Hafnium isotope evidence for the origin of Cenozoic basaltic lavas from the southwestern United States

Hafnium isotope analyses of 52 Cenozoic basalts from the southwestern United States document the differences and similarities of the mantle beneath the continents, as compared to the suboceanic mantle. One of the major conclusions of this work is documentation of a widespread Nd-Hf isotope al-ray that is oblique to that of the oceanic island basalt (GIB) array, indicating that the subcontinental mantle forms an important component to the Nd-Hf isotope balance of the Earth. Basalts from the Basin and Range province have Sr, Nd, and Pb isotope compositions that overlap those of GIB, and have been interpreted by many workers to have been derived from an GIB-like mantle plume [e.g., Fitton er al., 1991; Kempton et al., 1991]. However, the Hf isotope compositions of Basin and Range basalts do not overlap those of GIB, and are instead more similar to mid-ocean ridge basalt (MORE) mantle. Recognition of a MORB source for high-E-Nd Basin and Range lavas is possible only with addition of Hf isotope data. The Pb, Sr, Nd, and Hf isotope compositions and trace element contents of Basin and Range basalts can be matched by mixing melts that were derived from small degrees of partial melting of a depleted MORE mantle, with partial melts of incompatible element enriched pyroxenite veins. In order to match the low Lu/Hf ratios measured in these basalts, one of these components (pyroxenite veins or depleted peridotite) must have been garnet bearing. Moreover, to match the positive E-Nd and E-Hf values of Basin and Range basalts, these lithophile element-enriched pyroxenite veins must be relatively young; we suggest they reflect mantle veining during earlier widespread Mesozoic or Cenozoic magmatism. Basalts from the Rocky Mountains and western Great Basin have similar trace element contents, high lithophile element contents and high large-ion lithophile element (LILE) to high-field-strength element (HFSE) ratios. The Sr, Nd, and Pb isotope compositions of most western Great Basin samples plot along the enriched mantle (EM) II array of Zindler and Hart [1986], but basalts from the Rocky Mountains plot along the EM I array. The Hf isotope compositions of western Great Basin and Rocky Mountain samples overlap. However, these Hf isotope compositions are anomalous relative to the OIB array; most samples have higher E-Hf values at a given E-Nd value as compared to GIB. The lead isotope compositions of both the Rocky Mountain and western Great Basin samples plot significantly above the northern hemisphere reference line in terms of Pb-207/Pb-204 ratios, supporting models for input of crustal material into the mantle. Crustal recycling into the mantle can produce a mantle source region that has high LILE/HFSE ratios, which are a characteristic of the source of basalts from the western Great Basin and Rocky Mountains. To produce the negative E-Nd and E-Hf values, but high E-Hf values at a given E-Nd value (as compared to the OIB mantle), the crustal material input into the mantle must be a blend of pelagic and turbidite sediments. Ancient subduction and storage of pelagic sediments in the mantle will produce a source region with negative E-Nd values but positive E-Hf values. In contrast, subduction of turbidite sediments will produce a mantle that has negative E-Hf and E-Nd values, but low E-Hf values at a given E-Nd value as compared to the GIB mantle. A mixture of pelagic to turbidite sediments that has a ratio greater than 1:1. 2 can produce the negative E-Hf and E-Nd values, and high E-Hf values at a given E-Nd value (relative to OIB).