AMS–NRM interferences in the Deccan basalts: Toward an improved understanding of magnetic fabrics in flood basalts

Abstract

The evaluation of flow direction in volcanic rocks is among the most important applications of magnetic fabrics studies. A statistically significant sample set of titanomagnetite-bearing lava flows from the Malwa Plateau, the northern part of the Deccan traps in India, has been investigated for a possible interference of induced and natural remanent magnetization (NRM). The NRM alters the scalar anisotropy of magnetic susceptibility (AMS) parameter and the orientations of the AMS principal magnetic axes, which are crucial for the evaluation of the flow direction. For cleaning of the NRM component, the lava samples have been demagnetized by use of an alternating field (AF) tumbling demagnetizer (peak fields of 100 mT) as previous studies have shown that static AF demagnetization can bias the results. Samples with normal magnetic fabrics demonstrate a redistribution of their principal axes after the demagnetization. The evaluated flow directions show a more differentiated flow pattern of the Malwa area, which seems to fit better into the regional geological setting. In samples with inverse magnetic fabrics, carrying a higher portion of single-domain particles, AMS principal axes remain unchanged after the demagnetization, indicating that these samples with high coercivity of magnetic carriers are not suitable for geological interpretations. According to these results, we propose that the AMS measurements after tumbling demagnetization give a better reflection of the intrinsic anisotropy of magnetic carriers (at least for samples with normal magnetic fabrics) and therefore a more precise and better reflection of the "actual" mineral fabric.

Shock pressure estimates in target basalts of a pristine crater: A case study in the Lonar crater, India

“…The randomly striking and shallow-dipping magnetic fabrics in the Lonar crater basalt are similar to those reported in the pristine Deccan basalt from the Malwa Plateau (Schöbel and de Wall, 2014). Similar types of magnetic fabrics are also common in the pristine lava flows of other regions, e.g., Hawaii (Cañón-Tapia et al, 1997).…”

“…In undeformed lava flows, the titanomagnetite crystallizes within a silicate framework and reflects the direction of lava flow (e.g., Park et al, 1988;Schöbel and de Wall, 2014). The correlation of shape, distribution, and magnetocrystalline anisotropy with silicate fabric may reveal the undeformed nature of the igneous rocks (Hargraves et al, 1991) and the magma flow directions (e.g., Wada, 1992;Bascou, et al, 2005).…”

Influence of static alternating field demagnetization on anisotropy of magnetic susceptibility: Experiments and implications

“…These changes in AMS principal directions are potentially problematic for lava flow studies, which often rely on the orientation of k 1 [e.g., Cañón‐Tapia , , and references therein]. Changes in principal AMS directions as well as scalar AMS parameters related to AF treatment were also found in lava flows from the Deccan Traps [ Schöbel and de Wall , ], and in 26 out of 48 samples from the Henties Bay Outjo Dyke swarm [ Wiegand , ], interpreted there as an interaction between AMS and NRM.…”

“…These observations suggest that grain size strongly affects the size and shape of the impressed ellipsoid, with MD grains producing prolate ellipsoids, i.e., an increase in susceptibility parallel to the field axis, and SD grains oblate ellipsoids, i.e., a decrease in susceptibility in the direction of the field [ Potter and Stephenson , ; Stephenson et al ., ]. For the same reason, increases and decreases in susceptibility parallel to the last AF direction have been described for samples exhibiting normal (MD grains) or inverse magnetic fabrics (SD grains) [ Schöbel and de Wall , ; Schöbel et al ., ]. Jordanova et al .…”

“…While yet others treat field‐induced anisotropy as an artefact [ Bhathal and Stacey , ; Henry et al ., ; Jordanova et al ., ; Potter and Stephenson , ; Violat and Daly , ]. In line with these different interpretations, contradicting recommendations have been put forward as to whether or not samples should be AF demagnetized prior to AMS measurements: either (1) AMS should be measured before AF treatment, in order to avoid effects of artificial field‐induced anisotropy [ Jordanova et al ., ; Potter and Stephenson , ] or (2) samples should be demagnetized before the AMS is determined, to remove the effects of stress‐induced domain wall pinning, or to remove the effects of a strong natural remanent magnetization (NRM) [ Lanci , ; Liu et al ., ; Park et al ., ; Schöbel and de Wall , ; Schöbel et al ., ].…”