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Diagenetic sequences leading to the formation of deep-sea chert have been studied in drill and piston core samples taken in the Antarctic and Pacific Oceans. A pure white chert cored at the base of a Neogene diatom-radiolarian ooze sequence on the Kergulean Plateau (Southern Ocean) contains no volcanic alteration products and was derived solely from the solution and reprecipitation of biogenous opaline silica. The chert consists entirely of alpha-cristobalite spherules (5-10 microns in diameter) with some strongly etched fragments of siliceous microfossils.
Cristobalite spherules identical with those reported from the Antarctic, as well as the Atlantic and Caribbean Ocean basins, are present in Upper Cretaceous and Tertiary cherts and silicified limestones of the equatorial Pacific Ocean basin. Diagenetic sequences involving the replacement of carbonate microfossils and ooze matrix have been traced via scanning electron microscopy. A typical replacement sequence is: (1) cristobalite spherules are deposited within the interstices of coccolith oozes and chalks; (2) concomitantly, calcareous nannofossils begin to dissolve, thereby producing additional pore space (some calcite is lost to the rock, but some is reprecipitated within the interstices as euhedral crystals up to 10 microns in diameter); and (3) more cristobalite is precipitated wh le more calcite is dissolved. The process continues until the calcite matrix is completely replaced by cristobalite.
Foraminifera tests in the calcareous ooze apparently recrystallize or are dissolved outright. Cristobalite spherules are deposited within molds or within the recrystallized test. Finally, the recrystallized tests are dissolved leaving a cast of cristobalite.
In all samples studied, cristobalite was deposited as an authigenic mineral by means of inorganic chemical precipitation. In no place was a gel phase observed, and none is thought to exist in the formation of deep-sea chert.
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