T255 Fluids and Melts in Geologic Systems (Posters)

Tuesday, 24 October 2017
9:00 AM - 6:30 PM
Halls 4EF

REDOX PATHS OF GRANITES DEDUCED FROM THEIR TEXTURE AND MINERALS

Paper number: T255-4
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

  • Tetsuichi Takagi Presenting Author
    National Institute of Advanced Industrial Sci and Technology, Tsukuba
  • Katsuhiro Tsukimura
    National Institute of Advanced Industrial Sci and Technology, Tsukuba

Abstract

The texture and constituent minerals of granites are a useful indicator for estimating redox paths of magmas during cooling. The fO2 buffering systems of magmas are divided into gas-buffer and rock-buffer, and the former includes SO2-H2S and CO2-CO(CH4) buffers and the latter includes the buffer reactions involving redox of Fe such as fayalite-magnetite-quartz (FMQ). In the logfO2-T diagram, the inclination of SO2-H2S buffer lines are gentler than that of common rock-buffer lines, and the two lines intersect between the temperature range from 850 to 700°C (Takagi and Tsukimura, 1997 Econ Geol., 92, 81-86). SO2 gas acts as a strong oxidizing agent for rocks in the temperature range below 850°C. The temperature of the intersection becomes higher with rising oxidation states of magma. In contrast, CO2-CO(CH4) buffer lines are almost parallel to common rock-buffer lines and located the lower fO2 side than FMQ. Therefore, we can suppose the three redox paths as follows. PATH-1: following SO2-H2S buffer throughout magmatic and subsolidus stages, PATH-2: following SO2-H2S buffer in the early magmatic stage and rock-buffer in the late magmatic to subsolidus stages, and PATH-3: following CO2-CO(CH4) buffer throughout magmatic to subsolidus stages. In the magmas following PATH-1, early (primary) magnetite, euhedral titanite, zoned hornblende (increasing #mg outward), anhydrite and sulfides are present. In the magmas following PATH-2, secondary magnetite, ilmenite or anhedral titanite, zoned hornblende (increasing #mg outward), non-magmatic silicates (e.g., actinolite, chlorite) and sulfides with no anhydrite are present. In the magmas following PATH-3, no magnetite, zoned hornblende (constant or decreasing #mg outward), and pyrrhotite are present. The granites following rock-buffer throughout magmatic to subsolidus stages are rare, because sulfur is commonly abundant enough to exert SO2-H2S buffer system in highly oxidized magmas (e.g., Hattori, 1993 Geology 21, 1083-1086). Even in PATH-2, when the SO2/H2S ratios are very low (PATH-2b), the texture and minerals are similar to those of PATH-3. The redox paths are basically controlled by the volatiles in magmas, and the magmas following PATH-1 and -2 become magnetite-series, and PATH-2b and -3 ilmenite-series granites.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-301132

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