Fe-Mg interdiffusion rates in clinopyroxene: Experimental data and implications for Fe-Mg exchange geothermometers

Chemical interdiffusion of Fe-Mg along the c-axis [001] in natural diopside crystals (XDi = 0.93) was experimentally studied at ambient pressure, at temperatures ranging from 800 to 1,200 °C and oxygen fugacities from 10-11 to 10-17 bar. Diffusion couples were prepared by ablating an olivine (XFo = 0.3) target to deposit a thin film (20-100 nm) onto a polished surface of a natural, oriented diopside crystal using the pulsed laser deposition technique. After diffusion anneals, compositional depth profiles at the near surface region (~400 nm) were measured using Rutherford backscattering spectroscopy. In the experimental temperature and compositional range, no strong dependence of DFe-Mg on composition of clinopyroxene (Fe/Mg ratio between Di93-Di65) or oxygen fugacity could be detected within the resolution of the study. The lack of fO2-dependence may be related to the relatively high Al content of the crystals used in this study. Diffusion coefficients, DFe-Mg, can be described by a single Arrhenius relation with (Formula presented). DFe-Mg in clinopyroxene appears to be faster than diffusion involving Ca-species (e.g., DCa-Mg) while it is slower than DFe-Mg in other common mafic minerals (spinel, olivine, garnet, and orthopyroxene). As a consequence, diffusion in clinopyroxene may be the rate-limiting process for the freezing of many geothermometers, and compositional zoning in clinopyroxene may preserve records of a higher (compared to that preserved in other coexisting mafic minerals) temperature segment of the thermal history of a rock. In the absence of pervasive recrystallization, clinopyroxene grains will retain compositions from peak temperatures at their cores in most geological and planetary settings where peak temperatures did not exceed ~1,100 °C (e.g., resetting may be expected in slowly cooled mantle rocks, many plutonic mafic rocks, or ultra-high temperature metamorphic rocks)

Red light induced holographic storage in an azobenzene polymethacrylate at room temperature

9 figures.Holographic diffraction gratings have been induced at room temperature (RT) using red light (633 nm), in a liquid crystalline side chain cyano-azobenzene polymethacrylate polymer. Stable gratings have been obtained under simultaneous irradiation with the interference pattern created with two coherent linearly polarized red light beams and UV (around 350 nm) unpolarized light. The main contributions to the diffraction efficiency can come from the photoinduced orientation of the azo units and perhaps from changes in the light transmitted through the regions of the film that have been illuminated with both red and UV light and those only illuminated with the UV light. Higher efficiency is achieved if the red light polarization is perpendicular to the grating vector than if it is parallel to that vector. Stable gratings have also been induced with either sequential or simultaneous irradiation with blue (488 nm) linearly polarized light and the red interference pattern. The contributions to the efficiency can have the same origin as under UV and red light illumination. The highest efficiency is achieved under simultaneous illumination with both blue and red light polarizations perpendicular to the grating vector.The financial support from the CICYT, Spain, under project no. MAT2002-04118-C02 is gratefully acknowledged. This work has been performed within the COST P8 Action.Peer reviewe