Effect of redox conditions on the evolution of magmas of Changbaishan Tianchi volcano, China–North Korea

Abstract

In this paper we use data on composition of melt, fluid, and crystal inclusions in minerals to reconstruct the compositions and formation conditions of the parental melts for the trachybasalts, trachybasaltic andesites, comendites, and pantellerites of Changbaishan Tianchi (Paektusan) volcano on the border of China and North Korea. The melts crystallized within broad temperature and pressure ranges of 1200–700 °C and 3100–1000 bar. The redox conditions expressed as ΔNNO are +0.9 to +1.4 log units for the basalt melts, −1.6 to −2.1 log units for the trachybasaltic andesite melts, −2.6 to −1.9 log units for the comendite melts, and −1.6 to −3.2 log units for the pantellerite melts. All melts are rich in alkalis and FeO. The sulfur contents of the mafic melts vary from 0.06 to 0.13 wt%. The pantellerite melts are sulfur-free, but the groundmass of the pantellerites contain pyrite crystals. Mafic melts have a range in water contents from 0.1–1.0 wt%. The H2O contents of the comendite melts vary from 3.7 to 4.8 wt%, and the pantellerite melts contain 0.2–0.6 wt% H2O. It has been revealed that the magma crystallization was accompanied by changes in oxygen fugacity caused by the decompression of the magma chamber beneath Changbaishan Tianchi volcano. Such decompression resulted in open-system crystal fractionation during magma degassing. We propose that the degassing is responsible for variations in water and sulfur contents in the melts. As the pressure decreases, the gas phases become progressively enriched in H2O and S, and the S oxidation state is changed from S2− to S4+. Sulfur degassing in the form of SO2 at decompression and spinel precipitation during open-system magmatic differentiation resulted in melt reduction.