Variations in the 40*Ar of the whole rock and the <0.1-µm fraction were used to monitor the involvement of K2O in the inferred burial metamorphic reactions of argillaceous sediment from Texas Gulf Coast well CWRU (Case Western Reserve University) 6. These data strongly support the occurrence of the reactions involving the illite/smectite mixed-layer clay inferred from depth-dependent mineralogical and chemical changes reported in a companion paper by Hower and others (this issue). The 40Ar released from all samples was generally greater than 40 percent radiogenic (40*Ar), allowing precise measurement of differences of 40*Ar with depth.

The whole-rock K-Ar apparent age shows a decrease from about 150 m.y. to 75 m.y. over just the same depth interval (1,850 to 3,700 m) that the illite/smectite mixed-layer clay progressively changes from 20 percent illite layers to 80 percent. This decrease in apparent age of the total shale is due to loss of 40*Ar from the rock. The 40*Ar loss is not caused by outgassing from increasing temperature, because the finest (<0.1-µm) fraction, which is nearly pure illite/smectite, actually gains 40*Ar with depth. The coarser fraction, from which the 40*Ar loss is occurring, concentrates the older 40*Ar—rich phases of the rock such as K-feldspar and mica. These results strongly imply that the K2O for the new illite layers of the illite/smectite is derived by chemical decomposition of the K-feldspar and mica. The gain in 40*Ar of the finest fraction also just corresponds with depth to the change in mineralogy of the illite/smectite and a large gain of K2O in this fraction from 2 to 5 percent K2O. The 40*Ar gain indicates that the mean time of K2O gain, which is the mean time of burial metamorphism, was about 18 m.y. ago.

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