Rapid passage of a small-scale mantle heterogeneity through the melting regions of Kilauea and Mauna Loa Volcanoes
Rapid passage of a small-scale mantle heterogeneity through the melting regions of Kilauea and Mauna Loa Volcanoes
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小规模地幔异质性快速穿过基拉韦厄火山和莫纳罗亚火山的融化区域
DOI：
10.1016/j.epsl.2007.04.026
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发表时间：
2006
影响因子：
5.3
通讯作者：
M. Rhodes
中科院分区：
文献类型：
--
作者：
J. Marske;A. Pietruszka;D. Weis;Michael O. Garcia;M. Rhodes
最近的基拉韦厄和莫纳罗亚火山熔岩提供了一个快照的大小，形状和夏威夷地幔柱内的成分不均匀性的分布。在这里，我们提出了一个研究的Pb，Sr，Nd同位素比值的两套年轻的史前熔岩从这些火山：（1）基拉韦厄山顶熔岩喷发于公元900年至1400年，（2）14 C-日期冒纳罗亚流喷发于公元2580年至140年前（相对于公元1950年）。这些熔岩显示出系统的同位素波动，而基拉韦厄熔岩跨越了以前认为存在于这两座火山之间的铅同位素分界线。在公元250年至1400年的短暂时期内，基拉韦厄火山和莫纳罗亚火山熔岩的206 Pb/204 Pb和87 Sr/86 Sr同位素比值和ε Nd值偏离了每座火山的典型值（基于历史和其他年轻的史前熔岩），向中间成分移动，随后又恢复到典型值。这是唯一已知的时期，在这些火山喷发的历史上，当这样一个同时收敛的铅，锶，钕同位素比值发生。共同的同位素组成的熔岩喷发从基拉韦厄和冒纳罗亚山在这短暂的岩浆事件可能是由一个小规模的成分不均匀性，通过两个火山的熔融区的快速通过。这种不均匀性被认为是单一的物体（根据这些火山山顶之间的距离，长度为35公里）或羽状基质本身（预计在两座火山下面都存在）。这一事件的时间尺度（几个世纪）比以前注意到的夏威夷熔岩同位素组成的变化要短得多，这是由于羽流内的不均匀性上涌造成的（数千至数万年）。根据同位素会聚时间的计算表明，熔融区的最大厚度（因此，不均匀性）为1.5 -10 km。小尺寸的异质性表明，熔体可以从夏威夷羽流内的小区域中提取，随后的化学改性最小（超出晶体分馏的影响）。这将是最有效的，如果熔融运输在地幔夏威夷盾状火山下发生主要是在化学隔离通道。
Recent Kilauea and Mauna Loa lavas provide a snapshot of the size, shape, and distribution of compositional heterogeneities within the Hawaiian mantle plume. Here we present a study of the Pb, Sr, and Nd isotope ratios of two suites of young prehistoric lavas from these volcanoes: (1) Kilauea summit lavas erupted from AD 900 to 1400, and (2)14C-dated Mauna Loa flows erupted from ∼2580–140 yr before present (relative to AD 1950). These lavas display systematic isotopic fluctuations, and the Kilauea lavas span the Pb isotopic divide that was previously thought to exist between these two volcanoes. For a brief period from AD 250 to 1400, the206Pb/204Pb and87Sr/86Sr isotope ratios and εNdvalues of Kilauea and Mauna Loa lavas departed from values typical for each volcano (based on historical and other young prehistoric lavas), moved towards an intermediate composition, and subsequently returned to typical values. This is the only known period in the eruptive history of these volcanoes when such a simultaneous convergence of Pb, Sr, and Nd isotope ratios has occurred. The common isotopic composition of lavas erupted from both Kilauea and Mauna Loa during this transient magmatic event was probably caused by the rapid passage of a small-scale compositional heterogeneity through the melting regions of both volcanoes. This heterogeneity is thought to have been either a single body (∼35 km long based on the distance between the summits of these volcanoes) or the plume matrix itself (which would be expected to be present beneath both volcanoes). The time scale of this event (centuries) is much shorter than previously noted for variations in the isotopic composition of Hawaiian lavas due to the upwelling of heterogeneities within the plume (thousands to tens of thousands of years). Calculations based on the timing of the isotopic convergence suggest a maximum thickness for the melting region (and thus, the heterogeneity) of ∼5–10 km. The small size of the heterogeneity indicates that melt can be extracted from small regions within the Hawaiian plume with minimal subsequent chemical modification (beyond the effects of crystal fractionation). This would be most effective if melt transport in the mantle beneath Hawaiian shield volcanoes occurs mostly in chemically isolated channels.