Explosive volcanism from the Galapgos Hotspot: Evidence from Miocene marine tephras on top of the Cocos Ridge (IODP Exp. 334)

The Central American Volcanic Arc (CAVA) has generated numerous Plinian eruptions along its 1200 km extension. The best-preserved archive of this volcanism is found as tephra layers in marine sediments downwind from the eruption centers on the floor of the Pacific. Up to now no such deposits are known to exist from the Galapagos Hot Spot. During IODP expedition 334 "CRISP" offshore Costa Rica a total of 31 tephra layers have been recovered at Site U1381 on top of the Cocos Ridge near the Costa Rican Trench. In the first 45 mbsf five of these tephra layers are intercalated with hemiplegic Pleistocene sediments associated with a near trench depositional environment. In contrast, the 26 tephra layers recovered from the deeper parts of Site U1381 (>45 mbsf) are embedded in a succession of Miocene silicic and calcareous ooze dominated sediments. Plate reconstruction shows that these sediments have been deposited in proximity to the Galapagos Islands. Individual tephra layers range in thickness from 1 to 33 cm. Macroscopically the tephra layers are either pinkish/white to gray/brown (20 layers) or pinkish/greenish black (11 layers). Dark tephra beds account for ~36% of the total tephra-bed assemblage at Site U1381. The felsic tephra layers consist of fresh, transparent glass shards, rare plagioclase and traces of pyroxene. Grain size ranges from medium to coarse ash (up to mm size). Mafic ash layers consist predominantly of very coarse, dark to light brown sideromelane glass shards, rare tachylitic particles, and minor plagioclase and trace pyroxene. Most of the sideromelane glass shards have blocky shapes some show a tubular appearance and are medium to poorly vesicular. Glass shards are well preserved without obvious alteration features. Generally, tephras from the older Miocene sequence have a lower crystal content than the tephras of the younger one. First results of electron microprobe analysis show that the tephras have a highly variable composition, ranging from relatively primitive basalts to trachytes and rhyolites. A first provenance analysis conducted based on major elements suggests that some of the tephra layers within the Pleistocene unit of Site U1381 might be derived from the two largest Central American eruptions in the last 200ka, the 191 ka old L-Fall and the 84ka old Los Chocoyos eruption. Within the Miocene unit we can identify both, an evolved tephra with low Ti/K ratios (<2) comparable to Central American Arc tephras as well as a series of evolved and primitive tephras with high Ti/K ratios (>2.5) probably associated with the Galapagos hot spot volcanism. In future, trace element analyses will better constrain the possible source areas and the preliminary correlations shown above.