Experimental petrology constraints on the recycling of mafic cumulate: a focus on Cr-spinel from the Rum Eastern Layered Intrusion, Scotland

Reactive liquid flow is a common process in layered intrusions and more generally in episodically refilled magma chambers. Interaction between newly injected melt and cumulates, or crystal mushes, perturbs the liquid line of descent of the melt and modifies mineral chemistry and texture. We present insights into the effects of assimilation of mafic cumulate rocks (gabbro, troctolite) by cogenetic Mg-rich basalt liquid using one-atmosphere, controlled fO₂ phase equilibrium experiments on picritic parental liquid to the Rum layered intrusion, Scotland. For picrite-only experiments at fO₂ = QFM, Cr-spinel (Cr# = Cr/[Cr + Al + Fe³⁺] = 0.43; Fe# = Fe²⁺/[Mg + Fe²⁺] = 0.32) saturates at 1320 °C, olivine (Fo₈₈) at ~1290 °C, plagioclase (An₇₇) at 1200 °C, and clinopyroxene (Mg#: 0.81) at 1180 °C. In melting experiments on picrite + gabbro mixtures, plagioclase (1230 °C, An₈₀) and clinopyroxene (1200 °C, Mg#: 0.85) saturation temperature and mode are increased significantly. Cr-spinel in these experiments has a distinctive, low Fe#. In melting experiments on picrite + troctolite mixtures, plagioclase (An₈₆) saturates at 1240 °C and clinopyroxene (Mg#: 0.81) at 1170 °C. Al-rich spinel crystallizes at high temperature (>1220 °C) and becomes more Cr-rich upon cooling, reaching the highest Cr# = 0.47 at 1180 °C (0.54 at QFM-1.2). The experimental results confirm that plagioclase and clinopyroxene stability plays a major role in determining the composition of coexisting spinel. Comparing our experimental results to the Rum Eastern Layered Intrusion, we propose a model for the precipitation of spinel from picrite-troctolite hybrid melt that is compatible with the observed olivine, plagioclase, and clinopyroxene chemistry.