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

摘要

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 Mg[sbnd]Fe 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 dis-aggregation 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 min-eralogical 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 A1_2O_5 and Cr_2O_3. Multiple cores have anomalously low A1_2O_3 and Cr_2O_3 contents, with or without high P_2O_5, Al_2O_3, and Cr_2O_3 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. P[sbnd]AI zoning patterns in olivine can be used to evaluate the influence of mush processes on olivine mantle probes.