@article{fdi:010084441,
  title = {{V}olumetric changes in weathered profiles : iso-element mass balance method questioned by magnetic fabric},
  author = {{M}ath{\'e}, {P}.{E}. and {R}ochette, {P}. and {V}andamme, {D}. and {C}olin, {F}abrice},
  editor = {},
  language = {{ENG}},
  abstract = {{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. {I}soelement mass balance methods ({IMB}) are often used to estimate volume changes during progressive weathering, based on the assumption that a given element (either {T}i, {Z}r or {T}h) is not mobile. {T}he petrofabric of the weathered material is often characterized by a mimetic replacement of primary minerals that fully preserve the primary fabric. {T}his suggests an isovolume weathering as volume change must be associated with shape change due to boundary conditions not allowing horizontal strains. {S}o collapse or dilation should induce vertical compaction or constriction, respectively. {T}he weak petrofabric of weathered materials may be precisely quantified using the anisotropy of low-field magnetic susceptibility technique ({AMS}). {T}his paper reports, for the first time, a combined {IMB} and {AMS} study of two different environments: a complex lateritic sequence on {P}recambrian metamorphic rocks in {C}ameroon and a weathering profile on a {P}leistocene basaltic flow in {M}orocco. {T}he lateritic profile, divided into a lower saprolite zone (>12 m) and an upper nodular iron-rich unit (4 m), is characterized by neoformed magnetic minerals (goethite/hematite/spinels) and a weak but rather consistent magnetic fabric. {S}aprolitization, which induces low susceptibility values (50×10 -9 m 3 /kg), preserves the linear tectonic fabric of the parent gneiss. {AMS} evidence for isovolumetric weathering agrees with {T}h based {IMB}, while {T}i and {Z}r partly indicate apparent collapse and dilation (up to 50%), respectively. {F}erralitization in the nodular iron crust, which enhances susceptibilities (500 to 700×10 -9 m 3 /kg, due to spinel phases), induces drastic reductions in anisotropy due to multiple generation of neoformed minerals destroying primary fabric. {H}owever, a weak horizontal planar magnetic fabric suggests compaction, in agreement with {T}h and {T}i based {IMB} while {Z}r would indicate isovolume weathering. {C}onversely, in the one meter thick weathering profile on basalt, the strong susceptibility (1.5 to 3×10 -6 m 3 /kg) is dominated by inherited titanomaghemite grains which totally preserve the weak magmatic fabric of the fresh basalt. {T}his again suggests that volume change is minor, while {T}i and {Z}r based {IMB} indicate 20 to 50% of dilation. {I}n both examples, large volume changes inferred from the {IMB} in units showing preserved primary {AMS} fabric appear unrealistic. {O}ur {AMS} study, together with the large discrepancies between the {IMB} results produced by various elements, indicate that the assumption of immobility of a given element is probably not fulfilled all along these profiles. {I}sovolumetric weathering in the saprolitic zone may be more widespread than suggested by {IMB}.},
  keywords = {{MAROC} ; {CAMEROUN}},
  booktitle = {},
  journal = {{E}arth and {P}lanetary {S}cience {L}etters},
  volume = {167},
  numero = {3-4},
  pages = {255--267},
  year = {1999},
  DOI = {10.1016/{S}0012-821{X}(99)00024-2},
  URL = {https://www.documentation.ird.fr/hor/fdi:010084441},
}