Nd-142/Nd-144 inferences on the nature and origin of the source of high He-3/He-4 magmas

摘要

High-precision measurements of Nd-142/Nd-144 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 He-3/He-4 and W-182/W-184 ratios, but their Sr, Nd-142, Nd-143, Hf, and Pb isotopic signatures overlap the values measured in North Atlantic MORB. These features imply either that the mantle source region of high He-3/He-4 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 Nd-142-Nd-143 data are consistent with the magma source acquiring the incompatible element depletion during, or shortly after, Earth formation if the bulk-Earth has a Nd-142/Nd-144 ratio more similar to the average measured for enstatite chondrites than modern terrestrial rocks. If Earth's initial Nd-142/Nd-144 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 Sm-146. This explanation, however, fails to account for the high He-3/He-4. 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. (C) 2017 Elsevier B.V. All rights reserved.