HR: 1340h
AN: V13E-0600    [Abstracts]
TI: Extreme Geochemical Heterogeneity in Afro-Arabian Oligocene Tephras
AU: * Ukstins Peate, I
EM: ingrid-peate@uiowa.edu
AF: Department of Geoscience, 121 Trowbridge Hall University of Iowa , Iowa City, IA 52242 United States
AU: Kent, A J
V13E-0600 AF: Department of Geosciences, Oregon State University 104 Wilkinson Hall , Corvallis, OR 97331 United States
AU: Baker, J A
V13E-0600 AF: School of Earth Sciences, Victoria University of Wellington P.O. Box 600, Wellington, 00000 New Zealand
AU: Menzies, M A
V13E-0600 AF: Department of Geology, Royal Holloway University of London, Egham, Surrey, TW20OEX United Kingdom
AB: Oligocene Afro-Arabian bimodal flood volcanism produced a series of voluminous silicic pyroclastic eruptions (60 to >3000 km3 dense rock equivalent, ca. 29.5 Ma) that are geochemically and magnetostratigraphically correlated to four deep-sea Indian Ocean ash layers located ~2700 km to the SE. In excess of 1300 major element analyses on over 1050 shards from these tephras, coupled with ~400 in-situ LA-ICP-MS trace element analyses show that the distal tephras preserve extreme geochemical heterogeneity that is not extractable in the equivalent welded, on-land pyroclastic deposits (SiO2: 68 to 75 wt %). The upper and lowermost tephras (4W and 5W) preserve the largest major element variations so far observed within individual eruptions (4W - SiO2: 43.1 to 76.1 wt. % with a compositional gap between 60.4 and 66.4 wt. %; 5W - SiO2: 57.4 to 79.3 wt. %). Tephra layers are volumetrically dominated (>85%) by clear tricuspate high-silica rhyolite shards, with clear light grey pumice and translucent dark shards representing rhyolitic to intermediate compositions (ca. 13%) and a minor component (<2%) of dark opaque basaltic ash grains with vitreous luster and botryoidal, vesicular texture. In addition, an individual banded shard from 5W records close to the full compositional spectrum observed within the entire tephra (SiO2: 59.2 to 75.3 wt. %), preserving the chemical variations observed in a 1600 km3 eruption on the scale of <1 mm3. Incompatible trace elements such as Nb display tight, linear trends (Nb: 30 to 190 ppm in 4W; 60 to 145 ppm in 5W) that correlate well with increasing silica concentration. Compatible trace elements, and major elements, show continuous, curvilinear trends decreasing with increasing silica (e.g. Ba), indicating that fractional crystallization was the major process in generating compositional variations rather than mixing of two different melt batches. Basaltic shards from 4W have ratios of very incompatible to moderately incompatible trace elements (Nb/Zr and La/Yb) that uniquely link them to basaltic lavas intercalated with and disrupted by the main silicic sequence (upper series flood basalts) rather than the underlying, volumetrically dominant main flood basalt series (31 to 29.7 Ma). Using Nb as a fractionation index yields degrees of crystallization >60 % to explain the compositional range of glass shards from 4W and 5W with intermediate to rhyolitic compositions. Nd isotope and incompatible trace element ratio similarities between the flood basalts and rhyolites require even greater degrees of fractional crystallization (>90 %) to have been ultimately responsible for producing silicic volcanism. High silicic magma volatile contents resultant from extreme fractionation were responsible for the explosive emplacement of these large-volume silicic units, and the presence of co-existing basaltic and zoned intermediate to silicic ash shard compositions may indicate that an injection of the mafic magma acted as a trigger for the eruption. The detailed compositional variations of ash shards from these distal tephra layers preserves unique snapshots of zoned magma chambers at the instant of eruption, and reveal the formation and petrogenetic history of these large-volume, chemically zoned silicic magmatic systems.
DE: 1036 Magma chamber processes (3618)
DE: 1065 Major and trace element geochemistry
DE: 3619 Magma genesis and partial melting (1037)
DE: 3643 Layered magma chambers
DE: 8428 Explosive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005