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The redox state of arc mantle using Zn/Fe systematics
Cin-Ty A Lee1, Peter Luffi, Véronique Le Roux
1Department of Earth Sciences, Rice University, Houston, Texas 77005, USA.
Nature
|December 3, 2010
Summary
The zinc-to-iron ratio (Zn/Fe(T)) in primitive arc magmas suggests mantle oxidation is not solely from subduction. Shallow-level differentiation, not mantle wedge alteration, likely causes higher oxidation states in arc lavas.
Area of Science:
- Geochemistry
- Petrology
- Mantle Geodynamics
Background:
- Arc lavas are typically more oxidized than mid-ocean-ridge basalts.
- Subduction is thought to introduce oxidized components into the mantle, leading to an oxidized sub-arc mantle wedge.
- Directly determining the iron oxidation state in the sub-arc mantle is challenging, fueling debate about its intrinsic oxidation state.
Purpose of the Study:
- To investigate the redox state of the mantle source for arc magmas.
- To determine if subduction significantly alters the mantle wedge's oxidation state.
- To understand the processes controlling iron oxidation in arc lavas.
Main Methods:
- Utilizing the redox-sensitive Zn/Fe(T) ratio as a proxy for the iron valence state in primary arc basalts and their mantle sources.
- Analyzing the behavior of Fe(2+), Fe(3+), and Zn during mantle melting and magma differentiation.
- Comparing Zn/Fe(T) in arc magmas with mid-ocean-ridge basalts.
Main Results:
- Primitive arc magmas exhibit Zn/Fe(T) ratios similar to mid-ocean-ridge basalts, indicating comparable Fe oxidation states in primary mantle melts.
- The Zn/Fe(T) ratio remains constant during early olivine fractionation, implying low Fe(3+)/Fe(T) in the magma.
- Iron oxidation state (Fe(3+)/Fe(T)) increases and magnetite stabilizes only after progressive fractional crystallization.
Conclusions:
- Subduction of oxidized crustal material may not significantly alter the redox state of the mantle wedge.
- The higher oxidation states observed in arc lavas are likely a consequence of shallow-level magmatic differentiation processes.
- Further research is needed to fully understand these shallow-level differentiation processes.
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