Sulphide Sinking in Magma Conduits: Evidence from Mafic-Ultramafic Plugs on Rum and the Wider North Atlantic Igneous Province
Sulphide Sinking in Magma Conduits: Evidence from Mafic-Ultramafic Plugs on Rum and the Wider North Atlantic Igneous Province
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岩浆管道中的硫化物下沉：来自朗姆酒和更广泛的北大西洋火成岩省的镁铁质-超镁铁质塞子的证据
DOI：
10.1093/petrology/egw010
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发表时间：
2016
影响因子：
3.9
通讯作者：
Hughes H
中科院分区：
文献类型：
--
作者：
Hughes H
镍铜铂族元素硫化物矿化在岩浆管道系统中较为常见。在许多情况下，形成不混溶硫化物液体的触发因素涉及含硫地壳岩石的同化作用。从概念上讲，这种管道内硫化物液滴的流体动力学基本上是重力下沉和向上夹带之间的平衡。因此，在与污染物发生相互作用的地点向上和向下流动的硫化物中可能存在地壳污染特征。我们研究了一套超镁铁质火山岩塞的朗姆酒，苏格兰岛，破译硫化物积累的控制近地表岩浆管道侵入到一个变量的沉积地层。这些岩栓的全岩成分与朗姆酒分层杂岩内超镁铁质单元的成分大致重叠，尽管每个岩栓之间的细微差异凸显了它们的个性。硫化物矿物具有岩浆δ 34 S（-1 ~+2）和S/Se比值（平均值= 2299），表明导管岩浆已经饱和硫。然而，西北朗姆的两个岩塞含有明显较粗（有时呈网状结构）的硫化物，其δ 34 S异常轻（-14 statists 7至+0 statists 3 ‰），S/Se比值升高（平均值= 4457），而不是直接的寄主岩。根据赫布里底盆地的沉积地层学，火山管道很可能穿过一组δ 34 S特征较轻（-33 ‰至-14 ‰）的侏罗纪泥岩侵入。我们认为，一个次级地壳S污染事件发生在目前暴露的水平以上，这些硫化物下沉到目前的位置。模型表明，在活跃的岩浆运输停止后，硫化物液体可能在几天内通过管道下沉数百米。这种硫化物的“退出”过程可以在全球其他垂直或陡峭倾斜的岩浆管道中观察到，例如，在东格陵兰的巨岩脉中。硫化物液体下沉在一个非活动的管道或在岩浆的“吸回”可能有助于解释地壳S-同位素组成的岩浆管道，似乎缺乏适当的岩性，以支持这种污染，无论是本地或更深的系统。
Ni–Cu–PGE (platinum group element) sulphide mineralization is commonly found in magmatic conduit systems. In many cases the trigger for formation of an immiscible sulphide liquid involves assimilation of S-bearing crustal rocks. Conceptually, the fluid dynamics of sulphide liquid droplets within such conduits is essentially a balance between gravitational sinking and upwards entrainment. Thus, crustal contamination signatures may be present in sulphides preserved both up- and down-flow from the point of interaction with the contaminant. We examine a suite of ultramafic volcanic plugs on the Isle of Rum, Scotland, to decipher controls on sulphide accumulation in near-surface magma conduits intruded into a variable sedimentary stratigraphy. The whole-rock compositions of the plugs broadly overlap with the compositions of ultramafic units within the Rum Layered Complex, although subtle differences between each plug highlight their individuality. Interstitial base metal sulphide minerals occur in all ultramafic plugs on Rum. Sulphide minerals have magmatic δ34S (ranging from –1·3 to +2·1‰) and S/Se ratios (mean = 2299), and demonstrate that the conduit magmas were already S-saturated. However, two plugs in NW Rum contain substantially coarser (sometimes net-textured) sulphides with unusually light δ34S (–14·7 to +0·3‰) and elevated S/Se ratios (mean = 4457), not represented by the immediate host-rocks. Based on the Hebrides Basin sedimentary stratigraphy, it is likely that the volcanic con duits would have intruded through a package of Jurassic mudrocks with characteristically light δ34S (–33·8 to –14·7‰). We propose that a secondary crustal S contamination event took place at a level above that currently exposed, and that these sulphides sank back to their present position. Modelling suggests that upon the cessation of active magma transport, sulphide liquids could have sunk back through the conduit over a distance of several hundreds of metres, over a period of a few days. This sulphide ‘withdrawal’ process may be observed in other vertical or steeply inclined magma conduits globally; for example, in the macrodykes of East Greenland. Sulphide liquid sinking within a non-active conduit or during magma ‘suck-back’ may help to explain crustal S-isotopic compositions in magma conduits that appear to lack appropriate lithologies to support this contamination, either locally or deeper in the system.