Remanence in authigenic magnetite: Testing the hydrocarbon‐magnetite hypothesis

Paleomagnetic, rock magnetic, petrographic, and geochemical studies of hydrocarbon‐saturated speleothems in southern Oklahoma indicate there is a relation between hydrocarbons and a chemical magnetization that resides in magnetite. The speleothems, which are composed of light and dark calcite bands, occur in caves of karst origin in the Ordovician Kindblade Formation. Vertebrate fossils interbedded with the deposits indicate they are Permian in age. The dark bands contain primary hydrocarbon‐filled fluid inclusions. The dark calcites possess over an order of magnitude stronger natural remanent magnetization than the lighter bands which do not yield stable decay during demagnetization. Alternating field (AF) and thermal demagnetization of specimens of the dark bands reveal a Permian direction of magnetization (declination = 160, inclination = 3, k = 17, α95 = 5, n = 56). The results of rock magnetic experiments, and the fact that most maximum unblocking temperatures are below 580°C, suggest that the dominant component resides in magnetite. In some specimens stable decay to 640°C suggests the presence of a weak component residing in hematite. The presence of authigenic magnetite spheres in magnetic extracts of the dark calcites supports a chemical origin for the magnetization. Shallow burial depths probably eliminate the possibility of a thermoviscous magnetization. The occurrence of primary hydrocarbon inclusions suggests that hydrocarbons seeped into the caves during precipitation of the speleothems and were trapped in the calcite crystals. The relationship between intensity of magnetization and hydrocarbon abundance leads us to propose that chemical conditions created by the hydrocarbons caused precipitation of authigenic magnetite and acquisition of the associated chemical remanence.