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).