Geochemical Journal, Vol. 47, pp. 185 to 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) 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 con- straints 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 com- ponent 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 contami- nated 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. Keywords: Andean Arc, arc lava, boron, prompt γ-ray analysis, mantle source INTRODUCTION the contribution of slab-derived fluid to the mantle wedge. For example, boron abundance and its element ratios of The behavior of boron in arc volcanic rocks has re- arc volcanic rocks decrease from the volcanic front to ceived great attention because of its potential to trace the the rear arc, which suggests that inventories of slab- chemical flux to the mantle source of arc magma from derived boron in the mantle source decrease across the the descending oceanic plate (Leeman and Sisson, 1996), arc (Ryan et al., 1995; Ishikawa and Tera, 1997). Ratios because of the enrichment of boron both in altered oce- of boron to fluid immobile elements have also been ap- anic crust (AOC) and sea floor sediment (Ishikawa and plied to determine whether AOC or the sediment cover of Nakamura, 1993; Smith et al., 1995), and because of its the slab is the dominant source of the fluid (Sano et al., high fluid-mineral distribution coefficient during dehy- 2001). dration of the descending oceanic crust and sediment This paper presents new measurements of boron and (Moran et al., 1992; Bebout et al., 1999). Boron abun- other trace element concentrations in regional representa- dance and ratios of boron to other trace elements with tive lavas obtained from 13 volcanoes on the Quaternary similar melt-mineral distribution coefficients but much volcanic front of the Southern Volcanic Zone (SVZ) of lower fluid-mineral distribution coefficients (e.g., B/Be the Andean arc in Chile. Boron abundances of volcanic and B/Nb) have been applied widely for estimation of rocks in the SVZ were reported previously as part of the studies of the contribution of slab-derived component to *Corresponding author (e-mail: [email protected]) the magma source of volcanic rocks from circum-Pacific Copyright © 2013 by The Geochemical Society of Japan. island arcs (Morris et al., 1990; Noll et al., 1996). This 185 lava Cocos Plate stratovolcano Northern Volcanic Zone caldera Carnegie Ridge 45 Ma 7~9 cm/yr Central San José NSVZ Volcanic Zone Ridge azca N Nazca Plate Southern Hornitos Volcanic TSVZ 7~9 cm/yr Zone 35 Ma Austral Chillán Atlantic Plate 2 cm/yr Volcanic Zone 05001000 km Lonquimay Villarrica 26 Ma 26 Ma CSVZ 18 Ma 12 Ma Osorno 7 Ma Chile Trench Chile Huequi Michinmahuida 12 Ma Corcovad 7 Ma Melimoyu SSVZ Mentolat Cay Hudson 7 Ma 0 200 km Fig. 1. Index map of the Southern Volcanic Zones (modified from Stern, 2004). Names of volcanoes where analyzed samples were obtained are also shown. The inset shows locations of the main map (box). study was undertaken to present the overall along-arc GEOLOGY variation of the ratios of fluid-mobile elements including boron to fluid-immobile elements for volcanic rocks on The Andean arc, which extends > ca. 7,500 km along the volcanic front of SVZ, and to evaluate the contribu- the western margin of South America, is divided into four tion of the slab-derived component to the magma source volcanic regions: the Northern (NVZ), Central (CVZ), of SVZ. Southern (SVZ), and Austral (AVZ) Volcanic Zones 186 H. Shinjoe et al. (Stern, 2004; Fig. 1). The SVZ (33–46°S) is bordered by de Chillán, a large composite stratovolcanic complex in the Pampean volcanic gap corresponding to the subduc- TSVZ (Dixon et al., 1999). We obtained two samples tion of the Juan Fernández Ridge to the north, and by the (011501Y and 011505Y) of Holocene dacite lava flow Patagonian volcanic gap marked by the Chile Triple