Element Geochemistry Of Peridotites From The Southwest Indian Ridge:Implications For Mantle Dynamics

Ultraslow spreading Southwest Indian Ridge (SWIR) is characterized by low mantle temperature, low mantle melting degree, fertile mantle composition, and heterogeneous magmatism with the occurrence of amagmatic ridge segment, which makes it an ideal area to investigate the dynamic effects of source mantle composition under unique geological context. The 53°E segment is a typical amagmatic ridge segment with abnormal geophyical and geochemical features, representing a suitable area to study mantle dynamic referring to mantle composition and ridge magmatisim. In this study, we examine the mineralogy and geochemistry of abyssal peridotites from the 53°E segment, to constrain mantle composition, melt extraction, and melt-mantle interaction beneath the SWIR.1. All the studied peridotites are harzbergites with cpx content lower than 6%. Their geochemistry compositions show a variation range comparable to that of the SWIR peridotites. Some peridotites have lower Al, HREE and higher Mg#, Cr# than hotspot affected peridotites, representing the most depleted mantle beneath SWIR. Mantle melting models show that mantle melting degree of the 53°E segment can be higher than 20%, indicating that there may be extremely depleted mantle beneath the study area, because mantle temperature is expected to be in normal value without the affect of hotspot. The garnet mantle melting model shows that more than 35% of the total melting occurs in the garnet stability mantle, which means that melting start in mantle deeper than 80km.2. The presence of extremely depleted mantle and high garnet mantle melting degree indicate that mantle may experience ancient melting event. Some of the melting may occur in the continental mantle, leading to lower cpx content, lower ol/opx ratio, higher Mg# and coarse opx (>1cm) in the residual mantle.3. The enrichments of LREE, Na and Sr in the studied peridotites indicate that mantle is refertilized by migrating melt, which is supported by the occurences of textures indicating melt crystalization and melt-mantle reaction. Melt refertilization model with the addition of small fraction of melt to the residual mantle can explain the enrichment of the incompatible elements in the studied peridotites.4. Melt refertilization model suggests that melt extraction beneath the ultraslow spreading ridge is incomplete. Some melt may migrate in diffusive porous flow thus may result in pervasive melt-mantle reaction. The slow mantle convecting rate, cold mantle temperature and thick lithosphere mantle of the ultraslow spreading ridge may enhance this effect. The extremely depleted mantle with higher melt permeability will lead to higher melt refertilization degree. The combination of these effects should be the main reasons for the incomplete extraction of the melt.5. The results of this study provide direct evidence for the presence of extremely depleted mantle beneath the ultraslow spreading ridge, which may reduce the magma supply. Some melt trapping in mantle make no contribution to the formation of oceanic crust, leading to reduced magma flux. These two factors can be the main reasons for the occurence of amagmatic segment and heterogeneous magmatism in the ultraslow spreading ridge.