Development of a quartz-based geothermometer for biogenic chert

In the accretionary complexes of areas where erosion is inactive, only biogenic chert is sometimes exposed and estimating diagenetic/metamorphic temperature in such areas is nearly impossible. We investigated the correlation of quartz crystallinity (CI), grain size (GS), and diagenetic/metamorphic temperature (PT) of biogenic chert and developed a quartz-based geothermometer to contribute to the advancement of tectonics in such regions. Sediments of microbial remains comprising amorphous silica can transform into biogenic chert/metachert during diagenesis/metamorphism through an increase in CI and GS. However, the relationship between CI, GS, and PT remains underexplored. By examining Paleozoic-Mesozoic biogenic chert/metachert from four regions of East Asia, we identified a systematic relationship between CI, GS, and PT. In cryptocrystalline chert (GS ≤ 1.94 μm), CI and GS exhibit a strong logarithmic relationship, whereas in crystalline chert, CI approaches its upper limit and GS widely varies. These relationships are consistent across four studied regions, irrespective of their geological age and location. CI and GS increase with increasing PT, with a positive linear relationship between CI and PT and a logarithmic relationship between GS and PT. CI and GS strongly correlate with PT (R > 0.929), making them reliable geothermometers for estimating diagenetic/metamorphic temperature.