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Melting the hydrous, subarc mantle: the origin of primitive andesites

机译：融化含水的亚弧幔：原始安山岩的起源

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This experimental study is the first comprehensive investigation of the melting behavior of an olivine + orthopyroxene +/- spinel-bearing fertile mantle (FM) composition as a function of variable pressure and water content. The fertile composition was enriched with a metasomatic slab component of <= 0.5 % alkalis and investigated from 1135 to 1470 degrees C at 1.0-2.0 GPa. A depleted lherzolite with 0.4 % alkali addition was also studied from 1225 to 1240 degrees C at 1.2 GPa. Melts of both compositions were water-undersaturated: fertile lherzolite melts contained 0-6.4 wt% H2O, and depleted lherzolite melts contained similar to 2.5 wt% H2O. H2O contents of experimental glasses are measured using electron microprobe, secondary ion mass spectrometry, and synchrotron-source reflection Fourier transform infrared spectroscopy, a novel technique for analyzing H2O in petrologic experiments. Using this new dataset in conjunction with results from previous hydrous experimental studies, a thermobarometer and a hygrometer-thermometer are presented to determine the conditions under which primitive lavas were last in equilibration with the mantle. These predictive models are functions of H2O content and pressure, respectively. A predictive melting model is also presented that calculates melt compositions in equilibrium with an olivine + orthopyroxene +/- spinel residual assemblage (harzburgite). This model quantitatively predicts the following influences of H2O on mantle lherzolite melting: (1) As melting pressure increases, melt compositions become more olivine-normative, (2) as melting extent increases, melt compositions become depleted in the normative plagioclase component, and (3) as melt H2O content increases, melts become more quartz-normative. Natural high-Mg# [molar Mg/(Mg + Fe2+)], high-MgO basaltic andesite and andesite lavas-or primitive andesites (PAs)-contain high SiO2 contents at mantle-equilibrated Mg#s. Their compositional characteristics cannot be readily explained by melting of mantle lherzolite under anhydrous conditions. This study shows that experimental melts of a FM peridotite plus the addition of alkalis reproduce the compositions of natural PAs in SiO2, Al2O3, TiO2, Cr2O3, MgO, and Na2O at 1.0-1.2 GPa and H2O contents of 0-7 wt%. Our results also suggest that PAs form under a maximum range of extents of melting from F = 0.2-0.3. The CaO contents of the melts produced are 1-5 wt% higher than the natural samples. This is not a result of a depleted source composition or of extremely high extents of melt but is potentially caused by a very low CaO content contribution from deeper in the mantle wedge.
机译：这项实验研究是对橄榄石+邻苯二甲酚+/-尖晶石的肥沃地幔（FM）成分随压力和水含量变化的熔化行为的首次全面研究。该可育组合物富含≤0.5％碱的交代平板成分，并在1135至1470摄氏度和1.0-2.0 GPa的温度下进行了研究。还研究了从1225到1240摄氏度，1.2 GPa的添加了0.4％碱的贫化锂铁矿。两种组合物的熔体都是水不饱和的：可育的锂铁矿熔体包含0-6.4 wt％的H2O，贫化的锂铁矿熔体包含约2.5 wt％的H2O。使用电子探针，二次离子质谱和同步加速器-源反射傅立叶变换红外光谱法（一种在岩石学实验中分析H2O的新技术）测量实验玻璃中H2O的含量。使用这个新的数据集并结合以前的含水实验研究的结果，提出了一个热压计和一个湿度计-温度计来确定原始熔岩最后与地幔保持平衡的条件。这些预测模型分别是H2O含量和压力的函数。还提出了一种预测性熔融模型，该模型可计算出橄榄石+邻苯二甲酚+/-尖晶石残余组合物（harzburgite）平衡的熔融成分。该模型定量地预测了H2O对地幔锂沸石熔融的以下影响：（1）随着熔融压力的增加，熔融成分变得更加橄榄石规范;（2）随着熔融程度的增加，熔融斜长石成分中的熔融成分逐渐减少，和（ 3）随着熔体中H2O含量的增加，熔体变得更具石英规范性。天然高Mg＃[摩尔Mg /（Mg + Fe2 +）]，高MgO玄武质安山岩和安山岩熔岩或原始安山岩（PAs）在地幔平衡的Mg＃s上含有较高的SiO2含量。它们的组成特征不能通过无水条件下地幔锂沸石的熔融来容易地解释。这项研究表明，FM橄榄岩的实验熔体加上碱的添加可重现SiO，Al2O3，TiO2，Cr2O3，MgO和Na2O中1.0-1.2 GPa的天然PA的组成，H2O含量为0-7 wt％。我们的结果还表明，PA在F = 0.2-0.3的最大熔融程度范围内形成。产生的熔体的CaO含量比天然样品高1-5 wt％。这不是源组成耗尽或熔融程度极高的结果，而可能是由于地幔楔深处的CaO含量非常低而引起的。

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