Isotopic characteristics of Neogene‐Quaternary tephra from IODP Site U1438: A record of explosive volcanic activity in the Kyushu‐Ryukyu arc
Isotopic characteristics of Neogene‐Quaternary tephra from IODP Site U1438: A record of explosive volcanic activity in the Kyushu‐Ryukyu arc
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IODP站点U1438新近纪-第四纪火山灰的同位素特征：九州-琉球弧爆发性火山活动的记录
DOI：
10.1029/2019gc008267
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发表时间：
2019
期刊：
影响因子：
--
通讯作者：
O.
中科院分区：
文献类型：
--
作者：
McCarthy;A.;Yogodzinski;G.;Tepley;F. J.;III;Bizimis;M.;Arculus;R.;Ishizuka;O.
国际海洋发现计划(IODP)站点U1438位于Amami-Sankaku盆地内，九州-帕劳山脊西南约50公里，九州-琉球弧今火山前锋东南约500公里。在该遗址发现了最近4年沉积的38个流纹质特弗拉岩层(1-15厘米厚)。具有代表性的火山灰层以未改变的无色玻璃碎屑为主，大小为100-300μm，伴生有稀有的辉石、角闪石、黑云母和锆石，用于同位素分析。火山岩的同位素组成范围较窄，分别为87Sr/86Sr(0.704~0.706)、206Pb/204Pb18.32~18.46、207Pb15.57~15.62、208Pb38.46~38.75，而εNd3.5%~+4.8%、−Hf+2.0%~+13.3%变化较大。这一记录表明，火山灰不是来自伊豆-博宁-马里亚纳弧或日本中部，而是来自九州-琉球弧的长英质火山作用。Amami-Sankaku盆地深4.8公里，在过去4 Ma距离九州-琉球弧长达600-900公里，比其他已从日本西南部开采到广泛的Tephra层的钻探点要远得多。因此，U1438火山喷发地点可能与广泛分布的流纹岩喷发有关。我们认为，从U1438遗址采集到的独特的火山灰记录和高精度的火山灰同位素分析可以作为识别琉球弧内沉没火山口和/或约束九州-琉球弧内主要火山口形成喷发的活动史(>；1 Ma)的重要标志。
International Ocean Discovery program (IODP) Site U1438 is located within the Amami‐Sankaku Basin, ~50 km SW of the Kyushu‐Palau Ridge and ~500 km SE of the present‐day volcanic front of the Kyushu‐Ryukyu arc. Thirty‐eight rhyolitic tephra layers (1–15 cm thick) deposited over the last 4 Myr were recovered at this site. Representative ash layers dominated by unaltered, colorless glass shards reaching 100–300 μm in size, in association with rare pyroxene, amphibole, biotite, and zircon, were sampled for isotopic analysis. The isotopic compositions of the tephra have a narrow range in87Sr/86Sr (0.704 to 0.706),206Pb/204Pb (18.32 to 18.46),207Pb/204Pb (15.57 to 15.62), and208Pb/204Pb (38.46 to 38.75) and are more variable inεNd(−3.5 to +4.8) andεHf(+2.0 to +13.3). This record indicates the tephra were not sourced from the Izu‐Bonin‐Mariana arc or from Central Japan but derived from felsic volcanism from the Kyushu‐Ryukyu arc. The 4.8‐km‐deep Amami‐Sankaku Basin was located up to 600–900 km from the Kyushu‐Ryukyu arc over the last 4 Ma, which is significantly more distal than other drill sites from which extensive tephra layers sourced from SW Japan have been recovered. Site U1438 tephra are thus likely related to widely distributed (M> 6) rhyolitic eruptions. We propose the unique tephra record and high‐precision isotope analysis of recovered tephra from Site U1438 can be used as an important marker to identify submerged calderas within the Ryukyu arc and/or constrain the history of activity (>1 Ma) of major calderas‐forming eruptions within the Kyushu‐Ryukyu arc.