Plagioclase as recorder of magma chamber processes in the Deccan Traps: Sr-isotope zoning and implications for Deccan eruptive event

Plagioclase as recorder of magma chamber processes in the Deccan Traps: Sr-isotope zoning and implications for Deccan eruptive event

Deccan Trap flood basalt volcanism reached its peak 65.5(±1)millionyears ago, giving rise to a 2.7km thick lava pile that formed the Western Ghats range near Mumbai. The total duration of this activity cannot be constrained better than ±1Ma by conventional Ar-dating techniques. It is also difficult to know whether there were short bursts of tremendous volcanic activity within that period which could have significantly impacted global climate. Here we employ diffusion relaxation modeling of 87Sr/86Sr zoning in plagioclase to obtain a better resolution on the life cycle of a magma chamber in order to provide a minimum constraint on the duration of the Western Ghats volcanic episode.The analyzed crystals are 2-5cm long and occur in so-called Giant Plagioclase Basalt flows that mark the tops of the oldest formations of the Western Ghats. In the formation studied, such crystals may have recorded the life cycle of the magma chamber that fed the preceding flows. Our diffusion modeling yields 780years for the maximum residence time for 5cm long plagioclase crystals in the magma chamber and a mean growth rate of 2.03×10-10cms-1. Assuming continuous crystal growth while the underlying pile was being emplaced gives us a combined maximum rate of 0.13myr-1 for incubation and extrusion of the lava. Using this rate we estimate that the peak volcanic activity lasted a minimum of 21,100years. A short duration for the Deccan would preclude AFC contamination processes during emplacement of the bulk of Deccan lavas.

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