Mathematics – Logic
Scientific paper
Aug 1984
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984gecoa..48.1589s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 48, Issue 8, pp.1589-1608
Mathematics
Logic
46
Scientific paper
The volume chemical diffusion of trace amounts of Sr in diopside has been measured as a function of temperature (1100-1300°C), pressure (1 bar-20 kbar), crystallographic direction, and composition. Three experimental/analytical techniques were employed: radiotracer and sectioning; stable tracer and ion microprobe; and Rutherford back-scattering spectroscopy. Comparison of the three yielded excellent agreement. Both natural and synthetic single crystal samples were used with results in the natural diopside giving diffusivities approximately two orders of magnitude greater than those in the man-made crystals. Samarium diffusion in the synthetic crystals was also examined with the ion probe technique with results similar to Sr. Arrhenius relations for diffusion ( D = D 0 exp [- H a / RT ]) were calculated for different pressures and analytical techniques, and activation volumes (gD V a ) were derived from the equation D = D ' exp [- P V a / RT ]. Values of V a were negative for Sr diffusion. An empirical relation describing the temperature and pressure dependence of D for Sr in the c direction of the synthetic samples is: D ( P , T ) = 1200 ( cm 2 / sec ) exp [-122 ( Kcal / mol )/ RT (° K )] exp [- P ( bar )/(2.94 T - 4640) R ]. The expression for D Sr in the natural samples ( c direction) at one atmosphere is: D (0, T ) = 54 ( cm 2 / sec ) exp [-97 ( Kcal / mol )/ RT (° K )]. A single compensation trend for all the data was evident for all values of D 0 and H a in the synthetic crystals. A number of models of geologic processes were investigated in light of the present data. Closure temperatures ( T c ) were calculated for examples of mineral-mineral age-dating and trace element geothermometry. High values of T c indicate that pyroxenes record emplacement events and are generally not disturbed unless a fluid enters the system. Isotopic equilibration times were examined for lower crustal xenoliths and the mantle source region for MORB using the formula of Hofmann and Hart (1978). Equilibration was shown to be too fast for production of isotopic anomalies in MORB via disequilibrium melting. Also. reasonable residence times at lower crustal temperatures were shown to produce the mineralogical-scale isotopic homogeneity observed in a crustal xenolith from Kilbourne Hole, New Mexico.
Hart Stanley R.
Shimizu Nobumichi
Sneeringer Mark
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