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

de Leeuw, G.A.M., Ellam, R.M., Stuart, F.M. and Carlson, R.W. (2017) 142Nd/144Nd Inferences on the nature and origin of the source of high 3He/4He magmas. Earth and Planetary Science Letters, 472, pp. 62-68. (doi:10.1016/j.epsl.2017.05.005)

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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.

Item Type:Articles
Additional Information:This work was funded by the Carnegie Institution for Science and the Scottish Universities Environmental Research Centre under NERC grant NE/E015069/1.
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Ellam, Professor Robert and Stuart, Professor Finlay and de Leeuw, Dr Goverdina
Authors: de Leeuw, G.A.M., Ellam, R.M., Stuart, F.M., and Carlson, R.W.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Earth and Planetary Science Letters
Publisher:Elsevier
ISSN:0012-821X
ISSN (Online):1385-013X
Published Online:30 May 2017
Copyright Holders:Copyright © 2017 The Authors
First Published:First published in Earth and Planetary Science Letters 472: 62-68
Publisher Policy:Reproduced under a Creative Commons license

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