Cathodoluminescence petrography of P-type jadeitites from the New Idria serpentinite body, California

摘要

Jadeitite from the New Idria serpentinite body of California is a fluid precipitation-to-metasomatic product. Optical cathodoluminescence (CL) microscopy of the jadeitite revealed that vein-filling 'pure' jadeites (mostly 97-99.9 mol% jadeite) exhibit bright luminescence, whereas 'impure' jadeites (mostly 75-95 mol% jadeite) in pale-greenish matrix show dark luminescence. The 'pure' jadeites in the veins are composed of mixtures of red, blue and dull blue CL-colored domains, showing growth textures (oscillatory bands). The 'impure' jadeites in the pale-greenish matrix with dark luminescence have a higher augite component (up to 5.37 wt% FeO), implying that the CL property is due to significant amount of Fe~(2+) to act as a quencher. CL spectra of the blue CL-colored domains of the vein-filling 'pure' jadeite have a doublet broad emission peak centered at ～ 320 and ～ 360 nm in the ultraviolet (UV) to blue region. In the red CL-colored domains, a broad asymmetric emission peak at ～ 700 nm is also recognized together with the doublet UV-blue emission peak. Comparing monochromatic CL images in the UV-blue (300-400 nm) and red (650-750 nm) emission regions with X-ray elemental maps, luminescence centers contributing the UV-blue and red CL emission peaks were assigned. The red emission peak of the 'pure' jadeite with subtle augite component would be attributed to lattice defects related to Ca~(2+), Fe~(2+) (or Fe~(3+)) and Mg~(2+) deficiency and/or excess centers in Ml or M2 sites. Alternatively, transition metal ions (Mn~(2+) and Fe~(3+)) or rare earth elements in the M1 and M2 sites as impurity centers, might contribute to the red emission peak. As the UV-blue emissions correlate with Al~(3+) content, i.e. purity of jadeite component, they might be related to Na~+ and/or Al~(3+) defect centers.