ALBERT

Description: The Central American Volcanic Arc (CAVA) is, and has been, one of the most active volcanic regions and generated numerous Plinian eruptions along his 1200 km extension. The best preserved archive of this volcanism can be found as ash layers in the marine sediments downwind from the volcanic sources on the Pacific floor. Numerous ash layers up to 8 Mio old, which occur in ODP and DSDP cores of Legs 66, 67, and 202, originated in Central America and southern Mexico. The cores lie across the ash distribution areas expected from dominant wind directions as identified by mapped fallout deposits. We have chosen 145 ash layers of all three Legs for first detailed analysis of these sites to built up a data base for upcoming IODP cruise 334: Costa Rica Seismogenesis Project. The ash layers commonly have sharp contacts at the bottom and diffuse transitions to terrigenous and pelagic sediments at the top. Ash layer thickness ranges from 0.5 to 60 cm with typical grain sizes from medium silt to coarse sand. The mineral assemblages are typical for arc volcanism (plagioclase, pyroxene, hornblende, and olivine). The most evolved tephras also contain biotite. Electron microprobe analyses of 1300 glass shards yield compositions ranging from basaltic andesite to rhyolite and trachyte. Felsic ashes can be divided into seven compositional groups by means of silica and potassium contents. Correlations between marine ashes and on-land tephras are constrained by petrographical and stratigraphical criteria, major element geochemistry of glasses and minerals, and trace element data from LA-ICP-MS analyses. Due to limited exposure on land, such correlations with individual tephras are only possible for deposits of late Pleistocene to Holocene age. Older ash layers, however, can be correlated with regional arc segments making use of systematic along-arc variations of trace-element characteristics (Zr/Nb, Ba/La, Ce/Yb, La/Yb and Ba/Zr) of the arc rocks. Results show that source areas of the ash layers are distributed along the entire CAVA, as well as at the Southern Mexican Arc. The marine tephra record provides important data for ongoing studies of CAVA volcanism: (a) dating of undated land tephra by correlation with marine ashes and the ages derived by sedimentation rates; (b) stratigraphic correlations along the entire arc can be traced much more completely in the marine sediment cores than by limited onshore outcrops alone; (c) long-term changes in magmatic evolution of volcanic complexes can be reconstructed by using the marine archive of ash layers.

Description: Llaima is a large active stratovolcano in the Southern Volcanic Zone in Chile. Field work in 2011 revised the postglacial stratigraphy after Naranjo&Moreno (1991) and led to the subdivision into units I to V. Postglacial activity started 13.500 years ago with caldera-forming eruption of two mafic ignimbrites (unit I). These are overlain by a sequence of three basaltic-andesitic to two dacitic lapilli fallout deposits and reworked tuffaceous sediments (unit II). At ~8600 cal BC a large Plinian eruption emplaced a compositionally zoned dacitic to andesitic fallout tephra (unit III) that became capped by subsequent andesitic surge deposits (unit IV) when the eruption became unstable. The following unit V represents a time interval of ~7000 years during which at least 30 basaltic to andesitic ash and lapilli fallout deposits with intercalated tuffaceous sediments and paleosols were emplaced. Bulk-rock, mineral and glass chemical analyses constrain the vertical compositional changes of Llaima tephras. Tephra compositions switch between a calc-alkaline differentiation trend (unit I) and a more tholeiitic trend (units II-IV), with samples of unit V varying between both trends, indicating a strong control of f(O2) (and P(H2O)) on the relative timing of Fe-Ti oxide fractionation. Moreover, iron rich fayalites that are in equilibrium with the glass composition occur in units II and III with calculated T-f(O2) close to the FMQ suggesting that late-stage fayalite precipitation involved crossing of the FMQ boundary. The younger unit V tephras and historical compositions define a second differentiation trend relatively enriched in K2O, Rb, Ba and Zr; this is not the results of changing source conditions but can be explained by a stronger early olivine fractionation in the respective magmas. Thermobarometric calculations based on amph, cpx-liq, plag-liq, ol-liq and Fe-Ti-oxide compositions constrain changing magma chamber positions over time. Storage depths were 14 - 19 km for unit I andesite and and varied between 10 to 17 km for unit II andesites and dacites. The compositionally zoned eruption of units III and IV withdrew dacite magma from ~10 km depth but andesite from a deeper level of 13-15 km. Storage depths of unit V andesitic magmas ranged from 6 to 15 km. Based on temporally changing storage depths and differentiation paths, a 4-stage evolution of the postglacial magmatic system of the Llaima volcanic complex is proposed. Reference: Naranjo JA, Moreno H (1991) Actividad explosiva postglacial en el volcan Llaima, Andes del Sur (38°45’S). Revista Geologica de Chile 18: 69-80