ИСТИНА

We present mineralogical and magnetic data of eleven oceanic gabbros which were dredged from the Mid-Atlantic ridge at 13° to 17°N. Their natural remanent magnetization(NRM) is primarily controlled by two microstructural types of Fe-Ti-oxides. A first type is represented by several millimeters sized, isometric cumulus/intercumulus grains present in interstitial positions between the rock-forming silicates. The second type is represented by micro-inclusions hosted by the rock-forming pyroxenes and plagioclases. During the post magmatic evolution, the different Fe-Ti-oxide types behaved differently affecting the bulk-rock magnetic properties in different ways. Understanding the primary NRM and its transformation in the dynamic environment of the Mid-Atlantic ridge is important for paleomagnetic data interpretation and for assessing the information from marine magnetic anomalies. For all studied gabbros, the Curie temperature Tc is in the range of 570-575º C indicating that magnetite is the dominant magnetic carrier of the NRM. In some samples, additional magnetic phases with TC ~330-340ºC corresponding to either titano-magnetite or monoclinic pyrrhotite and with TC ~700ºC corresponding to hematite are distinguished. The intensity of the bulk rock NRM is 0.3 – 7 A/m and shows a positive correlation with both, the quantity of cumulus/intercumulus Fe-Ti-oxide grains and with the abundance of the Fe-Ti-oxide micro-inclusions in the rock-forming silicates. Based on the presence of the different microstructural Fe-Ti-oxide types, two groups are distinguished: the first group has both, cumulus/intercumulus grains and silicate-hosted Fe-Ti-oxide micro-inclusions. The second group only shows silicate-, mostly plagioclase-hosted Fe-Ti-oxide micro-inclusions In the Mrs/Ms vs Hcr/Hc diagram both groups fall into the field of pseudo-single domain (PSD) grains, whereby the first group plots closer to the multi-domain (MD) part of the diagrams , and the second group plots closer to the single-domain (SD) part. The NRM in the second group amounts to 0.3-0.8 A/m indicating that the micro-inclusions play a quite important role for the bulk-rock NRM. In the second group, the bulk coercitivity, BC is 15-28 mT, and the saturation magnetization ratio, Mrs/Ms is 0.15-0.25, which is substantially higher than in the first group. This gives evidence of a stable magnetization in the second group, which is probably due to the dominance of the micro-inclusions. Moreover, the samples of the second group have a much higher Fisher statistics parameter of the NRM vectors (k=100-160) than the first group, where k=3-46. It is important to note that in the second group almost all pyroxene grains have been replaced by late amphibole in the course of low-temperature hydrothermal alteration, which probably also destroyed the cumulus/intercumulus Fe-Ti-oxides. Plagioclase remained largely unaltered during the hydrothermal overprint, and the plagioclase-hosted micro-inclusions preserved the primary NRM. We infer that, even though the Fe-Ti-oxide micro-inclusions in the rock-forming silicates are present at low concentrations, they are robust carriers of the primary NRM. When cumulus/intercumulus Fe-Ti-oxide grains and inclusion-bearing pyroxene are missing the record of remanence is only provided by plagioclase-hosted micro-inclusions. In this case the rocks yield accurate information on the direction of the primary NRM.