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doi:10.2204/iodp.proc.320321.211.2013

Results and discussions

We used depth scales compiled by Westerhold et al. (2012): meters below seafloor (mbsf or core depth below seafloor, method A [CSF-A]) is the core depth below seafloor, actual length of the recovered core, and the drillers depth; revised meters composite depth (rmcd or revised core composite depth below seafloor, method A [CCSF-A]) is the new revised depth scale of the spliced section. In addition, the depth scale of Site U1333 cores are correlated to the corrected revised meters composite depth of Ocean Drilling Program (ODP) Site 1218. Sediment in the Eocene–Oligocene transition shows a clear lithologic change from dark-color radiolarian ooze in the upper Eocene interval to light-color calcareous/nannofossil ooze in the overlying lower Oligocene interval (see the “Site U1333” chapter [Expedition 320/321 Scientists, 2010]). We found that five samples contained volcanic glasses in the coarse fraction of sediments (>75 μm in size) (Table T1): four white nannofossil ooze samples from the lower Oligocene interval and one very pale brown calcareous radiolarian ooze sample from the uppermost Eocene. Volcanic glasses from these five samples are dominated by clear silicic glass shards 80–300 μm in size (Fig. F1). We cannot find other minerals such as quartz, feldspar, mica, hornblende, or pyroxene. Brownish glass shards that indicate dacitic composition were rarely observed (e.g., Sample 320-U1333A-11X-2W, 22–24 cm). The glass shards are classified into three types based on their morphology (e.g., Heiken, 1972; Furusawa, 1995, and references therein): flat to weakly concave (concoidal) shards without junctions (plate-type), shards forming elongated fibrous to tubular vesicle walls (tube-type), and fragments of globular bubbles (balloon-type) (Fig. F1). Both tube- and balloon-type glass shards show various fracture surfaces including T- and Y-shape sections. Vesicle walls are smooth, and alteration and argillation appear to be insignificant.

Electron microprobe analyses of 181 glass shards from Sample 320-U1333C-14H-6W, 92–94 cm, revealed that the glass shards have uniform rhyolitic compositions (SiO₂ = 73.4 ± 0.4 wt%; Al₂O₃ = 11.5 ± 0.1 wt%) enriched in alkaline elements (Na₂O = 3.02 ± 0.20 wt%; K₂O = 5.27 ± 0.22 wt%) but depleted in Fe, Mg, and Ca (FeO* = 0.66 ± 0.09 wt%; MgO = 0.05 ± 0.02 wt%; CaO = 0.40 ± 0.07 wt%) (Table T2). The uniform major element compositions of the volcanic glasses indicate either a single source or a mixture of multiple sources with similar compositions. Chemical composition and morphology suggest that the origin of these glass shards was explosive volcanism(s) of rhyolitic composition. The five sediment samples that contain volcanic glass shards show no clear correlation with magnetic susceptibility (Fig. F1), but these samples seem to coincide with potassium peaks taken by a high-resolution X-ray fluorescence core scanner (Westerhold et al., 2012; T. Westerhold et al., pers. comm., 2012)

An episode of enhanced silicic volcanism in Central America that began in the middle Eocene and terminated around the Eocene/Oligocene boundary has been reported at ODP Site 999 in Caribbean Kogi Rise (Shipboard Science Party, 1997). Scientists at the site interpreted the Eocene volcanic episode as probably ignimbrite-forming eruptions on the Chortis Block in the Central American arc. The initiation of the Eocene volcanic episode may be related to plate tectonic rearrangements in the Pacific Ocean due to the abrupt change in the direction of Pacific plate motion at 43 Ma to a more western direction as recorded by the bend in the Hawaiian hotspot chain, which is likely to have had effects on the relative motion of the opposing Farallon plate and thus on subduction in the Middle America Trench. Our results from the eastern equatorial Pacific (Site U1333) indicate that the volcanism started around the Eocene/Oligocene boundary and then lasted after the Eocene–Oligocene transition. We need further investigation to clarify whether the Oligocene volcanic glasses from Site U1333 indicate that the enhanced Eocene eruption episode lasted after the Eocene/Oligocene boundary.

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