Two groups of primitive melts are observed as inclusions in Cr-spinels in a
picrite (Fo(88-92.4)) from northern Iceland. Group I inclusions have 12.2-
13.8 wt% MgO, high CaO (14.6-15.8 wt%), low Al2O3 (10.1-11.8 wt%) and very
high CaO/Al2O3 (1.3-1.5); they occur in spinels with a cr# between 0.63 and
0.70. Group II inclusions occur in spinels with slightly lower cr# (0.55-0
.60) and they have 13.3-15.8 wt% MgO, 12.0-13.8 wt% CaO, 11.2-13.4 wt% Al2O
3 and CaO/Al2O3 0.9-1.1. Both groups have low concentrations of the incompa
tible minor elements (TiO2, K2O and P2O5) and relatively high SiO2. The two
groups converge at MgO approximately 10 wt%. A single inclusion in a low c
r# spinel could represent a more evolved member (9.00 wt% MgO) of either gr
oup. Group I inclusions are on a clinopyroxene control line while Group II
inclusions are close to being on an olivine control line. Groups I and II c
annot be derived one from the other. They require high degree of melting fr
om a refractory mantle source which probably is pyroxenitic to account for
the high CaO. Melts with such high CaO/Al2O3 cannot be generated by melting
commonly assumed lherzolites, so that pyroxenite or wehrlite mantle compon
ent is the suggested source. (C) 2000 Elsevier Science B.V. All rights rese
rved.