Abstract Quantifying the distribution of granitic melt at all scales in mid- to lower crustal migmatitic terranes is critical to understand crustal melting processes, chemical differentiation of the crust and its rheological behavior during deformation. We propose a new method to determine the fraction of frozen granitic melt on a hand specimen scale based on the relative volumes of newly precipitated to total zircon (FPZ = Fraction of newly Precipitated Zircon) as obtained by image analysis on dated zircon cores and rims. Using the calculated Zr-solubility [Zr]sat in the melt at the inferred melting temperature and the Zr concentration in the bulk sample [Zr]bulk, the fraction of melt Fmelt can be determined through Fmelt = FPZ × [Zr]bulk / [Zr]sat. The such obtained Fmelt corresponds to the melt fraction in the hand specimen at the time the system closed for melt mobility. Thermodynamic modelling further allows estimation of H2O-contents required to maintain the melt fraction obtained from the melt-o-meter in a molten stage. The applicability of this method has been tested on eight migmatitic samples with peak temperatures between 725 and 925 °C. Most of the lower temperature migmatites ( 800 °C) retained Fmelt of 0.35–0.50 (±0.07–0.10). At these melt fractions, melt extraction and melt migration from and within the source should be efficient. Consequently, these samples are likely open-system migmatites affected by melt accumulation or depletion processes. The melt-o-meter requires that the rock types under consideration produced a granitic melt that remained zircon-saturated and is therefore restricted to migmatitic meta-sediments and meta-granitoids. When applied carefully, this melt-o-meter offers a new and powerful tool to not only quantify melt distribution but also evaluate the extent of melt mobility in migmatites.

中文翻译：

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量化地壳岩石中的冻结熔体：一种基于锆石边缘体积的新型熔体测量仪

摘要 量化中下地壳混合岩体中所有尺度的花岗岩熔体分布对于了解地壳熔融过程、地壳化学分异及其变形过程中的流变行为至关重要。我们提出了一种新方法，根据新沉淀的锆石与总锆石的相对体积（FPZ = 新沉淀的锆石的分数），通过对过时的锆石核和边缘的图像分析获得，在手标本尺度上确定冷冻花岗岩熔体的分数。使用计算出的 Zr 溶解度 [Zr]sat 在推断的熔化温度下的熔体中的 Zr 浓度和散装样品 [Zr]bulk 中的 Zr 浓度，熔体 Fmelt 的分数可以通过 Fmelt = FPZ × [Zr]bulk / [ Zr] 饱和。这样获得的 Fm​​elt 对应于在系统关闭以实现熔体流动性时手样本中的熔体分数。热力学模型进一步允许估计在熔融阶段保持从熔体o-计获得的熔体分数所需的 H2O 含量。该方法的适用性已经在 8 个混合岩样品上进行了测试，峰值温度在 725 到 925 °C 之间。大多数低温混合岩 (800 °C) 保留了 0.35–0.50 (±0.07–0.10) 的 Fmelt。在这些熔体分数下，熔体提取和熔体从源内和源内迁移应该是有效的。因此，这些样品可能是受熔体积累或消耗过程影响的开放系统混合岩。熔体测量仪要求所考虑的岩石类型产生花岗岩熔体，该熔体保持锆石饱和，因此仅​​限于混合变质沉积物和变质花岗岩。小心使用时，这种熔体测量仪提供了一种新的强大工具，不仅可以量化熔体分布，还可以评估混合岩中熔体流动的程度。