Absence of a high time-integrated ³He/(U+Th) source in the mantle beneath continents

Abstract

Volcanic rocks from ocean island and continental flood basalt provinces can exhibit ³He/⁴He ratios greatly in excess of those of mid-oceanic-ridge basalts (MORB). High ³He/⁴He ratios must indicate derivation from a mantle source with high time-integrated ³He/(U+Th) relative to depleted MORB-source mantle. The location of the high ³He/⁴He mantle reservoir is a poorly resolved but important issue because of the constraints it places upon the structure and convective style of Earth's mantle. It has been proposed that the high ³He/⁴He reservoir resides in the upper mantle, rather than the lower mantle, because Earth should be volatile poor and highly differentiated, with incompatible elements (such as He) concentrated in the upper mantle and crust. This hypothesis can be tested using continental intraplate alkaline volcanics (CIAV) that are generated at or near the boundary between the conducting lithospheric and convecting asthenospheric mantle. Olivine and clinopyroxene phenocrysts from Cretaceous to Miocene CIAV from Canada, South Africa, and Uganda have ³He/⁴He ratios more radiogenic than MORB, strongly arguing against a widespread high ³He/⁴He source in the continental lithosphere or the underlying convecting upper mantle. Combined with a global data set of CIAV and continental lithosphere mantle xenoliths, these results provide no evidence for high ³He/⁴He in any samples known to originate from this environment. Therefore, volcanic rocks with ³He/⁴He greater than MORB ³He/⁴He are likely to sample a mantle source with high time-integrated ³He/(U+Th) that cannot exist within or below the continents. This reservoir is also unlikely to exist within the upper mantle as defined by the ³He/⁴He distribution in MORB.