Chalcophile element processing beneath a continental arc stratovolcano
Chalcophile element processing beneath a continental arc stratovolcano
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DOI：
10.1016/j.epsl.2019.06.017
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发表时间：
2019-09-15
影响因子：
5.3
通讯作者：
Hammond, Samantha J.
中科院分区：
文献类型：
--
作者：
Cox, Daniel;Watt, Sebastian F. L.;Hammond, Samantha J.
亲铜元素在其经济价值和作为岩浆作用的潜在示踪剂方面都是重要的。然而，由于分析困难，火山岩中亲铜元素浓度的综合数据集很少。在这里，我们对智利南部（安图科）一个典型大陆弧层状火山的火山岩样品进行了近乎完整的亲铜元素（S、Cu、Ag、Se、As、Sb、Sn、W、Mo、Pb、Bi、Tl、Zn、Ga、Co）分析。Pb、Bi、W、Tl、Sb和As元素相对于parent - morb富集，表明这些元素已经从俯冲板块移动到弧下地幔楔中，而Cu和Ag则相反。极低的Se浓度表明Se和S一样是在安图科岩浆共喷发脱气过程中丢失的。先前对海洋弧的研究表明，当高fO(2)俯冲相关岩浆通过上覆岩石圈上升时，磁铁矿分馏可能在结晶过程中引发硫化物分馏。如果这个过程是广泛的，并且有一个突然的开始，这将导致残余熔体中Cu， Se和Ag含量的急剧下降。在安图科，虽然Fe2O3(T)和TiO2浓度在接近55wt .% SiO2（接近3wt .% MgO）时的下降表明磁铁矿分选，但这与Cu含量的相应下降无关。相反，我们观察到随着SiO2的增加和MgO的减少，Cu和Cu/Ag普遍减少。此外，最原始的安图科岩石中的Cu/Ag低于全球MORB阵列，表明熔体在地壳演化的早期阶段是硫化物饱和的。通过模拟分数结晶，我们发现，正如在安图科样品中观察到的那样，只需要很小体积（0.5-0.6体积%）的分馏硫化物就可以产生Cu和Ag的发散趋势。结果表明，硫化物分馏作用发生在安图科岩浆地壳演化的早期阶段。我们推断，这是由下地壳的失速促进的，对于深度bbb20 km的氧化岩浆来说，这是在硫化物稳定场内。然而，与大洋岛弧岩浆相比，安图科岩浆的高Dy-N/Yb-N提供了另一种机制，即在上升之前通过初始石榴石分选在下地壳中诱导硫化物分选。在这个深度范围内的分数结晶意味着后来的磁铁矿分馏对亲铜元素的分配行为只有很小的影响。相比之下，穿过较薄地壳的弧岩浆可能直到其演化的后期才经历硫化物饱和，这是由磁铁矿分选引起的。结果表明，岩浆分选深度范围决定了岩浆硫化物初始饱和度的主导控制因素，从而决定了亲铜元素的初始分布。这意味着需要二次过程来解释浅层地壳中硫化物和亲铜元素的迁移和浓度。(C) 2019 Elsevier B.V.版权所有
The chalcophile elements are important both in terms of their economic value and as potential tracers of magmatic processes at convergent margins. However, because of analytical difficulties, comprehensive datasets of chalcophile element concentrations for volcanic rocks are rare. Here, we present analyses of a near complete suite of chalcophile elements (S, Cu, Ag, Se, As, Sb, Sn, W, Mo, Pb, Bi, Tl, Zn, Ga, Co) for volcanic rock samples collected from a typical continental arc stratovolcano in southern Chile (Antuco). Enrichment in Pb, Bi, W, Tl, Sb and As relative to Parental-MORB indicates that these elements have been mobilised from the subducting slab into the sub-arc mantle wedge, in contrast to Cu and Ag. Very low Se concentrations suggest that Se, like S, was lost during co-eruptive degassing of the Antuco magmas. Previous studies on oceanic arcs have demonstrated that as higher fO(2) subduction-related magmas ascend through the overlying lithosphere, magnetite fractionation may trigger sulfide fractionation during crystallisation. If such a process is extensive and has a sharp onset, this would result in a plummet in the Cu, Se and Ag contents of the residual melt. At Antuco, although a decrease in the Fe2O3(T) and TiO2 concentrations at similar to 55 wt.% SiO2 (similar to 3 wt.% MgO) indicates magnetite fractionation, this is not associated with a corresponding drop in Cu contents. Instead, we observe a general decrease in Cu and a decrease in Cu/Ag with increasing SiO2 and decreasing MgO. Furthermore, Cu/Ag in the most primitive Antuco rocks are lower than the global MORB array, indicating that the melts were sulfide saturated at an early stage in their crustal evolution. Through modelling fractional crystallisation, we show that only a minor volume (0.5-0.6 vol.%) of fractionating sulfide is needed to produce divergent trends in Cu and Ag, as observed in the Antuco samples. Our results show that sulfide fractionation occurred from an early stage during the crustal evolution of Antuco's magmas. We infer that this was promoted by stalling in the lower crust, which for oxidised magmas at depths >20 km is within the sulfide stability field. However, elevated Dy-N/Yb-N of the Antuco magmas compared to oceanic island arc magmas provides an additional, or alternate mechanism to inducing sulfide fractionation in the lower crust prior to ascent, through initial garnet fractionation. Fractional crystallisation within this depth range meant that later magnetite fractionation had only a minor impact on the partitioning behaviour of the chalcophile elements. In contrast, arc magmas transiting thinner crust may not experience sulfide saturation until a later stage in their evolution, induced by magnetite fractionation. Our results imply that convergent margin crustal thickness, and therefore the depth range of magmatic differentiation, determines the dominant control on initial magmatic sulfide saturation and therefore the primary distribution of chalcophile elements. This implies that secondary processes are required to explain the transport and concentration of sulfides and chalcophile elements at shallower crustal levels. (C) 2019 Elsevier B.V. All rights reserved.