Calcium-rich melt inclusions in Cr-spinels from Borgarhraun, northern Iceland

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% Al₂O₃ and CaO/Al₂O₃ 0.9-1.1. Both groups have low concentrations of the incompatible 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 cr⇆ spinel could represent a more evolved member (9.00 wt% MgO) of either group. 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 cannot be derived one from the other. They require high degree of melting from 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 component is the suggested source. (C) 2000 Elsevier Science B.V. All rights reserved.