ジルコンの放射能効果(地学団体研究會創立十周年記念号)

抄録

Many data on the radio-effects in zircon are examined with the following results: (1) Zircon is subdivided structurally into three groups : Group A-Non-metamict zircons, includimg normal zircon and transparent hyacinth (G>4.6 or 4.55); Group B-Partly metamict zircons (G=4.6-4.0); Group C-Completely metamict zircon or typical malacon (G=4.0-3.9 or less). (2) In the damage process in zircon are recongnized three stages: Stage I-a continuous change from normal zircon to transparent hyacinths up to a certain saturation of ionization by free elections, the degree of which is shown in the colour order: colourless-yellow-orange-pink-purple; Stage II-a biscontinuous change from Group A to Group B. This change involves the abrupt partaking of radiogenic dissociation (ZrSiO_4→ZrO_2+SiO_2) occurring in part within a single crystal. Megascopic translucency or nearly opaque character carrying with yellowish and/or greenish tinge is an indication of this change; StageIII- a continuous increase of the dissociation products in Group B. This stage ends with the formation of Group C, opaque and green or brownish-green. (3) Time has a becibebly greater effect than alpha-activity. Most of the post-Cambrian granite zircons do not show so wide a range of variation of 2θ as seen in the pre-Cambrian, and less damaged crystals are present among the Miocene zircons. (4) A series of zircon from the Middle Cretaceous granites in Japan exhibits the general tendency that the later members are of greater radioactivity. This may be a reflection of the evolution of granitic magma, which becomes more enriched in the radioelements in its later stages. (5) Radio-effects in zircon are gradually restored by heating. Naturally heated zircons are met with in granitic xenoliths often found within volcanic rocks. Colour of these zircons corresponds with the age of the volcanic rocks. This suggests a means of age-determination for volcanic rocks.