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首页> 外文期刊>Contributions to Mineralogy and Petrology >Phase relations and liquid lines of descent of an iron-rich peralkaline phonolitic melt: an experimental study
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Phase relations and liquid lines of descent of an iron-rich peralkaline phonolitic melt: an experimental study

机译：富铁的碱性碱金属氧化物熔体的相关系和下降的液体线：实验研究

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We experimentally investigated the phase relations of a peralkaline phonolitic dyke rock associated with the Ilímaussaq plutonic complex (South Greenland). The extremely evolved and iron-rich composition (magnesium number = 2, alkalinity index = 1.44, FeO* = 12 wt%) may represent the parental magma of the Ilímaussaq complex. This dyke rock is therefore perfectly suited for performing phase-equilibrium experiments, since in contrast to the plutonic rocks of the complex, no major cumulate formation processes complicate defining a reasonable starting composition. Experiments were carried out in hydrothermal rapid-quench cold-seal pressure vessels at P = 100 MPa and T = 950–750 °C. H2O contents ranging from anhydrous to H2O saturated (~5 wt% H2O) and varying fO2 (~ΔlogFMQ −3 to +1; where FMQ represents the fayalite–magnetite–quartz oxygen buffer) were applied. Reduced and dry conditions lead to substantial crystallization of alkali feldspar, nepheline, hedenbergite-rich clinopyroxene, fayalite-rich olivine and minor amounts of ulvøspinel-rich magnetite, which represent the phenocryst assemblage of the natural dyke rock. Oxidized and H2O-rich conditions, however, suppress the crystallization of olivine in favor of magnetite and clinopyroxene with less or no alkali feldspar and nepheline formation. Accordingly, combined low fO2 and aH2O force the evolution of the residual melt toward decreasing SiO2, increasing FeO* and alkalinity index (up to 3.55). On the contrary, high fO2 and aH2O produce residual melts with relatively low FeO*, high SiO2 and a relatively constant alkalinity index. We show that variations of aH2O and fO2 lead to contrasting trends regarding the liquid lines of descent of iron-rich silica-undersaturated peralkaline compositions. Moreover, the increase in FeO* and alkalinity index (reduced and dry conditions) in the residual melt is an important prerequisite to stabilize late-magmatic minerals of the dyke rock, for example, aenigmatite (Na2Fe5TiSi6O20), coexisting with the most evolved melts at 750 °C. Contrary to what might be expected, experiments with high aH2O and interlinked high fO2 exhibit higher liquidus T’s compared with experiments performed at low aH2O and fO2 for experiments where magnetite is liquidus phase. This is because ulvøspinel-poor magnetite crystallizes at higher fO2 and has a higher melting point than ulvøspinel-rich magnetite, which is favored at lower fO2.
机译：我们实验研究了与Ilímaussaq岩体复合体（南格陵兰岛）相关的高碱性音石堤坝的相位关系。高度演化且富含铁的成分（镁数= 2，碱度指数= 1.44，FeO * = 12 wt％）可能代表Ilímaussaq复合物的母岩浆。因此，该堤坝岩石非常适合进行相平衡实验，因为与该复合体的深成岩相比，没有主要的累积形成过程使定义合理的起始成分复杂化。实验是在P = 100 MPa和T = 950-750°C的热液快速淬冷冷密封压力容器中进行的。 H2O含量范围从无水到H2O饱和（〜5 wt％H2O）和变化的fO2（〜ΔlogFMQ-3到+1；其中FMQ表示铁橄榄石-磁铁矿-石英氧缓冲液）。干燥和干燥条件的降低导致碱长石，霞石，富含黑钙石的斜辉石，富含铁橄榄石的橄榄石和少量富含ulvøspinel的磁铁矿的大量结晶，这代表了天然脉石的斑晶组合。但是，氧化和富含H2O的条件抑制橄榄石的结晶，有利于磁铁矿和斜辉石的形成，而很少或没有碱长石和霞石形成。因此，低的fO2和aH2O的结合会迫使残留的熔体向减少的SiO2方向发展，从而增加FeO *和碱度指数（最高3.55）。相反，高的fO2和aH2O产生的残余熔体具有相对较低的FeO *，较高的SiO2和相对恒定的碱度指数。我们表明，aH2O和fO2的变化导致关于富含铁的二氧化硅-不饱和过碱性组合物下降的液体线的对比趋势。此外，残留熔体中FeO *和碱度指数的增加（干燥条件的降低和干燥）是稳定堤坝岩浆后期岩浆矿物（例如铜硅石（Na2Fe5TiSi6O20））与发展最快的熔体共存的重要前提。 750°C。与预期相反，在磁铁矿为液相的实验中，与在低aH2O和fO2下进行的实验相比，具有高aH2O和相互连接的高fO2的实验显示出更高的液相线T'。这是因为贫富尔硫平的磁铁矿在较高的fO2时会结晶，并且熔点高于富富尔硫平的磁铁矿，后者在较低的fO2中受到青睐。

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