Generation of granitic melt is believed to occur predominantly by melting through the breakdown of hydrous minerals. However, melting due to the influx of H₂O has been recognized in anatectic amphibolite facies tonalitic grey gneisses, metagreywackes and low‐P metapelites, and has consequently been proposed as an alternative mechanism for the generation of granitic melt. Melting induced by H₂O addition is recognized from voluminous melt production at relatively low temperature, where hydrous minerals are stable and anhydrous minerals are preferentially consumed during melting. Mineral equilibrium modelling to determine the P–T conditions, melt volumes, melting reactions and viable H₂O sources reveals that the process is not restricted to specific compositions or P–T conditions, although lower pressure and lithologies with a low hydrous mineral content are more favourable. Melting reactions in all lithologies primarily consume quartz and feldspars to yield 5–6 mol.% melt for each mol.% of H₂O added. remains constant at ~0.70 to 0.77 during progressive melting as long as alkali feldspar is present. Once alkali feldspar is exhausted, plagioclase becomes the main reactant, producing more tonalitic melt compositions with gradually higher . Our results demonstrate that, at the site of melting, melting is driven by diffusion of H₂O into the target rock along chemical potential gradients, rather than the advective flow of a mechanically distinct water‐rich fluid phase. Melting will initiate and proceed as long as a gradient exists between the H₂O source and target lithology. Our calculations show that an ordinary magma, such as an I‐type magma with typical H₂O content, has a high enough to be a viable H₂O source, allowing diffusive H₂O‐fluxed melting to produce melt proportions and fertility comparable to that of dehydration melting. However, high degrees of partial melting require a considerable amount of H₂O, which necessitates a continuously advecting H₂O source such as a magma conduit or melt‐bearing shear zone. A magmatic H₂O source at emplacement level will undergo a similar amount of crystallization as the melt fraction produced in the target rock such that there will be no net melt production. Considering that shear‐zone hosted magma conduits are localized features, diffusive H₂O‐fluxed melting is likely to only be viable in a small fraction of the anatectic orogenic crust. Although it may play an important role in locally raising melt volumes and modifying magma chemistry through mingling and hybridization, it does not appear to, of itself, be able to generate significant volumes of granitic melt.

中文翻译：

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矿物平衡限制了大陆壳中H2O扩散扩散熔融的可行性

据信，花岗岩熔体的产生主要是通过分解含水矿物质而发生的。然而，由于H ₂ O的流入而引起的熔融已在角质闪长岩片相，灰阶片麻岩，变质灰泥岩和低P变质岩中被认识到，因此被提出为花岗岩熔体生成的替代机制。从H ₂ O的添加引起的熔化可以从较低温度下的大量熔体生产中认识到，在该温度下，含水矿物稳定，熔化期间优先消耗无水矿物。矿物平衡进行建模，以确定P - Ť条件，熔体体积，熔化反应和存活ħ ₂O来源表明，该过程并不仅限于特定的成分或P – T条件，尽管较低的压力和含水矿物含量较低的岩性更为有利。在所有岩性中，熔融反应主要消耗石英和长石，每添加1 mol％的H ₂ O会产生5 -6 mol％的熔体。只要存在长石碱，在逐步熔化过程中保持在约0.70至0.77的恒定值。碱长石耗尽后，斜长石成为主要反应物，产生更多的tonalitic熔体成分，并逐渐升高。我们的结果表明，在熔化位置，熔化是由H ₂扩散驱动的O沿化学势梯度而不是机械上不同的富水液相的对流流动进入目标岩石。只要H ₂ O源和目标岩性之间存在梯度，熔化就会开始并继续进行。我们的计算表明，普通的岩浆（例如具有典型H ₂ O含量的I型岩浆）具有足够高的强度，可以用作可行的H ₂ O源，从而允许扩散的H ₂ O助熔剂熔化，从而产生熔体比例和可比性到脱水融化。但是，高度的部分熔融需要大量的H ₂ O，因此必须连续不断地吸附H _2。O源，例如岩浆导管或熔体剪切带。处于沉积水平的岩浆H ₂ O源将经历与目标岩石中产生的熔体分数相似的结晶量，因此不会产生净熔体。考虑到剪切带托管的岩浆导管是局部特征，扩散的H ₂ O助熔融化可能仅在一小块南极造山壳中才可行。尽管它可能在局部提高熔体体积并通过混合和杂交改变岩浆化学方面发挥重要作用，但它本身似乎无法产生大量的花岗岩熔体。