Average composition of basic magmas and mantle sources of island arcs and active continental margins estimated from the data on melt inclusions and quenched glasses of rocks |
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| Authors: | V I Kovalenko V B Naumov A V Girnis V A Dorofeeva V V Yarmolyuk |
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| Institution: | (1) Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan;(2) Institute for Geothermal Sciences, Kyoto University, Noguchibaru, Beppu 874-0903, Japan |
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| Abstract: | Based on the generalization of data on melt inclusions and quenched glasses, the average compositions of subduction (island
arc and active continental margin settings) basic magmas were estimated. The main geochemical features of the average composition
of these magmas are significant depletion in Nb and Ta, less significant depletion in Ti, Zr, and Sm, and enrichment in Cl,
H2O, F, and P in the primitive mantlenormalized patterns. The average normalized contents of moderately incompatible HREE in
these magmas are close to those in the basic magmas of other geodynamic settings. Subduction basic magmas exhibit negative
correlation of Li, Y, Dy, Er, Yb, Lu, and Ti contents with MgO content. Most of incompatible elements (Nb, Ta, U, Th, LREE)
do not correlate with MgO, but correlate with each other and K2O. Variations in element contents are related to crystallization differentiation, magma mixing, and possibly, participation
of several sources. The water content in the island arc basic magmas varies from almost zero value to more than 6 wt %. Most
compositions are characterized by weak negative correlation between H2O and MgO contents, but some compositions define a negative correlation close to that in magmas of mid-ocean ridges (MOR).
Considered magmas demonstrate distinct positive correlation between MgO content and homogenization temperature, practically
coinciding with that of MOR magmas. Modeling of phase equilibria revealed widening of crystallization field of olivine in
the magmas of subduction zones compared to MOR magmas. This can be related to the high water content in subduction magmas.
Simultaneous liquidus crystallization of olivine and clinopyroxene in subduction magmas occurs at pressure approximately 5
kbar higher than that of MOR magmas. Based on the average ratios of trace element to K2O content, we determined the average
compositions for subduction magma sources. Relative to depleted mantle, they are enriched in all incompatible elements, with
positive anomalies of U, Rb, Ba, B, Pb, Cl, H2O, F, and S, and negative anomalies of Th, K, Be, Nb, Ta, Li, Nd, Pb, and Ti. A general elevated content of incompatible elements
indicates a reworking of the rocks of mantle wedge by fluids and melts that were released from the upper layers of subducted
plate. |
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