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121.
Dexter Perkins Elizabeth Y. Anthony 《Contributions to Mineralogy and Petrology》2011,162(6):1139-1157
In peridotites, olivine, clinopyroxene, and orthopyroxene are complex solid solutions with wide stability fields. Depending
mostly on bulk composition and pressure, these minerals may be accompanied by plagioclase (low pressure), spinel (moderate
pressure), or garnet (high pressure), resulting in 4-phase and rarer 5-phase assemblages. Although a particular mineral assemblage
is stable over a range of P–T, the compositions of the individual minerals vary with changing P–T conditions. Application
of standard geothermobarometers to olivine–clinopyroxene–orthopyroxene–spinel peridotites is problematic. An alternative approach
is to use a bulk rock composition to calculate equilibrium phase diagrams to determine the conditions under which a particular
assemblage is stable. This requires consideration of the 7-component system SiO2–Al2O3–Cr2O3–FeO–MgO–CaO–Na2O, internally consistent thermodynamic data for end members, and reliable mixing models for all mineral solutions. Experimental
studies in simpler systems, and solution models from the literature, permit derivation of multicomponent thermodynamic mixing
models for the key minerals. The models, when applied to xenoliths from Kilbourne Hole, constrain P and T of equilibration
and are less sensitive to mineral compositional variations, or uncertainty in activity models, than standard thermobarometry.
Our modeling provides the first tightly constrained pressure estimates for Kilbourne Hole, placing the xenoliths in the spinel
stability field at depths (30–45 km) that correspond to the uppermost mantle beneath the Rio Grande Rift. The fine-grained
equigranular lherzolite, porphyroclastic lherzolite, and some harzburgite-dunite specimens equilibrated at average conditions
of 11.5 Kbar-930°C, 12 Kbar-990°C, and 13 Kbar-1,080°C, respectively. The mantle beneath the Rio Grande Rift is layered; the
fine-grained equigranular lherzolite derives from relatively shallow depth (35 km average), and the porphyroclastic lherzolite
from slightly deeper levels. Lying 5–10 km beneath both lherzolites, the harzburgite-dunite represents a depth where melt
extraction has significantly altered mantle chemistry and where local thermodynamic equilibrium has not been maintained. 相似文献
122.
N2 2P fluorescent intensity was monitored as a function of incident photon energy from 40 to 70 eV. A structure was seen near 52.2 eV. This feature is attributed to the scattering of photoelectrons originating from the N2 2σg molecular orbital with a binding energy of about 37.7 eV. The kinetic energy of these photoelectrons corresponds to the peak of the 2P excitation cross section at 14.5 eV. 相似文献