Junc- (LT5; Dixon et al., 1999) from the Chillán volcano. tion where the Chile Rise collides with the Taitao Penin- Six samples of Lonquimay volcano in CSVZ were sula to the east. The SVZ is a ca. 1400 km chain of more obtained from lava flow from the central cone of the vol- than 60 historically and potentially active volcanoes with cano. Two samples (LON-29, and 40) were collected from numerous small eruptive centers (Stern, 2004; Stern et lava flow of the 1988–1990 eruption on the northeastern al., 2007). SVZ is subdivided into the northern (NSVZ, flank of the Lonquimay stratocone (Naranjo et al., 1992). 33–34.5°S), transitional (TSVZ, 34.5–37°S), central We collected three samples (020506YA, 020602YA, and (CSVZ, 37–41.5°S), and southern (SSVZ, 41.5–46°S) 020602YC) from Villarrica volcano in CSVZ. Sample segments (Stern, 2004; Fig. 1) based on petrographical 020506YA is from Pleistocene interglacial lava flow and geochemical features of volcanic rocks and tectonic (Villarrica I; Hickey-Vargas et al., 1989) in the northern considerations (Futa and Stern, 1988; Tormay et al., 1991; skirt of Villarrica volcano. Samples 020602YC (Villarrica López-Escobar et al., 1993; Hickey-Vargas et al., 2002). II; Hickey-Vargas et al., 1989) and 020602YA (Villarrica The SVZ is formed by subduction of the Nazca plate III; Hickey-Vargas et al., 1989), which were collected at (0–45 Ma) beneath the South American plate at a rate of NNE foothill of the volcano, respectively derive from 7–9 cm/yr in a direction of 22–30° NE, orthogonal to the post-glacial lava flow and 1984-eruption lava flow. Two Chile trench. The subduction angle increases southward samples (OSO-11 and 35) were obtained from Osorno from ca. 20° to >25° (Stern, 2004). The thickness of the volcano, a Late Pleistocene to recent composite continental crust beneath the SVZ decreases southward. stratovolcano in CSVZ (López-Escobar et al., 1992). Crustal thickness of the NSVZ is inferred to be >55 km OSO-11 was collected from lava flow (Osorno3; López- (Stern, 2004). In the TSVZ, crustal thickness inferred from Escobar et al., 1992) at the southern hillside of the sum- gravity data decreases from 55 km at 34.5°S to 35 km at mit. OSO-35 was taken from lava flow (Osorno2; López- 37°S (Hildreth and Moorbath, 1988). For the CSVZ and Escobar et al., 1992) along the Rio Blanco in the north- SSVZ, south of 37°S, the crustal thickness is 30–35 km ern skirt of Osorno volcano. (Lowrie and Hey, 1981). General correlation between the The samples from Huequi, Michinmahuida, Corcovad, continental crust thickness and the predominant volcanic Melimoyu, Mentolat, and Cay volcanoes in SSVZ (Heu- rocks has been demonstrated. In the NSVZ, where the 1, Mic-1, Cor-3, Mel-3, Men-3, and Cay-4, respectively) continental crust is thick, the predominant magmatic prod- are the same as those from which López-Escobar et al. ucts are andesites and dacites. In contrast, in CSVZ and (1993) previously reported geochemical data with a brief SSVZ, basalts and basaltic andesites are prominent prod- petrographic description. Heu-1, Mic-1, and Cor-3 are ucts (López-Escobar et al., 1993). andesite; the others are basalt in composition (López- Escobar et al., 1993). From Hudson, the southernmost volcano of SSVZ, three samples (HD1E1, HD1F1, and METHODS H4) were analyzed. HD1E1 and HD1F1 are basalt ob- Sampling sites tained from terminal moraine of outlet glacier on NNE Samples analyzed in this study were obtained from flank of the edifice, and H4 is a basaltic andesite from 13 volcanoes of SVZ: San José, Hornitos, Chillán, pyroclastic flow deposit forming a lower part of the SE Lonquimay, Villarrica, Osorno, Huequi, Michinmahuida, caldera (Orihashi et al., 2004). Petrographic descriptions Corcovad, Melimoyu, Mentolat, Cay, and Hudson, from and K–Ar ages of the Hudson samples were presented by north to south (Fig.