Distal Tephra Deposits Research Articles

Multiple sources of Quaternary tephra layers in the Mariana Trough

Tephra layers in Quaternary sediments (<1 Ma) in the central Mariana Trough contain three different series of glass shards: (1) low-K basaltic to basaltic-andesitic series (LKB: SiO 2 = 47.3–53.6 wt.%; K 2O = 0.2–0.8 wt.%); (2) medium-to high-K basaltic to dacitic series (HKS: SiO 2 = 47.6–70.9 wt.%; K 2O = 0.8–3.2 wt.%); and (3) low-K dacitic to rhyolitic series (LKDR: SiO 2 = 63.9–77.9 wt.%; K 2O = 0.8–1.5 wt.%). LKB glasses are sideromelane shards which occur as minor component in two thirds of the tephra layers investigated. HKS glasses, mostly pyroclastically fragmented, dominate the discrete millimeter- to centimeter-thick fallout layers and are ubitiquous as dispersed shards. LKDR series comprise silicic pumice shards that prevail in decimeter-thick ash turbidites and crystal-poor, lapilli- to bomb-sized pumices. The composition of the LKB glass shards match those of coeval basalts and basaltic andesites of the Mariana Trough. LKB glasses are interpreted to have formed by the spalling of glassy rims of deep-submarine pillow and sheet lavas and subsequently became incorporated in the arc-derived tephra fallout and flow deposits. HKS and LKDR series both compositionally differ from coeval CIP lavas. Characteristic trace-element ratios, however, such as high large-ion-lithopile-elements (LILE)/high-field-strength elements (HFSE) ratios (Ba/La= 33–81; Cs/Nb= 0.3–0.8) and Nb depletion (La/Nb= 2–22) constrain their origin from explosive eruptions of the Central Island Province (CIP) of the Mariana arc. The relative enrichments of semi-incompatible elements preclude HKS and LKDR series from being linked by fractional crystallization and they are interpreted as derivative liquids from different magmatic series of the CIP. HKS glasses are suggested to be counterparts of CIP volcanos with medium-K trends (≈ 1.5–2 wt.% K 2O at 60 wt.% SiO 2), whereas the relatively LILE-depleted LKDR series are derived from CIP volcanic islands and seamounts with lower-K trends (≈; 0.5–1.5 wt.% K 2O at 60 wt.% SiO 2). The magmatic diversity of the CIP — and thus the variability of the distal tephra deposits — can be explained neither by crustal-level differentiation nor by variable degrees of partial melting of a homogenous mantle source. In view of the isotopic homogeneity of the CIP, it can best be explained by the Stolper and Newman (1994) model of binary mixing of source components, with one endmember being the NMORB source of Stolper and Newman (1994) and the other one being a compositionally variable slab-derived H 2O-rich component similar to the H 2O-component of Stolper and Newman (1994). This interpretation implies that the variation in K 2O-content in the arc-derived glasses (about a factor of 3 at 60 wt.% SiO 2) reflects a magmatic diversity that is inherent to the central Mariana arc and is not necessarily reflective of temporal changes of the composition of arc magma sources.

Tephra sources and correlations in Ethiopia: Application of elemental and neodymium isotope data

Combined elemental and neodymium (Nd) isotope data for proximal and distal pyroclastic deposits from the Ethiopian volcanic province illustrate the utility of these parameters for establishing potential source regions for distal tephra and for correlation of widely-separated distal deposits. In the Ethiopian volcanic province, Nd isotopic compositions and, to some extent, elemental characteristics of volcanic products from distinct geographic/tectonic sub-provinces allow first-order indentification of geochemical domains. These domains are used to target potential source regions for the numerous distal tephra deposits exposed within the fossiliferous sedimentary sequences of the Afar and Omo-Turkana Basins. Pliocene distal deposits from the Hadar and Middle Awash regions of Afar, Ethiopia and the Omo-Turkana region of southern Ethiopia-northern Kenya are examined. Combined elemental and Nd isotope data are consistent with derivation of these tephra from sources within or adjacent to the central sector of the Main Ethiopian Rift or within the central to northern Afar. The data presented in this paper also point to the need for a better understanding of the physical and chemical characteristics of the eruptions giving rise to many of the distal deposits. Until information regarding the range of chemical and isotopic variability in these magmatic systems is available, it is difficult to link a specific set of geochemical characteristics to one particular eruption or vent.

Compositional heterogeneity of distal tephra deposits from the 1912 eruption of Novarupta, Alaska

Physical and chemical analyses of distal tephra from the 1912 eruption of Novarupta, Alaska, show considerable variations in glass and mineral compositions. A combination of a 150°C range in temperature deduced from iron-titanium oxide geothermometry, and curved patterns in bivariant element plots of glass compositions indicate that a chamber of compositionally zoned magma existed prior to the eruption. Magma-mixing cannot explain these features. The magma chamber may have resembled the model recently proposed by McBirney (1980). A highly silicic, quartz-phyric magma with mean phenocryst compositions of An 25 plagioclase, Fs 42 orthopyroxene, at a temperature of 880°C and a water pressure of 1.4 kbar, was located above a more mafic, hotter magma, bearing phenocrysts of An 45 plagioclase and Fs 35, orthopyroxene. Our results on distal tephras compare favorably with those from a recently completed study at source by Hildreth (1983), suggesting that useful petrologic information about distant volcanoes can be obtained from both types of deposits. Compositionally heterogeneous abyssal tephra layers are common in the Gulf of Alaska. Eruptions from chambers of zoned magma may account for many of these layers.