Geochemical significance of lithium and boron isotopic heterogeneity evolving during the crystallization of granitic melts

We present Li and B isotope data for muscovite, biotite, and feldspar + quartz separated from two-mica granite and biotite granite samples from the Huayang-Wulong granite suite (south Qinling, central China). Our data demonstrate systematic differences in the Li and B isotopic compositions among these minerals. Our results indicate that early-crystallizing minerals have lower 67Li and 611B values than the original melt and that residual melts and late magmatic fluids may acquire anomalously high 67Li and 611B values. Furthermore, our data imply that (1) late melts and magmatic fluids do not reflect the composition of their source melt, (2) minerals that crystallized over a large segment of magma evolution may be isotopically zoned, and (3) mineral-selective alteration by late magmatic fluids camouflages the source of the fluid, whose 67Li and 611B values reflect the isotopic compositions of the altered minerals rather than the composition of the remaining rock.

Automated summary

We analyzed Li and B isotopes in muscovite, biotite, and feldspar + quartz from two types of granite samples collected in central China. Our results revealed systematic differences in isotopic compositions among these minerals. The study suggests that early-crystallizing minerals have lower 67Li and 611B values compared to the original melt, while residual melts and late magmatic fluids show anomalously high 67Li and 611B values. Additionally, our findings indicate that late melts and magmatic fluids do not reflect the composition of their source, and mineral-selective alteration by late magmatic fluids masks the source of the fluid.