Cryptic zoning in primitive olivine as an archive of mush fluidization at mid-ocean ridges

Abstract

Processes operating within magmatic crystal mushes exert a fundamental control on the physical and chemical evolution of mid-ocean ridge systems. It is widely recognized that mush fluidization in response to magma recharge could produce variable compositional zoning in crystals. However, olivines generally show relatively smooth MgFe zoning in comparison with complex and short-length scale zoning in plagioclases. It is still unclear about the proceedings of the olivine before the mush disaggregation immediately prior to eruption and unknown whether plagioclase zoning is genuinely more complex than olivine zoning. Here, using slow-diffusing elements X-ray maps, we identify complex compositional zoning in olivines from MORBs of the Southwest South China Sea (SCS) and thus provide mineralogical evidence for the re-organization within a mush that takes place over an extended period of time. Primitive olivines have homogeneous forsterite concentrations (Fo = 89.5 mol%), but preserve zoning in Al2O3 and Cr2O3. Multiple cores have anomalously low Al2O3 and Cr2O3 contents, with or without high P2O5, Al2O3, and Cr2O3 dendrites, and are surrounded by Al–Cr-rich but P-depleted mantles, in which boundaries between cores and mantles are sharply defined. This compositional and textural zoning indicates dissolution, re-equilibration, and re-growth of evolved olivine within primitive magmas, demonstrating a redistribution of crystals within a mush zone, consistent with mush fluidization. Therefore, primitive olivines, widely-used monitors of mantle processes, may also have undergone mush fluidization, such that apparently primitive olivines may result from the re-equilibration of evolved olivines with primitive melts. PAl zoning patterns in olivine can be used to evaluate the influence of mush processes on olivine mantle probes.