Didier Vandamme

Deccan flood basalts at the Cretaceous/Tertiary boundary?

Abstract Joint consideration of new paleomagnetic, paleontological and geochronological data from the Deccan continental flood basalts in India and critical discussion of earlier results lead us to suggest that volcanic activity may have lasted less than 1 Ma, thus possibly ranking as one of the largest volcanic catastrophes in the last 200 Ma. Available data are best satisfied if volcanism spanned the Cretaceous/Tertiary boundary, followed shortly afterwards by rifting of the Arabian Sea. These results point out the need for further work which may help in choosing between “external” and “internal” models of the Cretaceous/Tertiary boundary events.

Duration of Deccan trap volcanism: a statistical approach

Abstract We examine the completeness of the magnetostratigraphic record in sequences of lava flows where few reversals have been recorded. A case of particular interest is the Deccan traps in India, which appear to exhibit on the order of 50 flows, with a three chron (NRN) polarity sequence and a strong bias to reversed polarity (80%). K/Ar ages only weakly constrain the duration of volcanism between 50 and 70 Myr. Simple statistical experiments in the form of random sampling of the reversal time scale in that time window show that volcanism is unlikely to have lasted more than 3 Myr. The range of acceptable chrons can be reduced with paleontological arguments to the 29–32 sequence. We have tested the probability of losing short chrons in that time window and as a result observing only a NRN sequence, when actually sampling 4 or 5 chrons. This probability is around 35% for 50 flows and 20% for 75 flows. Only the shortest chrons 30R and 32.1N can be missed: as a consequence possible correlations of the Deccan main reversed sequence are restricted to 29R and 31R (the very short chron 30R being unlikely from geological considerations). Recent paleontological data actually seem to restrict the correlation to 29R. In all cases, the observed polarity bias can be reproduced only if volcanism lasted less than 1 Myr.

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

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. Iso-element mass balance methods (IMB) 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 mimetic 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 strains. So collapse or dilation should induce vertical compaction or constriction, respectively. The weak petrofabric of weathered materials may be precisely quantified using the anisotropy of low-field magnetic susceptibility technique (AMS). This paper reports, for the first time, a combined IMB and AMS study of two different environments: a complex lateritic sequence on Precambrian metamorphic rocks in Cameroon and a weathering profile on a Pleistocene 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 characterized by neoformed magnetic minerals (goethite/hematite/spinels) and a weak but rather consistent magnetic fabric. Saprolitization, which induces low susceptibility values (50×10−9 m3/kg), preserves the linear tectonic fabric of the parent gneiss. AMS evidence for isovolumetric weathering agrees with Th based IMB, while Ti and Zr partly indicate apparent collapse and dilation (up to 50%), respectively. Ferralitization in the nodular iron crust, which enhances susceptibilities (500 to 700×10−9 m3/kg, due to spinel phases), induces drastic reductions in anisotropy due to multiple generation 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 susceptibility (1.5 to 3×10−6 m3/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 inferred from the IMB in units showing preserved primary AMS fabric appear unrealistic. Our 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. Isovolumetric weathering in the saprolitic zone may be more widespread than suggested by IMB results.

Néel temperatures of synthetic substituted goethites and their rapid determination using low‐field susceptibility curves

The Neel temperatures (T N ) of synthetic goethites (α-FeOOH), substituted with diamagnetic elements (Al, Ga) have been determined from the temperature dependence of their low-field susceptibility using a Kappabrige AC bridge. These temperatures proved to be identical to those derived from more conventional measurements of high-field or remanent magnetization versus temperature, and in good agreement with those inferred from Mossbauer models. The low-field susceptibility based-technique appears to offer a precise and rapid method of determining T N While Neel temperature coincides with maximum unblocking temperature in Gagoethites, supporting the assumption that antiferromagnetism and parasitic ferromagnetism have a similar origin in goethite, the lack of clear correlation between saturation remanent magnetization and increasing diamagnetic substitution suggests that the weak ferromagnetism of goethite is due to substitution independent defects induced during crystal growth.

Reply to comments on “Deccan flood basalts at the Cretaceous/Tertiary boundary?” by H. Wensink

Reponse aux commentaires de Wensink H. parus dans Earth planet. Sci., Letters, NLD, 1987, 85, 326-328