Helium isotopes in lithospheric mantle: evidence from tertiary basalts of the western USA

Abstract

The isotopic compositions of He, Sr, and Nd were measured in Tertiary-age basalts from the Basin and Range province of the western USA to evaluate models for the He isotopic character of subcontinental mantle lithosphere (SCML) and assess the role of recycled SCML in models of mantle evolution. Previous isotopic and trace element measurements suggested that most of these basalts were formed by melting of SCML. 3He/ 4He ratios, measured by in-vacuo crushing of olivine phenocrysts, vary from 2.9 to 7.8 times the atmospheric value (2.9 to 7.8 Ra) consistently below the MORB value of 8.7 ± 0.5 Ra. The lowest R/Ra values, associated with low ε Nd, high 87Sr/ 86Sr, and high La/Nb, are attributable to lithospheric mantle, and indicate that SCML is not dominated by MORB-type He, nor by high R/Ra, plume-type He. Consideration of geographic variability indicates there are two, and possibly three, distinct regions of SCML with differing He isotopic characteristics. SCML beneath the eastern Sierra Nevada is inferred to have 3He/ 4He of ∼5.5 Ra and a He/Nd ratio slightly less than MORB-type mantle; SCML beneath the central Basin and Range has 3He/ 4He of ∼4 Ra and a higher He/Nd ratio than MORB-type mantle. The SCML under southwestern Utah shows less systematic correlation of He isotopes with other geochemical parameters, but also has a lower bound R/Ra value of about 4 Ra. The inferred SCML helium ratios are consistent with retention of radiogenic 4He over 800 Ma for the eastern Sierra Nevada and 1700 Ma for the other two regions. The results are not consistent with models of He infiltration from the underlying asthenosphere and suggest the lithosphere of the Basin and Range region was not delaminated during the early Tertiary. The He, Sr, Nd, and Pb isotopic compositions inferred for the SCML of the southwestern USA are a reasonably good match to the characteristics of the EMII component of mantle heterogeneity identified in oceanic island basalts. High R/Ra mantle reservoirs identified in these basalts are not likely to represent recycled SCML.