M. T. Mirabueno’s scientific contributions

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Publications (2)


6 ka tephra from Sibuyan Sea, Philippines: Geochemical characteristics and its effects on planktonic foraminifera
  • Conference Paper

December 2010

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84 Reads

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S. Catane

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J. Teves

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The study of marine ash layers is important for identifying explosive eruptions and their possible impact on the marine environment. In the Philippines, there is additional benefit because tephra are not well preserved on land due to extensive erosion. Here we focus on three tephra layers (1-2 cm thick) preserved in a 960 cm-core, MD06-3057, from the Sibuyan Sea taken during the 2006 Marco Polo II cruise. The topmost light brown layer overlies the second and third dark gray to black layers. Planktonic foraminifera, retrieved from sediments below the first and third tephra layers, were dated using AMS C14 yielding ages of 5840 ± 60 years and 5950 ± 65 years, respectively. Highly vesiculated glass fragments with bubble wall structures were obtained from all intervals and were analyzed by electron microprobe. Physical characteristics and preliminary major element analysis show that the glasses are constrained to two compositional groups. The glasses from the thicker, light-colored ash layer is rhyolitic (69-74 SiO2 wt%); those from the thinner, darker ash layers are andesitic to dacitic (55-64 SiO2 wt%). Data for each ash layer form relatively tight clusters on incompatible element discrimination diagrams implying an eruptive rather than turbidite origin for the layers. The rhyolitic ashes have compositional affinity with post-caldera deposits from the Bulusan Volcanic Complex in the Bicol segment of the Eastern Luzon arc, located ~190 km to the east of the core site. In contrast, the glass compositions of the andesitic to dacitic tephra are distinct from the major element compositions of the Eastern Luzon arc volcanoes. The occurrence of three tephra layers, within a short ~100 year interval suggests that very explosive eruptions have affected the Sibuyan Sea basin and adjacent islands, since ~6000 years ago. This could have had a significant impact on the marine environment, as shown by the change in the size of planktonic foraminifera before, and after the eruption (i.e., smaller planktonic foraminifera within the ash layer).


Eastern Luzon Arc Affinity of Benham Rise Tephras from Major Element Chemistry of MD06-3047 Glasses

December 2010

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138 Reads

Piston coring on the Benham Rise in the western Philippine Sea during the Marco Polo 2 cruise of R/V Marion Dufresne in 2006 provided a rare opportunity to study tephra deposits offshore the Eastern Luzon volcanic arc. Four tephra layers were analyzed from the upper 7 m of core MD06-3047 (~450 km from the arc, 2510 mbsl): A 6 cm-thick black ash overlies three, 2 cm-thick light grey to grey tephras. Sediment samples within 1 cm below each of the grey tephra yielded AMS 14C ages in excess of ~40,000 ybp. Electron microprobe analysis on tephra glass revealed that the black ash has andesitic-dacitic compositions (57-66 wt% SiO2), whereas the three grey ashes cluster to rhyolitic compositions (73-78 wt% SiO2). Furthermore, the rhyolitic tephras also exhibit geochemical distinctions from each other, forming unique fields in variation diagrams with K2O or MgO abscissae. Average major element concentrations of the tephras, except K2O, increase towards the bottom of the core section. Tight linear trends on FeOtotal, MgO and CaO plots, and the abundance of pumiceous shards and bubble wall fragments, imply primary magmatic sources for the glasses. Depositional distances reaching 600 km from volcanic provenances have been reported for andesitic to rhyolitic tephras from Pinatubo volcano and the Japan arc. A comparable distance from the core site would cover several active arcs in the Philippines. The EPMA data points to affinity only with volcanic centers from the Bicol segment of the Eastern Luzon arc (or Bicol arc), based on review of new and literature data. The andesitic glasses bear a strong geochemical resemblance to historical Mayon volcanics, deviating moderately in average Al2O3 and FeOtotal levels. However, correlation of the andesitic tephra with Mayon may be problematic, as the oldest age for this volcano is only 20 cal. ka [1]. The age (~35-41 cal.ka) [2,3] and compositional similarities of the rhyolitic tephras with the calderagenic eruption products of Irosin volcano, on the other hand, suggest that this could be the most probable source. The ~450 km reach of these tephras would therefore imply at least one very explosive eruption event from Irosin volcano. More accurate dating of these rhyolitic tephras may be helpful in investigating the calderagenic phase of Irosin volcano and its potential environmental impacts. References: [1] Mirabueno, M. H. T. et al., 2007, Bull. Volc. Soc. Japan 52, 241-242. [2] Delfin, F.G. Jr. et al., 1993, Geothermics 22, 417-434. [3] Mirabueno, M. H. T. et al., 2006, Bull. Volc. Soc. Japan 36, 23-28.