Acquisition of natural remanence in the basaltic laterites of Deccan volcanic province (India): Implications to palaeomagnetic studies in laterites

Abstract

Lateritization in the Deccan basalt province (India) represents a large spectrum of chemical weathering with up to 99% CIA values and complete ferri- to antiferromagnetic transformations. We combined rock magnetic, palaeomagnetic, XRD, and XRF analyses to investigate the mineralogical transformations and its relationship to the acquisition of natural remanent magnetization (NRM) by sampling a 25 m thick Deccan lateritic profile from the West Coast of India. Significant amount of amorphous hematite (ha), crystalline hematite (hc), goethite and their Al- substituted forms were detected along with gibbsite and kaolinite. The domain size association of ‘SP + SD and ‘SP + SD + MD’ in the ferri- as well as antiferromagnetic forms; and the combination of ha and hc yield strong NRMs amongst these laterites. Low field demagnetization (<5mT) allows removal of haNRM; and the secondary components due to ferrimagnetic oxides can be removed at < 25 mT. Thereafter, the hcNRM continued to decay up to > 150 mT. Palaeomagnetic tests (Koenigsberger ratio, MDF, intensity decay and Zijderveld diagrams) indicate stable Characteristic remanent magnetizations (ChRM) due to hc; and the Al-substitutions does not affect the ChRM stability, depicting these laterites as excellent palaeomagnetic recorders. The stable ChRMs due to Chemical/Crystallization RMs due to ‘hc’ also benchmarks the degree of maturity within the process of lateritization.