spe412-14 page 259 259 Geological Society of America Special Paper 412 2006 Recent volcanic history of Irazú volcano, Costa Rica: Alternation and mixing of two magma batches, and pervasive mixing Guillermo E. Alvarado* Área de Amenazas y Auscultación Sísmica y Volcánica, Instituto Costarricense de Electricidad, Apdo. 10032-1000, Costa Rica Michael J. Carr* Brent D. Turrin Carl C. Swisher III Department of Geological Sciences, Rutgers University, New Brunswick, New Jersey 08903, USA Hans-Ulrich Schmincke Leibniz Institute for Marine Science, IfM-GEOMAR (Leibniz Institute for Marine Sciences), Wischhofstr. 1-3, D-24148 Kiel, Germany Kenneth W. Hudnut U.S. Geological Survey, 525 South Wilson Ave., Pasadena, California 91106-3212, USA ABSTRACT 40 Ar/ 39 Ar dates, field observations, and geochemical data are reported for Irazú volcano, Costa Rica. Volcanism dates back to at least 854 ka, but has been episodic with lava shield construction peaks at ca. 570 ka and 136–0 ka. The recent volcanic record on Irazú volcano comprises lava flows and a variety of Strombolian and phre- atomagmatic deposits, with a long-term trend toward more hydrovolcanic deposits. Banded scorias and hybridized rocks reflect ubiquitous magma mixing and commin- gling. Two distinct magma batches have been identified. One magma type or batch, Haya, includes basalt with higher high field strength (HFS) and rare-earth element contents, suggesting a lower degree melt of a subduction modified mantle source. The second batch, Sapper, has greater enrichment of large ion lithophile elements (LILE) relative to HFS elements and rare-earth elements, suggesting a higher subduction signature. The recent volcanic history at Irazú records two and one half sequences of the following pattern: eruptions of the Haya batch; eruptions of the Sapper batch; and finally, an unusually clear unconformity, indicating a pause in eruptions. In the last two sequences, strongly hybridized magma erupted after the eruption of the Haya batch. The continuing presence of two distinct magma batches requires two active magma chambers. The common occurrence of hybrids is evidence for a small, nearer to the surface chamber for mixing the two batches. Estimated pre-eruptive tempera- tures based on two-pyroxene geothermometry range from ~1000–1176 °C in basalts Alvarado, G.E., Carr, M.J., Turrin, Brent D., Swisher, C.C., Schmincke, H.-U., and Hudnut, K.W., 2006, Recent volcanic history of Irazú volcano, Costa Rica: Alternation and mixing of two magma batches, and pervasive mixing, in Rose, W.I., Bluth, G.J.S., Carr, M.J., Ewert, J., Patino, L.C., and Vallance, J., Volcanic hazards in Central America: Geological Society of America Special Paper 412, p. 259–276, doi: 10.1130/2006.2412(14). For permission to copy, contact editing@ geosociety.org. ©2006 Geological Society of America. All rights reserved. *E-mails: Alvarado: galvaradoi@icelec.ice.go.cr; Carr: carr@rutgers.edu; Turrin: bturrin@rci.rutgers.edu; Swisher: cswish@rci.rutgers.edu; Schmincke: h- u.schmincke@t-online.de; Hudnut: hudnut@usgs.gov