Crust-Mantle Interactions at Different Depths in the Subduction Channel: Magnesium Isotope Records of Ultramafic Rocks from the Mantle Wedges
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摘要: 在不同的俯冲深度，俯冲板片会释放出不同来源和组成的熔/流体进入俯冲隧道中，并进而影响上覆地幔楔及衍生岛弧岩浆的地球化学组成.然而，如何识别俯冲隧道中不同深度熔/流体组分的来源一直是俯冲带研究中的难点.对不同深度来源的地幔楔超基性岩进行了Mg同位素研究，发现了Mg同位素具有示踪俯冲板块熔/流体来源的能力.首先，研究了美国加州Franciscan杂岩中一套经历了多期次流体交代作用的浅部来源（< ~60 km）的变质超基性岩.这些部分蛇纹石化的地幔楔超基性岩在蛇纹石脱水形成滑石的过程中会释放轻Mg同位素进入流体，而重Mg同位素更多地残留在滑石相中；随后进一步受俯冲板块来源流体的交代形成具有高CaO和轻Mg同位素组成的透闪石化变橄榄岩，暗示流体中含有源自俯冲板片的、富集轻Mg同位素的碳酸盐，说明在弧前~60 km深度，部分含Mg碳酸盐（方解石）可以在俯冲隧道中发生溶解并迁移交代上覆地幔楔橄榄岩.对深部地幔楔来源（~160 km）的大别造山带毛屋地区超镁铁质岩体岩相学和元素地球化学研究结果证实了其交代成因.结合多相包裹体、元素地球化学以及前人估计的温-压条件，推测交代介质更接近超临界流体.锆石U-Pb年代学研究揭示，交代作用主要发生在古生代洋壳俯冲阶段（454±58 Ma），超高压变质作用则发生在三叠纪陆壳俯冲阶段（232.8±7.9 Ma）.古生代锆石中大量的碳酸盐矿物包裹体和重O同位素特征说明古生代洋壳俯冲交代过程中有沉积碳酸盐组分加入.全岩和单矿物的Mg同位素组成均显著低于地幔值以及大别新元古代榴辉岩，说明交代的碳酸盐组分来源应为循环的沉积富Mg碳酸盐，暗示了在俯冲带深部富Mg沉积碳酸盐在超临界流体中会发生溶解迁移.由于沉积碳酸盐具有独特的、显著富集轻Mg同位素组成的特征，这种交代作用会造成地幔楔局部具有异常的Mg同位素组成，从而解释目前观察到的岛弧火山岩的Mg同位素特征.因此，Mg同位素是示踪俯冲碳酸盐与上覆地幔楔相互作用的有效工具.Abstract: At different depths, the subducted slabs could release melts/fluids with distinct chemical components from different reservoirs into the subduction channel. Such melts/fluids may then affect the geochemical compositions of the overlying mantle wedge and the island arc magmas. However, how to identify the sources of melts/fluids at different depths in the subduction channels remains a challenging issue in studies of the subduction zones. Based on the Mg isotope studies on the ultramafic rocks derived from the mantle wedge at different depths, Mg isotopes are proposed to be a useful tool to distinguish the sources of melts/fluids in the subduction channel.A set of metamorphic ultramafic rocks from the Franciscan complex in California that have undergone multiple stages of metasomatism at the shallow depth (< ~60 km) in the subduction channel was studied. During the dehydration reactions that produced talc from serpentine, light Mg isotopes were preferentially released into fluids whereas heavy Mg isotopes were retained in talc. The tremolite-dominated samples that metamorphosed further by slab-derived fluids have high CaO contents and light Mg isotopic compositions, implying that a certain amount of Mg-bearing calcites could be dissolved into fluids and participated in metamorphism of mantle wedge peridotites. The petrographic and elemental geochemical studies of the ultramafic rocks from the Maowu complex of the Dabie orogenic belt, which were derived from the deep mantle wedge (~160 km), confirmed the metasomatism genesis. Combined with multiphase inclusions, element geochemistry, and peak P-T condition, we speculate that the metasomatic fluid was supercritical. Zircon geochronology studies revealed that the metasomatism mainly occurred during the oceanic crust subduction at Paleozoic (454±58 Ma), and the ultra-high pressure metamorphism occurred during the continental crust subduction at Triassic (232.8±7.9 Ma). The large amount of carbonate mineral inclusions and heavy oxygen isotope characteristics of the Paleozoic zircon indicate the incorporations of carbonate components during the Paleozoic metasomatism. The lighter Mg isotope composition of whole rocks and individual minerals than that of the mantle and the Dabie eclogite, indicates that the carbonate components should be sedimentary Mg-rich carbonates, which was dissolved in the supercritical fluid.Due to that the sedimentary carbonate has a unique and significantly enriched light Mg isotope feature, the metasomatism will cause heterogeneous Mg isotopic compositions of the mantle wedge, which may account for the observed Mg isotope characteristics of the arc lavas. Magnesium isotopes thus could be a potentially useful tracer of crust-mantle interactions at subduction zones.
