Immiscible sulphide liquids: Insights into chalcophile element fractionation processes in the oceanic crust

在洋壳中以岩浆硫化物球体形式保存的不混溶硫化物液体记录了其寄主岩浆系统的S和亲硫元素演化。最近的研究结果表明，会聚和发散板块边缘的岩浆硫化物颗粒在矿物学和化学方面存在系统差异，但这些差异的来源以及对洋壳亲磷元素循环的影响尚不清楚。控制S在硅酸盐熔体中溶解度的参数有：(1)温度、(2)压力、(3)氧逸度和(4)分馏程度。尽管大洋中脊和俯冲带的上地幔熔融条件和地壳岩浆分异不同，但这些作用对硅酸盐熔体的S饱和极限及其亲硫元素收支的影响仍然知之甚少。更好地约束岩浆系统的S元素和亲硫元素的演化，对于了解大洋岩石圈的亲硫元素循环、海底热液硫化物的组成、陆壳的形成、火山气体的组成以及一些陆下浅成热液斑岩矿床的金属和类金属收支是至关重要的。该项目旨在通过研究大洋中脊和大洋俯冲带大洋岩石圈中的岩浆痕量金属和类金属通量(如Co、Ni、Cu、Se、Ag、Te、PGe、Au、Bi)来解决这些问题。将使用最先进的分析技术，提供关于板块构造背景和岩浆系统时间演化的来自大洋岩石圈所有部分的岩浆硫化物球体的第一个全球微量元素数据集。反映大陆解体时大洋扩张开始的样品和大洋中脊最近的轴向火山活动，以及从海底最年轻的熔岩到熔岩/岩墙过渡期最老的熔岩堆的完整序列，记录了高时间分辨率的岩浆亲磷微量元素循环的演化。为了实现这些目标，需要从上岩石圈地幔到最上地壳的连续样品谱，这只能由钻探岩芯提供，例如在钻探钻探、南极大洋钻探和IODP探险期间采集的岩芯。我们已经确定了合适的岩芯，这些岩芯与现代海底和古代大洋岩石圈的样品(如特罗多斯蛇绿岩)一起提供了从玄武岩到流纹岩成分的包括上地幔橄榄岩、下地壳辉长岩、席状岩墙和熔岩的全面样品集。岩浆硫化物球体已经在其中的许多样品中被发现。拟议的项目将使我们能够在汇聚和发散的板块边缘开发整个大洋岩石圈中岩浆亲磷微量元素循环的第一个模型。

Immiscible sulphide liquids preserved as magmatic sulphide globules in the oceanic crust record the S and chalcophile element evolution of their host magmatic systems. Recent results report systematic variations in the mineralogy and chemistry of magmatic sulphide globules from convergent and divergent plate margins, but the origin of these differences, and the implications for the chalcophile element cycle in the oceanic crust, are unknown. Parameters that control the solubility of S in silicate melts include: (1) temperature, (2) pressure, (3) oxygen fugacity and (4) the degree of fractionation. Although upper mantle melting conditions and magma differentiation in the crust differ between mid-ocean ridges and subduction zones, the effects of these processes on the S saturation limit of a silicate melt and its chalcophile element budget are still poorly understood. Better constraints on the S and chalcophile element evolution of magmatic systems are critical for understanding the chalcophile element cycle in the oceanic lithosphere, the composition of seafloor hydrothermal sulphides, the formation of the continental crust, the composition of volcanic gas and possibly the metal and metalloid budget of some subaerial epithermal-porphyry deposits.The project aims to address these questions by investigating the magmatic trace metal and metalloid flux (e.g., Co, Ni, Cu, Se, Ag, Te, PGE, Au, Bi) through the oceanic lithosphere at mid-ocean ridges and at oceanic subduction zones. State-of-the-art analytical techniques will be used to present the first global trace element data set of magmatic sulphide globules from all sections of the oceanic lithosphere, with respect to the plate-tectonic setting and the temporal evolution of the magmatic system. Samples reflecting the initiation of ocean spreading during continental break-up and recent on-axis volcanic activity at mid-ocean ridges together with a full succession of the lava pile from the youngest at the seafloor to the oldest at the lava/dyke transition record the evolution of the magmatic chalcophile trace element cycle at high temporal resolution. In order to address these objectives, a continuous sample spectrum from the upper lithospheric mantle to the uppermost crust is required, which can only be provided by drill cores, such as those recovered during DSDP, ODP and IODP expeditions. We have identified suitable cores, which together with samples from the modern seafloor and from ancient oceanic lithosphere (e.g., Troodos ophiolite) provide a comprehensive sample set including upper mantle peridotites, lower crustal gabbros, sheeted dykes and lavas from basaltic to rhyolitic composition. Magmatic sulphide globules have already been identified in many of these samples. The proposed project will allow us to develop the first models of the magmatic chalcophile trace element cycle through the entire oceanic lithosphere at both convergent and divergent plate margins.