Resetting of REE, and Nd and Sr isotopes during carbonitization of a komatiite flow from Finland

Abstract

New isotopic (Nd, Sr, C and O) and trace-element data are reported for a 10-m-thick komatiite flow unit from the 2660-Ma Tipasjärvi greenstone belt of eastern Finland. This unit was selected because of its anomalously variable REE distribution patterns. More specifically, while the lower cumulate zone of the flow exhibits nearly-flat LREE patterns [ ( La Sm ) N =0.93−1.10 ], the spinifex-textured upper zone shows strongly depleted LREE patterns [ ( La Sm ) N ≈0.40 ]. Such a fractionation in REE cannot result from low-pressure differentiation because the flow has MgO contents ranging from 25% to 30%, and only olivine, a phase which does not fractionate the REE, should have crystallized. In previous studies the REE patterns have been interpreted by the assumption that the flow had a composite origin: a LREE-depleted parental magma was inferred for the upper part; the lower part was thought to be derived from a parental magma with a chondritic distribution of LREE. In fact, in the present study, a reassessment of field and petrographic data indicates that the lower part of the flow is rich in secondary carbonate (10–20 wt.%). Determination of the chemical and isotopic composition of the carbonate, and further evaluation of the chemical and isotopic patterns of the flow in the light of the new data clearly show that the addition of carbonate is responsible for the variation in REE patterns. The Tipasjärvi flow is not a magmatically composite flow, but a single and uniformly depleted flow, whose lower portion has experienced severe alteration of primary chemical and isotopic signatures by invasion of secondary carbonate.