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首页> 外文期刊>Contributions to Mineralogy and Petrology >Dolomite-bearing orogenic garnet peridotites witness fluid-mediated carbon recycling in a mantle wedge (Ulten Zone, Eastern Alps, Italy)
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Dolomite-bearing orogenic garnet peridotites witness fluid-mediated carbon recycling in a mantle wedge (Ulten Zone, Eastern Alps, Italy)

机译：含白云石的人造石榴石橄榄岩见证了地幔楔中流体介导的碳循环（意大利东部阿尔卑斯山的乌尔滕区）

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摘要

We document the presence of dolomite ± apatite in orogenic peridotites from the Ulten Zone (UZ, Italian Alps), the remnants of a Variscan mantle wedge tectonically coupled with eclogitized continental crust. These dolomite peridotites are associated with dominant carbonate-free amphibole peridotites, which formed in response to infiltration of aqueous subduction fluids lost by the associated crustal rocks during high-pressure (HP) metamorphism and retrogression. Dolomite-free and dolomite-bearing peridotites share the same metamorphic evolution, from garnet- (HP) to spinel-facies (low-pressure, LP) conditions. Dolomite and the texturally coexisting phases display equilibrium redistribution of rare earth elements and of incompatible trace elements during HP and LP metamorphism; clinopyroxene and amphiboles from carbonate-free and carbonate-bearing peridotites have quite similar compositions. These features indicate that the UZ mantle rocks equilibrated with the same metasomatic agents: aqueous CO2-bearing fluids enriched in incompatible elements released by the crust. The P–T crystallization conditions of the dolomite peridotites (outside the field of carbonatite melt + amphibole peridotite coexistence), a lack of textures indicating quench of carbonic melts, a lack of increase in modal clinopyroxene by reaction with such melts and the observed amphibole increase at the expense of clinopyroxene, all suggest that dolomite formation was assisted by aqueous CO2-bearing fluids. A comparison of the trace element compositions of carbonates and amphiboles from the UZ peridotites and from peridotites metasomatized by carbonatite and/or carbon-bearing silicate melts does not help to unambiguously discriminate between the different agents (fluids or melts). The few observed differences (lower trace element contents in the fluid-related dolomite) may ultimately depend on the solute content of the metasomatic agent (CO2-bearing fluid versus carbonatite melt). This study provides strong evidence that C–O–H subduction fluids can produce ‘carbonatite-like’ assemblages in mantle rocks, thus being effective C carriers from the slab to the mantle wedge at relatively low P–T. If transported beyond the carbonate and amphibole solidus by further subduction, dolomite-bearing garnet + amphibole peridotites like the ones from Ulten can become sources of carbonatite and/or C-bearing silicate melts in the mantle wedge. Keywords Dolomite-bearing peridotites - Mantle wedge - C–O–H fluid - Subduction fluids - Ulten Zone Communicated by J. Hoefs.
机译：我们记录了来自Ulten地带（UZ，意大利阿尔卑斯山）的造山橄榄岩中白云岩±磷灰石的存在，该地壳是楔形残余地壳与构造成陆块的地壳构造耦合的。这些白云岩橄榄岩与主要的不含碳酸盐的闪石橄榄岩相关，后者是在高压（HP）变质和倒退过程中，由于相关地壳岩石损失的含水俯冲流体的渗透而形成的。从石榴石（HP）到尖晶石相（低压LP）条件，无白云岩和含白云岩的橄榄岩具有相同的变质演化。在HP和LP变质过程中，白云石和质地共存的相显示出稀土元素和不相容的痕量元素的平衡再分布。来自无碳酸盐岩和含碳酸盐橄榄岩的斜辉石和闪石具有非常相似的组成。这些特征表明，UZ地幔岩石与相同的交代剂平衡：富含CO 2 的含水流体富含地壳释放的不相容元素。白云岩橄榄岩的PT结晶条件（在碳酸盐岩熔体+闪石橄榄岩共存的领域之外），缺乏结构表明碳熔体淬灭，与这类熔体反应的模态斜辉石的增加以及观察到的闪石的增加所有这些都表明以白屈烯为代价，白云石的形成是由含水CO 2 流体辅助的。比较UZ橄榄岩和碳酸盐和/或含碳的硅酸盐熔体交代的橄榄岩中碳酸盐和闪石的痕量元素组成，无助于区分不同的物质（流体或熔体）。观察到的一些差异（与流体有关的白云石中微量元素的含量较低）最终可能取决于后代交联剂的溶质含量（含CO 2 的流体与碳酸盐岩熔体）。这项研究提供了有力的证据，证明C–O–H俯冲液可以在地幔岩石中产生“类碳酸盐”聚集体，因此是在相对较低的P–T下从板到地幔楔的有效C载体。如果通过进一步俯冲运出碳酸盐和闪石固相线，含白云石的石榴石+闪石橄榄石（如Ulten的橄榄石）可能成为碳酸盐和/或含C硅酸盐在地幔楔中熔融的来源。关键词含白云岩橄榄岩-地幔楔-C–O–H流体-俯冲流体-乌尔滕带J. Hoefs沟通。

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