Trace element behavior in moderately weathered boles from the Deccan volcanic province: Implications for paleoenvironment

Abstract

The present study examines the behavior of trace elements in moderately weathered boles of three different colors (i.e., red, brown and green) from the Deccan Traps in order to understand paleoenvironmental conditions during periods of quiescence between successive eruptions of Deccan flood volcanism (DFV). The minimal (<40%) losses/gains of trace elements, including rare earth elements (REE), in the brown and red boles indicate their low mobility either due to limited release from primary minerals and/or due to sorption on secondary clay minerals and Fe-Mn oxides under near neutral pH (~5.5 to 8) conditions. The green bole shows a greater loss of trace elements, on average ~40–70%, including the REE and the typically immobile elements V and Zr. The low concentrations of V and Zr in the green bole are in accordance with low Ti and high SiO2, K2O and Rb concentrations that suggest addition of petrogenetically evolved volcanic ash (e.g., andesitic composition) compared to the lower flow basalt.A negative Ce anomaly and enrichment of the light REE (LREE) in the green bole indicate precipitation of soluble trivalent LREE desorbed from Fe-Mn oxide surfaces preferentially to insoluble oxidized tetravalent Ce. This is interpreted to indicate more reducing conditions at depth, with Fe-Mn oxides in a more oxidized upper saprolite/soil horizon were eroded prior to trapping of the green bole by the upper lava flow. The light red bole, weathered under a more pervasively oxidative environment, shows loss of Mn oxides that might have initially scavenged cerianite leading to a negative Ce anomaly preserved in the residual bole. Positive Ce anomalies in the dark red and brown boles, are consistent with their gain and negligible loss of Mn, respectively, indicating Ce precipitation on the Mn oxide surfaces relative to the other trivalent LREE in an oxidizing environment. Overall, the boles show weathering signatures suggestive of near neutral pH conditions and locally changing redox, some of which may have largely escaped post-eruptive acidic precipitation possibly due to the transport of acidic gases into the upper atmosphere (stratosphere) under arid conditions.