Abstract

To provide new insights into the origin and evolution of kimberlitic magmas with different diamond concentrations from the Arkhangelsk diamond province in north-western Russia, we examined the major- and trace-element compositions of ilmenite from diamondiferous kimberlite of the Grib pipe and diamond-barren kimberlites from the Kepino cluster (Stepnaya and TsNIGRI–Arkhangelskaya pipes). Ilmenite from diamond-barren kimberlites shows lower Mg, Ti, Cr, Ni and Cu concentrations with increase in both Fe³⁺ and Fe²⁺ and Nb, Ta, Zr, Hf, Zn and V concentrations. The main differences between kimberlites with different diamond contents are the Nb and Zr concentrations and their correlation patterns with Mg and Cr concentrations. Ilmenite from the Grib kimberlite has Zr concentrations <110 ppm, whereas ilmenite from the Kepino kimberlites has Zr concentrations >300 ppm. Ilmenite crystallisation within the Grib kimberlite occurred under increasing oxygen fugacity (fO₂), which may reflect assimilation of mantle peridotite by the kimberlitic magmas. Ilmenite from the Kepino kimberlites suggests its crystallisation under constant fO₂, with the ilmenite composition being controlled by processes of fractional crystallisation of megacrystic minerals. These assumptions were confirmed with assimilation–fractional crystallisation calculations. On the basis of obtained data, we developed a model for the evolution of the kimberlitic magmas for both diamondiferous and barren kimberlites. The diamond-bearing kimberlitic magmas were generated under intense interaction of kimberlitic magmas with the surrounding lithospheric mantle. It may be that during early modification of the lithospheric mantle by kimberlitic magmas as well as with kimberlitic magmas’ local stretching and swift ascent, the capture of the mantle xenoliths was favoured over the crystallisation of phenocrysts. The formation of barren kimberlitic magmas may have occurred when the lithospheric mantle in the vicinity of ascending magmas was already geochemically equilibrated with them. It also is possible that the magma’s ascent slowed under conditions of dominantly compressive stresses with crystallisation of olivine and other megacrystic phases.

中文翻译：

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俄罗斯阿尔汉格尔斯克钻石省的钛铁矿：含钻石的金伯利岩的组成，成因和指示剂

摘要

为了提供来自俄罗斯西北部阿尔汉格尔斯克钻石省的不同钻石浓度的金伯利岩岩浆成因和演化的新见解，我们研究了格里布管和钻石贫瘠的含金刚石岩中钛铁矿的主要和微量元素组成。 Kepi​​no集群中的金伯利岩（Stepnaya和TsNIGRI–Arkhangelskaya管道）。金刚石贫化金伯利岩中的钛铁矿显示出较低的Mg，Ti，Cr，Ni和Cu浓度，同时Fe ³⁺和Fe ²⁺均增加以及Nb，Ta，Zr，Hf，Zn和V的浓度。金刚石含量不同的金伯利岩之间的主要区别在于Nb和Zr浓度以及它们与Mg和Cr浓度的相关关系。来自格里布金伯利岩的钛铁矿的Zr浓度小于110 ppm，而开普诺金伯利岩的钛铁矿的Zr浓度大于300 ppm。Grib金伯利岩中的钛铁矿结晶是在氧气逸度（f O ₂）增加的情况下发生的，这可能反映了金伯利岩岩浆对地幔橄榄岩的吸收。Kepi​​no金伯利岩中的钛铁矿表明其在恒定f O ₂下的结晶钛铁矿的组成是由大晶矿物的分步结晶过程控制的。这些假设已通过同化-分数结晶计算得到了证实。在获得的数据的基础上，我们开发了金伯利岩岩浆和贫金伯利岩的金伯利岩岩浆演化的模型。含金伯利岩岩浆是在金伯利岩岩浆与周围岩石圈地幔的强烈相互作用下产生的。可能是在金伯利岩岩浆对岩石圈地幔的早期改造以及金伯利岩岩浆的局部伸展和迅速上升的过程中，地幔异岩的捕获比表晶的结晶更有利。当上升岩浆附近的岩石圈地幔已经与地球化学平衡时，可能形成了贫瘠的金伯利岩岩浆。岩浆的上升也有可能在主要为压应力的条件下随着橄榄石和其他大晶相的结晶而减慢。