Discussion
Higher amounts of which of the following isotopes indicate plume related enriched mantle origin of early Deccan volcanic rocks.
Answer: C
The indicator is a high \(^{3}\)He/\(^{4}\)He ratio.
Helium-3 is primordial, left over from Earth's formation, while helium-4 is continually produced by radioactive decay of uranium and thorium. Mid-ocean ridge basalts have about 8 times the atmospheric ratio, and lavas derived from deep, less degassed mantle plumes have much higher values, as at Hawaii and Iceland. High \(^{3}\)He/\(^{4}\)He in the early Deccan lavas and associated rocks is therefore taken as a fingerprint of the Reunion plume.
Sr and Os isotope ratios are raised by crustal contamination as well as by enriched sources, so they are not specific to plumes, and \(^{87}\)Rb/\(^{86}\)Rb is not a meaningful isotope ratio.
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