Application of shuttle imaging radar to geologic mapping

Images from the Shuttle Imaging Radar - B (SIR-B) experiment covering the area of the Panamint Mountains, Death Valley, California, were examined in the field and in the laboratory to determine their usefulness as aids for geologic mapping. The covered area includes the region around Wildrose Canyon where rocks ranging in age from Precambrian to Cenozoic form a moderately rugged portion of the Panamint Mountains, including sharp ridges, broad alluviated upland valleys, and fault-bounded grabens. The results of the study indicate that the available SIR-B images of this area primarily illustrate variations in topography, except in the broadly alluviated areas of Panamint Valley and Death Valley where deposits of differing reflectivity can be recognized. Within the mountainous portion of the region, three textures can be discerned, each representing a different mode of topographic expression related to the erosion characteristics of the underlying bedrock. Regions of Precambrian bedrock have smooth slopes and sharp ridges with a low density of gullies. Tertiary monolithologic breccias have smooth, steep slopes with an intermediate density of gullies with rounded ridges. Tertiary fanglomerates have steep rugged slopes with numerous steep-sided gullies and knife-sharp ridges. The three topographic types reflect the consistancy and relative susceptibility to erosion of the bedrock; the three types can readily be recognized on topographic maps. At present, it has not been possible to distinguish on the SIR-B image of the mountainous terrain the type of bedrock, independent of the topographic expression

Excess Silica in Omphacite and the Formation of Free Silica in Eclogite

Silica lamellae in eclogitic clinopyroxene are widely interpreted as evidence of exsolution during decompression of eclogite. However, mechanisms other than exsolution might produce free silica, and the possible mechanisms depend in part on the nature and definition of excess silica. ‘Excess’ silica may occur in both stoichiometric and non-stoichiometric pyroxene. Although the issue has been debated, we show that all common definitions of excess silica in non-stoichiometric clinopyroxene are internally consistent, interchangeable, and therefore equivalent. The excess silica content of pyroxene is easily illustrated in a three-component, condensed composition space and may be plotted directly from a structural formula unit or recalculated end-members. In order to evaluate possible mechanisms for the formation of free silica in eclogite, we examined the net-transfer reactions in model eclogites using a Thompson reaction space. We show that there are at least three broad classes of reactions that release free silica in eclogite: (i) vacancy consumption in non-stoichiometric pyroxene; (ii) dissolution of Ti-phases in pyroxene or garnet; (iii) reactions between accessory phases and either pyroxene or garnet. We suggest that reliable interpretation of the significance of silica lamellae in natural clinopyroxene will require the evaluation not only of silica solubility, but also of titanium solubility, and the possible roles of accessory phases and inclusions on the balance of free silica

Diffusion of C and O in calcite from 0.1 to 200 MPa

We measured the diffusivity of C and O in calcite over the pressure range 0.1–200 MPa at 600–800 °C in a pure CO2 atmosphere. The experiments were conducted on single, preannealed crystals of Chihuahuan calcite in an isotopically labeled atmosphere, and the diffusion profiles were measured by secondary ionization mass spectrometry (SIMS). At 800 °C, DC and DO are identical at 0.1 MPa at a value of ~10–13.5 cm2/s. The value of DC decreases to ~10–16 cm2/s with an increase in pressure to ~50 MPa and remains at that value to 200 MPa, but DO remains nearly constant at a value of ~10–14 cm2/s to 200 MPa. The identical values at low pressure indicate that C and O are migrating together as a carbonate anion. A simple model relates the diffusivity of carbonate anions to the formation of vacancies at the crystal surface, which predicts that DC 1/fCO2. The prediction matches the observed decrease in DC with increasing pressure to 50 MPa. The shapes of the diffusion profiles for the low–pressure experiments indicate compositional dependence of D, which also suggests the influence of CO2 sorption on the diffusivity. The value of DC at 0.1 MPa can be fitted to the relation DC = 0.62 exp[(–291 kJ/mol)/RT]. The activation energy is nearly twice the value determined for DC at 100 MPa, ~166 kJ/mol. The change in slope for log DC vs. P and the change in Ea between 0.1 and 100 MPa suggest that the migrating C species changes from carbonate anions at low pressure to carbon atoms at P 50 MPa. The values of DO at 0.1 MPa can be fitted toDO = 0.017 exp[(–261 kJ/mol)/RT], approximately the same as for C at 0.1 MPa and similar to the relation for DO at 100 MPa: DO = 0.008 exp[(–242 kJ/mol)/RT]