Evidence for mantle metasomatism by hydrous silicic melts derived from subducted oceanic crust
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Summary
This summary is machine-generated.Slab-derived melts, not just fluids, can alter mantle rocks in subduction zones. These silica-rich melts explain low niobium, tantalum, and titanium in island-arc basalts, suggesting melt metasomatism is more common than previously believed.
Area Of Science
- Geochemistry
- Petrology
- Tectonic Processes
Background
- Island-arc basalts characteristically show low concentrations of niobium, tantalum, and titanium.
- This depletion is attributed to mantle modification by a metasomatic agent from subducted oceanic crust.
- The nature of this agent (hydrous fluid vs. silica-rich melt) and its link to subduction zone thermal regimes are debated.
Purpose Of The Study
- To investigate the composition of liquids generated from melting subducted basalt under various conditions.
- To determine if these liquids can explain the observed trace-element depletions in island-arc basalts.
- To assess the role of slab-derived melts in mantle metasomatism.
Main Methods
- Experimental melting of basalt under H2O-added and low-temperature conditions.
- Analysis of resulting melt compositions, focusing on alkali, titanium, niobium, and tantalum content.
- Comparison of experimental melt compositions with natural samples (mantle xenolith glasses, metamorphic veins) and olivine-mantle interaction products.
Main Results
- Melting of basalt, even with added H2O and at low temperatures, produces alkali-rich silicic liquids.
- Melt alkali content increases with pressure, while titanium, niobium, and tantalum concentrations remain low.
- These experimental melts closely match glasses from mantle xenoliths and veins in fossil convergent zones.
- Interaction with olivine generates mineral assemblages consistent with metasomatized peridotites.
Conclusions
- Mantle metasomatism can be effectively driven by silica-rich melts derived from subducted oceanic crust.
- Slab-derived melts, under specific conditions, can produce the trace-element signatures observed in island-arc basalts.
- Melt metasomatism is likely a more significant process in subduction zones than previously assumed.