Petrogenesis of isotopically unusual Pliocene olivine leucitites from Deep Springs Valley, California

High-K mafic alkalic lavas (5.4 to 3.2 wt% K(2)O) from Deep Springs Valley, California define good correlations of increasing incompatible element (e.g., Sr, Zr, Ba, LREE) and compatible element contents (e.g., Ni, Cr) with increasing MgO. Strontium and Nd isotope compositions are also correlated with MgO; (87)Sr/(86)Sr ratios decrease and epsilon(Nd) values increase with decreasing MgO. The Sr and Nd isotope compositions of these lavas are extreme compared to most other continental and oceanic rocks: (87)Sr/(86)Sr ratios range from 0.7121 to 0.7105 and epsilon(Nd) values range from -16.9 to -15.4. Lead isotope ratios are relatively constant, (206)Pb/(204)Pb similar to 17.2, (207)Pb/(204)Pb similar to 15.5, and (208)Pb/(204)Pb similar to 38.6. Depleted mantle model ages calculated using Sr and Nd isotopes imply that the reservoir these lavas were derived from has been distinct from the depleted mantle reservoir since the early Proterozoic. The Sr-Nd-Pb isotope variations of the Deep Springs Valley lavas are unique because they do not plot along either the EM I or EM II arrays. For example, most basalts that have low epsilon(Nd) values and unradiogenic (206)Pb/(204)Pb ratios have relatively low (87)Sr/(86)Sr ratios (the EM I array), whereas basalts with low epsilon(Nd) values and high (87)Sr/(86)Sr ratios have radiogenic (206)Pb/(204)Pb ratios (the EM II array). High-K lavas from Deep Springs Valley have EM II-like Sr and Nd isotope compositions, but EM I-like Pb isotope compositions. A simple method for producing the range of isotopic and major- and trace-element variations in the Deep Springs Valley lavas is by two-component mixing between this unusual K-rich mantle source and a more typical depleted mantle basalt. We favor passage of MORB-like magmas that partially fused and were contaminated by potassic magmas derived from melting high-K mantle veins that were stored in the lithospheric mantle. The origin of the anomalously high (87)Sr/(86)Sr and (208)Pb/(204)Pb ratios and low epsilon(Nd) values and (206)Pb/(204)Pb ratios requires addition of an old component with high Rb/Sr and Th/Pb ratios but low Sm/Nd and U/Pb ratios into the mantle source region from which these basalts were derived. This old component may be sediments that were introduced into the mantle, either during Proterozoic subduction, or by foundering of Proterozoic age crust into the mantle at some time prior to eruption of the lavas.