GENESIS OF AGATES IN FLOOD BASALTS - TWISTING OF CHALCEDONY FIBERS AND TRACE-ELEMENT GEOCHEMISTRY

Chalcedony layers in agates from the Parana flood basalt contain alter natively high and low trace contents of Al, Fe, K, and Na, as measured with an ion-probe. They also contain Ni and Mn but no significant Li, Mg, or Ca. These oscillatory profiles confirm oscillations predicted by a self-organizational crystallization model (developed earlier) bas ed on the joint operation of cation-enhanced quartz growth, which prod uces the banding, and morphologically unstable crystallization fronts, which generates the fibrous texture. Length-fast fibers that grow thr ough strong decreasing trace element gradients are forced to incorpora te a higher content of the trace elements (substituting for Si) along their peripheries than along their centers. Since these trace ions (Al 3+ and Fe3+) are larger in size than Si4+, the fibers have to grow twi sted to accommodate the additional peripheric trace-element content. T he twist period of a fiber, calculated such that the twisting produces just enough stretching to contain the peripheric trace element conten t, comes out proportional to the fiber radius and to the minus one-hal f power of the difference in cube roots of the trace-element contents along the periphery and along the center of the fiber. The ion-probe a nalyses also confirm this geometric constraint of the solid-solution t wisting mechanism proposed here. When plotted on an Al versus (Na+K+2N i+2Fe) diagram (a diagram of substitutional versus interstitial cation s), the ion-probe analyses of the agate oscillate along a unit-slope l ine and shift steadily toward the southeast of the diagram, a pattern consistent with the progressive oxidation of ferrous to ferric iron. T his oxidation (by H2O) Is inevitably driven by the increase in both Fe 2+ and H2O concentrations produced by the very growth of the quartz la yers. The Fe3+ so generated increasingly substitutes for silicon in qu artz, makes the latest, drusy quartz purple, and causes occasional gro wth of goethite needles along with the latest quartz. The same oxidati on also increases alkalinity ana thus promotes zeolite growth, which a ccounts for the well-known association of zeolites with agates.