Stable strontium isotope fractionation during crystal-melt separation in granitic magma evolution

Stable Sr isotopic composition (δ^88/86Sr) can be used to study magmatic processes, but their fractionation mechanism during magmatic evolution remains unclear. To understand the fractionation behaviors of the stable Sr isotopes during magmatism, we report the δ^88/86Sr values of the Huili granitic pluton, which was subjected to intensive crystal-melt separation. The Huili pluton consists of K-feldspar granite and more evolved albite granite, and the albite granite exhibits significantly higher δ^88/86Sr values (+ 0.36‰ to + 0.52‰) than that of K-feldspar granite (+ 0.11‰ to + 0.25‰). K-feldspar, which contributes most of the Sr budget of the K-feldspar granite, has slightly lower δ^88/86Sr values (− 0.01‰ to + 0.17‰) than the whole rock. The δ^88/86Sr variation of the Huili granites can be explained by separation of melt from K-feldspar-dominated crystals, because crystallization of K-feldspar can result in heavy Sr isotopic composition of the extracted interstitial melt. Stable Sr and Ba isotopic ratios in the Huili granites are highly coupled toward the heavy direction, reflecting their similar element partitioning and isotope fractionation behaviors between the crystalline K-feldspar and melt. This study indicates that melt extraction plays a key role in granitic magma evolution.