HIGH-TEMPERATURE, CLOSED-SYSTEM ORIGIN OF AGATES IN BASALTS: NEW EVIDENCE

Crystallization of the self-organizational chalcedony textures and trace-element compositions typical of many agates requires that the growth be from a glob of silica gel (Wang & Merino, GCA 1990, AJS 1995). Globs of silica gel must form instantaneously as a hot basalt flowing on the ground traps water puddles: the water goes supercritical, reacts with silica chains in the melt, and forms silanol-ended silica polymer chains that attach at T junctions -- a gel. (If gel blobs form by puddle trapping, agates should occur near the bottom of the basalt flow. Agates do occur only in the bottom 3 meters of a 14-meter-thick basalt flow/agate quarry in the Parana Basalt at Rio Jacui, NW of Porto Alegre, Brazil, whereas the basalt vesicles, which occur only in the top meter, are all empty.) Each glob of silica gel, as a closed system, quickly crystallizes into an agate upon cooling, as the surrounding basalt crystallizes too, at high temperature. The last quartz (often coarse amethyst) to grow inside the agate may contain coprecipitated fibrous goethite: The measured lack of 18-Oxygen fractionation between the amethyst and the goethite indicates that the crystallization of both is at several hundred degrees C. The water of the initial gel glob collects in the agate's central void, itself resulting from the density difference between gel and quartz. The water leaks or flashes out and isotopically alters a shell of basalt around the agate. In a sample of fresh Deccan basalt + agate we measure delta 18-Oxygen values of 22 %o at the edge of the agate, 15 %o in basalt at 2 mm from the agate, 10 %o at 1 cm, and 6 %o at 2 cm from the agate - that is, down to fresh basalt. The same 1995 theoretical quantitative model that accounts for the self-organizational textures and compositions of agates and for their fibrous, length-fast quartz also accounts for a score of geochemical, mineralogical, microstructural, isotopic, and chromatic features of agates.