Paleoweathering of Deccan Basalt: A Rock Magnetic Prospective

The paleoweathering surfaces (boles) developed during the quiescence periods in successive eruptions of the Deccan flood volcanism (DFV) contain significant information on surficial weathering processes and atmospheric conditions. We studied rock magnetic properties of four well-developed bole profiles of three different colors i.e., red (dark=10R-3/6 and light=10R-4/8), brown (5YR-4/4) and green (5G-6/2) from the Poladpur formation of the Wai Subgroup, Deccan Basalt. These boles are mainly composed of smectite clays and show moderate chemical weathering. The green bole shows an overall depletion in ferrimagnetic titano-(magnetite) concentrations with respect to the protolith basalt as well as absence of secondary antiferromagnetic minerals (e.g., hematite). The absence of hematite in green bole is consistent with the deeper/lower saprolitic weathering of basalt under reducing environment. The brown bole consists of pseudo single domain (PSD) ferrimagnetic particles. The dark red bole shows nearly complete oxidation of ferrimagnetic minerals into hematite. The high amount of hematite and low concentration of superparamagnetic (SP) ferrimagnetic particles in moderately weathered dark red bole suggests that high temperature heating from the overlying lava flow under aerobic conditions would have enhanced oxidation of ferrimagnetic minerals into hematite. The light red bole shows paragenesis of SP and single domain (SD) ferrimagnetic particles, and hematite indicating pedogenic alteration of protolith basalt in an oxidizing semi-arid climatic condition. Heating from the upper lava flow was restricted to top ~20 cm of the light red bole profile. We document a variety of surficial/deep alteration processes of basalts modified by secondary hydro-(thermal) alterations and addition of volcanic ash leading to genesis of different color boles. In all the studied boles, only light red bole showed minimal influence of these secondary processes and partly preserves the pedogenic signatures. The light red bole is also free from any signatures of acid rains (such as extensive dissolution of titanomagnetites) and endorse the “mock aridity” condition in the proximal areas of the Deccan Traps during the successive eruptions of DFV.