Evidence against an ancient non-chondritic mantle source for North Atlantic Igneous Province lavas

Citation:
Day, JMD.  2016.  Evidence against an ancient non-chondritic mantle source for North Atlantic Igneous Province lavas. Chemical Geology. 440:91-100.

Date Published:

2016/11

Abstract:

North Atlantic Igneous Province (NAIP) lavas host olivine with the highest 3He/4He ever measured for terrestrial
igneous rocks (up to 50 RA, or 4He/3He = ~15,300). The relationship of high-3He/4He with Pb isotope compositions
close to the terrestrial geochron and 143Nd/144Nd plausibly consistentwith supra-chondritic mantle Sm/Nd
in Baffin Island and West Greenland lavas has been interpreted to reflect an ancient ‘non-chondritic’ mantle
source signature. Alternatively, assimilation of continental crustal rocks with unradiogenic Pb isotope compositions
and low 143Nd/144Nd, into magmaswith high-3He/4He, and derived from variably depleted mantle sources,
could impart similar geochemical signatures. Radiogenic and stable isotope data for NAIP lavas are consistent
with origins as melts from upper mantle sources that contain low-18O/16O recycled lithosphere and/or hydrothermally
altered crust, or that have experienced pervasive contamination by crustal gneisses. Olivines from
NAIP lavas with 3He/4He spanning from 8 to 48 RA have δ18O ranging from 3.5 to 5.5‰. These compositions
are consistent with sources of ambient mantle and low-δ18O recycled lithosphere, or with concomitant crustal
assimilation and He-loss during fractional crystallization. Limited assimilation (≤1%) of incompatible element
rich crustal gneisses with low 206Pb/204Pb and 143Nd/144Nd by melts from variably depleted mantle sources
can explain Nd-Pb isotope compositions of Baffin Island and West Greenland picrites. Icelandic lavas provide
supporting evidence that the ancestral mantle plume responsible for generating NAIP magmatism sampled variably
enriched and depletedmantle,with no evidence for ancient non-chondriticmantle sources. Pervasive crustal
contamination and partial melting of heterogeneous mantle sources, generated by plate tectonic processes, can
account for the compositions of continental flood basalts (CFB),without the requirement of a non-chondritic terrestrial
reservoir. Combined with evidence that the 142Nd/144Nd composition of the bulk silicate Earth is due to
nucleosynthetic S-process deficits in chondrite meteorites, these observations cast doubt thatNAIP lavas sampled
a non-chondritic mantle source with Sm/Nd higher than in chondrites. If short-lived radiogenic (e.g.,
146Sm-142Nd, 182Hf-182W)isotope anomalies are found in CFB, theymust either reflect assimilation of isotopically
anomalous crustal materials, or partial melting of early-formed mantle heterogeneities produced by differentiation
and late accretion.

DOI:

10.1016/j.chemgeo.2016.07.002