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Oscillatory- and sector-zoned pyrochlore from carbonatites of the Kerimasi volcano, Gregory rift, Tanzania
Mineralogical Magazine ( IF 2.7 ) Pub Date : 2020-12-14 , DOI: 10.1180/mgm.2020.101
Anatoly N. Zaitsev , John Spratt , Alexander G. Shtukenberg , Andrei A. Zolotarev , Sergey N. Britvin , Sergey V. Petrov , Alina V. Kuptsova , Anton V. Antonov

The Quaternary carbonatite–nephelinite Kerimasi volcano is located within the Gregory rift in northern Tanzania. It is composed of nephelinitic and carbonatitic pyroclastic rocks, tuffs, tuff breccias and pyroclastic breccias, which contain blocks of different plutonic (predominantly ijolite) and volcanic (predominantly nephelinite) rocks including carbonatites. The plutonic and volcanic carbonatites both contain calcite as the major mineral with variable amounts of magnetite or magnesioferrite, apatite and forsterite. Carbonatites also contain accessory baddeleyite, kerimasite, pyrochlore and calzirtite. Zr and Nb minerals are rarely observed in rock samples, though they are abundant in eluvial deposits of carbonatite tuff/pyroclastic breccias in the Loluni and Kisete craters. Pyrochlore, ideally (CaNa)Nb2O6F, occurs as octahedral and cubo-octahedral crystals up to 300 μm in size. Compositionally, pyrochlore from Loluni and Kisete differs. The former is enriched in U (up to 19.4 wt.% UO2), light rare earth elements (up to 8.3 wt.% LREE2O3) and Zr (up to 14.4 wt.% ZrO2), and the latter contains elevated Ti (up to 7.3 wt.% TiO2). All the crystals investigated were crystalline, including those with high U content (a = 10.4152(1) Å for Loluni and a = 10.3763(1) Å for Kisete crystals). They have little or no subsolidus alteration nor low-temperature cation exchange (A-site vacancy up to 1.5% of the site), and are suitable for single-crystal X-ray diffraction analysis (R1 = 0.0206 and 0.0290; for all independent reflections for Loluni and Kisete crystals, respectively). Observed variations in the pyrochlore composition, particularly Zr content, from the Loluni and Kisete craters suggest crystallisation from compositionally different carbonatitic melts. The majority of pyrochlore crystals studied exhibit exceptionally well-preserved oscillatory- and sometimes sector-type zoning. The preferential incorporation of smaller and higher charged elements into more geometrically constrained sites on the growing surfaces explains the formation of the sector zoning. The oscillatory zoning can be rationalised by considering convectional instabilities of carbonatite magmas during their emplacement.



中文翻译:

来自坦桑尼亚格雷戈里裂谷 Kerimasi 火山碳酸盐岩的振荡和扇形分区烧绿石

第四纪碳酸岩-霞石克里马西火山位于坦桑尼亚北部的格雷戈里裂谷内。它由霞石质和碳酸质火山碎屑岩、凝灰岩、凝灰岩角砾岩和火山碎屑角砾岩组成,其中包含不同深成岩(主要是伊焦岩)和火山岩(主要是霞石)的块体,包括碳酸岩。深成岩和火山碳酸岩都含有方解石作为主要矿物,并含有不同数量的磁铁矿或镁铁矿、磷灰石和镁橄榄石。碳酸岩还含有副斜锆石、菱镁矿、烧绿石和钙锆石。Zr 和 Nb 矿物在岩石样品中很少观察到,尽管它们在 Loluni 和 Kisete 陨石坑的碳酸岩凝灰岩/火山碎屑角砾岩的残积沉积物中含量丰富。烧绿石,理想情况下 (CaNa)Nb 2 O6 F,以八面体和立方八面体晶体的形式出现,尺寸高达 300 μm。在成分上,来自 Loluni 和 Kisete 的烧绿石不同。前者富含 U(高达 19.4 wt.% UO 2)、轻稀土元素(高达 8.3 wt.% LREE 2 O 3)和 Zr(高达 14.4 wt.% ZrO 2),后者含有提高 Ti(高达 7.3 wt.% TiO 2)。所有研究的晶体都是晶体,包括那些具有高 U 含量晶体(Loluni 的a = 10.4152(1) Å 和 Kisete 晶体的a = 10.3763(1) Å)。它们几乎没有或没有亚固相线改变,也没有低温阳离子交换(A-位点空位高达位点的 1.5%),适用于单晶 X 射线衍射分析(R 1= 0.0206 和 0.0290;分别用于 Loluni 和 Kisete 晶体的所有独立反射)。观察到来自 Loluni 和 Kisete 陨石坑的烧绿石成分的变化,特别是 Zr 含量表明是从成分不同的碳酸岩熔体中结晶出来的。所研究的大多数烧绿石晶体都表现出保存完好的振荡型分区,有时甚至是扇形分区。更小和更高电荷的元素优先结合到生长表面上更多几何约束的位置解释了扇区分区的形成。考虑到碳酸岩岩浆在其侵位过程中的对流不稳定性,可以使振荡分带合理化。

更新日期:2020-12-14
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