Thermodetrital and crystallodetrital magnetization in an Icelandic hyaloclastite

An Icelandic hyaloclastite, mostly composed of millimetric fragments of bas altic glass, that is fresh at the bottom of the unit but largely palagoniti zed in the upper part, has been studied by petrologic, mineralogical, and m agnetic means, with the aim of determining the nature and characteristics o f the natural remanent magnetization (NRM). The NRM was generally found to consist of two components: a thermodetrital remanent magnetization (thermo- DRM) and a crystallodetrital remanent magnetization (crystallo-DRM). Thermo -DRM and crystallo-DRM are defined here as the remanences acquired as a res ult of the deposition of magnetic particles of detrital origin individually carrying either a TRM (thermoremanent magnetization) or a CRM (crystalliza tion remanent magnetization), respectively. Regardless of the chemistry and size and of these particles, Thellier experiments carried out on samples c arrying a thermo-DRM provide apparent paleointensities close to the expecte d geomagnetic paleointensity, which suggests that in the present case the f ractional alignment of individual magnetic moments is similar for DRM and T RM. In the upper part of this outcrop, grain growth CRM was acquired by ind ividual grains of magnetite which crystallized as a result of palagonitizat ion of basaltic glass at low temperature (<100 degrees C). No systematic di fference could be observed between the directions of characteristic remanen ce in the layers richest in magmatic magnetic grains and the layer where se condary magnetite is the main remanence carrier. In both cases, the overall remanence exhibits a large inclination error (similar to 20 degrees), and the samples have a marked anisotropy of magnetic susceptibility which is ty pical of sedimentary fabrics. Thus, secondary magnetite probably formed pri or to the deposition of particles, and the bulk remanence in the palagoniti zed layers is a crystallo-DRM rather than a CRM. Throughout the entire stra tigraphic thickness, Thellier paleointensity data are of good or excellent quality regardless of the nature of the primary remanence. In agreement wit h previous theoretical inferences and experimental results the layers carry ing a crystallo-DRM provide a much lower (by a factor of 2) apparent paleoi ntensity than the layers where the remanence is a thermo-DRM. This suggests that palagonitized basaltic glasses should not be used for paleointensity determinations. Our study shows that application of the Thellier thermal pa leointensity method to sedimentary rocks can be a useful tool for distingui shing crystalline versus thermal blocking of the magnetic moments of the in dividual particles which, after deposition, carry a DRM. More generally, di screpancies between relative paleointensities obtained from sediments can b e expected if rocks with different. proportions of thermo-DRM, crystallo-DR M, or CRM are compared.