The maximum value of Mg# [= 100Mg/(Mg + Fe2+ + Mn)] in chromium-bearing spinel-group minerals (Chr) in the Ultrabasic Core Zone (UCZ) of the Lyavaraka orthopyroxenite – harzburgite – dunite complex of the Serpentinite Belt in the Kola Peninsula is 54.5–67.5. Such highly magnesian compositions of spinel are associated with notable enrichments of ferric iron (Fe3+# 58–63). There are two generations of accessory Chr in the UCZ unit. The first generation occurs as inclusions in olivine that is not unusually magnesian (Mg# 90.3), and the second is closely associated with serpentine. The compositional series of Chr at Lyavaraka attains more aluminous compositions than was observed in nearby intrusive bodies. The anomalously high level of Mg in Chr, also manifest in ilmenite, is mainly a result of the high intrinsic fugacity of oxygen attained locally in the melt. A progressive buildup in H2O and increase in fO2 likely resulted from efficient vesiculation and selective loss of H2 from the Al-undepleted komatiitic magma crystallizing in a shallow setting. The chromian spinel forming in such a modified magma is virtually unzoned in Mn, and a minor quantity of Mn is also present in olivine and orthopyroxene. In contrast, zinc is strongly partitioned in the core of Chr, as it is relatively incompatible in the coexisting olivine and orthopyroxene at that stage. Zinc efficiently partitioned into the H2O-enriched melt, which crystallized as the pegmatitic orthopyroxenite near the contacts at Lyavaraka. A high potential of oxidation appears to be characteristic of all orthopyroxenite – harzburgite – dunite suites of the Serpentinite Belt formed from a primitive melt of komatiitic composition.

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