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首页> 外文期刊>Journal of Asian earth sciences >An experimental study of the role of partial melts of sediments versus mantle melts in the sources of potassic magmatism
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An experimental study of the role of partial melts of sediments versus mantle melts in the sources of potassic magmatism

机译：钾盐岩浆作用源中沉积物部分熔体与地幔熔体作用的实验研究

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Potassium-rich lavas with K/Na of 2 are common in orogenic and anorogenic intraplate magmatic provinces. However, in the primitive mantle, the concentration of Na exceeds that of K by 10 times. The source of K-rich lavas thus needs to be either K-enriched or Na-depleted to account for high K/Na ratios. The geochemical and isotopic compositions of high Sr-87/Sr-86 post-collisional lavas show that their mantle source contains a recycled crustal component. These highly K-enriched lavas with crustal like trace element patterns are termed "orogenic lamproites" and are compositionally distinct from K-rich "anorogenic lamproites" that show lower Sr-87/Sr-86 and a trace element pattern that resembles that of primary mantle melts. For both groups the processes of K-enrichment within their source are uncertain and are thought to be linked to melts of sedimentary rocks for "orogenic lamproites" and low-degree melts of ultramafic mantle rocks for "anorogenic lamproites". In both cases, metasomatism of the mantle lithosphere is the precursor to K-rich magmatism. In this study we experimentally determine the effects of mantle metasomatism by sediment- and hydrous mantle melts. The experiments simulate the interaction of refractory lithospheric mantle and metasomatizing melt in a 2-layer reaction experiment. The sediment/dunite reaction experiments lead to formation of a strongly K-enriched phlogopite pyroxenite layer sandwiched between the two starting materials. The low temperature of the sediment/dunite reaction runs at 1000 degrees C simulates a fore-arc subduction environment, in which the melts of sediment are consumed during interaction with dunite as the temperature is below the solidus of the produced phlogopite pyroxenites. The hydrous mantle melt/dunite reaction run is simulated by reacting a hydrated basanite with dunite. Since the temperature of the reaction is higher than the solidus of the resulting phlogopite-pyroxenites (1200 degrees C), the hydrous melt is not consumed but flows further, increasing in K2O and K/Na as it reacts with the refractory peridotite. In both cases, melts are enriched in K and K/Na increases by crystallizing a low K and low K/Na eclogitic residue. Compositions of glass and phlogopite from both types of reactions are comparable to glasses and phlogopites found within post-collisional lavas. Since the enrichment of K within the reaction zone is strongly controlled by the formation of low K/Na and low-K residues, metasomatic enrichment of the mantle lithosphere in K does not need a highly K-enriched metasomatic agent.
机译：K / Na> 2的富钾熔岩在造山带和厌食板内岩浆省很常见。但是，在原始地幔中，Na的浓度比K的浓度高10倍。因此，富钾熔岩的来源需要富钾或贫钠，以解决高钾/钠比问题。高Sr-87 / Sr-86碰撞后熔岩的地球化学和同位素组成表明，它们的地幔源含有可循环利用的地壳成分。这些高度含钾的熔岩具有类似地壳的痕量元素模式，被称为“造山红闪石”，其成分与富含钾的“造山性红闪石”不同，后者显示出较低的Sr-87 / Sr-86和痕量元素模式类似于原始地幔融化。对于这两个组，其来源内的钾富集过程是不确定的，并且被认为与“造山红闪石”与沉积岩的熔体有关，以及与“造山红闪石”的超镁铁质地幔岩石的低度熔体有关。在这两种情况下，地幔岩石圈的交代作用都是富钾岩浆作用的先兆。在这项研究中，我们通过实验确定沉积物和含水的地幔熔体对地幔交代作用的影响。该实验在2层反应实验中模拟了难熔岩石圈地幔与交代熔体的相互作用。沉积物/白云母反应实验导致形成了夹在两种起始材料之间的富含钾的金云母辉绿岩层。沉积物/蒙脱石反应的低温在<1000摄氏度下进行，模拟了前弧俯冲环境，在该环境中，由于与褐云母相互作用时温度低于所生成的金云母辉石的固相线，沉积物的熔体被消耗掉了。含水地幔熔体/钙with石反应过程是通过使水合玄武岩与榴辉石反应来模拟的。由于反应温度高于所得金云母-辉石的固相线（1200摄氏度），含水熔体没有被消耗，而是进一步流动，随着其与耐火橄榄岩的反应，其K2O和K / Na含量增加。在这两种情况下，熔体都富含K，并且通过使低K和低K / Na盐残渣结晶而增加K / Na。来自两种类型反应的玻璃和金云母的组成与碰撞后熔岩中发现的玻璃和金云母相当。由于反应区内钾的富集受低K / Na和低K残留物的形成强烈控制，因此K中地幔岩石圈的交代富集不需要高度富K的交代剂。

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