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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 4. (April)

First Page: 760

Last Page: 760

Title: Uranium in Volcanic Rocks: Progress: ABSTRACT

Author(s): John W. Gabelman

Article Type: Meeting abstract


Calc-alkaline rocks of crustal(?) derivation analyzed 0 to 5 ppm uranium in magmatic phases, but hundreds of ppm in epigenetic end-products. Alkaline rocks of mantle(?) derivation analyzed 0 to 22 ppm in magmatic phases, 30 to 45 ppm in deuterically modified phases and hundreds of ppm in final epigenetic modifications. Incompatibility with rock-forming silicates relegates large ions including uranium to residual gas or fluid. Mantle volatiles released during magma generation or crystallization mineralize rocks according to relative chemical reactivity.

In rocks with < 5 ppm U, U-Th were not detected by autoradiography or fluorescence. Uniform weak glow at 10 to 20 ppm U suggests uniform distribution at the eutectic. Selective distribution in more uraniferous rocks favors deuteric or pegmatitic biotite, zircon, complex silicates, and multiple oxides. U-Th uniformly distributed within grains are considered contemporaneous lattice substitutions. Some magmatic minerals in rocks with >60 ppm U show inclusions variably containing Th, REE, U, Ti, V, Ni, Cr, Cu, Pb, Zn, Ba, and Mn. Halogens, S, and P underscore the volatile role. Inclusion-laden asphaltite is common. Lattice inclusions may represent vapor trapped during crystallization. Those in cleavages, amygdules, or fractures appear epigenetic. Those lacking Th probably are of rel tively low temperature.

Surface tension would inhibit liquid entry into tight textures and lattices, so volatile transport and ionic lattice diffusion are inferred. Inclusions account for most Th-U distribution patterns suggesting dominant epigenetic enrichment of volcanics by residual volatiles.

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