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around 200-300 ppm when Earth was formed. Over the course of Earth's history, a significant amount of ⁴He has been generated by the alpha decay of uranium, thorium and other radioactive isotopes, to the point that only around 7% of the helium now in the mantle is primordial helium, thus lowering the total ³He:⁴He ratio to around 20 ppm. Ratios of ³He:⁴He in excess of the atmospheric ratio are indicative of a contribution of ³He from the

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