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The redox "filter" beneath magmatic orogens and the formation of continental
crust
#MMPMID29774235
Tang M
; Erdman M
; Eldridge G
; Lee CA
Sci Adv
2018[May]; 4
(5
): eaar4444
PMID29774235
show ga
The two most important magmatic differentiation series on Earth are the
Fe-enriching tholeiitic series, which dominates the oceanic crust and island
arcs, and the Fe-depleting calc-alkaline series, which dominates the continental
crust and continental arcs. It is well known that calc-alkaline magmas are more
oxidized when they erupt and are preferentially found in regions of thick crust,
but why these quantities should be related remains unexplained. We use the
redox-sensitive behavior of europium (Eu) in deep-seated, plagioclase-free arc
cumulates to directly constrain the redox evolution of arc magmas at depth.
Primitive arc cumulates have negative Eu anomalies, which, in the absence of
plagioclase, can only be explained by Eu being partly reduced. We show that
primitive arc magmas begin with low oxygen fugacities, similar to that of
mid-ocean ridge basalts, but increase in oxygen fugacity by over two orders of
magnitude during magmatic differentiation. This intracrustal oxidation is
attended by Fe depletion coupled with fractionation of Fe-rich garnet. We
conclude that garnet fractionation, owing to its preference for ferrous over
ferric iron, results in simultaneous oxidation and Fe depletion of the magma.
Favored at high pressure and water content, garnet fractionation explains the
correlation between crustal thickness, oxygen fugacity, and the calc-alkaline
character of arc magmas.
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