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Cr-spineland silic ate phases . or an intergrBnular phase . It has been postulated
that refractory Cr-spinel and olivine in s-cecrultrama fic rocks may act as a sink
for If (Crockel. 1979; Naldrett m.a.l., 1979 1. In addition . Mitchell and xeevs
" 9811 predicted that high -temperature If eucvs may exist under upper mantle
P·T conditions in environments of low t5 2 • Iridium in this form would remain
as a residual phase dur ing par tial melting of the mant le (M it chell and Keays ,
1981 ). In contrast to Ir, Pd is more vola tile {Arculus and Delano , 19811 and .
therefore, during part ial melting Pd fractionates from If which is left in the
residue (Henogen §1 .ilL 1980 ; Mitchell and Keays, 19811 . Hence, high ly
depleted and refractory mantle peridotites have low total PGE contents.
fractionated CNPGE patte rns with negative slopes from Ir to Pd. and Pd/lr< < 1.
Such patterns have been reported from the Thetford Mines Oph iolite. Quebec
IOshin and croeeet. 19821. and the Ronda Peridotite, Spain (Stockman, 19821.
and calculated for the Vourinos Oph iolite Complex . Greece tccceene III aI.•
19891.
The CNPGE pattern and Pdflr =0.6 of sample L225 is cons istent with it
being a residue from panial melting. Howe ver. more extreme fractionation of
Pd and Ir is expected for harzburgita as refractory as sample L225. The PGE­
depleted {lr :.:2. 1-2. 9 ppb . Pd"0.45 -0 .68 ppbl Black Lake harzburgites of the
Thetford Mines Oph iolite have average Pdllr =0.22 IOshin and Crocket, 19821.
which are expected to be more typical of harzburgites that are residues from
ext ensive part ial melting and effi cient extraction of magma . Despite this .
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sample L225 will be used as tho starting composition for discussing the PGE