Volumetric changes in weathered profiles: iso-element mass balance method questioned by magnetic fabric

A major issue in understanding weathering processes is to determine to what extent fabrics, structures and volumes from the parent rock are preserved through the weathering profile. Iso-element mass balance methods (LMB) are often used to estimate volume changes during progressive weathering, based on the assumption that a given element (either Ti, Zr or Th) is not mobile. The petrofabric of the weathered material is often characterized by a mime tic replacement of primary minerals that fully preserve the primary fabric. This suggests an isovolume weathering as volume change must be associated with shape change due to boundary conditions not allowing horizontal strain s. So collapse or dilation should induce vertical compaction or constrictio n, respectively. The weak petrofabric of weathered materials may be precise ly quantified using the anisotropy of low-field magnetic susceptibility tec hnique (AMS). This paper reports, for the first time, a combined IMB and AM S study of two different environments: a complex lateritic sequence on Prec ambrian metamorphic rocks in Cameroon and a weathering profile on a Pleisto cene basaltic flow in Morocco, The lateritic profile, divided into a lower saprolite zone (>12 m) and an upper nodular iron-rich unit (4 m), is charac terized by neoformed magnetic minerals (goethite/hematite/spinels) and a we ak but rather consistent magnetic fabric. Saprolitization, which induces lo w susceptibility values (50 x 10(-9) m(3)/kg), preserves the linear tectoni c fabric of the parent gneiss, AMS evidence for isovolumetric weathering ag rees with Th based IMB, while Ti and Zr partly indicate apparent collapse a nd dilation (up to 50%), respectively. Ferralitization in the nodular iron crust, which enhances susceptibilities (500 to 700 x 10(-9) m(3)/kg, due to spinel phases), induces drastic reductions in anisotropy due to multiple g eneration of neoformed minerals destroying primary fabric. However, a weak horizontal planar magnetic fabric suggests compaction, in agreement with Th and Ti based IMB while Zr would indicate isovolume weathering. Conversely, in the one meter thick weathering profile on basalt, the strong susceptibi lity (1.5 to 3 x 10(-6) m(3)/kg) is dominated by inherited titanomaghemite grains which totally preserve the weak magmatic fabric of the fresh basalt, This again suggests that volume change is minor, while Ti and Zr based IMB indicate 20 to 50% of dilation. In both examples, large volume changes inf erred from the LMB in units showing preserved primary AMS fabric appear unr ealistic. Our AMS study, together with the large discrepancies between the IMB results produced by various elements, indicate that the assumption of i mmobility of a given element is probably not fulfilled all along these prof iles. Isovolumetric weathering in the saprolitic zone may be more widesprea d than suggested by IMB results, (C) 1999 Elsevier Science B.V. All rights reserved.