Binary mixing of enriched and undegassed (primitive?) mantle components (He, Sr, Nd, Pb) in Samoan lavas

Abstract

We have measured He, Sr, Nd, and Pb isotope ratios and Rb, Sr, Sm, and Nd concentrations in stratigraphically controlled lavas from the Pago shield volcano on Tutuila, American Samoa. We interpret these lavas as products of mixing between two isotopically extreme mantle constituents. The first is a highly enriched component with very high Sr isotope ratios, low ^3He/^4He ratios, and high “Δ(7/4)” and “Δ(8/4)” Pb isotopic characteristics. This is probably recently recycled ( < 400 Ma) crustal material. The second component has high ^3He/^4He ratios ( > 24R_A) and intermediate Sr, Nd, and Pb isotopic ratios. This material was derived from a largely undegassed mantle source, and, in conjunction with data from several other ocean islands, provides strong evidence for the existence of a high ^3He/^4He ratio mantle end member (primitive helium mantle, PHEM) with consistent Sr, Nd, and Pb isotopic characteristics: near bulk-earth ^(87)Sr/^(86)Sr (0.7042–0.7052) and ^(143)Nd/^(144)Nd (0.51265–0.51280,ε_(Nd) = +0.2to+3.2), and radiogenic Pb (^(206)Pb/^(204)Pb∼ 18.5–19.0, ^(207)Pb/^(204)Pb∼ 15.5–15.57, ^(208)Pb/^(204)Pb∼ 38.4–39.2). Although this material cannot have been derived from a reservoir completely closed to elemental fractionation for the full 4.55 Ga duration of Earth's history, it may indicate the presence of a highly primitive mantle source. Erratic temporal variations in the isotopic composition of individual flows indicate sporadic and variable mixing of these two sources. We interpret these results using a model in which high ^3He/^4He ratio plume material, rising intermittently from a lower-mantle source, intercepts and melts recycled crustal matter in the upper mantle or lithosphere and erupts as a binary mixture of PHEM and the so-called “EM” components derived from this recycled material.