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首页> 外文期刊>Contributions to Mineralogy and Petrology >The evolution of calcite-bearing kimberlites by melt-rock reaction: evidence from polymineralic inclusions within clinopyroxene and garnet megacrysts from Lac de Gras kimberlites, Canada
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The evolution of calcite-bearing kimberlites by melt-rock reaction: evidence from polymineralic inclusions within clinopyroxene and garnet megacrysts from Lac de Gras kimberlites, Canada

机译：熔岩反应生成方解石的金伯利岩的演化：来自加拿大Lac de Gras金伯利岩的斜辉石和石榴石大晶体中的多矿物包裹体的证据

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摘要

Megacrystic (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) xenocrysts within kimberlites from Lac de Gras (Northwest Territories, Canada) contain fully crystallized melt inclusions. These 'polymineralic inclusions' have previously been interpreted to form by necking down of melts at mantle depths. We present a detailed petrographical and geochemical investigation of polymineralic inclusions and their host crystals to better understand how they form and what they reveal about the evolution of kimberlite melt. Genetically, the megacrysts are mantle xenocrysts with peridotitic chemical signatures indicating an origin within the lithospheric mantle (for the Cr-diopsides studied here similar to 4.6 GPa, 1015 degrees C). Textural evidence for disequilibrium between the host crystals and their polymineralic inclusions (spongy rims in Cr-diopside, kelyphite in Cr-pyrope) is consistent with measured Sr isotopic disequilibrium. The preservation of disequilibrium establishes a temporal link to kimberlite eruption. In Cr-diopsides, polymineralic inclusions contain phlogopite, olivine, chromite, serpentine, and calcite. Abundant fluid inclusion trails surround the inclusions. In Cr-pyropes, the inclusions additionally contain Al-spinel, clinopyroxene, and dolomite. The major and trace element compositions of the inclusion phases are generally consistent with the early stages of kimberlite differentiation trends. Extensive chemical exchange between the host phases and the inclusions is indicated by enrichment of the inclusions in major components of the host crystals, such as Cr2O3 and Al2O3. This chemical evidence, along with phase equilibria constraints, supports the proposal that the inclusions within Cr-diopside record the decarbonation reaction: dolomitic melt + diopside -> forsterite + calcite + CO2, yielding the observed inclusion mineralogy and producing associated (CO2-rich) fluid inclusions. Our study of polymineralic inclusions in megacrysts provides clear mineralogical and chemical evidence for an origin of kimberlite that involves the reaction of high-pressure dolomitic melt with diopside-bearing mantle assemblages producing a lower-pressure melt that crystallizes a calcite-dominated assemblage in the crust.
机译：来自Lac de Gras（加拿大西北地区，加拿大）的金伯岩中的大晶（> 1厘米）斜柏石（Cr-透辉石）和石榴石（Cr-pyrope）异晶含有完全结晶的熔体。这些“多矿物夹杂物”先前已被解释为通过在地幔深度处使熔体收缩而形成。我们对多矿物包裹体及其基质晶体进行了详细的岩石学和地球化学研究，以更好地了解它们的形成方式以及它们揭示的金伯利岩熔体演化的信息。从遗传学上讲，大晶是具有橄榄岩化学特征的地幔异晶，表明岩石圈地幔的起源（对于这里研究的Cr-二硅化物类似于4.6 GPa，1015摄氏度）。基质晶体与它们的多矿物包裹体（Cr-透辉石中的海绵状边缘，Cr-pyrope中的钾盐）之间的不平衡的组织学证据与测得的Sr同位素不平衡一致。不平衡的保持建立了与金伯利岩喷发的时间联系。在Cr-二叠氮化物中，多矿物夹杂物包含金云母，橄榄石，铬铁矿，蛇纹石和方解石。大量的流体包裹体痕迹环绕着包裹体。在Cr-吡咯中，夹杂物还包含Al-尖晶石，斜辉石和白云石。包裹相的主要和微量元素组成与金伯利岩分化趋势的早期阶段基本一致。主体相和夹杂物之间的广泛化学交换通过主体晶体主要成分（例如Cr2O3和Al2O3）中的夹杂物富集来表明。该化学证据以及相平衡约束条件支持以下建议：Cr-透辉石中的夹杂物记录了脱碳反应：白云岩熔体+透辉石->镁橄榄石+方解石+ CO2，产生观察到的夹杂物矿物学并产生相关的（富CO2）流体包裹体。我们对超大晶体中的多矿物包裹体的研究为金伯利岩的起源提供了明确的矿物学和化学证据，涉及到金云母熔体与含透辉石的地幔组合的反应，产生了一个低压熔体，该熔体在地壳中以方解石为主的组合物结晶。

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