Melt segregations in a Columbia River Basalt lava flow: A possible mechanism for the formation of highly evolved mafic magmas
Melt segregations in a Columbia River Basalt lava flow: A possible mechanism for the formation of highly evolved mafic magmas
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DOI：
10.1016/j.lithos.2009.04.003
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发表时间：
2009-10
期刊：
影响因子：
3.5
通讯作者：
M. Hartley;T. Thordarson
中科院分区：
文献类型：
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作者：
M. Hartley;T. Thordarson
Levering 熔岩位于哥伦比亚河玄武岩群的 Grande Ronde 地层内。在哨兵峡，它由 14-20 米厚的帕霍霍片叶组成，具有典型的膨胀帕霍霍叶片的三重垂直划分，分为基底区、熔岩核心和熔岩地壳。诸如水平囊泡片之类的分离结构在熔岩核心与熔岩地壳界面处异常发育。熔岩核心含有许多发育良好的囊泡圆柱体，这些囊泡圆柱体从基底区向上延伸穿过核心，并在与上地壳的界面处转变为水平的囊泡片。哨兵间隙波瓣内的宿主熔岩样本的成分随地层高度的变化没有统计学上的显着变化。然而，偏析结构中的物质明显更加演化，与主熔岩中的 MgO 含量约为 3 wt.% 相比，约为 5 wt.%。偏析中不相容元素（例如 TiO2、K2O、Ba、Zr）平均富集 1.6 倍。受全岩化学成分限制的建模表明，通过宿主熔岩的 30% 至 40% 的分级结晶，产生了分离材料。场关系表明，囊泡圆柱体由源自基底区的管囊泡供给营养。因此，偏析植根于基底地壳，这意味着熔体是通过从流底部附近的糊状层中提取富含挥发物的晶间熔体而产生的。 Grande Ronde 地层分为五个不同的化学组；三个高 MgO（>4.5 wt.% MgO）和两个低 MgO（<4.5 wt.% MgO）。 Levering 宿主熔岩属于高 MgO 组之一，而其偏析与低 MgO 组之一相同。这种相关性使我们提出，高 MgO 组和低 MgO 组通过分步结晶相关，并且 Levering 熔岩内形成分离的过程模拟了形成高度演化、非非晶体 Grande Ronde 熔岩的地下过程。
The Levering lava lies within the Grande Ronde Formation of the Columbia River Basalt Group. At Sentinel Gap it consists of a 14–20 m-thick pahoehoe sheet lobe featuring the threefold vertical division typical of inflated pahoehoe lobes into basal zone, lava core and lava crust. Segregation structures such as horizontal vesicle sheets are exceptionally well developed at the lava core to lava crust interface. The lava core contains numerous well-developed vesicle cylinders which extend from the basal zone up through the core and transform into horizontal vesicle sheets at the interface with the upper crust. The host lava samples show no statistically significant change in composition with stratigraphic height within the Sentinel Gap lobe. However, material in the segregation structures is markedly more evolved, with MgO content of ∼3 wt.% compared to ~5 wt.% in the host lava. The segregations are enriched in incompatible elements (e.g. TiO2, K2O, Ba, Zr) by an average factor of 1.6. Modelling, constrained by whole-rock chemical composition, indicates generation of segregated material by 30 to 40% fractional crystallisation of the host lava. Field relations show that vesicle cylinders are fed by pipe vesicles originating in the basal zone. The segregations are thus rooted in the basal crust, implying that the melt is derived by extraction of volatile-enriched inter-crystal melt from a mush horizon near the base of the flow. The Grande Ronde Formation is divided into five distinct chemical groups; three high-MgO (>4.5 wt.% MgO) and two low-MgO (<4.5 wt.% MgO). The Levering host lava belongs to one of the high-MgO groups, while its segregations are identical to one of the low-MgO groups. This correlation leads us to propose that the high-MgO and low-MgO groups are related by fractional crystallisation and the process by which segregations are formed within the Levering lava emulates the subsurface process that forms the highly evolved, aphyric Grande Ronde lavas.