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.