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The thermodynamic equilibrium dihedral angle at grain junctions in crystalline rocks is set by the grain boundary interfacial surface energies, but the long times required to attain equilibrium mean that the observed dihedral angles in igneous rocks are generally set by the kinetics of crystallisation. We distinguish three types of augite-plagioclase-plagioclase dihedral angle in mafic igneous rocks. In the first, augite grows in the pores of a pre-existing plagioclase framework accompanied by little to no inwards-growth of the plagioclase pore walls. In the second, the plagioclase pore walls grow inwards simultaneously with the augite, and the dihedral angle is generally larger than the original angle at which the two plagioclase grains impinged except when the impingement angle itself is large. The first type is seen in rapidly crystallised rocks, whereas the second is observed in slowly cooled rocks. The third type is highly asymmetric and resembles (and so we call) an eagle's beak: it is only seen in slowly cooled rocks. It is common in gabbroic cumulates, and is also present in strongly orthocumulate troctolites. Using the mode of interstitial phases to calculate the amount of interstitial liquid present in a series of mafic cumulates from the Rum and Skaergaard layered intrusions, we show that the asymmetry of three-grain junctions in troctolites increases as the rocks progress from adcumulate to orthocumulate (i.e. as the olivine-plagioclase crystal mush becomes more liquid-rich), with eagles' beaks becoming the dominant three-grain junction geometry for troctolitic mushes containing ~ 12 vol.% interstitial material (corresponding to ~ 30 vol.% liquid in the mush). The geometry of three-grain junctions in mafic rocks is thus a function not only of cooling rate, but also of the progression along the liquid line of descent during fractionation. The first two types of junction are formed in relatively primitive liquids, during which the crystal mushes on the margins of the solidifying magma body are formed predominantly of plagioclase and olivine, whereas the eagle's beak geometry occurs once augite forms an important component of the crystal framework in the accumulating mush, either because it is a framework-forming primocryst phase or because it grows from highly abundant interstitial liquid., Author(s): Marian B. Holness [sup.1], A. C. Fowler [sup.2] [sup.3] Author Affiliations: (1) grid.5335.0, 0000000121885934, Department of Earth Science, University of Cambridge, , Downing Street, CB2 3EQ, Cambridge, UK (2) [...]

How and what extent residual garnet in the source and garnet fractionation contribute to the heavy Fe isotopic compositions in high-silica igneous rocks remain unexplored. We here measured the Fe isotopic compositions of garnet-bearing leucosomes, garnet-free leucosomes, garnetites, garnet-amphibolites, eclogites and their constituent minerals in the North Qaidam ultrahigh pressure metamorphic belt. Garnet-amphibolites were derived from the retrogression of eclogites. Garnet-bearing leucosomes were formed by partial melting of eclogites with residual garnet in the source, while garnet-free leucosomes and garnetites originated from garnet-bearing leucosomes by garnet fractionation and accumulation, respectively. Iron isotopic measurements show that eclogites and garnet-amphibolites have similar Fe isotopic compositions with [delta].sup.56Fe of ~ 0.05â°, indicating that fluids involved in retrograde metamorphism are internally buffered and thus did not modify the Fe elemental and isotopic budget of metamorphic rocks. Garnet-bearing leucosomes have heavier Fe isotopic compositions ([delta].sup.56Fe ~ 0.09â°) than eclogites and garnet-amphibolites, consistent with residual isotopically light Fe-rich garnet in the source and the preferential extraction of isotopically heavy Fe-rich omphacite into melts during eclogite partial melting. The [delta].sup.56Fe values of garnet-bearing and garnet-free leucosomes increase with the increase of SiO.sub.2, Fe.sup.3+/ΣFe and (Na + K)/Al, and with the decrease of FeO.sub.T, suggesting that isotopically light Fe-rich garnet fractionation, together with compositional changes of magmas, controls the observed Fe isotopic variations. Rayleigh-fractionation modeling estimates Fe isotopic fractionation factors between garnet and felsic melts to be [alpha].sub.garnet-felsic melt = 0.999958-0.999988. Such small Fe isotopic fractionation factors suggest that except for garnet fractionation, fractionation of other isotopically light Fe-rich minerals (e.g., ulvospinel-titanomagnetite, ilmenite, biotite) is also required to explain the extremely heavy Fe isotopic enrichments observed in high-silica igneous rocks worldwide., Author(s): Wenli Liang [sup.1], Jian Huang [sup.1] [sup.2], Guibin Zhang [sup.3], Fang Huang [sup.1] [sup.2] Author Affiliations: (1) grid.59053.3a, 0000000121679639, CAS Key Laboratory of Crust-Mantle Materials and Environments, School of [...]