Barium isotope evidence for crystal-melt separation in granitic magma reservoirs

Abstract

Crystal-melt separation is important for understanding the evolution of granitic magma and the connection between plutonic and volcanic rocks. Crystal accumulation and melt extraction are the two key factors in the separation process. However, the existing geochemical indicators for crystal-melt separation in many granitic plutons are largely concealed by the remaining interstitial melt. Here, we report Ba isotope compositions of well-characterized Huili granitic pluton and associated mineral separates from the Jiaobei Terrane in the North China Craton. The Ba isotopes are significantly fractionated between the two types of granites in the Huili pluton. The more differentiated albite granites (AG) have heavier isotope compositions (δ138/134BaAG = 0.50–0.95‰) than the K-feldspar granites (KG) (δ138/134BaKG = −0.14 to 0.16‰). The δ138/134Ba of the whole-rocks change in the same manner as multiple element contents (such as K, Ba, and Sr). Trace element modeling suggests that such variations can be explained by K-feldspar-controlled crystal-melt separation in a K-rich granitic magma.As the K-feldspar granites represent the residual crystal mush, the Ba isotope data of the coexisting Ba-bearing minerals can impose critical constraints on crystal-melt separation. The K-feldspar shows the highest Ba content (387–1465 μg/g) and the lowest δ138/134Ba (−0.23 to 0.01‰) among all the investigated minerals, suggesting that its crystallization should drive the interstitial Na- and Si-rich melt towards an isotopically heavy composition. The apparent fractionation values of coexisting biotite and muscovite (Δ138/134BaBiotite-Muscovite) vary from −0.98 to 1.01‰, indicating that the coexisting minerals are not in Ba isotope equilibrium. Combined with microstructural features, the Ba isotope disequilibrium signatures of coexisting minerals most likely reflect crystal accumulation and repacking during crystal-melt separation, providing a novel geochemical indicator for identifying the granitic plutons that represent residual crystal mush in the mid- to upper-crust. This study indicates that Ba isotopes are useful for deciphering petrogenetic links between intrusive and extrusive rocks.