Zones of diagenetic silica record past geothermal gradients in Neogene diatomaceous sediments and siliceous rocks in the Circum-Pacific region and are useful in evaluating the petroleum potential of these important source rocks. In many areas burial diagenesis has produced the well-known lithologic and mineralogic progression: diatomite (opal-A) ^rarr chert and porcelanite (opal-CT) ^rarr chert and porcelanite (quartz). Temperature is an important control of these transformations. The range in temperature for the conversion of opal-A to opal-CT, calculated from measured values of heat flow and thermal conductivity, is 25 to 56°C. Opal-CT transforms to quartz in the range 43 to 81°C. Similar ranges of temperatures are calculated from oxygen isotopes of opal-CT an quartz from cherts and porcelanites using the experimental fractionation for quartz and water, assuming the transformations occurred in isotopic equilibrium with water whose oxygen isotopic composition differed only slightly from standard mean ocean water. Taking the top and base of the opal-CT zone as approximate isotherms, the past geothermal gradient for any area equals the difference between these isotherms divided by the thickness of the opal-CT zone. In addition, both the thickness and the depth (reconstructed maximum overburden) to the top of this zone decrease with increasing geothermal gradient. Because depth and thickness are related by a simple linear expression, either may be used to estimate past geothermal gradients in Neogene siliceous rocks.

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