We have found huge viscous magnetic components (HVC) in the Central Iberian Ranges (Northern Spain). This component can exceed 90% of the NRM and does not depend on the lithology nor on the age of the rocks. All sampled lithologies are sedimentary rocks, and most of them are marine carbonates. The HVC normally unblocks at 100°C (or less if we performed more detailed demag routines). The HVC mean is the present magnetic field plus some noise apparently related to the drilling direction.
We think this may be controlled by summer insolation and surface alteration (Goethite?); surface rocks in this area may reach 50°C or more in summer time. But we are not sure if this is enough to produce the huge viscous component (many of these rocks may exceed a few thousands of mA/m).
Does anybody know any references which may shed some light on the problem?
Goethite is a good bet for a component with a low blocking temperature and “recent” direction. However, goethite is weakly magnetic; to get a few thousand of mA/m in the NRM intensity there should be a lot of it in your samples. Large amount of goethite should give the rocks a bright yellow color.
Another candidate for the large VRM is very fine grains of magnetite(maghemite) – at around SP-SD boundary. Such grains can form by sulfides decomposition. Marine carbonates often contain pyrite and greigite, which are unstable in oxic conditions and can produce magnetite when exposed to air.
You can test for the presence of SP grains in your samples by measuring frequency dependence of susceptibility, or any other rock mag parameters that are sensitive to SP grains – e.g., hysteresis or a ratio of saturation magnetization to susceptibility. If you have accesses to a VSM or AGM, you can measure FORC diagrams; or, you can give your samples an IRM and monitor its decay with time (easy to do with the VSM) – SP-rich rocks will have an exponential decay of the IRM.
Hi Sasha,
The question is - how do you know that this is really VISCOUS component? Did you make measurements before and after keeping samples in the shielded space for weeks or months? Goethite potentially can be a source of very recent chemical remanence, but it has a huge coercivity, so it can hardly be a carrier of viscous remanence.
Cheers,
Sergei.
Does it have to be viscous? As far as I understand from the question, Emilio wants to find an explanation for a component with low unblocking temperatures and recent direction. Could be recent chemical alteration, in addition to viscous magnetization.
If so, I think, the easiest way to distinguish between goethite and maghemite is 3-axes Lowrie's method.
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