HIGH ND-143 ND-144 IN EXTREMELY DEPLETED MANTLE ROCKS/

Using an iron hydroxide precipitation to concentrate the rare earth el ements it was possible to obtain precise Nd isotopic data on ultramafi c rocks. Four samples of unserpentinized harzburgite from the Voykar o phiolite, Polar Urals, are characterized by extreme depletion in ferti le components including light rare earth elements down to similar to 1 .7 ppb Nd. These samples have f(Sm/Nd) values > +1 and epsilon(Nd)(0) values ranging from +21 to +47, the highest ever measured in terrestri al whole rock samples. Combined epsilon(Nd) and epsilon(Sr) values of the ultramafic and mafic rocks and a leach indicate that alteration fr om seawater and/or groundwater is orders of magnitude less than requir ed to produce a detectable change in the epsilon(Nd) values even in sa mples with > 50% serpentine. Three of the harzburgites plot on a Sm-Nd whole rock array along with samples of dunite, websterite, gabbro and diabase from the ophiolite. The slope of the array corresponds to an age of 387 +/- 34 Ma and an initial epsilon(Nd) = +8.6 +/- 1.8. These results indicate that the oceanic crustal rocks and the harzburgites a re complementary and come from a MORE source. While the linear Sm-147/ Nd-144-/Nd-143/Nd-144 correlation is consistent with this model, one s ample of harzburgite is far removed from the array, indicating the pre sence of older, depleted mantle material that is unrelated to the crus tal section, This must be locally available in the upper mantle for te ctonic occlusion during obduction of the ophiolite, Alternatively, thi s ''aberrant'' sample may represent the original ultrabasic section fr om older basalt extraction that was permeated by basalts derived from much deeper layers that provided the Voykar ophiolite crust. The major -element and low-REE-compositions of the Voykar harzburgites can be fo rmed by progressive extraction of melt from an undepleted mantle proto lith with the requirement that melt separation began in the garnet Ihe rzolite stability field. The extreme chemical and isotopic characteris tics of the Voykar harzburgites demonstrate that extraction of partial melt can generate an upper mantle with highly variable epsilon(Nd) an d very low Nd concentrations. The potential presence of large amounts of such material in the upper mantle poses an intriguing geochemical p roblem in that its bulk isotopic signature may not be detectably expre ssed in basalts, which are dominated by sources with higher average Nd concentrations and much lower epsilon(Nd) and f(Sm/Nd).