(a) Correlation of Ca# and Mg# of representative samples of harrisitic peridotite vein, magnesium-rich dykes, peridotites, troctolites, gabbro veins, and poikilo-macro-spherulithic plagioclase peridotites from the Central Series on Rum. The Mg# (100 x Mg/(Mg +Fe)) and Ca# (100 x Ca/(Ca + Na)) of all samples, except Dyke-RM 2, show that these rocks are primitive relative to fast spreading lower oceanic crustal rocks (light blue reference samples from Gillis et al., 2014b and references therein). The low Ca# of the poikilo-macro-spherulitic plagioclase peridotite is likely a result of presence or later infiltration of evolved alkaline fluids and melts (e.g. from more evolved feldspathic peridotite dykes and veins). (b) REE plot of Central Series peridotites compared to peridotite and troctolite from the Eastern Layered Series (ELS). The black dashed line shows the modelled REE pattern of a cumulate rock calculated via a Rayleigh fractionation model from a primary melt identical to the feldspathic peridotite dykes (Har-dyke 1 A) using 80% solidification of an olivine (0·7), plagioclase (0·2) and clinopyroxene (0·1) assemblage. The modelled REE pattern follows the measured pattern for the Eastern Layered Series Unit 9 peridotite well (normalizing values from McDonough & Sun, 1995 and modelling parameters from Meyer et al. 2009). The Central Series cumulate peridotites, on the other hand, are enriched in LREE compared to the Eastern Layered Series (ELS) peridotites and display REE patterns more similar to troctolite. Assuming the Central Series cumulate peridotites initially had REE compositions similar to the Eastern Layered Series peridotites, post-cumulus interaction/mixing between plagioclase-rich melt and the Central Series cumulate peridotites is inferred. Reference data in (a) and (b) from the Eastern Layered Series are from Emeleus (1997,) and Meyer et al. (2009).