TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 47 (No. 2), pp. 185-199, 2013

Boron and other trace element constraints on the slab-derived component in Quaternary volcanic rocks from the Southern Volcanic Zone of the Andes

Hironao Shinjoe,1 Yuji Orihashi,2 José A. Naranjo,3 Daiji Hirata,4 Toshiaki Hasenaka,5 Takaaki Fukuoka,6 Takashi Sano7 and Ryo Anma8

1Tokyo Keizai University, Japan
2Earthquake Research Institute, The University of Tokyo, Japan
3Servicio Nacional de Geología y Minería, Chile
4Kanagawa Prefectural Museum of Natural History, Japan
5Department of Earth and Environmental Sciences, Graduate School of Science and Technology, Kumamoto University, Japan
6Department of Environment Systems, Faculty of Geo-environmental Science, Rissho University, Japan
7Department of Geology and Paleontology, National Museum of Nature and Science, Japan
8Integrative Environmental Sciences, Graduate School of Life and Environmental Sciences, Tsukuba University, Japan

(Received April 18, 2012; Accepted December 25, 2012)

Abstract: We present a dataset for boron and other trace element contents obtained from samples from 13 volcanoes distributed along the Quaternary volcanic front of the Southern Volcanic Zone (SVZ) of the Chilean Andes. The dataset shows constraints on the nature of slab-derived component to mantle source. Analyzed samples show large negative Nb and Ta anomalies, and enrichment of alkaline earth elements and Pb, which are features of typical island arc volcanic rocks. Boron contents of SVZ volcanic rocks range 2.3-125.5 ppm, exhibiting marked enrichment relative to N-MORB and OIB. Both the boron contents and B/Nb ratios of the volcanic rocks increase from the southern SVZ (SSVZ) to central SVZ (CSVZ). Fluid mobile/immobile element ratios (B/Nb, Ba/Nb, Pb/Nb, and K/Nb) are used to examine slab-derived component to mantle source. Trace element compositions of altered oceanic crust (AOC)-derived fluid, sediment-derived fluid, and sediment melt are modeled. Mantle sources of volcanic rocks in CSVZ with high B/Nb ratios were contaminated by both AOC-derived and sediment-derived fluids. In contrast, mantle sources of volcanic rocks in SSVZ with a low B/Nb ratio were contaminated with ca. 3 wt% melt of subducted sediment, which had suffered from loss of boron during progressive devolatilization before melting.
Key words: Andean Arc, arc lava, boron, prompt γ-ray analysis, mantle source

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