Publication Date:
2022-05-26
Description:
© The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Earth and Planetary Science Letters 449 (2016): 145-154, doi:10.1016/j.epsl.2016.05.052.
Description:
The origin of volatiles on Earth is still a matter of debate. Noble gases are an efficient
geochemical tool to constrain Earth formation processes due to their inertness. Several studies have
focused on the neon isotopic composition of the lower mantle because the 20Ne/22Ne ratio is thought to
reflect that of Earth’s primordial components. Two models to explain the origin of light noble gases on
Earth have been proposed: either solar wind implantation onto the Earth's solid precursors or dissolution
into the mantle of a primordial atmosphere captured from solar nebula gas. In order to test these two
models, we analyzed the noble gas compositions (helium, neon and argon) of two submarine oceanic
island basalt glasses from Fernandina volcano (Galápagos archipelago), which have among the most
primitive/unradiogenic terrestrial helium and neon isotopic compositions. Several sample pieces are
studied both by step-crushing and by laser ablation analyses of single vesicles. Results of step-crushing
are consistent with those of laser ablation analyses, but the latter results provide new insights into the
origin of atmospheric contamination. The single-vesicle laser-ablation measurements overlap with the
step crushing results, but have systematically higher 40Ar/36Ar, and 3He/36Ar, suggesting less
atmospheric contamination using this method. The single vesicle data therefore suggest that atmospheric
contamination is introduced by exposure to the modern atmosphere, after sample collection. 3He/4He
values are about 23 times the atmospheric ratio (R/Ra) for the two Fernandina (Galápagos) samples, in
agreement with previous studies. We obtain 20Ne/22Ne and 40Ar/36Ar isotopic ratios as high as 12.91 and
9400, respectively, for the mantle source of the Galápagos hotspot. The new data show that step-crushing and laser ablation analyses are complementary methods that should be used together to derive
the noble gas ratios in uncontaminated samples. The results of neon compositions are consistent with
previous hotspot studies and support the model of solar wind implantation associated with sputtering to
explain helium and neon origins on Earth.
Description:
M.D.K. acknowledges support from NSF OCE in collecting the samples and
allowing his participation in this study (OCE-1259218 and OCE-1232985). M.M. acknowledges the
financial support from the UnivEarthS Labex program of Sorbonne Paris Cité (ANR-10-LABX-0023
and ANR-11-IDEX-0005-02).
Description:
2018-06-09
Keywords:
Oceanic island basalts
;
Galápagos
;
Lower mantle
;
Helium
;
Neon
;
Argon isotopes
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
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