Publication: Diagenetic processes and remagnetization
in Permo- Triassic reb beds
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1988
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Servicio de Publicaciones de la Universidad Complutense de Madrid
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Abstract
Palaeomagnetic methods of analysis have largely been concentrated on distinguishing between the relatively hard and soft components which make up the natural remanent magnetization (NRM) of sedimentary rocks. Ihis has been because of the widespread beliefthat, in general, the hard cornponents are primary in origin whule the soft components are secondary, having been acquired a long time afler deposition. Also it is generally assumed that the primary components originate from depositional processes rathcr than diagenetie (post-depositional) alteration. Magnetic components acquired by in situ alteration are usually altributed to chemical remanent magnetization (CRM) with no more specific reference to the processes of magnetization. Furthermore the general practice has been Lo assume that the magnetic components with higher blocking temperatures or coercivities faithfully record the ambient geomagnetie field at or near the time of deposition despite any direct geological evidence of the age of the components. This simplistie view of palaeomagnetism arises from the Neel equation which relates relaxation time to coercive force, particle volume and temperature for an assembly of non-interacting single-domain grains:
t-1 =C exp(—v Js Hc/2kT)
Where C is the characteristic frequency of thermal fluctuation, v is the particle volume, Js spontaneous magnetization, Hc the microscopic coercive force, k Boltzmann’s constant and T is absolute temperature. This equation holds good given Pie constraints indicated. However it is obvious that in nature many problems will rise. In sedimentary rocks these include changes in particle volume and composition and the precipitation of comptetely new magnetic minerais. Mosí of the problems arise directty as a result of diagenetie processes; they can be grouped into three ciasses:
1. Post-depositional chemical alteration of magnetie grains (phase transition).
2. Changes in the particle volume (grain growth).
3. Neoformation of magnetie minerais (authigenesis).
Using data from continental red beds examples of these proeesses will be used to demonstrate how they can produce secondary magnetizations. The role of the Neel equation in remagnetization of red beds in deeply buried sedimentary basins will also be described.