The natural magnetization of a 3‐kilometer section of Icelandic crust

Abstract

The magnetization and magnetic properties of samples from a 3 km vertical section of Icelandic crust, the Iceland Research Drilling Project section, are described. The section is located at the head of Reydarfjordur, eastern Iceland at 65°01′N, 14°21′W and extends vertically from about 0.5 km to 3.5 km depth beneath the original surface. The section consists of 1 km of exposed flows with minor dikes and volcaniclastics underlain by a 2‐km drill core component consisting of 60% flows and minor volcaniclastics and 40% dikes. The section is largely normally magnetized, corresponding in age and polarity to seafloor spreading anomaly 5 (geomagnetic polarity epoch 9). Flows which accumulated during anomaly 5 time extend vertically for 2145 m in contrast with 1000‐m‐thick equivalent sections which are exposed updip. This great increase in downdip thickening is consistent with observations elsewhere in eastern Iceland and models for Icelandic crustal formation involving the combination of volcanic activity, subsidence, and spreading. A combination of paleomagnetic and structural evidence is used to provide an age calibration for the section and to predict the local presence of magnetic overprinting in the lower part of the section. This overprinting is probably in part due to the production of secondary magnetite and is developed to a varying extent in epidote bearing flows at below 2.8‐km crustal (original) depth. Where moderate secondary magnetite development has occurred, flows frequently show both normal and reverse stable polarities, with the latter being the original. Secondary magnetite development is high where dike density in the epidote bearing flows is also high. Here very strong, well defined, pervasive normal magnetization characterizes the flows, this also being the dominant polarity of the dikes. Magnetic overprinting of this type, imposed shortly after crustal formation, could occur at 800‐ to 1200‐m depth in typical ocean crust and may provide a contribution to the source for the linear magnetic anomaly patterns of the ocean basins. The initial susceptibility of the flows shows the best‐defined depth trends over the section. Susceptibility increases with depth to about 2‐km crustal depth and then decreases systematically to the bottom of the section. The reason for the initial increase is not known, but the decrease at depth can be explained by transfer of iron from primary oxides to secondary oxides and silicates. The decrease is sufficiently linear to warrant extrapolation to a zero value at 4.3‐km crustal depth, or just below the estimated level of onset of greenschist facies metamorphic conditions. In typical oceanic crust the equivalent level is estimated to be at 1.2‐km depth. The net magnetization of the section, the combined remanent and induced contributions, when expressed as average values for 500‐m depth intervals, is everywhere positive, that is, is in the sense of the present field. Variation in net magnetization is related to the proportions of the normal and reversely magnetized units in each interval.