Phase relations and liquid lines of descent of an iron-rich peralkaline phonolitic melt: an experimental study
Phase relations and liquid lines of descent of an iron-rich peralkaline phonolitic melt: an experimental study
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富铁过碱性声硅熔体的相关系和液体下降线：实验研究
DOI：
10.1007/s00410-012-0809-6
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发表时间：
2013
影响因子：
3.5
通讯作者：
Nowak M
中科院分区：
文献类型：
--
作者：
Giehl C;Marks M;Nowak M
我们实验研究了与Ilímaussaq深成杂岩（南格陵兰）有关的过碱性响岩脉岩的相关系。极端演化和富铁的组成（镁值= 2，碱度指数= 1.44，FeO* = 12重量%）可能代表伊利毛萨克杂岩的母岩浆。因此，这种岩墙岩石非常适合进行相平衡实验，因为与复合体的深成岩相反，没有主要的堆积物形成过程使确定合理的起始成分变得复杂。实验在P = 100 MPa、T = 950-750 °C的水热急冷密封压力容器中进行。H2O含量范围从无水到H2O饱和（~5 wt% H2O），fO 2变化（~ΔlogFMQ −3到+1;其中FMQ代表铁橄榄石-磁铁矿-石英氧缓冲）。还原和干燥的条件导致大量的碱性长石、霞石、富含钙铁辉石的单斜辉石、富含铁橄榄石的橄榄石和少量的富含铁尖晶石的磁铁矿的结晶，它们代表了天然脉岩的斑晶组合。然而，氧化和富H2O的条件下，抑制有利于磁铁矿和单斜辉石的结晶橄榄石，很少或没有碱性长石和霞石的形成。因此，结合低fO 2和aH 2 O迫使剩余熔体的演变，减少SiO2，增加FeO* 和碱度指数（高达3.55）。相反，高fO 2和aH 2 O产生相对低的FeO*，高SiO2和相对恒定的碱度指数的残余熔体。我们表明，变化ofaH 2 O和fO 2导致对比的趋势，关于富铁二氧化硅不饱和过碱性组合物的下降液线。此外，残余熔体中FeO* 和碱度指数（还原和干燥条件）的增加是稳定岩脉岩石中晚期岩浆矿物的重要先决条件，例如，在750 °C下与最演化的熔体共存的aenigmatite（Na 2Fe 5 TiSi 6 O20）。与预期相反的是，highaH 2 O和互连的高fO 2的实验表现出较高的liquidusT的相比，在lowaH 2 O和fO 2的实验磁铁矿是液相线相。这是因为贫硅尖晶石磁铁矿在较高的fO 2下结晶，并且具有比富硅尖晶石磁铁矿更高的熔点，富硅尖晶石磁铁矿在较低的fO 2下受到青睐。
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 atP= 100 MPa andT= 950–750 °C. H2O contents ranging from anhydrous to H2O saturated (~5 wt% H2O) and varyingfO2(~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 lowfO2andaH2O force the evolution of the residual melt toward decreasing SiO2, increasing FeO* and alkalinity index (up to 3.55). On the contrary, highfO2andaH2O produce residual melts with relatively low FeO*, high SiO2and a relatively constant alkalinity index. We show that variations ofaH2O andfO2lead 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 highaH2O and interlinked highfO2exhibit higher liquidusT’s compared with experiments performed at lowaH2O andfO2for experiments where magnetite is liquidus phase. This is because ulvøspinel-poor magnetite crystallizes at higherfO2and has a higher melting point than ulvøspinel-rich magnetite, which is favored at lowerfO2.
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