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首页> 外文期刊>Contributions to Mineralogy and Petrology >Dynamics of melting beneath a small-scale basaltic system: a U-Th–Ra study from Rangitoto volcano, Auckland volcanic field, New Zealand
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Dynamics of melting beneath a small-scale basaltic system: a U-Th–Ra study from Rangitoto volcano, Auckland volcanic field, New Zealand

机译：小型玄武质系统下的融化动力学：来自新西兰奥克兰火山场朗伊托托火山的U-Th-Ra研究

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The Auckland volcanic field is a Quaternary monogenetic basaltic field of 50 volcanoes. Rangitoto is the most recent of these at ~500 year BP and may mark a change in the behaviour of the field as it is the largest by an order of magnitude and is unusual in that it erupted magmas of alkalic then subalkalic basaltic composition in discrete events separated by ≤50 years. Major and trace element geochemistry together with Sr–Nd and U-Th–Ra isotopes provides the basis for modelling the melting conditions that brought about the eruption of two chemically different lavas with very little spatial or temporal change. Sr–Nd isotopes suggest that the source for both eruptions is similar with a slight degree of heterogeneity. The basalts show high 230Th-excess compared with comparable continental volcanic fields. We show that the alkalic basalts give evidence for lower degrees of partial melting, higher amounts of residual garnet, a longer melting column and lower melting and upwelling rates compared with the subalkalic basalts. The low upwelling rates (0.1–1.5 cm/year) modelled for both magmas do not suggest a plume or major upwelling in the mantle region beneath Auckland; therefore, we suggest localised convection due to relict movement from the active subduction system situated 400 km to the southeast. A higher porosity for the initial alkalic basalt is based on 226Ra-excesses, suggesting movement of melt by two different porosities: the initial melt travelling in fast high porosity channels from greater depths preserving a high 230Th-excess and the subsequent subalkalic magma travelling from a shallower depth through lower porosity diffuse channels preserving a high 226Ra-excess; this creates a negative array in (226Ra/230Th) versus (230Th/238U) space previously only seen in mid ocean ridge Basalt data. This mechanism suggests the Auckland volcanic field may operate by the presence of discrete melt batches that are able to move at different depths and speeds giving the field its erratic spatial and temporal pattern of eruptions, a type of behaviour that may have implications for the evolution of other continental volcanic fields worldwide.
机译：奥克兰火山场是由50个火山组成的第四纪单系玄武岩场。朗伊托托（Rangitoto）是大约BP大约500年中最新的岩石，它可能标志着该油田行为的变化，因为它最大，幅度达一个数量级，而且不寻常，因为它在不连续的事件中喷出了碱性岩浆，然后形成了次碱性玄武岩浆相隔≤50年。主要和微量元素地球化学以及Sr-Nd和U-Th-Ra同位素为模拟熔融条件提供了基础，该熔融条件导致了两种化学上不同的熔岩的喷发，而它们的时空变化很小。 Sr–Nd同位素表明，两次爆发的来源都相似，但异质性略有不同。与可比的大陆性火山岩相比，玄武岩的 230 Th过量高。我们表明，与玄武岩玄武岩相比，碱性玄武岩提供了较低的部分熔融度，较高的残留石榴石数量，更长的熔融柱以及较低的熔融和上升速率的证据。两种岩浆模拟的低上升速度（0.1-1.5厘米/年）并不表明在奥克兰以下的地幔区域有羽状流或大的上升流。因此，我们建议进行局部对流，这是由于遗骸从位于东南400公里处的活跃俯冲系统中移动而来。初始碱性玄武岩的较高孔隙度基于 226 Ra过剩量，表明熔体以两种不同的孔隙度运动：初始熔体在较大的快速高孔隙度通道中从较大的深度传播，从而保持较高的 230 剩余量和随后的碱溶岩浆从较浅的深度穿过较低孔隙度的扩散通道，从而保持较高的 226 Ra剩余量；这会在（ 226 Ra / 230 Th）与（ 230 Th / 238 U）空间中创建负数组以前只能在洋中脊玄武岩资料中看到。这种机制表明，奥克兰火山场可能是由于存在离散的熔体批次而运行的，这些熔体能够以不同的深度和速度运动，从而赋予该场其不稳定的喷发时空格局，这种行为类型可能会对火山的演化产生影响。世界各地的其他大陆火山场。

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