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

Abstract

Hafnium isotope Hafnium 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 array that is oblique to that of the oceanic island basalt (OIB) 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 OIB, and have been interpreted by many workers to have been derived from an OIB‐like mantle plume [e.g., Fitton et al, 1991; Kempton et al., 1991]. However, the Hf isotope compositions of Basin and Range basalts do not overlap those of OIB, and are instead more similar to mid‐ocean ridge basalt (MORB) mantle. Recognition of a MORB source for high‐εNd 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 MORB 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 εNd and εHf 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 8nf values at a given εNd value as compared to OIB. 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 207Pb/204Pb 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 εNd and εHf values, but high εHf values at a given εNd 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 εNd values but positive εHf values. In contrast, subduction of turbidite sediments will produce a mantle that has negative εHf and εNd values, but low εHf values at a given εNd value as compared to the OIB mantle. A mixture of pelagic to turbidite sediments that has a ratio greater than 1:1.2 can produce the negative εHf and εNd values, and high εHf values at a given εNd value (relative to OIB).