fe-rich garnet lherzolite with major element composition similar to estimates of primitive mantle, but large-ion-lithophile and light-rare-earth element depletion relative to heavy-rare-earth elements, and with Sr, Nd, Pb, Os, and O isotopic compositions similar to MORB. In contrast, infertile, Mg-rich spinel harzburgite is predominant among low-temperature (<970°C) xenoliths. This implies a significant chemical gradient and increasing density with depth in the mantle section represented by the xenoliths, and the absence of a deep, low density, olivine-rich root below the southernmost South American crust such as has been inferred below Archean cratons. With respect to both temperature/rheology and chemistry/density, the subcontinental mantle lithosphere below southernmost South America is similar to that below oceanic crust. It is interpreted to have formed by tectonic capture, during the Paleozoic, of a segment of what had previously been oceanic lithosphere generated at a late Proterozoic mid-oceanic spreading ridge. 相似文献
174.
The study of particulate organic matter (OM) in Arctic Ocean sediments from the late Cretaceous to the Eocene (IODP Expedition 302) has revealed detailed information about the aquatic/marine OM fluxes, biological sources, preservation and export of terrestrial material. Here, we present detailed data from maceral analysis, vitrinite reflectance measurements and organic geochemistry. 相似文献