Experimental study of liquid immiscibility in the Kiruna-type Vergenoeg iron-fluorine deposit, South Africa
Experimental study of liquid immiscibility in the Kiruna-type Vergenoeg iron-fluorine deposit, South Africa
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DOI：
10.1016/j.gca.2017.01.025
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发表时间：
2017-04-15
影响因子：
5
通讯作者：
Holtz, Francois
中科院分区：
文献类型：
--
作者：
Hou, Tong;Charlier, Bernard;Holtz, Francois
在这项研究中，我们实验评估是否基律纳型铁氟Vergenoeg存款，南非（17重量%）的整体组成SiO2和55重量% FeOtot）可能对应于与其寄主流纹岩配对的不混溶的富Fe熔体。寄主流纹岩的合成粉末与镁铁质端元（矿石）以不同的比例混合。实验条件为1-2千巴和1010摄氏度，在起始组合物中具有一定范围的H2O和F含量。在实验中，在相对干燥的条件下，在对应于FMQ-1.4至FMQ+1.8（FMQ =铁橄榄石-磁铁矿-石英固体缓冲液）的氧逸度条件下，用萤石饱和，出现成对的不同的不混溶液体。所述富Si不混溶液体含有60.9-73.0重量% SiO2，9.1-12.5重量% FeOtot，2.4-4.2重量%富Na_2 O、K_2 O和Al_2 O_3。成对的富Fe不混溶熔体具有41.0- 49.5wt.% SiO2，20.6-36.1重量% FeOtot和4.5-6.0重量% F，并富含MgO，CaO和TiO 2。在富水（aH（2）O > 0.2; a =活性）和/或氧化（> FMQ+1.8）条件下进行的实验中，不溶性不会发展。在所有实验中，固相是磁铁矿、+/-铁橄榄石、萤石和鳞石英。我们的研究结果表明，从Vergenoeg管的岩石结晶在一个岩浆房托管两个不混溶的硅酸盐熔体。结晶管从富铁熔体解释其极端富集的Ca，F和Fe相比，主机流纹岩。然而，它的低体积二氧化硅含量相比，实验富铁熔体表明，管道形成的再活化的镁铁质晶体糊状物为主的磁铁矿和铁橄榄石。演化的残留液体以及共轭不混溶的富硅熔体的分离产生了主机流纹岩。Vergenoeg矿石中的大量氟（类似于12重量%）F）很难用萤石从富铁硅酸盐熔体（高达6 wt.%）中的简单结晶来解释F在萤石饱和）。相反，我们证实了以前的假设，即萤石富集，部分原因是由于热液流体的迁移。(C)2017爱思唯尔有限公司版权所有
In this study we experimentally assess whether the bulk composition of the Kiruna-type iron-fluorine Vergenoeg deposit, South Africa (17 wt.% SiO2 and 55 wt.% FeOtot) could correspond to an immiscible Fe-rich melt paired with its host rhyolite. Synthetic powder of the host rhyolite was mixed with mafic end-members (ore rocks) in variable proportions. Experimental conditions were 1-2 kbar and 1010 degrees C, with a range of H2O and F contents in the starting compositions. Pairs of distinct immiscible liquids occur in experiments saturated with fluorite, under relatively dry conditions, and at oxygen fugacity conditions corresponding to FMQ-1.4 to FMQ+1.8 (FMQ = fayalite-magnetite-quartz solid buffer). The Si-rich immiscible liquids contain 60.9-73.0 wt.% SiO2, 9.1-12.5 wt.% FeOtot, 2.4-4.2 wt.% F, and are enriched in Na2O, K2O and Al2O3. The paired Fe-rich immiscible melts have 41.0-49.5 wt.% SiO2, 20.6-36.1 wt.% FeOtot and 4.5-6.0 wt.% F, and are enriched in MgO, CaO and TiO2. Immiscibility does not develop in experiments performed under water-rich (aH(2)O > 0.2; a = activity) and/or oxidized (> FMQ+1.8) conditions. In all experiments, solid phases are magnetite, +/- fayalite, fluorite and tridymite. Our results indicate that the rocks from the Vergenoeg pipe crystallized in a magma chamber hosting two immiscible silicate melts. Crystallization of the pipe from the Fe-rich melt explains its extreme enrichment in Ca, F and Fe compared to the host rhyolitic rocks. However, its low bulk silica content compared to experimental Fe-rich melts indicates that the pipe formed by remobilization of a mafic crystal mush dominated by magnetite and fayalite. Segregation of evolved residual liquids as well as the conjugate immiscible Si-rich melt produced the host rhyolite. The huge amount of fluorine in Vergenoeg ores (similar to 12 wt.% F) can hardly be explained by simple crystallization of fluorite from the Fe-rich silicate melt (up to 6 wt.% F at fluorite saturation). Instead, we confirm a previous hypothesis that the fluorite enrichment is, in part, due to the migration of hydrothermal fluids. (C) 2017 Elsevier Ltd. All rights reserved.