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Key words:
- magnesium isotope /
- orogenic ultramafic rock /
- subduction channel /
- mantle wedge metasomatism /
- zircon /
- petrology
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图 1 美国加州Franciscan杂岩中变橄榄岩样品的δ26Mg-MgO含量图解
据Li et al.(2018).第一组为部分蛇纹石化橄榄岩，第二组样品为完全蛇纹石化橄榄岩，第三组样品为含滑石变橄榄岩，第四组样品为含透闪石变橄榄岩
Fig. 1. δ26Mg vs. MgO (%) of meta-peridotites from the Franciscan complex, USA
图 2 大别山围岩石英榴辉岩(a，b)和毛屋地区石榴辉石岩(c，d)锆石U-Pb年龄
图a中蓝色年龄点代表变质锆石区域定年结果，红色年龄点代表继承锆石区域定年结果；图c中蓝色年龄点代表变质锆石区域(Th/U < 0.1)定年结果，红色年龄点代表交代锆石区域(Th/U > 0.1)定年结果(据Shen et al., 2018)
Fig. 2. U-Pb dating of zircons from the quartz eclogites (a, b) within country gneisses and the Maowu garnet pyroxenites (c, d)
图 3 毛屋石榴辉石岩和大别山榴辉岩中全岩和单矿物Mg同位素组成比较(a)，模拟计算交代超临界流体和碳酸盐组分的比例(b)
a中δ26Mg=-0.25±0.07‰，2SD(Teng et al., 2010); b据Shen et al.(2018)
Fig. 3. (a) δ26Mg vs. whole-rock (WR) MgO (%) for widespread Dabie-Sulu eclogites (DE) and Maowu GC associated with garnet (Gt) and clinopyroxene (Cpx) separates from this and previous works, (b) mixing modeling for the different sources in terms of Mg/Ca (molar ratio) and δ26Mg
图 4 毛屋石榴辉石岩中组Ⅰ(a)和组Ⅱ锆石(b)U-Pb年龄和O同位素组成
c~h为组Ⅰ锆石中矿物包体拉曼图；i~k为组Ⅱ锆石中矿物包体拉曼图(据Shen et al., 2018)
Fig. 4. Panels (a) and (b) display cathodoluminescence (CL) images of type Ⅰ and type ⅠI zircons from Maowu GC, respectively, combined with in situ 206Pb/238U ages, O isotope compositions, and mineral inclusions
图 5 毛屋岩体中石榴单斜辉石岩和围岩片麻岩中石英榴辉岩的锆石稀土元素以及Nb-Ta元素含量相关图
年龄数据来自Shen et al.(2018)，微量元素数据为沈骥等未发表数据
Fig. 5. REE patterns and relationships between REE, HFSE concentrations and 206Pb/238U ages for zircons from the garnet clinopyroxenites within Maowu complex, as well as from quartz eclogites within the country gneisses
图 6 俯冲带碳酸盐循环和Mg同位素体系
Fig. 6. Illustration showing magnesium isotope systematics in subduction zones
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