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Evidence from high‐Ni olivines for a hybridized peridotite/pyroxenite source for orogenic andesites from the central Mexican Volcanic Belt
Author(s) -
Straub Susanne M.,
LaGatta Alexandra B.,
MartinDel Pozzo Ana Lillian,
Langmuir Charles H.
Publication year - 2008
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2007gc001583
Subject(s) - andesites , peridotite , lile , geology , geochemistry , andesite , adakite , basalt , mantle (geology) , basaltic andesite , mantle wedge , continental crust , partial melting , petrology , subduction , oceanic crust , volcanic rock , volcano , paleontology , tectonics
Subduction zone magmatism produces calc‐alkaline andesite melts that combine the high SiO 2 , Na 2 O, and K 2 O abundances of the differentiated continental crust with low FeO, FeO/MgO, and TiO 2 typical of melts from depleted mantle. Ni‐rich olivines in basaltic andesites and andesites of the central Mexican Volcanic Belt suggest that this dichotomy reflects a particular mechanism of mantle processing in the subduction environment. Hydrous slab components rich in Si, Na, and fluid mobile large‐ion lithophile elements (LILE) transform mantle olivine to “reaction orthopyroxene.” Along the ascent paths, and embedded into surrounding peridotite, secondary pyroxenite lithologies are created that are composed of “reaction orthopyroxene” next to mantle clinopyroxene and orthopyroxene. Partial melts from peridotite and pyroxenite then mix to produce primary calc‐alkaline basaltic andesites and andesites that are rich in Na and LILE. The steady slab flux maintains high levels of Na and LILE in the mantle source but also induces repetitive melting that steadily depletes the subarc mantle in FeO, TiO 2 , and other high field strength elements. If mantle processing thus creates primary basaltic andesite and andesite melts with the fractionated major element signature of the continental crust, the high magnesium number (Mg # (=Mg/Mg + Fe 2+ )) ∼60–70 of these melts still requires additional differentiation to arrive at the lower Mg # ∼55 of average continental crust.

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