Characterization and Correlation of Megascopic Tephras in Site 792 Cores from the Izu-Ogasawara Forearc Basin (Japan) by Trace Elements and 87Sr/86Sr and 143Nd/144Nd Isotopes

Abstract

Glass shards from 26 megascopic tephras (1-29 cm thick) were separated and analyzed for 26 elements and 87 Sr/ 86 Sr and 143 Nd/ 144 Nd isotope ratios. The isotope data allowed us to assess the degree of alteration of the samples. The separates contain less than 10% of palagonitized glass or secondary alteration minerals. Correlation diagrams and multivariate analyses have been used to characterize the glass shards in a tectonic setting and group the tephras according to their magmatic affinity. Four tholeiitic groups, two calc-alkaline groups, and one group with glass of shoshonitic affinity can be distinguished. The tholeiitic groups form a time series; however, judging from the rare-earth-element patterns and the temporal variations of the Ce/Yb ratios, the evolution is not unidirectional from strong to weak tholeiitic affinity.The tephras have been correlated with volcanism along the Izu-Ogasawara and Ryukyu arcs by means of trace element distribution, 87 Sr/ 86 Sr ratios, and their magmatic affinity. The tholeiitic layers were derived from volcanoes on the Izu-Ogasawara Arc, most probably from the volcanic island Hachijojima. Only one group of the calc-alkaline layers can have derived from the Izu-Ogasawara Arc. The other calc-alkaline layers and the two layers with shoshonitic affinity probably derive from volcanism along the Ryukyu Arc.

Regional‐scale input of dispersed and discrete volcanic ash to the Izu‐Bonin and Mariana subduction zones

“…Egeberg et al . [] interpreted trace element, Sr‐, and Nd isotopic signatures in certain ash layers at ODP Site 792 (located on the eastern margin of the Izu‐Bonin forearc basin between the active volcanic arc and the Izu‐Bonin Trench, Figure ) as variously deriving from the Ryukyu and Honshu arcs. Scudder et al .…”

One Million Years tephra record at IODP Sites U1436 and U1437: Insights into explosive volcanism from the Japan and Izu arcs

“…The IODP Expedition 350 took place in April and May 2014 entitled “IBM Reararc: The Missing Half of the Subduction Factory.” Previous drilling in the IBM (ODP Legs 125, 126, 132, 185; Figure ) focused on the arc front and the forearc, and used the geochemistry of sediments and volcanic deposits to study the local arc magmatic evolution, tectonic history, and subduction processes over the past 52 Myr (e.g. Bryant, Arculus, & Eggins, , ; Egeberg, Brunfelt, & Stabel, ; Gill, Hiscott & Vidal, ; Straub, ; Straub, Goldstein, Class, Schmidt, & Gomez‐Tuena, ; Straub, Layne, Schmidt, & Langmuir, ). IODP Expedition 350 was the first scientific drilling activity to investigate the history of the reararc (“the missing half”) (Tamura, Busby, & Blum, ).…”

Isotopic Characteristics of Neogene‐Quaternary Tephra From IODP Site U1438: A Record of Explosive Volcanic Activity in the Kyushu‐Ryukyu Arc

“…IBM Arc tephra from Straub et al (, , , ), Schmidt (), and Egeberg et al (). ODP Site 792 in the IBM forearc shows preserved tephra suggested to derived from the Ryukyu arc (Egeberg et al, ). Field of Quaternary volcanoes (basalts to dacites) from Notsu et al ().…”

“…Timing of South and Central Kyushu ignimbrites after Kamata and Kodama (, and references therein) and Kimura et al (). ODP Leg 792 in the Izu‐Bonin Mariana forearc shows preserved tephra suggested to derive from the Ryukyu arc (Egeberg et al, ). Hiatus in volcanism after Kimura et al (, ).…”

“…The tephra layers would therefore have been derived from eruptive centers located at distances of greater than 800 km. Though preservation of thick tephra deposits in ocean sediments is not unusual, as shown in the IBM forearc and in the Shikoku and Ryukyu sedimentary basins (e.g., Egeberg et al, ; Reagan et al, ; Straub et al, ; Tsuji et al, ), preservation of thick tephra layers from eruptive centers situated at such distances (>600 km) are significantly less common (e.g., Schindlbeck et al, ). Mahony et al () modeled the tephra thickness with distance to the volcanic source as a function of eruption magnitude, demonstrating that tephra deposited several hundreds of kilometers from the trench are most likely to be related to large (>M5) caldera‐forming eruptions.…”