Extreme incompatibility of helium during mantle melting: Evidence from undegassed mid-ocean ridge basalts

Abstract

We report total helium concentrations (vesicles+glass) for a suite of thirteen ultradepleted mid-ocean ridge basalts (UD-MORBs) that were previously studied for volatile contents (CO2, H2O) plus major and trace elements. The selected basalts are undersaturated in CO2 + H2O at their depths of eruption and represent rare cases of undegassed MORBs. Sample localities from the Atlantic (2), Indian (1) and Pacific (7) Oceans collectively show excellent linear correlations (r2=0.75–0.92) between the concentrations of helium and the highly incompatible elements C, K, Rb, Ba, Nb, Th and U. Three basalts from Gakkel Ridge in the Arctic were also studied but show anomalous behavior marked by excess lithophile trace element abundances.In the Atlantic–Pacific–Indian suite, incompatible element concentrations vary by factors of 3–4.3, while helium concentration varies by a factor of 13. The strong correlations between the concentrations of helium and incompatible elements are explained by helium behavior as the most incompatible element during mantle melting. Partial melting of an ultradepleted mantle source, formed as a residue of earlier melt extraction, accounts for the observed concentrations. The earlier melting event involved removal of a small degree melt (∼1%) at low but non-zero porosity (0.01–0.5%), leading to a small amount of melt retention that strongly leveraged the incompatible element budget of the ultradepleted mantle source. Equilibrium melting models that produce the range of trace element and helium concentrations from this source require a bulk solid/melt distribution coefficient for helium that is lower than that for other incompatible elements by about a factor of ten. Alternatively, the bulk solid/melt distribution coefficient for helium could be similar to or even larger than that for other incompatible elements, but the much larger diffusivity of helium in peridotite leads to its more effective incompatibility and efficient extraction from a larger volume of mantle during melting. In either case, partial melting leaves a mantle residue with elevated (U+Th)/3He. Consequently, peridotite residues of mantle melting cannot be the source of high 3He/4He observed at ocean island hotspots such as Hawaii and Iceland.The extreme effective incompatibility of helium entails that high 3He/4He mantle sources, isolated before 4.45 Ga based on Xe and W isotopes, have not experienced any melt extraction since they were isolated from convecting portions of the mantle.