Spherules with pure iron cores from Myanmar jadeitite: Type-I deep-sea spherules?

摘要

Here we report spherules in Myanmar jadeitite, a rock forming from jadeitic fluids within mantle-derived serpentinized rocks in subduction zones under high-pressure conditions (>1.0GPa) and rather low temperatures of about 250-370°C. The spherules have off-centre iron nuclei and dendritic wüstite cortexes, with tiny wüstite crystals perpendicular to the surface of iron core. Within the spherules are vesicles occupied by calcite, jadeite, albite? or mixtures of these phases, and the cortexes contain about 10wt.% SiO_2+Al_2O_3+Na_2O filling materials within wüstite. The spherules are in direct contact with jadeite crystals. Contrasting patterns of some individual spherules are obvious between a front area with a crowd of hill-like prominences and a rear zone with one or more rings on the surface. Such surface features and internal textures suggest that they experienced movement at high temperature and then rapid cooling. Chemical compositions of the nuclei are homogenous and consist of nearly pure iron with minor Cr (<0.05wt.%), Mn (<0.80wt.%), and Ni (0.142-0.23wt.%), and a trend of Ni decreasing and Cr increasing from core to cortex. Mn in the cortex (up to about 2.00wt.%) is far more enriched than the nucleus. The bulk ratios (average) of δ~(56)Fe and δ~(57)Fe in the core and cortex are 0.51and 0.78, respectively. Such features suggest that there is a very low possibility of origin associated with volcanic explosive eruption, impact ejecta, chemical reduction or oxidation of iron on seafloor. Since biological reduction processes are not significant under high P/T condition in subduction zones, this origin is excluded. Considering their low Ni contents, it is more likely that they belong to the minor type-I deep-sea cosmic spherules/dusts of low isotope fractionation. This discovery shows that such spherules could remain stable under low-temperature and high-pressure conditions during recycling processes, and therefore could be found in rocks related to slab-derived sediments within subduction zones. This also suggests that subducted oceanic slab sediments contribute to the formation of jadeitite, coupled with dehydration of sediments and altered oceanic crust.