Geochemical relationship between REE and PGE in mafic and ultramafic rocks and their petrogenetic significance - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 279

basaltic smelting formed during partial melting of non-depleted upper mantle the
overall balance of PGE should grow by about half-order compared to the original
mantle source, thus to be (ppb): Os - 17; Ir - 16; Ru - 25; Pt - 35; Pd - 20. When
extrapolating these values to a lower degree of partial melting of the mantle source,
for example, to 10%, the PGE contents in basaltic smelting, obviously, will be (ppb):
Os - 8.5; Ir - 8; Ru - 12.5; Pt - 17.5; Pd - 10. To add more, Barnes et al . [1985]
proposed to divide the PGE group into two subgroups 1) subgroup of iridium (Os, Ir,
Ru), and 2) subgroup of palladium (Rh, Pt, Pd); however, as they emphasized, such
division should not be considered as an analogy for the separation of REE group into
subgroups of LREE and HREE. These researchers also pointed at the possibility of
using Pd/Ir parameter as one of the geochemical indicators for discrimination and
classification of rocks of ultramafic and mafic composition. They estimated the aver-
age values of this parameter and found it increasing from peridotite komatiites (10)
to pyroxenite komatiites (30) and further to the basalts of continents and ocean floor
(100). Note that in plutonic varieties of ultramafic and mafic rocks the values of this
parameter vary over a wider range, for example, in some varieties of samples from
mafic-ultramafic Stillwater massif (Montana, USA) the value of Pd/Ir reached 865,
while in ore chromitites of ophiolite associations it is close to 0.1.
Taking into account the data mentioned above on the distribution of REE and
PGE in ultramafic and mafic rocks, as well as on correlations between the contents of
the elements of these two groups, one can assume that in the mantle basaltic smelt-
ing that is formed when the degree of partial melting was 20%, the concentration of
REE, which are elements that are relatively easy to transform into melt, should be
lower than their concentration in basaltic smelting, which arose at about 10% degree
of partial melting of a similar in composition mantle source. It is obvious that the
increase of degree of partial melting of mantle source was accompanied by a decrease
in REE contents of each subsequent portion of melt and simultaneous relative enrich-
ment of this portion with platinum group elements. However, as can be seen from the
analytical data mentioned above in this chapter, even at the highest degrees of partial
melting and resulting intense depletion of the mantle source by fusible components
the ultramafic restites formed as a result of that process were not fully sterilized with
respect to REE.
Summarizing the considered materials on REE and PGE relations in different
types of rocks and mafic-ultramafic associations, we can conclude that partial melt-
ing of non-depleted upper mantle, resulted in the formation of ultramafic restites
and basaltic melts complementary to them, in fact was the earliest stage of a multi-
stage fractionation and redistribution of PGE and REE within the mantle-crust sys-
tem. At the same time with increasing of degrees of partial melting of mantle sources
in the forming ultramafic restites, as well as in complementary mafic smelting, and
then - also in the products of their crystallization, an increase in PGE concentrations
was accompanied by a decrease in REE concentrations. As a result, in these rocks
we can observe an inverse relation between the contents of the elements from these
two groups contrasting in their properties. Apparently, the same type of mechanism
could provide a described by some specific examples 'phenomenon' of geochemical
antagonism between REE and PGE. However, such interpretation of the relations
between these elements in ultramafic and mafic rocks can be considered only as a
first approximation to the forthcoming solution of these complex petrological and
geochemical problems.