Volcanology and Petrology of Irazú Volcano, Costa Rica

Irazu volcano, the highest (3432 m a.s.l.) and one of the largest volcanoes (600 km3) at the southern end of the Central American Volcanic Front (CA VF), is a basaltic andesite shield volcano located in the Cordillera Central of Costa Rica. The eruptive history of Irazu is reconstructed through mapping and stratigraphic studies of the summit area and south flank. The rocks consist in high-K basaltic andesite to andesite and rare dacites (Si(h= 53-64%, FeQJMgO = 1.46, Cr= 18-145 ppm, Ni= 9-111 ppm) and high-Mg basalts (Si(h = 50-53%, FeOr'Mg 0= 1, Cr= 216-411, Ni = 115-211). The basaltic andesite lava flows and minor associated tephras a.re volumetrically dominant. The basaltic lava flows and tephra deposits are subordinate but very important in being the most prumuve magmas yet recogmzed m the Quaternary Costa Rican Volcanic Front (c;&VF). Irazu lava contain phenocrysts of plagioclase (A1140-s1) with variable roning, clinopyroxene (augite,W03344E1141.wss.1s), weakly zoned orthopyroxene (enstatite, En69-78), olivine (Fo,2.90_5) with Cr-spin el inclusions, and Petri-oxides (titanomagnetite and chromian magnetite). Biotite, apatite and hornblende phenocrysts and microphenocrysts a.re uncommon. Pre-eruption temperatures of the magmas, as determined by two-pyroxene thermometry, range from > 920°C to I 185°C for hornblende andesites to basalts, respectively. Irazu lavas have high REE abundances and radiogenic isotopic compositions (Sr and Nd) anomalous compared to the rest of the CA VF but are similar to other lavas in the Cordillera Central of Costa Rica. Basaltic lavas are interpreted to have formed by low degrees of melting of an enriched mantle source (E-type MORB or OIB-lik:e). Mostly high-K basaltic andesites and andesites are in general consistent with derivation from a calc-alkaline magma by fractional crystalliz.ation at different crustal levels. There is, however, good evidence that mixing of these magmas at different crustal levels played an important role in the petrogenesis of these lavas. Banded tephras are common and indicate incomplete 'low-pressure mixing' (p s9 in some cases coated by pyroxene and plagioclase, or with picotite inclusions in basaltic andesites, two or more generations of plagioclase and pyroxene phenocrysts, and plagioclase and augite with reverse roning record a complex history of magma mixing. Homogeous 'high-pressure mixing' events between high-Mg basalts and basaltic andesites (p � 3 kb) produced hybrid calcalkaline lavas with anomalous geochemical trends (e.g., high MgO, Cr, and Ni contents). Other processes, such as in situ crystallization and assimilation of subvolcanic rocks and deep-crustal igneous rocks could also have occurred. Generally, the main Holocene eruptive phases at the summit began with magmatic or "dry" eruptions followed by phreatomagmatic and/or phreatic eruptions. During these time tephras are predominantly of basaltic andesite composition. The deposits contain , however, evidence for the existence of basaltic to dacite magmas preserved in the mixing events. The earliest reported eruption of Irazu (phreatomagmatic and strombolian with rare phreatic phases) was in 1723. Other reports of volcanic eruptions during the 18th and 19th centuries are very doubtful and no deposits have been located. Several phreatomagmatic phases have occurred during this century (1917-1921, 1924, 1928, 1930, 1933, 1939-1940 and 1963-1965). The maximum Volcanic Explosivity Index CVEi Newhall and Self, 1982) was grade 3. A minimum of 0.5 km3 (DRE, dense rock equivalent) of basaltic andesite magma has been produced in historic time. According to historic documents, several volcanic explosions were preceded by earthquakes. The timing of explosive phases at Irazu in 1723, 1918, 1928, 1933, and 1963-1965 was comparated with earth tides (maximum or minimum tidal amplitude), but a correlation is not always evident. Historic strombolian, phreatic and phreatomagmatic eruptions generated ballistic fallout, pyroclastic surge and 'pyroclastic' flow deposits. Some thin ( < 1 m) pyroclastic-rich mass flow deposits and proximal, pyroclastic, "dry" and "wet" surge deposits were deposited from currents that flowed back into the crater. The explosive eruptions of lrazu and their products are governed by external ( erosion and collapse of the conduit walls, and recycling of tephras) and internal factors (magma discharge rate and rise rate of bubbles through the magma). The high degree of fragmentation of tephra at Irazu reflects the recycling tephras, and magma/water interaction during eruptions. The distribution of the pyroclastic deposits, the stratigraphy, and their relationships, together with eyewitness, grain siz.e analyses and SEM (scanning electron microscope) investigations support the interpretation of eruption slyle and the recycling of tephra. Volcaniclastic debris fans in the Reventado river, Cartago, have been the sites of repeated mass flows in prehistoric and historic time. Volcaniclastic mass flow events unrelated to eruptions have been triggered during high-intensity rain storms principally in October and November. In addition, very fine ash deposits from phreatomagmatic eruptions form an impermeable cover and cause the destruction of the forest resulting in unstable slope conditions (especially deep erosion and reactivation of landslides) in the Irazu drainage basins. This contributes to the formation of lahars during the rainy season (May to December). According to historic records, the recurrence interval of Irazu's main mass flows is 24.5 ± 12.5 years for the last 130 years, and an average of about 50 years from 1723 to 1965. Based on direct sediment concentration measurement of the 1963-1965 lahars and their comparison to the structure of the sediments, there is no strict correlation between the arbitrary rheological classification of Beverage and Culbertson (1964) and the sedimentological criteria of Smith (1986). Thus, the rheological behavior cannot always be inferred from the deposits, as can be seen from the 1963-1965-lahar historic events. The tectonic lineations at Irazu are likely to be the sites of future eruptions (summit and flank) or shallow (depth S 15 km) strong earthquakes (Modified Mercalli scale Intensity, MMI S VII; surface wave magnitude, Ms S 6.5). The greatest hazards at Irazu are related to thick fallout deposits, volcaniclastic mass flows (lahars), landslides and strong ground motion during earthquakes. The total estimated damage caused by eruptions of Irazu and mass flows that affect Cartago and other towns could be in the order of U.S. $ 100-200 million. This investigation presents information on the characteristic eruption styles at Irazu, patterns of its historic and prehistoric eruptive activity, and their effects (including socio-economical aspects) that are essential for an adequate short-term hazard assessment, and provide the basis for planning land use and emergency responses