Abstract
High-precision measurements of 142Nd/144Nd in picrites from the Baffin Bay region that contain the highest 3He/4He ratios yet measured in terrestrial mantle-derived rocks are indistinguishable from the value measured in the terrestrial standard and other modern mantle-derived rocks. The Baffin Island lavas are distinguished from other hotspot lavas by their unusually high 3He/4He and 182W/184W ratios, but their Sr, 142Nd, 143Nd, Hf, and Pb isotopic signatures overlap the values measured in North Atlantic MORB. These features imply either that the mantle source region of high 3He/4He magmas carries the lithophile isotopic signatures of incompatible element depletion, or that the He isotope signature of this source is decoupled from the lithophile isotope tracers in the magmas. The coupled 142Nd–143Nd data are consistent with the magma source acquiring the incompatible element depletion during, or shortly after, Earth formation if the bulk-Earth has a 142Nd/144Nd ratio more similar to the average measured for enstatite chondrites than modern terrestrial rocks. If Earth's initial 142Nd/144Nd was higher than the average of enstatite chondrites, the data are consistent with the traditional interpretation that the depleted-mantle reservoir was formed through the extraction of an incompatible-element-rich reservoir, such as continental crust, after the circa 4 Ga extinction of 146Sm. This explanation, however, fails to account for the high 3He/4He. The Nd isotopic composition of the picrites could reflect a dominant contribution from the incompatible element depleted source of North Atlantic MORB, overprinted by a small (10–20%) contribution from a mantle source with He concentrations at least ten times higher than the depleted mantle along with W isotopic compositions substantially higher than typical of mantle-derived rocks.
Highlights
The isotopic composition of helium in Earth’s interior is an important parameter that must be accounted for by models that seek to explain the differentiation history of the Earth
The high 3He/4He and lithophile element isotope systematics of the Baffin Island picrites can be explained in at least two ways: 1) The picrite melts are a mixture of depleted MORB mantle (DMM) and primitive mantle (PM)
The overlapping μ142Nd, but very different 3He/4He of the Baffin picrites and Mid-ocean ridge basalts (MORB) in this model would imply that DMM was developed by continent extraction from a portion of the early-formed mantle reservoir, increasing the incompatible element depletion and causing a dramatic reduction in noble gas concentrations
Summary
The isotopic composition of helium in Earth’s interior is an important parameter that must be accounted for by models that seek to explain the differentiation history of the Earth. Mid-ocean ridge basalts (MORB) derived from the convecting upper mantle typically have 3He/4He ratios of approximately 8Ra (where Ra is the atmospheric 3He/4He ratio of 1.39 × 10−6) (Graham, 2002). The source of high 3He/4He in Earth’s interior must have evolved in a reservoir with a time-integrated (U + Th)/3He ratio, and attendant capacity to generate 4He, that was lower than that of most of the convecting upper mantle Whether this reflects a mantle reservoir depleted in incompatible elements such as U and
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