Helium isotopes in lithospheric mantle: Evidence from Tertiary basalts of the western USA

The isotopic compositions of He, Sr, and Nd were measured in Tertiary-age b asalts from the Basin and Range province of the western USA to evaluate mod els for the He isotopic character of subcontinental mantle lithosphere (SCM L) and assess the role of recycled SCML in models of mantle evolution. Prev ious isotopic and trace element measurements suggested that most of these b asalts were formed by melting of SCML. He-3/He-4 ratios, measured by in-vac uo crushing of olivine phenocrysts, vary from 2.9 to 7.8 times the atmosphe ric value (2.9 to 7.8 Ra) consistently below the MORE value of 8.7 +/- 0.5 Ra. The lowest R/Ra values, associated with low epsilon(Nd), high Sr-87/Sr- 86, and high La/Nb, are attributable to lithospheric mantle, and indicate t hat SCML is not dominated by MORE-type He, nor by high R/Ra, plume-type He. Consideration of geographic variability indicates there are two, and possi bly three, distinct regions of SCML with differing He isotopic characterist ics. SCML beneath the eastern Sierra Nevada is inferred to have He-3/He-4 o f similar to 5.5 Ra and a He/Nd ratio slightly less than MORE-type mantle; SCML beneath the central Basin and Range has He-3/He-4 of similar to 4 Ra a nd a higher He/Nd ratio than MORE-type mantle. The SCML under southwestern Utah shows less systematic correlation of He isotopes with other geochemica l parameters, but also has a lower bound R/Ra value of about 4 Ra. The infe rred SCML helium ratios are consistent with retention of radiogenic He-4 ov er 800 Ma for the eastern Sierra Nevada and 1700 Ma for the other two regio ns. The results are not consistent with models of He infiltration from the unde rlying asthenosphere and suggest the lithosphere of the Basin and Range reg ion was not delaminated during the early Tertiary. The He, Sr, Nd, and Pb i sotopic compositions inferred for the SCML of the southwestern USA are a re asonably good match to the characteristics of the EMII component of mantle heterogeneity identified in oceanic island basalts. High R/Ra mantle reserv oirs identified in these basalts are not likely to represent recycled SCML. Copyright (C) 1998 Elsevier Science Ltd.