Diamonds from Dachine, French Guiana: A unique record of early Proterozoic subduction
Diamonds from Dachine, French Guiana: A unique record of early Proterozoic subduction
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DOI：
10.1016/j.lithos.2016.09.026
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发表时间：
2016-11
期刊：
影响因子：
3.5
通讯作者：
Christopher B. Smith;M. Walter;G. Bulanova;S. Mikhail;Antony D. Burnham;L. Gobbo;S. Kohn
中科院分区：
文献类型：
--
作者：
Christopher B. Smith;M. Walter;G. Bulanova;S. Mikhail;Antony D. Burnham;L. Gobbo;S. Kohn
来自法属圭亚那Dachine的钻石在全球钻石种群中独一无二。金刚石是在一种不寻常的超镁铁质喷出岩浆中被运送到地表的，这种岩浆与玻安岩或科马提岩有亲和力，它在~ 2.2 Ga的弧地质背景中侵位。达欣金刚石具有内部和外部形态，表明从碳过饱和流体或熔体中相对快速生长，并表现出与高应变环境中的居住一致的内部特征。基于氮（N）缺陷，金刚石被分类为Ib-IaA型。N异常低的聚集态对钻石的热历史产生了严格的限制，有效地排除了对流地幔中的衍生作用。达钦钻石的碳和氮同位素组成与沉积碳源一致，大多数钻石的δ 13 C值< − 25‰，δ 15 N值> + 4‰。原生碳可能沉积在早元古代的海底。硫化物包裹体具有低Ni和Cr，与岩石圈榴辉岩型硫化物包裹体相当。三个石榴石和一个单斜辉石包裹体的成分也是榴辉岩，其中一个石榴石包裹体的主要成分指示约250 km深度的起源。硅酸盐包裹体在许多不相容的微量元素（如轻稀土元素，铌，铪，锆）高度亏损，和建模表明榴辉岩源岩性，其中包含LREE富集的微量相，如绿帘石或褐帘石，和HFSE丰富的阶段，如金红石。五个包裹体中有四个异常富含Mn，以及Ni和Co，建模表明原岩具有俯冲正常MORB加上约10%的铁锰地壳成分的散装成分。我们提出了一个模型，其中Dachine金刚石沉淀的再活化沉积碳在板幔界面的液体最终由deserpentinization板橄榄岩在深度为200至250公里。这些流体也可能引发楔形橄榄岩的熔融，导致富含挥发物的超镁铁质熔体将钻石迅速运送到表面。金刚石的形成和从板幔界面折返的过程发生在古元古代俯冲带，时间跨度非常有限，可能不到一百万年。
Diamonds from Dachine, French Guiana, are unique among worldwide diamond populations. The diamonds were transported to the surface in an unusual ultramafic extrusive magma with an affinity to boninite or komatiite, which was emplaced within an arc geological setting at ~ 2.2 Ga. Dachine diamonds have internal and external morphologies indicative of relatively rapid growth from carbon oversaturated fluids or melts, and exhibit internal features consistent with residence in a high-strain environment. On the basis of nitrogen (N) defects the diamonds are categorized as Type Ib–IaA. The unusually low aggregation state of N places severe constraints on the thermal history of the diamonds, effectively ruling out derivation in convecting mantle. The carbon and N isotopic compositions of Dachine diamonds are consistent with a sedimentary source of carbon, with the majority of diamonds having δ13C values < − 25‰ and δ15N values > + 4‰. The primary carbon was presumably deposited on an early Proterozoic seafloor. Sulphide inclusions have low Ni and Cr and are comparable to lithospheric eclogitic-type sulphide inclusions. Three garnet and one clinopyroxene inclusion are also eclogitic in composition, and one garnet inclusion has a majorite component indicating an origin around 250 km depth. The silicate inclusions are highly depleted in many incompatible trace elements (e.g. LREE, Nb, Hf, Zr), and modelling indicates an eclogitic source lithology that contained a LREE-enriched trace phase such as epidote or allanite, and an HFSE-rich phase such as rutile. Four of the five inclusions are unusually enriched in Mn, as well as Ni and Co, and modelling suggests a protolith with a bulk composition of subducted normal MORB plus about 10% ferromanganese crust component. We suggest a model wherein Dachine diamonds precipitated from remobilized sedimentary carbon at the slab–mantle interface from liquids derived ultimately by deserpentinization of slab peridotite at depths of ~ 200 to 250 km. These fluids may also trigger melting in wedge peridotite, resulting in a volatile-rich ultramafic melt that transports the diamonds rapidly to the surface. The process of diamond formation and exhumation from the slab–mantle interface occurred in a Paleoproterozoic subduction zone and over a very limited timespan, likely less than a million years.