An SPO-induced CPO in composite mantle xenoliths correlated with increasing melt-rock interaction

Abstract

Melt-rock interaction, a general term describing various processes including cryptic and modal metasomatism and melt-rock reaction, is the main process responsible for modifying chemical composition of previously melt-depleted mantle peridotite. However, the underlying mechanisms of melt-rock interaction and its effect on mantle rheology, particularly in natural peridotites, is poorly constrained. Composite xenoliths are natural examples of melt-rock interaction as they contain pyroxenitic veins interpreted as evidence of passage of melt through peridotite at high pressures. Here, we present new mineral chemistry (major, trace element, and water contents of olivine and pyroxenes) and microstructural data on a suite of composite xenoliths from the Neogene Hannuoba basalt, North China Craton. We show that despite having experienced high melt/rock ratios, olivines and pyroxenes contain very low water contents (<10 ppm and <100 ppm, respectively). In contrast, melts calculated to be in equilibrium with clinopyroxene show enriched trace element signatures suggesting the infiltrating melt had a crustal origin. Microstructural data corroborate a key role for melt infiltration in causing a systematic shift in olivine crystallographic preferred orientation (CPO) from initially A-type to AG-type with increasing melt/rock ratio. By describing the olivine grain shape with respect to the crystal reference frame, we show that as pyroxene mode (and hence melt/rock ratio) increases, olivine grains appear to rotate with their flattest (0 1 0) faces aligning with the melt flow plane, resulting in an olivine CPO controlled by its shape-preferred orientation (SPO). Previously, such an SPO-induced CPO was only demonstrated in shallow magmatic environments such as mafic intrusions, in mafic lavas, and in high-pressure/high-temperature experiments. Such a finding in the deep lithosphere is important as it suggests that dislocation creep may not always play a major role in CPO development, particularly when melt is involved.