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

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.